Presynaptic Effects Research Articles

Dual synaptic sites of D1‐dopaminergic regulation of ethanol sensitivity of NMDA receptors in nucleus accumbens

Regulation of NMDAreceptor-mediated synaptic transmission onto accumbal medium spiny neurons (MSN) may constitute an important site in drug reward and reinforcement in mesolimbic structures. Previously, we reported that D(1)-like dopamine receptors activate a postsynaptic cAMP/PKA/DARPP-32 signaling cascade culminating in phosphorylation of SER897-NR1 subunits and a reduction in the sensitivity to ethanol of NMDA receptor-mediated synaptic transmission. Here, we use a detailed electrophysiological analysis of D(1)-like receptor regulation of the ethanol sensitivity of accumbal NMDA receptors (NMDARs) through recordings of quantal Sr(2+)-supported NMDA miniature synaptic currents (mEPSCs) in reduced Mg(2+) (0.6 mM) and report dual presynaptic and postsynaptic components of D(1)-like regulation of ethanol sensitivity of NMDARs. Ethanol inhibited NMDA mEPSC amplitude and frequency in a dose-dependent manner (25-75 mM), indicating inhibitory effects on presynaptic and postsynaptic components NMDA receptor-mediated synaptic transmission. The presynaptic inhibitory effect was corroborated by analysing the ratio of paired-pulse facilitation (PPF) of Ca(2+)-supported NMDA EPSCs. Activation of D(1) receptors with the agonist, SKF 38393 (25 microM), reversed ethanol suppression of NMDA mEPSC frequency and amplitude. Furthermore, the Mg(2+)-dependent decay off-rate of NMDA mEPSCs was substantially reduced by ethanol in a manner strongly reversed by the D(1) agonist. D(1) receptor-mediated attenuation of both the presynaptic and postsynaptic actions of ethanol was completely blocked by a D(1) selective antagonist (SCH 23390). These data suggest that D(1)-like receptors modulate both the presynaptic and postsynaptic effects of ethanol on NMDA receptor-mediated synaptic transmission in nucleus accumbens (NAc) and that these interactions may contribute to ethanol-induced neuroadaptation of the reward pathway.

Presynaptic Effects of Carbon Monoxide in the Myoneural Synapse of the Frog

Presynaptic Effects of Carbon Monoxide in the Myoneural Synapse of the Frog

The Estrogen Receptor Modulator Tamoxifen Enhances Spontaneous Glycinergic Synaptic Inhibition of Hypoglossal Motoneurons

Tamoxifen (Tam), a widely used anticancer agent, is now also used for healthy women with risk of breast cancer. Furthermore, it is the prototype of the selective estrogen receptor modulator family, with promise for neuroprotection. However, possible effects on neurotransmission have been little explored. Recently, Tam was shown to potentiate chloride responses to low concentrations of exogenous glycine in cultured spinal neurons from rat embryo. The present study investigates the possible modulation by Tam of the spontaneous synaptic glycinergic activity recorded from voltage-clamped hypoglossal motoneurons, using the whole-cell patch-clamp technique in brainstem slices from juvenile rat. Miniature currents were isolated with tetrodotoxin. Tam increased the mean amplitude of glycinergic miniature currents, by 68-79% at 2 microM (in nine of 10 cells) and by 47% at 0.5 microM (in four of nine cells). Furthermore, Tam markedly increased the frequency of glycinergic miniatures, by a factor reaching 15 in some neurons, even in the presence of the Ca2+ channel blocker Cd2+. Tam also increased the frequency of the total spontaneous glycinergic activity without tetrodotoxin. The increase in miniature amplitude is consistent with the increase in postsynaptic glycine receptor sensitivity previously reported. The increase in frequency indicates an additional presynaptic effect. Addition of exogenous glycine could also increase the frequency of glycinergic miniatures. Thus, one of the presynaptic effects of Tam might be potentiation of the basal activity of presynaptic glycine receptors facilitating glycine release. Possible risks related to modulation of glycinergic neurotransmission by Tam should be considered when recommending its use in healthy individuals.

