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

A cross-taxonomic perspective on the integration of temperature cues in vertebrate seasonal neuroendocrine pathways

The regulation of seasonality has been an area of interest for decades, yet global climate change has created extra urgency in the quest to understand how sensory circuits and neuroendocrine control systems interact to generate flexibility in biological timekeeping. The capacity of temperature to alter endogenous or photoperiod-regulated neuroendocrine mechanisms driving seasonality, either as a direct cue or through temperature-dependent effects on energy and metabolism, is at the heart of this phenological flexibility. However, until relatively recently, little research had been done on the integration of temperature information in canonical seasonal neuroendocrine pathways, particularly in vertebrates. We review recent advances from research in vertebrates that deepens our understanding of how temperature cues are perceived and integrated into seasonal hypothalamic thyroid hormone (TH) signaling, which is a critical regulator of downstream seasonal phenotypic changes such as those regulated by the BPG (brain-pituitary-gonadal) axis. Temperature perception occurs through cutaneous transient receptor potential (TRP) neurons, though sensitivity of these neurons varies markedly across taxa. Although photoperiod is the dominant cue used to trigger seasonal physiology or entrain circannual clocks, across birds, mammals, fish, reptiles and amphibians, seasonality appears to be temperature sensitive and in at least some cases this appears to be related to phylogenetically conserved TH signaling in the hypothalamus. Nevertheless, the exact mechanisms through which temperature modulates seasonal neuroendocrine pathways remains poorly understood.

Mapping the brain of the chicken (Gallus gallus), with emphasis on the septal-hypothalamic region

There has been remarkable progress in discoveries made in the avian brain, particularly over the past two decades. This review first highlights some of the discoveries made in the forebrain and credits the Avian Brain Nomenclature Forum, responsible for changing many of the terms found in the cerebrum and for stimulating collaborative research thereafter. The Forum facilitated communication among comparative neurobiologists by eliminating confusing and inaccurate names. The result over the past 15yearshas been a standardized use of avian forebrain terms. Nonetheless, additional changes are needed. The goal of the paper is to encourage a continuing effort to unify the nomenclature throughout the entire avian brain. To emphasize the need for consensus for a single name for each neural structure, I have selected specific structures in the septum and hypothalamus that our laboratory has been investigating, to demonstrate a lack of uniformity in names applied to conservative brain regions compared to the forebrain. The specific areas reviewed include the distributions of gonadotropin-releasing hormone neurons and their terminal fields in circumventricular organs, deep-brain photoreceptors, gonadotropin inhibitory neurons and a complex structure and function of the nucleus of the hippocampal commissure.

Constructing the suprachiasmatic nucleus: a watchmaker's perspective on the central clockworks

The circadian system constrains an organism's palette of behaviors to portions of the solar day appropriate to its ecological niche. The central light-entrained clock in the suprachiasmatic nucleus (SCN) of the mammalian circadian system has evolved a complex network of interdependent signaling mechanisms linking multiple distinct oscillators to serve this crucial function. However, studies of the mechanisms controlling SCN development have greatly lagged behind our understanding of its physiological functions. We review advances in the understanding of adult SCN function, what has been described about SCN development to date, and the potential of both current and future studies of SCN development to yield important insights into master clock function, dysfunction, and evolution.

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.

Deep-brain photoreceptors (DBPs) involved in the photoperiodic gonadal response in an avian species, Gallus gallus

