SEROTONIN-OPERATED POTASSIUM CURRENT IN CA1 NEURONS DISSOCIATED FROM RAT HIPPOCAMPUS

1. The intracellular mechanisms of serotonin (5-HT) response were investigated in dissociated rat hippocampal pyramidal neurons using the nystatin-perforated patch technique. 2. Under voltage-clamp conditions, 5-HT evoked outward currents (I5-HT) with an increase in membrane conductance at a holding potential of -40 mV. The outward current reversed at the K+ equilibrium potential, which shifted 59.4 mV with a 10-fold change in extracellular K+ concentration. 3. The first application of 5-HT on neurons perfused with Ca2+-free external solution induced outward currents of I5-HT but the amplitude was diminished dramatically with successive applications. Pretreatment with the membrane-permeant Ca2+ chelator 1,2-bis-(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA-AM) also diminished the I5-HT amplitude. 4. Pretreatment with pertussis toxin (PTX) had no effect on I5-HT5. 5. The I5-HT was not cross-desensitized with the caffeine-induced outward current but with outward current mediated by the muscarinic acetylcholine receptor. Pretreatment with Li+ significantly enhanced the I5-HT, indicating that I5-HT is involved in the elevation of intracellular free Ca2+ released from inositol triphosphate (IP3)-sensitive Ca2+ store sites but not from the caffeine-sensitive ones. 6. The calmodulin (CaM) antagonists, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), inhibited I5-HT in a concentration-dependent manner. 7. The Ca2+/CaM-dependent protein kinase II inhibitor 1-[N,O-Bis (5-isoquinolinesulfonyl)-N-methyl-L-tyrosil]-4-phenylpiperazine depressed the I5-HT. 8. Blockade of protein kinase C (PKC) by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) had no effect on I5-HT. However, the enhancement of PKC activity by phorbol myristate acetate diminished it. 9. The results suggest that 5-HT-induced K+ outward current on somata of hippocampal neurons is mediated by the following mechanisms. First, 5-HT activates 5-HT2 family receptors coupled to PTX-insensitive G-protein and may stimulate phosphatidylinositol breakdown by phospholipase C. Consequently, with the increase in metabolites, IP3 may release Ca2+ from caffeine-insensitive store sites. Secondly, the increased intracellular free Ca2+ may activate the Ca2+/CaM-dependent protein kinase II, which phosphorylates some proteins, and the K+ channels may open.