Previous studies have shown that the expression of glutamate receptor subunits is developmentally regulated and have been implicated in processes of cell differentiation during postnatal life. The tissue localization and developmental pattern of the glutamate receptor 2 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptor were investigated by means of immunohistochemistry and immunoblotting. Labeling of amacrine and ganglion cells and the inner plexiform layer appeared early during development, while glutamate receptor 2 subunit expression in the outer plexiform layer started after the first postnatal week. The distribution of labeling within the inner plexiform layer changed from nonorganized to laminated appearance prior to eye-opening. There was an increasing number of positive amacrine and ganglion cell somata during the first 2 weeks, but their number decreased considerably as the retina matured and were seen at least up to 35 days of postnatal development. Little labeling was found in the ganglion cell layer and in the inner plexiform layer of late postnatal and adult retina. Labeling in the outer plexiform layer and of bipolar cell somata appeared to increase in the developing retina. Glur2 labeling of these cells and the outer plexiform layer became discernible during the second postnatal week, and this labeling was present in the adult as well. Immunoblotting showed that GluR2 protein levels were similar at postnatal days 7 and 10, but slightly decreased between the second and fourth postnatal weeks. Our data imply that the immunological expression of glutamate receptor 2 subunit in the inner plexiform layer decreases as a function of age, and is correlated with developmental event(s) in the postnatal retina.

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.

We investigated the photocurrents from isolated rods of the South American anuran, Caudiverbera caudiverbera. Rod outer segments were on average 66.4 ± 11.2 μm (mean ± S.D., n = 104) in length and 6.6 ± 0.9 μm (mean ± S.D.) in diameter; 40 ± 22 photoisomerizations (mean ± S.D., range 10–99, n = 16) were required for eliciting a half-saturating photocurrent response. The time-to-peak was 911 ± 217 ms (mean ± S.D., n = 14, 20°C) in the linear range of the response and the integration time of the current response was 1744 ± 451 ms (mean ± S.D., n = 14). The time-to-peak appears to be slower and the integration time shorter in Caudiverbera than in Ambystoma tigrinum, Rana pipiens or Xenopus laevis rods under similar experimental conditions. The α-band of rod spectral sensitivity has a λmax at 520 ± 2.1 nm (mean ± S.D., range 516–525 nm, n = 24) and the bandwidth fits a porphyropsin visual pigment. The single-event response amplitude ranges from 0.31–0.51 pA, depending on the calculation method. The intrinsic dark current (variance at dark minus variance under bright light) was 0.045 ± 0.040 pA2 (mean ± S.D., n = 24). Our results support the presence of a dark-noise component below 1 Hz, with kinetics similar to the single-photon evoked response and a rate of 0.006 events s−1 (n = 9).

The second-order visual mechanisms perform the operation of integrating the spatially distributed local visual information. Their organization is traditionally considered within the framework of the filter-rectify-filter model. These are the second-order filters that provide the ability to detect texture gradients. However, the question of the mechanisms' selectivity to the modulation dimension remains open. The aim of this investigation is to answer the above question by using visual evoked potentials (VEPs). Stimuli were textures consisting of staggered Gabor patches. The base texture was nonmodulated (NM). Three other textures represented the base texture which was sinusoidally modulated in different dimensions: contrast, orientation, or spatial frequency. EEG was recorded with 20 electrodes. VEPs of 500 ms duration were obtained for each of the four textures. After that, VEP to the NM texture was subtracted from VEP to each modulated texture. As a result, three different waves (d-waves) were obtained for each electrode site. Each d-wave was then averaged across all the 48 observers. The revealed d-waves have a latency of about 200 ms and, in our opinion, reflect the second-order filters reactivation through the feedback connection. The d-waves for different modulation dimensions were compared with each other in time, amplitude, topography, and localization of the sources of activity that causes the d-wave (with sLORETA). We proceeded from the assumption that the d-wave (its first component) represents functioning of the second-order visual mechanisms and activity changes at the following processing stages. It was found that the d-waves for different modulation dimensions significantly differ in all parameters. The obtained results indicate that the spatial modulations of different texture parameters caused specific changes in the brain activity, which could be evidence supporting the specificity of the second-order visual mechanisms to modulation dimension.

