The use of convulsants in studying possible functions of amino acids in the toad spinal cord

Differences in the activation of inhibitory motoneuron receptors in the frog Rana ridibunda by GABA and glycine and their interaction

Intracellular recording of potentials was used in isolated spinal cord segments from the frog Rana ridibunda to compare the inhibitory effects of GABA and glycine on the motoneuron membrane. At equal concentrations, the response (a change in membrane potential) to application of glycine was 1.5-2 times greater than the response to GABA in terms of amplitude, and EC(50) values were 0.75 and 1.57 mM, respectively. The response to simultaneous application of GABA and glycine averaged 79.1 +/- 2.4% (n = 19) of the sum of the individual responses and 130.1 +/- 1.5% (n = 19) of the glycine response (partial occlusion). Preliminary application of glycine decreased the GABA response by 85.3 +/- 0.2% (n = 10), while preapplication of GABA decreased the glycine response by only 52.9 +/- 0.3% (n = 11). The glycine and GABA responses were specifically suppressed by strychnine and bicuculline. These results provide evidence that as in mammals, amphibian motoneurons have both glycine (predominantly) and GABA(A) receptors; they also show that asymmetrical cross inhibition can occur.

Involvement of GABA(B) receptors in presynaptic inhibition of fibers of the descending projections of the spinal cord in the frog Rana ridibunda

Isolated spinal cord preparations from Rana ridibunda frogs were used for studies of the effects of the GABA(B) receptor agonists (-)-baclofen (50 and 100 microM) and GABA (4-8 mM) and the specific GABA(B) receptor antagonist 2-hydroxysaclofen (100 microM) on the transmission of signals from fibers of the ventral columns monosynaptically connected with motoneurons in segments 9 and 10. These experiments showed that (-)-baclofen (50 and 100 microM) produced significant and dose-dependent suppression of excitatory postsynaptic potentials (EPSP) in motoneurons and ventral root potentials evoked by stimulation of fibers of the ipsi- and contralateral ventral columns. The inhibitory effect of (-)-baclofen (100 microM) on descending EPSP was 35-50% blocked by the GABA(B) receptor antagonist 2-hydroxysaclofen (0.2 mM). The inhibitory effect of GABA (4-8 mM) on descending EPSP was 60% blocked by the GABA(A) receptor antagonist picrotoxin (0.05 mM). (-)-Baclofen (50 microM) and GABA (4 and 6 mM) were found to have inhibitory effects on ventral root potentials evoked by stimulation of the ipsi- and contralateral ventral columns. The data obtained here, as well as data obtained by pharmacological analysis and conditioning by stimulation of the ipsi- and contralateral ventral columns, are regarded as a significant argument supporting the existence of GABA(B) receptor-mediated presynaptic inhibition of descending fibers connected monosynaptically to spinal cord motoneurons in the frog Rana ridibunda.

Effects of the epileptogenic agent strychnine on membrane currents elicited by agonists of the NMDA and non-NMDA receptors in Xenopus oocytes

The effects of strychnine (STRY) on ion channels activated by N-methyl-D-aspartate (NMDA), kainate (KA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolpropionate (AMPA) and quisqualate (QUIS) were studied using Xenopus oocytes, microinjected with mRNA from rats' brains. STRY reduced NMDA-, KA- and AMPA-induced membrane currents in a dose-dependent manner. The effect was more pronounced with NMDA than with KA and AMPA. QUIS-induced membrane currents were not affected by STRY. The depressive effect of STRY on NMDA responses was voltage dependent. The effect of STRY on the NMDA-induced membrane currents remains unchanged when the concentration of NMDA or glycine was increased. Intracellular injection of STRY did not alter the NMDA response.

