Two types of melanopsin retinal ganglion cell differentially innervate the hypothalamic suprachiasmatic nucleus and the olivary pretectal nucleus

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) innervate the hypothalamic suprachiasmatic nucleus (SCN) and the olivary pretectal nucleus (OPN), providing irradiance information for entrainment of circadian rhythms and for stimulating the pupillary light reflex. In this study, mice were used in which the melanopsin gene was replaced with the tau-lacZ gene. Heterozygous ( tau-lacZ ^+/– ) mice express both melanopsin and β-galactosidase. In tau-lacZ ^+/– mice, only ∼50% of melanopsin ipRGCs contain β-galactosidase, and these cells are specifically labeled with a C-terminus melanopsin antibody. Retrograde tracer injection into the SCN labels β-galactosidase-expressing ipRGCs (termed M1) that comprise ∼80% of the SCN-projecting ipRGCs. M1 ipRGCs and an additional set of ipRGCs (termed M2) are labeled with a melanopsin antiserum targeted against the N-terminus of the melanopsin protein; M2 ipRGCs do not contain detectable β-galactosidase, and these cells make up the remainder of the SCN-projecting RGCs. Tracer injection into the OPN labeled non-melanopsin RGCs and both types of melanopsin ipRGC: 45% M1 and 55% M2. Infection of the iris with pseudorabies virus (PRV) results in retrograde transneuronal label of OPN projection neurons that innervate preganglionic parasympathetic neurons of the Edinger-Westphal nucleus; PRV-labeled cells were located almost exclusively within the terminal field of M1 ipRGCs in the periphery (shell) of the OPN. The OPN core receives retinal input, and we hypothesize that the OPN core receives input from the M2 ipRGCs. Two subtypes of melanopsin ipRGCs project differentially to the SCN and OPN; the functional significance of ipRGCs subtypes is currently unknown.     

Intergeniculate leaflet: An anatomically and functionally distinct subdivision of the lateral geniculate complex

The intergeniculate leaflet (IGL) in the rat is a distinctive subdivision of the lateral geniculate complex that participates in the regulation of circadian function through its projections to the circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The present investigation was undertaken to provide a precise definition of the IGL and a characterization of its neuronal organization including neuronal morphology, chemical phenotype, connections, and synaptic organization. The IGL extends the entire rostrocaudal lenght of the geniculate complex and contains a distinct population of small to medium neurons. In Golgi preparations, the neurons are multipolar with dendrites largely confined to the IGL. The neurons can be subdivided into three groups on the basis of neurotransmitter content and projections: (1) neurons that contain GABA and neuropeptide Y and project to the SCN; (2) neurons that contain GABA and enkephalin and project to the contralateral IGL; and (3) a small group of neurons that projects to the SCN but not characterized as yet by neurotransmitter content. The IGL receives dense, bilateral input from retinal ganglion cells and dense substance P input of unknown origin. A number of neurons in the anterior hypothalamic area and, particualrly, the retrochiasmatic area project to the IGL, and there are sparse projections from brainstem monoamine and cholinergic neurons. The synaptic organization of the IGL is complex with afferents terminating in glomerular complexes that include axoaxonic synapatic interactions. Virtually all IGL afferents synapse upon dendrites and spines, with the densest synaptic input occurring on the distal portions of the dendritic arbor. The organization of the IGL and its connections as revealed in this analysis is in accord with its role in the integration of visual input with other information to provide feedback regulation of the SCN integration of visual input with other information to provide feedback regulation of the SCN pacemaker. © 1994 Wiley-Liss, Inc.     

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus,intergeniculate leaflet,and visual midbrain: bifurcation and melanopsin immunoreactivity

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.     

