Properties of tetraethylammonium ion-resistant K+ channels in the photoreceptor membrane of the giant barnacle

Single-channel properties of BK-type calcium-activated potassium channels at a cholinergic presynaptic nerve terminal

1. A high-conductance calcium-activated potassium channel (BK KCa) was characterized at a cholinergic presynaptic nerve terminal using the calyx synapse isolated from the chick ciliary ganglion. 2. The channel had a conductance of 210 pS in a 150 mM:150 mM K+ gradient, was highly selective for K+ over Na+, and was sensitive to block by external charybdotoxin or tetraethylammonium (TEA) and by internal Ba2+. At +60 mV it was activated by cytoplasmic calcium [Ca2+]i with a Kd of approximately 0.5 microM and a Hill coefficient of approximately 2.0. At 10 microM [Ca2+]i the channel was 50 % activated (V) at -8.0 mV with a voltage dependence (Boltzmann slope-factor) of 32.7 mV. The V values hyperpolarized with an increase in [Ca2+]i while the slope factors decreased. There were no overt differences in conductance or [Ca2+]i sensitivity between BK channels from the transmitter release face and the non-release face. 3. Open and closed times were fitted by two and three exponentials, respectively. The slow time constants were strongly affected by both [Ca2+]i and membrane potential changes. 4. In cell-attached patch recordings BK channel opening was enhanced by a prepulse permissive for calcium influx through the patch, suggesting that the channel can be activated by calcium ion influx through neighbouring calcium channels. 5. The properties of the presynaptic BK channel are well suited for rapid activation during the presynaptic depolarization and Ca2+ influx that are associated with transmitter release. This channel may play an important role in terminating release by rapid repolarization of the action potential.

Currents in the presynaptic terminal arbors of barnacle photoreceptors

We have described the currents flowing across the presynaptic membranes of the four median photoreceptors of the giant barnacle, Balanus nubilus, using a quasi-voltage clamp arrangement. Membrane potential, measured in the terminal region of one photoreceptor, was controlled in all four terminals by feedback current supplied through the nerve containing the photoreceptors' axons. The [Ca2+]o in the saline was reduced to decrease the Ca2+ current, enabling better voltage control, and tetraethylammonium ion (TEA, 20 mM) was added to block a fast voltage-dependent K+ conductance. Depolarizing voltage steps from the resting potential in the dark (-60 mV) evoked slow, inward Ca(2+)-dependent currents which could be blocked by Co2+, Mg2+, or Cd2+. The Ca2+ currents were followed by large outward currents that persisted for many seconds after the offset of moderate or large pulses. These tail currents increased in magnitude and duration with pulse duration and reversed at about -80 mV, consistent with previous evidence for a Ca(2+)-activated K+ conductance in this membrane. When the Ca(2+)-activated outward current was reduced to zero by increasing the [K+]o so as to set EK at -20 mV, and then stepping the voltage to this value, the step evoked a steady inward Ca2+ current. Thus, the Ca2+ current did not show voltage- or Ca(2+)-dependent inactivation. When Ba2+ was substituted for Ca2+, 500-ms depolarizing steps evoked steady inward currents but no outward currents. In any given experiment, the activation voltage of the Ca2+ or Ba2+ current did not depend on holding potential. At the barnacle photoreceptor's synapse, the postsynaptic cell adapts to maintained presynaptic voltage by a mechanism that is not understood. We conclude that neither Ca2+ current inactivation nor a shift in activation voltage with holding potential can account for this adaptation.