Glycine induces two distinct membrane currents in neonatal rat sympathetic preganglionic neurones in vitro

1. The effects of glycine (Gly) on neonatal (12- to 16-day-old) rat sympathetic preganglionic neurones (SPNs) in transverse (500 microns) thoracolumbar spinal cord slices were studied by whole-cell patch-clamp techniques. 2. Gly elicited three types of membrane currents when applied to SPNs by pressure ejection (100 mM; 20-180 ms pulse duration): (1) an outward current (20/94 cells); (2) an inward current (30/94); and (3) a biphasic response (44/94) consisting of an outward followed by an inward current. 3. The Gly-induced outward current (IGly,(out)) had a mean reversal potential of -67 mV, was reversed in a low (5.7 mM) chloride Krebs solution, and was reversibly eliminated by strychnine (0.1-1 microM). 4. The Gly-induced inward current (IGly,(in)) had a mean reversal potential of -41 mV, was reduced in a Na(+)-free and increased in a high (15 mM) K+ solution; strychnine at the high concentration of 1 microM reduced the response by an average of 53%. 5. The electrophysiological and pharmacological characteristics of the biphasic response suggest that it was a combined response of outward and inward currents. 6. The results show that Gly elicits two distinct membrane currents, a Cl(-)-dependent outward current and a cationic inward current, which are mediated by strychnine-sensitive and strychnine-resistant Gly receptors. Functionally, activation of both types of Gly receptors reduces neuronal excitability and attenuates synaptic transmission.

The effects of 4-aminopyridine on the cat spinal cord: rhythmic antidromic discharges recorded from the dorsal roots

In a previous paper, we have reported that 4-aminopyridine (4-AP, i.v., 10 mg/kg) induces in decerebrate spinal and paralyzed cats, a sustained rhythmic activity (2.5-8.5 Hz) in various muscle nerves. We describe here that similar discharges are recorded from the proximal stump of cut cutaneous nerves. The latter rhythmic activity arises from intense antidromic discharges in the dorsal roots. The rhythmic discharges are recorded from dorsal roots of both spinal cord enlargements as well as from thoracic roots. The rhythmic activity is highly synchronous among adjacent dorsal roots. Bilateral activity is also highly cross-correlated, but may be dissociated by unilateral stimulation of one dorsal root. It is not yet possible to determine the precise site where the antidromic discharges recorded from the dorsal roots are generated. 4-AP could act directly at the terminal level of the primary afferents or could activate interneurons impinging upon the terminals.

Glucocorticoid modulates the sensitivity of the GABAA receptor on primary afferent neurons of bullfrogs

With intracellular and voltage-clamp recording techniques, we have demonstrated that the glucocorticoids, prednisolone and hydrocortisone at a concentration of 5 microM to 1 mM, reversibly depressed gamma-aminobutyric acid (GABA)-induced responses on primary afferent neurons of bullfrogs. An analysis with dose-response curves revealed that the glucocorticoids decreased the sensitivity of the GABAA receptor in a non-competitive manner. We suggest that glucocorticoids act as an antagonist of the GABAA receptor on primary afferent neurons, probably by reducing the number of functional GABAA receptor ionic channel complexes.

Mechanical changes in the amphibian spinal cord produced by afferent volleys of nerve impulses

A volley of impulses entering the toad spinal cord via large myelinated fibers in the lumbar dorsal roots was shown to evoke a contraction of the cord, which lasted for about 100 ms or more. A volley entering the cord antidromically via the ventral roots produced only a small, brief contraction. When two electric shocks were delivered to the same dorsal roots at a short interval, the contraction associated with the second shock was small; a period of about 1 s was required for a full recovery. When two shocks were applied separately to two neighboring dorsal roots, the contraction associated with the second shock was partially or totally occluded. Electric polarization of the dorsal root fibers produced mechanical changes in the cord. The effects of magnesium salt, GABA, glutamate and several other neuropharmacological agents on the contractile process were investigated. The experimental findings suggest that the contractile process is related to the phenomenon of primary afferent depolarization.

