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Hypothesis
Relaxation: Molecular and physiological significance
Authors’ Contribution:
A Study Design
B Data Collection
C Statistical Analysis
D Data Interpretation
E Manuscript Preparation
F Literature Search
G Funds Collection
George B. Stefano1 DEG, Gregory L. Fricchione2 EF, Tobias Esch3 EF
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Received: 2006.06.19
Accepted: 2005.07.03
Published: 2006.09.01
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Neuroscience Research Institute, State University of New York College at Old Westbury, NY, U.S.A.
Department of Psychiatry, Massachusetts General Hospital, Boston, MA, U.S.A.
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Division of Integrative Health Promotion, Coburg University of Applied Sciences, Coburg, Germany
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Source of support: Supported in part by grants MH 47392 and DA 09010 (GBS)
Summary
There appears to be a molecular process for relaxation. Given this, we attempt to demonstrate
this phenomenon based on established molecular and physiological processes in light of our current understanding of central and peripheral nervous system mechanisms. Central to our hypothesis is the significance of norepinephrine, nitric oxide, dopamine and morphine signaling both
in the central and peripheral nervous system. We find that nitric oxide and morphine control catecholamine processes on many levels, including synthesis, release and actions. We conclude that
enough scientific information exists to support these phenomena as actual physical processes that
can be harnessed to provide better patient care.
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References:
relaxation response • nitric oxide • belief • limbic system • norepinephrine • morphine •
dopamine
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© Med Sci Monit, 2006; 12(9): HY21-31
PMID: 16940938
Author’s address:
Dr. George B. Stefano, Neuroscience Research Institute, State University of New York College at Old Westbury,
Old Westbury, NY 11568, U.S.A., e-mail: [email protected]
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Med Sci Monit, 2006; 12(9): HY21-31
One may speculate that cognitive abilities arose or evolved
because highly complex sensory-motor integration mechanisms were already in place (an organized brain) on which
this “highly developed” coping strategy could be based.
Moreover, cognitive coping uses this foundation as its operating platform. That is, cognition must be able to activate
normal non-cognitive stress phenomena as well as deactivate them at the appropriate time, i.e, relaxation. Indeed,
the subtleties of the integration would be hard to discern, as
would the exact stimulus of a complex sensory experience.
Yet, this would not take away from the existence of the connection/integration, it only further dramatizes its complexity. Thus, for example, we can predict that both cognitive
and non-cognitive coping would be able to influence immune phenomena via neuro-immune, vascular-immune and
neuro-vascular mechanisms. Based on the above, the mind
depends on the underlying neural substrates to manifest itself. Thus, both consciously and unconsciously, it should be
able to influence its underlying foundation.
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It is reasonable to propose that our bodies contain naturally occurring antibiosenescent, health maintaining and antistress processes, involving immune, vascular and neural systems, that serve to maintain our life for a reasonable period
of time, and that the vigor of these processes, in part, determine our mean life span. In most mammals, once these
protective systems diminish in their capacity, their reproductive life has also ended. In man, however, in part aided
by our integrative capacity, our life span is extended well
beyond our reproductive years. It would not be surprising, therefore, to find critical neuronal processes linked
to man’s cognitive ability that have the ability to promote
health. These constitutive processes would manifest themselves above background activity during times of need, e.g.,
stress, when an increase in a health-related cognitive stimulus initiates this innate, non-cognitive protective neural
process to become evident. We speculate that the ability to
relax is a major physiological process, which emerges when
“conditions” are appropriate.
or regulate the many individual neural processes that potentially summate a decision-making process. That is, the brain
represents only neural tissues organized into various neural
patterns that can work together or separately. Without a unifying component being able to cope with a focus, the significance and uniqueness of this coping strategy would be lost.
These individual processes (storing sensory information and
motor responses in many brain compartments, along with
the multitude of simultaneous integration processes) are extremely complex and varied. Moreover, a unified entity, a
“mind”, would only be involved with experience-related phenomena (both exteroceptive and interoceptive) since this is
the realm in which coping strategies are designed [4].
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BACKGROUND
WHAT IS RELAXATION?
For more than 30 years, Herbert Benson and colleagues, building on the work of Swiss Nobel Prize laureate Dr. Walter R.
Hess, have described a physiological response, termed the “relaxation response”, that they describe as being the opposite
of the stress response [1]. It results in decreased metabolism,
heart rate, blood pressure, and rate of breathing, as well as a
decrease in brain activity [2]. Recently, we have added nitric
oxide (NO) as a molecule involved in this learned response
[3–5]. Clearly, this response embodies actions that one would
associate with relaxation. We surmise the relaxation response
is part of a larger physiological process, which can simply be
referred to as relaxation. In this regard, we also surmise that
the ability of a practitioner or person to elicit the relaxation
response is strengthened by the trust or belief in the expected
outcomes as well as the belief that a particular environment
is safe or suitable to relax [6]. In fact, the strength of the fiduciary relationship between physician and patient appears
to play a direct role in the effectiveness of medical treatment
[4,6–10]. Thus, it would appear that there are larger physiological processes allowing for relaxation to occur. However,
the exact mechanism or combination of cascading mechanisms involved until now has escaped detection.
