Separation of ionic currents in the somatic membrane of frog sensory neurons

Voltage-gated calcium currents in whole-cell patch-clamped bullfrog dorsal root ganglion cells: effects of cell size and intracellular solutions

Acutely dissociated bullfrog dorsal root ganglion (DRG) cells could be divided into two classes by measurement of cell capacitance. A bimodal distribution of cell capacitance was found and a value of 75 pF was used to divide frog DRG cells into 'small' and 'large' types. Two distinct voltage-activated Ca2+ currents were evoked in both classes of cells: a rapidly inactivating, low-voltage-activated current and a slowly-inactivating, high-voltage-activated current. When the recording pipette contained CsCl, greater peak inward current values and densities were seen in large cells compared to small cells. No significant differences were observed in the distribution of low-and high-voltage-activated currents in small and large cells. Replacement of pipette solutions containing CsCl with solutions containing equimolar concentrations of Cs glutamate, L-arginine Cl, or N-methyl-D-glucamine significantly increased both the reversal potential and the maximum amplitude of the Ca2+ currents in both small and large DRG cells. These increases indicate that internal substitutions with organic ions suppresses outward currents more effectively than does CsCl. In contrast to findings with CsCl, when organic ions were used in the pipette solution a significantly higher proportion of low-threshold Ca2+ channels was observed in small cells compared to large cells. These observations indicate that when organic solutions were used internally, significant differences in the proportion of low-threshold to high-threshold Ca2+ channels were observed in small and large cells. The composition of the internal solution is a critical variable when determining the type and amount of inward Ca2+ current in different types of neurons.

Effects of lindane (gamma-BHC) and related convulsants on GABAA receptor-operated chloride channels in frog dorsal root ganglion neurons

Effects of lindane (gamma-benzenehexachloride; gamma-BHC) on GABA-evoked Cl- current (IGABA) in freshly dissociated frog sensory (dorsal root ganglion) neurons were studied and compared with those of tert-butylbicycloortho benzoate (TBOB) and picrotoxin by the use of the suction-pipette method [13]. Drugs were applied with a rapid drug-application method, "Concentration-clamp" technique. At concentration of GABA of > 3 x 10(-6) M, at least two components of the IGABA were recognized distinct degree of desensitization. Those were defined as the peak and plateau components in the text. At low concentration (3 x 10(-7) M) of gamma-BHC, only the plateau component of IGABA at 10(-5) M were depressed without changing the peak amplitude. While gamma-BHC at high concentration (3 x 10(-5) M) depressed both the peak and plateau current components. The gamma-BHC-induced depression of IGABA seemed to be IGABA-component-dependent. A detailed analysis of the gamma-BHC action in the concentration-response relationship for GABA revealed that the IGABA with strong desensitization was preferentially blocked by gamma-BHC (3 x 10(-5) M). The rate of recovery of the IGABA from gamma-BHC-induced block depended on the concentration of GABA. The lower the concentration of GABA, the slower the recovery. The GABAA receptor Cl- channels were proposed to be classified into two types of the gamma-BHC-sensitive and -resistant ones.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular ATP changes the voltage-dependence of delayed rectifier potassium current in bullfrog primary afferent neurons

Dissociated bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration to study the steady-state activation and inactivation curves for a delayed rectifier potassium current. The 50%-activation of the current occurred at +15 mV when measured with ATP (5 mM) in the pipette solution as opposed to -11 mV with 5'-adenylylimidodiphosphate (AMP-PNP, 5 mM) and -15 mV with adenosine 5'-O-(3-thiotriphosphate) (5 mM). The 50%-inactivation of the current occurred at -6 mV with ATP but at -31 mM with AMP-PNP. The results suggest that intracellular ATP modulates voltage-dependence of the delayed rectifier in amphibian afferent neurons.

C fiber generates a slow Na+ spike in the frog sciatic nerve

The present study was carried out to examine the properties of A and C fibers in the bullfrog sciatic nerves by applying several agents through perfusing solutions between stimulating and recording electrodes. The compound action potentials (CAPs) of A beta and A delta fibers were tetrodotoxin (TTX)-sensitive and were abolished in Na(+)-free solution. However, C fiber CAP was TTX-insensitive although CAP disappeared in Na(+)-free solution. Moreover, C fiber CAP was not blocked by Ca2+ channel blockers and its chronaxy (2 ms) and conduction velocity (0.70 m/s) indicate that the time constant of C fiber CAP is relatively large (2.88 ms). These suggest that a slow Na+ channel, which is TTX-resistant, contributes to C fiber action potentials.

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.

