A pharmacological study of the depression of spinal neurones by glycine and related amino acids

Comparison of the effects of central administration of serine and glycine on body temperature of the rabbit

In previous research, intracerebroventricular (ICV) injection of serine (1-4 mg) caused dose-related hypothermia in rabbits and reduced rises in body temperature caused by leukocytic pyrogen (LP) and prostaglandin E2 (PGEs). Since serine is the major precursor of the putative neurotransmitter glycine, these effects of serine may be due to its conversion to glycine. To assess this possibility, glycine was administered centrally to see if its effects on body temperature are similar to those of serine. ICV injections of glycine (0.25-1.0 mg) caused dose-related decreased in body temperature in a 10 degrees C environment but had no significant effect at 23 degrees C. Glycine (1 mg) delayed the normal rise in temperature in 30 degrees C environment and reduced LP fever and PGE2 hyperthermia. ICV glycine and serine in combination were, however, subadditive in producing hypothermia, which suggests that these amino acids act at different central sites. A difference in the interaction of serine and glycine with strychnine in producing hypothermia also suggests that the action of serine is not entirely mediated by glycine. Since serine and glycine have similar effects on normal body temperature and fever some portion of the effects of serine may be caused by its conversion to glycine. The subadditivity data and the difference in temperature effects when the amino acids are given with strychnine suggest that serine and glycine probably have separate central sites of action.

Further studies of the chemical sensitivity of the oscillatory potentials of the electroretinogram (ERG) I. GABA- and glycine antagonists

The oscillatory potentials (OPs) of the mudpuppy ERG were studied to evaluate the effects of GABA and glycine antagonists. Upon exposing the retina to bicuculline and picrotoxin, blocking agents of the putative inhibitory neurotransmitter GABA, all the OPs were selectively abolished. The earlier OPs (01-03) appeared more sensitive to the drug than the latter ones (04-05). There was no appreciable effect on the range of sensitivity and saturation level of the b-wave. Low concentrations of strychnine, blocking the effect of the putative inhibitory neurotransmitter glycine, produced a selective decrease of the amplitude of the OPs. The first (01) appeared less sensitive to the drug than the later ones. Higher concentrations extinguished all the OPs but also decreased the suprathreshold amplitude of the b-wave. In conclusion, the present results in agreement with previous work, suggest that the OPs appear to have a different origin from the b-wave and seem to be generated by inhibitory feed-back circuits within retina. The differential sensitivity of the individual oscillatory peaks indicate that perhaps chemically different synaptic activities might underlie the individual oscillatory potentials.

Electrophysiological analysis of taurine and glycine action on neurons of the midpuppy retina. I. Intracellular recording

Intracellular recordings were carried out in the prefused retine-eyecup preparation of the mudpuppy. Taurine and glycine were added to the bathing medium to study their effects on different retinal neurons. In a few cases, gamma-aminobutyric acid was exogenously applied to compare GABA vs taurine/glycine action. Receptors and horizontal cells were relatively insensitive to taurine/glycine, while amacrines and ganglion cells were comparatively more sensitive to these agents. Bipolar cells proved to be differentially effected by inhibitory amino acids: hyperpolarizing (OFF) bipolars were depressed by taurine/glycine and proved less sensitive to GABA; depolarizing (ON) bipolars were suppressed by GABA and were comparatively less sensitive to glycine/taurine. Taurine and glycine had identical actions on neurons and both were about equally effective at the same concentration. Strychnine blocked the action of taurine and glycine. The patterns of glycine/taurine sensitivity and their effects on second order neurons eliminate taurine as a photoreceptor transmitter; one or both of these agents may be utilized by a subclass of amacrine cells which interact with hyperpolarizing bipolars, other amacrine cells and ganglion cells. It appears that taurine or glycine or both may be selectively involved in OFF channel activity, while GABA may subserve an equivalent role for the ON channel.

