The effect of intrahypothalamic microinjection of hemicholinium (HC-3) on the hippocampal theta rhythm of cats

Abstract

Acute and chronic intact cat preparations were made with recording electrodes located in the dorsal hippocampus and cerebral cortex, stimulating electrodes in the mesencephalic reticular formation, and an “Injectrode” in the posterior hypothalamus through which both electrical and pharmacological stimulation could be applied. In all preparations 12–200 μg hemicholinium (HC-3), administered through the “Injectrode”, suppressed the hippocampal theta rhythm previously elicited by electrical stimulations of either the reticular formation or the posterior hypothalamus. The latency for this effect ranged from 90 min to 3 hours. Reticular-induced desynchronization of the cortical EEG disappeared 2–5 hours after HC-3 administration. Reversal of the HC-3 effect on the hippocampal theta rhythm was seen shortly after administration of carbachol, bethanechol and pilocarpine. No consistent behavioral effects accompanied these changes in EEG. These findings are discussed in light of the hypothesis that HC-3 exerts a post-synaptic action in the CNS, although a pre-synaptic effect through diffusion of the injected HC-3 to a distant site cannot be ruled out with certainty.