Naltrexone hydrochloride

All solutions were mixed using reverse osmosis purified drinking water. Ethyl alcohol (190 Proof; Pharmco-AAPER, Brookville, CT, USA) was diluted with reverse osmosis water to a concentration of 4% w/v alcohol. Baclofen and naltrexone hydrochloride (Sigma-Aldrich, St Louis, MO, USA) were dissolved in physiological saline and administered via the intramuscular route (2 ml volume/injection) as detailed below. Vehicle (saline) was administered using the same volume and route of injection for control conditions. Drug doses and vehicle were given in mixed order across baboons.

Drugs used in this study were: morphine hydrochloride (Merck & Co., Inc., Rahway, NJ); morphine-6-O-sulfate (sodium salt, prepared in our laboratory as described previously^{13 (link)}); the δ-OR selective antagonist naltrindole hydrochloride (NLD; Tocris Biosciences, Minneapolis, MN); the κ-OR selective antagonist norbinaltorphimine hydrochloride (nor-BNI; a generous gift from Dr. Philip Portoghese, University of Minnesota, Minneapolis, MN): and non-selective opioid antagonist naltrexone hydrochloride (NTX; Sigma Aldrich, St. Louis, MO). All drugs were prepared in physiological saline immediately prior to use. The volume of intraperitoneal (i.p.) injections for all animals was 2 mL/kg of rat body weight. The control group of animals received an equivalent volume of the vehicle only. Treatment of animals with OR antagonists was conducted as described previously. Drugs were injected i.p. 30 min (NTX 2 mg/kg) ^{14 (link),20 (link)} (NLD 1 mg/kg) ^{14 (link),21 (link)} or 18 hr (nor-BNI 5 mg/kg) ^{14 (link),22 (link)} prior the time of peak antinociceptive effect of M6S or morphine.

Dr. Yanan Zhang at Research Triangle Institute provided RO5263397 (purity > 98%). All doses of RO5263397 (3.2 and 5.6 mg/kg, i.p.) for rats were selected based on our previous studies (Liu et al., 2018b (link)). We chose 5.6 mg/kg as the highest dose of RO5263397 for rats in the present study since our previous study found that 10 mg/kg RO5266397 showed a slight inhibitory effect on rates of responding on a food-associated lever (Liu et al., 2018b (link)). Doses of RO5263397 for mice were based on previous study (Revel et al., 2013 (link)) and a locomotor activity test in the present study (Figure 1B). RO5263397 was dissolved in the vehicle, which contains 1 part absolute ethanol, 1 part Emulphor-620 (Rhodia), and 18 parts physiologic saline (Liu et al., 2018b (link)). Morphine sulfate was provided by Research Technology Branch, National Institute and Drug Abuse, National Institutes of Health (Rockville, MD, USA), and was dissolved in 0.9% saline. Naltrexone hydrochloride (0.1 and 1 mg/kg, i.p.) was purchased from Sigma-Aldrich and dissolved in saline. CFA was purchased from Difco (Detroit, MI, USA) and dissolved in paraffin oil (Siemian et al., 2018 (link)).

Ethyl alcohol 200 proof (Pharmco-AAPER, Brookfield, CT, USA) was purchased from the pharmacy at UAMS. All EtOH concentrations are expressed as percentage volume/volume (v/v) and were dissolved in tap water. Naltrexone hydrochloride and (R)(−)−2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Naltrexone and DOI doses are expressed as weight of the salt, were dissolved in 0.9% physiological saline (naltrexone) or sterile water (DOI), and were administered via subcutaneous injection (sc) in a 1 ml/kg injection volume.

Salt and enantiomeric forms of the compounds used in the present study were as follows: (−)-morphine sulfate pentahydrate (National Institute on Drug Abuse, Drug Supply Program, Rockville, MD, USA), (−)-naltrexone hydrochloride (Sigma-Aldrich Co., St. Louis, MO, USA), and corynoxine (1) hydrochloride (see General Experimental Procedures). Doses of the compounds used in the present study are expressed as the weight of the salt or free base form listed above. All the compounds were dissolved in a vehicle consisting of sterile water (HyClone water for injection, 1 L, Fisher Scientific, Waltham, MA, USA) containing 5% Tween 80 (polyoxyethylenesorbitanmonooleate, Fisher Scientific) and 5% propylene glycol (Sigma-Aldrich Co.). All the compounds and the vehicle were administered i.v. Each solution was filtered with a 0.2 μm pore size syringe filter (Millex-LG, 0.20 μm, SLLG025SS, Sigma-Aldrich Co.) prior to injections. The injection volumes were 1.0 mL/kg of subject body weight. The dose ranges and pretreatment times were referenced to our preliminary data and the literature.^{26 (link)}