Methylene blue inhibits stimulatory effect of sodium nitroprusside but not of 3-morpholino sydnonimine on prolactin secretion in freely moving male rats.

Previously we reported that intracerebroventricular (i.c.v.) administration of nitric oxide (NO) donors, sodium nitroprusside (SNP) (1-10 microg), and 3-morpholino-sydnonimine (SIN-1) (10-100 microg), induced dose-dependent increases in plasma prolactin levels of freely moving male rats, suggesting a role of NO in the control of prolactin secretion. The present results show that i.c.v. pretreatment with methylene blue (MB) (30 microg), a guanylyl cyclase inhibitor, significantly reduced the effects of microinjections of SNP (3 and 5 microg), however, this did not modify the stimulatory action of SIN-1 (30 microg) on plasma prolactin levels of conscious male rats. Alone, MB did not modify basal prolactin levels. These results suggest different mechanisms of action of SNP and SIN-1 to stimulate prolactin secretion in vivo. Activation of soluble guanylyl cyclase seems to mediate the neuroendocrine action of NO released from SNP but not of SIN-1. Different cellular distribution of NO generating activity from these donors as well as the possible generation of other radicals simultaneously with NO from SIN-1 could explain these differences.