Effects of monoamines on the intrinsic excitability of lateral orbitofrontal cortex neurons in alcohol-dependent and non-dependent female mice

Changes in brain reward and control systems of frontal cortical areas including the orbitofrontal cortex (OFC) are associated with alcohol use disorders (AUD). The OFC is extensively innervated by monoamines, and drugs that target monoamine receptors have been used to treat a number of neuropsychiatric diseases, including AUDs. Recent findings from this laboratory demonstrate that D2, α2-adrenergic and 5HT_1A receptors all decrease the intrinsic excitability of lateral OFC (lOFC) neurons in naïve male mice and that this effect is lost in mice exposed to repeated cycles of chronic intermittent ethanol (CIE) vapor. As biological sex differences may influence an individual's response to alcohol and contribute to the propensity to engage in addictive behaviors, we examined whether monoamines have similar effects on lOFC neurons in control and CIE exposed female mice. Dopamine, norepinephrine and serotonin all decreased spiking of lOFC neurons in naïve females via activation of G_iα-coupled D2, α2-adrenergic and 5HT_1A receptors, respectively. Firing was also inhibited by the direct GIRK channel activator ML297, while blocking these channels with barium eliminated the inhibitory actions of monoamines. Following CIE treatment, evoked spiking of lOFC neurons from female mice was significantly enhanced and monoamines and ML297 no longer inhibited firing. Unlike in male mice, the enhanced firing of neurons from CIE exposed female mice was not associated with changes in the after-hyperpolarization and the small-conductance potassium channel blocker apamin had no effect on current-evoked tail currents from either control or CIE exposed female mice. These results suggest that while CIE exposure alters monoamine regulation of OFC neuron firing similarly in males and female mice, there are sex-dependent differences in processes that regulate the intrinsic excitability of these neurons.

Tipepidine activates VTA dopamine neuron via inhibiting dopamine D₂ receptor-mediated inward rectifying K⁺ current

We previously reported that the novel antidepressant-like effect of tipepidine may be produced at least partly through the activation of mesolimbic dopamine (DA) neurons via inhibiting G protein-coupled inwardly rectifying potassium (GIRK) channels. In this study, we investigated the action of tipepidine on DA D2 receptor-mediated GIRK currents (IDA(GIRK)) and membrane excitability in DA neurons using the voltage clamp and current clamp modes of the patch-clamp techniques, respectively. DA neurons were acutely dissociated from the ventral tegmental area (VTA) in rats and identified by the presence of the hyperpolarization-activated currents. Tipepidine reversibly inhibited IDA(GIRK) with IC50 7.0 μM and also abolished IDA(GIRK) irreversibly activated in the presence of intracellular GTPγS. Then tipepidine depolarized membrane potential and generated action potentials in the neurons current-clamped. Furthermore, the drug at 40 mg/kg, i.p. increased the number of cells immunopositive both for c-Fos and tyrosine hydroxylase (TH) in the VTA. These results suggest that tipepidine may activate DA neurons in VTA through the inhibition of GIRK channel-activated currents.