Regulation of transmitter release at the squid giant synapse by presynaptic delayed rectifier potassium current

1. The three-microelectrode voltage clamp technique and pharmacological agents were used to examine the properties and functions of potassium currents in squid giant presynaptic terminals. 2. Outward currents consisted of two components: a slow component which activated over hundreds of milliseconds and was blocked by extracellular application of tetraethylammonium (TEA) ions and a more rapidly activating component which was relatively insensitive to extracellular TEA. 3. The more rapid component was studied in isolation by treating presynaptic terminals with extracellular TEA, as well as tetrodotoxin (to block sodium channel currents) and manganese (to block calcium channel currents). The magnitude of this current component was 1-2 mA cm-2 at 0 mV. Rates of activation and deactivation were voltage dependent and little evidence of inactivation was seen for depolarizations less than several seconds in duration. 4. The reversal potential of the current was -70 to -80 mV in normal saline and became more positive with elevated extracellular potassium concentrations, suggesting that potassium is the primary permeant ion. Accumulation of extracellular potassium appeared to be marked during depolarizations that produced significant activation of the current. 5. Extracellular application of 3,4-diaminopyridine (DAP) blocked the current with an apparent dissociation constant of 7 microM at 0 mV. Intracellular applications of DAP and TEA also were effective in reducing this current. These treatments, but not extracellular TEA application, broadened presynaptic action potentials and increased the magnitude and time-to-peak of postsynaptic currents elicited by the broadened presynaptic action potentials. Postsynaptic currents were a sensitive and linear function of action potential duration; a 30% increase in action potential duration increased postsynaptic current amplitude by 190%. 6. Estimation of the magnitude and time course of the presynaptic calcium current, based on previous measurements of calcium channel gating, indicated that action potential broadening produces a large increase in calcium current magnitude. These calculations predict that a 30% increase in presynaptic action potential duration will increase the peak amplitude of the calcium current by approximately 170% and the total amount of calcium entry by approximately 230%. This implies a linear relationship between transmitter release and calcium entry during an action potential and can be explained by assuming that calcium co-operatively triggers release within intracellular domains that do not overlap.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium activated potassium channels in cultured astrocytes

The patch clamp technique was used to analyze single channel currents in intact and excised patches of glial cell membranes grown in primary cultures from newborn rat brain. Glial cells were morphologically identified by immunohistochemical staining for glial fibrillary acidic protein. Outward currents due to single channels were observed in recordings from both intact and excised patches obtained from the cell body region. The channel responsible for these currents was preferentially permeable to K+ because the reversal potential for this current was correlated with changes in the potassium equilibrium potential, when experimentally altered. The single channel conductance was 25 pS when measured between -20 and +20 mV in solutions with physiological K+ concentrations (10 degrees C). Channel gating was dependent on both the internal Ca2+ concentration and the membrane potential. Either depolarization of the membrane patch, or the addition of increasing Ca2+ concentrations to the internal surface, increased the probability of channel opening. Tetraethylammonium reversibly blocked the channel whereas 4-aminopyridine had no effect. The characteristics exhibited by this channel indicate that a Ca2+-activated K+ channel is present in the membrane of astrocytes grown in culture. These results, combined with previous evidence for a voltage dependent Ca2+ channel, suggest a dynamic role for glial cells in controlling excitability in the central nervous system. Influx of Ca2+ upon depolarization would increase the membrane permeability to K+ and could increase the "buffering" capacity of glial cells for extracellular K+.

A calcium-activated potassium current in motor nerve terminals of the mouse

Local circuit currents involving presynaptic terminals were recorded by micro-electrodes inserted into the perineurium of nerves from the triangularis sterni muscle of the mouse. A transient outward current component was isolated by blocking the voltage-activated (delayed rectifier) K current by 3,4-diaminopyridine (3,4-DAP). The amplitude of this component depended on external K concentration and fell to zero at [K]o = 15 mM. Since it also depended on [Ca]o, it was identified as a Ca-activated K current (IK(Ca)). Tetraethylammonium (TEA) (2 mM), Ba (2 mM), Co (10 mM) and Mn (2.5 mM) blocked IK(Ca). IK(Ca) decayed to zero in approximately 12 ms and recovered from inactivation in about 100 ms. Ca current was enhanced in inverse proportion to the degree of IK(Ca) depression. The possible role of IK(Ca) in the process of neuromuscular facilitation is briefly discussed.

