Crossed and uncrossed retinal projections to the hamster circadian system

Distribution, development, and identity of retinal ganglion cells labeled in the Sert-Cre reporter mouse

The mouse retina contains over 40 types of retinal ganglion cells (RGCs) that differ in morphology, function, or gene expression. RGCs also differ by whether their axons target the brain.s ipsilateral or contralateral hemisphere. Contralaterally projecting RGCs (contraRGCs) are widespread in mouse retina, whereas ipsilateral projecting RGCs (ipsiRGCs) are confined to the ventro-temporal (VT) crescent of retina. In this study, we employed the Sert-Cre transgenic line, which had been reported to selectively label ipsiRGCs, to study ipsiRGCs during development. Although the number of Cre-expressing ipsiRGCs did not significantly increase with postnatal age, the region of retina that they occupied did, and by adulthood represented ~30% of the retinal surface. Unexpectedly, genetic ablation of Sert-Cre cells failed to fully disrupt ipsilateral projecting retinal axons, suggesting that not all ipsiRGCs generated Cre in Sert-Cre mice. To test this hypothesis, we retrogradely labeled ipsiRGCs in Sert-Cre mice which revealed that not all ipsiRGCs are labeled in Sert-Cre mice and a small population of contraRGCs flanking the VT crescent generates Cre in this line. These results do not negate the usefulness of the Sert-Cre mouse but do raise important caveats to the interpretation of such studies.

Retinal input is required for the maintenance of neuronal laminae in the ventral lateral geniculate nucleus

Retinal ganglion cell (RGC) axons provide direct input into several nuclei of the mouse visual thalamus, including the dorsal lateral geniculate nucleus (dLGN), which is important for classical image-forming vision, and the ventral lateral geniculate nucleus (vLGN), which is associated with non-image-forming vision. Through both activity- and morphogen-dependent mechanisms, retinal inputs play important roles in the development of dLGN, including the refinement of retinal projections, morphological development of thalamocortical relay cells (TRCs), the timing of corticogeniculate innervation, and the recruitment of inhibitory interneurons from progenitor zones. In contrast, little is known about the role of retinal inputs in the development of vLGN. Grossly, vLGN is divided into two domains, the retinorecipient external vLGN (vLGNe) and the non-retinorecipient internal vLGN (vLGNi). We previously found that vLGNe consists of transcriptionally distinct GABAergic subtypes that are distributed into at least four adjacent laminae. At present, it remains unclear whether retinal inputs influence the development of these cell-specific neuronal laminae in vLGNe. Here, we elucidated the developmental timeline for the formation and maintenance of these laminae in the mouse vLGNe and results indicate that these laminae are specified at or before birth, well before eye-opening and the emergence of experience-dependent visual activity. We observed that mutant mice without retinal inputs have a normal laminar distribution of GABAergic cells at birth; however, after the first week of postnatal development, these mutants exhibited a dramatic disruption in the laminar organization of inhibitory neurons and clear boundaries between vLGNe and vLGNi. Overall, our results show that while the formation of cell type-specific layers in vLGNe does not depend on RGC inputs, retinal signals are critical for their maintenance.

Decussating axons segregate within the anterior core of the primate optic chiasm

BACKGROUND

The axons of ganglion cells in the nasal retina decussate at the optic chiasm. It is unclear why tumours cause more injury to crossing nasal fibres, thereby giving rise to temporal visual field loss in each eye. To address this issue, the course of fibres through the optic chiasm was examined following injection of a different fluorescent tracer into each eye of a monkey.

METHODS

Under general anaesthesia, cholera toxin subunit B-Alexa Fluor 488 was injected into the right eye and cholera toxin subunit B-Alexa Fluor 594 was injected into the left eye of a single normal adult male rhesus monkey. After a week's survival for anterograde transport, serial coronal sections through the primary optic pathway were examined.

RESULTS

A zone within the core of the anterior and mid portions of the optic chiasm was comprised entirely of crossing fibres. This zone of decussation was delineated by segregated, interwoven sheets of green (right eye) and red (left eye) fibres. It expanded steadily to fill more of the optic chiasm as fibres coursed posteriorly towards the optic tracts. Eventually, crossed fibres became completely intermingled with uncrossed fibres, so that ocular separation was lost.

CONCLUSIONS

A distinct, central compartment located within the anterior two-thirds of the optic chiasm contains only crossing fibres. Sellar tumours focus their compressive force on this portion of the structure, explaining why they so often produce visual field loss in the temporal fields.

Retinal Input to the Primate Lateral Geniculate Nucleus Revealed by Injection of a Different Label Into Each Eye

ABSTRACT

The primate lateral geniculate nucleus has long been a favorite structure among anatomists because of its striking lamination. It has been shown that each lamina receives input from a different eye using various single label techniques but never by double labeling. Here, we illustrate the organization of retinal inputs to the lateral geniculate nucleus by injection of cholera toxin-B conjugated to Alexa Fluor-488 into the right eye and cholera toxin-B conjugated to Alexa Fluor-594 into the left eye.

Columnar and Laminar Segregation of Retinal Input to the Primate Superior Colliculus Revealed by Anterograde Tracer Injection Into Each Eye

Purpose

After the lateral geniculate nucleus, the superior colliculus is the richest target of retinal projections in primates. Hubel et al. used tritium autoradiography to show that axon terminals emanating from one eye form irregular columns in the stratum griseum superficiale. Unlabeled gaps were thought to be filled by the other eye, but this assumption was never tested directly.

Methods

Experiments were performed in two normal macaques. In monkey 1, [³H]proline was injected into the left eye and the pattern of radiolabeling was examined in serial cross-sections through the entire superior colliculus. In monkey 2, cholera toxin subunit B conjugated to Alexa 488 was injected into the right eye and cholera toxin subunit B - Alexa 594 was injected into the left eye. The two fluorescent labels were compared in a reconstruction of the superior colliculus prepared from serial sections.

Results

In monkey 1, irregular columns of axon terminals were present in the superficial grey. The projection from the peripheral retina was stronger than the projection from the macula. In monkey 2, the two fluorescent Alexa tracers mainly interdigitated: a conspicuous gap in one label was usually filled by a clump of the other label. There was also partial laminar segregation of ocular inputs. In the far peripheral field representation, the contralateral eye's input generally terminated closer to the tectal surface. In the midperiphery the eyes switched, bringing the ipsilateral input nearer the surface.

Conclusions

Direct retinal input to the macaque superior colliculus is segregated into alternating columns and strata, despite the fact that tectal cells respond robustly to stimulation of either eye.

