Theta-like activity in hippocampal formation slices: the effect of strong disinhibition of GABAA and GABAB receptors.

The involvement of GABAA and GABAB receptors in neural mechanisms responsible for the production of theta rhythms in hippocampal formation (HPC) slices is addressed in the present study. In a number of papers published in the last decade, we have demonstrated that theta-like activity can be successfully recorded in the limbic cortex maintained in vitro when the cholinergic agonists, acetylcholine, carbachol or muscarine, were added to the bath. Recently, we have also shown a strong GABAA modulation of the cholinergic-induced in vitro theta-like activity. This study presents a report of the first demonstration of in vitro theta-like field responses induced a consequence of simultaneously inhibiting hippocampal GABAA and GABAB receptors. HPC slices (350 microns) were maintained in a gas-liquid interface chamber (35 degrees C). Theta-like activity was induced in the presence of bath perfusion of bicuculline (GABAA antagonist) and 2-hydroxysaclophen (GABAB antagonist). This in vitro induced field response was antagonized both by muscimol (GABAA agonist) and baclophen (GABAB agonist). In addition, the experiments presented here revealed that bicuculline/2-hydroxysaclophen-induced in vitro theta-like activity also had a strong cholinergic M1 involvement: it was abolished by hemicholinium-3 (choline transport blocker) and pirenzepine (specific antagonist of M1 receptor), but not by gallamine (specific antagonist of M2 receptor). The results of the present study provided further evidence for a strong GABAergic/cholinergic interaction in the neural mechanism responsible for production of theta-like activity in the hippocampal formation slices.