Plasma melatonin rhythm lost in preglaucomatous chicks

Constant light in early life induces fear-related behavior in chickens with suppressed melatonin secretion and disrupted hippocampal expression of clock- and BDNF-associated genes

Background

Light management plays an important role in the growth and behavior of broiler chickens. Constant light in early post hatch stage has been a common practice in broiler industry for improving growth performance, while whether and how constant light in early life affects the behavior of broiler chickens is rarely reported.

Results

In this study, newly hatched chicks were kept in either constant (24 L:0 D, LL) or (12 L:12 D, LD) photoperiod for 7 d and then maintained in 12 L:12 D thereafter until 21 days of age. Constant light increased the average daily feed intake but not the body weight, which led to higher feed conversion ratio. Chickens in LL group exhibited fear-related behaviors, which was associated with higher corticosterone, lower melatonin and 5-HT levels. Concurrently, constant light exposure increased the mRNA expression of clock-related genes and suppressed the expression of antioxidative genes in the hippocampus. Moreover, brain derived neurotrophic factor/extracellular signal-regulated kinase (BDNF/ERK) pathway was suppressed in the hippocampus of chickens exposed to constant light in the first week post hatching.

Conclusions

These findings indicate that constant light exposure in early life suppress melatonin secretion and disrupts hippocampal expression of genes involved in circadian clock and BDNF/ERK pathway, thereby contributing to fear-related behaviors in the chicken.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00720-4.

The impact of dark exposure on broiler feeding behavior and weight of gastrointestinal tract segments and contents

Ross 308 broilers were observed at 2 ages to quantify how duration of darkness affects behavior and alters the gastrointestinal tract (GIT, segment and content weights) over 24 h. Four treatments provided 1 (1D), 4 (4D), 7 (7D), or 10 (10D) h of darkness. Birds (n = 4000) were housed in 8 rooms with 8 pens per room (2 replications per treatment and 4 replications per gender per room). The GIT data were collected on day 27 to 28 (6 males per treatment, euthanized at 2 h intervals for 24 h) and expressed as a percentage of body weight. Data were analyzed as a completely randomized design, with treatment nested within room. Production data were analyzed as a 4 (dark) x 2 (gender) factorial arrangement and GIT data as a 4 (dark) x 12 (time) factorial arrangement. Regression analyses established relationships between darkness and dependent variables. At 31 d, regression analyses showed no effect on body weight. The highest feed consumption was observed under 4D. Mortality was lowest under 10D. Birds on 10D were the most feed efficient and had the heaviest crops. Crop content interacted with time of day, with peaks prior to dark under 4D, 7D, and 10D. Empty gizzard weight increased linearly as dark increased (P < .01). Behavior was examined as a 4 (dark) x 2 (age) x 2 (gender) factorial arrangement of treatments. Five birds per gender per room were focally observed for 24 h. Dark data were examined using regression analyses and an analysis of variance assessed age and gender data. As dark increased, feeding bout frequency increased and total time spent at the feeder decreased linearly (P = 0.01 and P < .01, respectively). As birds aged, feeding frequency decreased and feed bout length increased. Males visited the feeder more frequently. Birds anticipated dark periods >4 h and increased feeding activity prior to dark. Broilers adapt their feeding behavior in response to dark exposure, which alters GIT segment and content weight and likely feed passage rate.

Anterior Segment Growth and Peripheral Neural Pathways in Chick

PURPOSE

To learn if peripheral nerve pathways are necessary for corneal expansion and anterior segment growth under a 12-hr light:dark cycle or for the inhibition of corneal expansion under constant light rearing.

