GABAA and GABAC receptors on mammalian rod bipolar cells

Light-evoked responses of bipolar cells in a mammalian retina

We recorded light-evoked responses from rod and cone bipolar cells using patch-clamp techniques in a slice preparation of the rat retina. Rod bipolar cells responded to light with a sustained depolarization (ON response) followed at light offset by a slight hyperpolarization. ON and OFF cone bipolar cells were encountered, both with diverse temporal properties. The responses of rod bipolar cells were composed primarily of two components, a nonspecific cation current and a chloride current. The chloride current was reduced greatly in axotomized cells and could be suppressed by coapplication of the GABA(A) antagonist bicuculline and the GABA(C) antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid. This suggests that it largely reflects feedback from GABAergic amacrine cells. The response latency of intact rod bipolar cells was shorter than that of the axotomized cells, and the sensitivity curve covered more than twice the dynamic range. Application of the GABA receptor antagonists partially mimicked the effects of axotomy. These findings suggest that functional properties of the axon terminal system-notably synaptic feedback from amacrine cells-play an important role in defining the response properties of mammalian bipolar cells.

Surround inhibition of mammalian AII amacrine cells is generated in the proximal retina

1. Intracellular recordings were obtained from neurons in the superfused retina-eyecup preparation of the rabbit under dark-adapted conditions. Neurotransmitter agonists and antagonists were applied exogenously via the superfusate to dissect the synaptic pathways pharmacologically and thereby determine those pathways responsible for the generation of the on-centre/off-surround receptive fields of AII amacrine cells. 2. Application of the metabotropic glutamate receptor agonist, APB, reversibly blocked both the on-centre and off-surround responses of AII cells. These data were consistent with the idea that both the centre- and surround-mediated responses are derived from inputs from the presynaptic rod bipolar cells. 3. Whereas rod bipolar cells showed on-receptive fields approximately 100 microm across, we found no evidence for an antagonistic off-surround response using light stimuli which effectively elicited the off-surrounds of AII amacrine cells. These results indicated that the surrounds of AII cells are not derived from rod bipolar cell inputs. 4. Application of the ionotropic glutamate receptor antagonists CNQX or DNQX enhanced the on-centre responses of AII cells but attenuated the off-surround responses. These data indicated that the centre- and surround-mediated responses could not both be derived from signals crossing the rod bipolar-to-AII cell synapse. 5. Application of the glycine antagonist, strychnine, had only minor and variable effects on AII cell responses. However, the GABA antagonists picrotoxin and bicuculline enhanced the on-centre response but attenuated or completely blocked the off-surround response of AII cells. The GABA antagonists had no effect on the responses of horizontal cells indicating that their effects on AII cell responses reflected actions on inner retinal circuitry rather than feedback circuitry in the outer plexiform layer. 6. Application of the voltage-gated sodium channel blocker TTX enhanced the on-centre responses of AII cells but attenuated or abolished their off-surround responses. 7. Taken together, our results suggest that the on-centre responses of AII cells result from the major excitatory drive from rod bipolar cells. However, the surround receptive fields of AII cells appear to be generated by lateral, inhibitory signals derived from neighbouring GABAergic, on-centre amacrine cells. A model is presented whereby the S1 amacrine cells produce the surround receptive fields of AII amacrine cells via inhibitory, feedback circuitry to the axon terminals of rod bipolar cells.

Immunocytochemical localization of cannabinoid CB1 receptor and fatty acid amide hydrolase in rat retina

