Glycine and GABAA receptor-mediated synaptic transmission in rat substantia gelatinosa: inhibition by mu-opioid and GABAB agonists

1. Bicuculline-sensitive and strychnine-sensitive inhibitory postsynaptic currents (IPSCs) could be evoked in neurones of the rat substantia gelatinosa of the spinal trigeminal nucleus pars caudalis. 2. Spontaneous tetrodotoxin (TTX)-insensitive-mediated miniature IPSCs (mIPSCs) blocked by strychnine or bicuculline were also present in many neurones. The decay of the glycine receptor-mediated mIPSCs was fitted by a single exponential, whereas the decay of the GABAA receptor-mediated mIPSCs could in some instances be fitted by a single exponential, but in other instances required two exponentials. 3. An increase in baseline current noise developed during the course of the recording. This noise was abolished by strychnine (1 microM) but was insensitive to bicuculline (10 microM), TTX (0.5 microM), [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 1 microM) or baclofen (30 microM). The single-channel conductance underlying the noise was estimated to be 21 pS. 4. The mu-opioid agonist DAMGO (1-10 microM) reduced the amplitude of the evoked glycine receptor-mediated IPSC and the evoked GABAA receptor-mediated IPSC. The mu-opioid antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP, 1 microM) reversed the DAMGO inhibition. 5. The GABAB agonist baclofen (30 microM) reduced the amplitude of the evoked glycine receptor-mediated IPSC and the GABAA receptor-mediated IPSC. The inhibition was reversed by the selective GABAB antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino- 2-(S)-hydroxypropyl-P-benzyl-phosphinic acid (CGP 55845A, 1 microM). 6. Both DAMGO and baclofen reduced the frequency of glycine and GABAA receptor-mediated mIPSCs without affecting average amplitude, and increased the percentage of failures of the evoked glycine and GABAA receptor-mediated IPSCs, suggesting a presynaptic site of action.

Changes in the properties of developing glycine receptors in cultured mouse spinal neurons

We studied several neurophysiological properties of in vitro maturing glycine receptors in mouse spinal cord neurons cultured for various times: 3-7 days (early), 10-12 days (intermediate), and 17-24 days (mature), using whole-cell and gramicidin-perforated techniques. The glycine-activated Cl- conductance increased about 6-fold during in vitro development, and the current density increased from 177+/-42 pA/pF in early to 504+/-74 pA/pF in mature neurons. The sensitivity to glycine increased transiently from 39+/-2.8 microM in early neurons to 29+/-1 microM in intermediate neurons. Using whole-cell recordings, we found that ECl did not change during development. With the gramicidin-perforated technique, on the other hand, ECl shifted from -27 to -52 mV with development. Thus, immature neurons were depolarized by the activation of glycine receptors, whereas mature neurons were hyperpolarized. The current decayed (desensitized) during the application of 500 microM glycine. The decay was single exponential and the time constant increased from 2,212+/-139 msec in early neurons to 4,580+/-1,071 msec in mature neurons. Picrotoxin (10 microM) inhibited the current to a larger extent in early neurons (46+/-6% of control), and the sensitivity of these receptors to strychnine (IC50) increased from 23+/-3 nM to 9+/-1 nM in mature neurons. In conclusion, several properties of spinal glycine receptors changed during in vitro neuronal maturation. This indicates that, similar to GABA(A) receptors, the functions of these receptors are developmentally regulated. These changes should affect the excitability of spinal neurons as well as other maturation processes.

[3H]5,7-dichlorokynurenic acid recognizes two binding sites in rat cerebral cortex membranes

Binding of [3H]5,7-dichlorokynurenic acid ([3H]DCKA), a competitive antagonist of the strychnine-insensitive glycine site of the N-methyl-D-aspartate (NMDA) receptor channel complex, was characterized in synaptic plasma membranes from rat cerebral cortex. Non linear curve fitting of [3H]DCKA saturation and homologous displacement isotherms indicated the existence of two binding sites: a specific, saturable, high affinity site, with a pKD value of 7.24 (KD = 57.5 nmol/l) and a maximum binding value (Bmax) of 6.9 pmol/mg of protein and a second site, with micromolar affinity. The pharmacological profile of both binding components was determined by studying the effect on [3H]DCKA and [3H]glycine binding of a series of compounds known to interact with different excitatory and inhibitory amino acid receptors. These studies confirmed the identity of the high affinity site of [3H]DCKA binding with the strychnine-insensitive glycine site of the NMDA receptor channel complex. 3-[2-(Phenylaminocarbonyl)ethenyl]-4,6-dichloroindole-2-carb oxylic acid sodium salt (GV 150526A), a new, high affinity, selective glycine site antagonist (1), was the most potent inhibitor of this component of binding (pKi = 8.24, Ki = 5.6 nmol/l). The low affinity component of [3H]DCKA binding was insensitive to the agonists glycine and D-serine and the partial agonist (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA 966), though recognised by glycine site antagonists. The precise nature of this second, low affinity [3H]DCKA binding site remains to be elucidated.

