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PubMed1999

Metabotropic GABA receptors facilitate L-type and inhibit N-type calcium channels in single salamander retinal neurons.

1. Whole-cell voltage clamp experiments were performed on isolated spiking retinal neurons from the salamander retina. Calcium channel currents were studied using barium as the charge carrier while potassium and sodium currents were suppressed with TEA and TTX, respectively. 2. Baclofen, a metabotropic GABA receptor agonist, both enhanced and suppressed high-voltage-activated calcium channel current. Baclofen facilitated an L-type channel current, and this effect was not voltage dependent. As reported previously, baclofen inhibited an N-type channel current and this action was voltage dependent. 3. While the suppressive effect was mediated by a fast-acting, direct G-protein action, the facilitatory effect was slower and was blocked by inhibitors of protein kinase C (PKC), either GF-109203x or the PKC (19-36) sequence fragment. 4. The pharmacology of the inhibitory and facilitatory responses differed. Commonly used antagonists of metabotropic GABA receptors, CGP35348 and CGP55845, were more potent antagonists of the inhibitory response. Similarly, a selective agonist at the metabotropic GABA receptor, APMPA, was also more effective in eliciting the inhibitory response. 5. These observations indicate that there may be two baclofen-sensitive metabotropic GABA receptors with opposing effects on calcium channel current. This is the first description of a facilitatory action of GABAB receptors and indicates that GABA may not function exclusively as an inhibitory transmitter.

Ambystoma↗
PubMed2001

Intracellular calcium reduces light-induced excitatory post-synaptic responses in salamander retinal ganglion cells.

The whole-cell patch clamp technique was used to study the effect of intracellular Ca2+ on light-evoked EPSCs in on-off ganglion cells in salamander retinal slices. Both AMPA and NMDA receptors contributed to the light-evoked responses. In the presence of strychnine and picrotoxin, ganglion cells responded to light onset and offset with transient inward currents at -70 mV. These currents were reduced by 35 +/- 3 % when the light stimulus was preceded by a depolarizing step from -70 to 0 mV. The inhibitory effect of depolarization on light-evoked EPSCs was strongly reduced in the presence of 10 mM BAPTA. The degree of EPSC inhibition by the prepulse holding potential followed the current-voltage relationship of the Ca2+ current found in the ganglion cell. In the presence of the NMDA receptor antagonist AP-7, glutamate-dependent current was nearly abolished when high Ca2+ was substituted for high Na+ solution. The release of Ca2+ from internal stores by caffeine or inositol trisphosphate reduced the EPSCs by 36 +/- 5 and 38 +/- 11 %, respectively, and abolished the inhibitory effect of depolarization. The inhibitory effect of depolarization on EPSCs was reduced 5-fold in the presence of AP-7, but was not reduced by the AMPA receptor antagonist CNQX. Neither inhibition of Ca2+-calmodulin-dependent enzymes, nor inhibition of protein kinase A or C had any significant effect on the depolarization-induced inhibition of EPSCs. Our data suggest that elevation of [Ca2+]i, through voltage-gated channels or by release from intracellular stores, reduced primarily the NMDA component of the light-evoked EPSCs.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed2001

Simultaneous recording of receptor current and intraciliary Ca2+ concentration in salamander olfactory receptor cells.

1. The suction pipette technique was combined with laser spot fluorescence microscopy to record simultaneously odour-induced current responses and intraciliary Ca2+ concentration from isolated salamander olfactory receptor cells loaded with the fluorescent Ca2+ indicator fluo-3. 2. When exposed for 1 s to increasing odour concentrations both the suction pipette current and fluo-3 fluorescence increased dynamically, rising with a similar time course. Thereafter, the fluorescence signal decayed more slowly, outlasting the current response by 0.56 +/- 0.12 s. 3. The fluo-3 fluorescence evoked by progressively increasing odour concentrations varied in an approximately linear manner with the magnitude of the suction pipette current. 4. Prolonged odour stimulation evoked synchronous oscillations in both suction pipette current and intraciliary calcium concentration with a mean period of 4.5 +/- 0.3 s. 5. When external Na+ was omitted from the stimulating solution the oscillation period for both the current and fluorescence signals was lengthened by a factor of 1.9 +/- 0.2 in comparison with the oscillation period when stimulated in Ringer solution. 6. These results support the currently accepted mechanism for Ca2+ homeostasis within the olfactory cilia, and are consistent with the notion that the oscillations induced by prolonged odour exposure represent the coupled oscillation of Ca2+ and cyclic nucleotide concentrations.