The main route of information flow through the vertebrate retina is from the photoreceptors towards the ganglion cells whose axons form the optic nerve. Bipolar cells of the frog have been so far reported to contact mostly amacrine cells and the majority of input to ganglion cells comes from the amacrines. In this study, ganglion cells of frogs from two species (Bufo marinus, Xenopus laevis) were filled retrogradely with horseradish peroxidase. After visualization of the tracer, light-microscopic cross sections showed massive labeling of the somata in the ganglion cell layer as well as their dendrites in the inner plexiform layer. In cross sections, bipolar output and ganglion cell input synapses were counted in the electron microscope. Each synapse was assigned to one of the five equal sublayers (SLs) of the inner plexiform layer. In both species, bipolar cells were most often seen to form their characteristic synaptic dyads with two amacrine cells. In some cases, however, the dyads were directed to one amacrine and one ganglion cell dendrite. This type of synapse was unevenly distributed within the inner plexiform layer with the highest occurrence in SL2 both in Bufo and Xenopus. In addition, SL4 contained also a high number of this type of synapse in Xenopus. In both species, we found no or few bipolar to ganglion cell synapses in the marginal sublayers (SLs 1 and 5). In Xenopus, 22% of the bipolar cell output synapses went onto ganglion cells, whereas in Bufo this was only 10%. We conclude that direct bipolar to ganglion cell information transfer exists also in frogs although its occurrence is not as obvious and regular as in mammals. The characteristic distribution of these synapses, however, suggests that specific type of the bipolar and ganglion cells participate in this process. These contacts may play a role in the formation of simple ganglion cell receptive fields.

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 (>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 μm and 10 μm that could be correlated with fast responses recorded from the optic nerve, and the remainder with axon diameters down to 0.2 μm 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.

Shapiro et al. (2004) introduced a new visual effect (the induced contrast asynchrony) that demonstrates a perceptual separation between the response to a modulated light and the response to contrast of the light relative to background. The effect is composed of two physically identical disks, one surrounded by a dark annulus and the other by a light annulus. The luminance levels of both central disks were modulated in time, producing a stimulus with in-phase luminance modulation and antiphase contrast modulation. Observers primarily perceived the disks to be modulating asynchronously (i.e. they perceived the contrast), but at low temporal frequencies could also track the luminance level. Here we document that the induced contrast asynchrony disappears when the surrounds are achromatic and the center lights are modulated near the equiluminant axis. Observers viewed 1-deg-diameter disks embedded 2-deg-diameter achromatic surrounds. The chromaticity of the disks was modulated in time (1 Hz) along lines in an S versus Luminance cardinal color plane and an L-M versus Luminance cardinal color plane; observers responded as to whether the modulation appeared in phase. For all observers and both color planes, the lights appeared in phase most frequently at angles near the standard observer's equiluminant line and out of phase at angles further away from that line. Observers differed in the range of angles that produce the appearance of in-phase modulation. The results suggest that induced contrast asynchronies may be useful as a technique for equating luminance of disparate lights.

This study tests the generality of previously demonstrated rod hue biases (red and blue biases at shorter wavelengths and a green bias at longer wavelengths) that cause the loci of the three spectral unique hues to shift to longer wavelengths. We found rod hue biases for 2-deg targets to be generally similar in magnitude and light-level dependence to those observed for 7.4-deg targets (the size most often studied) when measured at 7-deg eccentricity. The largest effects for both test sizes occurred at the lowest light levels tested, 1 log scotopic troland. All three rod hue biases were found with 0.6-deg targets, but were not reliably measurable at the lowest light levels and were reduced in magnitude and consistency across observers. The largest rod hue biases all occurred at the same scotopic light level, which corresponds to different photopic light levels for the three hue biases, because of differences in photopic and scotopic spectral sensitivity. This suggests that no single photopic light level will produce such large effects for all three rod hue biases. Finally, when the rod influence on a specific unique-hue locus was measured using photopically (rather than scotopically) constant stimuli, rod hue biases were still found but were more variable in magnitude and incidence across observers. We conclude that the rod hue biases we have previously described can be found with smaller stimuli, at somewhat higher light levels, and under photopically constant conditions, although our prior conditions tend to produce larger, more reliable rod hue biases.

