Ionic permeabilities of L-glutamate activated, excitatory synaptic channel in crayfish muscle

Kainate induces an intracellular Na+-activated current in cultured embryonic rat hippocampal neurones

1. In embryonic rat hippocampal neurones cultured for < 3 days, kainate induced an inward current at negative potentials that recovered to baseline levels immediately upon termination of agonist application. However, in neurones cultured for longer, the kainate-induced current was often followed by a long-lasting inward current that slowly recovered to baseline levels. The amplitude of the delayed current (Idelay) triggered by kainate was positively related both to the duration of application at constant agonist concentration and to concentration at constant application duration. 2. Idelay could last for several minutes and was accompanied by a conductance increase, which closely paralleled current amplitude. Depression of the kainate-induced current response at receptor level with CNQX or at ionic level with Na+-free solution eliminated Idelay. However, when applied during Idelay neither CNQX nor Na+-free solution had any effect on Idelay. Li+ effected the same response as Na+ in mediating kainate-induced Idelay. 3. GABA-activated Cl- current, which was associated with the same amount of inwardly directed charge flow at the same potential as that induced by kainate, did not trigger a long-lasting delayed current. 4. Idelay depended on the existence of extracellular K+ and its amplitude increased with the increase in K+ concentration. Neither applying Cl-- or Ca2+-free solutions nor increasing intracellular Ca2+ buffering speed and capacity altered Idelay. Exposure to the specific KCa channel blockers apamin and charybdotoxin also failed to alter Idelay. However, Idelay could be blocked by Cs+, Ba2+ and high concentrations of 4-aminopyridine (4-AP) and TEA. 5. Inside-out excised patch-clamp recordings revealed that low density or highly clustered Na+-activated K+ channels were expressed in the cell bodies of cultured embryonic rat hippocampal neurones. These could be the elementary channels underlying Idelay.

Neuromuscular glutamatergic and GABAergic channels

Our laboratory has worked extensively on glutamatergic and GABA-ergic channels, predominantly in crayfish, but also in locust, Drosophila and recently Ascaris. Channel currents were recorded in the different modes of the patch-clamp technique (Hamill et al., 1981). The opening kinetics of the channels were derived from open and closed time histograms obtained from single channel recordings. From these, channel conductances could also be evaluated. The most relevant data were obtained by very rapidly rising and falling pulses (time of change about 0.1 ms) of agonists applied to outside-out patches containing the respective channels (Franke et al., 1987). From such recordings we constructed dose-response curves for peak and steady-state currents, for the rise times of the currents and for the time constants of desensitization. In double-pulse experiments we measured recovery from desensitization and predesensitization due to low agonist concentrations. For most of the channel types, we succeeded in constructing a reaction scheme which in computer simulations mimicked channel behaviour to a good approximation.

Rapid activation, desensitization, and resensitization of synaptic channels of crayfish muscle after glutamate pulses

Completely desensitizing excitatory channels were activated in outside-out patches of crayfish muscle membrane by applying glutamate pulses with switching times of approximately 0.2 ms for concentration changes. Channels were almost completely activated with 10 mM glutamate. Maximum activation was reached within 0.4 ms with greater than or equal to 1 mM glutamate. Channel open probability decayed with a time constant of desensitization of 2 ms with 10 mM glutamate and more rapidly at lower glutamate concentrations. The rate of beginnings of bursts (average number of beginnings of bursts per time bin) decayed even faster but approximately in proportion to the glutamate concentration. The dose-response curve for the channel open probability and for the rate of bursts had a maximum double-logarithmic slope of 5.1 and 4.2, respectively. Channels desensitized completely without opening at very low or slowly rising glutamate concentrations. Desensitization thus originates from a closed channel state. Resensitization was tested by pairs of completely desensitizing glutamate pulses. Sensitivity to the second pulse returned rapidly at pulse intervals between 1 and 2 ms and was almost complete with an interval of 3 ms. Schemes of channel activation by up to five glutamate binding steps, with desensitization by glutamate binding from closed states, are discussed. At high agonist concentrations bursts are predominantly terminated by desensitization. Quantal currents are generated by pulses of greater than 1 mM glutamate, and their decay is determined by the duration of presence of glutamate and possibly by desensitization.

Blood flow and blood volume determinations in aorta and in coronary circulation by density dilution

Continuous blood mass-density measurements were performed in anesthetized dogs and injections of 0.7-1.4 ml/kg isotonic saline solution were applied. The resulting density dilution curves were used to compute blood volume, total flow in the aorta and local flow in the coronary circulation. Blood volume calculations were compared with blood volume determined by Evans blue injections and a close agreement was found. Blood flow determined by density dilution was independent from the investigated sites of injection or sampling. We conclude from these results that small volume injections of isotonic saline solution can be used to determine blood volume and flow by density dilution. In addition to these findings, a marked retention of the injected fluid was observed. Possible mechanisms to explain this retention include albumin deposition in the endothelial pores and/or variations of blood viscosity and capillary pressure.

Calcium dependent gating of the L-glutamate activated, excitatory synaptic channel on crayfish muscle

Excitatory, glutamate-activated single channel currents were measured in outside-out patches of crayfish muscle. The open times of single channel openings, and the durations and rates of bursts were evaluated. These kinetic parameters were not appreciably affected by replacement of extracellular Na+ by Li+ or choline. Changes in extracellular Ca2+ concentration Cao also did not influence the duration of single openings. However the mean burst duration decreased for Cao less than 13.5 mM and the rate of bursts declined with a power of almost 2 in low Cao. At Cao less than 1 mM practically no channel openings were observed in presence of glutamate. In order to exclude more rapid desensitization of the glutamate receptors in low Cao as the cause of disappearance of channel openings, glutamate was applied in short pulses with a liquid-filament switch. In 0 Cao also a glutamate pulse did not trigger channel openings. In presence of 13.5mM Cao, the inorganic Ca-channel blockers La3+ and Cd2+ diminished the duration and rate of bursts of channel openings in a similar manner as low Cao. The effects of low Cao and of Cd2+ were tested also on quantal postsynaptic currents, EPSCs, which were recorded through a perfused macro-patch-clamp electrode. At 1.4 mM Cao in the perfused electrode tip, spontaneous EPSCs were reduced at least by a factor of 4, and elicited EPSCs by a factor of 16. Application of Cd2+ had similarly strong effects on the EPSCs. Also the decay of EPSCs was shortened substantially in 1.4 mM Cao or 5 mM Cd2+. The inhibitory Cl(-)-channel of crayfish muscle, activated by glutamate or GABA, also was studied in outside-out patches. The openings of this channel persisted in 0 Cao solutions; the block of channel openings in low Cao thus is a specific property of the excitatory channel. The action of Cao on the excitatory channel may be described as that of a cofactor to glutamate. A possible reaction scheme is proposed.