Download Dorsal spinal cord stimulation obtunds the capacity of intrathoracic

Articles in PresS. Am J Physiol Regul Integr Comp Physiol (June 10, 2009). doi:10.1152/ajpregu.90821.2008
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Dorsal spinal cord stimulation obtunds the capacity of intrathoracic
extracardiac neurons to transduce myocardial ischemia
1
Jeffrey L. Ardell, 2René Cardinal, 2Michel Vermeulen and
2
1
J. Andrew Armour
Department of Pharmacology, East Tennessee State University, James H. Quillen College of
Medicine, Johnson City, Tennessee 37614 and 2 Centre de recherche, Hôpital du Sacré-Cœur de
Montréal, Montréal, Canada H4J 1C5
Correspondence should be addressed to:
Jeffrey L. Ardell, Ph.D.
Department of Pharmacology
East Tennessee State University
PO Box 70577
Johnson City, TN 37614-0577
Phone: 423-439-8866
Fax: 423-439-8773
Email: [email protected]
Running title: SCS modifies extracardiac neuronal transduction of myocardial ischemia
Key words: cardiac pacing; ventricular ischemia; middle cervical ganglion neuron;
cardiac nervous system; neurocardiology
1
Copyright © 2009 by the American Physiological Society.
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Abstract
Populations of intrathoracic extracardiac neurons transduce myocardial ischemia, thereby
contributing to sympathetic control of regional cardiac indices during such pathology. Our
objective was to determine whether electrical neuromodulation using spinal cord stimulation (SCS)
modulates such local reflex control. Methods. In 10 anesthetized canines, middle cervical ganglion
neurons were identified that transduce the ventricular milieu. Their capacity to transduce a global
(rapid ventricular pacing) versus regional (transient regional ischemia) ventricular stress was tested
before and during SCS (50 Hz, 0.2 msec duration at 90% motor threshold) applied to the dorsal
aspect of the T1 to T4 spinal cord. Rapid ventricular pacing and transient myocardial ischemia
both activated cardiac related middle cervical ganglion neurons. SCS obtunded their capacity to
reflexly respond to the regional ventricular ischemia, but not rapid ventricular pacing.
Conclusions. Spinal cord inputs to the intrathoracic extracardiac nervous system obtund the latter’s
capacity to transduce regional ventricular ischemia, but not global cardiac stress. Given the
substantial body of literature indicating the adverse consequences of excessive adrenergic neuronal
excitation on cardiac function, these data delineate the intrathoracic extracardiac nervous system as
a potential target for neuromodulation therapy in minimizing such effects.
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INTRODUCTION
Delivering high frequency electrical stimuli to the dorsal aspect of the T1-T4 thoracic spinal
cord (SCS) can alleviate the symptoms associated with chronic refractory angina pectoris (19;
23; 29; 32). SCS also stabilizes ventricular electrical alterations that attend regional ventricular
ischemia (14), reduces the size of infarcts induced by transient myocardial ischemia (37) and
protects against ischemia-induced ventricular tachycardia/ventricular fibrillation in animal
models with chronic myocardial infarction and heart failure (27).
Electrical neuromodulation impacts on the dynamic interactions among autonomic afferent
and efferent neurons mediated via as yet poorly defined spinal cord and peripheral neural circuits
(18; 20; 35; 43). Centrally, SCS reduces the release of neuropeptides from cardiac nociceptive
afferent neurons (17), reduces intrasegmental and supraspinal transmission of cardiac
nociceptive information (20; 35), and increases the activity of a sub-population of thoracic
sympathetic preganglionic efferent neurons (17; 18). Peripherally, SCS reduces the activity
generated by intrinsic cardiac neurons by ~70% at baseline as well as when they are activated by
transient ventricular ischemia (13; 21). Transection of the ansae subclavia eliminates the
suppressor effects of SCS on intrinsic cardiac neural activity, indicating that responses are due
primarily to the influence of spinal cord neurons acting via the sympathetic nervous system (21).
