The hippocampal opioidergic system: a comparative immunocytochemical study in four rodents

The occurrence and distribution of four opioid peptides (methionine-enkephalin /met-enk/, leucine-enkephalin /leu-enk/, dynorphin-A /dyn-A/, dynorphin-B /dyn-B/) were studied and compared in the subregions and layers of the hippocampi of four rodent species (rat, mouse, hamster and guinea-pig) by means of pre-embedding immunocytochemistry in order to reveal species-specific morphological characteristics. Concerning the mossy fibre system, in the rat met-enk and dyn-A, in the mouse met-enk, in the the hamster dyn-A, in the guinea-pig dyn-B was found to be the dominating immunocytochemically detectable hippocampal opioid peptide. Outside the mossy fibre system, in other hippocampal areas met-enk-immunopositive varicose nerve fibres were most frequently detected in the rat and mouse, whereas dyn-B was the most abundant in the guinea-pig. In the hamster hippocampus all the four studied opioid peptides occurred at least in few varicose fibres. In the rat and mouse enkephalinergic boutons formed pericellular baskets around non-principal cells, whereas dynorphin-immunopositive boutons were visualized in similar arrangement around principal cells (both granule and pyramidal cells) in three of the studied species, except the rat. Among other species-specific differences, the presence of leu-enk-immunopositive perikarya in the golden hamster hippocampus, and a highly ordered leu-enk-immunoreactive fibre system exclusively detected in the CA1 area of the guinea-pig hippocampus are noteworthy. The results confirm the high degree of species-specific heterogeneity characterizing the distribution of the opioid peptides in the hippocampal formation.