Weightlessness during spaceflight results in enhanced synapse formation ina fish brain vestibular nucleus

Synapse counts were undertaken by conventional electron microscopy in prima ry vestibular integration centers, (i.e. nucleus descendens and nucleus mag nocellularis of the brainstem area octavolateralis) and in the diencephalic visual nucleus corticalis of spaceflown neonate swordtail fish Xiphophorus helleri as well as in 1 g control siblings. Spaceflight (16 days micrograv ity, (mug), STS-90 Neurolab Mission) yielded an increase in synaptic contac ts within the vestibular nucleus descendens indicating that lack of input r esulted in compensation processes. No effect of mug, however, was observed in the visual nucleus corticalis and in the vestibular nucleus magnocellula ris which is situated in the close vicinity of the nucleus descendens. In c ontrast to the latter, the nucleus magnocellularis does not receive exclusi vely vestibular input, but inputs from the lateral line as well, possibly p roviding sufficient input at microgravity. (C) 2000 Published by Elsevier S cience Ireland Ltd.