GABA(B) RECEPTORS NEGATIVELY REGULATE TRANSCRIPTION IN CEREBELLAR GRANULAR NEURONS THROUGH CYCLIC-AMP RESPONSIVE ELEMENT-BINDING PROTEIN-DEPENDENT MECHANISMS

GABA(B) receptors affect short-term signalling in various cell types. However, nothing is known about possible long-term effects on transcri ption. To analyse such effects in the CNS, we studied GABA(B) receptor -mediated gene regulation in primary cultures of cerebellar granule ne urons. Transcription was followed using a chloramphenicol acetyl trans ferase reporter gene driven by the minimal cyclic AMP-responsive eleme nt (TGACGTCA). Transcription was stimulated by activation of both the cyclic AMP (forskolin: 5 x 10(-6) M) and the Ca2+ dependent (KCl: 30 m M) pathways (-)-Baclofen (10(-6) M to 10(-4) M), a specific GABA(B) re ceptor agonist, reduced by 50-70% the transcriptional stimulation evok ed by both forskolin and KCl, whereas isoguvacine, a GABA(A) receptor agonist, was without effect. Moreover, the GABA(B) antagonist CGP 3534 8 abrogated the inhibitory effects of both GABA and baclofen, indicati ng that GABA(B) receptors were specifically implicated in this respons e. Measurements of cyclic AMP levels suggested that (-) baclofen inhib its forskolin-initiated transcription by reducing cyclic AMP productio n. Direct transcriptional activation, via the cyclic AMP pathway, by o verexpression of the catalytic subunit of the cyclic AMP-dependent pro tein kinase, was not significantly altered by (-) baclofen. This indic ates again that (-) baclofen-dependent inhibitory mechanisms operate u pstream of cyclic AMP-dependent protein kinase at the level of second messenger formation. Further, we used a yeast transcriptional activato r GAL4-cyclic AMP-responsive element binding protein to analyse whethe r GABA(B) receptor-mediated inhibition of cyclic AMP-responsive elemen t transcription implicated the transacting factor cyclic AMP-responsiv e element binding protein. We show that the negative effects of (-) ba clofen implicate this transcription factor and this holds good for bot h the forskolin and KCl-stimulated pathways. The results indicate that GABA(B) receptors negatively regulate cyclic AMP-responsive element b inding protein-mediated transcription in the CNS.