Morphological and functional identifications of catfish retinal neurons. I. Classical morphology

Glycogen in retinal horizontal cells of the African mud catfish Clarias gariepinus (Burchell, 1822) and its physiological significance

This paper reports on glycogen store in the retinal horizontal cells (HC) of the African mud catfish Clarias gariepinus, as seen by histochemical reaction with periodic acid Schiff (PAS) and transmission electron microscopy in light- as well as dark-adapted state. Glycogen is abundant in the large somata and less in their axons, characterised ultrastructurally by many microtubules and extensive gap junctions interconnecting them. There was no apparent difference in glycogen content in HC somata between light- and dark adaptation, but the axons clearly showed absence of glycogen in dark condition. The HC somata (presynaptic) make synapses with dendrites in the outer plexiform layer. Müller cell inner processes, which contain more densely packed glycogen, invest the HC. Other cells of the inner nuclear layer do not show any appreciable content of glycogen. Rods, but not cones, contain abundant glycogen in their inner segments and synaptic terminals. It is likely that glycogen is used as energy substrate in hypoxia for this species that dwell muddy aquatic environment with low oxygen content. They appear to have high energy demand, and a high glycogen content in HC could act as a ready source to fulfil physiological processes, like microtubule-based transport of cargo from the large somata to axons and the maintenance of electrical activities across the gap junctions between the axonal processes. It is also likely that they can supplement glucose to the neighbouring inner nuclear layer neurons, which are clearly devoid of glycogen.

Intrinsic light response of retinal horizontal cells of teleosts

The discovery of intrinsically photosensitive retinal ganglion cells has overthrown the long-held belief that rods and cones are the exclusive retinal photoreceptors. Intrinsically photosensitive retinal ganglion cells use melanopsin as the photopigment, and mediate non-image-forming visual functions such as circadian photoentrainment. In fish, in situ hybridization studies indicated that melanopsin is present in retinal horizontal cells-lateral association neurons critical for creating the centre-surround receptive fields of visual neurons. This raises the question of whether fish horizontal cells are intrinsically photosensitive. This notion was examined previously in flat-mount roach retina, but all horizontal-cell light response disappeared after synaptic transmission was blocked, making any conclusion difficult to reach. To examine this question directly, we have now recorded from single, acutely dissociated horizontal cells from catfish and goldfish. We found that light induced a response in catfish cone horizontal cells, but not rod horizontal cells, consisting of a modulation of the nifedipine-sensitive, voltage-gated calcium current. The light response was extremely slow, lasting for many minutes. Similar light responses were observed in a high percentage of goldfish horizontal cells. We have cloned two melanopsin genes and one vertebrate ancient (VA) opsin gene from catfish. In situ hybridization indicated that melanopsin, but less likely VA opsin, was expressed in the horizontal-cell layer of catfish retina. This intrinsic light response may serve to modulate, over a long timescale, lateral inhibition mediated by these cells. Thus, at least in some vertebrates, there are retinal non-rod/non-cone photoreceptors involved primarily in image-forming vision.

Circuits and properties of signal transmission in the retina

This essay looks at the historical significance of three APS classic papers that are freely available online: Naka K-I and Nye PW. Role of horizontal cells in organization of the catfish retinal receptive field. J. Neurophysiol 34:785-801, 1971. Marmarelis PZ and Naka K-I. Nonlinear analysis and synthesis of receptive-field responses in the catfish retina. II. One-input white-noise analysis. J. Neurophysiol 36: 619-633, 1973. Naka K-I, Marmarelis PZ, and Chan RY. Morphological and functional identifications of catfish retinal neurons. III. Functional identification. J. Neurophysiol 38: 92-131, 1975.

Developmental switch in excitability, Ca(2+) and K(+) currents of retinal ganglion cells and their dendritic structure

In the retina of teleost fish, continuous neuronal development occurs at the margin, in the peripheral growth zone (PGZ). We prepared tissue slices from the retina of rainbow trout that include the PGZ and that comprise a time line of retinal development, in which cells at progressive stages of differentiation are present side by side. We studied the changes in dendritic structure and voltage-dependent Ca(2+), Na(+), and K(+) currents that occur as ganglion cells mature. The youngest ganglion cells form a distinct bulge. Cells in the bulge have spare and short dendritic trees. Only half express Ca(2+) currents and then only high-voltage-activated currents with slow inactivation (HVAslow). Bulge cells are rarely electrically excitable. They express a mixture of rapidly inactivating and noninactivating K(+) currents (IKA and IKdr). The ganglion cells next organize into a transition zone, consisting of a layered structure two to three nuclei thick, before forming the single layered structure characteristic of the mature retina. In the transition zone, the dendritic arbor is elaborately branched and extends over multiple laminae in the inner plexiform layer, without apparent stratification. The arbor of the mature cells is stratified, and the span of the dendritic arbor is well over five times the cell body's diameter. The electrical properties of cells in the transition and mature zones differ significantly from those in the bulge cells. Correlated with the more elaborate dendritic structures are the expression of both rapidly inactivating HVA (HVAfast) and of low-voltage-activated (LVA) Ca(2+) currents and of a high density of Na(+) currents that renders the cells electrically excitable. The older ganglion cells also express a slowly activating K(+) current (IKsa).

The open- and closed-loop gain-characteristics of the cone/horizontal cell synapse in goldfish retina

Under constant light-adapted conditions, vision seems to be rather linear. However, the processes underlying the synaptic transmission between cones and second-order neurons (bipolar cells and horizontal cells) are highly nonlinear. In this paper, the gain-characteristics of the transmission from cones to horizontal cells and from horizontal cells to cones are determined with and without negative feedback from horizontal cells to cones. It is shown that 1) the gain-characteristic from cones to horizontal cells is strongly nonlinear without feedback from horizontal cells, 2) the gain-characteristic between cones and horizontal cells becomes linear when feedback is active, and 3) horizontal cells feed back to cones via a linear mechanism. In a quantitative analysis, it will be shown that negative feedback linearizes the synaptic transmission between cones and horizontal cells. The physiological consequences are discussed.

