Substance P abolishes the facilitatory effect of ATP on spontaneous glycine release in neurons of the trigeminal nucleus pars caudalis

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

Participation of calbindin-D28K in nociception: results from calbindin-D28K knockout mice

Since calbindin-D(28K) (CB-D(28K))-positive neurons have been related to nociceptive sensory processing, we have hypothesized that altered CB-D(28K) expression could alter nociceptive transmission. We have used +/+ and -/- knockout (KO) mice for CB-D(28k) in different behavioral models of pain and sensory responses at the caudalis subdivision of the trigeminal spinal nucleus in order to understand how this protein may participate in nociception. Behavioral responses to formalin injection in the hind paw or at the whisker pad or in the hind paw glutamate or i.p. acetic acid tests showed an increase of the pain threshold in CB-D(28k) -/- mice. KO mice showed a diminution of the inhibitory activity at Sp5C nucleus and a marked reduction of GABA content. Sp5C neurons from CB-D(28k) -/- mice did not change their spontaneous activity or tactile response after formalin injection in the whisker pad. In contrast, Sp5C neurons increased their spontaneous firing rate and tactile response after formalin injection in their receptive field in CB-D(28k) +/+ mice. The results of this study demonstrate the active role played by CB-D(28k) in nociceptive sensory transmission. The lack of this calcium binding protein, associated to deficient GABAergic neurotransmission, translates into dysfunction of sensory processing of nociceptive stimuli.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

Ca2+/calmodulin-dependent kinase II signalling cascade mediates P2X7 receptor-dependent inhibition of neuritogenesis in neuroblastoma cells

ATP, via purinergic P2X receptors, acts as a neurotransmitter and modulator in both the central and peripheral nervous systems, and is also involved in many biological processes, including cell proliferation, differentiation and apoptosis. Previously, we have reported that P2X7 receptor inhibition promotes axonal growth and branching in cultured hippocampal neurons. In this article, we demonstrate that the P2X7 receptor negatively regulates neurite formation in mouse Neuro-2a neuroblastoma cells through a Ca2+/calmodulin-dependent kinase II-related mechanism. Using both molecular and immunocytochemical techniques, we characterized the presence of endogenous P2X1, P2X3, P2X4 and P2X7 subunits in these cells. Of these, the P2X7 receptor was the only functional receptor, as its activation induced intracellular calcium increments similar to those observed in primary neuronal cultures, exhibiting pharmacological properties characteristic of homomeric P2X7 receptors. Patch-clamp experiments were also conducted to fully demonstrate that ionotropic P2X7 receptors mediate nonselective cation currents in this cell line. Pharmacological inhibition of the P2X7 receptor and its knockdown by small hairpin RNA interference resulted in increased neuritogenesis in cells cultured in low serum-containing medium, whereas P2X7 overexpression significantly reduced the formation of neurites. Interestingly, P2X7 receptor inhibition also modified the phosphorylation state of focal adhesion kinase, Akt and glycogen synthase kinase 3, protein kinases that participate in the Ca2+/calmodulin-dependent kinase II signalling cascade and that have been related to neuronal differentiation and axonal growth. Taken together, our results provide the first mechanistic insight into P2X7 receptor-triggered signalling pathways that regulate neurite formation in neuroblastoma cells.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Purinergic P2X receptors facilitate inhibitory GABAergic and glycinergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

This study examined whether adenosine 5'-triphosphate (ATP) modulated inhibitory glycinergic and GABAergic neurotransmission to cardiac vagal neurons. Inhibitory activity to cardiac vagal neurons was isolated and examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. ATP (100 microM) evoked increases in the frequency of glycinergic and GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in cardiac vagal neurons which were blocked by the broad P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (100 microM). Application of the P2Y agonists uridine triphosphate (15 microM) and adenosine 5'-0-(Z-thiodiphosphate) (60 microM) did not enhance inhibitory neurotransmission to cardiac vagal neurons however, application of the selective P2X; receptor agonist, alpha, beta-methylene ATP (100 microM), increased glycinergic and GABAergic mIPSC neurotransmission to cardiac vagal neurons. The increase in inhibitory neurotransmission evoked by alpha, beta-methylene ATP was abolished by the selective P2X receptor antagonist 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (100 microM) indicating P2X receptors enhance the release of inhibitory neurotransmitters to cardiac neurons. The voltage-gated calcium channel blocker cadmium chloride did not alter the evoked increase in inhibitory mIPSCs. This work demonstrates that P2X receptor activation enhances inhibitory neurotransmission to parasympathetic cardiac vagal neurons and demonstrates an important functional role for ATP mediated purinergic signaling to cardiac vagal neurons.

