Termination Of Photorefractoriness Research Articles

Photorefractoriness in birds—photoperiodic and non-photoperiodic control

Avian breeding seasons vary in length and in the degree of asymmetry with respect to the annual cycle in photoperiod to suit species-specific food resources. Asymmetry is the result of photorefractoriness. The degree of photorefractoriness, absolute or relative, is related to the length and asymmetry of the breeding season. Absolute photorefractoriness is associated with a marked decrease in hypothalamic cGnRH-I. However, during the initiation of absolute photorefractoriness there is a transient period during which the gonads regress in advance of the decrease in cGnRH-I, and this stage may be analogous to relative photorefractoriness. Photoinduced prolactin secretion has an inhibitory modulatory role during the initiation of absolute photorefractoriness, but is unlikely to be the only factor involved, while a possible role for avian gonadotrophin inhibitory hormone is not established. The first stage in the termination of photorefractoriness is the resumption of cGnRH-I synthesis. The major environmental cue driving gonadal maturation, and the transitions between the photosensitive state and photorefractoriness is the annual cycle in photoperiod. A range of non-photoperiodic cues may also play a role: social cues, climatic factors (temperature, rainfall, etc.), food availability and nutritional state. There is considerable evidence that these cues can influence gonadal maturation and the timing of egg-laying. There is some evidence that non-photoperiodic cues (certainly temperature and possibly social cues and food availability) can affect the timing of the onset of photorefractoriness, but no evidence that they can influence the time of the end of photorefractoriness.

Transient hypothyroidism reinitiates egg laying in turkey breeder hens: termination of photorefractoriness by propylthiouracil

A study was conducted to determine the effects of transient hypothyroidism induced by propylthiouracil (PTU) on termination of photorefractoriness and reinitiation of lay in turkey breeder hens. The PTU was given for 6- or 8-wk periods via the feed and at various doses to yearling hens that had been continuously exposed to long photoperiods [16 h light (L):8 h dark (D)] for at least 25 wk. There was a dose-dependent cessation of lay as well as deletion of thyroxine (T4) and triiodothyronine (T3) during the treatment period. Hens receiving 0.1% PTU or more had little or no circulating thyroid hormones after 2 wk of treatment. Furthermore, resumption of a normal rate and duration of egg laying occurred following withdrawal of the PTU, without any changes in photoperiod. However, this effect only occurred in those hens that had received PTU doses of 0.1% or more and only when the treatment had been given for greater than 6 wk. The resumption of normal levels of egg laying occurred in the absence of a typical preceding molt. Body weights, livability, and fertility and hatchability of eggs from these hens were similar to those of controls. Clearly, turkey hens can be effectively recycled by pharmacological manipulation of the thyroid gland and the results are supportive of thyroid hormone(s) involvement in maintaining photorefractoriness in turkey hens.

Short-day experience is not a prerequisite for the termination of photorefractoriness in the reproductive cycle of baya weaver,Ploceus philippinus

In most photoperiodic avian forms (irrespective of temperate or tropical distribution) including the baya weaver,Ploceus philippinus, seasonal reproduction comes to an end due to the development of a photoperiodically controlled photorefractory phase when birds cease to respond to the stimulatory effect of long days. In the present paper photoperiodic control of the termination of photorefractory phase has been examined by studying the effect of short-day exposure lasting 4–6 months on long-day response of birds. Results indicate that unlike in other photoperiodic birds short-day exposure of winter is not a prerequisite for the termination of photorefractory phase in the reproductive cycle of baya weaver. Artificial long days on the other hand hasten the termination of this phase. Refractory phase in baya weaver, therefore, unlike that in temperate forms, is a temporary state resulting most likely from a sequel of physiological events triggered by long days of spring/summer which temporarily mask the photostimulatory response. Spontaneous termination of photorefractoriness in birds of tropical habitats may have a selective value imparting to the reproductive cycle the necessary elasticity for adaptation to diverse ecological conditions.

Photorefractoriness and Its Termination in the Subtropical House Sparrow, Passer Domesticus: Involvement of Circadian Rhythm

Groups of photorefractory female subtropical house sparrows, Passer domesticus, when treated with 6 weeks of a short photocycle (8L:16D) showed significant ovarian growth on their return to a long photocycle (15L:9D). A 6-hr photophase coupled with scotophase of varying durations does not terminate the refractory period under photoperiod cycles of 12 (6L:6D), 36 (6L:30D) and 60 (6L:54D) hr but the refractory period is terminated by light-dark cycles of 24 (6L:18D), 48 (6L:42D) and 72 (6L:66D) hr. These results are consistent with the Bünning hypothesis of coincidence between endogenous photosensitive rhythmicity and environmental photoperiod timing that an endogenous circadian rhythm is involved in the maintenance and termination of photorefractoriness.

