Inhibition of Heat shock protein 90 in the Spinal Cord Enhances Opioid Anti-Nociception by Disabling an Inhibitory GABA Circuit

Abstract

One strategy to enhance the clinical treatment of chronic and acute pain is the development of dose-reductive compounds that, when co-administered with palliatives, can reduce the dose of opioid required for treatment while retaining analgesic efficacy and decreasing side-effects. We previously showed that intrathecal (IT) administration of the heat shock protein 90 (Hsp90) inhibitor 17-AAG potentiated the efficacy of morphine induced antinociception while reducing or not changing opioid side effects. Proteomic analysis of the translational alterations induced by IT 17-AAG administration revealed an upregulation of GABA transporter 2 (GAT-2). To elucidate the role of GAT-2 in mediating the enhancement of morphine antinociception, 17-AAG treated mice were administered a GAT inhibitor, Nipecotic Acid (NA). NA effectively blocked the 17-AAG enhancement of antinociception compared to controls, while not changing responses in controls, suggesting that GAT-2 mediates 17-AAG's antinociceptive enhancement. We hypothesized that an inhibitory GABA circuit is present in the spinal cord which reduces opioid antinociception and is disabled by Hsp90. To test this, mice were treated with IT GABA-A or GABA-B agonists prior to systemic morphine administration. The GABA-A agonist reversed 17-AAG enhanced antinociception much like NA. Intriguingly, the GABA-B agonist reversed 17-AAG antinociception in males only, suggesting a GABAergic sex-specific difference. To further test the inhibitory GABA circuit hypothesis, naive mice (no 17-AAG) were injected with IT GABA-A or GABA-B antagonists prior to morphine and tail flick. Males showed enhanced morphine antinociception with either antagonist. However, neither drug had an impact on female mice, again suggesting mechanistic sex differences. These results taken together suggest that IT 17-AAG potentiates morphine-induced antinociception via upregulation of GAT-2, relieving an inhibitory GABAergic brake on opioid antinociception. This study provides mechanistic insight into how Hsp90 inhibitors can be used to boost opioid antinociception and may have identified a novel inhibitory circuit of opioid antinociception. This study was funded by an Arizona Biomedical Research Commission New Investigator Award #ADHS18-198875 and institutional funds from the University of Arizona.