The distribution of local preferred directions and motion sensitivities within the receptive fields of so-called tangential neurons in the fly visual system was previously found to match optic flow fields as induced by certain self-motions. The complex receptive-field organization of the tangential neurons and the recent evidence showing that the orderly development of the fly's peripheral visual system depends on visual experience led us to investigate whether or not early visual input is required to establish the functional receptive-field properties of such tangential neurons. In electrophysiological investigations of two identified tangential neurons, it turned out that dark-hatched flies which were kept in complete darkness for 2 days develop basically the same receptive-field organization as flies which were raised under seasonal light/dark conditions and were free to move in their cages. We did not find any evidence that the development of the sophisticated receptive-field organization of tangential neurons depends on sensory experience. Instead, the input to the tangential neurons seems to be “hardwired” and the specificity of these cells to optic flow induced during self-motions of the animal may have evolved on a phylogenetical time scale.

The visual cortex of freshwater turtles contains pyramidal cells, which have a regular spiking (RS) firing pattern, and several categories of aspiny, inhibitory interneurons. The interneurons show diverse firing patterns, including the fast spiking (FS) pattern. Postsynaptic potentials (PSPs) evoked in FS cells by visual stimulation of the retina reach their peak amplitudes as much as 200 ms before PSPs in RS cells (Mancilla et al., 1998). FS cells could, consequently, control the amplitudes of light-evoked PSPs in RS cells by producing disynaptic, feedforward inhibitory postsynaptic potentials (IPSPs) that overlap in time with geniculocortical excitatory postsynaptic potentials (EPSPs). Since FS cells receive recurrent, excitatory inputs from RS cells, they could also control the amplitudes of light-evoked PSPs in RS cells via polysynaptic, feedback inhibition. The in vitro geniculocortical preparation of Pseudemys scripta was used to characterize the temporal relationships of EPSPs and IPSPs produced in RS cells by electrical activation of geniculate afferents and by diffuse light flashes presented to the retina. GABAA receptor-mediated inhibition was blocked using extracellular application of bicuculline (3.5 mM) or intracellular perfusion of picrotoxin (1 μM) in individual RS cells. Electrical stimulation of thalamic afferents produced compound PSPs. Blockade of GABAA receptor-mediated IPSPs with either bicuculline or picrotoxin provided evidence for both early and late IPSPs in RS cells. Analysis of the apparent reversal potentials of light-evoked PSPs indicated the existence of early IPSPs during the first 140–300 ms following light onset. Light responses of cells perfused with picrotoxin diverged from control light responses at about 300 ms after light onset and had maximum amplitudes that were significantly different from control light responses. These experiments indicate that the responses of RS cells to both electrical and natural stimulation of geniculate afferents are controlled by both early and late IPSPs, consistent with activation of both feedforward and feedback pathways.

Based on cytoarchitectonic criteria, the primate pulvinar nucleus has been subdivided into medial (PM), lateral (PL), and inferior (PI) regions. However, these subdivisions show no correlation with those established by electrophysiological, immunocytochemical, or neuroanatomical tracer studies. In this work, we studied the connections of the pulvinar nucleus of Cebus monkey with visual areas V1, V2, V4, MT, and PO by means of retrograde fluorescent tracers injected into these areas. Based on the projection zones to cortical visual areas, the visual portion of the pulvinar of Cebus monkey was subdivided into three subregions: P1, P2, and P3, similar to those described in the macaque (Ungerleider et al., 1984). In Cebus, P1 includes the centrolateral portion of traditionally defined PI and adjacent portion of PL. P2 is located in the dorsal portion of PL and P3 includes the medial portion of PI and extends dorsally into adjacent PL and PM. In addition, we studied the histology of the pulvinar using multiple criteria, such as cytoarchitecture and myeloarchitecture; histochemistry for cytochrome oxidase, NADPH-diaphorase, and acetylcholinesterase; and immunocytochemistry for two calcium-binding proteins, calbindin and parvalbumin, and for a neurofilament recognized by the SMI-32 antibody. Some of these stains, mainly calbindin, showed additional subdivisions of the Cebus pulvinar, beyond the traditional PI, PL, and PM. Based on this immunohistochemical staining, the border of PI is moved dorsally above the brachium of the superior colliculus and PI can be subdivided in five regions (PIP, PIM, PIC, PIL, and PILS). Regions P1, P2, and P3 defined based on efferent connections with cortical visual areas are not architectonically/neurochemically homogeneous. Rather they appear to consist of further chemoarchitectonic subdivisions. These distinct histochemical regions might be related to different functional modules of visual processing within one connectional area.