Comparison of glycine- and GABA-evoked currents in two types of neurons isolated from the rat striatum

Strychnine-sensitive glycine-activated currents and gamma-aminobutyric acid (GABA)-activated currents were compared in two types of neurons acutely isolated from striatal slices by vibrodissociation: large cells, presumably cholinergic giant aspiny neurons (GAN) and medium sized cells, presumed medium spiny neurons (MSN). Whole cell voltage clamp and concentration jump techniques were used. All cells responded to glycine (10-1000 microM) and GABA (2-100 microM), in MSN and GAN the maximal responses to glycine were 50 and 120% of the GABA response, respectively. GABA- and glycine- responses were additive and blocked selectively by bicuculline (1 microM) and strychnine (50 nM), respectively. These results predict the presence of alpha- and beta-subunits of the glycine receptor in the striatum.

The effects of strychnine, bicuculline, and ketamine on 'immersion-inhibited' dorsal horn convergent neurons in intact and spinalized rats

In both intact and spinalized rats, this study examined the effects of strychnine (a glycine antagonist), bicuculline (a GABAA antagonist), and ketamine (a non-competitive NMDA receptor antagonist) on one particular class of lumbar dorsal horn convergent neurons. This group of convergent neurons are inhibited when a rat's entire ipsilateral hindpaw is immersed in 50 degrees C water and has a strong afterdischarge as soon as the paw is removed from the water. Strychnine (2 mg/kg, iv) increased ongoing activity and blocked the 'inhibition phase' in both intact and spinalized rats demonstrating that a spinal-related glycine mechanism was involved in the inhibition. However, only in intact rats did the firing rate of the 'afterdischarge phase' increase significantly from pre-drug levels, suggesting that supraspinal sites may be involved in modulating this phase. Ketamine (15 mg/kg, iv) depressed ongoing activity and the firing rate in the afterdischarge phase of these neurons. Additionally, ketamine reversed the strychnine-induced increase in ongoing activity. Bicuculline (2 mg/kg, iv) had no effect on the activity of this cell class. As shown previously, and replicated here, these 'immersion-inhibited' neurons invariably have both inhibitory and excitatory mechano-receptive fields on the ipsilateral hindpaw. Thus, the response of this class of convergent neurons to noxious stimulation may be a function of relative inputs of glycine and EAA's, each possibly triggered by the stimulation of different receptive fields/regions on the same paw. Furthermore, when both fields are co-stimulated during noxious immersion of the entire paw, glycine has a stronger influence on activity than does the EAA's.

Strychnine-sensitive stabilization of postsynaptic glycine receptor clusters

The cellular and molecular mechanisms underlying the postsynaptic aggregation of ionotropic receptors in the central nervous system are not understood. The glycine receptor (GlyR) and its cytoplasmic domain-associated protein, gephyrin, are clustered at the postsynaptic membrane and constitute a good model for addressing these questions. The glycine receptor is inhibited by strychnine. The effects of chronic strychnine treatment on the expression and cellular distribution of gephyrin and glycine receptor were therefore tested using primary cultures of spinal cord neurons. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that the glycine receptor alpha1, alpha2, beta subunits and gephyrin mRNAs were expressed at comparable levels in strychnine-treated and untreated cultures. The number of immunoreactive cells and the subcellular distribution of gephyrin and GlyR subunits was determined with standard and confocal immunofluorescence. The proportion of gephyrin and glycine receptor-immunoreactive (IR) cells was unaffected by strychnine treatment. Confocal microscopy revealed that the glycine receptor was mainly localized intracellularly near the nucleus. This cytoplasmic glycine receptor was not associated with the Golgi apparatus nor with the rough endoplasmic reticulum and therefore is not likely to correspond to neosynthesized proteins. The number of GlyR clusters on the somato-dendritic membrane was dramatically reduced on neurons displaying intracellular staining. In contrast, the subcellular distribution and the number of gephyrin clusters was not modified by the treatment. The fact that gephyrin postsynaptic localization was not modified by strychnine suggests that the aggregation of glycine receptor and gephyrin is governed by different mechanisms. The distribution of other cell surface molecules such as NCAM or GABAA receptor beta2/3 subunits was not modified by strychnine treatment. Chronic exposure of the cultures to tetrodotoxin did not affect gephyrin or glycine receptor cluster formation. Taken together, these results indicate that functional glycine receptor, but not electrical synaptic activity, is required for the formation of glycine receptor clusters.

Glycinergic synaptic inputs to bipolar cells in the salamander retina

1. Glycine activated strychnine-sensitive chloride conductances at both the dendrites and the axonal telodendria of most bipolar cells in the salamander retina. 2. The chloride equilibrium potential of bipolar cells was found to be negative to -50 mV, indicating that glycinergic synapses on bipolar cells are inhibitory. 3. Some bipolar cells exhibited discrete, strychnine-sensitive, chloride-mediated inhibitory postsynaptic currents (IPSCs). These were elicited by focal application of glutamate at the inner plexiform layer (IPL). Glycinergic synapses were localized using simultaneous focal application of calcium to retinal slices bathed in calcium-free media. Both dendritic and telodendritic glycinergic IPSCs were observed. 4. The decay of the telodendritic IPSCs was well fitted by a single exponential with a time constant of 17.7 +/- 8.7 ms. Similar kinetics were observed for dendritic IPSCs in some cells, but in one class of on-centre bipolar cell the decay of the dendritic IPSCs was better fitted by a sum of two exponentials with time constants 9.9 +/- 4.3 and 51.3 +/- 24.3 ms. 5. The dendritic IPSCs were best driven by application of glutamate at the distal IPL (the off sublamina), while the telodendritic IPSCs were driven best by application near the telodendria. These results suggest that bipolar cell dendrites receive inhibitory glycinergic inputs from interplexiform cells that are excited by off-centre bipolar cells, whereas bipolar cell telodendria receive glycinergic amacrine cell inputs that are antagonistic to the photoreceptor inputs. 6. Both inputs could be elicited in the presence of tetrodotoxin (TTX), but the dendritic IPSCs were sometimes abolished by TTX, suggesting that sodium-dependent spikes play an important role in the transmission of interplexiform cell signals to the outer plexiform layer.

