Microcircuits for night vision in mouse retina

Because the mouse retina has become an important model system, we have begun to identify its specific neuron types and their synaptic connections. Here, based on electron micrographs of serial sections, we report that the wild-type mouse retina expresses the standard rod pathways known in other mammals: (1) rod --> cone (via gap junctions) to inject rod signals into the cone bipolar circuit; and (2) rod --> rod bipolar --> AII amacrine --> cone bipolar --> ganglion cell. The mouse also expresses another rod circuit: a bipolar cell with cone input also receives rod input at symmetrical contacts that express ionotropic glutamate receptors (Hack et al., 1999, 2001). We show that this rod-cone bipolar cell sends an axon to the outer (OFF) strata of the inner plexiform layer to form ribbon synapses with ganglion and amacrine cells. This rod-cone bipolar cell receives direct contacts from only 20% of all rod terminals. However, we also found that rod terminals form gap junctions with each other and thus establish partial syncytia that could pool rod signals for direct chemical transmission to the OFF bipolar cell. This third rod pathway probably explains the rod responses that persist in OFF ganglion cells after the well known rod pathways are blocked (Soucy et al., 1998).

Impaired behavioral suppression by light in metabotropic glutamate receptor subtype 6-deficient mice

The metabotropic glutamate receptor subtype 6 is localized on the dendrites of ON bipolar cells in mammalian retina, and is responsible for synaptic transmission from photoreceptors to ON bipolar cells. We have previously provided electrophysiological evidence that metabotropic glutmate receptor subtype 6-deficient mice have an impairment in the ON visual pathway. In this study, we compared, between metabotropic glutamate receptor subtype 6-deficient (n=9) and wild-type mice (n=7), their daily wheel-running activity in constant dark and light-dark cycle environments. There was no difference in their free-running rhythmicity in a constant dark environment nor in their ability to entrain their active/rest phase to the phase-shifted light-dark cycle environment, indicating that the circadian system in mutant mice was functioning normally. However, the wheel-running activity was suppressed immediately after light onset of the light-dark cycle in wild-type mice (suppressive effect), whereas that of mutant mice was prolonged for several hours in spite of light onset (very weak suppressive effect). The suppression of activity in wild-type mice is a "masking effect" of the endogenous circadian rhythm in response to light stimuli. The results indicate that the failure of mutant mice to suppress their activity upon light onset is not due to abnormality in their circadian system, but to their lack of response to light stimuli. This study clearly demonstrates that the dysfunction of the ON visual pathway in metabotropic glutamate receptor subtype 6-deficient mice impairs their behavioral responsiveness to light and yet preserves their circadian system.

[Neurotransmitter ATP and cytokine release]

Microglia, the resident macrophages in the central nervous system (CNS), are rapidly activated upon trauma or ischemic injury, releasing cytokines and undertaking tissue repair. Recent studies have indicated that CNS immune cells express ionotropic P2X and metabotropic P2Y purinoceptors and undergo functional changes in response to extracellular ATP. Non-stimulated cultured rat retinal microglia expressed metabotropic P2U(P2Y2, P2Y4) and ionotropic P2Z(P2X7) purinoceptors equally, whereas in LPS-stimulated microglia, P2Z and its CA2+ response became dominant. Upon hypoxia (1% oxygen) activation, the P2U response became dominant, and proliferation was induced possibly via intracellular Ca2+ mobilization and/or capacitative Ca2+ entry. TNF-alpha and IL-1 beta were released in both LPS- and hypoxia-activated states, enhanced by the P2Z agonist BzATP and suppressed by the antagonist oATP, indicating P2Z involvement in their release. P2Z activation was simultaneously anti-mitotic and induced apoptosis of microglia. Release of cytokines may be induced via Ca2+ influx and activation of NFAT, NF-kappa B or p44/42 and p38 MAP kinases, switching off the mitotic signal transduction pathway and triggering the apoptotic cascade at the same time.

