Fluid swivel

Animals received a ketamine or saline infusion one hour following the CHIMERA procedure. As previously described [47 (link)], racemic (±) ketamine hydrochloride (100 mg/mL) (Covetrus, Dublin, OH, USA) was diluted in 0.9% sterile saline before the infusion. The ketamine infusion (0, 10, or 20 mg/kg, IV) took place over two hours. Animals were given a loading dose of IV bolus ketamine or saline before being put into the infusion chambers (Med Associates Inc., St. Albans, VT, USA). Each chamber (14” L × 12” W × 15” H) was equipped with an infusion pump (Harvard Pump 11 Elite, Holliston, MA, USA) that used a 5 mL plastic syringe connected to a fluid swivel (Instech, Plymouth Meeting, MA, USA) by polyurethane tubing. This tubing was encased in a metal spring-wire tether that was magnetically attached to the metal cannula on the exit port of the catheter between the rat scapulae. The animals were allowed free movement in the chambers during the infusion due to the tether. Chambers contained two infrared photobeams to measure the spontaneous horizontal activity of animals throughout the infusion for locomotor activity levels, whereby a computer recorded the number of photobeam breaks.

The heroin self-administration training paradigm and conditions were adapted from our previous studies (Ye et al., 2017 (link)). Operant chambers (AniLab Software and Instruments, Ningbo, China) were equipped with two nosepoke operandi (AniLab Software and Instruments, Ningbo, China) located 5 cm above the floor and with light stimuli. Rats were trained to self-administer heroin intravenously (0.05 mg/kg/infusion) during three 1-h daily training sessions separated by 5 min for 10 days. We used a fixed-ratio 1 (FR1) schedule of reinforcement with a 40-s time-out employed after each infusion. Briefly, rats were connected to a drug line consisting of a metal tether covering a polyethylene tubing which, through a fluid swivel (Instech, Plymouth Meeting, PA), was connected to a syringe pump loaded with a 10 ml syringe. The session began with the illumination of a house light that remained on for the entire session. Nosepokes into the active operandum led to a delivery of intravenous heroin accompanied by presentation of a 5-s tone-light cue. Nosepokes into the inactive operandum were counted but had no consequences. The heroin self-administration procedure was used in all four experiments. We excluded a total of eight rats from the experiments: three rats due to catheter patency failure and five rats due to failure to acquire heroin self-administration.

Racemic (±) ketamine hydrochloride (100 mg/mL) (Mylan Institutional LLC, Rockford, IL) was diluted in 0.9% sterile saline and was administered IV in operant conditioning chambers (Med Associates Inc., St. Albans, VT). Each chamber (14ʺ L × 12ʺ W × 15ʺ H) was equipped with an infusion pump (Harvard Pump 11 Elite, Holliston, MA) using a 5 mL Hamilton glass syringe connected to a fluid swivel (Instech, Plymouth Meeting, PA) by polyurethane tubing encased in a metal spring-wire tether to prevent chewing by the animals. The spring-wire tether was attached to the vascular access button on the rat using a luer-lock connection. All tethered animals had free mobility in the chambers during the infusion period. Each infusion chamber was equipped with two infrared photo-beams that monitored spontaneous horizontal activity of animals during the infusion period. The number of beam breaks was recorded by the computer during the infusion period.