Pharmacological analysis of excitatory and inhibitory synaptic transmission in horizontal brainstem slices preserving three subnuclei of spinal trigeminal nucleus

Spinal trigeminal nucleus (Vsp) consists of three subnuclei: oralis (Vo), interpolaris (Vi) and caudalis (Vc). Previous anatomical studies using antero-/retro-grade tracers have suggested that intersubnuclear ascending/descending synaptic transmissions exist between subnuclei. However, pharmacological properties of the intersubnuclear synaptic transmission have not been studied yet. Since three subnuclei are located in Vsp along rostro-caudal axis, it will be necessary to prepare horizontal brainstem slices to perform pharmacological analysis of the intersubnuclear synaptic transmission. We here show horizontal brainstem slices retaining three subnuclei, and that, using blind whole-cell recordings in the slices, synaptic transmission may be abundantly retained between subnuclei in the horizontal slices, except for the transmission from Vo to Vc. Finally, pharmacological analysis shows that excitatory and inhibitory synaptic responses, respectively, are mediated by AMPA and NMDA receptors and by GABA(A) and glycine receptors, with a differential contribution to the synaptic responses between subnuclei. We therefore conclude that horizontal brainstem slices will be a useful preparation for studies on intersubnuclear synaptic transmission, modulation and plasticity between subnuclei, as well as, further, other brainstem nuclei.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

P2Y1and P2X7receptors induce calcium/calmodulin-dependent protein kinase II phosphorylation in cerebellar granule neurons

The activation of nucleotide receptors-- both ionotropic, P2X, and most of metabotropic, P2Y-- increases intracellular calcium concentration, resulting in calcium/calmodulin-dependent protein kinase II (CaMKII) activation. Stimulation of cerebellar granule neurons in culture-- with different P2X and P2Y agonists and their effect on CaMKII phosphorylation-- was studied using immunocytochemical and microfluorimetrical techniques. P2X agonist: 2'-3'-o-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP) and diadenosine pentaphosphate (Ap(5)A); and P2Y agonists: 2-(methylthyo)-adenosine diphosphate (2MeSADP) and uridine 5'-bisphosphate (UDP); tested induced a CaMKII phosphorylation but with a different immunostaining pattern in each group. Stimulation with 2MeSADP induced a Ca(2+) release from intracellular stores and a significant CaMKII phosphorylation in cell somas and neurites. This agrees with the subcellular distribution of P2Y(1). MRS 2179, a specific P2Y(1) inhibitor, antagonized the 2MeSADP effect. On the other hand, cerebellar granule neuron stimulation with BzATP, in Mg(2+)-free conditions, produced extracellular calcium entrance and, as a result, a significant increase in CaMKII phosphorylation mostly in fibres, which correspond with P2X(7) subdistribution. Immunocytochemical and microfluorimetrical experiments, using Zn(2+) and Brilliant Blue G (BBG), as a specific P2X(7) antagonist, confirmed that BzATP was acting through the P2X(7) receptor. These results indicate that P2Y(1) and P2X(7) produce a significant increase in CaMKII phosphorylation, but show important differences in subcellular distribution and in effect duration. P2X(7) activation in granule neurons is not associated with pore formation, according to the absence of YO-PRO-1 fluorescence. The abundant presence of P2X(7) at the synaptic structures suggests a relevant role played by this receptor in synaptic plasticity.

Frequency-dependent modulation of glycine receptor activation recorded from the zebrafish larvae hindbrain

