Anomalous rectification in horizontal cells.

1. Horizontal cells are electrically coupled through gap junctions. This is a disadvantage in elucidating the membrane properties of the cells. In order to block gap junctions, adenosine 3',5'-cyclic monophosphate (cyclic AMP) or its analogues, dibutyryl cyclic AMP and 8-bromo cyclic AMP, were ionophoretically injected into horizontal cells of the carp or turtle retina. 2. Before injection of the chemicals the input resistance of the cell was so low as to be unmeasurable, because the applied current leaked through gap junctions. After injection, however, the input resistance was significantly increased. 3. After the injection dye-coupling between horizontal cells was not observed when examined by intracellular injection of Lucifer Yellow dye, supporting the idea that high concentrations of intracellular cyclic AMP block gap junctions. 4. In this situation responses to light delivered to the receptive field centre were increased in amplitude, while responses to light delivered to the receptive field surround were greatly diminished. 5. After injection horizontal cells were readily polarized by conventional intracellular current injection. The hyperpolarizing light responses in carp and turtle luminosity-type cells (H1 cells) could be reversed by depolarizing the horizontal cells, and the reversal potentials were estimated to be about 0 mV. In addition, the resistance increase which accompanied the hyperpolarizing light responses could be detected. 6. In turtle biphasic chromaticity-type horizontal cells (H2 cells), hyperpolarizing light responses to shorter wavelengths and depolarizing ones to longer wavelengths could be reversed by depolarizing the horizontal cells. Both responses have almost the same reversal potential at about 0 mV. The membrane resistance changes associated with light responses were also detected; the resistance increased during the hyperpolarizing response, while it decreased during the depolarizing response. These observations suggest that the ionic mechanisms of both responses are probably the same, irrespective of their polarities.

The influence of HEPES on light responses of rabbit horizontal cells

The effect of cellular dialysis associated with whole-cell recording was studied in 24 developing hypoglossal motoneurons in a rat brainstem slice preparation. In all cases, establishing whole-cell continuity with the electrode solution resulted in an increase in the input resistance measured in current clamp. The mean magnitude of this increase was 39.7% and the time course of the maximum effect was quite variable. There was no correlation found between the time to maximum effect and the magnitude of the increase in resistance. These data indicate that the passive membrane properties are not constant during whole-cell recording in mammalian motoneurons.

1. Simultaneous intracellular recordings of responses to light flashes were obtained from rod-horizontal cell and rod-hyperpolarizing bipolar cell pairs in isolated retinae of the toad. The gain and temporal filtering of synaptic transfer were characterized throughout the rods' range of light responses. 2. Paired rod-horizontal cell and rod-bipolar cell responses to dim flashes (less than 0.4 Rh*, where Rh* denotes effective photoisomerizations per rod per flash) exhibited nearly the same time course. Analysis of the onset of the horizontal cell responses revealed a temporal lag equivalent to a single stage of low-pass filtering (tau f = 75-200 ms). No filtering was discerned in the transfer of dim-flash responses from rods to bipolars. On average, horizontal cells were five times as sensitive (mV/Rh*) and hyperpolarizing bipolar cells 10.7 times as sensitive as their paired rods. 3. For brighter flashes, up to 1600 Rh*, the rising and return phases of bipolar responses appeared to be simple scaled versions of the rod responses. The scaling factor was equal to the ratio of flash sensitivities for dim flashes. Rod responses greater than about 2 mV produced a saturation of the bipolar cell response. 4. The return phases of the horizontal cell responses were kinetically similar, scaled versions of the rod responses for rod potentials less than about 5 mV. However, the rising phases lagged significantly behind those of the rod. The effective time constant of the lag increased proportionally with flash intensity. For the brighter flashes, the horizontal cell response peaked as much as a second after the rod response. 5. The linear scaling, minimal temporal filtering and saturation of the bipolar cell responses were satisfactorily reproduced by a model of synaptic transfer that assumed that the rate of transmitter release followed the rod voltage exponentially and that the postsynaptic conductance followed Michaelis-Menten saturation (Falk & Fatt, 1972). 6. The progressively longer lag in the horizontal cell responses to brighter flashes was satisfactorily simulated by a kinetically limited Falk and Fatt model which postulated that the effective electrical time constant of the horizontal cell membrane strongly depended on synaptic or voltage-modulated conductances. 7. Satisfactory model simulations of all postsynaptic responses required that an e-fold change in the release rate of transmitter from the rod be obtained with a 2 mV change in the rod potential.

