Further evidence for hypothalamic asymmetry in endocrine control of the ovary.

The differential release of FSH and LH associated with hemigonadectomy (hemi-x) of prepubertal male rats can be blocked by unilateral hypothalamic deafferentation located on the side ipsilateral, but not contralateral, to the hemi-x. Also, ovarian compensatory hypertrophy (OCH) in prepubertal female rats can be blocked by ipsilateral, but not contralateral, hypothalamic hemi-islands. Both these endocrine phenomenon are limited to knife cuts on a particular side of the brain. These results suggest that there are neural connections with the gonads which are involved in endocrine regulation and that hypothalamic control of the endocrine system may be asymmetrically organized. In support of this, the present study shows that in adult female rats, unilateral injections of the excitotoxin kainic acid (1.0 microgram in 1.0 microliter) into the retrochiasmatic area of the hypothalamus can block OCH if the injections are located on the side ipsilateral to the hemi-x. This phenomenon was more apparent if the lesions and hemi-x were located on the right side. In addition, neither hemi-x nor kainic acid injections alone had any effect on vaginal cycles, whereas the combination of these two treatments reduced the incidence of estrus. However, these effects on vaginal cycles were not specifically associated with lesions or hemi-x on a particular side, thus excluding this as an explanation of the different endocrine effects of kainic acid on the two sides of the brain. These results support the evidence for a direct neural contribution to endocrine control and further suggest a functional difference in the two-halves of the hypothalamus in neuroendocrine regulation.