A comparative study of the action of gamma-aminobutyric acid and piperazine on the lobster muscle fibre and the frog spinal cord

Mutations affecting the glycine receptor agonist transduction mechanism convert the competitive antagonist, picrotoxin, into an allosteric potentiator

Contrary to its effects on the gamma-aminobutyric acid type A receptor, picrotoxin antagonism of the alpha 1 subunit of the human glycine receptor is shown to be competitive, not use-dependent, and nonselective between the picrotoxin components, picrotin, and picrotoxinin. Competitive antagonism and non-use dependence are consistent with picrotoxin binding to a site in the extracellular domain. The mutations Arg-->Leu or Arg-->Gln at residue 271 of the glycine receptor alpha 1 subunit, which are both associated with human startle disease, have previously been demonstrated to disrupt the transduction process between agonist binding and channel activation. We show here that these mutations also transform picrotoxin from an allosterically acting competitive antagonist to an allosteric potentiator at low (0.01-3 microM) concentrations and to a noncompetitive antagonist at higher (> or = 3 microM) concentrations. This demonstrates that arginine 271 is involved in the transduction process between picrotoxin binding and its mechanism of action. Thus, the allosteric transduction pathways of both agonists and antagonists converge at a common residue prior to the activation gate of the channel, suggesting that this residue may act as an integration point for information from various extracellular ligand binding sites.

Mono N-aryl ethylenediamine and piperazine derivatives are GABAA receptor blockers: implications for psychiatry

Ethylenediamine (EDA) and piperazine are known GABA-A receptor agonists and this activity appears to reside in their carbamate adducts. In CO2-free incubation medium EDA and piperazine weakly reverse the inhibitory action of 1 microM GABA on specific [35S]t-butylbicyclophosphorothionate (35S-TBPS) binding to rat brain membranes in vitro. In 25 mM sodium bicarbonate buffer, EDA and piperazine much more potently inhibit 35S-TBPS binding in a way reversible by the GABA-A receptor blocker R5135. Thus, native EDA and piperazine are weak GABA-A receptor blockers, while their presumed carbamate adducts, formed by reaction with bicarbonate, are more potent GABA-A receptor agonists. Virtually all structural modifications of EDA or piperazine result in GABA-A receptor blockers, even in the presence of bicarbonate, judging from their abilities to fully or partially reverse the inhibitory effect of GABA on 35S-TBPS binding. Of 12 non-aromatic piperazine or EDA derivatives, the piperazine derivatives are the more potent GABA antagonists, although all are weak compared to the mono N-aryl derivatives. Nineteen mono N-aryl EDA derivatives are moderately potent GABA antagonists, including 10 with demonstrated or potential antidepressant activity. Most of the N-aryl piperazines are moderately to highly potent GABA antagonists, one (pitrazepin) being 4 to 5 times more potent than bicuculline. There are several clinically effective antidepressants (e.g. Amoxapine, Mianserine) and antipsychotics (Clothiapine, Loxapine, Metiapine, Clozapine and Fluperlapine) among the more potent N-aryl piperazine GABA antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

On the mechanism of action of picrotoxin on GABA receptor channels in dissociated sympathetic neurones of the rat

1. The mechanism of action of picrotoxin on GABA receptor channels in rat sympathetic neurones has been investigated with whole-cell clamp. In addition, the action of picrotoxin on single GABA channels has been examined in outside-out membrane patches from these cells. 2. Picrotoxin, at concentrations which dramatically reduced the amplitude of whole-cell GABA currents, did not alter the spectral time constants or single-channel conductance estimated by analysis of GABA-activated current noise. This was observed at potentials both negative and positive to the GABA reversal potential (i.e. for both inward and outward GABA currents). In control conditions, the slow and fast time constants from GABA noise were 40 +/- 14 ms and 2 +/- 0.4 ms, while the estimated single-channel conductance was 14 +/- 2 pS. In the presence of picrotoxin, the time constants and estimated single-channel conductance were 41 +/- 5 ms, 2.7 +/- 0.6 ms and 15 +/- 2.3 pS. 3. Picrotoxin did not alter the shape of the whole-cell GABA current-voltage relationship, indicating that the steady-state block was not voltage dependent. The lack of effect of picrotoxin on the GABA noise spectra and the lack of outward rectification makes it unlikely that picrotoxin acts by a simple voltage-dependent (or voltage-independent) channel blocking mechanism. In the presence of picrotoxin the reversal potential for GABA remained at approximately 0 mV in symmetrical chloride. 4. Distributions of total burst durations, obtained from single-channel records with low concentrations of GABA, were fitted with three or four exponential components. Picrotoxin had no consistent effect on the time constants of the total burst length distributions. It also did not alter the amplitude of the main conductance state. However, picrotoxin did reduce the frequency of channel openings. 5. The application of brief ionophoretic pulses of GABA, to cells under whole-cell voltage clamp, revealed that the rate of onset of block by picrotoxin was accelerated in the presence of GABA. In the absence of agonist, picrotoxin produced a more slowly equilibrating block. 6. Our data are consistent with a mechanism whereby picrotoxin binds preferentially to an agonist bound form of the receptor and stabilizes an agonist-bound shut state. This could, for example, mean that picrotoxin enhances the occurrence of a desensitized state or an allosterically blocked state.

