Single-unit recording and micropressure ejection techniques were used to test the effects of norepinephrine (NE) on the responses of neurons in the superficial layers (the stratum griseum superficiale and stratum opticum) of the hamster's superior colliculus (SC). Application of NE suppressed visually evoked responses by ≥30% in 75% of 40 neurons tested and produced ≥30% augmentation of responses in only 5%. The decrement in response strength was mimicked by application of the α2 adrenoceptor agonist, p-aminoclonidine, the nonspecific β agonist, isoproterenol, and the β1 agonist, dobutamine. These agents had similar effects on responses evoked by electrical stimulation of the optic chiasm and visual cortex. The α1 agonist, methoxamine, augmented the light-evoked responses of 53% of 49 SC cells by ≥30%, but had little effect on responses evoked by electrical stimulation of optic chiasm or visual cortex. The effects of adrenergic agonists upon the glutamate-evoked responses of SC cells that were synaptically “isolated” by concurrent application of Mg2+ were similar to those obtained during visual stimulation. Analysis of effects of NE on visually evoked and background activity indicated that application of this amine did not significantly enhance signal-to-noise ratios for most superficial layer SC neurons, and signal-to-noise ratios were in some cases reduced. These results indicate that NE acts primarily through α2 and β1 receptors to suppress the visual responses of SC neurons. Activation of either of these receptors reduces the responses of SC neurons to either of their two major visual inputs as well as to direct stimulation by glutamate, and it would thus appear that these effects are primarily postsynaptic.

The purpose of the present experiments was to evaluate the contribution of the glutamate-glutamine cycle in retinal glial (Müller) cells to photoreceptor cell synaptic transmission. Dark-adapted isolated rat retinas were superfused with oxygenated bicarbonate-buffered media. Recordings were made of the b-wave of the electroretinogram as a measure of light-induced photoreceptor to ON-bipolar neuron transmission. L-methionine sulfoximine (1–10 mM) was added to superfusion media to inhibit glutamine synthetase, a Müller cell specific enzyme, by more than 99% within 5–10 min, thereby disrupting the conversion of glutamate to glutamine in the Müller cells. Threo-hydroxyaspartic acid and D-aspartate were used to block glutamate transporters. The amplitude of the b-wave was well maintained for 1–2 h provided 0.25 mM glutamate or 0.25 mM glutamine was included in the media. Without exogenous glutamate or glutamine the amplitude of the b-wave declined by about 70% within 1 h. Inhibition of glutamate transporters led to a rapid (2–5 min) reversible loss of the b-wave in the presence and absence of the amino acids. In contrast, inhibition of glutamine synthetase did not alter significantly either the amplitude of the b-wave in the presence of glutamate or glutamine or the rate of decline of the b-wave found in the absence of these amino acids. Excellent recovery of the b-wave was found when 0.25 mM glutamate was resupplied to L-methionine sulfoximine–treated retinas. The results suggest that in the isolated rat retina uptake of released glutamate into photoreceptors plays a more important role in transmitter recycling than does uptake of glutamate into Müller cells and its subsequent conversion to glutamine.

The Cavalieri method and the optical fractionator were employed to estimate the volume and neuron numbers, respectively, of the dorsal lateral geniculate nucleus (dLGN) in seven adult male pigtail monkeys (Macaca nemestrina). Unbiased estimates were selectively obtained for the parvocellular (P), magnocellular (M), and interlaminar plus superficial (I + S) layers of the nucleus. The dLGN had a mean volume of 56.5 mm3, and contained on average 1.79 million neurons. The P layers contributed 64% of the volume and 83% of the neurons in the dLGN; the corresponding proportions for the other layers were 13% and 9% (M), and 23% and 8% (I + S). Interindividual variability was large for neuron counts, which varied within a two-fold range, and lower for volume estimates. Since no published data are available for the pigtail dLGN, the present results are compared with quantitative studies of the dLGN in other macaque species, placing special emphasis in the discussion of the methodologies used.