Light-induced synchronization of the SCN coupled oscillators and implications for entraining the HPA axis

The suprachiasmatic nucleus (SCN) synchronizes the physiological rhythms to the external light-dark cycle and tunes the dynamics of circadian rhythms to photoperiod fluctuations. Changes in the neuronal network topologies are suggested to cause adaptation of the SCN in different photoperiods, resulting in the broader phase distribution of neuron activities in long photoperiods (LP) compared to short photoperiods (SP). Regulated by the SCN output, the level of glucocorticoids is elevated in short photoperiod, which is associated with peak disease incidence. The underlying coupling mechanisms of the SCN and the interplay between the SCN and the HPA axis have yet to be fully elucidated. In this work, we propose a mathematical model including a multiple-cellular SCN compartment and the HPA axis to investigate the properties of the circadian timing system under photoperiod changes. Our model predicts that the probability-dependent network is more energy-efficient than the distance-dependent network. Coupling the SCN network by intra-subpopulation and inter-subpopulation forces, we identified the negative correlation between robustness and plasticity of the oscillatory network. The HPA rhythms were predicted to be strongly entrained to the SCN rhythms with a pro-inflammatory high-amplitude glucocorticoid profile under SP. The fast temporal topology switch of the SCN network was predicted to enhance synchronization when the synchronization is not complete. These synchronization and circadian dynamics alterations might govern the seasonal variation of disease incidence and its symptom severity.

Diverse GABAergic neurons organize into subtype-specific sublaminae in the ventral lateral geniculate nucleus

In the visual system, retinal axons convey visual information from the outside world to dozens of distinct retinorecipient brain regions and organize that information at several levels, including either at the level of retinal afferents, cytoarchitecture of intrinsic retinorecipient neurons, or a combination of the two. Two major retinorecipient nuclei which are densely innervated by retinal axons are the dorsal lateral geniculate nucleus, which is important for classical image-forming vision, and ventral LGN (vLGN), which is associated with non-image-forming vision. The neurochemistry, cytoarchitecture, and retinothalamic connectivity in vLGN remain unresolved, raising fundamental questions of how it receives and processes visual information. To shed light on these important questions, used in situ hybridization, immunohistochemistry, and genetic reporter lines to identify and characterize novel neuronal cell types in mouse vLGN. Not only were a high percentage of these cells GABAergic, we discovered transcriptomically distinct GABAergic cell types reside in the two major laminae of vLGN, the retinorecipient, external vLGN (vLGNe) and the non-retinorecipient, internal vLGN (vLGNi). Furthermore, within vLGNe, we identified transcriptionally distinct subtypes of GABAergic cells that are distributed into four adjacent sublaminae. Using trans-synaptic viral tracing and in vitro electrophysiology, we found cells in each these vLGNe sublaminae receive monosynaptic inputs from retina. These results not only identify novel subtypes of GABAergic cells in vLGN, they suggest the subtype-specific laminar distribution of retinorecipient cells in vLGNe may be important for receiving, processing, and transmitting light-derived signals in parallel channels of the subcortical visual system.

LRRTM1 underlies synaptic convergence in visual thalamus

It has long been thought that the mammalian visual system is organized into parallel pathways, with incoming visual signals being parsed in the retina based on feature (e.g. color, contrast and motion) and then transmitted to the brain in unmixed, feature-specific channels. To faithfully convey feature-specific information from retina to cortex, thalamic relay cells must receive inputs from only a small number of functionally similar retinal ganglion cells. However, recent studies challenged this by revealing substantial levels of retinal convergence onto relay cells. Here, we sought to identify mechanisms responsible for the assembly of such convergence. Using an unbiased transcriptomics approach and targeted mutant mice, we discovered a critical role for the synaptic adhesion molecule Leucine Rich Repeat Transmembrane Neuronal 1 (LRRTM1) in the emergence of retinothalamic convergence. Importantly, LRRTM1 mutant mice display impairment in visual behaviors, suggesting a functional role of retinothalamic convergence in vision.

F-spondin Is Essential for Maintaining Circadian Rhythms

The suprachiasmatic nucleus (SCN) is the master pacemaker that drives circadian behaviors. SCN neurons have intrinsic, self-sustained rhythmicity that is governed by transcription-translation feedback loops. Intrinsic rhythms within the SCN do not match the day-night cycle and are therefore entrained by light-derived cues. Such cues are transmitted to the SCN by a class of intrinsically photosensitive retinal ganglion cells (ipRGCs). In the present study, we sought to identify how axons from ipRGCs target the SCN. While none of the potential targeting cues identified appeared necessary for retinohypothalamic innervation, we unexpectedly identified a novel role for the extracellular matrix protein F-spondin in circadian behavior. In the absence of F-spondin, mice lost their ability to maintain typical intrinsic rhythmicity. Moreover, F-spondin loss results in the displacement of vasoactive intestinal peptide (VIP)-expressing neurons, a class of neurons that are essential for maintaining rhythmicity among SCN neurons. Thus, this study highlights a novel role for F-spondin in maintaining circadian rhythms.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

SCN Organization Reconsidered

The SCN has long had organizational schemas imposed on it. In most, the SCN is dichotomized, with one region typically associated with the presence of vasopressin cells and the other associated with cells containing vasoactive intestinal polypeptide and certain afferent terminal fields. If assumed to be accurate, the schemas that have been intended to simplify and conceptually organize the known anatomy may actually interfere with the understanding of how various cell types and input pathways contribute to circadian rhythm regulation. This review describes inadequacies of existing schemas and notes several practical difficulties that undermine their usefulness. These include “static” versus “dynamic” anatomy, generalizations about SCN organization in relation to the plane or level of section, and the concept of differential density, all of which contribute to a view in which the SCN is substantially more complex than typically depicted in oversimplified line drawings. The need for accurate topographical description is emphasized.

