The cells horizontal cells talk to

Vision

Printed

0042-6989/82/060653-08503.00/O PergamonPress Ltd

Vol. 22. pp. 653 to 660. 1982 in Great Britain

Res.

THE CELLS HORIZONTAL

CELLS TALK TO

KEN-ICHI NAKA

National Institute

for Basic Biology, Okazaki, Japan 444

INTRODlJ(JTION

In 1962 William A. H. Rushton, to whose memory this article is dedicated, asked me to come to his laboratory in Cambridge to perform experiments on the responses from the horizontal cells; the response was then known as the S-potential. At that time, the potential was implicated in a role in dark adaptation or bleaching adaptation in Rushton’s terminology (Rushton. 1965). The potential and the structure, the S-space, which generated it, were a reasonable candidate for the excitation pool through which gain of many receptors was to be controlled (Rushton, 1959). The S-space, a lamina formed by horizontal cells, seemed a physical realization of the hypothetical screen on which the real and dark lights were cast upon to be mixed together (Barlow and Sparrock, 1964). Our experiments on the teleost horizontal cells failed to yield any supportive evidence for the view (Naka and Rushton, 1968) and, in desperation, we wrote “(S-space is) a river perhaps, flowing through that crowded community into which all cells can empty their electrical effluence. One likes to have one’s electrode in the council chamber not in the sewer” (Naka and Rushton, 1967). It was Maksimova (1969) who published the initial evidence to show that current artificially injected into horizontal cells could evoke discharges from the ganglion cells. Her pioneering effort led to a series of our papers in which we tried to define the pathways of information flow leading from the horizontal cells to other neurons in the retina. In this article I will summarize our past results and introduce newer ones to substantiate our claim that the horizontal cells perform the most unique function ever found in the central nervous system and that the S-space is indeed the chamber of high council.

trodes were manipulated independently and their tips were separated by 0.3-0.4 mm. Electrodes were filled with 2-M potassium citrate. White-noise signal was produced by a signal generator (N. F. Circuit Design Block, WG-722) and correlation between the input and output was made by a spectrum analyser (Hewlett-Packard 3582A). References will be made on the results of white-noise current injection, details of which will be presented in full elsewhere (Sakuranaga, and Naka, in preparation). Identification of horizontal cells’ two structural parts, their soma and axon, was made based on the cells’ response to travelling random grating (Davis and Naka, 198Ob).

MATERIALS AND METHODS

Materials used were the “eye-cup” preparation of the channel catfish, Ictalurus punctatus, obtained from Happy Jack Trout Farm, Azusa, CA, Adams’ Catfish Farm, Angleton, TX and several catfish farms in Japan. Most of the methods involved were already described in our previous publications and the only new procedure used here was the two-(intracellulart electrode experiments in which one electrode was placed in the horizontal cells to inject extrinsic current and the other in other neurons to monitor responses evoked by the injected current. The two elec653

STRUCI-URE OF THE RETINA AND DEFINITION

OF TERMS

Catfish retina has very active photochemical movements (Walls, 1942). Under our experimental conditions rods are extended deep into the pigment-cell layer to be screened off from light and only cones are active. There is one dominating class of cones with their absorption maximum at around 625 nm. There is only one class of cone horizontal cells, which will be referred to simply as the horizontal cell in this article. The horizontal cells receive input from single class of cones and the cells’ distal processes can easily be traced back into the cone terminals. The horizontal cells have axons which face the inner synaptic layer and often come into direct contact with elements in the layer. The S-space formed by the axons have a much larger space constant than that of the somas (Marmarelis and Naka, 1972). All horizontal cells are luminosity type and light stimulus always hyperpolarizes them. All the bipolar cells in the retina formed concentric receptive fields and their surround had the same spatial dimension as that of the horizontal-cell soma’s receptive field (Davis and Naka, 1980b). Type C cells are those cells which are commonly called the (transient) amacrine cells (Naka et al., 1975). Some of our type C cells could have been a ganglion cell. We maintain that, unless a cell is morphologically identified, it is not possible to distinguish a transient amacrine cell from a transient ganglion cell because in the literature both cells are reported to produce spike discharges (Chan and Naka, 1976). Type N cells are those cells which are commonly known as the (sustained) amacrine cells (Kaneko, 1973). Type NA cells correspond to the depolarizing