Nitric oxide (NO) is a neuronal messenger molecule that mediates pathway refinement in some brain regions. We used nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry to examine the development of NO expression in the superior colliculus (SC) of kittens aged E28–E58 and P2–P57 and adults in order to determine if NO expression is correlated with pathway refinement. At E28, labeled cells were seen only within the subventricular zone (SVZ). At E36–E41, labeled cells were also found within the deep gray layer (DGL) of SC. At E51 and E58, a few labeled neurons were also present in the intermediate gray layer (IGL). These neurons already had extensive dendritic fields and well-developed morphologies at the time that they first expressed nitric oxide synthase (NOS). The number of neurons labeled in the DGL and IGL increased postnatally, reaching a peak density between P14 and P35. Neurons within the optic (OL) and superficial gray layers (SGL) were first visible at P7 and increased slightly in number until adulthood. However, SGL-labeled neurons were relatively limited in number and lightly labeled at all ages examined. We conclude that (1) NADPHd expression occurs in SC beginning in the second trimester in kittens and progresses in a ventral to dorsal pattern between E36–P35; (2) few neurons in kitten SGL are labeled by NADPHd and these appear relatively late in postnatal development; and (3) there is no correlation between NOS expression and retinocollicular pathway refinement in kittens, a result different from that seen in rodents.

The retina possesses subpopulations of amacrine cells, which utilize different transmitters, including acetylcholine (ACh), GABA, and dopamine. We have examined interactions between these neurones by studying the effects of nicotinic agonists on GABA and dopamine release. Isolated rabbit retinas were incubated with [3H]dopamine and then superfused. Fractions of the superfusate (2 min) were collected and the [3H]dopamine in each sample was measured. Endogenous GABA release was examined by incubating retinas in a small chamber. At 5-min intervals, the medium was changed and the GABA measured by high-pressure liquid chromatography (HPLC). Exposure of the retina to nicotine, epibatidine, and other nicotinic agonists increased the release of both GABA and dopamine. The effects of nicotine and epibatidine were blocked by mecamylamine, confirming an action on nicotinic receptors. The action of epibatidine on dopamine release was unaffected by glutamate antagonists but was blocked by picrotoxin and gabazine. These results suggested that nicotine might increase dopamine release indirectly by stimulating the release of GABA, which in turn inhibited the release of an inhibitory transmitter acting tonically on the dopaminergic amacrines. Exposure of the retina to GABA caused a small increase in dopamine release. This hypothetical inhibitory transmitter was not GABA, an opioid, adenosine, glycine, nociceptin, a cannabinoid, or nitric oxide because appropriate antagonists did not affect the resting release of dopamine. However, metergoline, a 5HT1/5HT2 receptor antagonist, and ketanserin, a 5HT2A receptor antagonist, but not the 5HT1A antagonist WAY100635, increased the resting release of dopamine and blocked the effects of nicotine. The 5HT1A/5HT7 agonist 8-hydroxy DPAT inhibited both the nicotine and GABA-evoked release of dopamine. We conclude that nicotinic agonists directly stimulate the release of GABA, but the evoked release of dopamine is indirect, and arises from GABA inhibiting the input of an inhibitory transmitter, which we tentatively identify as serotonin.