Three primitive photoreceptors [melanopsin (Opn4), neuropsin/opsin5 (Opn5) and vertebrate ancient opsin (VAOpn)] were reported as possible avian deep-brain photoreceptors (DBPs) involved in the perception of photoperiodic information affecting the onset and development of reproduction. The objective of this study was to determine the effect of long-day photostimulation and/or sulfamethazine treatment (SMZ, a compound known to advance light-induced testes development) on gene expression of DBPs and key hypothalamic and pituitary genes involved in avian reproductive function. Two-week old chicks were randomly selected into four experimental groups: short-day control (SC, LD8:16), short-day+SMZ (SS, LD8:16, 0.2% diet SMZ), long-day control (LC, LD16:8), and long-day+SMZ (LS, LD16:8, 0.2% diet SMZ). Birds were sampled on days 3, 7, and 28 after initiation of a long-day photoperiod and/or SMZ dietary treatments. Three brain regions [septal-preoptic, anterior hypothalamic (SepPre/Ant-Hypo) region, mid-hypothalamic (Mid-Hypo) region, posterior-hypothalamic (Post-Hypo) region], and anterior pituitary gland were dissected. Using quantitative real-time RT-PCR, we determined changes of expression levels of genes in distinct brain regions; Opn4 and Opn5 in SepPre/Ant-Hypo and Post-Hypo regions and, VAOpn in the Mid-Hypo region. Long-day treatment resulted in a significantly elevated testes weight on days 7 and 28 compared to controls, and SMZ augmented testes weight in both short- and long-day treatment after day 7 (P<0.05). Long-day photoperiodic treatment on the third day unexpectedly induced a large 8.4-fold increase of VAOpn expression in the Mid-Hypo region, a 15.4-fold increase of Opn4 and a 97.8-fold increase of Opn5 gene expression in the Post-Hypo region compared to SC birds (P<0.01). In contrast, on days 7 and 28, gene expression of the three DBPs was barely detectable. LC group showed a significant increase in GnRH-1 and TRH mRNA in the Mid-Hypo compared to SC on day 3. Pituitary LHβ and FSHβ mRNA were significantly elevated in LC and LS groups compared to SC on days 3 and 7 (P<0.05). On days 3 and 7, TSHβ mRNA level was significantly elevated by long-day treatment compared to the SC groups (P<0.05). Results suggest that long-day photoperiodic activation of DBPs is robust, transient, and temporally related with neuroendocrine genes involved in reproductive function. Additionally, results indicate that two subsets of GnRH-1 neurons exist based upon significantly different gene expression from long-day photostimulation and long-day plus SMZ administration. Taken together, the data indicate that within 3 days of a long-day photoperiod, an eminent activation of all three types of DBPs might be involved in priming the neuroendocrine system to activate reproductive function in birds.

Hypothalamic gene switches control transitions between seasonal life history states in a night-migratory photoperiodic songbird

This study investigated photoperiodic plasticity in hypothalamic expression of genes implicated in the photoperiodic light perception (rhodopsin, melanopsin, neuropsin and peropsin), transduction (pax6, bmal1, clock, per2 and casr), induction (eya3, tshβ, dio2 and dio3, gnrh and gnih) and metabolism (NPY, sirtuin1, foxO1, hmgcr, citrate synthase and dehydrogenases) in photosensitive and photorefractory redheaded buntings. There was a significant increase in eya3, tsh β, dio2, pax6 and rhodopsin and decrease in dio3 mRNA expression at hour 15 and/or 19 on the day photosensitive buntings were subjected to a 13- or 16 h, but not to 8- and 11 h light exposure. Downstream reproductive and metabolic gene expression was not altered, except for an increase in those genes coding for succinate and malate dehydrogenase enzymes involved in lipogenesis. Photorefractory buntings had high dio3 mRNA expression which significantly declined after 1 short day exposure, suggesting possible involvement of dio3 in the maintenance of photorefractoriness. Positive correlation of rhodopsin on eya 3 and tshβ indicates its role in photoperiodic timing, perhaps involving the peropsin and pax6 genes. These results suggest that rapid switching of hypothalamic gene expression underlies photoperiod-induced seasonal plasticity and regulates transitions from photosensitive to photostimulated and from photorefractory to photosensitive states in migratory songbirds.

A photoperiodic molecular response in migratory redheaded bunting exposed to a single long day

A long day response is triggered by the activation of EYA3 (eyes absent 3) and TSH-β (thyroid stimulating hormone beta subunit) genes in the pars tuberalis (PT). However, protein products of these genes are not yet shown in the hypothalamus of a photoperiodic species. Therefore, using the 'first long day paradigm', EYA3 and TSH-β along with c-FOS and GnRH peptides were immunohistochemically localized and measured in the hypothalamus of photoperiodic redheaded buntings that were maintained on short days (SD, LD 8/16) and subjected to one full long day (LD, LD 16/8). Following morning light remained turned off, and birds were sacrificed in the first hour of the day. Brains were collected and processed for immunohistochemistry of peptides. FOS-lir and GnRH-lir cells were significantly higher in the preoptic area (POA) in LD than in SD, which indicated photoperiod induced neuronal activation and downstream effects, respectively, under LD. In LD, EYA3-lir cells were significantly increased in septal lateralis (SL) with fibres extending to sub-septal organ (SSO); EYA3 fibres were very dense in median eminence. Similarly, there were significantly increased TSH-β-lir cells in the ventricular region with much abundance in the PT and TSH-β-lir fibres in the SSO (extending up to SL), inferior hypothalamic nucleus (IH) and infundibular nucleus (IN) in LD birds. Elevated EYA3, TSH-α and TSH-β mRNA levels further confirmed photoperiodic induction at the transcriptional level in buntings on the first long day. These are the first results showing localization of photoperiodically induced peptides in the hypothalamus of a songbird species, the redheaded bunting.