Recent work has indicated that cobalt, at sub-millimolar concentrations, blocks horizontal cell (HC) to cone feedback, without attenuating direct cone to second-order cell synaptic transmission. We utilized low concentrations (0.25–0.5 mM) of cobalt to test the contribution of the feedback circuit, and other possible cobalt-sensitive mechanisms, to the receptive-field surrounds of retinal neurons. In the great majority of cases, low cobalt blocked ganglion cell surrounds, and it invariably blocked driving the ganglion cell by extrinsic current injected into the HC network. Although low cobalt reduced the integrating area of the HC network, dopamine, which similarly constricted the HC receptive area, did not block ganglion cell surrounds. Low cobalt reduced a late depolarizing wave in the HC light-evoked waveform and selectively suppressed the depolarizing component of chromatic HCs, both signs of HC to cone feedback. Low cobalt also reduced or blocked completely the receptive-field surrounds of a small sample of bipolar and amacrine cells. These results implicate the HC to cone feedback synapse in the formation of the receptive-field surround of retinal neurons.

The retinal ganglion cells (RGCs) of the primate form at least two classes—M and P—that differ fundamentally in their functional properties. M cells have temporal-frequency response characteristics distinct from P cells (Benardete et al., 1992; Lee et al., 1994). In this paper, we elaborate on the temporal-frequency responses of M cells and focus in detail on the contrast gain control (Shapley & Victor, 1979a,b). Earlier data showed that the temporal-frequency response of M cells is altered by the level of stimulus contrast (Benardete et al., 1992). Higher contrast shifts the peak of the frequency-response curve to higher temporal frequency and produces a phase advance. In this paper, by fitting the data to a linear filter model, the effect of contrast on the temporal-frequency response is subsumed into a change in a single parameter in the model. Furthermore, the model fits are used to predict the response of M cells to steps of contrast, and these predictions demonstrate the dynamic effect of contrast on the M cells' response. We also present new data concerning the spatial organization of the contrast gain control in the primate and show that the signal that controls the contrast gain must come from a broadly distributed network of small subunits in the surround of the M-cell receptive field.

Intracellular recording techniques were used to evaluate the effects of norepinephrine (NE) on the membrane properties of superficial layer (stratum griseum superficiale and stratum opticum) superior colliculus (SC) cells. Of the 207 cells tested, 44.4% (N = 92) were hyperpolarized by ≥3 mV and 8.7% (N = 18) were depolarized by ≥3 mV by application of NE. Hyperpolarization induced by NE was dose dependent (EC50 = 8.1 μM) and was associated with decreased input resistance and outward current which had a reversal potential of −94.0 mV. Depolarization was associated with a very slight rise in input resistance and had a reversal potential of −93.1 mV for the single cell tested. Pharmacologic experiments demonstrated that isoproterenol, dobutamine, and p-aminoclonidine all hyperpolarized SC cells. These results are consistent with the conclusion that NE-induced hyperpolarization of SC cells is mediated by both α2 and β1 adrenoceptors. The α1 adrenoceptor agonists, methoxamine and phenylephrine, depolarized 35% (6 of 17) of the SC cells tested by ≥3 mV. Most of the SC cells tested exhibited responses indicative of expression of more than one adrenoceptor. Application of p-aminoclonidine or dobutamine inhibited transsynaptic responses in SC cells evoked by electrical stimulation of optic tract axons. Inhibition of evoked responses by these agents was usually, but not invariably, associated with a hyperpolarization of the cell membrane and a reduction in depolarizing potentials evoked by application of glutamate. The present in vitro results are consistent with those of the companion in vivo study which suggested that NE-induced response suppression in superficial layer SC neurons was primarily postsynaptic and chiefly mediated by both α2 and β1 adrenoceptors.

