Antinociceptive and toxic effects of (+)-epibatidine oxalate attributable to nicotinic agonist activity

1. Epibatidine is an analgesic substance, isolated from the skin of the poisonous frog Epipedobates tricolor, for which the mechanism of action was previously unknown. 2. The IC50 of synthetic (+)-epibatidine oxalate (the naturally occurring isomer) for [3H]-nicotine binding to rat whole-brain membranes was 0.1 nM. The (-)-isomer also exhibited high affinity (IC50 = 0.2 nM). 3. (+)- and (-)-Epibatidine exhibited much lower affinity for displacement of the muscarinic ligand [3H]-N-methylscopolamine binding to rat cortical membranes (Kapp = 6.9 microM and 16.0 microM respectively). The (+)-enantiomer of epibatidine had an antagonist/agonist (NMS/oxo-M) binding ratio of 4.2 This is consistent with a muscarinic antagonist profile. 4. (+)-Epibatidine oxalate (10 microM) did not cause significant (> 30%) displacement of radioligand binding to opioid, excitatory amino acid, benzodiazepine, 5-HT, dopamine, adrenaline or peptide receptors. 5. (+)- and (-)-Epibatidine (5-20 micrograms kg-1 s.c.) doubled response latency in the mouse hot-plate test. Antinociception and behavioural depression induced by (+)-epibatidine (5 micrograms kg-1) was fully blocked by the nicotinic antagonists mecamylamine (2 mg kg-1 s.c.) or dihydro-beta-erythroidine (2 mg kg-1 s.c.). The muscarinic antagonist scopolamine (0.4 and 10 mg kg-1 s.c.) caused partial reversal of antinociception induced by (+)-epibatidine in mice, but not in rats. 6. These findings demonstrate that (+)-epibatidine oxalate salt is a highly selective and potent nicotinic analgesic agent.