Ivermectin and piperazine toxicoses in dogs and cats

Review of all reports involving anthelmintics in dogs and cats to the IAPIC between January 1, 1986 and August 10, 1988, revealed that ivermectin (extra-label use) and piperazine accounted for over 50% of the calls assessed as toxicoses and suspected toxicoses. Both ivermectin and piperazine are gamma-aminobutyric acid (GABA) agonists and their major effects appear to be on the central nervous system. Ivermectin toxicoses at estimated doses of greater than or equal to 100-less than 500 micrograms/kg were reported more than once only in the collies (n = 25) and Australian shepherds (n = 10); these two breeds accounted for 46% (69 of 150) of the toxicoses and suspected toxicoses calls in dogs. Ataxia, behavioral disturbances, tremors, mydriasis, weakness/recumbency, apparent blindness, hypersalivation/drooling (dogs only), and coma were the most commonly reported clinical signs in dogs and cats with suspected ivermectin toxicoses. Shock, dyspnea, vomiting, and ataxia were the most common clinical signs attributed to the microfilaricidal activity of ivermectin. Piperazine was the anthelmintic with the greatest number of reports of toxicoses and suspected toxicoses in cats. Piperazine neurotoxicity in cats and dogs usually was manifested by muscle tremors, ataxia, and/or behavioral disturbances within 24 hours after estimated daily dose(s) between 20 and 110 mg/kg.

The action of oxygen and oxygen at high pressure on inhibitory transmission

The effect of 100% oxygen at ambient pressure, 100% oxygen at 1.7 Atmospheres Absolute (ATA), 100% oxygen at 5.1 ATA, helium at 1.7 ATA and helium at 5.1 ATA on inhibitory synaptic transmission was studied using the lobster walking leg neuromuscular preparation. Exposure to 100% oxygen at ambient pressure, at 1.7 ATA or at 5.1 ATA produced a decrease in inhibitory transmission manifest as a fall in inhibitory synaptic conductance (Ginh). The largest decrease in Ginh was seen in 100% oxygen at ambient pressure, while a progressively smaller decrease was seen in 100% oxygen at 1.7 ATA and 5.1 ATA, respectively. Also associated with 100% oxygen at ambient pressure was the disappearance of inhibitory junction potentials. Pressurization with helium produced a fall in Ginh at 5.1 ATA but no change or a slight increase at 1.7 ATA. The action of either 100% oxygen at ambient and at 1.7 or 5.1 ATA or helium at 1.7 or 5.1 ATA was shown to be on presynaptic parameters since the percent decrease in Ro induced by exogenous application of gamma-aminobutyric acid (GABA), the inhibitory transmitter, was the same in either 100% oxygen at ambient pressure, 100% oxygen or helium at 1.7 ATA and 5.1 ATA. The similarity in action of oxygen to the action of isoniazid, a known glutamic acid decarboxylase (the enzyme that catalyzes the production of GABA) inhibitor in the same preparation suggests that one possible site of oxygen action is on GABA production.