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Hypothesis
WHAT IS THE MIND?
Before going further we must define the term “mind.” For
us, it was enticing to think that the chance alteration of genetic or neural pathways leading to cognitive processes also
provided such endowed animals with an additional survival
coping strategy [11] and that these cognitive coping abilities provided such organisms with a competitive edge for
survival. The burgeoning of cognitive theory and therapy
in the recent past is testimony to the insight that altering
and improving cognitive coping mechanisms can help dissipate the emotional ravages attendant to the stress response.
Interestingly, cognition may also contribute to stress when
it cannot be turned off [12].
In order for cognitive ability to develop and succeed, however, there must first be a unifying consciousness to control
Belief has an emotional component in that the brain motivation and reward circuitry will be reinforced with a positive
emotional valence attached to the believed in person, idea or
thing [4,7–9,12,13]. This emotionalized memory, replete with
“somatic markers”, i.e., bodily sensations that accompany emotion and set the feeling tone, “feels right” to the person [14].
Clearly, emotion can be viewed as a process reinforcing a belief so that rationality cannot “weigh” the belief down into a
lack of activity (see [11,15]). Indeed, belief in regard to a therapy, doctor, or personal religion may stimulate physiological
processes, enhancing naturally occurring “healthy” processes
by augmenting their level of performance [11,15].
Furthermore, belief, trust, pleasure and love via limbic as
well as other central nervous system (CNS) and peripheral
nervous system (PNS) processes (see [4,7–9,13,16] for details on the hard wiring of these processes) are involved in
relaxation because they are critical factors in the conscious
or unconscious decision making that takes place in determining whether it is safe or not safe to relax (Figure 1). Given
this, can a person’s belief in an expected outcome actually
affect the expected outcome? We believe the answer is yes,
barring organic disease in the sensory, integrative or motor component of the processing pathway.
Thus, cognitive and non-cognitive neural processes originating in the brain/mind may communicate through simple signaling pathways to intimately link the CNS and the
diffuse immune system, including vascular components.
While many of the body’s processes occur without cogni-
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Stefano GB et al –Neural processes in relaxation
Cholinergic triggerNicotine
enhances
morphine
release
Dopamine
First and Always
Conscious and Unconscious
Limbic Involvement
Sympathetic Response
Flight or Fight
Stress Response
Norepinephrine
Rapidly Inhibits
Morphine
If appropriate – depends
Nitric oxide on safe environment-Activation
tive input, the use of cognitive intervention or awareness
as a coping strategy allows us to manipulate non-cognitive
processes, implying that the “mind” can intervene and impose change in physiological systems. Indeed, this change
can be both beneficial and pathological. Non-cognitive
processes, therefore, may exist to promote health and cognition itself may stimulate these health-promoting mechanisms when properly activated or called upon.
Clearly, this type of “hard” non-cognitive and “soft” cognitive
linkage is instrumental in the concept of the mind-body phenomenon. We believe it is also quite important in understanding relaxation, since it also may represent a physical manifestation of this phenomenon. Admittedly, these processes are
called on in many animals in a very unconscious way, indicating that they may be of primal origin, i.e., antibiosenescent.
STRESS
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Relaxation
Figure 1. As described in the text, it is surmised
that the adrenergic/sympathetic system
is always on, maintaining an alert
status, which provides high survival
value. However, after cognitive and
non cognitive limbic associated neural
processes have determined it is safe
to reduce the level of alertness, i.e.,
relax, relaxation can proceed. This
latter process may involve a cholinergic
trigger, switching to morphinergic
and subsequent nitric oxide signaling.
This signaling suppresses further
catecholamine metabolism to
norepinephrine while simultaneously
liming its signaling capabilities, i.e.,
nitric oxide inhibits dopamine beta
hydroxylase. Thus, relaxation appears
to result from general inhibition of the
active sympathetic tone.
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CNS
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Med Sci Monit, 2006; 12(9): HY21-31
Stress describes a challenge that forces biological organisms to react in order to adapt (keep balanced) and stay
“healthy”, i.e., survive [17]. Simply defined, stress represents an event or stimulus that alters the existing immediate organismic homeostasis or “allostasis” [18]. Through
an extremely complicated allostatic process, all living organisms maintain their survival in the face of both externally and internally generated “stressors”. This apparent
harmonization is constantly challenged often to the point
of threat [12,17,19–24]. Thus, the stress responses (physiological processes that occur in the face of stress, e.g., fightor-flight response) can be viewed as being highly protective. The broad spectrum of stimuli capable of engaging
the stress response is remarkable and reflects how well integrated our perceptions of the physical, psychological and
social worlds are [25]. Biochemical (neurotransmitter, peptides, steroids), physiological (heart rate, blood pressure)
and behavioral (anxiety, depression, tension) concomitants
of stress may co-mediate a disease response [26].
