Properties of centre-hyperpolarizing, red-sensitive bipolar cells in the turtle retina

Electrical coupling between bipolar cells in carp retina

Intracellular recordings were made simultaneously from pairs of neighboring bipolar cells by advancing two independent microelectrodes into retinas of carp (Cyprinus carpio). Bipolar cells were identified by their response properties and in several samples were verified by intracellular injection of Lucifer yellow. Current of either polarity injected into one member of the bipolar cell pair elicited a signconserving, sustained potential change in the other bipolar cell without any significant delay. This electrical coupling was reciprocal, and it was observed between cell types similar in function and in morphology. Our results strongly suggest that there is a spatial summation of signals at the level of bipolar cells, which makes central receptive field areas much larger than their dendritic fields.

A dopamine- and protein kinase A-dependent mechanism for network adaptation in retinal ganglion cells

Vertebrates can detect light intensity changes in vastly different photic environments, in part, because postreceptoral neurons undergo "network adaptation." Previous data implicated dopaminergic, cAMP-dependent inhibition of retinal ganglion cells in this process yet left unclear how this occurs and whether this occurs in darkness versus light. To test for light- and dopamine-dependent changes in ganglion cell cAMP levels in situ, we immunostained dark- and light-adapted retinas with anti-cAMP antisera in the presence and absence of various dopamine receptor ligands. To test for direct effects of dopamine receptor ligands and membrane-permeable protein kinase ligands on ganglion cell excitability, we recorded spikes from isolated ganglion cells in perforated-patch whole-cell mode before and during application of these agents by microperfusion. Our immunostainings show that light, endogenous dopamine, and exogenous dopamine elevate ganglion cell cAMP levels in situ by activating D1-type dopamine receptors. Our spike recordings show that D1-type agonists and 8-bromo cAMP reduce spike frequency and curtail sustained spike firing and that these effects entail protein kinase A activation. These effects resemble those of background light on ganglion cell responses to light flashes. Network adaptation could thus be produced, to some extent, by dopaminergic modulation of ganglion cell spike generation, a mechanism distinct from modulation of transmitter release onto ganglion cells or of transmitter-gated currents in ganglion cells. Combining these observations with results obtained in studies of photoreceptor, bipolar, and horizontal cells indicates that all three layers of neurons in the retina are equipped with mechanisms for adaptation to ambient light intensity.

A dopamine- and protein kinase A-dependent mechanism for network adaptation in retinal ganglion cells

Vertebrates can detect light intensity changes in vastly different photic environments, in part, because postreceptoral neurons undergo "network adaptation." Previous data implicated dopaminergic, cAMP-dependent inhibition of retinal ganglion cells in this process yet left unclear how this occurs and whether this occurs in darkness versus light. To test for light- and dopamine-dependent changes in ganglion cell cAMP levels in situ, we immunostained dark- and light-adapted retinas with anti-cAMP antisera in the presence and absence of various dopamine receptor ligands. To test for direct effects of dopamine receptor ligands and membrane-permeable protein kinase ligands on ganglion cell excitability, we recorded spikes from isolated ganglion cells in perforated-patch whole-cell mode before and during application of these agents by microperfusion. Our immunostainings show that light, endogenous dopamine, and exogenous dopamine elevate ganglion cell cAMP levels in situ by activating D1-type dopamine receptors. Our spike recordings show that D1-type agonists and 8-bromo cAMP reduce spike frequency and curtail sustained spike firing and that these effects entail protein kinase A activation. These effects resemble those of background light on ganglion cell responses to light flashes. Network adaptation could thus be produced, to some extent, by dopaminergic modulation of ganglion cell spike generation, a mechanism distinct from modulation of transmitter release onto ganglion cells or of transmitter-gated currents in ganglion cells. Combining these observations with results obtained in studies of photoreceptor, bipolar, and horizontal cells indicates that all three layers of neurons in the retina are equipped with mechanisms for adaptation to ambient light intensity.

Syncytial integration by a network of coupled bipolar cells in the retina

A model system for syncytial integration is the outer vertebrate retina, where graded signals or electrotonic potentials interact laterally via gap junctions to form an integrated response that is relayed by chemical synapses to the next layer of interconnected cells. Morphological and physiological experiments confirm that bipolar cells form quasisyncytial lattices, and so this review will aim to address two important issues: the function of coupling in visual information processing and the construction of a robust mathematical model that can adequately simulate signal spread in the bipolar cell syncytium. It is shown that the role of coupling in bipolar cells differs from that associated in the presynaptic networks, namely, loss in spatial resolution in order to increase the signal-to-noise ratio. The intrinsic membrane properties of bipolar cells which give rise to voltage-dependent currents are inactive over the normal in vivo operating range of membrane potential and may be shunted as a direct result of electrotonic coupling, suppressing any possibility of action potential propagation in the bipolar cell syncytium. It is therefore speculated that the mechanisms underlying processing of information in bipolar networks are dependent on the structure of bipolar cells and in particular, on the presence of gap junctions. It is proposed that a three-dimensional model which incorporates the spatial properties of each bipolar cell in the network in the form of a leaky cable is the most likely model to simulate signal spread in the bipolar cell syncytium in vivo. This is because discrete network models represent each bipolar cell in the syncytium as isopotential units without any spatial structure, and thus are unable to reproduce the temporal characteristics of electrotonic potential spread within the central receptive field of bipolar cells.

The organization of the turtle inner retina. I. ON- and OFF-center pathways

Intracellular recordings and dye injections of Lucifer yellow, horseradish peroxidase, or Neurobiotin were made in bipolar, amacrine, and ganglion cells of the Pseudemys turtle retina. By using a standard light-stimulation protocol in a sample of 375 labeled neurons, we were able to identify morphological and physiological characteristics of 11 types of bipolar cell, 37 types of amacrine cell, and 24 types of ganglion cell. To make sense of these data, we have chosen to group the 72 essentially different neuron types into traditional, functionally significant pathways. In this paper we look at the neuronal types in the inner plexiform layer (IPL) in terms of their contribution to generalized luminosity responses such as sustained ON- or OFF-center and transient ON-OFF ganglion cells; in the companion paper (J. Ammermüller, J.F. Muller, and H. Kolb, 1995, J. Comp. Neurol. 358:35-62) we look at them in terms of their involvement in color opponency and directional selectivity. A functional organization of the turtle IPL into OFF sublaminae (strata 1 and 2) and ON sublaminae (strata 3, 4, and 5), as has been described for other vertebrate retinas, was quite clear for two varieties of OFF-center bipolar cells (B4 and B5) and for all four types of sustained ON-center bipolar cell (B1, B2, B6, and B7). Thus, we found no sustained ON-center bipolar cell terminating in strata 1 and 2. We did, however, see three varieties of sustained OFF-center bipolar cells (B3, B9, and B10) having axon terminals in strata 3-5 (the ON sublamina) in addition to their terminations in stratum 1 or 2 (the OFF sublamina). Monostratified sustained ON- and OFF-center amacrine and ganglion cells rigidly obeyed the border of ON and OFF sublaminae. However, multistratified and diffuse sustained amacrine and ganglion cells could be either ON-center or OFF-center, and they did not strictly obey the border: such ON-center cells always had processes in one of the ON sublaminae (strata 3-5), and the equivalent OFF-center cells always had processes in one of the OFF sublaminae (strata 1 and 2). Monostratified transient amacrine and ganglion cells were concentrated in the middle of the IPL (around stratum 3), whereas bi-, tri-, or multistratified transient amacrine or ganglion cells always had processes in both the ON and the OFF sublaminae.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of the turtle inner retina. II. Analysis of color-coded and directionally selective cells

