The population of retinal ganglion cells which project ipsilaterally in the brain was examined in the fat-tailed dunnart, Sminthopsis crassicaudata, following injection of horseradish peroxidase into one optic tract. Retinae were examined as wholemounts and optic nerves as serial sections. In addition, visual fields were measured ophthalmoscopically. Ipsilaterally projecting ganglion cells were located temporal to a line which ran vertically through the middle of the area centralis and extended medially to define a ventrolateral crescent. Temporal to the naso-temporal division, a mean of 77% of ganglion cells projected ipsilaterally; these cells represented 20% of the total ganglion cell population. The magnitude and retinal location of the ipsilateral projection correlated with the extensive binocular field which measured 180 deg in the vertical (from 20 deg below the horizontal axis to 70 deg beyond the zenith) and 140 deg in horizontal meridian. Ipsilaterally projecting axons were restricted to the lateral third of the optic nerve along its length, sharing territory with contralaterally projecting axons.

We have recently developed a behavioral assay, based on the escape response of fish to a threatening object, to analyze quantitatively the visual sensitivity of zebrafish. During the course of dark adaptation, we measure the threshold light intensity required to evoke an escape response. Under a normal light–dark (LD) cycle, thresholds for both the cone and rod systems are considerably lower in late afternoon hours than in early morning hours. Over a period of 24 h, zebrafish are most sensitive to visual stimuli prior to light off and least sensitive prior to light on. Under conditions of constant illumination, this rhythm of visual sensitivity persists for several days but is gradually lost. In constant light (LL), the rhythm persists 1–2 days; thereafter, visual thresholds at all times of the day converge at a level similar to thresholds measured in late afternoon hours in control animals. In constant darkness (DD), the rhythm persists at least 5 days; thereafter, it dampens to a level about a half-log unit less sensitive to that measured in the late afternoon hours in control animals. These data suggest that visual sensitivity in zebrafish is regulated by an endogenous circadian clock which functions to decrease the visual sensitivity.

The purpose of the present study was to find out what particular stimulus features, in addition to the direction and velocity of motion, specifically activate neurons in the nucleus lentiformis mesencephali (nLM) in pigeons. Visual responses of 60 nLM cells to a variety of computer-generated stimuli were extracellularly recorded and quantitatively analyzed. Ten recording sites were histologically verified to be localized within nLM with cobalt sulfide markings. It was shown that the pigeon nLM cells were specifically sensitive to the leading edge moving at the optimal velocity in the preferred direction through their excitatory receptive fields (ERFs). Generally speaking, nLM cells preferred black edges to white ones. However, this preference cannot be explained by OFF-responses to a light spot. The edge sharpness was also an essential factor influencing the responsive strength, with blurred edges producing little or no visual responses at all. These neurons vigorously responded to black edge orientated perpendicular to, and moved in, the preferred direction; the magnitude of visual responses was reduced with changing orientation. The spatial summation occurred in all neurons tested, characterized by the finding that neuronal firings increased as the leading edge was lengthened until saturation was reached. On the other hand, it appeared that nLM neurons could not detect any differences in the shape and area of stimuli with an identical edge. These data suggested that feature extraction characteristics of nLM neurons may be specialized for detecting optokinetic stimuli, but not for realizing pattern recognition. This seems to be at least one of the reasons why large-field gratings or random-dot patterns have been used to study visual responses of accessory optic neurons and optokinetic nystagmus, because many high-contrast edges in these stimuli can activate a neuron to periodically discharge, or groups of neurons to simultaneously fire to elicit optokinetic reflex.

We measured the photopic spectral sensitivity of multiunit activity in the torus longitudinalis and optic tectum of goldfish. Since negative contrast stimuli are most effective for exciting torus longitudinalis, spiking activity was evoked by the shadow of a disc moving through a monochromatic light beam projected upon a screen. The amount of activity evoked in torus longitudinalis generally increased with the monochromatic stimulus radiance at the same rate for all wavelengths, indicating a univariant response. Spiking activity in tectum, however, increased at different rates across the spectrum, indicating color-dependent responses. The action spectra for torus longitudinalis were all similar and relatively flat as expected of a homogeneous, broad-band luminance processing system, and about 1 log unit more sensitive than the tectal action spectra. The latter generally displayed sharp peaks and dips in sensitivity indicative of opponent processing.