Another important element of stressful stimulation may be
the duration or time component of the noxious stimulus
[20,27]. A brief physical or mental “assault” may allow an
organism, through various detailed allostatic compensatory
mechanisms, to “deal” with both an appraised or perceived
stress. If the situation were to continue chronically, the organism might become vulnerable, susceptible to negative aspects of the stress response, such as in the case of prolonged
immune down-regulation [28–32]. Moreover, our physiological and psychological stress response “systems” plainly
function or were designed to function over the short term,
i.e., fight or flight, not for prolonged periods of time. Given
the signal molecule commonalties and similarities found
in diverse organisms during the course of evolution, not
to mention the common design of animal nervous systems
regardless of phyla [24,33–38], it is not surprising to learn
they also similarly exhibit stress responses [24].
The pathological effects of stress are those induced by longterm stress. This is what Hans Selye referred to as the “general adaptation syndrome”, when a particular stressor (highly emotional situation, physical abuse, etc.) remains for a
long period of time [39]. Here, the persistent elevated stress
response causes the system to function at its full capacity.
This cannot continue for extended periods of time without
metabolic detriment to the organism [17,22,23,40]. Thus,
short-term stress processes are beneficial because we can
overcome a particular obstacle. Long-term expression of
these processes can be viewed as detrimental. Our physiologic systems are not designed for long-term stress, such as
prolonged immune compromise. We may also bring upon
ourselves a long-term stress resulting from our perception of
the stressor itself. Perhaps, with the appearance of cognitive
appraisal capabilities, human beings were, as a side effect,
able to translate the short-term stress process into a longerterm stress process simply by thinking about it and moreover dwelling on it (such as contemplating a boss firing you
for several months, cognitive “constipation”) [12].
Furthermore, as just noted, chronic stress can impact many
physiological systems. In part, the reason for this may be that
at the core of many disorders one may find a proinflammatory situation that manifests itself in diverse tissues, differ-
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DA, Various cellular compartments
Requires
Excitation/Activation
NE
Norcocolarine
Reticuline
Salutaridine
Thebaine
Codeine
Morphine
Morphine 6 glucuronide
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E
Requires down regulation/calm,
restore homeostasis
Activation-Maintain a state
of high alertness, energy
Down regulate the activation,
restore homeostasis
ently masking the commonality [41]. In this regard, various
integrative medical techniques may be efficacious because
they too exhibit commonalities [7–9] not only in regard to
shared signal molecules, i.e., opioid, and neural circuit, i.e.,
limbic, components but their ability to depend on trust and
belief, inducing a state of relaxation. Moreover, it may be
the ability to induce relaxation that breaks the negative impact of chronic stress [12]. Thus, it is not surprising to find
that integrative/complementary medical techniques appear
to have a general global impact on diverse disorders [42–
44] given the relationship they have with stress, i.e., calming or stress reduction.
RELAXATION
Figure 2. The “Ying-Yang” nature of the excitation
and relaxation pathways manifests
itself in the biochemical pathway for
synthesizing dopamine and morphine,
both arising from common precursors.
Despite the common precursor one
pathway initiates activation whereas
the opiate pathway initiates down
regulation and mediates pleasure and
reward signaling that can only emerge
once no life threatening phenomena
are perceived. Given this common
component of both pathways we
surmise that endogenous auto regulators
determine which activity emerges. This
may also help explain the significance
of emotion as a trigger for this pathway
whose outcome depends on what
emotion is evoked. It also, based on
neuroanotomical data, provides a critical
understanding for profound behavioral
modifications that can occur in addiction,
for example
[7,8,13,42,43,112,117,118,143].
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DOPA Tyramine
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Med Sci Monit, 2006; 12(9): HY21-31
Tyrosine
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Hypothesis
(cNOS) is stimulated, NO release occurs for a short period
of time, but this level of NO can exert profound physiological actions for a longer period of time [32,45]. NO is not
only an immune, vascular and neural signaling molecule, it
is also antibacterial, antiviral, scavenges free radicals, and it
down-regulates endothelial and immunocyte activation and
adherence, thus performing vital physiological activities,
including vasodilation [32,46–48]. Thus, NO has the potential to protect an organism from microbes and physiologic
disorders such as hypertension, and also diminishes excessive immune and endothelial activation [3,46]. Indeed, its
constitutive presence may set the tone for the activation of
these cells and its absence or presence at lower levels may
set the stage for progressive deterioration, i.e., Alzheimer’s
Disease (see [3,45,49]).
