Changes in thyrotropin-releasing hormone levels in hippocampal subregions induced by a model of human temporal lobe epilepsy: effect of partial and complete kindling.
Endogenous thyrotropin-releasing hormone has been hypothesized to modulate seizure activity, possibly by subserving an anticonvulsant function in limbic brain. A specific and sensitive radioimmunoassay was utilized to quantitate thyrotropin-releasing hormone levels in dorsoventrally dissected hippocampal subregions after partially (an experimental paradigm of complex partial epilepsy) or fully kindled (repeated generalized) seizures, to define specific seizure-related limbic pathways that may contain thyrotropin-releasing hormone. Samples were taken from electrode controls and 1, 6, 24, 48 and 144 h after a fully kindled seizure or 24 h after the first occurrence of a stage 3-4 (partially kindled) seizure in rats. Thyrotropin-releasing hormone levels were below controls in all subregions taken 1 h after a fully kindled seizure. They resembled control values 6 h after seizure, were substantially elevated at 24 and 48 h, and then returned to control levels by 144h. Low thyrotropin-releasing hormone levels seen shortly after the seizure presumably indicate peptide depletion during the ictus. The higher levels seen at later times occurred during a postictal period coinciding with refraction to additional seizure-generating stimulation. These values probably reflect enhanced synthesis since the largest increases were seen in subregions (dentate gyrus, hilus/CA4, CA3) that contain perforant path terminals, and where previously observed intrinsic hippocampal thyrotropin-releasing hormone messenger RNA increases were seen. The thyrotropin-releasing hormone response was less robust in ventral hilus/CA4 and CA3 areas, leading to speculation that this smaller response could, in part, explain why the ventral (temporal) hippocampus may be more susceptible to seizure-induced damage. No changes in thyrotropin-releasing hormone were detected after partially kindled seizures, suggesting that thyrotropin-releasing hormone is not involved in epileptogenesis or its stereotypic motor behavior. The time-course and distribution of thyrotropin-releasing hormone elevations seen after a fully kindled (repeated generalized) seizure, and the lack of effect of partial kindling (complex partial seizure) are consistent with previous observations concerning postictal thyrotropin-releasing hormone messenger RNA expression. These neurochemical results support the hypothesis that endogenous thyrotropin-releasing hormone can serve an anticonvulsant neuromodulatory function in specific limbic pathways relevant to temporal lobe epilepsy.
Knoblach
S M
SM
Program in Medical Neurobiology, Indiana University School of Medicine, USA.
Kubek
M J
MJ
D000818:Animals; D004195:Disease Models, Animal; D004210:Dissection; D004833:Epilepsy, Temporal Lobe / Q000378:metabolism*; D006624:Hippocampus / Q000378:metabolism*; D007696:Kindling, Neurologic / Q000502:physiology*; D008297:Male; D011863:Radioimmunoassay; D051381:Rats; D011919:Rats, Inbred Strains; D013973:Thyrotropin-Releasing Hormone / Q000378:metabolism*; D014018:Tissue Distribution
D013973:Thyrotropin-Releasing Hormone