Binocular Interactions in the Entrainment and Phase Shifting of Locomotor Activity Rhythms in Syrian Hamsters

To assess binocular interactions and possible ocular dominance in entrainment of circadian rhythms, Syrian hamsters maintained in LL were subjected for several weeks to schedules of eye occlusion with opaque contact lenses. In separate groups, the opaque lens was inserted into the left or right eye for 12 h at the same clock time each day. The left and right eyes of other groups were alternately occluded for 12 h each day, with initial occlusion of either the left or right eye for different groups. Amajority of hamsters entrained their locomotor activity rhythm when 1 eye was occluded for 12 h. The modified visual input imposed by covering 1 eye is sufficient to induce entrainment. Locomotor rhythms of most animals in which the 2 eyes were alternately occluded for 12 h each day phasedelayed onset of activity during the 1st few days of the lensing procedure; activity onset then free ran with • < 24 h for several weeks until entraining with • of 24 h regardless of whether the left or right eye was initially occluded. Entrainment eventually occurred when activity onset coincided with occlusion of the eye contralateral to the one that was first lensed. Photic and nonphotic explanations for eventual entrainment of locomotor rhythms are discussed, and evidence for asymmetrical photic input from the 2 eyes to the SCN is considered

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.

A Path to Sleep Is through the Eye

Sleep is expressed as a circadian rhythm and the two phenomena exist in a poorly understood relationship. Light affects each, simultaneously influencing rhythm phase and rapidly inducing sleep.

Light has long been known to modulate sleep, but recent discoveries support its use as an effective nocturnal stimulus for eliciting sleep in certain rodents. “Photosomnolence” is mediated by classical and ganglion cell photoreceptors and occurs despite the ongoing high levels of locomotion at the time of stimulus onset. Brief photic stimuli trigger rapid locomotor suppression, sleep, and a large drop in core body temperature (Tc; Phase 1), followed by a relatively fixed duration interval of sleep (Phase 2) and recovery (Phase 3) to pre-sleep activity levels. Additional light can lengthen Phase 2. Potential retinal pathways through which the sleep system might be light-activated are described and the potential roles of orexin (hypocretin) and melanin-concentrating hormone are discussed. The visual input route is a practical avenue to follow in pursuit of the neural circuitry and mechanisms governing sleep and arousal in small nocturnal mammals and the organizational principles may be similar in diurnal humans. Photosomnolence studies are likely to be particularly advantageous because the timing of sleep is largely under experimenter control. Sleep can now be effectively studied using uncomplicated, nonintrusive methods with behavior evaluation software tools; surgery for EEG electrode placement is avoidable. The research protocol for light-induced sleep is easily implemented and useful for assessing the effects of experimental manipulations on the sleep induction pathway. Moreover, the experimental designs and associated results benefit from a substantial amount of existing neuroanatomical and pharmacological literature that provides a solid framework guiding the conduct and interpretation of future investigations.

Eye-specific visual processing in the mouse suprachiasmatic nuclei

Internal circadian clocks are important regulators of mammalian biology, acting to coordinate physiology and behaviour in line with daily changes in the environment. At present, synchronisation of the circadian system to the solar cycle is believed to rely on a quantitative assessment of total ambient illumination, provided by a bilateral projection from the retina to the suprachiasmatic nuclei (SCN). It is currently unclear, however, whether this photic integration occurs at the level of individual cells or within the SCN network. Here we use extracellular multielectrode recordings from the SCN of anaesthetised mice to show that most SCN neurons receive visual input from just one eye. While we find that binocular inputs to a subset of cells are important for rapid responses to changes in illumination, we find no evidence indicating that individual SCN cells are capable of reporting the average light intensity across the whole visual field. As a result of these local irradiance coding properties, our data establish that photic integration is primarily mediated at the level of the SCN network and suggest that accurate assessments of global light levels would be impaired by non-uniform illumination of either eye.

Retinofugal projections in the mouse

The laboratory mouse is increasingly a subject for visual system investigation, but there has been no comprehensive evaluation of this species' visual projections. Here, projections were visualized and mapped following intraocular injection of cholera toxin B subunit. Tissue was processed using standard procedures applied to 30 μm free-floating sections with diaminobenzidine as the chromogen. The mouse retina projects to ~46 brain regions, including 14 not previously described in this species. These include two amygdaloid nuclei, the horizontal limb of the diagonal band, the paraventricular hypothalamic nucleus, several visual thalamic nuclei, the paranigral nucleus, several pretectal nuclei, and the dorsal cortex of the inferior colliculus. Dense retinal patches were also observed in a narrow portion of the ipsilateral intermediate layer of the superior colliculus. The superior fasciculus of the accessory optic tract, which innervates the medial terminal nucleus, was also determined to be a terminal zone throughout its length. The results are compared with previous descriptions of projections from mouse intrinsically photoreceptive retinal ganglion cells, and with data from the hamster, Nile grass rat, and laboratory rat. The retinal projection patterns are similar in all four species, although there are many differences with respect to the details. The specific visual functions of most retinorecipient areas are unknown, but there is substantial convergence of retinal projections onto regions concerned with olfaction and audition.

Nocturnal Light and Nocturnal Rodents

Investigators typically study one function of the circadian visual system at a time, be it photoreception, transmission of photic information to the suprachiasmatic nucleus (SCN), light control of rhythm phase, locomotor activity, or gene expression. There are good reasons for such a focused approach, but sometimes it is advantageous to look at the broader picture, asking how all the parts and functions complete the whole. Here, several seemingly disparate functions of the circadian visual system are examined. They share common characteristics with respect to regulation by light and, to the extent known, share a common input neuroanatomy. The argument presented is that the 3 hypothalamically mediated effects of light for which there are the most data, circadian clock phase shifts, suppression of nocturnal locomotion (“negative masking”), and suppression of nocturnal pineal function, are regulated by a common photic input pathway terminating in the SCN. For each, light triggers a relatively fixed interval response that is irradiance-dependent, the effective stimulus can be very brief light exposure, and the response continues to completion in the absence of additional light. The presence of a triggered, fixed-length response interval is of particular importance to the understanding of the circuitry and mechanisms regulating circadian rhythm phase shifts because it implies that the SCN clock response to light is not instantaneous. It also may explain why certain stimuli (neuropeptide Y or novel wheel running) administered many minutes after light exposure are able to block light-induced phase shifts. The understanding of negative masking is complicated by the fact that it can be represented as a positive change, that is, light-induced sleep, not just as a reduction in locomotion. Acute nocturnal light exposure also induces adrenal hormone secretion and a rapid drop in body temperature, physiological responses that appear to be regulated similarly to the other light effects. The likelihood of a common regulatory basis for the several responses suggests that additional light-induced responses will be forthcoming and raises questions about the relationships between light, SCN cellular anatomy, the molecular clockworks of SCN neurons, and SCN throughput mechanisms for regulating disparate downstream activities.

