A distinct distribution of functional presynaptic 5-HT receptor subtypes on GABAergic nerve terminals projecting to single hippocampal CA1 pyramidal neurons

5-HT is known to modify the excitability of GABAergic interneurons projecting to hippocampal CA1 neurons. In this study we investigate the presence and functionally characterize the 5-HT receptor subtypes found on the presynaptic nerve terminals of these GABAergic neurons. Using conventional whole-cell patch recording, we confirmed that the 5-HT(1A) agonist, 8-hydroxy-2-dipropylaminotetralin, presynaptically decreased electrically evoked GABA release while the 5-HT(3) agonist, m-chlorophenylbiguanide (mCPBG), presynaptically facilitated release. Using the 'synaptic bouton preparation', where CA1 neurons are acutely isolated with functional nerve terminals/boutons remaining adherent, we next showed that these receptor subtypes are found presynaptically. We next used the technique of focal stimulation of a single bouton in this preparation to further investigate the distribution of these 5-HT receptor subtypes. We found that all boutons contained inhibitory 5-HT(1A) receptors while a subset of boutons showed both 5-HT(1A) and excitatory 5-HT(3) receptors. No boutons were detected which contained only 5-HT(3) receptors. Our studies show that presynaptic 5-HT receptor subtypes are found presynaptically and are not uniformly distributed. This provides another potential mechanism whereby 5-HT can modulate GABA release and hence the excitability of hippocampal neurons.

Noradrenaline receptor-mediated potentiation of caffeine-induced Ca( 2+)-activated K(+) currents in bovine ciliary muscle cells

PURPOSE

Adrenoceptor-mediated modulation of a caffeine (CAF)-induced [Ca(2+)](i) elevation and resulting Ca(2+)-activated K(+) current (I(CAF)) in bovine ciliary muscle (CM) cells were investigated.

METHODS

The nystatin-perforated patch clamp technique for the measurement of membrane currents and a microscope based fura-2 fluorescence imaging of [Ca(2+)](i) were applied to CM cells freshly dissociated with collagenase and identified with smooth muscle-specific alpha-isoactin.

RESULTS

Under voltage-clamped conditions, noradrenaline (NA) potentiated I(CAF) in a NA concentration-dependent manner without producing current responses to NA when NA was applied alone. NA-induced potentiation of I(CAF) occurred within 20 sec after the application of NA, while the NA-potentiated I(CAF) gradually recovered to the control level within 30 min after removal of NA. Despite the little current response to NA applied alone, NA elicited a [Ca(2+)](i) elevation in a manner similar to that induced by CAF although the NA-induced [Ca(2+ )](i) elevation was smaller than the CAF-induced [Ca(2+ )](i) elevation. In contrast to the significant potentiation of I(CAF) with NA, NA produced little potentiation of the CAF-induced [Ca(2+)](i) elevation. The NA-induced potentiation of I( CAF) was antagonized by an alpha(1) adrenoceptor antagonist, prazosin. Neither clonidine nor isoproterenol had an effect on I(CAF), suggesting that alpha(2) and beta adrenoceptor are not involved in the response to NA.

CONCLUSIONS

These results suggest that NA potentiates I( CAF) via alpha(1) adrenoceptor activation and that the NA-induced potentiation occurs at Ca(2+)-activated K(+) channels but not CAF-induced Ca(2+) releasing sites.

Alkalinization-induced K+ current of the mouse megakaryocyte

We have recently found that mouse megakaryocytes responded to extracellular alkalinization to pH > 8.0, generating a K+ current under voltage-clamped conditions with the whole cell recording mode of the patch-clamp technique. The purpose of this study was to physiologically and pharmacologically characterize the alkaline-dependent K+ conductance of the megakaryocyte membrane. The alkalinization-induced K+ current (I(ALK)) did not seem to be Ca2+-dependent since I(ALK) was allowed to be generated under intracellularly Ca2+-buffered conditions with 10 mM EGTA, which completely prevented the generation of caffeine-induced Ca2+-activated currents of mouse megakaryocytes; and no [Ca2+]i elevation was evoked by the alkalinization protocol in contrast to a significant increase in [Ca2+]i in response to caffeine when [Ca2+]i was measured with a fura 2 ratiometry. I(ALK) was strongly suppressed with tetraethylammonium (TEA), 4-aminopyridine (4-AP) and streptomycin (SM), but was completely resistant to quinidine (QND). The values of IC50 for the suppression of I(ALK) with TEA, 4-AP and SM were 5.6, 0.47 and 1.5 mM, respectively. Voltage-gated K+ currents (I(K)) of the same megakaryocyte preparation were weakly suppressed with TEA and 4-AP, while they were significantly suppressed with either SM or QND. These results suggest that mouse megakaryocytes possess K+ conductance that was activated by extracellular alkalinization and that probably differs from conventional K+ conductance in its pharmacological properties.

