Spatiotemporal specificity in cholinergic control of neocortical function

Abstract

Cholinergic actions are critical for normal cortical cognitive functions. The release of acetylcholine (ACh) in neocortex and the impact of this neuromodulator on cortical computations exhibit remarkable spatiotemporal precision, as required for the regulation of behavioral processes underlying attention and learning. We discuss how the organization of the cholinergic projections to the cortex and their release properties might contribute to this specificity. We also review recent studies suggesting that the modulatory influences of ACh on the properties of cortical neurons can have the necessary temporal dynamic range, emphasizing evidence of powerful interneuron subtype-specific effects. We discuss areas that require further investigation and point to technical advances in molecular and genetic manipulations that promise to make headway in understanding the neural bases of cholinergic modulation of cortical cognitive operations.

Figure 1. Factors that control the spatiotemporal specificity of cholinergic actions in the cortex

The spatiotemporal precision of the cholinergic system is a function of a number of factors, three of which are discussed throughout this review. (A) First, the functional organization of cholinergic projections to the cortex. The spatial extent and specificity of individual and collective cholinergic cell innervation of the cortex is a major determinant of the spatial range of ACh release and impact. Anatomical investigation of these issues led to two organizational models (diffuse and mosaic) of the cortical cholinergic projections from the basal forebrain. These models fit with classical notions of diffuse and global cholinergic broadcast, as activation of either model system would lead to widespread cortical ACh modulation. However, recent anatomical reexamination suggests that the functional organization of these projections might follow a different principle. In the proposed modular model, neighboring cholinergic cells project to distinct cortical areas that are, in turn, functionally interconnected. Such an organization could mediate localized ACh release in specific cortical regions, as well as coordinate modulatory influences across computational pathways in the cortex. Modern anatomical and genetic approaches offer opportunities to continue delving into the organization and specificity of the cholinergic projections. For instance, whether there is specificity in the cortical postsynaptic targets of individual cholinergic cells remains elusive, and could be investigated with new tools and approaches that take advantage of advances in molecular genetics. Moreover, although the anatomical terminal innervation of the cholinergic system sets upper and lower spatial boundaries for cholinergic actions, understanding the functional scale of ACh impact requires that we consider the functional inputs to the cholinergic projection system, an issue that is not discussed here. Whether functional inputs activate this structure diffusely, or whether they focally drive circumscribed cholinergic projection cell groups remains to be investigated. (B) A second factor determining the spatiotemporal precision of cholinergic action is the mode of transmission of ACh at cholinergic terminals in the cortex. It has been argued that ACh is released from non-junctional sites, mediating slow and diffuse activation of cholinergic receptors over large cortical spaces (volume transmission). However, recent anatomical and electrophysiological data suggest that the point-to-point precision of classical synaptic transmission can be observed in the cortical cholinergic system, circumscribing fast cholinergic signaling to contacted cortical elements. (C) The third factor determining spatiotemporal precision is the diversity and dynamics of the cell-type specific effects of ACh. The spatiotemporal profile of ACh concentration interacts with the particular sensitivity, kinetics, and localization of diverse cholinergic receptors in postsynaptic targets. Together with cell-type specific expression of these receptors and downstream signaling cascades, these interactions determine the spatiotemporal coordination, interplay and predominance of a diversity of ACh effects in specific cortical cells and networks.

Figure 2. Ionotropic (nicotinic) and metabotropic (muscarinic) receptors mediate the effects of Ach

