Shigang He

Large-scale morphological survey of mouse retinal ganglion cells

Five hundred twenty ganglion cells in an isolated whole‐mount preparation of the mouse retina were labeled using the “DiOlistic” method (Gan et al. [ 2000 ] Neuron 27:219–225) and were classified according to their morphological properties. Tungsten particles coated with a lipophilic dye (DiI) were propelled into the whole‐mount retina using a gene gun. When a dye‐coated particle contacted the cell membrane, the entire cell was labeled. The ganglion cells were classified into four groups based on their soma size, dendritic field size, and pattern and level of stratification. Broadly monostratified cells were classified into three groups: RGA cells (large soma, large dendritic field), RGB cells (small to medium‐sized soma, small to medium‐sized dendritic field), and RGC cells (small to medium‐sized size soma, medium‐sized to large dendritic field). Bistratified cells were classified as RGD. This study represents the most complete morphological classification of mouse retinal ganglion cells available to date and provides a foundation for further understanding of the correlation of physiology and morphology and ganglion cell function with genetically manipulated animals. J. Comp. Neurol. 451:115–126, 2002.

Retinal direction selectivity after targeted laser ablation of starburst amacrine cells

Directionally selective retinal ganglion cells respond strongly when a stimulus moves in their preferred direction, but respond little or not at all when it moves in the opposite direction. This selectivity represents a classic paradigm of computation by neural microcircuits, but its cellular mechanism remains obscure. The directionally selective ganglion cells receive many synapses from a type of amacrine cell termed ‘starburst’ because of its regularly spaced, evenly radiating dendrites. Starburst amacrine cells have a synaptic asymmetry that has been proposed as the source of the directional response in the ganglion cells. Here we report experiments that make this unlikely, and offer an alternative concept of the function of starburst cells. We labelled starburst cells in living retinas, then killed them by targeted laser ablation while recording from individual directionally selective ganglion cells. Ablating starburst cells revealed no asymmetric contribution to the ganglion cell response. Instead of being direction discriminators, the starburst cells appear to potentiate generically the responses of ganglion cells to moving stimuli. The origin of direction selectivity probably lies with another type of amacrine cell.

Identification of ON-OFF direction-selective ganglion cells in the mouse retina

We identified the ON–OFF direction‐selective ganglion cells (DSGCs) in the mouse retina and characterized their physiological, morphological and pharmacological properties. These cells showed transient responses to the onset and termination of a stationary flashing spot, and strong directional selectivity to a moving rectangle. Application of various pharmacological reagents demonstrated that the ON–OFF DSGCs in the mouse retina utilize a similar array of transmitters and receptors to compute motion direction to their counterparts in the rabbit retina. Voltage clamp recording showed that ON–OFF DSGCs in the mouse retina receive a larger inhibitory input when the stimulus is moving in the null direction and a larger excitatory input when the stimulus is moving in the preferred direction. Finally, intracellular infusion of neurobiotin revealed a bistratified dendritic field with recursive dendrites forming loop‐like structures, previously classified as RGD2 by morphology. Overall, the ON–OFF DSGCs in the mouse retina exhibit almost identical properties to their counterparts in the rabbit retina, indicating that the mechanisms for computing motion direction are conserved from mouse to rabbit, and probably also to higher mammals. This first detailed characterization of ON–OFF DSGCs in the mouse retina provides fundamental information for further study of maturation and regulation of the neuronal circuitry underlying computation of direction.

Change detection by thalamic reticular neurons

The thalamic reticular nucleus (TRN) is thought to function in the attentional searchlight. We analyzed the detection of deviant acoustic stimuli by TRN neurons and the consequences of deviance detection on the TRN target, the medial geniculate body (MGB) of the rat. TRN neurons responded more strongly to pure-tone stimuli presented as deviant stimuli (low appearance probability) than those presented as standard stimuli (high probability) (deviance-detection index = 0.321). MGB neurons also showed deviance detection in this procedure, albeit to a smaller extent (deviance-detection index = 0.154). TRN neuron deviance detection either enhanced (14 neurons) or suppressed (27 neurons) MGB neuronal responses to a probe stimulus. Both effects were neutralized by inactivation of the auditory TRN. Deviance modulation effects were cross-modal. Deviance detection probably causes TRN neurons to transiently deactivate surrounding TRN neurons in response to a fresh stimulus, altering auditory thalamus responses and inducing attention shift.

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.

