Mechanisms of stimulatory effects of mecamylamine on the dorsal raphe neurons

Previous studies showed that mecamylamine a noncompetitive and nonspecific blocker of nicotinic acetylcholine receptors (nAChRs), stimulates the activity of the dorsal raphe nucleus (DRN) serotonergic neurons and DRN serotonin (5-HT) release. In the present study, the mechanisms involved in these mecamylamine-induced effects were examined using electrophysiology and calcium-imaging studies, both performed in Wistar rat midbrain slices. Mecamylamine (0.5-9 mu M), bath administered, increased the firing frequency of identified 5-HT DRN neurons by a maximum of 5% at 3 mu M. This effect was accompanied by a 112 % increase in the frequency of spontaneous excitatory postsynaptic currents of 5-HT DRN neurons. It was blocked by the AMPA/kainate receptor blocker CNQX (10 mu M) and by the specific alpha 4 beta 2 nAChRs blocker dihydro-beta-erythroidine (100 nM) but was not affected by tetrodotoxin (TTX, 500 nM). Simultaneously, mecamylamine produced a 58 % decrease in the frequency of GABAergic spontaneous inhibitory postsynaptic currents, an effect that was not influenced by TTX. Calcium-imaging studies support the results obtained with the electrophysiological studies by showing that mecamylamine (3 mu M) increases the activity of a cell population located in the midline of the DRN, which was sensitive to the inhibitory effects of 8-OH-DPAT, an agonist at 5-HT1A receptors. It is assumed that mecamylamine, in low concentrations, acts as an agonist of alpha 4 beta 2 nAChRs present on the glutamatergic DRN terminals, thus increasing intra-raphe glutamate release. This stimulatory effect is reinforced by the decrease in DRN GABA release, which is dependent on the mecamylamine-induced blockade of alpha 7 nAChRs located on DRN GABAergic terminals. We hypothesize that at least a part of mecamylamine antidepressant effects described in animal models of depression are mediated by an increase in DRN 5-HT release.