Investigating the activation of nucleus accumbens projective paraventricular nucleus of the thalamus during cue-food reward conditioning

Abstract

When a previously neutral cue is learnt to predict a food reward, the presence of the cue itself can elicit the motivation to obtain the food. The nucleus accumbens shell (nAccSh) in the mesolimbic system is involved in generating motivation. It is known that paraventricular thalamic (PVT) neurons project to nAccSh. However, how it may interact with the nAccSh in conditioning is not well understood. Therefore, I investigated the activation of nAccSh-projective PVT neurons after the conditioning. It is hypothesised that nAccSh-projective PVT neurons are activated by conditioning.

Male Wistar rats were injected in nAccSh with a retrograde adeno-associated viral vector (AAV) containing a channelrhodopsin (ChR2)-green fluorescent protein (GFP) construct. This labelled the membrane of all neurons projecting to the nAccSh, including PVT neurons. The rats were then randomly divided into conditioning group (light stimulus predicting the release of a sugar pellet, n=4) and control group (light not predicting sugar pellet, n=3). After the conditioning experiment rats were euthanised, and the brains were sliced. The PVT was stained for GFP and cFos (an indirect marker for neuronal activation), using fluorescent immunohistochemistry.

The nuclear cFos staining counts were similar across the anterior, medial and posterior PVT [two-way ANOVA, F(2,15)=1.1, p=0.3458] without a group effect [F(1,15)=0.98, p=0.3376]. Co-localisation of GFP-labeled nAccSh-projective PVT neurons and cFos nuclei was seen in both groups across PVT regions. The analysis showed a trend towards a regional difference in the count of co-localised cells [F(2,15) =3.4, P=0.0584]. However, it was not significantly different between the groups [F(1,15)= 0.5, p=0.4574].

These findings suggest that different PVT regions can be activated by repetitive exposure to stimuli and food rewards, regardless of their predictive relation. Pavlovian conditioning may not necessarily activate nAccSh-projective PVT neurons. However, this preliminary study requires more replicates to fully test the hypothesis.