METHODS

Recently hatched White Leghorn chicks under anesthesia received unilateral ciliary ganglionectomy (CGx), cranial cervical ganglionectomy (Sx), or section of the ophthalmic nerve (TGx), along with sham-operated and/or never-operated control cohorts. Chicks were reared postoperatively under either a 12-hr light:dark cycle or under constant light. After 2 weeks and with the chicks under anesthesia, corneal radii of curvature and diameters were obtained with a photokeratoscope, refractometry and A-scan ultrasonography were performed, and the axial and equatorial dimensions of enucleated eyes were measured with digital calipers. Corneal areas were calculated from corneal curvatures and diameters.

RESULTS

Despite the rich peripheral innervation to the eye, the selective denervations performed here exerted remarkably limited effects on corneal expansion and anterior segment development in chicks reared under either lighting condition. Ophthalmic nerve section did reverse in large part the inhibition of equatorial expansion of the vitreous chamber occurring under constant light rearing.

CONCLUSIONS

The ciliary, sympathetic, or ophthalmic peripheral nerve pathways to the eye are not required either for corneal expansion and anterior segment development under a 12-hr light:dark cycle or for the inhibition of corneal expansion under constant light rearing. The ocular sensory innervation may be a means for regulating vitreous cavity shape.

Season of birth, natural light, and myopia

PURPOSE

To investigate the possible roles of season of birth and perinatal duration of daylight hours (photoperiod) in the development of myopia.

DESIGN

Retrospective, population-based, epidemiological study.

PARTICIPANTS

A total of 276 911 adolescents (157 663 male, 119 248 female) 16 to 22 years old. All were Israeli-born conscripts to the Israeli Defense Forces who were examined during the 5-year period 2000 through 2004.

METHODS

Noncycloplegic refraction was determined by autorefractometer and validated by qualified optometrists. Myopia, defined on the basis of right eye spherical equivalence, was classified as mild (-0.75 to -2.99 diopters [D]), moderate (-3.0 to -5.99 D), or severe (-6.0 D or worse). The photoperiod was recorded from astronomical tables and classified into 4 categories. Using multivariate logistic regression models, we calculated odds ratios (ORs) for several risk factors of myopia including season of birth.

MAIN OUTCOME MEASURE

The OR for photoperiod categories as risk factors for myopia.

RESULTS

Overall prevalences of mild, moderate, and severe myopia were 18.8%, 8.7%, and 2.4%, respectively. There were seasonal variations in moderate and severe myopia according to birth month, with prevalence highest for June/July births and lowest for December/January. On multivariate logistic regression, the ORs of photoperiod categories for moderate and severe myopia were highly significant and demonstrated a dose-response pattern. Odds ratios for severe myopia were highest for the shortest versus the longest photoperiods (1.24; 95% confidence interval, 1.15-1.33; P<0.001). Mild myopia was not associated with season of birth or perinatal light exposure. Other risk factors were gender (1.14 for female), education level (1.32 for age above 12), and father's origin (1.31 for Eastern vs. Israeli origin).

CONCLUSION

Myopia in this population is associated with birth during summer months. The exact associating mechanism is not known but might be related to exposure to natural light during the early perinatal period.

Continuous ambient lighting and lens compensation in infant monkeys

PURPOSE

Protracted daily lighting cycles do not promote abnormal ocular enlargement in infant monkeys as they do in a variety of avian species. However, observations in humans suggest that ambient lighting at night may reduce the efficiency of the emmetropization process in primates. To test this idea, we investigated the ability of infant monkeys reared with continuous light to compensate for optically imposed changes in refractive error.

METHODS

Beginning at about 3 weeks of age, a hyperopic or myopic anisometropia was imposed on 12 infant rhesus monkeys by securing either a -3 D or +3 D lenses in front of one eye and a zero-powered lens in front of the fellow eye. Six of these monkeys were reared with the normal vivarium lights on continuously, whereas the other six lens-reared monkeys were maintained on a 12-h-light/12-h-dark lighting cycle. The ocular effects of the lens-rearing procedures were assessed periodically during the treatment period by cycloplegic retinoscopy, keratometry, and A-scan ultrasonography.