Presynaptic Modulation of Synaptic Transmission by Pregnenolone Sulfate as Studied by Optical Recordings

The effects of pregnenolone sulfate (PREGS), a putative neurosteroid, on the transmission of perforant path-granule cell synapses were investigated with an optical recording technique in rat hippocampal slices stained with voltage-sensitive dyes. Application of PREGS to the bath solution resulted in an acute augmentation of EPSP in a dose-dependent manner. The PREGS effect was dependent on the extracellular Ca(2+) concentration ([Ca(2+)](o)), but independent of NMDA receptor activation. PREGS caused a decrease in paired-pulse facilitation, which implies that PREGS positively modulates presynaptic neurotransmitter releases. Firmer support for this mechanism was that PREGS augmented the synaptically induced glial depolarization (SIGD) that reflects the activity of electrogenic glutamate transporters in glial cells during the uptake of released glutamate. The selective alpha7nAChR antagonist alpha-BGT or MLA prevented the SIGD increase by PREGS. Furthermore DMXB, a selective alpha7nAChR agonist, mimicked the PREGS effect on SIGD and antagonized the effect of PREGS. The presynaptic effect of PREGS was partially attenuated by the L-type Ca(2+) channel (VGCC) blocker nifedipine. Based on these findings, we proposed a novel mechanism underlying the facilitated synaptic transmission by PREGS: this neurosteroid sensitizes presynaptic alpha7nAChR that is followed by an activation of L-type VGCC to increase the presynaptic glutamate release.

Short‐Term Facilitation and Depression in the Cerebellum: Some Observations on Wild‐Type and Mutant Rodents Deficient in the Extracellular Matrix Molecule Tenascin C

Short-term plasticity was studied on synapses to Purkinje cells (PC): paired-pulse facilitation in parallel fibers (PF) and paired-pulse depression in climbing fibers (CF). Both phenomena relate to synaptic strength. These forms of short-term plasticity were tested on cerebellar slices in rat by early postnatal synchronous stimulation of olivary neurons (i.e., CFs) with harmaline and by inhibition of a metabotropic glutamate receptor (mGluR) as well as in mice that were deficient in the extracellular matrix glycoprotein tenascin-C. Harmaline stimulation delayed the developmental competition between CF inputs and maintained multiple innervation. Paired-pulse depression of the CF-PC synapse after harmaline treatment was more expressed. However, paired-pulse facilitation in PF-PC synapses remained unchanged. Electrophysiological responses of postsynaptic mGluR1 in CF-PC synapses could be obtained only with AMPA receptors blocked and glutamate uptake impaired. The mGluR1-specific antagonist CPCCOEt suppressed the CF-mGluR EPSC in some PCs and potentiated it in other PCs. CF paired-pulse depression was not changed with CPCCOEt, thus excluding a presynaptic effect. The postsynaptic effect was underlined by CPCCOEt-induced rise in amplitude of EPSC and by a prolongation of its decay time. Tenascins are extracellular matrix glycoproteins that may restrict the regenerative capacity of the nervous tissue. Testing short-term presynaptic plasticity in tenascin-C-deficient mice showed that CF paired-pulse depression was less expressed while PF paired-pulse facilitation was augmented except in a group of cells where there was even depression. The results underline differences in forms of short-term plasticity with regard to susceptibility to diverse modulatory factors.

Inhibition of Perforant Path Input to the CA1 Region by Serotonin and Noradrenaline

