Evaluation of peripheral versus central effects of GABA(B) receptor activation using a novel, positive allosteric modulator of the GABA(B) receptor ADX71943, a pharmacological tool compound with a fully peripheral activity profile

Abstract

Background and purpose: The GABA(B) receptor agonist, baclofen, has shown promising effects in patients suffering from pain, post-traumatic stress disorder, alcoholism, overactive bladder and gastroesophageal reflux disease. However, baclofen's short duration of action and side effects limit its wider use. Here we characterized a novel, GABA(B) receptor positive allosteric modulator (PAM) ADX71943.

Experimental approach: In vitro, ADX71943 was assessed for pharmacological activity and selectivity using recombinant and native GABA(B) receptors. In vivo ADX71943 was assessed in the acetic acid-induced writhing (AAW) test in mice and formalin tests (FTs) in mice and rats. Marble burying (MB) and elevated plus maze (EPM) tests, rotarod, spontaneous locomotor activity (sLMA) and body temperature (BT) tests in mice and rats were used to investigate centrally-mediated effects.

Key results: In vitro, in the presence of GABA, ADX71943 increased the potency and efficacy of agonists and showed selectivity at the GABA(B) receptor. ADX71943 reduced pain-associated behaviours in AAW; an effect blocked by GABA(B) receptor antagonist CGP63360. ADX71943 reduced pain in the FT in mice and rats, but was inactive in the MB and EPM despite reaching high concentrations in plasma. ADX71943 had no effect on BT, rotarod and sLMA.

Conclusions and implications: ADX71943 showed consistent and target-related efficacy in tests of disorders that have a significant peripheral component (acute and chronic pain), while having no effect in those associated with centrally-mediated anxiety-like reactivity and side effects. Thus, ADX71943 is a useful pharmacological tool for delineation of peripherally- versus centrally-mediated effects of GABA(B) receptor activation.

Figure 1

chemical structure of ADX71943.

Figure 2

Concentration–response curve of ADX71943 on recombinant hGABA_B receptor in intracellular Ca²⁺ mobilization assay in the presence of an EC₂₀ of GABA (A). Concentration–response curve of ADX71943 on native GABA_B receptor expressed in rat (B, C) or in human (D) cortical membranes in [³⁵S]-GTPγS binding assay in the presence of an EC₅₀ of baclofen. Data are representative of at least three independent experiments performed in duplicate.

Figure 3

Effects of ADX71743 on the number of acetic acid-induced writhes (see text). In the dose–response experiment, (A) C57Bl6/J mice (n = 11–12 per group) were treated p.o. with PD, ADX71943 (0.3, 1, 3, 10 and 30 mg·kg⁻¹), 1% CMC or baclofen (3 mg·kg⁻¹). In the target engagement experiment, (B) C57Bl6/J mice (n = 11–14 per group) were treated p.o. with PD or ADX71943 (10 mg·kg⁻¹) as treatment 1 (Tmt 1) followed by p.o. treatment with CGP63360 (1 mg·kg⁻¹) or saline (Sal) as treatment 2 (Tmt 2; see text). Each point represents the observed mean (± SEM). **P < 0.01 compared with the PD/Sal-treated group.

Figure 4

Nociceptive behaviours in C57Bl6/J mice (A, B) and Sprague-Dawley rats (C, D) monitored during phase I (A, C) and phase II (B, D) of the formalin test (see text). Mice (n = 10–14 per group) were pretreated p.o. with PD, ADX71943 (0.3, 1, 3, 10, 30 and 100 mg·kg⁻¹), saline (Sal) or baclofen (6 mg·kg⁻¹). Rats (n = 10 per group) were pretreated p.o. with PD, ADX71943 (1, 5, 15 and 50 mg·kg⁻¹) or baclofen (3 mg·kg⁻¹). Each point represents the observed mean (± SEM). *P < 0.05, ***P < 0.001 compared with PD. ^###P < 0.001 compared with Sal.

Figure 5

Number of buried marbles in C57Bl6/J mice (n = 10 per group) following p.o. treatment with PD, ADX71943 (0.3, 1, 3, 10, 30 and 100 mg·kg⁻¹), saline (Sal) or chlordiazepoxide (CDZ; 30 mg·kg⁻¹). Each point represents the observed mean (± SEM). ^###P < 0.001 compared with Sal.