P.2.036 - Somatostatin-expressing neurons in the bed nucleus of stria terminalis in anxiety- and addiction-related behaviours in mice

Abstract

The bed nucleus of stria terminalis (BNST), part of the extended amygdala circuitry, is currently studied due to its role in the anxiety-, stress-, and fear-related behaviours, as well as in addiction [1]. The BNST is a highly heterogeneous area constituting of set of subnuclei and a variety of neuron populations, properties of which have only partially been revealed so far. One of the neuron populations, on which only a very little research has been conducted, is the somatostatin (Sst) -expressing neurons, highly abundant in the anterodorsal part of the BNST (adBNST), especially in oval and juxtacapsular nuclei of the BNST [2]. We aimed to elucidate the role of the Sst-neurons in behaviours such as anxiety and reward. To study the projections of the Sst-neurons in the adBNST, we injected Cre-inducible myristoylated green fluorescent protein (myr-GFP) to the adBNST of Sst-Cre mice (n=2). Following a whole-brain screening for fluorescent neurites, an abundancy of neurites were observed in the lateral hypothalamus (LH) and amygdaloidal nuclei, mostly central (CeA) and medial nucleus of amygdala (MeA). Neurites were also observed nucleus accumbens (NAc) as well as in midbrain structures like ventral tegmental area (VTA) and substantia nigra (SN). To follow up the tracing results we ran tests on amygdala and VTA associated behaviours. We used Sst-IRES-Cre mice (n=21) and targeted the Cre-expressing neurons using stereotaxic delivery of AAV-vector encoding Cre-inducible hM3Dq-DREADDs with Cre-inducible mCherry fluorescent protein as the control (n=20). The mice were treated with 1.0 mg/kg i.p. clozapine-N-oxide (CNO), to activate the Sst-neuron firing, 30 min prior to the behavioural tests. To assess acute anxiety-like behaviour, we used the elevated-plus maze paradigm and a modified open field test, in which a novel object is introduced to the arena in the middle of the trial [3]. To study the potential effect on reward-associated behaviours, we used the biased conditioned place preference (CPP) test. In the elevated-plus maze, the activation of the Sst-neurons did not affect the locomotor activity of the mice, the time spent in the open arms, nor the exploratory activities, like the frequency of the head dips or the stretch-attend postures, as compared to the controls. In the open field test, no effects on the movement were observed. The DREADD group spent less time in the centre of the field during the last 3 min of the test (t(39)=2.143 p=0.038), but no differences in the number of contacts with the novel object were observed. The CPP test failed to show any meaningful rewarding or aversive properties of CNO-induced activation of the Sst-neurons. During the conditioning trials, the animals in the DREADD and control groups moved similar distances after the activation. Chemogenetic activation of the adBNST Sst-neurons failed to show any significant behavioural effects in the chosen paradigm for the reward-related behaviour, but showed a minor change in anxiety-associated parameter in open field test. These data support the initial tracing results, but further research is needed to clarify the role of the Sst-subcircuitry of adBNST.