Effects of human placental extract on chemical and thermal nociception in mice.

Several reports indicate that pregnancy and parturition are associated with elevated maternal pain thresholds to noxious stimuli. The objective of this study was to examine whether the human placental extract, a clinically used preparation, can inhibit experimental nociception. Nociception was assessed in mice using acetic acid-induced writhing and hot-plate tests. The human placental extract (200 and 400 mg/kg, i.p.) elicited dose-related antinociception in the acetic acid-induced writhing test. Furthermore, it (200 mg/kg, i.p.) potentiated the morphine-induced antinociception (1.25 mg/kg, s.c.). In the hot-plate test, the human placental extract (100, 200 and 400 mg/kg, i.p.) per se, displayed no significant antinociception but potentiated the duration of morphine (10 mg/kg, s.c.) analgesia. The potentiation by the extract of the morphine-induced antinociception in both acetic acid and hot-plate tests was, however, found to be naloxone sensitive. Mice treated with the extract (400 mg/kg, i.p.) neither manifested any overt behavioural change in the open-field test nor demonstrated significant influence on pentobarbital sleeping time, suggesting that it has no central depressant or sedative activity. The data provide evidence to show that the human placental extract has a peripheral analgesic property possibly mediated by an opioid mechanism.