Cannabinoids have major effects on central nervous system function. Recent studies indicate that cannabinoid effects on the visual system have a retinal component. Immunocytochemical methods were used to localize cannabinoid CB1 receptor immunoreactivity (CB1R-IR) and an endocannabinoid (anandamide and 2-arachidonylglycerol) degradative enzyme, fatty acid amide hydrolase (FAAH)-IR, in the rat retina. Double labeling with neuron-specific markers permitted identification of cells that were labeled with CB1R-IR and FAAH-IR. CB1R-IR was observed in all cells that were protein kinase C-immunoreactive (rod bipolar cells and a subtype of GABA-amacrine cell) as well as horizontal cells (identified by calbindin-IR). There was also punctate CB1R-IR in the distal one-third of the inner plexiform layer (IPL) that could not be assigned to a cell type. FAAH-IR was most prominent in large ganglion cells, whose dendrites projected to a narrow band in the proximal IPL. Weaker FAAH-IR was observed in the soma of horizontal cells (identified by calbindin-IR); the soma of large, but not small, dopamine amacrine cells (identified by tyrosine hydroxylase-IR); and dendrites of orthotopic- and displaced-starburst amacrine cells (identified by choline acetyltransferase-IR) but in less than 50% of the starburst amacrine cell somata. The extensive distribution of CB1R-IR on horizontal cells and rod bipolar cells indicates a role of endocannabinoids in scotopic vision, whereas the more widespread distribution of FAAH-IR indicates a complex control of endocannabinoid release and degradation in the retina.

Expression of the voltage-gated chloride channel ClC-2 in rod bipolar cells of the rat retina

Voltage-gated chloride channels (ClC) are highly conserved during evolution and appear to participate in a variety of physiological functions. Recently, ClC-2 was proposed to play a role in stabilizing the chloride equilibrium potential near or below the resting membrane potential in neurons expressing ligand-gated chloride channels. Because rod bipolar cells in mammalian retina express three forms of inhibitory ligand-gated chloride channels, we decided to study ClC-2 localization and function in the rat retina. RNA encoding ClC-1, -2, -3, -4, and -5 was detected by reverse transcription-PCR in the rat retina. ClC-2-specific antibodies identified protein on cell bodies and in synaptic layers. Double-immunofluorescence staining revealed that intense ClC-2 immunoreactivity colocalized with PKC-stained rod bipolar cells. Patch-clamp experiments performed with individual rod bipolar cells demonstrated the presence of a time-dependent, inwardly rectified current activated at hyperpolarizing membrane potentials. This current demonstrated selectivity for different anions (Cl(-) > I(-) > gluconate), was inhibited by Cd(2+), and was minimally reduced by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid. These features are consistent with currents generated by ClC-2 channels. Our data indicate that functional ClC-2 channels are present in retinal rod bipolar cells and support a role for ClC-2 in maintaining Cl(-) homeostasis in neurons with ligand-gated chloride channels.

Indoleamine-accumulating amacrine cells are presynaptic to rod bipolar cells through GABA(C) receptors

gamma-Aminobutyric acid (GABA), is a main source of inhibitory modulation of the rod pathway in the mammalian retina. The authors previously showed that rod bipolar cells express at least three types of ionotropic GABA receptors. Here, the authors sought to determine which neurons are the presynaptic partners at these synapses in the rabbit retina. Indoleamine-accumulating amacrine cells (IACs) were immunolabeled with an antiserum against serotonin (5HT) in vertical sections and wholemounts of rabbit retinae that had been preloaded with 5HT. The tissue was double labeled for the rho subunits of the GABA(C) receptor or the alpha3 subunit of the GABA(A) receptor. Punctate immunofluorescence was observed for both receptor subunits and was found to coincide with the dendrites and varicosities of IACs. The localization of rho subunits was examined at the ultrastructural level by using postembedding techniques on slam-frozen, cryosubstituted tissue. Double labeling at the electron microscopic level revealed that 5HT-immunoreactive processes were presynaptic to rod bipolar cells through GABA(C) receptors. Intracellular injection of the two morphologic subclasses of IAC amacrine cells, S1 and S2, with Lucifer yellow followed by immunolabeling for the alpha3 or rho subunits revealed that varicosities on the dendrites of both cell types were in register with alpha3- and rho-immunoreactive puncta. Taken together, these results suggest that IACs are presynaptic to rod bipolar cells through GABA(C) receptors and possibly through GABA(A) receptors.