Extraretinal photocontrol of oviposition in pinealectomized domestic fowl

Avian photoreceptors and their role in the regulation of daily and seasonal physiology

Birds time their activities in synchronization with daily and seasonal periodicities in the environment, which is mainly provided by changes in day length (=photoperiod). Photoperiod appears to act at different levels than simply entraining the hypothalamic clock via eyes in birds. Photoreceptor cells that transmit light information to an avian brain are localized in three independent structures, the retina of eyes, pineal gland and hypothalamus, particularly in the paraventricular organ and lateral septal area. These hypothalamic photoreceptors are commonly referred to as encephalic or deep brain photoreceptors, DBPs. Eyes and pineal are known to contribute to the circadian regulation of behavior and physiology via rhythmic melatonin secretion in several birds. DBPs have been implicated in the regulation of seasonal physiology, particularly in photoperiod induced gonadal growth and development. Here, we briefly review limited evidence that is available on the roles of these photoreceptors in the regulation of circadian and seasonal physiology, with particular emphasis placed on the DBPs.

Exploring avian deep-brain photoreceptors and their role in activating the neuroendocrine regulation of gonadal development

In the eyes of mammals, specialized photoreceptors called intrinsically photosensitive retinal ganglion cells (ipRGC) have been identified that sense photoperiodic or daylight exposure, providing them over time with seasonal information. Detectors of photoperiods are critical in vertebrates, particularly for timing the onset of reproduction each year. In birds, the eyes do not appear to monitor photoperiodic information; rather, neurons within at least 4 different brain structures have been proposed to function in this capacity. Specialized neurons, called deep brain photoreceptors (DBP), have been found in the septum and 3 hypothalamic areas. Within each of the 4 brain loci, one or more of 3 unique photopigments, including melanopsin, neuropsin, and vertebrate ancient opsin, have been identified. An experiment was designed to characterize electrophysiological responses of neurons proposed to be avian DBP following light stimulation. A second study used immature chicks raised under short-day photoperiods and transferred to long day lengths. Gene expression of photopigments was then determined in 3 septal-hypothalamic regions. Preliminary electrophysiological data obtained from patch-clamping neurons in brain slices have shown that bipolar neurons in the lateral septal organ responded to photostimulation comparable with mammalian ipRGC, particularly by showing depolarization and a delayed, slow response to directed light stimulation. Utilizing real-time reverse-transcription PCR, it was found that all 3 photopigments showed significantly increased gene expression in the septal-hypothalamic regions in chicks on the third day after being transferred to long-day photoperiods. Each dissected region contained structures previously proposed to have DBP. The highly significant increased gene expression for all 3 photopigments on the third, long-day photoperiod in brain regions proposed to contain 4 structures with DBP suggests that all 3 types of DBP (melanopsin, neuropsin, and vertebrate ancient opsin) in more than one neural site in the septal-hypothalamic area are involved in reproductive function. The neural response to light of at least 2 of the proposed DBP in the septal/hypothalamic region resembles the primitive, functional, sensory ipRGC well characterized in mammals.

Light-induced reduction of cytoplasmic free calcium in neurons proposed to be encephalic photoreceptors in chick brain

A population of cerebrospinal fluid-contacting neurons (CSFcn) in the lateral septal organ (LSO) may serve as encephalic photoreceptors (EPRs) functioning to signal the onset of seasonal reproductive development in birds. Previous studies on CSFcn in the LSO have focused on identification of retinal protein components in fixed brain tissue. In order to understand better the mechanisms underlying the light-induced photosexual response in birds, a physiological characterization is required. In this study, changes of intracellular free calcium concentration ([Ca2+]i) were monitored during light stimulation of CSFcn in the LSO in live brain slices from embryonic chicks. Using the fluorescent calcium indicator fluo-4, a reduced [Ca2+]i within CSFcn was recorded in response to photostimulation, which is consistent with what has been demonstrated in rods and cones following illumination. Results support the hypothesis that CSFcn in the LSO function as EPRs in the avian brain.

The search for deep encephalic photoreceptors within the avian brain, using gonadal development as a primary indicator

A review of the literature was completed on central neural structures regarded to be the site of encephalic photoreceptors in avian species. The photoreceptors are thought to function as endogenous clocks, respond to certain lengths and characteristics of the photoperiod, and serve to activate important physiological events such as gonadal function at the optimal season or time each year. Three sites have been explored: eyes, pineal gland, and deep encephalic photoreceptors within the ventral forebrain. To date the evidence supports the latter as the best candidate for housing specialized neuroendocrine photoreceptors. Within the ventral forebrain, most studies have concentrated on the medial basal hypothalamus (also known as the infundibular tuberal complex), however a second locus, the lobus parolfactorius, has also been identified. Specifically, a group of cerebrospinal fluid (CSF)-contacting neurons in the medial portion of the lateral septal organ (LSO) within the lobus parolfactorius is a second viable candidate. The chick appears to be an excellent model to determine whether or not the CSF-contacting neurons of the medial LSO are deep encephalic photoreceptors.

