Transient and sustained depolarization of retinal ganglion cells by Ih

A single retinal circuit model for multiple computations

Vision is dependent on extracting intricate features of the visual information from the outside world, and complex visual computations begin to take place as soon as at the retinal level. In multiple studies on salamander retinas, the responses of a subtype of retinal ganglion cells, i.e., fast/biphasic-OFF ganglion cells, have been shown to be able to realize multiple functions, such as the segregation of a moving object from its background, motion anticipation, and rapid encoding of the spatial features of a new visual scene. For each of these visual functions, modeling approaches using extended linear-nonlinear cascade models suggest specific preceding retinal circuitries merging onto fast/biphasic-OFF ganglion cells. However, whether multiple visual functions can be accommodated together in a certain retinal circuitry and how specific mechanisms for each visual function interact with each other have not been investigated. Here, we propose a physiologically consistent, detailed computational model of the retinal circuit based on the spatiotemporal dynamics and connections of each class of retinal neurons to implement object motion sensitivity, motion anticipation, and rapid coding in the same circuit. Simulations suggest that multiple computations can be accommodated together, thereby implying that the fast/biphasic-OFF ganglion cell has potential to output a train of spikes carrying multiple pieces of information on distinct features of the visual stimuli.

Activation of group I metabotropic glutamate receptors regulates the excitability of rat retinal ganglion cells by suppressing Kir and I h

Group I metabotropic glutamate receptor (mGluR I) activation exerts a slow postsynaptic excitatory effect in the CNS. Here, the issues of whether and how this receptor is involved in regulating retinal ganglion cell (RGC) excitability were investigated in retinal slices using patch-clamp techniques. Under physiological conditions, RGCs displayed spontaneous firing. Extracellular application of LY367385 (10 µM)/MPEP (10 µM), selective mGluR1 and mGluR5 antagonists, respectively, significantly reduced the firing frequency, suggesting that glutamate endogenously released from bipolar cells constantly modulates RGC firing. DHPG (10 µM), an mGluR I agonist, significantly increased the firing and caused depolarization of the cells, which were reversed by LY367385, but not by MPEP, suggesting the involvement of the mGluR1 subtype. Intracellular Ca²⁺-dependent PI-PLC/PKC and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways mediated the DHPG-induced effects. In the presence of cocktail synaptic blockers (CNQX, D-AP5, bicuculline, and strychnine), which terminated the spontaneous firing in both ON and OFF RGCs, DHPG still induced depolarization and triggered the cells to fire. The DHPG-induced depolarization could not be blocked by TTX. In contrast, Ba²⁺, an inwardly rectifying potassium channel (Kir) blocker, and Cs⁺ and ZD7288, hyperpolarization-activated cation channel (I _h) blockers, mimicked the effect of DHPG. Furthermore, in the presence of Ba²⁺/ZD7288, DHPG did not show further effects. Moreover, Kir and I _h currents could be recorded in RGCs, and extracellular application of DHPG indeed suppressed these currents. Our results suggest that activation of mGluR I regulates the excitability of rat RGCs by inhibiting Kir and I _h.

Electrical activity of ON and OFF retinal ganglion cells: a modelling study

OBJECTIVE

Retinal ganglion cells (RGCs) demonstrate a large range of variation in their ionic channel properties and morphologies. Cell-specific properties are responsible for the unique way RGCs process synaptic inputs, as well as artificial electrical signals such as that from a visual prosthesis. A cell-specific computational modelling approach allows us to examine the functional significance of regional membrane channel expression and cell morphology.

APPROACH

In this study, an existing RGC ionic model was extended by including a hyperpolarization activated non-selective cationic current as well as a T-type calcium current identified in recent experimental findings. Biophysically-defined model parameters were simultaneously optimized against multiple experimental recordings from ON and OFF RGCs.

MAIN RESULTS

With well-defined cell-specific model parameters and the incorporation of detailed cell morphologies, these models were able to closely reconstruct and predict ON and OFF RGC response properties recorded experimentally.

SIGNIFICANCE

The resulting models were used to study the contribution of different ion channel properties and spatial structure of neurons to RGC activation. The techniques of this study are generally applicable to other excitable cell models, increasing the utility of theoretical models in accurately predicting the response of real biological neurons.

Intrinsic physiological properties of rat retinal ganglion cells with a comparative analysis

Mammalian retina contains 15–20 different retinal ganglion cell (RGC) types, each of which is responsible for encoding different aspects of the visual scene. The encoding is defined by a combination of RGC synaptic inputs, the neurotransmitter systems used, and their intrinsic physiological properties. Each cell's intrinsic properties are defined by its morphology and membrane characteristics, including the complement and localization of the ion channels expressed. In this study, we examined the hypothesis that the intrinsic properties of individual RGC types are conserved among mammalian species. To do so, we measured the intrinsic properties of 16 morphologically defined rat RGC types and compared these data with cat RGC types. Our data demonstrate that in the rat different morphologically defined RGC types have distinct patterns of intrinsic properties. Variation in these properties across cell types was comparable to that found for cat RGC types. When presumed morphological homologs in rat and cat retina were compared directly, some RGC types had very similar properties. The rat A2 cell exhibited patterns of intrinsic properties nearly identical to the cat alpha cell. In contrast, rat D2 cells (ON-OFF directionally selective) had a very different pattern of intrinsic properties than the cat iota cell. Our data suggest that the intrinsic properties of RGCs with similar morphology and suspected visual function may be subject to variation due to the behavioral needs of the species.

Electrical synaptic input to ganglion cells underlies differences in the output and absolute sensitivity of parallel retinal circuits

Parallel circuits throughout the CNS exhibit distinct sensitivities and responses to sensory stimuli. Ambiguities in the source and properties of signals elicited by physiological stimuli, however, frequently obscure the mechanisms underlying these distinctions. We found that differences in the degree to which activity in two classes of Off retinal ganglion cell (RGC) encode information about light stimuli near detection threshold were not due to obvious differences in the cells' intrinsic properties or the chemical synaptic input the cells received; indeed, differences in the cells' light responses were largely insensitive to block of fast ionotropic glutamate receptors. Instead, the distinct responses of the two types of RGCs likely reflect differences in light-evoked electrical synaptic input. These results highlight a surprising strategy by which the retina differentially processes and routes visual information and provide new insight into the circuits that underlie responses to stimuli near detection threshold.

Colocalization of hyperpolarization-activated, cyclic nucleotide-gated channel subunits in rat retinal ganglion cells

The current-passing pore of mammalian hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels is formed by subunit isoforms denoted HCN1-4. In various brain areas, antibodies directed against multiple isoforms bind to single neurons, and the current (I(h)) passed during hyperpolarizations differs from that of heterologously expressed homomeric channels. By contrast, retinal rod, cone, and bipolar cells appear to use homomeric HCN channels. Here, we assess the generality of this pattern by examining HCN1 and HCN4 immunoreactivity in rat retinal ganglion cells, measuring I(h) in dissociated cells, and testing whether HCN1 and HCN4 proteins coimmunoprecipitate. Nearly half of the ganglion cells in whole-mounted retinae bound antibodies against both isoforms. Consistent with colocalization and physical association, 8-bromo-cAMP shifted the voltage sensitivity of I(h) less than that of HCN4 channels and more than that of HCN1 channels, and HCN1 coimmunoprecipitated with HCN4 from membrane fraction proteins. Finally, the immunopositive somata ranged in diameter from the smallest to the largest in rat retina, the dendrites of immunopositive cells arborized at various levels of the inner plexiform layer and over fields of different diameters, and I(h) activated with similar kinetics and proportions of fast and slow components in small, medium, and large somata. These results show that different HCN subunits colocalize in single retinal ganglion cells, identify a subunit that can reconcile native I(h) properties with the previously reported presence of HCN4 in these cells, and indicate that I(h) is biophysically similar in morphologically diverse retinal ganglion cells and differs from I(h) in rods, cones, and bipolar cells.