The distributions of rod and cone photoreceptors have been determined in the retina of the California ground squirrel, Spermophilus beecheyi. Retinas were fixed by perfusion and the rods and cones were detected with indirect immunofluorescence using opsin antibodies. Local densities were determined at 2-mm intervals across the entire retina, from which total numbers of each receptor type were estimated and isodensity distributions were constructed. The ground squirrel retina contains 7.5 million cones and 1.27 million rods. The peak density for the cones (49,550/mm2) is found in a horizontal strip of central retina 2 mm ventral to the elongated optic nerve head, falling gradually to half this value in the dorsal and ventral retinal periphery. Of the cones, there are 14 M cones for every S cone. S cone density is relatively flat across most of the retina, reaching a peak (4500/mm2) at the temporal end of the visual streak. There is one exception to this, however: S cone density climbs dramatically at the extreme dorso-nasal retinal margin (20,000/mm2), where the local ratio of S to M cones equals 1. Rod density is lowest in the visual streak, where the rods comprise less than 5% of the local photoreceptor population, increasing conspicuously in the ventral retina, where the rods achieve 30% of the local photoreceptor population (13,000/mm2). The functional importance of the change in S to M cone ratio at the dorsal circumference of the retina is compromised by the extremely limited portion of the visual field subserved by this retinal region. The significance for vision, if any, remains to be determined. By contrast, the change in rod/cone ratio between the dorsal and ventral halves of the retina indicates a conspicuous asymmetry in the ground squirrel's visual system, suggesting a specialization for maximizing visual sensitivity under dim levels of illumination in the superior visual field.

We studied the GABA sensitivity of horizontal cells in the isolated goldfish retina. After the glutamatergic input to the horizontal cells was blocked with DNQX, GABA depolarized the monophasic and biphasic horizontal cells. The pharmacology of these GABA-induced depolarizations was tested with the GABA receptor antagonists bicuculline-methiodide and picrotoxin, the GABA transporter agonist nipecotic acid, and the GABA transporter antagonist SKF 89976-A. The GABA-induced responses of monophasic horizontal cells consisted of two components; one with characteristics of GABA-gated chloride channels, and one with characteristics of GABA transporters. In biphasic horizontal cells, we only found evidence for GABA-gated chloride channels. The results show that monophasic horizontal cells in goldfish contain the two components of a positive feedback loop (GABA transporters and GABA-gated chloride channels), as described in salamander. Furthermore, our results indicate that the monophasic horizontal cells may project directly to the biphasic horizontal cells, via an excitatory GABAergic pathway. We propose that the function of these GABAergic systems in horizontal cells is to abolish cone dominance in bipolar cells surround in the dark-adapted retina.

Choroidal blood flow (ChBF) in birds is regulated by a neural circuit whose components are the retina, the suprachiasmatic nucleus, the medial division of the Edinger-Westphal nucleus (EWM), the ciliary ganglion, and the choriod. We have previously shown that lesions of EWM appear to result in pathological alterations in the retina. To determine whether EWM lesions also lead to altered visual functions, we have examined the effects of EWM lesions on visual acuity in pigeons. Bilateral lesions of EWM were made electrolytically, and visual acuity for high-contrast, square-wave gratings was determined behaviorally about 1 year later and compared to that of a group of pigeons that had received sham lesions of EW about 1 year prior to acuity testing. Because lesions targeting EWM invariably resulted in damage to the adjoining lateral part of the Edinger-Westphal nucleus (EWL), which controls pupillary constriction and accommodation, two additional control groups were studied. In one such control group, bilateral lesions in the area pretectalis (AP), which innervates the pupillary control part of EWL and thereby controls pupillary constriction, were made and the effects on visual acuity determined about 1 year later. In the second such control group, the effects of acute accommodative and pupillary dysfunction on acuity were studied in pigeons made cycloplegic. The accuracy of all lesions was later confirmed histologically. The mean acuities of birds with AP lesions (9.1 ± 1.4 cycles/deg) and sham lesions (7.1 ± 1.5 cycles/deg) were not significantly different from normal, based on published normative data on pigeons. In contrast, pigeons with lesions that completely destroyed EW bilaterally showed visual acuity (2.7 ± 0.1 cycles/deg) that was well below the acuity of the sham and AP-lesion control groups. The acuity of the cycloplegic pigeons (4.8 ± 0.3 cycles/deg) and one pigeon with a nearly complete bilateral EWL but a unilateral EWM lesion (6.4 cycles/deg) indicated that only about half of the loss with a bilateral EW lesion could be attributed to accommodative dysfunction. Thus, bilateral destruction of EWM appears to have led to a loss in visual acuity. This conclusion suggests that disruption of adaptive neural regulation of ChBF may impair visual function. Destruction of EWM was, however, associated with damage to the somatic components of the oculomotor and trochlear nuclei. The possibility cannot be excluded that such damage also contributed to the acuity loss.