Electrophysiological effects of GABA on fish retinal horizontal cells are blocked by bicuculline but not by picrotoxin

We have studied electrophysiologically the actions of gamma-aminobutyric acid (GABA) and related pharmacological agents on fish retinal horizontal cells by recording intracellularly from isolated retinae perfused with Ringer containing the various drugs. We show that although GABA usually hyperpolarizes the membrane potential relative to its dark level, it sometimes and particularly at higher (greater than or equal to 5 mM) concentration produces membrane depolarization, with reduction in the light evoked responses (S-potentials) in both cases. These effects are reversed by bicuculline but not by picrotoxin, although both agents antagonize GABA in many other preparations [5, 25]. The GABA uptake blocker nipecotic acid [15] hyperpolarizes horizontal cells and reduces their light evoked responses, and again these effects are reversed by bicuculline but not by picrotoxin. beta-Alanine, which blocks glial GABA transport [29], and diaminobutyric acid (DABA), which blocks neuronal GABA transport [14, 29, 31], have effects similar to those of nipecotic acid. We discuss these actions of GABA and of the other related drugs and their differential sensitivity to bicuculline and picrotoxin.

Dendrobine, an antagonist of beta-alanine, taurine and of presynaptic inhibition in the frog spinal cord

1 The effects of dendrobine and nobiline, alkaloids isolated from Dendrobium nobile, on the electrical activity and on amino acid-induced depolarizations of primary afferent terminals were tested on the frog isolated spinal cord and were compared with those of picrotoxinin and strychnine. 2 Dendrobine (3 X 10(-5) M) caused a slight hyperpolarization in both dorsal and ventral roots and this hyperpolarization was accompanied by the augmentation of the dorsal root potential (DR-DRP) and the ventral root potential and reflex (DR-VRP and DR-VRR). The amplitude of the dorsal root reflex (DR-DRR) however, was reduced significantly. Nobiline (3 X 10(-5) M) had no significant effect on either the root potentials or the reflexes. 3 Dendrobine (3 X 10(-5) M) reduced the dorsal root potential induced by repetitive antidromic stimulation of ventral root (VR-DRP) as well as diminishing the maximum rate of rise of the dorsal root potential induced by the stimulation of adjacent dorsal roots (DR-DRP), during which time the amplitude of the DR-DRP was seen to be augmented. 4 Dendrobine (3 X 10(-5) M) reduced the beta-alanine- and taurine-induced depolarizations of primary afferent terminals, while having little effect upon GABA- and glycine-induced depolarizations. 5 Dendrobine (10(-5) M) reversibly blocked the presynaptic inhibition caused by antidromic conditioning stimulation of the ventral root. 6 These effects of dendrobine were qualitatively similar to those of strychnine but were somewhat different from those of picrotoxinin, a molecule having the same picrotoxane skeleton. 7 The present results are discussed with reference to the likely neurotransmitters involved in presynaptic inhibition in the frog spinal cord, and with respect to the structure-activity relationship of picrotoxane compounds as amino acid antagonists.

Sleep-related variations of membrane potential in the lateral geniculate body relay neurons of the cat

Membrane potential of lateral geniculate body relay neurons was monitored in chronic cats during the sleep-waking cycle. Neurons were tonically depolarized throughout paradoxical (P) sleep and the maximal level of polarization occurred during slow (S) sleep (mean difference of membrane potential between S and P sleep: + 10.2 +/- 1.3 mV, n = 6, range: 8-12 mV). Some features of the spontaneous activity of S and P sleep are briefly discussed in relation to the level of membrane potential. In particular it is suggested that the phasic depolarizations underlying the bursts of action potentials during S sleep, and which are reproduced retinal cell axons impinging upon the hyperpolarized membrane.