Suppression of serotonergic neuronal firing by alpha-adrenoceptor antagonists: evidence against GABA mediation

Recent pharmacological studies have shown that administration of alpha-adrenoceptor antagonists, either systemically or locally in the vicinity of 5-HT cells of the dorsal raphe, suppresses their firing activity. In light of the prominent NE innervation of the dorsal raphe nucleus, these findings suggest that blockade of NE transmission in the dorsal raphe by these drugs underlies the suppression produced. The finding that systemic administration of picrotoxin, a GABA antagonist, partially reverses the suppression of 5-HT cells produced by systemic application of alpha-adrenoceptor antagonists led to the proposal that GABA interneurons located within the dorsal raphe mediate this suppression of 5-HT cell firing. This proposal has been tested, in this study, by examining the ability of two GABA antagonists, picrotoxin and bicuculline methiodide, when applied iontophoretically to reverse the suppression produced by two alpha-adrenoceptor antagonists, WB-4101 and phentolamine. First, evidence is presented that WB-4101 and phentolamine suppress 5-HT cell firing specifically by their blockade of alpha-adrenoreceptors. Second, the inability of both GABA antagonists tested to interfere with the suppression produced by these alpha-adrenoceptor antagonists is reported. These findings provide evidence against the proposal that GABA mediates the suppression of 5-HT cells by alpha-adrenoceptor antagonists.

Actions of microiontophoretically applied oxytocin, and immunohistochemical localization of oxytocin, vasopressin and neurophysin in the rat caudal medulla

Oxytocin-, vasopressin- and neurophysin-containing axons were visualized within the rat caudal medulla using the immunoperoxidase technique. The highest densities of axons and terminals were found in the nucleus tractus solitarius, nucleus dorsalis vagus, nucleus commissuralis, nucleus reticularis lateralis and within the marginal layer of the nucleus trigeminalis. In these areas, oxytocin fibres predominated markedly over vasopressin fibres. In a series of electrophysiological experiments, neurones in these and surrounding areas were predominantly depressed following the iontophoretic application of oxytocin. This depression was seen on both spontaneous and glutamate-evoked neuronal firing.

Conductance changes during bath application of beta-alanine and taurine in giant interneurons of the isolated lamprey spinal cord

(1) Input conductances of giant interneurons in the isolated spinal cord of lampreys were measured with two separate intracellular electrodes. Bath applications of 0.4--3 mM beta-alanine and taurine produced large, reversible conductance increases which were Cl-dependent. (2) Strychnine at 1--2.5 microM might be a competitive antagonist of both amino acids, but had a stronger effect on taurine. Bicuculline and picrotoxin were weak antagonists of beta-alanine and taurine in some cells. (3) A few giant interneurons desensitized after repeated application of the amino acids, but most became more sensitive. Responses to the amino acids also increase at low temperatures in in Na-free fluid, suggesting effect of uptake mechanisms.

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.

Strychinine binding associated with synaptic glycine receptors in rat spinal cord membranes: ionic influences

Ammonium salts of some anions decrease the potency of glycine in inhibiting (3H)strychnine binding associated with synaptic glycine receptors. A correspondence exists between the ability of the ammonium salts of anions to increase the IC50 of glycine in inhibiting the (3H) strychnine binding, their capacity to reduce the (3H) strychnine binding itself, and their capacity to reverse inhibitory postsynaptic potentials. The decrease of (3H)strychnine binding in the presence of chloride is abolished by sodium, while the decrease of the potency of glycine in inhibiting (3H)strychnine is not. Binding of (3H)strychnine is influenced by monovalent cations in a biphasic fashion. Concentrations of Li+, K+, and Na+ up to 150mM decrease (3H)strychnine binding, while higher concentrations of the cations increase (3H)strychnine binding. Inhibition by glycine of (3H)strychnine binding is enhanced by low concentrations of these cations.

A comparison of the inhibitory effects of taurine and GABA on identified Purkinje cells and other neurons in the cerebellar cortex of the rat

The microiontophoretic application of taurine and GABA was studied in the cerebellar cortex of the rat. Both taurine and GABA produced a dose-dependent depression of spike frequency of cerebellar neurons. GABA (2-42 nA, mean 27 nA) induced an inhibition of spike discharge on all 138 cells tested, including 29 Purkinje cells. Taurine (60-200 nA, mean 108 nA) induced an inhibition of spike discharge on 93 of the 106 cerebellar neurons tested, including inhibition on 22 of 25 Purkinje cells. Iontophoretic application of bicuculline and picrotoxin antagonized the inhibitory effects of both GABA and taurine on Purkinje cells as well as on cerebellar neurons in general. Strychnine did not antagonize the inhibition of either GABA or taurine. Simultaneous application of taurine and GABA produced a synergistic inhibitory effect on the firing rate of Purkinje cells. Taurine, in contrast to GABA, appeared to be more depressant when applied in the Purkinje cell dendritic zone than when applied near the soma. The data are discussed in terms of taurine functioning as a neurotransmitter in the cerebellum of the rat and having receptor sites distinct from those for GABA.

