5-HT INHIBITS LATERAL ENTORHINAL CORTICAL-NEURONS OF THE RAT IN-VITROBY ACTIVATION OF POTASSIUM CHANNEL-COUPLED 5-HT1A RECEPTORS

Serotonin (1-40 mu M) reduced input resistance by 20.6 +/- 6% and hype rpolarized stellate and pyramidal neurons of layers two and three of t he lateral entorhinal cortex. 5-Carboxamidotryptamine, a 5-HT1 agonist , and the selective 5-HT1A agonist 8-hydroxy-dipropylaminotetralin mim icked the action of serotonin. The reversal potential of 5-HT-mediated hyperpolarizations was sensitive to the extracellular K+ concentratio n, indicating a potassium conductance change. Serotonin treatment supp ressed excitatory amino acid-mediated synaptic potentials (by 48%, K-d = 6.9 mu M) and responses to exogenously applied glutamate (70.1 +/- 17% of control, n = 7), but did not alter paired-pulse facilitation, i ndicating a postsynaptic site of action. Intracellular application of QX-314, a blocker of potassium conductance, significantly reduced depr ession of synaptic potentials by 5-HT agonists. In cells filled with Q X-314, responses to exogenously applied glutamate were not reduced by serotonin or 5-carboxamidotryptamine application. These results indica te that the observed conductance increase associated with 5-HT applica tion accounts for most if not all of the observed depressant effects o f 5-HT1A agonists on excitatory amino acid-mediated neurotransmission. (C) 1997 Elsevier Science B.V.