Distribution of opioid binding sites in the guinea pig hippocampus as compared to the rat: a quantitative analysis

In vitro autoradiography of cryostat sections revealed major differences between the distribution of opioid binding sites in the hippocampus of the guinea pig and the rat. Only very low binding was found in the pyramidal cell layer, the dentate granular cell layer and the commissural-associational zone of the dentate molecular layer of the guinea pig, whereas these areas were moderately to densely labeled in the rat. In the guinea pig an enrichment of sites was observed in the terminal field of the mossy fiber system in the hilus which was absent in the rat. Binding sites in the guinea pig were found to be mainly of the kappa and mu type. The distribution of [Leu]enkephalin immunoreactivity does not correlate well with the distribution of delta opioid binding sites in the hippocampus. Quantification of opioid binding sites in the hippocampus demonstrates that no one type of site can be assigned to a specific hippocampal subregion nor does the intensity or the pattern of distribution of binding types agree well with the distribution of endogenous opioid peptides.