CHANGES IN EXPLORATORY ACTIVITY FOLLOWING STIMULATION OF HIPPOCAMPAL 5-HT1A AND 5-HT1B RECEPTORS IN THE RAT

The object exploration task allows the measure of changes in locomotor and exploratory activities, habituation, and reaction to a spatial change and to novelty. The effects of intrahippocampal (dorsal CA1 field) microinjections of serotonin 1 receptor (5-HT1) agonists on these behavioral components were evaluated in the rat. 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, 5 mu g/mu l) was used as a 5-HT1A agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo[3,2-b]pyrid-5-one (CP 93,129, 16 mu g mu l) as a 5-HT1B agonist, and scopolamine (10 mu g/mu l) as a muscarinic cholinergic antagonist. Scopolamine induced a long-lasting increase in locomotor activity and a lack of reaction to spatial change; both these results are in agreement with the known crucial influence of the septo-hippocampal cholinergic system in hippocampal functioning. Stimulation of 5-HT1A and 5-HT1B receptors induced a decrease in object exploration and habituation without affecting the retrieval of spatial information, But stimulation of hippocampal 5-HT1B receptors induced a selective change in the animal's emotional state, i.e., an initial decrease in locomotor activity and a neophobic reaction in response to a new object; such effects did not occur following stimulation of 5HT1A receptors, These results have to be considered in the light of the anxiogenic property of 5-HT1B agonists. On the whole, they support the hypothesis of the involvement of the serotonergic system, via 5HT1A and 5-HT1B receptors, in the modulation of hippocampal functions. (C) 1995 Wiley-Liss, Inc.