Mefloquine effects on ventral tegmental area dopamine and GABA neuron inhibition: A physiologic role for Connexin-36 gap junctions

Abstract

Connexin-36 (Cx36) gap junctions (GJs) appear to be involved in the synchronization of GABA interneurons in many brain areas. We have previously identified a population of Cx36-connected ventral tegmental area (VTA) GABA neurons that may regulate mesolimbic dopamine (DA) neurotransmission, a system implicated in reward from both natural behaviors and drugs of abuse. The aim of this study was to determine the effect mefloquine (MFQ) has on midbrain DA and GABA neuron inhibition, and the role Cx36 GJs play in regulating midbrain VTA DA neuron activity in mice. In brain slices from adolescent wild-type (WT) mice the Cx36-selective GJ blocker mefloquine (MFQ, 25 μM) increased VTA DA neuron sIPSC frequency sixfold, and mIPSC frequency threefold. However, in Cx36 KO mice, MFQ only increased sIPSC and mIPSC frequency threefold. The nonselective GJ blocker carbenoxolone (CBX, 100 μM) increased DA neuron sIPSC frequency twofold in WT mice, did not affect Cx36 KO mouse sIPSCs, and did not affect mIPSCs in WT or Cx36 KO mice. Interestingly, MFQ had no effect on VTA GABA neuron sIPSC frequency. We also examined MFQ effects on VTA DA neuron firing rate and currentevoked spiking in WT and Cx36 KO mice, and found that MFQ decreased WT DA neuron firing rate and current-evoked spiking, but did not alter these measures in Cx36 KO mice. Taken together these findings suggest that blocking Cx36 GJs increases VTA DA neuron inhibition, and that GJs play in key role in regulating inhibition of VTA DA neurons.

Connexin-36 Knock-Out Mice have Increased Threshold for Kindled Seizures: Role of GABA Inhibition

“…Thus, MFQ has multiple effects that need to be considered with any interpretation regarding its effects. However, we have shown previously in midbrain slice studies that its GJ blocking effects predominate, and may be exclusively operational, after 1 hr of administration [55], which was the time delay we chose to study its effects on kindled seizures. Furthermore, in this previous study, we demonstrated that GABA inhibition to VTA dopamine neurons following MFQ treatment was markedly enhanced, nearly 5X over control or baseline conditions, suggesting that uncoupling GABA neurons from their electrotonic load increases their rate of firing and ultimately their inhibitory drive to dopamine neurons.…”

“…We have previously demonstrated that MFQ blocks electrical coupling between VTA GABA neurons in adult mice [25,55] and reduces brain stimulation reward [52]. While MFQ is selective for GJs, it has other physiologically-relevant non-specific effects.…”

“…Specifically, parvalbumin (PV)-containing GABAergic interneurons of the hippocampus have been suggested to have GJs, which likely facilitate synchronization of oscillatory neuronal networks [53,54]. We have also evaluated the role of Cx36 GJs in modulating ventral tegmental area (VTA) dopamine and GABA neuron excitability [25,55,56] as well as ethanol effects on these neurons [56,57]. It was in these studies that we noted that Cx36 KO mice did not exhibit ethanol withdrawal-induced seizures compared to WT mice (unpublished observation).…”

Nicotine Enhances the Excitability of Gaba Neurons in the Ventral Tegmental Area via Activation of Alpha 7 Nicotinic Receptors on Glutamate Terminals

“…All neurons classified as VTA GABA neurons in vivo were located in the VTA, and met the criteria established in previous studies for spike waveform characteristics in rats [24][25][26], and in mice [27,28]. Presumed VTA GABA neurons were characterized by short-duration (<200 µsec; measured at half-peak amplitude of the spike), initially negative-going, relatively fast-firing (10-80 Hz), non-bursting spikes.…”

“…Substantia nigra compacta (SNc) was then identified medial to SNr. GABA neurons in the VTA were studied by visualizing GAD+ neurons in an area medial to the glowing SNr, posterior to the fasciculus retroflexus and mammillothalamic tract, and anterior to the decussation of the superior cerebellar peduncle [27,28]. Neurons in the VTA of GAD-GFP mice that did not fluoresce but exhibited a non-cation specific inward rectifying current (I h ), in combination with relatively low input resistance and regular, slow spike activity were assumed to be DA neurons [25,27,28,30,31].…”