Color coding and directional selectivity (DS) of retinal neurons were studied in the Pseudemys turtle by using similar intracellular recording and staining techniques as in the preceding paper (J. Ammermüller and H. Kolb, 1995, J. Comp. Neuronal. 358:1-34). Color-coded responses were elicited by red (621 or 694 nm), green (525 or 514 nm), and blue (455 nm) light flashes. In addition to red/green and yellow/blue types of chromaticity horizontal cells, in our sample of 305 identified cells we found that 17% of bipolar cells, 6.5% of amacrine cells, and 18% of ganglion cells exhibit color-coded responses. DS responses were found in 37% of the tested ganglion cells and 41% of the tested amacrine cells. Two morphologically identified bipolar cell types, B10 and B11, were red-ON/blue-OFF and red-OFF/green, blue-ON, respectively. Of five identified amacrine cell types, three were red-OFF/blue-ON center (A1, A3, A23b), one was red-OFF/green-ON center (A32), and one (A33) was double color-opponent of red-ON/blue-OFF center:red-OFF/blue-ON surround. Five ganglion cell types had variously color-coded centers (G14 and G24) or surrounds (G3 and G18), including one type, G6, that was double color-opponent (red-OFF/green-ON center:red-ON/green-OFF surround). Responses to colors were found primarily in sustained responses of bipolar and ganglion cells. However, in amacrine cells, transient components of the response also showed color dependence. Red-OFF-center responses were found in ganglion cells that were in a position to make connections at the strata 2/3 border with the red-OFF bipolar cell (B11); red-ON-center responses occurred in ganglion cells with branches in stratum 4 of the IPL where the red-ON-center bipolar (B10) ended. Blue-ON-center signals appeared to be processed mainly in strata 1-2/3, and blue-OFF-center signals in strata 3-5 of the IPL, with contributions of amacrine cells and bipolar cells. Labeled DS amacrine cells could be identified as A9, A20, and A22, and ganglion cells as G19, G20, and G24. The latter type (G24) showed DS and color coding. All response types (ON-center, OFF-center, ON-OFF) were encountered. DS amacrine cells were monostratified near the middle of the IPL, whereas DS ganglion cells were mono-, bi-, and multistratified, although all DS ganglion cells had one feature in common: they had dendrites in stratum 1 of the IPL.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of kainic acid and piperidine dicarboxylic acid on displaced bipolar cells in the turtle retina

An immunoreaction against glutamate was used to visualize photoreceptors, bipolar, and ganglion cells in the turtle retina. Incubation of the retina prior to fixation in kainic acid (9 microM) led to selective loss of glutamate-like immunoreactivity in OFF-centre bipolar cells, as judged by the loss of staining in the distal half of the inner plexiform layer. In addition, displaced bipolar cells and ganglion cells lost their immunoreactivity. Incubation of the retina in 2,3-cis piperidine dicarboxylate (1 mM) did not result in noticeable glutamate depletion in any cell but enhanced labelling in displaced bipolar cells. These findings suggest that all displaced bipolar cells in the turtle retina are depolarized by kainic acid and hyperpolarized by 2,3-cis piperidine dicarboxylate.

Physiological and morphological properties of off- and on-center bipolar cells in the Xenopus retina: effects of glycine and GABA

We studied the morphology and center-surround organization of Lucifer Yellow injected OFF- and ON-center bipolar cells in the light-adapted Xenopus retina and the effects of glycine and GABA on their cone-mediated light responses. In both classes of cell, prominent antagonistic surround responses up to 20 mV in amplitude could be evoked without first suppressing the center responses with steady illumination. An additional feature of the light-evoked bipolar cell response was a pronounced (up to -24 mV) delayed hyperpolarizing after potential (DHAP) which followed the depolarizing responses of both classes of bipolar cell. The morphological features of dye-injected bipolar cells conformed to the general idea of segregation of ON and OFF pathways in the inner and outer interplexiform layer, however, the morphology of axonal arborizations was different for both classes. OFF-center cells ramified symmetrically around the primary branchpoint, whereas ON-center cells had a strongly asymmetrical arrangement of their axonal tree. The center and surround responses were differentially sensitive to glycine and GABA. Glycine eliminated the antagonistic surround responses in both OFF and ON cells; the center responses were reduced to some extent but were not eliminated. In contrast, GABA affected the hyperpolarizing responses much more strongly than the depolarizing response components. That is, the amplitude of the center response in the OFF cell and the surround response in the ON cell was reduced 80-90% during exposure to GABA, whereas the surround and center depolarizations of OFF and ON cells, respectively, were reduced only 0-10%. Our findings implicate a role for GABAergic and glycinergic pathways in the center-surround organization of bipolar cells in Xenopus retina. In addition, the results suggest that the pathways mediating center-surround antagonism may be different in OFF-bipolar cells vs. ON-bipolar cells.

