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

1. The intracellular mechanisms of serotonin (5-HT) response were inve stigated in dissociated rat hippocampal pyramidal neurons using the ny statin-perforated patch technique. 2. Under voltage-clamp conditions, 5-HT evoked outward currents (I5-HT) with an increase in membrane cond uctance 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 out ward currents of I5-HT but the amplitude was diminished dramatically w ith successive applications. Pretreatment with the membrane-permeant C a2+ chelator 2-bis-(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA-AM) also diminished the I5-HT amplitud e. 4. Pretreatment with pertussis toxin (PTX) had no effect on I5-HT5. 5. The I5-HT was not cross-desensitized with the caffeine-induced out ward current but with outward current mediated by the muscarinic acety lcholine receptor. Pretreatment with Li+ significantly enhanced the I5 -HT, indicating that I5-HT is involved in the elevation of intracellul ar free Ca2+ released from inositol triphosphate (IP3)-sensitive Ca2store sites but not from the caffeine-sensitive ones. 6. The calmoduli n (CaM) antagonists, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-n aphthalenesulfonamide (W-7), inhibited I5-HT in a concentration-depend ent manner. 7. The Ca2+/CaM-dependent protein kinase II inhibitor 1-[N ,O-Bis nesulfonyl)-N-methyl-L-tyrosil]-4-phenylpiperazine depressed th e I5-HT. 8. Blockade of protein kinase C (PKC) by 1-(5-isoquinolinesul fonyl)-2-methylpiperazine (H-7) had no effect on I5-HT. However, the e nhancement 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-p rotein and may stimulate phosphatidylinositol breakdown by phospholipa se C. Consequently, with the increase in metabolites, IP3 may release Ca2+ from caffeine-insensitive store sites. Secondly, the increased in tracellular free Ca2+ may activate the Ca2+/CaM-dependent protein kina se II, which phosphorylates some proteins, and the K+ channels may ope n.