Average Ca2+ signal (Δ F / F 0 ) of CeA µOR-expressing neurons in naive mice after intraperitoneal injection of naloxone (left) and independent mice after intraperitoneal injection of fentanyl (0.3 mg kg -1 ) plus saline or fentanyl (0.3 mg kg -1 ) plus naloxone (5 mg kg -1 ) (n = 7 mice).
