Site specificity of adrenalectomy-induced brain growth

Effects of maternal bilateral adrenalectomy on fetal rat cerebral cortex

We investigated the effects of maternal bilateral adrenalectomy (ADX) on fetal rat cerebral cortices by assessing morphological and morphometrical parameters in light and electron microscopy studies. Pregnant adult rats underwent ADX on day 4 of gestation. Embryonic brain tissue from control and ADX groups were examined at each of the 10 embryonic stages, days 11 through 20 (E11-E20). Control and adrenalectomized pregnant females were sacrificed at each fetal stage, fetuses were removed from the uterine horns, and their brain tissue was processed for light and electron microscopy examination. In the ADX fetuses, the cortical laminae were thicker and overall cortex thickness was greater, but structural differentiation in the primitive cortical layers was delayed compared with that observed in controls. Apart from the differences in embryonic cell development noted at the structural level, we found no noticeable ultrastructural differences between the cerebral cortices of ADX and control fetuses at any of the stages studied. Overall, the ADX group's cortical tissue exhibited a greater degree of cell migration, an extended proliferative period in the cortical layers with the highest proportion of mitotic activity, and a greater thickness than that of control specimens. We believe that by eliminating the inhibitor effects of glucocorticoids, ADX effectively induces mitotic activity and extends the proliferative periods in developing cerebral cortex.

Adrenalectomy-induced neuronal degeneration

The binding of glucocorticoids to CNS receptors results in the modulation of many processes, ranging from neurotransmission to cell birth and death. It is of no surprise, therefore, that the removal of these steroids following adrenalectomy disrupts a variety of physiological functions throughout the brain. It is the aim of this review to briefly describe the findings of research examining some of these glucocorticoid-mediated CNS effects; however, as many of these areas have been reviewed extensively by others, this review will focus on the recently described phenomenon, adrenalectomy-induced hippocampal cell death.

The relationship between adrenal steroids and enrichment-induced brain growth

The question of whether brain growth brought about by environmental enrichment is mediated by the adrenal cortex has not been answered. Accordingly, young male rats were either adrenalectomized (ADX) and infused with a constant maintenance dose of corticosterone (2 mg.kg-1.day-1) or sham-operated and implanted with a blank infusion device. Half of each surgical group was maintained in either impoverished (IC) or enriched conditions (EC). After 30 days, changes in forebrain growth and thickness of various cortical and subcortical regions were determined for each group. Enrichment and ADX independently increased forebrain weight and thickened cortical tissue at about the same anatomical sites. However, combined treatments were additive, not interactive. EC-induced brain growth is mimicked but not mediated by adrenalectomy.

Regional patterns of brain growth during the first three weeks following early adrenalectomy

Day 11 adrenalectomized (ADX) and sham-operated control rats were compared with respect to overall and regional brain growth and DNA content at 1, 2, and 3 weeks postsurgery. Possible treatment effects on the rate of postnatal cell loss were also assessed by injecting the animals with 3H-thymidine on day 2 postnatal (9 days prior to surgery) and subsequently measuring the amount of radiolabelled DNA remaining in various brain regions at each time point. Adrenalectomy led to reliable increases in cerebral cortex and midbrain-diencephalon weights within 1 week postsurgery, whereas cerebellum, hippocampus, and overall brain weights were not significantly elevated until 1 or 2 weeks later. The effects of adrenalectomy on tissue DNA content were likewise regionally dependent. According to the 3H-thymidine results, there was no significant influence of adrenalectomy on the loss of previously labelled DNA. In conclusion, the timing of brain growth acceleration following adrenal removal is regionally specific, with the most rapid effects occurring in the cortex and midbrain-diencephalon. Furthermore, adrenalectomy-induced increases in brain cell number appear to depend primarily on increased cellular genesis (as previously demonstrated) rather than decreased cell death.