Pentobarbitone interference with inhibitory synaptic transmission in crayfish stretch receptor neurones

Antagonism of the actions of glutamate by pentobarbitone or midazolam in the frog optic tectum in vitro

Excitatory synaptic transmission, induced by electrical stimulation of optic nerve fibres on relay neurones, was recorded from in vitro preparations of the optic tectum of the frog. Bath-applied glutamate (the putative excitatory transmitter of the optic nerve) produced transient enhancement of tectal field potentials, followed by a depression, presumably caused by sustained neuronal depolarization. Pentobarbitone potently antagonized the depressant effect of glutamate, producing an approximate 50% reduction in the response of the tectum to glutamate at 25 microM. Midazolam also decreased the effect of glutamate with an IC50 value of 5 nM. Since, in the optic tectum of the frog, neither pentobarbitone nor midazolam enhance responses to bath-applied GABA, it is suggested that this area of the brain is a useful preparation in which to investigate the interaction of barbiturates and benzodiazepines with glutamate receptor mechanisms, without concurrent interactions with GABAergic processes.

Frequency-dependent depression of inhibition in guinea-pig neocortex in vitro by GABAB receptor feed-back on GABA release

1. The mechanisms involved in the lability of inhibition at higher frequencies of stimulation were investigated in the guinea-pig in vitro neocortical slice preparation by intracellular recording techniques. We attempted to test the possibility of a feedback depression of GABA on subsequent release. 2. At resting membrane potential (Em, -75.8 +/- 5.2 mV) stimulation of either the pial surface or subcortical white matter evoked a sequence of depolarizing and hyperpolarizing synaptic components in most neurones. An early hyperpolarizing component (IPSPA) was usually only obvious as a pronounced termination of the EPSP, followed by a later hyperpolarizing event (IPSPB). Current-voltage relationships revealed two different conductances of about 200 and 20 nS and reversal potentials of -73.0 +/- 4.4 and -88.6 +/- 6.1 mV for the early and late component, respectively. 3. The conductances of IPSPA and IPSPB were fairly stable at a stimulus frequency of 0.1 Hz. At frequencies between 0.5 and 2 Hz both IPSPs were attenuated with the second stimulus and after about five stimuli a steady state was reached. Concomitantly IPSPs were shortened. The average decrease in synaptic conductance between 0.1 and 1 Hz was 80% for the IPSPA and 60% for the IPSPB. At these frequencies the reversal potentials decreased by 5 and 2 mV, respectively; Em and input resistance (Rin) were not consistently affected. 4. The amplitudes of field potentials, action potentials and EPSPs of pyramidal cells were attenuated less than 10% at stimulus frequencies up to 1 Hz, suggesting that alterations in local circuits between the stimulation site and excitatory input onto inhibitory interneurones may play only a minor role in the frequency-dependent decay of IPSPs. 5. Localized application of GABA produced multiphasic responses. With low concentrations and application near the soma an early hyperpolarization prevailed followed by a depolarizing late component. Brief application of GABA at low frequencies induced constant responses; at higher frequencies, the responses sometimes declined. The current-voltage relationships of the two GABA responses were similar to each other and to the early IPSP. An apparently fivefold higher conductance was estimated at lower Ems, suggesting that the GABA response had a voltage sensitivity. The slope conductance of IPSPs was decreased by up to 50% for tens of seconds after postsynaptically detectable effects of GABA had dissipated. 6. Application of the GABA uptake inhibitor nipecotic acid (50-500 microM) reduced the conductance of both components of orthodromically evoked inhibition and shortened the IPSP at low frequencies, but had no additional effects at higher stimulation rates.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Comparative extracellular current flow at dendrites and soma of dentate granule cells during long-term potentiation

In the study of long-term potentiation (LTP) in the dentate gyrus, the magnitude of the initial slope of the evoked synaptic potential (ESP) in the region of the granule cell layer, is commonly used as a measure of synaptic efficacy at the site of the perforant path (PP) input. To investigate the viability of this assumption, in rats anesthetized with either chloropent or urethane, LTP was induced by tetanic stimulation of the PP and ESPs were measured simultaneously at both the soma and dendritic site of the PP input. Following LTP, the two responses were decoupled, the somal response being greater than the dendritic in rats anesthetized with chloropent, the opposite being true in urethanized animals. We conclude that in investigating changes in synaptic efficacy following LTP, measurements should be taken at the site of the PP input.