Convulsant and possible anticholinergic actions of dendrotoxin in the amphibian spinal cord

1 Dendrotoxin (DTOX)6, 6a and 5,6-1, fractions of the venom isolated from the green mamba (Dendroaspis viridis) promoted both spontaneous and stimulus-coupled rhythmic activity and antagonized the cholinergically mediated ventral root-dorsal root potential (VR-DRP) of frog spinal cord. The different time course and reversibility of these two effects indicates that the toxin has two entirely separate sites of action on the frog spinal cord. 2 Since DTOX 6 neither blocked nor enhanced responses of ventral and dorsal roots to glutamate, gamma-aminobutyric acid (GABA), beta-alanine, glycine or aspartate, it is unlikely that its convulsant action resulted from an alteration of the postsynaptic actions of inhibitory or excitatory amino acids. 3 An alteration in the threshold for action potential generation could perhaps contribute to the convulsant action of DTOX 6, although other mechanisms such as blockade of the release of unspecified inhibitory substances cannot be excluded. 4 In addition to the lack of effect on amino acid responses, DTOX failed to block the polysynaptic DR-VRP or DR-DRP pathways, which are mediated at least in part by amino acid neurotransmitters. Although this would be consistent with a specific action of DTOX at the cholinergic synapse of the VR-DRP pathway, this site of action has not yet been demonstrated unequivocally. Other possible mechanisms whereby DTOX could block VR-DRP are discussed.

Post-synaptic conductance increase associated with presynaptic inhibition in cat lumbar motoneurones

1. Motoneurones were examined in which low-intensity p.b.s.t conditioning volleys caused a 5% or greater decrease of gastrocnemius monosynaptic e.p.s.p.s without evidence of long-lasting i.p.s.p.s on superimposed single sweeps. 2. Short constant current pulses were injected into these cells and in twenty-two of twenty-three cases the voltage decay was faster when preceded by the same p.b.s.t. conditioning stimuli which caused a decrease in the Ia e.p.s.p. 3. Comparing these decays to short pulse decays generated in a simple analogue neurone model suggested that after conditioning stimuli a tonic conductance increase had occurred which was located electrotonically remote from the soma in some cases or more diffusely in other cases. 4. Long-lasting i.p.s.p.s were brought out by averaging the baseline following conditioning stimuli in ten of fifteen cases, also suggesting a post-synaptic conductance increase. 5. Averaging the voltage response to long saturating constant current pulses showed a decreased motoneurone input resistance in three of eight cases. 6. The semilogarithmic decay of four of eleven conditioned e.p.s.p.s was more rapid than controls. 7. Although short pulse voltage decay analysis revealed consistent evidence for increased post-synaptic conductance following conditioning stimuli, it was not possible to decide if the location and extent of this conductance increase were sufficient to rule out presynaptic inhibition.

Is the pressor response to strychnine centrally mediated?

Intravenous administration of strychnine (0.003-0.300 mg . kg-1) to curarized, chloralosed dogs induced hypertension and tachycardia. In spinal cord-transected dogs, intravenous administration of strychnine no longer elicited a rise in blood pressure and heart rate. Intracisternal injections of strychnine also produced hypertension and tachycardia but at lower doses. Similar results were obtained after intrathecal administration of strychnine in doses significantly different from those effective on intravenous and intracisternal administration. These findings suggest that the central nervous system may be involved in the haemodynamic changes induced by strychnine, but did not allow the site of action to be located.

