βγ-mediated enhancement of corticotropin-releasing hormone-stimulated adenylyl cyclase activity by activation of γ-aminobutyric acid B receptors in membranes of rat frontal cortex

Abstract

A number of studies have shown that activation of γ-aminobutyric acid B (GABA B) receptors potentiates neurotransmitter-induced accumulation of cyclic AMP in brain slices, but the mechanisms involved in the facilitatory effect have not been fully elucidated. In the present study, we showed that in membranes of rat frontal cortex the GABA B receptor agonist (−)baclofen increased basal adenylyl cyclase activity and potentiated the maximal enzyme stimulation elicited by corticotropin-releasing hormone (CRH). The less active enantiomer (+)baclofen had no effect on cyclic AMP formation, whereas the natural agonist GABA mimicked the stimulatory action of (−)baclofen. In radioligand-binding experiments, the affinity and maximal binding capacity of 125I-Tyr-CRH was not affected by (−)baclofen. The GABA B receptor antagonist CGP 55845A competitively counteracted the (−)baclofen potentiation of CRH-stimulated adenylyl cyclase activity with a pA 2 value of 6.70. Moreover, both (−)baclofen and GABA, but not (+)baclofen, caused a concentration-dependent stimulation of [ 35S]GTPγS binding to membrane G-proteins. The intracerebral injection of pertussis toxin significantly reduced the facilitatory effects of (−)baclofen on both basal and CRH-stimulated adenylyl cyclase activities. Moreover, membrane incubation with the GDP-bound form of the α subunit of transducin, a scavenger of G protein βγ subunits, blocked the stimulatory effects of (−)baclofen. The data indicate that in rat frontal cortex activation of GABA B receptors potentiates the CRH stimulation of adenylyl cyclase activity through a mechanism involving the βγ subunits of the pertussis toxin-sensitive G protein G i/G o.