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Advanced Pharmacology-I
(PHR5001)
Lecture 8:
Analgesics (Centrally Acting)
Dr. M G Azam
Asstt. Professor
Dept. of Pharmacy, NSU
1
What is Pain?
□ Pain can be defined as a somatic sensation of acute
discomfort, a symptom of some physical hurt or disorder, or
even emotional distress.
□ Pain is a part of a rapid warning relay instruction the motor
neurons of the central nervous system to minimize detected
physical harm.
□ Pain can be classified into two types: □ Chronic pain is pain
that last much longer than pain normally would with a particular
injury.
Acute pain is short-term pain or pain with an easily identifiable
cause. Acute pain “is the body's warning of present damage to
tissue or disease.
Opioid Analgesics can be used to treat both types of pain
What Causes Pain?
 Pain is caused by the stimulation of
pain receptors which are free nerve
endings.
 “Nocireceptors are pain receptors that
are located outside the spinal column in
the dorsal root ganglion and are named
based upon their appearance at their
sensory ends. These sensory endings look
like the branches of small bushes.
 There are two types of nocireceptors
that mediate fast or slow pain signals
 The perception of pain is when these
receptors are stimulated and they
transmit signal to the central nervous
system via sensory neurons in the spinal
cord.
Pain Signaling
These neurons release excitatory neurotransmitters which
relay signals from one neuron to another.
“The signals are sent to the thalamus, in which pain
perception occurs. From the thalamus, the signal travels to
the somatosensory cortex in the cerebrum, at which point
the individual becomes fully aware of the pain .
Terminology
“Opium” is a Greek word meaning “juice,” or the exudate from
the poppy seeds (Paper somniforum)
“Opiate” is a drug extracted from the exudate of the poppy.
□ Analgesia simply means the absence of pain without loosing
consciousness.
□ “Opioid” is a natural or synthetic drug that binds
to opioid receptors producing agonist effects:
Used for thousands of years to produce: Euphoria,
Analgesia, Sedation, Relief from diarrhea, Cough
suppression
□ Opium contains over 20 distinct alkaloids
(morphine was the first alkaloid of opium to be isolated
in 1803). By the late 19th century use of these “pure”
opium derivatives spread throughout the medical world.
Endogenous Opioid Peptides
□ “Endogenous opioid peptides are the naturally occurring
ligands for opioid receptors. The term endorphin is used
synonymously with endogenous opioid peptides. These peptides
are produced by the pituitary gland and by the hypothalamus. are
found in the central nervous system mainly in limbic and brainstem
areas associated with pain reception, and the certain areas of the
spinal cord.
□ These natural peptides work as ligands that interact with their
specific receptors causing structural changes that result in other
changes in the effected neuron such as the opening or closing of
ion gated channels or the activation or deactivation of certain
enzymes.
□ Opioid peptides work by modulating the release and uptake of
specific neurotrasmitters in the neurons they are found. This
alteration of neurochemical balance creates a vast amount of
possible physiological effects, one of which is analgesia.
General Consideration of Narcotic Analgesics
【action mechanism】
ligands
opioids receptor
Gi
inhibiting adenylate cyclase
increasing potassium ion efflux or reducing
calcium ion influx
impeding neuronal firing and
transmitter release
Classification of drug
1. Origin
• natural opiates: morphine,
•
heroin, codeine, thebaine, paraverine
synthetic analgesics: meperidine, methadone, fentanyl,
anadol, etorphine, pentazocine.
2. potency
• strong agonist: morphine, heroin (diacetylmorphine ),
•
•
•
Fentanyl
meperidine, methadone,fentanyl
moderate agonist: codeine, propoxyphene.
mixed agonist-antagonist: buprenorphine, pentazocine
antagonist: naloxone, naltrexone
When codeine is administered ~10% is
converted to morphine by O-demethylation
(in the liver) cytochrome p450
Codeine
Opioid Antagonists
• Opioid Antagonists are used to treat
opioid overdose cases.
• Most are derived from Thebaine (an
alkaloid of Opium)
• The have strong binding affinity for the
mu receptors
• They work by competitive inhibition at
the binding site (It binds but does not
change the receptor while at the same
time blocking the agonist).
• Naloxone is administered intravenously.
• It can rapidly produce the withdrawal
symptoms associated with opioid
addiction.
• Naltrexone is another example of an
opioid antagonist. It is more potent than
Naloxone and is used in the treatment of
alcohol addiction but its mechanism in
this treatment is unknown.
