Muscarinic cholinergic receptors in the songbird and quail brain: A quantitative autoradiographic study

In order to clarify the neuroanatomical basis for postulated muscarinic cholinergic control of a wide array of physiological processes in birds, the distribution of muscarinic cholinergic receptors in the brain of three avian species was investigated by quantitative autoradiography. The species consisted of two passerines (songbirds), the European starling (Sturnus vulgaris) and the song sparrow (Melospiza melodia), and one galliform, the Japanese quail (Coturnix coturnix japonica). [³H] N-methyl scopolamine (NMS), a muscarinic cholinergic antagonist was used as the ligand to label the receptors. Initial experiments demonstrated that the binding of this ligand in the three species is saturable in the nanomolar range and has a high affinity (K_d = ±0.6 nM). Displacement experiments revealed that three muscarinic ligands competed ii an order of potency characteristic of the mammalian muscarinic receptor (i.e., atropine ≥ oxotremorine ≥ carbachol) for NMS binding in the avian brain.

In all three species, portions of the basal ganglia, such as the parolfactory lobe and the paleostriatum augmentatum, exhibited the highest density of binding. On the other hand, the paleostriatum primitivum, the avian homologue of the mammalian globus pallidus, contained very few binding sites. Other telencephalic sites, such as the ventral and dorsal hyperstriatum, also revealed relatively high receptor density. However, the neostriatum and especially the ectostriatum showed much lower levels. In the hypothalamus, in all three species, specific binding could be observed in the ventromedial nucleus and adjacent areas. The paraventricular nucleus also showed moderate levels of binding density, especially in the two songbird taxa. At a more rostral level, the preoptic area showed low levels of binding. In the quail, the sexually dimorphic nucleus of the preoptic area was clearly outlined in the autoradiograms by the low level of binding sites compared to the surrounding areas. In the two passerine species, nuclei of the song system were identified by either high or low levels of NMS binding. High binding defined area X and the mesencephalic nucleus, intercollicularis (ICo). In contrast, the robust nucleus of the archistriatum and the magnocellular nucleus of the anterior neostriatum showed low levels of binding in comparison with the surrounding tissue. None of these nuclei were visible in the quail autoradiograms except for ICo, which appeared as in the passerines as i heavily labelled area surrounding the lightly labelled nucleus mesencephalicus lateralis par dorsalis. In all three species, the hippocampal complex was devoid of NMS binding except for two lateral dark bands that were present along the entire rostral to caudal extent of the hippocampus. These results suggest general similarities between the distribution of muscarinic cholinergic binding sites in birds and mammals. Among the three species, the qualitative pattern of receptor density is similar, with the exception of the telencephalic nuclei controlling vocal behavior in songbirds, which have no apparent homologues in the quail.