Neuropharmacological specificity of brain structures involved in soman-induced seizures.

Pharmacological control of seizure activity following nerve agent exposure is critical in reducing neuropathology and improving survival in casualties. Three classes of drugs, anticholinergics, benzodiazepines and excitatory amino acid (EAA) antagonists, have been shown to be effective at moderating nerve agent-induced seizures. However, little is known about which brain structures are involved in producing the anticonvulsant response. This study evaluated drugs from each class, injected directly into one of three specific brain structures, the perirhinal cortex, the entorhinal cortex, or the mediodorsal thalamus, for their ability to modulate seizures induced by the nerve agent soman. The drugs evaluated were the anticholinergic scopolamine, the benzodiazepine midazolam, and the EAA antagonist MK-801. For each drug treatment in each brain area, anticonvulsant ED₅₀ values were calculated using an up-down dosing procedure over successive animals. There was no statistical difference in the anticonvulsant ED₅₀ values for scopolamine and MK-801 in the perirhinal and entorhinal cortices. MK-801 pretreatment in the mediodorsal thalamus had a significantly lower anticonvulsant ED₅₀ value than any other treatment/injection site combination. Midazolam required significantly higher doses than scopolamine and MK-801 in the perirhinal and entorhinal cortices to produce an anticonvulsant response and was ineffective in the mediodorsal thalamus. These findings support the contention that specific neuroanatomical pathways are activated during nerve agentinduced seizures and that the discrete brain structures involved have unique pharmacological thresholds for producing an anticonvulsant response. This study is also the first to show the involvement of the mediodorsal thalamus in the control of nerve agent-induced seizures.