The hypothalamo-hypophysial neurosecretory system of the white-crowned sparrow, Zonotrichia leucophrys gambelii.

1. The North American fringillid finch, Zonotrichia leucophrys gambelii, has a well developed hypothalamic neurosecretory system. 2. Compared with the nucleus supraopticus, the nucleus paraventricularis is poorly developed and poor in neurosecretory material. 3. The median eminence can contain so much neurosecretory material that it may be regarded as a depot of such material second only to the neurohypophysis. 4. The nucleus supraopticus contains already in the nestling a considerable quantity of neurosecretory material; however, the neurosecretory content of the neurohypophysis is quite low. In the median eminence of the nestling there are only scattered neurosecretory granules. 5. In early summer the neurosecretory hypothalamic nuclei and the median eminence contain only very small amounts of neurosecretory material. However in autumn, when the birds are refractory to photoperiodic stimulation, there are very extensive accumulations of neurosecretory material. The ganglionic cells, in which the numerous neurosecretory granules are stored, appear to be relatively inactive in late spring. 6. Birds which had been held since late-autumn or winter on short daily photoperiods of 8 hours, also in spring and early summer, had large amounts of neurosecretory material in the cells of the hypothalamic nuclei and in the median eminence. 7. During June–August, with birds held since previous autumn on 8-hour days, the increase of the daily photoperiod from 8 hours to 20 hours caused, in 20 days, a marked increase in testicular weight and body weight. Simultaneously there was a decrease in the amount of neurosecretory material in the cells of the hypothalamic nuclei and especially in the median eminence whereas the quantity of neurosecretory material in the neurohypophysis appeared to be unaffected. The reduced quantity of neurosecretory material in the median eminence was particularly noticeable during the second half of the daily photoperiod. During the dark period in the 20-hour birds, as well as in the 8-hour birds and birds on natural photoperiod, there was a reaccumulation of neurosecretory material. 8. From December to February the response to increases in daily photoperiod are not as pronounced as in spring; the 8-hour and 20-hour birds have no observable differences in the neurosecretory picture. 9. Osmotic stress by addition of NaCl to the drinking water causes in the nucleus supraopticus and the nucleus paraventricularis a marked increase in nuclear volume and an increase in the formation of neurosecretory material. Simultaneously there is a depletion of the store of neurosecretory material in the neurohypophysis. 10. The increase in body weight, which follows the increase in daily photoperiod, is the result primarily of an extensive deposition of subcutaneous and visceral fat. This is the result of a hyperphagia and an alteration in intermediary metabolism in which glycogen storage in the liver is extensively reduced. 11. In early summer (June) the median eminence of the castrates of both sexes had substantially more neurosecretory material than those of the control birds. However, in autumn there were no apparent differences between castrates and normal birds, because of the increased neurosecretory material in the latter. 12. The causes and significance of these observations in respect to hypothalamic control of the endocrine system are discussed.