Parvalbumin immunoreactivity in the reticular thalamic nucleus of developing rats.

Developmental changes in the distribution of parvalbumin immunoreactivity in the rat reticular thalamic nucleus from embryonic day 18 to adult were studied using immunohistochemistry. Neurons containing parvalbumin immunoreactivity in their cell somata were first detected at embryonic day 18. By embryonic day 20, many cells extended parvalbumin-positive fibers into the ventral thalamic nucleus. During development of the ventral thalamic nucleus, these fibers increased in number and thickness and ran in various directions. By postnatal day 16, these fibers became thinner and less well-stained. Neurons in the reticular thalamic nucleus extended their parvalbumin-positive axons into other nuclei of the thalamus, but in these other nuclei, parvalbumin-positive terminal and fibers developed primarily postnatally. In the reticular thalamic nucleus, the distribution of GABA-like immunoreactivity was very similar to that of parvalbumin at embryonic day 20 with cells extending fibers which contained immunoreactivity for GABA into the ventral thalamic nucleus. Thus parvalbumin immunoreactivity appears in the neurons of the reticular thalamic nucleus early in development of this nucleus, first in cell somata of neurons, then in their axons and dendrites. Apart from the ventral thalamic nucleus, the innervation by parvalbumin containing neurons in the reticular thalamic nucleus of the rest of the thalamus, showed a variable developmental pattern with different nuclei being innervated at different times, presumably reflecting the separate development of individual thalamic nuclei.