Early and late morphological effects of experimental HPNS--animal model of psychosis?

The high pressure neurological syndrome (HPNS), a neurological condition during elevated pressure especially in deep diving, has been simulated with experimental animals. Rats were subjected to 61 bars with slow pressure increase and one or two hours constant high pressure; subsequently the pressure was released to sea level within 20 seconds--leading to immediate oxygen depletion and death of animals--or with slow decompression rates allowing survival. In all animals, brains and partly other organs were investigated morphologically. In animals sacrificed immediately, subtle changes in different brain regions were found: symmetrical occurrence of dark neurons in the hippocampus formation, cortex and brain stem, reduced expression of tyrosin hydroxylase in the substantia nigra and enhanced expression of Bax protein in some of these regions. The dark neurons were only observed after aldehyde fixation, otherwise the brains were unaltered despite ultrarapid decrease of highly elevated pressure. In animals that were allowed to survive for different time periods, some of these subtle changes were equally noted by light and electron microscopy. Furthermore, the ventricles were enlarged, the astrocytic reaction in the hippocampus increased and some signs of the destruction of the adrenal gland were visible. We conclude, that HPNS leads to minimal changes within the nervous system. The behaviour of animals during pressure was slightly altered, the weights after the experiments reduced, but no lasting sequelae were noted. Since both in human and experimental deep diving conditions signs of psychosis were reported, this HPNS model must be considered as a tentative animal model of human psychosis.