Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

Abstract

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

Keywords: Ach receptors; acetylcholine; cellular excitability; neocortex; network activity; neuromodulation; synaptic transmission.

Figure 1

Effect of nicotinic acetylcholine receptors (nAChRs) and muscarinic ACh receptors (mAChRs) activation on the membrane potential of various neocortical cell types. The central schema represents the main cell types in the neocortex. Excitatory neurons are shown in red and inhibitory GABAergic neurons are shown in blue. The electrophysiological responses to the optogenetic activation of cholinergic fibers (in light blue) or the application of a cholinergic agonist (shown in green) or antagonist (shown in red) of each cell type are depicted in the inserts. Timing of cholinergic manipulation is shown as a vertical or horizontal bar. Muscarinic and nicotinic cholinergic receptors associated with the observed response, when known, are shown as four main subtypes: M1-M3-M5 like receptors (yellow and red), M2-M4 like receptors (violet and red), α4β2 heteromeric nAChRs (violet and blue) and α7 homomeric nAChRs (yellow and blue). All shown experimental traces reflect studies listed in Tables s 1, 2. Selected traces were recorded in sensory areas of the rodent neocortex. Inclusion criteria for the experimental traces comprise knowledge of the cell-types and the receptor subtype (nicotinic or muscarinic) involved in the electrophysiological response. Abbreviations: PC, pyramidal cell; SS, spiny-stellate cell; IN, interneuron; MC, Martinotti cell; BC, basket cell; DBC, double-bouquet cell; NGFC, neurogliaform cell; BPC, bipolar cell. Reproduced and adapted from: (left, top to bottom): (A). Brombas et al., ; (B) Arroyo et al., ; (C) Dasgupta et al., ; (D) Hedrick and Waters, ; (E) Kawaguchi, (Right, top to bottom): (F) Gulledge et al., ; (G) Kawaguchi, ; (H) Shalinsky et al., ; (I) Dasgupta et al., ; (J) Hedrick and Waters, . For more exhaustive information on agonist concentration, species and cortical area examined, see Tables s 1, 2.

Figure 2

Effect of nAChRs and mAChRs activation on neocortical synaptic dynamics. The central schema represents the main neocortical cell types and their synaptic connections. A fiber of subcortical provenance associated with cholinergic boutons is also shown. Excitatory neurons are shown in red and inhibitory GABAergic neurons are shown in blue. The electrophysiological responses to the application of a cholinergic agonist or antagonist or to basal forebrain (BF) optical stimulation are depicted in the inserts. Panels show the modulation of synaptic dynamics in terms of increase or decrease in PSP/PSC size. Muscarinic and nicotinic cholinergic receptors associated with the observed response, when known, are shown as four main subtypes: M1-M3-M5 like receptors (yellow and red), M2-M4 like receptors (violet and red), α4β2 heteromeric nAChRs (violet and blue) and α7 homomeric nAChRs (yellow and blue). All shown experimental traces reflect studies listed in Table 3. Selected traces were recorded in sensory areas of the rodent neocortex. Inclusion criteria for the experimental traces comprise knowledge of the pre and postsynaptic cell-types and the receptor subtype (nicotinic or muscarinic) involved in the response. Abbreviations: PC, pyramidal cell; TTPC, thick tufted pyramidal cell; STPC, slender tufted pyramidal cell; SS, spiny-stellate cell; MC, Martinotti cell; BC, basket cell; NGFC, neurogliaform cell; BPC, bipolar pyramidal cell; IPC, inverted pyramidal cell. Reproduced and adapted from: (left, top to bottom): (A) Brombas et al., ; (B) Urban-Ciecko et al., ; (C) Kruglikov and Rudy, ; (D) Dasgupta et al., ; (E) Yamamoto et al., ; (F) Salgado et al., ; (G,H) Eggermann and Feldmeyer, ; (I) Kruglikov and Rudy, ; (J) Markram et al., . For more exhaustive information on technique, species and cortical area examined, see Table 3.

Figure 3

Differential expression of cholinergic receptors in various neuronal compartments across cell-types. Heatmap matrices show the occurrence of cholinergic receptor subtypes at the level of different cell-types. The presence of a given subtype in a cellular compartment is classified as consistently expressed (consistent findings across experimental studies), sometimes expressed (evidence of its presence is only partial) and never expressed (presence of a given subtype is undetectable). Abbreviations: PC, pyramidal cell; M1, M2, M3, M4, muscarinic cholinergic receptors 1–4; nAChR, nicotinic acetylcholine (ACh) receptor.

Figure 4

Subcellular nicotinic and muscarinic signaling processes at the glutamatergic synapse being modulated by ACh. Only the main relevant pathways and components are shown. Receptor subtypes which are less expressed on pre and post-synaptic membranes and related downstream processes are shown in semi-transparent colors. Abbreviations: ACh, acetylcholine; ACh Esterase, acetylcholinesterase; M1-M5, muscarinic acetylcholine receptor types 1–5; nAChR (α7, α4β2), nicotinic acetylcholine receptor (types α7, α4β2); VGCC, voltage-gated calcium channel; KA, kainate receptor; GIRK, G-protein activated inward rectifier K⁺ channel; PKA, protein kinase A; CaM, calmodulin; AC, adenylyl cyclase; DAG, diacylglycerol; PKC, protein kinase C; NOS, NO-synthase; HO-2, heme oxygenase 2; sGC, soluble guanylyl cyclase; PKG, cGMP-dependent protein kinase; HCN, hyperpolarization-activated cyclic nucleotide-gated channel; TRPC1, transient receptor potential cation channel 1; mGluR, metabotropic glutamate receptor; Pyk2, protein-tyrosine kinase 2; PiP2, phosphoinositol-1,4,5-biphosphate; PLC β, phospholipase C β; IP₃, inositol triphosphate; IP₃R, IP₃ receptor; RyR, ryanodine receptor; SERCA, sarco-endoplasmic reticulum Ca²⁺-ATPase.

Figure 5

Differential expression of cholinergic receptors in transcriptome-derived cell types. (A) Excitatory cell types. (B) Interneurons in somatosensory cortex. Gene expression is normalized to a maximum of 1 on a gene-by-gene basis. (C) Correlation matrix (positive values of correlation matrix Pearson correlation coefficient matrix). (D) Anti-correlation matrix (negative values of correlation matrix). The data is from Zeisel et al. (2018) and was collected with high-throughput single-cell RNA sequencing, a method which counts individual RNA molecules. Abbreviations: PV, parvalbumin; SST, somatostatin; VIP, vasointestinal peptide; ChAT, choline acetyltransferase. (E) Expression of ACh receptor genes across the Frontal cortex cell-clusters identified in Saunders et al. (2018). The data was collected using Drop-seq (a method which allows the use of older animals and elimination of certain technical artifacts) to profile the RNA expression of individual cells. Semi-supervised independent component analysis was used to group cells into the sub-clusters using network-based clustering (ibid). Expression levels were normalized to the highest expression across all the selected genes. In this data set, receptor expression was particularly high in L23 and L5a PCs.