Antinociceptive Effect of Amitriptyline in Mice of Acute Pain Models

Indian Journal of Experimental Biology Vol. 45, June 2007, pp 529-531

Antinociceptive effect of amitriptyline in mice of acute pain models

Keshab Raj Paudel, Balbhadra Prasad Das, G P Rauniar, Himal Sangraula, Satish Deo & S K Bhattacharya Department of Pharmacology, B. P. Koirala Institute of Health Sciences, Dharan, Nepal Received 6 September 2006; revised 15 January 2007

Tricyclic antidepressant drugs induce antinociceptive effect and suggest that their analgesic action could be related to the monoaminergic activity of the drugs. The analgesic activity of amitriptyline was observed in mouse models of acute pain. Mice were divided into different groups and were given amitriptyline in different doses alone and in combination with morphine. Reaction time in Hot-Plate and Tail-Flick tests was observed. Results showed that amitriptyline had antinociceptive effect in acute pain state in experimental models. Amitriptyline in combination with morphine had better analgesic effect than the morphine alone in Hot-Plate test.

Keywords: Acute pain, Amitriptyline, Antinociception, Mice

Acute pain is a protective response to injury and most pain tolerance is influenced by psychological state. often it is nociceptive (i.e. resulting from injury or For example, the patients with acute pain show inflammation of somatic or visceral tissue)1. Studies normal personality profile, but the degree of pain indicate that specialized neural pathways are involved experienced is related to the degree of anxiety in transmission of pain and these pathways are present4. Psychotropic medication, particularly the sensitive to changes in stimulus features, such as tricyclic antidepressants, sometimes in combination intensity, quality and duration which are modulated with phenothiazines and anti-histamines, are effective by opioid peptides, serotonin and norepinephrine2. in many instances of central pain and help increase Transmission of painful stimulus through spinal the tolerance and decrease the need for the narcotics column and CNS is modulated by excitatory and in other pain states4. inhibitory neurotransmitters, as well as action at Metanalyses5,6 have confirmed the efficacy of sodium and potassium channels; norepinephrine and triclyclic antidepressants in the treatment of serotonin may be excitatory or inhibitory but they are neuropathic pain1. Tricyclic antidepressant drugs inhibitory on pain transmission1. induce antinociceptive effect and it is suggested that Nociceptive pain is usually treated with anti- their analgesic action could be related to the inflammatory or analgesic agents1. Non-steroidal anti- monoaminergic activity of the drugs7. However inflammatory drugs suppress noxious signals by studies on antinociceptive effect of these agents in reducing the sensitivity of peripheral nociceptors and acute pain management are lacking to support their opioid drugs are administered to provide long-lasting use in such a pain state, though the amitriptyline has pain relief2. Continued and prolonged use of narcotics shown to have some analgesic effect in patients with in patients with pain is not recommended because of acute pain8. Therefore the present study involves the serious behavioral consequences, the development of antinociceptive effect of amitriptyline in acute pain tolerance, and dependence liability. Long-term use of models of mice through Hot-Plate and Tail-Flick analgesics usually produces behavioral complications tests. that are more difficult to manage than the pain it was desired to eliminate3. Materials and Methods The recent literature on pain study shows that pain Animals⎯ Experiments were performed on adult threshold is relatively constant for an individual, but albino mice (n = 50) weighing 20-30 g. The mice were maintained under controlled room temperature ______(25°±2°C) and light and dark (12:12 hr) conditions Telephone- 977- 025-25555- 2483 (office) 977-9842021799 (residence) and were given food pellets and water ad libitum. E-mail: [email protected] Before conducting the experiment, ethical clearance 530 INDIAN J EXP BIOL, JUNE 2007

