GABA/Glycine Inhibition

 Cross references:  Lamprey GABA        Lamprey Neurotransmitters       
Inhibition of Locomotion in Lampreys     Neuromodulators vs Neurotransmitters     
Behavioral Disinhibition   Tonic Inhibition

The earlier experiments seem to be mostly about Neuromodulators  , but little by little, researchers seem to have learned to distinguish between neuromodulators and Neurotransmitters  .  

1973  143<150
Transmitters and reticulospinal neurons.  
No PubMed Abstract:   
but there is a Google Abstract:  
    "Glycine and GABA strongly depress the firing of reticulospinal neurons, glycine being more effective than GABA. The effects of strychnine and bicuculline methochloride on synaptic inhibition and on the actions of these amino acids are consistent with the earlier evidence that both glycine and GABA are inhibitory transmitters in the medullary reticular formation. However, although strychnine is consistently a relatively specific antagonist of glycine, bicuculline methochloride is far less specific in distinguishing between glycine and GABA on reticulospinal neurons. Studies of synaptic inhibition in the brain stem using bicuculline methochloride alone are therefore to be interpreted with caution. Both convulsants should be tested on the actions of glycine and GABA before conclusions about transmitter identity are made."  
    "Large proportions of reticulospinal neurons appear to be unaffected by ACh, 5-HT, NA, and DA. Of the amine-sensitive reticulospinal neurons, ACh and DA usually depress firing whereas 5-HT and NA usually cause excitation. These effects, however, are generally weak (especially excitation), and it is difficult to draw firm conclusions about the possible functional significance of these substances on reticulospinal neurons."  
    208 Related PubMed citations:    
    and one Cited by.  

Key Point
"Glycine and GABA strongly depress the firing of reticulospinal neurons, glycine being more effective than GABA." 

Glycine and GABA receptors on lamprey bulbar reticulospinal neurones
No PubMed Abstract
but there is a Google Abstract:  
1.  Intracellular recordings were made from the bulbar group of reticulospinal neurones of lamprey ammocoetes during the application of glycine and GABA to the bath.
    2.  Glycine and GABA produced a reversible dose-dependent hyperpolarization of the membrane potential. The hyperpolarizations were associated with an increase in the conductance of the cell membrane and an increase in the Cl permeability was implicated.
    3.  Strychnine antagonized the glycine potentials but not the GABA potentials. Picrotoxin anatagonized the GABA potentials but not the glycine potentials. It was concluded that there are two types of inhibitory receptor on the lamprey bulbar reticulospinal neurones.
There were also 99 PubMed Related citations:  

My comment
    Bath-applied neuromodulators.  Not neurotransmitters.     
Please see:    Neuromodulators vs Neurotransmitters    

Conductance increases produced by glycine and gamma-aminobutyric acid in lamprey interneurones.  

    1. Conductances of individual neurones in the isolated lamprey spinal cord were measured with separate intracellular electrodes for recording potentials and for passing current pulses during application of glycine or GABA (0.1-1.0 MM) in Ca-free bathing fluid. Large, reversible increases in conductance were produced in giant interneurones by both amino acids, but Müller axons and sensory dorsal cells were unaffected.
    2. Conductance increases produced by glycine and by GABA were selective for Cl. Both conductance increases were linearly related to external Cl concentrations and repeated exposure to the amino acids in Cl-free fluid progressively reduced the conductance increases to less than 1% of their values in normal Cl.  
    3. Strychnine was a competitive antagonist of glycine, while GABA was antagonized competitively by bicuculline and non-competitively by picrotoxin.  
    4. The sensitivity of giant interneurones to glycine and GABA increased at low temperatures, in Na-free fluid, and after repeated exposure to the amino acids. Sensitization may have been produced by inhibition of uptake mechanisms for glycine and GABA in the spinal cord.  
    5. Discharges of interneurones recorded extracellularly were inhibited by bath-applied glycine and GABA, but directly elicited action potentials of axons were unaffected.   Strychnine and Cl-free fluid in the presence of Ca produced seizures in lamprey spinal cord.  
    6. The conclusions of these experiments are that different receptors for glycine and for GABA are present on giant interneurones, that glycine is the better candidate for an inhibitory transmitter in the lamprey spinal cord, and that GABA produces effects similar to those which have been well studied in arthropod muscle."
    178 Related citations:   
    Click on the active link.  
My comment
    Bath-applied neuromodulators.  Not neurotransmitters.     
Please see:    Neuromodulators vs Neurotransmitters 

