Peter Sterling

Localization of mGluR6 to Dendrites of ON Bipolar Cells in Primate Retina

We prepared antibodies selective for the C‐terminus of the human mGluR6 receptor and used confocal and electron microscopy to study the patterns of immunostaining in retina of monkey, cat, and rabbit. In all three species punctate stain was restricted to the outer plexiform layer. In monkey, stain was always observed in the central element of the postsynaptic “triad” of rod and cone terminals. In monkey peripheral retina, stain was seen only in central elements, but in the fovea, stain was also observed in some dendrites contacting the base of the cone terminal. S‐cone terminals, identified by staining for S opsin, showed staining of postsynaptic dendrites. These were identified as dendrites of the ON S‐cone bipolar cell by immunostaining for the marker cholecystokinin precursor. The staining pattern suggests that all types of ON bipolar cells, despite their marked differences in function, express a single isoform of mGluR6. Ultrastructurally, mGluR6 was located not on the tip of the central element, near the site of vesicle release, but on its base at the mouth of the invagination, 400–800 nm from the release site. Thus, the mGluR6 receptors of ON bipolar cells lie at about the same distance from sites of vesicle release as the iGluR receptors of OFF bipolar cells at the basal contacts. J. Comp. Neurol. 423:402–412, 2000.

Anatomical organization of the brachial spinal cord of the cat. II. The motoneuron plexus

The present study has attempted to correlate the distribution and orientation of the motoneuron dendrites with the somatotopically organized distribution of their cell bodies. Despite local variations, the predominant dendritic orientation within the motoneuronal cell groups is longitudinal. Retrograde changes after section of various combinations of peripheral nerves show that motoneurons supplying the intrinsic flexor muscles of the forelimb are aligned longitudinally in the dorsolateral regions of the lateral motoneuronal cell groups from C6-T1. Extensor motoneurons are similarly aligned with each other but lie ventral to the flexor motoneurons. Motoneurons supplying the girdle musculature lie still more ventrally and medially. The cell bodies and dendrites intermingle, forming a longitudinal plexus which is supplied by afferents whose terminal arborizations also run longitudinally. The possible roles of these arrangements were discussed with reference to both the maintenance of specificity of the afferent supply to the motoneurons and the activation of synergistic motoneurons.

Receptive fields and synaptic organization of the superficial gray layer of the cat superior colliculus

Abstract Cells in the superficial gray layer of the cat superior colliculus show binocular interaction and respond selectively to certain features of a visual stimulus, for example, movement, direction, and size. Although the binocular and directional properties depend on input from the visual cortex, certain spatial properties of the receptive field do not. Neither can they depend exclusively on the retino-collicular input since this is mainly from the contralateral eye. Therefore, the spatial properties must arise from an intrinsic collicular mechanism that can be activated from either eye. Seen with the electron microscope the most striking feature of the synaptic organization is that the dendrites make extensive synaptic contacts with each other. Synapses from the retina and visual cortex terminate on the vesicle-free regions of the dendrites and also on the regions given rise to dendro-dendritic contacts. Retinal and cortical terminals are rare on cell bodies, and neither type makes, nor appears to receive an axo-axonic contact. If the dendro-dendritic contacts are inhibitory, they may represent the basis on which complicated receptive field properties are established in the colliculus using only limited number of cells.

Synaptic Ribbon: Conveyor Belt or Safety Belt?

The synaptic ribbon in neurons that release transmitter via graded potentials has been considered as a conveyor belt that actively moves vesicles toward their release sites. But evidence has accumulated to the contrary, and it now seems plausible that the ribbon serves instead as a safety belt to tether vesicles stably in mutual contact and thus facilitate multivesicular release by compound exocytosis.

Anatomical organization of the brachial spinal cord of the cat. III. The propriospinal connections

Summary The present study has demonstrated the existence of a topically organized projection from neurons in Rexeds laminae V–VIII to the motoneuroneal cell groups in the brachial cord of the cat. It was found that cells in the lateral parts of laminae V–VII project preferentially to the dorsolateral part of the lateral motoneuronal cell group and that cells in the central parts of lamina VII project preferentially to the ventrolateral and the medial parts of this cell group, both projections being primarily ipsilateral. Cells in the medial parts of lamina VII and in lamina VIII were found to project preferentially to the medial motoneuronal cell group, bilaterally. The projections to the lateral motoneuronal cell group appeared to arise primarily within the brachial cord, while the projections to the medial motoneuronal cell group were found to have a wider segmental origin. In addition, homologous areas of laminae V–VIII appeared to be interconnected throughout the brachial cord. The dendrites in the motoneuronal cell groups are oriented predominantly longitudinally, while those in laminae V–VIII appeared to be oriented primarily in the transverse plane. It was found that the terminal arborization of the propriospinal and brain stem fibers frequently tended to assume the orientation of the dendrites of the regions in which they terminated. The findings have been discussed in relation to both the intrinsic motor organization of the spinal cord and the organization of the descending pathways.

