Autoradiographic mapping of galanin receptors in the monkey brain

The distribution of specific binding sites for the peptide galanin was studied in the monkey brain by using 125I-galanin and the method of quantitative in vitro autoradiography. The binding to tissue sections was found to be rapid, reversible and saturable with a calculated Bmax of around 13 fmol mg-1 tissue in cortical regions and a KD ranging between 0.39 and 0.22 nM in different brain regions. Specifically bound 125I-galanin was detected in a number of brain regions throughout the rostrocaudal axis of the monkey brain. More specifically, the majority of the 125I-galanin binding sites were present in the basal forebrain (e.g. olfactory tubercle, lateral septal nuclei, nucleus accumbens, substantia innominata and the basal nucleus of Meynert), the amygdala (e.g. the lateral, medial accessory and central nuclei), the amygdala-hippocampal area, hypothalamus (e.g. the anterior hypothalamic area, the ventro-medial hypothalamus and the zona incerta), as well as in certain brain stem cell groups such as the substantia nigra (pars compacta), locus coeruleus, central grey substance, parabrachial nucleus, sensory and motor nuclei of the trigeminal nerve and the dorsal vagal complex. In the neocortex, highest binding density occurred in layer 4 of all neocortical fields, except in the visual cortex where the highest densities occurred in layers 4cb, 5a and 6. Major fibre tracts such as the fimbria and the stria terminalis also contained high densities of 125I-galanin binding sites. Areas poor in, or lacking, specific 125I-galanin binding sites included the basal ganglia, the thalamus, the mammillary bodies, the cerebellum, and the reticular formation of the brain stem. The pattern of galanin receptor distribution in the monkey brain reported here closely resembles that observed for galanin receptors and galanin-immunoreactive preterminal processes described previously in the rat brain. An exception, however, was neocortex, where in the rat neither 125I-galanin binding nor galanin-positive fibres have been observed. The present findings may indicate a close association between galanin receptors and nerve terminals containing galanin immunoreactivity in the monkey brain and implies important role(s) for galanin in neurotransmission in the monkey central nervous system.