KEN-ICHI NAKA

654

and type NB cells to the hyperpolarizing amacrine cells (Naka et al., 1975). In the catfish both cells form biphasic receptive fields (Davis and Naka, 1980b). We have consistently avoided the use of essenceindicating names because a cells response pattern is bound to depend upon the kind of stimulus, light or extrinsic current, used to evoke it (cf. Rowe and Stone, 1977). And terms such as transient or sustained are subjective. RESULTS Signal

transmission within the S-space

Naka and Rushton (1967) advance a concept that the horizontal cells in a given layer formed a lamina. Within it current spread freely based on simple mathematical rules (Naka and Rushton, 1967; Marmarelis and Naka, 1972). Kaneko (1971) confirmed the concept experimentally by injecting a step of current into one cell and recording resulting potential changes from a nearby cell. The past results, theoretical as well as experimental, were all concerned with the steadystate value of current spread, and no dynamics were taken into account. Here we inject white-noise modulated current into one point in the S-space formed by the somas and monitor potential changes due to the current spread from another point in the space. Because my main concern was the dynamics of the current spread, I measured the transfer gain and phase shift between the input and output. A set of typical results is shown

Amplituda

1

in Fig. 1, in which two electrodes in the space was separated by about 0.4 mm and the input signal had a flat power spectrum from near d.c. to 1OOHz. Two curves were for the current injected into one of the two electrodes. In the frequency range I was interested in, the transfer gains were constant and there was no shift in the phase; the S-space was a simple resistive network and there was no capacitive or inductive component. In a previous study, the horizontal-cell responses were shown to have little power beyond 30 Hz (Marmarelis and Naka, 1973). For all practical purposes, therefore, the S-space was a resistive network. As we would expect from the results shown in Fig. 1. the coherence function in the frequency range was close to 1: signal transmission within the space was linear. The S-space formed by the axons had the same dynamic: it was also a pure resistive network within the frequency range we were interested in. Signal transmission from horizon&d to ganglion cells

As in the pike retina, current injected -into the catfish horizontal cells could drive the on- and off-center ganglion cells (Maksimova, 1969; Naka and Nye, 1971). In catfish, there was no exception to this observation. A depolarization of the horizontal cells produced spike discharges from the on-center ganglion cells and a hyperpolarization discharges from the offcenter ganglion cells. It was possible to drive on- and off-center ganglion cells simultaneously. With sinusoidally modulated current each phase of the input pro-

c

Phase

Fig. 1. Results of an experiment in which two electrodes were placed in the S-space. White-noise modulated current was injected into one of them and resulting potential changes were recorded from the other. Two electrodes separated by about 0.4 mm and the injecting and recording electrodes were switched over for the two curves in the records. “Amplitude” was for the transfer gain with ticks at 20 dB intervals and “phase” was the phase shift between the input and output with ticks for 45’. Curves were calculated by Hewlett-Packard 3582A spectrum analyser. Also shown in the figure is a sketch of the S-space which is bounded by two parallel high-resistance membranes. Original of the sketch was made in 1964 by W. A. H. Rushton.

655

The cells horizontal cells talk to HORIZONTAL

BIPOLAR

TO GANGLION

TO

GANGLION

TYPE A ON-CENTER

TYPE B OFF-CENTE , 50msec, Fig. 2. First-order kernels for the horizontal cell (soma)-to-ganglion cell and for the bipolar cell-toganglion cell neuron chains. In each record kernels from four injection experiments are shown. The first-order kernels were computed by cross-correlating input signals (white-noise current) against the output, the spike discharges counted and transformed into an analog signal at a 2-msec bin. Kernels were the best linear approximation of the current waveform most likely to trigger a spike discharge. The ordinate, in the units of spikes/A per sec. is normalized. The records shown here are modifications of those published by Naka (1977).

