Internal perfusion studies demonstrating GABA-induced chloride responses in frog primary afferent neurons

Minimal and Maximal Models to Quantitate Glucose Metabolism: Tools to Measure, to Simulate and to Run in Silico Clinical Trials

Several models have been proposed to describe the glucose system at whole-body, organ/tissue and cellular level, designed to measure non-accessible parameters (minimal models), to simulate system behavior and run in silico clinical trials (maximal models). Here, we will review the authors' work, by putting it into a concise historical background. We will discuss first the parametric portrait provided by the oral minimal models-building on the classical intravenous glucose tolerance test minimal models-to measure otherwise non-accessible key parameters like insulin sensitivity and beta-cell responsivity from a physiological oral test, the mixed meal or the oral glucose tolerance tests, and what can be gained by adding a tracer to the oral glucose dose. These models were used in various pathophysiological studies, which we will briefly review. A deeper understanding of insulin sensitivity can be gained by measuring insulin action in the skeletal muscle. This requires the use of isotopic tracers: both the classical multiple-tracer dilution and the positron emission tomography techniques are discussed, which quantitate the effect of insulin on the individual steps of glucose metabolism, that is, bidirectional transport plasma-interstitium, and phosphorylation. Finally, we will present a cellular model of insulin secretion that, using a multiscale modeling approach, highlights the relations between minimal model indices and subcellular secretory events. In terms of maximal models, we will move from a parametric to a flux portrait of the system by discussing the triple tracer meal protocol implemented with the tracer-to-tracee clamp technique. This allows to arrive at quasi-model independent measurement of glucose rate of appearance (Ra), endogenous glucose production (EGP), and glucose rate of disappearance (Rd). Both the fast absorbing simple carbs and the slow absorbing complex carbs are discussed. This rich data base has allowed us to build the UVA/Padova Type 1 diabetes and the Padova Type 2 diabetes large scale simulators. In particular, the UVA/Padova Type 1 simulator proved to be a very useful tool to safely and effectively test in silico closed-loop control algorithms for an artificial pancreas (AP). This was the first and unique simulator of the glucose system accepted by the U.S. Food and Drug Administration as a substitute to animal trials for in silico testing AP algorithms. Recent uses of the simulator have looked at glucose sensors for non-adjunctive use and new insulin molecules.

Guidelines for the quality control of population pharmacokinetic-pharmacodynamic analyses: an industry perspective

Quality population modeling and simulation analyses and reports are something every modeler desires. However, little attention in the literature has been paid to what constitutes quality regarding population analyses. Very rarely do published manuscripts contain any statement about quality assurance of the modeling results contained therein. The purpose of this manuscript is to present guidelines for the quality assurance of population analyses, particularly with regards to the use of NONMEM from an industrial perspective. Quality guidelines are developed for the NONMEM installation itself, NONMEM data sets, control streams, output listings, output data files and resultant post-processing, reporting of results, and the review processes. These guidelines were developed to be thorough yet practical, though are not meant to be completely comprehensive. It is our desire to ensure that what is reported accurately reflects the collected data, the modeling process, and model outputs for a modeling project.

Molecular and functional expression of cation-chloride cotransporters in dorsal root ganglion neurons during postnatal maturation

GABA depolarizes and excites central neurons during early development, becoming inhibitory and hyperpolarizing with maturation. This "developmental shift" occurs abruptly, reflecting a decrease in intracellular Cl(-) concentration ([Cl(-)](i)) and a hyperpolarizing shift in Cl(-) equilibrium potential due to upregulation of the K(+)-Cl(-) cotransporter KCC2b, a neuron-specific Cl(-) extruder. In contrast, primary afferent neurons (PANs) are depolarized by GABA throughout adulthood because of expression of NKCC1, a Na(+)-K(+)-2Cl(-) cotransporter that accumulates Cl(-) above equilibrium. The GABA(A)-mediated depolarization of PANs determines presynaptic inhibition in the spinal cord, a key mechanism gating somatosensory information. Little is known about developmental changes in Cl(-) transporter expression and Cl(-) homeostasis in PANs. Whether NKCC1 is expressed in PANs of all phenotypes or is restricted to subpopulations (e.g., nociceptors) is debatable. Likewise, whether PANs express KCC2s is controversial. We investigated NKCC1 and K(+)-Cl(-) cotransporter expression in rat and mouse dorsal root ganglion (DRG) neurons with molecular methods. Using fluorescence imaging microscopy, we measured [Cl(-)](i) in acutely dissociated rat DRG neurons (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and classified with phenotypic markers. DRG neurons of all sizes express two NKCC1 mRNAs, one full-length and a shorter splice variant lacking exon 21. Immunolabeling with validated antibodies revealed ubiquitous expression of NKCC1 in DRG neurons irrespective of postnatal age and phenotype. As maturation progresses [Cl(-)](i) decreases gradually, persisting above equilibrium in >95% mature neurons. DRG neurons express mRNAs for KCC1, KCC3s, and KCC4, but not for KCC2s. Mechanisms underlying PANs' developmental changes in Cl(-) homeostasis are discussed and compared with those of central neurons.

Physiologically-based pharmacokinetic (PBPK) models in toxicity testing and risk assessment

Physiologically-based pharmacokinetic (PBPK) modeling offers a scientifically-sound framework for integrating mechanistic data on absorption, distribution, metabolism and elimination to predict the time-course of parent chemical, metabolite(s) or biomarkers in the exposed organism. A major advantage of PBPK models is their ability to forecast the impact of specific mechanistic processes and determinants on the tissue dose. In this regard, they facilitate integration of data obtained with in vitro and in silico methods, for making predictions of the tissue dosimetry in the whole animal, thus reducing and/or refining the use of animals in pharmacokinetic and toxicity studies. This chapter presents the principles and practice of PBPK modeling, as well as the application of these models in toxicity testing and health risk assessments.

Parameter estimation for linear compartmental models--a sensitivity analysis approach

Linear compartmental models are useful, explanatory tools, that have been widely used to represent the dynamic behavior of complex biological systems. This paper addresses the problem of the numerical identification of such models, i.e., the estimation of the parameter values that will generate predictions closest to experimental observations. Traditional local optimization techniques find it difficult to arrive at satisfactory solutions to such a parameter estimation problem, especially when the number of parameters is large and/or few data are available from experiments. We present herewith a method based on a prior sensitivity analysis, which enables division of a large optimization problem into several smaller and simpler subproblems, on which only sensitive parameters are estimated, before the whole optimization problem is tackled from starting points that are already close to the optimum values. This method has been applied successfully to a linear 13-compartment, 21-parameter model describing the postprandial metabolism of dietary nitrogen in humans. The effectiveness of the method has been demonstrated using simulated and real data obtained in the intestine, blood and urine of healthy humans after the ingestion of a [(15)N]-labeled protein meal.