The influence of bicarbonate ions on the GABA-mimetic activity of ethylenediamine

A study was performed to investigate the GABA-mimetic activity of ethylenediamine (EDA) and piperazine at mammalian gamma-aminobutyric acid (GABA) receptors using radioligand binding assays and in vitro isolated tissues. The potency of ethylenediamine and piperazine as inhibitors of the binding of GABA receptors to synaptic membranes from rat brain was measured in Tris-buffers and Krebs-Henseleit solution (KHS). The potency of ethylenediamine and piperazine at GABAA and ethylenediamine at GABAB receptors was raised if Krebs-Henseleit solution was used for the assay. Piperazine was inactive at GABAB receptors. The potency of the antagonist of GABAA receptors bicuculline methobromide, was also increased in Krebs-Henseleit when compared with Tris-citrate buffer. Of the ions present in Krebs-Henseleit, bicarbonate ions were responsible for the increase in the GABA-mimetic potency of ethylenediamine and piperazine. Addition of either NaHCO3 or KHCO3 (25 mM) to Tris-HCl buffer (for GABAA binding) or Tris-HCl plus 2.5 mM CaCl2 (for GABAB binding) yielded IC50 values similar to those measured in Krebs-Henseleit solution. Bicarbonate ions also enhanced the ability of ethylenediamine to potentiate the binding of [3H]diazepam to membranes from rat brain (raising both the potency of ethylenediamine and its maximum effect) in this system. In the absence of HCO-3 ions, ethylenediamine potentiated the binding of [3H]diazepam by raising the maximum binding capacity (Bmax) without changing the affinity (Kd) of the receptors. Potassium bicarbonate (25 mM) caused ethylenediamine to further potentiate the binding of [3H]diazepam by changing both Bmax and Kd.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiating action of midazolam on GABA-mediated responses and its antagonism by Ro 14-7437 in the frog spinal cord

The effect of midazolam, a new water-soluble benzodiazepine, on an in vitro slice preparation of the frog spinal cord was investigated using electrophysiological recordings. Midazolam potently (ED50 = 1 nM) enhanced the depolarizing action of GABA on primary afferent fibres while leaving the depolarizing effect of glutamate, glycine or high K+ solutions unchanged. Concentrations of midazolam higher than 100 nM had an antagonistic effect on GABA responses. Ro 14-7437 was a powerful and selective antagonist of the midazolam potentiation without affecting control responses to GABA, glutamate or high K+. The antagonism of GABA responses induced by high doses of midazolam was not sensitive to Ro 14-7437. Our data suggest that midazolam is a very potent and selective modulator of GABA responses: this finding illustrates that electrophysiological techniques can detect specific effects of very low concentrations of benzodiazepines on a CNS slice preparation with well preserved architectural organization.

Structure-activity studies on the potentiation of benzodiazepine receptor binding by ethylenediamine analogues and derivatives

The effect of ethylenediamine analogues on in vitro binding of [3H]-diazepam to crude cerebral cortical synaptosomal membranes in the rat was studied. Ethylenediamine significantly increased [3H]-diazepam binding to a maximum potentiation of 154% control (EC50 = 1.8 X 10(-4) M) and was the most active compound studied in terms of both potency and the maximum potentiation observed. Potentiation of [3H]-diazepam binding by ethylenediamine analogues is dependent on carbon-chain length, appears to require two terminal amino groups, and is not observed in the rigid analogues studied. Potentiation of [3H]-diazepam binding by ethylenediamine analogues is mediated largely by a change in receptor number and not receptor affinity. Results are discussed in terms of the possible nature of the ethylenediamine binding site.

Electrophysiological effects of piperazine and diethylcarbamazine on Ascaris suum somatic muscle

