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 µM), bath administered, increased the firing frequency of identified 5-HT DRN neurons by a maximum of 5% at 3 µM. 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 µM) and by the specific α4ß2 nAChRs blocker dihydro-ß-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 µM) 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 α4ß2 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 α7 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.

Nicotine Increases Spontaneous Glutamate Release in the Rostromedial Tegmental Nucleus.

The rostromedial tegmental nucleus (RMTg) is a bilateral structure localized in the brainstem and comprise of mainly GABAergic neurons. One of the main functions of the RMTg is to regulate the activity of dopamine neurons of the mesoaccumbens pathway. Therefore, the RMTg has been proposed as a modulator of the reward system and adaptive behaviors associated to reward learning. The RMTg receives an important glutamatergic input from the lateral habenula. Also, it receives cholinergic inputs from the laterodorsal and pedunculopontine tegmental nuclei. Previously, it was reported that nicotine increases glutamate release, evoked by electric stimulation, in the RMTg nucleus. However, the mechanisms by which nicotine induces this effect were not explored. In the present work, we performed electrophysiological experiments in brainstem slices to study the effect of nicotine on spontaneous excitatory postsynaptic currents recorded from immunocytochemically identified RMTg neurons. Also, we used calcium imaging techniques to explore the effects of nicotine on multiple RMTg neurons simultaneously. We found that nicotine promotes the persistent release of glutamate through the activation of α7 nicotinic acetylcholine receptors present on glutamatergic afferents and by a mechanism involving calcium release from intracellular stores. Through these mechanisms, nicotine increases the excitability and synchronizes the activity of RMTg neurons. Our results suggest that the RMTg nucleus mediates the noxious effects of the nicotine, and it could be a potential therapeutic target against tobacco addiction.

Insulin Regulates GABAA Receptor-Mediated Tonic Currents in the Prefrontal Cortex.

Recent studies, have shown that insulin increases extrasynaptic GABAA receptor-mediated currents in the hippocampus, causing alterations of neuronal excitability. The prefrontal cortex (PFC) is another brain area which is involved in cognition functions and expresses insulin receptors. Here, we used electrophysiological, molecular, and immunocytochemical techniques to examine the effect of insulin on the extrasynaptic GABAA receptor-mediated tonic currents in brain slices. We found that insulin (20-500 nM) increases GABAA-mediated tonic currents. Our results suggest that insulin promotes the trafficking of extrasynaptic GABAA receptors from the cytoplasm to the cell membrane. Western blot analysis and immunocytochemistry showed that PFC extrasynaptic GABAA receptors contain α-5 and δ subunits. Insulin effect on tonic currents decreased the firing rate and neuronal excitability in layer 5-6 PFC cells. These effects of insulin were dependent on the activation of the PI3K enzyme, a key mediator of the insulin response within the brain. Taken together, these results suggest that insulin modulation of the GABAA-mediated tonic currents can modify the activity of neural circuits within the PFC. These actions could help to explain the alterations of cognitive processes associated with changes in insulin signaling.