Inhibitory control of plateau properties in dorsal horn neurones in the turtle spinal cord in vitro

1. The role of inhibition in control of plateau-generating neurones in the dorsal horn was studied in an in vitro preparation of the spinal cord of the turtle. Ionotropic and metabotropic inhibition was found to condition the expression of plateau potentials. 2. Blockade of gamma-aminobutyric acid (GABAA) and glycine receptors by their selective antagonists bicuculline (10-50 microM) and strychnine (5-20 microM) enhanced the excitatory response to stimulation of the dorsal root and facilitated the expression of plateau potentials. 3. Bicuculline and strychnine also facilitated the generation of plateau potentials in response to depolarizing current pulses, suggesting the presence of tonic ionotropic inhibitory mechanisms in turtle spinal cord slices. 4. Activation of GABAB receptors also inhibited plateau-generating neurones. The selective agonist baclofen (5-50 microM) inhibited wind-up of the response to repeated depolarizations induced synaptically or by intracellular current pulses. 5. Baclofen reduced afferent synaptic input. This effect was not affected by bicuculline or strychnine and was blocked by the selective GABAB receptor antagonist 2-hydroxysaclofen (2-OH-saclofen, 100-400 microM). 6. Postsynaptically, baclofen inhibited plateau properties. Activation of GABAB receptors produced a hyperpolarization (7.0 +/- 0.5 mV, mean +/- S.E.M., n = 29) with an associated decrease in input resistance (22.7 +/- 3.1%, n = 24). These effects were blocked by extracellular Ba2+ (1-2 mM). 7. When the baclofen-induced hyperpolarization and shunt were compensated for by adjusting the bias current and the strength of the stimulus, baclofen still inhibited generation of plateau potentials. Wind-up and after-discharges were also inhibited by baclofen. These effects remained in the presence of tetrodotoxin (1 microM) and were antagonized by 2-OH-saclofen. 8. The inhibition of plateau properties was observed even when the baclofen-induced hyperpolarization and shunt were blocked by Ba2+ and when potassium channels were blocked by Ba2+ (3 mM), tetraethylammonium (TEA, 15 mM) and apamin (0.25-0.5 microM). The baclofen-sensitive component of the plateau potential was reduced by nifedipine (10 microM), suggesting a modulation of postsynaptic L-type Ca2+ channels. 9. We suggest that inhibitory regulation of plateau properties plays a role in somatosensory processing in the dorsal horn. The inhibitory control of wind-up and after-discharges may be particularly significant in physiological and therapeutic control of central sensitization to pain.

Blockade and recovery of spontaneous rhythmic activity after application of neurotransmitter antagonists to spinal networks of the chick embryo

We studied the regulation of spontaneous activity in the embryonic (day 10-11) chick spinal cord. After bath application of either an excitatory amino acid (AP-5 or CNQX) and a nicotinic cholinergic (DHbetaE or mecamylamine) antagonist, or glycine and GABA receptor (bicuculline, 2-hydroxysaclofen, and strychnine) antagonists, spontaneous activity was blocked for a period (30-90 min) but then reappeared in the presence of the drugs. The efficacy of the antagonists was assessed by their continued ability to block spinal reflex pathways during the reappearance of spontaneous activity. Spontaneous activity ceased over the 4-5 hour monitoring period when both sets of antagonists were applied together. After application of glycine and GABA receptor antagonists, the frequency of occurrence of spontaneous episodes slowed and became highly variable. By contrast, during glutamatergic and nicotinic cholinergic blockade, the frequency of occurrence of spontaneous episodes initially slowed and then recovered to stabilize near the predrug level of activity. Whole-cell recordings made from ventral spinal neurons revealed that this recovery was accompanied by an increase in the amplitude of spontaneously occurring synaptic events. We also measured changes in the apparent equilibrium potential of the rhythmic, synaptic drive of ventral spinal neurons using voltage or discontinuous current clamp. After excitatory blockade, the apparent equilibrium potential of the rhythmic synaptic drive shifted approximately 10 mV more negative to approximately -30 mV. In the presence of bicuculline, the apparent equilibrium potential of the synaptic drive shifted toward the glutamate equilibrium potential. Considered with other evidence, these findings suggest that spontaneous rhythmic output is a general property of developing spinal networks, and that GABA and glycinergic networks alter their function to compensate for the blockade of excitatory transmission.