Spatial organization of the bipolar cell's receptive field in the retina of the tiger salamander

1. The spatial properties of rods, horizontal cells and bipolar cells were studied by intracellular recording in the isolated, perfused retina of the tiger salamander, Ambystoma tigrinum. Low stimulus intensities were used in order to keep cell responses close to, or within, their linear intensity/response range. 2. Spatial properties of bipolar cell receptive fields, measured while perfusing with normal Ringer solution, were compared with those measured during exposure to agents that eliminated the bipolar cells' receptive field surround (RFS). In this way, the spatial properties of the receptive field centre (RFC) and those of the RFS could be characterized independently. 3. To a good approximation, the contribution to the horizontal cell's response of unit area of its receptive field declined exponentially with distance from the centre of the receptive field. The (apparent) length constant describing this decay was 200 microns. The one-dimensional length constant of the horizontal cell syncytium was thus 248 microns. The variation of response amplitude with the radius of a centred circular stimulus was consistent with this finding. 4. This was true also of the RFCs of bipolar cells. The one-dimensional length constant of the RFC of off-centre bipolar cells averaged 124 microns. That of the RFC of on-centre cells averaged 62 microns though values were more variable, the RFCs of some on-centre cells being comparable to those of off-centre cells. These values were independent of the class of photoreceptor driving the bipolar cell. 5. The large size of the RFCs of off-centre cells and many on-centre cells cannot by explained by light scatter within the retina or by voltage spread within the rod syncytium. We proposed that off-centre cells are tightly coupled in a syncytium. On-centre cells, on average, are less tightly coupled. 6. The spatial properties of the bipolar cell's RFS were consistent with the notion that the RFS represents a convolution of the horizontal cell's receptive field and the bipolar cell's RFC. 7. The spatial properties of bipolar cell receptive fields were reconstructed from the measured properties of their RFCs and the measured properties of horizontal cell receptive fields. Under the conditions of our experiments, the bipolar cell's response could be described by a linear difference between a component generated by the RFC and a component generated by the RFS. 8. The spatial filtering characteristics of the bipolar cells were calculated from our data.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of bipolar-bipolar coupling in the carp retina

ON and OFF bipolar cells were identified in the light-adapted carp retina by means of intracellular recording and Lucifer yellow dye injection. The receptive field centers, determined by measuring the response amplitudes obtained by centered spots of different diameters, were 0.3-1.0 mm for ON bipolar cells and 0.3-0.4 mm for OFF bipolar cells. These central receptive field values were much larger than the dendritic field diameters measured by histological methods. Simultaneous intracellular recordings were made from pairs of neighboring bipolar cells. Current of either polarity injected into one member of a bipolar cell pair elicited a sign-conserving, sustained potential change in the other bipolar cell. The coupling efficiency was nearly identical for both depolarizing and hyperpolarizing currents. The maximum separation of coupled bipolar cells was approximately 130 microns. This electrical coupling was reciprocal and summative, and it was observed in cell types of similar function and morphology. Dye coupling was observed in 4 out of 34 stained cells. These results strongly suggest that there is a spatial summation of signals at the level of bipolar cells, which makes their central receptive fields much larger than their dendritic fields.

Morphometric analysis of serotoninergic bipolar cells in the retina and its implications for retinal image processing

The entire population of retinal serotoninergic bipolar cells in the turtle Pseudemys scripta elegans was labeled by immunocytochemical methods. This allowed a systematic analysis to be made of the morphological variabilities among a functionally homogeneous neuronal population. The analyzed morphological characteristics included: size of the Landolt club, size of the soma, lateral extension of the ramification within the outer plexiform layer, course of the axon across the inner nuclear layer, pattern of the axonal ramification within the inner plexiform layer, lateral extension of these ramifications, and density of the cells. Whereas characteristics 1-4 and 7 show a morphological variability strictly related to the location of the bipolar cell with respect to the visual streak, a fovealike structure, characteristics 5 and 6 show no such correlation. The size of the soma increases by a factor of 4 from the visual streak toward the periphery. The area covered by the ramification in the OPL increases from 330 micron 2 at the visual streak to 50,000 microns 2 at the dorsal and ventral edges of the retina. The coverage factor remains the same throughout the retina, as well as the area covered by the ramifications in the IPL, which is about 2,000 microns 2. At the visual streak and at 110 degrees ventral and 68 degrees dorsal of the visual streak, the bipolar axons cross the INL perpendicularly. In between the axons take an oblique course leading to an axon-induced shift between input and output of up to 250 microns.(ABSTRACT TRUNCATED AT 250 WORDS)

Cone synapses with Golgi-stained bipolar cells that are morphologically similar to a center-hyperpolarizing and a center-depolarizing bipolar cell type in the turtle retina

Two types of Golgi-stained bipolar cells have been examined by light and electron microscopy to determine the ultrastructure of their synapses with photoreceptors and the spectral type of photoreceptor with which they connect in the retina of the turtle (Pseudemys scripta elegans). We have chosen bipolar cells that correspond in morphology to a center-hyperpolarizing type (H-bipolar) and a center-depolarizing type (D-bipolar) shown in Marchiafava and Weiler's paper (Proc. R. Soc. B 208:103-113, '80). The latter authors recorded intracellular responses and marked their cells with the fluorescent dye Procion yellow so that we have a clear picture of their morphology. These bipolars have been called B4 and B6, respectively, according to our recent classification scheme of Golgi-stained cells. Serial section electron microscopy of two B4 bipolars shows that they made wide-cleft, striated basal junctions with red and green cone pedicles. They connected to six or seven different cone pedicles. The three B6 bipolar cells studied made narrow-cleft, semi-invaginating basal junctions with cone pedicles. The dendrites of B6 bipolars did not become central elements at ribbon synapses although they invaginated toward the synaptic ribbon. Serial section electron microscopy indicated that B6 bipolars contacted three to five cone pedicles also of a red or green cone type. We suggest that in the case of these two varieties of bipolar cell in turtle retina, sign-conserving synapses with cones are wide-cleft basal junctions while sign-inverting synapses are narrow-cleft, semi-invaginating in morphology.

The pigeon pattern electroretinogram is not affected by massive loss of cell bodies in the ganglion layer induced by chronic section of the optic nerve

In pigeons, electroretinographic responses to contrast reversal of sinusoidal gratings (pattern ERGs) were recorded before and after section of the left optic nerve. Ninety and 270 days following optic nerve section the ganglion cell layer of the side that underwent the surgical procedure showed an 80% reduction in the number of cell bodies as compared with the intact side. At these times the pattern ERGs showed comparable amplitudes in both eyes. There is a possibility that the inner nuclear layer of the pigeon retina plays a major role in the generation of the pattern ERG.

Quantitative morphological study of the outer nuclear layer in the turtle retina

Types and densities of retinal cells with somata located in the outer nuclear layer of the turtle retina were studied with combined light and electron microscopic observations of serial sections. One third of the somata in the outer nuclear layer were found to be those of displaced bipolar cells, while the rest were of those of photoreceptors. Somata were 8-10 micrometers in horizontal diameter for both types of retinal cells.