Amino acid pharmacology of mammalian central neurones grown in tissue culture

1. Spinal and cerebellar-brainstem areas of fetal mouse were dissociated and grown in tissue culture until large enough to permit stable intracellular recording. 2. The tissue-cultured neurones, growing as a monolayer and accessible under direct vision using phase contrast optics, allowed precise placement of intracellular recording and extracellular ionophoretic pipettes. 3. Ionophoresis of GABA and glutamate revealed a non-uniform distribution of responses over the cell surface, with a lack of spatial coincidence in sensitivity between the two. GABA inhibited and glutamate excited all cells tested. 4. GABA responses evoked at the cell body and on nearby process membrane were almost uniformly hyperpolarizing, while those at some peripheral process membrane were either hyperpolarizing, depolarizing or a combination of both events. All responses were associated with an increase in membrane slope conductance. 5. Membrane polarization showed that all hyperpolarizing events extrapolated to about the same inversion potential, which averaged about 9 mV more negative than resting potential (n = 95 cells). The depolarizing phases of responses evoked at peripheral membranes extrapolated to about 0 mV (n = 5 cells). 6. The hyperpolarization and increase in membrane conductance of GABA responses at the cell body were dependent on Cl- ions and the inversion potential of the response was dependent on the Cl- ion concentration gradient. The inversion potentials of GABA, glycine and beta-alanine responses were identical. 7. When matched in magnitude for evoked conductance increase, glycine responses decayed more rapidly than GABA. Glycine and beta-alanine voltage responses both decayed faster than GABA responses of comparable size. 8. In about half the cells tested sustained or rapidly repeated application of GABA and glycine transformed hyperpolarizing responses into depolarizations which were associated with a maintained conductance increase. Results from conditioning-test experiments with pairs of GABA and glycine responses suggest that the reversal of response polarity is due to a rapid redistribution of Cl- ions. 9. The limiting slope of log-log dose-response curves for GABA-induced conductance averaged about 2, while those for glutamate-induced depolarizations averaged about 1. The results suggest that two molecules of GABA and one molecule of glutamate participate in the respective post-synaptic responses. 10. The observation indicate that mammalian C.N.S. tissue grown in culture is a suitable model to study C.N.S. membrane pharmacology with increasing precision.

The effects of amino acids and antagonists on the isolated hemisected spinal cord of the immature rat

1 Records of ventral and dorsal root polarity of the isolated hemisected spinal cord of the 3-9 day old rat showed that respective dose-dependent depolarizations of motoneurones (VR responses) and primary afferent terminals (DR responses) were produced by both acidic and neutral amino acids in the presence of procaine (1 mM) or tetrodotoxin (0.1 micron). 2 Of the four neutral amino acids, gamma-aminobutyrate (GABA), glycine, taurine, and beta-alanine, GABA was the most effective in producing DR responses and glycine the most effective in producing VR responses. Only taurine depressed the electrical activity recorded from ventral roots. 3 The DR responses produced by GABA, beta-alanine and taurine were all antagonized by bicuculline (5 micron) and picrotoxin (5 micron). Bicuculline was more selective than picrotoxin in antagonizing VR responses produced by GABA. 4 Strychnine (1 micron) antagonized VR responses produced by glycine beta-alanine and taurine without affecting responses produced by GABA. DR responses to the neutral amino acids were unaffected by strychnine.

Characterization and ionic basis of GABA-induced depolarizations recorded in vitro from cat primary afferent neurones

1. Responses of single cells in the isolated cat spinal ganglion to GABA applied by superfusion or by iontophoresis were recorded using intracellular micro-electrodes. 2. Of the twelve structurally related compounds investigated, GABA was the most effective in its ability to produce a depolarization of the cell membrane. 3. Studies determining concentration-response relationships indicate that two to three molecules of GABA are required to combine with the GABA receptor for activation. 4. Bicuculline and picrotoxin, each act in a non-competitive manner to antagonize the GABA-induced membrane current. 5. The equilibrium potential for iontophoretically induced GABA depolarizations (EGABA) was found to be -23.5 plus or minys 6.1 mV. EGABA was independent upon [cl-]o, but independent of [Na+]o, [K+], or [Ca2+]o. 6. Intracellular injection of twenty antions (Br-, I-, NO2-, NO3-, ClO4-, SCN-, Bf4-, HS-, OCN-, ClO3-, BrO3-, F-, HCO2-, HSO3-, HCO3-, CH3CO2-, SO42-, C6H5O73-) indicated that the activated GABA receptor membrane was permeable to those anions whose hydrated diameter is no larger than that of ClO-3. 7. Restoration of the GABA depolarization to its control level after augmentation by Cl- injection had a mean time constant of 27.8 plus or minus 2.6 min. Picrotoxin did not alter this value. 8. When foreign anions were exchanged for Cl- in the perfusion solution, the ten anaions smaller or equal to ClO3-, decreased the GABA depolarization by 50-90% and increased its time course 1.5-2.0 x control. The only exception having a small radius was Br- which augmented the amplitude 10-30%. 9. The ten anions larger than ClO3- produced a biphasic effect, i.e. an initial augmentation followed by a marked (up to 100%) depression of the response. Experiments with CH3COO-, CH3SO4-, or HOCH2CH2SO3-, indicated that this depression was non-competitive.