Central projections of intrinsically photosensitive retinal ganglion cells in the macaque monkey

Circadian rhythms generated by the suprachiasmatic nucleus (SCN) are entrained to the environmental light/dark cycle via intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin and the neuropeptide pituitary adenylate cyclase‐activating polypeptide (PACAP). The ipRGCs regulate other nonimage‐forming visual functions such as the pupillary light reflex, masking behavior, and light‐induced melatonin suppression. To evaluate whether PACAP‐immunoreactive retinal projections are useful as a marker for central projection of ipRGCs in the monkey brain, we characterized the occurrence of PACAP in melanopsin‐expressing ipRGCs and in the retinal target areas in the brain visualized by the anterograde tracer cholera toxin subunit B (CtB) in combination with PACAP staining. In the retina, PACAP and melanopsin were found to be costored in 99% of melanopsin‐expressing cells characterized as inner and outer stratifying melanopsin RGCs. Two macaque monkeys were anesthetized and received a unilateral intravitreal injection of CtB. Bilateral retinal projections containing colocalized CtB and PACAP immunostaining were identified in the SCN, the lateral geniculate complex including the pregeniculate nucleus, the pretectal olivary nucleus, the nucleus of the optic tract, the brachium of the superior colliculus, and the superior colliculus. In conclusion, PACAP‐immunoreactive projections with colocalized CtB represent retinal projections of ipRGCs in the macaque monkey, supporting previous retrograde tracer studies demonstrating that melanopsin‐containing retinal projections reach areas in the primate brain involved in both image‐ and nonimage‐forming visual processing. J. Comp. Neurol. 522:2231–2248, 2014. © 2014 Wiley Periodicals, Inc.     

Depletion of cortical target induced by prenatal ionizing irradiation: effects on the lateral geniculate nucleus and on the retinofugal pathways.

Studies using neonatal surgical lesions to reduce the target area of the retina have supported the idea that developing axons show only a limited specificity in their targeting. This investigation tested whether retinogeniculate axons adjust for partial target depletion by repositioning of axons. We used adult Swiss mice exposed to gamma rays at the time when layer IV cells are generated in the ventricular zone (16 days of gestation). Nissl-stained brain sections were used for histological analyses in thalamus and cortex. Retinal ganglion cells were backfilled from the optic tract with horseradish peroxidase. Intraocular injections of horseradish peroxidase were used to study the retinal projections. In the posterior cortex there was a nearly complete absence of layer IV. The irradiated animals showed a 75% reduction of the dorsal lateral geniculate nucleus. The ventral division, superior colliculus, and other visually related nuclei were not affected. The loss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%). Therefore, the shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, pattern of retinal projections did not differ from that of the controls. The effect on the dorsal lateral geniculate nucleus is discussed in the light of differences between prenatal and neonatal damage of the presumptive visual cortex. The absence of aberrant retinal projections suggests that repositioning of axons is not the first mechanism employed by retinal axons to match connections in numerically disparate populations.     

Single retinal ganglion cells projecting in bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals in the cat

In most mammals with frontalized eyes, retinal ganglion cells in the nasal or temporal retina send their axons to the contralateral or ipsilateral half, respectively, of the brain. Previous studies in the cat, however, have indicated a retinal region of “nasotemporal overlap” from which arise the retinal projections to both the contralateral and ipsilateral halves of the brain. The present study thus examined in the cat whether any retinal ganglion cells give rise to bifurcating axons that innervate both halves of the brain. By employing fluorescent retrograde double labeling, we investigated whether or not single retinal ganglion cells project bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. After Fast Blue was injected into the lateral geniculate nucleus on one side and Diamidino Yellow was injected contralaterally into the lateral geniculate nucleus, 100–200 ganglion cells in each retina were double labeled with both tracers. These double-labeled cells were distributed primarily in the temporal retina, including the region around the vertical meridian and, additionally, in the nasal retina. About 60–80% of the double-labeled cells had large cell bodies (more than 25 μm in diameter), and the others had medium-sized ones (15–25 μm in diameter). The pattern of distribution of double-labeled cells, which was observed after the combined injection into both superior colliculi, was similar to that seen after the combined injection into both lateral geniculate nuclei; more than 9% of double-labeled cells, however, were large. The results indicate that a certain population of ganglion cells in the cat retina send their axons bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. The bilaterally projecting ganglion cells are mostly large, corresponding probably to α cells (the morphological counterparts of Y cells). In comparison with the patterns of bilateral projections of single retinal ganglion cells in the rat and monkey, the pattern of the bilateral retinofugal projections in the cat could represent an intermediate between those in the rat and monkey. © 1994 Wiley-Liss, Inc.     