The mammalian visual cortex harbors a number of functional maps that represent distinct attributes of stimuli in the visual environment. How different functional maps are accommodated within the same cortical space, especially in species that show marked irregularities in one or more functional maps, remains poorly understood. We used optical imaging of intrinsic signals and electrophysiological techniques to investigate the organization of the maps of orientation preference, ocular dominance, and visual space in ferret. This species shows striking nonuniformity in the arrangement of ocular dominance domains and disruption of the mapping of visual space along the V1/V2 border. We asked whether these irregularities would be reflected in the organization of the map of orientation preference. The results show that orientation preference is mapped consistently within both V1 and V2, and across the interareal boundary, with no reflection of the irregularities in the other maps. These observations demonstrate the accommodation of multiple functional maps within the same cortical space without systematic geometrical relationships that necessarily constrain the organization of each representation. Furthermore, they imply that the structure of the map of orientation preference reflects the architecture and activity patterns of cortical circuits that are independent of other columnar systems established in layer 4.

We studied the visuomotor activity of corticotectal (CT) cells in two visual cortical areas [area 17 and the posteromedial lateral suprasylvian cortex (PMLS)] of the cat. The cats were trained in simple oculomotor tasks, and head position was fixed. Most CT cells in both cortical areas gave a vigorous discharge to a small stimulus used to control gaze when it fell within the retinotopically defined visual field. However, the vigor of the visual response did not predict latency to initiate a saccade, saccade velocity, amplitude, or even if a saccade would be made, minimizing any potential role these cells might have in premotor or attentional processes. Most CT cells in both areas were selective for direction of stimulus motion, and cells in PMLS showed a direction preference favoring motion away from points of central gaze. CT cells did not discharge with eye movements in the dark. During eye movements in the light, many CT cells in area 17 increased their activity. In contrast, cells in PMLS, including CT cells, were generally unresponsive during saccades. Paradoxically, cells in PMLS responded vigorously to stimuli moving at saccadic velocities, indicating that the oculomotor system suppresses visual activity elicited by moving the retina across an illuminated scene. Nearly all CT cells showed oscillatory activity in the frequency range of 20–90 Hz, especially in response to visual stimuli. However, this activity was capricious; strong oscillations in one trial could disappear in the next despite identical stimulus conditions. Although the CT cells in both of these regions share many characteristics, the direction anisotropy and the suppression of activity during eye movements which characterize the neurons in PMLS suggests that these two areas have different roles in facilitating perceptual/motor processes at the level of the superior colliculus.

As a part of ongoing efforts to understand the cholinergic circuitry in the mammalian retina, we studied the coexpression of nicotinic acetylcholine receptors (nAChRs) and gamma-aminobutyric acid (GABA), the GABA transporter 1 (GAT-1), or choline acetyltransferase (ChAT) immunoreactivity in the rabbit retina. Double-label experiments with monoclonal antibody 210 (mAb 210) against nAChRs and antibodies against GABA revealed that several populations of GABA-containing amacrine, displaced amacrine, and ganglion cells displayed nAChR immunoreactivity. Some of them also exhibited ChAT immunoreactivity and were identified as the cholinoceptive starburst cells. Other GABAergic amacrine cells positive for mAb 210 were not cholinergic. Simultaneous visualization of mAb 210 and GAT-1 immunoreactivity revealed that 10% of GAT-1 immunoreactive amacrine cells contained nAChRs. Ninety-nine percent of the GAT-1 labeled cells demonstrated GABA immunoreactivity, but only 75% of the GABAergic cells were outlined by GAT-1 staining. Neither population of starburst cells exhibited GAT-1 immunoreactivity. Thus, mAb 210 expressing, GAT-1 positive cells in the rabbit retina constitute a noncholinergic subset of GABAergic amacrine cells. Taken together, our results suggest that some GABAergic amacrine cells are cholinoceptive, raising the possibility that ACh, acting through nAChRs, can modulate the release of GABA in the rabbit retina.