Modelling intrinsic electrophysiological properties of ON and OFF retinal ganglion cells

ON and OFF retinal ganglion cells (RGCs) display differences in their intrinsic electrophysiology: OFF cells maintain spontaneous activity in the absence of any input, exhibit subthreshold membrane potential oscillations, rebound excitation and burst firing; ON cells require excitatory input to drive their activity and display none of the aforementioned phenomena. The goal of this study was to identify and characterize ionic currents that explain these intrinsic electrophysiological differences between ON and OFF RGCs. A mathematical model of the electrophysiological properties of ON and OFF RGCs was constructed and validated using published patch-clamp data from isolated intact mouse retina. The model incorporates three ionic currents hypothesized to play a role in generating behaviors that are different between ON and OFF RGCs. These currents are persistent Na( + ), I (NaP), hyperpolarization-activated, I (h), and low voltage activated Ca(2 + ), I (T), currents. Using computer simulations of Hodgkin-Huxley type neuron with a single compartment model we found two distinct sets of I (NaP), I (h), I (T) conductances that correspond to ON and OFF RGCs populations. Simulations indicated that special properties of I (T) explain the differences in intrinsic electrophysiology between ON and OFF RGCs examined here. The modelling shows that the maximum conductance of I (T) is higher in OFF than in ON cells, in agreement with recent experimental data.

Hyperpolarization-activated current (I(h)) in ganglion-cell photoreceptors

Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin and serve as the primary retinal drivers of non-image-forming visual functions such as circadian photoentrainment, the pupillary light reflex, and suppression of melatonin production in the pineal. Past electrophysiological studies of these cells have focused on their intrinsic photosensitivity and synaptic inputs. Much less is known about their voltage-gated channels and how these might shape their output to non-image-forming visual centers. Here, we show that rat ipRGCs retrolabeled from the suprachiasmatic nucleus (SCN) express a hyperpolarization-activated inwardly-rectifying current (I_h). This current is blocked by the known I_h blockers ZD7288 and extracellular cesium. As in other systems, including other retinal ganglion cells, I_h in ipRGCs is characterized by slow kinetics and a slightly greater permeability for K⁺ than for Na⁺. Unlike in other systems, however, I_h in ipRGCs apparently does not actively contribute to resting membrane potential. We also explore non-specific effects of the common I_h blocker ZD7288 on rebound depolarization and evoked spiking and discuss possible functional roles of I_h in non-image-forming vision. This study is the first to characterize I_h in a well-defined population of retinal ganglion cells, namely SCN-projecting ipRGCs.

Mechanisms and distribution of ion channels in retinal ganglion cells: using temperature as an independent variable

Trains of action potentials of rat and cat retinal ganglion cells (RGCs) were recorded intracellularly across a temperature range of 7-37 degrees C. Phase plots of the experimental impulse trains were precision fit using multicompartment simulations of anatomically reconstructed rat and cat RGCs. Action potential excitation was simulated with a "Five-channel model" [Na, K(delayed rectifier), Ca, K(A), and K(Ca-activated) channels] and the nonspace-clamped condition of the whole cell recording was exploited to determine the channels' distribution on the dendrites, soma, and proximal axon. At each temperature, optimal phase-plot fits for RGCs occurred with the same unique channel distribution. The "waveform" of the electrotonic current was found to be temperature dependent, which reflected the shape changes in the experimental action potentials and confirmed the channel distributions. The distributions are cell-type specific and adequate for soma and dendritic excitation with a safety margin. The highest Na-channel density was found on an axonal segment some 50-130 microm distal to the soma, as determined from the temperature-dependent "initial segment-somadendritic (IS-SD) break." The voltage dependence of the gating rate constants remains invariant between 7 and 23 degrees C and between 30 and 37 degrees C, but undergoes a transition between 23 and 30 degrees C. Both gating-kinetic and ion-permeability Q10s remain virtually constant between 23 and 37 degrees C (kinetic Q10s = 1.9-1.95; permeability Q10s = 1.49-1.64). The Q10s systematically increase for T <23 degrees C (kinetic Q10 = 8 at T = 8 degrees C). The Na channels were consistently "sleepy" (non-Arrhenius) for T <8 degrees C, with a loss of spiking for T <7 degrees C.

Slow excitation of cultured rat retinal ganglion cells by activating group I metabotropic glutamate receptors

As in many CNS neurons, retinal ganglion cells (RGCs) receive fast synaptic activation through postsynaptic ionotropic receptors. However, the potential role of postsynaptic group I metabotropic glutamate receptors (mGluRs) in these neurons is unknown. In this study we first demonstrated that the selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) increased intracellular calcium concentration in neurons within the ganglion cell layer of the rat retina. This prompted us to use an immunopanned-RGC and cortical astroglia coculture preparation to explore the effect of group I mGluR activation on the electrophysiological properties of cultured RGCs. Using perforated patch-clamp recordings in current-clamp configuration, we found that application of DHPG increased spontaneous spiking and depolarized the resting membrane potential of RGCs. This boosting effect was attributed to an increase in membrane resistance due to blockade of a background K(+) conductance. Further experiments showed that the group I mGluR-sensitive K(+) conductance was not blocked by 3 mM Cs(+), but was sensitive to acidification. Pharmacological studies indicated that the effect of DHPG on RGCs was mediated by the mGluR1 rather than the mGluR5 receptor subtype. Our results suggest a facilitatory role for group I mGluR activation in modulating RGC excitability in the mammalian inner retina.

Ectopic expression of multiple microbial rhodopsins restores ON and OFF light responses in retinas with photoreceptor degeneration

By expressing channel rhodopsin-2 (ChR2) in inner retinal neurons, previous studies have demonstrated restoration of ON responses in the retina after the death of rod and cone photoreceptors. In this study, we report that the expression of halorhodopsin (HaloR), a light-driven chloride pump, can effectively restore OFF responses in inner retinal neurons of mice with retinal degeneration. We show that HaloR-expressing retinal ganglion cells respond to light with rapid hyperpolarization and suppression of spike activity. After termination of the light stimulus, their membrane potential exhibits a rapid rebound overshoot with robust sustained or transient spike firing. Furthermore, we show that coexpression of ChR2/HaloR in retinal ganglion cells can produce ON, OFF, and even ON-OFF responses, depending on the wavelength of the light stimulus. Our results suggest that the expression of multiple microbial rhodopsins such as ChR2 and HaloR is a possible strategy to restore both ON and OFF light responses in the retina after the death of rod and cone photoreceptors.

HCN4-like immunoreactivity in rat retinal ganglion cells

Antisera directed against hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels bind to somata in the ganglion cell layer of rat and rabbit retinas, and mRNA for different HCN channel isoforms has been detected in the ganglion cell layer of mouse retina. However, previous studies neither provided evidence that any of the somata are ganglion cells (as opposed to displaced amacrine cells) nor quantified these cells. We therefore tested whether isoform-specific anti-HCN channel antisera bind to ganglion cells labeled by retrograde transport of fluorophore-coupled dextran. In flat-mounted adult rat retinas, the number of dextran-backfilled ganglion cells agreed with cell densities reported in previous studies, and anti-HCN4 antisera bound to the somata of approximately 40% of these cells. The diameter of these somata ranged from 7 to 30 microm. Consistent with localization to cell membranes, the immunoreactivity formed a thin line that circumscribed individual somata. Optic fiber layer axon fascicles, and the proximal dendrites of some ganglion cells, also displayed binding of anti-HCN4 antisera. These results suggest that the response of some mammalian retinal ganglion cells to hyperpolarization may be modulated by changes in intracellular cAMP levels, and could thus be more complex than expected from previous voltage and current recordings.

