Is electrical coupling involved in the generation of posterior hypothalamic theta rhythm?

Data obtained in in vitro experiments and urethane anaesthetized animals have revealed that the mechanisms responsible for the generation of hippocampal cholinergic theta rhythm are specifically affected by the administration of broad spectrum gap junctions (GJs) blocker - carbenoxolone (CBX). The aim of this study was to examine the effect of GJs modulation on the production of posterior hypothalamic theta. Specifically, we were interested in evaluating whether CBX could attenuate the theta rhythm recorded from the supramammillary nucleus and posterior hypothalamic nuclei, in both in vitro and in vivo preparations. The data we obtained from in vitro and in vivo preparations demonstrated that the administration of CBX did not suppress cholinergically induced theta in posterior hypothalamic area (PHa) slices nor the theta rhythm observed in the PHa of urethane anaesthetized rats. Moreover, the application of trimethylamine, while very effective in the enhancement of hippocampal theta rhythm, did not produce any changes in theta oscillations observed in either in vitro or in vivo posterior hypothalamic area preparations. These data show that electrical coupling via GJs is not involved in theta rhythm generation in the PHa. Surprisingly, we observed a significant enhancement of theta activity in response to the carbenoxolone administration in both in vitro and in vivo PHa preparations. The theta rhythm enhancement detected in those experiments was attenuated by the application of spironolactone (mineralocorticoid receptors antagonist). We suggest that the observed excitatory effects of CBX on posterior hypothalamic oscillatory activity in the theta band could be mediated by mineralocorticoid receptors.