Effects of preoptic injection of glucagon on luteinizing hormone secretion in ovariectomized rats with or without estrogen priming.

Effects of microinjection of glucagon or GIP into the medial preoptic area on luteinizing hormone (LH) and prolactin (PRL) release were examined in unanesthetized ovariectomized rats with or without estrogen priming. Microinjection of glucagon (1.0 microgram) into the medial preoptic area of ovariectomized estrogen-primed rats significantly facilitated the circadian afternoon rise in LH secretion as compared to the hormone values in control animals microinjected with physiological saline. The timing of the afternoon LH rise was not altered by glucagon and the circadian rise of PRL secretion was not altered by glucagon injected in the preoptic area. The injection of GIP did not have any significant effect on either LH or PRL secretion. In ovariectomized estrogen-unprimed rats, on the other hand, glucagon did not affect the pulsatile LH secretion, but it inhibited PRL secretion. GIP did not affect any hormone secretion. The results show that (1) glucagon, as other secretion family peptides such as secretion and PHI, can stimulate the preoptic LH secretory mechanism that undergoes the circadian clock mechanism under the influence of estrogen, and (2) without estrogen priming, glucagon in the preoptic area inhibits PRL secretion.