In the retina, somatostatin (SST), an inhibitory peptide that influences neuronal activity, is predominantly expressed by sparsely occurring amacrine cells. The SST subtype 2A receptor is expressed by rod bipolar cells, including their axonal terminals. We used Ca2+-imaging techniques and the ratiometric Ca2+ indicator dye fura-2 AM to investigate Ca2+ dynamics in rod bipolar cell terminals. Depolarization of rod bipolar cells by the addition of high K+ (50 or 100 mM) elicited a sustained increase in [Ca2+]i in rod bipolar terminals that returned to basal levels following K+ removal. The Ca2+ response was dependent on extracellular Ca2+, and was inhibited by the Ca2+ channel blocker Cd2+ and by the selective L-type Ca2+ channel blocker, nimodipine. SST inhibited a K+ depolarization-induced [Ca2+]i response in rod bipolar terminals. This inhibition was observed with 1 nM SST and was maximal with 1 μM SST. These findings indicate that SST may regulate transmitter release from rod bipolar terminals by activating the SST subtype 2A receptor through modulation of intracellular Ca2+.

We describe the temporal pattern of mRNA expression of some of the molecular components of the NO/cGMP second messenger system in the developing rat visual cortex and the effect of dark rearing on their expression levels using semiquantitative RT-PCR. mRNA expression for these molecules was altered by dark rearing in one of three ways: (1) no change—rod, olfactory, and cone/testis CNG channels, nonselective cation channels gated by cyclic nucleotides and highly permeable to Ca2+; (2) decrease—cyclic nucleotide phosphodiesterases which regulate cyclic nucleotide levels, and soluble guanylyl cyclase, the key synthetic enzyme producing cGMP and potently activated by nitric oxide; and (3) increase—cGMP kinase I, a key enzyme activated by cGMP to phosphorylate a variety of intracellular proteins including cytoskeletal elements. These data suggest important and distinct roles for the cGMP system in both early and late developmental events in the rat visual cortex.

Rod and cone photoreceptors send their signals to ON- and OFF-retinal ganglion cells through different pathways in the primate retina. We hypothesized that increments and decrements of light may be processed differently by the rod-bipolar pathway because of the funneling of the rod signal through the rod bipolar cell. We tested this hypothesis using a psychophysical adaptation paradigm, which has provided evidence that photopic increments and decrements of light are processed by ON- and OFF-pathways in the human visual system. We had observers adapt to either a rapid-on or rapid-off sawtooth waveform, under both photopic and scotopic conditions. We then measured detection thresholds for one cycle of a rapid-on or rapid-off sawtooth stimulus. For photopic stimuli, sawtooth adaptation asymmetrically raised thresholds for test stimuli in a manner that depended on the polarity of the adaptation stimulus. For scotopic stimuli, thresholds were raised, but no significant selective adaptation effect was found. By repeating the photopic condition with sawtooth stimuli which had been filtered using an impulse response function derived for the rod system, we demonstrated that the lack of selective adaptation was not a consequence of the sluggish temporal response of the rod-bipolar pathway. We conclude instead that the reduced effectiveness of sawtooth adaptation is due to channeling of rod photoreceptor signals through the rod bipolar cell before reaching ON- and OFF-ganglion cells.