The distribution of AMPA-selective glutamate receptor subunits was studied in the cat retina using antisera against GluR1 and GluR2/3. Both antisera were localized in postsynaptic sites in the outer plexiform layer (OPL) as well as the inner plexiform layer (IPL). Immunoreactivity for GluR1 was seen in a subpopulation of OFF cone bipolar cells and a number of amacrine and ganglion cells. Within the IPL, processes staining for GluR1 received input from OFF and ON cone bipolar cells but not from rod bipolars. Labeling for GluR2/3 was seen in horizontal cells, an occasional cone bipolar cell, and numerous amacrine and ganglion cells. In the IPL, GluR2/3 staining was postsynaptic to cone bipolar cells in both sublaminae. AII amacrine cells which receive rod bipolar input were also labeled for GluR2/3. With both antisera, staining was limited to a single member of the bipolar dyad complex, providing morphological evidence for functional diversity in glutamatergic pathways.

Immunocytochemistry was used to reveal a population of bipolar cells that contain γ-atrial natriuretic peptide 1–25 (γ-ANP) in turtle retina. This same antibody was also used in rat retina as a comparative control. The retinas were examined by both conventional light microscopy and confocal microscopy with double-labeling to determine whether protein kinase C-α-like immunoreactivity (PKC-α-LI) was colocalized with the γ-ANP-LI. Some thick sections of turtle retina immunostained with only the γ-ANP antibody were also examined by electron microscopy. In rat, a subpopulation of bipolar cells with axons terminating close to the ganglion cell layer was labeled. Double-labeling experiments indicated that the γ-ANP-LI and PKC-α-LI were colocalized in rat retina, and thus all the bipolar cells with γ-ANP-LI were rod bipolar cells. In turtle, the γ-ANP antibody labeled certain bipolar cells that were characterized by bistratified axon terminals arborizing on the borders of strata S2/3 and S3/4 in the inner plexiform layer (IPL). Double labeling with PKC-α antibody indicated that bipolar cells with γ-ANP-LI were not the same bipolar cell types with PKC-α-LI. Thus, γ-ANP-LI appears to be a new marker for a distinct type of bipolar cell in turtle retina. At the ultrastructural level, the γ-ANP-LI was visible throughout the cytoplasm of the bipolar cells from dendrites to axon terminals. In the outer plexiform layer (OPL), labeled dendrites contacted photoreceptor pedicles almost exclusively at narrow-cleft basal junctions, but infrequently formed the central element at a photoreceptor ribbon synapse. In the IPL, axon terminals with γ-ANP-LI made ribbon synapses onto a combination of amacrine and ganglion cells. Since narrow-cleft basal junctions and photoreceptor ribbon-related junctions are known to be associated with ON-center bipolar cells in turtle, and since the axon terminals of bipolars with γ-ANP-LI stratify primarily in the ON-strata of the IPL, we suggest that these cells are likely to be ON-center cells. It is possible that the γ-ANP may be involved in regulating the activity of Na+/K+ ATPase or in the modulation of cGMP levels.

To better understand the mechanisms of extracellular space volume regulation and their possible effects on retinal function, light-induced changes in the concentrations of the principal extracellular ions (Na+, K+, Ca2+, and Cl−) were measured with ion-sensitive microelectrodes in the chick retina–pigment epithelium–choroid preparation. Changes of extracellular space volume were assessed by measuring the concentration of an impermeant marker, tetramethylammonium. In the inner retina, transient ON/OFF Na+ decrease was about twice as large as K+ increase, and the charge difference was compensated by a decrease in Cl− concentration. The ion changes were accompanied by extracellular space-volume decreases here. In the subretinal space, [Na+]o increase was about twice as large as K+ decrease, yet [Cl−]o also decreased; this was accompanied by a sustained extracellular space-volume increase. The ionic changes in the inner retina are consistent with a model of extracellular space-volume regulation which assumes that neuronal depolarization causes net uptake of NaCl, cell swelling, and extracellular space shrinkage. However, to prevent the apparent violation of electroneutrality in the subretinal space, our simple model should be expanded to include the involvement of unidentified anion(s). Substantial changes in the subretinal space volume may influence interaction between the neural retina and pigment epithelium. Among ionic changes, only the light-induced [K+]o decrease around the photoreceptors and the [Ca2+]o increase near the photoreceptor bodies and synaptic terminals are large enough (−25% and 7.5%, respectively) to be likely candidates for integrated intercellular signaling.