Evidence of metabotropic glutamate receptor subtypes found on catfish horizontal and bipolar retinal neurons

We have used electrophysiological, pharmacological and immunological techniques to determine which classes of metabotropic glutamate receptors exist on cone horizontal cells in the catfish retina. Patch-clamp recordings in acutely dissociated cone horizontal cells provide evidence that group I and III metabotropic glutamate receptors exist, and are linked to modulation of a voltage-gated calcium current. Group II metabotropic glutamate receptor agonists did not affect the calcium current. Immunocytochemical techniques were used to study the localization of metabotropic glutamate receptor subtypes found in the catfish retina. Antibodies raised against group I (metabotropic glutamate receptor 1alpha, metabotropic glutamate receptor 5), group II (metabotropic glutamate receptor 2/3) and group III (metabotropic glutamate receptor 6) metabotropic glutamate receptor subtypes were used to label acutely dissociated horizontal, bipolar and Müller cells. Results from immunostaining provide evidence that cone horizontal cells express group I (metabotropic glutamate receptor 1alpha, metabotropic glutamate receptor 5) and group III (metabotropic glutamate receptor 6), but not group II (metabotropic glutamate receptor 2/3) receptor subtypes, consistent with our electrophysiological results. Cone horizontal cells exposed to anti-metabotropic glutamate receptor 1alpha, 5 or 6 antibodies all demonstrated diffuse overall staining, with patches of dark immunostaining found on both dendritic processes and cell somata. In catfish bipolar cells, all four of the anti-metabotropic glutamate receptor antibodies stained the processes and cell bodies of bipolar cells homogeneously. There was no evidence for a group of bipolar cells that did not stain with the antimetabotropic glutamate receptor antibodies, although the densest immunostaining occurred when bipolar cells were incubated with the anti-metabotropic glutamate receptor 6 antibody. Müller cells did not show immunostaining against any anti-metabotropic glutamate receptor antibody. Our non-immune controls confirmed that immunostaining was specific for the antigen, and immunoblots were performed to demonstrate the specificity of the antibodies in catfish retina. These results support the hypothesis that group I and III metabotropic glutamate receptor subtypes are found on catfish horizontal cells, and group I, II and III metabotropic glutamate receptor subtypes are expressed on catfish bipolar cells. The metabotropic glutamate receptors on catfish cone horizontal cells act to modulate the voltage-gated sustained calcium current found on these cells.

Metabotropic glutamate receptor-mediated suppression of an inward rectifier current is linked via a cGMP cascade

Glutamate, the neurotransmitter released by photoreceptors, excites horizontal cells and OFF-type bipolar cells by activating ionotropic receptors. This study investigated an additional action of glutamate in which it modulates a voltage-gated ion channel in horizontal cells. We find that glutamate and APB (2-amino-4-phosphonobutyrate) produce a delayed and moderately prolonged suppression of an inward rectifier current (IRK+). This effect is proposed to occur via an APB-sensitive metabotropic glutamate receptor (mGluR) because common agonists for the ionotropic or APB-insensitive mGluRs are ineffective and the APB-insensitive receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) does not block the actions of glutamate or APB. 8-Br-cGMP, 1-methyl-3-isobutylxanthine (IBMX), and atrial natriuretic peptide (ANP) but not 8-Br-cAMP mimic the suppression of IRK+. The effects of glutamate and APB are blocked by protein kinase inhibitors including Rp-8-pCPT-cGMPS, H-8, and H-7 as well as by ATPgammaS. We hypothesize that the APB receptor suppresses IRK+ via upregulation of cGMP and subsequent activation of a cGMP-dependent protein kinase. This pathway is likely regulated by an ATP-dependent phosphorylation. This is a novel signaling pathway for mGluRs and indicates that at least two distinct APB-activated pathways exist in the retina. Functionally, this APB receptor-mediated action found in horizontal cells would provide a means by which spatially restricted changes of glutamate, produced by local illumination of photoreceptors, could regulate IRK+ and consequently the response properties of these neurons. This would serve to adapt selectively retinal regions stimulated by small regions of the visual world.

Origins of horizontal cell spectral responses in the retina of marine teleosts (Centropomus and Mugil sp)

Intracellular recording and marking of horizontal cells (HCs) were carried out in isolated retinas of marine teleosts (Centropomus and Mugil sp.) to identify the cellular origins of different spectral types of S-potential. The spectral responses recorded under mesopic conditions were classified into four types: photopic L, biphasic C(R/G), biphasic C(G/R), and scotopic L(sL). Intracellular marking with Lucifer yellow (LY) or Procion yellow (PY) revealed that in the Centropomus retina an L-type response was recorded from H1 and H2 cells, C(R/G)-type from H3 cells, and sL-type from H4 cells. However, in 20% of microelectrode penetrations from the photoreceptor surface, the sequential order of response appearances was found to be L-, sL-, and C(R/G)-types. In this species, thick dendritic processes of H3 and H4 cells are distributed at the same level as the cell body. In the Mugil retina, on the other hand, L-type response was recorded from H1 cells, C(G/R)-type from H2 cells, C(R/G)-type from H3 cells, and sL-type from H4 cells, this sequential arrangement being very regular. In both species, the cone-connected H1 cells are small and possess an axon, while the two other cone HCs as well as rod-connected H4 cells are axonless. Earlier (Negishi et al., 1988) and present findings indicate that there is a wide variety of HC morphology and functional organization in the teleost retina.

Use-dependent and use-independent blocking actions of picrotoxin and zinc at the GABAC receptor in retinal horizontal cells

The inhibitory actions of picrotoxin (PTX) and zinc on the GABAC receptor in acutely isolated catfish cone horizontal cells were studied and compared using the whole-cell patch clamp technique. PTX blocked the GABAC current elicited by 30 microM GABA with IC50 = 0.64 microM. Over a PTX concentration range of 1-100 microM, simultaneous application of PTX with GABA (30 microM) produced current transients at both the onset and offset of the drug pulse. When the PTX concentration was maintained before, during, and after GABA application, the current transients at the onset and offset of GABA application disappeared. Thus, these transients seem to reflect a slower initial action of PTX at, and faster washout of PTX from, the GABAC receptor than GABA when they were co-applied. The full recovery from PTX inhibition required a second GABA application. Recovery could not be achieved by a prolonged wash in the absence of GABA. These results suggest that PTX effect is use-dependent. Zinc also potently blocked the GABAC current elicited by 30 microM GABA with an IC50 about an order of magnitude higher than that of PTX (IC50 = 8.2 microM). However, only the onset, but not the offset current transient was observed when zinc was simultaneously applied with GABA. The full recovery of the GABAC current from zinc inhibition was obtained after washing for 20 sec and did not require a subsequent GABA application. This indicates that the zinc effect is use-independent. Our findings suggest that: (1) the zinc binding site is on the surface of the GABAC receptor molecule; (2) there is a PTX binding site that is probably inside the receptor and its access requires GABA binding to the receptor.