from one of the two ganglion-cell types: signals in the horizontal cells were shared by the two ganglion-cell types. In Fig. 2 are shown results of a series of experiments in which white-noise modulated current injected into the horizontal cells evoked discharges from the ganglion cells. The discharges were crosscorrelated with the input to produce the first-order correlation or the first-order (Wiener) kernels. The correlograms were the waveform of the current which most likely produced a spike discharge from the ganglion cells: or more exactly the linear part of the waveform. We note that (1) there was little variation in the waveforms and (2) the waveforms for the on- and offcenter cells were very similar except their polarity. In Fig. 2 are also shown a set of kernels for the bipolar cell-to-ganglion cell neuron chains. The kernels were obtained by injecting white-noise modulated current into the bipolar cells to evoke discharges from the ganglion cells. In the experiments, I observed that currents injected into the on-center bipolar cells produced discharges from the on-center and not from off-center ganglion cells, and the same applied to the off-center pairs: two channels were functionally segregated. As shown in this figure, a depolarization of the bipolar cells always produced spike discharges from the ganglion cells and the waveforms’of the kernels for the on- and off-center cells were also almost identical. In catfish, as in other retinas, a spot of light depolarized the on-center bipolar cells and hyperpolarized the off-center bipolar cells: an annulus of light hyperpolarized the former and depolarized the latter cells (Kaneko, 1970; Toyoda, 1973; Schwartz, 1974; Naka and Ohtsuka, 1975; Yazulla, 1976). If the signal transmission from the horizontal to bipolar cells were quasi-linear, a hyper-polarization of the horizontal cell must produce spike discharges from the off-center ganglion cells and depress discharges from the onduced discharges

center ganglion cells and a depolarization must produce opposite responses in the cells. What I have described so far is summarized in a “truth table” (Table 1). The table shows that stimulation by a spot of light and a depolarization of the horizontal cells produced similar effects and an annulus of light and a hyperpolarization of the cells produced similar effects in the ganglion cells. Light always hyperpolarized the horizontal cells and only the depolarizations of bipolar cells produced spike discharges from the ganglion cells. Therefore, we come to a conclusion that (1) bipolar cells receive two independent inputs, one from the receptors and other from the horizontal cells and (2) the signals in the horizontal cells were transmitted to the bipolar cells without changing their signs in the on-center cells and with their signs inversed in the off-center cells. Experimental proof of this premise will be given in the next section. Signal transmissionfrom horizontal to bipolar cells

Current injected into the horizontal-cell soma produced potential changes in the on- and off-center bipolar cells (Fig. 3). In the on-center cells, a pulse of Table 1. Modes of ganglion-cell responses evoked either by photic inputs or by extrinsic current injected into the horizontal cells Ganglion cells

spot

On-center Off-center

On Off

Photic Annulus Off On

Current Depol f-fyper On Off

Off On

A spot of light stimulated a concentric field’s center and an annulus of light its surround. For the current response, a depolarizing kernel was represented as “on” and a hyperpolarising kernel as “off”. In a previous study (Naka and Nye, 1971). “on” dischargers were expressed as sustained discharges and “off” discharges as transient discharges.

656

KEN-ICHI

Horizontal Al

On - cmtw

A2

BI

82

cell to bipdw Off - C*“Ar

cell 83

84

--

Y+--0x0.43

1

Fig. 3. Signal transmission from the horizontal cell (soma) to the bipolar cells. The current injection electrode was in a soma and the recording electrode was in a bipolar cell. Records A were from an on-center and those in B were from an off-center bipolar cell. In this and in all other figures. sharp transients were the artifacts of current injection and upper curves were for the injected current and lower ones were for the responses. Vertical calibration was 5 mV for the response and 5 nA for the current. The 0.4-set time calibration applies to records B3 and 4.

