Ontogeny of dynorphin-like immunoreactivity in the hippocampal formation of the rat

Abstract

Light microscopic immunocytochemical techniques were used to analyze the ontogeny of dynorphin (A)-like immunoreactivity (DLI) in the hippocampal formation of the Sprague-Dawley rat. For comparison purposes, alternate sections of the same brains were processed for the localization of methionine enkephalin-like immunoreactivity (ELI). DLI was first detectable in CA3a stratum lucidum and the suprapyramidal hilus on postnatal day (P) 6. On P7, DLI was evenly present throughout the full extent of the mossy fiber system. From P8 to P19, DLI progressively increased in intensity and could be localized in the fine axons and spherical swellings. The mossy fiber system and occasional perikarya superficial to stratum granulosum were the only hippocampal elements that exhibited DLI. In corroboration with earlier results, stratum lucidum ELI was first detected within large spherical bouton-like swellings on P13. From these data it is concluded that DLI appears in morphologically immature mossy fibers soon after they reach their target fields. In contrast, enkephalin is first detected within morphologically elaborated mossy fiber boutons well after the establishment of functional synapses.