On the probable absence of GABA receptors on the terminations of motor axon collaterals in the cat spinal cord

When administered microelectrophoretically, GABA and the GABA-mimetic piperidine-4-sulphonic acid (P4S) appear to have no direct hyperpolarizing or depolarizing effect on the terminations of motor axon collaterals excited electrically in the ventral horn of the lumbar spinal cord of the cat. This lack of effect on axon terminals of motoneurones, which contrasts with the bicuculline-sensitive depolarization by P4S of the spinal terminals of primary afferent fibres, is consistent with previous reports of the probable absence of pharmacologically detectable GABA receptors on the spinal terminals of other central excitatory neurones, namely those of the red and lateral vestibular nuclei.

Pyridazinyl-GABA derivatives as GABA and glycine antagonists in the spinal cord of the cat

Of two arylaminopyridazine derivatives of gamma-aminobutyric acid (GABA) tested as antagonists of the inhibitory actions of glycine and GABA in the spinal cord of pentobarbitone-anaesthetized cats, one - SR95531 - was sufficiently selective to be of use in microelectrophoretic investigations of GABA-mediated synaptic transmission.

Glycine antagonists. Synthesis, structure, and biological effects of some bicyclic 5-isoxazolol zwitterions

The bicyclic 5-isoxazolol zwitterions 4,5,6,7-tetrahydroisoxazolo[4,3-c] pyridin-3-ol (3, iso-THPO), 5,6,7,8-tetrahydro-4H-isoxazolo [4,3-c]azepin-3-ol (12, iso-THAO), and 5,6,7,8-tetrahydro-4H-isoxazolo [3,4-c]azepin-3-ol (13, iso-THIA), which are structurally related to the glycine antagonist 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol (iso-THAZ), have been synthesized and tested biologically. All of these compounds were glycine antagonists approximately equipotent with iso-THAZ during microelectrophoretic ejection near cat spinal neurons. In contrast to iso-THAZ, which also interacts with 4-aminobutyric acid (GABA) receptors in rat brains, neither 12 nor 13 show any significant affinities for GABA binding or uptake mechanisms in vitro. The glycine antagonist 3 was, however, shown also to be a moderately potent inhibitor of GABA uptake. The structure of 12 was established by an X-ray analysis. The bond lengths of the 5-isoxazolol anionic moiety of 12 are in agreement with a pronounced delocalization of the negative charge of this compound.

Ibotenic acid analogues. Synthesis and biological testing of two bicyclic 3-isoxazolol amino acids

The bicyclic 3-isoxazolol amino acids (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-4-carboxylic acid (5, 4-HPCA) and (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-6-carboxylic acid (11, 6-HPCA) were synthesized as model compounds for studies of the structural requirements of central excitatory amino acid neurotransmitter receptors. 4-HPCA was synthesized via introduction of a methoxycarbonyl group into the 4-position of the lithiated N-nitroso intermediate 1. The key reaction in the synthesis of 6-HPCA is an intramolecular N-alkylation of the appropriately substituted acetamidomalonate derivative 7 using sodium hydride as a base. On the basis of the pKA values for 4-HPCA the existence of an intramolecular hydrogen bond in the zwitterionic form of this amino acid is proposed. 6-HPCA was shown by 1H NMR spectroscopy to adopt preferentially a conformation with the carboxylate group in an equatorial position. 4- and 6-HPCA were tested as agonists and antagonists at excitatory amino acid receptors on neurones in the cat spinal cord using microelectrophoretic techniques. Neither compound showed significant effects at these receptors.