High-pass filtering of input signals by the Ih current in a non-spiking neuron, the retinal rod bipolar cell

Hyperpolarization-activated cyclic nucleotide-sensitive (HCN) channels mediate the I(f) current in heart and I(h) throughout the nervous system. In spiking neurons I(h) participates primarily in different forms of rhythmic activity. Little is known, however, about its role in neurons operating with graded potentials as in the retina, where all four channel isoforms are expressed. Intriguing evidence for an involvement of I(h) in early visual processing are the side effects reported, in dim light or darkness, by cardiac patients treated with HCN inhibitors. Moreover, electroretinographic recordings indicate that these drugs affect temporal processing in the outer retina. Here we analyzed the functional role of HCN channels in rod bipolar cells (RBCs) of the mouse. Perforated-patch recordings in the dark-adapted slice found that RBCs exhibit I(h), and that this is sensitive to the specific blocker ZD7288. RBC input impedance, explored by sinusoidal frequency-modulated current stimuli (0.1-30 Hz), displays band-pass behavior in the range of I(h) activation. Theoretical modeling and pharmacological blockade demonstrate that high-pass filtering of input signals by I(h), in combination with low-pass filtering by passive properties, fully accounts for this frequency-tuning. Correcting for the depolarization introduced by shunting through the pipette-membrane seal, leads to predict that in darkness I(h) is tonically active in RBCs and quickens their responses to dim light stimuli. Immunohistochemistry targeting candidate subunit isoforms HCN1-2, in combination with markers of RBCs (PKC) and rod-RBC synaptic contacts (bassoon, mGluR6, Kv1.3), suggests that RBCs express HCN2 on the tip of their dendrites. The functional properties conferred by I(h) onto RBCs may contribute to shape the retina's light response and explain the visual side effects of HCN inhibitors.

Mechanism underlying rebound excitation in retinal ganglion cells

Retinal ganglion cells (RGCs) display the phenomenon of rebound excitation, which is observed as rebound sodium action potential firing initiated at the termination of a sustained hyperpolarization below the resting membrane potential (RMP). Rebound impulse firing, in contrast to corresponding firing elicited from rest, displayed a lower net voltage threshold, shorter latency and was invariably observed as a phasic burst-like doublet of spikes. The preceding hyperpolarization leads to the recruitment of a Tetrodotoxin-insensitive depolarizing voltage overshoot, termed as the net depolarizing overshoot (NDO). Based on pharmacological sensitivities, we provide evidence that the NDO is composed of two independent but interacting components, including (1) a regenerative low threshold calcium spike (LTCS) and (2) a non-regenerative overshoot (NRO). Using voltage and current clamp recordings, we demonstrate that amphibian RGCs possess the hyperpolarization activated mixed cation channels/current, Ih, and low voltage activated (LVA) calcium channels, which underlie the generation of the NRO and LTCS respectively. At the RMP, the Ih channels are closed and the LVA calcium channels are inactivated. A hyperpolarization of sufficient magnitude and duration activates Ih and removes the inactivation of the LVA calcium channels. On termination of the hyperpolarizing influence, Ih adds an immediate depolarizing influence that boosts the generation of the LTCS. The concerted action of both conductances results in a larger amplitude and shorter latency NDO than either mechanism could achieve on its own. The NDO boosts the generation of conventional sodium spikes which are triggered on its upstroke and crest, thus eliciting rebound excitation.

Hyperpolarization-activated channels in trigeminal ganglia innervating healthy and pulp-exposed teeth

AIM

To use immunocytochemistry for determining the expression of HCN1, HCN2 and HCN3 (three subunits of the hyperpolarization-activated cyclic nucleotide-gated current channel) in rodent trigeminal ganglia (TG) that innervate healthy teeth and determine if expression of HCN subunits is increased in TG following pulp exposure.

METHODOLOGY

Pulps were exposed in right maxillary incisors of male Sprague-Dawley rats. After fixation, TG were removed, cryostat sectioned, and immunocytochemistry was utilized to study the expression of HCN1-3 subunits. Immunoreactivity of individual neurons from the maxillary region of the TG was determined with ImageJ software. Differences in the number immunopositive neurons amongst groups were tested for statistical significance with either a Yates or Pearson's chi-square or Fisher's exact probability tests depending on neuron sample size. Differences in the intensity of immunoreactivity between groups were tested for statistical significance with a Student's t-test.

RESULTS

The majority of TG neurons were immunopositive for HCN1-3. Moreover, statistically significant increases in the number of TG neurons immunopositive for HCN1 and the intensity of HCN1-3 immunoreactivity were observed within hours of exposing the tooth pulp.

CONCLUSIONS

HCN1-3 expression, as determined by immunocytochemistry, is increased within hours after injury. Given that I(h) can facilitate neuronal excitability, results of the current study suggest that antagonists to HCN1-3 subunits could work as analgesics in the alleviation of orofacial pain.

Ih without Kir in adult rat retinal ganglion cells

Antisera directed against hyperpolarization-activated mixed-cation ("I(h)") and K(+) ("K(ir)") channels bind to some somata in the ganglion cell layer of rat and rabbit retina. Additionally, the termination of hyperpolarizing current injections can trigger spikes in some cat retinal ganglion cells, suggesting a rebound depolarization arising from activation of I(h). However, patch-clamp studies showed that rat ganglion cells lack inward rectification or present an inwardly rectifying K(+) current. We therefore tested whether hyperpolarization activates I(h) in dissociated, adult rat retinal ganglion cell somata. We report here that, although we found no inward rectification in some cells, and a K(ir)-like current in a few cells, hyperpolarization activated I(h) in roughly 75% of the cells we recorded from in voltage clamp. We show that this current is blocked by Cs(+) or ZD7288 and only slightly reduced by Ba(2+), that the current amplitude and reversal potential are sensitive to extracellular Na(+) and K(+), and that we found no evidence of K(ir) in cells presenting I(h). In current clamp, injecting hyperpolarizing current induced a slowly relaxing membrane hyperpolarization that rebounded to a few action potentials when the hyperpolarizing current was stopped; both the membrane potential relaxation and rebound spikes were blocked by ZD7288. These results provide the first measurement of I(h) in mammalian retinal ganglion cells and indicate that the ion channels of rat retinal ganglion cells may vary in ways not expected from previous voltage and current recordings.

Different mechanisms generate maintained activity in ON and OFF retinal ganglion cells

Neuronal discharge is driven by either synaptic input or cell-autonomous intrinsic pacemaker activity. It is commonly assumed that the resting spike activity of retinal ganglion cells (RGCs), the output cells of the retina, is driven synaptically, because retinal photoreceptors and second-order cells tonically release neurotransmitter. Here we show that ON and OFF RGCs generate maintained activity through different mechanisms: ON cells depend on tonic excitatory input to drive resting activity, whereas OFF cells continue to fire in the absence of synaptic input. In addition to spontaneous activity, OFF cells exhibit other properties of pacemaker neurons, including subthreshold oscillations, burst firing, and rebound excitation. Thus, variable weighting of synaptic mechanisms and intrinsic properties underlies differences in the generation of maintained activity in these parallel retinal pathways.

