2.5. Self-Administration Model

Addiction/dependence is a critical side effect of opioids and must be evaluated with any new member of this family. The most reliable test of abuse liability with translational relevance is a contingent drug self-administration model, in which rats are trained to press a lever for intravenous drug delivery (van Ree et al., 1978). Sixteen standard self-administration chambers (30×20×20 cm, Med Associates) were housed inside sound-attenuating cubicles fitted with a fan for airflow and to mask noise. Each chamber contained two retractable levers, two stimulus lights, a speaker for tone delivery, and a house light to provide general illumination. In addition, each chamber was equipped with a balanced metal arm and spring leash attached to a swivel (Instech). Tygon® tubing extended through the leash and was connected to a 10 ml syringe mounted on an infusion pump located outside the sound-attenuating cubicle. Rats were anesthetized with i.p. injections of ketamine (66 mg/kg; VedcoInc, St Joseph, MO, USA), xylazine (1.3 mg/kg; Lloyd Laboratories, Shenandoah, IA, USA), and equithesin (0.5 ml/kg); sodium pentobarbital (4 mg/kg), chloral hydrate (17 mg/kg), and 21.3 mg/kg magnesium sulfate heptahydrate dissolved in 44% propylene glycol, 10 % ethanol solution). Ketorolac (2.0 mg/kg, i.p. Sigma, St. Louis, MO, USA) was given just prior to surgery as an analgesic. One end of a silastic catheter was inserted 33 mm into the external right jugular and secured with 4.0 silk sutures. The other end ran subcutaneously and exited from a small incision just below the scapula. This end is attached to an infusion harness (Instech Solomon, Plymouth Meeting, PA, USA) that provides access to an external port for i.v. drug delivery. Following this surgical procedure, rats were given a subcutaneous injection of antibiotic solution of Cefazolin (10 mg/0.1 ml; Schein Pharmaceuticals, Florham Park, NJ, USA) and allowed to recover for 5 days.

During self-administration, rats received an i.v. infusion (0.1 ml) of 10 U/ml heparinized saline before each session. After each session, catheters were flushed with cefazolin and 0.1 ml 70 U/ml heparinized saline. Catheter patency was periodically verified with methohexital sodium (10 mg/ml dissolved in 0.9% physiological saline), a short-acting barbiturate that produces a rapid loss of muscle tone when administered i.v.. Daily 2-h sessions occurred in a fixed ratio schedule of reinforcement. The house light signaled the beginning of a session and remained on throughout the session. During the sessions, a response on the active lever resulted in activation of the pump for a 2-s infusion (50 μl bolus infusion) and presentation of a stimulus complex consisting of a 5-s tone (78 dB, 4.5 kHz) and a white stimulus light over the active lever, followed by a 20-s time out. Responses occurring during the time out and on the inactive lever were recorded, but had no scheduled consequences. Rats were trained initially to press a lever for a sucrose pellet (45mg) in a single 6 h session. The following day, a response on the lever no longer resulted in sucrose reward, but instead produced an i.v. infusion of JT09 (20 mg/kg/inf). After five days, JT09 was replaced with cocaine (50ug/50ul bolus infusion) and the lever replaced with a nose poke aperture. We chose cocaine to assess reward processing because it is readily administered to rats and is not considered an opioid compound. This way we can ensure that lever responding for cocaine was not impacted by prior experience with opioids (i.e., JT09).