Regional specificity of the long-term regulation of tyrosine hydroxylase in some catecholaminergic rat brainstem areas. I. Influence of long-term hypoxia.

The present study was carried out to investigate the influence of long-term hypoxia on tyrosine hydroxylase (TH) protein quantity in some catecholaminergic rat brainstem areas such as the dorsomedial medulla (DMM), the ventrolateral medulla (VLM) and the locus coeruleus (LC). TH protein quantity was also measured in a dopaminergic structure, the substantia nigra (SN). Male Sprague-Dawley rats were exposed to normobaric hypoxia (10% O2/90% N2) for 3, 7, 14 or 22 days. Controls were kept in normoxia for the same period. This study demonstrates that: (1) 3 days of hypoxia produced a 50% and a 26% increase in the quantity of TH protein in the rostral and caudal LC, respectively; (2) 14 days of hypoxia produced a 44% increase of TH protein content exclusively in the caudal part of DMM and a 31% increase in the VLM area; and (3) the stimulus failed to alter the TH protein quantity in the SN. After 14 and 22 days of hypoxia respectively, the TH protein content in the LC and DMM returned to the level of controls. To determine whether the increase in TH protein quantity could be related to a change in norepinephrine (NE) content, the rate constant of disappearance (k) of NE was measured in the catecholaminergic areas of intact or chemodenervated rats submitted to long-term hypoxia. Our results show that hypoxia causes an increase of TH protein quantity within the subpopulations of catecholaminergic areas additionally with an elevation in the NE content. These data suggest a selective response of the TH regulation to long-term hypoxia within the caudal DMM catecholaminergic area which receives chemosensory inputs.