Behavioral specificity of the bitter taste gene Soa

In mice, aversion to the bitter acetylated sugar sucrose octaacetate (SOA) is determined by a single genetic locus with three alleles. SWR/J (SW) inbred mice are SOA tasters: They avoid many compounds characterized as bitter-tasting by humans, at concentrations to which C3HeB/FeJ (C3:SOA demitasters) mice are less sensitive. C3.SW-Soa(a) congenic taster mice contain the taster allele transposed to a 99% C3 bitter-insensitive genetic background. SW, C3, C3.SW-Soa(a) congenic taster, and C3.SW demitaster mice were behaviorally tested with a series of 48-h two-bottle preference tests to determine the influence of the Soa(a) taster allele on sensitivity to a variety of bitter-tasting compounds. Soa allelic variation had a major effect on sensitivity to 0.003-1.0 mM SOA and several concentrations of the bitter-tasting alkaloids brucine, strychnine, and quinine. Effects were also found for 0.1 mM denatonium and 1 mM propylthiouracil. For caffeine, cycloheximide, thiamine, and two nonbitter compounds (NaCl and calcium hydroxide), the SW mice avoided lower concentrations than the other strains, but this avoidance was not due to the Soa(a) allele because both the C3 inbred and C3.SW-Soa(a) congenics were less sensitive. These results suggest the Soa gene product influences sensitivity to a subset of bitter-tasting compounds.

Crossed rhythmic synaptic input to motoneurons during selective activation of the contralateral spinal locomotor network

To investigate the cellular mechanisms underlying locomotor-related left-right coordination, we monitored the crossed synaptic input to lumbar motoneurons during contralateral ventral root rhythmicity in the neonatal rat spinal cord in vitro. Using a longitudinal split-bath setup, one hemicord was kept in normal solution, whereas the contralateral hemicord was exposed to 5-HT and NMDA. With this approach, rhythmic bursting could be induced in the ventral roots on the agonist-exposed side, whereas the ventral roots on the agonist-free side remained silent. Intracellular recordings were made from L1-L3 motoneurons on the silent agonist-free side during rhythmic activity in the contralateral ventral roots. At the resting membrane potential, the typical crossed synaptic input was a rhythmic barrage of depolarizing IPSPs. This input modulated the frequency of spikes induced with depolarizing direct current by inhibiting firing in phase with the contralateral bursts. Intracellular chloride loading increased the amplitude of the IPSPs, suggesting that they were chloride-dependent. Strychnine but not bicuculline generally blocked the rhythmic inhibitory input when added to the agonist-free side during contralateral rhythmicity. APV and CNQX on the agonist-free side abolished the rhythmic inhibitory input in most motoneurons but not in all. We suggest that rat spinal motoneurons receive a mainly glycinergic rhythmic inhibition from the contralateral half of the locomotor network. Unlike in simpler vertebrates, the crossed inhibition often appears to be at least disynaptic, involving inhibitory premotor neurons located on the same side as the receiving motoneurons. These premotor neurons are rhythmically excited via a crossed pathway that depends on glutamatergic transmission.

Chronic administration of NMDA glycine partial agonists induces tolerance in the Porsolt swim test

The Porsolt swim test (PST) was used to assess behavioral effects following acute or chronic treatment with two N-methyl-D-aspartate (NMDA) glycine partial agonists, 1-aminocyclopropanecarboxylic acid (ACPC), and D-cycloserine (DCS). Consistent with previous findings in mice, single intravenous doses of ACPC in rats produced a significant, dose-dependent reduction in immobility in the PST compared to saline. Single dose DCS also elicited significant dose-dependent reductions in PST immobility times. Single-dose ACPC or DCS (200 mg/kg) reduced immobility (p < 0.05) by 26 or 30%, respectively, compared to saline. However, multiple dosing with either ACPC or DCS (6 daily doses, 200 mg/kg) produced an apparent behavioral adaptation, as the immobility data were indistinguishable from chronic saline administration. Moreover, pretreatment with a 5-day course of ACPC or DCS promoted the development of a behavioral cross-tolerance following a sixth dose of DCS or ACPC, respectively. The development of a behavioral tolerance in the PST following chronic therapy of these drugs appears to be a general feature of glycine partial agonists. In toto, these findings support the hypothesis that chronic administration of NMDA glycine partial agonists produces a behavioral tolerance putatively through an adaptation of the NMDA receptor complex.

Glycine receptors in cultured chick sympathetic neurons are excitatory and trigger neurotransmitter release