We studied the development of multibank rod retinae by monitoring the size-related addition of new layers of rod inner and outer segments in four species of deep-sea fishes and found two different growth paradigms. In the mesopelagic Chauliodus sloani, new banks of rod inner and outer segments are added as long as the fish increases in size, as observed earlier by Locket (1980). By contrast, in three bathybenthic species (Antimora rostrata, Coryphaenoides (Coryphaenoides) guentheri, and Coryphaenoides (Nematonurus) armatus), the final complement of banks is reached when the specimens have grown to between 20 and 47% of their maximal size, suggesting that the visual system is mature only after this stage. Increase in retinal area, density of rod nuclei, and densities of rod inner and outer segments were also studied in these and additional species. Taken together with previous data on rod proliferation patterns and outer segment membrane synthesis, our findings indicate that at least in species with no continual addition of new banks, there is no major functional difference between the innermost and outermost banks of rod inner and outer segments. While Chauliodus spends all its life in the mesopelagic environment, the three bathybenthic species live in this environment during early development and descend towards greater depths only upon maturation. We speculate that this coincides with the stage when the full complement of rod banks is formed in the retina, as a possible prerequisite for a life outside the reach of sunlight.

Pharmacological studies have implicated retinal opiate pathways in the visual regulation of ocular growth. However, the effects of opiate receptor subtype-specific compounds on form-deprivation myopia (FDM) are inconsistent (Seltner et al., 1997), and may be mediated by non-opiate receptors. The purpose of this study was to test whether opiate receptor-inactive (D-) enantiomers elicit the same FDM-suppressing effect as their opiate receptor-active (L-) counterparts. Since some opiates are thought to act at NMDA receptors, we also tested whether NMDA receptor agonists and antagonists influence ocular growth or FDM. We found that both L- and D- enantiomers of morphine-like compounds (dextrorphanol and levorphanol, and D- and L-naloxone) were equally effective in blocking FDM. The NMDA receptor antagonists dextromethorphan, MK801, and AP5 also suppressed FDM. A single toxic dose of NMDA, that destroys many subtypes of amacrine cells (including those that synthesize the opioid peptide enkephalin), induced myopia and ocular enlargement in ungoggled eyes, and eliminated the ability of form-deprivation to enhance ocular growth. The NR-1 subunit of the NMDA receptor was localized to a narrowly stratified, intense stratum at approximately 50% depth in the inner plexiform layer, diffusely throughout the proximal inner plexiform layer, and to many somata in the amacrine and ganglion cell layers. These observations suggest that most effects of opiate receptor ligands on FDM in the chick are mediated by non-opiate receptors, which are likely to include NMDA receptors. NMDA as an excitotoxin transiently enhances ocular growth, but thereafter disables retinal mechanisms that promote emmetropization and FDM. These observations are consistent with a prominent role for pathways utilizing NMDA receptors in FDM and ocular growth-control.

ninaED1, a dominant allele of the major Drosophila rhodopsin gene, expresses a rhodopsin that is predominantly recovered in a 80-kD complex that likely represents rhodopsin dimers. By driving either ninaED1 or ninaE+ expression from a heat-shock promoter, we show that the 80-kD rhodopsin complex forms immediately after gene activation. In wild type, but not ninaED1, rhodopsin monomeric forms are detected at later times. The generation of monomeric forms of wild-type rhodopsin is suppressed in vitamin A-deprived flies or in flies heterozygous for the dominant rhodopsin mutation. We also show that ninaED1 expression does not affect the maturation of another Drosophila visual pigment, Rh3. These results are consistent with the view that the ninaED1 rhodopsin antagonizes an early posttranslation process that is specific for maturation of the ninaE-encoded rhodopsin.