A neurophysiological analysis of the effect of kainic acid on nerve fibres and terminals in the cat spinal cord

Kainic acid was administered micro-electrophoretically in relatively small amounts (approx. 0.15 nmol) near gastrocnemius motoneurones in the spinal cord of cats anaesthetized with sodium pentobarbitone. After initial excitation, extracellularly recorded orthodromic and antidromic field potentials were reduced. Such neurophysiological evidence for motoneuronal damage or death persisted for 5 h, the longest period of observation. At the site of administration, the terminations of gastrocnemius group Ia afferent fibres were electrically inexcitable for approximately 1 h. Subsequently, the number and excitability of these terminations appeared to be normal, as were the depolarizing actions at bicuculline-sensitive receptors of micro-electrophoretic piperidine-4-sulphonic acid and of gamma-aminobutyric acid (GABA) released at axoaxonic synapses on these terminations. Central myelinated and non-myelinated fibres and terminals of muscle group Ia afferent fibres, and the synaptic release of GABA on these terminals at axo-axonic synapses formed by certain spinal interneurones, thus appear to be relatively insensitive to kainic acid administered in amounts which damage or destroy motoneurones.

GABA agonists. Resolution, absolute stereochemistry, and enantioselectivity of (S)-(+)- and (R)-(-)-dihydromuscimol

(RS)-5-(Aminomethyl)-2-isoxazolin-3-ol (dihydromuscimol, DHM) is a potent 4-aminobutyric acid (GABA) agonist, the inhibitory effects of which on neurons are sensitive to the antagonist bicuculline methochloride (BMC), and it also interacts with the GABA uptake system in vitro. (S)-(+)-DHM (4) and (R)-(-)-DHM (5) were obtained in optically pure forms via resolution of tert-butyloxycarbonyl-protected DHM (1) using cinchonidine as the only resolving agent. The optical purity and absolute stereochemistry of 4 and 5 were established by chemical correlation to the (S)-(+) enantiomer of 3-hydroxy-4-aminobutyric acid (GABOB). While 4 was a specific and potent BMC-sensitive GABA agonist in vivo and in vitro, possibly the most potent GABA agonist so far described, the inhibition of GABA uptake by DHM proved to reside exclusively in the (R)-(-) enantiomer (5). The affinity of 5 for BMC-sensitive GABA receptor sites in vitro was some 50 times lower than that of 4. Compounds 4 and 5 can be considered semirigid isosteres of the conformationally flexible GABA analogues (S)-(+)- and (R)-(-)-GABOB, respectively, which show a very low degree of enantioselectivity with respect to GABA synaptic mechanisms. This correlation between the degree of enantioselectivity and conformational mobility of chiral GABA analogues might be of importance for the design of new drugs with specific actions at synapses at which GABA is the transmitter.

Synthesis and structure-activity studies on excitatory amino acids structurally related to ibotenic acid

With use of ibotenic acid as a lead, analogues of (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and of (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-7-carboxylic acid (7-HPCA) were synthesized and tested as excitants of neurons in the cat spinal cord by using microelectrophoretic techniques and as inhibitors of the binding of kainic acid in vitro. Like AMPA and 7-HPCA, (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridine-5-carboxylic acid (10, 5-HPCA) and (RS)-3-hydroxy-5-(bromomethyl)isoxazole-4-propionic acid (11, ABPA) proved to interact potently and selectively with central quisqualic acid receptors, assumed to represent physiological glutamic acid receptors. Analogues of 7-HPCA or 10, in which one or both of the acid groups were masked, were very weak or inactive as neuronal excitants and had no antagonistic effects at excitatory amino acid receptors. The structure of 7-HPCA in the crystalline state was established by X-ray analyses. The preferred conformation of 10 in aqueous solution was determined by 1H NMR spectroscopy. On the basis of these studies, 7-HPCA as well as 10 were shown to adopt preferentially conformations with the carboxylate groups in equatorial positions. It is suggested that AMPA, 7-HPCA, and 10 interact with quisqualic acid receptors in conformations essentially reflecting active conformation(s) of glutamic acid at these receptors.

Glycine antagonism by RU 5135

In the spinal cord of the pentobarbitone-anaesthetised cat, microelectrophoretic RU 5135, a steroid derivative, was a potent strychnine-like glycine antagonist, having less effect on the inhibition by GABA of neuronal firing.