1. Total RNA isolated from embryonic chick paravertebral sympathetic ganglia was used in a reverse transcription-polymerase chain reaction (RT-PCR) assay with a pair of degenerate oligonucleotide primers deduced from conserved regions of mammalian glycine receptor alpha-subunits. Three classes of cDNA were identified which encode portions of the chicken homologues of the mammalian glycine receptor alpha 1, alpha 2 and alpha 3 subunits. 2. The presence of functional glycine receptors was investigated in the whole-cell configuration of the patch-clamp technique in neurons dissociated from the ganglia and kept in culture for 7-8 days. In cells voltage clamped to -70 mV, glycine consistently induced inward currents in a concentration-dependent manner and elicited half-maximal peak current amplitudes at 43 microM. 3. The steady-state current-voltage relation for glycine-induced currents was linear between +80 and -60 mV, but showed outward rectification at more hyperpolarized potentials. Reversal potentials of these currents shifted with changes in intracellular chloride concentrations and matched the calculated Nernst potentials for chloride. 4. beta-Alanine and taurine were significantly less potent than glycine in triggering inward currents, with half-maximal responses at 79 and 86 microM, respectively. At maximally active concentrations, beta-alanine-evoked currents were identical in amplitude to those induced by glycine. Taurine-evoked currents, in contrast, never reached the same amplitude as glycine-induced currents. 5. The classical glycine receptor antagonist strychnine reversibly reduced glycine-induced currents, with half-maximal inhibition occurring at 62 nM. Two more recently characterized glycine receptor antagonists, isonipecotic acid (half-maximal inhibition at 2 mM) and 7-trifluoromethyl-4-hydroxyquinoline-3-carboxylic acid (half-maximal inhibition at 67 microM), also blocked glycine-evoked currents in a reversible manner. The chloride channel blocker picrotoxin reduced glycine-evoked currents, with half-maximal effects at 348 microM. Inhibition by the glycine receptor channel blocker cyanotriphenylborate was half-maximal at 4 microM. 6. Apart from evoking inward currents, glycine occasionally triggered short (< 100 ms) spike-like currents which were abolished by hexamethonium and thus reflected synaptic release of endogenous acetylcholine. In addition, glycine caused Ca(2+)-dependent and tetrodotoxin-sensitive tritium overflow from neurons previously labelled with [3H]noradrenaline. This stimulatory action of glycine was reduced in the presence of strychnine and after treatment with the chloride uptake inhibitor furosemide (frusemide). 7. In 65% of neurons loaded with the Ca2+ indicator fura-2 acetoxymethyl ester, glycine increased the ratio of the fluorescence signal obtained with excitation wavelengths of 340 and 380 nm, respectively, which indicates a rise in intracellular Ca2+ concentration. 8. The results show that sympathetic neurons contain transcripts for different glycine receptor alpha-subunits and carry functional heteromeric glycine receptors which depolarize the majority of neurons to trigger transmitter release.

Pharmacological and physiological characterization of murine homomeric beta3 GABA(A) receptors

Gamma-Aminobutyric acid (GABA[A]) receptor beta3 subunits were expressed in Xenopus laevis oocytes and studied using two-electrode voltage clamp. Injected oocytes exhibited an increased resting membrane conductance and more depolarized membrane potentials compared to uninjected control cells. Oocytes expressing beta3 subunits were insensitive to GABA and muscimol, but pentobarbitone increased the membrane conductance in a concentration-dependent manner. The membrane current response to pentobarbitone reversed at the Cl- equilibrium potential and at relatively high concentrations (> 500 microM), a rebound Cl- current was induced following the removal of pentobarbitone. In transfected human embryonic kidney (HEK) cells, the rebound current amplitude was reduced by desensitizing the beta3 receptor with increased durations of ligand application. Both picrotoxin (0.5 nM to 10 microM) and Zn2+ (10 nM to 100 microM) reduced the resting membrane conductance for beta3 cDNA-injected oocytes. These oocytes were insensitive to flurazepam (5 microM) and alphaxalone (10 microM), but responded with increased membrane conductance to propofol (10 microM) and pregnanolone (50 nM to 5 microM). The antagonists, bicuculline (10 microM) and strychnine (50 nM to 100 microM), also induced conductance increases in a concentration dependent manner; however, glycine (1 mM) was inactive. It was concluded that beta3 subunits form spontaneously opening ion channels that can be up-regulated by some allosteric modulators, principally by pentobarbitone and propofol and, surprisingly, by bicuculline and strychnine, whilst picrotoxin and Zn2+ acted as antagonists. Computer modelling of some kinetic schemes was used to describe the rebound current observed in transfected HEK cells. This indicated that pentobarbitone, after modulation of the conductance, is potentially capable of further binding to the beta3 receptor complex 'driving' the receptor into one or more desensitized states. This phenomenon may be of some importance for native neuronal GABA(A) receptors, where pentobarbitone can also evoke rebound current activation.

Respiratory rhythm generation in mammals: synaptic and membrane properties

Respiratory rhythm generation depends on a complex interaction between synaptic and membrane properties of functionally defined neurons. To gain a better understanding of how inhibitory and excitatory synaptic inputs lead to the generation of the respiratory rhythm we analyzed the depolarization pattern of respiratory neurons that were recorded in the transverse slice preparation of mice (P8-22) and the in vivo adult cat. Using voltage-calmp recordings from respiratory neurons and specific antagonists for inhibitory synaptic transmission we demonstrate under in vitro conditions, that inspiratory (n = 7) and post-inspiratory neurons (n = 13) received concurrent glycinergic and glutamatergic synaptic input during inspiration. A similar conclusion was gained with chloride injections into in vivo respiratory neurons. The inhibitory input was essential not only for generating the characteristic depolarization pattern of respiratory neurons, but also for switching the respiratory rhythm between inspiration and post-inspiration. The generation of the depolarization pattern depends also on intrinsic membrane properties. Negative current injections reveal that excitatory synaptic input was amplified by intrinsic bursting properties in some inspiratory neurons (n = 4) recorded in vitro. Although such properties have not been described under in vivo conditions our findings suggest that with respect to inspiratory, post-inspiratory and late-inspiratory neurons, the principle network organization is similar under both in vitro and in vivo conditions.