The second messenger cyclic guanosine monophosphate (cGMP) plays a role in many aspects of retinal processing. cGMP-gated channels function in photoreceptors, Müller, bipolar, and ganglion cells; and cGMP can modulate gap-junction conductivity. In the inner retina, both particulate and soluble guanylate cyclases can elevate levels of cGMP. The soluble isoform of guanylate cyclase is activated by nitric oxide (NO). In turtle retina, nitric oxide synthase, the enzyme that synthesizes NO, has been previously localized in discrete amacrine cells, somata in the ganglion cell layer, and in many processes in the inner plexiform layer. However, there have been no studies localizing soluble guanylate cyclase in the turtle retina. To functionally localize soluble guanylate cyclase, we stimulated retinas with the NO donors (±)-S-nitroso-N-acetylpenicillamine or spermine (nitric oxide) adduct, and then used immunocytochemistry to localize increases in cGMP-like immunoreactivity (cGMP-LI). The cells containing soluble guanylate cyclase should show cell autonomous increases in cGMP-LI in response to stimulation with NO. NO-stimulated increases in cGMP-LI occurred in many distinct amacrine cell types, select bipolar cells, some somata in the ganglion cell layer, and in discrete bands of processes in the inner plexiform layer. The pattern of cGMP-LI demonstrated qualitative dose response differences to the NO donors. This is the first localization of soluble guanylate cyclase in specific retinal neurons in the turtle; and the first functional activation of soluble guanylate cyclase in the amacrine cells of any species. The broad neuronal distribution of NO-stimulated cGMP-LI suggests that the NO/soluble guanylate cyclase/cGMP cascade is involved at several levels of visual processing in the inner retina.

The present study has identified a population of cone photoreceptors in the murine retina that are uniquely immunoreactive for protein kinase C (PKC). Wavelength-sensitive cone subtypes are segregated along the dorso-ventral axis in the mouse retina with ventral retina occupied exclusively by ultraviolet wavelength-sensitive (UVWS) cones, and dorsal retina dominated by middle wavelength-sensitive cones. PKC-positive cones are found primarily in the ventral retina, and double-label immunocytochemistry using a short wavelength-sensitive opsin antibody confirms that they specifically correspond to the UVWS cone subtype. The PKC antibody, as documented in other mammals, also identifies rod bipolar cells in the mouse retina. UVWS cones and bipolar cells have previously been shown to share transcriptional regulatory elements, as observed in transgenic mice encoding a portion of the human SWS-opsin promoter controlling the lacZ reporter gene. In such mice, the transgene product, β-galactosidase, is expressed in populations of both cones and bipolar cells. The present study confirms that lacZ-expressing photoreceptors are indeed PKC-positive photoreceptors, but that the lacZ-expressing bipolar cells are not the PKC-positive rod bipolar cells. These cells must correspond to a type of cone bipolar cell.

Cyclic GMP has been shown in recent years to directly activate ion channels in bipolar and ganglion cells, and to indirectly regulate coupling between horizontal cells, and between bipolar and amacrine cells. In all of these cases, the effects of cyclic GMP are mimicked by nitric oxide. An increase in calcium concentration stimulates the production of nitric oxide by neuronal and endothelial forms of nitric oxide synthase, which in turn activates soluble guanylate cyclases, enhancing the synthesis of cyclic GMP. Though some effects of nitric oxide do not involve cyclic GMP, the nitric oxide-cyclic GMP cascade is well recognized as a signaling mechanism in brain and other tissues. The widespread occurrence of nitric oxide/cyclic GMP-regulated ion channel activity in retinal neurons raises the possibility that nitric-oxide-sensitive soluble guanylate cyclases play an important role in cell–cell communication, and possibly, synaptic transmission. Immunohistochemical studies have indicated the presence of soluble guanylate cyclase in retinal synaptic layers, but such studies are not suitable for determination of the density or quantitative subcellular distribution of the enzyme. Microanalytical methods involving microdissection of frozen retina also showed the presence of cyclase activity in retinal plexiform layers but these methods did not permit distinction between nitric oxide-sensitive and insensitive cyclases. In this study, we fractionated retinal homogenate into the cytosolic and synaptosomal fractions and investigated the specific activity and distribution of soluble guanylate cyclase and nitric oxide synthase. The results show that both enzymes are present in the synaptosomal fractions derived from inner and outer plexiform layers. The synaptosomal fraction derived from inner retina was highly enriched in cyclase activity. Nitric oxide synthase activity was also higher in the inner than outer retinal synaptosomal fraction. The results suggest that the nitric oxide-cyclic GMP system is operational in both synaptic layers of retina and that it may play a more significant role in the inner retina.

