Six flight and six control rats were sacrificed and standard receptor binding assays for receptor number and affinity were performed. Brains were dissected so that the hippocampus and posterior cortex area could be saved for assays, including serotonin, dopamine, noradrenergic, cholinergic and GABA measurements. The Mg-dependent Na+/ K+ ATPase activity was determined by colorimetric assay of Pi formed from trisATP added to cortical membranes. Group sizes varied from three to six depending on the necessity of pooling tissues.

Results:

Data indicate that few receptor changes occurred in the microgravity. When receptor changes did occur, they appeared to be restricted to a particular terminal field, suggesting that microgravity affected terminal mechanisms (e.g., release and uptake) differently, rather than exerting a generalized effect on the projection neuron. The increase in 5HT1 receptor in the hippocampus may reflect altered neuromodulation in this area by serotonergic neurons originating in the raphe nuclei. As it has been suggested that one major function of the hippocampus is to serve as a spatial map of the environment, perhaps the transition to microgravity may necessitate major changes in any spatial map of the environment. The 5HT1 receptor may play a role in such a modification. Similarly, the flight-associated marginal decrease in D2 binding in the stratum might reflect a down-regulation induced by heightened dopaminergic activity in the nigra, associated with novel motor activity under microgravity.