Infusion pump

Testing was conducted in 16 standard operant response chambers (32 x 25 x 34 cm; Med Associates, St Albans, VT, USA), which were housed in sound-attenuating boxes (41 x 56 x 56 cm) equipped with a fan for masking noise and to provide ventilation. Each chamber was equipped with two retractable levers (1 active, 1 inactive) and a house light. The chambers also contained a metal arm with an adjustable weight and a spring connector, which were attached to a swivel. Polyethylene tubing threaded through the spring connector was connected to a 10 ml syringe attached to an infusion pump (Med Associates) located outside of the sound-attenuating chamber. The tubing exiting from the base of the spring connector was connected to the back mount of the intravenous catheter.
Four infrared photobeam detectors were also positioned on the sidewall of each operant chamber to measure locomotor activity. Active and inactive lever presses, number of infusions and locomotor activity was collected and recorded using MED-PC software.

Operant conditioning for oral MA reinforcement was conducted in cohorts of 12 mice of the same sex during the dark phase of the circadian cycle. On each self-administration day, mice were relocated in their home cages to a non-colony procedural room, approximately 30 min before testing. Mice were tested in 59.69 cm × 40.64 cm operant chambers (Med Associates, St. Albans, VT, USA), housed in a sound-attenuated cabinet and equipped with two nose-poke holes (one inactive and one active/MA-reinforced) and a liquid receptacle attached to an infusion pump (Med Associates, St. Albans, VT, USA), which delivered 20 µL of the assigned MA solution (0.8, 1.6, or 3.2 g/L) into the receptacle (respectively, MA-0.8, MA-1.6, and MA-3.2). MA-naïve controls responded to potable water only (MA-0). Mice were trained to self-administer water/MA for 7 consecutive days under a fixed ratio (FR) 1 schedule of reinforcement. As in our prior study [17 (link)], each reinforcer delivery was signaled by a compound light/tone cue and 20 s time-out period. At the end of each self-administration session, mice were returned to their home cages and relocated to the colony room. At twenty-four hours following the 7th self-administration session, mice underwent a behavioral test battery to assay the effects of oral MA on the indices of negative affect and sensorimotor processing.

The experiments were conducted in operant chamber boxes (25 × 27 × 30 cm). Each box has two levers located 9 cm above the floor, but only one lever (an “active”, retractable lever) activates the infusion pump (Med Associates Inc., St. Albans, VT, USA). METH-HCL was dissolved in sterile saline at a concentration 0.1 mg/kg/infusion and loaded into syringes. Each syringe was mounted above infusion pumps and connected via a rotating “swivel” to a back mounted cannula, which finally led to the jugular catheter of the rat. Swivel apparatuses are sufficiently long enough to allow rats to move freely in the behavioral chamber. Data collection and programming were conducted using PC computers with a Med-PC interface (Med Associates, Inc., St. Albans, VT, USA).

Behavioural testing was conducted in 16 standard operant response chambers (32 × 25 × 34 cm: Med Associates, St Albans, VT, USA). Operant response chambers were housed in sound-attenuating boxes (41 × 56 × 56 cm) equipped with a fan for ventilation. Each chamber was equipped with two retractable levers, two cue lights and a house light. Chambers also contained a metal arm and a spring connector which was attached to the animal’s catheter. Polyethylene tubing was attached to a 10-mL syringe, driven by an infusion pump (Med Associates). Four infrared detectors were used to measure locomotor activity during test sessions. Active and inactive lever presses, number of infusions and locomotor activity were recorded using MED-PC software (Med Associates).