Modeling the seasonal adaptation of circadian clocks by changes in the network structure of the suprachiasmatic nucleus

The dynamics of circadian rhythms needs to be adapted to day length changes between summer and winter. It has been observed experimentally, however, that the dynamics of individual neurons of the suprachiasmatic nucleus (SCN) does not change as the seasons change. Rather, the seasonal adaptation of the circadian clock is hypothesized to be a consequence of changes in the intercellular dynamics, which leads to a phase distribution of electrical activity of SCN neurons that is narrower in winter and broader during summer. Yet to understand this complex intercellular dynamics, a more thorough understanding of the impact of the network structure formed by the SCN neurons is needed. To that effect, we propose a mathematical model for the dynamics of the SCN neuronal architecture in which the structure of the network plays a pivotal role. Using our model we show that the fraction of long-range cell-to-cell connections and the seasonal changes in the daily rhythms may be tightly related. In particular, simulations of the proposed mathematical model indicate that the fraction of long-range connections between the cells adjusts the phase distribution and consequently the length of the behavioral activity as follows: dense long-range connections during winter lead to a narrow activity phase, while rare long-range connections during summer lead to a broad activity phase. Our model is also able to account for the experimental observations indicating a larger light-induced phase-shift of the circadian clock during winter, which we show to be a consequence of higher synchronization between neurons. Our model thus provides evidence that the variations in the seasonal dynamics of circadian clocks can in part also be understood and regulated by the plasticity of the SCN network structure.

Circadian clocks drive the temporal coordination of internal biological processes, which in turn determine daily rhythms in physiology and behavior in the most diverse organisms. In mammals, the 24-hour timing clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is a network of interconnected neurons that serves as a robust self-sustained circadian pacemaker. The electrical activity of these neurons and their synchronization with the 24-hour cycle is established via the environmental day and night cycles. Apart from daily luminance changes, mammals are exposed to seasonal day length changes as well. Remarkably, it has been shown experimentally that the seasonal adaptations to different photoperiods are related to the modifications of the neuronal activity of the SCN due to the plasticity of the network. In our paper, by developing a mathematical model of the SCN architecture, we explore in depth the role of the structure of this important neuronal network. We show that the redistribution of the neuronal activity during winter and summer can in part be explained by structural changes of the network. Interestingly, the alterations of the electrical activity patterns can be related with small-world properties of our proposed SCN network.

Neuroanatomy of the extended circadian rhythm system

The suprachiasmatic nucleus (SCN), site of the primary clock in the circadian rhythm system, has three major afferent connections. The most important consists of a retinohypothalamic projection through which photic information, received by classical rod/cone photoreceptors and intrinsically photoreceptive retinal ganglion cells, gains access to the clock. This information influences phase and period of circadian rhythms. The two other robust afferent projections are the median raphe serotonergic pathway and the geniculohypothalamic (GHT), NPY-containing pathway from the thalamic intergeniculate leaflet (IGL). Beyond this simple framework, the number of anatomical routes that could theoretically be involved in rhythm regulation is enormous, with the SCN projecting to 15 regions and being directly innervated by about 35. If multisynaptic afferents to the SCN are included, the number expands to approximately brain 85 areas providing input to the SCN. The IGL, a known contributor to circadian rhythm regulation, has a still greater level of complexity. This nucleus connects abundantly throughout the brain (to approximately 100 regions) by pathways that are largely bilateral and reciprocal. Few of these sites have been evaluated for their contributions to circadian rhythm regulation, although most have a theoretical possibility of doing so via the GHT. The anatomy of IGL connections suggests that one of its functions may be regulation of eye movements during sleep. Together, neural circuits of the SCN and IGL are complex and interconnected. As yet, few have been tested with respect to their involvement in rhythm regulation.

A neural model for cyclovertical eye movements and their disorders

Both see-saw nystagmus and dissociated vertical divergence are cyclovertical eye movements characterized by vertical disconjugation and torsional conjugation. See-saw nystagmus is known to occur with chiasmal disorders and bitemporal hemianopia. Dissociated vertical divergence is commonly encountered in the infantile strabismus syndrome. A hypothetical model is presented in which both conditions are explained. The basic organization of the oculomotor system is most likely monocular and synchronous eye movements may have developed by neuronal coupling of the symmetrical oculomotor structures. The vertical dissociation of both eye movement disorders is explained by insufficiently developed neuronal coupling between the superior colliculi. A functional differentiation between crossed and uncrossed retinal ganglion cells fibers is assumed to cause this diminished binocular coupling in the case of see-saw nystagmus. The interstitial nucleus of Cajal may well play a pivotal role in explaining the distinct torsional eye movements in both conditions.

Shedding light on class-specific wiring: development of intrinsically photosensitive retinal ganglion cell circuitry

Neural circuits associated with retinal ganglion cells have long been used as models for investigating the mechanisms that govern circuit development and function. Similar to neurons in the brain, retinal ganglion cells are subdivided into distinct classes based upon their morphology, physiology, and patterns of connectivity. Newly developed transgenic tools in which individual classes of retinal ganglion cells are labeled with reporter proteins have recently provided a method to study the development of their class-specific circuitry. Here, we examine a single class of intrinsically photosensitive retinal ganglion cells and discuss their class-specific circuitry, as well as the cellular and molecular mechanisms that govern assembly of this circuitry.

Neurons identified by NeuN/Fox-3 immunoreactivity have a novel distribution in the hamster and mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) has several structural characteristics and cell phenotypes shared across species. Here, we describe a novel feature of SCN anatomy that is seen in both hamster and mouse. Frozen sections through the SCN were obtained from fixed brains and stained for the presence of immunoreactivity to neuronal nuclear protein (NeuN-IR) using a mouse monoclonal antibody which is known to exclusively identify neurons. NeuN-IR did not identify all SCN neurons as medial NeuN-IR neurons were generally not present. In the hamster, NeuN-IR cells are present rostrally, scattered in the dorsal half of the nucleus. More caudally, the NeuN-IR cells are largely, but not exclusively, scattered inside the lateral and dorsolateral border. At mid- to mid-caudal SCN levels, a dense group of NeuN-IR cells extends from the dorsolateral border ventromedially to encompass the central subnucleus of the SCN (SCNce). The pattern is similar in the mouse SCN. NeuN-IR does not co-localize with either cholecystokinin- or vasoactive intestinal polypeptide, but does with vasopressin-IR in the caudal SCN. In the hamster SCNce, numerous cells contain both calbindin- and NeuN-IR. The distribution of NeuN-IR cells in the SCN is unique, especially with regard to its generally lateral location through the length of the nucleus. The distribution of NeuN-IR cells is not consistent with most schemas representing SCN organization or with terminology referring to its widely accepted subdivisions. NeuN has recently been identified as Fox-3 protein. Its function in the SCN is not known, nor is it known why a large proportion of SCN cells do not contain NeuN-IR.

