Previous studies have shown that indoleamine-accumulating cells (IACs) in the rabbit retina consist of two main cell types: S1 and S2 amacrine cells (Vaney, 1986; Sandell & Masland, 1986). Both cell types are wide-field GABA amacrine cells that make reciprocal synaptic contacts with rod bipolar cell terminals (Ehinger & Holmgren, 1979; Strettoi et al., 1990). We have examined the coupling pattern of S1 and S2 amacrine cells after the intracellular injection of Neurobiotin. Our results may be summarized as follows: (1) S1 amacrine cells were extensively coupled and their dendrites formed a network similar to but less dense than the matrix stained with an antibody to serotonin. (2) Morphological observations and cluster analysis, based on a scattergram, showed that the vast majority of coupled cells were S1 amacrine cells, accounting for approximately half of the total IACs. The rest of the uncoupled IACs were S2 amacrine cells. (3) Sometimes, two adjacent varicosities, one from an injected S1 and one from a coupled S1, contacted a single rod bipolar terminal. (4) S2 amacrine cells were also coupled but much less than the S1s. (5) Rarely, crossover coupling between S1 and S2 amacrine cells was observed. These results suggest that the extensive coupling between S1 amacrine cells, combined with a larger dendritic field, may contribute a wide-field component to the inhibitory surround of the rod pathway. By comparison, the smaller, weakly coupled S2 amacrine cells may provide a local component.

Contrary to the traditional view that receptive fields are limited in spatial extent, recent studies have indicated that the response of neurons to a local stimulus within the receptive field can be modulated by stimulation of the surrounding region. Here we quantified the nature of these contextual effects on visual motion responses of neurons in the pigeon's optic tectum using standard extracellular recording techniques. All of the cells tested responded well to a test spot moving across their receptive fields. When a background pattern was moved in the same or in a similar direction as that of the test spot, the responses of most deep tectal neurons to the test spot were maximally inhibited. Movement of the background in the opposite or near opposite direction produced minimal inhibition or even facilitation. For some deep tectal neurons, this directionally selective modulation by the moving background was maintained when the background motion was paired with different movement directions of the test spot (including both the preferred and least preferred directions). Thus, this selectivity for opposing motion was independent of the absolute direction of either the test spot or the background, a finding which is consistent with the results reported by Frost and Nakayama (1983), although they did not include all test spot directions. For some other neurons, identified here for the first time, the background movement selectively modulated the response only when the test spot moved in the neuron's preferred directions. These neurons lost selectivity for opposing motion when the test spot moved in nonpreferred directions. The significance of these contextual effects on the motion response of tectal neurons may be related to how the brain distinguishes self-induced motion from object motion and segregates figure from ground.

Directionally selective (DS) ganglion cells of rabbit retina are of two principal types. ON DS ganglion cells prefer low velocity in one of three directions of movement and project axons to the accessory optic system (AOS), whereas ON–OFF DS ganglion cells prefer higher velocity in one of four directions and project to tectum and thalamus. Each has a distinct, recognizable dendritic morphology, based upon the correlation of form, physiology, and central projections. In previous Golgi studies, ON and ON–OFF DS cells were found to be partly co-stratified, and ON–OFF DS cells were found to co-stratify with starburst amacrine (SA) cells, the cholinergic amacrine cells of the retina, which also contain elevated levels of GABA. SA cells are radially symmetrical, have synaptic boutons in a distal annular zone of its dendritic tree, are presynaptic primarily to ganglion cell dendrites, co-stratify with ON–OFF DS ganglion cells, and contain the neurotransmitters shown pharmacologically to be involved in DS responses. For these reasons, SA cells are thought to play a role in the DS mechanism. Several models of this mechanism have utilized SA cell dendritic geometry in a centrifugal, radial format to impose directional inputs on DS ganglion cells.

Whole-cell voltage-clamp recordings were performed to investigate voltage-dependent K+ currents in acutely isolated retinal cone bipolar cells (CBCs) from the rat. The physiological and pharmacological properties of the currents were compared with those in rod bipolar cells (RBCs). The K+ currents were found to be much larger in CBC than in RBCs. In addition, the currents in CBCs were activated and inactivated at more negative potentials. Based on the apparent inactivation property of the currents, CBCs were found to fall into two groups of cells that differed in the inactivation kinetics of IK(V) but did not correlate to the ON- and OFF-type. The IK(V) for the group of CBCs showing faster inactivation, as well as for all RBCs, contained two components with decay time constants around 0.1 and 1 s. The IK(V) for the group of CBCs showing slower inactivation only contained the slower component. Furthermore, three components of IK(V) were observed based on tetraethylammonium (TEA) sensitivity: high-sensitive, low-sensitive, and resistant component. The IK(V) for a portion of CBCs showing faster inactivation, as well as for all RBCs, contained all three components. The IK(V) for the remaining CBCs, including all of those CBCs showing slower inactivation, only contained the latter two components. This study reveals a differential expression of K+ currents in rat retinal bipolar cells, suggesting that K+ channels may play an important role in bipolar cell processing in mammalian retinas.

