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Thyrotropin-releasing hormone (TRH) is markedly increased in the rat brain following soman-induced convulsions
M J Kubek1, T M Shih, J L Meyerhoff
1Department of Anatomy, Indiana University School of Medicine, Indianapolis 46202-5120, USA.
Abstract:
Soman is an organophosphorus (OP) compound which irreversibly inhibits acetylcholinesterase (AChE), the primary synaptic inactivator of acetylcholine. Resultant excessive cholinergic activity elicits generalized convulsions and brain lesions. Recent evidence suggests that other neurotransmitter/neuromodulator systems may be affected by the OP compounds as well. Since we have shown that both electrically and chemically induced seizures cause significant and prolonged increases in the neuropeptide thyrotropin-releasing hormone (TRH) in epileptogenic sites, we examined soman-induced convulsion effects on CNS TRH. Rats were injected with either soman (100 microg/kg SC; equivalent to 0.9 LD50) or saline and observed for convulsive activity. Forty-eight hours post injection, dramatic increases of TRH over control levels were seen in frontal cortex (30-fold), pooled cortex (24-fold), hippocampus (16-fold), piriform cortex (14-fold), entorhinal cortex (11-fold), and amygdala (2-fold). No change was observed in either hypothalamus or pituitary. Our results demonstrate, for the first time, a substantial effect of an OP on a specific neuropeptide system in vivo. The neurochemical and behavioral consequences of the soman-induced increases in TRH, especially in the frontal cortex, are presently unknown. Clearly, much more work is required to discern the exact role TRH has following soman exposure.
Insights
Soman, an organophosphate, significantly increases thyrotropin-releasing hormone (TRH) in rat brains, particularly the frontal cortex, following convulsions. This study reveals a novel neurochemical effect of organophosphate exposure on neuropeptide systems.
Area of Science:
- Neuroscience
- Toxicology
- Biochemistry
Background:
- Organophosphorus (OP) compounds like soman inhibit acetylcholinesterase (AChE), leading to excessive cholinergic activity, convulsions, and brain lesions.
- Evidence suggests OP compounds may impact other neurotransmitter/neuromodulator systems.
- Previous research demonstrated that seizures increase neuropeptide thyrotropin-releasing hormone (TRH) in epileptogenic sites.
Purpose of the Study:
- To investigate the effects of soman-induced convulsions on central nervous system (CNS) TRH levels in rats.
- To determine if soman exposure alters TRH concentrations in specific brain regions.
Main Methods:
- Rats were administered a sub-lethal dose of soman (100 microg/kg SC) or saline.
- Animals were observed for convulsive activity.
- Forty-eight hours post-injection, TRH levels were measured in various brain regions using biochemical assays.
Main Results:
- Soman injection caused dramatic, dose-dependent increases in TRH levels across multiple brain regions.
- Frontal cortex showed a 30-fold increase, pooled cortex a 24-fold increase, and hippocampus a 16-fold increase.
- No significant changes in TRH levels were observed in the hypothalamus or pituitary gland.
Conclusions:
- This study provides the first in vivo evidence of a substantial effect of an organophosphate compound on a specific neuropeptide system.
- Soman exposure significantly elevates CNS TRH levels, suggesting a complex neurochemical response beyond AChE inhibition.
- The functional consequences of soman-induced TRH increases, particularly in the frontal cortex, remain unknown and require further investigation.