Seasonal and lactational effects on the prolactin response to a dopamine antagonist and TRH in the Bennett's wallaby (Macropus rufogriseus rufogriseus).

In the Bennett's wallaby prolactin is thought to maintain lactational and seasonal quiescence and is essential for early lactation. However, plasma prolactin concentrations determined in daily or weekly samples at these times are unchanged. In the present study, female Bennett's wallabies were blood sampled at 2-hr intervals over a 24-hr period during seasonal quiescence on either natural or artificial photoperiods to determine whether a diurnal rise of prolactin occurs at this time. Prolactin concentrations did not exhibit a diurnal change. Further experiments were aimed at determining whether there was an increase in the prolactin response to a dopamine antagonist or TRH during the transition to seasonal quiescence. Nonlactating and lactating female Bennett's wallabies were treated with saline, 0.5, 1, and 2 mg of the dopamine antagonist domperidone and 100 micrograms TRH in October, December, February, and April. In both groups there was a significant elevation in plasma prolactin concentration in response to domperidone with an increasing response at each successive month. In early (October and December) and peak (April) lactation the prolactin response was greater in lactating animals. There was no significant prolactin response to TRH in lactating animals. In nonlactating wallabies, the prolactin response to TRH was increased in February. At peak lactation (April), the response to 1 mg domperidone was blocked when the dose was administered 2 hr after temporary removal of pouch young (RPY). With either larger doses (20 mg) or a 1-mg dose injected 8 hr after RPY, a significant prolactin response was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoperiodic induction in quail as a function of the period of the light-dark cycle: implications for models of time measurement.

The earliest detectable event in the photoperiodic response of quail is a rise in luteinizing hormone (LH) secretion beginning at about hour 20 on the first long day. The timing of this rise was measured in castrated quail after entrainment to short daylengths which cause significant phase angle differences in the circadian system: (1) LD 2:22 and LD 10:14, and (2) LD 3:21 (T = 24 hr) and LD 3:24 (T = 27 hr). The quail were then exposed to 24 hr of light (by delaying lights-off), and the time of the first LH rise was measured; it was similar in all schedules. Quail were also entrained to LD 3:21 or LD 3:24 and then given a single 6-hr nightbreak 6-12, 7-13, or 13-19 hr after dawn. The earlier pulse was marginally more inductive in the 27-hr cycle. Thus the entrainment characteristics of the photoinducible rhythm (phi i) in quail appear very different from those of the locomotor circadian rhythm, and raise doubts as to whether phi i is a primary circadian oscillator.

Photoperiodic control of the development of the LHRH neurosecretory system of European starlings (Sturnus vulgaris) during puberty and the onset of photorefractoriness.

The development of the reproductive system was studied in juvenile starlings during the acquisition of photosensitivity, the attainment of sexual maturation after photostimulation and the subsequent onset of photorefractoriness, using immunohistochemistry for LHRH and radioimmunoassay measurements of hypothalamic, pituitary and plasma hormone concentrations. The first stage of sexual development induced by exposure of photorefractory immature starlings to short days (8 h light:16 h darkness; 8L:16D) was characterized by a decrease in pituitary prolactin content within 1 week and an increase in hypothalamic LHRH content, in the size of the LHRH perikarya and in the intensity of immunostaining in the median eminence in 4-6 weeks. Sexual maturation occurring after exposure to long days (18L:6D) was associated with further increases in LHRH content and cell size, and increases in LH and prolactin concentrations. During testicular regression, LHRH perikarya were reduced in size and staining intensity but LHRH immunostaining in the median eminence and content in the hypothalamus remained high until gonadal regression was almost complete. Prolactin levels were maximal during testicular regression. These results suggest that gonadal regression is initiated by a reduction in LHRH synthesis and possibly, in addition, an external inhibitory influence on LHRH release. Hypothalamic LHRH content eventually declined and LHRH immunostaining in the median eminence was much reduced in fully photorefractory starlings maintained under long days.