Seizures induce dramatic and distinctly different changes in enkephalin, dynorphin, and CCK immunoreactivities in mouse hippocampal mossy fibers.

Light microscopic immunocytochemical techniques were used to evaluate the influence of recurrent limbic seizure activity on the immunoreactivity for 3 neuropeptides--enkephalin, dynorphin, and cholecystokinin (CCK)--contained within the mouse hippocampal mossy fiber axonal system. Seizures were induced either by the placement of a small unilateral electrolytic lesion in the dentate gyrus hilus or by intraventricular injection of kainic acid. Both treatments induce epileptiform activity in hippocampus lasting several hours. Four days after either lesion placement or injection of 0.05-0.1 microgram kainic acid, immunoreactivity for all 3 peptides was altered throughout the intact mossy fiber system, bilaterally, but in distinctly different ways: enkephalin immunoreactivity (ENK-I) was dramatically elevated, dynorphin immunoreactivity was reduced, and CCK immunoreactivity (CCK-I) was either severely reduced or completely absent in the mossy fiber system. ENK-I was also clearly increased in other areas, including the lateral septum, the entorhinal cortex, and within the entorhinal (perforant path) efferents to temporal hippocampus. In contrast, the loss of CCK seemed restricted to the mossy fiber system in that immunostaining appeared normal in scattered hippocampal perikarya, within the dentate gyrus commissural system, as well as within other limbic structures. Four days after injections of 0.2 or 0.25 microgram kainic acid, mossy fiber ENK-I was greatly elevated, dynorphin immunoreactivity was reduced, but, unlike the situation with lower kainic acid doses, CCK-I was only modestly reduced in the mossy fibers and was clearly reduced in other hippocampal systems as well. These data indicate that epileptiform physiological activity differentially affects the regulation of 3 neuroactive peptides contained within the hippocampal mossy fiber system and suggest a mechanism through which seizurelike episodes can have a lasting influence on the operation of specific hippocampal circuitries.