Modulation of coupling between retinal horizontal cells by retinoic acid and endogenous dopamine

The regulation of electrical coupling between retinal neurons appears to be an important component of the neuronal mechanism of light adaptation, which enables the retina to operate efficiently over a broad range of light intensities. The information about the ambient light conditions has to be transmitted to the neuronal network of the retina and previous evidence has indicated that dopamine is an important neurochemical signal. In addition, recent studies suggest that another important chemical signal is retinoic acid, which is a light-correlated byproduct of the phototransduction cycle. This review summarizes the latest findings about the effects of dopamine and retinoic acid on gap junctional coupling in the retinas of mouse, rabbit and fish.

ON direction-selective ganglion cells in the mouse retina

Two types of ganglion cells (RGCs) compute motion direction in the retina: the ON–OFF direction‐selective ganglion cells (DSGCs) and the ON DSGCs. The ON DSGCs are much less studied mostly due to the low encounter rate. In this study, we investigated the physiology, dendritic morphology and synaptic inputs of the ON DSGCs in the mouse retina. When a visual stimulus moved back and forth in the preferred–null axis, we found that the ON DSGCs exhibited a larger EPSC when the visual stimulus moved in the preferred direction and a larger IPSC in the opposite, or null direction, similar to what has been found in ON–OFF DSGCs. This similar synaptic input pattern is in contrast to other well‐known differences, namely: profile of velocity sensitivity, distribution of preferred directions, and different central projection of the axons. Immunohistochemical staining showed that the dendrites of ON DSGCs exhibited tight cofasciculation with the cholinergic plexus. These findings suggest that cholinergic amacrine cells may play an important role in generating direction selectivity in the ON DSGCs, and that the mechanism for coding motion direction is probably similar for the two types of DSGCs in the retina.

Receptive field microstructure and dendritic geometry of retinal ganglion cells.

We studied the fine spatial structure of the receptive fields of retinal ganglion cells and its relationship to the dendritic geometry of these cells. Cells from which recordings had been made were microinjected with Lucifer yellow, so that responses generated at precise locations within the receptive field center could be directly compared with that cells dendritic structure. While many cells with small receptive fields had domeshaped sensitivity profiles, the majority of large receptive fields were composed of multiple regions of high sensitivity. The density of dendritic branches at any one location did not predict the regions of high sensitivity. Instead, the interactions between a ganglion cells dendritic tree and the local mosaic of bipolar cell axons seem to define the fine structure of the receptive field center.

RETINOIC ACID MODULATES GAP JUNCTIONAL PERMEABILITY BETWEEN HORIZONTAL CELLS OF THE MAMMALIAN RETINA

In the retina, all‐trans retinoic acid (at‐RA) could function as a light signal because its production increases with the level of illumination. Given the well‐established effects of retinoic acid on cell coupling in other tissues, it is possible that the changing levels of at‐RA modulate the gap junctional permeability between retinal neurons. This study examines the effects of retinoic acid on horizontal cell coupling, which is known to be modulated by the ambient light level. Single horizontal cells were injected under visual control with either Neurobiotin (mouse retina) or Lucifer yellow (rabbit retina) and the extent of tracer coupling or dye coupling was used to monitor the gap junctional permeability. In the mouse retina, the injection of Neurobiotin revealed a network of ≈ 150–250 tracer‐coupled horizontal cells. The tracer coupling was completely abolished by incubating the retina in 150 μm at‐RA for 35 min. In the rabbit retina, the injection of Lucifer yellow into A‐type horizontal cells revealed networks of ≈ 15–30 dye‐coupled horizontal cells. Incubation in 150 μm at‐RA reduced the dye coupling within 12 min and complete uncoupling was achieved after 35 min. The uncoupling effects of at‐RA in the mouse and rabbit retinas were concentration‐ and time‐dependent and they were reversible after washout. The coupling was not affected by either the 9‐cis form of retinoic acid or by at‐RA that had been isomerized by intensive light. The uncoupling effect of at‐RA persisted following treatment with a D1 receptor antagonist and thus was dopamine‐independent. This study has established that at‐RA is able to modulate the gap junctional permeability between horizontal cells in the mammalian retina, where its light‐dependent release has already been demonstrated.

ON direction-selective ganglion cells in the rabbit retina: Dendritic morphology and pattern of fasciculation

ON direction-selective (DS) ganglion cells were identified by electrophysiological recordings in DAPI labeled, isolated rabbit retinas. Their responses to a flashing spot were sustained. Their responses to moving stimuli were strong in the preferred direction and weak in the null direction. Injection of the recorded cells with Lucifer yellow revealed that the cells had a distinct dendritic morphology, consistent with that described previously (Buhl & Peichl, 1986; Amthor et al., 1989; Famiglietti, 1992a). When neighboring cells were injected, an extensive dendritic co-fasciculation was observed. The pattern of fasciculation restricts the possible synaptic connections of the ON DS cell.