Quantitative study of tyrosine hydroxylase protein levels within the somatic area of the rat locus coeruleus during postnatal development

Abstract

To date only global dosages of tyrosine hydroxylase (TH) protein have been realized in the locus coeruleus (LC) without discriminating the enzyme contained in the cell body area from the one in the surrounding neuropil. The preceding immunohistochemical study (Bezin et al., 1994) revealed a dramatic plasticity of the cellular expression of TH in the LC during the postnatal development of the rat. It was therefore necessary to develop a quantitative biochemical approach, strengthened by a great anatomical resolution, to follow the developmental evolution of TH levels exactly in the space containing the coerulean TH-immunoreactive perikarya. In the present work two biochemical parameters necessary for precisely defining the phenotypic characterization of TH expression within the rat LC have been established during the postnatal development at six different stages: postnatal day 4 (PND4), PND10, PND14, PND21, PND30, and PND42. TH tissue concentration and content were precisely determined along the caudorostral extent of the LC within the previously (Bezin et al., 1994) defined spaces delimited by the TH-containing perikarya. TH tissue concentration remained quite stable during the postnatal development. TH quantity exhibited few age-related variations with a transient peak at PND10 and followed the same evolution as the volume containing the TH-expressing perikarya. The mean cell contribution to the total quantity of TH measured in the whole LC showed important age-related fluctuations with a dramatic peak at PND10 followed by a drastic decrease until PND21. Such modifications of the steady-state levels of TH in the perikarya of coerulean neurons could be related to temporal requirements of this enzyme in their other cellular compartments such as dendrites or terminals, and/or alterations of the synthesis of the protein.