RESULTS

Five of six animals in each of the lighting groups demonstrated clear evidence for compensating anisometropic growth. In both lighting groups, eyes that experienced optically imposed hyperopic defocus (-3 D lenses) exhibited faster axial growth rates and became more myopic than their fellow eyes. In contrast, eyes treated with +3 D lenses showed relatively slower axial growth rates and developed more hyperopic refractive errors. The average amount of compensating anisometropia (continuous light, 1.6 +/- 0.5 D vs. control, 2.3 +/- 0.5 D), the structural basis for the refractive errors, and the ability to recover from the induced refractive errors were also not altered by continuous light exposure.

CONCLUSION

Ambient lighting at night does not appear to overtly compromise the functional integrity of the vision-dependent mechanisms that regulate emmetropization in higher primates.

Three avian eye enlargement protocols as myopia models: effects of pharmacological intervention

Myopia is a prevalent condition threatening vision in half or more of the individuals in some subpopulations and at some ages. The etiology is unknown. It is not even clear whether myopia has several possible causes or only one, whether the condition is inherited or acquired. Suggested counter measures have ranged from behavior modification during the growing years through various medications, to surgical intervention. It is suggested that answers to these questions must lie in research exploring how the eye develops, how it serves its owner. A common finding in most cases of myopia is that the eyeball is too long axially, thus the image from a distant object comes to sharp focus in front of, rather than at, the retina. Accommodation increases the refractive power of the eye and is counterproductive in a myope. Nonetheless, one suggested treatment has been to suppress accommodation by paralyzing the ciliary muscles. Another theory suggests that high intraocular pressure causes eye enlargement, thus the anti-glaucoma drugs have sometimes been recommended. We report here some trials involving drugs given to chicks with light-induced avian glaucoma, a syndrome characterized by moderate to severe myopia, and elevated intraocular pressure.

Melatonin reduction by lithium and albinism in quail and hamsters

Melatonin was measured by radioimmunoassay in several genetic strains of Japanese quail. Plasma melatonin (PM), measured at the nighttime peak, was highest in wild type quail reared on a diurnal lighting schedule; this PM peak was suppressed in continuous light. Albino quail had low melatonin levels, whether reared under diurnal conditions or in continuous light. Ocular melatonin was also suppressed in albinos and in dilute mutants. At midday sampling, melatonin was only half as high in albinos as in wild types. Intraocular pressure (IOP, daytime readings) was uniformly low in diurnal birds and was elevated in all quail reared under continuous light. Thus in pigmented birds, a high melatonin level was associated with high IOP, but in albinos displaying high IOP, ocular melatonin was not elevated. Lithium chloride, mixed in the feed, brought about a pronounced reduction in plasma, pineal, and ocular melatonin, in wild-type quail reared on a diurnal schedule. This confirms earlier findings in lithium-fed rats. Golden hamsters displayed a characteristic diurnal cycle of pineal melatonin, with a sharp middark peak; in albino hamsters, also kept on a diurnal schedule, this peak occurred at the same time, but albinos had melatonin levels only about one-third as high as those of pigmented animals.

Light-induced avian glaucoma as an animal model for human primary glaucoma

Glaucoma can be induced in domestic chicks at the will of the investigator, by the simple device of rearing the chicks under continuous light. This light-induced avian glaucoma (LIAG) is presented as an animal model system for human open-angle glaucoma. A number of morphological and physiological findings in LIAG are reviewed, and the LIAG system is compared with several other glaucoma model systems, in dogs, rabbits and monkeys. Intraocular pressure in LIAG has been demonstrated to be responsive to several anti-glaucoma drugs, and the system could be used for further drug testing. Thus it is suggested that LIAG may be especially useful in studies seeking to understand human glaucoma, and how to forestall it, or treat it. As well, a prolonged "pre-glaucoma" period is available to the investigator working with LIAG, during which a pathological course is already underway in the eye, but intraocular pressure has not yet gone up.