Much attention has been paid to the effect of various types of neonatally induced retinal insults on neurons of the cat lateral geniculate nucleus (LGN). Little is known about cellular adjustment to functional altertions commencing in the adult. The present study was aimed at examining the effect of monocular enucleation or retinal impulse blockade on mature neurons of the cat LGN and perigeniculate nucleus. In addition to labeling the relay neurons with cytochrome oxidase (CO) histochemistry and immunohistochemistry, and presumed interneurons with GABA immunohistochemistry, the two markers were also combined in the same section. The results showed that GABA-immunoreactive neurons in the LGN can be subdivided into two major groups: highly immunoreactive neurons (Hir) and moderately immunoreactive neurons (Mir). These two groups differed slightly in their size and CO levels. With monocular enucleation or TTX treatment, there was a reduction in the numerical density of Hir and a concomitant increase in Mir in the affected laminae. However, there was no evidence of a reduction in GABA immunoreactivity in neurons of the perigeniculate nucleus. With regard to relay cells our data were in agreement with our previous findings (Kageyama & Wong-Riley, 1985) that there was a statistically significant positive correlation between cell size and CO levels, so that neurons with large cross-sectional areas were predominately darkly reactive for CO (Dco), while medium-to-small neurons were mainly moderate to lightly reactive (M/Lco). After monocular enucleation or TTX injections, the numerical density and size of Dco were significantly reduced, while those of M/Lco were proportionally increased in the affected laminae, indicating a conversion of some Dco to M/Lco. The results indicate that the maintenance of CO levels in predominately the large relay cells and GABA levels in a subclass of small neurons of the mature cat LGN are activity-dependent and are highly sensitive to retinal insults.

Dark-adapted adults' electroretinographic b−wave thresholds are —2 log units below a−wave thresholds and ~3 log units above the perceptual threshold, and their perceptual and the visually evoked cortical potential (VEP) thresholds are similar. Dark-adapted infants' scotopic a− and b−wave thresholds for full-field stimuli are both about 0.5 log units above those of adults, but their scotopic VEP thresholds for such stimuli have not been studied. We obtained scotopic VEP thresholds for brief, full-field stimuli from dark-adapted, infants and adults to consider the relationships of the cortical responses to the responses of more distal structures, namely the rod photoreceptors (scotopic a−wave) and ON-bipolars (scotopic b−wave). The median VEP threshold of infants is 0.5 log unit above that of adults. Thus, the relationships of a− and b−wave and VEP thresholds in infants are similar to those in adults. These results are consistent with rod cell immaturities being the primary determinant of the difference between infants' and adults' thresholds.

We have investigated the role of calcium in the regulation of pigment granule migration in photoreceptors of the semi-terrestrial crab, Sesarma cinereum. Isolated crab eyes (eyecup plus eyestalk) were maintained in crustacean Ringer either prepared normally or calcium-free plus 50 mM EGTA. Pigment granule movement was indirectly observed by monitoring reflectance from the eye during light stimuli using intracellular optical physiological techniques. Electroretinograms (ERGs) were also measured during light stimuli. EGTA treatment caused gradual loss of centripetal migration of pigment granules (normally leading to pupillary closure), and photoreceptors eventually became locked in the open-pupil, dark-adapted state despite repeated stimuli. In contrast, ERG responses continued throughout EGTA treatment, although the size and shape ofthe response was altered. Normal ERG responses and pigment granule movements returned after replacing EGTA-Ringer with normal-calcium medium. These results suggest that centripetal migration of pigment granules in crustacean photoreceptors requires calcium.

The proline-23-histidine line 3 (P23H-3) transgenic rat carries a human opsin gene mutation leading to progressive photoreceptor loss characteristic of human autosomal dominant retinitis pigmentosa. The aim of the present study was to evaluate neurochemical modifications in the P23H-3 retina as a function of development and degeneration. Specifically, we investigated the ion channel permeability of photoreceptors by tracking an organic cation, agmatine (1-amino-4-guanidobutane, AGB), which permeates through nonspecific cation channels. We also investigated the activity of ionotropic glutamate receptors in distinct populations of bipolar, amacrine, and ganglion cells using AGB tracking in combination with macromolecular markers. We found elevated cation channel permeation in photoreceptors as early as postnatal day 12 (P12) suggesting that AGB labeling is an early indicator of impending photoreceptor degeneration. However, bipolar, amacrine, or ganglion cells displayed normal responses secondary to ionotropic glutamate receptor activation even at P138 when about one half of the photoreceptor layer was lost and apoptosis and gliosis were observed. These results suggest that possible therapeutic windows as downstream neurons in inner retina appear to retain normal function with regard to AGB permeation when photoreceptors are significantly reduced but not lost.

