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Biomedical subjects

E Gwinner

Publications and source records attributed to E Gwinner.

At least 37 records · Page 2Linked to original sources
PubMed1995

Endogenous gonadal, LH and molt rhythms in tropical stonechats: effect of pair bond on period, amplitude, and pattern of circannual cycles.

To investigate the effects of reproduction and associated stimuli on the circannual cycles of African stonechats Saxicola torquata axillaris birds were held for 29 months in aviaries under a constant equatorial (12.25 h) photoperiod, either singly (10 females and 10 males) or in 10 male/female pairs. The birds of all 3 groups went through circannual cycles in gonadal size, plasma LH and molt, but groups differed with regard to actual reproductive performance. During the second cycle, only one of the singly-held females laid eggs and incubated. In contrast, in the paired females egg-laying and incubation occurred in all but one bird. About 50% of the clutches from paired females contained fertilized eggs confirming the expectation of behavioral differences between the paired and unpaired birds. However, despite differences in reproductive performance there were no differences in either circannual period or duration of reproductive phases. Moreover, there was no correlation between number of broods produced per season and circannual parameters of the paired females. Therefore, the temporal course and, particularly, the period during which reproduction is possible is rigidly determined by an endogenous program that is not influenced by reproductive performance. A rigid program of this kind may be advantageous in the tropics because it prevents prolongation of the breeding season in years with favourable conditions which in turn could jeopardize optimal timing of breeding in the following year and thus reduce lifetime reproductive success.

Africa↗
PubMed1995

Continuous melatonin administration accelerates resynchronization following phase shifts of a light-dark cycle.

Circadian rhythms of most passerine birds and some reptiles depend to a considerable extent on the presence of periodic melatonin, which is considered to be part of the central pacemaking system. Recent results on house sparrows have suggested that melatonin, apart from its periodic effects on the circadian system, may also exert effects derived from a constant action: Continuous melatonin applied through subcutaneously implanted silastic tubing enhanced the synchronization response to a low-amplitude light-dark zeitgeber, indicating some kind of sensitization to zeitgeber stimuli. In the present study we tested the prediction derived from these results, that speed of resynchronization after a phase shift of a light-cycle should be enhanced if melatonin is continuously administered. We found that, indeed, house sparrows required less time to resynchronize to an 8 h advance or delay phase shift of a low-amplitude light-dark cycle while carrying a silastic implant filled with melatonin than while carrying an empty implant. The effect is suggested to result from either one or a combination of the following mechanisms: (i) An increased circadian visual sensitivity, (ii) a diminished amplitude of the circadian oscillation, iii) an altered feedback of the locomotor activity to the oscillatory system.

Animals↗
PubMed1995

Synchronization of circadian rhythms of house sparrows by oral melatonin: effects of changing period.

House sparrows (Passer domesticus) whose circadian rhythms of locomotor activity and feeding had been abolished by pinealectomy were held in constant dim light and periodically exposed to melatonin in the drinking water. By alternating 8 h of melatonin water with variable phases of tap water, rhythms with periods (T) ranging from 21 to 27 h were produced. When melatonin was administered in rhythms with periods of 23, 24, and 25 h, feeding and locomotion behavior of most birds were rhythmic and synchronized with the exogenous melatonin rhythm. The rest phase coincided approximately with the phase of melatonin availability. Under melatonin cycles < 23 h and > 25 h, fewer birds had synchronized rhythms. Nonsynchronized birds were either arrhythmic or they expressed free-running rhythms. Under melatonin rhythms with periods between 23 and 26 h, the phase-angle difference between defined phases of the behavioral rhythms and the melatonin rhythm became more positive with increasing T. These data are consistent with the hypothesis (a) that periodic exogenous melatonin can substitute, at least to a certain degree, for the endogenous plasma melatonin rhythm normally resulting from the periodic melatonin secretion by the pineal gland, and (b) that this melatonin rhythm acts on another oscillator, possibly the SCN, as part of the overall circadian pacemaking system.

