DEVELOPMENT OF TRIGEMINAL NUCLEUS PRINCIPALIS IN THE RAT - EFFECTS OFTARGET REMOVAL AT BIRTH

Little is known about how neurons develop in the trigeminal nucleus pr incipalis (PrV) despite their acknowledged role in establishing whiske r-related patterns in the thalamus and cortex. Golgi-impregnated PrV c ells were studied in newborn, 4-day-old and adult rats. Adult neurons typically had short dendrites that were confined to a hemisphere aroun d the soma. In contrast, at birth PrV neurons had radial trees and mor e primary dendrites than did adults, but adult-like numbers of dendrit ic spines. By day 4, most neurons had eccentric dendritic trees and th e numbers of primary dendrites per neuron were adult-like, yet spines were more prevalent than in adults and newborns. Thus, it appears that there is a pruning of the dendritic tree during the first postnatal w eek. To assess the role of retrograde signals from the thalamus on PrV development, the right thalamus was destroyed at birth. By postnatal day 6, the number of neurons in the left PrV was 59% of that in the ri ght PrV, PrV transverse area was reduced by 21%, cell density was redu ced by 48%, and somatic diameter was increased by 36%, relative to the intact right PrV. By contrast, in the left V subnucleus interpolaris, which has only a weak thalamic projection, these measures were unaffe cted. Thus, neonatal thalamic lesions selectively depopulated the PrV. The morphology of PrV neurons was affected by the thalamic lesions: e .g. the total dendritic length, the number of dendritic branch points and the total number of spines were increased. The number of primary d endrites and the tree's eccentricity, area, and volume of influence we re unaffected by the lesion. The structure of neurons in subnucleus in terpolaris was unaffected by the lesion. Thus, normal afferent pattern ing is insufficient for normal development of PrV cells. Interactions among dendrites and retrograde signals from a target are also importan t.