CHRONIC HYPOXIA CAUSES MORPHOLOGICAL ALTERATIONS IN ASTROGLIA IN THE PHRENIC NUCLEUS OF YOUNG-ADULT RATS

The present study was carried out to determine if chronic hypoxia in noninjured rats causes morphological alterations in the phrenic nucleus similar to those caused by spinal cord hemisection rostral to the nucleus. Normal rats were subjected to chronic hypoxia in an atmosphere chamber for 48 h. A Bioquant system was used to quantitatively analyze electron micrographs through the phrenic nuclei of chronically hypoxic and normal rats. The results indicated that chronic hypoxia caused astroglial processes to retract from between adjacent dendrites, leading to a significant increase in phrenic dendrodendritic membrane apposition. Unlike spinal cord injury, however, chronic hypoxia did not induce synaptogenesis in the phrenic nucleus. Specifically, the mean length of phrenic dendrodendritic appositions increased significantly from a normal value of 1.42 ± 0.09 to 1.82 ± 0.11 μm in hypoxic animals. Moreover, the mean number of dendrodendritic appositions in the normal phrenic nucleus (16.75 ± 1.11) increased markedly (24.00 ± 3.54) in the phrenic nucleus of hypoxic rats. There was no significant difference between the mean number of double (17.75 ± 1.03) or multiple (20.00 ± 1.47) synapses in normal rats as compared to the mean number of double (18.50 ± 3.10) or multiple (20.25 ± 2.84) synapses in the phrenic nucleus of hypoxic animals. From the present results, it is concluded that the rapid synaptogenesis in the phrenic nucleus following spinal cord hemisection is induced by the generalized effects of spinal cord injury and astroglial process retraction does not necessarily lead to synapse formation. The physiological significance of the astroglial movement, however, needs elucidation by additional experiments. © 1990.