current injected into a soma produced a response of the same polarity and in the off-center cells the polarity of the response was reversed. The response had no clearly defined latent period. In both cases the response had smooth rising and falling phases to indicate that high-frequency components were lost in the transmission. When the polarity of the injected current was reversed, polarity of the response reversed also and they were mirror images of each other. With step inputs of longer duration we observed an initial peak and a swing of potential toward the opposite direction at the cessation of the pulse. The (slow) transients were, however, symmetric at the on- and offsets of the pusle (Fig. 3, B3 and 4). As far as we could tell from step experiments the signal transmission between the horizontal and bipolar cells was quite linear. Indeed, white-noise analysis performed on the neuron chain showed that the polarity of the firstorder kernels depended upon the type of bipolar cells and they predicted a cell’s response with a MSE (mean square error) of about 20”/0 (Naka et al., 1975). The second-order kernels for the chain were very

small and noisy. A pulse of current injected into the bipolar cells did not produce any response in the horizontal cells. The transmission was one-way. The results described in this section confirmed two important points: (1) the signal transmission from horizontal cells to bipolar cells was quasi-linear to exclude such possibilities as gain control or lateral inhibition. (2) The transmission was through either sign-noninverting or sign-inverting synapses as predicted from the experiments in the previous section. Signal transmission from horizontal to type C cells A pulse of current of either polarity injected into the horizontal-cell axons produced brief depolarizing potential from the type C cells. There was no exception to this observation. Unlike the bipolar cell’s re-

NAKA

sponse, the transient depolarization had (1) a welldefined latency, (2) a fast rising phase and (3) a slower decay to the baseline. These response characteristics indicated the highly nonlinear nature of the signal transaction. I noted that a depolarizing pulse always evoked a response of a longer latency than a hyperpolarizing pulse. Amplitudes of type C cells’ response to a pulse of current of about 5 nA delivered into the axons could be as large as the response evoked by light stimulus. With longer steps of current, the re-

sponse often became on-off transients

like the resuch examples are shown in Fig. 5. In the figure, columns A and B show responses from two typical type C cells, one of them showing clear spike discharges (Fig. 5. B). The cell in column C could not be classified as a type C cell based on its light-evoked response: I would have judged it as an on-center ganglion cells. Step inputs of either polarity gave rise to transient depolarizations at the on- and offsets of the current. It was often observed that amplitude of the transient responses evoked by the negative-going edge of the current was larger than that of the responses evoked by the positive-going edge of the current. This observation was in variance with the records shown in Fig. 4 in which brief pulses of current of either polarity produced responses of similar amplitude. I have no explanation for this discrepancy except to note that the horizontal cells showed a hyperpolarizing swing at the onset of light stimulus and that the initial peak of the light-evoked on-ff response was always larger than the one seen at the offset of light stimulus. White-noise analysis performed on the neuron chain showed that the signal transmission was very nonlinear as one would expect from the results described in Figs 4 and 5. The first-order kernels were very small and noisy and they did predict the cell’s response with a MSE of SCr!JO”/,.This was similar to sponse

evoked

by

Horizontal

Al

light

cdl

stimulus.

to tyw

Several

C C*tt

81 n

A2

82 -

Fig 4. Signal transmission from the horizontal cell (axon) to the type C cell. The current injection electrode was in an axon and the recording electrode was in a type C cell. Current of either polarity produced a transient depolarization from the type C cell. In records Bl and 2, five traces were superimposed. Details were similar to those in Fig. 2. Vertical calibration was 10 mV for the response and 5 nA for the current. The 0.2~set time scale applies to records Bl and 2.

657

The cells horizontal cells talk to

White-noise analysis performed on the horizontalto-type N cell neuron chain showed that the firstorder kernels could predict the cell’s response with a MSE of about 30%. The neuron chain was quite linear. Unlike the bipolar cells, however, type N cells showed well-defined second-order kernels. DISCUSSION

Fig. 5. Responses from type C cells produced by light stimulus and steps of current injected into the horizontalcell axons. Records were from three different cells, A, B and C. Those in the top row were by light stimulus (timing of the light stimulus is not shown). The cell in B nroduced spike-discharges. Vertical calibration was 1OmV for the response and 5 nA for the current.