Glycine, GABA and synaptic inhibition of reticulospinal neurones of lamprey.    
Intracellular recordings were made from the cell bodies and axons of giant reticulospinal neurones (Müller cells) of the lamprey and the effects of a variety of putative neurotransmitters tested. Bath-applied acetylcholine, carbamylcholine, norepinephrine, dopamine, histamine and serotonin were without effect. Glycine and gamma-aminobutyric acid (GABA) hyperpolarized and reduced the input resistance of cell bodies but had no effect on the membrane conductance of axons."   
    126 Related citations:   
    Active link.  Click to read. 
My comment
    Bath-applied neuromodulators.  Not neurotransmitters.     
Please see:    Neuromodulators vs Neurotransmitters 

Evidence for a tonic GABAergic control of serotonin neurons in the median raphe nucleus.
    "We examined whether serotonin (5-HT)-containing neurons of the midbrain raphe nuclei are subject to an inhibitory control by GABA. We found that injection into the median raphe nucleus of the GABA antagonists picrotoxin and bicuculline and the GABA agonist muscimol increase and decrease, respectively, the 5-HT turnover and the steady-state content of 5-hydroxyindoleacetic acid. The results provide evidence of a tonic inhibition by GABA of 5-HT neuronal activity in the median raphe nucleus; this inhibitory effect is potentiated by benzodiazepines."
    394 Related citations:  

    6 Cited by's:

gamma-Aminobutyric acid and glycine activate Cl- channels having different characteristics in CNS neurones
    "We have studied the properties of Cl- channels in Müller cells of the lamprey brain, which are activated by two putative neurotransmitters, glycine and gamma-aminobutyric acid (GABA). Previous studies have shown that in these cells the two ligands act through different receptors, and it was of interest to determine whether or not the two receptors, in turn, activate separate channels having different properties. We have shown previously that the channels activated by glycine have large conductances which decrease rapidly as the intracellular Cl- concentration [Cl-]i is increased above normal. We show here that GABA-activated channels have much smaller conductances which are independent of [Cl-]i over the experimental range and also that the response to a saturating application of glycine is not occluded by simultaneous GABA application. These observations indicate that the glycine and GABA receptors are coupled to separate Cl- channels having different
    202 Related citations:
    and 3 Cited by's.

Peculiarities of receptor-channel complexes for inhibitory mediators in the membranes of lamprey spinal cord neurones.  
Effects of inhibitory mediators on the membranes of isolated lamprey spinal cord neurones were investigated by means of whole-cell recording and concentration clamp techniques. Glycine and gamma-aminobutyric acid (GABA) applications evoked desensitizing chloride currents.
    The concentrations at half-maximum effects were 16 microM for glycine- and 1.5 mM for GABA-activated currents. Ionic responses to applications of both amino acids demonstrated full cross-desensitization. Strychnine and bicuculline suppressed both glycine- and GABA-activated conductances to the same degree.  
    We suggest that in the membranes of lamprey spinal cord neurones glycine and GABA act on the same receptor-channel complexes.
    165 Related citations
    2 Cited by's:   
No link.  See PubMed abstract.     
My comment
    Bath-applied neuromodulators.  Not neurotransmitters.     
Please see:    Neuromodulators vs Neurotransmitters 

