The role of the brain catecholaminergic system in the control of gonadotropin secretion. II. The involvement of the brain stem in the control of gonadotropin secretion (author's transl)

Abstract

The studies on the role of the midbrain ascending pathways in the proestrous surges of gonadotropin and PRL have recently appeared, suggesting the importance of monoamine pathways in this process. In the preceeding study, it was suggested that the ascending noradrenergic system plays a stimulatory role in the induction of preovulatory surges of gonadotropin and PRL. The purpose of the present study was to investigate the noradrenergic mechanisms in the lower brain stem which are involved in preovulatory surges of gonadotropin and PRL.Adult female rats of Wistar strain served as experimental animals. Each surgical operation was stereotaxically performed under ether anesthesia. Blood samples were collected without anestheisa through the indwelling atraial cannulae for the determination of serum LH, FSH and PRL by radioimmunoassay.(1) The bilateral electrolytic lesions, which were made in the ventrolateral part of the medulla oblongata at 1100-1330 h on proestrus, blocked ovulation in 5 out of 7 animals (p<0.05) and significantly suppressed the proestrous surges of serum LH, FSH and PRL when compared to the sham-lesioned animals, while the lesions made in the dorsomedial part of the medulla oblongata blocked ovulation in only 1 out of 5 animals.(2) The bilateral implants of diethyldithiocarbamate (DDC), an inhibitor of norepinephrine synthesis, into the ventrolateral part of the medulla oblongata, which were made at 1100-1330 h on proestrus, blocked ovulation in 6 out of 9 animals (p<0.05) and significantly suppressed the proestrous surges of serum LH and FSH, while the DDC implants into the dorsomedial part of the medulla oblongata blocked ovulation in only 1 out of 5 animals.(3) The unilateral implants of ovarian steroids (estradiol benzoate or progesterone) into the ventrolateral part of the medulla oblongata failed to increase or decrease serum LH, FSH and PRL values at least within 6 hours in the ovariectomized, estrogen-primed rats.(4) The unilateral electrochemical stimulations of the ventrolateral part of the medulla oblongata failed to induce ovulation and the surges of serum LH and FSH in the pentobarbital-blocked proestrous rats, although the stimulations slightly suppressed the serum PRL value.The present study suggests that the ventrolateral part of the medulla oblongata is involved in the induction of the proestrous surges of LH, FSH and PRL. The finding that the DDC implants into the ventrolateral part of the medulla oblongata significantly suppressed the proestrous surges of LH and FSH indicates that the noradrenergic system arising in the Al NE cell group may play an important role in the induction of the proestrous gonadotropin surge. However, the induction of the gonadotropin surge was not obtained from the implants of ovarian steroids into, nor the electrochemical stimulations of, the ventrolateral part of the medulla oblongata.