Off-pathway synaptic transmission in the outer retina of the axolotl is mediated by a kainic acid-preferring receptor

Intracellular recordings were made from OFF-centre bipolar cells and horizontal cells in the superfused axolotl retina eyecup preparation. Bath-applied (+/-)cis-2,3-piperidine dicarboxylic acid (PDA), gamma-D-glutamylglycine (DGG), L-glutamic acid diethyl ester (GDEE), (+/-)2-amino-5-phosphonovaleric acid (2-APV) and magnesium ions were assessed as antagonists of the actions of the photoreceptor transmitter. The rank order of antagonist efficacy was PDA greater than DGG greater than greater than 2-APV = GDEE = Mg2+. The results indicate that transmission at OFF-pathway synapses in the outer retina of the axolotl is mediated by a kainic acid-preferring receptor.

Pigeon pattern electroretinogram: a response unaffected by chronic section of the optic nerve

Retinal evoked responses to sinusoidal gratings modulated in counterphase (pattern ERG) have been recorded from the pigeon eye. The pattern ERG amplitude depends upon the temporal frequency of the modulation, the contrast, the spatial frequency and the area of the stimulus. In 8 pigeons the pattern ERG has been recorded at different times after the unilateral section of the optic nerve. It has been found that the pattern ERG has a comparable amplitude in the two eyes as a function of the spatial frequency, 3 and 9 months after the section of the left optic nerve. At these times, histological evidence shows a drastic reduction in the density of the retinal ganglion cells on the operated side in comparison to the control one. These findings suggest that retinal sources other than the ganglion cells are responsible for te generation of the pigeon pattern ERG.

Electrical and morphological properties of off-center bipolar cells in the carp retina

Electrical membrane properties of carp off-center bipolar cells were studied by injecting a ramp of current (changing at a rate of about 0.3 nA/s) through one barrel of a double-barreled electrode and recording the voltage drop through the other barrel. The light-elicited response reversed the polarity at a positive membrane potential. The current-voltage curve showed inward and outward rectifications which modified the amplitude of the light-elicited response. The degree of membrane rectifications varied considerably among the cells sampled. Off-center bipolar cells were stained by iontophoretic injections of Lucifer-yellow dye after studying their electrical properties. There was a significant correlation between the difference in cell morphology and the degree of membrane rectification. The cells, which were characterized by small cell bodies lying close to the proximal part of the inner nuclear layer and by thin primary dendrites, tended to show rectifying membrane properties, compared to those characterized by large cell bodies lying close to the distal part of the inner nuclear layer and by thick primary dendrites. A site showing rectifying membrane properties and the origins of a large variability in membrane rectifications are discussed in detail.

An analysis of transmission from cones to hyperpolarizing bipolar cells in the retina of the turtle

Voltage noise was recorded from centre-hyperpolarizing bipolar cells in the retina of the snapping turtle. The identity of the cells was confirmed by intracellular staining. The variance of the voltage fluctuations of the membrane potential present in the dark was suppressed by up to 30-fold by 100 microns diameter light spot stimuli centred on the cell's receptive field. Such noise reduction is expected when light hyperpolarizes the photoreceptors and reduces the rate of release of transmitter from the terminals. The spectra of the fluctuations were analysed as the sum of two components: (a), a component with power band width limited to below approximately 10 Hz, and (b), a component Sh(f) of the form Sh(f) = Sh(0)/(1 + (f/f0)2)2, with f0 = 27 Hz. The two components were attributed (a) to the noise generated in the cones and transmitted through the synapse to the bipolar cells and (b) to the action of transmitter on the bipolar cell membrane. The component Sh(f) attributed to the action of transmitter on the bipolar cells corresponded to an event approximately 14 ms in duration. The event had a peak amplitude in the range 17.6-223 microV with a mean of 69.5 microV. It is estimated that, in the dark, the number of such events contributing to the noise is about 9200 s-1. It is estimated that each elementary noise event in the cones controls approximately thirty of the transmitter-related events at the synapse. Responses to flashes of darkness applied on steady illumination were analysed by a method of matched filtering. The responses fluctuated in amplitude, and the analysis of this fluctuation suggested an elementary event of approximately the same amplitude as found from the noise analysis. Enlarging the diameter of the stimulus spot to 1500 microns repolarized the bipolar cells with an associated increase in voltage noise. Implications for the synaptic mechanisms of the centre-surround organization are discussed.

Ionic mechanisms underlying the responses of off-center bipolar cells in the carp retina. I. Studies on responses evoked by light

Off-center bipolar cells show hyperpolarizing responses to spot illumination in the receptive field center and depolarization responses to an annulus in the surround. To understand the ionic mechanisms underlying these responses, we examined the current-voltage relationship of these bipolar cells, input resistance changes during their light-evoked responses, and the reversal potentials of these responses. Off-center bipolar cells generally showed inward rectification when they were hyperpolarized and outward rectification when they were strongly depolarized. The membrane potential at which the I-V relationship deviated from linearity varied in individual cells. Hyperpolarizing center responses were generally accompanied by a resistance increase, irrespective of signal inputs either from red-sensitive cones or from rods, and the response polarities reversed at greater than +50 mV. Depolarizing surround responses were accompanied by a resistance decrease with a reversal potential at about +28 mV (one case). From the above observations, it is suggested that the center responses are generated by a decrease in sodium conductance (gNa) and the surround response is generated by an increase in gNa.

The spatial frequency sensitivity of bipolar cells

Retinal bipolar cells constitute the output stage of the outer layer of the retina. There are several constraints on the ability of the bipolar cell array to respond to the different spatial frequency components of the visual image, including (i) electrical coupling in the dendritic tree receiving receptor input; (iii) the "lateral inhibition" mediated by horizontal cells. Using simple mathematical models, we derive analytical expressions for the spatial frequency response of the bipolar cell array for the case in which horizontal cells are presynaptic to bipolar cells (feedforward model) and also for the case in which horizontal cells are presynaptic to receptors (feedback model). The results illustrate the importance of the three factors mentioned in determining the bipolar cells' properties. The optimal spatial frequency for stimulating the bipolar cell array, and the range of spatial frequencies transmitted onward to the inner plexiform layer, are thus related to the anatomical and electrical properties of the cells in the outer plexiform layer.

