Firing pattern characteristics of tonically active neurons in rat striatum: context dependent or species divergent?

Tonically active neurons (TANs)--presumably, striatal cholinergic interneurons--exert a strong influence on striatal information processing. Primate studies often describe a characteristic TAN response comprising suppressed activity followed by rebound firing that occasionally is preceded by a brief activation. By contrast, studies in behaving rats report pronounced excitation during movement. These differences in firing patterns may be due to variations in behavioral conditions or could stem from the fact that TANs in rodents use different neuronal mechanisms. If similar/different task conditions yield similar/different activity patterns, then the two species may share neuronal mechanisms; however, if similar task conditions yield different activity patterns, then the two species use different neuronal mechanisms. To evaluate these possibilities, we recorded TAN activity in the ventral and dorsolateral striatal regions in rats performing a simple instrumental task similar in concept to one used in primate studies. We demonstrate that TAN activity is substantially influenced by event context; yet, under identical task conditions, primate and rat TANs display similar activity patterns, whereas under different conditions they do not. Our results suggest that the observed differences in firing patterns likely reflect dissimilarities in task attributes rather than species-dependent neuronal mechanisms and call for re-evaluation of the excitatory response in primate research.