Distribution of the inositol trisphosphate receptor in the catfish retina

Inositol 1,4,5-trisphosphate (InsP3) mobilizes intracellular stored Ca2+ by binding to specific receptors that are similar to the ryanodine receptor of skeletal and cardiac muscle. We have immunolocalized the InsP3 receptor to the inner nuclear layer and limiting membranes of the catfish retina. Immunocytochemistry on dissociated retinal cells further localized the receptor in the horizontal, bipolar and Müller glial cells. Immunostaining of the rat retina localized the InsP3 receptor to the plexiform layers. These data show a different distribution of InsP3 receptor in the catfish retina compared to that of other vertebrates, that may be suggestive of a different functional role for this receptor in different species.

Large retinal ganglion cells in the channel catfish (Ictalurus punctatus): three types with distinct dendritic stratification patterns form similar but independent mosaics

Retinal ganglion cells in the channel catfish (Ictalurus punctatus) were retrogradely labelled, and those with the largest somata and thickest primary dendrites were categorized by their levels of dendritic stratification. Three types were found, each forming a mosaic making up approximately 1% of the ganglion cell population. Using a system based on established sublaminar terminology, we call these the alpha-a (alpha a), alpha-b (alpha b), and alpha-c (alpha c) ganglion cell mosaics. Cells of the alpha a mosaic had large, sparsely branched trees in sublamina a at 10-30% of the depth of the inner plexiform layer (IPL), sclerad to those of all other large ganglion cells. Some alpha a somata were displaced into the IPL or inner nuclear layer (INL) but belonged to the same mosaic as their orthotopic counterparts. Cells of the alpha b mosaic had dendrites that branched a little more and arborized in sublamina b at 50-60% of the IPL depth. Many also sent fine branches into sublamina a, and some were fully bistratified in a and b. The alpha c cells arborized in the most vitread sublamina, sublamina c, at 80-95% of the IPL depth. The soma areas of the three types in the largest retina studied ranged between 139 microns 2 and 670 microns 2 with significant differences in the order alpha a > alpha c > or = alpha b. Analyses based on nearest-neighbour distance (NND) and on spatial auto- and cross-correlograms showed that each mosaic was statistically regular and independent of the others. Mosaic spacings were similar for each type, giving mean NNDs of 242-279 microns in the largest retina and 153-159 microns in a smaller one. Correspondences between these mosaics, previously defined large ganglion cell types in catfish, and other mosaic-forming large ganglion cells in fish and frogs are discussed along with their implications for neuronal classification, function, development, and evolution.

Response dynamics and receptive-field organization of catfish ganglion cells

Responses from catfish retinal ganglion cells were evoked by a spot or an annulus of light and were analyzed by a procedure identical to the one used previously to study catfish amacrine cells (Sakai H. M., and K.-I. Naka, 1992. Journal of Neurophysiology. 67:430-442.). In two-input white-noise experiments, a response evoked by simultaneous stimulation of the center and surround was decomposed into the components generated by the center and surround through a process of cross-correlation. The center and surround responses were also decomposed into their linear and nonlinear components so that the response dynamics of the linear and nonlinear components could be measured. We found that the concentric organization of the receptive field was determined by linear components, i.e., the first-order kernels generated by the center and surround were of opposite polarity. Both the center and surround generated second-order kernels with similar signatures, i.e., the second-order components formed a monotonic receptive field. The peak response time of the first- and second-order kernels from the surround was longer by approximately 20 ms than that of the center. Except for the DC potential present in the intracellular responses, almost identical first- and second-order kernels for the center and surround were obtained from both the intracellular response and spike discharges. Thus, information on concentric organization of a receptive field is translated into spike discharges with little loss of information. A train of spike discharges carries, simultaneously, at least four kinds of information: two linear and two nonlinear components, which originate in the receptive field center and the surround. A spike train is not a simple signaling device but is a carrier of complex and multiple signals. Victor, J. D., and R. M. Shapley (1979. Journal of General Physiology. 74:671-687.) discovered similarly that, in the cat retina, static second-order nonlinearity is encoded into spike trains. Results obtained in this study support the thesis that signals generated by the preganglionic cells are translated into spike discharges without major modification and that those signals can be recovered from the spike trains (Sakuranaga, M., Y. Ando, and K.-I. Naka. 1987. Journal of General Physiology. 90:229-259.; Korenberg, M. J., H. M. Sakai, and K.-I. Naka. 1989. Journal of Neurophysiology. 61:1110-1120.). Current injection studies have shown that such signal transmission is possible (Sakai, H. M., and K.-I. Naka, 1988a. Journal of Neurophysiology. 60:1549-1567.; 1990. Journal of Neurophysiology. 63:105-119.).

L-glutamate suppresses HVA calcium current in catfish horizontal cells by raising intracellular proton concentration

Catfish horizontal cells exhibit a high voltage-activated, slowly inactivating calcium (HVA) current. In isolated cells, this current underlies a sustained action potential that can be evoked by depolarization. L-Glutamate at concentrations as low as 1 microM suppressed HVA current and shortened the duration of action potentials. pH indicator dye measurements revealed that L-glutamate acidified cells by up to 0.3 pH units. pH/HVA current dose-response curves indicated that the EC50 for pH inhibition of HVA current was 7.1 and the Hill coefficient was 1.98. L-Glutamate's suppression of HVA current was eliminated when the patch pipette solution contained a high concentration of proton buffer, but was not affected when pipettes contained GTP gamma S. These results support the hypothesis that L-glutamate can modulate HVA current by changing intracellular pH.