The properties of caffeine- and carbachol-induced intracellular Ca2+ release in mouse bladder smooth muscle cells

Freshly dissociated bladder smooth muscle cells of mice developed spontaneous, caffeine- (ICAF) and carbachol-induced (ICCh) currents under voltage-clamped conditions. Spontaneous currents, ICAF and ICCh were blocked with tetraethylammonium at 3 x 10(-4)-10(-2) M but were resistant to both charybdotoxin (10(-7)-10(-6) M) and iberiotoxin (10(-7)-10(-6) M). The reversal potential for each current indicated that K+ channels play a major role in the generation of each current. Both spontaneous currents and ICAF but not ICCh were abolished in nominally Ca2+-free and nicardipine (10(-6) M)-containing media. These results suggest that the activity of L-type voltage-sensitive Ca2+ channels is important in the generation and maintenance of spontaneous currents and ICAF but not ICCh. Ryanodine (10(-6) M) prevented spontaneous currents, ICAF and caffeine-induced [Ca2+]i elevation but not ICCh and carbachol-induced [Ca2+]i elevation, suggesting that the response of bladder smooth muscle cells to carbachol may involve a Ca2+ store distinct from that for caffeine. Pretreatment with carbachol suppressed ICAF to 22 +/- 7% (n = 7) and the caffeine-induced [Ca2+]i elevation to 25 + 3% (n = 6). Similarly, caffeine suppressed ICCh to 23 +/- 4% (n = 9) and the carbachol-induced [Ca2+]i elevation to 24 +/- 6% (n = 6).

Inhibition of sodium current by chloride channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) in guinea pig cardiac ventricular cells

The effects of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a potent anion transport blocker, on transmembrane action potentials (APs) and the sodium current (I[Na]) of guinea pig ventricular myocytes were examined by using conventional microelectrode and whole-cell patch-clamp recording techniques. In papillary muscle preparations, DIDS (> or =0.1 mM) suppressed the maximal upstroke velocity (.v[max]) of the AP without significant changes in other AP parameters. Extracellular application of DIDS on single cardiomyocytes isolated from the guinea pig ventricle markedly reduced the peak amplitude of the tetrodotoxin (TTX)-sensitive and voltage-activated sodium current. The concentration-dependent block of DIDS could be expressed by the Hill equation with a Hill coefficient of 0.97 and a dissociation constant of 0.15 mM at a holding potential of (VH) -120 mV. DIDS (0.1 mM) shifted the steady-state inactivation curve for I(Na) toward more negative potentials by 6.0 +/- 0.5 mV and the activation curve to more positive potentials by 5.0 +/- 1.0 mV, although the slope factors were unaffected. With repetitive depolarizing pulses from -120 mV, DIDS produced a use-dependent block on the I(Na). Recovery of I(Na) from inactivation was slowed (time constant = 245 ms, compared with 10 ms of control) in the presence of 0.1 mM DIDS. In the two-pulse experiments, DIDS produced two distinct phases of development of I(Na) block, the rapid phase (tau = 5 ms) caused by an open channel block, and the slower phase (tau = 382 ms) induced by an inactivated channel block. These results suggest that the Cl- transport blocker DIDS has a direct inhibitory effect on the cardiac sodium channel. DIDS-induced use dependence of I(Na) block may result from the interaction of the drug with sodium channels in both the open and inactivated channel states.

