Extinction of hippocampal theta rhythm evoked by midbrain stimulation after intracerebroventricular administration of substance P fragment and enkephalin administration in rabbits

Abstract

Repetitive stimulation of the midbrain reticular formation with electrical stimuli causes extinction of the hippocampal theta rhythm. This phenomenon was used as an indicator of the effect of intracerebroventricular administration of the C-terminal hexapeptide substance P fragment (SP 6–11), Leu- and Met-enkephalin on hippocampal electrical activity in the experiments performed on rabbits. Intracerebroventricular administration of SP 6–11 caused an increase in the number of trains of pulses required to produce the extinction of the theta rhythm. Both of the enkephalins had opposite effects, the effect of Met-enkephalin being more than twice as strong as Leu-enkephalin. Control administration of C-terminal pentapeptide substance P fragment (SP 7–11) and 0.9% NaCl solution had no significant effect on the tested phenomenon.