Chronic administration of buprenorphine in combination with samidorphan produces sustained effects in olfactory bulbectomised rats and Wistar-Kyoto rats

Background: The combination of buprenorphine, a partial mu-opioid receptor agonist and a functional kappa-opioid receptor antagonist, with samidorphan, a functional mu-opioid receptor antagonist, is being developed as an adjunct therapy for major depressive disorder, in order to harness the mood-enhancing effects of opioids without unwanted side-effects such as a risk of addiction. Acute and subacute administration of the combination of buprenorphine and samidorphan is effective in reducing forced swim immobility in the Wistar-Kyoto rat, but the chronic effects have not been examined. Aims and methods: The purpose of this study was to assess if chronic (14-day) administration of buprenorphine (0.1 mg/kg, subcutaneous) alone or in combination with samidorphan (0.3 mg/kg, subcutaneous) maintains antidepressant-like activity in the olfactory bulbectomised rat model and the Wistar-Kyoto rat, two models that exhibit ongoing behavioural deficits in tests commonly used to study effects of antidepressants. Results: Olfactory bulbectomised-induced hyperactivity was attenuated by chronic administration of buprenorphine alone and in combination with samidorphan, to that of sham control activity levels. Neither buprenorphine nor samidorphan altered stress-associated defecation in sham or olfactory bulbectomised rats in the open field. In Wistar-Kyoto rats, buprenorphine alone significantly reduced forced swim immobility and increased locomotor activity three hours post-final dosing. Buprenorphine plus samidorphan significantly reduced forced swim immobility without changing locomotor activity at this time point. Buprenorphine alone also significantly reduced forced swim immobility 24 h post-final dosing. Conclusion: Chronic treatment of buprenorphine alone or buprenorphine plus samidorphan is effective in reversing behavioural deficits in distinct non-clinical paradigms. These non-clinical results complement the antidepressant effect of this combination observed in clinical studies.