Behavior during hippocampal microinfusions. IV. Transmitter interactions.

The interaction between noradrenergic, cholinergic, and GABAergic receptor stimulation in the dentate gyrus of the rat was investigated at the behavioral level. Awake, unrestrained rats in a holeboard/activity apparatus received continuous 40-min infusions into the dentate hilus of combined solutions of norepinephrine, the cholinergic agonist carbachol, and the GABA antagonist picrotoxin. Infusions into the dentate gyrus of either carbachol or picrotoxin have been found to produce a comparable locomotor activation of rats that is probably due to the excitation of dentate granule cells. Low dose hippocampal infusions of norepinephrine have been shown to significantly affect the quality but not the quantity of the locomotor activity or rats. Co-infusion of norepinephrine potentiated the effects of picrotoxin and blocked the effects of carbachol. It is hypothesized that the noradrenergic input to the area dentata increases the efficacy of extrinsic afferents while also facilitating recurrent granule cell inhibition mediated by GABAergic interneurons.