David I. Vaney

Many diverse types of retinal neurons show tracer coupling when injected with biocytin or Neurobiotin

This study demonstrates that the junctional connections between rod-signal interneurons in mammalian retina can be visualized by tracer coupling, following intracellular injection of the biotinylated compounds, biocytin and Neurobiotin. In addition, many other types of retinal neurons -including B-type horizontal cells and several types of retinal ganglion cells-show specific patterns of tracer coupling, usually to cells of the same neuronal type but occasionally to cells of other neuronal classes. These findings suggest that electronic transmission occurs commonly throughout the retina and, consequently, diverse types of retinal neurons may form functional networks of coupled cells.

Chapter 2 The mosaic of amacrine cells in the mammalian retina

Although only a minority of the estimated 40 to 50 amacrine types in mammalian retina have been characterized with respect to their dendritic morphology, neurotransmitter content and topographic distribution, the colour-by-colour tiling of the amacrine mosaic is sufficiently advanced that recurring patterns of amacrine organization are becoming evident. This is particularly so in rabbit and cat retinae and, throughout this review, the organization of each amacrine system is illustrated with reference to these two species

GABA-like immunoreactivity in cholinergic amacrine cells of the rabbit retina

In the ganglion cell layer of the rabbit retina, the inhibitory transmitter gamma-aminobutyric acid (GABA) and its analogues are accumulated by neurons that appear to match in size and number the population of displaced amacrine cells that synthesize the excitatory transmitter acetylcholine. In this double-label study, we have established directly that the cholinergic amacrine cells, selectively stained with diamidino-phenylindole, are strongly immunoreactive with GABA antisera. The coexistence of two classical transmitters, one excitatory and the other inhibitory, in this defined neuronal population, suggests that stimulation of the cholinergic amacrines may give rise to complex responses in their target neurons.

Direction selectivity in the retina: symmetry and asymmetry in structure and function

Visual information is processed in the retina to a remarkable degree before it is transmitted to higher visual centres. Several types of retinal ganglion cells (the output neurons of the retina) respond preferentially to image motion in a particular direction, and each type of direction-selective ganglion cell (DSGC) is comprised of multiple subtypes with different preferred directions. The direction selectivity of the cells is generated by diverse mechanisms operating within microcircuits that rely on independent neuronal processing in individual dendrites of both the DSGCs and the presynaptic neurons that innervate them.

The immunocytochemical detection of amino-acid neurotransmitters in paraformaldehyde-fixed tissues

In this study, we show that specific antibodies can be raised against paraformaldehyde conjugates of amino acids, including the neurotransmitters glycine, gamma-amino-butyric acid and glutamate, and a non-neuroactive amino acid, glutamine. These antibodies against paraformaldehyde conjugates specifically detect the above amino acids in paraformaldehyde-fixed tissues. The penetration of antibodies into paraformaldehyde-fixed tissues is much superior to the penetration of antibodies into glutaraldehyde-fixed tissues; hence good labeling can be observed through the depth of the tissues. Unlike glutaraldehyde, fixation with paraformaldehyde does not give rise to high levels of tissue autofluorescence and, thus, these antibodies are very effective for immunofluorescence studies. Furthermore we suggest that the ability of these antibodies to detect amino acids in paraformaldehyde-fixed tissues will permit their use in situations where it is necessary to detect other other fixation-sensitive antigens, such as neurotransmitter receptors and transporters.

GABA-like immunoreactivity in NADPH-diaphorase amacrine cells of the rabbit retina

NADPH-diaphorase histochemistry selectively stains discrete populations of retinal interneurons in diverse mammals, including two amacrine types in the rabbit retina. In this study, we have demonstrated that most of these neurons show GABA-like immunoreactivity by combining indirect immunofluorescence and diaphorase histochemistry on frozen retinal sections. The NADPH-diaphorase amacrines account for only a small proportion of the GABA-positive cells in rabbit retina, thus reinforcing the emerging consensus that GABAergic amacrines are remarkably diverse in their morphology and function.

pH-Gated dopaminergic modulation of horizontal cell gap junctions in mammalian retina