NO activity
Briefly, we surmise that relaxation represents an equally real
physiological process, allowing for physiological recovery
and promoting health by terminating the other physiological process normally associated with stress (Figure 1). In this
regard, it represents a naturally occurring proactive protective mechanism that, once stimulated, provides a beneficial
outcome for the individual invoking the process [42–44]. We
surmise that the molecular components of this process must
be constitutively expressed so that it can continually be “felt”
as well as be stimulated to rapidly increase its desired beneficial effect. This aspect of relaxation is important in terminating, for example, alert states such as found in the stress
response once a perceived threat ends (Figure 1). The inability to invoke relaxation may be an important factor in allowing chronic stress to gain a foothold in a person’s mind
translating into pathophysiological disorders [12].
Background molecular processes
We surmise constitutive NO signaling is critical in relaxation [12,32,40]. When constitutive nitric oxide synthase
The significance of cNOS-derived NO may also be ascertained by the number of chemical messengers that can stimulate its production. For example, the endocannabinoids,
anandamide and 2-arachidonyl-glycerol (2-AG), are naturally occurring cNOS-derived NO-stimulating signaling
molecules that are also constitutively expressed [32,50–61].
Anandamide, an endogenous endocannabinoid, can also
cause NO release from human immune cells, neural tissues
and human vascular endothelial cells [53,62]. Anandamide
can also initiate invertebrate immune cell cNOS-derived
NO [50].
Estrogen can also stimulate cNOS-derived NO in human immune and vascular cells [62–64]. It can also perform this
function in invertebrates, stimulating cNOS-derived NO release from neural tissues [65,66].
Recent work from our laboratory has revealed that morphine
can be made by normal healthy animal cells, including human cells [67,68] (Figure 2). This supports the previously found opiate receptor subtype μ3, cloned from human
immune, vascular and neural tissues, which is selective for
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Stefano GB et al –Neural processes in relaxation
Morphine
μ3
Blood Vessel
Endothelium
Nitric oxide
WBC
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Neuron
Vasodilation
Down regulate proinflammatory events
Antibacterial, antiviral, scavenges free radicals
Inhibits norepinephrine synthesis
Inhibits immunocyte adherence
Inhibits smooth muscle contraction
Activates Reward Centers - Feels Good
Calms Nervous tissues
Figure 3. Peripheral manifestations of molecular
components of relaxation. In order to
be effective and have the potential to
exert a therapeutic outcome relaxation
must be broad in its ability to induce
widespread down regulation while
simultaneously not completely altering
an animals excitatory alert processes
to the point where they are rendered
useless. Hence it not surprising to find
the mu3 opiate receptor, which is opiate
alkaloid selective and opioid peptide
insensitive, present on diverse tissue
types where it is coupled to constitutive
nitric oxide release, which we also have
linked to global down regulation and
thus, exerting therapeutic actions that
are equally diverse
[42,43,43,118,119,144,145].
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CNS and Adrenal Gland
Acetylcholine
the opiate alkaloids morphine and morphine-6-glucuronide and not opioid peptides [69], demonstrating a specific morphinergic signaling mechanism in animal tissues.
Furthermore, μ3 opiate receptors stimulated by morphine
are coupled to cNOS-derived NO release, resulting in the
down-regulation of immune, vascular, gut and neural tissues [3,70–74]. Thus, besides well known cNOS-derived NO
stimulators, i.e., acetylcholine (ACh) [75,76], newer chemical messengers have been discovered, which are also associated with NO coupling.
Why are there so many pathways that lead to cNOS-derived NO release? We believe that each signaling system
performs this common function under different circumstances. Endogenous morphine [32], given its long latency
before increases in its levels are detected, arises after trauma/inflammation and, through a NO mechanism, downregulates these processes in neural, vascular and immune
tissues [28,55,77]. Anandamide, as part of the ubiquitous
arachidonate and eicosanoid signaling cascade, serves to
maintain and augment tonal NO in vascular tissues [32,59].
Estrogen, through NO release, provides an additional pathway by which the system can down-regulate immunocyte and
vascular function in women [62]. This may be due to both
the immune and vascular trauma associated with cyclic reproductive activities, such as endometrial buildup, when a
high degree of vascular and immune activities are occurring. Given the extent of proliferative growth capacity during peak estrogen levels in this cycle, NO may function to
enhance down-regulation of the immune system to allow
for these changes. Clearly, therefore, enhanced cNOS activity is beneficial within the concept and time framework
of relaxation.
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Med Sci Monit, 2006; 12(9): HY21-31
Proposed signaling in relaxation
Individuals who are relaxing experience peripheral vasodilation, warming of the skin, a decrease in heart rate and an
overwhelming sense of well-being only when this can occur
in a safe and trusted environment (see [4]). Counter-intuitively, there may be initial sympathetic activation, as noted
by norepinephrine (NE) levels, which initially go up [78].