Expression and functional characterization of GABA transporters in crayfish neurosecretory cells

The effect of GABA on membrane potential and ionic currents of X-organ neurons isolated from the crayfish eyestalk was investigated. Under voltage-clamp conditions, GABA elicited an inward Na+ current followed by a sustained outward chloride current. Sodium current was partially blocked in a dose-dependent manner by antagonists of GABA plasma membrane transporters such as beta-alanine, nipecotic acid, 1-[2([(diphenylmethylene)imino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NO 711), and SKF89976-A at concentrations between 1 and 100 microm. This current was totally blocked by the combined application of NO 711 (5 microm) and beta-alanine (50 microm). We obtained an EC(50) of 5 microm and a Hill coefficient of 0.97 for the GABA transport mediated response. These results together with studies of immunolocalization using antibodies against neuronal vertebrate GABA transporters (GATs) indicate the presence of GAT-1- and GAT-3-like proteins in X-organ neurons. To isolate the sustained outward Cl- current, extracellular free sodium solution was used to minimize the contribution of GAT activity. We concluded that this current was caused by the activation of GABA(A)-like receptors with an EC50 of 10 microm and a Hill number of 1.7. To assign a functional role to the GATs in the X-organ sinus gland system, we determine the GABA concentration (0.46-0.15 microm) in hemolymph samples using HPLC. In summary, our results suggest that a sodium-dependent electrogenic GABA uptake mechanism has a direct influence on the excitability of the X-organ neurons, maintaining an excitatory tone that is dependent on the circulating GABA level.

Antagonistic action of pitrazepin on human and rat GABA(A) receptors

Pitrazepin, 3-(piperazinyl-1)-9H-dibenz(c,f) triazolo(4,5-a)azepin is a piperazine antagonist of GABA in a variety of electrophysiological and in vitro binding studies involving GABA and glycine receptors. In the present study we have investigated the effects of pitrazepin, and the GABA(A) antagonist bicuculline, on membrane currents elicited by GABA in Xenopus oocytes injected with rat cerebral cortex mRNA or cDNAs encoding alpha1beta2 or alpha1beta2gamma2s human GABA(A) receptor subunits. The three types of GABA(A) receptors expressed were reversibly antagonized by bicuculline and pitrazepin in a concentration-dependent manner. GABA dose-current response curves for the three types of receptors were shifted to the right, in a parallel manner, by increasing concentrations of pitrazepin. Schild analyses gave pA2 values of 6.42+/-0.62, n = 4, 6.41+/-1.2, n = 5 and 6.21+/-1.24, n = 6, in oocytes expressing rat cerebral cortex, alpha1beta2 or alpha1beta2gamma2s human GABA(A) receptors respectively (values are given as means +/- s.e. mean), and the Hill coefficients were all close to unity. All this is consistent with the notion that pitrazepin acts as a competitive antagonist of these GABA(A) receptors; and that their antagonism by pitrazepin is not strongly dependent on the subunit composition of the receptors here studied. Since pitrazepin has been reported to act also at the benzodiazepine binding site, we studied the effect of the benzodiazepine antagonist Ro 15-1788 (flumazenil) on the inhibition of alpha1beta2gamma2s receptors by pitrazepin. Co-application of Ro 15-1788 did not alter the inhibiting effect of pitrazepin. Moreover, pitrazepin did not antagonize the potentiation of GABA-currents by flunitrazepam. All this suggests that pitrazepin does not affect the GABA receptor-chloride channel by interacting with the benzodiazepine receptor site.

Compartmental models: theory and practice using the SAAM II software system

Understanding in vivo the functioning of metabolic systems at the whole-body or regional level requires one to make some assumptions on how the system works and to describe them mathematically, that is, to postulate a model of the system. Models of systems can have different characteristics depending on the properties of the system and the database available for their study; they can be deterministic or stochastic, dynamic or static, with lumped or distributed parameters. Metabolic systems are dynamic systems and we focus here on the most widely used class of dynamic (differential equation) models: compartmental models. This is a class of models for which the governing law is conservation of mass. It is a very attractive class to users because it formalizes physical intuition in a simple and reasonable way. Compartmental models are lumped parameter models, in that the events in the system are described by a finite number of changing variables, and are thus described by ordinary differential equations. While stochastic compartment models can also be defined, we discuss here the deterministic versions--those that can work with exact relationships between model variables. These are the models most widely used in discussions of endocrinology and metabolism. In this chapter, we will discuss the theory of compartmental models, and then discuss how the SAAM II software system, a system designed specifically to aid in the development and testing of multicompartmental models, can be used.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Development of compartmental models in stable-isotope experiments: application to lipid metabolism

Kinetic experiments are of great importance in lipid research because they further the understanding of lipid metabolism in vivo and help to explain the physiopathology of lipid disorders in humans. At present, due to species specificity, no valid animal model can efficiently replace a study in humans to explore lipid metabolism, and the use of radioactive tracers is restricted in humans. Thus, stable-isotope tracer kinetic studies have become an important component of research programs to achieve in humans a quantitative understanding of the dynamics of metabolic processes in vivo. The aim of this review is to describe the practical aspects of compartmental model development in stable-isotope experiments. The recent development of computer hardware and modeling software has dramatically facilitated the task of the modeler in his or her calculations. In the current review, we show that the model may be considered an integral component of the experimental design and that model development must obey strict rules to provide a rigorous solution. The main difficulties of model development in tracer experiments, such as experiment design, model identifiability, data expression, comparison of models, or tracer recycling, are presented with extensive references. We have paid particular attention to kinetic modeling in stable-isotope experiments because they have shown the greatest development in recent years.

Arterial wall mechanics in conscious dogs. Assessment of viscous, inertial, and elastic moduli to characterize aortic wall behavior

To evaluate arterial physiopathology, complete arterial wall mechanical characterization is necessary. This study presents a model for determining the elastic response of elastin (sigma E, where sigma is stress), collagen (sigma C), and smooth muscle (sigma SM) fibers and viscous (sigma eta) and inertial (sigma M) aortic wall behaviors. Our work assumes that the total stress developed by the wall to resist stretching is governed by the elastic modulus of elastin fibers (EE), the elastic modulus of collagen (EC) affected by the fraction of collagen fibers (fC) recruited to support wall stress, and the elastic modulus of the maximally contracted vascular smooth muscle (ESM) affected by an activation function (fA). We constructed the constitutive equation of the aortic wall on the basis of three different hookean materials and two nonlinear functions, fA and fC: sigma = sigma E + sigma C + sigma SM + sigma eta + sigma M = EE. (epsilon - epsilon 0E) + EC.fC.epsilon + ESM.fA.epsilon + eta. [equation: see text] + M.[equation: see text] where epsilon is strain and epsilon 0E is strain at zero stress. Stress-strain relations in the control state and during activation of smooth muscle (phenylephrine, 5 micrograms.kg-1.min-1 IV) were obtained by transient occlusions of the descending aorta and the inferior vena cava in 15 conscious dogs by using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. The fC was not linear with strain, and at the onset of significant collagen participation in the elastic response (break point of the stress-strain relation), 6.02 +/- 2.6% collagen fibers were recruited at 23% of stretching of the unstressed diameter. The fA exhibited a skewed unimodal curve with a maximum level of activation at 28.3 +/- 7.9% of stretching. The aortic wall dynamic behavior was modified by activation increasing viscous (eta) and inertial (M) moduli from the control to active state (viscous, 3.8 +/- 1.3 x 10(4) to 7.8 +/- 1.1 x 10(4) dyne.s.cm-2, P < .0005; inertial, 61 +/- 42 to 91 +/- 23 dyne.s2.cm-2, P < .05). Finally, the purely elastic stress-strain relation was assessed by subtracting the viscous and inertial behaviors.