P2 Purinoceptor expression and functional changes of hypoxia-activated cultured rat retinal microglia

P(2) purinoceptors appear to modulate microglia function, but their role in hypoxic microglia has not been investigated. We examined in postnatal rat retinal microglia cultured under hypoxic (1% oxygen) condition, their P2 expression, proliferation and cytokine release in the presence or absence of the P2 receptor agonists and antagonists. Fura-2 fluorescence measurements of intracellular Ca(2+) rises to P2 receptor agonists and antagonists indicated that both P(2U) and P(2Z) were expressed in hypoxic microglia. Hypoxia induced BrdU incorporation and release of interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) as well. The P(2U) agonist, UTP, maintained the BrdU incorporation, whereas the P(2Z) agonist, BzATP, suppressed it, but significantly enhanced IL-1beta and TNF-alpha release, suggesting that the P(2U) response may underlie the mitotic activity, and that of P(2Z), the IL-1beta and TNF-alpha release of hypoxia-activated microglia.

Differential expression of ionotropic glutamate receptor subunits in the outer retina

Ionotropic glutamate receptors (iGluRs) are extremely diverse in their subunit compositions. To understand the functional consequences of this diversity, it is necessary to know the subunits that are expressed by known cell types. By using immunocytochemistry with light and electron microscopy, we localized several subunits (GluR2/3, GluR4, and GluR6/7) in cat retinal neurons, postsynaptic to photoreceptors. Type A horizontal cells express all three subunits strongly, whereas type B horizontal cells express GluR2/3 strongly, GluR6/7 weakly, and do not express GluR4. When they are present, the subunits are expressed strongly throughout the cytoplasm of the somata and primary dendrites; however, in the terminals, they are concentrated at the postsynaptic region, just opposite the presumed site of photoreceptor glutamate release. Surprisingly, all bipolar cell classes (OFF cone bipolar cells, ON cone bipolar cells, and rod bipolar cells) express at least one iGluR subunit at their dendritic tips. Cone bipolar cells forming basal contacts with the cones (presumably OFF cells) express all three subunits in association with the electron-dense postsynaptic membrane. Invaginating dendrites of cone bipolar cells (presumably ON cells) express GluR2/3 and GluR4. Rod bipolar cells (ON cells) express GluR2/3 in their invaginating dendrites. The function of iGluRs in horizontal cells and OFF bipolar cells clearly is to mediate their light responses. GluR6/7 subunit in the receptor of these cells may be responsible for the dopamine-mediated enhancement of glutamate responses that have been observed previously in these cells. The function of iGluRs in ON bipolar cells remains an enigma.

Neurochemistry of the mammalian cone 'synaptic complex'

The cone 'synaptic complex' is a unique structure in which a single presynaptic axon secretes glutamate onto processes of bipolar cells (both ON and OFF) and horizontal cells. In turn, the horizontal cell processes antagonize cone and bipolar responses to glutamate (probably by GABA). What still remains largely unknown is the molecular identity of the postsynaptic receptors and their exact locations. We identified several subunits of the glutamate receptor and the GABAA receptor expressed at the cone synaptic complex and localized them ultrastructurally. Glutamate receptors: (i) Invaginating (probably ON) bipolar dendrites in the monkey and rat express the metabotropic glutamate receptor, mGluR6. The stain is intense on the dendritic membrane where it first enters the invagination, and weak at the tip nearest to the ribbon. The cone membrane is electron-dense where it apposes the intense stain for mGluR6. Surprisingly, invaginating bipolar dendrites in the cat also express the AMPA receptor subunits, GluR2/3 and GluR4. (ii) Dendrites forming basal contacts in the cat (probably OFF) express the AMPA subunits GluR2/3, GluR4, and also the kainate subunit, GluR6/7. The stain is especially intense at the dendritic tips in apposition to electron-dense regions of cone membrane. (iii) Horizontal cells in the cat express the AMPA subunits GluR2/3, GluR4 and the kainate subunit, GluR6/7. The stain is strongest in the cytosol of somas and primary dendrites, but is also present in the invaginating terminals where it localizes to the membrane subjacent to the ribbon. GABAA receptors: (i) ON and OFF bipolar dendrites in the monkey express the alpha 1 and beta 2/3 subunits. The stain is localized to the bipolar cell membrane in apposition to horizontal cell processes. (ii) Cones did not express the GABAA subunits tested by immunocytochemistry, but beta 3 mRNA was amplified by RT-PCR from rat photoreceptors.

