The effect of chronic administration of L-arginine and L-NAME on morphine-induced antinociception in ovariectomized rats

Objective : The role of ovarian hormones and nitric oxide on morphine-induced antinociception and their interaction have been widely investigated. The results of previous study showed that nitric oxide synthase inhibition differently affects morphine-induced antinociception in male and female rats. The present study was carried out to evaluate the different effects of chronic administration of L-arginine (LA) and L-NAME (LN) on morphine-induced antinociception in ovariectomized (OVX) and naive female rats.
Materials and Methods : Sixty female rats were randomly divided into six groups (n = 10) as follows: (1) sham, (2) OVX, (3) sham-LA (4) sham-LN (5) OVX-LA, and (6) OVX-LN. The animals of sham-LA and OVX-LA groups received daily injection of 200 mg/kg LA (i.p.) during 6 weeks, while in sham-LN and OVX-LN groups, the animals were treated with 10 mg/kg LN (i.p.). The animals of sham and OVX groups received 2 ml/kg saline (i.p.) instead of LA and LN. Finally, all the animals were tested on the hot plate test (52 ± 0.2°C; cut-off time 80 seconds) for evaluating the antinociceptive effects of morphine. The hot plate test was performed as three base records with a 15-min interval before the injection of morphine (10 mg/kg; s.c.) following which it was repeated every 15 min after injection. Analgesic effect of morphine was quantified as maximal percent effect (MPE). Base reaction latency times (seconds) before the injection of morphine and MPE after the injection of morphine were compared using repeated-measure analysis of variance (ANOVA) followed by post-hoc Tukey's test. Differences were considered statistically significant when P < 0.05.
Results : Before injection of morphine, there was no significant difference observed between sham and OVX groups in three recorded base reaction latency times. The base reaction latency times in sham-LA group were significantly higher than those of sham group (P < 0.001). In sham-LN group, the base reaction latency times were nonsignificantly lower than those of sham group (P = 0.095). There was no significant difference between OVX-LA group and OVX group. In OVX-LN group, three base reaction latency times were nonsignificantly lower than those of OVX group (P = 0.077). MPE in sham-LN group was higher than that of sham group (P < 0.05); however, there was no significant difference between sham-LA and sham groups.
Conclusion : It is concluded that NO has a role in pain perception and the analgesic effect of morphine. The effect of NO might be differing in the presence or absence of ovarian hormones, but further investigations need to be done.

Keywords
L-arginine, L-NAME, morphine, ovariectomy, pain, hot plate, rat, analgesia