Normal and rebound impulse firing in retinal ganglion cells

Given that the action potential output of retinal ganglion cells (RGCs) determines the nature of the visual information that is transmitted from the retina, an understanding of their intrinsic impulse firing characteristics is critical for an appreciation of the overall processing of visual information. Recordings from RGCs within an isolated whole-mount retina preparation showed that their normal impulse firing from the resting membrane potential (RMP) was linearly correlated in its frequency with the stimulus intensity. In addition to describing the relationship between the magnitude of the current injection and the resulting impulse frequency (F/I relationship), we have characterized the properties of individual action potentials when they are elicited from the RMP. In contrast, hyperpolarizing below the RMP revealed that RGCs displayed a time dependent anomalous rectification, manifested by the appearance of a depolarizing sag in their voltage response. When an adequate period of hyperpolarization was terminated, a fast phasic period of “rebound excitation” was observed, characterized by a brief phasic burst of impulse activity. When compared to equivalent action potential firing evoked by depolarizing from the RMP, rebound spiking was associated with a lower threshold and shorter latency for impulse activation as well as a prominent, phasic, burst-like doublet, or triplet of impulses. The rebound action potential had a more positive voltage overshoot and displayed a higher peak rate of rise in its upstroke than those correspondingly generated by depolarizing current pulses from the RMP. Blocking sodium spikes with TTX confirmed that the preceding hyperpolarization led to the recruitment and subsequent generation of a transient depolarizing voltage overshoot, which we have termed the net depolarizing overshoot (NDO). We propose that the NDO boosts the generation of sodium spikes by triggering rebound spikes on its upstroke and crest, thus accounting for the observed voltage dependent change in the firing pattern of RGCs.

Cellular mechanisms underlying the pharmacological induction of phosphenes

Visual sensations evoked by stimuli other than luminance changes are called phosphenes. Phosphenes may be an early symptom in a variety of diseases of the retina or of the visual pathways, but healthy individuals may perceive them as well. Phosphene-like phenomena are perhaps the most common side effect reported in clinical pharmacology. Ivabradine, a novel anti-anginal drug that reduces heart-rate by inhibiting the hyperpolarization activated current expressed in cardiac sinoatrial node cells (I(f)) induces phosphenes in some patients. One hypothesis is that ivabradine interacts with the visual system by inhibiting hyperpolarization-activated current in retinal cells (Ih). An Ih current with properties similar to cardiac I(f) has been reported in retinal neurones. Under normal circumstances most of the random fluctuations generated within the retinal circuits do not reach the level of conscious perception because they are filtered out. Presumably, filtering occurs mostly within the retina and one serious candidate for this action is the ability of Ih to act as a negative-feedback mechanism. Ih activation in the membrane of visual cells causes dampening of responses to slow noisy inputs thus tuning the visual system to perceptually more relevant signals of higher frequency. Ih inhibition, by altering at the retinal synapses the filtering of signals generated by thermal breakdown of rhodopsin or other fluctuations, is expected to increase the probability of phosphene occurrence. It is the purpose of the present paper to outline and discuss the features of the visual system and the pharmacological conditions relevant to phosphene perception.

The inhibitory effect of nilvadipine on calcium channels in retinal ganglion cells in goldfish

PURPOSE

Our aim was to examine the inhibitory effect of nilvadipine on voltage-gated calcium (Ca) channels in solitary ganglion cells.

METHODS

Eyes were excised from goldfish. Ganglion cells were enzymatically dissociated from isolated retina. Whole-cell currents were recorded with the perforated-patch clamp technique.

RESULTS

Depolarizing step pulses to more than -48 mV evoked a slowly inactivating inward Ca current. The current-voltage relation for the nilvadipine-sensitive current was bellshaped, and the peak current reached a maximum at -8 mV in the presence and absence of nilvadipine. Nilvadipine block of voltage-gated Ca current was dose-dependent between 1 and 100 microM. The half-maximum inhibitory dose was 35 microM.

CONCLUSIONS

The inhibitory effect of orally administered nilvadipine on Ca channels had a mild influence in ganglion cells.

Intrinsic ON responses of the retinal OFF pathway are suppressed by the ON pathway

Parallel ON and OFF pathways conduct visual signals from bipolar cells in the retina to higher centers in the brain. ON responses are thought to originate by exclusive use of metabotropic glutamate receptor 6 (mGluR6) expressed in retinal ON bipolar cells. Paradoxically, we find ON responses in retinal ganglion cells of mGluR6-null mice, but they occur at long latency. The long-latency ON responses are not blocked by metabotropic glutamate or cholinergic receptor antagonists and are not produced by activation of receptive field surrounds. We show that these longer-latency ON responses are initiated in the OFF pathways. Our results expose a previously unrecognized intrinsic property of OFF retinal pathways that generates responses to light onset. In mGluR6-null mice, long-latency ON responses are observed in the visual cortex, indicating that they can be conducted reliably to higher visual areas. In wild-type (WT) mice, APB (DL-2-amino-4-phosphonobutyric acid), an mGluR6 agonist, blocks normal, short-latency ON responses but unmasks longer-latency ones. We find that these potentially confusing ON responses in the OFF pathway are actively suppressed in WT mice via two pharmacologically separable retinal circuits that are activated by the ON system in the retina. Consequently, we propose that a major function of the signaling of the ON pathway to the OFF pathway is suppression of these mistimed, and therefore inappropriate, light-evoked responses.

Retinal bipolar cell types differ in their inventory of ion channels

Bipolar cells were recorded in rat retinal slices to study the distribution of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels. Patch-clamp whole cell measurements were combined with intracellular filling and recorded cells were morphologically identified. HCN channel isoforms HCN1-4 are differentially expressed in bipolar cells. Each bipolar cell type has a characteristic inventory of HCN channels. The combination of HCN channel currents and other voltage-gated currents can be used as a kind of “finger print” to electrophysiologically identify and classify bipolar cell types. Using this approach of combined electrophysiological and morphological classification we could identify a new ON-cone bipolar cell type.

An integrated approach to classifying neuronal phenotypes

Characterizing the functional phenotypes of neurons is essential for understanding how genotypes can be related to the neural basis of behaviour. Traditional classifications of neurons by single features (such as morphology or firing behaviour) are increasingly inadequate for reflecting functional phenotypes, as they do not integrate functions across different neuronal types. Here, we describe a set of rules for identifying and predicting functional phenotypes that combine morphology, intrinsic ion channel species and their distributions in dendrites, and functional properties. This more comprehensive neuronal classification should be an improvement on traditional classifications for relating genotype to functional phenotype.

Electrophysiological changes of retinal ganglion cells in Royal College of Surgeons rats during retinal degeneration

Retinitis pigmentosa is one of the main causes of blindness throughout the world and its pathogenesis remains unclear. In this study, patch-clamp recording was performed from ganglion cells in retinal dystrophic Royal College of Surgeons rats and normal rats during development. The three discharge patterns of action potential in ganglion cells of Royal College of Surgeons rats were single, transient, and sustained firing. The main discharge pattern was single firing in P1-2W, then transient and sustained firing. However, during later stages of retinal degeneration, action potential was reduced in amplitude and frequency, and was even lost in Royal College of Surgeons rats. These findings differed dramatically from those of rats without retinal degeneration, suggesting there are functional ganglion cells in Royal College of Surgeons rats early, but lose part function during retinal degeneration, even though morphological differences are not apparent. Understanding the changes of electrophysiological characteristics during retinal degeneration in detail may help explore the optimal time course for treating retinitis pigmentosa.

