Effects of limbic forebrain ablation on reproductive function were examined. Frontal cortex (Group A), septum (B), fornix (C), anterior limbic area (the area around anterior part of the hippocampus which located at rostral to the septal nuclei)(D), olfactory bulb (E), limbic cortex (F), and hippocampal formation (G) were ablated with suction, taking care not to damage the neighboring brain structures. Operations were performed under pentobarbital anesthesia on the day of estrus or diestrus I in 4-day cycling rats. Animals were autopsied 30 to 35 days postoperatively either on the day of proestrus or estrus. The preoptic basal hypothalamic region was intact in all examined rats.
Various degrees of septal ablation suppressed the appearance of vaginal cornification for about 10 days, although the cycle resumed eventually. However, vaginal cycles became irregular predominantly diestrous for 9/14 of the septal-ablated rats. Normal ovulation occurred in 6 of 7 rats and significant changes in serum or pituitary LH and FSH were not observed. Complete ablation of the septum, however, reduced the weight of the uterus and the amount of intraluminal fluid.
Ablation of the hippocampus induced slight increases in serum LH and FSH, and transection of the fornix increased serum LH, so that the hippocampus-fornix system seems to inhibit gonadotropin secretion. In these animals, cornified vaginal smears were absent for several days postoperatively, but the regular cycles were resumed subsequently. Normal ovulation as well as normal weights of ovaries, uterus and intraluminal fluid were observed.
Ablation of the anterior limbic area disturbed the vaginal cycle for a few days postoperatively. Marked increase in serum and pituitary LH and FSH as well as increases in ovarian, uterine and adrenal weights and greater number of ova in oviducts were observed in these animals. Thus, this area of the brain consistently appears to exert an inhibitory influence on pituitary ovarian activity.
No changes were observed in the rats with ablation of the frontal cortex, the olfactory bulb or the limbic cortex.
Therefore, limbic forebrain structures is not essential for ovulation and maitenance of the estrous cycle, but appear to exert a significant modulating influence on pituitary gonadal function.