Forskolin modulation of desensitization at GABA(A) and glycine receptors is not mediated by cAMP-dependent protein kinase in isolated carp amacrine-like cells

The effects of forskolin on gamma -aminobutyric acid type-A (GABA(A)) and g lycine receptors in amacrine-like cells of carp (Carassius auratus) retina were studied using patch-clamp techniques. Application of 50 etaM forskolin markedly accelerated the desensitization of whole-cell responses induced b y 100 muM GABA or glycine without changing the peak amplitude of the respon se. Both 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) and 3-isob utyl-1-methylxanthine (IBMX) (500 CIM) failed to accelerate the desensitiza tion of these two receptors. Protein kinase A (PKA) inhibitors, N-{2- [(rho -bromocinnamyl)amino]ethyl}-5-isoquinolinesulfonamide dihydrochloride (H-8 9) and N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride ( H-8), could not block these effects of forskolin. An inactive analogue of f orskolin, 1,9-dideoxyforskolin (DFSK), accelerated the desensitization effe ctively. These results suggest that forskolin's effects are not mediated by activation of the PKA pathway. Moreover, similar results were obtained usi ng excised outside-out patches of these cells, suggesting that forskolin ma y act on an extracellular site(s). The neurosteroids 5 alpha -pregnane-3 al pha ,21-diol-20-one (THDOC) and 5-pregnen-3 beta -ol-20-one sulfate sodium (PS), structural analogues of forskolin, accelerated the desensitization of these receptors without changing the peak amplitudes, thus mimicking forsk olin's effects. Furthermore, PS interacted with forskolin on these receptor s so as to slow down the responses. These results raise the possibility tha t forskolin acts directly on an extracellular site(s) of the GABA, and glyc ine receptors, shared with neurosteroids, in carp amacrine-like cells.