Characterization of cholinergic regulation of seizures by the midline thalamus

This study determined the effects of injections of different cholinergic agents in the central medial intralaminar nucleus of the thalamus on seizures induced by intravenous injection of pentylenetetrazol. Injections of the cholinesterase inhibitor, neostigmine bromide, induced a stiff, tremulous state and lowered myoclonic, clonic and tonic seizure thresholds. The nicotinic agonist, tartrate, depressed arousal and facilitated all types of seizure, while its antagonist, d-tubocurarine chloride, heightened arousal and transformed pentylenetetrazol-induced convulsions, with tonic seizures occurring at a very low threshold without preceding myoclonic or clonic seizures or EEG spikes. The muscarinic agonist (+/-)pilocarpine hydrochloride, in very large doses, induced slight hyperactivity and facilitated tonic seizures but did not affect myoclonic or clonic seizures. Its antagonist, (-)scopolamine hydrobromide, slightly depressed arousal and myoclonic and clonic seizure thresholds. Injections of mixtures of agonists and antagonists (d-tubocurarine chloride + nicotine tartrate or (+/-)pilocarpine hydrochloride + (-)scopolamine) had little effect on spontaneous behavior or seizures. These results suggest that the midline thalamus regulates seizures and arousal, under the control of cholinergic neurotransmission. Nicotinic and muscarinic receptors have opposing roles in mediating these functions.