1 Electrophysiological recordings were made from the bag region of Ascaris suum muscle. Membrane potential and input conductance or membrane current under voltage clamp were measured. 2 In high-Cl- Ringer, bath-applied piperazine, at concentrations greater than 10(-4)M, produced a dose-dependent and reversible increase in input conductance associated with a hyperpolarizing potential. The increase in input conductance was reduced when the preparations were bathed in low-Cl- Ringer. Gamma-Aminobutyric acid (GABA) and piperazine reversal potentials were measured with a voltage clamp on the same cells using iontophoretic application of the agonists. The reversal potentials were the same and close to the predicted Nernst Cl- potential (-65 mV). When GABA and piperazine were applied simultaneously piperazine reversibly reduced the amplitude of the control outward GABA current response. It was concluded that piperazine acts as a GABA agonist of low potency on the extra-synaptic GABA receptors of the bag, mediating an increase in Cl- conductance. 3 Acetylcholine was applied iontophoretically within 100 micron of the bag region while the preparation was bathed in a low-Ca2+, low-Cl- Ringer. The response under voltage clamp was a dose-dependent inward current associated with an increase in input conductance. This response was reversibly antagonized by 3 X 10(-5)M tubocurarine, high concentrations of diethylcarbamazine (10(-3) to 10(-2)M) but not high concentrations of piperazine (10(-3) to 10(-2)M). It was concluded that there are extra-synaptic acetylcholine receptors on the bag region of Ascaris muscle and that diethylcarbamazine but not piperazine acts as an antagonist. 4 Bath-applied diethylcarbamazine (10(-4) to 2 X 10(-3)M) produced a reversible dose-dependent depolarization of the membrane potential which was associated with an increase in the amplitude and frequency of spontaneous depolarizing potentials in active preparations at 32 degrees C to 35 degrees C in high-Cl- Ringer. The excitatory action of diethylcarbamazine was not blocked by 3 X 10(-5)M tubocurarine. Diethylcarbamazine (10(-4) to 10(-3)M) had no effect on the outward current response to GABA iontophoresis. Diethylcarbamazine (10(-4) to 10(-2)M) reversibly antagonized in a dose-dependent manner the delayed rectification of the bag membrane. In a low-Ca2+, low-Cl- Ringer, diethylcarbamazine (10(-4) to 2 X 10(-3)M) reversibly antagonized the voltage-sensitive outward current of the bag. This effect was mimicked by high-K+ Ringer or perfusion with 4-aminopyridine (10(-3) to 2 X 10(-3)M). It was concluded that diethylcarbamazine did not react with the GABA receptor but antagonized a voltage-sensitive K+ conductance.

Measurement of GABA-evoked conductance changes of lobster muscle fibres by a three-microelectrode voltage clamp technique

The effective membrane conductance and capacity of lobster muscle fibres was measured by a three-intracellular-microelectrode voltage clamp technique. Conductance values agreed well with those determined under current clamp, by means of the 'short' cable equations. Reversible increases in conductance evoked by gamma-aminobutyric acid (GABA) were reflected by differences (delta V) in electrotonic potential amplitude recorded at the centre, and midway between the centre and fibre end respectively. GABA dose--conductance curves derived from cable theory or from delta V measurements were virtually identical. The effective capacity (ceff), determined from the area beneath the 'on' delta V capacity transient, yielded values of the membrane time constant consistently lower than those obtained by the graphical method of E. Stefani & A.B. Steinbach (J. Physiol., London. 203, 383-401 (1969)); one possible explanation for this discrepancy is discussed. In the presence of GABA, the effective capacity was reduced in a dose-related manner. The results were interpreted in terms of an equivalent circuit in which surface membrane was arranged in parallel with cleft-tubular membrane of finite conductance, charged through an access resistance. GABA was though to be decreasing ceff by selectively increasing the conductance of the cleft-tubular membranes.

The effects of piperazine on rat sympathetic neurones

1. The neuronal effects of the anthelmintic piperazine (Pip) on rat sympathetic ganglia were studied in vitro by means of intracellular and extracellular recording techniques. 2. Surface potential recordings indicated that Pip (0.1-10 mM as citrate, 1-30 mM as hexahydrate) produced a sustained depolarization (reversible on washing) of rat ganglia. gamma-Aminobutyric acid (GABA, 1-100 micro M) also evoked reversible depolarizations but, unlike Pip, responses to the higher doses of GABA declined during a 2 min exposure. Depolarizations produced by Pip or carbachol (but not GABA) were markedly depressed by hexamethonium but only slightly by bicuculline or picrotoxin. 3. Intracellular recordings revealed that Pip-induced depolarizations were accompanied by an increase in membrane conductance and a broadening and depression of the directly-evoked spike. 4. In the presence of hexamethonium (1 mM), the responses to Pip hexahydrate and to cholinoceptor agonists were abolished, but Pip citrate still changed the spike configuration and induced membrane hyperpolarization with a small conductance increase. These residual effects were mimicked by superfusing with Na citrate or Ca2+-free medium, suggesting that significant Ca2+ binding by the citrate anion of the Pip salt was probably responsible for the observed activity of Pip citrate in the presence of hexamethonium. 5. It is concluded that on rat ganglia Pip is a nicotinic agonist, with no detectable GABA-mimetic activity.

