Stomatopod crustaceans possess apposition compound eyes that contain more photoreceptor types than any other animal described. While the anatomy and physiology of this complexity have been studied for more than two decades, few studies have investigated the molecular aspects underlying the stomatopod visual complexity. Based on previous studies of the structure and function of the different types of photoreceptors, stomatopod retinas are hypothesized to contain up to 16 different visual pigments, with 6 of these having sensitivity to middle or long wavelengths of light. We investigated stomatopod middle- and long-wavelength-sensitive opsin genes from five species with the hypothesis that each species investigated would express up to six different opsin genes. In order to understand the evolution of this class of stomatopod opsins, we examined the complement of expressed transcripts in the retinas of species representing a broad taxonomic range (four families and three superfamilies). A total of 54 unique retinal opsins were isolated, resulting in 6–15 different expressed transcripts in each species. Phylogenetically, these transcripts form six distinct clades, grouping with other crustacean opsins and sister to insect long-wavelength visual pigments. Within these stomatopod opsin groups, intra- and interspecific clusters of highly similar transcripts suggest that there has been rampant recent gene duplication. Some of the observed molecular diversity is also due to ancient gene duplication events within the stem crustacean lineage. Using evolutionary trace analysis, 10 amino acid sites were identified as functionally divergent among the six stomatopod opsin clades. These sites form tight clusters in two regions of the opsin protein known to be functionally important: six in the chromophore-binding pocket and four at the cytoplasmic surface in loops II and III. These two clusters of sites indicate that stomatopod opsins have diverged with respect to both spectral tuning and signal transduction.

Vision begins with photoisomerization of 11-cis retinal to the all-trans conformation within the chromophore-binding pocket of opsin, leading to activation of a biochemical cascade. Release of all-trans retinal from the binding pocket curtails but does not fully quench the ability of opsin to activate transducin. All-trans retinal and some other analogs, such as β-ionone, enhance opsin’s activity, presumably on binding the empty chromophore-binding pocket. By recording from isolated salamander photoreceptors and from patches of rod outer segment membrane, we now show that high concentrations of β-ionone suppressed circulating current in dark-adapted green-sensitive rods by inhibiting the cyclic nucleotide-gated channels. There were also decreases in circulating current and flash sensitivity, and accelerated flash response kinetics in dark-adapted blue-sensitive (BS) rods and cones, and in ultraviolet-sensitive cones, at concentrations too low to inhibit the channels. These effects persisted in BS rods even after incubation with 9-cis retinal to ensure complete regeneration of their visual pigment. After long exposures to high concentrations of β-ionone, recovery was incomplete unless 9-cis retinal was given, indicating that visual pigment had been bleached. Therefore, we propose that β-ionone activates and bleaches some types of visual pigments, mimicking the effects of light.

Nitric oxide (NO) is a gaseous neuromodulator that has physiological functions in every cell type in the retina. Evidence indicates that NO often plays a role in the processing of visual information in the retina through the second messenger cyclic guanosine monophosphate (cGMP). Despite numerous structural and functional studies of this signaling pathway in the retina, none have examined many of the elements of this pathway within a single study in a single species. In this study, the NO/cGMP pathway was localized to specific regions and cell types within the inner and outer retina. We have immunocytochemically localized nitric oxide synthase, the enzyme that produces NO, in photoreceptor ellipsoids, four distinct classes of amacrine cells, Müller and bipolar cells, somata in the ganglion cell layer, as well as in processes within both plexiform layers. Additionally, we localized NO production in specific cell types using the NO-sensitive dye diaminofluorescein. cGMP immunocytochemistry was used to functionally localize soluble guanylate cyclase that was activated by an NO donor in select amacrine and bipolar cell classes. Analysis of cGMP and its downstream target, cGMP-dependent protein kinase II (PKGII), showed colocalization within processes in the outer retina as well as in somata in the inner retina. The results of this study showed that the NO/cGMP signaling pathway was functional and its components were widely distributed throughout specific cell types in the outer and inner salamander retina.

A number of authors have observed amacrine cells containing high levels of immunoreactive parvalbumin in primate retinas. The experiments described here were designed to identify these cells morphologically, to determine their neurotransmitter, to record their light responses, and to describe the other cells that they contact. Macaque retinas were fixed in paraformaldehyde and labeled with antibodies to parvalbumin and one or two other markers, and this double- and triple-labeled material was analyzed by confocal microscopy. In their morphology and dendritic stratification patterns, the parvalbumin-positive cells closely resembled the knotty type 2 amacrine cells described using the Golgi method in macaques. They contained immunoreactive glycine transporter, but not immunoreactive γ-aminobutyric acid, and therefore, they use glycine as their neurotransmitter. Their spatial density was relatively high, roughly half that of AII amacrine cells. They contacted lobular dendrites of AII cells, and they are expected to be presynaptic to AII cells based on earlier ultrastructural studies. They also made extensive contacts with axon terminals of OFF midget bipolar cells whose polarity cannot be predicted with certainty. A macaque amacrine cell of the same morphological type depolarized at the onset of increments in light intensity, and it was well coupled to other amacrine cells. Previously, we described amacrine cells like these that contacted OFF parasol ganglion cells and OFF starburst amacrine cells. Taken together, these findings suggest that one function of these amacrine cells is to inhibit the transmission of signals from rods to OFF bipolar cells via AII amacrine cells. Another function may be inhibition of the OFF pathway following increments in light intensity.

