Centrifugal projections upon the retina: an anterograde tracing study in the pigeon (Columba livia)

The retinal projection to the nucleus lentiformis mesencephali in zebra finch (Taeniopygia guttata) and Anna's hummingbird (Calypte anna)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are retinal recipient nuclei involved in the analysis of optic flow and the generation of the optokinetic response. In both pigeons and chickens, retinal inputs to the nBOR arise from displaced ganglion cells (DGCs), which are found at the margin of the inner nuclear and inner plexiform layers. The LM receives afferents from retinal ganglion cells, but whether DGCs also project to LM is a matter of debate. Previous work in chickens had concluded that DGCs do not project to LM, but a recent study in pigeons found that both retinal ganglion cells and DGCs project to LM. These findings leave open the question of whether there are species differences with respect to the DGC projection to LM. In the present study, we made small injections of retrograde tracer into the LM in a zebra finch and an Anna's hummingbird. In both cases, retrogradely labeled retinal ganglion cells and DGCs were observed. These results suggest that a retinal input to the LM arising from DGCs is characteristic of most, if not all, birds.

The centrifugal visual system of a palaeognathous bird, the Chilean Tinamou (Nothoprocta perdicaria)

The avian centrifugal visual system, which projects from the brain to the retina, has been intensively studied in several Neognathous birds that have a distinct isthmo-optic nucleus (ION). However, birds of the order Palaeognathae seem to lack a proper ION in histologically stained brain sections. We had previously reported in the palaeognathous Chilean Tinamou (Nothoprocta perdicaria) that intraocular injections of Cholera Toxin B subunit retrogradely label a considerable number of neurons, which form a diffuse isthmo-optic complex (IOC). In order to better understand how this IOC-based centrifugal visual system is organized, we have studied its major components by means of in vivo and in vitro tracing experiments. Our results show that the IOC, though structurally less organized than an ION, possesses a dense core region consisting of multipolar neurons. It receives afferents from neurons in L10a of the optic tectum, which are distributed with a wider interneuronal spacing than in Neognathae. The tecto-IOC terminals are delicate and divergent, unlike the prominent convergent tecto-ION terminals in Neognathae. The centrifugal IOC terminals in the retina are exclusively divergent, resembling the terminals from "ectopic" centrifugal neurons in Neognathae. We conclude that the Tinamou's IOC participates in a comparable general IOC-retina-TeO-IOC circuitry as the neognathous ION. However, the connections between the components are structurally different and their divergent character suggests a lower spatial resolution. Our findings call for further comparative studies in a broad range of species for advancing our understanding of the evolution, plasticity and functional roles of the avian centrifugal visual system.

The effect of unilateral disruption of the centrifugal visual system on normal eye development in chicks raised under constant light conditions

The centrifugal visual system (CVS) comprises a visually driven isthmic feedback projection to the retina. While its function has remained elusive, we have previously shown that, under otherwise normal conditions, unilateral disconnection of centrifugal neurons in the chick affected eye development, inducing a reduced rate of axial elongation that resulted in a unilateral hyperopia in the eye contralateral to the lesion. Here, we further investigate the role of centrifugal neurons in ocular development in chicks reared in an abnormal visual environment, namely constant light. The baseline ocular phenotype of constant light-reared chicks (n = 8) with intact centrifugal neurons was assessed over a 3-week post-hatch time period and, subsequently, compared to chicks raised in normal diurnal lighting (n = 8). Lesions of the isthmo-optic tract or sham surgeries were performed in another seventeen chicks, all raised under constant light. Ocular phenotyping was performed over a 21-day postoperative period to assess changes in refractive state (streak retinoscopy) and ocular component dimensions (A-scan ultrasonography). A pathway-tracing paradigm was employed to quantify lesion success. Chicks raised in constant light conditions with an intact CVS developed shallower anterior chambers combined with elongated vitreous chambers relative to chicks raised in normal diurnal lighting. Seven days following surgery to disrupt centrifugal neurons, a significant positive correlation between refractive error asymmetry between the eyes and lesion success was evident, characterized by hyperopia in the eye contralateral to the lesion. By 21 days post-surgery, these contralateral eyes had become emmetropic, while ipsilateral eyes had developed relative axial hyperopia. Our results provide further support for the hypothesis that the centrifugal visual system can modulate eye development.

Retinal cross talk in the mammalian visual system

The existence and functional relevance of efferent optic nerve fibers in mammals have long been debated. While anatomical evidence for cortico-retinal and retino-retinal projections is substantial, physiological evidence is lacking, as efferent fibers are few in number and are severed in studies of excised retinal tissue. Here we show that interocular connections contribute to retinal bioelectrical activity in adult mammals. Full-field flash electroretinograms (ERGs) were recorded from one or both eyes of Brown-Norway rats under dark-adapted (n = 16) and light-adapted (n = 11) conditions. Flashes were confined to each eye by an opaque tube that blocked stray light. Monocular flashes evoked a small (5-15 μV) signal in the nonilluminated eye, which was named "crossed ERG" (xERG). The xERG began under dark-adapted conditions with a positive (xP1) wave that peaked at 70-90 ms and ended with slower negative (xN1) and positive (xP2) waves from 200 to 400 ms. xN1 was absent under light-adapted conditions. Injection of tetrodotoxin in either eye (n = 15) eliminated the xERG. Intraocular pressure elevation of the illuminated eye (n = 6) had the same effect. The treatments also altered the ERG b-wave in both eyes, and the alterations correlated with xERG disappearance. Optic nerve stimulation (n = 3) elicited a biphasic compound action potential in the nonstimulated nerve with 10- to 13-ms latency, implying that the xERG comes from slow-conducting (W type) fibers. Monocular dye application (n = 7) confirmed the presence of retino-retinal ganglion cells in adult rats. We conclude that mammalian eyes communicate directly with each other via a handful of optic nerve fibers. The cross talk alters retinal activity in rats, and perhaps other animals.