Currents evoked by GABA and glycine in acutely dissociated neurons from the rat medial preoptic nucleus

The responses of acutely dissociated medial preoptic neurons to application of GABA, and glycine were studied using the perforated-patch whole-cell recording technique under voltage-clamp conditions. GABA, at a concentration of 1 mM, evoked outward currents in all cells (n = 33) when studied at potentials positive to -80 mV. The I-V relation was roughly linear. The currents evoked by GABA were partially blocked by 25-75 microM picrotoxin and were also partially or completely blocked by 100-200 microM bicuculline. Glycine, at a concentration of 1 mM, did also evoke outward currents in all cells (n = 12) when studied at potentials positive to -75 mV. The I-V relation was roughly linear. The currents evoked by glycine were largely blocked by 1 microM strychnine. In conclusion, the present work demonstrates that neurons from the medial preoptic nucleus of rat directly respond to the inhibitory transmitters GABA and glycine with currents that can be attributed to GABAA receptors and glycine receptors respectively.

Use of differential normal pulse voltammetry for the measurement of locus coeruleus catecholaminergic metabolism in an acute anaesthetized rodent model of allodynia: effect of mexiletine

Neuropathic pain can be triggered by non-painful stimuli (e.g., light touch), a sensory abnormality termed allodynia. The acute blockade of spinal glycine receptors with intrathecal strychnine induces a reversible allodynia-like state in the rat. We describe the application of in vivo differential normal pulse voltammetry with carbon fibre micro-electrodes for monitoring the catechol oxidation current (CAOC) of the locus coeruleus (LC) in the strychnine model of allodynia. In addition, we tested the effect of mexiletine, a drug useful in the management of clinical neuropathic pain in this model. Our results show that somatosensory processing in the spinal cord of urethane-anaesthetized rats is radically altered during glycine receptor blockade such that the normally innocuous stimulus of hair deflection causes the marked activation of the LC as determined using in vivo differential normal pulse voltammetry. Mexiletine suppressed the LC and cardiovascular responses of strychnine induced allodynia. Results of this study indicate that LC CAOC, an index of LC neuronal activity: (a) is a sensitive biochemical index of strychnine-allodynia; (b) is temporally correlated with the cardiovascular and motor responses evoked by hair deflection during glycine receptor blockade; and (c) can be used to quantitate allodynia in the strychnine model.

Spinal strychnine alters response properties of nociceptive-specific neurons in rat medial thalamus

Experiments in both conscious and anesthetized animals indicate that intrathecal (i.t.) strychnine (STR; glycine receptor antagonist) produces acute, reversible allodynia, as evidenced by inappropriate behavioral and autonomic responses to cutaneous tactile stimuli. Although STR is known to produce disinhibition of afferent input to the spinal cord, changes in spinal reflexes cannot fully explain the complex behaviors observed following i.t. STR. Which supraspinal sites are involved in STR-dependent allodynia and how this abnormal somatosensory message is relayed to these sites remain to be determined. The medial thalamus contains many nociceptive-specific (NS) neurons and is believed to be involved in mediating the affective-motivational aspects of pain. It is thus important to determine whether spinally administered STR elicits changes in the responses of medial thalamic NS neurons. Extracellular single-unit recordings were conducted in urethan-anesthetized rats (290-490 g). A detailed characterization of 20 thalamic NS units (1 per rat; 2 in 1 case) was conducted before and immediately after i.t. STR (40 microg). Initially, all of the units in this study were classified as NS, because they were excited by noxious pinch but not by innocuous tactile stimuli. After i.t. STR, all (formerly NS) units exhibited significant responses to innocuous tactile stimuli (brush and/or air jet) applied to lumbar or sacral dermatomes. This effect of STR on thalamic NS neurons was acute and reversible. The majority of units (11 of 20) also exhibited an increase in spontaneous firing rate. Although the complete pinch receptive field (RF) could not be determined for all units, the available data indicate that the RFs for brush stimulation after i.t. STR were substantially different from the pre-STR pinch RFs for all but three units. The same i.t. STR injection that caused the observed changes in medial thalamus also produced allodynia, in the form of brush-evoked cardiovascular or motor responses, in 18 of the 19 rats. The ability of NS cells in medial thalamus to respond to tactile input after i.t. STR suggests that the STR lowers the threshold of nociceptive neurons that project directly and/or indirectly to medial thalamus. These observations suggest that ascending nociceptive pathways and medial thalamic structures contribute to the expression of STR-dependent allodynia.

Agonist action of taurine on glycine receptors in rat supraoptic magnocellular neurones: possible role in osmoregulation

1. To evaluate the implication of taurine in the physiology of supraoptic neurones, we (i) investigated the agonist properties of taurine on glycine and GABAA receptors of supraoptic magnocellular neurones acutely dissociated from adult rats, using whole-cell voltage clamp, (ii) studied the effects of taurine and strychnine in vivo by extracellular recordings of supraoptic vasopressin neurones in anaesthetized rats, and (iii) measured the osmolarity-dependent release of endogenous taurine from isolated supraoptic nuclei by HPLC. 2. GABA, glycine and taurine evoked rapidly activating currents that all reversed close to the equilibrium potential for Cl-, indicating activation of Cl(-)-selective channels. Glycine-activated currents were reversibly blocked by strychnine (IC50 of 35 nM with 100 microM glycine), but were unaffected by the GABAA antagonist gabazine (1-3 microM). GABA-activated currents were reversibly antagonized by 3 microM gabazine, but not by strychnine (up to 1 microM). 3. Responses to 1 mM taurine were blocked by strychnine but not by gabazine and showed no additivity with glycine-induced currents, indicating selective activation of glycine receptors. Responses to 10 mM taurine were partially antagonized by gabazine, the residual current being blocked by strychnine. Thus, taurine is also a weak agonist of GABAA receptors. 4. In the presence of gabazine, taurine activated glycine receptors with an EC50 of 406 microM. Taurine activated at most 70% of maximal glycine currents, suggesting that it is a partial agonist of glycine receptors. 5. In vivo, locally applied strychnine (300 nM) increased and taurine (1 mM) decreased the basal electrical activity of vasopressin neurones in normally hydrated rats. The effect of strychnine was markedly more pronounced in water-loaded rats. 6. Taurine, which is concentrated in supraoptic glial cells, could be released from isolated supraoptic nuclei upon hyposmotic stimulation. Decreases in osmolarity of 15 and 30% specifically enhanced basal release of taurine by 42 and 124%, respectively. 7. We conclude that supraoptic neurones express high amounts of glycine receptors, of which taurine may be regarded as a major natural agonist. We postulate that taurine, which can be released in hyposmotic situations, acts on glycine receptors to exert an inhibitory control on magnocellular neurones during alterations of body fluid homeostasis, implicating an active participation of glial cells in this neuroendocrine regulatory loop.