Slowly inactivating potassium current in cultured bull-frog primary afferent and sympathetic neurones

1. Cultured bull-frog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. The cells were superfused with a nominally calcium-free Ringer solution containing tetrodotoxin (3 microM), magnesium (10 mM), cobalt (1 mM), barium (2 mM), 4-aminopyridine (3 mM) and caesium (2 mM). 2. Step depolarizations (10-40 mV, 100-300 ms) from a holding potential close to the rest (typically -70 mV) evoked an outward current (IK) followed by an outward tail current. The peak amplitude of the current was reduced to less than 10% by tetraethylammonium (30 mM). 3. IK developed to its peak in 200 ms at -30 mV. Tail currents reversed at potentials that changed according to the logarithm of the extracellular potassium concentrations. 4. Tail currents declined to the baseline according to an exponential function of time (tau congruent to 40 ms at -60 mV) and its reciprocal time constant increased e-fold with a 13 mV hyperpolarization. 5. The current inactivated during sustained (1-20 s) depolarizing pulses according to a single exponential function (tau congruent to 3 s). 6. The peak amplitude of IK at -30 mV was progressively increased as the holding potential was made more negative than -70 mV reaching the maximum with step depolarizations from -120 mV. Reversed phenomenon was observed as the holding potential was made less negative than -70 mV. 7. The removal of the steady-state inactivation occurred along with a single exponential function and the time constant was decreased from 70 ms at -70 mV to 10 ms at -120 mV. 8. It is suggested that a slowly inactivating potassium current which we called IK in amphibian sensory neurones could be a class of a 'delayed' rectifier potassium current. A potassium current with properties indistinguishable from those which have been described for the sensory IK also occurred in cultured bull-frog sympathetic neurones. 9. Forskolin (1-30 microM) and 1,9-dideoxy forskolin (10 microM) reduced the amplitude of IK by up to 85% but these actions were not mimicked by any of 8-bromo-cyclic AMP (1 mM), dibutyryl cyclic AMP (1 mM) and 3-isobutyl-1-methylxanthine (1 mM). A hydrophilic forskolin analogue, 7-O-hemisuccinyl-7-deacetyl forskolin (10 microM), was about one-tenth as potent as forskolin (10 microM).

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+.

GABA affects the glutamate receptor-chloride channel complex in mechanically isolated and internally perfused Aplysia neurons

The effects of gamma-aminobutyric acid (GABA) on the glutamate receptor chloride ion (Cl-) channel complex were examined in mechanically isolated and internally perfused Aplysia neurons using a concentration clamp technique. GABA at concentrations of 3 x 10(-6) M or more, concentration dependently delayed the recovery of the glutamate response from desensitization. This effect was independent of the GABA response and Cl- redistribution. Muscimol (10(-4) M) mimicked the effect of GABA. However, this was not the case for baclofen (10(-3) M). In some isolated neurons, GABA at concentrations of more than 10(-4) M clearly induced an additional Cl- current, the current kinetics of which were different from those induced by lower concentrations of GABA. Even in the continued presence of 10(-4) M GABA, which desensitized the fast GABA response, higher concentrations of GABA (3 x 10(-4) M to 10(-2) M) elicited the additional current in a concentration-dependent manner. The presence of 10(-4) M glutamate completely abolished this current, indicating cross-desensitization between the glutamate and slow GABA responses. High concentrations of GABA (3 x 10(-2) M) did not activate the glutamate receptor coupled to the large cation channel. The results suggest that, in Aplysia neurons, the glutamate receptor-Cl- channel complex has some similarities to the GABA receptor-Cl- channel complex.

ATP regulates muscarine-sensitive potassium current in dissociated bull-frog primary afferent neurones

1. Bull-frog dorsal root ganglion cells in primary culture were voltage clamped in the whole-cell configuration. The pipette solution contained ATP (5 mM). 2. Step depolarizations (5-70 mV, 0.1-1 s) from a holding potential close to the resting potential (range, -64 to -79 mV) evoked a non-inactivating potassium current with properties indistinguishable from those which have been reported for the M-current of bull-frog sympathetic neurones. 3. An unhydrolysable ATP analogue APP(NH)P (5 mM), substitute with ATP in the pipette solution, did not support the M-current activation. 4. Bath application of ATP (30 nM-30 microM) reduced the amplitude of the M-current in a concentration-dependent manner, congruent to 50% inhibition of the current occurring with 1 microM-ATP. The main effect of ATP was to reduce the maximum M-conductance without changing the activation and deactivation kinetics of the M-current. 5. Essentially the same results were obtained with ADP (0.1-30 microM) and alpha, beta-methylene-ATP (10-30 microM). AMP (10-100 microM) and adenosine (10-30 microM) were without effect on the M-current. 6. The ATP-induced inhibition of the M-current was irreversible when an unhydrolysable GTP analogue GTP-gamma-S (10-30 microM) was present in the pipette solution. ATP (3 microM) reduced the amplitude of the M-current only by about 10% when GDP-beta-S (100 microM) was present in the pipette solution. Pre-treatment of the cells with pertussis toxin (IAP; 500 ng ml-1) for 24 h at 24 degrees C did not prevent the ATP-induced M-current inhibition. 7. Phorbol 12-myristate 13-acetate (PMA; 1-3 microM) reduced the amplitude of the M-current to about 50%. A reduction in the M-current amplitude by PMA (3 microM) and ATP (10 microM) was attenuated when staurosporine (200 nM) was present in the pipette solution. Forskolin (10 microM) was without effect on the M-current. 8. It is concluded that ATP acting at P2 receptors, associated with an IAP-insensitive GTP-binding protein, inhibits the M-current in amphibian primary afferent neurones.

