Effects of progesterone and estrogen on the electrical activity of the limbic system

Acute effects of 17 beta-estradiol on brain excitability studied in vitro and in vivo

The acute effects of 17 beta-estradiol on brain excitability were studied in vitro and in vivo utilizing rat hippocampal slices and a cat cerveau isolé preparation. The hippocampal slices were perfused with 17 beta-estradiol (10(-7)-10(-10) M) for 30 min. No effects were observed on synaptic activation and inhibition and on the response to iontophoretically applied GABA in intact and ovariectomized female rats (n = 43). In males (n = 32), however, a small (12%) but significant increase in population spike amplitude was observed after 30 min exposure to 10(-9) M 17 beta-estradiol. Higher and lower concentrations were ineffective. In vivo, no acute effects of 17 beta-estradiol on focal epileptic seizure thresholds, evoked potentials, or augmenting response were observed in the visual cortex of non-estrous female cats (n = 11; median dose 1 micrograms/kg, range 0.5 microgram/kg-10 mg/kg).

Autoradiographic localization of estradiol-binding neurons in the rat hippocampal formation and entorhinal cortex

This study has examined the distribution of [3H]estradiol and [1 alpha,2 alpha-3H]testosterone uptake in the hippocampal formation and entorhinal cortex of male and female rats. In both males and females, [3H]estradiol-binding neurons in Ammon's horn are located deep in stratum pyramidale and may correspond either to polymorphic interneurons or to early maturing pyramidal cells. Interneurons of strata oriens, lucidum and radiatum of Ammon's horn and of stratum moleculare of the subiculum also bind [3H]estradiol, as do basket cell interneurons in the polymorphic, infragranular layer of the dentate gyrus. While no granule cells appear to accumulate [3H]estradiol, these cells may be affected transsynaptically by gonadal steroids via their afferent contacts with the entorhinal cortex, which, of the areas examined, contains the greatest number of [3H]estradiol-binding neurons. While relatively few neurons concentrate [3H]estradiol in the hippocampal formation, these are localized to specific subpopulations, which may enhance their functional significance. Because there is no significant nuclear accumulation of [3H]-alpha-testosterone in either the entorhinal cortex or hippocampal formation, it appears that aromatase enzyme activity is not a major contributor to estrogen receptor occupancy in adult rats.

Hormonal regulation of axonal sprouting in the hippocampus

Removal of septal fibers to the rat hippocampal formation is followed by an ingrowth of sympathetic axons into the deafferented regions. We have shown previously that the pattern of sprouting is more restricted in males than in females when the lesions are made in mature animals, but is almost the same if lesions are made during the early postnatal period. In the present study, the relationship of circulating sex hormones to the sprouting response was investigated by comparing the extent of sympathetic axon ingrowth following fimbrial lesions in intact or gonadectomized adult male and female rats. The effects of manipulating sex steroids during development was examined by comparing sympathetic axonal sprouting after fimbrial lesions in rats which were castrated (male) or treated with testosterone (females and castrated males) on postnatal day 2. We find that (1) gonadectomy of either adult female or male rats does not affect the sprouting response, but (2) neonatal castration of male rats permits sprouting in a pattern similar to normal females, and neonatal testosterone treatment of females or castrated males results in the more limited sprouting response characteristic of normal males. These results indicate that the sex-related differential response to fimbrial lesions may be determined by developmental differences in endogenous steroid levels.

Gonadal steroids: effects on excitability of hippocampal pyramidal cells

Electrophysiological field potentials from hippocampal slices of rat brain show sex-linked differences in response to 1 X 10(-10)M concentrations of estradiol and testosterone added to the incubation medium. Slices from male rats show increased excitability to estradiol and not to testosterone. Slices from female rats are not affected by estradiol, but slices from female rats in diestrus show increased excitability in response to testosterone whereas slices from females in proestrus show decreased excitability.

Gonadal steroids: humoral modulators of nerve-cell function

The gonadal steroids, estradiol, testosterone and progesterone, alter neuroendocrine function and behavior in adult mammals and do so by acting on the central nervous system and pituitary gland. Besides intracellular receptor sites, various cellular and chemical actions of gonadal steroids are being studied within the brain and pituitary. The review summarizes the present state of such research and points as well to new advances in our understanding of the cellular basis of developmental events such as brain sexual differentiation and puberty, which are influenced by gonadal steroids.