Experience-independent development of the hamster circadian visual system

Experience-dependent functional plasticity is a hallmark of the primary visual system, but it is not known if analogous mechanisms govern development of the circadian visual system. Here we investigated molecular, anatomical, and behavioral consequences of complete monocular light deprivation during extended intervals of postnatal development in Syrian hamsters. Hamsters were raised in constant darkness and opaque contact lenses were applied shortly after eye opening and prior to the introduction of a light-dark cycle. In adulthood, previously-occluded eyes were challenged with visual stimuli. Whereas image-formation and motion-detection were markedly impaired by monocular occlusion, neither entrainment to a light-dark cycle, nor phase-resetting responses to shifts in the light-dark cycle were affected by prior monocular deprivation. Cholera toxin-b subunit fluorescent tract-tracing revealed that in monocularly-deprived hamsters the density of fibers projecting from the retina to the suprachiasmatic nucleus (SCN) was comparable regardless of whether such fibers originated from occluded or exposed eyes. In addition, long-term monocular deprivation did not attenuate light-induced c-Fos expression in the SCN. Thus, in contrast to the thalamocortical projections of the primary visual system, retinohypothalamic projections terminating in the SCN develop into normal adult patterns and mediate circadian responses to light largely independent of light experience during development. The data identify a categorical difference in the requirement for light input during postnatal development between circadian and non-circadian visual systems.

Daily and seasonal adaptation of the circadian clock requires plasticity of the SCN neuronal network

Circadian rhythms are an essential property of many living organisms, and arise from an internal pacemaker, or clock. In mammals, this clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus, and generates an intrinsic circadian rhythm that is transmitted to other parts of the CNS. We will review the evidence that basic adaptive functions of the circadian system rely on functional plasticity in the neuronal network organization, and involve a change in phase relation among oscillatory neurons. We will illustrate this for: (i) photic entrainment of the circadian clock to the light-dark cycle; and (ii) seasonal adaptation of the clock to changes in day length. Molecular studies have shown plasticity in the phase relation between the ventral and dorsal SCN during adjustment to a shifted environmental cycle. Seasonal adaptation relies predominantly on plasticity in the phase relation between the rostral and caudal SCN. Electrical activity is integrated in the SCN, and appears to reflect the sum of the differently phased molecular expression patterns. While both photic entrainment and seasonal adaptation arise from a redistribution of SCN oscillatory activity patterns, different neuronal coupling mechanisms are employed, which are reviewed in the present paper.

Reelin is required for class-specific retinogeniculate targeting

Development of visual system circuitry requires the formation of precise synaptic connections between neurons in the retina and brain. For example, axons from retinal ganglion cells (RGCs) form synapses onto neurons within subnuclei of the lateral geniculate nucleus (LGN) [i.e., the dorsal LGN (dLGN), ventral LGN (vLGN), and intergeniculate leaflet (IGL)]. Distinct classes of RGCs project to these subnuclei: the dLGN is innervated by image-forming RGCs, whereas the vLGN and IGL are innervated by non-image-forming RGCs. To explore potential mechanisms regulating class-specific LGN targeting, we sought to identify differentially expressed targeting molecules in these LGN subnuclei. One candidate targeting molecule enriched in the vLGN and IGL during retinogeniculate circuit formation was the extracellular matrix molecule reelin. Anterograde labeling of RGC axons in mutant mice lacking functional reelin (reln(rl/rl)) revealed reduced patterns of vLGN and IGL innervation and misrouted RGC axons in adjacent non-retino-recipient thalamic nuclei. Using genetic reporter mice, we further demonstrated that mistargeted axons were from non-image-forming, intrinsically photosensitive RGCs (ipRGCs). In contrast to mistargeted ipRGC axons, axons arising from image-forming RGCs and layer VI cortical neurons correctly targeted the dLGN in reln(rl/rl) mutants. Together, these data reveal that reelin is essential for the targeting of LGN subnuclei by functionally distinct classes of RGCs.

Phase misalignment between suprachiasmatic neuronal oscillators impairs photic behavioral phase shifts but not photic induction of gene expression

The ability of the circadian pacemaker within the suprachiasmatic nucleus (SCN) to respond to light stimulation in a phase-specific manner constitutes the basis for photic entrainment of circadian rhythms. The neural basis for this phase specificity is unclear. We asked whether a lack of synchrony between SCN neurons, as reflected in phase misalignment between dorsomedial (dmSCN) and ventrolateral (vlSCN) neuronal oscillators in the rat, would impact the ability of the pacemaker to respond to phase-resetting light pulses. Light pulses delivered at maximal phase misalignment between the vlSCN and dmSCN oscillators increased expression of Per1 mRNA, regardless of the circadian phase of the dmSCN. However, phase shifts of locomotor activity were only observed when the vlSCN and dmSCN were phase aligned at the time of stimulation. Our results fit a model in which a vlSCN oscillator phase gates its own response to light and in turn relays light information to a dmSCN oscillator. This model predicts that the phase misalignment that results from circadian internal desynchronization could preserve the ability of light to induce gene expression within the master circadian clock but impair its ability to induce behavioral phase shifts.

Basis of robustness and resilience in the suprachiasmatic nucleus: individual neurons form nodes in circuits that cycle daily

How the cellular elements of the SCN are synchronized to each other is not well understood. We explore circadian oscillations manifest at the level of the cell, the tissue, and the whole animal to better understand intra-SCN synchrony and master clock function of the nucleus. At each level of analysis, responses to variations in operating environment (robustness), and following damage to components of the system (resilience), provide insight into the mechanisms whereby the SCN orchestrates circadian timing. Tissue level rhythmicity reveals circuits associated with an orderly spatiotemporal daily pattern of activity that is not predictable from their cellular elements. Specifically, in stable state, some SCN regions express low amplitude or undetectable rhythms in clock gene expression while others produce high amplitude oscillations. Within the SCN, clock gene expression follows a spatially ordered, repeated pattern of activation and inactivation. This pattern of activation is plastic and subserves responses to changes in external and internal conditions. Just as daily rhythms at the cellular level depend on sequential expression and interaction of clock genes, so too do rhythms at the SCN tissue level depend on sequential activation of local nodes. We hypothesize that individual neurons are organized into nodes that are themselves sequentially activated across the volume of the SCN in a cycle that repeats on a daily basis. We further propose that robustness is expressed in the ability of the SCN to sustain rhythmicity over a wide range of internal and external conditions, and that this reflects plasticity of the underlying nodes and circuits. Resilience is expressed in the ability of SCN cells to oscillate and to sustain activity-related rhythms at the behavioral level. Importantly, other aspects of pacemaker function remain to be examined.

