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).
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 (mGluRo) 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 mGluRo, 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.
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.
(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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.