The ability SCS to reduce infarct size secondary to transient myocardial ischemia is blocked by
prior adrenergic blockade (37), likewise indicating a primary role for SCS mediated control of
sympathetic efferent neurons regulating the stressed heart.
Intrathoracic extracardiac neurons are in constant communication with intrinsic cardiac ones
in the overall coordination of regional cardiac indices (3; 11).
Neurons in intrathoracic
extracardiac ganglia receive major inputs from the spinal cord preganglionic neurons that are
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under the control of the intermediolateral cell column which, in turn, receives inputs from spinal
and supraspinal components of the cardiac neuronal hierarchy (2; 20). Furthermore, inputs
arising from spinal cord neurons are known to exert influences on middle cervical ganglion
neurons involved in short-loop intrathoracic cardio-cardiac reflexes (5; 6). It remains to be
established whether central neuronal inputs to the intrathoracic extracardiac nervous system
influence the latter’s capacity to transduce the cardiac milieu. Thus, the present experiments
were designed: 1) To determine whether spinal cord inputs influence the capacity of middle
cervical ganglion neurons to transduce the ventricular milieu; and 2) To determine whether SCS
influences how these neurons respond to regional versus global ventricular stress, thereby
impacting upon intrathoracic extracardiac sympathetic reflex function.
MATERIALS AND METHODS
Animal preparation. These experiments were performed in accordance with the guidelines
for animal experimentation described in the “Guiding Principles for Research Involving Animals
and Human Beings” (1) and in accordance with the Guide to the Care and Use of Experimental
Animals set up by the Canadian Council on Animal Care, under the regulation of the Animal
Care Committee of the University of Montreal.
Adult mongrel dogs (n = 10; 20-25 kg) of either sex were anesthetized with sodium
thiopental (25 mg/kg iv, supplemented as required), intubated and maintained under positivepressure ventilation. After surgery, anesthesia was maintained with α-chloralose (75 mg/kg, i.v.
bolus, with repeat doses of 12.5 mg/kg i.v. as required). Noxious stimuli were applied to a paw
periodically to ascertain the adequacy of anesthesia. Body temperature was maintained between
37.5-38.5°C by water-jacketed Micro-Temp heating units (Zimmer, Dover, OH, U.S.A.). Fluid
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replacement (physiological saline) and α-chloralose were administered via a femoral vein
catheter.
Spinal cord stimulation. After placing the animal in the prone position, the epidural space
was entered with a Touhy needle via a small skin incision in the caudal dorsal thorax. A fourpole lead (Octrode lead, Advanced Neuromodulation Systems, Plano, TX) was advanced
rostrally in the epidural space to the T1-T4 spinal cord level, utilizing anterior-posterior
fluoroscopy. The tip of the lead was positioned slightly to the left of midline, according to
current clinical practice (33). The most cranial pole of the lead was positioned at the T1 level
with the caudal pole at T3-T4 level. Electrical current was delivered via its rostral and caudal
poles to verify their functional position. Stimuli were delivered via a stimulus isolation unit and
constant current generator (Grass SIU 5B) connected to a stimulator (Grass S88 Stimulator,
Grass Instruments, Quincy, MA, U.S.A.). Increasing stimulus intensity (with the rostral pole as
cathode) to motor threshold intensity (MT) induced muscle contractions in the proximal forepaw
and shoulder. Stimulation with the caudal pole as cathode at MT activated thoracic paravertebral
muscles, resulting in a twisting movement of the trunk. Animals were rotated to the decubitus
position and motor threshold was re-established. In each experimental protocol, SCS was
delivered for 9 minutes at 50 Hz, 200 µsec duration and at a current intensity of 90% MT (range
0.19 and 0.78 mA; mean 0.43 mA). The rostral and caudal poles were chosen as cathode and
anode, respectively, according to current clinical practice (32; 33). Motor threshold was checked
periodically during the experiment and did not vary significantly from initial levels.
