Chick brain cholinergic receptors studied by antagonist labelling: distribution, ontogeny & function

Abstract

By means of an in vitro labelling technique and methods of light microscope autoradiography the distribution of/muscarinic cholinergic receptor, labelled L by the tritiated antagonists I-Quinuclidinyl benzilate or Propylbenzilylcholine mustard, and the distribution of the nicotinic cholinergic receptor, labelled by a-Bungarotoxin, was shown in thin tissue sections of the young post hatch chick brain. The distribution of muscarinic cholinergic receptor in the chick brain was found to be widespread. The highest density concentrations of muscarinic receptors, for example in the paleostriatum augmentatum and hyperstriatum ventrale of the forebrain, thalamic and mesencephalic relay nuclei, optic tectum and brain stem nuclei, were found to be highly regionally localised. In contrast, nicotinic receptors were found to be concentrated to mesencephalic and diencephalic regions of the brain, in particular to colliculi and the principal optic relay nuclei of the diencephalon. The density of nicotinic receptor in the forebrain, apart from the olfactory lobe, was found to be very low. The concentration of muscarinic receptor throughout the brain was found to be at least one order of magnitude greater than the concentration of nicotinic cholinergic receptor. Without exception, all regions populated by substantial concentrations: of nicotinic receptor were also populated by substantial concentrations of muscarinic receptor. On the other hand, a number of brain regions populated by high densities of muscarinic receptor were found to be almost devoid of specific a-bungarotoxin labelled nicotinic cholinergic receptor, eg. the hyperstriatum ventrale. H antagonist labelled muscarinic and nicotinic cholinergic receptors in chick brain tissue slices were shown to be similar to values given by other reports for alternative tissue preparations. The great majority of H muscarinic antagonist binding sites were shown to be specific, ie. atropine 'sensitive' or displaced. Evidence has been given which suggests that muscarinic antagonists are labelling a heterogeneous population of receptor. Questions concerning the identity of muscarinic antagonist labelled receptor are discussed. The distribution of antagonist labelled muscarinic receptor in the chick brain was compared with in vitro labelled autoradiographed brain sections of the 50 day post natal rat brain, with the objective of discovering whether homologous neurons of the rat and chick brain showed similar patterns of distribution and concentrations of muscarinic receptor. For the great majority of established and assumed homologous neuronal populations between these species of vertebrate brain, correspondence of distribution and density of receptor was shown. Where differences were apparent, eg. cell layers of the olfactory bulb, these differences have been suggested to reflect the increase, or alternatively decrease, of particular cholinoceptiye cell-types, possibly micro circuit interneurons, subserving the greater or lesser emphasis of particular sensory modalities between these, species of vertebrate. The distribution of antagonist labelled muscarinic receptor, again using in vitro labelling autoradiographic localisation proceedures, was measured in the in ovo and early post hatch chick brain. High densities of muscarinic receptor were shown in those regions of 10 days in ovo brains which, post hatch, are populated by (regionally comparative)high densities of receptor. In addition almost all regions of the post hatch, chick brain, shown to be populated by low densities of muscarinic receptor, were shown during the latter stages of in ovo development.to be populated by transient moderate to high densities of muscarinic receptor, eg. ectostriatum, hyperstriatum and intercalatus superior. Between 12 and 19 days in ovo, all regions of the developing chick brain, but in particular regions of the mid- and forebrain, were characterised by patterns of muscarinic receptor distribution which were found to occur only during in ovo brain development. Antagonist labelled muscarinic binding sites, apparently localised to parallel arrays of cells r disappeared around 19 days in ovo for reasons which may reflect a change in muscarinic ligand binding properties or a change in access of the radiolabelled ligand to receptor binding sites, restricted around this age by oligodendrogliogenesis and subsequent neuronal process myelination.