Blue-sensitive rod input to bipolar and ganglion cells of the Xenopus retina

Intracellular recordings were obtained from chromatic and non-chromatic bipolar cells, identified by Lucifer yellow injection in the Xenopus retina. The chromatic cells, which lacked center-surround organization, were short wavelength hyperpolarizing (lambda max 445 nm) and long wavelength depolarizing. Under photopic conditions the depolarizing component was driven by 612 nm cones, but under mesopic conditions it appeared that 524 nm rods also constituted an input to the response. The non-chromatic bipolars encountered were of the off-center (hyperpolarizing) variety, with an active antagonistic surround, and peak spectral sensitivity in the red portion of the spectrum. Extracellular recordings were obtained from color-coded ganglion cells classified as type 1 or 2 in frog retina by Maturana et al. (1966) [J. gen. Physiol. 43, 129-175] and Bäckström and Reuter (1975) [J. Physiol. 246, 79-107]. The spectral sensitivity of the long latency "on" component was matched by the density spectrum of the 445 nm rod. This response component lacked center-surround organization and showed a relatively broad area of spatial integration. In contrast, a short latency component had a spectral sensitivity matched by the 612 nm cone pigment under photopic conditions, was either "on" or "off" center, showed center-surround organization and had a relatively small area of spatial integration. We speculate that in Xenopus retina, both chromatic and non-chromatic bipolar cells provide synaptic input to the class 1,2 ganglion cell.

An "antagonistic" surround facilitates central responses by retinal ganglion cells

Ganglion cells in the turtle retina respond to increments of saturating illumination of the receptive field centre by progressively decreasing the number of spikes of the responses, perhaps as a result of membrane inactivation. Intracellular recording indicates that a simultaneous illumination of the receptive field surround greatly facilitates the "suprasaturated" central responses, while the expected centre-surround antagonism is still present between photoresponses below saturation. It is suggested that both pre- and post-synaptic mechanisms provide the ganglion cells with the unique possibility, among other retinal cells, to shift their full dynamic range across more than 3 log units of illumination intensity.

The morphology of the bipolar cells, amacrine cells and ganglion cells in the retina of the turtle Pseudemys scripta elegans

The morphology of the neurons that contribute to the inner plexiform layer of the retina of the turtle Pseudemys scripta elegans has been studied by light microscopy of whole-mount material stained by the method of Golgi. Cells have been distinguished on the basis of criteria that include dendritic branching patterns, dendritic morphology, dendritic tree sizes and stratification of processes in the inner plexiform layer. Many of the neurons have dendritic trees oriented parallel to and a few exhibit an orthogonal orientation with the linear visual streak present in the retina of this species. The neurons of the turtle retina have been compared, where possible, with the neurons of the lizard retina as described by Cajal. The findings are discussed in relation to other vertebrate retinas, and correlations are made with recent electrophysiological recordings of the turtle retina. Comments are made with regard to the significance of orientation of neurons relative to the linear visual streak.

Retinal ganglion cells: a functional interpretation of dendritic morphology

The electrical properties of the different anatomical types of retinal ganglion cells in the cat were calculated on the basis of passive cable theory from measurements made on histological material. Standard values for the electrical parameters were assumed (R1= 70 Ω cm, Cm= 2 μF cm-2,Rm= 2500 Ω cm2). We conclude that these neurons need not be equipotential despite their small dimensions, mainly because of their extensive branching. The interactions between excitation and inhibition when the inhibitory battery is near the resting potential can be strongly nonlinear in these cells. To characterize the different types of ganglion cells in terms of this property we introduce the factor by which the soma depolarization induced by a given excitatory input is reduced by inhibition. In this framework we analyse some of the integrative properties of an arbitrary passive dendritic tree and we then derive the functional properties that are characteristic for the various types of ganglion cells. Our main results are: (i) Nonlinear saturation at the synapses may be made effectively smaller by spreading the same (conductance) input among several subunits on the dendritic field. Subunits are defined as regions of the dendritic field that are somewhat isolated from each other and roughly equipotential within. (ii) Shunting inhibition can specifically veto an excitatory input, if it is located on the direct path to the soma. TheFvalues can then be very high even when the excitatory inputs are much larger than the inhibitory, as long as the absolute value of inhibition is not too small. Inhibition more distal than excitation is much less effective. (iii) Specific branching patterns coupled with suitable distribution of synapses are potentially able to support complex information processing operations on the incoming excitatory and inhibitory signals. The quantitative analysis of the morphology of cat retinal ganglion cells leads to the following specific conclusions: (i) None of the cells examined satisfies Rail’s equivalent cylinder condition. The dendritic tree cannot be satisfactorily approximated by a non-tapering cylinder. (ii) Under the assumption of a passive membrane, the dendritic architecture of the different types of retinal ganglion cells reflects characteristically different electrical properties, which are likely to be relevant for their physiological function and their information processing role: (a) α cells have spatially inhomogeneous electrical properties, with many subunits. Within each subunit nonlinear effects may take place; between subunits good linear summation is expected.Fvalues are relatively low. (b) β cells at small eccentricities have rather homogeneous electrical properties. Even distal inputs are weighted rather uniformly. Electrical inhomogeneities of the a type appear for P cells at larger eccentricities.Fvalues are low. (c) γ-like cells have few subunits, each with high input resistance underlying nonlinear saturation effects possibly related to a sluggish character.Fvalues are high: inhibition of the shunting type can interact in a strongly nonlinear way with excitatory conductance inputs. (d) δ-like cells show many subunits with a high input resistance, covering well the dendritic area. Within each subunit inhibition on the direct path to the soma can specifically veto a more distal excitation. It is conjectured that such a synaptic organization superimposed on the δ cell morphology underlies directional selectivity to motion. (iii) Most of our data refer to steady-state properties. They probably apply, however, to all light evoked signals, since transient inputs with time to peak of 30 ms or more can be treated in terms of steady-state properties of the ganglion cells studied. (iv) All our results are affected only slightly by varying the parameter values within reasonable ranges. If, however, the membrane resistance were very high, all ganglion cells would approach equipotentiality. ForRm= 8000 Ω cm2subunits essentially disappear in all types of ganglion cells (for steady state inputs). Our results concerning nonlinear interaction of excitation and inhibition ( values) would, however, remain valid even for much larger values ofRmand for any value ofR1larger than 30-50 Ω cm. The critical requirement is that peak inhibitory conductance changes must be sufficiently large (around 5 x 10-8S) with an equilibrium potential close to the resting potential. Underestimation of the diameters of the dendritic branches may affect these conclusions (Fcould be significantly lower).