Glycinergic and GABAergic inputs affect short-term suppression in the cochlear nucleus

Most cochlear nucleus (CN) neurons exhibit short-term response suppression to a second stimulus in a paired-pulse (click), forward-masking, paradigm. The magnitude of suppression, which appears to be greater than that observed in acoustic nerve, is dependent on the temporal separation and/or relative intensities of the two stimuli. Recent evidence suggests that inhibitory circuitry ending on CN neurons may mediate this response suppression. Using extracellular recordings from single CN neurons, suppression was evaluated using a forward-masking paradigm. Responses to paired acoustic clicks (i.e., a 'masker' followed by an identical 'probe' click) were measured while the time interval between the masker and probe was varied systematically. The role of inhibitory circuitry in forward-masking in the CN was assessed by pharmacologic manipulation of the GABA(A) and glycine(I) (strychnine-sensitive) receptors. Blockade of glycinergic or GABAergic receptors by iontophoretic application of the antagonists, strychnine and bicuculline methiodide, decreased the effects of forward-masking by shortening recovery times of the probe response in 2/3 of the neurons tested. Conversely, agonist application (glycine, and GABA or muscimol) increased the magnitude of suppression and delayed recovery of the probe response relative to control values. These findings suggest that known circuits releasing glycine and/or GABA mediate short-term response suppression in some CN neurons.

Both high- and low voltage-activated calcium currents contribute to the light-evoked responses of luminosity horizontal cells in the Xenopus retina

We examined the contribution of two intrinsic voltage-dependent calcium channels to the light-evoked responses of a non-spiking retinal neuron, the horizontal cell (HC). HC's isolated from the Xenopus retina were studied by the whole cell version of the patch clamp. In a mixture of agents which suppressed Na- and K-dependent currents, we identified a transient, low voltage-activated Ca current suppressed by Ba2+ and blocked by Ni2+ (T-type) and a sustained, high voltage-activated, dihydropyridine-sensitive Ca current that was enhanced by Ba2+ (L-type). We made simultaneous intracellular recordings from rods and HC's in the intact, dark-adapted Xenopus retina. Under certain stimulus conditions, transient oscillations appeared in HC responses but were absent in rod light-evoked waveforms. One type of transient was seen at relatively hyperpolarized potentials (< -45 mV), was enhanced by Sr2+ and inhibited by Ni2+. It thus appears to depend on a T-type Ca-current. A second type of oscillation was seen to be superimposed on a prolonged depolarizing wave following light off in the HC and as spike-like depolarizations in rods. These oscillations were enhanced by Ba2+ and Sr2+, but blocked by the dihydropyridine, nifedipine, indicating their dependence on an L-type calcium conductance. All calcium-dependent oscillations were suppressed by 0.05-0.5 mM Co2+. Suppression of glutamate neurotransmission with CNQX or kynurenate, or glycine neurotransmission with strychnine, enhanced the HC oscillations.

Rhythmic motor activity and interlimb co-ordination in the developing pouch young of a wallaby (Macropus eugenii)

1. The forelimb motor behaviour of developing wallaby was studied. A clock-like alternating movement was reactivated whenever the animal was removed from the pouch. 2. Forelimb stepping frequency increased during the first 3 weeks of development, while the phase relationship remained constant. Forelimb activity could be affected by altering the afferent feedback from the contralateral limb, or an increase in ambient temperature. 3. In vitro experiments were performed using an isolated brainstem-spinal cord preparation from animals up to 6 weeks postnatal. Fictive locomotor activity could be evoked by electrical stimulation or bath-applied NMDA (< 10 microM). 4. Bath-applied strychnine (10-25 microM) and bicuculline (10-50 microM) disrupted the phase relationship between motor pools, while rhythmic motor discharge remained in the absence of these inhibitory pathways. 5. The present findings indicate that the pattern generator that underlies the robust forelimb movement during the first journey to the pouch is retained for different motor functions during in-pouch development. The neural network that underlies such behaviour can be divided into two major components, a rhythm generator within each hemicord, and a pattern co-ordinating pathway which involve both glycinergic and GABAergic interneurones.