Cholinergic amacrine neurons of the dogfish retina

In the mammalian retina, the dendritic arbors of cholinergic amacrine neurons have a unique starburst shape; these arbors lie in narrow sublaminae within the inner plexiform layer, where they provide input to a wide variety of ganglion cell types. Immunocytochemistry has been used to identify cholinergic cells in one poikilotherm, the goldfish (Tumosa et al., 1984), but there has been no description of the detailed dendritic morphology of these cells in the lower vertebrates. In the present study, cholinergic neurons have been characterized, by immunocytochemistry and dye filling, in the retina of the Pacific Coast dogfish, Squalus acanthias. The inner nuclear layer contained two populations of choline acetyltransferase-immunoreactive amacrine cells, of different sizes (average soma diameters 12.2 vs. 16.3 microns); 70% of the immunoreactive cells were of the smaller type. Cholinergic dendrites from these two cell populations formed two narrow strata within the inner plexiform layer, at depths of 14% and 31%. In the ganglion cell layer, 40% of the cells were immunoreactive for choline acetyltransferase (ChAT); these cells were very homogeneous in size, had an average diameter of 12.6 microns, and appeared to represent a single class of cholinergic amacrine. The dendrites of these cells formed a single, narrow stratum within the inner plexiform layer, at a depth of 59%. In living preparations, the smallest cell bodies in the ganglion cell layer were filled iontophoretically with Lucifer Yellow, under microscopic control. Such cells invariably had a stellate morphology; in many cases, they appeared quite similar to the starburst cholinergic amacrine cells described in rabbit and rat (Vaney, 1984; Voigt, 1986). Although double-label experiments failed to demonstrate ChAT immunoreactivity in specific dye-filled cells, the dendritic arbors of individual dye-filled stellate dogfish amacrines did co-stratify precisely with the proximal ChAT-immunoreactive sublamina of the inner plexiform layer. In addition, dye injection and ChAT immunocytochemistry appeared to label the same population of dogfish neurons, as suggested by the close structural similarity, and similar numerical proportion, of the cells identified with these two techniques. Similarities between the displaced cholinergic amacrine neurons of the dogfish retina, and the cholinergic, "starburst" amacrine neurons of the rabbit retina, are discussed.

Afferent connections of the optic tectum in channel catfish Ictalurus punctatus

Horseradish peroxidase was injected unilaterally into the optic tectum of the channel catfish, Ictalurus punctatus. The sources of tectal afferents were thereby revealed by retrogradely labeled neurons in various brain centers. Retrogradely labeled cells were seen in both the ipsilateral and contralateral telencephalon. The superficial pretectal area was labeled on both sides of the brain. Ipsilateral projections were also observed coming from the entopeduncular nucleus. Both the anterior thalamic nucleus and the ventro-medial thalamic nucleus projected to the ipsilateral optic tectum. Cells in the ipsilateral nucleus of the posterior commissure were seen to project to the tectum. Labeled fibers were visualized in the lateral geniculate nucleus ipsilateral to the injected tectum, however, no labeled cell bodies were observed. Therefore, tectal cells project to the lateral geniculate nucleus, but this projection is not reciprocal. No labeled cells were found in the cerebellum. Labeled cells occurred in both the ipsilateral and contralateral medial reticular formation; they were also observed in the ipsilateral nucleus isthmi. A projection was seen coming from the dorsal funicular nucleus. Furthermore, labeled cells were shown in the inferior raphe nucleus.

A correlative study of the physiology and morphology of the retinotectal pathway of the perch

The anatomy and physiology of the retinotectal pathway of the perch was investigated using physiological and histological techniques. Massed responses of the optic nerve to single shocks exhibited five distinct peaks. Single-unit responses to shocks indicate two groups of fast fibers correlating well with peaks I and II of the massed response. The flash-evoked response in nerve and tectum has three major phases (PSPI-III), with a marked low-threshold fast component. Patterns of flash-evoked response from single fibers vary, but the responses of fast transient fibers coincide with the timing of PSPI, and longer latency groups with PSPII-III. Units reflexly activated by efferents were also seen, and 12% of units were photically inexcitable. Surprisingly, few fibers responded well to a scanned spot light, unlike tectal cells, and receptive fields were often large (greater than 70 deg). ON/OFF responses, evoked either by whole field or local illumination, were much commoner than pure ON or OFF responses. Effects of electrical stimulation or cautery of the tectum on the flash-evoked response of fiber bundles, via the efferents were marginal, but repetitive stimulation or section of the optic nerve produced clear-cut deficits in the slow components of the flash-evoked response of the nerve. Stimulation of the eighth nerve produced a complex long-latency, large-amplitude response in the optic nerve. The fiber spectrum of the optic nerve taken from electron micrographs revealed the presence of a relatively small group (less than 1%) of thick fibers with diameters between 3 microns and 10 microns that could be correlated with fast responses recorded from the optic nerve, and the remainder with axon diameters down to 0.2 microns providing the slow responses. The distribution of cell-body diameters from sectioned and wholemount material indicated a marked distinction between small and large ganglion cells. The total number of fibers in the nerve was estimated 868,840.

Dendritic growth of DAPI-accumulating amacrine cells in the retina of the goldfish

The dendritic growth of amacrine cells was studied in the retina of the goldfish. Those cells that accumulated the intraocularly injected fluorescent dye 4,6-diamidino-2-phenylindole (DAPI) were compared in retinal wholemounts from small and large animals. Dendritic arbors were stained by intracellular injections of Lucifer yellow. Based upon their dendritic morphology, two types were identified, radiate and fusiform. The dendritic field areas and the number of dendritic terminals were quantitatively compared for cells in small retinas from young fish and large retinas from old fish in the region of the retina that grows by expansion. For both cell types the dendritic fields were significantly larger in the large retinas. The radiate cells had the same number of dendritic terminals in retinas of both sizes, where the fusiform cells in the large retinas had significantly more dendritic terminals than those in the small retinas. This resulted from the addition of new dendrites to the proximal arbor. These data show that with retinal expansion the pattern of dendritic growth by a amacrine cells is cell-type specific.

Junctions form between catfish horizontal cells in culture

Cone horizontal cells from the catfish retina extend out processes after a few days in culture that sometimes contact adjacent cone horizontal cells. Two types of specialized junctions were observed by electron microscopy along the newly formed contact areas. One junctional type consisted of prominent electron-dense material along and just under the plasma membrane of one or both of the contacting elements. Sometimes vesicle clusters were associated with these junctions. The other type of junction showed some electron-dense material along the membranes of both processes and patchy areas of close membrane apposition resembling gap junctions. In about half of the cases tested, electrical coupling was detected between cone horizontal cells that had made contact in culture. In no case was the coupling as tight as is typically found between horizontal cells that had formed gap junctions in vivo.