Bath-applied monoamines-dopamine (DA), serotonin (5-HT), and noradrenaline (NE)-strongly suppress the perforant path (PP) input to CA1 hippocampal region with very little effect on the Schaffer collaterals (SC) input. The effect of DA action on PP field excitatory postsynaptic potential (fEPSP) has been characterized in detail, but relatively little is known about the NE and 5-HT effects. Here we show that the maximal inhibition of the PP fEPSP by NE is approximately 55%, whereas 5-HT inhibition is weaker ( approximately 35%). The half-maximal inhibitory concentration of both 5-HT and NE is approximately 1 muM. Neither NE nor 5-HT affected paired-pulse facilitation, suggesting that the effect is not presynaptic. This is in contrast to DA, which does have a presynaptic effect. The NE effect was blocked by alpha2 antagonists, whereas the alpha1 antagonist corynanthine and beta-antagonist propranolol were ineffective. The effect of 5-HT was mimicked by the agonist, 5-carboxamidotryptamine maleate (5-CT), and not affected by adrenergic and dopaminergic antagonists. To determine the 5-HT receptors involved, we tested a number of 5-HT antagonists, but none produced a complete suppression of the 5-HT effect. Of these, only the 5-HT7 and 5-HT2 antagonists produced weak but significant inhibition of 5-HT effect. We conclude that NE inhibits the PP fEPSP through postsynaptic action on alpha2-adrenoceptors and that 5-HT7, 5-HT2, and some other receptor may be involved in 5-HT action in PP.

Adenosine Mediation of Presynaptic Feedback Inhibition of Glutamate Release

Conditions of increased metabolic demand relative to metabolite availability are associated with increased extracellular adenosine in CNS tissue. Synaptic activation of postsynaptic NMDA receptors on neurons of the cholinergic brainstem arousal center can increase sufficient extracellular adenosine to act on presynaptic A1 adenosine receptors (A1ADRs) of glutamate terminals, reducing release from the readily releasable pool. The time course of the adenosine response to an increase in glutamate release is slow (tau > 10 min), consistent with the role of adenosine as a fatigue factor that inhibits the activity of cholinergic arousal centers to reduce arousal.

Selective α2-adrenergic properties of dexmedetomidine over clonidine in the human forearm

We tested the hypothesis that dexmedetomidine (Dex) has greater alpha(2)- vs. alpha(1) selectivity than clonidine and causes more alpha(2)-selective vasoconstriction in the human forearm. After local beta-adrenergic blockade with propranolol, forearm blood flow (plethysmography) responses to brachial artery administration of Dex, clonidine, and phenylephrine (alpha(1)-agonist) were determined in healthy young adults before and after alpha(2)-blockade with yohimbine (n = 10) or alpha(1)-blockade with prazosin (n = 9). Yohimbine had no effect on phenylephrine-mediated vasoconstriction but blunted Dex-mediated vasoconstriction (mean +/- SE: -41 +/- 5 vs. -11 +/- 2%; before vs. after yohimbine) more than clonidine-mediated vasoconstriction (-39 +/- 5 vs. -28 +/- 4%; before vs. after yohimbine) (P < 0.02). Prazosin blunted phenylephrine-mediated vasoconstriction (-39 +/- 4 vs. -8 +/- 2%; before vs. after prazosin) but had similar effects on both Dex- (-30 +/- 4 vs. -39 +/- 6%; before vs. after prazosin) and clonidine-mediated vasoconstriction (-29 +/- 3 vs. -41 +/- 7%; before vs. after prazosin) (P > 0.7). Both Dex and clonidine reduced deep forearm venous norepinephrine concentrations to a similar extent (-59 +/- 12 vs. -55 +/- 10 pg/ml; Dex vs. clonidine, P > 0.6); this effect was abolished by yohimbine and blunted by prazosin. These results suggest that Dex causes more alpha(2)-selective vasoconstriction in the forearm than clonidine. The similar vasoconstrictor responses to both drugs after prazosin might be explained by the presynaptic effects on norepinephrine release.

Brain-Derived Neurotrophic Factor Enhances GABA Release Probability and Nonuniform Distribution of N- and P/Q-Type Channels on Release Sites of Hippocampal Inhibitory Synapses

