On the mechanism of action of GABA in pelvic vesical ganglia: biphasic responses evoked by two opposing actions on membrane conductance

Excitatory GABAA receptor in autonomic pelvic ganglion neurons innervating bladder

Major pelvic ganglia (MPG) are relay centers for autonomic reflexes such as micturition and penile erection. MPG innervate the urogenital system, including bladder. γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system, and may also play an important role in some peripheral autonomic ganglia, including MPG. However, the electrophysiological properties and function of GABAA receptor in MPG neurons innervating bladder remain unknown. This study examined the electrophysiological properties and functional roles of GABAA receptors in bladder-innervating neurons identified by retrograde Dil tracing. Neurons innervating bladder showed previously established parasympathetic properties, including small membrane capacitance, lack of T-type Ca(2+) channel expression, and tyrosine-hydroxylase immunoreactivity. GABAA receptors were functionally expressed in bladder innervating neurons, but GABAC receptors were not. GABA elicited strong depolarization followed by increase of intracellular Ca(2+) in neurons innervating bladder, supporting the hypothesis GABA may play an important role in bladder function. These results provide useful information about the autonomic function of bladder in physiological and pathological conditions.

Interleukin-1beta causes a biphasic response in neurons of rat major pelvic ganglia

The effect of interleukin-1beta (IL-1beta) on peripheral autonomic neurons was examined with intracellular microelectrodes, in vitro. Recombinant human IL-1beta (6-300 pM) produced a depolarization, associated with decrease in input resistance, followed by a hyperpolarization, associated with increase in input resistance, in neurons of rat major pelvic ganglia (MPG). IL-1beta 163-171 (10-100 pM), the active domain of human IL-1beta, also produced a biphasic response. The IL-1beta-induced responses reversed polarity at the equilibrium potential for Cl-. The IL-1beta-induced responses were blocked by picrotoxin (100 microM) but not by bicuculline (20 microM). Imidazole-4-acetic acid (14AA, 100 microM), a GABA(C) receptor antagonist, reduced the IL-1beta-induced responses. The results suggest that the IL-1beta-induced biphasic response is mediated through GABA(C) receptors in rat MPG neurons.

Role of GABAA and GABAC receptors in the biphasic GABA responses in neurons of the rat major pelvic ganglia

The role of gamma-aminobutyric acid-A (GABAA) and GABAC receptors in the GABA-induced biphasic response in neurons of the rat major pelvic ganglia (MPG) were examined in vitro. Application of GABA (100 microM) to MPG neurons produced a biphasic response, an initial depolarization (GABAd) followed by a hyperpolarization (GABAh). The input resistance of the MPG neurons was decreased during the GABAd, whereas it was increased during the GABAh. The GABAd could be further separated into the early component (early GABAd) with a duration of 27 +/- 5 s (mean +/- SE; n = 11) and the late component (late GABAd) with a duration of 109 +/- 11 s (n = 11). The duration of the GABAh was 516 +/- 64 s (n = 11). The effects of GABA (5-500 microM) in producing the depolarization and the hyperpolarization were concentration-dependent. GABA (5-30 microM) induced only late depolarizations. The early component of the depolarization appeared when the concentration of GABA was >50 microM. Muscimol produced only early depolarizing responses. Baclofen (100 microM) had no effect on the membrane potential and input resistance of MPG neurons. Bicuculline (60 microM) blocked the early GABAd but not the late GABAd and the GABAh. Application of picrotoxin (100 microM) with bicuculline (60 microM) blocked both the late GABAd and the GABAh. CGP55845A (3 microM), a selective GABAB receptor antagonist, did not affect the GABA-induced responses. cis-4-Aminocrotonic acid (CACA, 1 mM) and trans-4-aminocrotonic acid (TACA, 1 mM), selective GABAC receptor agonists, produced late biphasic responses in the MPG neurons. The duration of the CACA responses was almost the same as those of the late GABAd and GABAh obtained in the presence of bicuculline. Imidazole-4-acetic acid (I4AA, 100 microM), a GABAC receptor antagonist, depressed the late GABAd and the GABAh but not the early GABAd. I4AA (100 microM) and picrotoxin (100 microM) also suppressed the biphasic response to CACA. The early GABAd and the late GABAd were reversed in polarity at -32 +/- 3 mV (n = 7) and -38 +/- 2 mV (n = 4), respectively, in the Krebs solution. The reversal potential of the GABAh was -34 +/- 2 mV (n = 4) in the Krebs solution. The reversal potentials of the late GABAd and the GABAh shifted to -20 +/- 3 mV (n = 5) and -22 +/- 3 mV (n = 5), respectively, in 85 mM Cl- solution. These results indicate that the late GABA(d) and the GABAh are mediated predominantly by bicuculline-insensitive, picrotoxin-sensitive GABA receptors, GABAC (or GABAAOr) receptors, in neurons of the rat MPG.