GABAergic activation of an inwardly rectifying K+ current in mouse cerebellar Purkinje cells

Cerebellar Purkinje cells integrate motor information conveyed by excitatory synaptic inputs from parallel and climbing fibres. Purkinje cells abundantly express B-type G-protein-coupled gamma-aminobutyric acid receptors (GABABR) that are assumed to mediate major responses, including postsynaptic modulation of the synaptic inputs. However, the identity and function of effectors operated by GABABR are not fully elucidated. Here we characterized an inwardly rectifying current activated by baclofen (Ibacl), a GABABR agonist, in cultured mouse Purkinje cells using a ruptured-patch whole-cell technique. Ibacl is operated by GABABR via Gi/o-proteins, as it is not inducible in pertussis-toxin-pretreated cells. Ibacl is carried by K+ because its reversal potential shifts with the equilibrium potential of K+. Ibacl is blocked by 10(-3) M Ba2+ or Cs+, and 10(-8) M tertiapin-Q. Upon the onset and offset of a hyperpolarizing step, Ibacl is activated and deactivated, respectively, with double-exponential time courses (time constants, <1 ms and 30-80 ms). Based on similarities in the above properties, G-protein-coupled inwardly rectifying K+ (GIRK) channels are thought to be responsible for Ibacl. Perforated-patch recordings from cultured Purkinje cells demonstrate that Ibacl hyperpolarizes the resting potential and the peak level achieved by glutamate-evoked potentials initiated in the dendrites. Moreover, cell-attached recordings from Purkinje cells in cerebellar slices demonstrate that Ibacl impedes spontaneous firing. Therefore, Ibacl may reduce the postsynaptic and intrinsic excitability of Purkinje cells under physiological conditions. These findings give a new insight into the role of GABABR signalling in cerebellar information processing.

Dendrodendritic electrical synapses between mammalian retinal ganglion cells

Electrical synapses between alpha-type ganglion cells were detected using combined techniques of dual patch-clamp recordings, intracellular labeling, electron microscopy, and channel subunit connexin immunocytochemistry in the albino rat retina. After intracellular injection of Neurobiotin into alpha-cells of inner (ON-center) and outer (OFF-center) ramifying types, measurement of tracer coupling resulted in a preferentially homologous occurrence among cells of the same morphological type (n = 19 of 24). In high-voltage as well as conventional electron microscopic analysis, direct dendrodendritic gap junctions (average size, 0.86 mum long) were present in contact sites between tracer-coupled alpha-cells. In simultaneous dual whole-cell recordings from pairs of neighboring alpha-cells, these cells generated TTX-sensitive sustained spiking against extrinsic current injection, and bidirectional electrical synapses (maximum coupling coefficient, 0.32) with symmetrical junction conductance (average, 1.35 nS) were observed in pairs with cells of the same morphological type. Precise temporal synchronization of spike activity (average time delay, 2.7 msec) was detected when depolarizing currents were simultaneously injected into the pairs. To address whether physiologically identified electrical synapses constitute gap junctional connectivity between cell pairs, identified neuronal connexin36 immunoreactivity was undertaken in Lucifer yellow-labeled cell pairs after patch-clamp recordings. All alpha-cells expressed connexin36, and confocal laser-scanning imaging demonstrated that connexin36 is primarily located at dendritic crossings between electrically coupled cells (seven sites in a pair, on average). These results give conclusive evidence for electrical synapses via dendrodendritic gap junctions involving connexin36 in alpha retinal ganglion cells of the same physiological type.

Inwardly rectifying potassium channels in rat retinal ganglion cells

Inwardly rectifying potassium channels (Kir channels) are important for neuronal signalling and membrane excitability. In the present work we characterized, for the first time, Kir channels in rat retinal ganglion cells (RGCs), the output neurons in the retina, using immunocytochemical and patch-clamp techniques. Various subunits of Kir channels (Kir1.1, 2.1, 2.3, 3.1, 3.2 and 3.3) were expressed in RGCs, but with distinct subcellular localization. Kir1.1 was mainly expressed in axons of RGCs. Kir2.1 and Kir2.3 were both present in somata of RGCs. Whereas staining for Kir3.1 was profoundly present in an endoplasmic reticulum-like structure and Kir3.2 was strongly expressed in the cytoplasm and the cytomembrane of somata, dendrites and axons of RGCs, faint, sparse labelling for Kir3.3 was seen in the cytomembrane. Immunoreactivity for Kir4.1 and Kir4.2 was not detectable in RGCs. Whole-cell currents mediated by Kir channels were recorded in isolated RGCs and they differed from hyperpolarization-activated currents (I(h)) by showing full activation in < 10 ms, no inactivation, and being significantly suppressed by 300 microM Ba2+. Unlike in retinal horizontal cells and bipolar cells, these currents were mainly mediated by G-protein-coupled Kir3 (GIRK) channels, as demonstrated by the fact that GDP(beta)S and GTP(gamma)S included in the pipette solution markedly decreased and increased the currents, respectively. Furthermore, the GIRK channels were probably coupled to GABA(B) receptors, because baclofen considerably increased the Kir currents and the increased currents were suppressed by Ba2+. These characteristics of the Kir currents provide more versatility for signalling of RGCs.

Dissociation of retinal ganglion cells without enzymes

We describe here methods for dissociating retinal ganglion cells from adult goldfish and rat without proteolytic enzymes, and show responses of ganglion cells isolated this way to step-wise voltage changes and fluctuating current injections. Taking advantage of the laminar organization of vertebrate retinas, photoreceptors and other cells were lifted away from the distal side of freshly isolated goldfish retinas, after contact with pieces of membrane filter. Likewise, cells were sliced away from the distal side of freshly isolated rat retinas, after these adhered to a membrane filter. The remaining portions of retina were incubated in an enzyme-free, low Ca2+ solution, and triturated. After aliquots of the resulting cell suspension were plated, ganglion cells could be identified by dye retrogradely transported via the optic nerve. These cells showed no obvious morphological degeneration for several days of culture. Perforated-patch whole-cell recordings showed that the goldfish ganglion cells spike tonically in response to depolarizing constant current injections, that these spikes are temporally precise in response to fluctuating current injections, and that the largest voltage-gated Na+ currents of these cells were larger than those of ganglion cells isolated with a neutral protease.

Availability of low-threshold Ca2+ current in retinal ganglion cells

Spiking in central neurons depends on the availability of inward and outward currents activated by depolarization and on the activation and priming of currents by hyperpolarization. Of these processes, priming by hyperpolarization is the least described. In the case of T-type Ca2+ current availability, the interplay of hyperpolarization and depolarization has been studied most completely in expression systems, in part because of the difficulty of pharmacologically separating the Ca2+ currents of native neurons. To facilitate understanding of this current under physiological conditions, we measured T-type current of isolated goldfish retinal ganglion cells with perforated-patch voltage-clamp methods in solutions containing a normal extracellular Ca2+ concentration. The voltage sensitivities and rates of current activation, inactivation, deactivation, and recovery from inactivation were similar to those of expressed alpha1G (CaV3.1) Ca2+ channel clones, except that the rate of deactivation was significantly faster. We reproduced the amplitude and kinetics of measured T currents with a numerical simulation based on a kinetic model developed for an alpha1G Ca2+ channel. Finally, we show that this model predicts the increase of T-type current made available between resting potential and spike threshold by repetitive hyperpolarizations presented at rates that are within the bandwidth of signals processed in situ by these neurons.