Sexual maturation in starlings raised on long or short days: changes in hypothalamic gonadotrophin-releasing hormone and plasma LH concentrations

Is the extended prepubertal period which occurs in most birds due to a juvenile photorefractoriness analogous to photorefractoriness in adults? Starlings were taken from nest boxes 4 days after hatching and hand-reared on long days or short days. In females on long days, hypothalamic gonadotrophin-releasing hormone (GnRH) content and plasma LH concentration remained low. In intact males on long days, plasma LH remained low and the testes remained small, and in castrated birds, plasma LH also remained low. In females reared on short days, hypothalamic GnRH content began to increase dramatically from 4 weeks of age and plasma LH increased from 6 weeks of age. In intact males on short days, there was a slight increase in plasma LH and testicular size, and in castrated males, plasma LH increased markedly from six weeks of age. All birds on long days moulted into adult plumage, whereas those on short days retained juvenile plumage. The changes in birds reared on short days were similar to those found when photorefractory adult birds are transferred to short days. This adds to the evidence that juvenile photorefractoriness is analogous to photorefractoriness in adults, and therefore that the seasonal termination of photorefractoriness is an annual puberty. However, the responses to short days occur later in birds raised on short days than in older birds transferred to short days, which suggests either that short day-induced changes occur more slowly in young birds, or that birds only respond to short days after a certain age.

Photoperiodic time measurement during the termination of photorefractoriness in the starling ( Sturnus vulgaris L.)

Groups of photorefractory male European starlings have been kept under Nanda-Hamner LD schedules ranging from LD 6:6 to LD 6:54 for about 6 weeks. Subsequently, when transferred to LD 13:11, all birds of groups LD 6:18 and LD 6:42 went through a complete cycle of testicular growth and regression. Furthermore, they carried out a postnuptial molt, indicating that they have been able to terminate photorefractoriness under these LD schedules. In contrast, no positive responses could be detected in groups LD 6:6, LD 6:30, LD 6:36, and LD 6:54. Groups LD 6:12, LD 6:24, and LD 6:48 were intermediate in their reactions. Only part of these birds showed gonadal recrudescence when tested for photosensitivity. Changes in plasma testosterone levels were closely related to variations in testicular diameter. These results clearly suggest a strong circadian component in photoperiodic time measurement during the termination of photorefractoriness in the starling. Positive or negative responses were completely in line with predictions derived from the model of external coincidence. A comparison with the results of a similar experiment previously done with photosensitive birds failed to detect basic differences in the interpretation of the various LD cycles. These findings suggest that the same photoperiodic time measurement system is used at both phases of the starling's annual cycle.

Endocrine changes associated with the termination of photorefractoriness by short daylengths and thyroidectomy in starlings (Sturnus vulgaris).

When starlings (Sturnus vulgaris) are transferred from short to long days, hypothalamic content of gonadotrophin-releasing hormone (GnRH) and pituitary gonadotrophin content eventually decrease, as birds become photorefractory, to values lower than they were on short days. This implies that both should increase as photorefractoriness is terminated some time after transfer from long to short days. Further, since thyroidectomy causes termination of photorefractoriness in birds held on long days, this should also result in an increase in hypothalamic GnRH and pituitary gonadotrophin contents. This study tests these hypotheses. Male starlings were transferred from short to long days for 10 weeks, by which time they should have become photorefractory. One group of birds was then killed; blood was collected, hypothalami and pituitaries were excised, and the stage of moult and testicular weights recorded. The remaining birds were then kept on long days, transferred to short days or thyroidectomized and kept on long days. Groups of birds in each treatment group were killed and sampled 2, 8 and 14 weeks later. Hypothalamic content of GnRH, and pituitary and plasma FSH and prolactin concentrations, were measured by radioimmunoassay. In birds sampled after 10 weeks of long days, hypothalamic content of GnRH was quite low, pituitary and plasma levels of FSH were very low, testes were small and pituitary and plasma prolactin levels were high. In intact birds kept on long days, hypothalamic GnRH content decreased further and remained low. Pituitary and plasma FSH levels remained low, testes remained small and pituitary and plasma prolactin levels decreased slowly.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian periodicity in termination of photorefractoriness in the yellow-throated sparrow

Photorefractory male yellow-throated sparrows (Gymnorhis xanthocollis) were subjected to six resonance light cycles, consisting of 6 h basic photophase coupled with scotophases of longer durations (in cycles of 12 (6L∶6D) h, 24 (6L∶18D) h, 36 (6L∶30D) h, 48 (6L∶42D) h, 60 (6L∶54D) h and 72 (6L∶66D) h). Termination of photorefractoriness was evident in cycles of 24, 48 and 72 h, but not in 12, 36 and 60 h. The results are consistent with the ‘external-coincidence’ model (Bünning hypothesis) and suggest that endogenous circadian rhythmicity is involved in the termination of photorefractoriness.

Thyroidectomy results in termination of photorefractoriness in starlings (Sturnus vulgaris) kept in long daylengths.

