The GABAB receptor is a therapeutic target for CNS and neuropathic disorders; however, few preclinical studies have explored effects of chronic stimulation. This study evaluated acute and chronic baclofen treatments on GABAB-activated G-proteins and signaling protein phosphorylation as indicators of GABAB signaling capacity. Brain sections from rats acutely administered baclofen (5 mg/kg, i.p.) showed no significant differences from controls in GABAB-stimulated GTPγS binding in any brain region, but displayed significantly greater phosphorylation/activation of focal adhesion kinase (pFAKTyr397) in mesocorticolimbic regions (caudate putamen, cortex, hippocampus, thalamus) and elevated phosphorylated/activated glycogen synthase kinase 3-β (pGSK3βTyr216) in the prefrontal cortex, cerebral cortex, caudate putamen, nucleus accumbens, thalamus, septum, and globus pallidus. In rats administered chronic baclofen (5 mg/kg, t.i.d. for five days), GABAB-stimulated GTPγS binding was significantly diminished in the prefrontal cortex, septum, amygdala, and parabrachial nucleus compared to controls. This effect was specific to GABAB receptors: there was no effect of chronic baclofen treatment on adenosine A1-stimulated GTPγS binding in any region. Chronically-treated rats also exhibited increases in pFAKTyr397 and pGSK3βTyr216 compared to controls, and displayed wide-spread elevations in phosphorylated dopamine- and cAMP-regulated phosphoprotein-32 (pDARPP-32Thr34) compared to acutely-treated or control rats. We postulate that those neuroadaptive effects of GABAB stimulation mediated by G-proteins and their sequelae correlate with tolerance to several of baclofen's effects, whereas sustained signaling via kinase cascades points to cross-talk between GABAB receptors and alternative mechanisms that are resistant to desensitization. Both desensitized and sustained signaling pathways should be considered in the development of pharmacotherapies targeting the GABA system.

The GABAB receptor agonist baclofen is being explored as a potential treatment for cocaine addiction, as therapeutic activation of the GABAergic system is theorized to counterbalance over‐stimulation of the dopaminergic system in brain regions involved in reward. Rats acutely administered i.p. baclofen (5 mg/kg, single dose 15 min. prior to sacking) showed no immediate changes in GABAB‐stimulated GTPγS binding in any brain region but displayed significantly greater phosphorylation of focal adhesion kinase (FAK) in key mesocorticolimbic regions (caudate putamen, cortex, hippocampus, thalamus, and prefrontal cortex) compared to controls, as well as elevated levels of glycogen synthase kinase 3‐beta (GSK3β) phosphorylation in these and other regions (nucleus accumbens, septum, and globus pallidus). In contrast, GABAB‐stimulated GTPγS binding in rats administered chronic (5 mg/kg, t.i.d. for five days) baclofen was significantly lower in the prefrontal cortex, septum, amygdala, and parabrachial nucleus than in the controls. While chronically‐treated rats exhibited similar increases in FAK phosphorylation, they displayed attenuated levels of GSK3β phosphorylation and wide‐spread elevations in levels of DARPP32 phosphorylation compared to those treated acutely or with vehicle. These findings indicate a desensitization of GABAB‐mediated signal transduction in rats following chronic administration of baclofen. Grant Funding Source: Supported by the National Institute on Drug Abuse: R01‐DA03690 and P50‐DA006634.