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Volume 16, Number 15, Issue of August 1, 1996 pp. 4799-4809
Copyright ©1996 Society for Neuroscience

Protein Kinase and G-Protein Regulation of Ca²⁺ Currents in Hermissenda Photoreceptors by 5-HT and GABA

Received Feb. 21, 1996; revised April 30, 1996; accepted May 3, 1996.

Ebenezer N. Yamoah and Terry Crow

Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, Texas 77225

The effects of serotonin (5-HT) and GABA on two Ca²⁺ currents, a transient low-voltage-activated current (tLVA) and a sustained high-voltage-activated current (sHVA) were examined in isolated photoreceptors of Hermissenda. The sHVA current was blocked by 5-HT and reduced by activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate. The effects of 5-HT were transiently reversed by staurosporine and partially blocked by the PKC inhibitor peptide [PKC(19-36)]. GABA enhanced both the tLVA and sHVA currents at low concentrations (5 nM to 5 µM) and reduced the sHVA current at high concentrations (>10 µM). The GABA-mediated enhancement of the Ca²⁺ current at low concentrations was sensitive to block by picrotoxin. The protein kinase A (PKA) inhibitor peptide [PKI(6-22)amide] blocked enhancement of both Ca²⁺ currents produced by cAMP analogs and GABA, suggesting that the effects at low concentrations may be PKA mediated. Caged GTP--S released by flash photolysis reduced the sHVA current, and pretreatment of the photoreceptors with pertussis toxin blocked the effects of higher concentrations of GABA, indicating that at higher concentrations, the effects may be G-protein mediated.

Key words: calcium current; -aminobutyric acid; neuromodulation; cellular plasticity; Hermissenda; serotonin

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