Presynaptic GABAA and GABAB Receptor-mediated Phasic Modulation in Axons of Spinal Motor Interneurons.  
The lamprey spinal cord has been utilized to investigate the role of presynaptic inhibition in the control of the spinal motor system.  
    Axons of the lamprey spinal cord are comparatively large because of their lack of myelination. Axons impaled with microelectrodes demonstrate depolarizing responses to the application of GABAA and GABAB receptor agonists, muscimol and baclofen. These depolarizing effects are counteracted by the specific GABAA and GABAB receptor antagonists, bicuculline and phaclofen.  
    GABAA receptor activation leads to a gating of Cl- channels on the axons. However, the ionic mechanism leading to axonal depolarization following GABAB receptor activation is unknown. After initiation of fictive locomotion, these axons demonstrate oscillations in axonal membrane potential related to the locomotor cycle. During ficitive locomotion they depolarize in phase with the bursting of the ipsilateral ventral root of the same segment.
    These axonal membrane potential oscillations are due to a phasic GABAA and GABAB receptor-mediated gating of ion channels on the axonal membrane. Fictive locomotion in the lamprey spinal cord is largely unaffected by antagonism of one or other GABA receptor subtype alone, but is severely disrupted by simultaneous antagonism of both subtypes.  
    In conclusions, we demonstrate, for the first time, an agonist-gated depolarization of a vertebrate presynaptic element measured by direct impalement of the axon under study. We also demonstrate that GABA-mediated presynaptic inhibition occurs in axons of spinal interneurons. It is not limited to the primary afferents as has previously been believed."     
    81 Related citations
    4 Cited by's:   
No link.  See PubMed abstract.     
My comment
    Bath-applied neuromodulators.  Not neurotransmitters.     
Please see:    Neuromodulators vs Neurotransmitters  . I think that the term "presynaptic" probably always indicates neuromodulators rather than neurotransmitters. 

The involvement of GABAB receptors and coupled G-proteins in spinal GABAergic presynaptic inhibition.  
GABA acts as a presynaptic inhibitory transmitter in the spinal cord.  In the lamprey, it has recently been shown that it acts in this way at both primary sensory and motor system synapses and is important in the generation of a locomotor rhythm. Both GABAA and GABAB receptors are activated at these sites by GABA released during physiological activity. In some systems, GABAB receptor activation has been shown to lead to modulation of ion channel function indirectly through the action of a pertussis toxin (PTX)-sensitive G-protein.  
    Here we have studied the mechanism of action of the presynaptic GABAB receptor in this system. GABAB receptor activation leads to a decrease in axonal membrane impedance and also to a reduction in the axonal action potential duration. The ionic basis for this response remains unknown, though it is not, unlike the response to GABAA receptor activation, mediated by an increase in conductance to Cl-. The effects of GABAB receptor antagonism with phaclofen are mimicked by pretreatment of the spinal cord with PTX. Because this procedure inactivates certain classes of G-proteins, it seemed likely that the GABAB receptor-mediated effects are initiated via a presynaptic population of PTX-sensitive G-proteins. Experiments in which only presynaptic G-proteins were interfered with indicate that this is so. Stable analogs of GTP and GDP were used to activate permanently or to antagonize, respectively, the GTP binding site in the presynaptic component of these spinal synapses.  
    We conclude that GABAB receptor-mediated synaptic suppression in the spinal cord is caused by GTP binding to presynaptic G-proteins linked to the GABAB receptor.
    146 Related citations  
My comment
    Bath-applied neuromodulators.  Not neurotransmitters.     
Please see:    Neuromodulators vs Neurotransmitters I think that the term "presynaptic" probably always indicates neuromodulators rather than neurotransmitters. 

