Beta‐adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs: Autoradiographic and immunohistochemical studies

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Abstract
Species differences in the distribution of β‐adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs were examined using in vitro autoradiographic techniques. β1‐receptors were found in the hippocampal area CA1 of both species, although guinea pigs had significantly lower receptor densities in comparison to rats. In guinea pigs, β2‐adrenergic receptors were predominant in hippocmpal area CA1. Hippocampal area CA3 had very low levels of β1‐ and β2‐receptors in both species. The retrohippocampal area was also found to have a distinct topographic distribution of β‐receptors. In rats, the subiculum and parasubiculum (layers II‐III) were heavily labeled for β1‐receptors; in contrast, guinea pigs had few receptors in these regions. β2‐receptors were particularly prominent in the parasubicular region in rats. The entorhinal cortex laminae was found to contain β‐receptors in both rats and guinea pigs. Immunohistochemical techniques were used to compare the pattern of catecholaminergic innervation with the receptor distribution within each hippocampal subregion. Despite the general lack of β‐receptors in area CA3, abundant catecholamine immunoreactive fibers were observed in CA3 of rat and guinea pig hippocampus. Significant species differences were found in the distribution of hippocampal β‐adrenergic receptor subtypes, and moreover, in both species the distribution of hippocampal β‐adrenergic receptors did not coincide with the pattern of hippocampal adrenergic innervation.

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