Ambystoma↗
PubMed2001

A light-dependent increase in free Ca2+ concentration in the salamander rod outer segment.

1. The Ca(2+) indicator dye fluo-5F was excited by an argon ion laser to measure changes in free Ca(2+) concentration ([Ca2+]i) in the outer segments of isolated salamander rods rapidly exposed to a 0 Ca(2+), 0 Na(+) solution designed to minimise surface membrane Ca(2+) fluxes. Over 30-60 s of laser illumination, the fluorescence first increased rapidly and then declined at a rate that was much slower than in Ringer solution and consistent with previous physiological evidence that 0 Ca(2+), 0 Na(+) solution greatly retards light-induced changes in [Ca(2+)]i. 2. The initial increase in fluorescence was investigated with a sequence of 100 ms laser flashes presented at 5 s intervals. The fluorescence evoked by the second laser flash was on average 30 % larger than the first, and subsequent responses exhibited a slow decline like that measured with continuous laser exposures. The initial increase in fluorescence did not depend upon the timing of exposure to 0 Ca(2+), 0 Na(+) solution but appeared to be evoked by exposure to the laser light. 3. Both the increase and subsequent decline in fluorescence measured with brief laser flashes could be reduced by incorporation of the Ca(2+) chelator BAPTA. This and other results indicate that the fluorescence increase was unlikely to have been caused by a change in the affinity of fluo-5F for Ca(2+) or an increase in the quantity of incorporated dye available to bind Ca(2+) but reflects an actual release of intracellular Ca(2+) within the outer segment. 4. The pool of Ca(2+) available to be released could be decreased if, before the first laser flash, the rod was exposed to light bright enough to bleach a substantial fraction of the photopigment. The releasable pool could also be depleted by exposure to saturating light of much lower intensity if delivered in Ringer solution but not if delivered in 0 Ca(2+), 0 Na(+) solution. We conclude that Ca(2+) can be released within the outer segment both by the bleaching of rhodopsin and by the reduction in [Ca(2+)]i which normally accompanies illumination in Ringer solution. 5. The activation of rhodopsin appears somehow to induce the release of Ca(2+) from a binding site or store within the outer segment. Substantial release, however, required stimulating light of an intensity sufficient to bleach a considerable fraction of the visual pigment. It therefore seems unlikely that such release contributes to the normal Ca(2+)-mediated modulation of transduction during light adaptation. The mechanism and physiological function of light-induced Ca(2+) release are unknown.

Ambystoma↗
PubMed1991

Pigmentary system of the adult alpine salamander Salamandra atra atra (Laur., 1768).

The pigmentary system of the skin from adult specimens of the black alpine salamander Salamandra atra atra was investigated by light microscope, electron microscope, and biochemical studies. Results were compared with those obtained in previous study of the subspecies Salamandra atra aurorae. Unlike Salamandra atra aurorae, which presents epidermal xanthophores and iridophores, Salamandra atra atra is completely melanized, presenting only epidermal and dermal melanophores. The melanosomes in both the epidermis and the dermis appear to derive from a multivesicular premelanosome similar to that in the goldfish, and the epidermal melanosomes are smaller than those in the dermis. Premelanosomes with an internal lamellar matrix were not observed. The biochemical results have shown that in the ethanol extracts obtained from the skin in toto and from the melanosomes, pteridines and flavins are always present and are the same as those extracted from the black skin areas of Salamandra atra aurorae.

Animals↗
PubMed1974

Light-induced resistance changes in retinal rods and cones of the tiger salamander.

