Intracerebroventricular administration was performed as previously described [12 (link)]. After removal of the dust cap and dummy cannula, a 33-gauge infusion cannula that extended 1 mm beyond the guide cannula was inserted. The infusion cannula was attached to a polyethylene tube (PE-50; Plastics One, Roanoke, VA, USA) that was connected to a 10 μl Hamilton syringe driven by a microinfusion pump (KD Scientific, Holliston, MA, USA). Solutions were infused in a volume of 1-3 μl at a rate of 0.5 μl/min. The infusion cannula was left in place for at least 2 min to allow drug diffusion.
Vehicle (10 mL/kg saline) or naloxone (1, 2 or 4 mg/kg/10 mL) was administered subcutaneously 5 min prior to intracerebroventricular infusion of vehicle (saline in a 2 μL volume) or oxytocin (500 ng in a 2 μL volume) for Experiments 1 and 4. Vehicle (artificial cerebrospinal fluid in a 3 μL volume) or CTAP (1, 2 or 3 μg in a 1, 2 or 3 μL volume, respectively) was administered intracerebroventricularly 45 min prior to intracerebroventricular infusion of vehicle or oxytocin, as described above, for Experiment 2. Norbinaltorphimine is not selective for κ-opioid receptors until 4-24 h after administration [42 ]. Thus, vehicle (10 mL/kg saline) or norbinaltorphimine (10, 20, or 30 mg/kg/10 mL) was administered intraperitoneally 24 h prior to intracerebroventricular infusion of vehicle or oxytocin for Experiment 3. Mice were tested on the elevated zero maze or the tail suspension test 15 min after intracerebroventricular administration of oxytocin. Experimental drugs or controls were assigned to separate groups (i.e., a between-subjects design) of randomly selected mice (see Fig.1 ).Experimental timeline. ![]()
Vehicle (10 mL/kg saline) or naloxone (1, 2 or 4 mg/kg/10 mL) was administered subcutaneously 5 min prior to intracerebroventricular infusion of vehicle (saline in a 2 μL volume) or oxytocin (500 ng in a 2 μL volume) for Experiments 1 and 4. Vehicle (artificial cerebrospinal fluid in a 3 μL volume) or CTAP (1, 2 or 3 μg in a 1, 2 or 3 μL volume, respectively) was administered intracerebroventricularly 45 min prior to intracerebroventricular infusion of vehicle or oxytocin, as described above, for Experiment 2. Norbinaltorphimine is not selective for κ-opioid receptors until 4-24 h after administration [42 ]. Thus, vehicle (10 mL/kg saline) or norbinaltorphimine (10, 20, or 30 mg/kg/10 mL) was administered intraperitoneally 24 h prior to intracerebroventricular infusion of vehicle or oxytocin for Experiment 3. Mice were tested on the elevated zero maze or the tail suspension test 15 min after intracerebroventricular administration of oxytocin. Experimental drugs or controls were assigned to separate groups (i.e., a between-subjects design) of randomly selected mice (see Fig.
At least 5 days following intracerebroventricular cannula implantation, mice were tested on the elevated zero maze. Before testing, mice were pretreated with vehicle (VEH) or an opioid antagonist: naloxone (NLX), CTAP, or norbinaltorphimine (norBNI) and treated with vehicle (VEH) or oxytocin (OXT). At least 7 days after the elevated zero maze tests, mice previously exposed to vehicle or naloxone and vehicle or oxytocin were then tested on the tail suspension test. On the day of the experiment, mice were pretreated and treated with the same drug combination.
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