Integrating brain, behavior, and phylogeny to understand the evolution of sensory systems in birds

The comparative anatomy of sensory systems has played a major role in developing theories and principles central to evolutionary neuroscience. This includes the central tenet of many comparative studies, the principle of proper mass, which states that the size of a neural structure reflects its processing capacity. The size of structures within the sensory system is not, however, the only salient variable in sensory evolution. Further, the evolution of the brain and behavior are intimately tied to phylogenetic history, requiring studies to integrate neuroanatomy with behavior and phylogeny to gain a more holistic view of brain evolution. Birds have proven to be a useful group for these studies because of widespread interest in their phylogenetic relationships and a wealth of information on the functional organization of most of their sensory pathways. In this review, we examine the principle of proper mass in relation differences in the sensory capabilities among birds. We discuss how neuroanatomy, behavior, and phylogeny can be integrated to understand the evolution of sensory systems in birds providing evidence from visual, auditory, and somatosensory systems. We also consider the concept of a "trade-off," whereby one sensory system (or subpathway within a sensory system), may be expanded in size, at the expense of others, which are reduced in size.

Retinal projection to the pretectal nucleus lentiformis mesencephali in pigeons (Columba livia)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are retinal-recipient nuclei involved in the analysis of optic flow and the generation of the optokinetic response. The nBOR receives retinal input from displaced ganglion cells (DGCs), which are found at the margin of the inner nuclear and inner plexiform layers, rather than the ganglion cell layer. The LM receives afferents from retinal ganglion cells, but whether DGCs also project to LM remains unclear. To resolve this issue, we made small injections of retrograde tracer into LM and examined horizontal sections through the retina. For comparison, we also had cases with injections in nBOR, the optic tectum, and the anterior dorsolateral thalamus (the equivalent to the mammalian lateral geniculate nucleus). From all LM injections both retinal ganglion cells and DGCs were labeled. The percentage of DGCs, as a proportion of all labeled cells, varied from 2-28%, and these were not different in morphology or size compared to those labeled from nBOR, in which the proportion of DGCs was much higher (84-93%). DGCs were also labeled after injections into the anterior dorsolateral thalamus. The proportion was small (2-3%), and these DGCs were smaller in size than those projecting to the nBOR and LM. No DGCs were labeled from an injection in the optic tectum. Based on an analysis of size, we suggest that different populations of retinal ganglion cells are involved in the projections to LM, nBOR, the optic tectum, and the anterior dorsolateral thalamus.

Mosaic and concerted evolution in the visual system of birds

Two main models have been proposed to explain how the relative size of neural structures varies through evolution. In the mosaic evolution model, individual brain structures vary in size independently of each other, whereas in the concerted evolution model developmental constraints result in different parts of the brain varying in size in a coordinated manner. Several studies have shown variation of the relative size of individual nuclei in the vertebrate brain, but it is currently not known if nuclei belonging to the same functional pathway vary independently of each other or in a concerted manner. The visual system of birds offers an ideal opportunity to specifically test which of the two models apply to an entire sensory pathway. Here, we examine the relative size of 9 different visual nuclei across 98 species of birds. This includes data on interspecific variation in the cytoarchitecture and relative size of the isthmal nuclei, which has not been previously reported. We also use a combination of statistical analyses, phylogenetically corrected principal component analysis and evolutionary rates of change on the absolute and relative size of the nine nuclei, to test if visual nuclei evolved in a concerted or mosaic manner. Our results strongly indicate a combination of mosaic and concerted evolution (in the relative size of nine nuclei) within the avian visual system. Specifically, the relative size of the isthmal nuclei and parts of the tectofugal pathway covary across species in a concerted fashion, whereas the relative volume of the other visual nuclei measured vary independently of one another, such as that predicted by the mosaic model. Our results suggest the covariation of different neural structures depends not only on the functional connectivity of each nucleus, but also on the diversity of afferents and efferents of each nucleus.