Anesthetics produce differential actions on the discharge activity of a single neuron

The effects of 26 anesthetic agents were studied on the rhythmical discharge activity of a single isolated neuron (crayfish stretch receptor). Many of these agents produced concentration-dependent biphasic responses (excitation and depression), and some also induced altered discharge patterns (burst activity). The dominant effect of a few of the anesthetics was excitation (e.g. alphaxolone); depression (e.g. decanol); or burst activity (e.g. benzocaine). A correlation was found to exist between equieffective concentrations in the perfusate and membrane/buffer partition coefficients; however, this general phenomenon does not provide an explanation for the biphasic or differential responses. These results demonstrate that selective interactions occur at the level of the single neuron, and suggest the existence of recognition sites in neuronal membranes which can discriminate structural differences of anesthetics.

Role of uptake in gamma-aminobutyric acid (GABA)-mediated responses in guinea pig hippocampal neurons

Intracellular recordings were obtained from hippocampal pyramidal neurons maintained in vitro. Measurements were made of the conductance change induced by iontophoretically applied gamma-aminobutyric acid (GABA) and, using voltage-clamp techniques, of inhibitory postsynaptic currents resulting from activation of inhibitory pathways. Analysis of GABA iontophoretic charge-response curves indicated that there was considerable variation among neurons with respect to the slope of this relation. The placement of the GABA-containing pipette did not appear to be responsible for the observed variation, since vertical repositioning of the pipette did not alter the slope of the charge-response relationship. Steady iontophoresis of GABA from one barrel of a double-barreled pipette markedly affected the charge-response relation obtained when short pulses were applied to the other barrel. The curve was shifted to the left, and the slope was decreased. Concomitantly, the enhanced GABA-induced responses were prolonged. Similar alterations in GABA responsiveness were observed when the uptake blocker, nipecotic acid, was iontophoretically applied. Furthermore, bath application of saline containing a reduced sodium concentration (25% of control) also produced a prolongation of GABA-mediated responses. Under voltage clamp, inhibitory postsynaptic currents were observed to have biphasic decays. The initial, fast decay was prolonged by an average of 18% by nipecotic acid, whereas the later, slow phase was prolonged by 23%. The results of these studies support the hypothesis that a saturable GABA uptake system is responsible for the observed variation in the charge-response curves and, in turn, underlies the apparent sensitizing effect of excess GABA application. The results also suggest that a reduction of transmitter uptake affects the time course of inhibitory postsynaptic currents in the hippocampus.

Prolongation of inhibitory postsynaptic currents by pentobarbitone, halothane and ketamine in CA1 pyramidal cells in rat hippocampus

Spontaneous inhibitory postsynaptic currents (i.p.s.cs) were recorded in voltage-clamped CA1 neurones in rat hippocampal slices. The exponential decay of i.p.s.cs was prolonged by concentrations of sodium pentobarbitone as low as 50 microM. With concentrations up to 100 microM, there was no change in the amplitude or rise time of the currents but current amplitude was depressed at 200 microM. The prolongation of currents increased with drug concentration within the range tested (50 to 200 microM). Halothane, at concentrations from 1 to 5%, also increased the time constant of decay of i.p.s.cs. The effect increased with concentration and was fully reversible. Ketamine, at a concentration of 0.5 mM, increased the time constant of decay of i.p.s.cs by 50 to 80% and the effect was reversible. Ethanol (10-200 mM), nitrous oxide (75-80%), and caffeine (10 microM-5 mM) had no detectable effect on the i.p.s.cs. It is suggested that pentobarbitone, halothane and ketamine increase the time constant of decay of the i.p.s.cs by stabilizing the open state of channels activated by gamma-aminobutyric acid.

Dose effects of halothane on sensory evoked responses obtained from the cortex, reticular formation and central gray

Sensory evoked field potentials were recorded from the mesencephalic reticular formation (MRF), central gray (CG) and somatosensory cortex (SCX), following incremental doses of halothane in freely-moving rats. Halothane concentrations of 0.25%, 0.5% 1.0% and 2.0% were used. In general, the responses from each structure were affected in dose response manner. The averaged acoustic evoked responses (AAER) exhibit more sensitivity to halothane than the averaged visual evoked responses (AVER). The evoked response and its components obtained from each structure were affected differently by halothane mainly following the initial two halothane doses, (0.25% and 0.5%); mainly increase in amplitude was observed in the recording obtained from the MRF, decrease in the CG, and mixed (increase and/or decrease) in SCX. The degree of the depression of the sensory evoked responses was directly correlated to the level of anesthesia as assessed by sural nerve stimulation.