Decrease in synapsin I staining in the hypogastric ganglion of aged rats

The rat hypogastric ganglion (HG) contains populations of both sympathetic and parasympathetic postganglionic neurons which supply the lower pelvic viscera. These neuron populations can be identified by tyrosine hydroxylase (TH) and NADPH-diaphorase (NADPH-d) staining, respectively. The effects of age on the distribution of synapsin I, a nerve terminal marker, in relation to these neuron populations has been investigated in young adult and aged rats. Most synapsin staining was axosomatic and was markedly reduced in the aged animals particularly in relation to sympathetic (NADPH-d-negative/TH-positive) neurons. Image analysis of synapsin I staining in relation to individual sympathetic neurons confirmed that there was a reduction with age of about 50% but no change in synapsin I staining in relation to parasympathetic neurons. These results suggest that synaptic transmission and peripheral integration may be affected in old age and that the autonomic control of the pelvic viscera may be compromised as a result, particularly with regard to the sympathetic innervation. Other autonomic ganglia were also studied for comparison but no such age-related differences were observed.

Activation of calcium-dependent chloride channels causes post-tetanic depolarization in rabbit parasympathetic neurons

Intracellular recordings were made from neurons in rabbit and feline vesical parasympathetic ganglia in vitro. In response to cathodal current injection (0.1-1 nA for 2-20 ms) the majority of rabbit neurons (229 out of 250) exhibited a single action potential that was followed by a fast and slow after-hyperpolarization (sAHP neuron). The remainder of the cells exhibited an action potential followed by only a fast after-hyperpolarization (fAHP neuron). fAHP neurons did not exhibit anomalous rectification and a spontaneous rhythmic hyperpolarization, which were common membrane properties in sAHP neurons. In response to a train of cathodal current pulses (5-20 Hz for 0.1-10 s), fAHP neurons exhibited action potentials followed by a post-tetanic depolarization (PTD). The PTD was associated with a decrease in membrane input resistance. The amplitude and duration of the PTD were a function of the number of action potentials in the train. The amplitude of the PTD was increased by membrane hyperpolarization and its estimated reversal potential was approximately -30 mV. Low-chloride solution and intracellular injection of chloride ions augmented the amplitude and duration of the PTD, whereas low-sodium, high-potassium and low-potassium solutions did not affect them. Tetraethylammonium (5-10 mM) and barium (0.5-1 mM) increased the amplitude and duration of the PTD. Nominal calcium-free solutions and omega-conotoxin (500 nM) abolished the PTD. The data suggest that activation of chloride channels by calcium influx through omega-conotoxin-sensitive calcium channels mediates the PTD. Repetitive stimulation of the pelvic nerve evoked a train of orthodromic action potentials followed by the PTD of fAHP neurons. (+)-Tubocurarine (10 microM) and hexamethonium (200 microM), but not atropine (1 microM), abolished orthodromic action potentials and the PTD, whereas these cholinergic antagonists did not depress the PTD evoked by direct action potentials. In summary, the data suggest that the PTD may function as a slow synaptic potential in fAHP neurons. This appears likely because neither slow excitatory nor inhibitory postsynaptic potentials are present in neurons of rabbit vesical parasympathetic ganglia. In contrast, slow inhibitory and excitatory postsynaptic potentials were recorded from neurons in feline vesical parasympathetic ganglia.

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.

GABAA receptor-mediated increase in membrane chloride conductance in rat paratracheal neurones