Long-lasting exposures to brain-derived neurotrophic factor (BDNF) accelerate the functional maturation of GABAergic transmission in embryonic hippocampal neurons, but the molecular bases of this phenomenon are still debated. Evidence in favor of a postsynaptic site of action has been accumulated, but most of the data support a presynaptic site effect. A crucial issue is whether the enhancement of evoked IPSCs (eIPSCs) induced by BDNF is attributable to an increase in any of the elementary parameters controlling neurosecretion, namely the probability of release, the number of release sites, the readily releasable pool (RRP), and the quantal size. Here, using peak-scaled variance analysis of miniature IPSCs, multiple probability fluctuation analysis, and cumulative amplitude analysis of action potential-evoked postsynaptic currents, we show that BDNF increases release probability and vesicle replenishment with little or no effect on the quantal size, the number of release sites, the RRP, and the Ca2+ dependence of eIPSCs. BDNF treatment changes markedly the distribution of Ca2+ channels controlling neurotransmitter release. It enhances markedly the contribution of N- and P/Q-type channels, which summed to >100% ("supra-additivity"), and deletes the contribution of R-type channels. BDNF accelerates the switch of presynaptic Ca2+ channel distribution from "segregated" to "nonuniform" distribution. This maturation effect was accompanied by an uncovered increased control of N-type channels on paired-pulse depression, otherwise dominated by P/Q-type channels in untreated neurons. Nevertheless, BDNF preserved the fast recovery from depression associated with N-type channels. These novel presynaptic BDNF actions derive mostly from an enhanced overlapping and better colocalization of N- and P/Q-type channels to vesicle release sites.

The General Anesthetic Isoflurane Depresses Synaptic Vesicle Exocytosis

General anesthetics have marked effects on synaptic transmission, but the mechanisms of their presynaptic actions are unclear. We used quantitative laser-scanning fluorescence microscopy to analyze the effects of the volatile anesthetic isoflurane on synaptic vesicle cycling in cultured neonatal rat hippocampal neurons monitored using either transfection of a pH-sensitive form of green fluorescent protein fused to the luminal domain of VAMP (vesicle-associated membrane protein), (synapto-pHluorin) or vesicle loading with the fluorescent dye FM 1-43. Isoflurane reversibly inhibited action potential-evoked exocytosis over a range of concentrations, with little effect on vesicle pool size. In contrast, exocytosis evoked by depolarization in response to an elevated extracellular concentration of KCl, which is insensitive to the selective Na+ channel blocker tetrodotoxin, was relatively insensitive to isoflurane. Inhibition of exocytosis by isoflurane was resistant to bicuculline, indicating that this presynaptic effect is not caused by the well known GABA(A) receptor modulation by volatile anesthetics. Depression of exocytosis was mimicked by a reduction in stimulus frequency, suggesting a reduction in action potential initiation, conduction, or coupling to Ca2+ channel activation. There was no evidence for a direct effect on endocytosis. The effects of isoflurane on synaptic transmission are thus caused primarily by inhibition of action potential-evoked synaptic vesicle exocytosis at a site upstream of Ca2+ entry and exocytosis, possibly as a result of Na+ channel blockade and/or K+ channel activation, with the possibility of lesser contributions from Ca2+ channel blockade and/or soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated vesicle fusion.

Presynaptic and Postsynaptic Roles of NO, cGK, and RhoA in Long-Lasting Potentiation and Aggregation of Synaptic Proteins

Recent results suggest that long-lasting potentiation at hippocampal synapses involves the rapid formation of clusters or puncta of presynaptic as well as postsynaptic proteins, both of which are blocked by antagonists of NMDA receptors and an inhibitor of actin polymerization. We have investigated whether the increase in puncta involves retrograde signaling through the NO-cGMP-cGK pathway and also examined the possible roles of two classes of molecules that regulate the actin cytoskeleton: Ena/VASP proteins and Rho GTPases. Our results suggest that NO, cGMP, cGK, actin, and Rho GTPases including RhoA play important roles in the potentiation and act directly in both the presynaptic and postsynaptic neurons, where they contribute to the increase in puncta of synaptic proteins. cGK phosphorylates synaptic VASP during the potentiation, whereas Rho GTPases act both in parallel and upstream of cGMP, in part by maintaining the synaptic localization of soluble guanylyl cyclase.