Action of 5-hydroxytryptamine on isolated spinal cord of bullfrogs

Slow depolarizations of dorsal root nerve terminals and motoneurons, which were produced by 5-hydroxytryptamine (5-HT) applied directly to isolated bullfrog spinal cords, were recorded by the sucrose-gap method. These depolarizations were eliminated in the Ca-deficient Ringer's solution containing Mg, suggesting that these 5-HT depolarizations were not caused by a direct action of 5-HT on dorsal root nerve terminals or motoneurons but rather by actions of transmitters released from interneurons. Indeed, mephenesin, which is a selective blocker of polysynaptic transmission in the spinal cord, inhibited more markedly the 5-HT depolarization than the L-glutamate or GABA depolarization. The transmitter directly responsible for the generation of the 5-HT depolarization of dorsal root nerve terminals was not considered to be GABA as the 5-HT depolarization was not antagonized by picrotoxin. It would thus appear that 5-HT stimulates interneurons in the amphibian spinal cord and unknown transmitters released from these interneurons depolarize the dorsal root nerve terminals.

Bimodal action of glycine on frog spinal motoneurones

In the presence of procaine the changes in electrical potential caused by glycine in the ventral root of the isolated hemisected spinal cord of the frog have been shown to be compounded of both hyperpolarizing and depolarizing responses to the amino acid. From a comparison of the effects of changes in the perfusion medium on the potentials produced by glycine, beta-alanine and L-glutamate in the presence and absence of strychnine, it was concluded that glycine acts on a similar receptor to beta-alanine and causes a hyperpolarizing response which is blocked by strychnine. However, glycine has an additional, depolarizing, action which is usually the major effect and masks the hyperpolarizing response. The depolarizations produced by L-glutamate and glycine could be differentiated by their different ionic dependencies. The glycine depolarization was selectively decreased by a lowered sodium ion concentration in the medium while L-glutamate depolarizations were selectively enhanced by lowered potassium ion concentration.

The interaction of four putative glutamate antagonists with glutamate and their effects on the toad spinal cord

1. Four putative glutamate antagonists (L-glutamate diethyl ester, L-glutamate dimethyl ester, L-proline and 1-hydroxy-3-amino-pyrrolidone-2) were tested on the isolated hemisected toad spinal cord. 2. 1-hydroxy-3-amino-pyrrolidone-2 (10(-3)-10(-2) M) selectively antagonized the depolarizations evoked in both dorsal and ventral roots by applications of L-glutamate (5 X 10(-4) M). 3 1-hydroxy-3-amino-pyrrolidone-2 also antagonized the depolarizations evoked in both dorsal and ventral roots by stimulation of the adjacent dorsal root. 4. The dimethyl and diethyl esters of L-glutamate and L-proline had their own depolarizing actions on the dorsal and ventral roots, and neither potentiated nor antagonized the effects of L-glutamate. 5. The results with 1-hydroxy-3-amino-pyrrolidone-2 offer further evidence for the involvement of L-glutamate and L-aspartate in synaptic transmission in the amphibian spinal cord.

