Effects of fluoxetine and PCPA on isolation-induced morphine self-administration and startle reactivity

The present study investigated the effects of the SSRI fluoxetine and the serotonin synthesis blocker — parachlorophenylalanine (PCPA) on morphine self-administration and startle reactivity in rats subjected to social isolation during adulthood. Adult Wistar rats were housed individually or in pairs for 21 days. They were treated with fluoxetine, PCPA, or vehicle and tested for their startle response and intake of a morphine solution (0.5 mg/ml). Socially restricted rats consumed significantly more morphine solution (but not water) than rats living in pairs, in both one-bottle and in two-bottle tests. They also showed significantly higher startle response amplitude. Daily fluoxetine treatment (5 mg/kg i.p.) counteracted these behavioral alterations induced by isolation housing while PCPA treatment (200 mg/kg for 3 consecutive days) further exacerbated it. Social isolation may increase morphine self-administration and emotional reactivity in the startle box by affecting serotonin. Antidepressants (such as fluoxetine) may normalize or stabilize serotonin function and restore the behavioral changes produced by isolation.