THE EFFECT OF SEROTONIN DEPLETION ON RAT ADAPTIVE-BEHAVIOR IN HIGH AND LOW RESPONDERS TO NOVELTY

Adaptive behavioral responses to repeated daily testing in the open fi eld test (OFT) were studied in rats that had a different locomotor res ponse to a novel environment (ambulatory scores above and below the me dian, respectively). A clear-cut progressive habituation of explorator y activity was observed over the 5 daily sessions, but was more pronou nced in HR thigh responders) than in LR (low responders). p-Chlorophen ylalanine-induced serotonin depletion, confirmed biochemically by high -performance liquid chromatography (HPLC), attenuated the rate of beha vioral habituation in the LR and HR (to a minor degree), and abolished the between group differences in motor activity in the OFT. Autoradio graphy revealed a significantly greater specific binding of the GABA(A ) receptor agonist, [H-3]muscimol, to the entorhinal cortex of LR than to the entorhinal cortex of HR rats, and a near-significant trend wit hin the dentate gyrus of the hippocampus, but not within other brain s tructures. In conclusion, serotonin was found to contribute in a compl ex way to the habituation of locomotor activity, an effect that depend ed on the initial level of locomotor activity. The intrinsic mechanism underlying the observed effects might involve changes in central GABA (A) receptors.