1. The electrical properties of retinal rods and cones of the larval tiger salamander were investigated with intracellular electrodes, and the cells identified by means of dye injections.2. Both types of photoreceptors are hyperpolarized by illumination. Following stimulation with brief flashes of dim light, rod responses show a slower time course than cone responses; with bright flashes, rod responses can be recognized because of their long recovery time.3. Values of input resistance were derived from the voltage displacement induced by constant current pulses in darkness or at the peak of the photoresponse. The input resistance following illumination was also calculated from the effect of steady polarizing currents on the amplitude of the photoresponse.4. In darkness, the input resistance of the rod cells is time- and voltage-dependent, but the voltage-current relations of most cells have a linear region which includes the physiological limits of membrane potential. At the peak of the photoresponse, the input resistance (slope of the linear region of the v-i relations) is decreased.5. Cone cells show approximately linear v-i relations. As reported by previous authors, illumination increases the input resistance.6. These results support the current view that the cone photoresponse is the consequence of a reduction in the permeability of channels which in darkness shunt the membrane. In rods, however, it appears that the main effect of illumination is to increase the permeability of the membrane to ions for which the equilibrium potential is more negative than the membrane potential in darkness.

Animals↗
PubMed1974

Responses of olfactory bulb neurones to odour stimulation of small nasal areas in the salamander.

1. Previous experiments have suggested that one way odours may be discriminated is by different spatial patterns of response at both the olfactory bulb and receptor level. The present experiments were designed to test to what extent the position of an odour on the receptor mucosa can influence the activity of olfactory bulb neurones.2. To deliver odours to small areas on the nasal receptor sheet a new method for local application of odour was developed. The flow rate, concentration, and time course of the odour were controlled using the olfactometer described in the preceding paper.3. In thirty olfactory bulb units in the salamander it was found that if the response of a unit to odour delivered to the entire exposed receptor epithelium were suppression (type S), then the unit tended to be suppressed when odour was delivered to a number of localized epithelial regions. If the response were excitation (type E) to stimulation of the entire epithelium, then stimulation to only one or two localized regions would elicit the maximum response.4. Different epithelial regions had the ability to cause excitation in the same bulbar unit depending on the odour being used. Two odours, camphor and amyl acetate, elicited maximum excitation when they were presented to different mucosal areas. The areas at which presentation of these odours gave excitation were surprisingly consistent from unit to unit and animal to animal.5. The data presented here suggest the presence of restricted excitatory receptive fields for some olfactory bulb neurones for a particular odour.6. The presence of spatial response patterns using odour delivery to small nasal receptor regions and thus the presence of receptive fields is discussed with reference to bulbar neuronal circuitry.

Action Potentials↗
PubMed1975

Physiological properties of junctions between nerve and muscle developing during salamander limb regeneration.

1. Physiological properties of developing nerve-muscle junctions were studied in regenerating limbs of adult salamanders. 2. During the period of synapse formation the muscle fibres had diameters of 4-10 mum, resting potentials of minus 90 to minus 100 mV and input resistances of 10-50 Momega. Some, but not all, pairs of adjacent muscle fibres were electrically coupled. 3. At the stage when muscle fibres could first be identified, some of them were not innervated, at least as determined by electrophysiological criteria. 4. During muscle innervation the neuromuscular synapses were encountered in several intermediate phases of maturity. (i) At the least mature junctions small spontaneous synaptic potentials occurred, but stimulation of the motor nerve trunk did not evoke synchronous transmitter release. (ii) At other junctions maximal nerve stimulation evoked only a single end-plate potential of low quantum content. (iii) More mature fibres received synaptic input from as many as four motor neurons, which could be distinguished by their discrete stimulus thresholds. 5. During this period of synapse development the fibres lacked an action potential but often showed a prolonged response to depolarization. 6. Fibres in normal adult muscles had from one to three synaptic inputs, were not electrically coupled, and responded to depolarization with an action potential.

Action Potentials↗
PubMed1975

Horizontal cell responses in the retina of the larval tiger salamander.

