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

Synthetic omega-conotoxin: a potent calcium channel blocking neurotoxin

The Ca2+ channel blocking action of synthetic omega-conotoxin (omega CTX) was studied on isolated frog dorsal root ganglion neurons using a 'concentration clamp' technique which enabled internal perfusion and rapid external solution change. At 100 nM, omega CTX showed a time-dependent depression of Ca2+ current (ICa). At higher concentrations, omega CTX exhibited a dose-dependent depression of ICa amplitude without changing the current-voltage relationship. Increases in external Ca2+ concentration partly overcame the inhibitory action of omega CTX on the ICa amplitude. At 10 microM omega CTX totally blocked ICa without effect on the Na+ current. It was likely that omega CTX had high selectivity for the Ca2+ channel.

Contribution of chloride shifts to the fade of gamma-aminobutyric acid-gated currents in frog dorsal root ganglion cells

1. The contribution of Cl- redistribution to the decay phase of the GABA (gamma-aminobutyric acid) response was investigated in isolated frog sensory neurones, using a suction-pipette technique which allows for internal perfusion under conditions of voltage clamp. 2. In neurones perfused with 120 mM [Cl-]i and [Cl-]o at driving forces (delta VH) of less than 15 mV, no shift of GABA equilibrium potential (EGABA) occurred during a continuous application of GABA, at various concentrations. However, increases of delta VH towards negative or positive potentials over 15 mV induced EGABA shifts. 3. The degree of EGABA shift was governed by the total amount of Cl- flux across the soma membrane, an event which depends upon delta VH, GABA concentration and drug application time. 4. The time-dependent EGABA shift due to Cl- redistribution during GABA application induced a current run-down resulting from a decreased Cl- gradient and a diminished Cl- conductance (gCl), the latter brought about by a drop in the intracellular ionic density of Cl-. 5. The EGABA shift during a continuous GABA application was also affected by [Cl-]i; e.g. the shift more readily occurred at lower [Cl-]i. 6. In neurones perfused with internal and external solutions containing 120 mM-Cl- at a delta VH of less than 10 mV, the change of gCl occurred with no shift of EGABA during the continuous application of GABA at concentrations over 6 x 10(-5) M, thereby indicating a 'real' GABA receptor desensitization. The desensitization depended solely upon the agonist concentrations but not upon the amount of ICl. Under these conditions, the time course of recovery from GABA desensitization was estimated. The decrease of gCl at the desensitization phase was a single exponential. 7. At a delta VH greater than 15 mV, therefore, the decay of ICl induced by GABA concentrations over 6 x 10(-6) M consists of the sum of both the 'real' GABA receptor desensitization and the current run-down brought about by Cl- shifts. The gCl at the current decay phase consisted of a double exponential. In the present experiments we chose experimental conditions with which Cl- shift become negligible. 8. The 'pure' GABA receptor desensitization during a continuous application of GABA developed rapidly at GABA concentrations over 10(-5) M. The speed of desensitization was facilitated by increasing the magnitude of desensitization.

Selective effects of enzyme treatment used for dissociating single cells on their GABA-receptor activities

Frog dorsal root ganglion (DRG) cells treated with collagenase and trypsin were dissociated mechanically. The characteristics of dose-response curves of GABA-induced Cl- currents (ICl) were mainly studied using the "concentration-clamp" technique, by which the external solution around isolated single neurons was rapidly changed within a few milliseconds. The saturating maximum currents induced by various concentrations of GABA normalized to the peak ICl induced by 10(-5) M GABA on the same neuron varied from 1.5 to 5.5 among the neurons, though the absolute maximum currents varied little. The Hill coefficients (n = 1.92 +/- 0.09, mean +/- S.D.) also varied little on the neurons. The time constants of the desensitization phase of fully activated GABA responses were not so markedly different among the neurons. The present results suggested a possibility that the enzyme treatment produced a selective inactivation of alpha 1 unit, a high affinity subtype of the GABA-receptor.