Modulation of an electrical synapse between solitary pairs of catfish horizontal cells by dopamine and second messengers

1. Retinas from channel catfish were dissociated and the cells maintained in culture. Horizontal cells that normally receive input from cone photoreceptors were identified. The conductance of the electrical junction formed between a pair of 'cone' horizontal cells was measured by controlling the membrane voltage of each cell with a voltage clamp maintained through either a micropipette or a patch pipette. The two techniques yielded similar results. 2. Transjunctional current was measured while transjunctional voltage was stepped to values between +/- 60 mV. The current (measured 5 ms after a step) was proportional to voltage over the range tested. For steps to voltages greater than +/- 45 mV, the current exhibited a slight time-dependent decline. 3. Dopamine decreased junctional conductance in a dose-dependent fashion. A 50% reduction was obtained with 10 nM-dopamine. The D1 agonist fenoldopam (100 nM) also decreased junctional conductance. The uncoupling produced by either agent was rapid and reversible. 4. The introduction of 100 microM-cyclic AMP into one cell of a pair decreased junctional conductance by, on average, 40%. Forskolin (1-10 microM), an activator of adenylate cyclase, decreased junctional conductance 50-90%. 5. The introduction of 80 microM-cyclic GMP into one cell of a pair decreased junctional conductance by, on average, 40%. Nitroprusside (1-10 microM), an activator of guanylate cyclase, reduced junctional conductance 40-65%. 6. The introduction of a peptide inhibitor specific for the cyclic AMP-dependent protein kinase reversed a decrease in junctional conductance produced by superfusion with either dopamine (1 microM), fenoldopam (100 nM) or forskolin (5-10 microM). 7. Intracellular Ca2+ concentration was measured with the fluorescent indicator Fura-2. The intracellular Ca2+ concentration was increased by activation of a Ca2+ current. Junctional conductance remained constant as the internal Ca2+ concentration changed from 100 to 700 nM. 8. Intracellular pH was measured with the fluorescent indicator bis-carboxyethylcarboxyfluorescein. The application of acetate (2.5 mM) reduced intracellular pH by 0.2-0.3 units and decreased junctional conductance by approximately 50%. A subsequent application of fenoldopam did not alter intracellular pH, but decreased junctional conductance by more than 50%. 9. The sensitivity of the junctional conductance between isolated horizontal cells to dopamine is consistent with dopamine having a direct effect on coupling in intact retina. Dopamine regulates the activity of a cyclic AMP-dependent protein kinase which in turn modulates junctional conductance. Changes in intracellular pH and Ca2+ concentration are not involved in mediating the effect of dopamine on coupling. Cyclic GMP and intracellular pH may participate in regulatory pathways independent of that used by cyclic AMP.

Topography and morphology of retinal ganglion cells in the coral trout Plectropoma leopardus (Serranidae): a retrograde cobaltous-lysine study

The retinal topography of the adult coral trout Plectropoma leopardus (Serranidae, Perciformes) is examined in Nissl-stained material and confirmed by means of retrograde labelling with cobalt-lysine from the optic nerve. Concentric isodensity contours surround a temporoventral area centralis of over 1.5 x 10(4) cells per mm2 which lies ventral of the elongated optic nerve head. Peripheral densities of ganglion cells fall to less than 0.25 x 10(4) cells per mm2. A total of 1.17 x 10(5) ganglion cells is found over the entire retina. Silver intensifications of cobalt-lysine-stained cells allow a detailed analysis of ganglion cell morphology. Soma and dendritic field size increase with eccentricity, and within the confines of the area centralis only the smallest cell size is found. Dendritic arbors of peripheral ganglion cells are all oriented parallel to the retinal margin with an axon lying close to the vitreous humor. Nine classes of cells are characterized on the criteria of soma size, dendritic field size, primary and secondary branching patterns, soma position, and dendritic termination areas. Possible morphological homologies with ganglion cells of other teleosts are also discussed.

Pharmacological characterization of voltage-clamped catfish rod horizontal cell responses to kainic acid

Excitatory amino acid-induced currents were examined in voltage-clamped rod horizontal cells dissociated from the catfish retina. The cells responded to glutamate (GLU) and the GLU analogues kainate (KA), quisqualate (QA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), while N-methyl-D-aspartate (NMDA) produced inconsistent responses. Of the effective agonists, only KA produced large, concentration-dependent current responses. While QA, AMPA, GLU, and NMDA were poor agonists, these compounds were able to block rod horizontal cell responses to KA. The rank order potency for this inhibition was: QA greater than AMPA greater than or equal to L-GLU much greater than D-GLU = NMDA. Several excitatory amino acid receptor antagonists were also able to inhibit rod horizontal cell responses to KA. The rank order potency for the inhibition by the compounds tested was: kynurenate greater than cis-piperidine-dicarboxylic acid much greater than D,L-alpha-amino-adipate. Comparison of the potency of several ligands to inhibit rod and cone horizontal cell responses to KA suggested similarities in the KA binding sites of both cell types.

Amacrine cells in the retina of a teleost fish, the roach (Rutilus rutilus): a Golgi study on differentiation and layering

We studied 250 amacrine cells in the retina of the tetrachromatic cyprinid Rutilus rutilus (roach) after rapid Golgi impregnation. All cells were recorded in camera lucida drawings from 50 -80 μm sections. For classifications we used independent criteria of presumed functional relevance, most of which could be quantified. These included ‘gross morphological’ features such as size, symmetry and orientation of the dendritic field, pattern of branching and number of ramification points as well as fine structural details like process diameter and the occurrence of spines and varicosities. We also took into account the pattern of radial distribution of dendrites. To obtain information about the subdivision of the inner plexiform layer, we used the relative stratification levels of stratified amacrine cells to plot a frequency distribution diagram showing that the dendrites of these cells are clustered in seven discrete sublayers of unequal width; four sublayers occupy the distal half of the inner plexiform layer (sublamina a) and three sublayers are present in the proximal half (sublamina b). The subdivision was compared with densitometric data of the inner plexiform layer after various staining methods and with previous observations about the location of bipolar terminals and neurochemical bandings. Our findings suggest that this layer is composed of complementary structural components, each of which is subject to a specific layering pattern. On this basis we could distinguish 43 different types of amacrine cell. If individual types occurred in more than one sublayer, they were considered as subtypes; of these, we found 70 different ones. Among the 43 types, 6 were observed only once. In comparison with amacrine cells described in other species, six ‘new’ types were identified. For each individual type, an identity chart was prepared summarizing camera lucida drawings of tangential views at low and high magnification, a semischematical drawing of the radial location of the dendrites, and the most relevant quantitative data. Our observations are discussed in the context of available evidence about light-evoked responses of identified amacrine types in other species, and possible transmitter content. They substantiate a functional concept according to which amacrine cells provide (i) a multicellular aggregate for coupled membrane potential; (ii) unit activity by the action of entire individual cells; and (iii) local microcircuits caused by isolated activation of single dendrites or parts thereof. The great variety of morphological differentiation, and the numerous transmitters found, suggest that within this basic framework individual amacrine types serve highly complex and sophisticated roles in retinal information processing. Our attempt towards a detailed classification and description of amacrine cell types is intended to provide a reference for future intracellular and neurochemical work, to facilitate precise identification.