Adaptation in the input-output relation of the synapse made by the barnacle's photoreceptor

A study was made of synaptic transmission between the four median photoreceptors of the giant barnacle (Balanus nubilus) and their post-synaptic cells (I-cells). Simultaneous intracellular recordings were made from the presynaptic terminal region of a photoreceptor and from the soma of an I-cell. The photoreceptor's membrane potential provided feed-back to bath electrodes that passed current into the receptors' axons, permitting the voltage to be controlled at the point of arborization of their presynaptic terminals. Simultaneous recordings from a second photoreceptor showed that its voltage tracked the first. Step depolarizations of the receptors from their dark resting potential (about -60 mV) caused hyperpolarizations of the I-cell that reached a peak, then decayed to a plateau value. The amplitude of the I-cell's response grew with presynaptic depolarizations, saturating at presynaptic values 10-20 mV depolarized from dark rest. Step hyperpolarizations of the receptors from dark rest evoked depolarizations of the I-cell consisting of an initial peak, which varied greatly in amplitude and wave form from preparation to preparation, followed by a plateau. The presence of this post-synaptic response indicates that transmitter is released continuously from the receptors at their dark resting potential. An input-output relation of the synapse was obtained by presenting step depolarizations from a holding potential of -80 mV, where steady-state transmitter release is shut off. The relation is sigmoidal; in the exponentially rising phase of the curve, a 5-11 mV presynaptic change produces a 10-fold change in post-synaptic response. When the presynaptic holding potential was set at values ranging from -80 to -40 mV, the relation between the I-cell's response and the absolute potential to which the receptor was stepped shifted along the presynaptic voltage axis. The slopes of the input-output relations were roughly parallel or increased as the photoreceptors were held more depolarized. This observation limits the possible mechanisms of the shift.

Localized Ca2+ and calcium-activated potassium conductances in terminals of a barnacle photoreceptor

Calcium channels are found in the presynaptic terminals of neurones, where they have a key role in synaptic transmission. They are also found in the somata of many cells, in dendrites and along a few axons. In no cell is the actual distribution of these channels known in detail, because there are no known toxins or other agents suitable for labelling calcium channels, and the current through these channels is usually too small to be quantified with extracellular electrodes. However, several experiments have suggested that the density of the channels is less in the axon than in the cell body or terminal region. Here we have used the indicator dye Arsenazo III in conjunction with an array of photodetectors to examine the spatial influx of calcium in the presynaptic terminal region of the giant barnacle, Balanus nubilus. In these cells, calcium entry occurs in a restricted region less than 50 micron in length, which corresponds closely to the region of synaptic contact with second-order cells. Outside this area the magnitude of calcium entry is reduced at least 50-fold. With reasonable assumptions it follows that the calcium channel density is equally localized. In addition, we demonstrate that these cells have a calcium-activated potassium conductance. Since calcium entry is restricted to the synaptic zone, this conductance must be effective only in this region.

Oxygen uptake occurs faster than sodium pumping in bee retina after a light flash

When neurones are active there is an entry of Na+, which must subsequently be pumped out, and an increase in their oxygen consumption rate (Qo2). The Na+ pump derives its energy from ATP, splitting it into ADP and Pi, and it has reasonably been proposed that the changes in concentrations of ATP, ADP and Pi lead to a stimulation of the O2 consumption by the mitochondria and hence to a restoration of the stock of ATP. Here we present evidence suggesting that Qo2 must be controlled differently in the retinal photoreceptor cells of the honeybee drone. Stimulation of drone photoreceptors with a flash of light causes an entry of Na+ (ref. 4) and a transient increase in Qo2 that indicates respiration of the right order of magnitude to provide ATP to pump the Na+ out. We report intracellular recordings of changes in intracellular sodium (Nai+) and potassium (Ki+) in response to single light flashes and have compared the time course of extra oxygen consumption (delta Qo2) with these ion changes and other indices of Na+ pumping. We found that the time course of pumping seems to lag behind the time course of delta Qo2. It follows that the mitochondrial respiration must be stimulated by some signal which is generated earlier than the rise in ADP produced by the Na+ pump.