Numerical approximation of mathematical model for absorption of subcutaneously injected insulin

A pharmacokinetic model is modified to enable quantitation of subcutaneous insulin absorption following insulin injections of soluble insulin and monomeric insulin analogues. The model for soluble insulin includes diffusion, equilibration between hexameric and dimeric insulin and absorption of dimeric insulin molecules. Numerical approximation is carried out by modelling the whole system as a capacitor-resistor network with lumped elements and discrete sources and sinks. By means of the analytical solution for monomeric-insulin absorption, it can be shown that the approximation scheme yields sufficiently accurate results. The modified model for soluble insulin demonstrates dose- and concentration-dependent insulin absorption within the range of therapeutic concentrations and volumes. Additionally, parameters are estimated from published glucose-clamp data. The results of the data fitting indicate that the model presented is adequate for pharmacological studies. The model is suitable for individual parameter estimation from the time course of plasma insulin or from the disappearance curves of radiolabelled injected insulin.

Penicillin-induced triphasic modulation of GABAA receptor-operated chloride current in frog sensory neuron

Effects of penicillin-G (PCN) on GABA-evoked Cl- current (IGABA) were investigated in freshly dissociated frog sensory neurons by the use of the concentration-clamp technique combined with the suction-pipette method. Under conditions where the internal and external solutions allowed only Cl- permeability, PCN elicited triphasic modulation on IGABA, consisting of two modes of blockade on IGABA and a following rebound (rebound-like transient IGABA). Simultaneously applied PCN and GABA depressed IGABA immediately (phasic blockade), with the depressed IGABA slightly recovering in amplitude to achieve a stable level of blockade (tonic blockade). When a solution containing a mixture or PCN and GABA was quickly replaced by one containing GABA alone, a rebound-like transient Cl- current (IR) was evoked. Each component of the PCN actions on IGABA was PCN- and GABA-concentration-dependent. The reversal potential for each component of the PCN actions on IGABA was close to the chloride equilibrium potential (ECl) calculated using the Nernst equation. The current-voltage (I-V) relations for both the phasic and tonic blockade revealed inward rectification, while I-V curves for the control IGABA and the IR were outwardly rectified. The degree of IGABA-desensitization and the amplitude of the IR correlated well. The data suggest that partial removal of the GABAA receptor-desensitization may result in generation of the IR.

Blockade of 5-HT3 receptor-mediated currents in dissociated frog sensory neurones by benzoxazine derivative, Y-25130

1. The effect of Y-25130, ((+-)-N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dih ydr o- 2H-1,4-benzoxazine-8-carboxamide hydrochloride), a high affinity 5-hydroxytryptamine3 (5-HT3) receptor ligand, was examined on the 5-HT-induced response in dissociated frog dorsal root ganglion (DRG) neurones by use of the extremely rapid concentration-jump ('concentration-clamp') and the conventional whole-cell patch-clamp techniques. 2. 5-HT induced a rapid transient inward current associated with an increase in membrane conductance at a holding potential of -70 mV. The current amplitude increased sigmoidally as 5-HT concentration increased. The half-maximum value (Ka) and the Hill coefficient estimated from the concentration-response curve were 1.7 x 10(-5) M and 1.7, respectively. 3. The current-voltage (I-V) relationship of 5-HT-induced current (I5-HT) showed inward rectification at potentials more positive than -40 mV. The reversal potential (E5-HT) was -11 mV. The E5-HT value was unaffected by total replacement of intracellular K+ by Cs+, indicating that the 5-HT-gated channels might be large cation channels. 4. Both the activation and inactivation phases of I5-HT were single exponentials. The time constants of activation and inactivation (tau a and tau i) decreased with increasing 5-HT concentration. 5. The 5-HT response was mimicked by a selective 5-HT3 receptor agonist, 2-methyl-5-HT, but the maximum response induced was approximately 25% that of 5-HT. The 5-HT response was reversibly antagonized by the 5-HT3 receptor antagonists, ICS 205-930, metoclopramide and Y-25130, but not by a 5-HTIA receptor antagonist, spiperone, and a 5-HT2 receptor antagonist, ketanserin. The half-inhibition concentrations (IC50) were 4.9 x 10-10 M for Y-25130, 4.8 x 10-10 M for ICS 205-930 and 8.6 x 10-9 M for metoclopramide.6. Y-25130 (5 x 10-10 M) caused a rightward shift of the concentration-response curve for 5-HT while decreasing the maximum response.7. The results suggest that Y-25130 is a potent antagonist of the 5-HT3 receptor-channel complex.

Non-competitive inhibition of GABAA responses by a new class of quinolones and non-steroidal anti-inflammatories in dissociated frog sensory neurones

1. The interaction of a new class of quinolone antimicrobials (new quinolones) and non-steroidal anti-inflammatory agents (NSAIDs) with the GABAA receptor-Cl- channel complex was investigated in frog sensory neurones by use of the internal perfusion and 'concentration clamp' techniques. 2. The new quinolones and the NSAIDs (both 10(-6)-10(-5) M) had little effect on the GABA-induced chloride current (ICI) when applied separately. At a concentration of 10(-4) M the new quinolones, and to a lesser degree the NSAIDs, produced some suppression of the GABA response. 3. The co-administration of new quinolones and some NSAIDs (10(-6)-10(-14) M) resulted in a marked suppression of the GABA response. The size of this inhibition was dependent on the concentration of either the new quinolone or the NSAID tested. The inhibitory potency of new quinolones in combination with 4-biphenylacetic acid (BPAA) was in rank order norfloxacin (NFLX) much greater than enoxacin (ENX) greater than ciprofloxancin (CPFX) much greater than ofloxacin (OFLX), and that of NSAIDs in combination with ENX was BPAA much greater than indomethacin = ketoprofen greater than naproxen greater than ibuprofen greater than pranoprofen. Diclofenac, piroxicam and acetaminophen did not affect GABA responses in the presence of ENX. 4. In the presence of ENX or BPAA, there was a small shift to the right of the concentration-response curve for GABA without any effect on the maximum response. However, the co-administration of these drugs suppressed the maximum of the GABA concentration-response curve, indicating a non-competitive inhibition, for which no voltage-dependency was observed.5. Simultaneous administration of ENX and BPAA also suppressed pentobarbitone (PB)-gated Icl. On the other hand, both PB and phenobarbitone reversed the inhibition of GABA-induced Ic, by coadministration of ENX and BPAA.6. The effect on GABAA responses of co-administration of new quinolones and NSAIDs was not via an interaction with benzodiazepine receptors coupled to the GABAA receptor, since this effect was not reversed by Rol5-1788 or diazepam.7. It is concluded that the co-administration of new quinolones and some of the NSAIDs inhibit GABAergic transmission, and could result in convulsions.

