Centrally administered vasopressin cross-sensitizes rats to amphetamine and drinking hypertonic NaCl.

Prior sodium restriction cross-sensitizes rats to the psychomotor effects of amphetamines and vice versa. Repeated central injections of vasopressin (VP) induce a psychomotor sensitization similar to amphetamine sensitization and repeated sodium deficiency. Thus brain VP signaling may be a common mechanism involved in mediating these two motivational systems. In experiment 1, we tested the hypothesis that rats previously sensitized to central VP would show enhanced psychomotor responses to amphetamine. Rats were administered saline, VP (50 ng), or amphetamine (1 mg/kg or 3 mg/kg) on days 1 and 2, and given saline or amphetamine on day 3. Amphetamine produced psychomotor arousal in all groups. However, amphetamine on day 3 elicited a significantly greater psychomotor response in rats that had prior injections of amphetamine or VP than in rats previously treated with saline. In experiment 2, the hypothesis that prior experience with central VP would cross-sensitize rats to drinking hypertonic sodium (NaCl) solutions was tested. Rats were administered VP (50 ng) or saline for 3 days. On the fourth day, nondeprived rats were given access to 0.3 M NaCl and water for 1 h. Control and saline-treated rats only drank 1 ml of 0.3 M NaCl, but rats previously exposed to central VP drank significantly more hypertonic saline (4 ml). These results show that prior experience with central VP cross-sensitizes rats to the psychomotor stimulant effects of amphetamine and the ingestion of concentrated NaCl solutions. This pattern of cross-sensitization links central VP signaling, amphetamine, and sodium deficiency, and therefore it may play a role in the cross-sensitization between sodium appetite and amphetamines.