Functional implications of species differences in the size and morphology of the isthmo optic nucleus (ION) in birds

In birds, there is a retinofugal projection from the brain to the retina originating from the isthmo optic nucleus (ION) in the midbrain. Despite a large number of anatomical, physiological and histochemical studies, the function of this retinofugal system remains unclear. Several functions have been proposed including: gaze stabilization, pecking behavior, dark adaptation, shifting attention, and detection of aerial predators. This nucleus varies in size and organization among some species, but the relative size and morphology of the ION has not been systematically studied. Here, we present a comparison of the relative size and morphology of the ION in 81 species of birds, representing 17 different orders. Our results show that several orders of birds, besides those previously reported, have a large, well-organized ION, including: hummingbirds, woodpeckers, coots and allies, and kingfishers. At the other end of the spectrum, parrots, herons, waterfowl, owls and diurnal raptors have relatively small ION volumes. ION also appears to be absent or unrecognizable is several taxa, including one of the basal avian groups, the tinamous, which suggests that the ION may have evolved only in the more modern group of birds, Neognathae. Finally, we demonstrate that evolutionary changes in the relative size and the cytoarchitectonic organization of ION have occurred largely independent of phylogeny. The large relative size of the ION in orders with very different lifestyles and feeding behaviors suggest there is no clear association with pecking behavior or predator detection. Instead, our results suggest that the ION is more complex and enlarged in birds that have eyes that are emmetropic in some parts of the visual field and myopic in others. We therefore posit that the ION is involved in switching attention between two parts of the retina i.e. from an emmetropic to a myopic part of the retina.

What the bird's brain tells the bird's eye: the function of descending input to the avian retina

As Cajal discovered in the late 19th century, the bird retina receives a substantial input from the brain. Approximately 10,000 fibers originating in a small midbrain nucleus, the isthmo-optic nucleus (ION), terminate in each retina. The input to the ION is chiefly from the optic tectum which, in the bird, is the primary recipient of retinal input. These neural elements constitute a closed loop, the centrifugal visual system (CVS), beginning and ending in the retina, that delivers positive feedback to active ganglion cells. Several features of the system are puzzling. All fibers from the ION terminate in the ventral retina and an unusual axon-bearing amacrine cell, the target cell, is the postsynaptic partner of these fibers. While the rest of the CVS is orderly and retinotopic, target cell axons project seemingly at random, mostly to distant parts of the retina. We review here the most significant features of the anatomy and physiology of the CVS with a view to understanding its function. We suggest that many of the facts about this system, including some that are otherwise difficult to explain, can be accommodated within the hypothesis that the images of shadows cast on the ground or on objects in the environment, initiate a rapid and parallel search of the sky for a possible aerial predator. If a predator is located, shadow and predator would be temporarily linked together and tracked by the CVS.

Distribution and structure of efferent synapses in the chicken retina

The visual system of birds includes an efferent projection from a visual area, the isthmo-optic nucleus in the midbrain, back to the retina. Using a combination of anterograde labeling of efferent fibers, reconstruction of dye-filled neurons, NADPH-diaphorase staining, and transmission electron microscopy, we have examined the distribution of efferent fibers and their synaptic structures in the chicken retina. We show that efferent fibers terminate strictly within the ventral retina. In two completely mapped retinas, only 2 fibers from a total of 15,359 terminated in the dorsal retina. The major synapse made by each efferent fiber is with a single efferent target amacrine cell (TC). This synapse consists of 5-25 boutons of 2 microm diameter, each with multiple active zones, pressed into the TC soma or synapsing with a basketwork of rudimentary TC dendrites in the inner nuclear layer (INL). This basketwork, which is sheathed by Muller cell processes, defines a private neuropil in the INL within which TCs were also seen to receive input from retinal neurons. In addition to the major synapse, efferent fibers typically produce several very thin processes that terminate nearby in single small boutons and for which the soma of a local amacrine cell is one of the likely postsynaptic partners. A minority of efferent fibers also give rise to a thicker process, terminating in a strongly diaphorase-positive ball about 5 microm in diameter.

The area centralis in the chicken retina contains efferent target amacrine cells

The retinas of birds receive a substantial efferent, or centrifugal, input from a midbrain nucleus. The function of this input is presently unclear, but previous work in the pigeon has shown that efferent input is excluded from the area centralis, suggesting that the functions of the area centralis and the efferent system are incompatible. Using an antibody specific to rods, we have identified the area centralis in another species, the chicken, and mapped the distribution of the unique amacrine cells that are the postsynaptic partners of efferent fibers. Efferent target amacrine cells are found within the chicken area centralis and their density is continuous across the border of the area centralis. In contrast to the pigeon retina then, we conclude that the chicken area centralis receives efferent input. We suggest that the difference between the two species is attributable to the presence of a fovea within the area centralis of the pigeon and its absence from that of the chicken.

Breaking the balance: ocular BDNF-injections induce visual asymmetry in pigeons

In pigeons, asymmetric photic stimulation around hatch induces functional visual asymmetries that are accompanied by left-right differences in tectal cell sizes. Different aspects of light-dependent neuronal differentiation are known to be mediated by the brain-derived neurotrophic factor (BDNF). Therefore, we investigated by means of single or triple BDNF- or saline-injections into the right eye of dark-incubated pigeon hatchlings if ocular BDNF enrichment mimics the effects of biased visual input. As adults, the birds were tested in a grit-grain discrimination task to estimate the degree and direction of visual lateralization followed by a morphometric analysis of retinal and tectal cells. The grit-grain discrimination task demonstrated that triple BDNF-injections enhanced visuoperceptual and visuomotor functioning of the left eye system. Morphometric analysis showed bilateral cell-type dependent effects within the optic tectum. While single-BDNF injections increased cell body sizes of calbindin-positive efferent neurons, triple-injections decreased cell sizes of parvalbumin-positive cells. Moreover, single BDNF-injections increased retinal cell sizes within the contralateral eye. Analysis of BDNF-induced intracellular signaling demonstrated enhanced downstream Ras activation for at least 24 h within both tectal halves whereas activity changes within the contralateral retina could not be detected. This points to primarily tectal effects of ocular BDNF. In sum, exogenous BDNF modulates the differentiation of retinotectal circuitries and dose-dependently shifts lateralized visuomotor processing towards the noninjected side. Since these effects are opposite to embryonic light stimulation, it is unlikely that the impact of light onto asymmetry formation is mediated by retinal BDNF.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Association amacrine cells of Ramón y Cajal: Rediscovery and reinterpretation

