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Freeze-fracture methodology was used to study the organization of the neuronal plasma membrane in the rat arcuate nucleus, an oestrogen sensitive area of the hypothalamus. The qualitative evaluation of freeze-fracture replicas of the perikarya, dendritic shafts and dendritic spines revealed that the numerical density intramembranous particles varied during the ovarian cycle. The number of small (< 10 nm) particles was decreased, and the number of the large (> 10 nm) particles was increased, in the P-face of the perikaryal plasma membranes in prooestrus and oestrus when compared to dioestrus. This change was associated with a significant increase in the number of exo-endocytotic images in the perikaryal plasma membrane in prooestrus. Changes in intramembranous particles during the ovarian cycle were not detected in arcuate dendritic shafts and dendritic spines.

Sex steroids during the perinatal period are able to modify the postnatal development of neurons within steroid-sensitive areas in the rat brain. This study was designed to test the possible influence of the early postnatal levels of sex steroids on the morphology of the astrocytes. The experimental manipulation of the neonatal levels of sex steroids was performed by the androgenization of females with a single injection of testosterone propionate and by the orchidectomy of males on the day of birth. Control females received a single injection of vehicle and control males were sham operated. All the animals were sacrificed at 3 months of age postnatally. The immunohistochemical distribution of the glial fibrillary acidic protein (GFAP), a marker of astrocytic filaments, was studied on coronal sections of the dorsal hippocampus, the globus pallidus and the hypothalamic arcuate nucleus. The number of GFAP immunoreactive cells, the number of GFAP immunoreactive primary processes per cell and the surface density of the GFAP immunoreactive material were evaluated. This morphometric evaluation revealed a decreased surface density of GFAP immunoreactive material in the hippocampus, globus pallidus and the ventral part of the arcuate nucleus of orchidectomized males when compared to control males. Sex differences in the distribution of GFAP immunoreactivity were detected in the hippocampus and globus pallidus. These differences were abolished by the androgenization of females. The number of GFAP immunoreactive cells was similar in all the experimental groups, indicating that the differences in surface density represent an effect of sex steroids on the growth of astrocytic processes rather than on the proliferation of astrocytes.

Adult cycling female rats were injected with estradiol valerate (2 mg/100 g body wt.), a treatment which has previously been shown to result in synaptic remodelling in the arcuate nucleus and constant vaginal estrus. During the 32 weeks following estrogen treatment, arcuate nucleus neuronal plasma membranes were quantatively assessed for intramembrane particle (IMP) number and size using freeze-fracture techniques. Neuronal membranes from untreated cycling females, females injected with oil and untreated males were also studied. Untreated rats had dimorphic sexual phenotypes in membrane organization; female rats had more IMP than males, mainly due to greater numbers of small (<10 nm) particles. These sex differences were observed in perikarya and dendritic shafts, but not in dendritic spines. Following estrogen treatment the density of IMP in membranes from females decreased. The IMP changes were found only in neuronal perikarya and dendritic shafts, not in dendritic spines, and were mainly due to a massive decrease in the number of small (< 10 nm) IMP which was only partially offset by an increase in the number of large (≥10 nm) IMP. Thus, by 32 weeks after estradiol valerate treatment, the number and size of IMP in neuronal membranes from females were not different from those seen in normal males. These results strengthen the idea that estradiol may affect the turnover of certain neuronal membrane components in sex-steroid sensitive areas of the brain.

Adult female rats showing regular vaginal cycles were studied in order to determine the number of axosomatic synapses in thin sections of the arcuate nucleus. The number of synapses per length of perikaryal membrane was significantly decreased in estrus, compared to other days of the estrous cycle (P < 0.05). The reduction in the number of synapses in estrus was accompanied by a decrease in the percentage of the average length of perikaryal membrane covered by presynaptic terminals and by an increase in the percentage of membrane in close apposition of glial processes. Since the average perikaryal perimeter was not significantly changed during the estrous cycle, these results indicate a net decrease in the number of arcuate nucleus axosomatic synapses between proestrus and estrus, with a reinnervation of arcuate neurons between estrus and metestrus. These results suggest that there is a physiological synaptic turnover in the arcuate nucleus of the rat during the estrous cycle.