GABA(B) receptor positive allosteric modulators with different efficacies affect neuroadaptation to and self-administration of alcohol and cocaine

Drugs of abuse induce widespread synaptic adaptations in the mesolimbic dopamine (DA) neurons. Such drug-induced neuroadaptations may constitute an initial cellular mechanism eventually leading to compulsive drug-seeking behavior. To evaluate the impact of GABA(B) receptors on addiction-related persistent neuroplasticity, we tested the ability of orthosteric agonist baclofen and two positive allosteric modulators (PAMs) of GABA(B) receptors to suppress neuroadaptations in the ventral tegmental area (VTA) and reward-related behaviors induced by ethanol and cocaine. A novel compound (S)-1-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-4-methyl-6,7,8,9-tetrahydro-[1,2,4]triazolo[4,3-a]quinazolin-5(4H)-one (ORM-27669) was found to be a GABA(B) PAM of low efficacy as agonist, whereas the reference compound (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF) had a different allosteric profile being a more potent PAM in the calcium-based assay and an agonist, coupled with potent PAM activity, in the [S-35] GTP gamma S binding assay in rat and human recombinant receptors. Using autoradiography, the high-efficacy rac-BHFF and the low-efficacy ORM-27669 potentiated the effects of baclofen on [S-35] GTP gamma S binding with identical brain regional distribution. Treatment of mice with baclofen, rac-BHFF, or ORM-27669 failed to induce glutamate receptor neuroplasticity in the VTA DA neurons. Pretreatment with rac-BHFF at non-sedative doses effectively reversed both ethanol- and cocaine-induced plasticity and attenuated cocaine i.v. self-administration and ethanol drinking. Pretreatment with ORM-27669 only reversed ethanol-induced neuroplasticity and attenuated ethanol drinking but had no effects on cocaine-induced neuroplasticity or self-administration. These findings encourage further investigation of GABA(B) receptor PAMs with different efficacies in addiction models to develop novel treatment strategies for drug addiction.Peer reviewe

GABAB receptor-mediated G-protein activation in brain sections : Effects of baclofen and a positive allosteric modulator

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system. It acts upon two classes of GABA receptors: ionotropic and metabotropic. The GABAB receptor is a metabotropic G protein-coupled receptor (GPCR) that associates with the Gαi/o family of G-proteins causing inhibitory actions via the inhibition of adenylyl cyclase. Baclofen, a GABAB receptor agonist, has recently been found to be a potential treatment of alcohol addiction by suppressing the drug craving stage of the addiction cycle. However, the adverse side effects of baclofen including sedation and hypothermia, as well as its narrow therapeutic index, have limited its clinical use in the treatment. Positive allosteric modulators may be the answer to providing more selective and personalised treatments. These modulators function by enhancing or attenuating the response activated by the agonists, while having little inherent intrinsic activity. Therefore, they are less likely to cause adverse effects while also reducing the dosage of the agonistic drugs, essentially diminishing the adverse effects of the agonist. Mu-opioid receptor antagonists have been available for the treatment of alcohol and drug addiction for over 20 years. Preclinical studies have found that a continuous infusion of naltrexone has led to the increased consumption of ethanol after the drug has been metabolised out of the system. Furthermore, nalmefene, a specific and potent opioid antagonist has been developed and shown to have therapeutic advantages over naltrexone. Our study wanted to compare sex-specific brain-wide distribution of G-protein coupling after activation of GABAB receptors by baclofen and the positive allosteric modulator RacBHFF in ten RccHan®:WIST albino male and female rats, and compare species differences between the rats and five wildtype C57BL/6J male mice. Furthermore, we wanted to determine whether G-protein coupling of GABAB receptors is altered in sixty female AA (Alko, Alcohol) alcohol-preferring rats with and without various modes of treatments with naltrexone and nalmefene (7 daily injections or 7-day continuous infusion using osmotic minipumps at equipotent low and moderate doses). We did this using [35S]GTPγS autoradiography as means to measure the efficacy and potency of different concentrations of RacBHFF combined with 30 µM baclofen. Our results showed that RacBHFF does have positive allosteric activity on baclofen-stimulated GABAB receptor-mediated G-protein activation and the extent of it was regionally dependent. RacBHFF had greatest efficacy in the cerebellum, cortex and thalamus in all the samples studied. Furthermore, 30 µM baclofen alone and 10 µM RacBHFF alone are not enough to produce a statistically significant enhancement in GABAB activity in all brain regions, but RacBHFF combined with baclofen does. High concentrations of RacBHFF (100 and 300 µM) led to the complete inactivation of the receptors with G-protein coupling levels falling below non-specific binding levels, thus indicating tolerance. This was noticed in both of the sexes, as well as both of the species, indicating high system stability. Furthermore, there were no statistically significant differences between the species, and only minor regional statistically significant differences between the sexes. Comparing the regional baclofen-stimulated GABAB receptor-mediated G-protein activation between the two rat models showed that there are slight statistically significant differences, most of which are present in the nucleus accumbens, a part of the mesolimbic dopamine reward pathway. In addition, continuous 7-day infusions of a moderate dose (4 mg/kg) of nalmefene produced the greatest statistically significant enhancement of receptor stimulation by RacBHFF and baclofen when compared to the saline-treated samples. Seven daily injections of the same dose of nalmefene also produced statistically significant RacBHFF and baclofen-stimulated GABAB receptor activation in the amygdala and medial habenular nucleus. Therefore, a moderate dose of nalmefene (either as a continuous infusion or repeated injections) seems to produce the greatest overall enhancement of RacBHFF-induced stimulation of GABAB receptors. In conclusion, our results reveal that depending on the concentration of RacBHFF and the location of the receptors, RacBHFF differs in its efficacy to GABAB receptors in both wildtype and treated animal models. Furthermore, our results provide the first preclinical evidence that in the presence of opioid receptor tolerance, the GABAB receptor remains rather unaltered although the signal transductions of opioid and GABAB receptors are identical. This suggests that a combinational therapy of the GABAB receptor PAMs with a mu-opioid antagonist may potentially enhance the anti-craving, anti-alcohol-consuming habits of alcohol-addicted individuals