Four groups of 10 male starlings were transferred from short daylengths (8 h light/day) to long daylengths (18 h light/day), which caused the tests to develop rapidly to maximum size and then to decrease to minimal size as birds became photorefractory. Birds were surgically thyroidectomized at 8, 16 or 28 weeks. A fourth group was left intact. Testicular volume and plasma FSH and prolactin concentrations were measured. After 42 weeks all birds were castrated and plasma FSH was measured during the next 6 weeks. Testicular growth began in all thyroidectomized birds between 4 and 8 weeks after thyroidectomy. By 42 weeks, the testes of all thyroidectomized birds were large, whereas those of intact birds were still of minimal size. Plasma FSH concentrations remained low in all birds and plasma prolactin values, originally elevated by long daylengths, decreased at a similar rate in thyroidectomized and intact birds. After castration at 42 weeks, plasma FSH values increased rapidly in all thyroidectomized birds but remained low in non-thyroidectomized birds. The results demonstrate that thyroidectomy of photorefractory starlings does not induce immediate testicular growth but may initiate a process which eventually terminates photorefractoriness in a way similar to that caused by return to short daylengths.

Circadian Rhythmicity and the Termination of Photorefractoriness in the Black-Headed Bunting

Groups of photorefractory male Black-headed Buntings (Emberiza melanocephala), previously maintained on long daylengths (15L:9D), were exposed to photoperiods in which a 6-h light period was coupled with dark periods of 6, 18, 30, 42, or 54 h; a sixth control group was kept on 15L:9D. Cycles with lengths of 24 (6L:18D) and 48 h (6L:42D) terminated photorefractoriness, but cycles of 12 (6L:6D), 36 (6L:30D), and 60 h (6L:54D) did not; refractoriness persisted in the birds on 15L:9D. The results demonstrate that the termination and maintenance of the refractory period in Black-headed Buntings, a palearctic migratory passerine, are mediated by an endogenous circadian system of photoperiodic time measurement similar to that involved in the initiation and maintenance of gonadal growth, as is the case for several nearctic migratory species.

Role of the pineal and its hormone melatonin in the termination of photorefractoriness in golden hamsters.

Continuous exposure of male hamsters to short day lengths induces testicular regression. This is followed many weeks later by spontaneous recrudescence of the testes with reinitiation of spermatogenesis and function of the accessory sexual glands. Hamsters at this stage of the annual reproductive cycle are refractory to short photoperiods--even continuous darkness will not induce another bout of testicular regression. Animals refractory to short days are also refractory to the pineal hormone melatonin and a number of investigators attribute spontaneous recrudescence and photo and melatonin refractoriness to a developed target cell insensitivity to endogenous melatonin from the pineal. Refractoriness is terminated by exposure to long days for at least 11 weeks. The pineal gland is reported to be essential for this process. We report here the effects of pinealectomy, daily melatonin injections, and constant-release melatonin implants on the ability of male hamsters to recover from the refractory state. In the absence of the pineal gland, refractory male hamsters did not discriminate (count?) 15 weeks of long days to terminate refractoriness. Daily melatonin injections at 1900 h, but not at 1200 h (lights 0600-2000 h) during the 15 weeks of long-day exposure blocked the recovery from refractoriness. Constant-release melatonin implants abolished the animals ability to measure 12 and 15 weeks of long days to terminate refractoriness. These results demonstrate that general target tissue insensitivity to melatonin cannot account for the refractory state in hamsters, that a multiplicity of target tissues may exist for melatonin to account for its varied roles throughout the annual reproductive cycle in hamsters, and that the pineal gland is intimately involved in the animals' ability to measure a prescribed duration of long days to terminate refractoriness.

Termination of photorefractoriness in the chukar partridge (alectoris graeca chukar) by low light intensity.

Adult females were maintained under controlled conditions (22 degrees C, 60% relative humidity and 16 h light/day) within bioclimatic chambers throughout the test. For 6 weeks prior to photostimulation photorefractory birds were exposed to a light intensity of 1, 3, 5 or 7 lux. The intensity was then increased to 50 lux. Control birds were kept in 8L : 16D and changed to 16L : 8D, the light intensity remaining at 50 lux. The time to first egg and the rate of egg production were recorded. A decrease in light intensity alone was able to terminate photorefractoriness in chukar partridge and the threshold light intensity required for a complete termination of refractoriness was less than 1 lux. Light intensities of 1, 3 or 5 lux resulted in a partial termination of refractoriness but 7 lux was completely ineffective in terminating photorefractoriness. The termination of photorefractoriness was therefore not an all-or-nothing response.

Termination of photorefractoriness in golden hamsters—photoperiodic requirements

The photorefractory period of golden hamsters is characterized by the failure of short daylengths (less than 12.5 hours per 24) to induce gonadal regression. Photorefractoriness is terminated by prolonged exposure to long daylengths (greater than 12.5 hours per 24). This study was designed to determine the precise duration of long day (LD 14 hours light/24) exposure necessary to terminate refractoriness. The data indicate that 11 weeks of LD 14:10 terminate refractoriness in nearly every hamster. Exposure for fewer than 11 weeks is less effective whereas exposure for more than 11 weeks is no more effective. The data show that hamsters begin measuring long days to terminate photorefractoriness irrespective of the physiological state of the reproductive system, and, therefore, that photorefractoriness is induced some time during the period of testicular regression (during the first 10-12 weeks of short day exposure).