Cortical ChAT⁺neurons co-transmit acetylcholine and GABA in a target- and brain-region specific manner

Abstract

18The cerebral cortex contains neurons that express choline acetyltransferase (ChAT) and are 19 a potential local source of acetylcholine. However, the neurotransmitters released by cortical 20ChAT + neurons and their synaptic connectivity are unknown. We show that the nearly all 21cortical ChAT + neurons are specialized VIP + interneurons that release GABA strongly onto 22other inhibitory interneurons and acetylcholine sparsely onto layer 1 interneurons and other 23 VIP + /ChAT + interneurons. This differential transmission of ACh and GABA based on the 24 postsynaptic target neuron is reflected in VIP + /ChAT + interneuron pre-synaptic terminals, as 25 quantitative molecular analysis shows that only a subset of these are specialized to release 26acetylcholine. In addition, we identify a separate, sparse population of non-VIP ChAT + 27 neurons in the medial prefrontal cortex with a distinct developmental origin that robustly 28 release acetylcholine in layer 1. These results demonstrate both cortex-region heterogeneity 29in cortical ChAT + interneurons and target-specific co-release of acetylcholine and GABA. 30 of VIP, a marker gene for a cardinal class of GABAergic interneurons, one would expect cortical 63ChAT + neurons to be GABAergic, but this has not been definitively shown. 64We previously reported that co-transmission of GABA is a common feature of cholinergic 65 neurons in the mouse forebrain [31][32][33] . Because GABA and ACh have opposite effects on 66 membrane voltage through ionotropic receptors, the functional consequences of their co-67 transmission on cortical circuits is unknown. One possibility is that they each transmit onto the 68 same post-synaptic targets and have competing effects, similar to the co-release of GABA and 69 glutamate in the habenula from entopeduncular neurons 34,35 or co-release of ACh and GABA from 70 starburst amacrine cells onto direction-selective retinal ganglion cells 36,37 . Another possibility is 71 that they target different post-synaptic cells, which could allow them to have complementary 72 network effects. To differentiate between these alternatives requires determining the molecular 73 competency of cortical ChAT + neurons to release GABA and ACh from their pre-synaptic 74 terminals, and systematic examination of their synaptic connectivity. 75To answer these many unknowns, we molecularly and functionally characterized cortical 76ChAT + neurons and describe two classes of cortical ChAT + neurons. The first is a subset of VIP 77 interneurons, and expresses the necessary cellular machinery to synthesize and release both 78ACh and GABA. A systematic survey of synaptic connectivity shows that, for these cells, most 79 synaptic output is GABAergic. Specifically, GABA release is robust onto somatostatin (Sst)-80 expressing interneurons, similar to the larger population of VIP interneurons. However, these cells 81 are capable of releasing ACh, with sparse and highly specific targeting of ACh mostly onto layer 82 1 interneurons and other cortical VIP + /ChAT + neuron...

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Lack of Hyperinhibition of Oriens Lacunosum-Moleculare Cells by Vasoactive Intestinal Peptide-Expressing Cells in a Model of Temporal Lobe Epilepsy

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