GABAB receptor-positive modulators: enhancement of GABAB receptor agonist effects in vivo

Abstract

In vivo effects of GABA(B) receptor-positive modulators suggest that they have therapeutic potential for treating central nervous system disorders such as anxiety, depression, and drug abuse. Although these effects generally are thought to be mediated by positive modulation of GABA(B) receptors, such modulation has been examined primarily in vitro. The present study was aimed at further examining the in vivo positive modulatory properties of the GABA(B) receptor-positive modulators, 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl) phenol (CGP7930) and (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF). Both compounds enhanced loss of righting induced by baclofen in mice. However, CGP7930 was less effective and rac-BHFF was less potent for enhancing loss of righting induced by γ-hydroxybutyrate (GHB), which, like baclofen, has GABA(B) receptor agonist properties. In contrast with baclofen- and GHB-induced loss of righting, the hypothermic effects of baclofen and GHB were not enhanced by rac-BHFF but were enhanced by CGP7930 only at doses that produced hypothermia when given alone. CGP7930-induced hypothermia was not attenuated by the GABA(B) receptor antagonist 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348), at doses that blocked baclofen-induced hypothermia, and was not increased by the nitric-oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester, at doses that increased the hypothermic effects of baclofen and GHB. The results provide evidence that CGP7930 and rac-BHFF act in vivo as positive modulators at GABA(B) receptors mediating loss of righting, but not at GABA(B) receptors mediating hypothermia. Conceivably, CGP7930, but not rac-BHFF, acts as an allosteric agonist at these latter receptors. Taken together, the results provide further evidence of pharmacologically distinct GABA(B) receptor subtypes, possibly allowing for a more selective therapeutic interference with the GABA(B) system.

Fig. 1.

Effects of the GABA_B receptor agonist baclofen, GHB, and the GABA_B receptor-positive modulators CGP7930 and rac-BHFF on righting in C57BL/6J mice (n = 8 per dose). Left, symbols represent the percentage of animals showing loss of righting at different times after intraperitoneal administration of baclofen or GHB. Right, symbols represent mean ± S.E.M righting scores, totaled for each animal across the six different times after injection. Open symbols in the upper right represent data obtained when baclofen and GHB were given together with 320 mg/kg of the GABA_B receptor antagonist CGP35348.

Fig. 2.

Top and middle, effects of the GABA_B receptor-positive modulators CGP7930 (top) and rac-BHFF (middle), administered intraperitoneally, on loss of righting produced by baclofen (top and middle left) and GHB (top and middle right). Symbols represent mean ± S.E.M righting scores, totaled for each animal (n = 8 mice per dose) across the six different times after injection. Data obtained in the presence of 320 mg/kg CGP35348 are indicated by ▴. Bottom, Schild-like plot of dose ratios for baclofen and GHB, calculated from the ED₅₀ values of the dose-response curves shown in the top and middle, as a function of the dose of the positive modulator.

Fig. 3.

Effects of baclofen, GHB, CGP7930, and rac-BHFF on body temperature in C57BL/6J mice. Symbols represent mean ± S.E.M. (n = 4–6 mice per dose). Left and center, gray-filled symbols represent data that are significantly different from vehicle control (Dunnett's test, P < 0.05). Right, AUC values shown were calculated for each dose from the time response data shown (left and center). Open symbols in the top right represent data obtained when baclofen and GHB were given together with 320 mg/kg of the GABA_B receptor antagonist CGP35348.

Fig. 4.

Effects of CGP7930 (top) and rac-BHFF (bottom) on hypothermia induced by baclofen (left) and GHB (right). Symbols represent mean ± S.E.M. AUC values (n = 4–6 mice per dose).

Fig. 5.

Effects of CGP35348 (top) and l-NAME (bottom) on hypothermia induced by CGP7930 (left), baclofen (center), and GHB (right). Symbols represent mean ± S.E.M. AUC values (n = 4–6 mice per dose).