In previous work, we have shown that sudden image displacements well outside the classical receptive field modulate the visual sensitivity of LGN relay cells. Here we report the effect of image displacements on the response versus contrast function. The stimuli consisted of a central spot of optimal size and polarity (contrast range: 3–98%), flashed alone or in the presence of a peripheral annulus (radii: 5–15 deg) containing a low spatial-frequency grating displaced at saccade-like velocities (shift). The most consistent effect of the shift on the response to a central spot was to reduce the responsiveness of Y relay cells and, to a lesser extent, of X relay cells. The reduction in responsiveness was primarily a divisive rather than a subtractive effect and could be modelled by assuming that a greater contrast was required to produce a given excitatory response. In the absence of a central spot, remote motion had inhibitory effects on the firing rates of the majority of relay cells, but its effect on retinal ganglion cells was mainly excitatory. When the shifting grating covered the classical receptive field and its periphery, facilitatory effects or suppressive effects, depending on the spatial phase of the pattern, were observed in both retinal and geniculate cells. Remote motion strongly suppresses the responsiveness of relay cells to stimuli within the classical receptive field. This suppressive effect involves intrageniculate processing and is primarily associated with a reduction in contrast gain. It is likely that shift suppression contributes to the loss of visual sensitivity observed in saccadic suppression.

Glycine is a major inhibitory neurotransmitter in the mammalian retina and has been shown to influence the responses of ganglion cells. Midget and parasol ganglion cells serve distinct physiological roles in the primate retina and show differences in their response characteristics to light stimuli. In the present study, we addressed the question of whether the expression of glycine receptors differs in midget and parasol ganglion cells. Ganglion cells in the retinae of marmoset and macaque monkeys were injected with Neurobiotin in a live in vitro retinal whole-mount preparation. Retinal pieces were then processed with an antibody against the α1 subunit of the glycine receptor. Strong punctate immunoreactivity indicative of synaptic localization is present in the ON and OFF sublamina of the inner plexiform layer. Many of the immunoreactive puncta coincide with the dendrites of both midget and parasol ganglion cells. Immunoreactive puncta are present on distal and proximal dendrites of ON and OFF cells. These results suggest that ON and OFF midget and parasol cells do not differ with respect to the distribution of the α1 subunit of the glycine receptor.

Glutamate and γ-aminobutyric acid (GABA) are the dominant amino acids in the retina and brain. The manufacturing and degradation pathways of both of these amino acids are intricately linked with the tricarboxylic acid cycle leading to rapid redistribution of these amino acids after metabolic insult. Postmortem ischemia in mammalian retina predominantly results in a loss of glutamate and GABA from neurons and accumulation of these amino acids within Müller cells. This accumulation of glutamate and GABA in Müller cells may occur as a result of increased release of these neurotransmitters from neurons, and decreased degradation. Quantification of the semisaturation value (half-maximal response) for glutamate and GABA Müller cell loading during postmortem ischemia indicated a shorter semisaturation value for GABA than glutamate. Such changes are consistent with a single aerobically dependent GABA-degradation pathway, and the existence of multiple glutamate-degradation pathways. Comparison with the in vitro ischemic model showed similar qualitative characteristics, but a markedly increased semisaturation time for glutamate and GABA Müller cell loading (a factor of 5–10) in the postmortem ischemia model. We interpret these differences to indicate that the in vitro condition provides a more immediate and/or severe ischemic insult. In the postmortem ischemia model, the delayed glial cell loading implies the availability of internal stores of both glucose and/or oxygen. Increased glial and neuronal immunoreactivity for the amino acids involved in transamination reactions, aspartate, alanine, leucine, and ornithine was observed, indicating a potential shift in the equilibrium of transamination reactions associated with glutamate production. These findings provide evidence that, in the rat retina, there are multiple pathways subserving glutamate production/degradation that include a multitude of transamination reactions. Further evidence is therefore provided to support a role for all four amino acids in glutamate metabolism within a variety of retinal neurons and glia.