This appears also to be the case for falling in love and enjoying pleasurable experiences, since this does, for example, represent the risk of rejection [8,13,79,80]. Regarding
the vasodilator peripheral heat-warming processes, we
speculate that this involves NO [4]. In regard to the sense
of well-being, we can assume that this process may also involve opioid signaling molecules, which are involved with
reward processes [8].
In examining a potential mechanism for relaxation, besides
the over-riding CNS output via the autonomic nervous system, peripheral neuro-vascular processes would appear to
be important [4] (Figure 3). We surmise NO to be of fundamental importance because of the increase in peripheral temperature, i.e., vasodilation [81]. With reference to
the peripheral vasculature, we find nerve terminals in the
vessels that when stimulated by nicotine, result in vasoconstriction followed by vasodilation [82], indicating a cholinergic mechanism. Clearly, this phenomenon is in line with
new data demonstrating nicotine stimulation of endogenous
morphine from nervous tissue, resulting in morphine NO
coupling and vasodilation, i.e., relaxation [78,83–86]. The
vasoconstriction component of the biphasic nicotine effect
is mediated by alpha-1-adrenoceptors stimulated by NE liberated from peripheral sympathetic adrenergic nerves [4].
Studies suggest that, because of insensitivity to atropine, ACh
does not mediate the nicotine-induced vasodilation [82].
Instead it is mediated from nerve endings in which a NO
generating system and ACh may coexist [87]. Thus, nicotine
stimulates the adrenergic and nitroxidergic nerves innervating, for example, the temporal arterial wall of denuded
endothelium in superficial dog tissue, resulting in contraction and a rapidly developing vascular relaxation, the latter
being mediated by cyclic GMP (cGMP) [88]. Slow relaxation caused by nicotine is associated with the elevation of
cGMP production via activation of guanylate cyclase, which
appears to be mediated by prostaglandin I2 [88].
However, newer experimental results allow us to modify this
interpretation. Namely, nicotine can stimulate the release of
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Med Sci Monit, 2006; 12(9): HY21-31
ly, the relaxation response [78] may, in a sense, represent
the left over flow of the earlier mild sympathetic stimulation, i.e., also representing work initiation in a different situation, i.e., stress response.
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Complicating this matter is the data indicating that ACh
inhibits, acting via prejunctional muscarinic, not nicotinic receptors, the synthesis and release of NO while concurrently antagonizing the release of NE [92,95]. However, the
response to exogenous NO is not influenced by ACh [93].
In addition, the secondary vasodilation to electrical nerve
stimulation appears to be attenuated by treatment with ACh
in a concentration-dependent manner [93]. These findings
suggest that ACh plays an essential role in vascular NO regulation (Figure 3). It may be the factor adjusting the interaction of NO and NE, simultaneously exerting its own vascular
action, i.e., stimulating endothelial NO release maybe via the
release of endogenous morphine [83,89,106,107] (Figure
3). Others found that beta(2)-adrenoceptor antagonists
blocked the relaxation induced by nicotine. Furthermore,
they demonstrated that beta(2)-adrenoceptor immunoreactivities and NADPH diaphorase reactivities were colocalized in the same nerve fibers in basilar and middle cerebral
arteries [103]. The authors speculate that NE acts on presynaptic beta(2)-adrenoceptors located on the NO nerve terminals to release NO resulting in vasodilation.
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endogenous morphine from adrenal tissues and white blood
cells, adding this opiate alkaloid into the immediate environment [83,89,90]. It is critical to note that, as documented
earlier, endogenous morphine signaling is coupled to cNOSderived NO release in these tissues. In individuals shown to
relax via lowering their blood pressure by listening to music, higher opiate alkaloid levels were found in their plasma
along with a significant alteration of the μ3 receptor levels
on blood cells and a lowering of plasma proinflammatory cytokine levels (i.e., immune down-regulation) [86,91], demonstrating the involvement of opiate alkaloids in this ability
to relax at the peripheral level. Supporting this hypothesis
further is the vasodilator effect, which requires NO, resulting
in the lowering of blood pressure and the ability of morphine
to release NO from immune and vascular cells as discussed
earlier. This release may be the actual event contributing to
the observed vasodilation and increase in peripheral skin
temperature associated with relaxing (Figure 3).
Taken together, we surmise that NE initially promotes a slight
vasoconstriction of the artery during the initial phase of relaxation (“getting in the mood”), indicating a slight enhancement of sympathetic activity upon stimulation (Figure 1).
This may also serve to demonstrate that sympathetic tone always manifests itself until its influence is diminished. This is
immediately followed by the release of NO from the nitroxidergic nerve or from an immune and/or vascular source,
which mediates a concentration-dependent vasodilation. In
monkeys, the cerebral arterial diameter under resting conditions is maintained by tonic release of NO from the nerve
(10–20%) or from the nerve and endothelium (30%) [92].
