Pentazocine, a kappa opioid receptor agonist, was tested for its analgesic effects using a tail flick analgesiometer in rats. Rats were divided into two groups - one treated with saline (control) and one treated with pentazocine. Tail flick latency was measured before and after treatment. Pentazocine significantly increased tail flick latency and percentage analgesia at 30, 60, 90, and 120 minutes post-treatment compared to controls, demonstrating its narcotic analgesic effects. Pentazocine is believed to relieve pain through agonism of kappa receptors in the peripheral and central nervous systems with fewer side effects than mu receptor agonists like morphine.

1. Pharmacology IPracticals- Expt No 4 PESU Page 1
Exp No 4 Date- 3-9-2019
Study of narcotic analgesic activity by tail flick Analgesiometer
Aim- To study the narcotic analgesic activity of kappa receptor agonist (pentazocine) using
tail flick analgesiometer.
Theory- Pain is an unpleasant sensation that can be either acute or chronic and that is a
consequence of complex neurochemical processes in the peripheral and central nervous
system. Drugs used to relieve pain are opioid (morphine like) and nonopioid (aspirin like)
analgesic group of drugs.
Pentazocine- It is morphine like opioid analgesic. Its analgesic action is due to the agonistic
action at kappa opioid receptors. Opioid analgesics can act through three different types of
opioid receptors, called mu, delta, and kappa. Morphine, the most widely used opioid
analgesic, acts primarily via activation of the mu opioid receptor located in the central
nervous system (CNS). This CNS action induces pain relief but is also associated with a wide
array of CNS-mediated side effects including nausea/vomiting, sedation, respiratory
depression, and abuse liability. As a way to avoid these undesirable CNS and mu opioid
mediated side effects, there has been an effort to develop opioids which activate peripheral
kappa opioid receptors present on sensory nerves, immune cells and the dorsal root ganglion
(DRG). Such compounds, Kappa Opioid Receptor Agonists (KORAs), are thought to have
the potential to provide pain relief (peripheral opioid analgesia) without producing significant
CNS and mu-opioid mediated side effects.
Tail-flick analgesiometer (Radiant heat analgesiometer)- It provides pain stimulus by
heated nichrome wire in a rat’s tail to determine analgesic activity of drugs. The tail flick
latency was obtained thrice before the administration of the drug, and mean was used as pre
drug latency. The tail was placed on the platform in such a way that the middle portion of the
tail remained just above the hot wire but without touching it. Radiant heat was directed to the
proximal third of the tail through a hot nichrome wire of the analgesiometer. The strength of
the current passing through the naked nichrome wire was kept constant at 2-4 Amps. The
latency period (reaction time) was noted when the animal responded with a sudden and
characteristic flick or tail lifting. A cut off time of 8-10 sec was planned to avoid any tissue
damage in the animal. The cut off time was considered when the animals do not respond up
to 10 seconds. Reaction time in seconds was used as the unit for measurement of pain and an
increase in reaction time was indicative of analgesia. Time between placing the tail of the rat
on the radiant heat source and sharp withdrawal of the tail was recorded as “reaction time”.
Cut off time of ten seconds was imposed in all sets of experiments taken as maximum latency

2. Pharmacology IPracticals- Expt No 4 PESU Page 2
so as to rule out thermal injury while noting down the reaction time. Animals that showed a
mean reaction time outside the range of five-six seconds, were discarded. In all the groups,
tail-flick test was performed prior to drug administration, and at 30, 60, 90 and 120 minutes
after drug administration, and the reaction time at each time interval (test latency) was
calculated. 1
Percentage analgesia was calculated using the following formula:
% Analgesia = MPE=TL-BL / ML-BL × 100
Where, M.P.E. = Maximum possible effect, M.L. = Maximum latency or cut off time T.L. =
Test latency, B.L. = Basal latency or control latency
Study Design- Study of analgesic activity using tail flick (radiant heat) analgesiometer in
albino Wistar rats.

