DIVERSITY OF NICOTINIC ACETYLCHOLINE-RECEPTORS IN RAT HIPPOCAMPAL-NEURONS .3. AGONIST ACTIONS OF THE NOVEL ALKALOID EPIBATIDINE AND ANALYSIS OF TYPE-II CURRENT

The nicotinic-agonist properties of epibatidine, an alkaloid originall y isolated from the Ecuadorian frog Epipedobates tricolor, was investi gated in rat hippocampal neurons grown in culture. The ability of epib atidine enantiomers to evoke nicotinic whole-cell currents of type IA, sensitive to blockade by alpha-bungarotoxin and methyllycaconitine an d presumably subserved by an alpha-7-based nAChR, and of type II, sens itive to blockade by dihydra-beta-erythroidine and presumably subserve d by an alpha(4) beta(2)-based nAChR, was studied and compared with th at of other nicotinic agonists. Epibatidine elicited type IA currents with EC(50) values of 2.9 and 4.3 mu M for the (-)- and (+)-enantiomer s, respectively. The potency of the agonists in evoking type IA curren ts was as follows: (-)-epibatidine > (+)anatoxin-a > (+)-epibatidine > (-)-nicotine > DMPP > cytisine > acetylcholine. The EC(50) values of (-)- and (+)-epibatidine in eliciting type II currents were 19 and 15 nM, respectively. The order of potency of the agonists in evoking type II currents was: (+)-epibatidine > (-)-epibatidine > cytisine > (+)-a natoxin-a > (-)-nicotine > acetylcholine. DMPP. Although these agonist s produced nearly identical maximal type IA responses, the size of the maximal type II responses varied with the agonist. Cytisine and (+)-a natoxin-a were the least efficacious agonists in inducing type II curr ents. Enantiomers of epibatidine showed the least stereoselectivity, t hose of nicotine showed a moderate stereoselectivity, and those of ana toxin-a exhibited the highest stereoselectivity in inducing either typ e of currents. In summary, these results suggest that epibatidine can be used as a novel nicotinic agonist for the study of type II currents , and that the nicotinic acetylcholine receptor related to type II cur rents may be one of the key target sites through which agonists such a s epibatidine and nicotine elicit their behavioral actions in vivo.