Morphological Changes and Characteristics of the Expression of Serine Racemase in the Hippocampus of Rats Exposed to Multiple Negative Gravitational Overloads

Objectives. To study the morphometric parameters and levels of expression of serine racemase in the hippocampus of rats repeatedly exposed to gravitational overloads in the caudocranial direction (–9 Gz). Materials and methods. Rat aged 12 months were subjected to 9- g forces for 5 min twice a day with 12-h intervals for 28 days (group 2, gravity group, n = 10); controls were not exposed to hypergravitation (group 1, controls, n = 10). Morphometric mean areas of the nuclei and perikarya of pyramidal cells were determined, along with the specific number of neurons with signs of damage, the relative areas of pyramidal neuron and neuroglial cell perikarya. Immunohistochemical studies addressed the level of expression of serine racemase in all zones of the hippocampus. Results. Signs of impaired blood supply in the microcirculatory component were seen in the form of spongiosis, thickening of capillary walls, and diapedesis of erythrocytes. All hippocampal zones in experimental rats showed sharp increases in the numbers of pyramidal neurons with signs of damage. Atrophic changes consisting of decreases in the mean and relative areas of neuron perikarya were identified. Immunohistochemical studies showed increased expression of serine racemase in field CA1, along with translocation of immunoreactive material into the processes of pyramidal neurons. Fields CA3 and CA4 showed a tendency to decreases in serine racemase content. Conclusions. Prolonged gravitational exposure in the caudocranial direction led to the appearance of signs of severe damage in all fields of the rat hippocampus, combined with increased expression and accumulation of serine racemase in the dendrites of pyramidal neurons in field CA1, which may be evidence that excitotoxicity is involved in the mechanisms of secondary damage in this zone.