The action of ammonium on postsynaptic inhibition of cat spinal motoneurons

Chloride transporters controlling neuronal excitability

Synaptic inhibition plays a crucial role in regulating neuronal excitability, which is the foundation of nervous system function. This inhibition is largely mediated by the neurotransmitters GABA and glycine that activate Cl^--permeable ion channels, which means that the strength of inhibition depends on the Cl^- gradient across the membrane. In neurons, the Cl^- gradient is primarily mediated by two secondarily active cation-chloride cotransporters (CCCs), NKCC1 and KCC2. CCC-mediated regulation of the neuronal Cl^- gradient is critical for healthy brain function, as dysregulation of CCCs has emerged as a key mechanism underlying neurological disorders including epilepsy, neuropathic pain, and autism spectrum disorder. This review begins with an overview of neuronal chloride transporters before explaining the dependent relationship between these CCCs, Cl^- regulation, and inhibitory synaptic transmission. We then discuss the evidence for how CCCs can be regulated, including by activity and their protein interactions, which underlie inhibitory synaptic plasticity. For readers who may be interested in conducting experiments on CCCs and neuronal excitability, we have included a section on techniques for estimating and recording intracellular Cl^-, including their advantages and limitations. Although the focus of this review is on neurons, we also examine how Cl^- is regulated in glial cells, which in turn regulate neuronal excitability through the tight relationship between this nonneuronal cell type and synapses. Finally, we discuss the relatively extensive and growing literature on how CCC-mediated neuronal excitability contributes to neurological disorders.

Chloride distribution in Aplysia neurones

1. The intracellular Cl(-) concentration (Cl(i)) and the membrane potential (E(m)) were measured in the medial pleural neurones of Aplysia under various experimental conditions designed to determine the Cl(-) conductance of the neurones and investigate the possibility of an active Cl(-) transport.2. The magnitude of the Cl(-) conductance of the cell depends on the experimental conditions.3. In normal sea water, large changes of E(m) produced by passing current across the cell membrane caused no change of Cl(i), suggesting that the Cl(-) conductance was low. Similarly, moderate changes of E(Cl) produced by decreasing Cl(o) or increasing Cl(i) had little or no effect on E(m).4. A high Cl(-) conductance was observed in high K(o) or very low Cl(o). It was greatly reduced if the external Ca(2+) was replaced by Co(2+), or in the presence of tubocurarine, or if the experiment was performed on an isolated cell soma. The high Cl(-) conductance is therefore attributed to the release of ACh and perhaps other transmitters from synaptic terminals.5. High concentrations of tetraethylammonium ions or procaine induced a depolarization of the cell, but a decrease of Cl(i). The rate of fall of Cl(i) was increased by lowering external K(+) or raising external Ca(2+), and was decreased by replacing external Ca(2+) by Co(2+).6. NH(4) (+) ions applied externally had effects similar to those of K(+) ions. In situations in which intracellular NH(4) (+) might be increased a fall in Cl(i) was observed.7. The changes of Cl(i) caused by TEA, procaine, or internal NH(4) (+) occur against the driving force for passive Cl(-) movements. They are still observed in isolated cell bodies, and cannot be attributed to the activation of synaptic channels.8. Some interpretations of these anomalous Cl(-) movements are discussed which could also account for the difference between E(Cl) and E(m) observed under normal conditions.

Cation transport by the neuronal K(+)-Cl(-) cotransporter KCC2: thermodynamics and kinetics of alternate transport modes

Both Cs(+) and NH(4)(+) alter neuronal Cl(-) homeostasis, yet the mechanisms have not been clearly elucidated. We hypothesized that these two cations altered the operation of the neuronal K(+)-Cl(-) cotransporter (KCC2). Using exogenously expressed KCC2 protein, we first examined the interaction of cations at the transport site of KCC2 by monitoring furosemide-sensitive (86)Rb(+) influx as a function of external Rb(+) concentration at different fixed external cation concentrations (Na(+), Li(+), K(+), Cs(+), and NH(4)(+)). Neither Na(+) nor Li(+) affected furosemide-sensitive (86)Rb(+) influx, indicating their inability to interact at the cation translocation site of KCC2. As expected for an enzyme that accepts Rb(+) and K(+) as alternate substrates, K(+) was a competitive inhibitor of Rb(+) transport by KCC2. Like K(+), both Cs(+) and NH(4)(+) behaved as competitive inhibitors of Rb(+) transport by KCC2, indicating their potential as transport substrates. Using ion chromatography to measure unidirectional Rb(+) and Cs(+) influxes, we determined that although KCC2 was capable of transporting Cs(+), it did so with a lower apparent affinity and maximal velocity compared with Rb(+). To assess NH(4)(+) transport by KCC2, we monitored intracellular pH (pH(i)) with a pH-sensitive fluorescent dye after an NH(4)(+)-induced alkaline load. Cells expressing KCC2 protein recovered pH(i) much more rapidly than untransfected cells, indicating that KCC2 can mediate net NH(4)(+) uptake. Consistent with KCC2-mediated NH(4)(+) transport, pH(i) recovery in KCC2-expressing cells could be inhibited by furosemide (200 microM) or removal of external [Cl(-)]. Thermodynamic and kinetic considerations of KCC2 operating in alternate transport modes can explain altered neuronal Cl(-) homeostasis in the presence of Cs(+) and NH(4)(+).

How does a little acronym become a big transmitter?

After an overview of the early, chequered history of the discovery of GABA and its gradual acceptance as inhibitory synaptic transmitter in the brain, the article lists and discusses some of the more unexpected later developments in studies of GABA, especially its role as excitatory transmitter in the immature brain.

Methylamine, but not ammonia, is hypophagic in mouse by interaction with brain Kv1.6 channel subtype

Ammonia and methylamine (MET) are endogenous compounds increased during liver and renal failure, Alzheimer's disease, vascular dementia and diabetes, where they alter some neurobehavioural functions probably acting as potassium channel blockers. We have already described that potassium channel blockers including tetraethylammonium (TEA), ammonia and MET are hypophagic in mice. Antisense oligonucleotides (aODNs) against Shaker-like Kv1.1 gene abolished the effect of TEA but not of ammonia and MET. The central effects elicited in fasted mice by ammonia and MET were further studied. For MET, an ED(50) value 71.4+/-1.8 nmol mouse(-1) was calculated. The slope of the dose-response curves for these two compounds and the partial hypophagic effect elicited by ammonia indicated a different action mechanism for these amines. The aODNs pretreatments capable of temporarily reducing the expression of all seven known subtypes of Shaker-like gene or to inactivate specifically the Kv1.6 subtype abolished the hypophagic effect of MET but not that of ammonia. Reverse transcription-polymerase chain reaction, Western blot and immunohistochemical results indicate that a full expression in the brain of Kv1.6 is required only for the activity of MET, and confirms the different action mechanism of ammonia and MET.