Kinetic properties of the caffeine-induced transient outward current in bull-frog sympathetic neurones

1. The kinetic properties of the caffeine-induced transient outward current (ICaff) of the bull-frog sympathetic neurone were investigated using the extremely rapid concentration-jump technique. By setting the holding potential at the equilibrium potential for Cl- (-50 mV), the involvement of the Ca(2+)-activated Cl- current was suppressed. Using a Na(+)-free (Tris) external solution, the involvement of the Na(+)-dependent sustained outward current was eliminated. The 'M' conductance was also occluded by pre-treatment with muscarine. Under these experimental conditions, ICaff consisted of a TEA-sensitive Ca(2+)-activated K+ current. 2. When the latent period from the application of caffeine until the onset of ICaff (termed the ICaff latency) was measured, 10 mM-caffeine gave a latency of 10.5 +/- 0.7 ms (n = 14, mean +/- S.E.M.) at 22 degrees C. The latency was independent of caffeine concentration between 3 and 30 mM. 3. The ICaff latency was temperature-dependent; it was shortened when the temperature was elevated. 4. Both the time to peak and half-decay time of ICaff were decreased with increasing caffeine concentration. In each cell, these parameters decreased by increasing the amplitude of ICaff. 5. At 22 degrees C, the time to peak and the half-decay time of ICaff elicited by 10 mM-caffeine showed a linear relationship, and this relationship was preserved on either elevating or lowering the temperature. On lowering the temperature (12 degrees C), the time to peak shortened whereas the half-decay time was prolonged. On elevating the temperature (32 degrees C), the time to peak was prolonged whereas the half-decay time was shortened. 6. When EGTA in the intracellular solution was replaced by equimolar BAPTA, the time to peak was prolonged while the half-decay time was shortened. 7. It is concluded that caffeine can activate ICaff, with a time course in the order of milliseconds, and that the kinetics of activation and inactivation of ICaff reflect the time-dependent change in the total amount of intracellular free Ca2+.

Inhibition of nicotinic acetylcholine response by atropine in frog isolated sympathetic neurons

The effect of atropine on nicotinic acetylcholine (ACh) response was studied in frog isolated sympathetic ganglion cells using a 'concentration clamp' technique which combines intra-cellular perfusion with a rapid external solution change within 3 ms. When atropine (more than 6 x 10(-7) M) was simultaneously applied to neurons with ACh (6 x 10(-6) M), the current amplitude was instantaneously reduced in a dose-dependent fashion. Further decreases in the current amplitude were observed in a time-dependent manner during about 20 min after the start of drug-application. Thereafter the current amplitude gradually restored toward the control level in the following 90-120 min in spite of the continuous presence of atropine. However, because d-tubocurarine greatly inhibited the 'restored' current, the last-mentioned is suggested to be also mediated by nicotinic ACh receptor-ionophore complex. Therefore, the inhibitory action of atropine on the peak amplitude of ACh response was examined at 15-20 min after adding the agent. It was dose-dependent but not voltage-dependent. Respective concentrations of atropine and d-tubocurarine causing half the maximum inhibition (IC50) of the peak current evoked by ACh (6 x 10(-6) M) were 1.8 x 10(-5) M and 1.8 x 10(-6) M. Thus, the inhibitory potency was 10 times less than that of d-tubocurarine, an antagonist of nicotinic ACh receptors. The blockade of ACh response by d-tubocurarine was competitive while that by atropine was non-competitive. The current elicited by ACh consisted of two (fast and slow) exponential components plus a steady-state one in the control period.(ABSTRACT TRUNCATED AT 250 WORDS)

Piperidine discriminates between the transient and the persistent components of the ACh-induced chloride current in Aplysia neurons

ACh-induced Cl- -current (ICl) is well known to desensitize with two components: an initial fast phase followed by a second, more slowly developing phase. In the present study, the influence of piperidine, a normal constituent in vertebrates and invertebrates, on ACh-induced ICl in isolated neurons of Aplysia was investigated by using the concentration clamp in combination with the voltage clamp technique. Pretreatment with piperidine in doses greater than 2 X 10(-4)M depressed the transient ACh-induced ICl but had little effect on the persistent ICl. Kinetic study of the desensitization phase of ACh-induced ICl showed that the slow time constant of the desensitization phase of ACh-induced ICl was not altered by pretreatment with piperidine. The present results indicate that piperidine can discriminate between the fast transient and slow persistent components of ACh-induced ICl in Aplysia neurons, and also suggest that two components of the desensitization phase of ACh-induced ICl function in an independent manner.

Effect of temperature on the inhibition of the GABA-gated response by intracellular calcium

The effects of the voltage-dependent Ca2+ inward current (ICa) on gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) in isolated frog sensory neurons were examined at different temperatures and with different concentrations of external Ca2+ using the 'concentration-clamp' technique. The total amount of inhibition of GABA-induced ICl by a preceding ICa increased in a hyperbolic manner with increasing Ca2+ influx. The time course of recovery of the GABA response after inhibition by Ca2+ influx followed a single exponential and was facilitated by warming but slowed dramatically by a slight cooling from 20 to 15 degrees C in spite of a decrease in Ca2+ influx. It is discussed that the energy-dependent, temperature-sensitive ionic pump and exchange systems at the surface membrane and intracellular organelles regulate the cytoplasmic free Ca2+, thus explaining the quantitative effects of Ca2+ influx and temperature on the inhibition of the GABA-gated Cl- channel.

Influences of external Ca2+ on the GABA-induced chloride current and the efficacy of diazepam in internally perfused frog sensory neurons

The effects of extracellular Ca2+ on the gamma-aminobutyric acid (GABA)-induced Cl- current and the efficacy of diazepam in the facilitation of GABA response were studied in frog isolated sensory neurons, using a 'concentration clamp' technique which combines a suction pipette (internal perfusion and voltage clamp) and a rapid drug application system. When nominal Ca2+-free external solution was changed to the solution containing 2 mM Ca2+, the response elicited by 1 x 10(-5) M GABA was reduced by about 40% of the control obtained in nominal Ca2+-free solution. The dose-response curve for GABA was shifted to the right without affecting the maximum response. It can be suggested that the application of external Ca2+ modulates the affinity of the GABA receptor to its agonist, GABA. Diazepam at the concentration of 3 x 10(-6) M shifted the dose-response curve for GABA to the left without changing the maximum response with or without external Ca2+. However, the augmentatory action of diazepam on the GABA response was reduced in the presence of external Ca2+. Possible mechanisms for inhibitory action of external Ca2+ on the GABA-gated response and the reduced efficacy of diazepam are discussed.

Effects of benzodiazepines and non-benzodiazepine compounds on the GABA-induced response in frog isolated sensory neurones

1. The effects of benzodiazepines and non-benzodiazepine compounds on the gamma-aminobutyric acid (GABA)-induced chloride current (ICl) were studied in frog isolated sensory neurones by use of a concentration-jump (termed 'concentration-clamp') technique, under single-electrode voltage-clamp conditions. The drugs used were classified into four categories as follows: full benzodiazepine receptor agonists (diazepam, clonazepam, nitrazepam, midazolam, clotiazepam and etizolam), partial agonists (CL 218,872, Ro 16-6028, Ro 17-1812 and Ro 23-0364), inverse agonists (Ro 15-3505, FG 7142 and beta-CCE) and a benzodiazepine receptor antagonist, Ro 15-1788 (flumazenil). 2. All full agonists at concentrations of 3 x 10(-6) M or less increased dose-dependently the peak amplitude of ICl elicited by 3 x 10(-6) M GABA to twice to three times larger than the control. However, no further augmentation of the GABA response was observed at concentrations of 1 x 10(-5) M or higher. Partial agonists also showed a dose-dependent augmentation of the GABA response at concentrations ranging from 3 x 10(-8) M to 3 x 10(-5) M, but their efficacies of augmentation of the GABA response were only about half or less of those of full agonists. Of the inverse agonists, beta-CCE had a unique dose-dependent effect on the GABA response. Beta-CCE reduced dose-dependently the GABA response at concentrations of less than 3 x 10(-6) M, but augmented it at concentrations of 3 x 10(-5) M and 6 x 10(-5) M. The inverse agonists reduced dose-dependently the GABA response. The benzodiazepine antagonist, flumazenil, slightly augmented the GABA response at concentrations between 3 x 10 7M and 3 x 10 5 M. 3. These results show clear differences in the effects on the GABA response between these four categories of compounds known to affect the benzodiazepine recognition site of the GABA/ benzodiazepine receptor-chloride channel complex. Our experimental system of frog isolated sensory neurones and a 'concentration-clamp' technique appears to be useful for evaluating efficacy of compounds on responses mediated by the GABA/benzodiazepine receptor-chloride channel complex.