Mammalian Retinal Bipolar Cells Express Inwardly Rectifying K+ Currents (IKir) With a Different Distribution Than That of Ih

Retinal bipolar cells comprise multiple subtypes that are well known for the diversity of their physiological properties. We investigated the properties and functional roles of the hyperpolarization-activated currents in mammalian retinal bipolar cells using whole cell patch-clamp recording techniques. We report that bipolar cells express inwardly rectifying K+ currents ( IKir) in addition to the hyperpolarization-activated cationic currents ( Ih) previously reported. Furthermore, these two currents are differentially expressed among different subtypes of bipolar cells. One group of cone bipolar cells in particular displayed mainly IKir. A second group of cone bipolar cells displayed both currents but with a much larger Ih. Rod bipolar cells, on the other hand, showed primarily Ih. Moreover, we showed that IKir and Ih differentially influence the voltage responses of bipolar cells: Ih facilitates and/or accelerates the membrane potential rebound, whereas IKir counteracts or prevents such rebound. The findings of the expression of IKir and the differential expression of Ih and IKir in bipolar cells may provide new insights into an understanding of the physiological properties of bipolar cells.

Effects of noise on the spike timing precision of retinal ganglion cells

Information in a spike train is limited by variability in the spike timing. This variability is caused by noise from several sources including synapses and membrane channels; but how deleterious each noise source is and how they affect spike train coding is unknown. Combining physiology and a multicompartment model, we studied the effect of synaptic input noise and voltage-gated channel noise on spike train reliability for a mammalian ganglion cell. For tonic stimuli, the SD of the interspike intervals increased supralinearly with increasing interspike interval. When the cell was driven by current injection, voltage-gated channel noise and background synaptic noise caused fluctuations in the interspike interval of comparable amplitude. Spikes initiated on the dendrites could cause additional spike timing fluctuations. For transient stimuli, synaptic noise was dominant and spontaneous background activity strongly increased fluctuations in spike timing but decreased the latency of the first spike.

Intrinsic physiological properties of cat retinal ganglion cells

Retinal ganglion cells (RGCs) are the output neurons of the retina, sending their signals via the optic nerve to many different targets in the thalamus and brainstem. These cells are divisible into more than a dozen types, differing in receptive field properties and morphology. Light responses of individual RGCs are in large part determined by the exact nature of the retinal synaptic network in which they participate. Synaptic inputs, however, are greatly influenced by the intrinsic membrane properties of each cell. While it has been demonstrated clearly that RGCs vary in their intrinsic properties, it remains unclear whether this variation is systematically related to RGC type. To learn whether membrane properties contribute to the functional differentiation of RGC types, we made whole-cell current clamp recordings of RGC responses to injected current of identified cat RGCs. The data collected demonstrated that RGC types clearly differed from one another in their intrinsic properties. One of the most striking differences we observed was that individual cell types had membrane time constants that varied widely from approximately 4 ms (alpha cells) to more than 80 ms (zeta cells). Perhaps not surprisingly, we also observed that RGCs varied greatly in their maximum spike frequencies (kappa cells 48 Hz-alpha cells 262 Hz) and sustained spike frequencies (kappa cells 23 Hz-alpha cells 67 Hz). Interestingly, however, most RGC types exhibited similar amounts of spike frequency adaptation. Finally, RGC types also differed in their responses to injection of hyperpolarizing current. Most cell types exhibited anomalous rectification in response to sufficiently strong hyperpolarization, although alpha and beta RGCs showed only minimal, if any, rectification under similar conditions. The differences we observed in RGC intrinsic properties were striking and robust. Such differences are certain to affect how each type responds to synaptic input and may help tune each cell type appropriately for their individual roles in visual processing.

Heterogeneous intrinsic firing properties of vertebrate retinal ganglion cells

Retinal ganglion cells (RGCs) use their characteristic firing patterns to encode various aspects of visual information and carry them to the brain. It has been thought that the firing pattern of an RGC's light response is determined primarily by the time course and spatiotemporal interaction of the synaptic inputs. However, it is unclear whether there is a difference in intrinsic firing properties among RGCs that could contribute to the cell-to-cell distinction of the light response firing pattern. We investigated the intrinsic firing properties of isolated goldfish RGCs, minimizing cytoplasmic disturbance with a perforated-patch, whole-cell recording technique. In response to a 1-s depolarizing current step, the majority of the examined RGCs (n = 84) displayed sustained firing that lasted over 800 ms (n = 24; tonic RGCs) or transient firing accommodated within 200 ms of the step onset (n = 47; phasic RGCs). Tonic and phasic RGCs also differed in their firing frequency-current intensity dynamics. There was a significant difference in the soma sizes of phasic and tonic RGCs, indicating that some parts of these groups originate from distinct morphological subtypes. In the presence of extracellular Ba(2+) (1 mM), phasic RGCs displayed sustained firing and firing frequency-current intensity dynamics similar to those of tonic RGCs. Thus a Ba(2+)-sensitive ion current (I(Ba-s)) underlies the firing characteristics of phasic RGCs. Under voltage-clamp conditions, I(Ba-s) was identified as a low-threshold, noninactivating voltage-dependent K(+) current. Because of its slow kinetics (time constant of activation, approximately 100 ms), I(Ba-s) may confer a gradually increasing hyperpolarizing driving force during maintained excitatory stimulus, which eventually would result in firing accommodation. These findings suggest that RGCs have heterogeneous intrinsic firing properties that could aid synaptic inputs in shaping light responses.

A dopamine- and protein kinase A-dependent mechanism for network adaptation in retinal ganglion cells

Vertebrates can detect light intensity changes in vastly different photic environments, in part, because postreceptoral neurons undergo "network adaptation." Previous data implicated dopaminergic, cAMP-dependent inhibition of retinal ganglion cells in this process yet left unclear how this occurs and whether this occurs in darkness versus light. To test for light- and dopamine-dependent changes in ganglion cell cAMP levels in situ, we immunostained dark- and light-adapted retinas with anti-cAMP antisera in the presence and absence of various dopamine receptor ligands. To test for direct effects of dopamine receptor ligands and membrane-permeable protein kinase ligands on ganglion cell excitability, we recorded spikes from isolated ganglion cells in perforated-patch whole-cell mode before and during application of these agents by microperfusion. Our immunostainings show that light, endogenous dopamine, and exogenous dopamine elevate ganglion cell cAMP levels in situ by activating D1-type dopamine receptors. Our spike recordings show that D1-type agonists and 8-bromo cAMP reduce spike frequency and curtail sustained spike firing and that these effects entail protein kinase A activation. These effects resemble those of background light on ganglion cell responses to light flashes. Network adaptation could thus be produced, to some extent, by dopaminergic modulation of ganglion cell spike generation, a mechanism distinct from modulation of transmitter release onto ganglion cells or of transmitter-gated currents in ganglion cells. Combining these observations with results obtained in studies of photoreceptor, bipolar, and horizontal cells indicates that all three layers of neurons in the retina are equipped with mechanisms for adaptation to ambient light intensity.