There are two types of horizontal cells, the luminosity and the chromaticity cells, in the retina of the kissing gourami, Helostoma rudolfi. Luminosity cells occupy the outermost layer proximal to the receptor terminals, whereas chromaticity cells form a layer proximal to the layer of luminosity cells. Neither type of cell has axons. Responses were evoked by light from red and green light-emitting diodes. The two stimuli were modulated either by a pulsatile or a white-noise signal. The luminosity cell always produced a hyperpolarizing response. The chromaticity cell produced a hyperpolarizing response when stimulated by only one color. However, in the presence of a steady or modulated green input, a red stimulus produced a depolarizing response. Such chromaticity cells were similar to the (spectral) biphasic chromaticity horizontal cells observed in other retinae. The depolarizing phase of the red response was produced by the balance of intensity of the two inputs, red and green. We used white-noise methodology to identify the dynamics of the horizontal cell's modulation response by taking advantage of the fact that a Wiener kernel is a measure of a cell's incremental sensitivity, which includes its response dynamics. Under all conditions, a steady state modulation response by both luminosity and chromaticity cells always was related linearly to the input modulation. The average mean square error (MSE) of the model predicted by the first-order kernel was approximately 8% for both luminosity (n = 116) and chromaticity (n = 23) cells. In some cases, the MSE was a few percent even when the peak-to-peak response amplitude was nearly 30 mV. The ratio of inputs from red and green cones to both types of horizontal cells was variable; the major input for luminosity cells came from red cones, whereas the major input for chromaticity cells came from green cones. First-order kernels generated by the major input were robust in terms of waveform in the sense that the waveform remained unchanged whether or not there was a steady or modulated illumination by the opposing color. The results reported here do not address the question of the neural circuitry that generates horizontal cell responses, in particular, the depolarizing response. However, whatever that circuitry might be, the high degree of linearity of the modulation response by both types of cell under various stimulus conditions imposes restrictions on the performance of any proposed model as well as on mechanisms that underlie the generation of the horizontal cell response.

Horizontal. cell dynamics: What are the main factors?

Postsynaptic to photoreceptors, horizontal cells face prolonged exposure to glutamate in the dark. Therefore, efficient hyperpolarizing mechanisms are crucial to keep horizontal cells within an operating range and to reduce glutamate-induced excitotoxicity. Combining electrophysiology, single-cell reverse transcriptase polymerase chain reaction, and immunocytochemistry, we found that horizontal cell bodies but not their axon terminals express the ether-à-gogo-related gene isoform 1 (erg1) K(+) channel. Erg1-mediated outward currents displayed voltage-dependent activation and C-type inactivation. Recovery from inactivation involved a transient open state. Gating of erg1 channels kept the voltage response to glutamate brief and at physiological amplitudes. With erg1 channels blocked, the response of horizontal cells to the onset of darkness was significantly enhanced. These results indicate a functional dichotomy between horizontal cell bodies and axon terminals in the processing of photoreceptor signals. The dark response thus reflects a finely tuned balance determined by the successive gating of ionotropic glutamate receptors and erg1 channels.

Horizontal cells from the retinas of white perch were isolated and maintained in cell culture for 3 days to 3 wk. Four morphologically distinct types of horizontal cells could be identified in culture and were labeled types H1, H2, H3, and H4. Whole-cell patch-clamp techniques were used to study the ionic currents present in the four cell types. In all cells, depolarizing commands above threshold elicited a fast-inward current followed by an outward current. The fast-inward current was abolished by tetrodotoxin (TTX) or 0 Na+ Ringer's, indicating the current was carried by Na+. In H1, H2, and H3 cells, the outward current, carried by K+, consisted of two components: a transient current (IA), blockable with 4-aminopyridine (4-AP), tetraethylammonium (TEA), or intracellular cesium and a sustained current that could be blocked with TEA. The H4 cell had only the sustained current. An inward rectifying K+ current (anomalous rectifier) was observed in the four cell types. The current was sensitive to the extracellular K+ concentration. Its activation showed two components: an instantaneous component and a slower component. The slow component becomes faster with greater hyperpolarizations. The four cell types possessed a small, sustained Ca2+ current that, under normal conditions, was masked by the inward Na+ current and outward K+ currents.

