EARLY PRENATAL CRITICAL PERIOD FOR CHORDA TYMPANI NERVE-TERMINAL FIELD DEVELOPMENT

In order to determine whether the developing central gustatory system responds to dietary manipulation during restricted developmental perio ds, terminal fields of the chorda tympani nerve within the nucleus of the solitary tract were investigated via anterograde transport of hors eradish peroxidase in control rats and in rats in which a low sodium d iet was systematically fed during specific periods of development. Rat s fed a low sodium diet (0.03% NaCl) from embryonic day 3 (E3) to day E12 and then fed a sodium replete diet to at least 60 days postnatal e xhibited enlarged and irregularly shaped chorda tympani terminal field s. Specifically, the dorsal zone of the field was the smallest in cont rols, whereas it was the largest in restricted rats, occupying more te rritory within the nucleus. This alteration in the terminal field was apparent in all groups of rats fed the low-NaCl diet beginning at E3, and continuing beyond E12. In contrast, no effects of the dietary mani pulation on the developing chorda tympani field was evident when it oc curred from E3 to day E9, from EO to day E9 or when it occurred at adu lthood only. Therefore, only 9 days of maternal exposure to a sodium-r estricted diet is required for a permanent expansion of the chorda tym pani terminal field in the offspring. Moreover, a brief period from E9 to E12 must be included within the g-day dietary restriction to yield the expanded field. Since this period is before taste receptors appea r on the tongue, it is likely that nonactivity-dependent factors deter mine the formation of the chorda tympani terminal field during later d evelopment. (C) 1997 Wiley-Liss, Inc.