Novel fast adapting interneurons mediate cholinergic-induced fast GABAA inhibitory postsynaptic currents in striatal spiny neurons

Previous work suggests that neostriatal cholinergic interneurons control the activity of several classes of GABAergic interneurons through fast nicotinic receptor‐mediated synaptic inputs. Although indirect evidence has suggested the existence of several classes of interneurons controlled by this mechanism, only one such cell type, the neuropeptide‐Y‐expressing neurogliaform neuron, has been identified to date. Here we tested the hypothesis that in addition to the neurogliaform neurons that elicit slow GABAergic inhibitory responses, another interneuron type exists in the striatum that receives strong nicotinic cholinergic input and elicits conventional fast GABAergic synaptic responses in projection neurons. We obtained in vitro slice recordings from double transgenic mice in which Channelrhodopsin‐2 was natively expressed in cholinergic neurons and a population of serotonin receptor‐3a‐Cre‐expressing GABAergic interneurons were visualized with tdTomato. We show that among the targeted GABAergic interneurons a novel type of interneuron, termed the fast‐adapting interneuron, can be identified that is distinct from previously known interneurons based on immunocytochemical and electrophysiological criteria. We show using optogenetic activation of cholinergic inputs that fast‐adapting interneurons receive a powerful supra‐threshold nicotinic cholinergic input in vitro. Moreover, fast adapting neurons are densely connected to projection neurons and elicit fast, GABAA receptor‐mediated inhibitory postsynaptic current responses. The nicotinic receptor‐mediated activation of fast‐adapting interneurons may constitute an important mechanism through which cholinergic interneurons control the activity of projection neurons and perhaps the plasticity of their synaptic inputs when animals encounter reinforcing or otherwise salient stimuli. Neostriatal cholinergic interneurons (CIN) activate GABAergic circuits in the neostriatum. One circuit provides feedback inhibition to CINs through unidentified interneurons. Another involves neurogliaform neurons (NGF) that elicit slow GABAergic inhibition is projection cells (SPN). We show that a new cell type termed the fast‐adapting interneuron (FAI) which also receives strong nicotinic excitation from CINs or the PPN provides fast GABAergic inputs to SPNs through facilitating synapses.