CENTRAL ANALGESIC ACTIVITY OF Litsea polyantha Juss . BARK EXTRACT

The sensation of pain is initiated in peripheral pain receptors (nociceptors) and its purpose is to draw attention to tissue damage. In order to test analgesic activity, it is obviously necessary to induce pain in the subject and then modify the response to, or perception of, this pain. Analgesic studies of the methanol (90% v/v) extract (MELP) of Litsea polyantha Juss. bark (Yield: 11.79% w/w) was carried out using healthy adult Swiss albino mice of either sex weighing between 20 to 25 g respectively. The experiment protocols were approved by the Institutional Animal Ethical Committee (621/02/ac/CPCSEA) prior to the conduct of the animal experiments. The animals were divided into 6 groups (n=6). Group I and II were used as control, received 10% v/v propylene glycol (PG) and distilled water (DW) at the dose of 10 ml/kg b.w. Group III, IV & V were treated with MELP (50, 75 and 100 mg/kg b.w., i.p.), respectively; Group VI received Morphine sulphate (10 mg/kg b.w., s.c.) an opioid analgesic as standard drug. A reduction in the tail withdrawal as compared to the control group was considered as evidence for the presence of analgesia. Tail flick latency was measured 30 min after the drug administration and Pain Inhibition Percentage (PIP) was calculated. MELP given by intraperitoneal route in mice showed significant and dose-dependent central analgesic activity (P<0.001) at all dose levels. MELP showed 22.2% – 60.4% increase in PIP in tail flick test and 21.2% – 67.8% increase in PIP in tail immersion method. 3


INTRODUCTION
The sensation of pain is initiated in peripheral pain receptors (nociceptors) and its purpose is to draw attention to tissue damage.The impulses are transmitted to the dorsal horn, spinal cord, reticular formation and thalamus and finally to the cerebral cortex.Thus many parts of the brain are involved in the perception of pain.
Analgesics can therefore work in several ways, and it is for this reason that they are often used in combination, mainly a narcotic-type with an anti-inflammatory or paracetamol.Narcotic analgesics work by mimicking natural neurotransmitter peptides known as endorphins and encephalin and others.There are several opioid receptors known, the main CNS receptors being the  (delta), k (kappa) and  (mu), with others including the  (sigma) and  (epsilon) receptors.
Morphine, the oldest and one of the most widely used of the opiate analgesics, is known to act primarily at  receptors.Naloxone antagonizes drug action at ,  and k receptors.In order to test for analgesic activity it is obviously necessary to induce pain in the subject and then modify the response to, or perception of, this pain (anesthesia, where the passage of pain impulses to the CNS is inhibited).This causes some difficulty in animal experiments; it is assumed that the animal responds to a pain stimulus in a similar manner to that which a human would, which cannot be proved.

Plant Materials
The methanol (90% v/v) extract (MELP) of Litsea polyantha Juss.bark (Yield: 11.79% w/w) was concentrated in rotary evaporator followed by lyophilization.The completely dried sample was then reconstituted with 10% v/v propylene glycol (PG) for pharmacological activities.

Methods
Analgesic activity of the methanol (90%) extract of Litsea polyantha Juss.was determined by both thermal and chemical methods in mice.

Results
MELP given by intraperitoneal route in mice showed significant and dose dependent central analgesic activity (P<0.001) at all dose levels (Table 1 and Figure 1).MELP showed 22.2% -60.4% increase in PIP in tail flick test.

Tail Immersion Method
The animals were divided into 6 groups (n=6).Group I and II were used as control, received 10% v/v propylene glycol (PG) and distilled water (DW) at the dose of 10 ml/kg b.w.
Before administration of the test compound or the standard drug the normal reaction time was determined.The water in a beaker was kept at a temperature of 55 ± 0.5 C. Mice are held in position in a suitable restrainer with the tail protruding out.The tail up to 5 cm was dipped in the beaker of hot water.The time taken to withdraw the tail clearly out of water is taken as the reaction time.Tail withdrawal response was measured starting 30 min after the challenge with the treatments.A cut off period was kept 15-18 s to prevent the damage of the tail.Tail withdrawal latency was measured 30 min after the drug administration and Pain Inhibition Percentage (PIP) was calculated.

Result
MELP given by intraperitoneal route in mice showed significant and dose dependent central analgesic activity (P<0.001) at all dose levels (Table 2 and Figure 2).MELP showed 21.2% -67.8% increase in PIP in tail immersion method.

Results
MELP given by intraperitoneal route in mice showed significant and dose dependent central analgesic activity (P<0.001) at all dose levels (Table 3 and Figure 3).MELP showed 39.9% -100% increase in PIP in hot plate method.

1 Central Analgesic Activity 1 . 1
Thermal method: Central analgesic activity o Tail flick method o Tail immersion method o Eddy's hot plate method Tail Flick MethodThe animals were divided into 6 groups (n=6).Group I and II were used as control, received 10% v/v propylene glycol (PG) and distilled water (DW) at the dose of 10 ml/kg b.w.Group III, IV & V were treated with MELP (50, 75 and 100 mg/kg b.w., i.p.), respectively; Group VI received Morphine sulphate (10 mg/kg b.w., s.c.) an opioid analgesic as standard drug.Before administration of the test compound or the standard drug the normal reaction time was determined.Basal reaction time of all the animals to radiant heat was recorded by placing the tip of the tail on the radiant heat source.A reduction in the tail withdrawal as compared to the control group was considered as evidence for the presence of analgesia.Tail flick latency was measured 30 min after the drug administration and Pain Inhibition Percentage (PIP) was calculated according to the following equation: PIP = [(T 1 -T 0 )/ T 0 ] x 100; where, T 1 is post drug latency and T 0 is predrug latency.

Figure 1 :
Figure 1: Effect of MELP on tail flick response in Swiss albino mice

Figure 2 : 1 . 3
Figure 2: Effect of MELP on Tail immersion response in Swiss albino mice

Figure 3 :
Figure 3: Effect of MELP on hot plate response in Swiss albino mice

Table 1 : Effect of MELP on tail flick response in Swiss albino mice
Values reported as Mean ± SEM (n=6).The data were analyzed by two way ANOVA followed by Bonferroni's Multiple Comparison Test.Asterisk indicated statistically significant values from control.* P<0.001.PG: Propylene Glycol; DW: Distilled Water; MELP: Methanol (90% v/v) extract of Litsea polyantha Juss.bark; PIP: Pain Inhibition Percentage.