Pharmacological analysis of slow potentials recorded in frog olfactory bulb during natural stimulation

Pharmacological agents (strychnine, picrotoxin, pentobarbital, chloralose, GABA, penicillin, morphine) were used to investigate the nature of the slow potential recorded in the frog olfactory bulb in response to natural stimulation. Three possible hypotheses were tested: 1) The slow potential is neuroglial in nature; 2) it is the analog of the dorsal-root potential of the spinal cord and reflects depolarization of primary afferents arising in the terminals of the olfactory nerve and responsible for presynaptic inhibition in the frog olfactory bulb; 3) the slow potential reflects postsynaptic processes. The results showed great similarity between changes in the slow and dorsal-root potentials of the spinal cord in response to the action of pharmacological agents. However, the slow potential is evidently a complex response and incorporates at least one other component - depolarization of the dendrites of unknown nature.

Inhibitory postsynaptic actions of taurine, GABA and other amino acids on motoneurons of the isolated frog spinal cord

The actions of glycine, GABA, alpha-alanine, beta-alanine and taurine were studied by intracellular recordings from lumbar motoneurons of the isolated spinal cord of the frog. All amino acids tested produced a reduction in the amplitude of postsynaptic potentials, a blockade of the antidromic action potential and an increase of membrane conductance. Furthermore, membrane polarizations occurred, which were always in the same direction as the IPSP. All these effects indicate a postsynaptic inhibitory action of these amino acids. When the relative strength of different amino acids was compared, taurine had the strongest inhibitory potency, followed by beta-alanine, alpha-alanine, GABA and glycine. Topically applied strychnine and picrotoxin induced different changes of post-synaptic potentials, indicating that distinct inhibitory systems might be influenced by these two convulsants. Interactions with amino acids showed that picrotoxin seletively diminished the postsymaptic actions of GABA, while strychnine reduced the effects of taurine, glycine, alpha- and beta-alanine. But differences in the susceptibility of these amino acid actions to strychnine could be detected: the action of taurine was more sensitively blocked by strychnine compared with glycine, alpha- and beta-alanine. With regard to these results the importance of taurine and GABA as transmitters of postsynaptic inhibition on motoneurons in the spinal cord of the frog is discussed.

Ventral root responses of the hemisected amphibian spinal cord to perfused amino acids in the presence of procaine

The use of the procaine-blocked hemisected spinal cord preparation to identify the primary action of amino acids and their antagonists on amphibian mononeurones is described. 2 Apart from an anomalous effect of glycine, the responses of frog spinal motoneurones to amino acids were shown to be similar to those of mammalian spinal neurones. 3 In the presence of procaine, gamma-aminobutyrate (GABA), taurine and beta-alanine caused a hyperpolarizing response, measured in ventral roots, whereas L-glutamate and, to a lesser extent, glycine caused depolarization. 4 Picrotoxin and bicuculline specifically blocked ventral root responses to GABA; strychnine blocked responses to taurine and beta-alanine but not responses to L-glutamate, glycine or GABA.

The effect of intravenously administered gamma-aminobutyric acid on afferent fiber polarization

(1) The effect of intravenously administered gamma-aminobutyric acid (GABA) on afferent fiber polarization in the feline spinal cord was ascertained from fluctuations induced in the DC level of dorsal root filaments. (2) A dose-related depolarization of the filament, with a concomitant reduction in the magnitude of the dorsal root potential, was observed after 50 and 100 mg/kg GABA. (3) GABA also depolarized filaments of preparations in which interneuronal activity was suppressed by pretreatment with tetrodotoxin. Since the magnitude of the depolarization induced in these preparations was equal to that observed in nonpretreated animals, it is likely that the depolarization in the latter preparations reflects a direct effect on afferent terminals or fibers rather than an action on interneurons. (4) GABA failed to depolarize filaments in animals pretreated with bicuculline. This suggests that intravenously administered GABA interacted with receptors that are identical with or similar to those involved in neurally evoked primary afferent depolarization (PAD). (5) The direct depolarization of afferent fibers by intravenous GABA and the blockade thereof by bicuculline are characteristics compatible with those of the endogenous axo-axonic transmitter operating in pathways mediating neurally evoked PAD. These data, therefore, support the involvement of GABA at this synapse in the mammalian spinal cord.

