Basal forebrain cholinergic signalling: development, connectivity and roles in cognition

Abstract

Acetylcholine plays an essential role in fundamental aspects of cognition. Studies that have mapped the activity and functional connectivity of cholinergic neurons have shown that the axons of basal forebrain cholinergic neurons innervate the pallium with far more topographical and functional organization than was historically appreciated. Together with the results of studies using new probes that allow release of acetylcholine to be detected with high spatial and temporal resolution, these findings have implicated cholinergic networks in 'binding' diverse behaviours that contribute to cognition. Here, we review recent findings on the developmental origins, connectivity and function of cholinergic neurons, and explore the participation of cholinergic signalling in the encoding of cognition-related behaviours.

Fig. 1 |. A model for BFCN fate specification.

A proposed model for cholinergic neuron fate commitment based on evidence from genetic studies (Table 1), common developmental mechanisms among cholinergic cell populations and trends in developmental regulation^–. a, The model proposes that lineage-specific transcription factors ‘organize’ the genome to regulate the expression of genes involved in the migration (radial or tangential) of immature neurons and/or neuroblasts and the expression of receptors for morphogens and trophic factors. Exposure to morphogens present in the basal forebrain or (later in development) to trophic factors present in the terminal fields ‘activates’ the genome. For example, the cholinergic gene locus has been shown to contain cis-regulatory elements that respond to retinoic acid (RA) signalling and a ciliary neurotrophic factor (CNTF) response element, STAT, that responds to CNTF signalling, meaning that treatment with either RA or CNTF increases choline acetyltransferase (CHAT) and vesicular acetylcholine transporter (VACHT) expression. b, The different germinal zones in the embryonic brain that give rise to basal forebrain cholinergic neurons (BFCNs) and our current understanding of the timing of the birth of the neurons in each of these zones and some of the contributing regulators. These regulators shown were selected on the basis of their expression in the progenitor zones and may represent master regulators. We propose that the earliest-born BFCNs (shown in blue) are generated from progenitors in the medial ganglionic eminence (MGE) and rely on the transcription factors NKX2.1, LHX8 and ISL1 for their specification, whereas subsequent generations of neurons born in this domain (shown in turquoise and purple) do not rely strongly on LHX8. The latest population to be derived from this zone is proposed to require OLIG2; however, it is also possible that this population is derived from a neighbouring germinal zone such as the anterior entopeduncular area or preoptic area. Most late-born BFCNs (shown in yellow) are generated in the septum (SEP) from NKX2.1-expressing and ZIC4-expressing progenitors and rely on NKX2.1 expression for their specification. However, one late-born population that expresses TBR1 (shown in green) migrates into the basal forebrain from the ventral pallium (vPAL). c, The proposed final location of the BFCNs derived from the progenitor populations proposed in part b. We propose that early-born MGE-derived progenitors produce the large cholinergic neurons that reside in the substantia innominata (SI), nucleus basalis of Meynert (nBM) and ventral pallidum (VP) (blue), whereas subsequently generated progenitors might produce the smaller cholinergic neurons in these regions (turquoise). The OLIG2-expressing lineage might generate the more medially located cholinergic neurons in these regions (purple). Late-born septal progenitors differentiate into large neurons that continue to populate the medial septum (MS) and diagonal band (DB) and migrate tangentially to populate the basal forebrain and striatum (yellow). Finally, the TBR1-expressing population (green) that arrives from the vPAL represents the most ventrally located neurons within the horizontal diagonal band (hDB), SI, nBM and VP. CNTFR, ciliary neurotrophic factor receptor; NGF, nerve growth factor; NT3, neurotrophin 3; SHH, sonic hedgehog protein.

Fig. 2 |. Spatial localization and projection patterns of BFCNs.