The photoresponses of structurally identified amacrine cells in the turtle retina

Intracellular recordings were obtained from amacrine cells afterwards identified morphologically by horseradish peroxidase injection. There is a correlation between the time course of the photoresponses and the distribution of the cell processes across the inner plexiform layer (i.p.l.). Cells producing the shortest duration, transient 'on-off' photoresponses branched in a single, narrow stratum of the i.p.l. (3-7 microns across). Transient photoresponses with a longer time course were recorded from cells branching in a thicker stratum of i.p.l. (up to 20 microns), or from bistratified cells. Amacrine cells producing sustained centre-on or centre-off photoresponses were radially diffused across the whole i.p.l.; therefore this type of photoresponse need not be associated with a specific cellular stratification within the i.p.l. It is concluded that the two main functional types of amacrine cell, i.e. transient on-off and sustained centre-on and centre-off, are subject to different structural organization of inputs than are the homologous physiological types of ganglion cells in this species, in the cat and in the carp. In a summary diagram the observed characteristics of the photoresponses are tentatively explained in terms of a non-homogeneous distribution of bipolar synaptic inputs along amacrine cell processes.

Connections of the small bipolar cells with the photoreceptors in the turtle. An electron microscope study of Golgi-impregnated, gold-toned retinas

The connections between photoreceptors and the small bipolar cells with a Landolt club in the turtle retina (Pseudemys scripta elegans) were analyzed with the electron microscope after Golgi impregnation and gold toning. The dendritic tree of several bipolars was partially or completely reconstructed from continuous series of thin sections. All bipolar cells studied established both basal and invaginating junctions with cone pedicles; in invaginating junctions, their dendrites invariably ended as the central process of triads. Dendrites of this bipolar cell type make invaginating synapses, basal junctions, or a mixture of both. In turn, each pedicle found within the dendritic field of the bipolar cell contacts it with a mixture of invaginating and basal junctions; furthermore, each pedicle synapses with more than one dendrite from the same bipolar cell. All small bipolar cells examined also synapse with rod pedicles; the geometry of the connections and the types of synapses are the same for both rods and cones. The fact that small turtle bipolars can form both basal and invaginating contacts with a single cone indicates that the polarity of the postsynaptic response can not be predicted on the basis of the structure of the photoreceptor-to-bipolar cell synapse.

Membrane specializations in the outer plexiform layer of the turtle retina

The internal organization of the plasma membrane at specialized contacts in the outer plexiform layer of the turtle, Pseudemys scripta elegans, was analyzed with the aid of the freeze-fracturing technique. In the invaginating synapse of cone pedicles the plasma membrane of the photoreceptor ending contains an aggregate of P-face particles, images of synaptic vesicle exocytosis, and rows of forming coated vesicles which are arranged in sequence from apex to base of the synaptic ridge. Thus, freeze-fracturing provides positive evidence that the synaptic ridge represents the active zone at the surface of the photoreceptor endings. Horizontal cell processes of dyads and triads have an aggregate of P-face particles opposite the apex of the ridge, but lack images of vesicle exocytosis. Deep-etching and rotary-shadowing demonstrate that an array of minute protrusions decorates the true outer surface of the horizontal cell membrane at the site of the intramembrane particle aggregate. The membrane of the invaginating bipolar dendrite is unspecialized. At basal junctions, the cone pedicle membrane has a sparse complement of P-face particles, but images of vesicle exocytosis are absent. The adjoining bipolar membrane is characterized by a prominent aggregate of E-face particles, often arranged in an orthogonal lattice. The freeze-fracture profile therefore suggests the existence of (1) a sign-conserving cone-to-horizontal cell synapse; (2) a sign-inverting synapse between cones and invaginating bipolar dendrites; and (3) a sign-conserving synapse between cones and bipolar dendrites at basal junctions. No freeze-fracture evidence was found for a horizontal-to-cone or horizontal-to-bipolar cell synapse within the synaptic invaginations.

Lateral interactions in the control of visual sensitivity

Threshold vs intensity curves for cone vision, measured in the parafoveal retina, quickly saturate if the adapting background is made small (e.g. 19' at 5 degrees eccentricity). Log increment threshold increases at a rate of about 3:1 with log background illuminance at levels as low as 10 td. This shows that lateral interactions are an important process in preserving differential sensitivity in cone vision across the wide range of illuminances over which it normally operates. Parallels between light and dark adaptation in the effect of field size were explored, since effects of comparable magnitude are observed in both. Backgrounds and bleaches equated for their effects at one field size do not have equal effects on threshold at other field sizes, however, with small-area bleaches raising threshold more than predicted. This failure of equivalence was also revealed in a second experiment, in which recovery of sensitivity following small area bleaches was measured in the presence of large steady background fields, which have the effect of lowering threshold. Thresholds following the small bleach were lowered less than expected on the basis of the "equivalent background" hypothesis, a result which we take to mean that signals from bleached cones exceed those produced by a background which has an equivalent effect on threshold (the "equivalent background"). Control experiments examined whether rods contribute to the overloading of cone response by small fields and the possible contribution of such central adaptation processes as spatial frequency adaptation.

Ratfish retina--intracellular recordings and HRP injections in an isolated, superfused all-rod retina

Hyperpolarizing responses to light were studied by intracellular recording in the isolated, superfused retina of the ratfish (Hydrolagus colliei). Two of these hyperpolarizing units were identified as horizontal cells by iontophoretic injection of horseradish peroxidase (HRP). These cells had rather large cell bodies (15 x 30 micrometer), elliptical dendritic arborizations measuring 150 x 300 micrometers and no axons. Since their physiological receptive fields were found to be at least 2.15 mm in diameter, it appears likely that either the photoreceptors or the horizontal cells are electrically coupled. Electron microscopy of HRP-injected horizontal cells showed their dendrites to end laterally in ribbon synapses of rods only and revealed dendro-dendritic contacts resembling gap junctions between injected and uninjected horizontal cells. The spectral sensitivity function of a dark-adapted horizontal cell can be described by a Dartnall nomogram based on a retinene pigment with lambda max = 473 nm. These findings are consistent with the histological observation that the ratfish retina appears to contain only rod photoreceptors.

An intracellular electrophysiological study of the ontogeny of functional synapses in the rabbit retina. I. Receptors, horizontal, and bipolar cells

An isolated perfused retina-eyecup of the rabbit neonate was used to study the functional maturation of synaptic interactions in the outer plexiform layer. Intracellular recordings from receptors, horizontal cells, hyperpolarizing and depolarizing bipolar cells were obtained in the adult and at different stages of maturation. Three types of horizontal cells could be distinguished based on the relative amount of rod and cone input, response wave form, receptive field diameter, and amplitude-intensity relationships. Two of the horizontal cell types were encountered with sufficient frequency such that maturation of these response characteristics could be followed over the developmental period of this study (8 days-adult). The growth rate of the amplitude-intensity relationships was different for the two commonly encountered types of horizontal cells; the small diameter receptive field type achieved an adultlike amplitude-intensity range at an earlier age than the large field type. Immature bipolar cell responses were initially monophasic potentials with transient-sustained components appearing at a later stage. Center surround antagonism of bipolar cells developed after center-mediated responses were first observed. This suggests some secondary modifications during synaptogenesis are responsible for the late maturation of center surround antagonism. The morphological appearance of synaptic contacts during different stages of synaptogenesis is discussed in reference to the different phases of functional, synaptic interactions of this study. It appears that some synapses in the vertebrate retina are functional at a time when synaptic structure is incomplete, based on ultrastructural observation. Maturation of receptive field diameter and amplitude-intensity function is discussed in relationship to possible presynaptic and postsynaptic mechanisms which may influence this growth.