Monocular deprivation early in postnatal development leads to anatomical and physiological changes in the lateral geniculate nucleus (LGN) and visual cortex. Many of these changes are dependent upon activation of the NMDA receptor. We have examined the role of visual experience in modifying NMDAR1 subunit expression in the LGN of animals reared with various forms of visual deprivation. Following monocular deprivation initiated either at eye opening or at the peak of the critical period, there were approximately 20% fewer NMDAR1-immunopositive neurons in the deprived laminae of the LGN. The loss of NMDAR1-immunopositive neurons was found throughout both the binocular and monocular segments of the LGN and after monocular deprivation until just 3 weeks of age. These results indicate that the loss of NMDAR1 in the LGN following monocular deprivation does not simply reflect changes in the visual cortex. The loss of NMDAR1 expression was not necessarily permanent. Initiation of binocular vision at the peak of the critical period ameliorated the effect of monocular deprivation and the introduction of a period of reverse occlusion led to a complete reversal. Taken together, the results show that the expression of the NMDAR1 subunit in the LGN can be modified by the pattern of visual experience during postnatal development.

GABAergic responses of acutely dissociated rat retinal neurons, including both bipolar cells (BCs) and other, morphologically round cells (RCs), were assayed with the fluorescent (FL), voltage-sensitive probe oxonol DiBaC4(5). Using intensified video microscopy and simultaneous recording, GABA responses were identified in one-third of cells in a typical microscope field; of these 85% hyperpolarized (0.05–0.3 log unit FL decreases) while the remainder depolarized (0.05–0.2 log unit FL increases). GABA-sensitive cells were also TACA-sensitive (trans-4-Aminocrotonic acid), and these ligands appeared interchangeable in ability to evoke responses. In RCs, an asymmetric co-responsive pattern was observed between GABA- and muscimol-evoked events. Muscimol-sensitive RCs responded well to GABA, but not all GABA-sensitive RCs responded to muscimol. In GABA-sensitive BCs, muscimol responses were typically weak or absent. Few BCs or RCs responded to CACA (cis-4-Aminocrotonic acid). Bicuculline-resistant GABA responses occurred in ∼80% of GABA-responsive RCs and BCs. Both bicuculline-sensitive (GABAA-like) and bicuculline-insensitive (GABAC-like) responses were resistant to picrotoxin. Although a small minority of GABA-sensitive cells hyperpolarized in response to R(+)baclofen, bicuculline-insensitive responses were not antagonized by 2-hydroxysaclofen, and were abolished in low [Cl−]o. Results suggested (1) that bicuculline-insensitive, Cl−-dependent, GABAC-like responses were broadly distributed and predominant among dissociated rat retinal neurons; (2) that muscimol was a particularly weak agonist for rat retinal BCs; and (3) that oxonol was a sensitive probe for retinal GABA responses.

The technique of current source-density analysis was applied to several components of the light-evoked field potentials (electroretinogram) from the retina of the superfused eyecup of rabbit. The depth distributions of the major current sources and sinks were: b-wave—sink at outer plexiform layer, source at inner plexiform layer; M-wave—sink at inner plexiform layer, source at retinal surface; and slow PIII—source near outer plexiform layer, sink at retinal surface. These distributions, along with the sensitivities of these responses to certain pharmacological agents, support earlier studies that Müller cells generate the M-wave and slow PIII, but that depolarizing bipolar cells directly generate the b-wave.

