The Effect of Oxygen at High Pressure on the Histology of the Central Nervous System and Sympathetic and Endocrine Cells

Rats were exposed to oxygen at 6 atm daily for about 8 weeks in one series and for 4 weeks in a second series. The exposure times were so adjusted as to give a low incidence of convulsions and lung symptoms. The treated animals did not show any evidence of lung damage at autopsy. Slight motor symptoms of a paralytic nature developed in some of the rats of the first series after about 2 weeks. The symptoms regressed during the continued treatment and had disappeared after further 5 weeks. Histological investigation of the nervous system after 7 to 9 weeks showed no degeneration. The rats of the second series exhibited no neurogenic symptoms. The exposures to high oxygen tension were found to have an action on the sympathetic nervous system resulting in an increased size of the nucleolus of neurons of the stellate ganglion and the adrenal medulla. In both series there was a decrease in the weight of the thymus, but the reduction was significant in the second series only. In agreement with earlier investigators a weight increase of the adrenals was found (P < 0.05 for the first series), and histologically, a thickening of the zonal fasciculata. A weight increase of the same statistical significance was obtained also for the thyroid. The histology of the pituitary was unchanged. Determinations of ribonucleic acid and size of nucleolus in the supraoptic nucleus did not reveal any changes. The investigation, consequently, could not provide any evidence for a direct degenerative action of OHP on the central nervous system. The observed changes are characteristic for a stress reaction. However, the weight increase of the thyroid is not typical for stress and may be an expression of a specific OHP effect.