Glutamate current noise: post‐synaptic channel kinetics investigated under voltage clamp.

SUMMARY1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na+ (i.e. in normal medium, Ca2+-free medium and Cl--free medium). M.e.j.c.s were also examined in isotonic Ca2+ where the inward post-synaptic current was carried mainly by Ca2+.2. In normal medium, mean m.e.j.c. amplitude = 2-34+0-05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) wasTm.e.j.c. = 2-62+0-11 msec (Vm = -80 mV, T = 22 0(). Decay-time was not markedly changed in Ca2+-free or Cl--free medium. Tm e jc approaches the life-time of glutamate activated junctional channels.3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.4. Tm.ej.c decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182X1 + 24X8 mV change in Vm.5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca2+-free and Cl--free solutions.6. The kinetics of junctional channels are altered in isotonic Ca2+. M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j .c. = 0 74 + 0 03 nA.The decay time becomes markedly briefer, Tm e jic = 101 +0-08 msec, indicating a reduction in mean channel life-time (Vm = -80 mV, T = 22 C).7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca2+, thus producing a prolongation in mean Tm ej c. This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of Tm e. c. of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca2+. The equilibrium potential for transmitter action may be slightly more positive than normal.8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.

Section: Methodssupporting

confidence: 78%

Section: Cs In Isotonic Ca2+supporting

confidence: 76%

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Properties of miniature excitatory junctional currents at the locust nerve‐muscle junction

Cull-Candy

Miledi

1982

“…5A, approximately 144 channels must be open at any time (and since the probability of opening is low, there must be far more channels than this actually in the cell membrane). In six cells studied with 30 mM-Cl-in the patch pipette (three cells with solution C and three cells with solution E), the average single-channel conductance found in this way was 5-4 + 1-3 pS, roughly twice the value found for the channels opened by glutamate in goldfish horizontal cells (Ishida & Neyton, 1985) but much smaller than the 120 pS glutamate-gated channel in locust muscle (Anderson, Cull-Candy & Miledi, 1978). There was no significant difference between the values obtained with low or high internal calcium concentration (solutions C or E, respectively).…”

Section: Noise Changes Associated With the Glutamate-induced Conductamentioning

confidence: 93%

Neurotransmitter‐induced currents in retinal bipolar cells of the axolotl, Ambystoma mexicanum.

Attwell

Mobbs

Tessier‐Lavigne

et al. 1987

116

SUMMARY1. Whole-cell patch clamping was used to study the membrane properties of isolated bipolar cells and the currents evoked in them by putative retinal neurotransmitters.2. Isolated bipolar cells show an approximately ohmic response to voltage steps over most of the physiological response range, with an average input resistance of 1-3 GfS and resting potential of -35 mV. These values are underestimates because of the shunting effect of the seal between the patch electrode and the cell membrane. Depolarization beyond -30 mV produces rapid activation (10-100 ms) ofan outward current (carried largely by potassium ions), which then inactivates slowly (05-2 s).3. Of five candidates for the photoreceptor transmitter, four (aspartate, Nacetylhistidine, cadaverine, putrescine) had no effect on bipolar cells. The fifth substance, L-glutamate, opened ionic channels with a mean reversal potential of -12 mV in some cells (presumed hyperpolarizing bipolar cells), and closed channels with a mean reversal potential, of -13 mV in other cells (presumed depolarizing bipolar cells); 4. The conductance increase induced by glutamate in presumed hyperpolarizing bipolar cells was associated with an increase in membrane current noise. Noise analysis suggested a single-channel conductance for the glutamate-gated channel of 5.4 pS. The power spectrum of the noise increase required the sum of two Lorentzian curves to fit it, suggesting that the channel can exist in three states.5. The conductance decrease induced by glutamate in presumed depolarizing bipolar cells was associated with a decrease in membrane current noise that could be described as the sum of two Lorentzian spectra, and which suggested a singlechannel conductance of 11 pS. The noise decrease implies that the channels closed by glutamate are not all open in the absence of the transmitter.6. GABA (y-aminobutyric acid) and glycine, transmitters believed to mediate lateral inhibition in the retina, open chloride channels in isolated bipolar cells, and increase the membrane current noise. Noise analysis suggested that the channels gated by GABA and glycine have conductances of 4-4 and 7-5 pS respectively. The noise spectra required the sum of two Lorentzian curves to fit them.t To whom all reprint requests should be sent.

“…Subsequent work (cf. Neher & Stevens, 1977;Cull-Candy, Miledi & Trautwein, 1979;Anderson, Cull-Candy & Miledi, 1978;Barker & McBurney. 1979) has demonstrated the wide applicability and usefulness of the method.…”

Section: Introductionmentioning

confidence: 99%

The kinetics of slow muscle acetylcholine‐operated channels in the garter snake.

Dionne¹

1981

SUMMARY1. Slow muscle synaptic responses were modelled kinetically in an attempt to define the mechanism by which slow fibre acetylcholine-operated channels differ from those in twitch fibres.2. Three kinetically distinguishable states were necessary. 3. All applicable three-state kinetic schemes were considered in an attempt to identify the simplest description of the data. Experimental tests eliminated several models. Two models were not tested because they contained an excessive number of adjustable parameters.4. The data were not fitted by kinetic schemes which postulated (i) channels which opened with one as well as two bound agonist molecules, (ii) channels which became blocked after opening, or (iii) separate populations of synaptic and extrasynaptic channels.5. The three-state kinetic model of del Castillo & Katz (1957) accurately described all the data. This sequential model relates a closed channel state with no agonist bound to its receptors, an intermediate state (also closed) with agonist bound, and an open channel state. It is the same model which has been used to describe synaptic responses in twitch fibres.6. The variation which allows, this model to describe both twitch and slow fibre synaptic responses is the lifetime of the intermediate state. In twitch fibres the intermediate state lifetime is undetectably brief by electrophysiological methods. However, in slow fibres this lifetime appears to be 1-2 msec, varying with voltage.7. Three of the four transition rates in this three-state kinetic scheme may be estimated by fitting the model to the data. These are the channel opening rate, the channel closing rate and the rate at which closed channels lose their bound agonist molecules. The latter two rates appear to depend exponentially on voltage. The channel opening rate was not detectably voltage-sensitive.