EFFECT OF 5-HYDROXYTRYPTOPHANE ON BEHAVIOR AND HIPPOCAMPAL PHYSIOLOGYIN YOUNG AND OLD RATS

Spatial memory ability, tested in a water maze, was severely impaired in control, 24-month-old hooded rats. A daily injection of the seroton in precursor, 5-hydroxytryptophane (5-HTP), prior to training sessions , had no effect on the behavior of the young rats but improved conside rably the performance of the old rats in the watermaze. In the same gr oups of young and aged rats, the response properties of the hippocampa l dentate gyrus to perforant path stimulation was assessed before and after parenteral administration of 5-HTP. The dentate gyrus of aged ra ts produces a smaller EPSPs in response to perforant path stimulation but a larger population spike for a given EPSP than that produced in y oung rat brains. These differences are not affected by 5-HTP. In young brains, priming commissural stimulation suppresses subsequent reactiv ity to perforant path stimulation. This priming effect is nearly absen t in aged rat hippocampus but reappears when the rat is injected with 5-HTP. It is suggested that the serotonergic innervation of the rat hi ppocampus plays a major role in regulation of the excitability of the hippocampus and in behavioral functions associated with this structure .