Alpha-2 adrenoceptor agonists have previously been shown to enhance neuronal survival in an optic nerve mechanical injury model and to protect photoreceptors in a light-induced degeneration model. The purpose of this study was to examine the effect of the alpha-2 adrenoceptor agonist in a pressure-induced retinal ischemia model. Brown-Norway rats were treated systemically or topically with alpha-2 adrenoceptor specific agonist brimonidine. Retinal ischemia was induced by increasing the intraocular pressure to 110 mm Hg for 50 min. The effect of brimonidine on retinal ischemic injury was functionally assessed in the rats 7 d later using electroretinography (ERG). Ischemia-induced retinal cell death was studied using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining. We found that brimonidine treatment significantly protected the retina from retinal ischemic injury in a dose- and time-dependent manner. This protection can be achieved either by systemic or topical application and can be blocked by pretreatment with the alpha-2 adrenoceptor antagonist, yohimbine. Using reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis, we found that brimonidine can up-regulate the expression of basic fibroblast growth factor, bcl-2 and bcl-xl in the retina. The drug also can activate two major cell survival signaling pathways in the retina: the extracellular-signal-regulated kinases (ERKs) and phosphatidylinositol-3′ kinase/protein kinase Akt pathways. All these aforementioned factors may potentially contribute in mediating brimonidine's protective effect in this acute retinal ischemia model.

We used qualitative tests to assess the sensitivity of 1043 V2 neurons (predominantly multiunits) in anesthetised macaque monkeys to direction, length, orientation, and color of moving bar stimuli. Spectral sensitivity was additionally tested by noting ON or OFF responses to flashed stimuli of varied size and color. The location of 649 units was identified with respect to cycles of cytochrome oxidase stripes (thick-inter-thin-inter) and cortical layer. We used an initial 8-way stripe classification (4 stripes, and 4 “marginal” zones at interstripes boundaries), and a 9-way layer classification (5 standard layers (2–6), and 4 “marginal” strata at layer boundaries). These classes were collapsed differently for particular analyses of functional distribution; the main stripe-by-layer analysis was performed on 18 compartments (3 stripes × 6 layers). We found direction sensitivity only within thick stripes, orientation sensitivity mainly in thick stripes and interstripes, and spectral sensitivity mainly in thin stripes. Positive length summation was relatively more frequent in thick stripes and interstripes, and negative length/size summation in thin stripes. All these “majority” characteristics of stripes were most prominent in layers 3A and 3B. By contrast, “minority” characteristics (e.g. spectral sensitivity in thick stripes; positive size summation in thin stripes) tended to be most frequent in the outer layers, that is, layers 2 and 6. In consequence, going by the four functions tested, the distinctions between stripes were maximal in layer 3, moderate in layer 2, and minimal in layer 6.

We have examined the visuotopic organization of area V2 of macaque monkeys in relation to its modular construction, comprising repetitive cycles of stripes running perpendicular to the border with area V1. Receptive fields were plotted in anesthetised animals, mainly using long penetrations parallel to the V1 border crossing several stripes in dorsal V2 within the representation of paracentral, inferior visual field. We confirm that each set of modules (thick, thin, and interstripes) mounts an unbroken coverage of the visual field, since there is almost invariably some overlap between the aggregate fields recorded in successive stripes of the same class, at intervals of one cycle. Also as expected, penetrations perpendicular to the stripes record changes in eccentricity along an isopolar visual meridian. We measured the size of the point image along such an isopolar meridian in nine cases, and showed that on average it exceeds the length of a typical cycle; again, this implies that no point in space escapes analysis by any of the functional modules. The representation of eccentricity across a cycle of stripes resembles a “ratchet” model, in which the gradient of eccentricity across a single stripe exceeds the gradient across the full cycle, leading to discontinuities (“switchbacks”) at the borders between stripes. The shift in eccentricity across the width of a stripe is sufficient to maintain a virtually continuous map across successive stripes of the same class; when coupled to receptive field scatter about the mean trend, this creates the overlap of aggregate fields.