D-600 blocks open Ca2+ channels more profoundly than closed ones

The open channel blocking action of the Ca2+ antagonists, D-600 and Cd2+, was investigated in single neurons isolated by enzymatic treatment from dorsal root ganglia of frog. Using a 'concentration clamp' (jump) technique Ca2+ antagonists were applied to the preparation just before (less than 100 ms) or during a depolarizing step which induced the maximal peak amplitude of Ca2+ current (ICa). The inhibitory action of D-600 was more pronounced when applied during a depolarizing step than when applied just before a depolarizing step. In contrast, Cd2+ was equally inhibitory, whether applied before or during the depolarization. Thus, D-600 affects open channels more quickly or more profoundly than closed ones, while Cd2+ acts equally on both open and closed Ca2+ channels.

Multiple mechanisms of antagonism of gamma-aminobutyric acid (GABA) responses

Gamma-aminobutyric acid (GABA) is one of the most important neurotransmitters in the brain. In an effort to understand the operation of the GABA receptor-ionophore complex, the antagonism of GABA responses by four substances was studied in bullfrog dorsal root ganglion cells by concentration-clamp and internal-perfusion techniques. Two antagonists (bicuculline and Zn2+) were competitive; two (picrotoxin and penicillin) were noncompetitive. However, significant changes in the kinetics of activation and inactivation were produced by the antagonists, including those that were competitive. The causes of these changes may be important clues to the structure and operation of the GABA receptor-ionophore complex.

Quantification of lysine dynamics in the Japanese quail

To cockerels of the Japanese quail four graduated flooding doses of 14C-labelled L-lysine (20-160 mg) in starch gel were administered into the crop. The animals were slaughtered between 40 and 210 min thereafter. The TCA soluble fraction of the gastrointestinal tract, of blood plasma, liver and muscle were analyzed for their lysine content and its specific radioactivity. Incorporation of radioactivity into the TCA insoluble fraction of the same organs was also followed. After quantification of lysine losses by catabolism and excretion a multicompartment model of lysine dynamics in the quail was set up. Model parameter values in general agree with independently obtained data. The necessity of model validation and extension as well as possibilities for model applications are pointed out.

Intracellular calcium ions decrease the affinity of the GABA receptor

Intracellular free Ca2+ [( Ca2+]i) plays a crucial role in the transduction of extracellular signals. It has been implicated in the modulation of light sensitivity in Limulus photoreceptors and in the efficacy of synaptic transmission; calcium ion fluxes are also involved in the postsynaptic facilitation of nicotinic transmission seen in sympathetic ganglia, and in activation of the acetylcholine (ACh) receptor. [Ca2+]i is also a second messenger for many biologically active substances. We recorded neuronal activities of sensory neurones from the bullfrog (Rana catesbiana), using the suction pipette method and a 'concentration clamp' technique to apply gamma-aminobutyric acid (GABA) to the cell. We report the first evidence that [Ca2+]i suppresses the GABA-activated Cl- conductance, by decreasing the apparent affinity of the GABA receptor.

A pharmacodynamic model for the activity of antibiotics against microorganisms under nonsaturable conditions

An exact mathematical solution was derived to a pharmacodynamic model which illustrates bacterial survival in the presence of antibiotics. In this report the survival of Pseudomonas aeruginosa in the medium of an initial concentration of 0.64 mM (320 mg/L) of piperacillin [(2S,5R,6R)-6-[(R)-2-(4-ethyl-2,3-dioxo-1- piperazinecarboxyamido)-2-phenylacetamido]-3,3-dimethyl-7- oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylate] was well described by the derived model for up to 24 h. The bacterial killing by the antibiotic and apparent natural growth rate constants were 2955.3 h-1 X mol-1 and 0.5698 h-1, respectively. The functional equation was also fit to the data of ampicillin against Escherichia coli under simulated in vivo conditions. The optimal multiple dosing time and the minimum critical concentration to achieve antimicrobial action can be readily calculated from the developed model. Computer simulations were made to examine the effect on microbial survival of such factors as initial antibiotic concentration (Co), elimination half-life (t1/2), kill rate constant (K) of the antibiotic, and apparent growth rate constant (Kapp) of the test organism.