We examined cytochrome oxidase (CO) activity in striate cortex of four macaque monkeys after monocular enucleation at ages 1, 1, 5, and 12 weeks. These animal experiments were performed to guide our interpretation of CO patterns in occipital lobe specimens obtained from two children who died several years after monocular enucleation during infancy for tumor. In the macaques, the ocular dominance columns were labelled by injecting [3H]proline into the remaining eye. After enucleation at age 1 week, ocular dominance columns were eliminated in layer IVcβ, resulting in a uniform pattern of autoradiographic label and CO staining. However, columns could still be seen in wet, unstained sections and with the Liesegang silver stain. Autoradiographs through layers IVcα and IVa showed residual, shrunken columns belonging to the missing eye, indicating that enucleation has less drastic effects in these layers. In the two human cases, enucleation at age 1 week also resulted in uniform CO staining in layer IVc. In the macaque after enucleation at age 5 weeks, ocular dominance columns belonging to the missing eye were severely narrowed, but still occupied 20% of layer IVcβ. CO revealed wide, dark columns alternating with thin, pale columns in layer IVcβ. The CO pattern and the columns labelled by autoradiography matched perfectly. After enucleation at age 12 weeks, only mild shrinkage of ocular dominance columns occurred. Enucleation at ages 1, 5, and 12 weeks did not alter the pattern of thin-pale–thick-pale stripes in V2. The main findings from this study were that (1) CO histochemistry accurately labels the boundaries of columns in layer IVcβ of macaque striate cortex after early monocular enucleation, making it a suitable technique for defining the critical period for plasticity of ocular dominance columns in human striate cortex; (2) enucleation causes more severe shrinkage of ocular dominance columns than eyelid suture; (3) early monocular enucleation obliterates ocular dominance columns in layer IVcβ, but their pattern remains visible in wet sections and with the Liesegang stain; and (4) enucleation does not affect CO staining in V2.

Extrastriate cortical area MT is thought to process behaviorally important visual motion signals. Psychophysical studies suggest that visual motion signals may be analyzed by multiple mechanisms, a “first-order” one based on luminance, and a “second-order” one based upon higher level cues (e.g. contrast, flicker). Second-order motion is visible to human observers, but should be invisible to first-order motion sensors. To learn if area MT is involved in the analysis of second-order motion, we measured responses to first- and second-order gratings of single neurons in area MT (and in one experiment, in area V1) in anesthetized, paralyzed macaque monkeys. For each neuron, we measured directional and spatio-temporal tuning with conventional first-order gratings and with second-order gratings created by spatial modulation of the flicker rate of a random texture. A minority of MT and V1 neurons exhibited significant selectivity for direction or orientation of second-order gratings. In nearly all cells, response to second-order motion was weaker than response to first-order motion. MT cells with significant selectivity for second-order motion tended to be more responsive and more sensitive to luminance contrast, but were in other respects similar to the remaining MT neurons; they did not appear to represent a distinct subpopulation. For those cells selective for second-order motion, we found a correlation between the preferred directions of first- and second-order motion, and weak correlations in preferred spatial frequency. These cells preferred lower temporal frequencies for second-order motion than for first-order motion. A small proportion of MT cells seemed to remain selective and responsive for second-order motion. None of our small sample of V1 cells did. Cells in this small population, but not others, may perform “form-cue invariant” motion processing (Albright, 1992).

The sudden displacement of the retinal image during a saccade raises the visual threshold of human observers to foveal stimuli. The fall in visual sensitivity observed during this phenomenon, known as saccadic suppression, seems to occur very early in the visual processing chain. The lateral geniculate nucleus (LGN) is a likely locus for the multiple retinal and extraretinal interactions occurring during saccadic eye movements, therefore we used the responses of relay cells of adult cats to simulate a pychophysical experiment. We first measured the responses of X and Y relay cells (27 X and 13 Y) to central spots of optimal size and different contrasts. The spots were presented either alone or time locked with the rapid movement of a large, high-contrast peripheral pattern, referred to as shift. We measured the percentage of trials on which the relay cell fired more spikes when the spot (contrast: 0.03–1.0) was present than when it was absent. In experiments with human observers the task was to indicate, by a keypress, which of two otherwise identical temporal intervals contained the spot. The shift reduces the sensitivity (raises the contrast threshold) of neurones in the cat relay cells to brief, stationary targets presented to the receptive-field center. The suppression of visual sensitivity is significantly greater in Y cells than in X cells (average sensitivity ratios 5.6 ± 5.4 in Y cells, 1.59 ± 0.9 in X cells: P < 0.001, U test). The shift also reduces the sensitivity of human observers to the same target. This suggests that the LGN is a potential locus for the modulation of visual responses that leads to saccadic suppression.