This observation is supported by other data from our laboratory since basal NO is cNOS-derived and keeps particular types of cells in a state of inhibition [3]. Endogenous
superoxide dismutase (SOD) in the cerebral artery appears
to protect the relaxation, induced by NO from perivascular
nerves, from the NO scavenger action of superoxide anion
[93]. We surmise that this NO then produces the longer-lived
phenomenon of smooth muscle relaxation. In another report, it was found that NE vascular hyper-responsiveness in
hypertension is dependent on an impairment of NO activity that is realized through NE-induced oxygen free radical
production [94], demonstrating an important contribution
to the understanding of this regulatory process.
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Hypothesis
In regard to a nervous origin of relaxation, the location of
the NO-releasing nerve, nitroxidergic nerve, has continued
to be a subject of debate. Various findings lead us to believe that the nitroxidergic nerve is located in the proximity of the adrenergic sympathetic nerve bundle [88,95–101].
Furthermore, the NE-stimulated vasoconstriction is followed
by relaxation that is suppressed by L-NA, a NOS inhibitor [102], supporting its interaction via a NE mechanism
[103]. Secondly, nicotine-induced relaxation is abolished
by guanethidine, tetrodotoxin, and pretreatment with 6hydroxydopamine, all causing destruction of the sympathetic nerve, and demonstrating again the NE component
[104]. Furthermore, we have demonstrated that eNOS-derived NO can inhibit NE neural release in animal vasculature [53]. Recently, we have also demonstrated that, once
NO is present, smooth muscle cells from rat and human
arteries fail to contract in response to NE [105], demonstrating that when the balance shifts to NO, NE cannot initiate vasoconstriction. Thus, the NE reported in, specifical-
We also surmise, based on current studies, that endothelialderived NO, released through normal pulsations due to vascular dynamics responding to the heart beat [32,73,108], as
well as ACh-stimulated endothelial NO release (via nicotinic
processes), may contribute to the effect of NO in inhibiting
NE processes as well as inducing smooth muscle relaxation.
Furthermore, vascular pulsations may be of sufficient strength
to also stimulate neuronal NOS-derived NO release, limiting
any basal NE actions. Interestingly, nitrosative stress, mediated by involvement of the reactive nitrogen oxide species
N2O3, does inhibit dopamine-b-hydroxylase, inhibiting NE
synthesis and contributing to the regulation of neurotransmission and vasodilatation [109,110]. This system may provide an autoregulatory mechanism involved in the neuronal control of peripheral vasomotor responses. Furthermore,
the ability of nicotine to release morphine from adrenal and
immune tissues helps explain the source of NO [83,89,90],
which results from the newly released morphine and μ3 coupling to NO release on these critical tissues.
In conclusion, relaxation peripherally appears to be mediated by a system of regulation involving NO, NE, ACh and morphine as neurotransmitters and local hormones. Contingent
on the preliminary vasoconstriction and depolarization of
the membrane initiated by the release of NE, vasodilation is
mediated by NO liberated from vasodilator nerves and maybe
other tissues that activate guanylate cyclase in smooth muscle and produce cGMP. During this stage, NO and NE exist simultaneously. Due to the signaling cascade of NO, NE
no longer mediates vasoconstriction. Instead, NO activates
guanylate cyclase, which produces vasodilation and the relaxation under a depolarized membrane state.
CNS involvement
The CNS regulating pathways and processes integral to mediating this process stimulate NE release either alone or in
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ated release of NO from rat brain hippocampus and amygdala and endogenous morphine’s presence in these tissues
[117]. Furthermore, DA can serve as a endogenous morphine precursor [67,68] (Figure 2).
HY
Supporting this hypothesis are other studies linking relaxation characteristics with that of pain. Once individuals are
exposed to painful stimuli such as a penetrating needle they
experience peripheral vasodilation, warming of the skin
[123–126], an increase in heart rate and a sense of agitation, all as a potential result of circulating catecholamines
and/or NO [127]. It is the function of the amygdala and
related subcortical regions to aid in the relief of these pain
states [113,128,129]. In examining a potential mechanism
for this relief, besides the over-riding CNS output via the
autonomic nervous system, peripheral neuro-vascular processes would appear to be important. Again, we surmise that
NO and its relation with prostaglandins are of fundamental importance in this response because of the increase in
peripheral temperature, i.e., vasodilation, in the ensuing
inflammatory response [123–126]. Indeed this is precisely the pathway, which is inhibited with non-steroidal antiinflammatory drugs (NSAID). Interestingly, as noted earlier, N2O3 does inhibit dopamine hydroxylase, inhibiting
NE synthesis and contributing to the regulation of neurotransmission and vasodilation [4,110]. This system may provide an autoregulatory mechanism involved in the neuronal
control of peripheral vasomotor responses as they relate to
pain control, where induction of a catecholamine-inhibiting substance may autoregulate the vasomotor response in
response to nitrosative stress (see [130] for the role of nitrosative stress in neuropathic pain).