Novel GABA analogues as hypotensive agents

The actions of four analogues of gamma-aminobutyric acid (GABA) on blood pressure and heart rate were measured in the cat after intracerebroventricular administration. These compounds were previously found to inhibit binding to GABAA receptors of neuronal membranes from the CNS of the rat. Each of the drugs, together with GABA, produced an average maximum reduction in blood pressure of 27.63% +/- 12.5. However, aminoethanethiosulfonic acid (AETS) was the most potent (ED50 = 2.24 X 10(-10) mol/kg) of the drugs, followed by 5-phenyl-2-pyrrole propionic acid (PPP), urocanic acid (UCA), m-aminobenzoic acid (MABA) and GABA. None of the compounds produced a significant effect on heart rate. The fact that these analogues mimicked the action of GABA on the cardiovascular system of the cat and that they were able to inhibit binding to GABAA receptors, indicates that they may be GABA agonists.

'Concentration-clamp' study of gamma-aminobutyric-acid-induced chloride current kinetics in frog sensory neurones

Kinetics of the activation and desensitization phases of gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) were studied in single frog sensory neurones using the 'concentration-clamp' technique which enables perfusion of drugs with the time constant of about 3 ms. Both activation and desensitization phases of GABA response consisted of a single exponential at low concentrations and a double exponential at high concentrations. The time constant of the fast kinetic component in each phase was relatively stable, about 5 ms for activation and 3 s for desensitization over concentrations from 3 X 10(-5) to 3 X 10(-4) M, whereas those of the slow kinetic component decreased with increasing concentrations. The two kinetic components in both phases showed the same reversal potential. The slow and fast activation components recovered sensitivity from desensitization with different time courses: the recovery rate of the fast activation component was slow and that of the slow one, rapid. The peak ICl elicited at GABA concentrations below 10(-5) M increased disproportionally at more negative membrane potentials, thereby suggesting that the activation kinetics is voltage dependent. The steady-state ICl-voltage relationship obtained with less than 10(-5) M-GABA showed a non-linearity, probably due to voltage dependence of activation rather than that of desensitization kinetics. These results suggest the presence of at least two different GABA receptor-Cl- ionophore complexes with a different affinity and kinetics.

Diazepam action on gamma-aminobutyric acid-activated chloride currents in internally perfused frog sensory neurons

The Cl- current (ICl) in gamma-aminobutyric acid (GABA)-sensitive frog sensory neuron was separated from other Na+, Ca2+, and K+ currents using a suction pipette technique which allows internal perfusion under a single-electrode voltage clamp. Diazepam (DZP) itself evoked no response but facilitated the dose- and time-dependently GABA-induced ICl without changing the GABA equilibrium potential (EGABA) at concentrations ranging widely, from 3 X 10(-9) to 10(-4) M. In the presence of DZP, the GABA dose-response curve shifted to the left without changing the maximum current, indicating that DZP modifies the interaction between GABA and its receptor rather than affecting directly the channel activation step. The enhancement of the GABA-induced ICl by DZP depended neither on the membrane voltage nor on the inward or outward direction of the ICl. DZP also potentiated the ICl elicited by GABA agonists such as beta-alanine, taurine, homotaurine, 5-aminovaleric acid, l-GABOB, d-GABOB, glycine, and muscimol. The GABA response enhanced by pentobarbital (PB) was further enhanced by adding DZP, indicating that DZP and PB do not act in the same way. Ro5-3663, a diazepam analogue, enhanced the GABA-induced ICl only in a narrow range of the concentrations but inhibited the current at concentrations higher than 2 X 10(-6) M.

The anion selectivity of the gamma-aminobutyric acid controlled chloride channel in the perfused spinal ganglion cell of frog

The selectivity of the GABA-controlled Cl- channel in the membrane of the dorsal root ganglion cell of the frog has been measured in internally perfused cells by means of current and voltage clamp. When Cl- was replaced by various anions, the 10(-5) or 10(-4) M GABA-induced reversal potentials (EGABA) for Br-, I-, NO3-, ClO4-, SCN-, BF4- and ClO3- were more negative than that for Cl-, despite the fact that, in solution, the test anions are either larger than or similar in size to Cl-.