Horizontal cells mediate lateral inhibition in the outer retina, and this process is dependent on electrical coupling through gap junctions, giving rise to receptive fields that are much wider than the dendritic fields. This study on rabbit retina shows that the permeability of the gap junctions between A-type horizontal cells, as assessed by Lucifer yellow dye coupling, is modulated by dopamine through a D1 receptor linked to adenylate cyclase. Both exogenously applied dopamine and endogenously released dopamine uncoupled the horizontal cells, but the effect was pH-gated whereby it occurred only at an extracellular pH 7.2±0.05. The horizontal cells also uncoupled in acidic media (pH 7.0 or below) in the absence of dopamine. Our results show that horizontal cell coupling in the mammalian retina is regulated by both dopamine and pH. Given that the pH in the outer retina varies with the metabolic activity of the photoreceptors, these results suggest that ambient light conditions could gate the activity of neurotransmitters through pH-sensitive mechanisms.

Almost all ganglion cells in the rabbit retina project to the superior colliculus

After a localized injection of horseradish peroxidase into the superior colliculus of the rabbit almost all of the ganglion cells in the monocular portion of the contralateral retina were retrogradely labelled with the enzyme. Of the few ganglion cells which were not labelled, most had large cell bodies.

Endogenous dopaminergic regulation of horizontal cell coupling in the mammalian retina

Horizontal cells in an isolated wholemount preparation of the mouse retina were injected with Lucifer yellow and neurobiotin to characterize both the pattern of gap junctional connectivity and its regulation by dopamine. The injected horizontal cells had a uniform morphology of a round cell body, a compact dendritic tree, and an axon, which could sometimes be traced to an expansive terminal system. The dendro‐dendritic gap junctions between neighboring cells mediated both weak Lucifer yellow dye coupling and strong neurobiotin tracer coupling. The extent of the tracer coupling was decreased by either exogenous dopamine (100 μM) or cyclic adenosine monophosphate (cAMP) analogs and was significantly increased by the D1 antagonist SCH 23390 (10 μM). These results provide the first evidence in the mammalian retina that the gap junctions between horizontal cells are endogenously regulated by dopamine, which acts through D1 receptors to increase the intracellular cAMP. It has been proposed that the gap junctional coupling between horizontal cells is mediated by connexin 32 (Cx32), but the pattern and dopaminergic regulation of horizontal cell coupling were unaffected in Cx32‐knockout mice, ruling out the possible involvement of Cx32. Every tracer‐coupled horizontal cell showed calbindin immunoreactivity, and vice versa, providing strong evidence that the horizontal cells in the mouse retina comprise a single cell type. Like the axonless horizontal cells in other mammalian retinas, the axon‐bearing horizontal cells in the mouse retina are coupled by gap junctions that are permeable to Lucifer yellow and dopamine sensitive, suggesting that the mouse horizontal cells have hybrid properties to compensate for the absence of axonless horizontal cells. J. Comp. Neurol. 418:33–40, 2000.

Local Edge Detectors: A Substrate for Fine Spatial Vision at Low Temporal Frequencies in Rabbit Retina

Visual acuity is limited by the size and density of the smallest retinal ganglion cells, which correspond to the midget ganglion cells in primate retina and the β-ganglion cells in cat retina, both of which have concentric receptive fields that respond at either light-On or light-Off. In contrast, the smallest ganglion cells in the rabbit retina are the local edge detectors (LEDs), which respond to spot illumination at both light-On and light-Off. However, the LEDs do not predominate in the rabbit retina and the question arises, what role do they play in fine spatial vision? We studied the morphology and physiology of LEDs in the isolated rabbit retina and examined how their response properties are shaped by the excitatory and inhibitory inputs. Although the LEDs comprise only ∼15% of the ganglion cells, neighboring LEDs are separated by 30–40 μm on the visual streak, which is sufficient to account for the grating acuity of the rabbit. The spatial and temporal receptive-field properties of LEDs are generated by distinct inhibitory mechanisms. The strong inhibitory surround acts presynaptically to suppress both the excitation and the inhibition elicited by center stimulation. The temporal properties, characterized by sluggish onset, sustained firing, and low bandwidth, are mediated by the temporal properties of the bipolar cells and by postsynaptic interactions between the excitatory and inhibitory inputs. We propose that the LEDs signal fine spatial detail during visual fixation, when high temporal frequencies are minimal.