Pentobarbital induces a naloxone-reversible decrease in mesolimbic self-stimulation threshold

The effects of sodium pentobarbital and naloxone were tested on intracranial self-stimulation (ICSS) in rats implanted with electrodes in the ventral tegmental area. Threshold for ICSS was determined using a rate-independent current titration paradigm. A low dose of pentobarbital (5 mg/kg) did not have a significant effect on ICSS thresholds, while a high dose (20 mg/kg) rendered the subjects too ataxic to respond reliably in the operant task. An intermediate dose (10 mg/kg) induced a highly significant lowering of threshold (17% below saline baseline levels) without apparent deterioration in response capability. The concurrent administration of naloxone (2 mg/kg) significantly reversed the pentobarbital-induced threshold decrease, while naloxone treatment alone had no effect on the ICSS threshold.