Avian circadian organization: a chorus of clocks

In birds, biological clock function pervades all aspects of biology, controlling daily changes in sleep: wake, visual function, song, migratory patterns and orientation, as well as seasonal patterns of reproduction, song and migration. The molecular bases for circadian clocks are highly conserved, and it is likely the avian molecular mechanisms are similar to those expressed in mammals, including humans. The central pacemakers in the avian pineal gland, retinae and SCN dynamically interact to maintain stable phase relationships and then influence downstream rhythms through entrainment of peripheral oscillators in the brain controlling behavior and peripheral tissues. Birds represent an excellent model for the role played by biological clocks in human neurobiology; unlike most rodent models, they are diurnal, they exhibit cognitively complex social interactions, and their circadian clocks are more sensitive to the hormone melatonin than are those of nocturnal rodents.

A possible neural cascade involving the photoneuroendocrine system (PNES) responsible for regulating gonadal development in an avian species, Gallus gallus

Neurons located in the lateral septal organ (LSO) and medial basal hypothalamus (MBH) have been proposed to be encephalic photoreceptors (EPRs), which sense photoperiodic time and initiate avian gonadal development. Controversy continues regarding the location of EPRs serving the PNES and their signal transduction pathway. Using quantitative real-time RT-PCR we determined activation of key genes following prolonged light periods and sulfamethethazine (compound known to advance light-induced testes development) in 21-day old chicks. Earliest activation occurred in genes of vasoactive intestinal polypeptide (VIP) and type 6 phosphodiesterase beta subunit (PDE-6 beta) in the LSO at 4 and 6h, respectively, after onset of light and sulfamethazine intake. In contrast, no change was detected in the MBH during the first 8h of that treatment. Thereafter, significant increases in gonadotropin releasing hormone (GnRH-1) and VIP receptor (VIPR) mRNA transcripts were detected in the bed nucleus of the pallial commissure (NCPa). Hours later, activation of all four genes (VIP, PDE-6 beta, GnRH-1, VIPR) were induced solely by photostimulation. Deiodinase 2 and tyrosine hydroxylase in the MBH did not show increased gene expression until 12h of photostimulation. Prolactin mRNA transcripts showed significant increases at 4h due to SMZ intake and at 24, 36 and 48 h due to long-day photoperiodic effects. Data suggest that VIP neurons in the LSO may serve as EPRs and utilize PDE, present in the phototransduction cascade of known photoreceptors. Additionally, VIP released from the LSO may modulate GnRH-1 neurons in the NCPa via VIP receptors by increasing GnRH-1 gene expression.

Reprogramming progeny cells of embryonic RPE to produce photoreceptors: development of advanced photoreceptor traits under the induction of neuroD

PURPOSE

In examining the prospect of producing functional photoreceptors by reprogramming the differentiation of RPE progeny cells, this study was conducted to investigate whether reprogrammed cells can develop highly specialized ultrastructural and physiological traits that characterize retinal photoreceptors.

METHODS

Cultured chick RPE cells were reprogrammed to differentiate along the photoreceptor pathway by ectopic expression of neuroD. Cellular ultrastructure was examined with electron microscopy. Cellular physiology was studied by monitoring cellular free calcium (Ca(2+)) levels in dark-adapted cells in response to light and in light-bleached cells in response to 9-cis-retinal.

RESULTS

Reprogrammed cells were found to localize red opsin protein appropriately to the apex. These cells developed inner segments rich in mitochondria, and while in culture, some formed rudimentary outer segments, analogous to those of developing photoreceptors in the retina. In response to light, reprogrammed cells reduced their Ca(2+) levels, as observed with developing retinal photoreceptors in culture. Further, on exposure to 9-cis-retinal, the light-bleached, reprogrammed cells increased their Ca(2+) levels, reminiscent of visual cycle recovery.

CONCLUSIONS

These results indicate the potential of reprogrammed cells to develop advanced ultrastructural and physiological traits of photoreceptors and point to reprogramming progeny cells of embryonic RPE as a possible alternative in producing developing photoreceptors.