Zebrafish (Danio rerio) represents an excellent genetic model for vertebrate visual system studies. Because the opsin proteins are ideal markers of specific photoreceptor cell types, we cloned six different zebrafish opsin cDNAs. Based on pairwise alignments and phylogenetic comparisons between the predicted zebrafish opsin amino acid sequences and other vertebrate opsins, the cDNAs encode rhodopsin, two different green opsins (zfgr1 and zfgr2), a red, a blue, and an ultraviolet opsin. Phylogenetic analysis indicates the zfgr1 protein occupies a well-resolved dendrogram branch separate from the other green opsins examined, while zebrafish ultraviolet opsin is closely related to the human blue- and chicken violet-sensitive proteins. Polyclonal antisera were generated against individual bacterial fusion proteins containing either the red, blue, or ultraviolet amino termini or the rod or green opsin carboxyl termini. Immunolocalization on adult zebrafish frozen sections demonstrates the green and red opsins are each expressed in different members of the double cone cell pair, the blue opsin is detected in long single cones, and the ultraviolet opsin protein is expressed in the short single cones. In 120-h postfertilization wholemounts, green, red, blue, and ultraviolet opsin-positive cells are detected in an orderly arrangement throughout the entire retina. The antibodies' photoreceptor-type specificity indicates they will be useful for characterizing both wild-type and mutant zebrafish retinas.

Responses of striate neurons to line textures were investigated in anesthetized and paralyzed adult cats. Light bars centered over the excitatory receptive field (RF) were presented with different texture surrounds composed of many similar bars. In two test series, responses of 169 neurons to textures with orientation contrast (surrounding bars orthogonal to the center bar) or motion contrast (surrounding bars moving opposite to the center bar) were compared to the responses to the corresponding uniform texture conditions (all lines parallel, coherent motion) and to the center bar alone. In the majority of neurons center bar responses were suppressed by the texture surrounds. Two main effects were found. Some neurons were generally suppressed by either texture surround. Other neurons were less suppressed by texture displaying orientation or motion (i.e. feature) contrast than by the respective uniform texture, so that their responses to orientation or motion contrast appeared to be relatively enhanced (preference for feature contrast). General suppression was obtained in 33% of neurons tested for orientation and in 19% of neurons tested for motion. Preference for orientation or motion contrast was obtained in 22% and 34% of the neurons, respectively, and was also seen in the mean response of the population. One hundred nineteen neurons were studied in both orientation and motion tests. General suppression was correlated across the orientation and motion dimension, but not preference for feature contrast. We also distinguished modulatory effects from end-zones and flanks using butterfly-configured texture patterns. Both regions contributed to the generally suppressive effects. Preference for orientation or motion contrast was not generated from either end-zones or flanks exclusively. Neurons with preference for feature contrast may form the physiological basis of the perceptual saliency of pop-out elements in line textures. If so, pop-out of motion and pop-out of orientation would be encoded in different pools of neurons at the level of striate cortex.

We investigated the synaptic connectivity of two populations of recoverin-labeled bipolar cells and GABAergic neurons in the inner plexiform layer (IPL) of the rat retina. Two types of cone bipolar cells, type 2 and type 8, were stained with anti-recoverin antibodies, and GABAergic neurons were stained with anti-glutamic acid decarboxylase (GAD) antibodies. Type 2 cone bipolar axons received synaptic input from amacrine cell processes in 177 cases; among these amacrine cell processes, 92 processes (52.0%) were GAD-like immunoreactive. A total of 159 amacrine cell processes, which are presynaptic to type 8 cone bipolar cells, were observed. Among these processes, 117 processes (73.6%) were GAD-like immunoreactive. The postsynaptic elements at the ribbon synapses of recoverin-labeled cone bipolar cells were observed in 482 processes. In both type 2 and type 8 cone bipolar cells, the major output was to amacrine cell processes. At the ribbon synapses of the type 2 cone bipolar cells, 224 of the postsynaptic profiles were amacrine cell processes, 97 processes (43.3%) were GAD-like immunoreactive. In type 8 cone bipolar cells, 45 processes (30.2%) of 149 amacrine cell processes were GAD-like immunoreactive. Our results provide morphological evidence that GABA is a major transmitter involved in the visual processing of type 2 and 8 cone bipolar cells and GABA may have a stronger influence on type 8 cone bipolar cells than type 2 cone bipolar cells in the IPL of the rat retina.