In vertebrates, most glycinergic inhibitory neurons discharge phasically at a relatively low frequency. Such a pattern of glycine liberation from presynaptic terminals may affect the kinetics of post-synaptic glycine receptors. To examine this influence, we have analyzed the behavior of glycine receptors in response to repetitive stimulation at frequencies at which consecutive outside-out currents did not superimpose (0.5-4 Hz). Neurotransmitter release was mimicked on outside-out patches from zebrafish hindbrain Mauthner cells using fast flow application techniques. The amplitude of outside-out currents evoked by short (1 ms) repetitive applications of a saturating concentration (3 mM) of glycine remained unchanged for application frequencies<or=1 Hz. When the application frequency was increased from 1 to 4 Hz, the amplitude of the outside-out currents decreased with time to reach a steady state level. This decrease in current amplitude was larger and occurred faster with increasing application frequencies. Recovery occurred when the stimulation frequency was decreased back to 1 Hz. The recovery time constant was independent on the application frequency. This frequency-dependent inhibition was also observed for non-saturating glycine concentrations. Our results indicate that glycine receptor activity is down-regulated when the stimulation frequency increases to values>1 Hz. Glycine-evoked current simulations using a simple Markov model describing zebrafish glycine receptor kinetic behavior, indicates that this down-regulation of glycine receptor efficacy is due to a progressive accumulation of the receptors in a long lasting desensitization state. Our simulations suggest that this down-regulation can occur even when spontaneous inhibitory currents were generated randomly at a frequency>1 Hz.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

Adenosine inhibits GABAergic and glycinergic transmission in adult rat substantia gelatinosa neurons

The effect of adenosine on inhibitory postsynaptic currents (IPSCs) was examined in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. Adenosine reversibly reduced the amplitude of GABAergic and glycinergic electrically evoked IPSCs (eIPSCs) in a dose-dependent manner (EC50 = 14.5 and 19.1 microM, respectively). The A1 adenosine-receptor agonist N6-cyclopentyladenosine also reduced the eIPSCs, whereas the A1 antagonist 8-cyclopentyl-1,3-dimethylxanthine reversed the inhibition produced by adenosine. In paired-pulse experiments, the ratio of the second to first GABAergic or glycinergic eIPSC amplitude was increased by adenosine, whereas the response of SG neurons to exogenous GABA or glycine was unaffected. Adenosine reduced the frequency of GABAergic and glycinergic spontaneous IPSCs without changing their amplitude. This reduction in frequency disappeared in the presence of a K+ -channel blocker (4-aminopyridine) but not in the absence of Ca2+. The inhibition by adenosine disappeared in the presence of cyclic-AMP analog (8-Br-cyclic AMP) and adenylate-cyclase activator (forskolin) but not protein-kinase C (PKC) activator (phorbol-12,13-dibutyrate). We conclude that adenosine suppresses inhibitory transmission in SG neurons by activating presynaptic A1 receptors and that this action is mediated by K+ channels and cyclic AMP but not by Ca2+ channels and PKC. This inhibitory action of adenosine probably contributes to the modulation of pain transmission in the SG.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Mechanisms of H+ modulation of glycinergic response in rat sacral dorsal commissural neurons

Many ionotropic receptors are modulated by extracellular H+. So far, few studies have directly addressed the role of such modulation at synapses. In the present study, we investigated the effects of changes in extracellular pH on glycinergic miniature inhibitory postsynaptic currents (mIPSCs) as well as glycine-evoked currents (IGly) in mechanically dissociated spinal neurons with native synaptic boutons preserved. H+ modulated both the mIPSCs and IGly biphasically, although it activated an amiloride-sensitive inward current by itself. Decreasing extracellular pH reversibly inhibited the amplitude of the mIPSCs and IGly, while increasing external pH reversibly potentiated these parameters. Blockade of acid-sensing ion channels (ASICs) with amiloride, the selective antagonist of ASICs, or decreasing intracellular pH did not alter the modulatory effect of H+ on either mIPSCs or IGly. H+ shifted the EC50 of the glycine concentration-response curve from 49.3 +/- 5.7 microM at external pH 7.4 to 131.5 +/- 8.1 microM at pH 5.5, without altering the Cl- selectivity of the glycine receptor (GlyR), the Hill coefficient and the maximal IGly, suggesting a competitive inhibition of IGly by H+. Both Zn2+ and H+ inhibited IGly. However, H+ induced no further inhibition of IGly in the presence of a saturating concentration of Zn2+. In addition, H+ significantly affected the kinetics of glycinergic mIPSCs and IGly. It is proposed that H+ and/or Zn2+ compete with glycine binding and inhibit the amplitude of glycinergic mIPSCs and IGly. Moreover, binding of H+ induces a global conformational change in GlyR, which closes the GlyR Cl- channel and results in the acceleration of the seeming desensitization of IGly as well as speeding up the decay time constant of glycinergic mIPSCs. However, the deprotonation rate is faster than the unbinding rate of glycine from the GlyR, leading to reactivation of the undesensitized GlyR after washout of agonist and the appearance of a rebound IGly. H+ also modulated the glycine cotransmitter, GABA-activated current (IGABA). Taken together, the results support a "conformational coupling" model for H+ modulation of the GlyR and suggest that H+ may act as a novel modulator for inhibitory neurotransmission in the mammalian spinal cord.