Different postsynaptic events in two types of retinal bipolar cell

The first synapse in the vertebrate visual system is made between the photoreceptors and the biopolar cells. Bioplar cells fall into two distinct classes according to whether the cell hyperpolarizes or depolarizes to small centred spots of light. Most evidence indicates that the light-induced hyperpolarization of the photoreceptprs suppresses transmitter release from the synaptic terminals, and it is probable that the differences between the two bipolar cell classes results from the different actions of the photoreceptor transmitter. In analysing the membrane potential fluctuations in both types of bipolar cell we find that the voltage noise spectra differ. It is to be expected that postsynaptic noise would be composed of the sum of noise generated in and transmitted from the cones and the noise arising from the statistical nature of synaptic transmission. We report here evidence for two such components in the voltage noise spectra recorded from each type of bipolar cell. The differences in the frequency distribution of the presumed transmitter-related components indicates that the transmitter generates events of longer duration in the depolarizing biopolar cells.

Rod and cone inputs to bipolar cells in goldfish retina

Five morphological types of bipolar cells which make synaptic contact with rods and cones are distinguished in the retina of adult goldfish (Carassius auratus) by characteristics readily observable in the light microscope. Cells were designated type a or type b according to whether their axons terminate in the distal part (sublamina a) or proximal part (sublamina b) of the inner plexoform layer, respectively. Analysis of serial semi-thin sections of Golgi-impregnated cells demonstrates that each subtype of bipolar contacts rods and a characteristic set of chromatic subtypes of cones: types a1 and b1 cells contact rods and red-sensitive cones, while types a2, b2 and b3 contact rods and red- and green-sensitive cones. Comparison with published descriptions of cells stained with Procion Yellow after intracellular recordings had been made suggests that type a cells should be off-center types and type b on-center. Furthermore, it is suggested that the receptive fields of cell types a1 and b1 should be non-color-coded, and those of a2, b2, and b3 color-coded.

Responses of rod bipolar cells in the dark-adapted retina of the dogfish, Scyliorhinus canicula

1. Responses to light were recorded from bipolar cells in the retina of the dogfish, Scyliorhinus canicula, under dark-adapted conditions. The identity of the cells was confirmed by Procion Yellow staining.2. More than 95% of the bipolar cells sampled were of the type which depolarized to a spot of light. These are termed depolarizing bipolar cells. In most cells, illumination of the surround had little effect on the responses elicited from the central receptive field.3. The mean flash sensitivity of the depolarizing bipolar cells was 270 mV/Rh(**) (where Rh(**) signifies rhodopsin photoisomerization per rod for full field illumination).4. The mean flash sensitivity of horizontal cells under the same conditions was 8 mV/Rh(**). In a limited sample of hyperpolarizing bipolar cells the highest flash sensitivity was 42 mV/Rh(**).5. The high flash sensitivity of the depolarizing bipolar cells indicates a large voltage gain at its synapse with rods. On the assumption of a rod flash sensitivity of 2 mV/Rh(**) the mean gain at the synapse was 135, but for some cells the gain was in excess of 500.6. Responses of depolarizing bipolar cells to dim flashes could be approximated by the impulse response of a 12-16 stage low-pass filter, whereas horizontal cell responses could be fitted by a low-pass filter of six sections. The implied filter at the rod-bipolar cell synapse is tuned to the higher frequency components of rod signals, thereby improving temporal resolution in the rod pathway.7. Depolarizing bipolar cell responses to test flashes are reduced by weak background illumination (less than 0.1 Rh(**)/sec). This desensitization, which would not be expected to affect rod responses, could be explained by a shift in the operating point to a less sensitive region of the intensity-response curve as a result of the large depolarization elicited by the background.8. The results of current injection into the cell in darkness and during the response to light are consistent with the release by rod terminals of a transmitter which closes ionic channels in a conductance path having a reversal potential of - 8 mV, transmitter release being suppressed by light.

Lateral interactions in human cone dark adaptation

1. The course of cone dark adaptation after exposure to a strong bleaching light depends on the size of the bleached region. Threshold for brief, tiny test flash centred in the bleached region is elevated more, and recovery is retarded by a small bleach. This effect has its parallel in the sensitization effect observed with steady backgrounds. 2. Previous results, that a similar sensitization effect is not observed in rod dark adaptation, are confirmed. 3. This sensitization effect in cone dark adaptation does not transfer binocularly, and is unaffected by pressure blinding during the bleaching exposure. 4. Threshold following a small bleach may be lowered by adding a steady annular background to the region surrounding the bleached patch. Conversely, bleaching the area surrounding a small, steady background can lower threshold for a test flash centred on the background. 5. These interactions between backgrounds and bleaches may be explained if bleaches produce long-lasting signals from neurones in the bleached area, which then lead into a spatially opponent stage of processing. 6. It is likely that the persisting signals come from the cone receptors, since the Bunsen-Roscoe Law (intensity-time reciprocity) holds for small bleaches as well as large, for durations up to about 3 sec.

Synaptic inputs to the ganglion cells in the tiger salamander retina

The postsynaptic potentials (PSPs) that form the ganglion cell light response were isolated by polarizing the cell membrane with extrinsic currents while stimulating at either the center or surround of the cell's receptive field. The time-course and receptive field properties of the PSPs were correlated with those of the bipolar and amacrine cells. The tiger salamander retina contains four main types of ganglion cell: "on" center, "off" center, "on-off", and a "hybrid" cell that responds transiently to center, but sustainedly, to surround illumination. The results lead to these inferences. The on-ganglion cell receives excitatory synpatic input from the on bipolars and that synapse is "silent" in the dark. The off-ganglion cell receives excitatory synaptic input from the off bipolars with this synapse tonically active in the dark. The on-off and hybrid ganglion cells receive a transient excitatory input with narrow receptive field, not simply correlated with the activity of any presynaptic cell. All cell types receive a broad field transient inhibitory input, which apparently originates in the transient amacrine cells. Thus, most, but not all, ganglion cell responses can be explained in terms of synaptic inputs from bipolar and amacrine cells, integrated at the ganglion cell membrane.