The physiological function of GABA transporters and GABA receptors in retinal horizontal cells (HCs) under dark- and light-adapted conditions were studied by whole-cell voltage clamp and intracellular recording techniques in retinal slices and whole-mounted isolated retinas of the larval tiger salamander. Puff application of GABA in picrotoxin elicited a NO-711 (a potent GABA transporter blocker)-sensitive inward current that did not exhibit a reversal potential in the physiological range, consistent with the idea that these HCs contain electrogenic GABA transporters. Application of GABA in NO-711 elicited a chloride current in HCs; about half of the current was suppressed by bicuculline or I4AA (a GABAC receptor antagonist), and the remaining half was suppressed by bicuculline + I4AA or picrotoxin. In whole-mount retinas, NO-711, bicuculline, I4AA, or picrotoxin hyperpolarized the HCs and enhanced the light responses under dark-adapted conditions, and blocked the time-dependent recovery of HC membrane potential and light responses during background illumination. Based on the parallel conductance model, GABA released in darkness mediates a chloride conductance about three times greater than the leak conductance or the glutamate-gated cation conductance. About half of this chloride conductance is mediated by GABAA receptors, and the other half is mediated by GABAC receptors. These results suggest that GABA released from HCs through the NO-711-sensitive GABA transporters activates GABAA and GABAC receptors, resulting in chloride conductance increase which leads to a HC depolarization and reduction of the light response. Additionally, GABA transporters also mediate GABA release in background light that is responsible for the recovery of HC membrane potential and light responses.

We have investigated the morphology of the NOS-like immunoreactive neurons and their synaptic connectivity in the rat retina by immunocytochemistry using antisera against nitric oxide synthase (NOS). In the present study, several types of amacrine cells were labeled with anti-NOS antisera. Type 1 cells had large somata located in the inner nuclear layer (INL) with long and sparsely branched processes ramifying mainly in stratum 3 of the inner plexiform layer (IPL). Somata of type 2 cells with smaller diameters were also located in the INL. Their fine processes branched mostly in stratum 3 of the IPL. A third population showing NOS-like immunoreactivity was a class of displaced amacrine cells in the ganglion cell layer (GCL). Their soma size was similar to that of the type 1 cells; however, their processes stratified mainly in strata 4 and 5 of the IPL. Labeled neurons were evenly distributed throughout the retina, and the mean densities were 57.0 ± 9.7 cells/mm2 for the type 1 cells, 239.3 ± 43.4 cells/mm2 for the type 2 cells and 121.2 ± 27.5 cells/mm2 cells for the displaced amacrine cells. The synaptic connectivity of NOS-like immunoreactive amacrine cells was identified in the IPL by electron microscopy. NOS-labeled amacrine cell processes received synaptic input from other amacrine cell processes and bipolar cell axon terminals in all strata of the IPL. The most frequent postsynaptic targets of NOS-immunoreactive amacrine cells were other amacrine cell processes. Ganglion cell dendrites were also postsynaptic to NOS-like immunoreactive neurons in both sublaminae of the IPL. Synaptic outputs onto bipolar cells were observed in sublamina b of the IPL. In addition, a few synaptic contacts between labeled cell processes were observed. Our results suggest that NOS immunoreactive cells may be modulated by other amacrine cells and ON cone bipolar cells, and act preferentially on other amacrine cells.