Changes in multiunit electrical activity (MUA) in rat brain during the estrous cycle and after adminstration of sex steroids

Effects of novelty and pain on behavior and hippocampal extracellular ACh levels in male and female rats

In vivo microdialysis was used to assess the effects of novelty and pain on hippocampal ACh release in male and female rats. Experiments were carried out during the dark phase and consisted of 2 days of tests: on Day 1, after Baseline 1, animals were exposed to a new cage (Novelty) to which, 30 min later, a plastic cylinder (Object) was introduced. On Day 2, after Baseline 2, the Formalin test (50 microl of formalin 10%, s.c. injected in the dorsal hindpaw) was carried out in the animal's home cage. All behaviors were recorded. The extracellular levels of ACh in the dorsal hippocampus were estimated, in 10-min samples, by assay of ACh in the dialysates by HPLC. On Day 1 the raw values of ACh were higher in females than in males, but no sex difference was present when the percentage of change was considered. In both sexes the Novelty and Object tests induced an increase in ACh levels with respect to Baseline. Higher levels of exploration were present in females than males during the first 10 min of Novelty. On Day 2, ACh release increased in both sexes during the Formalin test. No sex difference in either ACh raw values or the percentages of change were found. Females showed higher levels of licking and lower levels of activity than males. The present study shows that novelty and pain induce similar hippocampal cholinergic activation in male and female rats but different behaviors. The results are discussed in light of the several anatomical and functional sex differences present in the hippocampus.

Insulin-induced hypoglycemia does not impair the surge of luteinizing hormone secretion in the proestrous rat

To know whether insulin-induced hypoglycemia (IIH), which has been shown to inhibit pulsatile luteinizing hormone (LH) secretion, also affects the surge of LH secretion, adult female rats were injected with insulin (5 units/rat) or saline intravenously at 1300 or 1600 h on the day of proestrus and serum concentrations of LH and blood glucose were determined during the period from 1100 to 2100 h. The injection of insulin at neither 1300 nor 1600 h affected the surge of LH secretion, but it significantly decreased the blood glucose. Together with our recent hypothesis that the pulsatile and surge of LH secretion are controlled by separate gonadotropin-releasing hormone (GnRH) mechanisms, we conclude that the GnRH surge generator is less sensitive to IIH than the GnRH pulse generator.

Changes in catecholamine content in discrete brain nuclei during the estrous cycle of the rat

Norepinephrine (NE) and dopomine (DA) concentrations were measured in discrete brain nuclei in female rats at different stages of the 4-day estrous cycle. In the medial preoptic and paraventricular nuclei, NE levels were reduced on the days of proestrus and estrus as compared to metestrus-diestrus. NE levels sharply increased between estrus and metestrus in the lateral septal nucleus but did not vary significantly at other times. In the mesencephalic central gray catecholamine area, NE levels were elevated on metestrus as compared to all other days of the cycle. DA concentrations also varied in certain nuclei during the estrous cycle. In the lateral septum, DA levels were highest during diestrus, and in the nucleus of the tractus diagonalis, DA levels were highest on metestrus. The DA concentration of the caudate nucleus decreased between proestrus and estrus and remained low through metestrus. DA in the medial preoptic nucleus showed a similar pattern of change as did NE. In the median eminence DA levels increased significantly from proestrus afternoon to estrus, but did not vary on the other days. It is concluded that cyclic activity of catecholamines in a few discrete areas of the brain may be related to the fluctuations of the gonadotropins and ovarian hormones to influence ovulation and mating behavior.