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.2014 Nov 25:5:256.
doi: 10.3389/fphar.2014.00256. eCollection 2014.

GABAA receptor drugs and neuronal plasticity in reward and aversion: focus on the ventral tegmental area

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GABAA receptor drugs and neuronal plasticity in reward and aversion: focus on the ventral tegmental area

Elena Vashchinkina et al. Front Pharmacol..

Abstract

GABAA receptors are the main fast inhibitory neurotransmitter receptors in the mammalian brain, and targets for many clinically important drugs widely used in the treatment of anxiety disorders, insomnia and in anesthesia. Nonetheless, there are significant risks associated with the long-term use of these drugs particularly related to development of tolerance and addiction. Addictive mechanisms of GABAA receptor drugs are poorly known, but recent findings suggest that those drugs may induce aberrant neuroadaptations in the brain reward circuitry. Recently, benzodiazepines, acting on synaptic GABAA receptors, and modulators of extrasynaptic GABAA receptors (THIP and neurosteroids) have been found to induce plasticity in the ventral tegmental area (VTA) dopamine neurons and their main target projections. Furthermore, depending whether synaptic or extrasynaptic GABAA receptor populations are activated, the behavioral outcome of repeated administration seems to correlate with rewarding or aversive behavioral responses, respectively. The VTA dopamine neurons project to forebrain centers such as the nucleus accumbens and medial prefrontal cortex, and receive afferent projections from these brain regions and especially from the extended amygdala and lateral habenula, forming the major part of the reward and aversion circuitry. Both synaptic and extrasynaptic GABAA drugs inhibit the VTA GABAergic interneurons, thus activating the VTA DA neurons by disinhibition and this way inducing glutamatergic synaptic plasticity. However, the GABAA drugs failed to alter synaptic spine numbers as studied from Golgi-Cox-stained VTA dendrites. Since the GABAergic drugs are known to depress the brain metabolism and gene expression, their likely way of inducing neuroplasticity in mature neurons is by disinhibiting the principal neurons, which remains to be rigorously tested for a number of clinically important anxiolytics, sedatives and anesthetics in different parts of the circuitry.

Keywords: GABAA receptor; THIP; benzodiazepines; dendritic spines; dopamine neurons; neuroadaptation; neurosteroids.

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Figures

FIGURE 1
FIGURE 1
Pyramidal cells of hippocampal CA1 region exhibit distinct morphological responses to diazepam and THIP treatment. THIP increased the spine density on the basal secondary dendrites of CA1 pyramidal cells (F2,15 = 9.69,p < 0.01), but not in CA3 or VTA (p > 0.05). In contrast, diazepam (DZ) did not show any changes in spine density of the basal nor apical dendritic spines in these areas (p > 0.05).On the left, schematic neurons depict the spines either secondary or basal dendritic branches that were assessed here.In the middle, representative images of Golgi-stained neurons on horizontal sections. Insets are dendrites showing spines. Scale bar is 10 μm.On the right, plots showing the spine densities 24 h after a single injection of DZ or THIP. Mean ± SEM forn = 10, 5, and 5 mice vehicle (Veh), DZ, and THIP groups, respectively. **p < 0.01, (one-way ANOVA followed by Bonferronipost hoc test). C57BL/6J mice (21–26 days old) were decapitated at 24 h after treatment with DZ (5 mg/kg, ip), THIP (6 mg/kg, ip) or vehicle (Heikkinen et al., 2009; Vashchinkina et al., 2012; All animal procedures were approved by the Southern Finland Provincial Government). Brains were rapidly dissected and immediately placed into Golgi-Cox fixative and processed using the FD Rapid GolgiStain Kit (FD Neuro Technologies). Impregnated brains were then cut in serial horizontal 80-μm-thick sections using a cryostat (Leica Microsystems, Nussloch, Germany) and stained using the kit protocol. For each brain area, spine density per 10 μm was assessed in five cells. Notably, in VTA cells spines were determined on secondary dendrites, in CA1 and CA3 pyramidal cells on basal dendritic branches, using the 100× objective of Zeiss Axioplan microscope (Carl Zeiss MicroImaging GmbH, NA = 1.3, Ph3, DIC, oil), and later analyzed by National Institutes of Health ImageJ software (http://rsb.info.nih.gov/ij). Spine counting was performed blind concerning the treatments. Only protrusions that show continuity with the dendritic shaft were considered as spines.
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