Two Types of Nicotinic Receptors Mediate an Excitation of Neocortical Layer I Interneurons

Abstract

Nicotinic acetylcholine receptors are widely expressed in the neocortex but their functional roles remain largely unknown. Here we investigated the effect of nicotinic receptor activation on interneurons of layer I, which contains a high density of cholinergic fiber terminals. Ninety-seven of 101 neurons recorded in whole cell configuration in rat acute slices were excited by local pressure application of nicotinic agonists, acetylcholine (500 μM), 1,1-dimethyl-4-phenyl-piperazinium (500 μM) or choline (10 mM). Biocytin labeling confirmed that our sample included different morphological types of layer I interneurons. The responses to nicotinic agonists persisted in presence of glutamate and muscarinic receptor antagonists and on further addition of Cd2+ or tetrodotoxin, indicating that they were mediated by direct activation of postsynaptic nicotinic receptors. The kinetics of the currents and their sensitivity to nicotinic receptor antagonists, methyllycaconitine (1–10 nM) or dihydro-β-erythroidine (500 nM), suggested that early and late components of the responses were mediated by α7 and non-α7 types of receptors. Both components had inwardly rectifying I-V curves, which differed when intracellular spermine was omitted. Single-cell RT-PCR experiments identified α4, α7, and β2 as the predominantly expressed mRNAs, suggesting that the receptors consisted of α7 homomers and α4β2 heteromers. Finally, selective excitation of layer I interneurons through activation of their nicotinic receptors resulted in a tetrodotoxin-sensitive increase of inhibitory synaptic currents recorded in nonpyramidal cells but not in pyramidal cells of layer II/III. These results suggest that acetylcholine released in layer I may induce a disinhibition of the cortical network through activation of nicotinic receptors expressed by layer I interneurons.