Retinal Müller glial cells have been shown to undergo reactive gliosis in a variety of retinal diseases. Upregulation of glial fibrillary acidic protein (GFAP) is a hallmark of Müller cell activation. Reactive gliosis after retinal detachment or ischemia/reperfusion is characterized by hypertrophy and downregulation of inwardly rectifying K+ (Kir) currents. However, this kind of physiological alteration could not be detected in slowly progressing retinal degenerations. The photoreceptor toxin N-methyl-N-nitrosourea (MNU) leads to the rapid loss of cells in the outer nuclear layer and subsequent Müller cell activation. Here, we investigated whether Müller cells from MNU-treated mice exhibit reactive gliosis. We found that Müller cells showed increased GFAP expression and increased membrane capacitance, indicating hypertrophy. Membrane potential and Kir channel-mediated K+ currents were not significantly altered whereas Kir4.1 mRNA expression and Kir-mediated inward current densities were markedly decreased. This suggests that MNU-induced Müller cell gliosis is characterized by plasma membrane increase without alteration in the membrane content of Kir channels. Taken together, our findings show that Müller cells of MNU-treated mice are reactive and respond with a form of gliosis which is characterized by cellular hypertrophy but no changes in Kir current amplitudes.

Photoreceptors have high energy demands and densely packed mitochondria through which light passes before phototransduction. Old world primates including humans have three cone photoreceptor types mediating color vision with short (S blue), medium (M green), and long (L red) wavelength sensitivities. However, S-cones are enigmatic. They comprise <10% of the total cone population, their responses saturate early, and they are susceptible in aging and disease. Here, we show that primate S-cones actually have few mitochondria and are fueled by glycolysis, not by mitochondrial respiration. Glycolysis has a limited ability to sustain activity, potentially explaining early S-cone saturation. Mitochondria act as optical filters showing reduced light transmission at 400–450 nm where S-cones are most sensitive (420 nm). This absorbance is likely to arise in a mitochondrial porphyrin that absorbs strongly in the Soret band. Hence, reducing mitochondria will improve S-cone sensitivity but result in increased glycolysis as an alternative energy source, potentially increasing diabetic vulnerability due to restricted glucose access. Further, glycolysis carries a price resulting in premature functional decline as seen in aged S-cones. Soret band absorption may also impact on mitochondrial rich M and L cones by reducing sensitivity at the lower end of their spectral sensitivity range resulting in increased differentiation from S-cone responses. These data add to the list of unique characteristic of S-cones and may also explain aspects of their vulnerability.

In this study, we examined procedures that alter saccadic latencies and target selection to visual stimuli and electrical stimulation of area V1 in the monkey. It has been shown that saccadic eye movement latencies to singly presented visual targets form a bimodal distribution when the fixation spot is turned off a number of milliseconds prior to the appearance of the target (the gap period); the first mode has been termed express saccades and the second regular saccades. When the termination of the fixation spot is coincident with the appearance of the target (0 ms gap), express saccades are rarely generated. We show here that a bimodal distribution of saccadic latencies can also be obtained when an array of visual stimuli is presented prior to the appearance of the visual target, provided the elements of the array overlap spatially with the visual target. The overall latency of the saccadic eye movements elicited by electrical stimulation of area V1 is significantly shortened both when a gap is introduced between the termination of the fixation spot and the stimulation and when an array is presented. However, under these conditions, the distribution of saccadic latencies is unimodal. When two visual targets are presented after the fixation spot, introducing a gap has no effect on which target is chosen. By contrast, when electrical stimulation is paired with a visual target, introducing a gap greatly increases the frequency with which the electrical stimulation site is chosen.

A novel noise-masking technique was used to test D'Zmura and Knoblauch's (1998) idea that subjects employ off-channel looking in detecting chromatic test stimuli embedded in spatiotemporal chromatic noise. Detection thresholds were obtained for stationary, isoluminant, Gaussian-windowed (σx = σy = 2.25 deg; σt = 0.25 s), 135 deg (yellow/blue) or 160 deg (orange/blue–green), sinusoidal test gratings (11 deg × 11 deg; 0.75 cycle/deg) superimposed on each of a series of dynamic, random-check chromatic noise masks varying in azimuth in DKL space. Thresholds for detecting the test in the presence of these variable masks were again measured in the presence of an additional (auxiliary) noise mask created from colors falling on azimuths of 0 deg or 90 deg (135-deg test) or 0 deg or 135 deg (160-deg test). The effectiveness, kvar, of the variable noise masks in elevating grating detection thresholds was determined by fitting the detection data to the Pelli-Legge equation relating test detection energy to variable noise-mask energy: Et = K + kvarNvar. Differences in the calculated values of kvar for detection data obtained with and without the auxiliary masks were consistent with off-channel looking and were well accounted for by a simple model based on the idea that subjects possess a multichannel array of linear chromatic detectors spanning the isoluminant plane of DKL space, and they can choose the channel that has the highest signal-to-noise ratio.