Influences of trypsin and collagenase on acetylcholine responses of physically isolated single neurons of Aplysia californica

1. The influences of enzyme treatments (trypsin and collagenase) on responses to perfused acetylcholine were examined on physically isolated single Aplysia neurons, using the voltage-clamp, internal perfusion, and rapid external perfusion technique. 2. During treatment with trypsin (0.025 to 0.1%) for 10 to 30 min at room temperature (22 to 25 degrees C), the peak amplitude of the Na current induced by acetylcholine increased in a time- and dose-dependent manner, and the decay in the continued presence of acetylcholine was slowed. This effect of trypsin treatment was irreversible after washing for 60 min without enzyme. 3. Edrophonium, a cholinesterase inhibitor, has previously been shown to augment the Na acetylcholine response in this preparation by inhibition of acetylcholinesterase. After treatment of the neuron with trypsin, the augmentation after edrophonium was abolished. Furthermore, in the presence of edrophonium, trypsin also failed to increase the response. The dose-response curve for acetylcholine after treatment of trypsin was similar to that in the presence of edrophonium. These results suggest that the modification of the current response by trypsin is a result of removal of cholinesterase activity from the membrane. 4. In contrast to the effects of trypsin, collagenase (0.03 to 0.1%) for 10 to 60 min did not change the current amplitude of the acetylcholine response. However, collagenase treatment did alter the kinetics of the acetylcholine response in a dose-dependent manner, in that the rate of decay was accelerated. A similar acceleration was seen in the acetylcholine responses on other neurons which were due to Cl or K currents, suggesting that the effect was independent on the type of channel. This effect of collagenase was reversible after 30 to 60 min of washing of the neuron. 5. In the presence of edrophonium or after the treatment with trypsin, collagenase still accelerated the current kinetics of the acetylcholine response, indicating that cholinesterase activity is not related to this effect. Furthermore, heated collagenase (presumably inactivated) had a similar action, suggesting that the enzymatic activity of collagenase is not related to the modification of the response. 6. These results suggest that Aplysia acetylcholinesterase is sensitive to trypsin but not to collagenase. However, the preparation of a collagenase used in these studies contains some factor which alters the response to acetylcholine, but this effect is reversible and unrelated to enzymatic activity.

Hypothalamic warm-sensitive neurons possess a tetrodotoxin-sensitive sodium channel with a high Q10

We have investigated the ionic current responses to temperature of dissociated cells from the preoptic and anterior hypothalamus (PO/AH) of rat, using the 'whole-cell' configuration. The majority of the recorded neurons showed a linear increase in a non-inactivating inward current during warming (30-40 degrees C), and the Q10 was about 2. However, about 24% of PO/AH neurons were markedly sensitive to warming and the increase in non-inactivating inward current to a rise in temperature in the hyperthermic range (35-40 degrees C) had a high Q10 (4.3-7.0). This increase in current in the hyperthermic range was reversibly blocked by tetrodotoxin (TTX). The inward current in neurons with a Q10 of 2 was not affected by TTX. The results show that some neurons in the PO/AH possess a non-inactivating sodium channel that is highly temperature-sensitive in the hyperthermic range. These neurons are presumably the 'primary' warm-sensitive neurons.

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.

Tetraethylammonium stimulates adrenomedullary secretion by causing fluctuations in a cytosolic free Ca concentration

Tetraethylammonium (TEA) caused marked fluctuations in a cytosolic free Ca concentration, which was measured by fura-2 microfluorimetry in bovine adrenal chromaffin cells. At the same time. TEA stimulated catecholamine secretion from cat and bovine chromaffin cells. Present results suggest that TEA indirectly increases the Ca2+ influx by depolarizing the cell membrane and consequently stimulates catecholamine secretion.

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.