Direct and indirect enhancement of GABAergic neurotransmission by ammonia: implications for the pathogenesis of hyperammonemic syndromes

Experimental evidence indicates that ammonia causes neuroexcitation and seizures. This contrasts with the lethargy, confusion and other manifestations of global CNS depression commonly considered to be major components of hyperammonemic encephalopathies. Substantial data now indicates that ammonia can modulate GABAergic neurotransmission through direct and indirect mechanisms. This modulation consists of an enhancement of GABAergic neurotransmission at concentrations commonly encountered in hyperammonemic states and precedes the suppression of inhibitory neuronal function observed at higher (>1mM) ammonia concentrations. Not only is this increase in GABAergic neurotransmission consistent with the clinical picture of lethargy, ataxia and cognitive deficits associated with liver failure and congenital hyperammonemia, but it also provides a mechanism for testing new therapeutic modalities for the treatment of hyperammonemic encephalopathy.

Chloride concentration in cultured hippocampal neurons increases during long-term exposure to ammonia through enhanced expression of an anion exchanger

The effects of long-term exposure to ammonia on [Cl-]i in cultured hippocampal neurons were examined. Ammonia increased the [Cl-]i time- (>/=24 h) and concentration- (>/=2 mM) dependently, resulting in a depolarizing shift of the equilibrium potential of the GABAA receptor-Cl- channel opening (EGABA). Such an effect of ammonia was diminished by the inhibitors of Cl-/HCO3- exchangers, 0.1 mM 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and a carbonic anhydrase inhibitor, 2 mM acetazolamide, but not by a Na+/K+/2Cl-cotransport inhibitor, 50 microM bumetanide, suggesting an enhanced Cl-/HCO3- exchange activity by ammonia. The ammonia-induced increase in [Cl-]i was also abolished by the inhibitors of protein kinase C (PKC), 0.1 microM calphostin C and 10 microM 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine dihydrochloride (H-7), and of transcription and de novo protein synthesis, 1 microM actinomycin D and 0.5 microg/ml cycloheximide, while a PKC activator, 0.1 h microM phorbor 12-myristate 13-acetate (PMA), increased the [Cl-]i. The mRNA level of the AE3 Cl-/HCO3- exchanger was increased by ammonia in a calphostin C- and H-7-sensitive manner. The AE3-like immunoreactivity was also increased by ammonia. These findings suggest that long-term exposure to ammonia increases the expression of AE3 through the activation of PKC, resulting in an increase in [Cl-]i in neurons and a reduction of inhibitory postsynaptic potentials.

Effect of ammonia on motor function in adult rats

Changes in motor function were assessed in male rats after injecting graded doses (100, 200, 400, and 800 mg/kg, IP) of ammonium chloride and ammonium acetate. The effects were correlated with the concentrations of ammonia and glucose in the brain and blood. Spontaneous motor activity and motor coordination were inhibited after injecting 100 and 200 mg/kg, whereas with 400 and 800 mg/kg the animals exhibited convulsive movements. A dose-dependent increase was found in the concentrations of ammonia and glucose in both blood and brain. These were restored, 25 min after treatment, to control levels in the blood and not in the brain. A correlation was found between the time courses of inhibitory motor events and a rise in brain ammonia levels. Convulsant action of ammonium salts was accompanied by a marked elevation of ammonia and glucose concentration in the brain. The findings suggest that detoxication of diffused ammonia is a rate-limiting process in the brain and that ammonia, at toxic concentrations, decreases glucose utilization in the brain, resulting in an inhibition of motor function. A very high concentration of ammonia in the brain, although inhibiting glucose utilization, produces clonic convulsions probably by activating directly the motor neurons.

Modulation of ligand binding to components of the GABAA receptor complex by ammonia: implications for the pathogenesis of hyperammonemic syndromes

The effects of 5-2500 microM concentrations of neutral ammonium salts on the binding of ligands to components of the GABAA receptor complex were investigated. [3H]Flunitrazepam binding to the benzodiazepine receptor was enhanced by ammonium (10-500 microM), but not sodium tartrate with EC50 = 98 microM and Emax = 31%. Further increasing ammonium tartrate concentrations (500-2500 microM) decreased [3H]flunitrazepam binding to control levels. The ammonium tartrate-induced increase in [3H]flunitrazepam binding was manifested as a 50% decrease in Kd. Furthermore, GABA increased the potency of ammonium tartrate in enhancing [3H]flunitrazepam binding by 63%. [3H]Ro 15-1788 and [3H]Ro 15-4513 binding to the benzodiazepine receptor was not significantly enhanced by ammonium tartrate (Emax approximately 13%). Ammonium tartrate also increased, then decreased the binding of 500 nM [3H]muscimol to the GABAA receptor (EC50 = 52 microM, Emax = 30%) in a concentration-dependent manner, but had no effect on [3H]SR 95-531 binding (Emax < 16%). The ammonium tartrate-induced alterations in [3H]muscimol binding were demonstrated in saturation assays as the loss of the high affinity binding site and a 27% increase in the Bmax of the low affinity binding site. These results indicate that ammonia biphasically enhances, then returns ligand binding to both the GABA and benzodiazepine receptor components of the GABAA receptor complex to control levels in a barbiturate-like fashion. This suggests that ammonia may enhance GABAergic neurotransmission at concentrations commonly encountered in hepatic failure, an event preceding the suppression of inhibitory neuronal function observed at higher (> 1 mM) ammonia concentrations. This increase in GABAergic neurotransmission is consistent with the clinical picture of lethargy, ataxia and cognitive deficits associated with liver failure and congenital hyperammonemia.

Outward chloride/potassium co-transport in insect neurosecretory cells (DUM neurones)

The mechanism underlying outward chloride transport in the cell body and in the neuritic field of cockroach Dorsal Unpaired Median (DUM) neurones was assessed using the intracellular microelectrode technique. The chloride equilibrium potential was indirectly estimated from the reversal potentials of responses to gamma-aminobutyric acid (GABA) pressure ejections and of inhibitory postsynaptic potential (IPSP) evoked by electrical stimulation of the anterior connectives. Changes in intracellular chloride concentration [Cl-]i following various treatments were estimated from the amplitude changes of soma GABA responses and IPSP. Decreasing external Cl- concentration reduced the amplitude of GABA-mediated inhibitory events without affecting the membrane potential. Cl-/K+ co-transport was assessed by increasing external K+ concentration. The rate of outward Cl- movement was reduced furosemide but not by SITS or DIDS. All these results suggest that Cl- is not passively distributed in DUM neurones and that an active outwardly directed Cl-/K+ co-transport is implicated in the regulation of [Cl-]i.

