Synaptic Ribbon Plasticity in Utricular and Saccular Maculae: a Paradox

Research into the effects of weightlessness on rat vestibular maculae has consistently shown that ribbon synapses in hair cells of the utricular maculae exhibit statistically significant changes in number, kind, and distribution when rats are exposed to space flight (Ross, 1993, 1994, 2000). Synaptic plasticity was most evident when all type II hair cells of these maculae were considered and was confined to type II cells in just the complete hair cells. Analysis of the co-variance of the multiple variables (number, rod or sphere, pairs and groups) by the MANOVA feature of SuperANOVA software demonstrated further that day and weightlessness both had statistically significant effects on type II hair cells (Ross, 2000). These results were obtained from the posterior portion of the utricular macula and did not include the striola. For Neurolab, the striola and parastriolar area internal to the striola (pars interna) were studied. The Neurolab striolar data in general support the findings in hair cells of the utricular maculae, but effects in the saccular maculae differed. In saccular maculae, ribbon synapses in type I cells fluctuated while synapses in type II hair cells remained relatively stable throughout the flight and up to postflight day 2. Only differences between flight days 2 and 14 in ribbon synapses of type I hair cells were statistically significant. It cannot be argued that the lack of significant differences in type II hair cells was due to inner ears utilized since utricular and saccular samples were matched for the same rats in two cases.