In 1895, by means of the Golgi method, Santiago Ramón y Cajal discovered a cell having a unique morphology in the avian retina. This cell had its cell body in the amacrine cell level of the inner nuclear layer, only a few rudimentary dendrites at the outermost level of the inner plexiform layer (IPL), and a long axon coursing horizontally and terminating in the IPL. Despite having defined amacrine cells as cells without axons, Cajal named this cell type “association amacrine cell” (AAC). This discovery was not confirmed by other investigators for nearly a century. Very recently, however, isthmo-optic target cells (IOTCs), which receive the terminals of centrifugal fibers emanating from the isthmo-optic nucleus, have been identified as one type of AAC. As summarized and discussed in this review, the morphology of the AACs as described by Cajal has been completely confirmed. However, since these cells appear to be classical polarized, monoaxonal neurons and lack the dendritic interactions that are typical of amacrine cells, they should be regarded as a distinct type of retinal interneuron and not as amacrine cells.

Identification of neurons with acetilcholinesterase and NADPH-diaphorase activities in the centrifugal visual system of the chick

The isthmo-optic nuclei (ION) and ectopic neurons, which constitute the centrifugal visual system (CVS), are thought to be cholinoceptive and nitrergic. However, it is not clear which neurons express these markers, namely the ones that project to the retina rather than in neurons that only participate in a local circuit. Therefore, to characterize the neurochemical patterns of the centrifugal visual system in the post-hatched chick, retinopetal cells of the isthmo-optic nuclei and the ectopic region were identified via immunolabeling for cholera toxin, a neuronal tracer, which has been injected in the ocular globe. Then, double labeled with acetylcholinesterase histochemistry to reveal cholinergic synapses, or NADPH-diaphorase histochemistry as a nitrergic marker. Briefly, acetylcholinesterase activity was present mainly in cholera toxin labeled cell bodies of the isthmo-optic nucleus and the ectopic region indicating that retinal projecting neurons of centrifugal visual system comprise a cholinoceptive pathway. On the other hand, NADPH-diaphorase histochemistry was present in the neuropile and sparse cell bodies inside of the isthmo-optic nucleus and in ectopic neurons which were not cholera toxin positive suggesting their role in an intrinsic circuit of the centrifugal visual system. These data support the idea that these two neurochemical systems are present in distinct neuronal populations in the centrifugal visual system.

Morphology and connections of nucleus isthmi pars magnocellularis in chicks (Gallus gallus)

The nucleus isthmi pars magnocellularis (Imc) and pars parvocellularis (Ipc) influence the receptive field structure of neurons in the optic tectum (TeO). To understand better the anatomical substrate of isthmotectal interactions, neuronal morphology and connections of Imc were examined in chicks (Gallus gallus). Cholera toxin B injection into TeO demonstrated a coarse topographical projection from TeO upon Imc. Retrogradely labeled neurons were scattered throughout Imc and in low density within the zone of anterogradely labeled terminals, suggesting a heterotopic projection from Imc upon TeO. This organization differed from the precise homotopic reciprocal connections of Ipc and the nucleus isthmi pars semilunaris (SLu) with TeO. By using slice preparations, extracellular biotinylated dextran amine injections demonstrated a dense projection from most neurons in Imc upon both Ipc and SLu. Intracellular filling of Imc neurons with biocytin revealed two cell types. The most common, Imc-Is, formed a widely ramifying axonal field in both Ipc and SLu, without obvious topography. A less frequently observed cell type, Imc-Te, formed a widely ramifying terminal field in layers 10-12 of TeO. No neurons were found to project upon both Ipc/SLu and TeO. Both types possessed local axon collaterals and flat dendritic fields oriented parallel to the long axis of Imc. Imc neurons contain glutamic acid decarboxylase, which is consistent with Imc participating in center-surround or other wide-field inhibitory isthmotectal interactions. The laminar and columnar pattern of isthmotectal terminals also suggests a means of interacting with multiple tectofugal pathways, including the stratified subpopulations of tectorotundal neurons participating in motion detection.