Cytoplasmic Ca2+ mobilization and Ca(2+)-dependent membrane currents in dispersed bovine ciliary muscle cells

PURPOSE

The dependence of plasmalemma Ca2+ influx and Ca2+ release from intracellular stores on Ca2+ activated K+ channels of bovine ciliary muscle (CM) cells were examined.

METHODS

The nystatin-perforated patch clamp technique for the measurement of membrane currents and a microscope based fura-2 fluorescence imaging of [Ca2+]i were applied to CM cells freshly dissociated with collagenase and identified with smooth muscle-specific alpha-isoactin.

RESULTS

At holding voltages (VH) of > -60 mV, CM cells showed spontaneous transient outward currents (STOCs) and caffeine (> 10(-4) M) induced large transient outward currents (ICAF). Both STOCs and ICAF were abolished by tetraethylammonium chloride (10(-3) M) and charybdotoxin (10(-7) M), but not by apamin (10(-6) M), suggesting that both currents are mediated by Ca(2+)-activated K+ channels similar to those with medium (MK) or large (BK-type) conductance. Both STOCs and ICAF were gradually abolished in the nominally Ca(2+)-free and Co(2+)-containing solutions but were resistant to L-type Ca2+ channel blockers, including nicardipine, verapamil and diltiazem and a N-type channel blocker, omega-contoxin. The [Ca2+]i-elevation during high K+ (100 mM)-depolarization was prevented by Ca(2+)-free and Co(2+)-containing solutions but not by nicardipine.

CONCLUSIONS

These results suggest that CM cells possess MK or BK type-like Ca(2+)-activated K+ channels and that L-type Ca2+ channels play minor roles for the maintenance of Ca(2+)-dependent responses in contrast to other types of smooth muscle cells.

Impairment of Kit-dependent development of interstitial cells alters contractile responses of murine intestinal tract

We examined developmental changes in responses of the isolated segment of the ileum of BALB/c mice treated with a monoclonal antibody (ACK2) to the receptor tyrosine kinase (Kit) for 4 days postnatally to pharmacological agents in vitro. Rhythmic contraction and relaxation of the isolated ileum started to appear on day 4 postpartum, and the sensitivity to acetylcholine (ACh) decreased gradually after birth. Treatment with ACK2 induced augmentation of contractile responses and receptor sensitivity of the longitudinal muscle of the ileum to ACh, bradykinin, and prostaglandin F2 alpha. ACh induced larger depolarization in smooth muscle cells of the ileum in the ACK2-treated mice than in the control. Circular muscle responses to these substances, as measured by changes in intraluminal pressure, were not altered by ACK2 treatment. Results suggest that interstitial cells play an important role not only in the development of the pacemaking system of the small intestine but also in the functional development of the contractile properties of the intestinal smooth muscle.

Different effects of baclofen and GTP gamma S on voltage-activated Ca2+ currents in rat hippocampal neurons in vitro

Reduction of voltage-activated Ca2+ currents by intracellular application of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) through ultraviolet (UV) photolysis of the caged compound, is followed by a re-augmentation to control levels within 10 min, independently of the divalent cation used. The Ca2+ current inhibition by the gamma-aminobutyric acid type B (GABAB) receptor agonist baclofen, which is also thought to be mediated by a GTP-binding protein (G-protein), is potentiated when GTP gamma S is uncaged during agonist superfusion. The authors suggest that GTP gamma S activates G-protein-dependent pathways that are not activated by the baclofen receptor.

Rhythmic Cl- current and physiological roles of the intestinal c-kit-positive cells

