Electrophysiological and morpho-histochemical study of action of adrenalectomy on hippocampal neurons

Chronic insufficiency of adrenal hormones is a pathology leading to brain dysfunction. By electrophysiological approach there were studied mechanisms of adaptation of neural networks to chronic hormonal deprivation by extracellular recording of the single spike activity of hippocampal neurons (HN), which was caused by high-frequency stimulation of the entorhinal cortex (EC) in rats with unilateral removal of adrenal (adrenalectomy—AE). The balance of excitatory and inhibitory responses recorded in intact rat HN underwent characteristic changes in dynamics of development of neurodegeneration: the inhibitory responses dominating in norm were decreased in all AE terms (from 42% to 25% by 18 weeks). On the contrary, the minimal percentage of excitatory responses in norm was sharply increased at 25–27 days after AE (from 17% to 60%), by indicating a possible increase in cholinergic neurotransmission. The high level of the mean frequency of peristimulus spiking was recorded from the 25–27th day to the 18th week after AE, which indicates the presence of the high level of glutamate or the expressed activation of NMDA receptors. On the whole, the ratio of the excitatory/inhibitory HN responses suggests discrepancy of neural activity in EC HN chains under the AE conditions. Histochemical analysis has shown an increased sensitivity to AE in the CA1 area neurons. After disruption of neuronal structure by the 5th day of AE, 25–27 days after AE, proliferation of cellular elements was observed in the CA1 area, due which the complete filling of the “devastated” areas of hippocampus and a sharp enhancement of phosphatase activity occurred by 8-10 weeks in neuronal nuclei of the dentate gyrus. By 18 weeks after AE, most neurons in the CA1 area were subjected to chromatolysis with a fall of phosphatase activity. The presented make certain contribution to understanding of mechanisms of control of cognitive function and brain plasticity with interconnection with hormonal factors.