The differential effects of baclofen on segmental and descending excitation of spinal interneurones in the cat

Intravenous baclofen (1-6.25 mg kg-1) substantially reduced the monosynaptic excitation of neurones in the intermediate nucleus of the cat spinal cord by impulses in group I extensor muscle primary afferent fibres, but had little or no effect on excitation by stimulating fibres of the ipsilateral dorsolateral funiculus or the contralateral red nucleus. Relatively low concentrations of baclofen thus appear not to influence the release of excitatory transmitter from the terminals of rubrospinal, corticospinal and long descending propriospinal fibres, in contrast to the reduction of the release of primary afferent transmitters.

Securinine alkaloids: a new class of GABA receptor antagonist

Experiments were undertaken to determine the site of action of securinine and related convulsant indolizidines. All of these compounds induced tonic seizures in mice, with CD50 values ranging from 11 to 87 mg/kg. The CD50 for bicuculline was found to be 8 mg/kg. Equilibrium binding assays revealed that securinine and dihydrosecurinine inhibit [3H]GABA binding to rat brain membranes with an IC50 of approximately 50 microM, which is some 7 times less potent than bicuculline. Allosecurinine and virosecurinine have IC50 values greater than 1 mM. Both dihydrosecurinine and securinine inhibited GABA-stimulated benzodiazepine binding in rat brain membranes, though they were somewhat weaker than bicuculline in this respect. Other binding assays revealed that securinine and its analogs were inactive as inhibitors of bicuculline-insensitive GABA binding, benzodiazepine, cholinergic muscarinic, and beta-adrenergic receptor binding. In addition, while thiocyanate ion increased the apparent binding potency of bicuculline 10-fold, it had little effect on that of securinine. Extracellular electrophysiological studies on neurons in the cat spinal cord indicated that securinine and dihydrosecurinine blocked the inhibitory action of GABA while having no effect on that of glycine. Allo- and virosecurinine were much less active as GABA receptor antagonists in this test. These results suggest that, like bicuculline, securinine and dihydrosecurinine are selective antagonists of GABA recognition sites on mammalian central neurons.

GABA-mimetic activity and effects on diazepam binding of aminosulphonic acids structurally related to piperidine-4-sulphonic acid

The relationship between structure, in vivo activity, and in vitro activity of some analogues of the gamma-aminobutyric acid (GABA) agonist piperidine-4-sulphonic acid (P4S) was studied. The syntheses of 1,2,3,6-tetrahydropyridine-4-sulphonic acid (DH-P4S) and (RS)-pyrrolidin-3-yl-methanesulphonamide (PMSA-amide) are described. Like P4S, its unsaturated analogue DH-P4S and the five-ring isomer (RS)-pyrrolidin-3-yl-methanesulphonic acid (PMSA) were bicuculline methochloride (BMC)-sensitive inhibitors of the firing of neurones in the cat spinal cord. Whereas isonipecotic acid was less potent than its unsaturated analogue isoguvacine as a GABA-mimetic and as an inhibitor of GABA binding, the opposite relative potencies of P4S and DH-P4S were observed, P4S being proportionally more potent than DH-P4S. In contrast with P4S and DH-P4S, PMSA, which is an analogue of the potent GABA uptake inhibitor and BMC-sensitive GABA-mimetic homo-beta-proline, was a relatively weak inhibitor of GABA uptake in vitro. PMSA-amide was more than two orders of magnitude weaker than PMSA as an inhibitor of GABA binding and did not significantly affect GABA uptake in vitro. The effects of 3-aminopropanesulphonic acid (3-APS), PMSA, P4S, and DH-P4S on the binding of [3H]diazepam in vitro at 30 degrees C, in the presence or absence of chloride ions, were studied and compared with those of the structurally related amino acids GABA, homo-beta-proline, isonipecotic acid, and isoguvacine. Under these conditions the aminosulphonic acids were weaker than the respective amino acids in enhancing [3H]diazepam binding, the difference being more pronounced in the absence of chloride.

