Vasopressin (AVP) levels were measured in plasma and cerebrospinal fluid (CSF) of rats during acquisition and retention of a passive avoidance response. Only 5 min after the onset of the retention session a significantly higher level of AVP was found in plasma of animals which displayed a long latency, as compared with the levels of animals which showed a weak passive avoidance response (short latencies), or no passive avoidance behavior at all (controls). Moreover no changes in plasma AVP levels were found in plasma of rats submitted to acquisition or extinction of an active avoidance response. It is suggested that, although an elevated plasma AVP level is associated with strong retention of a passive avoidance response the peripheral circulation as well as the CSF are of minor importance for the transport of this neuropeptide to its site of behavioral action
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