Antidiuretic effect of bremazocine and U-50,488 in rats after α2-adrenoceptor blockade

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Abstract
The role of α2-adrenoceptors and κ-opioid receptors in urination was studied in rats. In water-loaded rats (40 mL kg−1 p.o.) the κ-opioid agonist bremazocine (0.05-0.2 mg kg−1 i.p.) induced a dose-related diuretic response in the second hour after administration, but had no effect in the first hour. When rats were pretreated with the α2-adrenoceptor antagonist idazoxan (1 mg kg−1 s.c.), bremazocine induced a dose-related antidiuretic response in the first hour; thereafter the rats showed an increase of urination similar to that with bremazocine alone. The antidiuretic effect of bremazocine was dependent on the dose of idazoxan with maximal response after 1–3 mg kg−1. Similar results were obtained with bremazocine in the presence of yohimbine (1 mg kg−1 s.c). The antidiuretic profile of bremazocine after idazoxan was shared by U-50,488 (2.5–10 mg kg−1 i.p.), although this compound alone at the high dose reduces urine output in the first hour. The antidiuresis induced by bremazocine in the presence of idazoxan in water-loaded rats was completely antagonized by 10 but not 2 mg kg−1 i.p. of the opioid antagonist naloxone. Thus, κ-opioid agonists, in addition to their diuretic effect, also produce an antidiuretic response which may be mediated by α2-adrenoceptors.