Chronic injection of an anti-c-KIT receptor tyrosine kinase monoclonal antibody (ACK2) results in the disruption of the normal motility patterns of young BALB/c mice intestine. This effect is accompanied by a drastic decrease in the number of intestinal c-kit-expressing (c-kit+) cells when studied immunohistochemically with the fluorescence-labelled antibody. In order to clarify the mechanism underlying the ACK2 action and the physiological roles of intestinal c-kit+ cells, we studied the excitability of intestinal c-kit+ cells in primary culture by use of the nystatin perforated-patch-clamp technique. Under voltage-clamp at -40 mV, the majority of c-kit+ cells tested (59/70) elicited rhythmic current waves with an amplitude and frequency of 263 +/- 24 pA and 2.30 +/- 0.25 cycles/min (mean +/- SEM), respectively. Intracellular perfusion of the c-kit+ cells with ethylenebis (okonitrilo) tetraacetate (EGTA) as well as a nominally Ca(2+)-free external solution or low holding voltage (< -60 mV) prevented the rhythmic current. The reversal potential of the rhythmic current was close to the equilibrium potential for Cl-(ECl). Moreover the rhythmic current was depressed by a Cl- channel blocker, 4-acetoamido-4-isothiocyanat-ostilbene-2,2'-disulphoni c acid (SITS). The smooth muscle cells freshly dissociated from the same intestinal specimen revealed a Ca(2+)-activated K+ current, as has been described in a variety of smooth muscle cells. Cultured smooth muscle cells from the ileum preparation lacked neither the Ca(2+)-activated K+ nor rhythmic Cl- currents. Smooth muscle cells freshly dissociated from the same ileum preparation and those in culture showed no immunoreactivity with the labelled ACK2, which was consistent with our previous in situ study. Results provided direct evidence that the intestinal c-kit+ cells, but not the smooth muscle cells, possess a rhythmic Cl- current oscillation, suggesting their participation in pacemaker activity for the peristaltic gut movement.

Effects of plant diterpenes on the neuronal GABAA receptor-operated chloride current

The effects of plant diterpenes, horminone (HMN) and taxodione (TXN), on the GABAA receptor-operated Cl-current (IGABA) were investigated in voltage-clamped and internally perfused neurones dissociated from frog dorsal root ganglia. Both diterpenes depressed IGABA in a concentration-dependent manner similar to that of picrotoxin. Concentrations required to elicit 50% depression of the IGABA were 10(-6) M for picrotoxin, 10(-5) M for HMN and 10(-4) M for TXN. Blocking and restoration kinetics of the IGABA with HMN were also similar to those of picrotoxin. Time constants for both the blockade and restoration of the IGABA with TXN were more than five times greater than those with HMN.

Bradykinin-evoked non-specific cationic current in neuroblastoma-glioma hybrid (NG108-15) cells and its down-regulation through differentiation

Effects of bradykinin (BK) on the membrane conductance and level of cytoplasmic free Ca2+ in undifferentiated and differentiated neuroblastoma-glioma hybrid (NG108-15) cells were studied using the nystatin-perforated patch-clamp technique and fura-2 fluorometry. Under voltage clamp at -20 mV, undifferentiated cells responded to BK at > 10(-9) M, producing a biphasic current composed of an apamin-sensitive Ca(2+)-activated K+ outward current and non-specific cationic inward current. Both current components corresponding to a biphasic elevation of [Ca2+]i were completely prevented by an intracellular perfusion with EGTA (1 mM) under conventional whole cell recording condition. Undifferentiated cells revealed almost no voltage sensitive Ca2+ current. In NG108-15 cells differentiated with 8-Br-cyclic AMP (1 mM) or rolipram (1 mM), an inhibitor of type IV phosphodiesterase, BK concentration required for the non-specific cationic current with amplitude of > 100 pA was much greater than that of undifferentiated cells. This suggests that the differentiated cells decreased BK-sensitivity in induction of the non-specific cationic current. The non-specific cationic channel is suggested to play roles as a source of Ca2+ entry in undifferentiated NG108-15 cells.

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)

Protein kinase A-mediated phosphorylation reduces only the fast desensitizing glycine current in acutely dissociated ventromedial hypothalamic neurons

Modulation of glycine receptor-ionophore complex by internally perfused cyclic AMP was investigated and compared to that of GABA in the acutely dissociated ventromedial hypothalamic neurons using whole-cell and outside-out patch-clamp techniques. Cyclic AMP significantly reduced both GABA- and glycine-gated chloride currents. The reduction in glycine-induced chloride current was specific in that only the fast-desensitizing one gated by high concentrations of glycine (30-100 microM) was affected. Cyclic AMP did not modulate the non-desensitizing current induced by lower concentrations (6-10 microM). Addition of N-[-2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, a protein kinase A inhibitor, did not have a significant effect on its own but prevented the attenuation of fast desensitizing glycine current induced by cyclic AMP. Both the reversal potential and inactivation kinetics of glycine current were not affected by the activation of protein kinase A, suggesting that cyclic AMP-mediated attenuation is not due to an enhancement of desensitization. In outside-out patch studies intracellular perfusion of cyclic AMP reduced the open probability of the 100 microM glycine-activated channels without affecting that of the 6 microM glycine-activated channels. In conclusion, cyclic AMP selectively modulates the channel open frequency of the glycine receptor when activated at higher concentrations through a protein kinase A-mediated phosphorylation.