1. The actions of gamma-aminobutyric acid (GABA) on the intramural neurones of 14-18 day old rats were studied in situ by use of intracellular current- and voltage-clamp techniques. The ionic conductance changes and the effects of various GABA-receptor agonists and antagonists on these neurones were also investigated. 2. Prolonged application of GABA either by ionophoresis (10 pC-10 nC) or superfusion (10-100 microM), evoked a biphasic membrane depolarization in over 90% of all paratracheal neurones studied. Typically, the response consisted of an initial rapid depolarization (18-45 ms) that subsequently faded over a period of 15-25 s to reveal a second smaller depolarization which was maintained for the duration of GABA application. Both components of the evoked response resulted in an increase in membrane conductance and an inward flow of current. 3. The amplitude of the transient inward current, recorded during the initial phase of the response, was linearly related to the membrane potential at which it was elicited and reversed symmetrically at a membrane potential of -32.7 mV. The underlying increase in conductance was largely independent of membrane potential. The equilibrium potential for the sustained inward current was -38.7 mV. Replacement of extracellular chloride with gluconate ions initially enhanced the GABA-evoked inward current. With successive applications of GABA in low chloride, the evoked current and conductance changes declined markedly. 4. Muscimol superfusion (1-10 microM) or ionophoresis (10 pC-10 nC) mimicked both the initial and late phases of the GABA-induced conductance change and inward current. Baclofen (1-100 microM) had no effect upon either resting membrane potential or conductance in any of the cells tested. 5. The large transient initial phase of the GABA-evoked inward current and depolarization were potently inhibited by picrotoxin (1-50 microM), whereas the smaller sustained inward current was largely resistant to picrotoxin. 6. All of the observed actions of GABA and muscimol were antagonized by bicuculline (0.1-10 microM) in an apparently competitive manner. 7. It is concluded that GABA acts via GABAA receptors present on the soma of paratracheal neurones to produce an increase in membrane chloride conductance. Prolonged application of GABA results in a decline in the observed current due to a combination of two processes: receptor desensitization and shifts in the chloride equilibrium potential. The possible roles for GABA in neural regulation of airway excitability are discussed.

Slow inward and late slow outward currents induced by hyperpolarizing pre-pulses in cat bladder parasympathetic neurones

Single-electrode voltage-clamp techniques were used to examine membrane currents recorded as a result of hyperpolarizing pre-pulses in cat bladder parasympathetic neurones. In 84 ganglion cells examined, two types of current were observed in response to hyperpolarizing pre-pulses of 10 ms to 1 s duration from holding potentials of -30 to -60 mV to test potentials of -90 to -130 mV. In 46 cells, a short-duration pulse induced a slow inward current (SIC); with longer pulse durations, an outward current was superimposed on the SIC, resulting in a late slow outward current (LSOC). In the remaining cells, either a SIC (n = 12) or an LSOC (n = 26) was recorded over a range of hyperpolarizing pre-pulse durations. The more depolarized the holding potential, the more hyperpolarized the test potential and the longer the pulse duration, the larger the amplitude of the SIC and LSOC. The SIC and LSOC were associated with an increase in input conductance. The extrapolated reversal potential (Vrev) for the LSOC obtained at a holding potential of -60 mV (where the LSOC seemed to be less contaminated with the SIC) was -89 +/- 4 mV (mean +/- standard error of the mean; n = 5), which is close to the equilibrium potential for the K ion. The LSOC was depressed by a high-K (10-20 mM) solution and potentiated by a low-K (0.47 mM) solution. The SIC was depressed by a low-Na (26.2 mM) solution, but was not affected significantly by a low-Cl (12.2 mM) solution. A low-Ca (0.1 mM)/high-Mg (5 mM) solution depressed the LSOC, while a high-Ca (5 mM) solution potentiated it. Cd (0.5 mM) blocked the SIC almost completely, and suppressed the LSOC. The LSOC but not the SIC was suppressed by tetraethylammonium chloride (10 mM). Superfusing Cs (3 mM) did not affect either the LSOC or the SIC. 4-Aminopyridine (1 mM) and muscarine (10 microM) depressed or replaced the SIC with an outward current, while potentiating the LSOC. These results suggest that a hyperpolarizing pre-pulse induces slow inward Na- and late slow outward Ca-dependent K currents, which are inactivated at depolarized potentials and are de-inactivated by hyperpolarizing pulses in a time-dependent manner.

Fast hyperpolarization following an excitatory postsynaptic potential in cat bladder parasympathetic neurons