The efficacy of the fluorescent conjugates of cholera toxin subunit B for multiple retrograde tract tracing in the central nervous system

Cholera toxin subunit B (CTB) is a sensitive neuroanatomical tracer that generally transports retrogradely in the nervous system, and has been used extensively in brightfield microscopy. Recently, Alexa Fluor (AF) conjugates of CTB have been made available, which now allows multiple tracing with CTB. In this study, we examined the efficacy of these new AF-CTB conjugates when injected into the brain, and compared the results to our previous experiences using fluorescent 3k dextran amines. To test this, we injected AF 488 and AF 594 CTB into the anterior cingulate cortex and the medial agranular cortex in the rat, and examined the retrograde transport to the lateral posterior nucleus of the thalamus. We found that CTB was very viscous but yet very sensitive: small injection sites revealed very intense and detailed retrograde labeling. Anterograde transport was seen only when tissue at the injection site was damaged. These findings suggest that AF-CTB is a very reliable and sensitive retrograde tracer, and should be the first choice retrograde tracer for experiments examining multiple pathways within the same brain.

Day-length encoding through tonic photic effects in the retinorecipient SCN region

The circadian clock in the suprachiasmatic nucleus (SCN) plays a critical role in seasonal processes by sensing ambient photoperiod. To explore how it measures day-length, we assessed the state of SCN oscillators using markers for neuronal activity (c-FOS) and the clock protein (PER1) in Syrian hamsters housed in long (LD, 16 : 8 h light : dark) vs. short days (SD, 8 : 16 h light : dark). During SD, there was no detectable phase dispersion across the rostrocaudal extent of the nucleus. In contrast, during LD, rhythms in the caudal SCN phase led those in the mid- and rostral SCN by 4-8 h and 8-12 h, respectively. Importantly, some neurons in the retinorecipient core SCN were unique in that they were FOS-positive during the dark phase in LD, but not SD. Transfer of LD animals to constant darkness or skeleton photoperiod revealed that dark-phase FOS expression depends on tonic light exposure rather than on intrinsic clock properties. By transferring animals from SD to LD, we next discovered that there are two separate populations of SCN cells, one responding to acute and the other to tonic light exposure. The results suggest that the seasonal encoding of day-length by the SCN entails reorganization of its constituent oscillators by a subgroup of neurons in the SCN core that respond to tonic photic cues.

Mechanisms underlying development of visual maps and receptive fields

Patterns of synaptic connections in the visual system are remarkably precise. These connections dictate the receptive field properties of individual visual neurons and ultimately determine the quality of visual perception. Spontaneous neural activity is necessary for the development of various receptive field properties and visual feature maps. In recent years, attention has shifted to understanding the mechanisms by which spontaneous activity in the developing retina, lateral geniculate nucleus, and visual cortex instruct the axonal and dendritic refinements that give rise to orderly connections in the visual system. Axon guidance cues and a growing list of other molecules, including immune system factors, have also recently been implicated in visual circuit wiring. A major goal now is to determine how these molecules cooperate with spontaneous and visually evoked activity to give rise to the circuits underlying precise receptive field tuning and orderly visual maps.

Absence of normal photic integration in the circadian visual system: response to millisecond light flashes

Light is the most prominent synchronizing stimulus for circadian rhythms. The circadian visual system responds in accordance with the energy content of photic stimuli longer than a few seconds. Here, as few as three flashes (2 ms each delivered to hamsters over 5 or 60 min at circadian time 19) elicited large phase advances. Ten or more flashes were required to induce FOS protein in the suprachiasmatic nucleus (SCN), and such induction occurred throughout the entire SCN, as well as outside the nucleus. High-density flash stimulation (0.5 s interflash interval) was ineffective, but response increased as the interval increased up to 4 s. In an irradiance response test, phase shifts appeared to be all-or-none with threshold irradiance between 140 and 1070 microW/cm2, implying lack of stimulus energy summation. Nevertheless, an irradiance ineffective when delivered as 10 flashes induced phase shifts when given as 100 flashes, but the response was substantially smaller than elicited by 10 flashes, each with approximately 1 log unit more irradiance. The results also show reduced sensitivity of flash-induced FOS response in the intergeniculate leaflet compared with the SCN, contrary to studies using longer light stimuli. Masking was robust and prolonged in response to 10 flashes. The data demonstrate that the circadian visual system responds markedly to brief, intense light stimuli without normal photic integration. This may involve a second input pathway different from that mediating the effects of longer, dimmer photic stimuli.

Central projections of melanopsin-expressing retinal ganglion cells in the mouse

A rare type of ganglion cell in mammalian retina is directly photosensitive. These novel retinal photoreceptors express the photopigment melanopsin. They send axons directly to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus (OPN), thereby contributing to photic synchronization of circadian rhythms and the pupillary light reflex. Here, we sought to characterize more fully the projections of these cells to the brain. By targeting tau-lacZ to the melanopsin gene locus in mice, ganglion cells that would normally express melanopsin were induced to express, instead, the marker enzyme beta-galactosidase. Their axons were visualized by X-gal histochemistry or anti-beta-galactosidase immunofluorescence. Established targets were confirmed, including the SCN, IGL, OPN, ventral division of the lateral geniculate nucleus (LGv), and preoptic area, but the overall projections were more widespread than previously recognized. Targets included the lateral nucleus, peri-supraoptic nucleus, and subparaventricular zone of the hypothalamus, medial amygdala, margin of the lateral habenula, posterior limitans nucleus, superior colliculus, and periaqueductal gray. There were also weak projections to the margins of the dorsal lateral geniculate nucleus. Co-staining with the cholera toxin B subunit to label all retinal afferents showed that melanopsin ganglion cells provide most of the retinal input to the SCN, IGL, and lateral habenula and much of that to the OPN, but that other ganglion cells do contribute at least some retinal input to these targets. Staining patterns after monocular enucleation revealed that the projections of these cells are overwhelmingly crossed except for the projection to the SCN, which is bilaterally symmetrical.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Binocular contributions to the responsiveness and integrative capacity of the circadian rhythm system to light