The responses to light of horizontal cells were recorded intracellularly in the retina of the larval tiger salamander. 2. All the units studied had a large summation area and were hyperpolarized by circles of light of any wave-length centred on the recording electrode, but two types could be distinguished according to the properties of their receptive fields. Type A units were hyperpolarized following illumination of any portion of their receptive field, while type B units were not hyperpolarized by illumination of their surround unless the centre was simultaneously illuminated, stimulation of the surround alone resulting in either a small depolarization or virtually no response. 3. Procion yellow injections showed that type A responses are recorded from thick and long processes not directly continuous with an identifiable cell body, while type B responses originate from the cell body of cells that send very fine and tortuous processes towards the receptors. The histological observations also suggested that the type A units represent expansions or swellings of one or more of the fine processes originating from the type B units. Therefore, it seems possible that both types of units are just different parts of a single kind of horizontal cell, and that a majority of the dye injections failed to stain them simultaneously because of the small diameter of the connecting process. 4. The large summation area of type A units can be explained, just as for horizontal cells in other retinae, by supposing that they are electrically coupled to other units of the same type. The receptive field properties of type B units, however, can only be partly explained by electrical coupling, and then only if the existence of voltage-dependent junctions is postulated. Instead, the reversal of the polarity of responses to an annulus of light during steady illumination of the centre, plus the available electron microscopic evidence, suggest that the effect of the surround on the type B units is due to a chemical synaptic impingement from the type A units.

Ambystoma↗
PubMed1977

The effects of nerve section and of colchicine treatment on the density of mechanosensory nerve endings in salamander skin.

We have shown that when one of the spinal nerves supplying the salamander hind limb is cut or treated with colchicine, the fields of the remaining nerves enlarge in area; whereas nerve section produces Wallerian degeneration, the colchicine-treated nerves conducted action potentials normally and their peripheral fields remained unchanged in area (Aguilar, Bisby, Cooper & Diamond, 1973). Since colchicine-treatment reduced neuronal transport, and nerve-section eliminated it, we proposed that nerve sprouting is regulated by factors normally conveyed to the endings by axoplasmic transport. 1. We have now investigated the effects of colchicine on the thresholds and distribution of individual mechanosensory endings in the skin. If reduction of neuronal transport were enough to cause the threshold to be increased to the point of total unresponsiveness, then this could be a sign of an early stage of degernation in those terminals. It could then be hypothesized that products of degeneration were providing a stimulus for adjacent nerves to sprout. 2. Quantitative physiological studies of the effects of colchicine doses known to interfere with fast axoplasmic transport, indicate that in some experiments the terminal field of the treated nerve was invaded by sprouting fibres from neighbouring axons, when its own endings were unchanged in number, distribution and sensory thresholds. In other experiments the colchicine-treated nerve endings showed an increase in threshold but their function was otherwise unchanged; a similar adjacent nerve sprouting occurred. In a final group, colchicine caused total unresponsiveness of some endings of the treated nerve. 3. When a region of skin was partially denervated by nerve section, the physiological analysis indicated that the number of new mechanosensory endings which sprouted from the remaining axons exactly matched the number lost by nerve degeneration: furthermore the distribution of the endings was normal. It therefore appears that sprouting ceased when the original density of mechanosensory endings in the skin was restored. 4. The possibility that the drug induced sprouting as a consequense of a direct action on the skin is unlikely. With [3H]colchicine, we found that the accumulation of label in the skin of the untreated limb, in which sprouting did not occur, equalled that of the opposite limb. 5. The present results lend support to the original hypothesis of Aguilar et al. (1973), which proposed that collateral sprouting of intact nerves occurs when the supply of neuronally transported factors becomes inadequate to balance out the effects of a postulated target-tissue stimulus. In the Discussion other examples of collateral nerve sprouting, such as that following adjacent denervation, are shown to be explainable by this hypothesis.

Action Potentials↗
PubMed1978

Responses of olfactory receptor cells to step pulses of odour at different concentrations in the salamander.

1. The response properties of single olfactory receptor cells in the salamander have been analysed in unitary recordings obtained with platinum-black metal-filled micro-electrodes. 2. Stimulation has been carried out using an apparatus which delivers odour pulses of abrupt onset, steady plateau and abrupt termination. The pulses have been monitored near the site of stimulation on the olfactory epithelium during the experiments. 3. The main type of response was a discharge of impulses that was time locked to the stimulus pulse. The pattern of the responses consisted of a relatively brief latency of onset, a rapid rise in impulse frequency, a continuation of impulse firing during the plateau of the pulse, and an abrupt termination of the discharge correlated with the termination of the pulse. 4. There was a clear relationship between the receptor responses and odour concentration. In general, impulse firing frequency increased with increasing odour concentration. The firing frequency ranged from approximately 1--3 impulses/sec at threshold, up to 20 impulses/sec at the highest concentration. 5. Two types of reduced impulse activity were observed. One occurred after the termination of the pulse and lasted 1--3 sec; this was a common occurrence. The other type was seen during a pulse as a reduction of impulse activity compared to the background level; this type was rarely observed. 6. The receptor responses resembled those of mitral cells in the olfactory bulb to odour pulses in their sensitivity to odour concentration. They differed in that mitral cells show primary response categories consisting of brief excitation followed by suppression, and pure suppression, that are rarely seen at the receptor level. These differences may be ascribed to synaptic interactions in the olfactory bulb. 7. It is concluded that the majority of receptor cells have a stereotyped discharge response pattern and a systematic relation to odour concentration. These properties appear to reflect the simple time course of the odour pulses used in these experiments. This represents an initial step toward analysing olfactory coding at the receptor level using stimuli controlled in a manner similar to that used in other sensory systems.