a, Overlapping pools of cholinergic neurons are located along the rostrocaudal extent of the basal forebrain in a manner that corresponds to their birthdate and site of origin^, (Fig. 1b,c). The axons of the basal forebrain cholinergic neurons (BFCNs) that make up these different rostrocaudal pools take four distinct projection paths to innervate their targets: the rostromedial pathway, the septal pathway, the dorsolateral pathway and the caudolateral pathway. In the schematic, arrows depict the relative trajectories of each of these fibre paths. Solid projections indicate fibres that traverse the sagittal plane, whereas those depicted with dashed lines are tracts that break from the sagittal plane and move laterally^,. Developmentally diverse populations and their expected projection paths are shown in colours that correspond to their relative birth order (Fig. 1b,c). The major targets of each of the projections are noted. b, BFCNs innervate cortical layers distinctly on the basis of their rostrocaudal location in the basal forebrain^,. Rostrally located, cortically projecting cholinergic neurons innervate both superficial and deeper layers of the cortex (purple and turquoise projection populations), whereas caudally located, cortically projecting cholinergic neurons primarily innervate deep layers of the cortex^, (blue and green). In both cases, fibre bundles traverse superficial or deep layers of the cortex as they find their targets^,. c, An input and output wiring diagram for BFCN populations, derived from studies on the connectivity of cholinergic neurons^,,–. The centre grey box organizes BFCNs beside horizontal arrows denoting their varying rostral (left) to caudal (right) extent. Regions shown to the left of the central BFCN box broadly innervate all BFCNs. Regions above and below the central BFCN box connect to specific BFCN population(s) denoted by the vertical arrows, highlighting approximate regional specificity in wiring. Brackets denote broad (rostrocaudal) overlap in connectivity across BFCN populations. The double-headed arrows denote reciprocal projections to a region, and the single-headed arrows denote unidirectional projections. EC, entorhinal cortex; hDB, horizontal subdivision of the diagonal band; HPC, hippocampus; L, layer; MS, medial septum; nBM, nucleus basalis of Meynert; PFC, prefrontal cortex, SI, substantia innominata; vDB, vertical diagonal band; VP, ventral pallidum.

Fig. 3 |. Improvements in measurement of in vivo ACh dynamics.

a, Different techniques used to measure acetylcholine (ACh) levels or dynamics in vivo are listed and are accompanied by schematic representations of typical measurements and a comparison of the spatiotemporal resolution and specificity of these measurements. Microdialysis can be used to measure extracellular ACh concentrations^,. Choline-sensitive microelectrodes indirectly measure ACh levels using electrochemistry^,. Optogenetic tagging uses Cre-dependent opsins to identify cholinergic neurons, and in vivo recordings from those neurons monitor their activity. Finally, ACh biosensors directly measure ACh release using fibre photometry^,,. More recently, ACh biosensors have been used in conjunction with widefield imaging to visualize ACh dynamics across the cortex during behaviour. For spatiotemporal resolutions, a single plus sign represents slow (minutes) measurements over a wide area (more than 250 μm), two plus signs represents measurements over seconds in a smaller area (~10–100 μm) and three plus signs represents fast (milliseconds) single-unit recordings. For specificity, a single plus sign indicates an indirect measure of ACh and three plus signs represents a direct measure of ACh. b, Schematics highlighting possible directions of future study using ACh biosensors and high-resolution or dynamic imaging techniques, which will allow us to better understand behaviourally relevant ACh dynamics. The left image depicts possible experiments in freely moving animals with a miniscope (a miniature microscope that can be head-fixed to a moving animal) that allows cell type-specific ‘sensing’ of ACh release as in fibre photometry, with the added potential for single-cell resolution. ACh measurements can be made in specific cell types (represented by green glowing neurons) (top). Alternatively, cell type-specific calcium dynamics can be measured with a calcium indicator (red) in response to ACh release (bottom). The right image shows possible experiments in head-fixed animals using two-photon imaging. This method offers the potential for deeper imaging at higher resolution to detect compartment-specific ACh dynamics. This includes measurement of ACh release by cholinergic axons (top) or ‘sensing’ of ACh release by dendrites (represented by green glowing circles, middle image). In addition, cell type-specific responses to compartment-specific ACh release can be measured. c, An example of the application of fibre photometry to measure ACh dynamics in a target brain region^,. An ACh biosensor (GRAB_ACh3.0) is injected into the basolateral amygdala (BLA). A cannula is implanted in the BLA to measure in vivo time-locked changes in ACh release. The traces to the right are schematized representations of the findings of two recent studies which demonstrated initial increases in ACh release in the BLA in response to an unconditioned stimulus (UCS) (including both positive valence and negative valence stimuli) but not the conditioned stimulus (CS) (left). After repeated CS–UCS presentations, however, ACh release in the BLA shifted from the UCS to the CS^,. nBM, nucleus basalis of Meynert; SI, substantia innominata.