It is widely accepted that human color vision is based on two types of cone-opponent mechanism, one differencing L and M cone types (loosely termed “red–green”), and the other differencing S with the L and M cones (loosely termed “blue–yellow”). The traditional view of the early processing of human color vision suggests that each of these cone-opponent mechanisms respond in a bipolar fashion to signal two opponent colors (red vs. green, blue vs. yellow). An alternative possibility is that each cone-opponent response, as well as the luminance response, is rectified, so producing separable signals for each pole (red, green, blue, yellow, light, and dark). In this study, we use psychophysical noise masking to determine whether the rectified model applies to detection by the postreceptoral mechanisms. We measured the contrast-detection thresholds of six test stimuli (red, green, blue, yellow, light, and dark), corresponding to the two poles of each of the three postreceptoral mechanisms. For each test, we determined whether noise presented to the cross pole had the same masking effect as noise presented to the same pole (e.g. comparing masking of luminance increments by luminance decrement noise (cross pole) and luminance increment noise (same pole)). To avoid stimulus cancellation, the test and mask were presented asynchronously in a “sandwich” arrangement (mask-test-mask). For the six test stimuli, we observed that noise masks presented to the cross pole did not raise the detection thresholds of the test, whereas noise presented to the same pole produced a substantial masking. This result suggests that each color signal (red, green, blue, and yellow) and luminance signal (light and dark) is subserved by a separable mechanism. We suggest that the cone-opponent and luminance mechanisms have similar physiological bases, since a functional separation of the processing of cone increments and cone decrements could underlie both the separation of the luminance system into ON and OFF pathways as well as the splitting of the cone-opponent mechanisms into separable color poles.

We have previously reported that a small number of retinal ganglion cells (RGCs) of adult cats survive 2 months after transection of the optic nerve (ON) and that α cells have the greatest ability to survive among different types of RGCs (Watanabe et al., 1995). Here we report the time course of RGC survival within 15 days after ON transection using retrograde labeling with DiI injected into the bilateral lateral geniculate nuclei of cats. The density of DiI-labeled RGCs in the central retina as well as in the periphery did not change until day 3 after ON transection, then decreased rapidly, to 43% of the original density on day 7, and falling to 19% by day 14. We then intracellularly injected Lucifer yellow into the DiI-labeled RGCs to examine the difference in the time course between surviving α and β cells. Similar to the density change in total surviving RGCs, the proportion of surviving β cells did not change until day 3, then decreased rapidly to 65% of the original density on day 4, falling to 12% by day 14. By contrast, 64% of α cells survived for 14 days after axotomy. Analysis of regression lines for survival time courses indicated that death of β cells was characterized with a rapid period phase from day 3 to day 7 after axotomy whereas that of α cells lacked it. Axon-like sprouting from surviving β cells was first recognized in the nerve fiber layer on day 3, and were later more conspicuous.

The neuronal generators of the b- and d-waves of the electroretinogram (ERG) were investigated in the tiger salamander retina to determine if amacrine and ganglion cells contribute to this field potential. Several agents were used that affect third-order neurons, such as tetrodotoxin, baclofen, and NMDA agonists and antagonists. Baclofen, an agent that enhances light responses in third-order neurons, increased the d-wave and reduced the b-wave. In contrast, agents that decrease light responses in third-order neurons had the opposite effect of enhancing the b-wave and depressing the d-wave. The effect on the d-wave was particularly pronounced. The results indicate that third-order neuronal activity influences b- and d-waves of the ERG. The opposing actions suggest that the b-wave to d-wave ratio might serve as an measure of ganglion cell function.

The present study has examined the spatial and temporal expression patterns of various proteins associated with the structure and function of mature photoreceptor outer segments in the developing ferret's retina using immunocytochemistry and RT-PCR. One set of proteins, including rod opsin, arrestin, and recoverin, was detected progressively in photoreceptors as they became postmitotic, being expressed well before the differentiation of outer segments. A second set of proteins, including β- and γ-transducin, cGMP-phosphodiesterase, phosducin, rhodopsin kinase, rod cGMP-gated cation channel protein, and peripherin, displayed a contrasting temporal onset and pattern of spatial emergence. These latter proteins first became detectable either shortly before or coincident with outer segment formation, and were expressed simultaneously in both older and younger photoreceptor cells. A third set, the short wavelength-sensitive (SWS) and medium wavelength-sensitive (MWS) cone opsin proteins, was the last to be detected, but materialized in a spatio-temporal pattern reminiscent of the neurogenetic gradient of the cones. These different spatial and temporal patterns indicate that cellular maturation must play a primary role in regulating the onset of expression of some of these proteins, while extrinsic signals must act to coordinate the expression of other proteins across photoreceptors of different ages.