Presynaptic effects of immunoglobulin G from patients with Lambert–Eaton myasthenic syndrome: Their neutralization by intravenous immunoglobulins

Intravenous immunoglobulin (IVIg) treatment improves muscle strength in Lambert-Eaton myasthenic syndrome (LEMS), but its specific mode of action is unknown. We have delineated its mode of action on neuromuscular blocking properties of LEMS IgG. The effect of sera and purified IgG from six patients with LEMS on evoked quantal release was investigated after direct application to the motor nerve terminal by the perfused macro-patch-clamp electrode in mouse hemidiaphragms. The effect of LEMS IgG was analyzed alone and after coincubation with different concentrations of IVIg or its Fab fragments. All LEMS sera and purified LEMS IgG fractions taken before IVIg treatment inhibited evoked quantal release in a dose-dependent manner. When LEMS IgG was coincubated with a therapeutic IVIg preparation, presynaptic inhibitory activity of LEMS IgG was diminished in a dose-dependent fashion. Monovalent Fab fragments were as effective in neutralizing the activity of LEMS IgG as whole IVIg. These direct neutralizing effects of IVIg may explain its therapeutic efficacy.

Deletion of presynaptic adenosine A 1 receptors impairs the recovery of synaptic transmission after hypoxia

Adenosine protects neurons during hypoxia by inhibiting excitatory synaptic transmission and preventing NMDA receptor activation. Using an adeno-associated viral (AAV) vector containing Cre recombinase, we have focally deleted adenosine A 1 receptors in specific hippocampal regions of adult mice. Recently, we found that deletion of A 1 receptors in the CA1 area blocks the postsynaptic responses to adenosine in CA1 pyramidal neurons, and deletion of A 1 receptors in CA3 neurons abolishes the presynaptic effects of adenosine on the Schaffer collateral input [J Neurosci 23 (2003) 5762]. In the current study, we used this technique to delete A 1 receptors focally from CA3 neurons to investigate whether presynaptic A 1 receptors protect synaptic transmission from hypoxia. We studied the effects of prolonged (1 h) hypoxia on the evoked field excitatory postsynaptic potentials (fEPSPs) in the CA1 region using in vitro slices. Focal deletion of the presynaptic A 1 receptors on the Schaffer collateral input slowed the depression of the fEPSPs in response to hypoxia and impaired the recovery of the fEPSPs after hypoxia. Delayed responses to hypoxia linearly correlated with impaired recovery. These findings provide direct evidence that the neuroprotective role of adenosine during hypoxia depends on the rapid inhibition of synaptic transmission by the activation of presynaptic A 1 receptors.

Selective subunit antagonists suggest an inhibitory relationship between NR2B and NR2A-subunit containing N-methyl-d-aspartate receptors in hippocampal slices

Glutamate receptors responding to N-methyl-D: -aspartate (NMDA) are involved in neural development, excitotoxicity and neuronal plasticity. Each receptor includes at least two NR2 subunits. Here, we have examined the effects of selective antagonists of NR2A and NR2B subunits (NVP-AAM07 and Ro25-6981 respectively) on the effects of NMDA in the CA1 field of rat hippocampal slices. We have observed that Ro25-6981 potentiates, rather than blocks, the effects of NMD on field EPSPs and paired-pulse interactions (indicators of presynaptic effects) and on postsynaptic depolarisation in hippocampal slices. The NR2A subunit antagonist NVP-AAM077 blocks the effects of NMDA alone, or after potentiation by Ro25-6981. The potentiation of NMDA by Ro25-6981 was not prevented by staurosporine (protein kinase inhibitor), okadaic acid (an inhibitor of serine/threonine protein phosphatases) or anisomycin (protein synthesis inhibitor), but was prevented by cyclosporin A, which inhibits Ca2+/calmodulin-dependent phosphatase 2B [calcineurin]. NMDA-dependent long-term potentiation (LTP) induced by electrical stimulation was not prevented by Ro25-6981 but was prevented by selective blockade of the NR2A subunit. The results suggest that, at both presynaptic and postsynaptic sites in the rat hippocampus, NR2B-subunit-containing receptors limit NMDA receptor function by inhibitory restraint over NR2A-subunit-containing receptors, via calcineurin activation, and that LTP induction critically involves primarily receptors containing the NR2A subunit. Endogenous factors or drugs that modify this NR2B/NR2A interaction could have a major influence on synaptic transmission and plasticity in the brain.