Naloxone
Naltrexone
Mechanism of action
• Activation of peripheral nociceptive fibers causes release
of substance P and other pain-signaling neurotransmitters
from nerve terminals in the dorsal horn of the spinal cord
• Release of pain-signaling neurotransmitters is regulated
by endogenous endorphins or by exogenous opioid
agonists by acting presynaptically to inhibit substance P
release, causing analgesia
Primary Effect of Opioid Receptor Activation
• Reduction or inhibition of neurotransmission, due
largely to opioid-induced presynaptic inhibition of
neurotransmitter release
• Involves changes in transmembrane ion conductance
– Increase potassium conductance (hyperpolarization)
– Inactivation of calcium channels
The Opioid Receptors
There are 3 main opioid receptors, the mu receptor, the delta
receptor, the kappa receptor.
The receptors are found on cell membranes of cells in the
nervous system (neurons) and have different effects.
μ-receptor
Morphine and its analogues (most opioids) bind to the mureceptor it produces the effects of analgesia (μ1-receptors). The
mu-receptor is also associated with other effects such as “sedation,
reduced blood pressure, nausea, euphoria, decreased respiration,
miosis (constricted pupils) and decreased bowel motility often
leading to constipation (μ2-receptors).
 When an opioid binds to the mu-receptor it induces a change in
shape which in turn induces a change in the ion channels of the
associated cell membrane.
μ-receptor
The mu-receptor opens up the ion channel allowing K+
to flow out of the cell causing hyperpolarization of the
membrane potential. This hyperpolarization causes it to
become extremely difficult for an action potential to be
reached and therefore the firing of the neuron become far
less frequent and the neurons excitability decreases .
The release of K+ also causes less calcium ions to enter the
terminal end of the neuron. This is where neurotransmitters
are stored and as a result this significantly reduces
neurotransmitter release.
These effects of a ligand binding to a mu-receptor essentially
turn off the neuron and in doing so block the relaying of pain
signals from pain receptors.
 Respiratory depression is considered the deadly side effect of
opioid analgesic drugs. It is the cause of death in all overdose
cases.
δ-receptor
The delta receptor is the strongest binding site of the
body’s natural pain killer, the class of opioid peptides called
the enkephalins.
 Morphine and other commonly used opioid analgesics
also bind to this receptor strongly and act as an agonist
much like they do with the mu receptor.
 The delta receptor is a G-protein linked receptor. When
an agonist binds to the delta receptor is induces a
conformational change that causes the activation of a
specific G-protein.
This G-protein “inhibits the membrane bound enzyme
adenylate cyclase and prevents the synthesis of cAMP. The
transmission of the pain signal requires cAMP to act as a
secondary messenger, and so inhibition of this enzyme
blocks the signal.
κ-receptor
 The kappa receptor is associated directly with analgesia
and sedation but with none of the undesired side effects
associated with the mu receptor.
 Because of this, it is an area of focus in current research
and shows promise in the development of a safer
analgesic.
When and agonist or ligand binds to the kappa receptor
it induces a conformational change that results directly in
the closing of the calcium ion channels in the terminal of
the neuron and the neuron can not relay pain messages.
 There are not many significant agonist of the kappa receptor
known. K-receptors only effect nerves that relay “pain
produced by non-thermal stimuli and mu receptors inhibit all
pain signals.
In vivo method for evaluating Analgesics
(centrally acting)
1. Haffner’s tail clip method in mice
2. Tail flick or other radiant heat methods
3. Hot Plate Method
4. Tail immersion tests
5. Formalin test in rats
1. Haffner’s Tail Clip Method
Purpose and Rationale:
The method was described by Haffner who
observed that raise of tail in mice treated
with morphine or similar opioid like drugs is less sensitive to
noxious stimuli.
Procedure:
The test compounds & the drug are usually administer S.C or
orally to fasted male mice (weight 18-25 g) 15, 30, 60 min prior
testing.
An artery clip is applied to the root of the tail of mice to
induce pain (The animal quickly responds to this noxious
stimuli by biting the clip).
The time between the stimulation onset and response is
measured by a stopwatch in 1/10 sec increments.
Haffner’s Tail Clip Method
Evaluation
A cut-off time is determined by taking the average reaction
time + 3 x STDEV of combined latencies of the control
mice.
Any reaction time of the test animals which is greater than
cut-off time is called a positive response indicative of
analgesic activity.
Critical Assessment of this test
The test does not need any sophisticated equipment but
a skilled observer is required.