Endogenous Na(+)-K+ (or NH4+)-2Cl- cotransport in Rana oocytes; anomalous effect of external NH4+ on pHi

1. In Rana oocytes, measurements with chloride-sensitive microelectrodes show that the mean intracellular chloride activity (34.8 +/- 6.3 mM, n = 79) is three times higher than that expected for the passive distribution of chloride ions across the outer membrane (12.4 mM, mean membrane potential -43 +/- 8.8 mV, n = 79). 2. Reuptake of chloride into oocytes depleted by prolonged exposure to chloride-free saline takes place against the electrochemical gradient. 3. Chloride reuptake does not take place in sodium-free solution or in a sodium-substituted potassium-free solution. It is inhibited by bumetanide (10(-5) M) in the bathing medium. 4. The overall stoichiometry of the transport mechanism deduced from simultaneous measurements of intracellular sodium and chloride using ion-selective electrodes is 1Na+:1K+:2Cl-. 5. Ammonium ions substitute for potassium on the cotransporter. 6. In oocytes smaller than 0.9 mm in diameter, exposure to external ammonium causes an alkaline shift in intracellular pH as the NH3 enters and takes up H+ to form NH4+. We propose that chloride-dependent NH4+ transport contributes to the accumulation of NH4+ and causes the 'postexposure' acidification as the intracellular NH4+ releases H+ to form NH3 which is then lost from the cell. 7. In larger oocytes ammonium exposure produces a rapid reduction in pHi which may be explained in part by cotransport-mediated uptake of NH4+. Evidence is also provided for a second chloride-dependent NH4+ transport mechanism and a chloride-independent process.

Effects of ammonium ions on synaptic transmission and on responses to quisqualate and N-methyl-D-aspartate in hippocampal CA1 pyramidal neurons in vitro

Effects of NH4Cl on CA1 pyramidal neurons and synaptic transmission were investigated with intracellular recording in fully submerged rat hippocampal slices. Superfusion with 1-4 mM NH4Cl reversibly depolarized the membrane by 15.1 +/- 1.4 mV, reduced the amplitude and broadened the duration of action potentials due to a slower rate of repolarization, without significant change in membrane conductance. When membrane potential was returned to control level by the injection of a steady outward current, action potential amplitude recovered but repolarization remained slow. The extent of depolarization was not dependent on the concentration of NH4Cl between 1 and 4 mM. NH4Cl greatly depressed orthodromic transmission evoked by the stimulation of Schaffer collateral/commissural fibers several minutes after depolarizing the CA1 neuron. Interruption of transmission began with a decrease in excitatory postsynaptic potential (EPSP) amplitude and eventually EPSPs were almost eliminated. When NH4Cl was removed, it took 2-3 min for membrane potential and 10-15 min for transmission to recover. Inward currents induced by bath application of quisqualate acting on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors were also depressed. In contrast, NH4Cl enhanced N-methyl-D-aspartate (NMDA)-induced currents. This potentiation disappeared in the absence of added Mg2+. A reduction in quisqualate-induced responses provided a possible explanation for the inhibition of excitatory transmission by NH4Cl.

Ammonia-independent modifications of the background EEG signal and paradoxical enhancement of epileptic abnormalities in EEG after acute administration of valproate to epileptic patients

The effects on the background quantitative EEG (power spectral analysis) and concentration of valproate in plasma were studied after single-dose (14.3-33.3 mg/kg) oral administration in 12 epileptic patients with generalized nonconvulsive or partial seizures. An increase of the amplitude of the background EEG (diffuse and preponderant on anterior scalp areas) and a decrease of the 12.5-32.0 Hz relative power (limited to the posterior electrode deviations) were observed; the increase in the EEG total power was paralleled by a definite increment in incidence of epileptic phenomena in the EEG. Both effects proved unrelated to shifts in vigilance or changes in the concentration of ammonia or serum glucose in plasma and confirm previous observations from superimposable study designs. These findings are qualitatively opposite to those observed during long-term treatment at comparable doses and are suggested to reflect a direct CNS action of acute administration of valproate.

Enhanced endogenous ornithine concentrations protect against tonic seizures and coma in acute ammonia intoxication

Pretreatment of mice with 5-fluoromethylornithine (5FMOrn), a selective inactivator of ornithine aminotransferase, diminishes the accumulation of ammonia in the brain after administration of ammonium acetate, and antagonizes ammonia-induced fatal tonic extensor convulsions. In about 50% of the treated animals the loss of the righting reflex and coma is prevented. Presumably these effects are based on the enhancement of urea formation by the increased liver ornithine concentrations. However, since brain ornithine concentrations are greatly enhanced by 5FMOrn, it is not excluded that ornithine has direct effects on cellular events involved in ammonia-induced seizure generation, even though 5FMOrn had no anticonvulsant properties in a series of established animal seizure models, including N-methyl-D,L-aspartate-induced convulsions. NMDA receptor antagonists are capable of preventing death, but do not protect against the generation of coma and tonic extensor convulsions in ammonium acetate intoxicated mice. Since no evidence was found for ammonia-induced glutamate release from rat hippocampus, there is no convincing evidence for the idea that the tonic convulsions are mediated by NMDA receptors. L-Methionine-D, L-sulfoximine (MSO)-induced seizures can be partially antagonized by pretreatment with 5FMOrn. However, the effect is considerably smaller than against ammonia-induced convulsions, although at the time of seizure onset brain ammonia levels of MSO-intoxicated mice were lower than in the animals receiving ammonium acetate. This suggests that MSO-convulsions are not entirely due to the elevation of brain ammonia concentrations, even though MSO administration mimics effects of ammonia on cortical inhibitory neuronal interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of GABA-mediated, chloride-dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices

1. gamma-Aminobutyric acid (GABA)-mediated, Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole-cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2- to 5- (PN2-5), 8- to 13-(PN8-13) and 15- to 20-(PN15-20)day-old CA1 neurones. In the presence of glutamate receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2-5 neurones, but 13 and 25 mV more negative than the RMP in PN8-13 and PN15-20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl-] ([Cl-]p). indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, EGABA agreed with the value predicted by the Goldman-Hodgkin-Katz equation at given concentrations of internal and external anions permeable through GABA-activated Cl- channels, whereas EGABA in older neurones was 8-18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl-]p and resting conductance indicated that the membrane of the PN2-5 neurones was readily permeable to Cl- which followed a passive Donnan equilibrium. Passive distribution of Cl- played a decreasing role in PN8-13 neurones and in PN15-20 neurones. 5. To assess the contribution of outward Cl- co-transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15-20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABAA-mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl- ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of GABA-gated bicarbonate conductance on potential, current and intracellular chloride in crayfish muscle fibres

