Hypothalamic neurosecretion in the refractory and post-refractory periods and its relationship to the rate of photoperiodically induced testicular growth in Zonotrichia leucophrys gambelii.

1. In late summer and autum many temperate-zone birds pass through a refractory period during which testicular growth cannot be induced experimentally with long daily photoperiods. This phenomenon is caused naturally by the long days of spring and early summer. It involves a decrease in release of gonadotropins by the adenohypophysis. Since previous investigations indicate that the release of gonadotropin is mediated by the hypothalamic neurosecretory system, a study of neurosecretory activity during refractoriness is basic for the understanding of this phenomenon. 2. Preliminary morphological studies on refractory birds revealed lateral divisions of neurosecretory cells lying between the supraoptic and paraventricular nuclei regions previously described (Okscheet al. 1959). Neurosecretory axons from these divisions join the tracts which pass to the median eminence. 3. Birds maintained in outdoor aviaries and killed from May through September showed a pronounced increase of neurosecretory material in the median eminence in late June, when testes regressed. This was accompanied by increases in neurosecretory material in the tracts and lateral division of neurosecretory cells described above, with reduction in sizes of cells and cell nuclei. The amount of neurosecretory material in the supraoptic nucleus was variable, but showed some increase, and the cells, again, decreased strikingly in size. 4. A series of birds was killed in the field from September through December. In the first part of this period there were considerable quantities of neurosecretory material in the median eminence. A pronounced reduction occurred in mid-October shortly before the earliest date at which positive testicular responses to long daily photoperiods have been obtained experimentally. The pathways from the lateral divisions of neurosecretory cells contained material initially (September) and at the end of the period (December) but were empty in late October and early November. The lateral divisions of neurosecretory cells contained minimal quantities of neurosecretory material at this time; the cells and nuclei appeared larger. Cells of the nucleus supraopticus contained variable amounts of neurosecretory material, but cells and nuclei reached a maximum in size, and presumably in activity, in late October and early November. 5. In studies of photoperiodic testicular response, birds were tested with 15-hour and 20-hour photoperiods at intervals from September through May. A positive response to long daily photoperiods first appeared in late October and early November. In adult birds the rate of response to 15-hour photoperiods was initially low, but increased significantly over November–December. The response of first-year birds (i.e. birds hatched during the previous summer) to 15-hour photoperiods proved difficult to interpret. On 20-hour photoperiods, however, the testicular growth rate was initially low, and increased significantly over November, December and January to an apparent maximum for this day length. 6. It is suggested that during refractoriness a factor required for the synthesis and/or release of gonadotropins cannot be transferred from the axons of the median eminence via the portal capillaries to the adenohypophysis in sufficient quantities to cause a testicular response. The inhibition at the level of the median eminence expresses itself histologically as an accumulation of neurosecretory material in the hypothalamus and a reduction in the amount of cytoplasm and the size of cell nuclei in neurosecretory regions. This inhibition sinks below an effective level for complete refractoriness in middle and late October, while days are still long enough to cause some activity of the neurosecretory system. This minor resumption of activity in autumn is probably associated with the autumnal gonadal recrudescence observed in some species. The reduced rates of testicular response obtained experimentally in November–December are attributed to persistent low-level activity of the inhibitory process. The reduced activity of the neurosecretory system after late autumn in wild birds is apparently attributable to the essentially nonstimulating effect of the shorter day lengths of this season.