Centrifugal visual system of Crocodylus niloticus: a hodological, histochemical, and immunocytochemical study

The retinopetal neurons of Crocodylus niloticus were visualized by retrograde transport of rhodamine beta-isothiocyanate or Fast Blue administered by intraocular injection. Approximately 6,000 in number, these neurons are distributed in seven regions extending from the mesencephalic tegmentum to the rostral rhombencephalon, approximately 70% being located contralaterally to the injected eye. None of the centrifugal neurons projects to both retinae. The retinopetal neurons are located in rostrocaudal sequence in seven regions: the formatio reticularis lateralis mesencephali, the substantia nigra, the griseum centralis tectalis, the nucleus subcoeruleus dorsalis, the nucleus isthmi parvocellularis, the locus coeruleus, and the commissura nervi trochlearis. The greatest number of cells (approximately 93%) is found in the nucleus subcoeruleus dorsalis. The majority are multipolar or bipolar in shape and resemble the ectopic centrifugal visual neurons of birds, although a small number of monopolar neurons resembling those of the avian isthmo-optic nucleus may also be observed. A few retinopetal neurons in the griseum centralis tectalis were tyrosine hydroxylase (TH) immunoreactive. Moreover, in the nuclei subcoeruleus dorsalis and isthmi parvocellularis, both ipsilaterally and contralaterally, approximately one retinopetal neuron in three (35%) was immunoreactive to nitric oxide synthase (NOS), and a slightly higher proportion (38%) of retinopetal neurons were immunoreactive for choline acetyltransferase (ChAT). Some of them contained colocalized ChAT and NOS/reduced nicotinamide adenine dinucleotide phosphate-diaphorase. Fibers immunoreactive to TH, serotonin (5-HT), neuropeptide Y (NPY), or Phe-Met-Arg-Phe-amide (FMRF-amide) were frequently observed to make intimate contact with rhodamine-labeled retinopetal neurons. These findings are discussed in relation to previous results obtained in other reptilian species and in birds.

Tecto-isthmo-optic transmission in pigeons is mediated by glutamate and nitric oxide

The isthmo-optic nucleus of the centrifugal system in birds receives primarily input from the ipsilateral tectum and projects to the contralateral retina. The present study using brain slices and microiontophoresis shows that synaptic transmission from the tectum to the centrifugal nucleus in pigeons is excitatory. About 75% of tecto-isthmo-optic fibers are glutamatergic, mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid but not N-methyl-D-aspartate-receptors, and 25% of others may use nitric oxide as a transmitter or modulator. On the other hand, about 60% of isthmo-optic cells receive glutamatergic afferents, 20% receive nitric oxidergic afferents, and 20% of others receive both glutamatergic and nitric oxidergic afferents from the tectum. In the last group, it is more likely that both glutamate and nitric oxide may co-release from the same tecto-isthmo-optic terminals. All the isthmo-optic cells examined in the present study also receive gamma-aminobutyric acid (GABA)ergic afferents via GABA(A) and GABA(B) receptors probably from some extratectal structures.

Sleep modifies retinal ganglion cell responses in the normal rat

Recordings were obtained from the visual system of rats as they cycled normally between waking (W), slow-wave sleep (SWS), and rapid eye movement (REM) sleep. Responses to flashes delivered by a light-emitting diode attached permanently to the skull were recorded through electrodes implanted on the cornea, in the chiasm, and on the cortex. The chiasm response reveals the temporal order in which the activated ganglion cell population exits the eyeball; as reported, this triphasic event is invariably short in latency (5--10 ms) and around 300 ms in duration, called the histogram. Here we describe the differences in the histograms recorded during W, SWS, and REM. SWS histograms are always larger than W histograms, and an REM histogram can resemble either. In other words, the optic nerve response to a given stimulus is labile; its configuration depends on whether the rat is asleep or awake. We link this physiological information with the anatomical fact that the brain dorsal raphe region, which is known to have a sleep regulatory role, sends fibers to the rat retina and receives fibers from it. At the cortical electrode, the visual cortical response amplitudes also vary, being largest during SWS. This well known phenomenon often is explained by changes taking place at the thalamic level. However, in the rat, the labile cortical response covaries with the labile optic nerve response, which suggests the cortical response enhancement during SWS is determined more by what happens in the retina than by what happens in the thalamus.

Evidence of muscarinic acetylcholine receptors in the retinal centrifugal system of the chick

In this study we characterize the presence of muscarinic acetylcholine receptors (mAChR) in the isthmo-optic nucleus (ION) of chicks by immunohistochemistry with the M35 antibody. Some M35-immunoreactive fibers were observed emerging from the retinal optic nerve insertion, suggesting that they could be centrifugal fibers. Indeed, intraocular injections of cholera toxin B (CTb), a retrograde tracer, and double-labeling with M35 and CTb in the ION confirmed this hypothesis. The presence of M35-immunoreactive cells and the possible mAChR expression in ION and ectopic neuron cells in the chick brain strongly suggest the existence of such a cholinergic system in this nucleus and that acetylcholine release from amacrine cells may mediate interactions between retinal cells and ION terminals.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Electrical interaction between neurons in the pigeon isthmo-optic nucleus

The present study used brain slices to investigate interneuronal communication in the isthmo-optic nucleus in pigeons. Electrical stimulation of the isthmo-optic tract generated a transmembrane potential in isthmo-optic cells that was obtained by subtracting the extracellular potential from the intracellular potential. This transmembrane potential resulted in enhancement of excitability and/or in production of spikes in 42 (63%) cells. In most cases, proximal axons marked in brain slices by Lucifer yellow were too short to reach the stimulation site, indicating that spikes were evoked by electrical field effect or ephaptic interaction produced by nearby cells whose axons were activated by stimulation. Eleven (16%) cells discharged a spikelet, or spike that was abolished by hyperpolarizing current injection leaving a spikelet. Markings of five of these cells all indicated the presence of dye-couplings, each of which consisted of a pair of cells. Fourteen (21%) cells only produced antidromic spikes with a short and constant latency. Four of these cells were marked and their axons passed through the stimulation site, implying that their nearby cells' axons might be cut too short to be electrically stimulated or they were in a sparse-cell area. The present results provide electrophysiological and neuroanatomical evidence that both electrical field effect and electrical coupling may play important roles in interneuronal communication within the pigeon isthmo-optic nucleus. These findings are supported by anatomical arrangement of densely packed cells and their oriented dendrites in this centrifugal nucleus.