Alpha-chloralose opens the chloride channel of frog isolated sensory neurons

The effect of alpha-chloralose on the sensory neurons isolated enzymatically and mechanically from frog dorsal root ganglia was studied using a suction-pipette technique. The threshold concentration of alpha-chloralose was around 3 x 10(-5) M and the current produced by alpha-chloralose saturated at the concentration of 3 x 10(-3) M or more. The dose-response curve for alpha-chloralose provided a Ka value of 6 x 10(-4) M and a Hill coefficient of 1.8. The reversal potential of the response elicited by alpha-chloralose was close to the equilibrium potential for Cl- (ECl), indicating that the current was carried through Cl- channels. The current-voltage relationship indicated that there was little voltage dependence in the alpha-chloralose-induced response. The analysis of the variance of the alpha-chloralose-induced Cl- current fluctuations showed two types of the receptor-ionophore complexes with different channel conductances.

A method for making solution changes in the sub-millisecond range at the tip of a patch pipette

A method is described here for making multiple fast external solution changes at the tip of a patch pipette. The time for the change, 0.2 ms, has been established by measuring changes in liquid junction potential at the tip of an open patch pipette. This technique of producing an abrupt change in solution allows agonist/receptor reactions to be studied under non-equilibrium conditions. We have applied this technique to the nicotinic receptors in outside-out patches from skeletal muscle cell line C2 (Jaffe and Saxel 1979) and from bovine adrenal chromaffin cells. The application of step changes in acetylcholine concentration produces current traces with a characteristic shape, which may be compared with the predictions of established models for the activation and desensitisation of the nicotinic receptor. The results of making single steps and also short pulses in acetylcholine concentration are demonstrated. The direct comparison of two different cholinergic agonists is demonstrated.

What confers specificity on glycine for its receptor site?

1. The structural requirements for activation of the glycine receptor were studied in isolated ventromedial hypothalamic neurones of rats by use of a 'concentration-clamp' technique under single-electrode voltage-clamp conditions. 2. alpha-Amino acids (L-alpha-alanine, and D-alpha-alanine, and L-serine), and glycine-methylester, glycine-ethylester and beta-amino acids (beta-alanine and taurine) produced a transient inward Cl- current, which was similar to that induced by glycine. 3. The responses to individual alpha- and beta-amino acids were selectively antagonized by strychnine, but were not affected by bicuculline, picrotoxin or the taurine antagonist, TAG (6-aminomethyl-3-methyl-4H,1,2,4-benzothiadiazine-1,1-dioxide hydrochloride), suggesting that alpha- and beta-amino acids activate the same glycine receptor. 4. beta-Amino acids were slightly more potent than the alpha-amino acids in causing cross-desensitization of the glycine response. 5. From the results of the structure-activity analysis of the optical isomers of alpha-alanine, serine and cysteine, a tentative structure of the glycine receptor is proposed.

Comparative study on barbiturates using isolated single neurons: GABA-mimetic action and augmentatory action on GABA response

The comparisons among barbiturate derivatives in both GABA-mimetic action and augmentatory effect on GABA response were made. Experiments were performed on single neurones isolated from frog dorsal root ganglia, using a concentration clamp technique which combines a suction pipette technique (internal perfusion and voltage-clamp) with a rapid external solution exchange within 2 ms. Barbiturates employed here were secobarbital, pentobarbital, hexobarbital and phenobarbital. All barbiturates at the concentrations over 1 x 10(-4) M evoked the chloride current (ICl) dose-dependently. In the GABA-mimetic action of barbiturates the potency was in the order of secobarbital greater than pentobarbital greater than hexobarbital greater than phenobarbital. All barbiturates at the concentrations over 1 x 10(-6) M (but up to 1 x 10(-3) M) augmented the GABA-induced ICl. Potency order of barbiturates in augmenting GABA response was the same as that in their GABA-mimetic actions when the concentration was 1 x 10(-4) M or less. However at the concentration of 1 x 10(-3) M the efficacies of secobarbital and pentobarbital in augmenting GABA response were greatly reduced while the other two barbiturates produced further augmentation.

Caffeine affects four different ionic currents in the bull-frog sympathetic neurone

1. Ionic mechanisms related to the caffeine-induced current (Icaffeine) were examined in the single isolated sympathetic neurones of the bull-frog. We used the 'concentration-jump' technique in combination with intracellular perfusion and a rapid external solution change, under single-electrode voltage-clamp conditions. 2. Icaffeine was pharmacologically separated into a tetraethylammonium (TEA)-sensitive transient outward current (ITO), a picrotoxin (PTX)-sensitive transient inward current (ITI) and a TEA- and PTX-insensitive sustained inward current (ISI). At low concentrations of caffeine, a sustained outward current (ISO) was observed instead of ISI. 3. All components of Icaffeine were abolished by intracellular perfusion of 30 mM-EGTA. Pre-treatment with A23187 or ryanodine or the simultaneous application of procaine either reduced or abolished all the components of Icaffeine in a dose-dependent manner. The concentration causing 50% inhibition (IC50) was 10(-8) M for A23187 and 2 mM for procaine. 4. The peak response of ITO increased abruptly at caffeine concentrations between 3 and 6 mM followed by saturation above 30 mM. A notch was observed on the rising phase of ITO. 5. The reversal potential (Ecaffeine) of ITO shifted 58 mV for a tenfold change of the extracellular K+ concentration. External application of TEA blocked ITO with an IC50 of 1 mM. ITO was relatively insensitive to apamin, 4-aminopyridine and muscarine. 6. In external solution containing 2 mM-Ca2+, ITO induced by 10 mM-caffeine recovered completely within 3 min from a previous exposure to caffeine. In the absence of extracellular Ca2+, there was little such recovery. A 5 min treatment in a Ca2+-free solution reduced ITO induced by the first application of caffeine by 5%. With a continuous application of 3 mM-caffeine, the amplitude of ITO induced by 10 mM-caffeine reduced in 1 min, and showed a partial recovery in 3 min. The amplitude of ITO increased by increasing the concentration of intracellular Cl-. 7. ITI was activated around the peak of ITO and was rapidly inactivated. ITI was evoked at caffeine concentrations of about 6-10 mM. When the intracellular Cl- concentration was changed, the amplitude of ITI behaved like a Cl- electrode. The Ecaffeine of ITI was close to the Cl- equilibrium potential (ECl). 8. ISI was a 'plateau' response and persisted for over 3 min. ISI was due to a decrease in K+ conductance. In the presence of muscarine (3 x 10(-5) M), ISI was occluded.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of the voltage-dependent calcium currents in isolated frog sensory neurons by GABA-related agonistic compounds