Intrinsic determinants of retinal axon collateralization and arborization patterns.

Permanent, novel retinal projections to the principal thalamic somatosensory (ventrobasal) or auditory (medial geniculate) nuclei can be produced in adult hamsters if the superior colliculus is ablated bilaterally and the somatosensory and auditory lemniscal axons are transected unilaterally on the day of birth. We studied the development of those novel projections by labeling retinal axons with the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate to examine the relative roles of intrinsic factors and axon-target interactions in the specification of retinal axon connections. Our principal findings are as follows: (1) In hamsters operated on the day of birth to produce the novel retinal projections, retinal ganglion cell axons projecting to the ventrobasal or medial geniculate nuclei develop in three morphologically distinct stages, i.e., elongation, collateralization, and arborization, as do retinal axons projecting to the dorsal lateral geniculate nucleus, the principal thalamic visual nucleus, in normal hamsters. (2) In both the ventrobasal and medial geniculate nuclei of operated hamsters, as in the dorsal lateral geniculate nucleus of normal hamsters, collateral branches were initially formed by retinal ganglion cell axons in both the superficial and internal components of the optic tract and only collaterals from the superficial component formed permanent projections. (3) The retinofugal axon terminal arbors in the ventrobasal and medial geniculate nuclei of mature, operated hamsters resemble the same three morphologic classes that are observed in the lateral geniculate nucleus of normal hamsters, although their absolute size appears to be altered. These data suggest that both superficial and internal optic tract axons can produce thalamic collaterals during development but that only superficial optic tract axons can permanently retain thalamic collaterals. Furthermore, the same morphologic types of retinofugal axons appear to contribute to normal and surgically induced retinal projections.     

Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the rat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (VLG) are ventral thalamic derivatives within the lateral geniculate complex. In this study, IGL and VLG efferent projections were compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the geniculate in four pathways. A dorsal pathway innervates the thalamic lateral dorsal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculohypothalamic tract to the suprachiasmatic nucleus and the anterior hypothalamus (IGL). A medial pathway innervates the zona incerta and dorsal hypothalamus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); and the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and subparaventricular zone (IGL). A caudal pathway projects medially to the posterior hypothalamic area and periaqueductal gray and caudally along the brachium of the superior colliculus to the medial pretectal area and the nucleus of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterior pretectal nucleus, the intermediate and deep gray of the superior colliculus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medial and lateral pontine gray, the parabrachial region, and the accessory inferior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives.     

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.     

Brainstem inputs to the ferret medial geniculate nucleus and the effect of early deafferentation on novel retinal projections to the auditory thalamus

Following specific neonatal brain lesions in rodents and ferrets, retinal axons have been induced to innervate the medial geniculate nucleus (MGN). Previous studies have suggested that reduction of normal retinal targets along with deafferentation of the MGN are two concurrent factors required for the induction of novel retino-MGN projections. We have examined, in ferrets, the relative influence of these two factors on the extent of the novel retinal projection. We first characterized the inputs to the normal MGN, and the most effective combination of neonatal lesions to deafferent this nucleus, by injecting retrograde tracers into the MGN of normal and neonatally operated adult ferrets, respectively. In a second group of experiments, newborn ferrets received different combinations of lesions of normal retinal targets and MGN afferents. The resulting extent of retino-MGN projections was estimated for each case at adulthood, by using intraocular injections of anterograde tracers. We found that the extent of retino-MGN projections correlates well with the extent of MGN deafferentation, but not with extent of removal of normal retinal targets. Indeed, the presence of at least some normal retinal targets seems necessary for the formation of retino-MGN connections. The diameters of retino-MGN axons suggest that more than one type of retinal ganglion cells innervate the MGN under a lesion paradigm that spares the visual cortex and lateral geniculate nucleus. We also found that, after extensive deafferentation of MGN, other axonal systems in addition to retinal axons project ectopically to the MGN. These data are consistent with the idea that ectopic retino-MGN projections develop by sprouting of axon collaterals in response to signals arising from the deafferented nucleus, and that these axons compete with other sets of axons for terminal space in the MGN. J. Comp. Neurol. 400:417–439, 1998. © 1998 Wiley-Liss, Inc.     

