1. Allosteric modulators of the δ GABAA receptor subtype demonstrate a therapeutic effect in morphine-antinociceptive tolerance and withdrawal in mice.
- Author
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Genaro, K, Doan, B, Johnstone, T, Gee, Kelvin, Yoshimura, Ryan, and Hogenkamp, Derk
- Subjects
Morphine-antinociceptive tolerance ,Opioid ,Opioid withdrawal symptoms ,Positive allosteric modulators ,δ-subunit GABA(A) receptor ,Analgesics ,Analgesics ,Opioid ,Animals ,Dose-Response Relationship ,Drug ,Female ,Humans ,Male ,Mice ,Mice ,Knockout ,Morphine ,Receptors ,GABA-A ,Receptors ,Opioid ,delta ,Water ,gamma-Aminobutyric Acid - Abstract
The present study evaluated the effects of compounds targeting extrasynaptic δ subunit-containing γ-aminobutyric acid type A receptors (δ*-GABAARs) to interrogate the role of tonic inhibition in the development of antinociceptive tolerance caused by repeated morphine administration. We investigated the effect of subchronic or acute treatment with non-steroidal positive allosteric modulators (PAMs) of δ*-GABAARs, such as 2-261, on the morphine-antinociceptive tolerance. Mice were treated twice daily with morphine for 9 days and antinociception was measured using the hot water tail immersion test. Co-treatment with 2-261 and morphine prevented morphine-antinociceptive tolerance and acute administration of 2-261 on day 9 was sufficient to reverse the tolerance. Other compounds with activity at δ*-GABAARs also reversed morphine tolerance, whereas an enaminone that lacked activity at δ*-GABAARs did not. Acute administration of 2-261 did not cause an additive or synergistic antinociceptive effect when combined with an acute submaximal dose of morphine. We then used Cre/LoxP recombination to generate GABAA δ-subunit knockout mice to corroborate the pharmacological results. Observations of male δ-knockout mice demonstrated that the δ*-GABAARs was necessary for 2-261 modulation of both analgesic tolerance and somatic withdrawal symptoms produced by subchronic morphine. While female mice still benefited from the positive effects of 2-261, the δ-subunit was not necessary for these effects, highlighting a distinction of the different pathways that could have implications for some of the sex-related differences seen in human opioid-induced outcomes. Consequently, subtype-specific allosteric modulators of GABAARs may warrant further investigation as pharmacological targets to manage tolerance and withdrawal from opioids.
- Published
- 2022