Evidence for muscarinic acetylcholine receptor subtypes in the pigeon telencephalon

At least five subtypes of muscarinic acetylcholine receptors are expressed in various mammalian tissue preparations. The following experiment, through the use of direct binding assays (using tritiated quinuclidinyl benzilate), competitive binding assays (using tritiated quinuclidinyl benzilate and unlabeled pirenzepine or AF-DX 116), and autoradiographic techniques, examined whether two of these five putative muscarinic acetylcholine receptor subtypes can be found in avian brain. Accordingly, autoradiographic mapping of pirenzepine-sensitive (M1-like) and AF-DX 116-sensitive (M2-like) muscarinic acetylcholine receptor subtypes in the pigeon telencephalon was conducted. Although both ligands bound throughout the brain, most telencephalic regions, including the archistriatum, the neostriatum, and basal ganglia structures like lobus paraolfactorius, nucleus accumbens, and paleostriatum, showed a higher density of M1-like sites. The exception to this finding was the nucleus basalis which appeared as a region where M2-like sites predominated. Moreover, the telencephalic region with the largest ratio of M1-like to M2-like sites was the lateral portion of the parahippocampus; a characteristic shared with the mammalian dentate gyrus. The findings reported here are generally consistent with previous reports of mammalian M1/M2 receptor distributions.