Effects of GABAA-, barbiturate- and benzodiazepine receptor agonists and GABAB agonist, baclofen, on voltage-dependent Ca2+ current (ICa) were studied in isolated frog sensory neurons after suppression of Na+ and K+ currents using single-electrode voltage-clamp. GABA, muscimol, taurine and pentobarbital (PB) dose-dependently induced a transient Cl- current (ICl), while baclofen and diazepam (DZP) did not elicit any currents. With GABAA agonists such as GABA, muscimol and taurine, ICa was suppressed transiently, and the maximum inhibition of ICa occurred within 1 min. The suppression of ICa by all GABAA agonists was neither voltage dependent nor attenuated in the presence of either bicuculline or picrotoxin. In addition, there was no correlation between GABA- and baclofen-induced suppressions of ICa. The results suggest that the inhibition of ICa by GABAA receptor agonists is not due to either GABAA or GABAB receptor activation at least. The inhibition of ICa by baclofen, PB and DZP was persistent. PB suppressed the amplitude of ICa and also facilitated the inactivation process, suggesting that PB behaves as a Ca channel blocker. However, the mechanisms of ICa suppression by baclofen and DZP are the subject for a future study. The potency order of the drugs in reducing ICa was muscimol greater than GABA = DZP greater than baclofen greater than PB greater than taurine.

Kinetic and pharmacological properties of the GABA-induced chloride current in Aplysia neurones: a 'concentration clamp' study

1. gamma-Aminobutyric acid (GABA) was applied by the 'concentration clamp' technique to isolated neurones of Aplysia. GABA induced a chloride current (ICl) due to activation of a single class of chloride-channel. 2. The concentration-response curve for the peak ICl gave an apparent dissociation constant of 6.4 X 10(-5) M and a Hill coefficient of 0.88. The current-voltage relationship was linear in the voltage range examined (-40 to +10 mV). 3. The activation phase of the ICl could be fitted to a single exponential function and desensitization followed the sum of two exponential functions. The time constants of activation and desensitization decreased with increasing concentrations of GABA but were voltage-independent. The recovery process from desensitization also followed the sum of two exponential functions. 4. As for the rate-limiting step of the channel activation, the hyperbolic relationship between the activation rate and GABA concentration showed that the rapid binding assumption holds, suggesting that the isomerization step is rate-limiting. The apparent channel closing rate constant was estimated to be 10 s-1 from the ordinate intercept of the linear part of the above relationship at lower concentrations. 5. Muscimol and beta-alanine induced a ICl, which cross-desensitized with that evoked by GABA. The GABA-ICl was not enhanced by diazepam (10(-6) M) or alpha-chloralose (10(-3) M), in fact depressant effects were evident. 6. Pentobarbitone decreased the GABA-ICl non-competitively without altering activation or desensitization kinetics. The concentration-inhibition curve gave a KD value of 8.9 x 10(-5) M and a Hill coefficient of 1.0. 7. These results suggest that GABA activates a single class of Cl channel in Aplysia neurones, which have one binding site for the agonist. The GABA receptor-Cl channel complex in Aplysia is pharmacologically and perhaps structurally different from that in vertebrates.

Characterization of an inward current elicited by edrophonium in physically isolated and internally perfused Aplysia neurons

We have studied the ionic and pharmacological properties of an inward current elicited by edrophonium, a cholinesterase inhibitor, on physically isolated and internally perfused Aplysia neurons using the voltage clamp, internal perfusion and rapid external perfusion techniques. The current amplitude was dependent on the external Na concentration [(Na)o] in an almost linear manner. However, complete replacement of (Na)o with Tris or sucrose failed to abolish the current. Internal application of Na [increased (Na)i] reduced the current amplitude. In normal (Na)o, changing (Ca)o (both increases and decreases in (Ca)o) reduced the current amplitude. In the sucrose-substituted (Na)o-free condition, edrophonium still could cause a small current (less than 5% of the control). However, an increased (Ca)o did not augment this residual current. Cs and Li carried the edrophonium-activated current when substituted for (Na)o. With sucrose-substituted Na-free sea water outside, edrophonium elicited an outward current when the neuron was internally perfused with Cs, but not when the neuron was internally perfused with K. Therefore, it is unlikely that K is permeant. External application of tetrodotoxin, a blocker of voltage-dependent Na channels, external application of Cd and internal application of F did not affect the current. The edrophonium response was most sensitive to strychnine, which was about 10 times more potent than D-tubocurarine. Hexamethonium, however, had no effect. The local anesthetics, lidocaine and procaine, inhibited the response over the same concentration range as D-tubocurarine. We conclude that edrophonium opens a monocationic channel (presumably a type of Na channel) which is sensitive to (Ca)o.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular picrotoxin blocks pentobarbital-gated Cl- conductance

Using the 'inside-out' configuration of frog sensory neurons, we studied the effect of intracellular picrotoxin on the pentobarbital-gated single channel response of Cl- -current (iCl). The pentobarbital-induced iCl showed no voltage-dependency and the single channel conductance (gamma Cl) was 16 +/- 3.1 pS (n = 6). Picrotoxin caused the pentobarbital-gated Cl- channels to react in a flickering pattern and then finally caused them to cease their opening altogether. This inhibitory action of picrotoxin was reversible.

Kinetic analysis of acetylcholine-induced chloride current in isolated snail neurons

1. Kinetics of activation and desensitization phases of the acetylcholine (ACh)-induced chloride current (ICI) were studied using isolated single neurons of Japanese land snail and the "concentration clamp" technique. 2. The dose-response curve for the peak ICI gave a dissociation constant of 7.1 x 10(-6) M and a Hill coefficient of 1.8. 3. The current-voltage relationship was linear in the voltage range examined (-60 to +10 mV) and the reversal potential (EACh) was -7.2 +/- 1.5 mV (N = 10). The value was close to the calculated equilibrium potential for chloride ions (ECI). 4. Both activation and desensitization phases of the ACh-induced ICI consisted of a single exponential at concentrations less than 3 x 10(-6) M and a double exponential at higher concentrations. The time constants of both phases decreased with increasing ACh concentrations but showed no potential dependency. 5. The recovery from desensitization of the ICI induced by 5 x 10(-6) M ACh proceeded double exponentially, with time constants of 11 and 114 sec at a holding potential of -30 mV. 6. Noise analysis was performed on a steady-state current induced by 3 x 10(-7) to 2 x 10(-6) M ACh. The mean open time was about 60 msec at 10(-6) M ACh and the single-channel conductance was 14 PS. 7. These results suggest that the ACh receptor-Cl channel complex in snail neurons has two binding sites with the dissociation constant of 7.1 x 10(-6) M and is rapidly activated and desensitized to a steady level in the presence of the agonist.

Strychnine decreases the voltage-dependent Ca2+ current of both Aplysia and frog ganglion neurons

1. The effects of strychnine on the voltage-dependent Ca2+ current (ICa) were studied in physically isolated Aplysia neurons and enzymatically dissociated frog sensory neurons of the dorsal root ganglion. Neurons were studied under the internal perfusion and the voltage clamp condition. 2. Strychnine decreased the ICa with threshold concentrations for effect at 1 to 10 microM. The depression of ICa increased with strychnine dose without effects on the current-voltage relation of ICa. The effects of low concentrations of strychnine were reversible, but recovery was incomplete at higher concentrations. The potency of strychnine was about 10 times less than that of diltiazem, an organic Ca2+ antagonist. At 100 microM the ICa of Aplysia neurons was reduced to about half of the control. This concentration of strychnine also reduced the peak amplitude of ICa of frog sensory neurons. 3. These results indicate that, in addition to its actions on transmitter responses and on Na+ and K+ currents, strychnine has effects on ICa that have not previously been appreciated.

