Nucleus accumbens GABAA receptor-mediated inhibitory transmission

: Relevance to early life stress and addiction

Abstract

The nucleus accumbens (NAc) consists primarily of GABAergic neurons, and is an essential integration site within the natural reward-pathway, which is ‘hijacked’ by drugs of abuse. We have previously shown for accumbal medium spiny neurons (MSNs) that α2-subunit containing GABAA receptors (α2-GABAARs), mediate phasic inhibition and that their genetic inactivation (α2-/-) abolishes behavioural sensitisation to cocaine. Recently, linkage association studies in humans revealed a genetic association of GABRA2 haplotypes with cocaine addiction, which was evident only in individuals who had experienced childhood trauma. Collectively, these studies indicate an association of childhood trauma, drug addiction and the GABRA2 haplotype. Here, I have characterised the NAcore MSN GABAARs, identifying two distinct synaptic populations namely α1- and α2-GABAARs.

Having established the GABAAR isoforms in control mice I then employed a mouse model of early life stress (ELS) to investigate the impact of early-life adverse experiences on cocaine-induced behaviours and the putative role of NAcore GABAARs in this interaction. An established fragmented maternal care paradigm was implemented to produce ELS. Adult wild type (WT) control mice received a daily i.p injection of cocaine (10 mg/kg) for 10 days, which resulted in behavioural sensitization, manifested as enhanced cocaine-induced increase of locomotor activity c.f. saline-injected controls. In contrast, adult ELS mice did not sensitise to cocaine, but exhibited a significantly increased acute locomotor response to a single cocaine injection (10 mg/kg) c.f. WT control. Interestingly, both features were exhibited by non-stressed α2-/- mice. Whole-cell voltage-clamp recordings of NAc MSNs of adult mice, previously exposed to the ELS paradigm, revealed a significant reduction in the amplitude and frequency of miniature inhibitory post-synaptic currents (mIPSCs) c.f. WT controls. Non-stressed α2-/- mice exhibited similar alterations of mIPSCs properties c.f. WT controls. Complementary immunohistochemical analysis revealed a significant and selective reduction of the staining for the α2-, but not the α1-GABAAR subunit staining in the NAcore of adult ELS c.f. WT control mice, indicating ELS selectively decreases α2-GABAAR expression. In conclusion, ELS and α2-/- mice share a selective α2-GABAAR mediated reduction of inhibitory phasic transmission, which is accompanied, under these experimental conditions, by an increased acute cocaine locomotor response and blunting of further behavioural sensitisation to cocaine. Collectively, these findings complement the human studies and suggest that such mouse models may prove useful in permitting a better understanding of the complex association of cocaine abuse, childhood trauma and the GABRA2 gene.

Certain neurosteroids are known to bind to and enhance the function of GABAARs. Furthermore, the brain levels of such steroids are changed by acute and chronic stress. I have identified an endogenous capacity to synthesise these GABAAR-active neurosteroids within NAcore coronal brain slice preparation. Additionally, in NAcore MSNs I discovered a novel effect of 5α-DHP and progesterone to increase the magnitude of a tonic current mediated by extrasynaptic α4βδ GABAARs. This effect of 5α-DHP was due in part to it being metabolised to the GABAAR active 5α3α, but also involved an additional action. The effect of progesterone on the tonic current was independent of a GABAAR-modulatory effect. Experiments are described to elucidate the mechanism whereby progesterone acts. Given the magnitude by which progesterone and 5α-DHP increase the tonic current, it will greatly impact neuronal excitability. Therefore, these studies should improve our understanding of how neurosteroids influence neural communication in this “reward pathway”.

Date of Award	2016
Original language	English
Awarding Institution	University of Dundee
Sponsors	Medical Research Council
Supervisor	Jeremy Lambert (Supervisor) & Delia Belelli (Supervisor)