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.

Penicillin-induced potentiation of glycine receptor-operated chloride current in rat ventro-medial hypothalamic neurones

1. Effects of penicillin G (PCN) on glycine (Gly)-evoked Cl- current (IGly) were investigated in acutely dissociated rat ventro-medial hypothalamic (VMH) neurones by the whole cell mode of patch clamp technique. 2. When PCN was applied simultaneously with Gly, PCN depressed IGly like a Cl- channel blocker. 3. The PCN-induced blocking action was clearly observed at a low PCN concentration (30 u), while the maximal blockade was achieved by 600 u (units per 10 ml) PCN. 4. When tested solution containing both PCN and Gly was quickly substituted with one containing Gly only, a new rebound-like transient current (I(T)) which also passed through Cl- channel, was elicited. 5. The peak amplitude of I(T) induced by PCN at concentrations higher than 100 u was greater than that induced by glycine alone. We termed this phenomenon PCN-induced potentiation of IGly. In all cells tested, PCN potentiated IGly. 6. At a lower PCN concentration below 30 u, I(T) generation was not clear in the presence of 10(-5) M gamma-aminobutyric acid. With PCN a higher concentration than 300 u, I(T) amplitude was greater than that of the original peak IGly. This was observed in 18 neurones out of 21. The maximal amplitude of the I(T) was achieved with 600 u PCN.

Augmentation of GABA-induced current in frog sensory neurons by pentobarbital

The effect of pentobarbital sodium (PB) on the gamma-aminobutyric acid (GABA)-induced macroscopic and microscopic Cl- currents (ICl and iCl, respectively) was studied in the soma membrane of isolated frog sensory neurons using a rapid concentration-jump and patch-clamp technique. The GABA-induced ICl was composed of a transient peak and a steady plateau. PB shifted the concentration-response curve of the GABA-induced peak ICl to the left without affecting the maximum value. The apparent dissociation constant (KD) decreased from 13 microM at control to 8.0, 4.8, and 2.9 microM by adding 10, 30, and 100 microM PB, respectively. PB also shifted the concentration-response curve for the plateau ICl to the left and augmented the maximum value of the plateau, indicating an increase in the available receptor-channel complex. The Hill coefficient (n = 2) in concentration-response curves of both peak and plateau responses was not changed by adding PB. Both the activation and desensitization phases of GABA-induced ICl consisted of two exponential components. PB significantly increased the time constant of slow desensitization component at all concentrations of GABA used. In the "inside-out" configuration, PB markedly increased the open probability (Po) of a GABA-gated single Cl- channel having a conductance of 14.57 +/- 2.3 pS (n = 123) without affecting the single-channel conductance. The increase of Po was due to the prolongation of mean open time (tau of the tau os) and shortening of mean closed time (tau cf and tau cs), resulting in the increase of channel-opening events.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Kinetic analysis of acetylcholine-induced current in isolated frog sympathetic ganglion cells

1. Kinetic properties of activation and inactivation phases of the ACh-gated nicotinic current were investigated in isolated frog sympathetic ganglion cells using "concentration-clamp" technique under voltage-clamp conditions. This technique combines internal perfusion with a rapid change of the external solution within a few milliseconds. 2. The dose-response curve for the peak current induced by ACh showed a sigmoidal increase, in which the apparent dissociation constant Kd and Hill coefficient were 2.6 x 10(-5) M and 2.0, respectively. 3. The current-voltage relationship of ACh-induced currents were linear at potentials more negative than the reversal potential (EACh) of -5.5 +/- 1.3 mV (mean +/- SE) but showed a slight inward rectification at positive potentials over +20 mV. Since this current could be generated predominantly by an increase of Na+ and K+ conductances, the value of EACh was close to the theoretical potential, -1.3 mV, for the total amount of both Na+ and K+ or Cs+ in the extracellular and intracellular solutions. 4. There was no difference between the dose-response curves of ACh- and nicotine-induced currents. The ACh-induced current was suppressed in a competitive manner by the nicotinic antagonists, d-tubocurarine and hexamethonium, but muscarine did not induce any response, indicating that the ACh-gated current might be mediated by the nicotinic ACh receptor-ionophore complex. 5. There was a latent period of the order of milliseconds in the ACh receptor activation phase before attaining exponential increase of activation process.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Intracellular picrotoxin blocks pentobarbital-gated Cl- conductance