Animals↗
PubMed1993

Twenty-four hour melatonin profiles in a nocturnally migrating bird during and between migratory seasons.

The garden warbler Sylvia borin is a long-distance migrating bird that travels exclusively at night. During the migratory seasons caged warblers develop intense nocturnal activity which may become even more intense than that during the day. It is demonstrated that in spite of dramatic seasonal changes in the 24-h pattern of locomotor activity measured in caged garden warblers, the corresponding pattern of plasma melatonin changed only very little. As in other species melatonin levels were generally low during the day (below 50 pg/ml) and high at night (350 to 650 pg/ml). A slight reduction in the nocturnal melatonin peak (from 650 pg/ml to between 350 and 400 pg/ml) observed during the autumn and spring migratory seasons, was possibly due to an increased light perception of the birds moving around while being exposed to dim night lights. In general the results show that the 24-h plasma melatonin profiles of these birds are rather robust and that locomotor activity does not depend in a simple and direct way on plasma melatonin levels.

Animals↗
PubMed1992

Development of melatonin rhythm in the pineal gland and eyes of chick embryo.

A melatonin rhythm was observed in the pineals of 18-day-old chick embryos incubated under a light-dark regime of 18: 6 h. A low pineal melatonin content was found during the light phase of the day. Concentrations started to increase 2 h after dark onset and reached maximum levels after 4 h of darkness. The amplitude of the pineal melatonin rhythm increased considerably after 2 days and night-time concentrations in 20-day-old embryos were more than 5 times higher than in 18-day-old ones. Significant day/night differences in melatonin production were found both in pineals and eyes. Exposure of eggs to 1 h of light during the dark period decreased the high melatonin concentrations in the eyes but not in the pineals of the 20-day-old chick embryo. The results suggest that in this precocial bird at least part of the circadian system may already operate during embryonic life.

Animals↗
PubMed1992

Circadian entrainment by feeding cycles in house sparrows, Passer domesticus.

We studied the potential zeitgeber qualities of periodic food availability on the circadian rhythms of locomotor and feeding activity of house sparrows. The birds were initially held in a LD-cycle of 12:12 h, with food restricted to the light phase. After transfer to constant dim light, the birds remained entrained by the restricted feeding schedule. Following an exposure to food ad libitum conditions, the rhythms could be resynchronized by the feeding cycle. Shortening of the zeitgeber period to 23.5 h resulted in the loss of entrainment in most birds, whereas a longer zeitgeber period of 25 h re-entrained the rhythms of most birds. Although these results prove that periodic food availability can act as a zeitgeber for the circadian rhythms of house sparrows, several features of our data indicate that restricted feeding is only a weak zeitgeber. The pattern of feeding activity prior to the daily time of food access shown under some experimental conditions suggests that anticipation is due to a positive phase-angle difference of the birds' normal circadian system rather than being caused by a separate pacemaker.

Animals↗
PubMed1992

The effect of pinealectomy on circadian plasma melatonin levels in house sparrows and European starlings.

We determined 24-hr plasma melatonin profiles in intact, sham-pinealectomized, and pinealectomized European starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) in a light-dark (LD) cycle and in constant darkness (DD). In the intact and sham-pinealectomized birds of both species, a melatonin rhythm was found, with low levels during the day and high levels during the night. Pinealectomy abolished the nighttime peak of melatonin in both species; hence, levels were low at all times sampled. This uniform response of plasma melatonin to pinealectomy contrasts with the differential response of circadian activity rhythms to pinealectomy for these two species. In DD, locomotor activity in pinealectomized house sparrows is usually arrhythmic, whereas in starlings a rhythm usually persists. This suggests that in the latter species free-running circadian rhythms are not necessarily dependent on a rhythm in plasma melatonin. The same is true for the synchronized activity rhythm observed in pinealectomized birds of both species in LD, as well as for the damped rhythm that persists in pinealectomized house sparrows following an LD-to-DD transfer. The results are consistent with the hypothesis that the pineal and its periodic output of melatonin constitute only one component in a system of at least two coupled pacemakers. They also suggest that there are species differences in the relative role played by the pineal and other pacemakers in controlling circadian rhythms in behavior.