The retinohypothalamic tract (RHT), a monosynaptic retinal projection to the SCN, is the major path by which light entrains the circadian system to the external photoperiod. The circadian system of rodents effectively integrates or counts photons, and the magnitude of the rhythm phase response is proportional to the total energy of the photic stimulus. In the present studies, responsiveness to light and integrative capacity of the circadian system were tested in hamsters after reduction of retinal photoreceptor input by 50%. At CT 19, animals in constant darkness with or without unilateral retinal occlusion were exposed to 1 of 6 irradiances of 5-min white-light pulses ranging from 0.0011 to 70 microW/cm(2) or 5 white-light pulses of 0.6 microW/cm(2) with durations ranging from 0.25 to 150.0 min. Assessment of light-induced circadian rhythm phase response and Fos expression in the SCN by these animals revealed that a 50% reduction in input from photoreceptors stimulated directly with light caused a decrease in responsiveness to the longest duration and highest irradiance pulses presented. Despite this effect, both the magnitude of Fos induction in the SCN and phase-shift response remained directly proportional to the total energy in the photic stimuli. The results support the view that a reciprocal relationship between stimulus irradiance and duration persists despite the 50% reduction in retinal photoreceptor input. The mechanism of integration neither resides in the retina nor in the RHT.

Intergeniculate leaflet: contributions to photic and non-photic responsiveness of the hamster circadian system

The circadian visual system is able to integrate light energy over time, enabling phase response and Fos induction in the suprachiasmatic nucleus to increase in proportion to the total energy of the photic stimulus. In the present studies, the contribution of the intergeniculate leaflet to light energy integration by the hamster circadian rhythm system was evaluated. Fos protein is induced in intergeniculate leaflet neurons at much lower irradiance levels than seen in suprachiasmatic nucleus neurons. Bilateral N-methyl-d-aspartate lesions of the intergeniculate leaflet decreased phase response of the circadian locomotor rhythm to high irradiance and, in animals exposed to long duration light stimuli, reduced Fos induction in the suprachiasmatic nucleus. Normal photon integration, as indicated by attenuated rhythm phase shifts and Fos induction in suprachiasmatic nucleus cells in response to the energy in light stimuli, does not occur in the absence of the intergeniculate leaflet and is likely to be a property of the circadian rhythm system, rather than solely of the suprachiasmatic nucleus. Anatomical analysis showed that virtually no intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus contain Fos induced by either light or locomotion in a novel wheel. However, cells projecting to the pretectum were found to contain novel-wheel induced Fos. The intergeniculate leaflet is implicated in the normal assessment of light by the circadian rhythm system, but the circuitry by which either photic or non-photic information gains access to the suprachiasmatic nucleus may be more complex than previously thought.

Differential response of Period 1 expression within the suprachiasmatic nucleus

The suprachiasmatic nuclei (SCNs) of the hypothalamus contain a circadian clock that exerts profound control over rhythmic physiology and behavior. The clock consists of multiple autonomous cellular pacemakers distributed throughout the rat SCN. In response to a shift in the light schedule, the SCN rapidly changes phase to achieve the appropriate phase relationship with the shifted light schedule. Through use of a transgenic rat in which rhythmicity in transcription of the Period 1 gene was measured with a luciferase reporter (Per1-luc), we have been successful in tracking the time course of molecular rhythm phase readjustments in different regions of the SCN that occur in response to a shift in the light schedule. We find that different regions of the SCN phase adjust at different rates, leading to transient internal desynchrony in Per1-luc expression among SCN regions. This desynchrony among regions is most pronounced and prolonged when the light schedule is advanced compared with light schedule delays. A similar asymmetry in the speed of phase resetting is observed with locomotor behavior, suggesting that phase shifting kinetics within the SCN may underlay the differences observed in behavioral resetting to advances or delays in the light schedule.

Early and rapid targeting of eye-specific axonal projections to the dorsal lateral geniculate nucleus in the fetal macaque

The emergence of eye-specific axonal projections to the dorsal lateral geniculate nucleus (dLGN) is a well established model system for exploring the mechanisms underlying afferent targeting during development. Using modern tract tracing methods, we examined the development of this feature in the macaque, an Old World Primate with a visual system similar to that of humans. Cholera toxin beta fragment conjugated to Alexa 488 was injected into the vitreous of one eye, and CTbeta conjugated to Alexa 594 into the other eye of embryos at known gestational ages. On embryonic day 69 (E69), which is approximately 100 d before birth, inputs from the two eyes were extensively intermingled in the dLGN. However, even at this early age, portions of the dLGN were preferentially innervated by the right or left eye, and segregation is complete within the dorsalmost layers 5 and 6. By E78, eye-specific segregation is clearly established throughout the parvocellular division of the dLGN, and substantial ocular segregation is present in the magnocellular division. By E84, segregation of left and right eye axons is essentially complete, and the six eye-specific domains that characterize the mature macaque dLGN are clearly discernable. These findings reveal that targeting of eye-specific axonal projections in the macaque occurs much earlier and more rapidly than previously reported. This segregation process is completed before the reported onset of ganglion cell axon loss and retino-dLGN synapse elimination, suggesting that, in the primate, eye-specific targeting occurs independent of traditional forms of synaptic plasticity.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

c-Fos expression in the brains of behaviorally "split" hamsters in constant light: calling attention to a dorsolateral region of the suprachiasmatic nucleus and the medial division of the lateral habenula

"Splitting" of circadian activity rhythms in Syrian hamsters maintained in constant light appears to be the consequence of a reorganized SCN, with left and right halves oscillating in antiphase; in split hamsters, high mRNA levels characteristic of day and night are simultaneously expressed on opposite sides of the paired SCN. To visualize the splitting phenomenon at a cellular level, immunohistochemical c-Fos protein expression in the SCN and brains of split hamsters was analyzed. One side of the split SCN exhibited relatively high c-Fos levels, in a pattern resembling that seen in normal, unsplit hamsters during subjective day in constant darkness; the opposite side was labeled only within a central-dorsolateral area of the caudal SCN, in a region that likely coincides with a photo-responsive, glutamate receptor antagonist-insensitive, pERK-expressing cluster of cells previously identified by other laboratories. Outside the SCN, visual inspection revealed an obvious left-right asymmetry of c-Fos expression in the medial preoptic nucleus and subparaventricular zone of split hamsters killed during the inactive phase and in the medial division of the lateral habenula during the active phase (when the hamsters were running in their wheels). Roles for the dorsolateral SCN and the mediolateral habenula in circadian timekeeping are not yet understood.