1. The effects of gamma-aminobutyric acid (GABA) on membrane potential and conductance as well as on the intracellular Cl- activity (aiCl) and intracellular pH (pHi) were studied in crayfish muscle fibres using a three-microelectrode voltage clamp and ion-selective microelectrodes. In the presence of CO2-HCO3-, the intracellular HCO3- activity (aiHCO3) was estimated from pHi. 2. In a nominally HCO3(-)-free solution, a near-saturating concentration of GABA (0.2 mM) produced a marked increase in membrane conductance but little change in potential. In a solution containing 30 mM-HCO3- (equilibrated with 5% CO2 + 95% air; pH 7.4), the GABA-induced increase in conductance was associated with a depolarization of about 15 mV, with an increase in aiCl and with a decrease in aiHCO3. All these effects were blocked by picrotoxin (PTX). The depolarizing action of GABA was augmented following depletion of extracellular and intracellular Cl-. 3. The GABA-induced increase in aiCl which took place in the presence of HCO3- was blocked by clamping the membrane potential at its resting level. This indicates that the increase in aiCl was due to passive redistribution of Cl-. In both the presence and absence of HCO3-, the GABA-activated transmembrane flux of Cl- showed reversal at the level of the resting potential, which indicates that under steady-state conditions the Cl- equilibrium potential (ECl) is identical to the resting potential. 4. In a Cl(-)-free, 30 mM-HCO3(-)-containing solution, 0.5 mM-GABA produced a PTX-sensitive increase in conductance which amounted to 15% of the conductance activated in the presence of Cl-. In the absence of both Cl- and HCO3-, the respective figure was 2.8%. Assuming constant-field conditions, the conductance data yielded a permeability ratio PHCO3/PCl of 0.42 for the GABA-activated channels. 5. In a Cl(-)-containing, HCO3(-)-free solution, the reversal potential of the GABA-activated current (EGABA) was, by about 1 mV, less negative than the resting membrane potential (RP). In a solution containing Cl- and 30 mM-HCO3-, EGABA-RP was 12 mV. Simultaneous measurements of EGABA, aiCl and aiHCO3 (pHi) gave a PHCO3/PCl value of 0.33. 6. In a Cl(-)-free, HCO3(-)-containing solution EGABA was close to the HCO3- equilibrium potential (EHCO3) and an experimental acidosis which produced a negative shift in EHCO3 was associated with a similar shift in EGABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Observation of cerebral metabolites in an animal model of acute liver failure in vivo: a 1H and 31P nuclear magnetic resonance study

Acute liver failure was induced in rats by a single intragastric dose of carbon tetrachloride. This causes hepatic centrilobular necrosis, as indicated by histological examinations, and produces a large increase in the activity of serum alanine aminotransferase. The plasma NH4+ level (mean +/- SEM) was 123 +/- 10 microM in the control group and 564 +/- 41 microM in animals with acute liver failure (each n = 5). 31P nuclear magnetic resonance (NMR) was used to monitor brain cortical high-energy phosphate compounds, Pi, and intracellular pH. 1H NMR spectroscopy was utilised to detect additional metabolites, including glutamate, glutamine, and lactate. The results show that the forebrain is capable of maintaining normal phosphorus energy metabolite ratios and intracellular pH despite the metabolic challenge by an elevated blood NH4+ level. There was a significant increase in the brain glutamine level and a concomitant decrease in the glutamate level during hyperammonaemia. The brain lactate level increased twofold in rats with acute liver failure. The results indicate that 1H NMR can be used to detect cerebral metabolic changes in this model of hyperammonaemia, and our observations are discussed in relation to compartmentation of NH4+ metabolism.

Inhibitory postsynaptic potentials in neonatal rat sympathetic preganglionic neurones in vitro

1. Intracellular recordings were made from antidromically identified sympathetic preganglionic neurones (SPNs) in transverse sections of thoraco-lumbar spinal cord from neonatal (12-22 day) rats. 2. Two types of hyperpolarizing (inhibitory) postsynaptic potentials (IPSPs) were recorded in the SPNs. The first type, which we have termed unitary IPSPs, were small, discrete IPSPs that occurred spontaneously and also following chemical or electrical stimulation applied to the spinal cord slices. The second type IPSP was a hyperpolarizing response evoked by either dorsal or ventral root stimulation. 3. Spontaneously occurring unitary IPSPs had an amplitude of 1 to 5 mV, and reversal potential of -60 to -75 mV; they were reversibly abolished by low Ca2+, tetrodotoxin (TTX) or strychnine but not by bicuculline and picrotoxin. 4. Pressure application of N-methyl-D-aspartate (NMDA), an excitatory amino SPNs; these were abolished by either strychnine or by the NMDA receptor antagonist D-2-amino-5-phosphonovalerate. Furthermore, electrical stimulation of dorsal rootlets elicited in several SPNs the discharge of strychnine-sensitive unitary IPSPs. 5. Electrical stimulation applied to dorsal or ventral rootlets elicited in nineteen and eight SPNs, respectively, an IPSP of larger amplitude (5 to 15 mV). The IPSP exhibited a reversal potential of -60 to 75 mV; it was changed to a depolarizing response in a low [Cl-]o solution, but was not significantly affected in a low [K+]o. Strychnine but not bicuculline or picrotoxin reversibly blocked the IPSPs in nearly all the SPNs. Additionally, hexamethonium and d-tubocurarine antagonized the IPSPs evoked by ventral but not by dorsal root stimulations. 6. Our results suggest that unitary and evoked IPSPs recorded in SPNs are due primarily to an increase of Cl- conductance by glycine or a glycine-like substance, released from interneurones, that can be activated by NMDA. Furthermore, IPSPs evoked by ventral root stimulation appear to represent a disynaptic event whereby nicotinic activation of a glycine-releasing interneurone results in a release of the inhibitory transmitter; this is then analogous to the Renshaw cell circuitry of the spinal motoneurones.

