Chronic nicotine administration does not increase nicotinic receptors labeled by [125 I]epibatidine in adrenal gland, superior cervical ganglia, pineal or retina

Neuronal nicotinic acetylcholine receptors (nAChRs) were measured in CNS and peripheral tissues following continuous exposure to saline or nicotine hydrogen tartrate (3.3 or 10 mg/kg/day) for 14 days via osmotic pumps. Initially, binding of [(3) H](-)nicotine, [(3) H]cytisine and [(3) H]epibatidine to nAChRs was compared to determine the suitability of each for these kinds of studies. The predominant nAChR labeled by agonists in the cerebral cortex is an alpha4beta2 subtype, whereas the predominant nicotinic receptors in the adrenal gland, superior cervical ganglia and pineal gland contain an alpha3 subunit, and they do not bind either [(3) H](-)nicotine or [(3) H]cytisine with high affinity. In retina some nAChRs bind all three ligands with high affinity, and others appear to bind only [(3) H]epibatidine. Thus, only [(3) H]epibatidine had high enough affinity to be useful for measuring the nAChRs in all of the tissues. The receptors from nicotine-treated rats were then measured using [(125) I]epibatidine, which has binding characteristics very similar to [(3) H]epibatidine. Treatment with the two doses of nicotine hydrogen tartrate increased binding sites in the cerebral cortex by 40% and 70%, respectively. In contrast, no significant changes in the density of receptor binding sites were found in the adrenal gland, superior cervical ganglia, pineal gland or retina. These data indicate that chronic administration of nicotine even at high doses does not increase all nicotinic receptor subtypes, and that receptors containing alpha3 subunits may be particularly resistant to this nicotine-induced change.