During prolonged application of GABA to cultured neurons from the chick embryo cerebrum, whole-cell voltage-clamp recordings show a decline in GABAgated chloride currents due to desensitization. At a holding potential of -60 mV, desensitization can normally be described as a single exponential process with a time constant (7) of 7-10 sec at a GABA concentration of 100 pM. After exposure to 50 pM forskolin, the peak amplitude of the GABA-induced currents declined and ii fast component of desensitization (T = 0.92 sec) appeared, whereas the slow component was essentially unchanged. This effect of forskolin was reversible after washing. Similar, although less robust effects were produced by incubation with 8-Br-cAMF', but not by 1,9-dideoxyforskolin. The desensitization process could also be measured by the GABAdependent uptake of 36Cl-into the cultured neurons. A 20-sec incubation with 10 pM GABA in physiological saline produced a 29% inhibition of subsequent GABA-gated 36Cluptake in the presence of 40 mM K' . This inhibition was increased to 64% by the addition of 100 pM forskolin to the preincubation medium. This effect of forskolin was prevented by 100 pM 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase. A similar acceleration of desensitization was produced by 0.5 mM 8-Br-CAMP or by 0.5 mM isobutylmethylxanthine, but these compounds themselves failed to produce desensitization in the absence of exogenous GABA. In the presence of GABA, CAMP analogs were effective in the order 8-(4-chlorophenylthio)-cAMP > 8-Br-CAMP > N6,02'-dibutyryl-cAMP. The results suggest that a CAMP-dependent phosphorylation converts the GABA receptor into a more rapidly desensitizing form.
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