Ammonia causes a drop in intracellular pH in metabolizing cortical brain slices. A [31P]- and [1H]nuclear magnetic resonance study

[31P]- and [1H]Nuclear magnetic resonance spectroscopy were used to study metabolism in cortical brain slices in the guinea-pig during acute exposure to pathophysiological concentrations of ammonia. Intracellular acidification, measured from the chemical shift of endogenous inorganic phosphate, was observed without any change in cellular energy status or concentrations of lactate, glutamate and glutamine. The initial acidification, which developed over a period of 9 min appeared to be heterogeneous, on the basis of a splitting of the inorganic phosphate resonance in a number of experiments, corresponding to pH changes of 0.07 and 0.27 pH units. Subsequently a homogeneous acidification, of 0.15 pH units, developed by 23 min following exposure to ammonia. Intracellular pH recovered within 6 min after discontinuation of the ammonia load. In the absence of external bicarbonate, intracellular pH was 0.12 units more acidic than in the bicarbonate buffer and ammonia caused a further acidification by 0.16 units. When glutamine synthase inhibitor, methionine sulphoximine, was added, there was a slow fall in intracellular pH. Under these conditions, subsequent addition of ammonia failed to cause acidification directly. Thus acute elevation of ammonia does not lead to a change in cerebral high-energy phosphate or lactate metabolism, but may be associated with a fall in cortical intracellular pH.

The dependence of motoneurone membrane potential on extracellular ion concentrations studied in isolated rat spinal cord

1. Intracellular recordings from ninety-nine motoneurones have been made in an in vitro hemisected spinal cord preparation. Their mean resting membrane potential in normal artificial cerebrospinal fluid (CSF) was -71 +/- 0.5 mV (+/- S.E.M.). The mean amplitude of the action potential was 84.0 +/- 1.4 mV (n = 50), and the mean input conductance was 101 +/- 7 nS (n = 49). 2. Both membrane potential and input conductance were sensitive to changes in [K+]o, [Na+]o, [Cl-]o and [Ca2+]o. 3. Replacement of extracellular Ca2+ by Mn2+ resulted in less than 1 mV hyperpolarization and a decrease in input conductance from 102 +/- 7 to 93 +/- 6 nS (n = 15). 4. At high [K+]o (greater than 10 mM) the membrane potential followed the potential predicted by the Nernst equation for K+ ions with a slope of 58 mV per 10-fold change in [K+]o. At low [K+]o (less than 10 mM) there was significant deviation from K+ equilibrium potential (EK). 5. [K+]i was found to be 106 mM when estimated from the reversal potential of the after-hyperpolarization of the antidromic action potential. 6. The reversal potential of the recurrent inhibitory postsynaptic potential (IPSP) in normal CSF was used to calculate [Cl-]i. This was 6.6 mM, which is less than would be expected if Cl- was passively distributed, indicating the presence of an outwardly directed Cl- pump. 7. Decreasing [Cl-]o from control (134 mM) to 4 mM resulted in a depolarization of 6.9 +/- 0.9 mV and a decrease in input conductance from 102 +/- 5 to 90 +/- 5 nS (n = 14) in 3 mM [K+]o. 8. Decreasing [Na+]o from 156 to 26 mM by substitution with choline resulted in a 6.2 +/- 0.5 mV hyperpolarization and a decrease in input conductance from from 102 +/- 4 to 76 +/- 4 nS (n = 5) in 3 mM [K+]o. 9. The input conductances for Na+, Cl- and K+ at the resting potential were calculated. After allowing for a microelectrode leak conductance, the relative input conductances were gNa/gK = 0.13 and gCl/gK = 0.25.

"Intracellular" GABA affects the equilibrium distribution of Cl- across the plasma membrane of a GABA acceptive neuron

The permeability of Cl- ions through single microdissected plasma membrane from Deiters' neurons was studied by a microtechnique. In particular, the time course of the passage of 36Cl- ions from a microchamber, M1, to another one, M2, across the membrane was followed. This study was performed with or without gamma-amino-butyric acid (GABA) in the two microchambers. The results suggest that in basal conditions the high intracellular concentration normally present in these neurons, 3.3 mM (1), causes a higher permeability of Cl- in the direction inside----outside in the respect of the plasma membrane. "Extracellular" GABA, 0.1 mM, is able to abolish this imbalance in Cl- permeability in the two opposite directions. This event appears to be the basis for GABA induced hyperpolarization of these neurons.

The H-reflex in the encephalopathy due to ammonia intoxication

Ammonia intoxication has been shown to decrease excitatory synaptic transmission in several regions of the central nervous system. To investigate the relation between an effect of ammonia on excitatory synaptic transmission and the behavioral depression in the encephalopathy due to ammonia intoxication, this study examined in the rat the effects of ammonia intoxication on the H-reflex, the behavioral and neurological signs of the encephalopathy due to ammonia intoxication, and correlated the effects on the H-reflex with the signs of encephalopathy. Ammonia intoxication abolished the H-reflex without affecting the M-response. This indicated that ammonia intoxication decreased spinal excitatory synaptic transmission without affecting neuromuscular excitatory synaptic transmission. In the encephalopathy due to ammonia intoxication, the H-reflex disappeared only during a very advanced stage of behavioral depression, i.e., coma. During early stages of behavioral depression, i.e., during a decrease of reactions to sensory stimuli, the H-reflex was not affected by ammonia intoxication. Therefore, mechanisms other than a decrease of excitatory synaptic transmission in the central nervous system may be responsible for the behavioral depression seen in early stages of the encephalopathy due to ammonia intoxication.

Inhibitory potentials in neurons of the deep layers of the in vitro neocortical slice

Neocortical neurons in slices of the rat sensorimotor region maintained in vitro generate postsynaptic potentials (PSPs) in response to focal extracellular stimulation. These PSPs are mainly depolarizing at the resting membrane potential (Vm) but a sequence of depolarizing-hyperpolarizing potentials is often disclosed by depolarizing the Vm. The stimulus-induced hyperpolarization can last up to 1000 ms and show two components: the early one (peak latency 10-20 ms), is inverted by diffusion of Cl- into the cell; the late one is diminished by augmenting [K+]o. The membrane conductance is increased throughout the stimulus-induced hyperpolarization, mainly during the first 10-60 ms. A decrease in excitability results from both the hyperpolarizing trend and the conductance increase. The latter is more effective in decreasing depolarizing than hyperpolarizing pulses of current injected intracellularly.