Glycine high affinity uptake and strychnine binding associated with glycine receptors in the frog central nervous system

Accumulation of [3H]glycine into synaptosomal fractions occurs by high affinity systems in cerebral cortex, optic tectum, brain stem and spinal cord of the frog. Specific [3H]strychnine binding which appears associated with postsynaptic glycine receptors is also demonstrable in these regions. By contrast, only very low levels of strychnine binding and high affinity glycine uptake occur in higher centers of the rat central nervous system. The relative potencies of small neutral amino acids in competing for [3H]strychnine binding are similar in frog brain and spinal cord. No evidence for a high affinity accumulation of [3H]taurine by synaptosomal fractions of frog spinal cord can be demonstrated. These observations favor glycine rather than taurine as an inhibitory transmitter in frog spinal cord. Moreover, these findings suggest that glycine may have a synaptic role in higher brain centers in the frog.

Supraspinal regulation of spinal reflex discharge into cardiac sympathetic nerves

(1) In chloralose anaesthetized cats, reflex responses were recorded in inferior cardiac nerves following stimulation of intercostal nerves and hind limb afferent nerves. (2) In 80% of cats, a long latency reflex response alone was recorded, whereas, in the others, a short and long latency response was present to intercostal nerve stimulation. (3) In cats displaying only a long latency somatocardiac reflex response, damage to the ventral quadrant of the ipsilateral cervical spinal cord, through which runs a bulbospinal inhibitory pathway, resulted in the appearance of shorter latency reflexes to intercostal nerve stimulation. Lesions elsewhere in the cervical cord did not do this. (4) The characteristics of the early responses indicated that they were somatosympathetic reflexes and not dorsal root reflexes. (5) The early reflexes remained and the late reflex disappeared on subsequent complete transection of the spinal cord. The early reflexes were therefore spinal reflexes, and suppressed in the animal with cord intact. (6) Lesions at C4, which included a contralateral hemisection and a section of dorsal columns extending into the dorsal part of the lateral funiculus, abolished the inhibition of a sympathetic reflex that followed stimulation of some somatic afferent nerve fibres. These sections did not release the spinal reflex. Therefore, this reflex inhibition was not responsible for the suppression of the spinal somatosympathetic reflex. (7) The descending inhibitory influence on the segmental reflex pathway was not antagonized by strychnine, bicuculline or picrotoxin. (8) The possibility is discussed that the spinal reflex pathway into cardiac sympathetic nerves is tonically inhibited by a bulbospinal pathway originating from the classical depressor region of the ventromedial reticular formation.

Effects of picrotoxin and strychnine on rabbit retinal ganglion cells: lateral interactions for cells with more complex receptive fields

1. The effects of picrotoxin and strychnine were tested on the receptive fields of direction sensitive cells, orientation sensitive cells, local edge detectors, uniformity detectors and large field units in the rabbit retina. 2. Picrotoxin eliminated the direction specificity and size specificity of 'on-off' and 'on' directionally sensitive cells for both black and white objects. Picrotoxin also made 'on' directionally sensitive cells responsive to faster velocities. 3. Picrotoxin eliminated the orientation specificity of orientation sensitive cells, and changed the bar-flank arrangement of the receptive field into a centre surround arrangement. Thus, the orientation specificity is due to inhibitory rather than excitatory mechanisms. 4. Picrotoxin altered the speed sensitivity of large field units so that they responded to slow speeds as well as fast ones, like centre surround Y cells. 5. Strychnine abolished the size specificity of local edge detectors and changed their speed specificity so that they responded to faster speeds. 6. Picrotoxin changed a uniformity detector into a sustained on centre cell. 7. Strychnine did not effect the direction specificity of directionally sensitive cells, the orientation specificity of orientation sensitive cells, or the speed specificity of large field units. Picrotoxin did not affect the size specificity of local edge detectors. 8. Picrotoxin and strychnine usually had opposing effects on the transient responses of these units to spots and annuli. In general picrotoxin prolonged and enhanced these responses at both on and off, and strychnine shortened them. 9. The effect of these drugs for every type of ganglion cell with complex receptive field properties was to make the receptive field more simple. The orientation selective cells, large field cells, 'on' direction selective cells and uniformity detectors seem to be centre surround cells with special properties that are abolished by these drugs. The 'on-off' direction selective cells and local edge detectors still on-off receptive fields, but in each case one of the drugs abolished the feature that was the basis for the cell's name.

