Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the se rotonergic system. Neurons in the hippocampus and in the EC receive a stron g serotonergic projection from the raphe nuclei and express serotonin (5-HT ) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application hav e been reported for neurons of the entorhinal cortex and the hippocampus, t he effects of serotonin on synaptic transmission are contrary in both areas . Serotonin mainly depresses fast and slow inhibition of the principal out- put cells of the hippocampus, whereas it selectively suppresses the excitat ion in the entorhinal cortex. On the basis of these data, we discuss the po ssible role of serotonin under physiological and pathophysiological circums tances.