Neurosteroid Agonist at GABAA Receptor Induces Persistent Neuroplasticity in VTA Dopamine Neurons

Abstract

The main fast-acting inhibitory receptors in the mammalian brain are g-aminobutyric acid type-A (GABA A ) receptors for which neurosteroids, a subclass of steroids synthesized de novo in the brain, constitute a group of endogenous ligands with the most potent positive modulatory actions known. Neurosteroids can act on all subtypes of GABA A receptors, with a preference for d-subunitcontaining receptors that mediate extrasynaptic tonic inhibition. Pathological conditions characterized by emotional and motivational disturbances are often associated with perturbation in the levels of endogenous neurosteroids. We studied the effects of ganaxolone (GAN)-a synthetic analog of endogenous allopregnanolone that lacks activity on nuclear steroid receptors-on the mesolimbic dopamine (DA) system involved in emotions and motivation. A single dose of GAN in young mice induced a dose-dependent, longlasting neuroplasticity of glutamate synapses of DA neurons ex vivo in the ventral tegmental area (VTA). Increased a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate ratio and rectification of AMPA receptor responses even at 6 days after GAN administration suggested persistent synaptic targeting of GluA2-lacking AMPA receptors. This glutamate neuroplasticity was not observed in GABA A receptor d-subunit-knockout (d-KO) mice. GAN (500 nM) applied locally to VTA selectively increased tonic inhibition of GABA interneurons and triggered potentiation of DA neurons within 4 h in vitro. Place-conditioning experiments in adult wild-type C57BL/6J and d-KO mice revealed aversive properties of repeated GAN administration that were dependent on the d-subunits. Prolonged neuroadaptation to neurosteroids in the VTA might contribute to both the physiology and pathophysiology underlying processes and changes in motivation, mood, cognition, and drug addiction.

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

“…In vitro incubation of midbrain slices with 0.5 mM ganaxolone for .4 hours also induced increased AMPAR/NMDAR current ratio in DA neurons. In agreement with the preference for d subunit-containing receptors, the neuroplasticity effects of gaboxadol and ganaxolone were abolished in d-KO mice (Vashchinkina et al, 2012(Vashchinkina et al, , 2014a. However, the level of d subunit protein expression in VTA GABA neurons is not very high, at least compared with the cerebellar granule neurons, and, therefore, direct immunohistochemical demonstration of its presence has failed because of background labeling even in the d-KO mice by available d subunit antibodies (unpublished).These results indicate that drugs potentiating tonic inhibitory conductance also target primarily GABAergic interneurons in the VTA and induce neuroplasticity effects in DA neurons by disinhibition.…”

“…Second, this finding was extended to a neurosteroid agonist ganaxolone, which does not have genomic effects like endogenous neurosteroids (Carter et al, 1997). A single mildly sedative dose of ganaxolone induced a similar long-lasting neuroplasticity in VTA DA neurons ex vivo as a dose of gaboxadol (Vashchinkina et al, 2014a). By using a GAD67-GFP knock-in mouse line, in which GABA neurons are fluorescently labeled, ganaxolone in vitro provoked a fourfold increase in tonic GABA Amediated conductance in VTA GABA neurons, but not in DA neurons of the TH-EFGP transgenic mice.…”

“…Surprisingly, both gaboxadol and ganaxolone failed to induce CPP, and if anything they induced aversion (Vashchinkina et al, 2012(Vashchinkina et al, , 2014a. Gaboxadol failed to sustain the acute nose-poking response in mice using the yoked control paradigm of intravenous administration over a wide dose range nor was it self-administered in cocaine-primed baboons, although the BZ agonist triazolam could replace cocaine (Vashchinkina et al, 2012).…”

“…This question was studied by Vashchinkina et al (2012Vashchinkina et al ( , 2014a. First, the GABAsite agonist gaboxadol (4,5,6,7-tetrahydroisoxazolo(5,4-c) pyridin-3-ol), which is a superagonist at d subunitcontaining receptors, whereas GABA is a partial agonist (Storustovu and Ebert, 2006;Saarelainen et al, 2008), induced prolonged (.6 days) increase in AMPAR/ NMDAR current ratios and AMPAR current rectification in VTA DA neurons 24-144 hours after a single sedative dose in a dose-dependent manner (Vashchinkina et al, 2012).…”

Role of GABA-active neurosteroids in the efficacy of metyrapone against cocaine addiction

“…Neurosteroids may also interfere with cocaine-induced associative learning, having important implications for cue-associated reinstatement [72,73]. The latest published research indicates that extrasynaptic GABA A receptors may be especially important in modulating tonic and reward-related dopamine within the mesolimbic circuit [74][75][76][77][78], which supports a role for neurosteroids in these effects since extrasynaptic GABA A receptors are a sensitive target for allopregnanolone and THDOC [75,77,79,80].…”