Effects of selected direct acting cholinergic drugs on pain behaviour in the naked mole-rat (heterocephalus glaber)
Various endogenous substances are involved in the control of nociception both at the segmental and at the higher levels of central nervous system. These substances include acetylcholine, which modify pain processing in a wide variety of experimentally induced or clinically related pain states by interacting with specific receptors. Acetylcholine is the endogenous ligand for the cholinergic receptor system with muscarinic and nicotinic receptors, and systemic or intrathecal stimulation of these receptors results in modulation of pain responses in animals and humans. In this study, the role of cholinergic system in nociception in the naked mole-rat (Heterocephalus glaber) was evaluated. The study explored the antinociceptive effects of the muscarinic receptor agonist, oxotremorine (10, 20, 50 and 100 ug/kg body weight) and the nicotinic receptor agonist, epibatidine (0.5, 1,2 and 3 ug/kg body weight) using three commonly used nociceptive tests. These were the formalin (20 Ill, 10%), the hot plate (60°C) and the tail flick (56 °C) tests. To elucidate possible interaction with opioidergic system, a general opioid receptor antagonist, naloxone, was simultaneously administered with the cholinergic agonists. Muscarinic (atropine) and nicotinic (mecamylamine) blockers were used for antagonistic reactions. In the formalin test, the duration the animal took lickinglbiting the injected paw was scored in blocks of 5 minutes for a duration of 60 minutes, whereas in the hot plate test the latency (s) the animal took to react to the thermal pain was recorded. In the tail flick test, tailflick withdrawal latency (s) was scored. The selected high doses (20, 50 or 100 ug/kg) of oxotremorine induced a statistically significant (P<0.05) dose-dependent reduction in the mean time spent licking/biting the injected paw in both the first and second phases of the formalin test. In both early and late phases of formalin test, the effect of oxotremorine on the mean time spent lickinglbiting the injected paw was reversed by atropine. The animals treated with epibatidine (1, 2, or 3 ug/kg) showed a statistically significant (P<0.05) reduction in the mean time spent in licking/biting the injected paw in both early and late phases of the formalin test. When mecamylamine was administered together with epibatidine (2 ug /kg), it significantly increased the mean time spent in lickinglbiting the injected paw in both phases of the formalin test. In the hot-plate test, oxotremorine (20, 30 or 50 ug /kg) and epibatidine (1, 2 or 31lg/kg) caused a significantly different dose-dependant increase in the hotplate response latency. The effects of oxotremorine and epibatidine were blocked by atropine and mecamylamine, respectively. In the tail-flick test, oxotremorine (20, 30 or 50 ug /kg) and epibatidine (1, 2 or 3Ilg/kg) caused a significantly different dose-dependant increase in the tailflick response latency. In the same test, the effects of oxotremorine and epibatidine were blocked by atropine and mecamylamine, respectively. In all the three nociceptive tests, naloxone in combination with oxotremorine or epibatidine exhibited synergism of their effects. Administration of atropine, mecamylamine or 'naloxone alone did not show any significant effect on the nociceptive behaviour in any of the three tests. In conclusion, the study showed that oxotremorine and epibatidine are effective antinociceptive drugs in the naked mole-rat and that naloxone is able to potentiate their antinociceptive effects. The data further reveals that the cholinergic system is crucial in pain regulation in the naked mole-rat.