Under awake and idling conditions, spontaneous intracellular membrane voltage is characterized by large, synchronous, low-frequency fluctuations. Although these properties reflect correlations in synaptic inputs, intrinsic membrane properties often indicate voltage-dependent changes in membrane resistance and time constant values that can amplify and help to generate low-frequency voltage fluctuations. The specific contribution of intrinsic and synaptic factors to the generation of spontaneous fluctuations, however, remains poorly understood. Using visually guided intracellular recordings of somatosensory layer 2/3 pyramidal cells and interneurons in awake male and female mice, we measured the spectrum and size of voltage fluctuation and intrinsic cellular properties at different voltages. In both cell types, depolarizing neurons increased the size of voltage fluctuations. Amplitude changes scaled with voltage-dependent changes in membrane input resistance. Because of the small membrane time constants observed in both pyramidal cells and interneuron cell bodies, the low-frequency content of membrane fluctuations reflects correlations in the synaptic current inputs rather than significant filtering associated with membrane capacitance. Further, blocking synaptic inputs minimally altered somatic membrane resistance and time constant values. Overall, these results indicate that spontaneous synaptic inputs generate a low-conductance state in which the amplitude, but not frequency structure, is influenced by intrinsic membrane properties.SIGNIFICANCE STATEMENT In the absence of sensory drive, cortical activity in awake animals is associated with self-generated and seemingly random membrane voltage fluctuations characterized by large amplitude and low frequency. Partially, these properties reflect correlations in synaptic input. Nonetheless, neurons express voltage-dependent intrinsic properties that can potentially influence the amplitude and frequency of spontaneous activity. Using visually guided intracellular recordings of cortical neurons in awake mice, we measured the voltage dependence of spontaneous voltage fluctuations and intrinsic membrane properties. We show that voltage-dependent changes in membrane resistance amplify synaptic activity, whereas the frequency of voltage fluctuations reflects correlations in synaptic inputs. Last, synaptic activity has a small impact on intrinsic membrane properties in both pyramidal cells and interneurons.

Effects of the major glutamate receptor agonists, kainate (KA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), quisqualate (QA), N-methyl-D-aspartate (NMDA), L-alpha-amino-4-phosphonobutyrate (L-AP4), and trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on horizontal cells (HCs) were studied in superfused larval tiger salamander retina. 20 microM of KA, AMPA, and QA mimicked the action of 3 mM glutamate in the absence and presence of 1 mM Co2+. 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the actions of KA and AMPA, but not those of QA and glutamate, indicative of the existence of CNQX-resistant QA receptors in the tiger salamander HCs. Prolonged application of ACPD hyperpolarized the HCs and enhanced the light responses, probably by shifting the resting HC voltage (Er) to a more hyperpolarized position. It is possible that the KA, AMPA, and CNQX-resistant QA receptors are involved in mediating the postsynaptic light responses in HCs, and ACPD receptors are involved in sensitivity adjustment of the HC responses.

Both high- and low voltage-activated calcium currents contribute to the light-evoked responses of luminosity horizontal cells in the Xenopus retina

The striatum of the lamprey, the first vertebrate group to appear in evolution, shows striking similarities to that of mammals with respect to histochemical markers, afferent and efferent projections and the effect of dopamine depletion, which leads to hypokinetic motor symptoms. The cellular properties of lamprey striatal neurons were studied here using patch-clamp recordings in acute striatal slices. Sixty-five per cent of recorded neurons were characterised by a prominent inward rectification due to a K+ conductance of the Kir type. They had a ramping response with a long delay to the first action potential due to activation of a low-voltage-activated A-type K+ current. Many such inwardly rectifying neurons (IRNs) had a hyperpolarised resting membrane potential and some had spiny dendrites. The remaining 35% of the neurons (non-IRNs) represent a heterogeneous group, including some with characteristics similar to the fast-spiking interneuron of the mammalian striatum. They showed short-lasting, large after hyperpolarisations (AHPs) and discharged action potentials at high frequency. None of the recorded neurons were spontaneously active but they received GABAergic and glutamatergic synaptic input. The fact that most lamprey striatal neurons display inward rectification indicates that this is a conserved characteristic of striatal neurons throughout vertebrate phylogeny. This is a cellular property of critical importance for the operations of the striatum in mammals.

The off-overshoot responses of photoreceptors and horizontal cells in the light-adapted retinas of the tiger salamander

In mature animals histamine infusion typically causes an H1-mediated increase and H2-mediated decrease in pulmonary vascular resistance (PVR). Moreover, low histamine concentrations can cause H1-mediated relaxation of vascular strips in mature animals, and in newborn animals histamine infusion causes only H1-mediated decreases in PVR. The mechanisms responsible for the different H1-mediated responses are unknown. We used an inflow-outflow occlusion technique to identify the sites of H1- and H2-mediated responses in lungs of developing lambs. Histamine was infused at 1.0 and 10.0 micrograms.kg-1.min-1 in control and H1- and H2-blocked lungs of newborn and juvenile lambs under "normoxic" and hypoxic conditions and in hypoxic H2-blocked lungs of mature sheep. In newborns histamine caused significant H1-mediated decreases in resistance across the arterial (delta Pa) and middle (delta Pm) segments of the circuit during both normoxia and hypoxia. In normoxic juveniles low-dose histamine caused H1-mediated decreases in the resistance across delta Pa and delta Pm, but the resistances across delta Pm rose above baseline at the higher dose. The venous segment exhibited only a high-dose increase in resistance. During hypoxia, the high-dose H1-mediated pressor response of delta Pm was attenuated compared with that during normoxia; however, the increase in venous resistance was unaffected. In hypoxic mature sheep, no low dose H1-mediated decrease in segmental resistances was seen, but at the higher dose an increase in all resistances occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Bipolar-amacrine transmission in the carp retina