The presynaptic effects of valproic acid in the isolated frog spinal cord

The effects of the anticonvulsant valproic acid (n-dipropylacetate, DPA) on frog primary afferent fibers was examined with sucrose gap recordings from the dorsal roots. Addition of DPA to the superfusate consistently reduced the amplitude and duration of the dorsal root potential. In contrast, DPA augmented the depolarization of dorsal roots produced by GABA, beta-alanine and taurine. It also decreased afferent fiber 'desensitization' to GABA. DPA depressed the ability of K+ and the excitatory amino acids glutamate and aspartate to depolarize afferent fibers. In addition, the compounds decreased the amount of K+ released by tetanic stimulation of the dorsal root. The K+-evoked release of tritiated GABA from cord slices was initially reduced by exposure to DPA, but was then unaffected after a longer application of the anticonvulsant. On the other hand, the high affinity uptake of tritiated GABA and glycine were almost totally blocked by the addition of DPA to the incubating medium. In sum, DPA has complex effects on neuronal membranes. Some of these effects may serve to explain the anticonvulsant actions of this drug.

Excitatory and inhibitory action of ibotenic acid on frog spinal motoneurones in vitro

Ibotenic acid (IBO), a conformationally-restricted analogue of the putative excitatory transmitter glutamate, produced biphasic effects on frog spinal motoneurones recorded wih DC-coupled Ag/AgCl electrodes placed on the central end of ventral roots. The initial effect consisted of a motoneuronal depolarization with an increase in excitability tested with low-intensity dorsal root stimulation (the depolarizing activity of IBO was about 6 times greater than that of glutamate). On washout, the depolarizing effect of IBO subsided and complete motoneuronal repolarization occurred; however, motoneuronal responses to glutamate and to trans-synaptic stimulation remained depressed for prolonged periods. This depressant effect was not reproduced following prolonged administrations of depolarizing doses of glutamate but it was similar to that produced by muscimol, an agonist on inhibitory GABA receptors. Unlike the muscimol-evoked depression, the IBO-induced depression was insensitive to bicuculline and was antagonized by local anaesthetics or high Mg2+ solutions. It is suggested that a substantial part of the IBO depressant action is likely due to release of long-acting inhibitory transmitters onto motoneurones.

The effect of gamma-aminobutyric acid on the input conductance and membrane potential of Ascaris muscle

1 Twin intracellular recordings were made from the bag region of Ascaris muscle in order to make conductance measurements. The preparation was bathed in a cool (22 degrees C) Ringer solution to abolish the large spontaneous depolarizing potentials and to improve stability for recording.2 The resting membrane potential was -31 +/- 1 mV, mean +/- s.e. mean (n = 17). The current-voltage plots were linear in the hyperpolarizing direction but showed evidence of delayed rectification during the application of depolarizing currents. The input conductance of the bag was measured from the slope of these plots during the application of hyperpolarizing current. The resting conductance of the bags was 2.4 +/- 0.2 muS, mean +/- s.e. mean (n = 12).3 When the preparation was perfused with gamma-aminobutyric acid (GABA) in concentrations greater than 3 muM, a dose-dependent increase in conductance associated with a hyperpolarizing potential was recorded. The log dose-response relationship obtained from 6 preparations was sigmoidal and had an ED(50) of 13 muM.4 When Cl(-) in the Ringer was replaced by SO(4) (2-), the GABA-induced conductance changes decreased and were associated with depolarizing potentials.5 Voltage responses were recorded in the bag region during the iontophoretic application of GABA to different regions of the muscle cell. The largest responses were recorded when GABA was applied to the bag region. Smaller responses were recorded when GABA was applied to the arms and syncytial regions. The responses of the bags were dose-dependent and were antagonized by bath-applied picrotoxin.6 An extrapolation method using 10(-5) M GABA suggested that the GABA reversal potential was about 30 mV more negative than the resting membrane potential. This was confirmed by means of a two microelectrode voltage clamp technique. The reversal potential was thus estimated as -61.2 +/- 2.2 mV mean +/- s.e. mean (n = 27).

