HAL (0.25 mg/kg/day; Sandoz Canada Inc., QC, Canada) was administered subcutaneously (SC) using Alzet osmotic minipumps

(model: 2002, Bafilomycin A1 chemical structure 14-day delivery, at a rate of 0.5 μL/h; Durect, Cupertino, CA, USA). This dose, when administered chronically, has been previously shown to result in ~ 75% D2R occupancy in the striatum of rats (Samaha et al., 2007, 2008), similar to D2R striatal occupancies observed following effective antipsychotic doses in humans (Kapur et al., 2000). AMPH (1 mg/kg, 0.5 mg/kg or 0.25 mg/kg; Sigma–Aldrich) was dissolved in 0.9% saline and administered intraperitoneally (IP). These doses were selected based on previous studies inducing behavioural sensitization to AMPH as well as studies examining the efficacy of antipsychotics in response to an AMPH challenge (e.g. Samaha et al., 2007). All rats were implanted with subcutaneous E2 pellets to provide a chronic low dose of E2 (0.36 mg/pellet, 90-day release; Innovative Research of

America, Sarasota, FL, USA). Additionally, half of the animals received a subcutaneous injection of E2 every second day (20 μg/kg dissolved in sesame seed oil) in a volume of 0.5 mL/kg body weight, providing an intermittent phasic high dose. The low-E2 rats also received an injection of sesame oil vehicle every second day as a control. These doses were chosen to mimic the levels of E2 in estrous and proestrous young cycling rats learn more (Overpeck et al., 1978; Quinlan et al., 2008). Rats were anesthetized using isoflurane (Inhalation

Anaesthetic, Richmond Hill, ON, Canada), and two 8.3-mm stainless steel cannulae (21-gauge; Plastics-One, Ronaoke, VA, USA) were stereotaxically implanted, bilaterally, toward both the left and PIK3C2G right NAcc at the following coordinates from bregma: anteroposterior (AP), 1.8 mm, lateral–medial (LM), 3.0 mm and dorsoventral (DV), 5.4 mm, at a 10° angle. Cannulae were anchored into place with skull-screws using dental cement. Obturators (26-gauge; Plastics-One) were inserted into each cannula. Following surgery, animals were single-housed for the remainder of the experiment and were handled every day for ~ 5 min/day. All surgical procedures, i.e. OVX and E2 pellet and cannula implantations, were performed at the same time in order to avoid multiple sessions of general anesthesia. All rats were ovariectomized via bilateral lumbar incisions (1 cm). Post-ovariectomy, rats were implanted with E2 pellets in the nape region. They were administered the analgesic drug Anafen (0.1 mL/rat, SC; Merial Canada Inc., Morgan Baie d’Urfe, QC, Canada) and the antibiotic penicillin G (0.2 mL/rat, intramuscular; CDMV, St Hyscinthe, QC, Canada). Antibiotic ointment (By/Par Pharmaceuticals Inc., Brampton, ON, Canada) was also applied to the incision. Rats were allowed a week to recover in their home cages following surgery.