Chronic baclofen desensitizes GABAB-mediated G-protein activation and stimulates phosphorylation of kinases in mesocorticolimbic rat brain

Abstract

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.

Paradoxical effect of baclofen on social behavior in the fragile X syndrome mouse model

“…The precise mechanism of tolerance induction is not known, as only few studies have focused on chronic systemic baclofen treatment and results in the literature are conflicting. One proposed mechanism is downregulation of the GABA B receptor-binding sites or phosphorylation of downstream signaling regulators (Keegan et al, 2015;Lehmann et al, 2003;Pacey, Tharmalingam, & Hampson, 2011). In addition, evidence has been published on cross talk between GABA B receptor and mGluR5 receptor after chronic treatment (Pacey et al, 2011).…”

A Review of the Potential Mechanisms of Action of Baclofen in Alcohol Use Disorder

“…Chronic baclofen produces many changes in the brain that could impact connectivity. We have previously mentioned that chronic baclofen produces plastic changes in regions of the reward system, including desensitization in G-protein-dependent systems and alterations in signaling of several kinase cascades (FAK, GSK3ß, DARPP-32) that are resistant to desensitization ( 37 ). In addition, AUD is associated with marked brain neuro-immune alterations ( 110 ); and studies have shown that baclofen has anti-inflammatory and neuroprotective effects on the brain.…”

“…Preclinical studies have highlighted such plastic effects of baclofen or other GABA-B agonists on brain systems after chronic treatment. Keegan et al have shown that rats treated chronically with baclofen have significant decreases in G-protein-dependent signal transduction (measured by GTP-gamma-S binding) in the frontal cortex, septum, amygdala, and parabrachial nucleus ( 37 ). Such decreases demonstrate a general desensitization of G-protein-dependent systems.…”

Elucidation of a CGP7930 in vitro metabolite by liquid chromatography/electrospray ionization quadrupole time‐of‐flight tandem mass spectrometry

“…Because GABA regulates neuronal excitability and GABA receptors are widely distributed throughout the brain and the nervous system, drugs targeting GABA receptors can profoundly regulate behaviour . GABA B , one of the major GABA receptors, belong to the G protein‐coupled receptor family and are generally considered to be excellent drug targets . The GABA B receptors play an important role in the treatment of depression and anxiety .…”

“…[1] GABA B , one of the major GABA receptors, belong to the G protein-coupled receptor family and are generally considered to be excellent drug targets. [2] The GABA B receptors play an important role in the treatment of depression and anxiety. [3] However, conventional drug development targeting GABA B receptors mainly focuses on agonists and antagonists which have the limitations of side effects, poor pharmacokinetics, and the development of tolerance.…”