Kinetic analysis of acetylcholine-induced chloride current in isolated Aplysia neurones

(1) Kinetics of activation and desensitization phases of the ACh-induced chloride current (ICl) were studied in isolated single neurones of Aplysia kurodai, using the 'concentration clamp' technique which combines internal perfusion and rapid exchange of the external solution within a few milliseconds (2) The dose-response curve for the peak ICl gave a dissociation constant of 6.7 X 10(-6) M and a Hill coefficient of 1.7. (3) The current-voltage relationship was linear in the voltage range examined (-70 to +30 mV). The reversal potential (EACh) was -7.1 +/- 1.8 mV (n = 14). The value was close to the calculated equilibrium potential for chloride ions (ECl). (4) The activation phase of the ICl was single exponential and the desensitization proceeded double exponentially to a steady state level. The time constants of both phases decreased with increasing concentrations of ACh but showed no potential dependency. The desensitizing component of the ICl was generated by activation of a single population of the receptor-channel complex. (5) The recovery from desensitization of the ICl induced by 6 X 10(-6) M ACh proceeded double exponentially, with time constants of 6.5 and 43 s at a holding potential of -30 mV. (6) Noise analysis performed on the steady state of ICl induced by low concentrations of ACh (3 X 10(-7) M to 3 X 10(-6) M) showed that the steady ICl was due to activation of a single population of the receptor-channel complex with a single channel conductance of 23.3 +/- 4.3 pS (n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of drug-gated chloride currents by calcium influx in frog sensory neurons

In isolated and internally perfused frog sensory neurons, muscimol-, pentobarbital- and alpha-chloralose-induced Cl- currents were suppressed by voltage-dependent Ca2+ currents. The amount of chloride currents inhibited by Ca2+ influx developed in a hyperbolic manner as a function of Ca2+ influx. The increase in intracellular Ca2+ concentration also suppressed the currents carried by monovalent anions, such as Br-, I-, NO3- and HCOO-.

Neurosteroid pregnenolone sulfate antagonizes electrophysiological responses to GABA in neurons

Our earlier biochemical studies suggested that the neurosteroid pregnenolone sulfate (PS) may reduce gamma-aminobutyric acid (GABA) action at the Cl- channel associated with GABAA receptors. In the present electrophysiological study the interaction of PS with the GABAA receptor was tested, using whole-cell voltage-clamp recordings from isolated cerebral cortical neurons of neonatal rats. At micromolar concentrations PS reversibly inhibited GABA-induced current, behaving as an allosteric receptor antagonist.

Kinetic analysis of glutamate-induced chloride current in Aplysia neurones: a 'concentration clamp' study

L-Glutamate (Glu) applied by the 'concentration clamp' technique to isolated neurones of Aplysia induced a chloride current (ICl) by activating a single population of the channel. The concentration-response curve for the peak ICl gave a dissociation constant of 1.3 x 10(-4) M and a Hill coefficient of 1.8. The current-voltage relationship was linear in the voltage range examined (-60 to +10 mV). The activation phase of the ICl followed a single-exponential time course and desensitization was complete with a double-exponential time course. The time constants for activation and desensitization decreased with increasing concentrations of Glu but were voltage-independent. The process of recovery from desensitization was also double-exponential. The single-channel conductance estimated by ensemble noise analysis was 50 +/- 4.7 pS (n = 4). These results suggest that the Glu receptor-Cl channel complex in Aplysia neurones consists of a single population with two binding sites for the agonist.

Effects of neostigmine and physostigmine on the acetylcholine receptor-ionophore complex in frog isolated sympathetic neurones

1. The effects of neostigmine and physostigmine, reversible carbamate acetylcholinesterase (AChE)-inhibitors, on nicotinic acetylcholine-induced inward currents (IACh) were investigated in enzymatically isolated single sympathetic ganglion cells from the bullfrog. The 'concentration clamp' technique which combines intracellular perfusion with a rapid external solution change under single electrode voltage-clamp conditions was used. 2. Pretreatment with neostigmine and physostigmine did not enhance IACh at any concentrations, suggesting that AChE activity had already disappeared during the enzymatic treatment of the preparation. 3. Both neostigmine and physostigmine inhibited IACh in a dose-dependent manner with IC50 values of 7.0 x 10(-4) M and 1.1 x 10(-4) M, respectively. The blockade by neostigmine was competitive, while that by physostigmine was non-competitive. 4. The inhibition of IACh by neostigmine and physostigmine showed no apparent voltage dependency. 5. Neostigmine did not cause obvious changes of the kinetics of IACh. However, physostigmine reduced both the fast and slow time constants of inactivation of IACh, thus facilitating the rate of inactivation without affecting the activation kinetics of IACh. 6. These results suggest that neostigmine and physostigmine have different direct actions on the ACh receptor-ionophore complex. Neostigmine may act on the ACh-receptor (the binding site of ACh) while physostigmine may interact with the ACh-gated cation channels.

Blockade of gamma-aminobutyric acid-gated chloride current in frog sensory neurons by picrotoxin

The mechanism of picrotoxin (PTX) suppression of the gamma-aminobutyric acid (GABA)-gated Cl- current (ICl) was examined in frog sensory neurons using the 'concentration-clamp' technique. The activation phase of GABA-induced ICl showed little change with the concomitant application of 10(-5) M PTX; however, the inactivation phase was markedly facilitated. ICl produced by simultaneous application of GABA and PTX was non-competitively suppressed while the plateau current showed a mixed type of competitive and non-competitive inhibitions. When the neuron was pretreated with PTX, the peak current of ICl produced by the simultaneous application of PTX and GABA was gradually suppressed, but the suppression of plateau level of ICl was rapid and not affected by pretreatment time. Recovery was not influenced by the length of pretreatment but depended only slightly on wash time. A major part of recovery occurred through the reactivation of the inhibited GABA receptor-ionophore complex by GABA. The first application of 3 x 10(-6) M GABA, after the response to 3 x 10(-6) M GABA was suppressed by the concomitant application of PTX, produced ICl consisting of the rapid phase and the slowly developing phase. ICl produced by simultaneous application of 10(-5) M GABA and 10(-5) M PTX and by the first application of 10(-5) M GABA after the inhibition showed a similar voltage dependence to the control ICl. These results indicate that PTX has access to the binding site inside the Cl- channel either through open channels or with different efficacies through closed channels, and thereby inhibits the GABA response by modifying the gating process.

Effects of diuretics on GABA-gated chloride current in frog isolated sensory neurones

1. Effects of three diuretics (furosemide, amiloride and alpha-human atrial natriuretic polypeptide (alpha-hANP] on GABA-activated chloride current (ICl) were investigated in frog isolated sensory neurones, following suppression of Na+, K+ and Ca2+ currents, by use of a 'concentration-clamp' technique. 2. Furosemide inhibited the GABA-activated ICl in a non-competitive manner and facilitated the inactivation phase, while amiloride inhibited the GABA response in a competitive manner, both inhibitions being concentration-dependent. Alpha-hANP had no effects on the GABA-induced ICl. 3. The reversal potential of GABA-activated ICl (EGABA) was not shifted in the presence of amiloride or furosemide. 4. The results suggest that amiloride may act at the GABA binding site while furosemide may act on the GABA-gated chloride channel.