GABAergic integration of progesterone and androgen feedback to gonadotropin-releasing hormone neurons.

Steroid feedback regulates GnRH secretion and previous work has implicated gamma-aminobutyric acid (GABA)ergic neurons as a mediator of these effects. We examined GABAergic postsynaptic currents (PSCs) in green fluorescent protein-identified GnRH neurons from mice exposed to different steroid milieus in vivo. Adult mice were ovariectomized and treated with estradiol (OVX+E, controls) or E plus progesterone (P, OVX+E+P). P decreased PSC frequency, a presynaptic effect, and PSC size, which could be via pre- and/or postsynaptic mechanisms. In contrast, dihydrotestosterone (DHT, OVX+E+DHT) increased both GABAergic PSC frequency and size in GnRH neurons. Tetrodotoxin (TTX), which eliminates action-potential-dependent presynaptic effects, did not alter frequency, suggesting DHT may have increased PSC frequency by increasing connectivity between GABAergic and GnRH neurons. TTX reduced PSC size below control values, indicating DHT may augment presynaptic GABA release but inhibits the postsynaptic GnRH neuron response. In mice treated with both P and DHT (OVX+E+P+DHT), PSC frequency and size were similar to controls, suggesting these steroids counteract one another. These results demonstrate GABAergic neurons participate in integrating and conveying steroid feedback to GnRH neurons, defining a potential central mechanism for steroid regulation of GnRH neurons during the reproductive cycle, and providing one possible mechanism for increased activity of these cells in hyperandrogenic females.

Modulation of presynaptic activity by phosphorylation in cultured rat spinal dorsal horn neurons

Abstract Phosphorylation, in particular by protein kinase C (PKC), modulates spinal sensory transmission and nociceptive behaviors. Whereas PKC's postsynaptic actions are well established, its presynaptic effects in spinal sensory neurons are mostly inferred from postsynaptic recordings. Here we first show that the amphipathic styryl dye FM 1-43 can be used to image presynaptic activity in cultured spinal dorsal horn cultures and then test whether PKC modulates presynaptic activity in cultured spinal dorsal horn neurons. Pretreatment with the broad-spectrum kinase inhibitor staurosporine (2 μmol/L) inhibited dye release. Bisindolylmaleimide I, a PKC inhibitor, potentiated dye release at low doses (200 nmol/L and 1 μmol/L), while inhibiting it at a higher dose (5 μmol/L). Activating PKC with phorbol dibutyrate (0.5 μmol/L) induced an increase in exocytosis, which is partially blocked by bisindolylmaleimide I. These results indicate that styryl dyes can be used to observe presynaptic regulation of spinal dorsal horn neurons, and that PKC acts presynaptically to modulate spinal sensory transmission. Perspective: With dye imaging technique, we demonstrate here that PKC presynaptically regulates sensory transmission in spinal dorsal horn neurons. In combination with conventional whole-cell patch-clamp recording technique, the present study provides a new methodology for studying spinal sensory transmission and modulation and facilitates our understanding of pain mechanism.

Blockade of CGRP receptors in the intracranial vasculature: a new target in the treatment of headache.

In primary headaches, there is a clear association between the headache and the release of calcitonin gene-related peptide (CGRP) but not with any of the other neuronal messengers. The purpose of this review is to describe the role of CGRP in the intracranial circulation and to elucidate a possible role for a specific CGRP receptor antagonist in the treatment of primary headaches. Acute treatment with a 5-HT(1B/1D) agonist (triptan) results in alleviation of the headache and normalization of the cranial venous CGRP levels, in part due to a presynaptic inhibitory effect on sensory nerves. The central role of CGRP in migraine and cluster headache pathophysiology has led to the search for small molecule CGRP antagonists with few cardiovascular side-effects. The initial pharmacological profile of such a group of compounds has recently been disclosed. One of these compounds has been found to be efficacious in the relief of acute attacks of migraine.

