Quantitative microchemical changes reflecting functional activity in supraoptic nerve cells of the rat
Watt, Robert M.
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1. Neurones and nuclei, isolated from the supraoptic regions of the rat's hypothalamus, have been examined by phase contrast and interference microscopy and by ultraviolet absorption microspectrography, in a quantitative study of their metabolic response to stimulation. 2. Vhen isolated from rats dehydrated for up to five days, fresh supraoptic nuclei were larger and more likely to contain marginated nucleoli. The dry mass of fixed nuclei did not change. The mean dry mass and nucleic acid content of supra¬ optic nucleoli and cell bodies increased significantly with progressive osmotic stimulation. Nucleolar changes preceded those in the cell body by 24 to 48 hours. 3. The significance of these changes is discussed in relation to published data concerning neuronal responses to injury and prolonged stimulation. 4. The distribution of dry mass among supraoptic nucleol may be bimodal after moderate osmotic stress, but the presence of two populations of neurones within the supraoptic nucleus could not be confirmed. 5. The increases in dry mass and nucleic acid content of supraoptic nucleoli and cell bodies were both greater and faster during five days dehydration at a raised environmental temperature (24°C). Thus the metabolic response can be graded according to the functional load placed on the neurone. 6. Neurohypophyses which have been partially depleted of their "stores" of neurohormone contained some factor which, when injected into the lateral cerebral ventricle of normal rats, resulted in an increase in the mean dry mass and nucleic acid content of supraoptic nucleoli. These nucleolar changes seemed independent of neurohormone secretion or serum osmolality in the recipient animals. 7. Extracts of normal ("non-depleted") neurohypophyses significantly inhibited the dry mass response of supraoptic nucleoli to osmotic stress. 8. "Hormone-depleted" posterior lobe extracts did not alter the mean dry mass or nucleic acid content of nucleoli from the posterior part of the arcuate nucleus. 9. Extracts of tissue from the parietal cortex had no demonstrable effect on supraoptic nucleoli. 10. The possibility is discussed that "active" substances derived from the neurohypophysis may serve to match synthetic activity to secretory losses in supraoptic neurones. 11. The mean dry mass of pituicytes increases rapidly during the first 48 hours of dehydration. This response is as fast as any supraoptic changes detected in this investigation.