Intracellular recording techniques were used to study a fast hyperpolarizing potential following the fast excitatory postsynaptic potential evoked by an orthodromic nerve stimulation in cat bladder parasympathetic ganglion cells. In the 61 ganglion cells examined, two types of responses were recorded on stimulating the preganglionic nerve; one had only a fast excitatory postsynaptic potential (type SI, n = 20) and the other had a fast excitatory postsynaptic potential followed by a fast hyperpolarizing potential (type SII, n = 41). In type SII neurons, the half-maximum duration of the afterhyperpolarizing potential following an orthodromic spike was longer than that of a direct spike produced by injecting a depolarizing current pulse through the recording electrode; the half-maximum durations for afterhyperpolarizing potentials following orthodromic and direct action potentials were comparable in type SI cells. Blocking the initiation of an orthodromic spike by hyperpolarizing the membrane in type SII cells revealed a fast excitatory postsynaptic potential followed by a fast hyperpolarizing potential which was similar to that observed at the resting potential. The fast hyperpolarizing potential had a duration comparable to that of an afterhyperpolarizing potential following an orthodromic action potential. The fast excitatory postsynaptic potential-fast hyperpolarizing potential sequence was blocked completely and reversibly by nicotinic receptor antagonists (hexamethonium and D-tubocurarine). Atropine, alpha-2 noradrenergic (yohimbine and phentolamine), and purinergic (caffeine) antagonists had no effect on the fast hyperpolarizing potential. In cells which show type SII responses, spontaneous excitatory postsynaptic potentials were not followed by a hyperpolarization. Depolarizing the membrane (by passing a cathodal current through the recording electrode) to an amplitude comparable to that of a fast excitatory postsynaptic potential also did not elicit a membrane hyperpolarization in type SII cells. In some cells, stimulating one preganglionic nerve trunk elicited a fast hyperpolarizing potential, but activating another nerve trunk innervating the same ganglion cell did not. There was no correlation between the variations in the amplitudes of the fast excitatory postsynaptic potential and the fast hyperpolarizing potential in type SII cells, but increasing the stimulus intensity applied to the presynaptic nerve fiber potentiated the amplitude of the fast excitatory postsynaptic potential and the fast hyperpolarizing potential. The fast hyperpolarizing potential was not associated with appreciable changes in input resistance.(ABSTRACT TRUNCATED AT 400 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.

Calcium-activated chloride conductance in parasympathetic neurons of the rabbit urinary bladder

Intracellular recordings were made from vesical pelvic ganglion cells of the rabbit in a Krebs solution containing tetrodotoxin (1 microM). Experiments were carried out during complete suppression of the calcium-dependent potassium conductance by tetraethylammonium (greater than or equal to 20 mM) and/or intracellular injection of cesium ions. The action potential was followed by a depolarizing afterpotential which lasted for 0.3-10 s and had a peak amplitude of 5-20 mV at about -50 mV. The afterdepolarization (ADP) could not be observed when the preceding calcium-dependent action potential was blocked in a nominally calcium-free solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA) or total substitution of extracellular calcium ions with barium ions selectively blocked the ADP. The ADP, associated with an increased membrane conductance, reversed its polarity at -17 mV, when ganglion cells were impaled with microelectrodes filled with potassium chloride or cesium chloride. This reversal level was similar to that of the depolarization induced by gamma-aminobutyric acid. The reversal potential shifted to about -50 mV when acetate or sulphate were injected as counter anions. The peak amplitude and the total duration of the ADP was increased by substitution of external sodium chloride with sucrose or sodium isethionate. These results suggest that the ADP results from calcium entry during the spike and subsequent opening of chloride channels in parasympathetic neurons of the rabbit.

Effect of lithium on veratridine-induced quantal and non-quantal release from inhibitory nerve terminals in crayfish muscle

Muscle fibres of small crayfish were voltage clamped and superfused for about 10 min with Li+ saline (Na+ replaced by Li+) which contained 5 mmol/l glutamate to desensitize excitatory postsynaptic receptors. Then 100 mumol/l veratridine were added to the superfusate which caused strong asynchronous quantal release of inhibitory transmitter. However, in the presence of Li+ strong inhibitory quantal release was only transient. It could be activated a second time by removal of Li+ and readministration of Na+. From the total of 0.7 to 1.1 million quanta released by veratridine only about 30-35% could be released in Li+ saline. The voltage clamp DC-currents recorded during veratridine-induced quantal release suggested that a non-quantal release component is additionally involved. This non-quantal release component was most prominent during the period of quantal release in Li+ superfusate while it was less obvious during the second enhancement of quantal release in normal saline. Together with previous results (Martin and Finger 1988) it may be concluded that quantal release, but not non-quantal release, is decreased by Li+ in the nerve terminals.

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.

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.

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

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

Characterization of gamma-aminobutyric acid responses with sulfate loading in cat bladder neurons

In some cells in cat bladder ganglia gamma-aminobutyric acid (GABA) applied iontophoretically produced a hyperpolarizing response accompanied by an increase in input conductance when recorded with potassium sulfate-filled microelectrodes. This GABA-induced hyperpolarization was blocked by bicuculline and was converted to a depolarizing GABA response when extracellular chloride concentration was low suggesting that the hyperpolarizing GABA response was mediated by the opening of chloride channels. In other cells, continuous passage of a small negative current converted a depolarizing GABA response to a hyperpolarizing response with time. This effect was accompanied by a negative shift of the reversal potential. These data indicated that injection of impermeable sulfate ions decreased the intracellular chloride concentration.