Animals↗
PubMed1991

Variation in hypothalamic gonadotrophin-releasing hormone content, plasma and pituitary LH, and in-vitro testosterone release in a long-distance migratory bird, the garden warbler (Sylvia borin), under constant photoperiods.

Changes in concentrations of hypothalamic gonadotrophin-releasing hormone (GnRH), pituitary and plasma LH, testicular mass, in-vitro release of testosterone, body mass and migratory activity were measured in male garden warblers (Sylvia borin) kept from November to June under a constant photo-period of 12.8 h. Under such conditions garden warblers gradually change from the photorefractory to the photosensitive state and gonadal recrudescence then occurs. Hypothalamic GnRH content was low from December to March, but increased in April to reach the highest levels in June. The spontaneous increase in GnRH was paralleled by increases in pituitary LH content, testicular mass and in-vitro testosterone release. Body mass decreased 1 month and nocturnal activity 2 months before the spontaneous increase in GnRH. Ovine LH increased in-vitro testosterone release over basal release at all times. The results suggest that in garden warblers (1) changes in hypothalamic GnRH content can occur under constant photoperiodic condition, (2) the gradual change from the photorefractory to the photosensitive state is not characterized by a gradual increase in hypothalamic content of GnRH (cf. starlings), and (3) Leydig cells are capable of testosterone release even during the photorefractory state.

Animals↗
PubMed1989

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.

Animals↗
PubMed1989

Photoperiod as a modifying and limiting factor in the expression of avian circannual rhythms.

In three species of birds that migrate long distances, the annual rhythms of gonadal activity, molt, and migratory restlessness (Zugunruhe) persist for more than 1 year under certain constant conditions. The most important zeitgeber for these circannual rhythms is the annual cycle of photoperiod, which adjusts the overall period of circannual rhythms to exactly 1 year and also provides for the appropriate adjustment of seasonal activities to the temporal structure of the environment. This is illustrated by results on garden warblers (Sylvia borin) indicating that the longer photoperiods experienced by individuals wintering far south in the African wintering area phase-advance spring migration and the accompanying gonadal development, relative to those of individuals wintering further north. The rate of acceleration is, however, slow enough to prevent a reproductive cycle during the Southern Hemisphere summer. Hence, endogenous circannual components and zeitgeber stimuli constitute a functional entity that provides as a whole for adaptive temporal programming. This idea is further supported by findings in the pied flycatcher (Ficedula hypoleuca), in which a circannual rhythmicity persists only if photoperiod in winter is at least as short as that normally encountered by the species in its wintering grounds slightly north of the equator. In collared flycatchers (Ficedula albicollis), in contrast, rhythmicity continues under much longer photoperiods, consistent with the fact that the wintering area of this species extends to latitudes far south of the equator. It is proposed that the adaptive function of circannual rhythms can be properly understood only if their interactions with environmental factors, particularly those that play a role as zeitgebers, are analyzed in sufficient detail. The biological significance of circannual rhythms may be more apparent in the context of the environmental constraints limiting their expression than in the often rather restricted set of conditions sustaining spontaneous annual cyclicity.

Adaptation, Physiological↗
PubMed1988

Timing of photorefractoriness in the European starling: significance of photoperiod early and late in the reproductive cycle.