Action Potentials↗
PubMed1979

A voltage-clamp study of the light response in solitary rods of the tiger salamander.

1. Single, isolated, rod photoreceptors were obtained by enzymatic dissociation of the tiger salamander (Ambystoma tigrinum) retina. These solitary cells retained the morphological features of rods of the intact retina and could be maintained in culture for several days. Solitary cells were penetrated with one or two micropipettes and their electrophysiology was studied by the voltage-clamp technique. 2. Intracellular recording with two micropipettes demonstrated that the inner segment of a solitary rod was effectively isopotential with the outer segment. 3. The time course of the voltage response to a flash resembled that of responses observed in rods in the intact retina. At low light intensities the response reached a peak in approximately 0.7 sec and then slowly declined. At high light intensities the time to peak response decreased and an initial transient arose as the response, after reaching the peak, quickly decreased to a less polarized plateau. 4. The normal voltage response could be compared with the current observed during a voltage clamp. At low light intensities the time course of the current response resembled the time course of the voltage response. When light intensity was increased the time course of the current response differed from the voltage response in that the time to peak amplitude remained relatively constant and an initial transient did not occur. It was possible to predict the current response produced by any intensity of light by using (i) an empirical equation which reproduced the time course of a dim response and (ii) the Michaelis-Menten equation. 5. The time course of the voltage-clamp current produced by a flash was the same at different values of maintained voltage. 6. The maximum amplitude of the voltage-clamp current produced by a flash or step of light was a non-linear function of membrane potential. It was relatively constant within the physiological range, decreased as the membrane potential was moved toward 0 mV, reversed polarity between 0 and 10 mV, and rapidly increased in magnitude as membrane potential was made more positive. Although this current was voltage dependent, no time dependence was evident (recording resolution greater than or equal to 5 msec). 7. Voltage-clamp experiments demonstrated an inward current which slowly developed after a hyperpolarizing voltage step. The effect of this voltage and time dependent current was to reduce, after a delay, the polarization initiated by light.

Ambystoma↗
PubMed1980

Rod and cone signals in the horizontal cells of the tiger salamander retina.

1. Intracellular recordings of horizontal cell responses to monochromatic lights of various wave-lengths and intensities were made in the retina of the larval tiger salamander in order to determine the contributions of rod and cone activities to horizontal cell responses. 2. Under conditions of extensive dark adaptation, and with dim light stimulation, the horizontal cell responses reflected mainly rod activity. In the light-adapted state or at high light intensities the cone contribution was dominant. 3. Bright adapting flashes selectively suppressed the rod component of horizontal cell responses. 4. Intracellular recordings from rods and cones showed that interactions between the two receptor types are very small and cannot account for the large rod--cone mixed input observed in horizontal cells. It is concluded that this input is mediated by direct connexions between receptors and horizontal cells.

Adaptation, Ocular↗
PubMed1980

Lateral contacts and interactions of horizontal cell dendrites in the retina of the larval tiger salamander.