Further observations on the interaction between glutamate and aspartate on lobster muscle

1 The ability of bath-applied L-glutamate to enhance subsequent depolarizations produced by bath-applied L-aspartate on lobster muscle was further investigated by means of intracellular recording techniques. 2. Increasing the conditioning glutamate concentration or exposure time produced a greater enhancement of aspartate responses. Enhancement was also dependent on the time interval between glutamate and aspartate doses and was not prevented by overnight storage of preparations in vitro. 3. The dose-depolarization curve for enhanced aspartate responses (measured at a fixed time following a given dose of glutamate) was displaced to the left along the abscissa scale relative to control, with no detectable change in limiting log-log slope. 4. Conditioning doses of kainate or domoate (but not quisqualate, aspartate, or KCl) also enhanced aspartate responses; however, their conditioning effect was little affected by increasing the concentration, exposure time, or time interval before applying aspartate. The rate of onset and decline of the enhanced aspartate response always resembled that of the previous conditioning agonist. 5. D and L-Aspartate were approximately equieffective depolarizing agents whereas D-glutamate was approximately 1/40 as potent as L-glutamate. After a conditioning dose of D or L-glutamate, responses to D or L-aspartate were enhanced. 6. In a Na+-free (Li+) medium, both the glutamate depolarization and the conditioning effect towards aspartate were largely abolished. With kainate however, Na+ was not apparently important either for evoking the kainate response or for producing the conditioning effect. 7. Bath-applied glutamate greatly enhanced and prolonged the time course of the iontophoretic aspartate potential with only a small effect on the glutamate potential; however, these effects were not maintained after washout of glutamate. In contrast, bath-application of aspartate depressed the aspartate potential while enhancing the glutamate potential. Some sites that were insensitive to iontophoretically-applied aspartate became clearly responsive to this agent during a bath-application of glutamate. 8. It is proposed that during conditioning with bath-applied glutamate, kainate or domoate, some agonist is trapped by extrajunctional sites and is subsequently displaced by bath-applied aspartate to produce the long-term enhancement effect.

Effect of piperazine on central and peripheral cholinergic synapses of the frog

The anthelmintic agent piperazine did not antagonize carbachol-induced depolarizations of the frog sartorius muscle but depressed root potentials and acetylcholine release from the frog spinal cord. We suggest that piperazine has a presynaptic site of action at central cholinergic nerve-terminals.

Receptors for gamma-aminobutyric acid (GABA) on Aplysia neurons

Aplysia neurons show 5 different types of response (three excitatory and two inhibitory) to iontophoretic application of gamma-aminobutyric acid (GABA). Four of these are associated with a membrane conductance increase, but one is associated with a conductance decrease. The most common response is a fast hyperpolarization which reverses at about--58 mV and is sensitive to manipulation of external Cl- concentration, and thus is due to a specific increase in Cl- conductance. There is an infrequent, slower hyperpolarizing response which does not reverse above about--80 mV and is insensitive to external Cl-. This response appears to result from a conductance increase to K+. Two types of depolarizing responses are associated with conductance increases. These responses differ in their latency, duration and sensitivity to curare. The more frequent is relatively rapid (peak at 1-2 sec) and is depressed by curare at high concentrations. In other neurons, GABA causes a slower response, peaking at 6-10 sec, which is not curare-sensitive. Usually for both types of response, the voltage and conductance changes are completely abolished by perfusion with Na+-free seawater, and the responses cannot be reversed with depolarization. In other neurons such as L11, the response can be reversed with depolarization, and appears to result from a conductance increase to both Na+ and Cl-. In neuron R15, GABA causes a slow depolarizing response (peak at about 9 sec) which is associated with a decreased membrane conductance, probably to K+. The classical GABA antagonists, picrotoxin and bicuculline, block Cl- responses but no others, while the fast Na+ and Cl- responses are depressed by curare. Strychnine does not affect any GABA response. The multiplicity of GABA responses, the specificity of their organization and the fact that only some neurons have receptors for GABA, argue that GABA may have a role as a neurotransmitter in Aplysia. Furthermore, the existence of several types of excitatory GABA response suggests that GABA may function both as an inhibitory and excitatory neurotransmitter.

A study of the interactions between glutamate and aspartate at the lobster neuromuscular junction

