Neurotransmitter receptors in the avian brain. II. Muscarinic cholinergic receptors.

The characteristics and distribution of muscarinic cholinergic receptors were examined in the pigeon brain using in vitro receptor autoradiography. The antagonist N-[3H]methylscopolamine was used as ligand and the presence of the putative subtypes M1 and M2 of the muscarinic cholinergic receptors examined using carbachol and pirenzepine as displacers. The highest densities of muscarinic cholinergic receptors in the pigeon brain were localized in the paleostriatum augmentatum and the lobus parolfactorius, areas homologous to the mammalian corpus striatum. In contrast, the paleostriatum primitivum, corresponding to the mammalian globus pallidus, was poor in muscarinic cholinergic receptors. The rest of the telencephalon was also rich in muscarinic cholinergic receptors, while thalamic, hypothalamic and brainstem areas, as well as the tectum, presented intermediate densities, similar to the mammalian brain. Exceptions to that were the hippocampus, which was poorly labeled in the bird brain and the cerebellum, which presented intermediate to high densities of muscarinic cholinergic receptors in the bird brain. Preliminary pharmacological studies suggest differences between avian and mammalian receptor subtypes, since carbachol and pirenzepine apparently did not recognize two different receptor subpopulations.