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.

RU24722 induces spatially organized phenotypic plasticity in the locus coeruleus.

The plasticity of tyrosine hydroxylase (TH) expression in rat locus coeruleus (LC) was evaluated after RU24722 TH induction using, as a new parameter of characterization, the quantitative topology of LC defined by TH-positive cells. This new phenotype was spatially organized into cell subpopulations in the medial LC, dorsal and ventro-lateral to the initial perikaryal space. Reserpine and parachlorophenylalanine, which elicited a similar increase in the TH content, failed to induce a significant change in the number of TH-expressing cells. Activation of TH expression is not sufficient to reveal the existence of such a plasticity and some original but still unknown mechanism(s) of control of TH expression is affected by RU24722.

Somatotopic organization of tyrosine hydroxylase expression in the rat locus coeruleus: long term effect of RU24722.

Tyrosine hydroxylase (TH) tissue concentration was determined by immunostaining of tissue sections directly transferred onto nitrocellulose membranes in the restricted region of the noradrenergic perikarya of the locus coeruleus (LC) along its postero-anterior axis. TH containing cells were systematically counted on adjacent post fixed sections stained by immunohistochemistry. The absolute quantity of TH was estimated in each section and was found to be linearly related to the number of TH immuno-positive cells found in the adjacent section. The ratio between these two parameters was thus used as an index of the cellular concentration of TH in noradrenergic cells. In the LC of control rats, the TH cellular concentration was lower (-39%) in the anterior than in the posterior half of the structure. Three days after an injection of 20 mg/kg of RU24722, an eburnamine derivative known to increase the quantity of TH in the LC, increases in quantities of TH were found in both portions of the LC. Moreover in the posterior LC the increase in the amount of TH resulted from a significant increase in the number of TH-immunopositive cells. In the anterior part, however, it was primarily the result of a significant increase in TH cellular concentration. Throughout the LC there was an increase in the cellular concentration of TH which was inversely proportional to the concentrations found in control animals. TH mRNA content was measured by a quantitative in situ hybridization in sections of both the posterior and anterior LC one day after a single injection of RU24722 at the same dose. The quantity of TH mRNA was significantly increased in both parts. The number of TH mRNA-expressing neurons also increased, especially in the anterior LC. Thus the effects at the level of TH protein and TH mRNA were strikingly parallel though increase in TH protein occurred later than the increase in the TH mRNA. These results suggest that in the rat LC: (1) there is a significant population of 'sleeping cells' in which TH expression is either inactivated or, at a low level of activation; (2) TH cellular concentration could exert a retrocontrol on its own expression in cells of the LC that contained TH and (3) TH expression appears to be regulated by different selective mechanisms in these two different subpopulations of noradrenergic cells within the LC.