Vasoactive intestinal polypeptide depolarizations in cat bladder parasympathetic ganglia

The effect of vasoactive intestinal polypeptide (VIP) on the neuronal membranes of isolated cat vesical pelvic ganglia and its underlying ionic mechanism were examined by means of intracellular recording and voltage-clamp techniques. Application of VIP (0.05-50 microM) to the neurones by pressure 'puff' ejection through a micropipette placed close to the neurones produced a depolarizing response (2-15 mV) in 83% of neurones tested; this effect was concentration dependent. The VIP-induced depolarization frequently evoked spontaneous action potentials in quiescent neurones and increased the frequency of action potentials in spontaneously firing neurones. The VIP depolarization was not blocked in a Ca2+-free, high-Mg2+ solution or in a solution containing hexamethonium (1 mM) and atropine (1 microM). Tetrodotoxin (TTX; 1 microM) also did not affect the VIP depolarization. The VIP depolarization was associated with an increase in membrane resistance and the slope of a current-voltage relation (I-V curve) was increased by VIP. Conditioning hyperpolarization and depolarization of the membrane increased and decreased the amplitude of the VIP depolarization, respectively. The VIP depolarization reversed polarity around--100 mV. The reversal potential shifted about 20 mV to a more positive level in a high-K+ (10 mM) solution in accord with the Nernst equation. Substituting Cl- with isethionate in the superfusate did not affect the reversal potential of the VIP depolarization. Closure of M-channels does not underlie VIP action since the VIP depolarization was enhanced by muscarine (10 microM) and unchanged in the presence of Ba (5 mM), or intracellular or extracellular Cs+, conditions known to block the M-channels (Adams, Brown & Constanti, 1982a, b). Tetraethylammonium (TEA; 20 mM) also did not affect the VIP depolarization. Voltage-clamp analyses showed that VIP applied by pressure ejection produced an inward current of 80-110 pA associated with a decrease in membrane conductance (from 2.8 to 3.5 nS) at a holding potential of--60 mV. VIP inward current was diminished by either repetitive or continuous application of VIP (5 microM) suggesting desensitization of the VIP receptor. It is concluded that VIP produces a depolarization in neurones of bladder parasympathetic ganglia by decreasing a K+ conductance, the pharmacological characteristics of which are unlike previously described K+ conductance mechanisms.

Noradrenaline hyperpolarization and depolarization in cat vesical parasympathetic neurones

Responses to noradrenaline (NA) applied by superfusion, ionophoresis or pressure pulse were analysed using conventional intracellular recording and voltage-clamp methods in cat vesical parasympathetic ganglia. NA (1 microM) hyperpolarized 60% of the neurones, depolarized 25%, and produced a biphasic potential, which comprised a membrane hyperpolarization followed by a membrane depolarization, in 10%. About 5% of the neurones did not respond to NA. The NA hyperpolarization was blocked by yohimbine (1 microM), an alpha 2-adrenoceptor antagonist, whereas the NA depolarization was blocked by prazosin (0.1-1 microM), an alpha 1-adrenoceptor antagonist. These data indicated that the NA hyperpolarization was mediated through alpha 2-adrenoceptors and the NA depolarization through alpha 1-adrenoceptors. The NA hyperpolarization was accompanied by an increase in conductance, while the NA depolarization was associated with a decrease in conductance measured under manual-clamp conditions. Similar conductance changes were observed under voltage clamp. NA hyperpolarizations became smaller as the membrane was hyperpolarized and reversed polarity beyond -100 mV. NA depolarizations also became smaller at hyperpolarized membrane potentials and reversed polarity around -90 mV. The NA responses were enhanced in low-K media and depressed in high-K Krebs solution. The NA hyperpolarization was blocked by the Ca antagonists, Cd, Mn and Co. Intracellular injection of EGTA caused a slowly developing, progressive block of the NA hyperpolarization. The NA depolarization was not affected by low Ca concentrations, Ca antagonists or intracellular injection of EGTA. In some neurones the NA depolarization was unmasked in solutions containing Ca antagonists and after intracellular EGTA injection. The NA hyperpolarization was depressed by intracellular injection and extracellular superfusion of Cs but not by TEA. Ba (10-100 microM) depressed the NA hyperpolarization by 30%. The NA depolarization persisted in the presence of muscarine (10 microM) and was not blocked by Cs or TEA but was depressed 70% by Ba (10 microM). These data are consistent with the hypotheses that alpha 2-adrenoceptor activation produces a membrane hyperpolarization that is mediated through a Ca-dependent K conductance, and that alpha 1-adrenoceptor activation produces a membrane depolarization through closure of a voltage-insensitive K channel.