the value for the kernels computed from light-evoked responses (Naka et al., 1975). The second-order kernels were well defined and did predict the responses with a MSE of about 30”/,. The configuration of the second-order kernels was very similar to that seen in the light-evoked experiments; the only difference was their time course. Signal transmissionfrom horizontal to type N cells

A step of current injected into the horizontal-cell axons evoked complex responses from type N cells (Fig. 6). Responses from type N cells were characterized by three features: (1) The response had a sustained component whose polarity depended upon that of the inputs and also upon the type of cells. A step of depolarizing. current produced a sustained depolarization __ from type NA cells and a sustained hyperpolarization from type NB cells: a hyperpolarizing current produced responses of opposing polarity. (2j Symmetric overshoots were seen at the on- and offset of the injected current. (3) A depolarizing, transient peak appeared when the cells were depolarized. The former two features were already seen in the bipolar cells. For example, the responses in Fig. 6, 3B, were very similar to those in Fig. 3, B3. The third feature was shared by the types C and N cells. Indeed, white-noise modulated current produced very similar responses with transient depolarizing peaks from types N and C cells. The responses from type N cells, therefore, had two components, one linear and sustained and the other nonlinear and transient. The former was responsible for the cells’ bipolar-cell-like behaviour and the latter the cells’ transient, depolarizing peaks. The responses from two type N cells, NA and NB cells, were complementary as seen from records Al and B4 and A2 and B3 in Fig. 6. Similar complementary features of the two type N cells were seen in their light-evoked responses (Davis and Naka, 1980b).

The horizontal cells whose response was first recorded by Gunner Svaetichin in 1952 had the most unique features, structural as well as functional, ever found in the central nervous system (Svaetichin, 1953). The cells did not produce any spike discharges and were coupled electrically together to form a lamina, the S-space (Naka and Rushton, 1967). These exceptional properties led earlier investigators to suggest the cells’ role as a modifier or controller of signal transmission within the retina (Svaetichin, 1961), and the cells were often referred to as glia-like. Rushton (1962) thought that the cells might be an instrument through which the gain of many receptors was controlled in the process of bleaching adaptation. If so, the horizontal cells’ dark potential must reflect the state of the adaptation. Our effort to establish this possible relationship failed, and evidence was overwhelming to rule out the suggested role of the cell. There was a period of time when we did not know what to make of the cells’ function except to say that the S-space formed by the cells could be a sewer (Naka and Rushton, 1967). In 1969, Maksimova injected current into the pike horizontal cells and produced spike discharges from the ganglion cells to shed the first light on the confused state on the nature and function of the horizontal cells (Maksimova, 1969). Her experiment identified the cells, beyond any doubt, as a neuron and as an element involved directly in the information processing within the retina. Before then, the view generally held was that the horizontal cell could not interpose in the direct neural pathway because the S-space the cell formed would smudge the finer details of the images cast on the retina. The cell, therefore, had to

HWiZOWOl Cd to Al ---

NA

El --

NB

typa N

cdl 83 _-_

-WV A2 --

82 -_

84

Fig. 6. Responses from type N cells produced by steps of current injected into the horizontal-cell axons. Records in B were from the same cell. Details were similar to those in Fig. 2. Vertical calibration was 5 mV for the response and 5 nA for the current. The 0.Cset time scale applies to records B3 and 4.

