POSTNATAL-DEVELOPMENT OF THE TYROSINE HYDROXYLASE-CONTAINING CELL-POPULATION WITHIN THE RAT LOCUS-COERULEUS - TOPOLOGICAL ORGANIZATION AND PHENOTYPIC PLASTICITY

The cellular phenotypic characteristics of tyrosine hydroxylase (TH) e xpression have been studied within the rat locus coeruleus (LC) during postnatal development at six different stages: postnatal day 4 (PND4) , PND10, PND14, PND21, PND30, and PND42. Coronal brain sections were s elected at intervals of 80 mu m along the caudorostral extent of the L C and processed for TH immunohistochemistry. At each anatomical level we (1) reconstructed the mean space of the LC delineated by the TH pos itive cell bodies, (2) enumerated the mean number of these cell bodies , and (3) determined the mean volume circumscribed by these cell bodie s and their density. The topological study revealed a steady remodelin g of the structure until the third week, with a progressive reducing o f a ventral cellular group in the anterior LC, which was no longer obs ervable at PND21, concomitant to the stretch of the structure toward i ts caudal limit. We have noted invariant and variant cellular phenotyp ic characteristics of TH expression. At any stage, the LC could be sep arated into a posterior and an anterior subregion and its total volume remained quite stable during the studied period. At PND14 and PND21, we observed a transient 33% increase in the total number of TH positiv e perikarya as compared to PND42. Conjoint analysis of the topological reconstruction and the density of TH positive cells suggested there w ere three distinct and precisely localized subsets of ''quiescent'' ne urons. TH gene expression in these cells would have lowered between PN D14 and PND21 inside two subsets and between PND21 and PND30 inside th e last one. So topologically defined populations of cells could be inv olved in specific functions. If they have not definitively lost their TH expression capacity, they could contribute to increasing TH levels in LC occurring in response to physiological perturbations or pharmaco logical treatments.