Acute exposure to either prolonged intermittent foot shock (PIFS) or brief continuous foot shock, (BCFS) decreases the sensitivity of rats to noxious stimuli, but differ in their mechanisms of actions. Since the peptide vasopressin (VP) has been implicated in analgesic and stress-related processes, the present study examined whether antagonism of central VP receptors with dPTyr(Me)AVP would alter the analgesic responses following PIFS or BCFS. While intracerebroventricular administration of dPTyr(Me)AVP, a V1 receptor antagonist, significantly attenuated the analgesic response to PIFS, it potentiated the analgesic response to BCFS. It should be noted that the form of PIFS employed in the present study was not blocked by naloxone. These results are discussed in terms of multiple forms of pain-inhibitory systems that may utilize collateral inhibition as a means of providing selective activation.
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