Porta-caval shunting changes neuronal sensitivity to ammonia

The relations between an effect of ammonia on postsynaptic inhibition, the amount of ammonium acetate i.v. to obtain this effect, and the tissue concentrations of NH4+ and glutamine were investigated in the cerebral cortex of cats without and with portacaval shunts. Normal cats required 2.43 mmol/kg ammonium acetate to affect postsynaptic inhibition. Cerebral NH4+ and glutamine increased from 0.21 mumol/g to 0.77 mumol/g and from 2.92 mumol/g to 5.54 mumol/g, respectively. In portacaval shunted cats, postsynaptic inhibition was normal in spite of increases of NH4+ and glutamine to 1.37 mumol/g and 14.28 mumol/g, respectively. Only 0.7 mmol/kg of ammonium acetate were sufficient to affect postsynaptic inhibition. This was associated with a statistically insignificant increase of NH4+ to 1.61 mumol/g and no change of glutamine. A chronic portasystemic shunt markedly increases the tolerance of postsynaptic inhibition to NH4+. However, postsynaptic inhibition becomes very sensitive to an acute systemic ammonia load and the associated increase of tissue NH4+ in the cerebral cortex. These observations help to understand the pathogenesis of the encephalopathy precipitated in patients with portasystemic shunts by an acute systemic ammonia load such as resulting from a gastrointestinal hemorrhage.

Comparative pharmacological effects on visual cortical neurons in monocularly deprived cats

Monocularly deprived (MD) cats show a loss of responsiveness to visual stimulation of the deprived eye among visual cortical neurons. Several lines of evidence suggest that this effect involves, at least in part, a suppression of deprived eye input, possibly mediated by GABA inhibition. In order to better understand the nature of this suppression we have evaluated the effectiveness of different types of disinhibitory and excitatory agents to reverse the effects of MD. We investigated bicuculline (a GABA antagonist); picrotoxin (a GABA antagonist with a different mechanism of action from bicuculline); strychnine (a glycine antagonist); ammonium ion (a blocker of membrane chloride channels); physostigmine (a cholinesterase inhibitor); and naloxone (an opiate antagonist and also a GABA antagonist). All drugs were given intravenously. Bicuculline restored binocularity to 50% of the visual cortical neurons tested and naloxone to 36%. With both drugs, receptive fields of the normal eye tended to lose specificity. The emergent deprived eye receptive fields were usually similar to those of the normal eye after drug administration. Ammonium ion produced binocular responses in 27% of neurons tested, but receptive fields were grossly abnormal; moreover, ammonium infusion tended to depress neuronal responsiveness. All other drugs tested failed to restore binocularity. These experiments lend further credence to the hypothesis that GABA inhibition contributes to the cortical effects of MD, since only drugs with GABA antagonistic action were effective in restoring neuronal responsiveness to the deprived eye.

Effects of ammonium salts on synaptic transmission to hippocampal CA1 and CA3 pyramidal cells in vivo

The effects of ammonium acetate or chloride, perfused through the lateral ventricle, were studied on the hippocampal formation of the rat. During perfusion with ammonia, the population spikes, evoked by stimuli delivered to the fimbria, were first increased and then reduced. On the other hand, the late positive wave gradually decreased throughout the application of ammonia. The inhibition, studied by the paired-pulse test, was found to be reduced when the population spike was transiently enhanced, indicating that disinhibition could be responsible for the enhancement of synaptically evoked responses. Neither antidromically evoked population spikes nor the typical effects of iontophoretically applied glutamate, aspartate or gamma-aminobutyrate were changed by ammonia. These findings can be accounted for by a single action of ammonia, a depression of excitatory synaptic transmission, the excitatory synapses on inhibitory interneurons being more readily depressed than those on the pyramidal cells. Both effects, early hyperexcitability and late depression, are probably due to a reduction in the release of the excitatory neurotransmitter, glutamate and/or aspartate. We tentatively suggest that these mechanisms are responsible for some of the symptoms observed during the development of hyperammonemic encephalopathies.

Pathophysiology of ammonia intoxication

Ammonia intoxication affects postsynaptic inhibition and disturbs inhibitory neuronal interactions. This study investigated whether or not the effect of ammonia on postsynaptic inhibition was associated with a change of the EEG, i.e., a change in the function of the central nervous system such as in an encephalopathy. We showed that the effect of ammonia on postsynaptic inhibition was associated with a marked change of the EEG, and that this change was not due to an effect of ammonia on the brain stem reticular activating system. In addition, it was shown that in the central nervous system a NH+4 concentration of about 1 mumol/g affected postsynaptic inhibition. Because ammonia simultaneously affected postsynaptic inhibition and the EEG at a NH+4 tissue concentration comparable to that observed in encephalopathy, it is proposed that a dysfunction of postsynaptic inhibition caused the encephalopathy due to ammonia intoxication by simultaneously disturbing inhibitory neuronal interactions in many regions of the central nervous system.

The ionic mechanism of postsynaptic inhibition in motoneurones of the frog spinal cord

The ionic mechanism of postsynaptic inhibition in frog spinal motoneurones was studied with conventional and with ion-sensitive microelectrodes. In these neurones the inhibitory postsynaptic potential was depolarizing, its reversal potential being 15 mV less negative than the resting membrane potential. During the inhibitory postsynaptic potential the input resistance of the motoneurones was reduced to 20% of the resting value, indicating a strong increase of membrane conductance. The Cl- equilibrium potential calculated from intra- and extracellular Cl- activity measurements coincided with the reversal potential of the inhibitory postsynaptic potential to within a few millivolts. During repetitive inhibitory postsynaptic activity the intracellular Cl- activity decreased markedly, while the extracellular Cl- activity increased slightly. These changes of intra- and extracellular Cl- activities were no longer found after suppression of the inhibitory postsynaptic potential by strychnine. Blockade of an active, inward-going Cl- transport system in motoneurones by NH+4 led to a shift of the Cl- equilibrium potential and the reversal potential of the inhibitory postsynaptic potential towards the resting membrane potential. After prolonged action of NH+4, the Cl- equilibrium potential approached the membrane potential to within 5 mV, while the reversal potential of the inhibitory postsynaptic potential and resting membrane potential coincided. The difference between Cl- equilibrium potential and membrane potential after blockade of the Cl- pump is traced back to interfering intracellular ions, such as HCO-3 or SO42-, leading to an overestimation of intracellular Cl- activity and to the calculation of an erroneous Cl- equilibrium potential. Inhibitory amino acids like gamma-aminobutyrate or beta-alanine evoked depolarizations with reversal potentials similar to that of the inhibitory postsynaptic potential. These depolarizations were associated with a marked decrease of neuronal input resistance during inhibition. During the actions of these compounds a decrease of intracellular and a small increase of extracellular Cl- activity were found. The activities of other ions (K+, Ca2+ and Na+) did not change significantly, with the exception of extracellular K+ activity, which was slightly increased. Evidence is presented that the inhibitory postsynaptic potential, as well as the depolarizing action of inhibitory amino acids in motoneurones, is the result of an increase in membrane Cl- permeability and an efflux of Cl- from these cells, while other ions do not seem to be involved.