Spatiotemporal profile of synaptic activation produced by the electrical and visual stimulation of retinal inputs to the optic tectum: a current source density analysis in the pigeon (Columba livia)

The optic tectum of the pigeon is a highly organized, multilayered structure that receives a massive polystratified afference of at least five different populations of retinal ganglion cells and gives rise to various anatomically segregated efferent systems. The synaptic organization of retino-tectal circuitry is, at present, mostly unknown. To investigate the spatiotemporal profile of synaptic activation produced by differential (electrical and visual) stimulation of the retinal inputs, we performed a high-spatial-resolution current source density analysis in the optic tectum of the anaesthetized pigeon. Electrical stimuli consisted of brief pulses of different durations applied to the optic nerve head, while visual stimuli consisted of light flashes of different intensities. Electrical stimulation generated sinks confined to retinorecipient layers. The temporal structure, spatial location and thresholds of these sinks indicated that they are all due to primary tectal synapses of retinal fibers with different conduction velocities. Sinks evoked by the fastest retinal axons were more superficially located than sinks produced by slower retinal fibers. Visual stimulation, on the other hand, resulted in a more complex pattern of current sinks, with various sinks located in the retinorecipient layers and also well below. Visual stimulation induced action potentials at superficial as well as deep tectal levels. We conclude that electrical stimulation activates most of the populations of ganglion cells as well as their primary tectal synapses, but is unable to elicit a significant activation of secondary tectal synapses. Visual stimulation, on the contrary, activates just some of the incoming retinal populations, but in a way that produces noticeable secondary activation of intratectal circuits. Laminar segregation of retinally evoked tectal activity, as reported here, has also been found in other vertebrates. Similarities and differences with previous studies are discussed.

Visual-field-specific heterogeneity within the tecto-rotundal projection of the pigeon

The organization of the tecto-rotundal projection of the pigeon was investigated by means of anterograde and retrograde tracing techniques. Besides the known organization in tecto-rotundal connectivity, this study additionally demonstrates major variations in the ascending projections of different tectal subfields. We show that the ventral tectum opticum (TO) has significantly more projections onto the nucleus rotundus (Rt) than dorsal tectal areas. This difference coincides with differential innervation densities of afferent fibres within rotundal subregions. While ventral tectal efferents project onto the ventral and central Rt, dorsal tectal efferents mainly arborize within limited areas between the central Rt and its dorsal cap, the nucleus triangularis. Thus, the ventral TO, representing the lower and frontal field of view, exhibits a quantitatively and spatially enhanced projection onto the Rt, as compared with the dorsal TO. The data presented here demonstrate a visual field-dependent projection pattern of ascending tectal outputs onto different rotundal domains. The data are consistent with behavioural studies, demonstrating tectofugal lesions to suppress visual stimulus analysis mainly within the frontal field of view.

Tectal afferents monosynaptically activate neurons in the pigeon isthmo-optic nucleus

Postsynaptic responses of 105 neurons in brain slices were intracellularly recorded from the isthmo-optic nucleus (ION) in pigeons, and 18 of these neurons were labeled with Lucifer yellow. Excitatory postsynaptic potentials (EPSPs) or spikes were produced in 93 cells, inhibitory postsynaptic potentials (IPSPs) in 10 cells, and EPSPs followed by IPSPs in two cells following electrical stimulation of the tecto-isthmooptic tract. The EPSPs occurred in an all-or-none fashion, with short latencies (1.3 +/- 0.6 ms). Repetitive stimulation increased their amplitude and duration, demonstrating that temporal summation was involved. Neurons producing excitatory responses were distributed throughout cellular layers of the nucleus. Pure IPSPs had a latency of 3.9 +/- 2.3 ms, and cells that responded in this manner were only distributed in the rostral portion of the nucleus. In the remaining two cells with EPSP-IPSP responses, the latency of excitatory responses was 1.5 ms in one cell and 1.4 ms in the other, and that of inhibitory responses was, respectively, 5.1 and 4.1 ms. Thus, it appeared that excitation was monosynaptic, whereas inhibition may be polysynaptic. Four single injections resulted in dye-coupled labeling, and two pairs of closely apposed cells fired spikes, probably resulting from spatial summation of their excitatory responses. The present study suggests that tectal cells directly activate ION neurons and that tectal fibers contact isthmo-optic neurons in a one-to-one fashion. Taken together with previous studies, it appears that the entire tecto-ION-retinal pathway is excitatory.