Amacrine cells with neurotensin- and somatostatin-like immunoreactivities in three species of teleosts with different color vision

Neurotensin- and somatostatin-like immunoreactivities were localized by pre-embedding techniques in retinal whole-mounts and radial sections of a monochromatic glass catfish (Kryptopterus bicirrhis), a dichromatic cichlid species (Aequidens pulcher), and the tetrachromatic roach (Rutilus rutilus). Both neuropeptides were observed in perikarya and processes of amacrine cells. For a precise identification of cell types, tangential and radial views were correlated with Golgi-impregnated material. The dendritic pattern defining the morphological subtype of amacrine cells was determined by the given neuropeptide or by the species-specific degrees of complexity of retinal structure and function. Neurotensin-like immunoreactivity was localized in amacrine cells of intermediate size, radial symmetry and dendrites with numerous varicosities; they were monostratified in sublayer 3 of the inner plexiform layer. This cell type was common to all three species. In the mono- and dichromatic retinas, a single type of amacrine cell with somatostatin-like immunoreactivity was found with radially oriented, varicose dendrites in sublayer 5. In the tetrachromatic roach retina, two somatostatin-positive amacrine cell types were found with very different patterns of ramification; furthermore, both of these types occurred in more than one sublayer. Possible functional implications for color vision of neuropeptide-specific amacrine cells with uniform morphology in all three species and those with a more varied morphology in the tetrachromatic roach are discussed.

Visual system of the channel catfish (Ictalurus punctatus): II. The morphology associated with the multiple optic papillae and retinal ganglion cell distribution

The retinal organization associated with the multiple optic papillae of the catfish Ictalurus punctatus was examined. In each retina from ten to 17 papillae form an oval ring (which is wider dorsoventrally than mediolaterally). The dorsalmost papilla in this ring lies at the center of the retina. In addition, up to seven small papillae lie within the ring. Bundles of fibers leave the neural retina via the papillae. These bundles remain separate as they pass through the distal portions of the neural retina and then merge before passing through the choroid. Bundles running through dorsal papillae receive fibers from a roughly wedge-shaped retinal area; bundles running through ventral papillae receive fibers from a small area of central retina and a disproportionately large area of peripheral retina. A band of high ganglion cell density was observed extending between the nasal and temporal poles of the retina. No correlation was found between the retinal areas contributing fibers to the bundles of axons emerging from individual papillae and the areas of high cell density. Furthermore, no correlation was found between the average area of the retinal ganglion cells and the ganglion cell density. From HRP preparations and axon counts we estimate that each retina of 95-mm catfish contains about 50,000 ganglion cells.

Morphological and physiological studies of rod-driven horizontal cells with special reference to the question of whether they have axons and axon terminals

Rod-driven (intermediate) horizontal cells were examined in the carp retina to determine whether they bear axons and axon terminals. These cells were injected with HRP after physiological identification of the response type; which consisted of a higher sensitivity to light and a slower response time course than cone-driven (external) horizontal cells, and spectral sensitivity peaking at 520 nm. The labeled cells were further identified morphologically by tracing their dendrites to rod photoreceptors by light and electron microscopy. About two-thirds of the labeled cells (18/30) had a slender, axonlike process (less than 1 micron in diameter, 70-300 micron in length) running horizontally from the soma. No axonlike process was found in the remaining cells. Unlike the axons of external horizontal cells, this axonlike process was short and did not form a long fusiform expansion. No membrane specialization was found along the axonlike process. Since it has been reported that the syncytium made of axon terminals of external horizontal cells serves as a signal bypass of the syncytium made of the somata, it was asked, in separate experiments, whether the intermediate horizontal cells also had such a double syncytial layer. Response amplitudes to a slit of light were measured by placing the slit at various distances from the recording electrode. The response amplitude decayed with distance with a single exponential function, indicating that the syncytium of intermediate horizontal cells consists of a monolayer. These physiological data are consistent with the morphological observations.

The synaptic ultrastructure in the outer plexiform layer of the catfish retina: a three-dimensional study with HVEM and conventional EM of Golgi-impregnated bipolar and horizontal cells

Synaptic structures between receptors and horizontal and bipolar cells in the outer plexiform layer (OPL) of Golgi-impregnated catfish retina were examined by conventional electron microscopy of serial ultrathin sections and by high-voltage electron microscopy (HVEM) of thick sections. Cone terminals contained multiple synaptic ribbons and rod terminals contained single synaptic ribbons. This observation was used to identify these two types of photoreceptors. The cone horizontal cell, located in the most distal part of the inner nuclear layer (INL), invaginated only cone terminals, whereas the rod horizontal cell, located in the proximal part of the INL, invaginated only rod terminals. Both lateral elements of the triad in the rod terminal originated from a single rod horizontal cell whereas the same structures in the cone terminal were often derived from several cone horizontal cells. Golgi-impregnated catfish bipolar cells were classified into two types based on the differences in their axonal arborization as described by Famiglietti et al. ('77). Axonal endings of type a bipolar cells were located in the distal part, sublamina a, of the inner plexiform layer (IPL), and axonal endings of type b cells were located in the proximal part, sublamina b, of the IPL. Dendrites from type a bipolar cells made direct contact with the synaptic ribbons in both rod and cone terminals whereas those from type b cells made indirect contact with the ribbons in both rod and cone terminals, but rare direct contact with the ribbon in rod terminals were also seen. In addition, bipolar cells made basal junctions or superficial contacts in both rod and cone terminals. The "lateral" processes of bipolar cells invaginating rods penetrated between the rod terminal and rod horizontal cell processes, and made basal junctions with both rod terminals and rod horizontal cells. There was no definitive morphological feature that could be associated with sign-conserving and sign-inverting signal transmission.

The visual system of the channel catfish (Ictalurus punctatus). I. Retinal ganglion cell morphology

Horseradish peroxidase was applied to lesions in the optic nerve of catfish (Ictalurus punctatus). The retinae were processed to reveal HRP-labelled ganglion cells. The histochemical techniques employed allowed fine details of the dendritic arbor to be resolved. Flat-mounted retinae were examined and the following characteristics were noted in individual ganglion cells: Soma area, shape, and depth; number and diameter of major dendrites; shape, area, and depth(s) within the inner plexiform layer (ipl) of the dendritic arbor; origin of the axon (from the soma or a dendrite). On the basis of these characteristics, eleven classes of ganglion cells were delineated: four classes of giant cells (G1-G4) and seven classes of smaller cells (S1-S7). G1 cells had dendrites arborizing in the most distal sublamina of the ipl. G1 cells in the dorsal retina had nasotemporally elongated dendritic arbors. G2 cells had dendrites in the proximal portion of the ipl. G3 cells were almost completely confined to a band running between the nasal and temporal retinal poles, through the center of the retina. In this location, the cells had dorsoventrally elongated dendritic arbors, which were bistratified in the ipl. G4 cells were displaced into the inner nuclear layer. S1 and S4 cells had axons arising from their somata, and dendrites arborizing in the distal and the proximal ipl, respectively. S2 cells were typified by their unstratified dendritic arbors. Similarly, S3 cells were characterised by their bistratified arbors. S5 cells arborized in the most proximal ipl sublamina. S6 cells were small ganglion cells with their somata lying in the inner nuclear layer. S7 cells tended to have complex dendritic arbors, and their axons arose from dendrites.

