MEDIATION BY PROTEIN-KINASE-C AND PROTEIN-KINASE-A OF G(0)-LINKED SLOW RESPONSES OF ENTERIC NEURONS TO 5-HT

5-HT activates the peristaltic reflex and is the neurotransmitter of a subset of myenteric interneurons. Hyperpolarizing afterpotential (AH) /type 2 neurons respond to 5-HT with a long-lived depolarization that is caused by the inhibition of a Ca2+-activated K+ conductance (gK(Ca) ). This effect is mediated by a G-protein-coupled receptor, 5-HT1P. 5- HT1P agonists specifically activate G alpha(o), the immunoreactivity o f which was found to be highly abundant and membrane-associated in alm ost all enteric neurons. Responses of hyperpolarizing AH/type 2 neuron s to 5-HT were inhibited by intracellular injection of GDP beta S or a nti-G alpha(o) Fab fragments but were potentiated and prolonged by int racellular GTP gamma S. Responses to 5-HT were antagonized by pertussi s toxin, downregulation of protein kinase C (PKC) and inhibitors of ph osphatidylcholine phospholipase C (PC-PLC), PKC (including pseudosubst rate peptides, chelerythrine, and the alpha/beta isoform-specific inhi bitor G(o) double over dot 6976), protein kinase A (PKA), and adenylat e cyclase. Responses to 5-HT were mimicked by activators of PKC, and 5 -HT induced a concentration-dependent increase in the membrane-associa ted PKC activity in isolated myenteric ganglia. Immunocytochemical stu dies suggested that the most abundant isoforms of PKC in enteric neuro ns are alpha and delta. These data suggest that signal transduction of the 5-HT1P-mediated slow response to 5-HT involves activation of PC-P LC by G alpha(o) to liberate diacylglycerol, which stimulates PKC (mos t likely alpha). PKC probably activates adenylate cyclase, which throu gh cAMP, activates PKA. Activation of both PKA and PKC lead to closure of gK(Ca).