Membrane recycling in the cone cell endings of the turtle retina

The ultrastructural effects of dark, light, and low temperature were investigated in the cone cell endings of the red-eared turtle (Pseudemys scripta elegans). Thin sections revealed that in dark-adapted retinas maintained at 22 degrees C, the neural processes which contact the cone cells at the invaginating synapses penetrated deeply into the photoreceptor endings. When dark-adapted retinas were illuminated for 1 h at 22 degrees C, the invaginating processes were apparently extruded from the synaptic endings. On the other hand, 1-h exposure to a temperature of 4 degrees C in the dark caused the invaginating processes to become much more strikingly inserted than at room temperature. A morphometric analysis showed that the ratio between the synaptic surface density of the endings and their total surface density decreased in the light and increased in the dark and cold. Freeze-fracturing documented fusion of synaptic vesicles with the presynaptic membrane in all conditions tested. These observations suggest that the changes in configuration of the pedicles in the light, dark, and cold reflect a different balance between addition and retrieval of synaptic vesicle membrane from the plasmalemma; in the dark, the rate of vesicle fusion is increased, whereas in the cold, membrane retrieval is blocked. When the eyecups were warmed up and illuminated for 30-45 min after cold exposure, a striking number of vacuoles and cisterns appeared in the cytoplasm and coated vesicles were commonly seen budding from the plasmalemma. 60-90 min after returning to room temperature, the endings had reverted to their normal configuration, and the vast majority of vacuoles, cisterns, and coated vesicles had disappeared. When horseradish peroxidase was included in the incubation medium, very few synaptic vesicles were labeled at the end of the period of cold exposure. 30-45 min after returning to 22 degrees C, vacuoles and cisterns contained peroxidase, whereas most synaptic vesicles were devoid of reaction product. 2 h after returning to 22 degrees C, coated vesicles, vacuoles, and cisterns had disappeared and a number of synaptic vesicles were labeled. These experiments suggest that vacuoles, cisterns, and coated vesicles mediate the retrieval of the synaptic vesicle membrane that has been added to the plasmalemma during cold exposure.

The responses of amacrine cells to light and intracellularly applied currents

1. Intracellular responses to light were recorded from amacrine cells in the retina of the turtle Pseudemys scripta elegans. 2. The recorded responses were identified on the basis of physiological criteria reported previously (Marchiafava, 1976). Amacrine cells produced transient 'on' and 'off' depolarizing responses irrespective of the retinal area illuminated and of wavelength. 3. The transient depolarizing responses increased by enlarging the illuminated circle up to 120 micrometer in radius. Circles covering larger areas, up to 200 micrometer, produced a relative decrease of the response amplitude. Thus, amacrine cells' receptive fields appear as a central 'excitatory' area of about 120 micrometer radius, surrounded by a 'suppressor' area. 4. Amacrine cells' photoresponses were associated with an increase in membrane conductance. The responses to illumination of central or peripheral areas of the receptive field, however, showed different reversal potentials. The responses to peripheral illumination reversed at about 15 mV above resting potential, while the equilibrium potential of the centre-photoresponses was indicated by extrapolation at about +30 mV. No conductance chance was detectable during steady lights. 5. Repetitive stimulation of the optic nerve invariably reduced amacrine cells' photoresponses, but not those recorded from bipolar cells. It follows then that only ganglion cell photoresponses originating from amacrines' input would be depressed by the nerve stimulation, which thus becomes a reliable test to discriminate whether ganglion cell photoresponses originate from amacrine or bipolar inputs.

Transmission from photoreceptors to ganglion cells in turtle retina

1. Synaptic transfer between photoreceptors and impulse-generating cells was studied in isolated eyecups from turtles. Single red-sensitive cones or rods were stimulated by current passed through an intracellular electrode, and impulses generated by the resulting synaptic action were recorded with an external micro-electrode. This technique permits study of retinal transmission without the operation of the visual transduction mechanism. Antidromic stimulation of the optic nerve indicated that most of the impulse-generating cells were ganglion cells.2. Individual ganglion cells responded transiently to changes in the membrane potential of a receptor and could be classified into three groups on the basis of the direction of the effective change in potential. Off centre ganglion cells responded selectively to depolarizations of a receptor, while on centre ganglion cells responded selectively to hyperpolarizations. On-off ganglion cells responded to both depolarizations and hyperpolarizations of a receptor.3. Ganglion cells gave the same pattern of response to electrical hyperpolarization of a receptor and to light in the centre of their receptive fields. Subthreshold depolarizing currents passed in a receptor antagonized the ganglion cell's response to light, and subthreshold hyperpolarizing currents reinforced the response. These observations are consistent with the view that the hyperpolarization generated by visual transduction is responsible for regulating the release of transmitter at the first retinal synapse.4. When a receptor was stimulated with weak current pulses of fixed intensity the number and latency of the ganglion cell impulses fluctuated randomly in successive trials. The relation between the fraction of trials yielding a response and the stimulus intensity was broad. These results indicate that the link between retinal input and output is noisy.5. In the most sensitive pairs of cells, a response of one or more impulses could be obtained in half the trials with a current of about 2 x 10(-11) A, which changed the potential of the receptor by 1-2 mV. A current of similar magnitude would be developed by about 130 photoisomerizations in a red-sensitive cone or 50 photoisomerizations in a rod.6. Dim background light producing a steady hyperpolarization of a few millivolts in the rods raised the threshold for electrically-evoked transmission from a rod to a ganglion cell. In experiments on red-sensitive cones, background light raised the threshold in the off pathway, in which depolarization was the effective stimulus, and lowered the threshold in the on pathway, in which hyperpolarization was the effective stimulus. These changes in sensitivity were not accompanied by obvious changes in the input resistance of the stimulated receptor. Regulation of retinal sensitivity in background light thus involves changes in synaptic transfer as well as changes in the sensitivity of the visual transduction mechanism.

The organization of the outer plexiform layer in the retina of the cat: electron microscopic observations

The outer plexiform layer of the cat retina has been examined by electron microscopy of random and serial ultrathin sections in order that neural profiles might be positively identified and their synaptic relationships studied. Photoreceptors are interconnected by means of gap junctions as are the A horizontal cells. B horizontal cells and axon terminals do not appear to be engaged in any synapses apart from those with photoreceptors, while A horizontal cells make rare 'junctions' with cone bipolars only. Interplexiform cell processes probably account for all the conventional chemical synapses in the outer plexiform layer of cat retina.