In European starlings, as in many other birds inhabiting higher latitudes, gonads develop in response to the increasing daylengths in early spring. Later in the year, however, the hypothalamo-pituitary-gonadal axis becomes refractory to the previously stimulatory long photoperiods and the gonads regress in summer. The present study addresses the question of when during the gonadal growth phase photorefractoriness is determined. A 13-h photoperiod induces testicular development and subsequent testicular regression associated with refractoriness in male starlings. An 11-h photoperiod, in contrast, induces only testicular development, and photorefractoriness never develops. When starlings were transferred to an 11-h photoperiod, either 12 or 25 days following exposure to a 13-h photoperiod, their testes developed to full size, but remained large to the end of the experiment, i.e. refractoriness did not develop. The same was even true of most birds in a third group that were transferred to an 11-h photoperiod after 46 days of the 13-h photoperiod, when gonads had developed to near maximal size. These data show that, in contrast to some other species of passerine birds, the onset of photorefractoriness does not become fixed before the testes have undergone considerable development, and that the photoperiodic conditions experienced at the end of the testicular growth phase are still effective in determining the precise time of onset of photorefractoriness. It is suggested that this peculiarity of the starling is related to the fact that its gonadal development begins rather early in spring and, hence, under much shorter photoperiods than the other species studied.

Animals↗
PubMed1987

Differential effects of pinealectomy on circadian rhythms of feeding and perch hopping in the European starling.

To study the effects of pinealectomy on the circadian rhythms of locomotor activity and feeding. European starlings (Sturnus vulgaris) were held in constant light (0.2 lux and 200 lux) and under constant temperature conditions. Locomotor activity was measured by means of perches with microswitches mounted underneath, and feeding with an infrared photocell system at the feeder. Pinealectomy consistently led to disturbances in perch-hopping rhythms and often to a complete loss of rhythmicity as revealed by periodogram analysis. In some birds, perch-hopping rhythms recovered following a period of initial arrhythmicity. When a perch-hopping rhythm was present, its period was usually shorter than it had been before pinealectomy. In contrast to its effects on perch hopping, pinealectomy had no effect on the persistence of feeding rhythmicity, although its period, like that of the hopping rhythm, decreased after this operation. These results support the hypothesis derived from previous studies that the circadian organization of feeding is different from that of perch hopping. Different circadian pacemakers may be involved, but other models may possibly explain the data just as well.

Animals↗
PubMed1987

Circannual variations in plasma luteinizing hormone levels in castrated male European starlings (Sturnus vulgaris).

Intact and castrated male European starlings were held for about 2 years in a constant 12-hr photoperiod and constant temperature conditions. At 1- to 2-month intervals, testicular width was measured by laparotomy, and blood samples were taken for analysis of plasma luteinizing hormone (LH). Most of the control birds went through at least one circannual cycle of testicular width and plasma LH concentration. In the castrates, a similar proportion of birds went through circannual LH cycles with periods indistinguishable from those of the controls. It is concluded that the testes and their hormones are not essential components of the mechanism that generates circannual gonadal cycles in male European starlings.

Animals↗
PubMed1981

[Circannual rhythms in animals and their photoperiodic synchronization].

During the last years, evidence has been accumulated indicating that the annual cycles of physiological and behavioral functions in animals are often preprogrammed as endogenous circannual rhythms. Under seasonally constant environmental conditions these rhythms persist for several cycles with periods deviating from year, whereas under natural conditions they are usually synchronized by seasonal changes in the environment, particularly that of photoperiod. Hence, the annual rhythm of photoperiod constitutes a Zeitgeber for the endogenous rhythmicity rather than providing causal stimuli that release particular activities at particular times of the year. On the basis of this new concept several peculiarities of annual biological rhythms under the influence of photoperiodic cycles with different properties can be better understood, at least at a formal level.

Animals↗
PubMed1980

The effect of celestial cues on the ontogeny of non-visual orientation in the garden warbler (Sylvia borin).

It was the aim of this study to find out whether additional information from the sun and the stars during early development leads to an improvement in the young birds' non-visual orientation performance during their first autumn migration period. The results indicate that the birds' ability to determine their migratory direction can mature independently of celestial information. Celestial cues, however, appear to be involved in the normal maturation process of migratory orientation, since the availability of celestial information during ontogeny tends to impair the birds' non-visual orientation.

Age Factors↗