A brief period of anoxia in vivo causes a transitory decrease in the size of the adenylate pool in the rat brain. This is probably caused by feedback inhibition by AMP of adenine nucleotide synthesis. Exposing rats to various degrees of hypoxia suggests that the sensitivity of the brain to lack of O(2) results from the brain's limited ability to maintain an adequate energy charge in unfavourable circumstances.

No abstract

1. The cytochrome-oxidase activity in eleven structures of the rabbit brain has been investigated. 2. Kinetic data suggest that the same enzyme occurs in all brain structures but that the activity on a tissue-weight basis differs from structure to structure, being in general higher in the phylogenetically newer structures. 3. When the kinetic data are related to cell density (as measured by DNA concentration) all the neuron-containing structures of the cerebrum and brain stem show the same activity but the corpus callosum and the cerebellum show much less. 4. A special study was made of the distribution of activity and cell density within the diencephalon. 5. The results suggest that all neurons in the cerebrum and brain stem have a similar cytochrome-oxidase activity, which is about 80 times that of glia, and that the cerebellum is relatively richer in glia or that it contains neurons with a much lower cytochrome-oxidase activity, or both.

THE study made by Scholz (1950Scholz ( , 1953 of the distribution of sodium between the aqueous humour and the plasma of the guinea-pig brought to light two interesting results. First, it showed that the aqueous-plasma distribution of sodium in the guinea-pig was markedly different from that found in other species which had been investigated. The value of the steady-state ratio of sodium in the guinea-pig aqueous to that in the plasma was found to be 1 024±0 0055 (23) (Scholz, 1950), compared with 0-936 in the horse; 0-945 in the steer; 0 943 sheep; 0 935 pig; 0 940 goat; 0 943 rabbit; 0 965 dog; and 0 960 cat (Davson, 1953). Secondly, Scholz (1953) showed that in the scorbutic guinea-pig the steady-state ratio fell to 0-948±0-01 (21), a value very close to that for plasma and its dialysate; this suggested to Scholz " that ascorbic acid is essential for the secretion of sodium into the aqueous from the plasma". This result was therefore considered to lend support to the hypothesis of Friedenwald and others (1943) regarding the part played by ascorbic acid in the formation of the aqueous humour. Since this finding has such a direct bearing on one of the central problems of aqueous humour dynamics, namely the factors responsible for the formation of the aqueous, it was decided to submit it to further test.Let us assume that scurvy is without effect on the plasma sodium concentration (an assumption which is justified below), and that throughout Scholz's experiments it remained constant at about 140 mEq./kg. H20, then the ratio of 1 024 for the normal animal indicates that the aqueous sodium concentration was 143 36 mEq./kg. H20. In the scorbutic animal, with a ratio of 0-948, the aqueous sodium concentration must have been about 132-72 mEq./kg. H2O. These figures show that scurvy is accompanied by a decrease in the aqueous sodium concentration of about 10-64 mEq./kg. H2O, and further that to maintain electrical neutrality there must have been either an entrance of an equivalent amount of other cations, or an equivalent decrease of anion, or compensatory changes in both.It is extremely unlikely on a priori grounds that such a large loss of sodium could be made good by other cations without profound alterations in the character of the blood-aqueous barrier and there are no indications at the moment that such alterations have occurred. It seems much more reasonable to suppose that any necessary adjustment would manifest itself in the

1. The respiratory rates in vitro of ten structures of the rabbit brain were measured in the presence of a normal (5mm) and an elevated (50mm) concentration of K(+) ions. The results were expressed on a dry-weight basis and in terms of cell density. 2. On a dry-weight basis, with a normal concentration of K(+), there was a steady decrease in respiratory rate on passing from the cerebral cortex through successively lower centres to the brain-stem, i.e. respiration was negatively correlated with phylogenetic age. 3. When the resting respiratory rates were expressed in terms of cell density there was no correlation with phylogenetic age. The neuron-containing structures of the cerebrum and brain-stem had identical respiratory rates with the exception of the cerebral cortex which had a higher rate. 4. The K(+)-stimulated respiratory rates/cell also showed a negative correlation with phylogenetic age; the higher centres responded with a greater percentage increase than the lower. 5. The results from the cerebellar cortex were anomalous. 6. There were variations of respiratory rate within the diencephalon. The thalamus had a relatively high respiratory rate, the anterior and ventral regions relatively low and the hypothalamus intermediate. 7. The oxygen consumption of white matter was not increased by a high external concentration of K(+).