Synaptic organization of the cone horizontal cells in the catfish retina

Horizontal cells of the vertebrate retina are known to contribute to the formation of the receptive field surrounds of photoreceptor and bipolar cells. However, few synapses have been described anatomically that might mediate these interactions. We have observed in the catfish retina that cone horizontal cell perikarya and dendrites make conventional chemical synapses onto photoreceptor terminal telodendria and onto bipolar cell dendrites, while horizontal cell axon terminals make chemical synapses onto the perikarya and processes of amacrine cells. The synapses are characterized by clusters of round vesicles aggregated close to the site of contact, as well as by electron-dense material associated with both pre- and postsynaptic membranes. The three kinds of synapses observed anatomically correspond to the synaptic pathways involving cone horizontal cells that have been suggested by the physiology of these cells.

Retinal GABA neurons: localization in vertebrate species using an antiserum to rabbit brain glutamate decarboxylase

Retinal gamma-aminobutyric acid (GABA) neurons have been localized immunocytochemically using a new antiserum against rabbit brain glutamate decarboxylase (GAD). The animals examined were: dogfish, ratfish, goldfish, catfish, turtle, chick, mouse, rat, pig, rabbit, cat and New World monkey. GAD-containing processes, observed as punctate deposits of immunochemical reaction product, formed discrete bands within the inner plexiform layers of all retinas examined. Immunoreactive, and therefore presumably GABAergic, amacrine cells were observed in all species. Displaced GABAergic amacrine cells were observed in the retinas of goldfish, catfish, turtle and chick, and sparsely in the rabbit as well. GABAergic horizontal cells were detected in catfish, goldfish, chick and turtle. Interplexiform cells in the cat and the rat were clearly immunoreactive for glutamate decarboxylase; this is the first report of GABAergic interplexiform cells in the rat.

Novel pathway connecting the outer and inner vertebrate retina

In many fish retinas, thin axons from the external horizontal cells extend through the inner nuclear layer and expand into large terminal processes that lie along the border of the inner nuclear and inner plexiform layers. Although the horizontal-cell axon terminals are structurally very prominent, their function is unknown. Here we report morphological and functional evidence that signals from catfish (Ictalurus punctatus) horizontal-cell axon terminals can be transmitted directly to amacrine cells. Current injected into horizontal-cell axon terminals produces responses from both transient and sustained amacrine cells very similar to those elicited by light stimuli. Electron microscope observations show chemical synapses from the axon terminals onto amacrine cell perikarya and processes. These data suggest that amacrine cells in the catfish retina receive two inputs, one from bipolar cells and the other from horizontal-cell axon terminals.

Effects of background and spatial pattern on incremental sensitivity of catfish horizontal cells

Incremental sensitivity of cones and horizontal cells was examined in the retina of the channel catfish (Ictalurus punctatus). In horizontal cells, steady full-field background illumination made the spot-evoked dynamic response larger and faster. The "enhancement" of incremental sensitivity was maximal at background levels of 20-50 microW/cm2. Similar changes in response were obtained by enlarging the diameter of the test spot (to 4 mm) without background illumination. In cones, no such enhancement with background was observed and small (0.4 mm) and large (4.0 mm) spots produced similar responses. Similar enhancement by a steady background has been observed also in the proximal neurons of catfish retina.

Influence of amacrine cells on receptive field organization of ganglion cells of the generalized vertebrate cone retina: electronic simulation

Two classes of amacrine cells are simulated, small-field and large-field. Small-field amacrine cells are formed by input from a single bipolar cell, while large-field amacrine cell is formed by inputs from same 7 bipolar cells that form the ganglion cell. Only tonic amacrine cells are studied with both chromatic and luminosity types as well as double- and single-opponent receptive fields. Amacrine cells are used in both feedforward to ganglion cells and feedback to bipolar and horizontal cells. Feedback to bipolar cells or feedforward to ganglion cells affected steady state levels in a predictable fashion. Negative feedback to bipolar cells and positive feedforward to ganglion cells does not introduce transients to ganglion cells while negative feedback to horizontal cells and negative feedforward does. Feedback to horizontal cells produces complex effects on bipolar, amacrine and ganglion cells dependent on such factors as center-surround field balance and negative feedback from luminosity type of horizontal cell to cones.

Effects of dopamine on photopic L-type S-potentials in the catfish retina

Photopic L-type responses were recorded from the soma and the axon terminal of horizontal cells of the catfish (Ictalurus punctatus) retina in eye-cup preparations. The responses were produced by a spot of light with 100-micron diameter (intensity, 10 microW/cm2), which was flashed or steadily illuminated and swept along a 6-mm length over the retinal surface at a speed of 0.95 mm/sec. In some experiments, a traveling random bar stimulus was used instead of the sweeping spot. While recording the responses, dopamine (DA) was applied in a jet form via a nebulizer over the retinal preparation or as superfusate to the eye-cup preparation. DA increased the response amplitude by about 50% and markedly narrowed the spatial profile of the responses from the soma but not from the axon terminal. These DA effects were observed in both normal retinas and those from which DA cells had been deprived by prior intraocular injection of 6-hydroxydopamine. Deprival of DA cells from the retina resulted in a slightly wider spatial profile of the soma and axon terminal responses than that in normal retinas. The results indicate that the spatial properties of photopic L-type responses are modulated by DA at the soma level of horizontal cells.

Synaptic organization involving receptor, horizontal and on- and off-center bipolar cells in the catfish retina

HRP-injected bipolar cells were analyzed electron microscopically in catfish retina. The dendrites of the two types of bipolar cells (on-center and off-center) invaginated into the photoreceptor terminals in distinct patterns. Those from off-center bipolars ended as the central element of triads in such a way that every synaptic ridge examined was contacted by a single process along its apex. In contrast, labeled processes from on-center bipolars never made direct contact with the synaptic ridges of any photoreceptors. In addition, chemical synapses made by external horizontal cells onto photoreceptors were observed. The postsynaptic sites was always a telodendrial process of a photoreceptor.