Cardiovascular instrumentation. Heart rate was monitored via a lead II ECG. Left
ventricular chamber pressure was recorded via a Millar catheter introduced into the left
ventricular cavity via the right femoral artery. A transthoracic incision exposed the heart and the
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left middle cervical ganglia associated with the left thoracic vagosympathetic trunk. After
opening the pericardial sac, 2 miniature solid-state pressure transducers (Konigsberg Instruments
Inc., Pasadena Ca., model P19) were inserted into the left ventricle mid-wall in the perfusion
beds of the ventral descending and circumflex coronary arteries. Aortic pressure was monitored
via a Bentley Trantec model 800 transducer connected to a Cordis # 6 catheter that was inserted
into the ascending aorta via a femoral artery. These cardiac indices were monitored throughout
the experiments via an eight channel rectilinear recorder (Nihon Kohden, Tokyo, Japan),
digitized (Cambridge Electronics Design, power 1401) and analyzed (Spike 2, Cambridge
Electronics Design) to determine mean hemodynamic values at baseline and in response to each
stressor.
Coronary artery occlusion. A 3-0 silk thread was placed around the ventral descending
coronary artery about 2 cm from its origin, distal to the site of origin of the first major branch of
that artery. Another silk thread was placed around the circumflex coronary artery about 3 cm
from its origin, distal to the origin of its first major branch. These threads were led through short
polyethylene tubing segments for subsequent snaring of each artery for 30 seconds. These
occlusion sites were chosen to obviate inducing too large a ventricular ischemic zone in order to
minimize subsequent ventricular arrhythmia induction.
Cardiac pacing protocol. Rapid ventricular pacing was performed by delivering trains of
super-threshold electrical stimuli at a rate of 180 pulses per minute to the right ventricle for 5minute periods of time. This was accomplished via bipolar electrodes that had been sutured to
the epicardium of the right ventricular outflow tract.
Middle cervical ganglion neuronal activity. The left middle cervical ganglion was identified
and left in place, attached to surrounding tissues to maintain its stability. The activity generated
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by neurons in that ganglion was identified using methods that have been described previously (6;
11). The recording microelectrode utilized had a 250-µm shank diameter, an exposed tip of 10
µm and an impedance of 9-11 MΩ at 1,000 Hz (Frederick Haer & Co., model #ME 25-10-2).
The indifferent electrode was attached to the mediastinum adjacent to that middle cervical
ganglion.
This ganglion was explored with the electrode tip placed from its ventral surface to its
underlying regions such that the activity generated by neurons in various loci within that
ganglion could be identified. Action potentials generated by the somata and/or dendrites of
individual neurons can be recorded from a locus within a middle cervical ganglion for hours
using this methodology (6). Action potential so identified were differentially amplified by
means of a Princeton Applied Research model 113 amplifier that had bandpass filters set at 300
Hz to 10 kHz and an amplification range of 100-500X. The output of this device was further
amplified (50-200X) and filtered (band width 100 Hz-2 kHz) by means of an optically isolated
amplifier (Applied Microelectronics Institute, Halifax, N.S., Canada). The amplified neuronal
signals, together with the cardiovascular signals, were digitized (Cambridge Electronics Design,
power 1401 data acquisition system) and analyzed using the Spike 2 software package
(Cambridge Electronics Design). Ganglionic loci were identified from which action potentials
with signal to noise ratios greater than 3:1 could be recorded. The activity generated by
individual neuronal somata was identified by the amplitude and configuration (waveform
recognition) of the recorded action potentials via the Spike 2 program. Using these techniques
and criteria, action potentials generated by individual cell bodies and/or dendrites rather than
axons of passage can be recorded for extended period of time (6; 11). As such, once an active
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locus was identified the activity generated by its spontaneously active neurons was studied
throughout the duration of the various protocols outlined below (cardiac stressors).