Extra- and intracellular amino acids in the hippocampus during development of hepatic encephalopathy

Fulminant hepatic failure was induced in rabbits by intravenous administration of galactosamine hydrochloride. The animals were sacrificed after 45 h and the hippocampus analyzed for free amino acids. In addition, free amino acids were measured in plasma and in the extracellular fluid of the hippocampus 20, 30 and 45 h after galactosamine injection. The extracellular fluid compartment was analyzed by slow perfusion of a thin dialysis tube which was implanted in the hippocampus one day prior to galactosamine administration. The amino acid concentration in the extracellular fluid agreed fairly well with that of the cerebrospinal fluid in the control situation. During development of hepatic failure, the plasma concentration of all amino acids increased. The changes in extracellular amino acids were smaller, except for phosphoethanolamine and glutamate. The concentration ratio intra/extracellular amino acids decreased in the hippocampus for amino acids with a normally high concentration gradient.

Mechanisms of chloride transport in crayfish stretch receptor neurones and guinea pig vas deferens: implications for inhibition mediated by GABA

Since activation of GABAA receptors is believed to open an associated Cl- channel, the intracellular Cl- activity (aiCl) must be lower than that predicted from a passive distribution, to account for hyperpolarizing responses, or higher, to account for depolarizing responses. The physiological and pharmacological properties of the implied Cl- extruding and accumulating mechanisms have been investigated by direct measurements of aiCl. A coupled K+-Cl- co-transport has been found in crayfish stretch receptor neurones and a predominating Cl(-)HCO3(-) exchange in guinea pig vas deferens. From the different ionic mechanisms involved in Cl- extrusion and accumulation, it is proposed that drugs which affect Cl- transport mechanisms will reduce GABA responses of both polarities only if their action is via interference with the Cl- recognition site, but not if it is via interference with the co- or counter-ion recognition site.

Ammonia, postsynaptic inhibition and CNS-energy state

Ammonia intoxication decreases the hyperpolarizing action of postsynaptic inhibition. This study examines the metabolic state of the spinal cord during this effect of ammonia intoxication on spinal motoneurons. ATP, ADP, AMP, the adenylate energy charge, glucose, PCr, pyruvate, alpha-ketoglutarate and glutamate were unchanged during the effect of ammonia on the hyperpolarizing action of postsynaptic inhibition. NH4+, glutamine and lactate were increased. Ammonia intoxication affected postsynaptic inhibition without changes of the resting membrane potential, the neuron input resistance, the action potential and EPSPs. The encephalopathy caused by ammonia intoxication is known to occur without an alteration of the tissue energy state. The effect of ammonia intoxication on postsynaptic inhibition can be considered as a cause of the encephalopathy because postsynaptic inhibition is altered without a change of the tissue energy state, the resting membrane potential, the whole neuron resistance, the action potential and EPSPs.

Effects of ammonia on synaptosomal membranes

Acute and sustained hyperammonemia in mice resulted in a decrease of the transition temperature of Arrhenium plots of synaptosomal (Na+-K+)ATPase. The activation energies in both phases of the plots were increased. "In vitro" addition of ammonia produced similar changes. This seems to indicate that ammonia alters the physical properties of synaptosomal membranes. The "in vitro" interaction of ammonia and ethanol at the membrane level was also investigated. Both agents together produced a further shift in the transition temperature and affected the activation energies. The relevance of these findings regarding the mechanism of ammonia toxicity and the protective effect of ethanol thereon is discussed.

The metabolic basis for the genesis of seizures: the role of the potassium-ammonia axis

A conceptual approach to the understanding of the pathogenesis of idiopathic seizures is presented. Hypokalemia and/or alkalosis promotes the elaboration of an alkaline urine, which increases the renal return of ammonia and exposes the brain to chronically higher concentrations of ammonia. In the brain, ammonia is preferentially detoxified to glutamine and therefore depletes the available glutamic acid, which is also a precursor of GABA, the major mediator of central inhibition. Mild chronic elevations of ammonia may also result in long-term nutritional alterations of amino-acid precursors of other brain neurotransmitters. A linkage thus exists for the metabolic basis of seizures: the role of the potassium-ammonia axis may be important in the selective depletion of GABA, the major mediator of central inhibition.

Post-synaptic inhibition of bulbar inspiratory neurones in the cat

Stable intracellular recordings from thirty-six bulbar inspiratory neurones revealed three centrally originating, rhythmic patterns of synaptic inhibition (i.p.s.p.s). (i) A declining pattern of i.p.s.p.s accompanying the early stages of inspiration (early inspiratory inhibition) was identified in a total of twenty neurones representing examples of each of the functional classes of bulbar neurones examined, i.e. six R alpha- and two R beta-neurones of the dorsal respiratory group and twelve R alpha-neurones of the ventral respiratory group. (ii) A transient pattern of i.p.s.p.s just preceding or coinciding with the cessation of inspiration (late inspiratory inhibition) was present in the remaining sixteen neurones which were tested, representing six R alpha-neurones and three R beta-neurones of the dorsal respiratory group and seven R alpha-neurones of the ventral respiratory group. (iii) An augmenting pattern of expiratory i.p.s.p.s was present in all thirty-six neurones. Late inspiratory and expiratory i.p.s.p.s in the same neurones showed a similar time course of reversal when chloride was injected or allowed to diffuse into the cells and were associated with similar increases in input conductance. Both patterns of i.p.s.p.s appear to arise at or close to the cell soma. Early inspiratory i.p.s.p.s required a relatively longer period of chloride injection for reversal to be accomplished. Input conductance changes were either absent or smaller than those associated with late inspiratory or expiratory inhibition. These i.p.s.p.s appear to arise at more distal dendritic sites. These patterns of i.p.s.p.s are discussed in relation to the mechanisms shaping the growth of central inspiratory activity, bringing this activity to an end, and suppressing its redevelopment during expiration.

Ammonia inhibits protein synthesis in slices from young rat brain

Ammonium chloride, widely used as an inhibitor of lysosomal protein degradation, was shown to inhibit strongly protein synthesis in neocortical slices from 10-day-old rats at a concentration of 10 mmol/L. Its usefulness in experiments with brain tissue is doubted. The possible meaning of the finding with respect to ammonia toxicity is briefly discussed.

The metabolic basis for the genesis of seizures: the role of the potassium-ammonia axis

A conceptual approach to the understanding of the pathogenesis of idiopathic seizures is presented. Hypokalemia and/or alkalosis promotes the elaboration of an alkaline urine, which increases the renal return of ammonia and exposes the brain to chronically higher concentrations of ammonia. In the brain, ammonia is preferentially detoxified to glutamine and therefore depletes the available glutamic acid, which is also a precursor of GABA, the major mediator of central inhibition. Mild chronic elevations of ammonia may also result in long-term nutritional alterations of amino-acid precursors of other brain neurotransmitters. A linkage thus exists for the metabolic basis of seizures: the role of the potassium-ammonia axis may be important in the selective depletion of GABA, the major mediator of central inhibition.