The effect of GABA on lumbar terminations of rubrospinal neurons in the cat spinal cord

Although GABA and piperidine-4-sulphonic acid depolarize I a afferent terminations in the cat spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for a bicuculline-sensitive alteration by either gabamimetic of the electrical threshold of rubrospinal terminations in the spinal intermediate nucleus. The terminal axonal arborizations in the spinal cord of neurons in the red nucleus thus do not have GABA receptors similar to those on the cell bodies. The results are discussed in relation to the depolarizing action of GABA on some central neurons, and on neurons with peripheral cell bodies, and to probable differences in the intracellular chloride content of neurons having peripheral or central cell bodies, and thus of different embryological origin. A presynaptic depolarizing inhibitory process mediated by GABA appears to be confined to the terminals of primary afferent fibres in the mammalian central nervous system.

The effects of naloxone, morphine and methionine enkephalinamide on Ia afferent terminations in the cat spinal cord

Naloxone, morphine, Met5-enkephalinamide (MENKA) and procaine were administered microelectrophoretically near extracellularly stimulated extensor muscle group Ia afferent fibres and terminations in the lumbar spinal cord of cats anaesthetized with pentobarbitone sodium. Observations were made of effects on the electrical threshold, on the depolarizing action of GABA or piperidine-4-sulphonate (P4S), and on bicuculline-sensitive primary afferent depolarization (PAD) generated by tetanic stimulation of flexor muscle low threshold afferents. All 4 agents reversibly elevated the threshold of Ia fibres in the dorsal column and Ia terminations in the ventral horn. The depolarizations of terminations by GABA or P4S were also reduced, an effect, which for all except MENKA, probably accounted for a concomitant reduction in PAD. In the absence of a consistent effect on either threshold or depolarization by GABAmimetics, MENKA reversibly diminished PAD, an action blocked by naloxone. Intravenously administered naloxone, in doses known to enhance spinal monosynaptic excitation in the cat, had no effect on GABAergic PAD and little or no effect on Ia termination threshold. The results are discussed in relation to a naloxone-sensitive effect of MENKA which reduces transmitter release from GABAergic axo-axonic synapses on Ia terminals, but which does not account for the enhancement of spinal reflexes by naloxone.

Depolarization of feline primary afferent fibres by acidic amino acids

When administered micro-electrophoretically into the spinal grey matter of cats anaesthetized with pentobarbitone, acidic amino acids known to be neuronal excitants lower the threshold of electrically stimulated muscle and cutaneous primary afferent fibres and terminations. This depolarizing effect was not observed with fibres stimulated in the white matter. Depolarization by micro-electrophoretic potassium and excitant amino acids appeared not be be associated with an alteration in terminal membrane conductance since there was no change in synaptically evoked primary afferent depolarization. Excitant amino acid depolarization was not blocked by the gamma-aminobutyric acid antagonist bicuculline methochloride, but was reduced by selective excitant amino acid antagonists. The results are discussed in relation to the probable absence of specific excitant amino acid receptors on afferent terminals, the depolarizing effect of the amino acids on myelinated fibres and non-myelinated terminals being more likely a consequence of changes in the extracellular medium associated with the depolarization and firing of neurones.

Ibotenic acid analogues. Synthesis and biological and in vitro activity of conformationally restricted agonists at central excitatory amino acid receptors

A number of analogues of ibotenic acid [(RS)-3-hydroxy-5- isoxazoleglycine ] were synthesized; they were tested as excitants on neurons in the cat spinal cord, by using microelectrophoretic techniques, and as inhibitors of the binding of kainic acid (KA) in vitro, by using synaptic membranes prepared from rat brains. The excitatory effects of the 3- isoxazolol amino acids (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5, 4-c]pyridine-7-carboxylic acid (4, 7- HPCA ), (RS)-alpha-amino-3-hydroxy-5,6-dihydro-4H- cyclohept [1,2-d] isoxa zole - 8-propionic acid (8, 8- AHCP ), (RS)-alpha-amino-3- hydroxy-7,8-dihydro-6H- cyclohept [1,2-d] isoxazole -4-propionic acid (12, 4- AHCP ), and (RS)-alpha-(methylamino)-3-hydroxy-5-methyl- 4- isoxazolepropionic acid (15, N-Me-AMPA) were shown to be sensitive to (S)-glutamic acid diethyl ester (GDEE), an antagonist at quisqualic acid ( QUIS ) receptors, and insensitive to (RS)-2-amino-5-phosphonovaleric acid ( 2APV ), an antagonist at N-methyl-(R)-aspartic acid (NMDA) receptors. The compounds 4 and 12 proved to be particularly potent agonists at the former class of receptor, assumed to represent physiological glutamic acid receptors. The amino acids (RS)-beta-(2-carboxyphenyl)alanine (19), an analogue of 12, and (RS)-2-(3-carboxyphenyl) glycine were weak GDEE-sensitive excitants with potencies comparable with that of 8. All of the compounds were tested as inhibitors of KA binding. With the exception of 12 and 19, which showed very low affinity for the KA binding sites, the compounds studied were inactive in this in vitro test system.

