Ascorbic acid concentration in the lateral hypothalamus is related to plasma osmolality

Microdialysis was used to measure extracellular ascorbic and uric acid concentrations in the lateral hypothalamus of water-restricted rats as they drank distilled water or 1.5% NaCl. Other water-restricted rats, not implanted with microdialysis probes, were decapitated 2 h after beginning to drink these fluids. Rats were inverted and their blood was collected for measurements of plasma osmolality and percent hematocrit. Results showed that drinking distilled water produced a significant increase in the ascorbic acid concentration but not in the uric acid concentration. Drinking 1.5% NaCl produced a significant decrease in the uric acid concentration but not in the ascorbic acid concentration. Drinking distilled water decreased mean osmolality from 306.0 to 291.5 mOsm/kg, whereas drinking 1.5% NaCl maintained mean osmolality at water-restricted levels. These results indicate that the extracellular fluid concentration of ascorbic acid in the lateral hypothalamus rises in response to a fall in plasma osmolality.