Catechol: a potent and specific inhibitor of the fast potassium channel in frog primary afferent neurones

The effects of catechol on various ionic channels of isolated primary afferent neurones of the bull-frog were examined by a single-suction-electrode clamp system, which combined internal perfusion and current or voltage clamp using an electronic switching circuit. Catechol was found to inhibit rather specifically the fast K+ current as does 4-aminopyridine (4-AP). Ca2+, Na+ and slow K+ currents were not affected. Although both 4-AP and catechol were inhibitors of the fast K+ channels, their sites of action were quite different. Catechol was effective when applied on the external surface of the cell membrane whereas 4-AP acted preferably internally. We assumed that a single fast K+ channel has two distinct sites for blockers: the catechol site is exposed to the external medium or situated at the outer orifice of the pore, and the 4-AP site is located within the same channel but is more easily accessible from inside the nerve cell than outside. The 4-AP and catechol sites were not, however, completely separate and independent of each other since a synergistic interaction was observed between catechol and 4-AP.

Pharmacology of amino acid receptors on vertebrate primary afferent nerve fibres

Structure-activity of primary afferent depolarising action (PAD) mediated by gamma-aminobutyrate (GABA) analogues suggests a difference between subsynaptic receptors located at fibre terminations within the dorsal horn and axonal receptors which are distributed throughout non-synaptic regions. The interaction of the bicuculline-sensitive GABA receptor (GABA A) ionophore complex with barbiturates and benzodiazepines suggests that at least three binding sites are required to explain the independent GABA-mimetic, GABA-potentiating and picrotoxin-reversing effects of such agents. Difficulties with explanation of the depressant effects of baclofen on spinal transmission, in terms of the bicuculline-resistant GABA (GABA B) receptor hypothesis, are mentioned. Glutamate-induced PAD of low threshold afferents is mediated indirectly through release of potassium. However, such terminals possess receptors (possibly autoreceptors for L-glutamate), activated by (+)2-amino-4-phosphonobutyrate, which cause depression of transmitter release. Primary afferent C-fibres possess receptors which are selectively activated by kainate and which mediate picrotoxin-resistant PAD. Such receptors may be involved in the presynaptic conditioning of C-fibre transmitter release. The peripheral terminals of vestibular primary afferents, in amphibia, possess excitatory amino acid receptors which are probably activated by the transmitter released from hair cells.

Model identification and estimation of organ-function parameters using radioactive tracers and the impulse-response function

Examination of the input-output events in functioning organs by the use of the impulse-response function (IRF) for a radioactive tracer is gaining more and more ground in nuclear medicine. This study summarizes the development of deconvolution analysis, laying special stress on the 'model-free' approach. System linearity and time invariance are discussed, and means of eliminating noise in IRFs originating from the input and organ-time-activity curves are outlined. Typical IRFs are illustrated by flow diagrams, time-domain curves, and their representation by Laplace transforms. The cases of nondiffusible and diffusible tracers as well as parenchymally extracted and transported substances are discussed. Methods for the derivation of models and for the calculation of physiologically important parameters from the IRFs are suggested.

GABA activates different types of chloride-conducting receptor-ionophore complexes in a dose-dependent manner

We report here evidence for 3 new subtypes (alpha 1, alpha 2 and beta) of type-A GABA receptor-channel complexes that conduct chloride ions. The chloride current, ICl, was isolated in the frog sensory neuron by a combination of voltage clamp and internal perfusion. Analysis of the variance of GABA-induced ICl fluctuations shows that the channel population N decreases exponentially with single-channel conductance gamma in such a way that alpha 2 less than alpha 1 less than beta for gamma and alpha 2 much greater than alpha 1 greater than beta for N, and that the population-rank plot fits Zipf's law. Various aspects of the GABA-induced ICl are understood from dose-dependent activation and inactivation of these functionally distinct receptor-channel types. The steady-state ICl is mediated by alpha 1 at low but by beta units at high GABA concentrations, and the pronounced ICl peak at intermediate and high doses reflects the desensitization of alpha 1 and alpha 2 receptors, respectively. Picrotoxin blocks alpha 1 and alpha 2 and has no effect on beta channels. Patch-clamp recordings indicate two distinct classes of GABA-gated chloride conductances that appear to correspond to the alpha 1 and beta types. The presence of these different ICl components explains why the dose-response relationship cannot be fitted well by a single Hill equation; the fitting requires a synthesis of 3 suitable Hill equations.