The effect of GABA on the terminations of vestibulospinal neurons in the cat spinal cord

Although GABA and piperidine-4-sulphonate depolarize Ia afferent terminations in the lumbar spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for a bicuculline-sensitive alteration by either GABAmimetic of the excitability of vestibulospinal terminations. This suggests that the terminal arborizations of vestibulospinal fibers, unlike their cell bodies, are devoid of GABA receptors having properties similar to those on either central neuron cell bodies, primary afferent cell bodies or their central terminations.

Actions of midazolam in the spinal cord of the cat

Midazolam, which induces anaesthesia in humans at intravenous doses of 0.3 mg/kg, did not anaesthetize cats at doses of 20 mg/kg. Nevertheless, intravenous doses as small as 0.3 mg/kg enhanced spinal primary afferent depolarization and presynaptic inhibition of spinal monosynaptic reflexes, and both intravenous and microelectrophoretic administration of midazolam enhanced the inhibitory effect of GABA on spinal neurones and the depolarization of Ia afferent terminations by GABA and piperidine-4-sulphonate. Some degree of specificity was demonstrated for the inhibitory effect of GABA in relation to those of glycine and noradrenaline, and the enhancement by midazolam of inhibition by GABA was blocked by R015-1788, which alone was inactive. Although these results are consistent with proposals that depressant benzodiazepines enhance the effectiveness of GABA as a central transmitter, such an effect alone may not fully account for the anaesthesia produced by midazolam in humans.

Preparation of peptides containing any desired amino acid: methionyl peptides of bovine rhodopsin

A general method is described which allows the identification and preparation of peptides containing any amino acid of interest. The method has been applied to isolation of the methionyl peptides from a peptic digest of oxidized bovine rhodopsin. The peptide digestion mixture is first partially separated by ion exchange column chromatography. Location of peptides containing the desired amino acid is performed by amino acid analysis of acid hydrolyzed column fractions by high voltage paper electrophoresis. Peptides are further purified and prepared by peptide mapping, elution, and amino acid analysis using inexpensive high capacity techniques. Peptide sequencing is performed by a manual dansyl-Edman method well adapted for rapidly processing large numbers of samples. The methods are particularly well suited for detection and preparation of peptides containing amino acids for which there is no specific detection method.

4,5,6,7-Tetrahydroisothiazolo[5,4-c]pyridin-3-ol and related analogues of THIP. Synthesis and biological activity

The thio analogues of the GABA (gamma-aminobutyric acid) agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol), the GABA uptake inhibitor THPO (4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol), and the glycine antagonist THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol) have been synthesized and tested biologically on single neurons in the cat spinal cord and in vitro by using synaptic membrane preparations obtained from rat brains. In contrast to THIP, thio-THIP (4,5,6,7-tetrahydroisothiazolo[5,4-c]pyridin-3-ol, 5) was only a weak GABA agonist. Thio-THPO (4,5,6,7-tetrahydroisothiazolo[4,5-c]pyridin-3-ol, 10) was slightly weaker than THPO as an inhibitor of GABA uptake in vitro, and these two compounds were approximately equipotent in enhancing the inhibition of the firing of cat spinal neurons by GABA. Like THAZ and structurally related bicyclic isoxazole zwitterions, thio-THAZ (5,6,7,8-tetrahydro-4H-isothiazolo[4,5-d]azepin-3-ol, 15) was an antagonist at glycine receptors on cat spinal neurons. The I/U ratios, which reflect the ability of neutral amino acids to penetrate the blood-brain barrier (BBB), were calculated for 5 (I/U = 16), 10 (63), and 15 (200). These low I/U ratios, compared with the findings that THIP (I/U = 500 or 1500) and THPO (I/U = 2500) enter the brain after systemic administration, suggest that the thio analogues may penetrate the BBB very easily.

