Postnatal development of the tyrosine hydroxylase-containing cell population within the rat locus coeruleus: topological organization andphenotypic plasticity.

The cellular phenotypic characteristics of tyrosine hydroxylase (TH) expression 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 selected at intervals of 80 microns along the caudorostral extent of the LC and processed for TH immunohistochemistry. At each anatomical level we (1) reconstructed the mean space of the LC delineated by the TH positive cell bodies, (2) enumerated the mean number of these cell bodies, and (3) determined the mean volume circumscribed by these cell bodies and their density. The topological study revealed a steady remodeling of the structure until the third week, with a progressive reducing of a ventral cellular group in the anterior LC, which was no longer observable at PND21, concomitant to the stretch of the structure toward its caudal limit. We have noted invariant and variant cellular phenotypic characteristics of TH expression. At any stage, the LC could be separated 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 positive perikarya as compared to PND42. Conjoint analysis of the topological reconstruction and the density of TH positive cells suggested there were three distinct and precisely localized subsets of "quiescent" neurons. TH gene expression in these cells would have lowered between PND14 and PND21 inside two subsets and between PND21 and PND30 inside the last one. So topologically defined populations of cells could be involved 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 pharmacological treatments.