Functional anatomy of the avian centrifugal visual system

Although first described over a century ago, the centrifugal visual system (CVS) projecting to the retina still remains somewhat of an enigma with regard to its functional role in visually-guided behavior. The highly developed avian CVS has been the most extensively investigated and the anatomical organization of its two component centrifugal structures, the n. isthmo-opticus (NIO) and ectopic neurons (EN), including its afferent brainstem projections is reviewed. The results of double-labeling studies combining axonal tracing techniques and immunohistofluorescence have demonstrated GABA immunoreactivity (-ir) of interneurons within the neuropilar zone of the NIO, choline acetyltransferase (ChAT)-ir and nitric oxide synthase (NOS)-ir in the centrifugal cells of the NIO and EN as well as in the afferent projection neurons of layers 9/10 of the optic tectum. The data are discussed in terms of neurochemical and excitatory/inhibitory mechanisms within the different components of the avian CVS in relation to hypotheses which have implicated this system in visual attention and ground-feeding behavior.

An immunohistochemical study of putative neuromodulators and transmitters in the centrifugal visual system of the quail (Coturnix japonica)

The aim of the present study was to analyze the neurochemical properties of the centrifugal visual system (CVS) of the quail using an immunohistochemical approach by testing 16 neuropeptides (angiotensin: ANG, bradykinin: BK, cholecystokinin, dynorphin, L and M-enkephalin, beta-endorphin: beta-END, galanin, alpha-neoendorphin, neurokinin A, neuropeptide Y (NPY), ocytocin, somatostatin, substance P, vasopressin, vasoactive intestinal polypeptide) and three neurotransmitters or their synthetic enzymes (choline acetyltransferase: ChAT, tyrosine hydroxylase: TH, serotonin: 5-HT and nitric oxide synthase: NOS, including the histochemical nicotinamide adenine dinucleotide phosphate diaphorase technique). For each substance, the somatic and afferent fiber and terminal labeling was analyzed within the nucleus isthmo-opticus (NIO) and the ectopic area (EA) and compared with that of retinopetal cell bodies labeled retrogradely with RITC following its intraocular injection (double-labeling procedure). The results showed that none of the centrifugal neurons were reactive to any of the substances tested. In contrast, all with the exception of ANG, BK and beta-END, labeled fibers and terminals within the EA and only four (ChAT, 5-HT, NPY and NOS) within the NIO. Possible sources of these immunoreactive fibers terminating in the NIO and EA were investigated by mapping the somatic immunolabeling of the different substances within brainstem regions previously shown by Miceli and other authors to project upon the centrifugal neurons. The data suggests that, besides the rapid retino-tecto-NIO-retinal loop, which facilitates the transfer of meaningful or more relevant information within particular portions of the visual field, the multiple afferent input which stems from various brainstem regions utilizes a wide range of neuroactive substances. Some of these afferent projections upon the centrifugal neurons appear to belong to nonspecific systems which might play a role in modulating the excitability of centrifugal neurons as a function of arousal.

The differential distribution of AMPA-receptor subunits in the tectofugal system of the pigeon

The tectofugal system of the pigeon was examined for the distribution of several glutamate-receptor subunits (AMPA Glu R1, Glu R2/3, Glu R4) and the calcium binding protein parvalbumin. With respect to the different antigens, a heterogeneous distribution was observed. Within the optic tectum, the Glu R1 like immunoreactivity was limited to the layers 2-5, 9, 10, and sparsely in layer 13, whereas the antibody to Glu R2/3 stained cell bodies in layers 9, 10, and very heavily in layer 13. In the rotundus only the Glu R4 antigen was expressed, while within the ectostriatal complex a large number of Glu R2/3 and a smaller contingent of Glu R4 positive neurons were stained. Quantitative analysis proved significant heterogeneities of these antigens in the mesencephalic as well as the diencephalic centre of the tectofugal pathway. The number of Glu R2/3 positive neurons undergoes a two-fold increase from the dorsal to the ventral lamina 13 of the optic tectum. Alterations in the amount of immunoreactive neurons were also observed within the rotundus, since the number of Glu R4 positive cells decreased from dorsal to ventral. Morphological differences and their correlation with functional specializations in visual information processing are discussed.

Serotonergic projections to the retina of rat and goldfish

Serotonin amacrine cells have been described in the retina of vertebrates, except mouse and rat. Moreover, serotonin immunoreactive fibers have been reported in the optic nerve of rodents, frog and stingray. The purpose of this work was to study serotonin dorsal raphe nucleus projections to the retina, and to determine whether this pathway occurs in teleosts. The existence of specific connections was investigated in the rat and in the goldfish by the use of the neurotoxic substance 5,7-dihydroxytryptamine followed by monoamines determination in the retina by HPLC. The administration of 5,7-dihydroxytryptamine into the central or the lateral area of the rat dorsal raphe nucleus decreased serotonin levels in the raphe area and in the hippocampus, but only the bilateral injection into the dorsal raphe nucleus decreased it in the retina. In the goldfish, the injection of 5,7-dihydroxytryptamine into the optic tectum decreased serotonin concentration in situ and in the retina. The binding of [3H]paroxetine, a marker of serotonin transporter, was reduced in the retina of both species after the central treatment with the neurotoxic substance. In addition, the administration of the serotonin precursor 5-hydroxytryptophan into the optic tectum increased serotonin levels in the site of the injection and in the retina. The intraocular administration of 5,7-dihydroxytryptamine produced a big decrease in the content of retinal serotonin. This indoleamine and 5-hydroxyindoleacetic acid were detected in the optic nerves of rat and goldfish. The results indicate the existence of serotonergic retinopetal fibers in the rat and in the goldfish, a pathway that was not specifically demonstrated for the rat and was not previously proposed for the fish. The study of these serotonergic projections from the brain to the retina could be of interest in the understanding of the functional role of serotonin in the retina.