The retinohypothalamic tract originates from a distinct subset of retinal ganglion cells

The retinal ganglion cells giving rise to retinohypothalamic projections in the rat were identified using retrograde transport of horseradish peroxidase (HRP) or Fluoro Gold injected into the suprachiasmatic nucleus (SCN), and using transneuronal transport of the Bartha strain of the swine herpesvirus (PRV-Bartha). When PRV-Bartha is injected into one eye, it is taken up by retinal ganglion cells, replicated, transported to axon terminals in the SCN, and released. There the virus may take one, or both, of two paths to retinal ganglion cells in the contralateral eye: (1) uptake by SCN neurons, replication, and release from the neurons with uptake and retrograde transport in retinal afferents originating in the contralateral retina; (2) transneuronal passage through axo-axonic appositions between retinal afferents in the SCN with subsequent retrograde transport of virus to the contralateral retina. The ganglion cells thus labeled are a homogeneous population of small neurons (mean diameter, 12.8 ± 2.2 μm an mean area, 81.8 ± 21.8 μm²) with sparsely branching dendrites that are widely distributed over the retina. This population is best identified when virus labeling of retinal projections in areas beyond the hypothalamus is eliminated by lateral geniculate lesions that transect the optic tract at its entry into the geniculate complex. The same population is labeled with retrograde tracers but, with both HRP and Fluoro Gold, other ganglion cells are labeled, presumably from uptake by fibers of passage, indicating that the virus is a more reliable marker for ganglion cells giving rise to retinohypothalamic projections. The ganglion cells identified correspond to a subset of type III, or W, cells. © 1995 Wiley-Liss, Inc.     

The contribution of inner and outer retinal photoreceptors to infra‐slow oscillations in the rat olivary pretectal nucleus

A subpopulation of olivary pretectal nucleus (OPN) neurons discharges action potentials in an oscillatory manner, with a period of approximately two minutes. This ‘infra‐slow’ oscillatory activity depends on synaptic excitation originating in the retina. Signals from rod‐cone photoreceptors reach the OPN via the axons of either classic retinal ganglion cells or intrinsically photosensitive retinal ganglion cells (ipRGCs), which use melanopsin for photon capturing. Although both cell types convey light information, their physiological functions differ considerably. The aim of the present study was to disentangle how rod‐cone and melanopsin photoresponses contribute to generation of oscillatory activity. Pharmacological manipulations of specific phototransduction cascades were used whilst recording extracellular single‐unit activity in the OPN of anaesthetized rats. The results show that under photopic conditions (bright light), ipRGCs play a major role in driving infra‐slow oscillations, as blocking melanopsin phototransmission abolishes or transiently disturbs oscillatory firing of the OPN neurons. On the other hand, blocking rod‐cone phototransmission does not change firing patterns in photopic conditions. However, under mesopic conditions (moderate light), when melanopsin phototransmission is absent, blocking rod‐cone signalling causes disturbances or even the disappearance of oscillations implying that classic photoreceptors are of greater importance under moderate light. Evidence is provided that all photoreceptors are required for the generation of oscillations in the OPN, although their roles in driving the rhythm are determined by the lighting conditions, consistent with their relative sensitivities. The results further suggest that maintained retinal activity is crucial to observe infra‐slow oscillatory activity in the OPN.     

