Different responses of nitric oxide synthase inhibition on morphine-induced antinociception in male and female rats

The roles of gonadal hormones and nitric oxide on pain perception and their interaction have been widely investigated. In the present study the chronic effect of l-NAME (NOS inhibitor) on morphine-induced antinociception in male and female rats was investigated. Forty rats were divided into four groups: (1) female (2) female-LN (3) male (4) and male-LN. The animals of groups 2 and 4 received daily injection of l-NAME (10 mg/kg) during 3 weeks. The animals of control groups 1 and 3 received 2 ml/kg saline instead of l-NAME. Finally, all animals were tested on the hot plate test (52 ± 0.2 °C; Cut-off 80 s) to evaluate the antinociceptive effects of morphine. The hot plate test was performed for base record 15 min before the injection of morphine (10 mg/kg; s.c.) and consequently it was repeated every 15 min after the injection. There were no significant differences in baseline latencies among all groups. Reaction times after injection of morphine in female-LN were higher than in the female control group (p < 0.01). There was, however, no significant difference between male control and male-LN groups. Reaction times in the female-LN group were significantly higher than in the male-LN group. Reaction times after injection of morphine in the male group was longer than in the female group (p < 0.01). It is concluded that sex hormones such as testosterone and estrogen have a role in pain perception and analgesia. NO has a modulatory effects on functions of sex hormones in pain perception and analgesic effects of opioids.