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

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-.

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.

Ca2+-mediated suppression of the GABA-response through modulation of chloride channel gating in frog sensory neurones

A suppressant effect of intracellular free Ca2+ on the gamma-aminobutyric acid (GABA)-induced chloride inward current (ICI(GABA)) was studied in isolated frog sensory neurones under whole cell voltage clamp. Voltage-dependent Ca2+ influx elicited during the steady state of ICI(GABA)-induced a fast, slowly recovering current relaxation in the outward direction, the amplitude of which was dependent on total Ca2+ influx. This suppressant effect showed specificity for different divalent cations, suggesting action at a specific intracellular effector. Single channel recording revealed a Ca2+-dependent decrease in the duration of the open time of the GABA-gated Cl- channel without change in single channel conductance.

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

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

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

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

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

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

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

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

Antagonists of GABA responses, studied using internally perfused frog dorsal root ganglion neurons

Responses of frog dorsal root ganglion neurons to GABA were studied under conditions of internal perfusion. Conductances to Na, Ca and K were pharmacologically blocked, C1 concentrations were maintained equal on both sides of the membrane and a small holding potential was used. Under these conditions GABA-induced C1 currents could be studied in isolation without shifts in EC1 occurring after GABA application. GABA currents were blocked by a variety of agents. The blockade by bicuculline and Zn was competitive, while that to penicillin was competitive at low concentrations (6 x 10(-5) M) and non-competitive at high concentrations (3 x 10(-4) M). Picrotoxin was non-competitive at all concentrations studied. The time course of the GABA-induced currents was changed in the presence of antagonists, including those that were competitive. These actions appear to be due to a change in the rates of receptor desensitization rather than shifts in EC1. Pretreatment with antagonists increased the degree of inhibition only for picrotoxin as compared to simultaneous application of GABA plus antagonist. The voltage dependence of the GABA response was altered by penicillin but not by other antagonists. GABA responses on frog dorsal root ganglion cell were also depressed by a variety of other metal ions (Cd, Ni, Cu, Co, Mn) and other drugs (strychnine, curare, 4-acetamide, 4'-isothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt, 4,4'-diisothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt trihydrate, bemegride and folic acid). In this preparation bicuculline and the heavy metal ions appear to block at or very near to the agonist binding site, while penicillin probably blocks the ion channel. The non-competitive action of picrotoxin appears not to be channel blockade, but to be due to a slowly equilibrating action at a site different from either the agonist binding site or the channel.

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

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

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

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

Modulation of the gamma-aminobutyric acid-gated chloride current by intracellular calcium in frog sensory neurones

The effects of the voltage-dependent Ca2+ inward current (ICa) on gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) in isolated frog sensory neurones were examined using the "concentration clamp" technique, an approach which combines internal perfusion by a suction pipette with a rapid external solution change method. A preceding ICa suppressed the GABA-activated ICl. While the inhibition was not influenced by changes of membrane potential between -40 and -100 mV, it was reduced when the internal EGTA concentration was increased. A preceding ICa shifted the GABA dose-response curve to the right without affecting the maximum current or the Hill coefficient (n = 2). Inhibition of the GABA-activated ICl by ICa showed a recovery in a single exponential manner, the time constant of which depended upon the extent of Ca2+ influx. At a low temperature (10 degrees C), however, the recovery from Ca2+-mediated inhibition was only slight. We conclude that, as a result of the voltage-dependent Ca2+ influx, a subsequent increase of intracellular Ca2+ concentration modulates the GABA-gated ICl in the soma membrane. Possible mechanisms are discussed.