Intracellular chloride in retinal neurons: measurement and meaning

Intracellular chloride activity measurements were obtained from mudpuppy retinal neurons using dual microelectrodes, one of which was made chloride-selective by filling the tip with chloride liquid ion exchange resin. In addition ionic substitution experiments were carried out in the perfused retina-eyecup preparation of the mudpuppy. A comparison of the membrane potential and calculated chloride equilibrium potential shows that retinal neurons differ in relative transmembrane chloride distribution. Ganglion cells have an ECl more negative than the resting membrane potential, whereas amacrine cells have a passive distribution of chloride. Horizontal cells have chloride distributed such that an increase in chloride conductance is depolarizing. On-bipolars have a chloride distribution similar to that of horizontal cells whereas off-bipolars show either passive distribution or some chloride accumulation. These findings are consistent with the idea that chloride ions may play a role as a depolarizing driving force for electrogenic activity of horizontal cells and on-bipolars. Results with Cl substitution experiments are consistent with this interpretation.

Ammonia and methionine sulfoximine intoxication

Intoxication with ammonium acetate abolished the suppression of action potential generation by cortical postsynaptic inhibition, i.e. produced 'disinhibition', due to the inactivation of neuronal Cl- extrusion. With the occurrence of disinhibition cerebral ammonia increased to 445% of normal; glutamine increased to 170%. Methionine sulfoximine (MSO), an inhibitor of glutamine synthetase, produced disinhibition about 3 h after administration; at this time cerebral ammonia was increased to 290% of normal, glutamine was unchanged. Intoxication with MSO for less than 3 h significantly decreased the amount of ammonium acetate needed to produce disinhibition at cerebral ammonia concentrations ot 340-430% of normal. MSO produces an endogenous ammonia intoxication which: (i) decreases the amount of exogenous ammonia required to affect cortical postsynaptic inhibitions; and (ii) eventually becomes sufficiently severe to disturb cortical inhibitory neuronal interactions by itself.

Regulation of glutamate biosynthesis and release in vitro by low levels of ammonium ions

The content and release of endogenous amino acids from isolated rat hippocampal slices were measured. The tissue was perfused with control media and pulsed with high potassium media in order to induce synaptic release. Pathophysiological concentrations of ammonium ions (3--5 mM) were added to the control medium for 60 min prior to the induced release. Amino acids belonging to the putative transmitter group were released extensively during potassium perfusion and, except for glutamate, even after ammonium ion perfusion. The spontaneous secretion of glutamate increased, however, slowly after the addition of ammonia. The incorporation of 14C from radiolabelled glucose and acetate into the amino acid fraction was studied in the presence of ammonia-containing media. Glucose was utilized to a moderately increasing extent, but acetate-derived radioactivity was strikingly decreased in the amino acid fraction during ammonia perfusion. The decreased acetate incorporation into amino acids was mainly due to an inhibition by ammonia of the accumulation of acetate by the CNS tissue.

Effectiveness of some anions in sustaining the efferent inhibition in the frog labyrinth

Efferent inhibition in the frog labyrinth is sustained by the release of acetylcholine (ACh) which opens a Cl(-)-channel in the hair cell membrane. To investigate more closely the nature of the permeability change underlying the ACh reaction, the external Cl(-) was replaced by anions of increasing hydrated size, and to test the possible role of a Cl(-)-pump in the sensory cells, drugs were applied which are known to block active cl(-) pumping in other systems. Experiments indicate that the ACh-operated inhibitory channel of the hair cell is larger than at other inhibitory synapses (or approximately 0.7 nm), while pharmacological treatments (DNP, NaN3, acetazolamide, ammonium acetate, DIDS) fail to demonstrate any active distribution of Cl(-) across the hair cell membrane.

The role of intracellular chloride in hyperpolarizing post-synaptic inhibition of crayfish stretch receptor neurones

1. The intracellular Cl(-) activity (a(Cl) (i)) of isolated crayfish stretch receptor neurones was measured using liquid ion exchanger Cl(-)-selective micro-electrodes. The potential developed due to the difference between the normal extracellular Cl(-) activity (a(Cl) (o)) and a(Cl) (i) (V(Cl)) was compared with the simultaneously measured reversal potential of the inhibitory post-synaptic potential (E(i.p.s.p.)) to further clarify the ionic basis of the i.p.s.p..2. In normal Ringer solution, V(Cl) (63.3 +/- 2.3 mV) was found to be close to the resting membrane potential (E(m), 62.6 +/- 3.9 mV) while E(i.p.s.p.) (74.5 +/- 1.9 mV) was more negative than either. The V(Cl) value corresponds to an apparent a(Cl) (i) of 12.7 +/- 1.3 mM, which is about 4 mM more than required for a Cl(-) governed E(i.p.s.p.) of 74.5 mV.3. Reducing a(Cl) (o) caused smaller changes in V(Cl) than predicted for passive Cl(-) re-distributions. On complete removal of extracellular Cl(-) (Cl(o) (-)), V(Cl) increased to 84.6 +/- 2.7 mV, equivalent to an apparent a(Cl) (i) of about 5 mM-Cl(-). This value can be used as an estimate of the level of intracellular interference on the Cl(-)-selective micro-electrode.4. Increasing extracellular K(+) (K(0) (+)) decreased both V(Cl) and E(i.p.s.p.). Decreasing K(o) (+) had the converse effect. The time course of the changes in V(Cl) and E(i.p.s.p.) was much the same. The difference between V(Cl) and E(i.p.s.p.) decreased to about 3 mV in high K(o) (+), and increased to about 30 mV in low K(o) (+). This variation in the difference between E(i.p.s.p.) and V(Cl) is consistent with the assumption that anions other than Cl(-) contribute to the recorded V(Cl) rather than another ion contributes to the inhibitory current.5. Application of 5 mM-NH(4) (+) or of frusemide (6 x 10(-4) M) decreased V(Cl) and E(i.p.s.p.). The difference between V(Cl) and E(i.p.s.p.) was also decreased.6. We conclude that a(Cl) (i) is lower than predicted from a passive distribution and thus the chloride equilibrium potential (E(Cl)) is more negative than E(m). If a constant intracellular interference equivalent to about 4 mM-Cl(-) is assumed to contribute to the recorded V(Cl), E(Cl) was approximately equal to E(i.p.s.p.) in all the experimental conditions. Therefore we suggest that the i.p.s.p. is solely generated by Cl(-) ions.