Primary receptor for inhibitory transmitters in lamprey spinal cord neurons.     
     "The action of glycine and GABA on isolated lamprey spinal cord neurons was investigated by means of intracellular perfusion and concentration clamp techniques. These amino acids activated desensitizing chloride ionic conductances."   
    "Increase in the transmitter concentration led to a decrease in the time constant of desensitization. Current-voltage relationships of glycine- and GABA-activated currents were strongly dose-dependent. At low agonist concentrations the time constant of activation decreased with membrane hyperpolarization.  
    Glycine- and GABA-activated currents exhibited complete cross-desensitization. The specific glycine antagonist, strychnine, suppressed both glycine- and GABA-activated currents to the same degree. Selective antagonists of GABA receptors, bicuculline and picrotoxin, produced equal blocking effects on glycine- as well as GABA-evoked responses.
    In the cells studied, taurine activated desensitizing ionic conductance. Responses evoked by taurine and glycine applications demonstrated complete cross-desensitization. Taurine-activated currents were sensitive to strychnine, bicuculline and picrotoxin.  
    These results suggest the existence of one receptor-channel complex for the main inhibitory transmitters in lamprey spinal cord neurons."  
My comments: 
    I think that the phrase "taurine and glycine applications" indicates neuromodulators rather than neurotransmitters.  Please see:    Neuromodulators vs Neurotransmitters  .  
    219 Related citations

Possible morphological substrates for GABA-mediated presynaptic inhibition in the lamprey spinal cord.  
"Gamma-aminobutyric acid (GABA) neurons intrinsic to the lamprey spinal cord are known to modulate synaptic transmission from interneurons active during locomotion and from mechanosensory dorsal cells. Many of these physiological effects are presynaptic.  
    To establish the morphological substrates for these axo-axonic interactions, an ultrastructural analysis was performed with an antiserum to fixed GABA. The GABA immunoreactivity (ir) was detected by postembedding peroxidase-antiperoxidase and immunogold techniques.  
    GABA-ir terminals were found to make appositions with unlabelled axons located in the dorsal columns and in the ventrolateral aspect of the spinal cord.  
    In the ventrolateral part of the cord, similar appositions between different GABA-ir terminals were also observed. The immunolabelled terminals contained spherical to pleomorphic synaptic vesicles, and also glycogen granules and dense core vesicles.  
    In some cases, the fine structure of the contacts between immunogold-labelled terminals and unlabelled axons suggested a synaptic relationship. Such a relation was found in a relatively small proportion (2-3%) of the appositions studied. These specializations were always observed in close relation to an output synapse of the postsynaptic axon. It is suggested that the axo-axonal contacts described may provide an effective modulation of the synaptic transmission from axons in the lamprey spinal cord."    
My comment
I think that the term "presynaptic" probably always indicates neuromodulators rather than neurotransmitters.  Please see:    Neuromodulators vs Neurotransmitters  .
    285 Related citations:   
See PubMed abstract.     

    Reticulospinal neurones provide monosynaptic glycinergic inhibition of spinal neurones in lamprey.
    "In lamprey, distinct groups of reticulospinal neurones utilize different neurotransmitters such as glutamate or serotonin. The present study demonstrates that a group of reticulospinal neurones inhibit their target neurones by an action on glycine receptors. Simultaneous intracellular recordings from a reticulospinal neurone and spinal target neurone shows that the former may evoke an IPSP in the latter. These IPSPs are elicited at a constant latency and amplitude, and follow high frequency stimulation (100 Hz). Furthermore, the IPSPs are maintained when the excitatory amino acid synaptic transmission is blocked, suggesting that the effects are not elicited via a powerful disynaptic pathway. These data taken together establish the monosynaptic nature of the pathway. IPSPs elicited from single reticulospinal neurones or from electrical stimulation of the reticular formation are suppressed by administration of strychnine, suggesting that glycine is the neurotransmitter of these inhibitory reticulospinal neurones."  
My comment
    Although "simultaneous intracellular recordings from a reticulospinal neurone and spinal target neurone" clearly shows the transmission from the reticulospinal neuron to the target neuron, the fact that the "administration" of strychnine suppresses the IPSP strongly suggests that the glycine is acting as a neuromodulator rather than a neurotransmitter. Please see:  Neuromodulators vs Neurotransmitters  .      
    147 Related citations
See PubMed abstract.        

Neural networks for vertebrate locomotion
    No PubMed Abstract, but full length paper on Google
    " Under resting conditions, the basal ganglia continuously inhibit the brain’s sundry motor centers so that no movements occur. But when the active inhibition is released, coordinated motions may begin. "
My comment:   
    This is a Scientific American article written for a general, non-technological, audience.  It wasn't very helpful.  Many of what he calls "neural networks" would probably qualify as "central pattern generators".    PubMed sorted them by Link but I rearranged them by Publication Date. 
    126 Related citations and 2 Cited by's.