Electrophysiological properties of subpopulations of rat dorsal root ganglion neurons in vitro

Intracellular recordings were made from rat dorsal root ganglion neurons in vitro. Action potentials with an inflection on the falling phase occurred in all cells conducting up to 5.2 m/s and in a proportion of faster conducting cells which decreased with increasing conduction velocity, until no cells conducting faster than 31 m/s had an inflection. Overall, all C-cells (less than 1.3 m/s), 61% of A delta-cells (1.3-12 m/s) and 23% of A alpha/beta-cells (greater than 12 m/s) had inflections. A-cells with inflections were found to be electrophysiologically distinct from those without as they differed in the mean and distribution of every action potential and afterhyperpolarization parameter measured. C-cells differed from all A-cell groups, but the means and distributions of most parameters were closer to those of A-cells with inflections than of A-cells without. In addition, all A- and C-cell action potentials with inflections were tetrodotoxin resistant, while all those without were sensitive. The only parameters whose means differed between A alpha/beta- and A delta-neurons were ones which correlated with conduction velocity (action potential duration, overshoot and maximum rate of rise and fall). The response pattern to prolonged current injection did not correlate with conduction velocity, but slightly more A-cells with inflections were single firing. A-cells with long afterhyperpolarizations always fired singly, while those with shorter durations fired singly or multiply. Somatic following frequency was most strongly limited by long afterhyperpolarization duration; it was also slightly lower in A delta- than in A alpha/beta-cells, and lower in A-cells with inflections than in those without. Fibre following frequencies were highest in the fastest conduction neurons.

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.

Activation kinetics of the acetylcholine-gated potassium current in isolated atrial cells

Processes involved in activation of the acetylcholine (ACh) receptor-operated K+ current (IK) were examined in atrial cells isolated from guinea pig using a "concentration-clamp" technique. This approach allows for the intracellular perfusion and the rapid change of external solution (time constant 4 ms) with cell-attached condition under a single-electrode voltage-clamp condition. The ACh-induced IK increased in a sigmoidal fashion with increasing concentrations of ACh. The Ka value estimated from the concentration-response curve was 3 X 10(-7) M, and the Hill coefficient was 1.0. The activation phase was accelerated not only by increasing the concentration of ACh but also by elevating the temperature. Before activation of the current, there was a brief latent period after the application of ACh. The latent period was shortened considerably with the increase in ACh concentration and in temperature, i.e., 267 +/- 20 ms at 18 degrees C, 98 +/- 11 ms at 26 degrees C, and 44 +/- 6 ms at 37 degrees C for 10(-6) M ACh. These results suggest that the latent period of ACh response seems to be the time lag needed for the activation of K+ channels using remote sensor.

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.

The low-threshold Ca current in isolated amygdaloid neurons in the rat

Two types of voltage-dependent Ca currents were recorded from isolated rat amygdaloid neurons under single-electrode voltage-clamp. A low-threshold Ca current was elicited at -60 mV or more positive potentials and inactivated rapidly. At -30 mV or more positive potentials, a high-threshold Ca current was also activated. In steady-state inactivation curve of the low-threshold Ca current, the half-inhibition value (h0.5) was -71 mV. The low-threshold Ca current was inhibited by organic and inorganic Ca blockers in a dose-dependent manner. The inhibitory effect of these Ca blockers was completely reversible while that of flunarizine was partly so. It is concluded that the membrane and pharmacological properties of the low-threshold Ca channel in the rat amygdaloid neurons are quite similar to those in the hypothalamic neurons.

Different time courses of the blockade of sodium current by lignocaine and SUN 1165 in single myocytes isolated from guinea-pig atrium

1. The time course of the blockade of sodium currents (INa) by the antiarrhythmic agents, lignocaine and SUN 1165, was studied in single myocytes isolated enzymatically from guinea-pig atrium, by a new concentration-jump termed as a 'concentration-clamp' technique. This technique combines an intracellular perfusion and a rapid exchange of external solution surrounding the voltage-clamped myocyte within 2 ms. 2. Lignocaine (3.7 x 10(-5) M to 3.7 x 10(-4) M) inhibited the peak amplitude of INa in a concentration-dependent fashion. It took 2 to 5s to reach apparent steady-state inhibition at the concentrations used. Complete recovery from the inhibition also took 2 to 5s after washing out the agent. In contrast, the inhibitory effect of SUN 1165 (1 x 10(-5) M) on the peak INa gradually progressed and reached a steady-state level about 2 min after the start of drug-application. The recovery required more than 10 min after washing out of the agent. 3. In cardiomyocytes treated with scorpion toxin (5 micrograms ml-1), the inactivation of INa was greatly inhibited, resulting in the relatively persistent Na inward current (persistent INa) during the depolarizing pulse. Lignocaine (1.1 x 10(-4) M) applied during the depolarizing pulse, reduced in a single-exponential fashion the amplitude of the persistent INa in milliseconds. On the other hand, lignocaine applied several tens of milliseconds before the depolarizing pulse induced only a small reduction of the peak amplitude of the first persistent INa. When SUN 1165 (1 x 10- M) was applied during the depolarizing pulse, there was only a small instantaneous reduction of the amplitude of the persistent I'N, although it did inhibit time-dependently, the peak IN.. Both agents accelerated the decay phase of the persistent IN in a time-dependent manner. 4. These results suggest that lignocaine and SUN 1165 may preferentially interact with the openstate of the sodium channel rather than with the rested one, although SUN 1165 does so much more slowly.