In order to identify middle cervical ganglion neurons that transduced the left ventricular
milieu, the activity generated by neurons within various loci of the left middle cervical ganglion
was recorded as gentle mechanical stimuli (delivered by a saline soaked cotton applicator) were
applied for 2-3 seconds to various loci on the ventral and ventro-lateral surface of the left
ventricle. It is known that most middle cervical ganglion neurons that transduce the ventricular
mechanical milieu also transduce its local chemical milieu (12). Once a neuron was identified
that responded to repeat (3x) local mechanical deformation of epicardial regions adjacent to
either the LAD or circumflex coronary artery, its spontaneous activity was recorded during 5-10
minutes control states in order to establish baseline values for that index.
Cardiac stressors. The ventricles were then paced at 180 beats per minute for 5 minutes.
After waiting for at least 5 minutes, the artery that perfused the identified sensory field was
occluded 3-4 times. That is, the ventral descending or circumflex coronary artery was occluded
individually for 30 seconds – depending on which vessels perfused its identified sensory field.
At least 5 minutes occurred between these interventions for preparation stabilization. The order
that these vessels were occluded varied among animals. The order of applying these two
protocols (ventricular pacing versus coronary artery occlusion) also varied among animals.
Spinal cord stimulation protocol. In order to test the effects of activating the dorsal aspect of
the thoracic spinal cord (SCS) on the activity generated by cardiac related middle cervical
ganglion neurons, in 4 preliminary animals 2, 5, 10 and 15 minute periods of spinal cord
stimulation were studied in the absence of stressors. SCS yielded similar neuronal activity
changes at up to 15 min of SCS (27±23 impulses/min [ipm] baseline to 38±27 ipm at SCS
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termination, p=0.36). To minimize the potential for long-term effects on neuronal activity postSCS (13; 21), SCS was instituted in each experimental protocol for only 9 minutes thereafter.
For the global stress of rapid cardiac pacing, SCS was started 2 minutes before and maintained
for 2 minutes after the allocated 5-minute periods of cardiac pacing (total 9 min). These data
were compared to those obtained during 5-minutes of pacing alone. With respect to coronary
artery occlusion (duration 30 secs), occlusions were performed throughout the final 30 seconds
of SCS (i.e., the occlusion and SCS terminated simultaneously) and compared to the response to
30 sec coronary artery occlusion by itself. One hour elapsed between each protocol in which no
interventions were instituted.
Data analysis. Spontaneous cardiodynamic fluctuations were minimal during control
periods, heart rate varying less than 5 beats/min and systolic pressure fluctuating less than 5 mm
Hg. Thus, thresholds for classifying induced cardiovascular changes were chosen to be greater
than these ranges. Action potentials arising from a locus within each middle cervical ganglion
studied were characterized by means of their differing configurations (waveform recognition) by
employing the Spike 2 program. Neuronal activity and cardiovascular indices recorded
simultaneously before each intervention were compared to those indices derived during each
intervention, average activity representing the same population recorded throughout all
interventions. The grouped data so derived are expressed as means ± s.d. SigmaStat 3.1 (Systat
Software) with one-way analysis of variance with post hoc comparisons (Holm-Sidak test) was
used to test for differences within and between groups. A significance of P < 0.05 was used.
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RESULTS
Neuronal activity. Spontaneously active neurons identified in one locus of each middle
cervical ganglion were analyzed by means of their unique action potential waveform
configurations (employing Spike-2 program). Only those that were activated by gently
mechanical stimuli applied to circumscribed epicardial loci on either the left ventricular ventral
or lateral epicardial wall entered this study. As a consequence we were able to able to identify in
control states 3-5 neurons in an active ganglionic locus. Collectively, neurons so identified
generated about 40 impulses per minute (Table 1). Although most neurons generated activity
throughout the cardiac cycle (Fig. 1), limited numbers generated cardiac-related activity usually
during ventricular isovolumetric contraction (Fig. 2). Application of focal mechanical stimuli to
these select ventricular epicardial loci activated these neurons 2-3 fold, doing so similarly upon
repeat local mechanical stimulus application. Neurons so identified became the subject of
subsequent investigation.