1. The contacts of horizontal cell dendrites with processes of other second order neurones were studied at the level of the electron microscope in serial sections of the salamander retina. Intracellular recordings of the responses to light of horizontal and bipolar cells were used to investigate the possible significance of some of the morphological findings.2. Horizontal cell dendrites make close membrane appositions (gap junctions) with one another and are post-synaptic to bipolar cell dendrites at presumed chemical synapses. On the other hand, there is no clear evidence that horizontal cell dendrites are presynaptic to any other neuronal processes at the outer plexiform layer, so that the output connexions of horizontal cell bodies remain a matter of speculation.3. It is suggested that the bipolar cell input and the gap junctions between dendrites contribute, respectively, depolarizing and hyperpolarizing components to the responses of horizontal cell bodies to surround illumination. In addition, the facilitatory effect of central illumination on the surround response of horizontal cell bodies may result, although perhaps only partly, from observed properties of the surround response of bipolar cells.4. In the course of these observations, bipolar cells were found to be presynaptic at the outer plexiform layer not only to horizontal cell dendrites, but also to other bipolar cells, horizontal cell axon terminals and certain processes belonging to an as yet unidentified neurone.

Ambystoma↗
PubMed1980

Behaviour of the rod network in the tiger salamander retina mediated by membrane properties of individual rods.

1. The spread of electrical signals between rods in the salamander retina was examined by passing current into one rod and recording the voltage responses in nearby rods. Rod network behaviour, measured in this way, was simulated from data on rod membrane properties gathered in voltage-clamp experiments on single isolated rods.2. The network voltage responses to square current pulses became smaller, more transient, and had a longer time-to-peak, for rods further away from the site of current injection. Depolarizing currents produced smaller responses than hyperpolarizing currents of the same magnitude.3. Neighbouring rods and cones were coupled less strongly than neighbouring rods.4. The response of the rod network to current injection was unaffected by 2 mm-aspartate(-), which eliminates transmission from receptors to horizontal cells.5. The input resistance of single isolated rods, measured at the resting potential, varied between 100 and 680 MOmega. The lower values were probably due to damage by the micro-electrodes. Electrical coupling was found to be very strong between the rod inner and outer segments.6. A strong ;instantaneous' outward rectification seen in isolated rods at potentials positive to -35 mV was reduced, but not abolished, by 15 mm-TEA.7. In normal solution, isolated rods exhibited a voltage- and time-dependent current, I(A), whose kinetics were approximated by a single first-order gating variable, and whose activation curve spanned the range between -40 and -80 mV. The time constant for the current varied with voltage and was 60-200 msec between -140 and -40 mV.8. A reversal potential for I(A) could not be found between -140 and -40 mV in normal solution, and the fully activated current, I(A), was approximately voltage-independent, with a magnitude of approximately 0.1 nA over this potential range.9. By several criteria, I(A) behaved as a single inward current activated by hyperpolarization. Pharmacological studies suggest, however, that it is the sum of at least two currents with very similar kinetics.10. Most isolated rods exhibited a very slow (tau approximately 3 sec) increase in net outward current on depolarizing beyond -35 mV. The magnitude of this current varied considerably between cells.11. Assuming that the rod network can be approximated by a square lattice of individual rods, resistively coupled together, the voltage-clamp data on isolated rods were used to predict the response of the network to current injection at one cell. The theoretical and observed network behaviour were in good agreement. The resistance coupling neighbouring rods was estimated to be approximately 300 MOmega. The current I(A) plays a major role in determining the behaviour of the rod network.12. The time-dependent current, I(A), is responsible for the peak-plateau wave form of the response to a bright flash. A current similar to I(A) could also account for the negative propagation velocity of the peak of the dim flash response, through the rod network of the turtle, observed by Detwiler, Hodgkin & McNaughton (1978).

Animals↗
PubMed1981

Synaptic action mediating cone responses to annular illumination in the retina of the larval tiger salamander.

1. Responses of salamander cones to steps of light on their surrounding area were intracellularly recorded through micropipettes filled with 2 M-potassium acetate or 2 M-potassium chloride. 2. Using 2 M-potassium acetate, cone step responses showed a larger relaxation for a 1100 micrometers diameter spot than for a 100 micrometers spot, but dim annular illumination failed to evoke any detectable response. With 2 M-potassium chloride the larger spot or an anulus evoked responses that were mainly or purely depolarizing, respectively, while the responses to the smaller spot were still hyperpolarizing. 3. The depolarizing response to annular illumination had an increased amplitude immediately after discontinuing the injection of inward current through a micropipette filled with 2 M-potassium chloride. 4. Depolarization by extrinsic current reversed the polarity of the depolarizing response to annular illumination. 5. It is suggested that the enhancement of the depolarizing influence of the surround is due to an increase in the intracellular concentration of chloride ions, which issue from the electrodes by passive diffusion or electrophoresis. Taking into consideration the effect of depolarizing current, it would follow that surround illumination induces an increase in the chloride conductance of the cone membrane. 6. Preliminary observations on the properties of the enhanced surround response reveal a slow time course and a receptive field of more than 270 micrometers radius.