gamma-Aminobutyric acid-induced depression of calcium currents of chick sensory neurons

The effects of gamma-aminobutyric acid (GABA) on calcium currents were investigated in avian dorsal root ganglion (DRG) cells. GABA was applied to the vicinity of the cells by ejection pipettes using constant-pressure pulses. GABA concentrations between 5 and 100 mu M reduced and slowed the calcium current in a dose-dependent manner. A contribution of K and Cl outward currents to the reduction of the inward current was minimized by using identical caesium chloride concentrations on both sides of the membrane. The onset of the effect was rapid and 80% of the effect was observed within 1 s. The attenuation of the Ca slope conductance by GABA was found to be independent of the membrane potential between -50 and +50 mV.

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.

Inhibitory post-synaptic currents in rat hippocampal CA1 neurones

Spontaneous synaptic currents were recorded in voltage-clamped CA1 neurones in rat hippocampal slices at room temperature (21-25 degrees C). The currents, which could be seen at the resting membrane potential only when cells were loaded with chloride ions, were blocked by bicuculline. It was concluded that they were inhibitory post-synaptic currents (i.p.s.c.s) generated by the opening of chloride-selective channels activated by gamma-aminobutyric acid (GABA). In twenty-nine cells, ten to forty-five i.p.s.c.s were recorded at a potential between -75 and -85 mV. In every cell, i.p.s.c.s varied widely in amplitude with an average coefficient of variation of 28.7%. The mean amplitude varied from 0.22 to 1.16 nA with an average of 0.52 nA. Reversed (outward) currents could be recorded from cells voltage clamped at positive potentials. Null potentials varied between -20 and +20 mV, the variability being attributed to differences in chloride loading. Most currents had a rapid growth phase with a mean growth time (20-80% of peak) of 1.1 ms followed by a slower decay phase. The decay phase was exponential with a single time constant. The average decay time constant (tau D) ranged from 8.3 to 16.2 ms with a mean value of 11.0 ms. The rate of decay of currents was affected by membrane potential. tau D decreased exponentially with hyperpolarization in the range from +40 to -120 mV with an average volt constant (H value) of 146 +/- 9.6 mV (mean +/- 1 S.E. of mean, n = 17). The mean value of tau D at 0 mV was 19 ms. In some cells, growth times also decreased with hyperpolarization. The decay of currents was faster at higher temperatures but remained exponential. At 32 degrees C, the average tau D at 0 mV was 8.3 ms (n = 5) giving a Q10 value of 3.3 for the decay time constant at 0 mV. The frequency and mean amplitude of i.p.s.c.s were reduced by tetrodotoxin (TTX) or cadmium, indicating that many of the currents were generated by action potentials in presynaptic terminals. The spontaneous 'miniatures' remaining had the same time course and voltage sensitivity as currents recorded in normal solutions. Pentobarbitone (50-100 microM) greatly prolonged the decay of i.p.s.c.s but had no discernible effect on their amplitude or growth phase.

The time course of GABA action on the crayfish stretch receptor: evidence for a saturable GABA uptake

The conductance increase induced by bath application of GABA has been measured in voltage-clamped stretch-receptor neurones of crayfish. A rapid conductance increase was obtained only at GABA concentrations above 3.3 X 10(-4) M. The response to lower GABA concentrations (between 10(-4) and 10(-6) M) developed slowly over 30-60 min. Repetitive application of intermediate GABA concentrations induced postsynaptic conductance changes which were progressively enhanced in their onset and magnitude. In the presence of nipecotic acid or in Na+-free Ringer solutions, the response to all GABA concentrations was rapid and constant for each concentration. The time course of inhibitory postsynaptic currents was unaffected by nipecotic acid. These results suggest the presence of a saturable GABA uptake system which limits the access of bath-applied GABA to postsynaptic receptors. This system has little if any effect on the termination of response to synaptically released GABA.

Anesthetics block excitation with various effects on inhibition in MRF neurons

The effects of isoflurane and halothane on the behavior of mesencephalic reticular neurons in the rat were studied by long-term extracellular microelectrode recording. Both anesthetics always suppressed the excitatory responses with simultaneous blocking or augmenting effects on the inhibitory responses of the reticular neurons. The blocking effects on the inhibitory responses were more frequently noticed during light anesthesia than during the deep stage.