The depolarization produced by bath-applied or iontophoretically applied glutamate and aspartate were recorded from lobster muscle fibres by means of intracellular microelectrodes. 2 Bath-applied glutamate or aspartate evoked reversible, membrane depolarizations; however, responses to repeated applications of aspartate decreased progressively in amplitude until a plateau level was attained. Repeated applications of glutamate, kainate, domoate or quisqualate did not produce a similar effect. 3 After a dose of glutamate, responses to bath-applied aspartate were enhanced. Responses to other depolarizing agonists were little affected by previous administration of glutamate. Aspartate dose-depolarization curves were therefore constructed after initial aspartate responses had stabilized. The log-log transforms of the aspartate and glutamate curves had limiting slopes of 0.8 and 2.1 respectively. 4 Iontophoretic application of aspartate to single glutamate-sensitive sites produced small depolarizations with slow time course, compared with the glutamate potentials. When aspartate and glutamate were pulsed simultaneously from a twin-barrelled pipette, the resultant glutamate potential was enhanced. It is suggested that this potentiation was due to summation of agonist concentrations in the receptor region interacting with a second-order dose-response relationship. 5 Bath-applied aspartate increased the amplitude and prolonged the half-decay time of the glutamate potential. This effect was particularly noticeable when the glutamate potential was of slow time course. 6 It is proposed that bath-applied aspartate has an agonist effect whose magnitude is possibly exaggerated by concomitant release of glutamate and/or inhibition by glutamate of aspartate uptake. This agonist action of aspartate is thought to be exerted mainly on extrajunctional areas of the glutamate-sensitive sites.

The nature of potassium chloride-induced relaxations of the rat anococcygeus muscle

1. The nature of KCl-induced relaxations of the rat anococcygeus muscle was investigated. 2. The relaxations were mimicked by other K+ salts, but not by NaCl. 3. The muscle was more susceptible to the relaxant effects of KCl than the contractile effects. 4. Addition of ouabain (100 micron) had no effect on the relaxations. 5. The relaxations were abolished by tetrodotoxin (5 microgram/ml), procaine (500 micron), and by section of the inhibitory nerves. 6. The results suggest that KCl-induced relaxations are due to stimulation of the inhibitory nerves by K+.

Antagonism by some antihistamines of the amino acid-evoked responses recorded from the lobster muscle fibre and the frog spinal cord

1 The effects of some antihistamines on the lobster muscle fibre and the frog spinal cord were investigated using intracellular and extracellular recordings, respectively. 2. On lobster muscle, histamine H1-blockers reversibly antagonized responses to bath-applied glutamate, aspartate and quisqualate but not responses to gamma-aminobutyric acid (GABA). Iontophoretic glutamate potentials were also reduced. Histamine (up to 1 mM) had no effect on this preparation. 3 The H1-antagonists produced a small increase in muscle membrane conductance and a slight hyperpolarization. These effects were largely unchanged in a low C1- bathing solution. Procaine (1 mM) decreased membrane conductance and did not affect responses to GABA or glutamate. 4 The H2-antagonist burimamide blocked both glutamate and GABA-evoked responses on the lobster muscle without affecting resting potential or conductance. 5 In the frog cord, bath-applied histamine produced ventral root depolarizations and dorsal root hyperpolarizations (sometimes biphasic responses). These effects were reduced by tetrodotoxin (TTX) but not by antazoline (H1-blocker) or burimamide; the latter reversibly antagonized responses to both glutamate and GABA on TTX-treated cords while antazoline was ineffective. 6 It is suggested that antihistamines can act as non-specific amino acid antagonists by interacting at the level of the receptor-coupled ionophores.

Effect of atropine and oxotremorine on the release of acetylcholine from the frog spinal cord

The in vitro frog spinal cord has been used to study acetylcholine (ACh) release and spinal root potentials. The preparation bathed in an eserine-containing medium spontaneously released ACh into the bathing fluid. This release was enhanced by atropine in a dose-related and long-lasting manner and transiently by oxotremorine. The release rate of ACh was increased by low frequency ventral root stimulation; this increase was found to be proportionally the same after application of atropine. Oxotremorine did not modify the elctrically-evoked ACh release but blocked or reduced the effect of atropine. It is concluded that the stimulatory action of atropine on ACh output cannot be entirely explained by an interaction of atropine with presynaptic cholinergic receptors and that other indirect mechanisms (via interneurones) may play a role in this effect.

A comparative study of the effects of glutamate and kainate on the lobster muscle fibre and the frog spinal cord

1 The depolarizing actions of glutamate and its conformationally restricted analogue kainate were investigated on the lobster muscle fibre and the frog spinal cord using intracellular and extracellular recordings, respectively. 2 Bath-applied kainate was less potent than glutamate on the lobster fibre but more potent on the frog cord. From the log-log transformation of dose-response curves it was proposed that more than one glutamate molecule was necessary to activate both the lobster and the frog receptor sites. In the frog, at least three kainate molecules were thought to be required for receptor activation. 3 The ionic dependence of glutamate and kainate responses appeared different for the two tissues. 4 Some possible explanations of the differential tissue sensitivity to kainate are discussed.