Photostimulation alters c-Fos expression in the dorsal raphe nucleus

Retinal afferents to the dorsal raphe nucleus (DRN) have been described in a number of species, including Mongolian gerbils, but functional correlates of this optic pathway are unknown at present. To determine whether temporally modulated photostimulation can affect c-Fos expression in the gerbil DRN, quantitative analysis of c-Fos-immunoreactive (c-Fos-ir) neurons was conducted following 60-min exposure to pulsed (2 Hz) photostimulation at selected times over the 12:12 h light/dark cycle. For comparison, c-Fos expression was also analyzed in the subnuclei of the lateral geniculate complex and in the suprachiasmatic nucleus (SCN). In the DRN, a substantial reduction was observed in the number of c-Fos immunoreactive (c-Fos-ir) neurons during the light period and early dark period in photostimulated vs. control animals. Similar results were obtained in the intergeniculate leaflet (IGL) and ventral lateral geniculate (VLG). However, no significant changes were observed in the number of c-Fos-ir neurons in the dorsal lateral geniculate nucleus or suprachiasmatic nucleus (SCN) following photostimulation, except for an increase in the middle of the dark period. These findings indicate that photic stimulation can lead to a suppression or down-regulation of c-Fos expression in the DRN that is probably mediated via the direct retinal pathway to the DRN in this species. The similarity between c-Fos expression profiles in the DRN and IGL/VGL suggest that efferent projections from the DRN may modulate c-Fos expression to visual stimulation in these subnuclei of the lateral geniculate complex.     

Neuromodulator content of hamster intergeniculate leaflet neurons and their projection to the suprachiasmatic nucleus or visual midbrain

The intergeniculate leaflet (IGL) of the lateral geniculate complex has widespread, bilateral, and reciprocal connections with nuclei in the subcortical visual shell. Its function is poorly understood with respect to its role in visual processing. The most well-known IGL projection, and the only one with a clear function, is the geniculohypothalamic tract (GHT) that terminates in the suprachiasmatic nucleus (SCN), site of the primary circadian clock. The hamster GHT is derived, in part, from IGL neurons containing neuropeptide Y and enkephalin. IGL neurons containing these peptides also project to the pretectal region. The present studies used a combination of immunohistochemical, lesion, and retrograde tracing techniques to study neuron types in the IGL and their projections to hamster SCN and pretectum. Two additional neuromodulators, gamma-aminobutyric acid (GABA) and neurotensin, are shown to be present in IGL neurons. The GABA- and neurotensin-immunoreactive neurons project to the SCN with terminal field patterns very similar to those for neuropeptide Y and enkephalin. IGL neurons of all four types also send projections to the pretectum, but rarely do individual cells project to both the SCN and the pretectum. Nearly all neurotensin is colocalized with neuropeptide Y in IGL neurons, although about half of the neuropeptide Y cells do not contain neurotensin. Otherwise, the extent to which the four neuromodulators are colocalized varies from 6% to 54%. Nearly every SCN neuron appears to contain GABA. In the IGL, the majority of cells studied are not identifiable by GABA immunoreactivity. Putative functions of the various neuromodulator projections from the IGL to pretectum or SCN are discussed.     

The projections of different morphological types of ganglion cells in the cat retina.

The central projections of the retinal ganglion cells of the cat were examined using the method of retrograde transport of horseradish peroxidase. Peroxidase was injected into the lateral geniculate nucleus and into the superior colliculus by means of a recording micropipette. After injections at retinotopically homologous points in these two structures in separate animals, tha patterns of retinal ganglion cell labeling were compared. We found that there were three populations of ganglion cells: small cells, that projected predominantly to the superior colliculus; medium-sized cells, that projected predominantly to the lateral geniculate nucleus; and large cells, some of which projected to both structures, and some of which projected to the lateral geniculate nucleus alons. Quantitative studies showed that the average size of the cells in each population was smaller at the area centralis than in the periphery. These results could be directly related to physiological classifications of retinal ganglion cells proposed by other authors.     