Inhibition in the central nervous system.
    No abstract but 97 Related citations.  Sorted by link. 

GABA- and glycine-immunoreactive terminals contacting motoneurons in lamprey spinal cord.  
The presence of terminals immunoreactive to either GABA or glycine contacting the motoneurons suggests that there is a morphological substrate for both GABAergic and glycinergic postsynaptic inhibition of motoneurons in the lamprey spinal cord."  
My comment
presence of terminals immunoreactive to either GABA or glycine contacting the motoneurons ..." is strong evidence that GABA and/or glycine are acting as neurotransmitters rather than neuromodulators. 
    Please see:  Neuromodulators vs Neurotransmitters  .    
    367 Related citations


The spinal GABAergic system is a strong modulator of burst frequency in the lamprey locomotor network. 
The spinal network coordinating locomotion is comprised of a core of glutamate and glycine interneurons. This network is modulated by several transmitter systems including spinal GABA interneurons."  
Thus the spinal GABA system does play a prominent role in burst frequency regulation in that it reduces the burst frequency by < or =50%, presumably due to presynaptic and soma-dendritic effects documented previously."  
    428 Related citations
    3 Cited by's
(No overall link.  See PubMed abstract.)    
    - Free full text -   Many active links.    
My comment:   
    Although I didn't find an explicit statement to this effect, reducing burst frequency probably reduces locomotion.  

Mechanisms and significance of reduced activity and responsiveness in resting frog tadpoles.   
Hatchling Xenopus laevis tadpoles spend most of their time attached to objects or the water surface by mucus secreted by a gland on the head. While attached, swimming activity and responsiveness to swim-initiating stimuli are reduced over long periods of time. We have investigated the mechanisms and significance of this apparent long-term inhibition. In behavioural experiments we show, firstly, that innervation of the cement gland and GABA(A)-mediated inhibition are necessary for attachment to reduce responsiveness, and secondly, that denervation of the cement gland increases tadpole activity and increases their predation by damselfly nymphs (Zygoptera). To investigate the neuronal pathway from the cement gland to GABA(A) inhibition, we have devised an immobilized, inverted tadpole preparation where a weight attached to the mucus simulates the force as it hangs. Simulated attachment reduces responsiveness and spontaneous fictive swimming activity. We have recorded the activity and responses of trigeminal neurons innervating the cement gland. They are spontaneously active and simulating attachment results in a sustained increase in this activity. We propose that hanging from a mucus strand increases firing in cement gland afferents. This leads to tonic GABA inhibition that reduces tadpole activity and responses, and leads to fewer attacks by predators."  
    88 Related citations: 
    4 Cited by's: 
    Free Full Text:   

Clustering of extrasynaptic GABA(A) receptors modulates tonic inhibition in cultured hippocampal neurons.  
Tonic inhibition plays a crucial role in regulating neuronal excitability because it sets the threshold for action potential generation and integrates excitatory signals. Tonic currents are known to be largely mediated by extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors that are persistently activated by submicromolar concentrations of ambient GABA. We recently reported that, in cultured hippocampal neurons, the clustering of synaptic GABA(A) receptors significantly affects synaptic transmission. In this work, we demonstrated that the clustering of extrasynaptic GABA(A) receptors modulated tonic inhibition. Depolymerization of the cytoskeleton with nocodazole promoted the disassembly of extrasynaptic clusters of delta and gamma(2) subunit-containing GABA(A) receptors. This effect was associated with a reduction in the amplitude of tonic currents and diminished shunting inhibition. Moreover, diffuse GABA(A) receptors were less sensitive to the GAT-1 inhibitor NO-711 and to flurazepam. Quantitative analysis of GABA-evoked currents after prolonged exposure to submicromolar concentrations of GABA and model simulations suggest that clustering affects the gating properties of extrasynaptic GABA(A) receptors. In particular, a larger occupancy of the singly and doubly bound desensitized states can account for the modulation of tonic inhibition recorded after nocodazole treatment. Moreover, comparison of tonic currents recorded during spontaneous activity and those elicited by exogenously applied low agonist concentrations allows estimation of the concentration of ambient GABA. In conclusion, receptor clustering appears to be an additional regulating factor for tonic inhibition."     