Comparison of low-threshold Ca2+ currents in the hippocampal CA1 neurons among the newborn, adult and aged rats

The electrical and pharmacological properties of the low-threshold Ca2+ current were compared among the newborn, adult and aged rats using the isolated hippocampal CA1 pyramidal neurons. The current-voltage relationship and the time constant of the decay phase of the low-threshold Ca2+ current in adult and aged rats were similar to those in newborn rats. The low-threshold Ca2+ current of adult and aged rats was also sensitive to nicardipine, a dihydropyridine derivative, same as that of newborn rats. We concluded that the nature of low-threshold Ca2+ current in the rat hippocampal CA1 pyramidal neurons is not affected by the aging.

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.

GABA-induced chloride current in rat isolated Purkinje cells

Electrical and pharmacological properties of the gamma-aminobutyric acid (GABA)-induced current in the rat isolated cerebellar Purkinje cell bodies were studied using a concentration-jump method, which is termed as a "concentration-clamp" technique. This technique enables the rapid exchange of external solution around the neurons to be perfused internally under a voltage-clamp condition. The peak amplitude of GABA response increased sigmoidally with the increase of the concentration of GABA. The value of the GABA concentration that evokes a half-maximal response (Ka) was 5 X 10(-5) M, and the Hill coefficient was 1.8. The current-voltage relationship for the GABA response showed nonlinearity at membrane potentials more negative than -40 mV. The reversal potential of GABA-evoked current was close to the equilibrium potential of Cl- (ECl), indicating that the current elicited by GABA is carried by Cl-. Both the activation and inactivation phases of GABA-induced Cl- current (ICl) consisted of fast and slow components. These time constants in both phases decreased as the concentration of GABA increased. Strychnine and bicuculline inhibited the GABA-induced ICl in a dose-dependent manner, and the inhibition of the GABA response by bicuculline was competitive. Pentobarbital sodium augmented the GABA response and modified the inactivation phase. The augmentation of the GABA response by pentobarbital was more profound at lower concentrations of GABA and was accompanied by a change in the Hill coefficient from 2 to 1. The properties of the GABA response in cerebellar Purkinje cells were thought to be basically similar to those previously reported in other preparations.

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)

Blockade of the voltage-dependent sodium current in isolated rat hippocampal neurons by tetrodotoxin and lidocaine

The effects of tetrodotoxin and lidocaine on the voltage-dependent sodium current (INa) were studied in the CA1 pyramidal neurons isolated acutely from rat hippocampus using a 'concentration-clamp' technique which combines the intracellular perfusion with a rapid external solution change within a few ms. Tetrodotoxin (TTX) exerted its inhibitory action in time- and dose-dependent manner on the peak amplitude of INa without any apparent effects on both the current activation and inactivation processes of the current. The time course for reaching a steady-state of the inhibitory action shortened with increasing TTX concentration, but the time course of recovery from the inhibition after washing out the toxin was quite the same at any concentrations used. Lidocaine also inhibited dose-dependently the INa, though with slightly accelerating both the activation and inactivation processes. The time courses for reaching the steady-state inhibition and the recovery from the inhibition were much shorter than those in the case of TTX. The results indicate that the voltage-dependent sodium channel of mammalian brain neuron is TTX-sensitive as well as that of peripheral neuron and that the mode of TTX inhibition on the INa is quite different from that of lidocaine.

Augmentation of GABA-induced chloride current in frog sensory neurons by diazepam

