Posterior hypothalamic GABAergic mediation of hippocampal theta in the cat

The effect of posterior hypothalamic (PH) microinjections of GABAA and GABAB agonists (muscimol and baclophen, respectively) and antagonists (bicuculline and 2-OH saclophen) on spontaneous, sensory and electrically induced hippocampal formation (HPC) theta EEG activity was investigated in freely behaving cats. Administration of GABAergic agonists abolished the theta rhythm recorded from HPC. This effect was reversible. A substantial difference in the recovery time course between frequency versus amplitude and power of hippocampal theta was observed. While theta frequency exhibited a rapid reappearance with a shallow slope, the power and amplitude showed a gradual recovery with a steeper slope. The PH injection of GABAergic antagonists produced HPC theta with increased power. These results demonstrate that both types of GABAergic receptors localized in PH are engaged in mechanisms responsible for generating hippocampal theta oscillations in freely behaving cats. The present study provides additional support for the essential difference between rats and cats in the programming of HPC theta amplitude and frequency. While the PH in rats is involved in programming the frequency of theta rhythm, the same region in cats mainly determines theta amplitude.