Interactions of gamma-aminobutyric acid (GABA), pentobarbital, and homopantothenic acid (HOPA) on internally perfused frog sensory neurons

Augmentatory actions among Cl- currents (ICl) induced by gamma-aminobutyric acid (GABA), pentobarbital (PB), and homopantothenic acid (HOPA) were investigated in isolated frog sensory neurons after suppression of Na+, K+, and Ca2+ currents using a suction pipette technique which combines internal perfusion with voltage clamp. GABA-sensitive neurons responded to both PB and HOPA, and the responses behaved as a simple Cl- electrode and reversed at the Cl- equilibrium potential (ECl). The dose-response curve for GABA-induced Cl- conductance was sigmoidal with the GABA concentration producing a half-maximum response (4.2 X 10(-5) M). Both GABA and HOPA dose-response curves shifted to the left in the presence of PB, though the facilitatory action of PB on GABA- and HOPA-induced ICl was more effective in the former. There was a significant facilitatory interaction between GABA- and HOPA-induced ICl. It is concluded that HOPA affects the GABA-GABA or PB-PB receptor interactions.

A carrier-protein receptor is not a prerequisite for avid hepatic elimination of highly bound compounds: a study of propranolol elimination by the isolated perfused rat liver

The highly efficient hepatic extraction of propranolol by the isolated perfused rat liver does not diminish when albumin binding is increased from 30 to 75%. One possible explanation of this insensitivity of propranolol uptake to changes in albumin binding is the mediation of uptake of bound ligand by an albumin receptor on the hepatocyte as postulated for oleate, taurocholic acid and rose bengal. To test this hypothesis, the hepatic extraction of propranolol was studied in the isolated perfused rat liver using alpha 1-acid glycoprotein, which lacks a hepatocyte receptor, as the carrier protein in the perfusate rather than albumin. Livers were perfused with a medium containing propranolol (4 microM) and varying concentrations of alpha 1-acid glycoprotein (0 to 25 microM). Hepatic extraction of propranolol was very high (0.990 +/- 0.006; mean +/- S.D.) and did not alter significantly despite an increase in bound fraction from 0.2 to 0.8, thus closely paralleling the findings when albumin is the carrier protein. This result indicates that bound propranolol is efficiently cleared by the liver, presumably by a "free intermediate" mechanism, in the absence of a specific carrier-protein receptor on the hepatocyte. This study does not, therefore, support the albumin receptor hypothesis.

An arylaminopyridazine derivative of gamma-aminobutyric acid (GABA) is a selective and competitive antagonist at the GABAA receptor site

In view of finding a new gamma-aminobutyric acid (GABA) receptor ligand we synthesized an arylaminopyridazine derivative of GABA, SR 95103 [2-(carboxy-3'-propyl)-3-amino-4-methyl-6-phenylpyridazinium chloride]. SR 95103 displaced [3H]GABA from rat brain membranes with an apparent Ki of 2.2 microM and a Hill number near 1.0. SR 95103 (1-100 microM) antagonized the GABA-mediated enhancement of [3H]diazepam binding in a concentration-dependent manner without affecting [3H]diazepam binding per se. SR 95103 competitively antagonized GABA-induced membrane depolarization in rat spinal ganglia. In all these experiments, the potency of SR 95103 was close to that of bicuculline. SR 95103 (100 microM) did not interact with a variety of central receptors--in particular the GABAB, the strychnine, and the glutamate receptors--did not inhibit Na+-dependent synaptosomal GABA uptake, and did not affect GABA-transaminase and glutamic acid decarboxylase activities. Intraperitoneally administered SR 95103 elicited clonicotonic seizures in mice (ED50 = 180 mg/kg). On the basis of these results it is postulated that St 95103 is a competitive antagonist of GABA at the GABAA receptor site. In addition to being an interesting lead structure for the search of GABA ligands, SR 95103 could also be a useful tool to investigate GABA receptor subtypes because it is freely soluble in water and chemically stable.

