Isolation rearing-induced facilitation of Pavlovian learning: abolition by postsession intra-amygdala nafadotride

It has been shown previously in this laboratory that rats reared in social isolation acquire a Pavlovian-conditioned approach task much more rapidly than their respective controls. This study assessed the involvement specifically of the mesoamygdaloid dopamine pathway in this facilitated learning of isolates. Thus, animals were required to associate arbitrary stimuli with a pulsed light stimulus (unconditioned stimulus, US). The US, while without biological significance, was nevertheless capable of eliciting an intrinsic and sustained alerting response. Procedures ensured that the arbitrary stimuli (tone or clicker) did not elicit a response in the first instance, and were presented either paired (CS+) or unpaired (CS-) with the US. Isolates and socially reared controls received intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle immediately following the end of each training session. The conditioned response increased over sessions in both groups of vehicle-infused rats during presentations of the CS+ stimulus, but not CS-, and isolates acquired this association more rapidly than controls. However, acquisition of this association was abolished by postsession intra-amygdala L-nafadotride. Responding to the US was largely unaffected by drug or rearing conditions. Hence, these data provide strong evidence for the specific involvement of the mesoamygdaloid dopamine projection in the facilitation of associative learning by isolation rearing.