Temporal resolution is a basic property of the visual system and critically depends upon retinal temporal coding properties which are also of importance for directional coding. Whether the temporal coding properties for directional coding derive form inherent properties or critically depend upon the temporal coding mechanisms is unclear. Here, the influence of acetylcholine and GABA upon photopic temporal coding was investigated in goldfish, using flicker stimuli, in a behavioral and an electrophysiological (ERG) approach. The goldfish temporal resolution ability decreased from more than 90% correct choices at 20 Hz flicker frequency to about 65% at 45 Hz flicker frequency with a flicker fusion frequency of approximately 39 Hz. Blockade of GABAa-receptors reduced the flicker fusion frequency to about 23 Hz, not affecting temporal resolution below 20 Hz flicker frequency. Partial blockade of nicotinic acetylcholine receptors reduced the flicker fusion frequency slightly and lowered the temporal resolution ability in the 25–30 Hz range. Blockade of muscarinic acetylcholine receptors had a smaller effect than the partial blockade of nicotinic acetylcholine receptors. In ERG-recordings, blocking GABAa-receptors increased the a- and b-wave amplitude, induced a delay, an increase and a slow fall-off of the d-wave. Blocking GABAc-receptors had little effect. Blocking GABAa- or GABAa/c-receptors changed the temporal resolution, when expressed as a linear filter, from a 3rd degree filter with resonance to a low order low-pass filter with a low upper limit frequency. The temporal transfer properties were barely changed by blocking either nicotinic or muscarinic acteylcholine receptors, although ERG-components increased in amplitude to varying degrees. The behavioral and electrophysiological data indicate the important role of GABA for temporal processing but little involvement of the cholinergic system. It is proposed that the interaction of the GABAergic amacrine cell network and bipolar cells determines the gain of the retinal temporal coding in the upper frequency range.

Recovery from bright light was studied in macaque rods by measuring the membrane current of single outer segments. The recovery phase of some responses displayed a plateau current of about one picoampere lasting for several seconds. The following evidence suggests these “steps” are single photon responses of abnormally long duration. (1) Over a limited range of intensities, step amplitude remained constant and summed linearly with intensity. The collecting area for step generation was about 2.6 × 10−3 μm2. (2) Step duration varied exponentially with a mean duration of about 6.5 s. (3) Fluctuation analysis of the tail currents was consistent with the idea that a step is evoked by isomerization of a single rhodopsin molecule, and that only 1 in 400 isomerizations leads to a response with a step-like waveform. (4) With only the distal portion of the outer segment in the electrode, the polarity of the step response reversed when the proximal portion of the outer segment was illuminated, indicating that step generation results from a local change in outer segment conductance near the site of photon absorption. (5) The probability of eliciting a step varied with the wavelength of light in the manner expected from the absorption spectrum of rhodopsin.

The cellular connectivity of the lacertilian parietal eye is not well understood. Because the intercellular connections establish the foundation for information processing, we have investigated cellular connectivity of one cell type in this simple vertebrate retina. We also developed an in vitro preparation to study the anatomy of the parietal eye visual system. Horseradish peroxidase transport in the in vitro preparation revealed a class of displaced ganglion cells occupying positions among the photoreceptors, in a location where the presence of interneurons had been suggested. Three-dimensional reconstruction at the electron-microscopic level showed that the morphology and synaptic input of these displaced ganglion cells is different from that of the previously known ganglion cells. The displaced ganglion cells receive an average of about 13 ribbon synapses from photoreceptors. The ribbon input is equally distributed between the soma and dendritic arbor. Junctional membrane measurement and ethnolic phosphotungstic acid-staining provided evidence for the existence of non-ribbon synaptic contacts (synaptoid junctions). Displaced ganglion cells make about 20 synaptoid junctions, 65% of which are on the dendritic arbor. The morphology of the displaced ganglion cell is such that a significant measure of synaptic input to the dendritic arbor will be transmitted to the soma.

Neonatal subcutaneous administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to hamsters results in a marked depletion of serotonin (5-HT) in cortex and an increase in the concentration of this amine in the superior colliculus (SC). To determine whether this increase was associated with an alteration in the synaptic organization of 5-HT-containing axons in the superficial gray layer of the SC, immunocytochemistry was combined with electron microscopy. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles make synaptic contacts in the superficial gray layer of the hamster's SC. In 5,7-DHT-treated animals, examination of 400 individual profiles indicated that 25.5% of 5-HT-positive profiles made synaptic contacts (P < 0.05). Given the recently demonstrated effect of 5-HT on retinotectal transmission in this species, the present results suggest that the functional organization of the SC may also be markedly altered in animals that sustain neonatal 5,7-DHT administration.