Lack of change in neurochemical markers during the postepileptic phase of intrahippocampal tetanus toxin syndrome in rats. The chronic epileptic syndrome induced by injecting tetanus toxin into rat hippocampus causes functional changes that essentially are permanent, outlasting the period of active seizures by at least 1 year. These long-term changes have been characterized by an impaired performance on a range of behavioral tasks, which in turn have been associated with a physiologic depression of hippocampal evoked responses but not with any discernible histopathology. In the present study, we examined the hippocampi of rats in the postseizure phase of the tetanus toxin model and observed no significant changes in the concentration of neurochemical markers for six neurotransmitters. Therefore, the long-term reduction in hippocampal excitability cannot be attributed to any major loss of afferents or hippocampal neurons using aspartate, acetylcholine, gamma-aminobutyric acid (GABA), glutamate, norepinephrine (NE), or serotonin as their transmitters. 