Muscarinic receptor M2 in cat visual cortex: Laminar distribution, relationship to γ‐aminobutyric acidergic neurons, and effect of cingulate lesions

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Abstract
Acetylcholine can have diverse effects on visual cortical neurons as a result of variations in postsynaptic receptor subtypes as well as the types of neurons and subcellular sites targeted. This study examines the cellular basis for cholinergic activation in visual cortex via M2 type muscarinic receptors in γ‐aminobutyric acid (GABA)‐ergic and non‐GABAergic cells, using immunocytochemical techniques. At light microscopic resolution, M2 immunoreactivity (‐ir) was seen in all layers except area and sublayer specific bands in layer 4. Subcellularly, M2‐ir occurred in both dendrites and terminals that form symmetric and asymmetric junctions. Layers 5 and 6 were characterized by axosomatic contacts that displayed labeling in the presynaptic component, and layer 6 displayed perikaryal postsynaptic staining, suggesting that corticofugal output neurons may be modulated particularly strongly via M2. Infragranular layers differed from the supragranular layers in that more labeled profiles were axonal than dendritic, indicating a dominant presynaptic effect by acetylcholine via M2 there. Unilateral cingulate cortex cuts caused reduction of cholinergic and noradrenergic fibers in the lesioned hemisphere at light microscopic resolution; at electron microscopic resolution, the synapse density and axonal M2 labeling were reduced, suggesting that M2 was localized presynaptically on extrathalamic modulatory inputs. Dual labeling with GABA in visual cortex layer 5 showed that half of M2‐labeled dendrites originated from GABAergic neurons. Given that only one‐fifth of all cortical dendritic profiles are GABAergic, this prevalence of dual labeling indicates an enrichment of M2 within GABAergic dendrites and, thus, implicates abundant postsynaptic action on GABAergic neurons via M2. In contrast, only one‐tenth of M2‐labeled terminals originated from GABAergic neurons, suggesting that the presynaptic action of acetylcholine via M2 receptors would be more selective for non‐GABAergic terminals. J. Comp. Neurol. 441:168–185, 2001.

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