Thyrotropin-releasing hormone receptor binding sites: autoradiographic distribution in the rat and guinea pig brain

Thyrotropin-releasing hormone (TRH) binding sites were labeled in vitro in mounted brain tissue sections from rat and guinea pig brains with [3H]methyl TRH and localized autoradiographically using 3H-sensitive film. Regional densities of TRH binding sites were measured by computer-assisted microdensitometry. The distribution of sites in both species was highly heterogeneous. In both guinea pig and rat brains, the highest densities of binding sites were seen in the amygdaloid nuclei and the perirhinal cortex. In contrast, in other brain areas, a clear difference between the distribution of sites in rat and guinea pig was found. The temporal cortex, pontine nuclei, and interpeduncular nucleus, which contained high densities of binding in the guinea pig, were scarcely labeled in the rat. The accessory olfactory bulb and the septohippocampal area presented in the rat higher concentrations of binding sites than in the guinea pig. Other brain areas showing intermediate to low densities in both species were accumbens nucleus, bed nucleus of the stria terminalis, dentate gyrus, facial and hypoglossal nuclei, and gelatinosus subnucleus of the trigeminal nerve, among others. The anterior pituitary also presented low to intermediate concentrations of receptors. The distribution of TRH sites here described does not completely correlate with that of endogenous TRH, but is in good agreement with previous biochemical data. The results are discussed in correlation to the physiological effects that appear to be mediated by TRH.