CONCLUSIONS

(i) mGluR6 receptors concentrate on dendrites at the base of the invagination rather than at the apex. This implies that receptors at both 'invaginating' and 'basal' contacts lie roughly equidistant from the release sites and should therefore receive similar spatiotemporal concentrations of glutamate. (ii) The 'cone' membrane is electron-dense opposite to the receptor sites on both ON and OFF bipolar cells. This suggests a special role for this region in synaptic transmission. Possibly, these densities signify a transporter that would regulate glutamate concentration at sites remote (> 200 nm) from the locus of vesicle release.

ON cone bipolar cells in rat express the metabotropic receptor mGluR6

The rod bipolar cell and about five types of ON cone bipolar cells depolarize to light by employing a sign-reversing metabotropic glutamate receptor. Glutamate responses are similar in both rod bipolar and cone bipolar cells, but the receptor mediating this response (mGluR6) was so far demonstrated only in rod bipolar cells. To test if ON cone bipolar cells also express mGluR6, we immunoreacted rat retina with an antibody specific for mGluR6, and studied the staining from serial ultrathin sections. We demonstrate that mGluR6 is indeed expressed in the dendritic tips of cone bipolar cells, the majority of which receive a ribbon synapse, and thus probably are ON cone bipolar cells. We further show that half of the dendritic tips contacting the cones stain for mGluR6, thus implying that all ON cone bipolar cell types express mGluR6.

Functional and morphological restoration of intracranial brachial lesion of the retinocollicular pathway by peripheral nerve autografts in adult hamsters

Axons of adult mammals can regenerate through peripheral nerve grafts and restore the retinocollicular pathway if lesioned proximal to the retinal ganglion cell somata. Whether the grafting and subsequent reinnervation of the superior colliculus (SC) is possible in distal axotomy in the brain is a question of clinical relevance. We have deafferented the SC of adult hamsters at its brachium thus axotomizing the retinal ganglion cell axons rostral to its synaptic contact with the SC neurons. After unilateral brachium transection, a short segment of the autologous sciatic nerve was grafted to bridge the lesioned site to the SC (n = 28). As controls the brachium was transected and left ungrafted (n = 12). Functional restoration was examined 3 to 75 weeks later in grafted (n = 16) and control (n = 5) animals by recording visual evoked responses from the collicular cells. Prior to recording the grafts were visually evaluated and categorized into successfully (n = 8) and unsuccessfully (n = 8) grafted groups. To diffuse flash stimuli applied to the contralateral eye, visual evoked field potentials were recorded from all successfully grafted, but not in unsuccessfully grafted (with the exception of one animal) nor control animals. Unitary spike responses to diffuse flash stimuli were recorded exclusively from three successfully grafted animals. Morphological reinnervation was examined in the remaining grafted (n = 12) and control (n = 7) animals by anterogradely labeling the regenerating retinal axons with WGA-HRP. Axons in the grafts and their terminals in the superficial layers of the SC were clearly labeled in 8 of the grafted and none of the controls. From these results we conclude that the brachium of the SC is conducive to axonal regeneration and the peripheral nerve graft is indeed effective in restoring distally axotomized visual pathway in adult mammals.