Chicken lateral septal organ and other circumventricular organs form in a striatal subdomain abutting the molecular striatopallidal border

The avian lateral septal organ (LSO) is a telencephalic circumventricular specialization with liquor-contacting neurons (Kuenzel and van Tienhoven [1982] J. Comp. Neurol. 206:293-313). We studied the topological position of the chicken LSO relative to molecular borders defined previously within the telencephalic subpallium (Puelles et al. [2000] J. Comp. Neurol. 424:409-438). Differential expression of Dlx5 and Nkx2.1 homeobox genes, or the Shh gene encoding a secreted morphogen, allows distinction of striatal, pallidal, and preoptic subpallial sectors. The chicken LSO complex was characterized chemoarchitectonically from embryonic to posthatching stages, by using immunohistochemistry for calbindin, tyrosine hydroxylase, NKX2.1, and BEN proteins and in situ hybridization for Nkx2.1, Nkx2.2, Nkx6.1, Shh, and Dlx5 mRNA. Medial and lateral parts of LSO appear, respectively, at the striatal part of the septum and adjacent bottom of the lateral ventricle (accumbens), in lateral continuity with another circumventricular organ that forms along a thin subregion of the entire striatum, abutting the molecular striatopallidal boundary; we called this the "striatopallidal organ" (SPO). The SPO displays associated distal periventricular cells, which are lacking in the LSO. Moreover, the SPO is continuous caudomedially with a thin, linear ependymal specialization found around the extended amygdala and preoptic areas. This differs from SPO and LSO in some molecular aspects. We tentatively identified this structure as being composed of an "extended amygdala organ" (EAO) and a "preoptohypothalamic organ" (PHO). The position of LSO, SPO, EAO, and PHO within a linear Dlx5-expressing ventricular domain that surrounds the Nkx2.1-expressing pallidopreoptic domain provides an unexpected insight into possible common and differential causal mechanisms underlying their formation.

Chicken suprachiasmatic nuclei: I. Efferent and afferent connections

The avian circadian system is composed of multiple inputs, oscillators, and outputs. Among its oscillators are the pineal gland, retinae, and a hypothalamic structure assumed to be homologous to the mammalian suprachiasmatic nucleus (SCN). Two structures have been suggested as this homolog -- the medial SCN (mSCN) and the visual SCN (vSCN). The present study employed biotin dextran amine (BDA) and cholera toxin B subunit (CTB) as anterograde and retrograde tracers to investigate the connectivity of the mSCN and vSCN in order to address this issue. Intravitreal injections of CTB were used to determine whether one or both of these structures receives afferent input from retinal ganglion cells. Both the vSCN and mSCN receive terminal retinal input, with the strongest input terminating in the vSCN. Precise iontophoretic injections of BDA and CTB in the mSCN and vSCN were used to identify efferents and afferents. The avian mSCN and vSCN collectively express more efferents and afferents than does the mammalian SCN. A subset of these connections matches the connections that have been established in rodent species. Individually, both the mSCN and vSCN are similar to the mammalian SCN in terms of their connections. Based on these data and other studies, we present a working model of the avian SCN that includes both the mSCN and vSCN as hypothalamic oscillators. We contend that both structures are involved in a suprachiasmatic complex that, as a functional group, may be homologous to the mammalian SCN.

Attenuation of gonadal response to photostimulation following ablation of neurons in the lateral septal organ of chicks

Many avian species in temperate zones respond to long photoperiods by showing recrudescence of gonads. Compelling evidence show that non-retinal, non-pineal photoreceptors exist in the avian brain. Within the ventral forebrain are specialized neurons that respond to light found in the medial portion of a circumventricular organ called the lateral septal organ (LSO). The objective of this study was to examine whether the integrity of the LSO was critical for rapid gonadal development in young male chicks placed under long day photostimulation. Birds were initially kept on a short photoperiod (LD 8:16) until 2 weeks of age, at which time bilateral electrolytic lesions were administered to the LSO. After surgery, birds were transferred to a long photoperiod (LD 16:8) and fed a chick starter diet containing 0.2% sulfamethazine (SMZ). The combination of a long day plus SMZ is known to stimulate sexual maturation in male chicks. In four separate experiments conducted to date, bilateral lesions directed to the LSO and lesions that missed and were placed caudal and ventral to the target resulted in a significant decrease in testes weight, compared to sham-operated controls (P<0.05). Results suggest that neurons in the LSO may be directly involved in responding to long photoperiods and stimulating gonadal development in broiler chicks.