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