Ambystoma↗
PubMed1984

Effect of changes in intra- and extracellular sodium on the inward (anomalous) rectification in salamander photoreceptors.

Solitary rod inner segments were obtained by enzymic dissociation of the tiger salamander retina. Ih, an inward current activated by membrane hyperpolarization, was studied using the single-pipette voltage-clamp technique with patch pipettes. In order to investigate Ih in isolation from voltage-dependent potassium and calcium currents, it was necessary to superfuse with a solution containing TEA and cobalt. When the solution in the patch pipette contained 45 mM-KCl and 50 mM-NaCl, the characteristics of Ih were indistinguishable from those previously described with fine-tip micro-electrodes: the reversal potential was near-30 mV and Ih was blocked by extracellular caesium and enhanced by an increase in the extracellular potassium concentration. The increase in Ih observed when the extracellular potassium concentration is raised is due to an increase in conductance and in driving force. Replacement of sodium in the patch pipette with choline caused a 15 mV displacement of the reversal potential for Ih in the depolarized direction. When using sodium-free patch pipettes, replacement of extracellular sodium displaced the reversal potential for Ih to -74 mV, a value in the range of the potassium equilibrium potential in solitary inner segments. Intracellular or intra- and extracellular sodium substitution affected neither the activation range of Ih nor the maximum conductance. From points 3-6 it can be concluded that Ih is carried mainly, if not exclusively, by sodium and potassium and that the channel responsible for Ih is insensitive to modifications of the intra- or extracellular sodium concentration. The results of long-term hyperpolarization, of partial block with caesium and of total sodium substitution are consistent with sodium and potassium permeating the same type of channel.

Action Potentials↗
PubMed1984

A quantitative analysis of interactions between photoreceptors in the salamander (Ambystoma) retina.

A quantitative description of the electrical properties of the photoreceptor layer in the salamander retina was obtained from earlier data on the characteristics of isolated rods and cones and on rod-rod coupling, and from new data on rod-cone and cone-cone coupling and on the rod photocurrent. Injecting -1 nA current into a rod elicits hyperpolarizations of about 20 mV in an adjacent rod and 4 mV in an adjacent cone. Responses of more distant receptors are smaller. Injecting -1 nA into a cone elicits hyperpolarizations of about 4 mV in an adjacent rod and 0.4 mV in a nearby cone. Depolarizing current evokes smaller responses. Assuming, in agreement with anatomical evidence, that each rod is electrically coupled to four rods and to four cones around it, and that there is no direct electrical coupling between cones, we found these results could be predicted from the properties of isolated rods and cones if adjacent rods are coupled by a resistance of 300 M omega and adjacent rods and cones are coupled by a resistance of 5000 M omega. The small cone-cone coupling seen is due to coupling via intervening rods. The two halves of double cones are not electrically coupled. The spectral sensitivity of both halves is a maximum around 620 nm wave-length. The rod photocurrent has been characterized by voltage-clamping rods isolated from the retina. In agreement with Bader, MacLeish & Schwartz (1979) we found the time course of the photocurrent to be approximately independent of voltage between -35 and -85 mV. The voltage responses of rods, single cones and double cones isolated from the retina obey the principle of univariance. Responses of receptors in the retina do not obey univariance. The main deviations from univariance observed can be explained if adjacent rods and cones are coupled by a resistance of 5000 M omega. Our data demonstrate that rod-cone coupling is relatively weak. We simplified our description of the photoreceptor network, by omitting cones, to investigate the spatiotemporal processing that the rod network is capable of. Computer simulations predict, as is found experimentally, that the rod voltage response to a large spot of bright light should show a much more pronounced initial transient hyperpolarization than the response to a small spot of light of the same intensity. This difference is produced by the combination of electrical coupling of the rods with the existence of a voltage-gated current, IA, in the rod membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