Effects of chemical and surgical lesions on levels of chromatographically identified enkephalin-like peptides in rat hippocampus.

The present study quantitates the content of Met- and Leu-enkephalin in the rat hippocampus, and provides information on the localization of the enkephalins within the hippocampal neuronal circuitry. Several enkephalins were identified in rat hippocampus, two of which are shown to be Met- and Leu-enkephalin. The levels of these enkephalins, and of other unidentified enkephalin-related peptides, were not depleted by intrahippocampal colchicine, which destroyed the great majority of the hippocampal granule cells and the associated mossy fiber pathway. Entorhinal cortical lesions ablating the perforant pathway input to the hippocampus also did not significantly lower enkephalin levels in the hippocampus. Unilateral fimbrial transection caused a significant bilateral increase in both Met- and Leu-enkephalin levels. This may result from loss of a stimulatory input to putative enkephalin containing interneurons within the hippocampus. The extents of all lesions were verified histologically in hippocampi used for biochemical analysis. No evidence was seen for the presence of enkephalins in the perforant pathway, nor in nerve fibers in the fimbria/fornix, which provide the other main source of hippocampal efferents. The enkephalins are likely to be intrinsic to the hippocampus, in which neuronal cell bodies containing enkephalin-like immunoreactivity have been extensively reported.