Hypothalamic neurosecretion and phosphatase activity in relation to the photoperiodic control of the testicular cycle of Zonotrichia leucophrys gambelii

Abstract

Our experiments and our observations on birds taken under natural conditions through the course of the year indicate that neurosecretory material accumulates in the neurosecretory cells of the supraoptic nucleus and in the axons of the median eminence during periods of short day lengths and that the neurosecretory cells become relatively inactive during such periods. When photosensitive birds are subjected to long daily photoperiods, the neurosecretory cells become much more active and the tendency to accumulate neurosecretory material in these cells and in the median eminence is sharply reduced. During the refractory period extensive accumulation occurs, both in the neurosecretory cells and in the median eminence even in birds subjected to long daily photoperiods. These observations lead us to the following working hypothesis: It is proposed that the neurosecretory cell produces at all times neurosecretory material but never in excess of a steady-state storage equilibrium and that, in photosensitive birds, long daily photoperiods, through unknown neural mechanisms, cause an alteration and removal of this material via the terminal parts of the axons in the stratum glandulare. The neurohumoral substance thus released passes to the adenohypophysis via the hypophysial portal system. The rate of removal during long daily photoperiods is too great to allow the cells to synthesize neurosecretory material sufficiently rapidly to permit accumulation of equilibrium amounts of aldehydefuchsin positive granules characteristic of short-day and refractory birds. The accumulation during the refractory period is presumably the result of a failure of the mechanism by which long daily photoperiods cause the alteration and removal of neurosecretory material. This hypothesis is also consistent with our observations of the neurosecretory system through its “daily” changes in activity, both in hemeral and ahemeral cycles.