Rapid ventricular pacing. Cardiac pacing increased the activity generated by identified
neurons, whether pacing was tested for 1-minute periods (Fig 1) or for the 5-minute periods used
to subsequently test the effects of SCS (Fig. 3). Pacing the ventricles at 180 beats per minute for
5 minutes enhanced neuronal activity by ~117% (range of neuronal activity from 1.4 – 68.4
impulses/min at baseline to 4.8 – 128.1 impulses/min during pace) (Table 1). All identified
neurons responded with increased activity that occurred in excess of that related to increasing the
heart rate (50% increase rate; 120 beats/min→180 beats/min). Furthermore, in some instances
pacing also initiated cyclic bursts of activity that persisted after terminating pacing (Fig. 3A).
Left ventricular chamber and aortic systolic pressures fell with pace onset. These indices
gradually returned to baseline values as pacing continued such that at the end of pacing (when
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neuronal activity was quantified) aortic pressure (128+7/100+6 mm Hg) was comparable to that
of control states (130+7/100+6 mm Hg).
Coronary artery occlusion. The activity generated by identified neurons increased during
transient occlusion of either the left anterior descending or circumflex coronary artery,
depending on whether the location of the mechanosensory field associated with identified
neurons in individual animals was perfused by one or the other artery (Fig. 4A; Table 1).
Repeated artery occlusion induced similar neuronal responses. Intra-myocardial systolic
pressure in the non-occluded ventricular zone remained unchanged overall (106+11 to 98+8 mm
Hg; n.s.); corresponding intramyocardial pressures in the risk zone fell ~18% during the transient
coronary occlusion. Transient coronary artery occlusion did not alter heart rate (121+8 to
120+10 beats per minute) or aortic pressure (130+10/98+7 to 130+11/95+8 mm Hg) overall.
Spinal cord stimulation. Nine minutes of spinal cord stimulation did not alter heart rate, left
ventricular chamber pressure or aortic pressure overall (data not shown). SCS did not change
neuronal activity overall (Table 1), even though in some instances activity of a few neurons was
changed (Fig 2).
Stressors in the presence of spinal cord stimulation. When ventricular pacing was instituted
in the presence of SCS, the enhancement of neuronal activity was similar to that which had
occurred during that intervention in the absence of SCS (Table 1). However, in two of the
animal’s spinal cord stimulation reduced the period of time that pacing modified neuronal
activity (Fig. 3). In contrast, regional ventricular ischemia no longer enhanced the activity
generated by identified MCG neurons when instituted in the presence of SCS (Fig. 4; Table 1).
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DISCUSSION
In basal states, SCS is known to suppress the activity generated by populations of intrinsic
cardiac neurons (13; 21). It also reduces the responsiveness of such neurons to regional
ventricular ischemia (13; 21). Data obtained from the current study indicate that SCS modified
populations of neuronal somata located in intrathoracic extracardiac ganglia. Furthermore, it
targets neurons in classical peripheral sympathetic ganglia that in turn target cardiac indices.
Although SCS did not suppress overall basal activity generated by the subpopulations of
intrathoracic sympathetic extracardiac neurons involved in transducing the ventricular milieu
identified in this study, it did obtund their capacity to transduce regional ventricular ischemia.
Reflex control of heart function is dependent upon complex interactions within both
peripheral and central neuronal elements of the cardiac neuronal hierarchy (2; 3; 20).
Neuromodulation, targeted at various levels of the cardiac neuronal hierarchy, has the capacity to
modify reflex function within the hierarchy (17; 22) and the cardiomyocytes themselves (37).
