Summary
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 µg) 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 secretin family peptides such as secretin 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.
Key words
Glucagon - Preoptic area - Luteinizing hormone
