Responses of putative medium spiny neurons and fast-spiking interneurons to reward-related sensory signals in Wistar and genetically hypertensive rats.

Medium spiny neurons (MSN) are the primary output neurons of the striatum. Their activity is modulated by exogenous afferents and local circuit inputs, including fast-spiking interneurons (FSI). Altered responses of MSN and FSI may account for altered reward-driven behaviour in hyperactive rat strains, such as the genetically hypertensive (GH) rat. To investigate whether striatal neuron responses differ between GH and Wistar rats, we recorded putative MSNs (pMSN) and FSI (pFSI) from freely moving GH and Wistar rats in a classically conditioned (Pavlovian) cue-reward association paradigm. Here, the same auditory cue signal predicted reward delivery in one block of trials, but was not followed by reward in another. The significance of the cue as a reward predictor was indicated during each block by an environmental context provided by the house light. The results showed that pMSN in GH rats, but not Wistar rats, were more sensitive to the auditory signal in the context indicating no-reward, than in the reward context. Such enhanced sensitivity to cues in a no-reward context may contribute to a specific deficit in instrumental behaviour seen in GH rats, which maintain higher levels of instrumental responding in a context that indicates responding will not be rewarded. In addition, pFSI also responded to auditory signals, but there was no significant effect of reward context. Surprisingly, given their known feed-forward role, pFSI responded at longer latency than pMSN, suggesting that relative timing of activity in the two populations may be task specific.