Some properties of inhibitory action of lidocaine on the Ca2+ current of single isolated frog sensory neurons

The action of lidocaine on the Ca2+ current (ICa) was studied on single isolated neurons of frog dorsal root ganglia using a 'concentration-clamp' technique which combines intracellular perfusion with a rapid external solution change. Lidocaine decreased the peak amplitude of ICa at a threshold concentration of 10 microM. Higher concentrations gave a dose-dependent increase in inhibition of ICa. Lidocaine also depressed the Na+ current (INa) at a threshold concentration 10 times lower than that for decreasing the amplitude of ICa of neurons isolated from same ganglia. The rate of inhibitory action on ICa was slowed at more negative holding potentials (VH). Lidocaine appears to block both closed and open Ca2+ channels, but acts more profoundly on open channels.

Separation of current- and voltage-dependent inactivation of calcium current in frog sensory neuron

The inactivation process of the calcium current was studied in frog dorsal root ganglion cells using a newly developed 'concentration clamp' technique, and an inactivation component independent of Ca influx was separated from the Ca-dependent component. The neuron was first placed in Ca-free solution, and pre-pulses of various amplitude were applied, thereby inducing depolarization without the Ca influx. Successive test pulses were applied immediately after a rapid exchange to a solution containing normal Ca. The amount of inactivation under this condition was much smaller than that measured when both pre- and test pulses were applied in the normal solution, and reached a steady level on pre-pulses over +50 mV. We suggest that the inactivation process of the Ca current consists of two components, i.e. a dominant component which is dependent on Ca influx, and a smaller one which is presumably voltage-dependent.

Kinetic properties of the pentobarbitone-gated chloride current in frog sensory neurones

1. The kinetic properties of the activation and inactivation (desensitization) phases of pentobarbitone (PB)-induced inward Cl- current (ICl) were studied in isolated frog sensory neurones, following suppression of Na+, K+ and Ca2+ currents, using the concentration jump technique which combines the internal perfusion and the rapid exchange of the external solutions surrounding a neurone with time constants of 2-3 ms. The results were compared with those of the gamma-aminobutyric acid (GABA)-gated ICl. 2. The PB dose-response curve was bell-shaped and the maximum peak value was less than the current induced by 1.7 X 1.5(-5) M-GABA, the concentration at which GABA evoked a half-maximum response. 3. The activation and inactivation phases of PB-induced ICl consisted of double-exponential, fast and slow components, respectively. The time constant of the fast component (tau af) of the activation was relatively stable in a concentration range between 3 X 10(-4) and 6 X 10(-3) M. The time constant of the slow component (tau as) of the activation decreased with increasing PB concentrations. Both the fast and slow components (tau if and tau is) of the inactivation decreased with increasing PB concentrations. 4. Over a wide range of concentrations the tau af and tau as values of the PB-induced ICl were 10-30 times greater than the respective values of GABA-induced ICl. 5. At concentrations below 10(-3) M the PB-induced ICl was voltage dependent at more negative potentials than -20 mV. 6. The PB-induced ICl was blocked by bicuculline and by picrotoxin, but in a different manner. Bicuculline increased the time constants of the activation and inactivation. Picrotoxin had little effect on the activation phase but markedly facilitated the inactivation phase. 7. High concentrations of PB (over 10(-3) M) led to a decline in both the peak and plateau currents of the PB-induced ICl. A transient 'hump' current appeared with wash-out of the external solutions containing high concentrations of PB. This hump current was blocked by bicuculline in a dose-dependent manner. 8. The results suggest the possibilities that the PB receptor-ionophore complexes consist of at least two different components having different affinities and kinetics and that the PB and GABA binding sites are closely located.

Sodium-dependent suppression of gamma-aminobutyric-acid-gated chloride currents in internally perfused frog sensory neurones

1. The effects of the Na+ electrochemical potential gradient on gamma-aminobutyric acid (GABA)-induced Cl- currents (ICl) in frog sensory neurones were studied, using a suction pipette technique with which internal perfusion can be accomplished under current- and voltage-clamp conditions. 2. Under current clamp, the depolarizing response to GABA decreased in the presence of external Na+. A similar external Na+-dependent reduction in the GABA-induced inward ICl was observed under voltage clamp. The reversal potential of GABA-induced ICl (EGABA) was nearly equal to the Cl- equilibrium potential (ECl), irrespective of the presence or absence of external Na+. 3. Varying the Na+ influx by changing the holding membrane potential (VH) altered the GABA response: the GABA-induced ICl decreased progressively as VH became more negative. 4. The effects of changing the external and internal Na+ concentrations ([Na+]o and [Na+]i) on the GABA-induced ICl were also studied. Increasing [Na+]o at a constant [Na+]i reduced this current while increasing [Na+]i at a fixed [Na+]o facilitated it. 5. A high temperature coefficient of about 3 was estimated with respect to the percentage reduction in GABA-induced ICl due to [Na+]o. 6. These results indicate that the [Na+]o-dependent suppression of GABA-induced ICl was mediated chiefly by the uptake of GABA subserved by a Na-GABA co-transport mechanism. 7. GABA dose-response measurements were made with and without external Na+. The [Na+]o-induced suppression was more pronounced in relative amount at lower concentrations and in absolute amount at intermediate concentrations. Analysis of these data indicates, however, that the Na+-coupled GABA influx kept increasing at GABA concentrations high enough to nearly saturate GABA-induced ICl, and the same saturating level was observed as in the Na+-free case. This indicates that the electrogenic co-transport current was much smaller so that our measurements of GABA-induced ICl' were contaminated very little. Thus, the present method based on recording of GABA-induced ICl was legitimate for the analysis of the Na-GABA co-transport. 8. By analysing the [Na+]o-dependent suppression of GABA-induced ICl, the stoichiometric ratio of the underlying co-transport was estimated to be one: one Na+ ion per GABA molecule. 9. The ICl induced by GABA agonists such as beta-alanine, taurine, l-GABOB (l-gamma-amino-beta-hydroxybutyric acid) and muscimol was not affected by the amount of external Na+ present, suggesting difference in the affinity between receptor and transport carrier.

Inhibition of Na+-dependent GABA uptake in isolated frog sensory nerve cell bodies by extra- and intracellular Li+

The effect of Li+ on the Na+-dependent gamma-aminobutyric acid (GABA) uptake was investigated by examining the effects of external and internal Li+ ([Li]o and [Li]i) on GABA-gated Cl- current (ICl) in the frog dorsal root ganglion cells using the suction pipette technique which allows the internal perfusion under voltage-clamp condition. The suppression of GABA responses in the presence of external Na+ ([Na]o) was larger at lower than at higher GABA concentration. Replacement of [Na]o with Li+ completely removed the Na+ suppression, and GABA dose-response curve in Li+ external solution agreed well with that in Na+-free (Tris+) external solution. Either increasing [Li]i) or internal Na+ ([Na]i) at a constant [Na]o equally reduced the Na+-dependent suppression of GABA-gated ICl. The results indicate that both the [Li]o and [Li]i remove in different manner the [Na]o-dependent suppression of GABA-induced ICl:i.e. the [Li]o acts as a blocker of Na+-GABA co-transport mechanism while the increase of [Li]i decreases the Na+ electrochemical potential gradient across the soma membrane as well as [Na]i does.