Against the stress of acute myocardial ischemia, SCS reduces electrical instability of the
ventricle (27) and infarct size (37), the later at least being dependent on modulation of adrenergic
nerve function (37). In animal models of reduced coronary reserve, SCS mitigates the ST
segment deviations associated with chemical activation of intrathoracic adrenergic neurons (14).
SCS by itself (18), or in combination with coronary artery occlusion (17), increases cFos
expression within sub-populations of high thoracic sympathetic preganglionic neurons. In the
current study, while the average response of middle cervical ganglia neurons to SCS did not
change over the stimulation times evaluated (up to 15 min), a sub-set of these neurons were
augmented – an effect that outlived the stimulation (Fig 2). Considering the heterogeneous
neuroanatomical organization of the extracardiac intrathoracic autonomic ganglia (24; 25),
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including afferent, efferent and local circuit neurons, it is not unexpected that the integrated
reflex response to stressors is partially reflective of the characteristics of their imposed afferent
and efferent neuronal inputs. As the vast majority (~70%) of neurons in the middle cervical
ganglia that respond to local epicardial mechanical deformation or chemical stimuli are local
circuit in nature (5; 6), presumably this population of polymodal neurons represented the
principal neuronal population evaluated in this study. Presumably that is in part why most
identified neurons were not directly activated by SCS. Moreover, SCS exerts its
cardioprotective effects (14; 27; 37) without interfering with the direct flow-through pre- to postganglionic efferent neuronal projections (34). We have proposed instead, that SCS exerts it
cardioprotective effects in part by modulating reflex processing within intrathoracic ganglia (13;
14).
Significant subpopulations of neurons within intrathoracic extracardiac and intrinsic cardiac
ganglia perform a major processing function - interconnecting neurons located within individual
or separate intrathoracic ganglia. Local circuit neurons perform this function (3; 8; 10; 41). This
neuron population forms the substrate for the processing of cardiovascular afferent information
within peripheral autonomic ganglia, even when such ganglia are disconnected from the
influence of central neurons (4; 7). Local circuit neurons transduce cardiovascular afferent
information reflexly to modulate the sympathetic and parasympathetic efferent outflows to
cardiac effector sites (11). This population also receives significant descending inputs from
central neurons (9; 28; 41). These neurons are crucial for overall information processing within
the intrathoracic nervous system and, as such, represent a principal modulator of the final
efferent neuronal outflows to different cardiac regions (3; 10).
We hypothesize that the
population of local circuit neurons represents the preferential target for SCS derived
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neuromodulation and that such inhibition limits excessive reflex responses of the cardiac nervous
system responding to the stress of acute myocardial ischemia. It is known that the inputs so
activated in the spinal cord project to intrathoracic extracardiac neurons via the ansae (8,9).
Whether this occurs via the efferent or afferent axonal projections from the spinal cord remains
to be explored, although the former is the most likely occurrence. In conjunction with SCS induced effects to increase myocyte resistance to ischemic stress (14; 37), we propose that within
the risk zone the supply/demand imbalance is thereby reduced, as is sensory activation within
that zone and this is fundamental to the clinical experience of angina relief associated with SCS
neuromodulation therapy (32).
Middle cervical ganglion neurons involved in transducing the cardiac mechanical and/or
chemical milieu (11) become excited by global cardiac stresses such as that attending rapid
ventricular pacing. Such an intervention alters the mechanical and chemical milieu of both
ventricles, thereby enhancing sensory inputs derived from multiple cardiac afferent neurons that
influence populations of middle cervical cardiac neurons (6) such that they become activated
(Table 1). In preliminary experiments in which relatively short-term ventricular pacing was
tested, activity increased soon after initiating pacing (Fig. 1). These data indicate that cardiac
pacing influences the intrathoracic cardiac nervous system, an influence that extinguishes with
time once pacing is terminated (Figs 1 and 3).
