Chronic baclofen desensitizes GABA(B)-mediated G-protein activation and stimulates phosphorylation of kinases in mesocorticolimbic rat brain.
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 (pFAK(Tyr397)) in mesocorticolimbic regions (caudate putamen, cortex, hippocampus, thalamus) and elevated phosphorylated/activated glycogen synthase kinase 3-β (pGSK3β(Tyr216)) 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 pFAK(Tyr397) and pGSK3β(Tyr216) compared to controls, and displayed wide-spread elevations in phosphorylated dopamine- and cAMP-regulated phosphoprotein-32 (pDARPP-32(Thr34)) 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.
Center for the Neurobiology of Addiction Treatment, Winston-Salem, NC 27157, USA; Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA.
Bradley M T
BM
Keegan
Center for the Neurobiology of Addiction Treatment, Winston-Salem, NC 27157, USA; Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA.
Thomas J R
TJ
Beveridge
Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA; Department of Chemistry, Wake Forest University, Winston-Salem, NC 27157, USA.
Jeffrey J
JJ
Pezor
Center for the Neurobiology of Addiction Treatment, Winston-Salem, NC 27157, USA; Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA.
Ruoyu
R
Xiao
Center for the Neurobiology of Addiction Treatment, Winston-Salem, NC 27157, USA; Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA.
Tammy
T
Sexton
Center for the Neurobiology of Addiction Treatment, Winston-Salem, NC 27157, USA; Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA.
Steven R
SR
Childers
Center for the Neurobiology of Addiction Treatment, Winston-Salem, NC 27157, USA; Department of Physiology and Pharmacology, Wake Forest School of Medicine, Winston-Salem, NC 27157, USA. Electronic address: ahowlett@wakehealth.edu.
Allyn C
AC
Howlett
D000818:Animals; D001345:Autoradiography; D001418:Baclofen / Q000494:pharmacology*; D001921:Brain / Q000187:drug effects* / Q000201:enzymology*; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32 / Q000378:metabolism; D051417:Focal Adhesion Kinase 1 / Q000378:metabolism; D058786:GABA-B Receptor Agonists / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D008297:Male; D010766:Phosphorylation / Q000187:drug effects; D011897:Random Allocation; D017207:Rats, Sprague-Dawley; D018080:Receptors, GABA-B / Q000378:metabolism*; D013997:Time Factors; D014443:Tyrosine / Q000378:metabolism
D051937:Dopamine and cAMP-Regulated Phosphoprotein 32; D058786:GABA-B Receptor Agonists; C497930:Ppp1r1b protein, rat; D018080:Receptors, GABA-B; D014443:Tyrosine; D051417:Focal Adhesion Kinase 1; C496052:Ptk2 protein, rat; D000071679:Glycogen Synthase Kinase 3 beta; D038362:Glycogen Synthase Kinase 3; D019204:GTP-Binding Proteins; D001418:Baclofen
Baclofen; Baclofen (PubChem CID: 2284); Dopamine and cAMP-regulated phosphoprotein-32 (DARPP-32)/protein phosphatase 1 regulatory subunit 1B (PP1RB); Focal adhesion kinase (FAK)/protein tyrosine kinase 2 (PTK2); GABA(B) receptor; Glycogen synthase kinase 3 (GSK3)