Aim: Substantial colocalization of functionally independent alpha4 nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors on presynaptic terminals has been observed in brain. The present study was aimed at addressing whether nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors interact on the same presynaptic terminal, suggesting a convergence of cholinergic and serotonergic regulation.
Methods: Ca(2+) responses in individual, isolated nerve endings purified from rat striatum were measured using confocal imaging.
Results: Application of 500 nmol/L nicotine following sustained stimulation with the highly selective 5-HT(3) receptor agonist m-chlorophenylbiguanide at 100 nmol/L resulted in markedly reduced Ca(2+) responses (28% of control) in only those striatal nerve endings that originally responded to m-chlorophenylbiguanide. The cross-regulation developed over several minutes. Presynaptic nerve endings that had not responded to m-chlorophenylbiguanide, indicating that 5-HT(3) receptors were not present, displayed typical responses to nicotine. Application of m-chlorophenylbiguanide following sustained stimulation with nicotine resulted in partially attenuated Ca(2+) responses (49% of control). Application of m-chlorophenylbiguanide following sustained stimulation with m-chlorophenylbiguanide also resulted in a strong attenuation of Ca(2+) responses (12% of control), whereas nicotine-induced Ca(2+) responses following sustained stimulation with nicotine were not significantly different from control.
Conclusion: These results indicate that the presynaptic Ca(2+) increases evoked by either 5-HT(3) receptor or nicotinic acetylcholine receptor activation regulate subsequent responses to 5-HT(3) receptor activation, but that only 5-HT(3) receptors cross-regulate subsequent nicotinic acetylcholine receptor-mediated responses. The findings suggest a specific interaction between the two receptor systems in the same striatal nerve terminal, likely involving Ca(2+)-dependent intracellular pathways that regulate these signaling systems at one or more levels.