Ionic currents and spontaneous firing in neurons isolated from the cerebellar nuclei

Neurons of the cerebellar nuclei fire spontaneous action potentials both in vitro, with synaptic transmission blocked, and in vivo, in resting animals, despite ongoing inhibition from spontaneously active Purkinje neurons. We have studied the intrinsic currents of cerebellar nuclear neurons isolated from the mouse, with an interest in understanding how these currents generate spontaneous activity in the absence of synaptic input as well as how they allow firing to continue during basal levels of inhibition. Current-clamped isolated neurons fired regularly ( approximately 20 Hz), with shallow interspike hyperpolarizations (approximately -60 mV), much like neurons in more intact preparations. The spontaneous firing frequency lay in the middle of the dynamic range of the neurons and could be modulated up or down with small current injections. During step or action potential waveform voltage-clamp commands, the primary current active at interspike potentials was a tetrodotoxin-insensitive (TTX), cesium-insensitive, voltage-independent, cationic flux carried mainly by sodium ions. Although small, this cation current could depolarize neurons above threshold voltages. Voltage- and current-clamp recordings suggested a high level of inactivation of the TTX-sensitive transient sodium currents that supported action potentials. Blocking calcium currents terminated firing by preventing repolarization to normal interspike potentials, suggesting a significant role for K(Ca) currents. Potassium currents that flowed during action potential waveform voltage commands had high activation thresholds and were sensitive to 1 mm TEA. We propose that, after the decay of high-threshold potassium currents, the tonic cation current contributes strongly to the depolarization of neurons above threshold, thus maintaining the cycle of firing.

Origin of transient and sustained responses in ganglion cells of the retina

Phasic and tonic light responses provide a fundamental division of visual information that is thought to originate in the inner retina. However, evidence presented here indicates that this duality originates in the outer retina. In response to a steady light stimulus, the temporal responses of On-bipolar cells fell into two groups. In one group, the light response peaked and then rapidly declined (tau approximately 400 msec) close to the resting membrane potential. At light offset, these cells exhibited a transient afterhyperpolarization. In the second group of On-bipolar cells, the light response declined 10-fold more slowly and reached a steady depolarization that was approximately 40% of the peak response. These neurons had a slowly decaying afterhyperpolarization at light offset. A metabotropic glutamate antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked light responses in both types of On-bipolar cell. CPPG only slightly depolarized transient On-bipolar cells, whereas sustained On-bipolar cells were significantly depolarized. Inorganic calcium channel blockers disclosed that these distinct On-bipolar responses were inherent to the bipolar cell and not attributable to synaptic feedback. CPPG had distinct effects on sustained and transient ganglion cells, similar to its action on bipolar cells. The antagonist depolarized and blocked the light responses of sustained ganglion cells. In transient ganglion cells, CPPG suppressed the On light response but did not depolarize the cell or block the Off light response. These results suggest that transient and sustained light responses in ganglion cells result from selective bipolar cell input and that these two fundamental visual channels originate at the dendritic terminals of bipolar cells.

Deactivation, recovery from inactivation, and modulation of extra-synaptic ion currents in fish retinal ganglion cells

As is shown magnificently by Heron Island's reef, the visual environment of many fishes includes various light intensities, hues and shapes that can change on large and small scales in space and time. Several articles in this issue address why fishes are sensitive to some of these properties, and how fishes and other aquatic species have acquired or fostered these sensitivities. This article discusses contributions of extrasynaptic ion currents, in a specific population of neurons, to the detection of ambient light levels, the appearance of certain visual stimuli and the disappearance of others.

Cloning and functional expression of human retinal kir2.4, a pH-sensitive inwardly rectifying K(+) channel

To identify novel potassium channel genes expressed in the retina, we screened a human retina cDNA library with an EST sequence showing partial homology to inwardly rectifying potassium (Kir) channel genes. The isolated cDNA yielded a 2,961-base pair sequence with the predicted open reading frame showing strong homology to the rat Kir2. 4 (rKir2.4). Northern analysis of mRNA from human and bovine tissues showed preferential expression of Kir2.4 in the neural retina. In situ hybridization to sections of monkey retina detected Kir2.4 transcript in most retinal neurons. Somatic hybridization analysis and dual-color in situ hybridization to metaphase chromosomes mapped Kir2.4 to human chromosome 19 q13.1-q13.3. Expression of human Kir2. 4 cRNA in Xenopus oocytes generated strong, inwardly rectifying K(+) currents that were enhanced by extracellular alkalinization. We conclude that human Kir2.4 encodes an inwardly rectifying K(+) channel that is preferentially expressed in the neural retina and that is sensitive to physiological changes in extracellular pH.

Contributions of ion conductances to the onset responses of octopus cells in the ventral cochlear nucleus: simulation results

The onset response pattern displayed by octopus cells has been attributed to intrinsic membrane properties, low membrane impedance, and/or synaptic inputs. Although the importance of a low membrane impedance generally is acknowledged as an essential component, views differ on the role that ion channels play in producing the onset response. In this study, we use a computer model to investigate the contributions of ion channels to the responses of octopus cells. Simulations using current ramps indicate that, during the "ramp-up" stage, the membrane depolarizes, activating a low-threshold K(+) channel, K(LT), which increases membrane conductance and dynamically increases the current required to evoke an action potential. As a result, the model is sensitive to the rate that membrane potential changes when initiating an action potential. Results obtained when experimentally recorded spike trains of auditory-nerve fibers served as model inputs (simulating acoustic stimulation) demonstrate that a model with K(LT) conductance as the dominant conductance produces realistic onset response patterns. Systematically replacing the K(LT) conductance by a h-type conductance (which corresponds to a hyperpolarization-activated inward rectifier current, I(h)) or by a leakage conductance reduces the model's sensitivity to rate of change in membrane potential, and the model's response to "acoustic stimulation" becomes more chopper-like. Increasing the h-type conductance while maintaining a large K(LT) conductance causes an increase in threshold to both current steps and acoustic stimulation but does not significantly affect the model's sensitivity to rate of change in membrane potential and the onset response pattern under acoustic stimulation. These findings support the idea that K(LT), which is activated during depolarization, is the primary membrane conductance determining the response properties of octopus cells, and its dynamic role cannot be provided by a static membrane conductance. On the other hand, I(h), which is activated during hyperpolarization, does not play a large role in the basic onset response pattern but may regulate response threshold through its contribution to the membrane conductance.

A zinc-dependent Cl- current in neuronal somata

Extracellular Zn2+ modulates current passage through voltage- and neurotransmitter-gated ion channels, at concentrations less than, or near, those produced by release at certain synapses. Electrophysiological effects of cytoplasmic Zn2+ are less well understood, and effects have been observed at concentrations that are orders of magnitude greater than those found in resting and stimulated neurons. To examine whether and how neurons are affected by lower levels of cytoplasmic Zn2+, we tested the effect of Zn2+-selective chelators, Zn2+-preferring ionophores, and exogenous Zn2+ on neuronal somata during whole-cell patch-clamp recordings. We report here that cytoplasmic zinc facilitates the downward regulation of a background Cl- conductance by an endogenous protein kinase C (PKC) in fish retinal ganglion cell somata and that this regulation is maintained if nanomolar levels of free Zn2+ are available. This regulation has not been described previously in any tissue, as other Cl- currents have been described as reduced by PKC alone, reduced by Zn2+ alone, or reduced by both independently. Moreover, control of cation currents by a zinc-dependent PKC has not been reported previously. The regulation we have observed thus provides the first electrophysiological measurements consistent with biochemical measurements of zinc-dependent PKC activity in other systems. These results suggest that contributions of background Cl- conductances to electrical properties of neurons are susceptible to modulation.