65X

KEN-ICHI NAKA

function as a modifier of signal transmission within the retina (Rushton. 1962). Our analysis of the responses from teleost horizontal cells revealed several important features which are: (I 1 The cells formed a lamina. the S-space. Functionally the horizontal cells in a given layer were coupled together through passages of low electrical resistance so that the layer looked as if it was a space bounded by a pair of parallel rubber sheets (Naka and Rushton. 1967). Results shown in Fig. 1 showed that the space also looked as if it was a pure resistive network for signals of up to 100 Hz. The frequency was high enough for any perturbation of the cells’ potential by light stimulus. The horizontal cells, through S-space. compute. so to speak. the average luminance level of the environment. We referred to the signal in the space as the integrating or global signal (Naka and Nye, 1970). Similar lamina structures were described in the horizontal cells (Lamb, 1976) and photoreceptors (Gold. 1979) in other retinas. Such a structure may turn out to be an important instrument in the information transformation in the vertebrate retina. (2) Michaelis-Menten equation described the relationship between the light input and the amplitude of the cell’s response. The same equation has been shown to fit the input-output relationship of most of the receptors (Baylor and Fuortes. 1970). The local slope of the equation was the cell’s incremental sensitivity and was defined by the amplitude of first-order kernels for a given mean intensity (Naka. er al.. 1979). The cell’s incremental sensitivity was not the WeberFechner relationship which was to be found in the more proximal neurons. One problem William Rushton tried to solve was to reconcile the two relationships. Michaelis-Menten and Weber-Fechner. The latter relationship was established itself. in Rushton’s mind. in the receptors. White-noise analysis performed on the horizontal cell showed that the two relationships did not coexist in the cell and theoretically they must not (Williams and Gale, 1977). (3) The cells formed the surround of the bipolar cell’s concentric receptive field. Current injection experiments. examples of which were shown in Fig. 2, gave direct evidence in favor of the thesis that the surround of bipolar cell’s concentric receptive field was provided by the horizontal cells. In our scheme, bipolar cells received two independent inputs; one input was the local signal from the receptors and the other input the integrating (global) signal from the horizontal cells (Naka and Nye, 1971). The signal transaction was through either sign non-inverting or sign inverting but not through excitatory or inhibitory synapses (Naka. 1976). Signals in the S-space was not excitatory or inhibitory: the signal was neutral. Although the horizontal cells are thought to provide the bipolar cells’ surround (Thibos and Werblin, 1978), a commonly held view proposes that the former cells formed the latter’s surround through the circuitous route of feedback synapse (from the hori-

zontal cell) to the receptors (cf. Toyoda and Tonosaki, 1978). Experimental proof of this argument came from the turtle experiments in which current injected into the horizontal cells produced responses in the receptors (Baylor et a/.. 1971). (4) Signals in the cells’ axon were transmitted to the neurons in the inner synaptic layer. In the experiments shown in Figs46. I showed that the axon’s signal was transmitted to types C and N ceils. The most simple explanation of this observation would be to have the signal transmitted directly to the cells in the proximal layer. Now I have given experimental proof that signals in the axons were indeed transmitted to the proximal cells and the main issue would be to identify the pathways through which signals in the axons were sent to the two proximal neurons. There are three means through which answer for this problem can be obtained. First. morphological contracts or synapses can be sought so that we will be able to follow the path of signal transmission. Secondly, another set of functional tests can be undertaken to provide further evidence for or against each proposed pathway. Thirdly. analysis of nonlinearity in the signal transmission might lead us to a clue to the problem. In Fig. 7 is shown evidence which belongs to the second category. In this experiment, 1 used a bar of light which swept through a fish’s field of view at a constant speed. The bar of light was oriented vertically and moved horizontally to and fro across the

t

Imm

I

Fig. 7. Responses evoked by a moving bar of light from a type C (upper record) and from a horizontal-cell axons (lower record). Two traces in each record were for the right-to-left and left-to-right sweeps of the bar of light which had a half-width of about 6Oym measured at the retinal surface aqd was swept across it at 2.6 mm/set. Two sets of records were obtained successively in a single electrode penetration. Procion-dye injected image of the type C cell is also shown. Calibration applies to all records.

