Ganglionic nicotinic receptor agonists exhibit anti-muscarinic effects in guinea-pig olfactory cortical brain slices

Abstract

The action of some nicotinic acetylcholine receptor agonists was re-examined on the surface field potentials (N-waves) evoked by electrical stimulation of the lateral olfactory tract in guinea-pig olfactory cortical brain slices. Bath superfusion of nicotine or the nicotinic stimulants dimethylphenylpiperazinium (DMPP), lobeline, cytisine, tetramethylammonium or suberyldicholine (up to 100 microM) had little or no effect on the extracellular N-wave amplitude, or the membrane potential, input resistance or excitability of olfactory neurones recorded intracellularly. In contrast, the muscarinic agonists, carbachol or oxotremorine-M consistently depressed the field in a reversible dose-dependent manner. Interestingly, in the presence of the ganglionic stimulants DMPP (n = 6 slices) or lobeline (n = 5 slices) (10-50 microM), the effects of carbachol or oxotremorine-M were antagonized in a weak competitive-type manner (pA2 values = 5.58 and 5.63 respectively, estimated from Schild plots, constrained to unity slope). This anti-muscarinic action was unaffected by d-tubocurarine or hexamethonium. Nicotine, cytisine, tetramethylammonium and suberyldicholine showed much weaker and inconsistent carbachol-blocking effects. Combination of DMPP with atropine produced dose ratio shifts close to those predicted for a common-site interaction of two competitive antagonists. In conclusion, consistent pre- or postsynaptic nicotinic agonist actions could not be detected in olfactory cortex slices; however, some ganglionic nicotinic agonists were shown to exhibit significant anti-muscarinic effects on this preparation. We suggest this action might be due to a direct atropine-like mechanism.