Morphological comparisons between outer and inner ramifying alpha cells of the albino rat retina

The somato-dendritic morphologies of large ganglion cells were studied by intracellular injections of Lucifer yellow in perfused in vitro preparations of the albino rat retina. The ganglion cells were prelabeled with retrogradely transported granular blue or labeled with acridine orange dropped into the perfusate of in vitro preparations. After the dye injection, somato-dendritic morphologies were successfully studied for 210 cells, the majority of which had a large soma more than 20 microns in diameter and were identified as alpha cells. According to the level of dendritic extensions within the inner plexiform layer (IPL) these alpha cells were further classified into inner ramifying (inner) and outer ramifying (outer) cells. Both qualitative and quantitative observations led us to conclude the following: 1) The outer cells have a spherical soma with relatively few primary dendrites, while inner cells have a large polygonal soma with more primary dendrites. 2) The dendritic field of inner cells was always larger than that of outer cells at every retinal location. The dendritic field diameter tended to increase as a function of retinal eccentricity from the optic disk, the tendency being more clear among inner cells. 3) The dendrites of outer cells branch more frequently in the proximal part of the dendritic field while those of inner cells branch more distally. 4) Total dendritic length of outer cells increases linearly with eccentricity whereas that of the inner cells does not change much irrespective of retinal location.

Optic nerve regeneration by peripheral nerve transplant

We studied the morphology of regenerated retinal ganglion cells and their axons in adult rodents after axotomy and autologous transplantation of the sciatic nerve. Regenerated ganglion cells, backlabeled with rhodamine dextran, were of similar size to or larger than those of intact cells in control animals. Dendrites and occasionally axons as well showed abnormal morphologies in most cells, though some cells appeared quite normal. Cross-sections of the regenerated axons, observed by electron microscopy, were always attached to either the Schwann cell cytoplasm or the basal lamina. The immunoreactive structures to anti-laminin antibody were quite irregular in the cross-sectioned graft and, compared with those of the intact sciatic nerve, they were generally smaller. Their appearance closely resembled that of the basal lamina in the graft observed by electron microscopy. These observations, taken together, suggest that the laminin-rich basal laminae of Schwann cells are essentially important for the regeneration of retinal axons in adult rodents.

Nasotemporal overlap of crossed and uncrossed retinal ganglion cell projections in the Japanese monkey (Macaca fuscata)

The nasotemporal overlap of crossed and uncrossed retinal ganglion cell projections were studied in 11 Japanese monkeys (Macaca fuscata) using HRP and fluorescent dyes (DAPI and RITC) as retrograde tracers and by physiological recordings of antidromic field potentials. A strip of nasotemporal overlap ran orthogonal to the horizontal meridian in all the whole-mount retinas studied. In HRP-labeled retinas of 6 monkeys, the width of the overlap gradually increased from 0.6 degrees in the central retina up to 5 degrees at eccentricity of 5 mm, and to 15 degrees at the extreme periphery. We also noted a clear asymmetric distribution of crossed and uncrossed retinal ganglion cell projections particularly in the perifoveal region; ipsilaterally projecting cells encircled the nasal edge of the fovea, whereas few contralaterally projecting cells were observed in the temporal edge. Soma-size analysis revealed that crossed projections in the temporal portion of the overlap arose mainly from large and small cells (presumably P alpha and P gamma cells, respectively); uncrossed projections in its nasal portion arose from medium-sized cells (presumably P beta cell). Direct evidence of the overlap as well as of the asymmetry was obtained in subsequent fluorescent dye experiments in 3 monkeys. Physiological studies on 2 additional monkeys confirmed the widening of the nasotemporal overlap towards the upper and lower parts of the retina. Moreover, in the nasal portion of the overlap, only slow potentials, which presumably reflect activities of P beta cells, were recorded after stimulation of the ipsilateral LGN as expected from the morphological study. The findings are discussed in relation to clinical observations of macular sparing and splitting, and with regard to the functional differences between P alpha and P beta cell systems on which binocular stereoscopic vision along the midsagittal plane may be based.

