The hypothalamo-hypophysial neuro secretory system of the zebra finch, Taeniopygia castanotis

General Résumé

The investigations described herein were made primarily to obtain information for an adequate description of the hypothalamic neurosecretory system of the Zebra Finch, Taeniopygia castanotis, but further, to obtain some indication of the morphologic variations associated with functional changes. Comparisons have been made with a similar previous study of the White-crowned Sparrow, Zonotrichia leucophrys gambelii.

• 1. The neurosecretory system of the Zebra Finch differs from that of Zonotrichia leucophrys gambelii as follows: a) The median division of the supraoptic nucleus is relatively poorly developed and lacks the very large cells that are so characteristic of Z. l. gambelii. The most rostral part of the preoptic recess of the Zebra Finch is neither as thin-walled nor as strongly extended rostrally as in Z. l. gambelii. b) In the perikarya of the neurosecretory ganglionic cells of the Zebra Finch the neurosecretory material is predominantly in the form of droplets and globules in contrast to the predominance of fine granules in Z. l. gambelii. c) The median eminence has a somewhat different structure than that of Z. l. gambelii. In silver preparations the looping fibers, characteristic of the posterior division of the median eminence and especially of the infundibular stem in Z. l. gambelii, are less prominent; the fine neural structure is somewhat reticular, consisting of fine endings whose relationships to the supraoptico-hypophysial tract and tubero-hypophysial tract must be investigated more closely. The neuroglia of the median eminence of the Zebra Finch show cytologic indications of activity. Selectively stainable ependymal and glial loops are lacking. d) The neurosecretory tract, which passes in a rostro-caudal direction through the zona interna, is especially conspicuous. Its repletion with neurosecretory material is in contrast to the neurosecretory content of the zona externa. This suggests that the zona externa, with its palisade layer, has a functional role that is independent of that of the fibers leading to the neurohypophysis. e) The neurohypophysis of the Zebra Finch is much more variable than that of Z. l. gambelii; there are sac-like, diverticular, and compact types.

• 2. Among wild Zebra Finches there are extensive differences in amount of neurosecretory material. The density of neurosecretory material in the palisade layer of the median eminence appears to have an inverse relationship to gonadal development.

• 3. The neurosecretory system is well differentiated in nestlings. The neurosecretory ganglionic cells contain extensive amounts of neurosecretory material. There is also some neurosecretory material in the median eminence whereas the neurohypophysis contains the smallest amounts.

• 4. The neurosecretory system of Zebra Finches in captivity with water ad libitum is relatively rich in neurosecretory material. In the neurosecretory cells the droplet form is most prevalent. When Zebra Finches are subjected to restricted water intake by permitting 1. a single two-minute drink per day (approximately 5 ml intake per week) or 2. a single two-minute drink per week (0.5–1.0 ml intake per week) the neurosecretory system becomes more active with enlargement of the neurosecretory cells, their nuclei, and their nucleoli. In the first group the occurrence of neurosecretory droplets increases significantly. Large neurosecretory globules become common. In the second group fine granular neurosecretory material and paranuclear cap-like accumulations of granules appear. Herring bodies develop frequently in the infundibular stem and neural lobe. Water restriction does not appear to affect the amount of neurosecretory material in the palisade layer of the median eminence. When Zebra Finches are given solution of NaCl up to 0.5 M in concentration as the sole source of drinking fluid, there is a moderate activation of the system characterized by the appearance of fine granular neurosecretory material. Birds that are able to tolerate 0.7 or 0.8 M NaCl have extremely enlarged neurosecretory cells with conspicuous fine granular neurosecretory material although homogeneous globules of neurosecretory material continue to be present. Herring bodies appear. The neurohypophyses are not completely depleted.

• 5. Many Zebra Finches maintain normal body weight with 0.6 M NaCl as the only drinking fluid. With 0.5 M the daily volume intake is of the order of 1 to 2.5 times body weight. Some individuals survive in apparently good health with 0.7–0.8 M although fluid intake is drastically reduced and body weight decreases somewhat. NaCl intake as high as 70 mg per gram body weight per day may occur in birds drinking hypertonic NaCl solutions. The ability of the Zebra Finch to tolerate high concentrations of NaCl in drinking water exceeds that of other passerine species studied thus far. Similarly the ability to survive in cages in a dry, hot environment with a water intake of ca. 1 ml per week is remarkable for a small bird.

• 6. Increasing the duration of the daily photoperiod from 9 to 18 hours neither depletes the neurosecretory content of the median eminence nor causes gonadal development. This is consistent with field studies that indicate that the reproductive activities of this species are not timed photoperiodically.