Measurement of nonuniform current densities and current kinetics in Aplysia neurons using a large patch method

An age-related decline in the cholinergic synaptic response may cause the firing pattern in the jaw-closing motor neurons, which resembles the aversive taste response in the feeding behavior of old Aplysia kurodai

Anorexia due to aging is recognized as a syndrome of animal feeding behavior. Age-related functional disorders of the brain often cause behavioral changes. We used Aplysia kurodai to study this neural mechanism, following our previous study on food preference behaviors. The age of each wild animal was defined by a previously described method, and a significant age-related decline in food intake was observed. In this study, we explored the effects of aging on a specific inhibitory synaptic response in jaw-closing (JC) motor neurons produced by cholinergic multiaction (MA) neurons, the size of which determines the delay between MA and JC firings and this delay is reduced during aversive taste responses; in our analyses, we found a significant age-related decline in the synaptic response. Thereafter, we further explored whether such functional decline affects the JC firing pattern during the normal feeding response. During the feeding-like rhythmic responses induced by electrical nerve stimulation, the firing of the JC motor neurons advanced toward that of the MA burst, which typically happens during aversive taste responses. These results suggest that the age-related decline in the cholinergic synaptic response may partly cause the JC firing patterns that resemble the aversive taste response in old animals.

Current- and Voltage-Clamp Recordings and Computer Simulations of Kenyon Cells in the Honeybee

The mushroom body of the insect brain is an important locus for olfactory information processing and associative learning. The present study investigated the biophysical properties of Kenyon cells, which form the mushroom body. Current- and voltage-clamp analyses were performed on cultured Kenyon cells from honeybees. Current-clamp analyses indicated that Kenyon cells did not spike spontaneously in vitro. However, spikes could be elicited by current injection in approximately 85% of the cells. Of the cells that produced spikes during a 1-s depolarizing current pulse, approximately 60% exhibited repetitive spiking, whereas the remaining approximately 40% fired a single spike. Cells that spiked repetitively showed little frequency adaptation. However, spikes consistently became broader and smaller during repetitive activity. Voltage-clamp analyses characterized a fast transient Na+current ( INa), a delayed rectifier K+current ( IK,V), and a fast transient K+current ( IK,A). Using the neurosimulator SNNAP, a Hodgkin–Huxley-type model was developed and used to investigate the roles of the different currents during spiking. The model led to the prediction of a slow transient outward current ( IK,ST) that was subsequently identified by reevaluating the voltage-clamp data. Simulations indicated that the primary currents that underlie spiking are INaand IK,V, whereas IK,Aand IK,STprimarily determined the responsiveness of the model to stimuli such as constant or oscillatory injections of current.

An improved method for real-time monitoring of membrane capacitance in Xenopus laevis oocytes

Measurements of membrane capacitance (C(m)) in Xenopus laevis oocytes offer unique experimental possibilities but are difficult to perform with current methods. To improve C(m) measurements in the two-electrode voltage clamp (TEVC) mode, we developed a paired-ramp protocol and tested its performance in a model circuit (with tunable C(m), membrane resistance R(m), and series resistance R(s)) and in Xenopus oocytes. In the cell model and with R(s) = 0 Omega, inaccuracy of C(m) estimates was <1% under widely varying conditions (R(m) ranging from 100 to 2000 kOmega, and C(m) from 50 to 1000 nF). With R(s) > 0 Omega, C(m) was underestimated by a relative error epsilon closely approximated as epsilon approximate 2 x R(s)/(R(s) + R(m)), in keeping with the theoretical prediction. Thus, epsilon may be neglected under standard conditions or, under extreme conditions, corrected for if R(s) is known. Relative imprecision of C(m) estimates was small, independent of R(s), and inversely related to C(m) (<1.5% at 50 nF, <0.4% at 200 nF). Averaging allowed reliable detection of C(m) deviations from 200 nF of 0.1 nF, i.e., 0.05%. In Xenopus oocytes, we could resolve C(m) changes that were small (e.g., DeltaC(m) approximate 2 nF upon 100 muM 8-Br-cAMP), fast (e.g., DeltaC(m)/Deltat approximate 20nF/30s upon 1 muM phorbol myristate acetate (PMA)) or extended and complex (e.g., fast increase, followed by prolonged C(m) decrease upon 1 muM PMA). Rapidly alternating between paired ramps and a second, step protocol allowed quasi-simultaneous monitoring of additional electrical parameters such as R(m), slope conductance g(m), and reversal potential E(rev). Taken together, our method is suited to monitor C(m) in Xenopus oocytes conveniently, with high temporal resolution, accuracy and precision, and in parallel with other electrical parameters. Thus, it may be useful for the study of endo- and exocytosis and of membrane protein regulation and for electrophysiological high-throughput screening.

