PHOSPHORYLATION OF DARPP-32 IS REGULATED BY GABA IN RAT STRIATUM AND SUBSTANTIA-NIGRA

In the medium-sized spiny neurons of the striatonigral pathway, a casc ade of events involving the activation of dopamine D-1 receptors, an i ncrease in cyclic AMP, and activation of cyclic AMP-dependent protein kinase causes the phosphorylation of DARPP-32 on Thr(34), converting D ARPP-32 into a powerful inhibitor of protein phosphatase-1. In the pre sent study, the incubation of striatal or substantia nigra slices with GABA also increased the phosphorylation of DARPP-32 on Thr(34). GABA did not significantly increase cyclic AMP levels in slices. The phosph orylation of DARPP-32 by GABA was blocked in both brain regions by pre treatment of slices with the GABA(A) receptor antagonist, bicuculline, but not with the GABA(B) receptor antagonist, phaclofen. Moreover, th e threonine phosphorylation of DARPP-32 produced by maximally effectiv e doses of either forskolin (in striatum) or L-3,4-dihydroxyphenylalan ine (in substantia nigra) was increased further by GABA. The data are consistent with a model in which GABA increases the phosphorylation st ate of DARPP-32 by inhibiting dephosphorylation of the protein by the calcium/calmodulin-dependent protein phosphatase, calcineurin.