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): II. Topography after rotation of an eye prior to retinal innervation of the brain

Retinal projections to visual centers in a marsupial mammal, the tammar wallaby (Macropus eugenii), have been investigated after an eye rotation prior to retinal innervation of the brain. Retinal topography to the superior colliculus and dorsal lateral geniculate nucleus was mapped by using laser lesions of the retina and horseradish peroxidase histochemistry. Despite the change in orientation of optic axon outgrowth from the developing eye after rotation, retinal ganglion cells made orderly connections in the colliculus and geniculate according to their original retinal position within the eye and not their rotated position. Axons must have corrected their pathways at some point between the back of the eye and their targets. The optic chiasm was one such site. Optic axons from the rotated eye took an abnormal course at the caudal end of the chiasm. Growth of optic axons through aberrant pathways in the brain did not preclude specific innervation of targets. When by chance optic axons entered through the oculomotor nerve root they specifically innervated their correct visual centers, albeit in reduced density, and did not innervate inappropriate targets. These results support the idea of specific interactions between growing axons, the pathways they grow along, and their targets.     

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.     

Melanopsin retinal ganglion cells receive bipolar and amacrine cell synapses

Melanopsin is a novel opsin synthesized in a small subset of retinal ganglion cells. Ganglion cells expressing melanopsin are capable of depolarizing in response to light in the absence of rod or cone input and are thus intrinsically light sensitive. Melanopsin ganglion cells convey information regarding general levels of environmental illumination to the suprachiasmatic nucleus, the intergeniculate leaflet, and the pretectum. Typically, retinal ganglion cells communicate information to central visual structures by receiving input from retinal photoreceptors via bipolar and amacrine cells. Because melanopsin ganglion cells do not require synaptic input to generate light-induced signals, these cells need not receive synapses from other neurons in the retina. In this study, we examined the ultrastructure of melanopsin ganglion cells in the mouse retina to determine the type (if any) of synaptic input these cells receive. Melanopsin immunoreaction product was associated primarily with the plasma membrane of (1) perikarya in the ganglion cell layer, (2) dendritic processes in the inner plexiform layer (IPL), and (3) axons in the optic fiber layer. Melanopsin-immunoreactive dendrites in the inner (ON) region of the IPL were postsynaptic to bipolar and amacrine terminals, whereas melanopsin dendrites stratifying in the outer (OFF) region of the IPL received only amacrine terminals. These observations suggested that rod and/or cone signals may be capable of modifying the intrinsic light response in melanopsin-expressing retinal ganglion cells.     

The M6 cell: A small-field bistratified photosensitive retinal ganglion cell

We have identified a novel, sixth type of intrinsically photosensitive retinal ganglion cell (ipRGC) in the mouse—the M6 cell. Its spiny, highly branched dendritic arbor is bistratified, with dendrites restricted to the inner and outer margins of the inner plexiform layer, co-stratifying with the processes of other ipRGC types. We show that M6 cells are by far the most abundant ganglion cell type labeled in adult pigmented Cdh3-GFP BAC transgenic mice. A few M5 ipRGCs are also labeled, but no other RGC types were encountered. Several distinct subnuclei in the geniculate complex and the pretectum contain labeled retinofugal axons in the Cdh3-GFP mouse. These are presumably the principle central targets of M6 cells (as well as M5 cells). Projections from M6 cells to the dorsal lateral geniculate nucleus were confirmed by retrograde tracing, suggesting they contribute to pattern vision. M6 cells have low levels of melanopsin expression and relatively weak melanopsin-dependent light responses. They also exhibit strong synaptically driven light responses. Their dendritic fields are the smallest and most abundantly branched of all ipRGCs. They have small receptive fields and strong antagonistic surrounds. Despite deploying dendrites partly in the OFF sublamina, M6 cells appear to be driven exclusively by the ON pathway, suggesting that their OFF arbor, like those of certain other ipRGCs, may receive ectopic input from passing ON bipolar cells axons in the OFF sublayer.