Agouti-related peptide prevents steroid-induced luteinizing hormone and prolactin surges in female rats

We studied the effect of Agrp (agouti-related peptide) on LH (luteinizing hormone) and PRL (prolactin) surges in ovariectomized rats primed with estradiol and progesterone. The rats displayed characteristic LH and PRL surges that were completely abolished by starving. Injection of either 1 nmol or 3 nmol Agrp (83-132), a potent antagonist of the orexigenic MC3 and MC4 receptors, completely prevented both the LH and PRL surges. We also investigated the effects of either a single or double injection of anti-Agrp serum to fasted animals, which were without LH and PRL surges. A single injection of the antiserum was without effect, but the rats that received double injection of anti-Agrp serum partially reinstated both the LH and PRL surges. Although the onset of LH and PRL surges was significantly delayed in the double treated group, the highest levels of the surges for both hormones were statistically indistinguishable compared with the control group. These data give a clear indication that endogenous Agrp may be involved in LH and PRL surges during starvation, providing further evidence that the melanocortin system is important for these hormonal surges in female rats.