 Peripheral analgesic ,such as Salicylate type are not
detected by this test.
2. Radiant Heat method
Purpose and rationale
This test was developed by Wolff et al for quantitative
measurement of pain threshold against thermal radiation
and evaluation of analgesic activity of opiates.
 This method is very useful for discriminating between
centrally acting morphine like analgesics and non-opiate
analgesics.
 Mice are placed into the cages leaving the tail exposed. A
light beam is focused to the proximal third of the tail.
Within a few seconds the animal flick the tail to escape. The
time until this reaction occurred is measured.
Radiant Heat method
Procedure
Groups of 10 mice of both sexes (weight:18 to 22g) are
used for each dose. Before administration of the test
compounds or standard (SC/orally), the reaction time is
determined.
 The animal is put into a small cage with an opening for
the tail at the rear wall. The tail is held gently by the
investigator.
 By the opening of the shutter, a light beam exerting
radiant heat is directed to the proximal third of the tail. For
about 6 s the reaction of the animal is observed.
The mouse try to pull the tail away and turns the head.
With a switch the shutter is closed immediately upon
reaction is observed.
The animal are submitted to the same Radiant Heat method
testing procedure after 30, 60 and
eventually 120 min. For each individual
animal reaction time is noted
Evaluation:
The average value of reaction time
after each intervals are calculated and
compared with the pretest value by
analysis of significance.
 At each time interval only those
animals which show a reaction time
twice as high or heigher as the pretest
value regarded as positive
3. Hot Plate Method
Purpose and Rational
The paw of mice and rats are very sensitive to
heat at the temperature which do not damage
the skin.
 The reaction to heat is characterized by jumping, withdrawal
and licking of the paws.
 The time until these response occur is prolonged after
administration of centrally acting analgesic
Procedure ( by Woolfe and Mac Donald, 1944):
The animal are placed in the hot plate consisting of electrically
heat surface (Temp.: 55 to 56 oC) and time until licking or
jumping occur recorded by stop watch.
The latency is recorded before and after 20, 60 and 90 min
following oral and s.c administration of standard or test
compounds.
Cont- Hot plate Method
Evaluation
The prolongation of latency times comparing the values
before and after administration of the test compounds or
values of the control with experimental groups cab be used
The hot plate tefor comparison using t-test.
Critical Assessment of the test
st has been used by many investigators and has been found to
be suitable for evaluation of centrally acting analgesic.
Limitation
Sedative, muscle relaxant and psycho mimetic drugs cause
false positive test.
4. Tail Immersion Methods
Purpose and rationale
The method has been developed to be selective for
morphine like compounds which are selectively capable to
prolong the reaction time of the typical tail- withdrawal
reflex in rats induced by immersing the end of the tail in the
warm water of 55 oC.
Procedure
Young Female Wister rats (170-210 g) are placed into the
individual cages leaving the tail hanging out freely. The animals
are allowed to adopt to the cages for 30 min before testing.
 The lower 5 cm of the tail marked. This part of the tail is
immersed in a cup of freshly filled water of exactly 55oC. Within
few second rats react by withdrawing the tail. The reaction time
is recorded in 0.5 s units by stopwatch.
Cont- Tail Immersion method
The reaction time is determined before and periodically
after either oral or subcutaneous administration of test
substances. Eg. after 30, 60, 120, 180, 240 and 360 mins.
 The cut-off time of immersion is 15 s.
 The withdrawal time of untreated animal is
between 1 to 5.5 s. A withdrawal time more
than 6s is regarded as positive response.
Evaluation: ED50 values can be calculated for each compounds
and time response curves (onset, peak and duration of effect) be
measured. All the morphine like analgesic have been shown to be
active at dose which do not change the behavior.
Critical Assessment of the test: The test is useful to differentiate
central opioid like analgesic from peripheral analgesics.
5. Formalin test in mice
Purpose and Rational
Formalin test consider as chronic pain model
which is sensitive to centrally acting analgesic agents.
Procedure: Male Wister rats (180-300 g) are administered
0.5ml 10% formalin solution into the dorsal portion of the
front paw. The test drug simultaneously administered S.C or
orally. Each individual rat placed into the clear plastic cage for
observation.
 Pain response are observed, at 30 and 60 min, and
indicated by elevation or favoring of paw or excessive licking
or biting of the paw.
Evaluation: Analgesic response or protection is indicated if
both paws are resting on the floor with no obvious favoring
of the injected paw.
The End