Complex organization of mouse and rat suprachiasmatic nucleus

The suprachiasmatic nucleus, site of the dominant mammalian circadian clock, contains a variety of different neurons that tend to form groups within the nucleus. The present investigation used single and multiple label tract tracing and immunofluorescence methods to evaluate the relative locations of the neuron groups and to compare them with the distributions of the three major afferent projections, the retinohypothalamic tract, geniculohypothalamic tract and the serotonergic pathway from the median raphe nucleus. The suprachiasmatic nucleus has a complex order characterized by peptidergic cell groups (vasopressin, gastrin releasing peptide, vasoactive intestinal polypeptide, calbindin, calretinin, corticotrophin releasing factor and enkephalin) that, in most cases, substantially overlap. The retinohypothalamic tract projects bilaterally to virtually all the suprachiasmatic nucleus in both rat (predominantly contralateral) and mouse (symmetric) and its terminal field overlaps that for the geniculohypothalamic tract, but with distinctions visible according to density criteria; neither provides more than sparse innervation of the dorsomedial suprachiasmatic nucleus. In the mouse, the serotonergic terminal field is densest medially and ventrally, but is also distributed elsewhere with varying density. The serotonergic terminal plexus in the rat is densest centromedially and largely, but not completely, overlaps the complete distribution of retinal terminals with density much reduced in the lateral suprachiasmatic nucleus. The locations of vasopressin neurons, retinohypothalamic tract terminals and serotonergic (mouse, rat) or geniculohypothalamic tract (rat) provide evidence for three clear, but not exclusionary, sectors of the suprachiasmatic nucleus. The data, in conjunction with emerging knowledge concerning rhythmically dynamic changes in the size of regions of neuropeptide gene expression in suprachiasmatic nucleus cells, support the view that suprachiasmatic nucleus organization is more complex than a simple "core" and "shell" arrangement. While generalizations about suprachiasmatic nucleus organization can be made with respect to location of cell phenotypes or terminal fields, oversimplification may hinder, rather than facilitate, understanding of suprachiasmatic nucleus structure-function relationships.

Descending projections of the hamster intergeniculate leaflet: relationship to the sleep/arousal and visuomotor systems

The intergeniculate leaflet (IGL), homolog of the primate pregeniculate nucleus, modulates circadian rhythms. However, its extensive anatomical connections suggest that it may regulate other systems, particularly those for visuomotor function and sleep/arousal. Here, descending IGL-efferent pathways are identified with the anterograde tracer, Phaseolus vulgaris leucoagglutinin, with projections to over 50 brain stem nuclei. Projections of the ventral lateral geniculate are similar, but more limited. Many of the nuclei with IGL afferents contribute to circuitry governing visuomotor function. These include the oculomotor, trochlear, anterior pretectal, Edinger-Westphal, and the terminal nuclei; all layers of the superior colliculus, interstitial nucleus of the medial longitudinal fasciculus, supraoculomotor periaqueductal gray, nucleus of the optic tract, the inferior olive, and raphe interpositus. Other target nuclei are known to be involved in the regulation of sleep, including the lateral dorsal and pedunculopontine tegmentum. The dorsal raphe also receives projections from the IGL and may contribute to both sleep/arousal and visuomotor function. However, the locus coeruleus and medial vestibular nucleus, which contribute to sleep and eye movement regulation and which send projections to the IGL, do not receive reciprocal projections from it. The potential involvement of the IGL with the sleep/arousal system is further buttressed by existing evidence showing IGL-efferent projections to the ventrolateral preoptic area, dorsomedial, and medial tuberal hypothalamus. In addition, the great majority of all regions receiving IGL projections also receive input from the orexin/hypocretin system, suggesting that this system contributes not only to the regulation of sleep, but to eye movement control as well.

Functional analysis of the role of the median raphe as a regulator of hamster circadian system sensitivity to light

The retinohypothalamic tract, a monosynaptic retinal projection to the suprachiasmatic nucleus (SCN), is the path by which light entrains the circadian system to the external photoperiod. Serotonergic neurons in the mesencephalic median raphe nucleus (MnR) also give rise to a major SCN afferent projection. The present study was designed to determine the extent to which MnR serotonergic projections regulate sensitivity of the circadian rhythm system to light. Serotonergic neurons in the MnR were destroyed by the direct application of the neurotoxin, 5,7-dihydroxytryptamine. Animals in constant darkness were given 5-min white light pulses at circadian time 19. Light intensity varied from 0.0011 to 70 microW/cm2. Assessment of rhythm phase response to light by lesioned and control animals revealed that animals lacking the MnR serotonergic projection are considerably more sensitive to light at high irradiances. The results are consistent with behavioral and physiological evidence implicating serotonin as an inhibitory modulator of the effects of light on circadian rhythmicity.

Intergeniculate leaflet and ventral lateral geniculate nucleus afferent connections: An anatomical substrate for functional input from the vestibulo-visuomotor system

The intergeniculate leaflet (IGL) has widespread projections to the basal forebrain and visual midbrain, including the suprachiasmatic nucleus (SCN). Here we describe IGL-afferent connections with cells in the ventral midbrain and hindbrain. Cholera toxin B subunit (CTB) injected into the IGL retrogradely labels neurons in a set of brain nuclei most of which are known to influence visuomotor function. These include the retinorecipient medial, lateral and dorsal terminal nuclei, the nucleus of Darkschewitsch, the oculomotor central gray, the cuneiform, and the lateral dorsal, pedunculopontine, and subpeduncular pontine tegmental nuclei. Intraocular CTB labeled a retinal terminal field in the medial terminal nucleus that extends dorsally into the pararubral nucleus, a location also containing cells projecting to the IGL. Distinct clusters of IGL-afferent neurons are also located in the medial vestibular nucleus. Vestibular projections to the IGL were confirmed by using anterograde tracer injection into the medial vestibular nucleus. Other IGL-afferent neurons are evident in Barrington's nucleus, the dorsal raphe, locus coeruleus, and retrorubral nucleus. Injection of a retrograde, trans-synaptic, viral tracer into the SCN demonstrated transport to cells as far caudal as the vestibular system and, when combined with IGL injection of CTB, confirmed that some in the medial vestibular nucleus polysynaptically project to the SCN and monosynaptically to the IGL, as do cells in other brain regions. The results suggest that the IGL may be part of the circuitry governing visuomotor activity and further indicate that circadian rhythmicity might be influenced by head motion or visual stimuli that affect the vestibular system.