gamma-Aminobutyric-acid- and pentobarbitone-gated chloride currents in internally perfused frog sensory neurones

gamma-Aminobutyric-acid- (GABA) and pentobarbitone-induced Cl- currents (ICl) were studied in isolated frog sensory neurones after suppression of Na+, K+ and Ca2+ currents using a suction-pipette technique combining internal perfusion with voltage clamp. All GABA-sensitive neurones responded to pentobarbitone. Both GABA- and pentobarbitone-induced ICl reversed at the Cl- equilibrium potential (ECl). The dose-response curve for maxima of GABA-induced ICl was sigmoidal with a mean concentration producing a half-maximum response, Ka of 2 X 10(-5) M at a Hill coefficient of 1.8. In the presence of pentobarbitone, the GABA dose-response curve shifted to the left without affecting the saturating maximum current. At high concentrations, both GABA and pentobarbitone could also potentiate the pentobarbitone- and GABA-induced ICl respectively, while pre-treatment with one of the two markedly attenuated currents induced by the other, indicating a 'cross-desensitization'. In the presence of pentobarbitone, the augmented response was voltage dependent and this augmentation was much greater in the inward-current direction than outward. In producing ICl, pentobarbitone and its stereoisomers were potent in the order of (-) isomer greater than (+/-) racemic mixture greater than (+) isomer. A stereospecific facilitatory action of pentobarbitone on GABA responses was also found in the same order. Responses to GABA, homotaurine, taurine, beta-alanine, 5-aminovaleric acid, (+)- and (-)-gamma-amino-beta-hydroxybutyric acid and muscimol were equally enhanced by pentobarbitone, though its action on glycine-induced ICl was less effective. Picrotoxin inhibited the GABA- and pentobarbitone-induced ICl from either side of membrane, while internal application of GABA and pentobarbitone did not exert any effect. It was concluded that pentobarbitone binds to the 'barbiturate receptors' located close to the GABA receptor-Cl- channel complex, and directly affects the GABA-GABA receptor interactions rather than the ionic channels.

Contribution of restricted extracellular space to the inactivation of calcium current in the snail neuron

The mechanisms underlying the inactivation of calcium current (ICa) were investigated in isolated nerve cell bodies of Helix aspersa using a suction pipette technique that allowed voltage clamp and internal perfusion at the same time. ICa was recorded after eliminating the Na and K currents by removing Na+ and K+ both in external and internal solutions, and ICa inactivation due to intracellular Ca2+ accumulation was blocked by 5-25 mM EGTA. The inactivation rates of ICa, IBa and ISr corresponded to two exponential processes. The inactivation rates of the inward currents (IMn, ICd and IZn) less than 1/5 of ICa fitted a single exponential. However, when neurons were superfused with hypertonic external solution by adding 100 mM sucrose together with internal EGTA, the steady-state inactivation of ICa, IBa and ISr was reduced, and the inactivation processes changed to a single exponential similar to that of IMn, ICd and IZn. In contrast, internal perfusion with the hypertonic solution had no effect on the inactivation of ICa, IBa and ISr. Therefore, it was concluded that the inactivation process of ICa is dependent not only on the membrane voltage and the intracellular Ca2+ accumulation as described previously, but is also affected by the rapid fall in the concentration of Ca2+ in the restricted extracellular spaces (RES) which gets enlarged by the hypertonic external solution. The same is also true for IBa and ISr.

Bicuculline and picrotoxin block gamma-aminobutyric acid-gated Cl- conductance by different mechanisms

Using isolated, internally perfused bullfrog dorsal root ganglion cells we have studied the dose-response curves for gamma-aminobutyric acid (GABA) in the presence of internally or externally applied GABA antagonists. With external application of antagonists the inhibition of the GABA current by bicuculline was competitive and that by picrotoxin was noncompetitive. Picrotoxin but not bicuculline blocked when internally perfused.