Stress and morphine analgesia: alterations following p-chlorophenylalanine

Recent studies have shown that while the analgesic responses induced by certain stressors appear to be related to morphine analgesia, the analgesic responses to other stressors do not. Para-chlorophenylalanine (PCPA), a potent tryptophan-hydroxylase inhibitor has been shown to decrease both basal pain thresholds and morphine analgesia on the flinch-jump test. To assess further the relationship between morphine and stress-induced analgesia, PCPA's effect upon the analgesic responses to cold-water swims, 2-deoxy-D-glucose, inescapable foot shock and morphine were determined using the flinch-jump and tail-flick tests. PCPA, which produced an 85% depletion of brain serotonin, significantly decreased jump thresholds while significantly increasing tail-flick latencies. Similarly, while morphine analgesia was decreased by PCPA on the flinch-jump test, it was not affected on the tail-flick test. The analgesic jump thresholds induced by cold-water swims and 2-deoxy-D-glucose as well as the increase tail-flick latencies induced by foot shock were unaffected by PCPA. These results are discussed in terms of PCPA's differential effects upon basal nociception and morphine analgesia and in terms of further dissociation between morphine and stress-induced analgesia.