Acetylcholine responses of identified neurons in Helix pomatia--I. Interactions between acetylcholine-induced and potential-dependent membrane conductances

The influence of potential-dependent membrane conductances on amplitude and time course of acetylcholine (ACh) responses was studied. The investigations were performed on the identified neurons B1 and B3 of the buccal ganglion of Helix pomatia. The neurons B1 and B3 were depolarized by ACh. The depolarization was accompanied by a decrease of membrane resistance. An inward rectification occurring negative to the resting membrane potential (RMP) reduced the amplitude of the ACh depolarizations. An outward rectification occurring positive to the RMP consisted of two parts and ceiled the ACh responses. The early outward current reduced the amplitude and modified the time course of ACh responses. Local responses or axonal action potentials increased the amplitude of the ACh depolarizations.

A voltage-clamp analysis of inward (anomalous) rectification in mouse spinal sensory ganglion neurones

Mouse embryo dorsal root ganglion neurones were grown in tissue culture and voltage-clamped with two micro-electrodes. Hyperpolarizing voltage commands from holding potentials of -50 to -60 mV evoked slow inward current relaxations which were followed by inward tail currents on repolarization to the holding potential. These relaxations are due to the presence of a time- and voltage-dependent conductance provisionally termed Gh. Gh activates over the membrane potential range -60 to -120 mV. The presence of Gh causes time-dependent rectification in the current-voltage relationship measured between -60 and -120 mV. Gh does not inactivate within this range and thus generates a steady inward current at hyperpolarized membrane potentials. The current carried by Gh increases when the extracellular K+ concentration is raised, and is greatly reduced in Na+-free solutions. Current-voltage plots show considerably less inward rectification in Na+-free solution; conversely inward rectification is markedly enhanced when the extracellular K+ concentration is raised. The reversal potential of Ih is close to -30 mV in media of physiological composition. Tail-current measurement suggests that Ih is a mixed Na+-K+ current. Low concentrations of Cs+ reversibly block Ih and produce outward rectification in the steady-state current-voltage relationship recorded between membrane potentials of -60 and -120 mV. Cs+ also reversibly abolishes the sag and depolarizing overshoot that distort hyperpolarizing electrotonic potentials recorded in current-clamp experiments. Impermeant anion substitutes reversibly block Ih; this block is different from that produced by Cs+ or Na+-free solutions: Cs+ produces outward rectification in the steady-state current-voltage relationship recorded over the Ih activation range; in Na+-free solutions inward rectification, of reduced amplitude, can still be recorded since Ih is a mixed Na+-K+ current; in anion-substituted solutions the current-voltage relationship becomes approximately linear. It is concluded that in dorsal root ganglion neurones anomalous rectification is generated by the time-and voltage-dependent current Ih. The possible function of Ih in sensory neurones is discussed.

The effects of temperature, pH and Cl-pump inhibitors on GABA responses recorded from cat dorsal root ganglia

GABA applied by iontophoresis produced GABA-induced currents (GCs) and GABA-induced depolarizations (GDs) which were recorded intracellularly from cat dorsal root ganglia (DRG). Lowering the temperature (37 to 27 degrees C) of the preparation depressed the amplitude of GCs while prolonging their rise-time and decay time. This depressant action was mainly due to a hyperpolarizing shift in the GABA equilibrium potential (EGABA). GABA responses could also be depressed by alkalinization of the superfusion solution or addition of putative chloride pump inhibitors, e.g. SITS, furosemide or bumetanide. However, the mechanism by which these latter procedures depressed GABA responses was not due to a shift in EGABA as occurred with lowered temperature. Instead we suggest that alkalinization or the putative chloride pump inhibitors affect the chloride channel or some other site associated with the GABA receptor complex and cause the depression we observed. GABA responses could be facilitated by lowering the pH of the superfusion solution or by injecting ammonium ion into a DRG. These results suggest that a temperature-sensitive, inwardly directed chloride pump that is resistant to SITS, furosemide or bumetanide, operates in cat DRG.