Altered daily rhythms of brain and pituitary indolamines and neuropeptides in long-term hypoxic rats.

To determine whether sustained hypoxia alters daily rhythms in brain and pituitary neurotransmitters, the daily variations in vasoactive intestinal peptide-like immunoreactivity (VIP-LI), neuropeptide Y-like immunoreactivity (NPY-LI), serotonin (5-HT), and 5-hydroxyindole-3-acetic acid (5-HIAA) content were determined in discrete brain regions, pineal gland and anterior pituitary of hypoxic (10% O(2); 14 days) and normoxic rats. Hypoxia suppressed daily variations in VIP-LI in the suprachiasmatic nuclei (SCN) and the anterior pituitary, enhanced the daily rhythmicity in serotonergic elements of the caudal part of the dorsomedial medulla oblongata (DMMc), and even induced daily variations in NPY-LI in the DMMc as well as in the ventrolateral medulla oblongata. In addition, punctual alterations in the rhythmicity of 5-HT and 5-HIAA in the pineal gland and of plasma corticosterone were observed in hypoxic rats. Thus results of this study indicate that a permanent nonphotic stimulus, such as sustained hypoxia, may affect the functioning of the internal clock located in the SCN and may alter the daily rhythmicity in neurotransmitter content of some brain nuclei and the pituitary gland.