Brain-stem afferents upon retinal projecting isthmo-optic and ectopic neurons of the pigeon centrifugal visual system demonstrated by retrograde transneuronal transport of rhodamine beta-isothiocyanate

Brain-stem afferents to the n. isthmo-opticus (NIO) and ectopic neurons (EN) of the centrifugal visual system (CVS) were determined in the pigeon using the retrograde transneuronal transport of the fluorescent dye Rhodamine beta-isothiocyanate (RITC) after its intraocular injection. In other experiments, either RITC was injected into various periocular tissues (controls) or the retrograde tracer Fluoro-gold (FG) was injected stereotaxically in the NIO. Following intraocular injections, the RITC retrograde labeling of cell bodies was observed contralaterally in the NIO and EN and transneuronally in layers 9/10 of the optic tectum, area ventralis-Tsai, zona peri-NIII, mesencephalic and pontine reticular formation (PRF), n. linearis caudalis-raphe, and bilaterally within a region referred to as zona peri-n.NVI (Zp-n.NVI) immediately underlying the abducens nerve nucleus. None of the above structures were labeled after RITC periocular injections. The FG labeling revealed that the tectal efferent neurons were mainly medium-sized, multipolar cells whose dendrites extended superficially to retino-recipient tectal layers 6 and 5. Quantitative measurements of the distribution of layers 9/10 RITC-labeled neurons indicated the highest densities to be localized within the ventral tectum corresponding to the representation of the dorsal retina and inferior visual field. We suggest that visual and nonvisual brain-stem afferents upon NIO and EN may play a role in the proposed mechanism of the avian CVS in attention, ground-feeding behavior, and modulation of retinal sensitivity.

A centrifugally controlled circuit in the avian retina and its possible role in visual attention switching

The isthmo-optic nucleus (ION) is the main source of efferents to the retina in birds. Isthmo-optic neurons project in topographical order on amacrine cells in the ventral parts of the retina, and a subclass of these known as proprioretinal neurons project onto the dorsal retina. We propose that, through the intermediary of the amacrine target cells, activity in the isthmo-optic pathway excites ganglion cells locally in the ventral retina but inhibits those in dorsal regions. This circuit would thereby mediate centrifugally controlled switches in attention between the dorsal retina, involved in feeding, and the more ventral parts, involved in scanning for predators. This hypothesis accounts for a wide range of disparate data from behavior, comparative anatomy, endocrinology, hodology, and neurophysiology.

Tectal neurons that participate in centrifugal control of the quail retina: a morphological study by means of retrograde labeling with biocytin

An avian retinopetal nucleus, the isthmo-optic nucleus (ION), is known to receive predominant inputs from the ipsilateral optic tectum. We injected biocytin into the ION in the Japanese quail, and retrogradely labeled tectal neurons projecting to the isthmo-optic (IO) neurons, or the tecto-IO neurons, with an extraordinary Golgi-like quality. Somata of the tecto-IO neurons were located in layer 9 of the tectum. The tecto-IO neurons did not have apical dendrites extending into superficial retino-recipient layers (layers 2-7), but had descending dendrites ramified in layers 9-12. They also possessed short ascending dendrites ramifying in the upper half of layer 9. This dendritic morphology suggests that main input to the tecto-IO neurons may not be of retinal origin. The tecto-IO neurons were spatially arranged in a regular pattern. Distances between neighboring tecto-IO neurons were 50-100 microns. The dendrites of each tecto-IO neuron were not widely dispersed in the horizontal plane, and were confined in a vertically oriented column of 100-200 microns diameter. They possessed axon collaterals extending horizontally in layers 12 and 13. The estimated total number of the tecto-IO neurons was approximately 7000-10,000, which is almost identical to the total cell number of the IO neurons. To label a small number of the tecto-IO terminals, biocytin was injected into a confined area of the optic tectum. The tecto-IO fibers densely arborized in a restricted space of the ION, which is comparable to the dimension of dendritic arborization of individual IO neurons. It is suggested that single tecto-IO neurons may make contact with single IO neurons. IO neurons are known to make synaptic contact with single target cells (association cells of Cajal) in the retina (Uchiyama & Ito, 1993; Uchiyama et al., 1995). The arborization pattern of the tecto-IO neurons' dendrites indicates that the tecto-IO neurons receive very local information in sensory and sensorimotor coordinate space. The morphology of the tecto-IO terminals suggests that the spatially confined information that drives the tecto-IO neurons is sent in parallel to single IO neurons, and then further to single retinal association cells of Cajal. Cellular-level restricted projection patterns of the tecto-isthmo-retinal system may reflect its function for centrifugal control of retinal function.