Macaca mulatta retinas were superfused, in vitro, to measure the efflux of dopamine. Steady light, in the low photopic range, stimulated dopamine release slightly. Flashing light (3 Hz) superimposed over the steady background increased dopamine efflux significantly. This increase was completely blocked by the addition of d,l-2-amino-4-phosphonobutyric acid (d,l-APB, 10–100 μM) to the superfusion medium, but not by the addition of the inactive enantiomer d-APB (10 μM). The results suggest that ON bipolar cells provide the excitatory drive to dopaminergic amacrine cells in primates, as in other species.

Damage to primary visual cortex (VC) in young cats leads to severe retrograde degeneration of the dorsal lateral geniculate nucleus (dLGN) and selective transneuronal retrograde degeneration of a class of retinal ganglion cells (RGCs) that have a medium-size soma. Previous studies have shown that “programmed” RGC death associated with normal development in one eye can be attenuated by removal of the other eye, suggesting that binocular interactions can influence developmental RGC death. The present study investigated whether removal of one eye also attenuates the ganglion cell loss that accompanies an early VC lesion. Five one-week-old cats received a unilateral VC lesion (areas 17, 18, and 19), and three of these cats also underwent monocular enucleation at the same time. Two normal control animals also were examined. RGC measurements were made from flat-mounted retinae when the animals were 5 weeks old. Sampling was restricted to a retinal area corresponding to the retinotopic representation included in the VC lesion. Results indicate that there is a marked loss of medium-size RGCs in the hemiretinae projecting to the damaged hemisphere in cats that received a VC lesion alone. However, there is no such loss in VC-lesion animals that also have a monocular enucleation. These results indicate that the transneuronal RGC loss that occurs after an early visual cortex lesion can be influenced by binocular interactions.

Melatonin synthesis in retinal photoreceptors is stimulated at night by a circadian oscillator and suppressed acutely by light. To identify photoreceptor mechanisms involved in the acute suppression of melatonin synthesis, an action spectrum was measured for dark-adapted Xenopus laevis eyecups at night. Intensity–response curves at six wavelengths from 400 to 650 nm were parallel, suggesting that a single photopigment predominates in melatonin suppression. Half-saturating intensities at 400, 440, 480, and 533 nm were not significantly different from one another, at 1–2 × 108 quanta cm−2 s−1. Significantly higher intensities of 580- and 650-nm light were required for melatonin suppression. These results indicate a predominant role for the principal green-absorbing rods in acute regulation of retinal melatonin synthesis in response to light, and argue against an important role for the red-absorbing cones. Higher than expected sensitivity at short wavelengths suggests that photoreceptors sensitive to blue and/or violet light may also contribute to melatonin suppression.

The common brush-tailed possum Trichosurus vulpecula is a small diprotodont marsupial common to both urban and natural environments. This is the first analysis of the neurotransmitter content of its retinal cells and, as the possum is a nocturnal forager, it was appropriate to begin with the dopaminergic amacrine cells that form an essential link in the modulation of the rod pathways subserving nocturnal vision. These results were compared with those from another diprotodont, the marsupial wallaby or quokka (Dann, 1996) to establish whether the dopaminergic systems were similar between these two diprotodont marsupials and also to compare these findings with those of other mammals. This study describes a population of amacrine cells in the possum retina that were immunolabelled with an antibody raised against tyrosine hydroxylase (TH). These TH-immunoreactive (IR) cells were located within the inner nuclear layer (INL) and their dendrites predominantly ramified within the most sclerad layers of the inner plexiform layer (IPL). The TH-IR amacrines formed a sparse cell population, of around 2400 cells, distributed over the entire retina. There was little evidence of a concentration gradient except for a slight elevation in density in the naso-temporal axis in dorsal retina. The formation of rings within the dendritic plexus, a feature common to TH-IR cells in other species, was also present in the possum and these appeared relatively frequently. This latter finding was rather unexpected since, in the marsupial quokka (Dann, 1996), the TH-IR dendrites formed fewer rings despite having the same density of TH-IR amacrines as the possum. This suggests that there may be subtle differences in the way the rod pathways are interconnected even within the same marsupial group and may also be a reflection of relative rod dominance across species.