Presynaptic action of barbiturates in the frog spinal cord

The action of pentobarbital on primary afferents of the isolated frog spinal cord was analyzed with sucrose gap and intracellular recordings techniques. Pentobarbital in concentrations generally considered to be in the anesthetic range greatly prolonged presynaptic inhibition and also depolarized primary afferents. The depolarization was accompanied by an increase in excitability and resulted from activation of gamma-aminobutyric acid receptors, possibly by a direct action on these receptors, since the depolarization was reversibly blocked by gamma-aminobutyric acid, but not by glycine, antagonists, and magnesium ions. Furthermore, dorsal root ganglion cells exhibited a reduced sensitivity to both gamma-aminobutyric acid and pentobarbital after a "desensitizing" dose of gamma aminobutyric acid. The prolongation of presynaptic inhibition and the activation of gamma-aminobutyric acid receptors on primary afferents by pentobarbital should act to reduce the amount of transmitter released from the first synapse in sensory pathways.

Studies on convulsants in the isolated frog spinal cord. I. Antagonism of amino acid responses

1. The isolated frog spinal cord was used to study the effects of picrotoxin, bicuculline, and strychnine on the responses of primary afferents to amino acids. Recording was by sucrose gap technique. 2. A series of neutral amino acids was found to depolarize primary afferents. Optimal activity was obtained by an amino acid whose carboxyl and amino groups were separated by a three-carbon chain length (i.e. GABA). Amino acids with shorter (i.e. beta-alanine, glycine) or longer (i.e. delta-aminovaleric acid, epsilon-aminocaproic acid) distances between the charged groups were less potent. Imidazoleacetic acid was the most potent depolarizing agent tested. 3. Picrotoxin and bicuculline antagonized the primary afferent depolarizations of a number of amino acids tested with equal specificity. Depolarizing responses to standard (10- minus 3 M) concentrations of beta-alanine and taurine were completely blocked by these convulsants, while depolarizations to 10- minus 3 gamma-aminobutyric acid (GABA) were only partially antagonized. Glycine responses were unaffected by these agentsk; Strychnine completely blocked beta-alanine and taurine depolarizations and incompletely antagonized several other neutral amino acids. GABA, glutamate, and glycine depolarizations were not affected. 5. These results suggest that there are at least three distinct populations of neutral amino acid receptors on primary afferent terminals: a GABA-like receptor, a taurine/beta-alanine receptor, and a glycine-like receptor. The strychnine resistance of the glycine responses indictaes that the primary afferent receptors for glycine differ from those on the somata of spinal neurones.

Bicuculline and the frog spinal cord

1 The effects of bicuculline on dorsal and ventral root activity and upon the depressant effect of gamma-aminobutyric acid (GABA) and glycine on ventral root responses have been studied on the isolated spinal cord of the frog.2 In the absence of stimulation, the alkaloid induced a variety of activity of which the most notable was phasic simultaneous slow wave depolarization in the dorsal and ventral roots which could be reduced or suppressed by magnesium.3 With low concentrations of bicuculline, the adjacent dorsal root response evoked by a single stimulus was depressed maximally before an increase in the ventral root response could be discerned.4 The bicuculline-induced dorsal root activity (in the absence of stimulation) was still apparent at times when the evoked dorsal root response was reduced.5 Bicuculline did not differentiate between the depressant effects of GABA and glycine on the evoked ventral root responses.6 The excitant effects of bicuculline reported here did not appear to be attributable to specific antagonism of the postsynaptic depressant action of GABA.