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There are several key areas of the CNS involved in relaxation processing. Subsequent to thalamic routing, there is
quick transit of crude data to the amygdala and a slower
more considerate flow of information to the primary sensory cortex, with subsequent routing of more refined information to the amygdala and the hippocampus, areas classically involved in emotion and memory – critical functions
of the so-called limbic system of the brain (Figure 1). The
quick route allows the organism to be rapidly responsive to
a fearful prompt. The more sophisticated appraisal route
permits the limbic system to operate with more reliable information that may allow the amygdala to stand down and
for relaxation to re-emerge if the decisions to be made are
safe and clear cut ones. If the decisions to be made are not
clear cut and are conflicted in some way the anterior cingulate cortex must make a response selection based on the
cognitive data from cortical regions and the emotional data
from the limbic system [111]. Hyperactivation of the anterior cingulate as seen in functional neuroimaging of depression and in pain states can be modified when the prefrontal cortex is stimulated by conscious positive expectation
and the relaxation that accompanies it [112].
Stefano GB et al –Neural processes in relaxation
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combination with NO (see [4]). If NE is released alone, vasoconstriction occurs. If NE is released with NO, we surmise
an initial short-lived vasoconstriction occurs followed by a
prolonged vasodilation mediated directly by NO. In this regard, if basal/tonal NE is present, NO overrides this effect
(Figure 1). If ACh is present, we surmise that its inhibition
of NE, causing loss of sympathetic tone, and its stimulation
of endothelial-derived NO, may also result in vasodilation.
The important point at this stage of explaining relaxation
is that, at last, we are beginning to see a mechanism that
can explain its characteristics and simultaneously provide
an explanation for its health benefits, i.e., via cNOS-derived
NO (see [3,32,45,47,48]).
Hence, it is no surprise that these limbic and paralimbic
areas are of great importance to relaxation, as well as pain
modulation. We surmise the proximity and shared “wiring”
of these two functions represents a critical point whereby
evaluations are made on whether or not the situation is appropriate to relax. This proximity and sharing of common
neural circuits is a critical survival element in animal evolution, allowing excitation to take over rapidly should the
situation arise. Importantly, the central nucleus of the amygdala is most strongly modulated by dopamine (DA), NE,
epinephrine and serotonin [113,114]. The basal nuclei
receive moderately high inputs of DA, NE and serotonin
[113,114], each of which has been demonstrated to exert
their desired effect, in part, via NO [115–119].
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Med Sci Monit, 2006; 12(9): HY21-31
We surmise that the centrally released NE exerts effects in
the periphery via cardiovascular circuitry [4] by initially promoting a slight vasoconstriction of the peripheral artery during the amygdalar response to relaxation input as part of the
limbic system’s inherent mechanism to maintain homeostasis and decrease pain perception via descending spinal pathways resulting in endogenous opiate analgesia [115,116].
In fact, reports have found endogenous morphine within
the structure of the hippocampus and amygdala, i.e., limbic system [117,120–122]. In addition, this morphine may
activate pleasure pathways via NO in rat brain hippocampus [112]. Studies from our laboratory confirm the medi-
This line of research allows us to further speculate that physiological relaxation emerged from this pathway, once the
proinflammatory element, under the right circumstances,
was removed. This probably was associated first with physiological states requiring a less alert state, such as sleep-like
activity. Later, when cognition evolved, it also became associated with our ability to determine a safe environment in
which relaxation may emerge. In either event, it would be
critical to have endogenous morphine involved because of
its analgesic action and ability to down-regulate immune
processes [30,32], effectively separating the two functions,
which would be critical to relaxation. Indeed, cognitive functions and devices that show their performance (EEG, fMRI,
SPECT, etc.) reveal a clear distinction between sleep and relaxation, and relaxation not only leads to an overall decrease
in brain activity but simultaneously to an increase in specific parts of the brain, e.g., related to cognition [131–133].
Another critical component of this determination, whether
to be alert or relax, is having morphine synthesis take place
via the catecholamine pathway in animals, including man
[67,68]. Again, this pathway behaviorally allows for excitation to occur as synthesis proceeds via DA and secondarily NE, followed by down-regulation via morphine and morphine-6-glucuronide signaling, as it goes to its end at opiate alkaloid synthesis [67,68].
THE HYPOTHESIS
cNOS-derived NO appears to be critical for relaxation because it has the ability to block a sympathetic response simply by having its release occur beyond the basal level, which
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Med Sci Monit, 2006; 12(9): HY21-31
In speculating on an overall theory as to what constitutes
relaxation, we offer the following hypothesis: Neural processes (CNS integrating limbic circuits), involving NO and
morphine, activate downstream signaling molecules that
stimulate cNOS-derived NO release from immune, neural
and vascular tissues. Prior to NO release this process also invokes the release of NE and opioid peptides. The presence
of NO is deduced by its vasodilating actions and the lack of
vascular sensitivity toward NE and by direct measurement
[5]. Relaxation can be cognitively learned, e.g., relaxation
response. Taken together, this potential is always present
and only after removing the sympathetic and stress reactions (e.g., disinhibition [3]), does it emerge. It is probably
this process along with many others that provide for mammalian longevity, i.e., it is antibiosenescent.