The effect of diazepam (DZP) on the GABA-induced macroscopic and microscopic Cl- current was investigated in isolated frog sensory neurons using both 'concentration-clamp' and patch-clamp techniques. At concentration range between 10(-9) and 10(-4) M, DZP itself evoked no response but potentiated time- and dose-dependently the subthreshold GABA responses, though at high DZP concentrations beyond 10(-5) M the potentiation ratio decreased. The potentiation effect was long-lasting and desensitized slowly over the course of several 10 minutes after washing-out of DZP. DZP potentiated GABA response without shifting the GABA reversal potential. The entire GABA dose-response curve was shifted in a parallel manner to the left by adding DZP without changing cooperatively: the Hill slope was 2.0. The potentiation of GABA response by DZP did not depend on either inward or outward direction of the Cl- current but slightly on the membrane potential. The time constants of activation of desensitization of GABA-gated Cl- current consisted of fast and slow components, respectively. The slow components were concentration-dependent, and significantly changed in the presence of DZP, while DZP had little effects on fast components. In the 'inside-out' configuration, the addition of DZP activated GABA-receptor ionophore complexes under subthreshold without changing the single Cl- channel conductance. It is concluded that DZP may act at a site to modulate GABA binding, in which DZP increases GABA binding affinity and also affects the kinetics of GABA-gated Cl- channels, indicating that DZP has dual action on the GABA-induced responses.

Electrophysiological estimation of the actions of acetylcholinesterase inhibitors on acetylcholine receptor and cholinesterase in physically isolated Aplysia neurones

1. The actions of representative cholinesterase inhibitors on the acetylcholine responses of physically isolated single neurones from the pedal ganglion of Aplysia californica were studied, using electrophysiological techniques and rapid agonist application to analyse both the inhibitory actions on the acetylcholine receptor-channel complex and the degree of inhibition of acetylcholinesterase activity on the same neurone. The inhibitors used were physostigmine, edrophonium and diisopropylfluorophosphate (DFP). 2. When selected neurones were suddenly exposed to 50 microM acetylcholine by a 'concentration clamp' technique a large Na-dependent inward current was initiated, and decayed in the continued presence of acetylcholine without external perfusion. However, if perfusion of the acetylcholine solution was reinitiated the current increased somewhat, indicating that the decay of current was due to some combination of receptor desensitization and local depletion of acetylcholine at the membrane by acetylcholinesterase. 3. With simultaneous application of acetylcholine (50 microM) and physostigmine (0.1 to 100 microM) there was a dose-dependent reduction of peak amplitude of the acetylcholine response. However, physostigmine at low concentrations (0.1 to 10 microM) caused a time-dependent increase in the current amplitude alone with a time- and dose-dependent inhibition of acetylcholinesterase activity. At the highest concentration of physostigmine (100 microM) acetylcholinesterase activity was abolished but the current peak was very depressed. After removal of physostigmine from the bathing solution, the current amplitude decreased toward the control at the two lower concentrations as the inhibitory actions on acetylcholinesterase activity were almost reversible, while at the two higher concentrations (10 and 100 microM) the current increased and the inhibition of acethylcholinesterase remained. 4. When acetylcholine (50 microM) and edrophonium (0.1 to 10 microM) were applied simultaneously, edrophonium caused a dose-dependent increase in the peak amplitude that was correlated with a dose-dependent inhibition of acetylcholinesterase activity. Prolonged exposure to edrophonium did not change the peak amplitude and there was no time-dependent change in the inhibition of acetylcholinesterase activity. At the highest concentration of edrophonium used (100 microM), simultaneous application with acetylcholine augmented the peak amplitude relative to control, but to a lesser extent than 10 microM. Prolonged exposure to the highest concentration of edrophonium caused a time-dependent reduction in the peak amplitude. The effects of edrophonium were quickly reversible after the removal of the drug from the bathing solution. 5. DFP (1 and 10mM), similar to 1OO microM physostigmine, caused a dramatic reduction of the peak current on simultaneous application with ftetylcholine. During exposure to DFP the current amplitude and acetylcholinesterase activity were very depressed. After removing DFP from the bathing solution the current amplitude increased to more than the control level after 1 mm DFP, while it did not recover to the control level after 10mM DFP. The inhibition of acetylcholinesterase activity remained at both concentrations. 6. These results indicate that all three cholinesterase inhibitors have dose-dependent actions both at the acetylcholine receptor-channel complex and at acetylcholinesterase. The methods we have developed may be useful in the evaluation of various cholinesterase inhibitors.

Ryanodine inhibits the Ca-dependent K current after depletion of Ca stored in smooth muscle cells of the rabbit ileal longitudinal muscle