Why Do Axons Differ in Caliber

CNS axons differ in diameter (d) by nearly 100-fold (∼0.1–10 μm); therefore, they differ in cross-sectional area (d2) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d2. We asked, given constraints on space and energy, what functional requirements set an axons diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ∼1 to >100 Hz; (3) a tracts distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises ≥d2. To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axons volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate.

Anatomical organization of the brachial spinal cord of the cat. I. The distribution of dorsal root fibers

Abstract As part of a larger study of the anatomical organization of the brachial cord in the cat, the distribution patterns and orientations of degenerating dorsal root fibers were examined in segments C6-T1. Dorsal root fibers entered Rexeds lamina I from the dorsal funiculus and coursed longitudinally. Few fibers were found in lamina II, but many were found in lamine III and IV. The fibers in laminae III and IV ran longitudinally and displayed a topical organization. There was a dense distribution of degenerating fibers to laminae V and VI, primarily in the central regions, and the fibers in these laminae were vertically oriented. The quantities of degenerating fibers were more modest in most of laminae VII and VIII, but as in laminae V and VI, the fibers were vertically oriented. There was a substantial projection of dorsal root fibers to the motoneuronal cell groups, and these fibers had a predominantly longitudinal course. The orientations of the terminal arborizations of the dorsal root fiber were discussed in terms of the predominant dendritic orientations in the various laminae.

Visualizing Synaptic Ribbons in the Living Cell

Visual and auditory information is encoded by sensory neurons that tonically release neurotransmitter at high rates. The synaptic ribbon is an essential organelle in nerve terminals of these neurons. Its precise function is unknown, but if the ribbon could be visualized in a living terminal, both its own dynamics and its relation to calcium and vesicle dynamics could be studied. We designed a short fluorescent peptide with affinity for a known binding domain of RIBEYE, a protein unique to the ribbon. When introduced via a whole-cell patch pipette, the peptide labeled structures at the presynaptic plasma membrane of ribbon-type terminals. The fluorescent spots match in size, location, number, and distribution the known features of synaptic ribbons. Furthermore, fluorescent spots mapped by confocal microscopy directly match the ribbons identified by electron microscopy in the same cell. Clearly the peptide binds to the synaptic ribbon, but even at saturating concentrations it affects neither the morphology of the ribbon nor its tethering of synaptic vesicles. It also does not inhibit exocytosis. Using the peptide label, we observed that the ribbon is immobile over minutes and that calcium influx is concentrated at the ribbon. Finally, we find that each ribbon in a retinal bipolar cell contains ∼4000 molecules of RIBEYE, indicating that it is the major component of the synaptic ribbon.

Streamlined synaptic vesicle cycle in cone photoreceptor terminals

Cone photoreceptors tonically release neurotransmitter in the dark through a continuous cycle of exocytosis and endocytosis. Here, using the synaptic vesicle marker FM1-43, we elucidate specialized features of the vesicle cycle. Unlike retinal bipolar cell terminals, where stimulation triggers bulk membrane retrieval, cone terminals appear to exclusively endocytose small vesicles. These retain their integrity until exocytosis, without pooling their membranes in endosomes. Endocytosed vesicles rapidly disperse through the terminal and are reused with no apparent delay. Unlike other synapses where most vesicles are immobilized and held in reserve, only a small fraction (<15%) becomes immobilized in cones. Photobleaching experiments suggest that vesicles move by diffusion and not by molecular motors on the cytoskeleton and that vesicle movement is not rate limiting for release. The huge reservoir of vesicles that move rapidly throughout cone terminals and the lack of a reserve pool are unique features, providing cones with a steady supply for continuous release.

Electrical Coupling between Mammalian Cones

BACKGROUND Cone photoreceptors are noisy because of random fluctuations of photon absorption, signaling molecules, and ion channels. However, each cones noise is independent of the others, whereas their signals are partially shared. Therefore, electrically coupling the synaptic terminals prior to forward transmission and subsequent nonlinear processing can appreciably reduce noise relative to the signal. This signal-processing strategy has been demonstrated in lower vertebrates with rather coarse vision, but its occurrence in mammals with fine acuity has been doubted (even though gap junctions are present) because coupling would blur the neural image. RESULTS In ground squirrel retina, whose triangular cone lattice resembles the human fovea, paired electrical recordings from adjacent cones demonstrated electrical coupling with an average conductance of approximately 320 pS. Blur caused by this degree of coupling had a space constant of approximately 0.5 cone diameters. Psychophysical measurements employing laser interferometry to bypass the eyes optics suggest that human foveal cones experience a similar degree of neural blur and that it is invariant with light intensity. This neural blur is narrower than the eyes optical blur, and we calculate that it should improve the signal-to-noise ratio at the cone terminal by about 77%. CONCLUSIONS We conclude that the gap junctions observed between mammalian cones, including those in the human fovea, represent genuine electrical coupling. Because the space constant of the resulting neural blur is less than that of the optical blur, the signal-to-noise ratio can be markedly improved before the nonlinear stages with little compromise to visual acuity.