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Additionally, endogenous morphine signaling appears to play
a major role in relaxation. Many immune processes perpetuate and become embellished with time by the recruitment
of cells, and through beneficial yet sometimes harmful signaling molecules such as the proinflammatory cytokines.
These molecules can all be down-regulated by morphine,
which is released following stress or trauma [30,32], specifically through cNOS-derived NO under certain circumstances. Thus, morphine may help overcome over-stimulated immune, vascular and neural tissues [32,68]. As such, it may be
part of the immune system regulation that prevents the all
too common ravages of what Bone called “immunologic dissonance”, including the systemic inflammatory response syndrome (SIRS) which sometimes culminates in the often lethal
multiple organ dysfunction syndrome (MODS) [134].
burst of NO, we can enhance the inhibition of NF-kB activation, limiting the extent and the severity of a proinflammatory situation. Does this occur during long-term stress or does
the proinflammatory mechanism become desensitized and
over-stimulated by a relentless insult, so that NO is formed
through inducible NOS, which not only becomes detrimental to the cells, but to the entire organism? We believe the
latter situation emerges with chronic stress and may give rise
to certain degenerative diseases [17,22,23,40,139]. For example, if a proinflammatory challenge is overwhelming or
persistent, iNOS production may reflect a last ditch attempt
to overcome antigenic stress while also trying to dampen a
potentially destructive proinflammatory blaze. The resulting
overproduction of NO is unfortunately toxic [4].
SE
will lead to vasodilation and peripheral sense of warmth. Its
presence can also explain the paradox of the presence of NE
in plasma while vasodilation is taking place [78]. Recently,
we demonstrated that during the relaxation response NO
levels are increased [5], further supporting the critical role
of NO in this process.
It is possible that some individuals may be deficient in this
regulatory process, leading, when challenged, to the unregulated and potentially damaging immune responses of SIRS
and MODS. We have found, for example, that in the immune
disorder histiocytic medullary reticulosis or malignant histiocytosis [135] a morphine-selective receptor, μ3, was not expressed, and granulocytes and monocytes cultured from this
patient could not be down-regulated when exposed to morphine. The foundation of the concept that morphine is critical to relaxation or a down-regulation of excitatory processes
is supported by these preliminary clinical findings, as well as
the fact that it can be made by human white blood cells and
the μ3 opiate receptor subtype is found on these tissues as well
[68,69]. Furthermore, high morphine levels have been found
in rat limbic tissues associated with NO release [117].
ASSOCIATED MOLECULAR MECHANISMS
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Hypothesis
The physiological significance of cNOS-derived NO, and
for that matter morphine-stimulating NO release, is that it
can influence proinflammatory and stress situations, presumably bringing both the “acute phase response” and the
“acute stress response” under control [45,54,55]. We propose that cNOS-derived NO initiates these events in part by
its ability to modify the function of the transcription factors,
i.e., NF-kB [136–138]. NF-kB binding sites are present in
the promoter regions of proinflammatory genes such as tumor necrosis factor (TNF), interleukin (IL)-1 and IL-6 (see
[136]). Interestingly, stimulation of cells with proinflammatory cytokines leads to the degradation of the NF-kB inhibitor IkBa, liberating NF-kB. NF-kB is then free to activate the
transcription of some of these very same proinflammatory
cytokines. NO inhibits the expression of proinflammatory
cytokines by stabilizing the NF-kB – IkBa complex, therefore preventing translocation of NF-kB into the nucleus
[136,138]. NO can also interfere with the binding of NF-kB
to the promoter region of proinflammatory cytokines such
as IL-6, which causes T-cell proliferation [136,138].
Putting this in perspective, if we liberate molecules that have
the ability to stimulate constitutive NOS, producing a quick
Its expression probably also differs among individuals, and
the strength of one’s beliefs or an animal’s individual history may also exert a profound impact on the heightened
expression of these proactive innate protective responses
that originate from the CNS.
Relaxation takes place in that emotionalized transitional relationship found in a person when we are in a trusted and
safe environment. Here, belief is critical. Once this condition is met then, and only then, will relaxation occur. This
attachment-based solace exhibits salutary effects on the limbic neural processes in the brain’s motivation and reward
circuitry responsible for regulating the stress response and
immune response systems via constitutive NO pathways
[7–9,13,16,32,140–142]. Hence, relaxation is real and an
important physiological process. It can be integrated in
self-care-oriented medical settings, i.e, stress management,
where it has demonstrated its value.
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