Pineal gland and ocular influences on turkey breeder hens. 1. Reproductive performance

The influence of the pineal gland and the eyes on reproductive performance of turkey hens was examined. Photosensitive, yearling Large White turkey breeder hens underwent pinealectomy (PX) or bilateral ocular enucleation (EX) or both while held on 8 hr of light per day (8L:16D) and were then photoinduced into a typical 20-week egg laying period by exposing the birds to 16 hr of light per day (16L:8D). Data were obtained for the time to first egg (sexual maturation), rate of egg laying, percent fertility, percent hatchability, weight of poults at hatching, and plasma melatonin levels. A significant interaction between PX and EX treatments occurred in the sexual maturation data only. Pinealectomy significantly delayed (P less than .05) the onset of egg laying (by 4.1 to 7.4 days) compared with the other treatment groups. No other significant differences in onset of lay occurred. Pinealectomy significantly depressed egg production during weeks 11 to 20 of lay and this resulted in fewer eggs per PX hen over the 20-week study (85.4 vs. 93.3 eggs in the PX and pineal intact groups, respectively). Enucleation did not significantly affect egg production. Neither the PX nor the EX treatment significantly affected fertility or hatchability determined at 4-week intervals over the 20-week study. However, poult weights were significantly greater as a result of either PX (P less than .01) or EX (P less than .05) treatment. Nocturnal elevation of plasma melatonin was significantly reduced by PX but not by EX. It was concluded that an extraretinal, extrapineal photoreceptive mechanism is operative in turkey breeder hens but both the pineal gland and the eyes can influence certain reproductive parameters.

Pineal gland and ocular influences on turkey breeder hens. 2. Body weight, feed intake, and egg characteristics

Yearling Large White turkey breeder hens underwent pinealectomy (PX), bilateral ocular enucleation (EX), or both and were then photo-induced into reproductive activity. During the subsequent 20-week reproductive period, data were recorded for body weight, change in body weight, feed intake, egg weight, eggshell thickness, and percent settable eggs. A significant interaction between PX and EX treatments did not occur for any of the parameters of this study. Absolute body weight, feed intake, shell thickness, and percent settable eggs were not significantly affected by PX during the study, but PX significantly increased egg weights. Feed intake was significantly reduced by EX, but other parameters were not affected. Both PX and EX reduced body weight losses typically observed in breeder hens during an in-season egg laying cycle. It was concluded that the pineal gland and the eyes can influence body weight changes, egg size, or feed intake in breeder turkey hens during a reproductive cycle.

Rhodopsin-like photosensitivity of isolated chicken pineal gland

The pineal gland of lower vertebrates such as fish, amphibians and reptiles is primarily a photoreceptor organ and its electrical activity changes in response to environmental lighting, whereas the mammalian pineal gland is a secretory organ and no longer responds to direct illumination. It is uncertain whether the avian pineal gland retains a photoreceptive capability, and also whether the gland is a photosensor involved in the photo-periodic control of reproductive systems and circadian rhythms in birds. Recently, I and other workers have shown that the circadian rhythm of serotonin N-acetyltransferase activity persisted in organ cultures of chicken pineals, indicating the presence of an endogenous circadian oscillator in the gland. Also, the observation that direct illumination of cultured chicken pineals suppressed the night-time increase of N-acetyl-transferase activity suggests that the gland contains a photoreceptor. I report here that the action spectrum of the photosensitivity of the isolated chicken pineal resembles the absorption spectrum of rhodopsin.

Participation of the eyes in the photostimulation of chickens

This study examined the participation of the eyes in the photosexual response of chickens to incandescent and fluorescent light. Intact and blinded female chickens were exposed to short photoperiods (8L:16D) of incandescent and fluorescent light and the age at first egg and the early rate of egg production. Blinding prevented this response. It was concluded that the eye is not essential to the photosexual response of chickens but that it may modify the response to light in certain conditions.

Pinealectomy and light environment effects on testicular and comb development in the 46-day-old broiler cockerel

Four replicate experiments were conducted to determine the effects of pinealectomy and environmental lighting on testes and comb weights in the broiler cockerel. Birds were housed in brooder batteries under a 14L:10D fluorescent white light regime for 2 weeks and then allotted to light-controlled chambers. Surgery was performed when the chicks were 3 to 5 days old. The fluorescent light treatments were 14L:10D green (narrow-band, 545 nm peak), 14L:10D cool white, and constant darkness. At the end of the 46-day experimental period, testes weights were determined and comb weight recorded. Pinealectomy did not affect testes weights or comb development. Darkness significantly (P less than .05) depressed testes and comb weight. This suggests that lack of light, but not pinealectomy, affects circulating hormone levels and/or tissue responsiveness in the young cockerel.

Multichannel digiral recording for oviposition in Japanese quail

A multichannel recording system is described which offers the advantages of accuracy and low cost. The system was a commercially digital printer and clock mechanism. Switch decoding circuitry is presented in the article. The system described will reocrd from 200 switches connected to egg trip levers on bird cages. An example of data obtained from two Japanese quail illustrates the smooth patterns of lay seen when recorded to one minute accuracy.

Eyes Have a Role in Photoperiodic Control of Sexual Activity of Coturnix

In blinded Japanese quail (Coturnix coturnix japonica) encephalic photoreception of the stimulus from long photoperiods is sufficient to induce and maintain normal gonadal function in females (egg laying) and in males (enlargement of the cloacal gland). However, the termination of sexual activity by short days is dependent on these birds having experienced long days at the time of blinding.