Deltorphin 2

The chemicals used in this study and their sources were as follows: morphine hydrochloride – Polfa (Kutno, Poland); DAMGO (Sigma-Aldrich, USA), Deltorphin II (Sigma-Aldrich, USA), DPDPE (Sigma-Aldrich, USA), U50,488H (Tocris, UK), SNC80 (Sigma-Aldrich, USA) and minocycline hydrochloride (Sigma-Aldrich, USA). Minocycline (MC; 30 mg/kg; i.p.) was dissolved in sterile water and pre-emptively administered intraperitoneally 16 h and 1 h before CCI and then twice daily for 7 days. This method of minocycline administration was used throughout the work and is referred to in the text as “repeated administration”. The control groups received a vehicle (water for injection) according to the same schedule. One hour after the last morning of minocycline or vehicle administration on day 7 after CCI, morphine (M; 20; 40 µg; 62.3 nM; 124.6 nM), DAMGO (MOR selective ligand; 1; 2 µg; 1.94 nM; 3.9 nM), U50,488H (KOR selective ligand; 25; 50 µg; 61.6 nM; 123.2 nM), DPDPE (DOR I selective ligand; 10; 20 µg; 15.48 nM; 30.96 nM), Deltorphin II (del II; DOR II selective ligand; 1.5; 15 µg; 1.9 nM; 19 nM), SNC80 (10; 20 µg; 22.24 nM; 44.48 nM) or a vehicle was intrathecally injected. The von Frey test (25 and 45 min later) and cold plate tests (30 and 50 min later) were carried out after vehicle or opioid agonist administration (Figure 1).

Deltorphin-II and naltriben methanesulfonate hydrate were obtained from Sigma-Aldrich. Deltorphin-II was solubilised in normal saline and naltriben methanesulfonate hydrate was solubilised in 10% DMSO in normal saline solution. Wright’s stain was obtained from Himedia Laboratories (Mumbai, India). Isopropyl alcohol, disodium hydrogen phosphate and potassium dihydrogen phosphate were obtained from Merck (India).