was obtained from the Local Ethical Committee on did morphine alone in Hot-Plate test (Table 1). In Animal Research and ethical guidelines for Tail-Flick test, high dose amitryptyline had investigations of experimental pain in conscious antinociceptive activity which was significant when animals were followed in accordance with IASP9 compared to saline but it differed significantly from (International Association for the Study of Pain). The morphine. Study showed that combination of mice were randomly divided into following five amitriptyline with morphine was more effective than groups of 10 each: group A (control – saline), group B morphine alone in Hot-Plate test. This better result of (standard control –morphine, 10mg/kg), group C (low the combined drugs (antidepressant and morphine) dose amitriptyline, 10mg/kg), group D (high dose can be explained by the fact that combined re-uptake amitriptyline, 50mg/kg), group E (morphine, 5mg/kg inhibition of serotonin and noradrenaline, and the + amitriptyline, 25mg/kg). Drugs were injected modulation of pain transmission by opioids appears to intraperitoneally 30 min prior to the experiment. Pain confer a greater degree of antinociception in animal was induced by placing the mice in the Hot-Plate models of experimental pain than the single agent12. meter and Tail-flick instrument. Morphine remains the reference compound among Hot-Plate test⎯ The thermal noxious stimulus was centrally acting analgesics both for acute and chronic produced in the mice by placing them on the hot-plate pain. Its mechanism of action and the role of opiate (Hot-plate, UGO BASILE, Italy) maintained at 55°C receptors are well known13. It has also been and the reaction time was recorded. Reaction time established that there are three major families of was taken as the period between placing the mice on opioid peptides in the brain; the enkephalins, the the hot-plate and time when they licked their paws. A dynorphins, and the endorphins2 associated with the cut-off time of 30 sec was used to prevent any thermal neurobiology of pain and its modulation. Besides injury to mice10, 11. these opioid receptors, serotonin and norepinephrine Tail-Flick test⎯ For the tail flick method pain was play a role in the modulation of signals related to 2 induced by giving infrared light on the tail of the mice tissue damage . (Tail-Flick Unit, UGO BASILE, Italy) 5 cm away Antinociceptive effect of high dose amitriptyline from the tip of the tail. Reaction time was noted by did not differ significantly from morphine (P>0.05) in observing the interval between placing the tail on the Hot-Plate test but it differed significantly (P<0.01) infrared light and the withdrawal of the tail. A cut off from morphine though it had antinociceptive effect time of 30 sec was used11. (P< 0.05) when compared to saline in Tail Flick test. 14 Statistical analysis⎯ The non-parametric Ardid et al. explained that antinociceptive potency Wilcoxon unpaired signed ranks test was used for the of re-uptake inhibitors varies according to their comparison of control and test groups. This test was monoamine specificity and the nature of stimuli. selected as the data did not have equal variance and a Serotonergic inhibitors were more effective in Hot- normal distribution. P value < 0.05 was considered Plate test. However, analgesic responses and the significant. mechanisms implicated were dependent in analgesiometer test used, and antinociceptive Results and Discussion responses were more potent in formaline test than in Amitriptyline had antinociceptive activity both in Tail-Flick test15. low and high doses in acute thermal nociception Valverde et al.15 have shown that tricyclic which was statistically significant and in combination antidepressants produce antinociception partly via the with morphine it produced more superior results than participation of endogenous opioid system and partly

Table 1⎯ Reaction time of different groups in Hot- Plate and Tail- Flick tests [Values are mean ± SD from 10 animals in each group] Parameters Group A Group B Group C Group D Group E Reaction time (sec) in Hot-Plate test 6.4±1.78 18.6±7.04b 12.5±2.28a 17.10±4.2b 26.6±3.75b,d Reaction time (sec) in Tail-Flick test 1.6±0.84 11.2±7.15b 1.8±0.42d 2.6±0.97a,d 5.6±3.03a,c P values: a <0.05, b < 0.01 compared to control c < 0.05, d < 0.01 compared to standard control Group A- control group (saline), Group B- standard control (morphine 10mg/kg), Group C (low dose amitriptyline, 10 mg/kg), Group D (high dose amitriptyline, 50 mg/kg), Group E (morphine + amitriptyline, 5mg/kg + 25 mg/kg). PAUDEL et al.: AMITRIPTYLINE AND ANTI-NOCICEPTION 531

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