Inhibition of dopamine release in the cat caudate nucleus by nigral application of glycine

"Encéphale isolé" cats were implanted with two push-pull cannulae, one in the left caudate nucleus and the other in the ipsilateral substantia nigra. L-3,5(-3)H-Tyrosine was introduced continuously into the caudate nucleus to study the release of 3H-DA. Glycine (10(-5)M) added to the superfusing medium of the push-pull cannula inserted into the substantia nigra reduced (25%) the spontaneous release of 3H-DA. Conversely, strychnine (10(-5)M) slightly stimulated the 3H-transmitter release. The inhibiting effect of glycine (10(-5)M) on 3H-DA release was no longer seen in the presence of strychnine (10(-5) M). The results support the hypothesis of a tonic glycinergic inhibitory control of the activity of the nigrostriatal dopaminergic neurons.

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.

Structure-activity relations of excitatory amino acids on frog and rat spinal neurones

1 A series of compounds structurally related to glutamic acid has been tested on frog and rat spinal neurones. The substances were added to procaine-containing medium bathing the isolated hemiscected spinal cord of the frog, and their potencies in depolarizing motoneurones were assessed by the magnitude of the potential produced in the ventral root. The electrophoretic technique was used to administer the substances around single interneurones of the rat spinal cord and the relative potencies of the compounds as excitants assessed by the magnitude of the currents required to produce similar rates of neuronal firing. 2 Parallel structure-activity relations were observed in the two series of experiments, suggesting that the receptors for excitatory amino acids on frog and rat spinal neurones are similar. 3 Quisqualate, domoate and kainate were the strongest excitants in both animals, with potencies around two orders of magnitude higher than that of L-glutamate. 4 2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) was a stronger excitant and L-3,4-dihydroxyphenylalanine (L-dopa) a weaker excotamt than L-glutamate on frog spinal motoneurones. The former compounds was also a more potent convulsant than L-glutamate on intraventricular injection into mouse brain. The lack of activity of 6-OH-DOPA on electrophoretic administration was attributed to oxidation. 5 Unlike the majority of amino acid excitants, several of the compounds shown in the present work to have moderate excitatory activity are not anionic at physiological pH. This indicates either that two negatively charged groups are not essential for interaction with a common excitatory receptor, or that more than one type of receptor is involved in the actions demonstrated.

Effect of antipsychotic drugs on the firing of dorsal raphe cells. II. Reversal by picrotoxin

As reported in the preceding study, the ability of certain antipsychotic and adrenolytic agents to inhibit the spontaneous firing of serotonergic 5HT neurons in the dorsal raphe nucleus appeared to be related to adrenergic blocking efficacy. However, the interaction between adrenergic and serotonergic systems was apparently indirect. In this phase of the study we investigated the hypothesis that another transmitter system could mediate this interaction. We examined the effects of two inhibitory amino acid transmitters (GABA and glycine) for possible effects on dorsal raphe cell firing using single cell recording and microiontophoretic techniques. In addition, the ability of the GABA antagonist, picrotoxin and the glycine antagonist, strychnine to reverse the effects of the antipsychotic and alpha-blocking drugs on dorsal raphe firing was tested. Both GABA and glycine were found to inhibit raphe cell firing selectively, allowing for a possible neurotransmitter function for these amino acids within the dorsal raphe nucleus. However, picrotoxin but not strychnine was found to reverse the effects of the antipsychotic and alpha-blocking drugs on raphe firing. Based on these results, we propose that the adrenergic input may influence 5HT neurons indirectly via a GABAergic interneuron or interposed GABA neuron.

Light-evoked release of glycine from cat and rabbit retina

The light-evoked release of [3H]glycine from retina in cat (in vivo) after pre-retinal perfusion and in rabbit (in vitro) after intravitreal injection was studied. The site of uptake of [3H]glycine into retina was checked by autoradiography and was found to be almost exclusively in a type of amacrine cells. If the retina loaded with [3H]glycine was stimulated by light flashes the release increased significantly in both in vivo and in vitro experiments. When the flashing light was exchanged for continuous light there was not change in the spontaneous efflux of radioactivity. Chromatographic experiments showed that the main part of the radioactivity released by light was glycine. The light-evoked release of glycine from retina was dependent on temperature and Ca2+. Low temperature (+2 to +4 degrees C) abolished the increased release. If Ca2+ was ommitted from the perfusion medium and EDTA was added there was no light-inducable change in the efflux of radioactivity in retinas loaded with [3H]valine. The present results, that light stimulation will release glycine from the retina both in vivo and in vitro, are further criterion for it as neurotransmitter.