Spinal interneurone depression by DS103-282

The depressant effect of 5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiodiazole (DS103-282) on the polysynaptic excitation of interneurones in the cat spinal cord appears to be related to a postsynaptic reduction in the effectiveness of excitatory transmitters than to interference with their presynaptic release.

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.

Post-excitatory depression of neuronal firing by acidic amino acids and acetylcholine in the cat spinal cord

In the spinal cord of cats anaesthetized with pentobarbitone, the excitation of interneurones and Renshaw cells by acidic amino acids or acetylcholine (Renshaw cells) is followed by a period of depressed excitability. This depression appears to be the consequence of prolonged repetitive firing rather than of the enzymic or chemical conversion of the excitants to neuronal depressants.

Effects of noradrenaline and 5-hydroxytryptamine on spinal Ia afferent terminations

When administered microelectrophoretically, noradrenaline (NA) and 5-hydroxytryptamine (5-HT) increased the thresholds of the terminal portions of extensor muscle Ia afferents stimulated extracellularly near lumbar motoneurons of anesthetized cats. This effect, and the concomitant increase in the electrical resistance of the extracellular medium near the orifices of multibarrel micropipettes, could be reversibly altered by ouabain. The results suggest further evidence is required of a direct effect of these amines at transmitter-related receptors on Ia terminations since the observed increase in threshold may be indirect, resulting from the sodium-dependent uptake of the administered amines by neurons and glia.

The structure of bovine rhodopsin

We have isolated 16 peptides from a cyanogen bromide digest of rhodopsin. These cyanogen bromide peptides account for the complete composition of the protein. Methionine-containing peptides from other chemical and enzymatic digests of rhodopsin have allowed us to place the cyanogen bromide peptides in order, yielding the sequence of the protein. We have completed the sequence of most of the cyanogen bromide peptides. This information, in conjunction with that from other laboratories, forms the basis for our prediction of the secondary structure of the protein and how it may be arranged in the disk membrane.

The dual effects of GABA and related amino acids on the electrical threshold of ventral horn group Ia afferent terminations in the cat

Amino acids were administered microelectrophoretically near the unmyelinated terminations of extensor muscle Ia afferent terminations stimulated electrically in the vicinity of lumbar motoneurones in anaesthetized cats. The predominant effect of one group (structurally related to GABA, poor substrates for in vitro amino acid uptake systems) was a reduction in the threshold (depolarization). The second group (including GABA and structural analogues which are substrates for GABA transport systems in vitro) had biphasic effects, an initial reduction being followed by an increase in threshold. The third group (structurally unrelated to GABA, substrates for amino acid uptake systems) only increased Ia termination thresholds. Reductions in termination thresholds, but not increases, were associated with diminution of synaptically evoked primary afferent depolarization, and were decreased by bicuculline methochloride. Many amino acids increased the electrical resistance of the intraspinal medium near the orifices of the barrels of seven barrel micropipettes, and for L-histidine, one of the third group of amino acids, both this effect and the increased threshold of terminations were reversibly modified by microelectrophoretic ouabain. These observations suggest that GABA-mimetics depolarize Ia terminations by interacting with bicuculline-sensitive receptors similar to those at hyperpolarizing gabergic synapses upon spinal interneurones. In addition, under the experimental conditions used, these and other amino acids increase termination thresholds, probably in the absence of any change in membrane conductance, an effect resulting from alterations in the ionic constitution of the extracellular medium around the orifices of micropipettes ejecting amino acids consequent upon the ouabain-sensitive co-transport of amino acids and sodium ions into neurones and glia.