659

The cells horizontal cells talk to fish’s field of view (actually the bar of light swept across the retinal surface). The upper traces were from a type C cell and the lower ones from a horizontalcell axon. The C cell’s morphology defined by intracellular dye injection is shown in the figure. Receptive field profiles of the two cells, defined by a moving bar of light, were strikingly similar except their polarity. In the experiments shown in Fig. 5, I showed that artificial polarization of the axons could mimic type C cells’ light-evoked response. The two sets of evidence lead me to suggest that signals in the axons must be sent to type C cells and that the transmission must be direct. If this were the case, the nonlinearity in the signal transmission must be attributed to the postsynaptic membrane; the nonlinearity was due to the intrinsic properties of the type C cell. It is generally believed that the on-off transient response from the amacrine cell (type C cell in catfish) was produced by an interaction of signals from two bipolar cells (Toyoda, 1974). Pathways of signal transmission from the axons to types NA and NB cells were more difficult to define because of the complexity of the signal transaction and also of the cells’ receptive field organization (Davis and Naka, 1980b). The transaction could not be as simple as in the type C cells. The scheme I published before, in which axons communicated with type N cells through sign-noninverting and signinverting synapses was an oversimplification of the cells’ synaptic structure (Naka, 1976). Further structural as well as functional analysis must be undertaken to solve this problem. Maksimova (1969) injected extrinsic current into the pike horizontal cells and evoked discharges from the ganglion cells. She noted that, in most of the cells, on-discharges became off-discharges and vice versa when the current’s polarity was reversed. In some others, injected current evoked onoff discharges. Schwartz (1973) recorded similar on-off discharges from the turtle ganglion cells by polarizing the horizontal cells. Negishi et al. (1977) noted three carp response types: their common type was our off-center cell, their reverse type was our on-center cell and their transient type was our type C cell. There are two main points which catfish results are in variance with those from other retinas: (1) In other retinas, results reported were not as straightforward as in catfish. For example, Negishi et al. reported that their common and reverse types were shared by both the on- and off-center cells. In catfish their common type was always found in the off-center cells and their reverse type always in the on-center cells. I have found no exception to this rule. The variance might have been due to the fact that in all other retinas there were more than one type of receptors and horizontal cells were present. The only exception was Naka and Witkovsky’s study on the dogfish retinas (Naka and Witkovsky, 1973). (2) In catfish no experiment was performed on the \.R ?26--D

on-off ganglion cells. A metal electrode placed on the catfish retina recorded only on- and off-center cells. A deep-penetrating glass pipette had to be used to record spikes (extracellularly) from onoff ganglion cells (Chan and Naka, 1976). My results on the type C cells shown in Fig. 5 were very similar to the other investigators’ results on the on-off ganglion cells. All the dissimilarities between catfish and other retinas, however, can be explained if former retina is assumed to be less complex. There are not many reports on two-electrodes intracellular current injection. A notable exception in the paper by Toyoda and Tonosaki (1978) in which they injected current into intermediate horizontal cells in carp retina and recorded “sign-non-inverting” responses from the on-center bipolar cells. This was what I found in the catfish retina. SUMMARY

In the outer synaptic layer, the horizontal-cell soma produces integrating signal and transmits it to the bipolar ceils where difference between the local (from the receptors) and integrating (or global) signals is made. Through this mechanism, signals for the average illuminance are removed in the bipolar cells. This is similar to the common-mode rejection in electronic circuits. This is exactly the reason why the d.c. components in the receptors are transmitted to the horizontal cells and why the two cells have exactly the same (static) input-output relationship, the Michaelis-Menten equation. A by-product of this arrangement is the concentric receptive field in the bipolar cells. Another function of the horizontal-cell soma is to transmit its signal back to the receptors to “improve” their frequency-response range (Marmarelis and Naka, 1973b). When the feedback is active the cell’s response follows much higher input frequency. Although the presumed transmitter was identified (Lam et al., 1978) and synapses made by the horizontal cells back to the receptors were observed (Davis and Naka, 1980a, and unpublished observations), quantitative description of the feedback is yet to be made. As of now we have to be contented with a statement that the feedback was a zeroth order in which the d.c. level of the cell’s potential was the most critical factor (Krausz and Naka, 1981). If this synapse and its function were proved to be true, they are radicallly different from the function of other synapses so far known to us. In the inner synaptic layer, as I described in this article, the horizontal-cell axon communicates with elements in the layer. As the d.c. components in the light signal are lost in the bipolar cells, the axon is the only pathway through which information on the dc. level of the incoming light is transmitted to the inner synaptic layer. I do not know why the information had to be brought down to the inner synaptic layer. Mathematical analysis of signal transmission between

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