The distribution of M1 and M2 muscarinic acetylcholine receptor subtypes in the developing cat visual cortex

Abstract

The binding site characteristics and ontogenesis of [3H]pirenzepine ([3H]PZ) (M1 receptor) and [3H]oxotremorine-M ([3H]OXO-M) (M2 receptor) binding sites were investigated in the cat visual cortex. Scatchard analysis of [3H]PZ binding in adult cat visual cortex revealed a single site with a Kd of 17.3 nm and a Bmax of 352.45 fmol/mg protein. [3H]OXO-M also bound to a single site with a Kd of 7.1 nM and a Bmax of 256.39 fmol/mg protein. Receptor autoradiography revealed that [3H]PZ binding sites were present only in telencephalic structures while [3H]OXO-M sites were distributed heterogeneously throughout the brain. [3H]PZ binding sites in adult visual cortex were present in the superficial and deep cortical layers with the densest labeling in layer I and a distinct band in layer V. [3H]OXO-M sites also avoided the middle cortical layers, but were most prominent in layers V and VI with less pronounced binding in layers I and II. Deafferentation of extrinsic inputs to the visual cortex did not reduce [3H]PZ nor [3H]OZO-M binding, but neuron-specific excitotoxic lesions of visual cortex abolished both populations of binding sites. This indicates that both populations of binding sites are located on cells intrinsic to the cortex. In early postnatal life, both [3H]PZ and [3H]OXO-M binding sites were localized to intermediate cortical layers. Following this, the laminar distribution of both populations redistributed; each with its own idiosyncratic profile. By postnatal day 49, [3H]PZ binding sites redistributed into the superficial and deep layers, the pattern of adult animals, while [3H]OXO-M sites maintained a pattern similar to younger animals, with substantial binding persisting in layer IV. As late as postnatal day 70, well after [3H]PZ binding sites had achieved their mature laminar pattern, [3H]OXO-M binding sites in visual cortex had not achieved their characteristic adult pattern. In addition, the normal laminar redistribution of both [3H]PZ and [3H]OXO-M binding sites during postnatal development of the cat visual cortex was prevented by eliminating cortical afferents in early postnatal life. This indicates that muscarinic receptor rearrangement in development is dependent upon cortical input or output.