A number of physiological responses to the microgravity environment have been
previously described. For example, temperature regulation, fluid volume and
water intake, calcium metabolism and the neuromuscular control of movement are
altered under microgravity. Such adaptations to microgravity may be medi- ated
by corresponding changes in brain neurotransmitter dynamics. This study examined
the effect of weightlessness on the neurotransmitter receptors in selected areas
of the brain of space-flown rats.
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.
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 projec-
tion 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 hippo- campus
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 environ- ment. 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.
Miller, J.D. et al.: Effects of Weightlessness on Neurotransmitter Receptors
in Selected Brain Areas. Physiologist, supl., vol. 28, no. 6, 1985, pp. S203-S204.
Miller, J.D. et al.: Effects of Weightlessness on Neurotransmitter Receptors
in Selected Brain Areas. Abstract S-203. Proceedings of the Seventh Annual Meeting
on the IUPS Commission on Gravitational Physiology, Niagara Falls, N.Y., October
13-18, 1985.
Miller, J.D. et al.: Effects of Weightlessness on Neurotransmitter Receptors
in Selected Brain Areas. Abstract 83.12. 36th Annual Fall Meeting of the American
Physiological Society, Buffalo, N.Y., October 13-18, 1985, Physiologist, vol.
28, no. 4, 1985, p. 377.
¥ = publication of related ground-based study