(a and b) Nicotinic AChRs are pentameric proteins consisting of a large variety of subunits. The subunit composition dictates channel function. Those expressed in the brain primarily exist as α7 homopentamers (a) or α4β2 heteropentamers, usually with a 2α, 3β stoichiometry (b) [38]. Shown are renderings of the side and top view of the receptors based on the closed structure of the Torpedo AChR (PDB code: 2bg9). The α4 subunit is colored blue, the β2 subunit yellow, and the α7 subunit red. These two nAChR subtypes display dramatically distinct kinetics and pharmacological properties [38]. Nicotinic responses are generally excitatory. Receptor activation produces transient depolarization due to the permeability of the ligand-gated channel with an equilibrium potential close to 0 mV. However, in addition to providing depolarization, α7 nAChRs can mediate slower cellular responses by virtue of the especially large Ca²⁺ permeability of these receptors. (c) Muscarinic receptors are G-protein coupled receptors (GPCRs), with the typical seven transmembrane domain structure of these proteins. Five different subtypes are known (M1-M5), of which four subtypes M1-M4 are the predominant ones in neocortex. Their functions depend on the signaling cascades that are initiated by the binding of ACh, which in turn largely depend on the subtype of heterotrimeric G protein associated with the receptor. M1, M3 and M5 (often referred to as M1-type) couple to Gq/11 G proteins, while M2 and M4 (often referred to as M2-type) are Gi/o-coupled receptors [57]. Upon binding of ACh to the receptor, the GDP associated with the G protein is exchanged for GTP. (d) Association with GTP produces the dissociation of the G protein. In the case of M1-type receptors the αq/11 subunit activates the enzyme phospholipase C β (PLC), which hydrolyses the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) resulting in the loss of PIP2 from the membrane and the production of diacyl glycerol (DAG) and inositol triphosphate (IP3). IP3 produces Ca²⁺ release from intracellular stores. DAG and Ca²⁺ activate protein kinase C (PKC). The loss of PIP2 (a), PKC-mediated phosphorylation of channels and other downstream targets (b) and Ca²⁺-mediated signaling (c) produce downstream effects. In the case of M2-type receptors the αi/o subunit produces inhibition of adenyl cyclase resulting in a decrease of cAMP levels. The βγ subunit complex diffuses through the membrane and binds to G-protein activated inward rectifier K⁺ (GIRK) channels activating them, or to N or P/Q type Ca²⁺ channels inhibiting them, or to other targets. Muscarinic effects can be excitatory or inhibitory depending on the targets of the signaling pathways activated by the receptor and can vary in different cells. They can also vary in a given cell at different times depending on the state of the cell. For instance to obtain IP3-dependent Ca²⁺ release, the Ca²⁺ stores must be full.

Figure 3. Nicotinic and muscarinic responses in neocortical neurons and their synapses

Cholinergic agonists regulate the function of neocortical neurons and their synapses in a cell-specific fashion. M1-type muscarinic modulation produces a sustained increase in the excitability of pyramidal neurons (PC) in supragranular and infragranular layers by inhibiting several types of K⁺ channels (see Figure 2, Table 1). In addition, perisomatic ACh produces a transient hyperpolarization as a result of the activation of SK Ca²⁺-activated K⁺ channels that precedes the sustained depolarization and is seen predominantly in layer V PCs. A nicotinic mediated depolarization, capable of eliciting spiking, has also been reported in L5 PCs in some cortical areas (see Table 1). In contrast to the sustained activation of PCs, ACh produces a sustained hyperpolarization of spiny stellate cells (SS) in layer 4 of somatosensory cortex. It has been suggested that this hyperpolarization may serve to filter weak thalamocortical inputs and favor the activation of spiny stellate cells by stronger, more synchronous inputs. Muscarinic and nicotinic responses have also been observed on GABAergic neurons. Muscarinic agonists powerfully depolarize and increase the activity of SOM-expressing Martinotti (mSOM) cells in layers II/III and V/VI. These neurons have an ascending axon that targets and inhibits the distal dendrites of PCs. SOM-cells in L4 of somatosensory cortex (xSOM) differ in morphology and intrinsic firing properties from Martinotti cells. Their axons don’t innervate L1 but profusely branch in L4, where they target local FS basket cells and are thus capable of disinhibiting this layer. These cells are also powerfully excited by muscarinic action [46]. Although there are contradictory observations regarding the effects of cholinergic modulation of the excitability of parvalbumin (PV)-expressing FS basket cells (bPV), most investigators agree there is no effect. The effect of ACh on chandelier cells (cPV), the second subtype of PV-interneuron has not been studied. In spite of their heterogeneity, all 5HT3aR IN subtypes are depolarized by ACh via nicotinic receptors. These interneuron group includes several subtypes, of which the two most prominent are illustrated in the figure: the neurogliaform cells (NGFC) which inhibit PCs and the bipolar/bitufted VIP INs (bVIP), which inhibit SOM INs and thus mediate disinhibition of excitatory neurons. There is evidence of muscarinic responses in some subpopulations within this family (Table 1), but they have not been studied in detail. In addition to these effects of cholinergic agonists on the excitability of cortical neurons, they have also been shown to modulate neurotransmitter release of several types of cortical synapses. Cholinergic agonists inhibit glutamate release from intracortical recurrent excitatory axons and GABA release from the terminals of FS basket cells (bPV) on excitatory neurons via activation of M2-type receptors; presumably by inhibiting N and P/Q type Ca²⁺ channels at the terminal. In contrast, cholinergic modulation increases thalamocortical inputs onto principal cells via nicotinic receptor modulation.