The present study indicated that SCS is ineffective at overcoming excessive sensory inputs
arising from a global cardiac stress such as that initiated by rapid ventricular pacing. In accord
with that observation, such spinal cord inputs are likewise ineffectual in reducing pacing-induced
ST segment deviations in animal models with reduced coronary reserve (14). This may reflect
an overriding direct local metabolic effect of cardiac pacing on the interstitial milieu and
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resultant global increase in cardiac afferent neuronal transduction. Against such pacing-induced
afferent input barrage, the capacity for higher centers to modulate intrathoracic reflex processing
may be limited. However, while maximal reflex responses within the middle cervical ganglia
neurons to an imposed pacing stress is maintained during SCS (Table 1), the duration of
sympatho-excitation can in some instances be truncated (Fig 3).
Regional ventricular ischemia creates boundary conditions whereby more limited ventricular
sensory neurites fields are affected relative to the surrounding interstitium. Such a cardiac
substrate would initiate differential sensory inputs to middle cervical ganglion cardiac neurons to
enhance the activity of intrathoracic extracardiac local circuit neurons (3; 6). Differential
transduction of regional boundary conditions likely represents a substantially different neuronal
substrate on which descending neuronal projections might act, especially when considering the
processing elements within the intrathoracic extracardiac ganglia - the local circuit neurons (12).
Presumably that is why the enhancement of neuronal activity subsequent to ischemia
transduction rendered this population more sensitive to SCS. Taken together, these data indicate
that spinal cord inputs to the intrathoracic extracardiac nervous system are capable of altering its
capacity to transduce the ischemic myocardium by primarily targeting neuronal processing
elements within its ganglia, the local circuit neurons.
Perspectives and Significance. The reflex response of the cardiac neuronal hierarchy to
imposed cardiac stress is a primary determinant of resultant cardiac responses. During acute
myocardial ischemia, individuals that exhibit excessive and imbalanced sympatho-excitation are
at increased risk for sudden cardiac death and/or progression into congestive heart failure (16;
30; 40). While beta adrenergic blockade has documented efficacy to reduce adverse
consequences of such cardio-pathology (16; 38), there are multiple untoward effects associated
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with its use (42). The cardiac neuronal hierarchy has emerged as a novel therapeutic target for
managing cardiac disease via pharmacological (36; 39), physical (26; 31) and/or electrical (15;
37; 43) means to target specific nexus points within the associated neural networks. That SCS
blunts/stabilizes intrathoracic extracardiac neuronal transduction of regional ventricular ischemia
supports the concept that SCS imparts is cardioprotective effects on cardiac electrical stability
(27) and myocyte viability (37) via multiple intrathoracic neuronal populations.
Acknowledgments: The authors gratefully acknowledge the technical assistance of Caroline
Bouchard. Supported by the Canada Institutes of Health Research (RC and JAA), the Quebec
Heart and Stroke Foundation (JAA and RC), and the NIH (HL71830, JLA).
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Figure legends
Figure 1. Ventricular pacing modified cardiac-related middle cervical ganglion neuronal
activity. Upper panel: Rapid right ventricular pacing (initiated throughout the RVP signal on
upper trace) induced a fall in left ventricular pressure (LVP) while activating 3 neurons identified
from raw MCG neuronal activity (line below) by their unique action potential configurations
(green, blue and black vertical lines). Lower panels depict raster sweep plots of the action
potentials generated by these 3 neurons during consecutive cardiac cycles for 60 sec during
control states (baseline) and right ventricular pacing (RV pace). Time 0 is defined by QRS
onset. Activity increased during pacing, rapidly returning to control values once pacing ceased
(c.f., unit activity).
Timing bars are located below each panel.
Figure 2. SCS modified the activity generated by middle cervical ganglion neurons
associated with left ventricular mechanosensory neurites. Panel A (Lead II electrocardiogram,
ECG) and panel B (left ventricular intramyocardial pressure; LV IMP) show QRS averaged
values (dark line) recorded during each of 1792 consecutive cardiac cycles (± 1 SD thin lines).
