Cortical inputs to m2-immunoreactive striatal interneurons in rat and monkey

Previous anatomical studies have been unsuccessful in demonstrating signifi cant cortical inputs to cholinergic and somatostatinergic striatal interneu rons in rats. On the other hand, electrophysiological studies have shown th at cortical stimulation induces monosynaptic EPSPs in cholinergic interneur ons. It has been proposed that the negative anatomical findings might have been the result of incomplete labeling of distal dendrites. In the present study, we reinvestigated this issue using m2 muscarinic receptor antibodies as a selective marker for cholinergic and somatostatinergic interneurons i n the striatum. This was combined with injections of either the anterograde tracer biotinylated dextran amine (BDA) in the monkey prefrontal cortex or aspiration lesion of the sensorimotor cortex in rats. The results showed t hat, in both species, a small percentage (1-2%) of cortical terminals make asymmetric synaptic contacts with m2-immunoreactive interneurons in the str iatum. Interestingly, the majority of these synapses are onto small dendrit ic spines or spine-like appendages, as opposed to dendritic shafts and/or c ell bodies. Thus, m2-containing striatal interneurons do receive direct cor tical inputs and can, therefore, integrate and modulate cortical informatio n flow through the striatum. Although the density of cortical terminals in contact with individual striatal interneurons is likely to be relatively lo w compared to the massive cortical input to projection neurons, both cholin ergic and somatostatinergic interneurons display intrinsic properties that allow even small and distal inputs to influence their overall state of neur onal activity. (C) 2000 Wiley-Liss, Inc.