Mapping fungal ion channel locations

Ion channel mapping techniques are described and the results for two fungal organisms, Saprolegnia ferax and Neurospora crassa, are presented. In these species, two channel types have been characterized, stretch-activated channels exhibiting significant calcium permeability and spontaneous channels having significant potassium permeability. Two distinct analyses of patch clamp data, analysis of channel self-clustering and association between different channel types, and localization along the hyphae, reveal significant differences between the two organisms. S. ferax maintains a tip-high gradient of both channel types which is lost after disruption of the actin cytoskeleton. There is significant self-clustering of the channels, as well as interactions between channel types. N. crassa on the other hand does not maintain tip-high gradients, and clustered distributions are observed only for the stretch-activated channels. In terms of physiological roles, evidence is quite strong that the stretch-activated channels function as a growth sensor in S. ferax, but have an unknown function in N. crassa. In both organisms, the potassium permeable channels presumably function in potassium uptake. The differences between these two organisms may be due, in part, to differences in their normal environment: aquatic versus terrestrial. Copyright 1998 Academic Press.

A-type potassium channel clusters revealed using a new statistical analysis of loose patch data

The spatial distribution of ion channels over the surface of a neuron is an important determinant of its excitable properties. We introduce two measures of channel clustering for use in patch-clamp experiments: a normalized chi-squared statistic (eta) and the number of zero-channel patches in a data set (Z). These statistics were calculated for data sets describing the distribution of A-type potassium channels on neurons of the nudibranch Doriopsilla and measurements of Ca-dependent outward current channels on bullfrog hair cells, as well as simulated channel distributions. When channels are clustered, eta is approximately equal to the amount of current in a cluster. The analysis shows that somatic A-channels in the nudibranch are distributed in clusters of approximately 50 channels each. The clusters are < 2 microns wide and are separated, on average, by 3.2 microns. Outward current channels on hair cells occur in clusters of approximately 27 channels each, in agreement with the original analysis. Channel clustering may reflect properties of the insertion or regulation of channels in the membrane.

Enzymatic gating of voltage-activated calcium channels

The model of calcium-channel gating described above, although almost certainly too simple, suggests a direct role for protein kinases and phosphatases in determining the kinetics of calcium channel gating on a subsecond time scale. In addition, it provides a unique perspective for understanding studies of calcium channel gating under widely different metabolic and pharmacological conditions. Although many of these effects may be specific to the dihydropyridine-sensitive or L-type calcium channel, they give an indication of the range of possibilities for integrating calcium-channel activity with cellular biochemistry.

Calpastatin and nucleotides stabilize cardiac calcium channel activity in excised patches

The activity of single L-type Ca2+ channels is rapidly lost (run-down) when contact between the membrane and cytosol is interrupted. We have now achieved the stabilization of cardiac Ca2+ channel activity of guinea-pig ventricular myocytes by using either cytosol or defined components added to excised patches. The endogenous protease inhibitor, calpastatin, together with nucleotides, ATP + GTP, was found to prevent run-down as effectively as cardiac cytosolic solution. These results suggest the involvement of proteolysis by calpain in run-down of channel activity and enable the study of cardiac Ca2+ channel regulation with free access to both sides of the membrane.

Multiple conductance levels of the dihydropyridine-sensitive calcium channel in GH3 cells

Calcium channels in GH3 cells exhibit at least five conductance levels when examined in cell-attached or outside-out patches. These channels resemble the high threshold Ca2+ current in their range of activation and inactivation, and in their sensitivity to dihydropyridines (DHP). Mean open times for the five levels were brief (less than 1 msec) in control solutions but increased in the presence of BAY K 8644. In 100 mM Ba2+ and BAY K 8644, the five predominant slope conductances were 8-9, 12-13, 16-18, 23-24, and 28 pS. The present study is the first report of multiple levels of the DHP-sensitive Ca2+ channel occurring with high frequency in native membranes. The range of conductance levels that we observed encompasses the range of conductances found for two other different types of Ca2+ channels and indicates that unit conductance should be used with caution as a distinguishing characteristic for identification of different channel types.