1. Effects of ryanodine on the membrane currents were investigated on dispersed smooth muscle cells of rabbit ileal longitudinal layer using voltage and patch clamp procedures. 2. With voltage clamp, membrane depolarization to 0 mV from the holding potential of -60 mV produced an inward Ca current (ICa) which was followed by transient and sustained outward currents (ITO and ISO, respectively). Prolonged depolarization of the membrane produced spontaneous oscillations of the outward current (oscillatory outward current; IOO) on ISO. 3. Ryanodine (30 microM) modified neither the basal membrane current recorded at the holding potential (-60 mV) nor ISO. Ryanodine inhibited both ITO and IOO in a concentration-dependent manner (IC50 = 5.5 and 4.5 microM, respectively, measured 12 min after application of ryanodine). These values were much higher than that observed in skeletal muscle for Ca release. 4. The time course of the ryanodine-induced inhibition of IOO was slow and the inhibition was irreversible. Caffeine (3 mM) enhanced the amplitudes of ITO and IOO in the presence of Ca, and only transiently enhanced IOO in the absence of Ca. However, following application of 10 microM ryanodine, 3 mM caffeine did not increase IOO. 5. Ryanodine (3-30 microM) slightly enhanced the amplitude of ICa evoked by depolarization pulses at potentials more negative than O mV but not that induced by larger depolarizations (positive potentials). 6. With patch clamp procedure, single Ca-dependent K channel currents were recorded in cell free and cell attached configurations. Application of 30 microM ryanodine transiently enhanced the Ca-dependent K current without any detectable changes in the amplitude of the single channel current recorded in the cell attached condition. In the inside-out membrane patch, when the intracellular membrane side was superfused with 1 microM Ca buffered with 10 mM EGTA, bath application of 10 microM ryanodine had no effect on the Ca-dependent K current. 7. It was concluded that both ITO and IOO are generated by Ca released from intracellular stores, mainly sarcoplasmic reticulum. Ryanodine appears to open irreversibly the Ca channel in the store and to inhibit the Ca-dependent K channel due to depletion of the stored Ca.

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.

Prolonged depolarization in turtle cones evoked by current injection and stimulation of the receptive field surround

1. Responses evoked by stimulation of the receptive field surround were recorded intracellularly from cone photoreceptors in the retina of the turtle (Pseudemys scripta elegans). 2. A distinctive depolarizing response was evoked by flashing an annulus of light while steadily illuminating the centre of the receptive field. The response, here called 'the prolonged depolarization', was found in 67% of a sample of 125 cones and could reach some 20 mV in amplitude. 3. The prolonged depolarization is characterized by a set of properties which include: the capacity to persist up to 17 s after the flash, a stereotypical waveform, a long period of temporal facilitation, a very narrow dynamic range, and a long refractory period (30-45 s). 4. Depolarizing current pulses (0.01-0.1 nA) evoke a prolonged depolarization which is similar to and functionally interchangeable with that evoked by light. The prolonged depolarization is thus apparently generated by a voltage-sensitive mechanism intrinsic to the cone. 5. Brief depolarizing spikes were recorded in a small fraction of cones. The spikes appear to be dissociable from the prolonged depolarization although both might arise for similar regenerative mechanisms. 6. The prolonged depolarization is typically preceded by a graded, stimulus-locked depolarization which can also be recorded in isolation by flashing annuli of low intensity. The graded depolarization is probably a manifestation of the depolarizing influence arising from synaptic feed-back from horizontal cells first described by Baylor, Fuortes & O'Bryan (1971). 7. It is suggested that the graded depolarization triggers the prolonged depolarization and that complex responses arise from the interaction of these disparate components.

The effect of temperature on the GABA-induced chloride current in isolated sensory neurones of the frog

1. The effect of temperature on the kinetics of the activation and inactivation phases of gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) was examined in frog isolated sensory neurones. 2. The peak ICl was reversibly reduced on changing the temperature and temperature-dependent coefficients were shown to exist, with the highest Q10 (1.58) occurring between 5-15 degrees C. 3. At both room temperature (20 degrees C) and 10 degrees C, the GABA dose-response curve was sigmoidal with a Hill coefficient of 2 and half-maximal responses to GABA, Kd, of 1.3 x 10(-5)M and 1.1 x 10(-5)M, respectively. Thus, indicating no change in the binding affinity of GABA when the temperature was decreased. 4. At GABA concentrations greater than 10(-5)M, both the activation and inactivation phases of the GABA-induced ICl consisted of double exponentials, fast and slow components respectively, in the temperature range of 10 to 30 degrees C. 5. The fast (tau af) and slow (tau as) activation time constants decreased with an increase in temperature and increased with a reduction in temperature. With an increased temperature, the reduction in peak ICl was due to a reduction in the slow time constant with no significant change in the fast time constant. 6. Both the fast (tau if) and slow (tau is) inactivation time constants were also increased by cooling to 10 degrees C; heating to 30 degrees C had little effect. 7. The concentration-dependence (10(-5) to 10(-3)M) of the slow activation (tau as) and inactivation (tau is) time constants was unaltered by the change in temperature. Similarly, the lack of concentration-dependence shown by the fast activation (tau af) and inactivation (tau if) time constants was unaltered by the temperature change. 8. From recordings made with 'inside-out' patches, the probability of opening of the GABA-induced Cl- channels showed a marked increase with cooling to 10 degrees C compared to room temperature (20 degrees C), with no change in channel conductance. 9. The change in the GABA-induced ICl at different temperatures is, therefore, not due to changes in binding but to subsequent channel activation. Possible mechanisms whereby this occurs are discussed.