Descending GABAergic projections to the mesencephalic locomotor region in the lamprey Petromyzon marinus.  
    See:  Lamprey GABA  for full Abstract, Similar articles and Cited by's. 

Diencephalic locomotor region in the lamprey--afferents and efferent control.   Free Article   
    In vertebrates, locomotion can be initiated by stimulation of the diencephalic locomotor region (DLR). Little is known of the different forebrain regions that provide input to the neurons in DLR.  
    In the lamprey, it had been shown previously that DLR provides monosynaptic input to reticulospinal neurons, which in turn elicit rhythmic ventral root activity at the spinal level.  
    To show that actual locomotor movements are produced from DLR, we use a semi-intact preparation in which the brain stem is exposed and the head fixed, while the body is left to generate actual swimming movements. DLR stimulation induced symmetric locomotor movements with an undulatory wave transmitted along the body. 
    To explore if DLR is under tonic GABAergic input under resting conditions, as in mammals, GABAergic antagonists and agonists were locally administered into DLR. Injections of GABA agonists inhibited locomotion, whereas GABA antagonists facilitated the induction of locomotion. These findings suggest that GABAergic projections provide tonic inhibition that once turned off can release locomotion. Double-labeling experiments were carried out to identify GABAergic projections to the DLR.  
    Populations of GABAergic projection neurons to DLR originated in the caudoventral portion of the medial pallium, the lateral and dorsal pallium, and the striatal area. These different GABAergic projection neurons, which also project to other brain stem motor centers, may represent the basal ganglia output to DLR. Moreover, electrical stimulation of striatum induced long-lasting plateau potentials in reticulospinal cells and associated locomotor episodes dependent on DLR being intact, suggesting that striatum may act via the basal ganglia output identified here."

Inhibitory descending rhombencephalic projections in larval sea lamprey.   
Lampreys are jawless vertebrates, the most basal group of extant vertebrates. This phylogenetic position makes them invaluable models in comparative studies of the vertebrate central nervous system. Lampreys have been used as vertebrate models to study the neuronal circuits underlying locomotion control and axonal regeneration after spinal cord injury. Inhibitory inputs are key elements in the networks controlling locomotor behaviour, but very little is known about the descending inhibitory projections in lampreys. The aim of this study was to investigate the presence of brain-spinal descending inhibitory pathways in larval stages of the sea lamprey Petromyzon marinus by means of tract-tracing with neurobiotin, combined with immunofluorescence triple-labeling methods.  
Neurobiotin was applied in the rostral spinal cord at the level of the third gill, and inhibitory populations were identified by the use of cocktails of antibodies raised against glycine and GABA.  
    Glycine-immunoreactive (-ir) neurons that project to the spinal cord were observed in three rhombencephalic reticular nuclei: anterior, middle and posterior. Spinal-projecting GABA-ir neurons were observed in the anterior and posterior reticular nuclei. Double glycine-ir/GABA-ir spinal cord-projecting neurons were only observed in the posterior reticular nucleus, and most glycine-ir neurons did not display GABA immunoreactivity.  
    The present results reveal the existence of inhibitory descending projections from brainstem reticular neurons to the spinal cord, which were analyzed in comparative and functional contexts."  
My comment:   
    Considering this reference, together with the one above it, suggests that locomotion is inhibited at at least two levels of the neuroaxis.  Judging from just these two references, GABA, mostly from the medial Pallium , inhibits the Mesencephalic Locomotor Region  and glycine from the Reticular Activating Sytem inhibits the  Spinal Locomotor Generator
         124 Related citations
    No Cited by's.