EPIBATIDINE BINDING-SITES AND ACTIVITY IN THE SPINAL-CORD

Epibatidine has been shown to be the most potent nicotinic agonist in several neuronal nicotinic receptor preparations. Similar to other nic otinic agonists, intrathecal (-)-epibatidine elicits dose-dependent in creases in presser, heart rate and pain responses in rats, as well as an increase in latency to withdraw from a noxious thermal stimulus. Th e latter response requires higher doses and is of shorter duration, su ggesting interaction with multiple subtypes of spinal nicotinic recept ors. In the present study, we relate the binding properties of (+/-)-[ H-3]epibatidine in spinal cord membrane preparations to the cardiovasc ular and behavioral responses. Unlike (-)-[H-3]cytisine or (-)-[H-3]ni cotine, (+/-)-[H-3]epibatidine reveals two sites; the ratio of high af finity to low affinity sites is 2:1. The rank ordering of potencies of the nicotinic agonists in displacing (+/-)-[H-3]epibatidine binding f rom spinal cord membranes correlates with the potencies in eliciting c ardiovascular and behavioral responses upon spinal administration. The nicotinic receptor antagonists, alpha-lobeline, dihydro-beta- erythro idine and methyllycaconitine, also displayed similar rank ordering of potencies in displacing (+/-)-[H-3]epibatidine, (-)-[H-3]cytisine or(- )-[H-3]nicotine binding to spinal nicotinic receptors. Virtually all t he nicotinic analogs exhibited a Hill coefficient of less than one in competing with (+/-)-[H-3]epibatidine to spinal cord membranes indicat ing their interaction with at least two classes of binding sites. Intr athecal(-)-epibatidine, in addition to eliciting an initial and subseq uently a sustained presser and tachycardic response, also exhibited a transient intervening bradycardia which coincided temporally with the duration of the analgesia. Repeated administration of (-)-epibatidine desensitized its responses as well as the cardiovascular and behaviora l responses to spinal nicotine and cytisine. Intrathecal alpha-lobelin e showed selectivity for blocking the analgesic response, whereas meth yllycaconitine exhibited selectivity for the presser and irritation re sponses. The NMDA receptor antagonist, AP-5, inhibited the presser, ta chycardic and irritation responses elicited by intrathecal (-)-epibati dine, confirming a role for spinal excitatory amino acids released by the nicotinic agonist. (C) 1997 Elsevier Science B.V.