Intraretinal axons of ganglion cells in the Japanese monkey (Macaca fuscata): conduction velocity and diameter distribution

In anesthetized and immobilized Japanese monkeys (Macaca fuscata), intraretinal conduction velocities of the ganglion cell axons were measured. The field potentials elicited by optic chiasm shocks consisted of fast and slow components with estimated conduction velocities of 1.19 and 0.72 m/s in recordings from the optic nerve fiber layer, and 1.65 and 1.00 m/s in recordings from the ganglion cell layer. Single cell recordings verified that the time course of the fast component corresponded to the antidromic spike latencies of Y-like cells, whereas that of the slow component covered the latency range of both X-like and W-like cells. In an electron microscopic study of the cross-sections of the intraretinal optic nerve fiber bundles, the axon diameter histograms of large samples (n = 3000-6000) all showed a unimodal distribution with a sharp peak at 0.3-0.6 micron and a long tail extending to 2-3 micron. The mean diameter was largest in the ventral and nasal bundles, smallest in the papillomacular bundle and intermediate in the dorsal, upper arcuate and lower arcuate bundles. However, diameter histograms of a small number of regional axons (n = 255-300) showed a broad tail distinct from the peak at 0.3-0.6 micron, enabling us to segregate a group of larger axons from the medium-sized to small axons. From such regional axon diameter histograms we estimated the mean relative occurrences of the larger axons (7.1-11.3%) and their mean diameters (0.9-1.3 micron). We further applied this relative frequency to the unimodal distribution of the histograms with larger samples in the upper and lower arcuate bundles and estimated the mean axon diameter of the large axons (1.1 micron) and that of the medium-sized to small axons (slightly below 0.5 micron). Finally, in studying the relation between axon diameter and conduction velocity in the two arcuate fiber bundles, we found it to be somewhat different from that previously reported for the cat retina.

Effects of EEG synchronization on visual responses of the cat's geniculate relay cells: a comparison among Y, X and W cells

Single unit activities of Y, X and W cells were recorded in the cat lateral geniculate nucleus to study how their responses to stationary light spots change with the cortical EEG. The shift from desynchronized to synchronized EEG drastically suppressed W cell activities, depressed both transient and sustained components of X cell responses, but did not affect the transient component of Y cell responses. This was ascribed to different inhibitory circuitries in the three parallel pathways and their differential modulation by the ascending brainstem activities.

Retinal inputs and laminar distributions of the dorsal lateral geniculate nucleus relay cells in the eastern chipmunk (Tamias sibiricus asiaticus)

Retinal inputs and their laminar distributions in the dorsal lateral geniculate nucleus (LGNd) of the eastern chipmunk (Tamias sibiricus asiaticus) were studied using histological and microelectrode recording techniques. A previous anatomical study (Fukuda et al. 1986a) indicated that the chipmunk LGNd had five laminae: contralaterally (contra) innervated lamina 1 and ipsilaterally (ipsi) innervated lamina 2 in its ventromedial part; laminae 3a (contra), 3b (ipsi) and 3c (contra) in its dorsolateral part. We have confirmed this finding in our present anatomical study and have also noted another ipsilaterally innervated thin lamina 0, medial to lamina 1. In our electrophysiological study, however, we were unable to record units from lamina 0 and to investigate it functionally. We recorded 232 units from laminae 1, 2 and the 3 complex, of which 95 were identified as Y-like, 46 as W-like, 15 as X-like, and 8 as mixed Y/W-like cells; the rest were either unclassified or visually unresponsive. In laminae 1 and 2, only Y-like and X-like cells were recorded, whereas in the laminae 3 complex W-like cells were recorded as well. The results suggest that the chipmunk laminae 1, 2 and 3 complex correspond relatively well to the cat laminae A, A1 and C complex, respectively. In the chipmunk LGNd, however, there were more Y-like cells in laminae 1 and 2, and a few X-like cells of which some were color sensitive. Also, lamina 3a had a concentration of mixed-type cells with Y-like receptive field properties and W-like OX latencies. As for retinotopy, the dorsoventral transition of the contralateral visual field (laminae 1, 3a, 3c) is represented along the dorsoventral dimension of the chipmunk LGNd, whereas the temporonasal transition is represented in the rostrocaudal direction. Receptive field positions of the ipsilaterally innervated relay cells are limited to the central overlapping field of the contralateral visual fields of both eyes. Relay cells with visual fields having elevations of below -20 degrees had relatively fast latency range and Y-like properties.