3. Pharmacology IPracticals- Expt No 4 PESU Page 3
Group I (Normal control group) Vehicle – normal saline
Table No 1- Dosing schedule and reaction time (in sec) in normal vehicle rats (Group I)
Vehicle- water- 2ml/kg BW IP
SN Markings Wt in gm Water Reaction time (in sec)
mg ml 0min 30min 60min 90min 120min
1 H 135 - 0.27 1.15 1.26 1.15 1.29 1.30
2 B 180 - 0.36 1.50 1.87 1.50 1.78 1.75
3 HB 160 - 0.32 1.66 1.66 1.70 1.72 1.71
4 TB 170 - 0.34 1.71 1.76 1.72 1.74 1.74
5 TT 160 - 0.32 1.65 1.66 1.68 1.65 1.70
6 UM 150 - 0.30 1.61 1.63 1.62 1.63 1.64
SD ± Mean 1.54±0.2068 1.64±0.2062 1.56±0.2167 1.63±0.1781 1.64±0.1710
Table 2-Group II- Dosing schedule and reaction time (in sec) in Pentazocine treated rats
Dose of Pentazocine- 10mg/kg BW IP Stock Sol- 10mg/ml
SN Markings Wt in gm Pentazocine Reaction time (in sec)
mg ml 0min 30min 60min 90min 120min
1 H 180 1.80 0.18 1.77 3.00 3.24 3.45 3.10
2 B 156 1.56 0.15 1.65 2.50 2.46 2.70 2.90
3 HB 150 1.50 0.15 1.62 2.43 2.42 2.64 2.83
4 TB 170 1.70 0.17 1.71 2.88 3.11 3.22 2.96
5 TT 180 1.80 0.18 1.76 2.97 3.20 3.40 3.02
6 UM 160 1.60 0.16 1.68 2.60 2.66 2.67 2.92
SD ± Mean 1.69±0.0598 2.73±0.2501 2.84±0.3782 3.01±0.3843 2.95±0.0950
Table No 3- Maximum possible effect of drugs in tail flick model of analgesia in group I
Normal rats
SN Markings Wt in gm % Analgesia Maximum possible effect (MPE)
After 30min After 60min After 90min After 120min
1 H 135 1.24 0 1.58 1.69
2 B 180 4.35 0 3.29 2.94
3 HB 160 0 0.47 0.71 0.59
4 TB 170 0.60 0.12 0.36 0.36
5 TT 160 0.11 0.35 0 0.59
6 UM 150 0.23 0.11 0.23 0.35
SD ± Mean 1.66±1.0883 0.19±0.1750 1.23±1.0283 1.08±1.0357

4. Pharmacology IPracticals- Expt No 4 PESU Page 4
Table No 4- Maximum possible effect of pentazocinein group II- Pentazocine treatedrats.
SN Markings Wt in gm % Analgesia Maximum possible effect (MPE)
After 30min After 60min After 90min After 120min
1 H 180 14.94 17.86 20.41 16.16
2 B 156 10.17 9.70 12.57 14.97
3 HB 150 9.66 9.54 12.17 14.43
4 TB 170 14.11 16.88 18.21 15.07
5 TT 180 14.68 17.47 19.90 15.29
6 UM 160 11.05 11.77 11.89 14.90`
SD ± Mean 12.43±2.4029 13.87±3.9620 15.85±4.0681 15.13±0.5759
Statistical analysis-Done by student t test
Table No 5- showing reaction time in sec and percentage analgesic activity (MPE) in Group I
(vehicle control) rats
Time Reaction time in sec % Analgesic activity P value Level of significance
At 0 min 1.54±0.2068 -- --
At 30 min 1.64±0.2062 1.66±1.0883 0.4518 Non-significant
At 60 min 1.56±0.2167 0.19±0.1750 0.9048 Non-significant
At 90 min 1.63±0.1781 1.23±1.0283 0.4462 Non-significant
At 120 min 1.64±0.1710 1.08±1.0357 0.4141 Non-significant
Table No 6- Showing reaction time in sec and percentage analgesic activity (MPE) in Group
II (Pentazocine treated) rats
Time Reaction time in sec %Analgesic activity P value Level of significance
At 0 min 1.69±0.0598 0 - -
At 30 min 2.73±0.2501*** 12.43±2.4029*** <0.0001 Significant
At 60 min 2.84±0.3782*** 13.87±3.9620*** <0.0001 Significant
At 90 min 3.01±0.3843*** 15.85±4.0681*** <0.0001 Significant
At 120 min 2.95±0.0950*** 15.13±0.5759*** <0.0001 Significant
Results and discussion-
Pentazocine is an opioid analgesic possessing mixed agonist-antagonist activity at opiate
receptors. Opioids are believed to exert their analgesic effects by stimulating specific opiate
receptors, designated as mu, kappa, and delta. Pentazocine is an agonist at kappa receptors
but is a weak antagonist or partial agonist at mu-receptors. Actions at kappa receptors are
believed to produce alterations in the perception of pain as well as emotional response to
pain. Since pentazocine is less active at the mu-receptor, it produces less respiratory
depression and posses a lower risk of physical dependence than morphine.

5. Pharmacology IPracticals- Expt No 4 PESU Page 5
In Group I (vehicle treated rats), there is no significant increase in reaction time after 30, 60,
90 and 120 minutes on comparison with reaction time at 0 minutes as the vehicle does not
possess any analgesic activity.
In Group II (pentazocine treated rats), a significant increase in reaction time after 30, 60, 90
and 120 minutes is observed on comparison with reaction time at 0 minutes.
Conclusion- It is concluded that pentazocine has shown narcotic analgesic activity in rats.
The rats treated with pentazocin have shown a significant increase in reaction time as well as
% analgesic effect when compared to vehicle treated rats.