Neuroprotective effects of the strychnine-insensitive glycine site NMDA antagonist (R)-HA-966 in an experimental model of Parkinson's disease

The neuroprotective effects of (R)-HA-966 and (S)-HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) were examined in an MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced animal model of Parkinson's disease. Systemic pretreatment of C57 black mice with the strychnine-insensitive glycine site antagonist, (R)-HA-966 (3-30 mg/kg, i.p.), dose-dependently attenuated MPTP-induced depletion of striatal dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC). Pretreatment with (R)-HA-966 also significantly protected the degeneration of tyrosine hydroxylase-positive neurons in the substantia nigra of mice treated with MPTP and alleviated the acute behavioral changes caused by the neurotoxin. In contrast, the other racemic form, (S)-HA-966, neither prevented the neurochemical depletions nor the neuronal injury caused by MPTP. These results indicate that excitatory mechanisms of neurodegeneration are involved in the pathophysiology of Parkinson's disease, and that strychnine-insensitive glycine site NMDA antagonists may serve as dopaminoprotective agents which intervene in the progressive neurodegeneration in Parkinson's disease.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus: whole-cell patch-clamp recordings from rat brain slice

The synaptic events underlying the excitation of neurons in the rat's dorsal nucleus of the lateral lemniscus were studied by whole-cell patch-clamp recordings in a brain slice preparation of the auditory midbrain. Both current-clamp and voltage-clamp data were obtained with the brain slice submerged in artificial cerebrospinal fluid. The rats were between 21 and 35 days of age at the time the recordings were made. Synaptic responses were evoked by a bipolar stimulating electrode placed on the lateral lemniscus just ventral to the dorsal nucleus. To eliminate glycinergic inhibitory responses, all physiological data were gathered with 0.5 microM strychnine added to the saline bath. Under current-clamp conditions, excitatory postsynaptic potentials could be subdivided into early and late components. The early component produced a single, highly reliable, short-latency spike and the later component produced a more variable, long-latency spike or train of spikes. The non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, completely blocked the early excitatory postsynaptic potential and its associated action potential. The N-methyl-D-aspartate antagonist, D,L-2-amino-5-phosphonovaleric acid, blocked the later excitatory postsynaptic potential and its action potentials. Typically, both early and late excitatory postsynaptic potentials could be recorded from the same cell, but the early excitatory postsynaptic potential was evoked at lower stimulus levels and had a larger amplitude than the later excitatory postsynaptic potential. Under voltage-clamp conditions, dorsal nucleus of the lateral lemniscus neurons responded to stimulation of the lateral lemniscus with excitatory postsynaptic currents. Outward excitatory postsynaptic currents were recorded with holding potentials that depolarized the cell membrane and inward currents were seen when the cell was hyperpolarized. The current-voltage (I-V) relation of the early peak portion of the excitatory postsynaptic current was nearly linear, whereas the I-V relation of the later excitatory postsynaptic current (12 ms after the peak) was non-linear over the range between -50 and - 100 mV. The outward excitatory postsynaptic current consisted of an early current that was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later current that was blocked by D,L-2-amino-5-phosphonovaleric acid. In artificial cerebrospinal fluid with normal concentrations of Mg2+, the inward excitatory postsynaptic current was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, but was not affected by D,L-2-amino-5-phosphonovaleric acid. In Mg2+-free artificial cerebrospinal fluid. however, the early component of the inward excitatory postsynaptic current was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later component was blocked by D,L-2-amino-5-phosphonovaleric acid. The results indicate that both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated synaptic responses are present in dorsal nucleus of the lateral lemniscus neurons of rats at 21-35 days of age. The N-methyl-D-aspartate component had a longer time-course and a higher threshold than the non-N-methyl-D-aspartate component, and was subject to a voltage-dependent Mg2+ block when the cell's membrane was hyperpolarized. The long-duration N-methyl-D-aspartate component is probably responsible for the prolonged inhibitory effect of dorsal nucleus of the lateral lemniscus neurons on physiological responses in the rat's inferior colliculus.

The glycine antagonist GV150526 protects somatosensory evoked potentials and reduces the infarct area in the MCAo model of focal ischemia in the rat

The neuroprotective activity of the novel, selective glycine antagonist GV150526 was assessed in the middle artery occlusion (MCAo) model of focal ischemia. Postischemia administration of GV150526 (3 mg/kg i.v.) up to 6 h post-MCAo resulted in a significant reduction of the infarct volume measured histologically 24 h later. The neuronal protection by GV150526 was accompanied by functionally significant protection determined by somatosensory evoked potential (SEP) responses recorded from the primary somatosensory cortex of rats under urethane anesthesia. Experimental occlusion of the MCA 7 days prior to electrophysiological testing induced a clear reduction in the SEP amplitude. GV150526 (3mg/kg, i.v.) was able to protect SEP responses recorded from the hindpaw cortical field in two groups of animals treated either 1 (n = 9) or 6 h (n = 10) post-MCAo. SEP responses recorded from the forepaw cortical field, an area closer to the core of the ischemic damage, were significantly protected only in the group treated 1 h post-MCAo. Histological evaluation of the rat brain regions showed a correlated decrease in the ischemic area of GV150526-treated groups. The volumes of the ischemic brains of both GV150526 groups were statistically different from the MCAo group (P < 0.05). These findings demonstrate that GV150526 is able to prevent the ischemic damage assessed histologically and affect the functional correlates of the ischemia evaluated by the electrophysiological SEP measurements.