Abnormal response of dopaminergic neurons to nicotine without perturbation of nicotinic receptors in αCGRP knock-out mice

Alpha-calcitonin gene-related peptide (αCGRP) is a neuropeptide with multiple biological properties, including the regulation of nicotinic acetylcholine receptors (nAChRs). We have previously reported a reduction of somatic withdrawal symptoms in αCGRP knock-out mice exposed to chronic nicotine, leading us to investigate the contribution of αCGRP to the regulations of ventral tegmental area (VTA) neurons and their response to nicotine. The electrophysiological activity of VTA dopaminergic (DA) neurons was recorded in vivo, under anesthesia. These neurons displayed identical spontaneous electrophysiogical activities in wild-type and αCGRP−/− mice. However, we found that intravenous administration of nicotine (30 μg/kg) had no significant effect on the activity of DA neurons in αCGRP−/− mice, whereas it induced a doubling of the firing rate in wild-type animals. A higher dose (90 μg/kg) produced a significant excitation in both strains, but this effect remained smaller in the mutants. To investigate this difference, we have studied the functional state of nAChRs in wild-type and αCGRP−/− mice. Both strains exhibited identical expression of α7 and α4β2 nAChRs as revealed by autoradiographical studies in the VTA. In addition, focal application of acetylcholine on DA neurons recorded by patch-clamp revealed identical currents mediated by nAChRs in mutant animals, as compared to wild-type mice. These data outline the possibility of a contribution of αCGRP to the effects of nicotine on DA neurons, by a physiological pathway independent of VTA nicotinic receptors.