Early postnatal development of substantia nigra neurons in rat midbrain slices: hyperpolarization-activated inward current and dopamine-activated current

Whole-cell voltage-clamp recordings in rat midbrain slices were used to determine the characteristics and postnatal development of a hyperpolarization-activated inward current (Ih) and a dopamine receptor-activated current of neurons in the substantia nigra pars compacta (SNC). Ih was activated by membrane hyperpolarization beyond the resting membrane potentials, and displayed strong inward rectification. No sign of time- and voltage-dependent inactivation was apparent. The reversal potential of Ih in mature (P12-P13) SNC neurons was -39.2 +/- 4.3 mV, and the activation curve of Ih was well fitted by a sigmoidal function with half-maximal activation at -114.0 +/- 5.6 mV. Those values are essentially the same in immature (P3-P5) neurons. The rate of Ih activation was best approximated by a single exponential function. The time constants were voltage dependent, ranging from 692.3 ms at -110 mV to 342.8 ms at -150 mV at P12 neurons, and 1142.5 ms at -110 mV to 593.2 ms at -150 mV at P3 neurons. There was a marked increase in the amplitude of Ih during postnatal development of SNC neurons, while the membrane capacitance did not alter after P5. The increase in Ih after P5 thus appears to result from an increase in Ih current density. Application of dopamine produced a hyperpolarization of mature SNC neurons, leading to suppression of action potentials. Dopamine-activated outward current was negligible during the first postnatal week, and increased rapidly after the second week. Our results indicate that the SNC neuron of the rat midbrain is not fully mature at the time of birth in terms of voltage-gated ion channels and neurotransmitter receptors, but develops over the first two postnatal weeks.

Dopamine modulates inwardly rectifying hyperpolarization-activated current (Ih) in cultured rat olfactory receptor neurons

The presence of dopamine receptors in olfactory receptor neurons (ORNs) suggests that odor sensitivity may be modulated by neurotransmitters at the level of primary sensory neurons. Using standard patch-clamp techniques on rat ORNs, we found that 1 microM dopamine, 500 microM SQ 22536 (SQ, an adenylyl cyclase inhibitor), 20 and 50 microM quinpirole (a selective dopamine D2 receptor agonist), and 1 mM adenosine 3', 5'-cyclic monophosphate (cAMP) modulate the hyperpolarization-activated current Ih. On hyperpolarizing from a holding potential of -58 mV, a small Cs+-sensitive inwardly rectifying current (Ih) was observed. Increases in extracellular K+ increased Ih amplitude without shifting its voltage dependence of activation, whereas increases in temperature produced an increase in Ih amplitude and a hyperpolarizing shift in the activation curve. Application of 1 microM dopamine reversibly shifted Ih activation to more negative potentials and decreased Ih current amplitudes. These effects were blocked by concomitant application of dopamine with sulpiride, a selective dopamine D2 receptor antagonist. The effects of dopamine were mimicked by quinpirole. Quinpirole (20 microM) decreased Ih current amplitude, but was without effect on Ih voltage dependence of activation. However, 50 microM quinpirole produced both a reduction of Ih peak currents and a hyperpolarizing shift in the activation curve for Ih. External application of the adenylyl cyclase inhibitor SQ 22536 produced a reversible decrease in peak currents but had no effect on Ih voltage dependence of activation, whereas internal application of cAMP shifted Ih activation to more depolarized potentials. Because Ih modulates cell excitability and spike frequency adaptation, our findings support a role for dopamine in modulating the sensitivity and output of rat ORNs to odorants.

Mechanisms of onset responses in octopus cells of the cochlear nucleus: implications of a model

The octopus cells of the posteroventral cochlear nucleus receive inputs from auditory-nerve fibers and form one of the major ascending auditory pathways. They respond to acoustic and electrical stimulation transiently and are believed to carry temporal information in the precise timing of their action potentials. The mechanism whereby onset responses are generated is not clear. Proposals aimed at elucidating the mechanism range from neural circuitry and/or inhibition, "depolarization block" (or inactivation of Na+ channels), and the involvement of a 4-aminopyridine (4-AP)-sensitive, low-threshold channel (K(LT)). In the present study, we used a compartment model to investigate possible mechanisms. The model cell contains a soma, an axon, and four passive dendrites. Four kinds of ionic channels were included in the soma compartment: the Hodgkin-Huxley-like Na+ and K+ channels, a 4-AP-sensitive, low-threshold channel, K(LT), and a Cs+-sensitive, hyperpolarization-activated inward rectifier, Ih. DC currents and half-wave-rectified sine waves were used as stimuli. Our results showed that an onset response can be generated in the absence of neuronal circuitry of any form, thus suggesting that the onset response in octopus cells is regulated intrinsically. Among the many factors involved, low-input impedance, partly contributed by Ih, appears to be essential to the basic onset response pattern; also, the K(LT) conductance plays a major role, whereas the inactivation of Na+ channels probably plays only a secondary role. The dynamics of Ih also can modify the response pattern, but due to its slow kinetics, its role is probably limited to longer-term regulation under the conditions simulated in this study.

Synchronized oscillations in the inferior olive are controlled by the hyperpolarization-activated cation current I(h)

The participation of a hyperpolarization-activated cationic current in the generation of oscillations in single inferior olive neurons and in the generation of ensemble oscillations in the inferior olive nucleus (IO) of the guinea pig and ferret was investigated in slices maintained in vitro. Intracellular recordings in guinea pig or ferret 10 neurons revealed that these cells could generate sustained endogenous oscillations (4-10 Hz) at hyperpolarized membrane potentials (-60 to -67 mV) after the intracellular injection of a brief hyperpolarizing current pulse. These oscillations appeared as the rhythmic generation of a low-threshold Ca2+ spike that typically initiated one or two fast Na+-dependent action potentials. Between low-threshold Ca2+ spikes was an afterhyperpolarization that formed a "pacemaker" potential. Local application of apamin resulted in a large reduction in the amplitude of the afterhyperpolarization, indicating that a Ca2+-activated K+ current makes a strong contribution to its generation. However, even in the presence of apamin, hyperpolarization of IO neurons results in a "depolarizing sag" of the membrane potential that was blocked by local application of Cs+ or partial replacement of extracellular Na+ with choline+ or N-methyl-D-glucamine+, suggesting that I(h) also contributes to the generation of the afterhyperpolarization. Extracellular application of low concentrations of cesium resulted in hyperpolarization of the membrane potential of IO neurons and spontaneous 5- to 6-Hz oscillations in single, as well as networks, of IO neurons. Application of larger concentrations of cesium reduced the frequency of oscillation to 2-3 Hz or blocked the oscillation entirely. On the basis of these results, we propose that I(h) contributes to single and ensemble oscillations in the IO in two ways: 1) I(h) contributes to the determination of the resting membrane potential such that reduction of I(h) results in hyperpolarization of the membrane potential and an increased propensity of oscillation through removal of inactivation of the low-threshold Ca2+ current; and 2) I(h) contributes to the generation of the afterhyperpolarization and the pacemaker potential between low-threshold Ca2+ spikes.

Omega-conotoxin-MVIID blocks an omega-conotoxin-GVIA-sensitive, high-threshold Ca2+ current in fish retinal ganglion cells

Reduction of Ca2+ current amplitude by the Conus peptide omega-conotoxin-MVIID (omega-CTx-MVIID) was measured in voltage-clamped, goldfish retinal ganglion cells. Effects of depolarizing shifts in holding potential, and sequential applications of omega-CTx-MVIID, omega-CTx-GVIA, and BAY-K-8644, together with effects of Ni2+ and omega-Aga-IIIA, indicated that omega-CTx-MVIID may target Ca2+ channels differing from those termed T, L, N, P < Q and R.