Comparison of GABA analogues at the crayfish stretch receptor neurone

The conductance increase induced by GABA and structurally related compounds has been measured in voltage clamped stretch receptor neurones of crayfish. GABA induced only at 10(-3) M a rapid conductance increase. The response to lower concentrations between 10(-6) and 10(-4) M developed slowly (20-60 min). The postsynaptic conductance increase induced by repetitive application of the same GABA concentration was progressively enhanced in the speed and magnitude. In the presence of nipecotic acid or in Na+-free Ringer solutions, the response to all GABA concentrations was instantaneous and constant for each concentration. Muscimol between 10(-6) and 10(-3) M caused instantaneous dose-dependent conductance increases. These results suggest the presence of a saturable GABA uptake system limiting the access of bath applied GABA, but not of muscimol, to postsynaptic receptor sites.

Effect of barbiturates on the GABA receptor of cat primary afferent neurones

1. The effects of the barbiturate anaesthetics, pentobarbitone and thiopentone, on the membrane properties and the gamma-aminobutyric acid (GABA)-induced responses of cat primary afferent neurones were studied with intracellular recording and voltageclamp techniques.2. At low concentrations (10(-7)-10(-5) M) both barbiturates slightly enhanced and prolonged GABA-induced depolarizations or currents without affecting the membrane properties. At these concentrations, barbiturates have no effect on the apparent dissociation constant of the GABA-GABA receptor interaction or the reversal potential for GABA-induced depolarizations or currents.3. At high concentrations (10(-4)-10(-3) M) barbiturates produced a few millivolts reduction in the resting membrane potential. Voltage-clamp analysis revealed that the depolarization was associated with one of the three types of conductance change, i.e., an initial increase followed by a decrease (40% of neurones examined), only an increase (40%) and only a decrease (20%).4. Analysis in different ionic media indicated that the depolarization with a reduced membrane resistance is associated with an increased chloride conductance and that the one with an increased membrane resistance is accompanied by a reduction in potassium conductance. Bath-application of GABA (10(-3) M) or picrotoxin (10(-5) M) inhibited the increase in chloride conductance but not the reduction in potassium conductance.5. Barbiturates at these high concentrations initially caused a marked augmentation and prolongation of GABA responses; this was followed by a depression. The depressant action did not appear to be voltage-dependent. These actions of barbiturates were not accompanied by changes in the apparent dissociation constant of the GABA-current dose-response curve or the reversal potential for GABA currents. In addition, the single exponential decay of GABA current was not changed despite a marked prolongation of its decay time.6. Picrotoxin (10(-5) M) antagonized the depressant effect of barbiturates at high concentrations on GABA currents, and barbiturates (5 x 10(-6) M) reduced the inhibitory action of picrotoxin (5 x 10(-6) M) on the GABA-currents.7. From all these results, it is suggested that the site of barbiturate actions on GABA-responses is mainly the allosteric site (the ionic conductance regulatory subunit) but not the agonist recognition site or the chloride channels linked with GABA receptors.

On the action of the anticonvulsant 5,5-diphenylhydantoin and the convulsant picrotoxin in crayfish stretch receptor

1. The effects of the anticonvulsant drug 5,5-diphenylhydantoin (DPH) and the convulsant drug picrotoxin (PTX) on various membrane properties and GABA-ergic inhibition were investigated in the slowly adapting neurone of the isolated crayfish stretch receptor. The soma was penetrated with two micro-electrodes to allow accurate determination of membrane conductances. 2. Neither DPH nor PTX at 10(-4) M had any significant effect on parameters of the anti- or orthodromic action potential or on the amplitude and duration of post-tetanic hyperpolarization. This suggests that the pharmacological properties of the two drugs are unlikely to be mediated by effects on cationic movements in this preparation. 3. DPH increased the amplitude and duration of the inhibitory post-synaptic potential (i.p.s.p.) within the range 10(-9) to 10(-4) M. The response to ionophoretically applied GABA was similarly prolonged. 4. PTX decreased the amplitude of the i.p.s.p. and prolonged its rising phase within the range 10(-8) to 10(-4) M. The response to ionophoretically applied GABA was similarly depressed. 5. A slow component of fluctuations in the resting potential was accentuated by DPH at 10(-4) M and eliminated by PTX 10(-4) M. This may reflect effects on the random opening and closing of inhibitory channels. 6. We conclude that the action of both drugs is post-synaptic and suggest that DPH decreases the probability of closing, and PTX the probability of opening, of the transmitter-activated channels. 7. The lack of any structural similarity between the two drugs suggests that they modify post-synaptic inhibition at separate sites. These sites appear to be interdependent since analysis of the shift in the DPH dose-response curve by PTX and vice versa, showed neither truly non-competitive nor competitive interaction.