Retinofugal pathways in juvenile and adult channel catfish, Ictalurus (Ameiurus) punctatus: an HRP and autoradiographic study

The retinal projections of the juvenile and adult channel catfish, Ictalurus (Ameiurus) punctatus, were studied by using horseradish peroxidase (HRP) and autoradiography. The contralateral optic tract sends fibers to the suprachiasmatic nucleus (SCN) and divides into lateral (LOT) and medial optic tracts (MOT). In the adult fish, the former is thicker than the latter, whereas in the juvenile form, the reverse is true. The MOT curves laterally and divides into eight to 15 medial fascicles of the optic tract (MFOT). The contralateral optic fibers project to the nucleus opticus dorsolateralis, nucleus of the posterior commissure, nucleus geniculatus lateralis, pretectal nuclear complex, nucleus corticalis, stratum fibrosum et griseum superficiale (SFGS), and a few optic fibers extend into the stratum griseum centrale. The tractus opticus accessorius arises from the posterodorsal margin of the LOT and extends ventromedially to project to the nucleus opticus accessorius. At the optic chiasm a few fibers do not decussate, and these fibers project to almost all ipsilateral sites similar to those of the contralateral side, including the optic tectum. The autoradiographic observations substantiated the analysis of optic fiber projections provided by the HRP technique.

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.

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.

Synaptic contacts of the horizontal cells in the retina of the marine teleost, Callionymus lyra L

The horizontal cell system in the retina of the fish Callionymus lyra L. was investigated light microscopically and electron microscopically. One type of rod horizontal cells, which are only found in the mixed ventral part, exclusively contacts spherules with lateral endknobs in the triads. Three types of cone horizontal cells occur. The first type, c-H1, contacts pedicles of pale and dark double cones and single cones. The processes always have a lateral position in the triads. This type has a constant contact pattern with the photoreceptor cells. The second type, c-H2, selectively contacts pedicles of pale double cones and the endbuds occupy a central position in the triads. In the single cone pedicles in the dendritic field of the c-H2 horizontal cell, the endbuds never seem to reach the triads. The third type, c-H3, only sends processes to pedicles of single cones where the endbuds occupy a central position in the triads. The synaptic connections of the horizontal cells of Callionymus differ from those observed in Nannacara anomala and Carassius auratus. The difference from the results obtained from Carassius is such that the information transfer model proposed for Carassius cannot apply to Callionymus.

Horizontal cells of the turtle retina. II. Analysis of interconnections between photoreceptor cells and horizontal cells by light microscopy

Criteria were established whereby the chief and accessory members of double cones, red-, green-, and blue-sensitive single cones, and rods could be distinguished from one another in 1-micrometer sections through the retinas of Pseudemys scripta elegans and Chelydra serpentina. Criteria included the presence of absence of an oil droplet, the size, color, and radial position of the oil droplet, the size and shape of the inner and outer segments, and the shape orientation of the connection between the perikaryon and synaptic terminal of each receptor cell. Using these criteria, the color-specific interconnections between horizontal cells and receptor cells were analyzed using 1-micrometer serial sections through Golgi-stained examples of horizontal cells of each type. The results of this analysis are: (a) H1 cells bodies contact both the red- and green-sensitive members of double cones, as well as red- and green-sensitive single cones. (b) H1 axon terminals contact only the red-sensitive chief members of double cones, red-sensitive single cones, and rods. (c) H2-type cells contact only green- and blue-sensitive single cones. (d) H3-type cells contact only blue-sensitive single cones. (e) H4-type cells contact only the green-sensitive accessory members of double cones. With the exception of H1 cell bodies, all horizontal cells generally contacted every appropriate receptor cell type within their receptive field. A model for the generation of complex horizontal cell light responses is proposed and discussed.

Retinal ganglion cells in the crucian carp (Carassius carassius). II. Overlap, shape and tangential orientation of dendritic trees

Ganglion cells were studied in methylene blue stained flat-mounted retinas. Three categories of cells are described: small (S) and large (L) ganglion cells in the main ganglion cell layer, and large ganglion cells (LD) with somata more or less displaced into the inner plexiform layer. These LD cells have two to four very thick primary dendrites and are identifiable as ganglion cells by their axons. An analysis of published data reveals that the large ganglion cells of the crucian carp (type L and LD) have several striking characteristics in common with the large ganglion cells of the dogfish, the frog and the cat: (1) they are selectively stained by methylene blue; (2) they comprise only 2-5% of all the ganglion cells; (3) the large cells can be divided into two or three subtypes, and within each subtype the dendritic trees usually cover the retinal surface with a two- or threefold overlap. New ganglion cells are formed from neuroblasts at the retinal margin and most dendrites first grow along this neuroblastic zone. Thus the main dendrites of the L and LD cells tend to be oriented parallel to the margin all around the periphery of a crucian carp retina. Independent of the size of the eye this parallel orientation disappears at the same relative distance from the margin (about one-third of the distance from the margin to the optic disc). If all L and LD cells are formed at the retinal margin and first develop oriented dendrites, we have to assume that the more randomly oriented dendritic trees in the central retina have undergone a reorganization.

Synaptic organization of the inner plexiform layer of the retina of Xenopus laevis

The inner plexiform layer (i. p. l.) of the retina of the South African clawed frog, Xenopus laevis , was studied by electron microscopy. Photomicrographs of single sections revealed synaptic morphologies comparable to those in other vertebrate retinae. In a partial serial reconstruction of a bipolar terminal, however, some unusual arrangements were found. The bipolar terminal made some synapses that at first examination appeared much like conventional synapses, but subsequent sections always revealed an extremely small ribbon. Many of the ribbon synapses were found to contact more than two postsynaptic processes; up to six pro­cesses postsynaptic to one ribbon contact were seen. A reciprocal synapse was not evident at each ribbon synapse. Montages of the entire width of the inner plexiform layer were constructed from sections cut from four different locations across the retina. The numbers of conventional and ribbon synapses per unit volume of tissue were determined. The synaptic densities found in Xenopus were much lower than those reported for other frogs. Differences in synaptic densities from the four locations were found to be statistically insignificant. The overall amacrine/bipolar synapse ratio was 6.8/1. The synaptic den­sities in the inner plexiform layer did not change when the tissue was stained with lead citrate alone rather than with uranyl acetate and lead citrate. The functional significance of the morphological and quantitative synaptic arrangements in Xenopus i. p. l. is discussed, and the synaptic organization is compared to that of other amphibia and vertebrates.