Presynaptic GABAB receptors modulate thalamic excitation of inhibitory and excitatory neurons in the mouse barrel cortex.

Cortical inhibition plays an important role in the processing of sensory information, and the enlargement of receptive fields by the in vivo application of GABAB receptor antagonists indicates that GABAB receptors mediate some of this cortical inhibition. Although there is evidence of postsynaptic GABAB receptors on cortical neurons, there is no evidence of GABAB receptors on thalamocortical terminals. Therefore to determine if presynaptic GABAB receptors modulate the thalamic excitation of layer IV inhibitory neurons and excitatory neurons in layers II-III and IV of the somatosensory "barrel" cortex of mice, we used a thalamocortical slice preparation and patch-clamp electrophysiology. Stimulation of the ventrobasal thalamus elicited excitatory postsynaptic currents (EPSCs) in cortical neurons. Bath application of baclofen, a selective GABAB receptor agonist, reversibly decreased AMPA receptor-mediated and N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs in inhibitory and excitatory neurons. The GABAB receptor antagonist, CGP 35348, reversed the inhibition produced by baclofen. Blocking the postsynaptic GABAB receptor-mediated effects with a Cs+ -based recording solution did not affect the inhibition, suggesting a presynaptic effect of baclofen. Baclofen reversibly increased the paired-pulse ratio and the coefficient of variation, consistent with the presynaptic inhibition of glutamate release. Our results indicate that the presynaptic activation of GABAB receptors modulates thalamocortical excitation of inhibitory and excitatory neurons and provide another mechanism by which cortical inhibition can modulate the processing of sensory information.

The mechanism of inhibition by xanthine of adenosine A 1-receptor responses in rat hippocampus

We have recently observed that the free radical-generating mixture of xanthine and xanthine oxidase (X/XO) can suppress the inhibitory effects of adenosine on synaptic transmission in the hippocampus, but that this action can be mimicked by xanthine alone. We have now clarified the mechanism of these interactions by using the new, potent and highly selective inhibitor of xanthine oxidase, 1-(3-cyano-4-neopentyloxyphenyl)pyrazole-4-carboxylic acid (Y-700). Field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region of rat hippocampal slices. X/XO induced a long-lasting increase of fEPSP slope and significantly reduced the presynaptic inhibitory effect of adenosine. Both these actions were prevented by Y-700 at a concentration of only 200 nM. Similarly the superfusion of xanthine alone increased fEPSP slope and reduced sensitivity to adenosine but these effects were also prevented by Y-700. The results indicate that the antagonism of adenosine responses by X/XO or by xanthine alone are entirely attributable to the activity of the added or endogenous XO activity, probably generating free radicals, and are not likely to be caused by a direct antagonistic action at the xanthine-sensitive site on the adenosine receptor.

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. The purpose of this study is to explore the contribution of GABAergic neurons to the regulation of locomotor burst frequency in the lamprey model. Using gabazine, a competitive GABAA antagonist more specific than bicuculline, the goal was to provide a detailed analysis of the influence of an endogenous activation of GABAA receptors on fictive locomotion, as well as their possible interaction with GABAB and involvement of GABAC receptors. During N-methyl-D-aspartate (NMDA)-induced fictive locomotion (ventral root recordings in the isolated spinal cord), gabazine (0.1-100 microM) significantly increased the burst rate up to twofold, without changes in regularity or "burst quality." Gabazine had a proportionately greater effect at higher initial burst rates. Picrotoxin (1-7.5 microM), a less selective GABAA antagonist, also produced a pronounced increase in frequency, but at higher concentrations, the rhythm deteriorated, likely due to the unspecific effects on glycine receptors. The selective GABAB antagonist CGP55845 also increased the frequency, and this effect was markedly enhanced when combined with the GABAA antagonist gabazine. The GABAC antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA) had no effect on locomotor bursting. 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. It is not required for burst generation, but acts as a powerful modulator.