There are two subclasses of alpha cell in the mammalian retina, which are morphologically identical in plain view but have opposite responses to a luminance change: one is ON center and the other is OFF center. Recent studies have shown that the neural circuitries, which underlie light responses in such ON- and OFF-ganglion cell pairs, are not mirror symmetric with respect to the ON and OFF pathways (Pang et al., 2003; Zaghloul et al., 2003; Murphy & Rieke, 2006). This study examines alpha-cell homologues in the mouse retina and elucidates the synaptic mechanisms that generate their light responses. Morphological analysis of recorded cells revealed three subclasses that were essentially identical in plan view but had distinct vertical stratification levels. We refer to these cells as the sustained ON (ON-S), sustained OFF (OFF-S), and transient OFF (OFF-T) cells (Murphy & Rieke, 2006; Margolis & Detwiler, 2007). Both ON-S and OFF-S cells were largely driven through the ON pathway via changes in excitatory and inhibitory inputs, respectively. Light responses of OFF-T cells were driven by transient changes in excitatory and inhibitory inputs. Light responses of OFF-S cells were also measured in connexin 36 knockout mice in order to dissect glycinergic input arising from AII amacrine cells. At photopic/mesopic intensities, peak glycinergic input to OFF-S cells in the connexin 36 knockout mouse was reduced by ~85% compared to OFF-S cells in the wild-type retina. This is consistent with the idea that AII cells receive their input from ON-cone bipolar cells through gap junctions and in turn provide glycinergic inhibition to OFF-S cells.

The pond turtle (Trachemys scripta elegans) exhibits a notably sluggish pupillary light reflex (PLR), with pupil constriction developing over several minutes following light onset. In the present study, we examined the dynamics of the efferent branch of the reflex in vitro using preparations consisting of either the isolated head or the enucleated eye. Stimulation of the oculomotor nerve (nIII) using 100-Hz current trains resulted in a maximal pupil constriction of 17.4% compared to 27.1% observed in the intact animal in response to light. When current amplitude was systematically increased from 1 to 400 μA, mean response latency decreased from 64 to 45 ms, but this change was not statistically significant. Hill equations fitted to these responses indicated a current threshold of 3.8 μA. Stimulation using single pulses evoked a smaller constriction (3.8%) with response latencies and threshold similar to that obtained using train stimulation. The response evoked by postganglionic stimulation of the ciliary nerve using 100-Hz trains was largely indistinguishable from that of train stimulation of nIII. However, application of single-pulse stimulation postganglionically resulted in smaller pupil constriction at all current levels relative to that of nIII stimulation, suggesting that there is amplification of efferent drive at the ganglion. Time constants for constrictions ranged from 88 to 154 ms with relaxations occurring more slowly at 174–361 ms. These values for timing from in vitro are much faster than the time constant 1.66 min obtained for the light response in the intact animal. The rapid dynamics of pupil constriction observed here suggest that the slow PLR of the turtle observed in vivo is not due to limitations of the efferent pathway. Rather, the sluggish response probably results from photoreceptive mechanisms or central processing.

Monocular deprivation early in development produces considerable change in the organization of connections within the central mammalian visual system. In the dorsal lateral geniculate nucleus, the soma, dendrites, and axon terminal fields of deprived cells become considerably smaller than nondeprived counterparts. We have examined the possibility that subcellular events enabling structural modification of deprived neurons include modification of proteins comprising the cytoskeleton. We examined the integrity of the cytoskeleton by measuring the response of a subset of its proteins to varying durations of monocular deprivation. Loss of all three neurofilament subunits (light, medium, and heavy) within deprived layers was observed to parallel changes in neuron gross structure. Monocular deprivation initiated beyond early life produced neither a change in structure nor a loss of neurofilament labeling.

Recently, we showed that contrast sensitivity for color and high–spatial frequency luminance stimuli is enhanced during smooth pursuit eye movements (Schütz et al., 2008). In this study, we investigated the enhancement over a wide range of temporal and spatial frequencies. In Experiment 1, we measured the temporal impulse response function (TIRF) for colored stimuli. The TIRF for pursuit and fixation differed mostly with respect to the gain but not with respect to the natural temporal frequency. Hence, the sensitivity enhancement seems to be rather independent of the temporal frequency of the stimuli. In Experiment 2, we measured the spatial contrast sensitivity function for luminance-defined Gabor patches with spatial frequencies ranging from 0.2 to 7 cpd. We found a sensitivity improvement during pursuit for spatial frequencies above 2–3 cpd. Between 0.5 and 3 cpd, sensitivity was impaired by smooth pursuit eye movements, but no consistent difference was observed below 0.5 cpd. The results of both experiments are consistent with an increased contrast gain of the parvocellular retinogeniculate pathway.

In the avian brain, the optokinetic response is controlled by two retinal-recipient nuclei: the nucleus of the basal optic root (nBOR) of the accessory optic system and the pretectal nucleus lentiformis mesencephali (LM). Although considered sister nuclei because of their similar response properties and function, there are both similarities and differences with respect to efferent projections and neurochemistry. Both nBOR and LM project to the cerebellum (Cb) directly as mossy fibers but also indirectly via the inferior olive (IO). In a previous report, we showed that the cerebellar- and inferior olivary-projecting neurons in nBOR of pigeons differentially express the calcium-binding proteins calretinin (CR) and parvalbumin (PV). Both CR and PV are expressed in the somata of LM neurons, although the latter is not as prevalent, and whether expression of CR and PV reflects cerebellar and IO projections is not known. In this report, by combining retrograde neuronal tracing from the Cb and IO with fluorescent immunohistochemistry, we examined the expression of these calcium-binding proteins in the pigeon LM. Half (52%) of the cerebellar-projecting neurons were CR+ve, but only 15% were PV+ve. Almost all (>95%) these PV+ve cells also expressed CR. In contrast, few of the IO-projecting neurons expressed CR or PV (≤5%). This is strikingly similar to what we observed in nBOR and reveals that calcium-binding protein expression is concordant with projection patterns in two nuclei that share similar functions.