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)

Effects of Ca2+ on the transient outward current of single isolated Helix central neurones

1. Single Helix neurones were studied under voltage-clamp conditions with internal perfusion in order to examine the contribution of internal and external Ca2+ and the effects of 4-aminopyridine (4-AP) on the transient outward current (IA). 2. In Na+ -free snail Ringer, replacement of external Ca2+ [( Ca2+]o) with equimolar Co2+ reduced the maximum amplitude of IA and induced a shift of the steady-state part of the IA inactivation curve (I-V curve), in a positive direction along the voltage axis when the neurone was internally perfused with K-aspartate. 3. In Ca2+ -free solutions, precipitation or chelation of internal Ca2+ [( Ca2+]i) by internal perfusion with KF or EGTA shifted the curve in a more negative direction without affecting the maximum amplitude of IA. Thus, the kinetics of IA are dependent not only upon [Ca2+]o, as previously suggested, but also upon [Ca2+]i. 4. In the presence of 4-AP the I-V curve for IA shifted in a hyperpolarizing direction and the maximal amplitude was reduced when the neurone was internally perfused with K-aspartate. However, 4-AP had little or no effect on the I-V relationship of IA when the neurone was internally perfused with the Ca2+ precipitating or chelating agent, KF or EGTA. These results suggest that the actions of 4-AP on IA are at least partly dependent upon [Ca2+]i.

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.

Effects of inositol phosphates on the membrane activity of smooth muscle cells of the rabbit portal vein

The effects of intracellular perfusion of inositol 1,4,5-trisphosphate (InsP3) or inositol 1,3,4,5-tetrakisphosphate (InsP4) on electrical responses of smooth muscle cell membranes of the rabbit portal vein were studied using the whole cell voltage clamp technique. Depolarisation to 0 mV from a holding potential of -60 mV, evoked inward Ca (Ica), transient outward (ISO), oscillatory outward (IOO) and sustained outward (ISO) currents. Generation of IOO was dependent on the [Ca]o, but it was also generated in 0 mM Ca solution for over 10 min. From amplitude histograms, IOO was divided into two components. Reduction in [Ca]o inhibited the appearance of but not the amplitudes of both IOO components. However, the larger component of IOO was more resistant to a reduction in [Ca]o than the smaller one. InsP3 (10 microM) increased the frequency of both IOO components to a greater extent than their amplitude, but the larger component was more sensitive to InsP3 than the smaller one. The increase in the occurrence of IOO induced by InsP3 did not occur following pretreatment with 3 mM caffeine or 1 nM A23187. In normal PSS, InsP3 was evoked by a depolarising pulse positive to -40 mV, whereas following perfusion with InsP3 (10 microM), IOO was evoked at -60 mV. In normal PSS, intracellular perfusion with 10 microM InsP4 changed neither the frequency nor the amplitude of IOO, and the amplitudes of ICa, ITO and ISO were also unchanged. However, in 10 mM Ca solution, 10 microM InsP4 generated IOO at a membrane potential of -60 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

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-.

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.

Permeability to various cations of the voltage-dependent sodium channel of isolated rat hippocampal pyramidal neurons

The permeability to various cations of the voltage-dependent sodium channel of isolated rat hippocampal pyramidal neurons was investigated under the voltage-clamp condition. The neurons were dispersed enzymatically and mechanically and perfused internally using a suction pipette technique. The permeability sequence, estimated from the reveral potential of the inward current was Li+ greater than Na+ greater than hydrazine+ greater than formamidine+ greater than guanidine+ greater than methylguanidine+ greater than monomethylamine+. Thus the ionic selectivity of the voltage-dependent sodium channel of rat central nervous system neurons is similar to that in the squid axon, myelinated frog nerve fiber and rat ventricle muscle.

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.

Blockade of N-methyl-D-aspartate response in enzyme-treated rat hippocampal neurons

The responses to excitatory and inhibitory amino acids have been investigated in isolated pyramidal cells from young and adult rat hippocampus using internal perfusion and 'concentration-clamp' techniques. The neurons dissociated in a purely mechanical way were sensitive to all excitatory amino acids (glutamate, kainate, quisqualate and N-methyl-D-aspartate (NMDA] and inhibitory amino acids (glycine, taurine and gamma-aminobutyric acid). The NMDA response was dramatically potentiated by adding glycine at threshold concentration (10(-6) M). The enzyme treatment of hippocampal slices selectively removed the NMDA sensitivity but did not alter all other pharmacological properties of voltage- and agonist-gated ion channels.