Panel C: dots in the raster sweep plot represent action potentials generated by 4 neurons recorded
during the 1792 cardiac cycles. This included activity recorded two min before (basal) and
during 9 min of continuous SCS (SCS on = gray shaded area), as well as 4 min following SCS
(top of raster plot). The horizontal time zero was set to the R wave of the ECG. Activity
increased as SCS persisted (gray zone in raster plot), including after SCS termination (top of this
plot C); increased activity demonstrated less phase-locking to the cardiac cycle. Panel D
represents a cumulative index of all action potentials recorded during these 1792 consecutive
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cardiac cycles demonstrating that activity was maximal during maximum local isovolumetric
dynamics of their receptor fields.
Figure 3. A. Rapid ventricular pacing (panel A, horizontal bar below) increased
neuronal activity. A maximum of 5 identified middle cervical ganglion neurons were recorded
that generated increased (cyclic bursts) activity during pacing, something that persisted for some
time after discontinuing pacing. (B) Nine minutes of SCS (SCS started 2 minutes prior to
pacing; horizontal bar below) induced a lesser response to pacing (lowest horizontal bar, B) and
one that extinguished soon after discontinuing SCS. In each panel, from above downward, are:
neuronal activity averaged over 5-second sequential intervals; discriminated neuronal activity
(activated neurons); intramyocardial pressure (LV IMP) recorded adjacent to the identified
sensory neurite field. The horizontal bar below represents SCS application time (applied 2
minutes before and continuing for 2 minutes after pacing).
Figure 4. Effects of left anterior descending coronary artery occlusion (30 seconds
duration; horizontal bars below each panel) on middle cervical ganglion neuronal activity (A)
before (Pre-SCS) and (B) during (SCS) spinal cord stimulation. The time scales of these 2
records differ in order emphasize the lack of long-term effects of ischemia after SCS. Neuronal
activity and activated neuron traces are as defined in Fig 1.
Table 1. The activity generated by 3-5 identified ventricular middle cervical ganglion
neurons per animal is averaged (impulses/min; mean + S.D.). Neuronal activity increased during
the rapid ventricular pacing, as well as brief periods of regional ventricular ischemia. SCS did
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not change neuronal activity overall. It suppressed neuronal responses to regional ventricular
ischemia but not those initiated by ventricular pacing. * p ≤ 0.02 comparing control versus
intervention; + p≤0.02 SCS versus SCS & intervention; #p≤0.01 intervention versus SCS &
intervention.
26
A. ECG
B. LV IMP
80
60
1
mmHg
40
20
0
C. Spikes
1500
consecutive 1000
cardiac cycles
500
D. Cumulative
SCS
on
basal
0
200
spike count
per bin
100
0
-0.1
0.0
0.1
0.2
Time (s)
0.3
0.4
A
40
30
Neuronal
activity
/5 s
20
10
0
Activated
neurons
LV
IMP
(mmHg)
150
100
50
Rapid
ventricular
pacing
B
Neuronal
activity
/5 s
0
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
40
30
20
10
0
Activated
neurons
LV
IMP
(mmHg)
SCS
Rapid
ventricular
pacing
150
100
50
0
Time (s)
Table 1: Middle cervical ganglion neuronal activity (impulses/min)
Intervention
Control
Intervention
ventricular pacing
36.7 ± 28.1
79.8 ± 47.5 *
coronary artery occlusion
41.5 ± 35.6
111.0 ± 61.4 *
Control
SCS
SCS & intervention
Ventricular pacing
30.8 ± 24.4
26.8 ± 19.7
72.5 ± 36.9 *+
Coronary artery occlusion
19.4 ± 16.2
19.0 ± 14.0
29.5 ± 20.2
(n = 10 dogs)
Control Stressors
Stressor with SCS
#