GABAB receptor-mediated presynaptic potentiation of ATP ionotropic receptors in rat midbrain synaptosomes

Nucleotides can activate ionotropic P2X receptors that induce calcium-responses in rat midbrain synaptosomes. In this report, we show that ATP elicits Ca(2+) responses producing a monophasic dose-response curve with an EC(50) value of 24.24+/-1.42 micro M. In the presence of gamma-aminobutyric acid (GABA), the ATP dose-response curve becomes biphasic with EC(50) values of 3.69+/-0.44 nM and 59.65+/-8.32 micro M. Moreover, the maximal calcium response induced by ATP is 52.1% higher than the control. This effect is mimicked or blocked by the specific GABA(B) receptor agonist and antagonist, baclofen and saclofen, respectively. Presynaptic GABA(B) receptors, identified by immunocytochemistry are present in 62% of the total synaptosomal population. Adenylate cyclase and protein kinase A cascades are involved in the potentiatory effects mediated by baclofen and their activation or inhibition modifies calcium signalling and synaptosomal cAMP levels. The potentiatory action of baclofen was confirmed by microfluorimetry performed on single synaptic terminals. In its presence, 86% of the terminals responding to 100 micro M ATP, are also able to respond to nanomolar concentrations (100 nM) of this nucleotide. This potentiatory effect is reduced to 32% in the presence of pertussis toxin. Our data suggest that the activity of P2X receptors is modulated by GABA(B) receptors in midbrain synaptosomes.

The general anesthetic pentobarbital slows desensitization and deactivation of the glycine receptor in the rat spinal dorsal horn neurons

Although many general anesthetics have been found to produce anesthetic and analgesic effects by augmenting GABA(A) receptor (GABA(A)R) function, the role of the glycine receptor (GlyR) in this process is not fully understood at the neuronal level in the spinal cord. We investigated the effects of a barbiturate general anesthetic, pentobarbital (PB), on the glycinergic miniature inhibitory postsynaptic currents (mIPSCs) and the responses to exogenously applied glycine, or taurine, a low affinity GlyR agonist, by using the whole-cell patch-clamp technique in the rat spinal dorsal horn neurons isolated using a novel mechanical method. Bath application of 30 microm PB significantly prolonged the decay time constant of the spontaneous glycinergic mIPSC without changing its amplitude and frequency. Co-application of 0.3 mm PB reduced the peak amplitude, affected the macroscopic desensitization and deactivation of the response to externally applied Gly in a concentration-dependent manner. In addition, the recovery of Gly response from desensitization was also prolonged by PB. However, PB did not change the desensitization and deactivation kinetics of the taurine-induced response. The GABA(A)R antagonist bicuculline (10 microm) did not affect the effect of PB on the Gly response. Thus, PB prolonged the spinal glycinergic mIPSCs by slowing desensitization and deactivation of GlyR. Two other structurally different intravenous anesthetics, i.e. propofol (10 microm) and etomidate (3 microm), prolonged the duration of the glycinergic mIPSC in the rat spinal dorsal horn neurons. In conclusion, on GlyR-Cl(-) channel complexes there may exist action site(s) of intravenous general anesthetics. GlyR and glycinergic neurotransmission may play an important role in the modulation of general anesthesia in the mammalian spinal cord.

Kinetics of etomidate actions on GABA(A) receptors in the rat spinal dorsal horn neurons

Electrophysiological properties of etomidate (ET)-induced current (I(ET)) at different concentrations and effects of ET at clinically relevant concentrations (1-10 microM) on postsynaptic GABA(A) receptor function were investigated using whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. The results showed that ET actions were concentration-dependent: low concentrations (10 microM) of ET potentiated GABA-activated current (I(GABA)), slowed activation, desensitization and deactivation of GABA(A) receptors; moderate concentrations (10-1,000 microM) of ET directly activated and desensitized GABA(A) receptors; high concentrations (>1,000 microM) of ET produced an inhibitory effect on I(ET). In addition, ET prolonged the duration of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the mechanically dissociated rat dorsal horn neurons. These results suggest that general anesthetics-induced changes at spinal level could significantly contribute to analgesia and general anesthesia.

The actions of propofol on gamma-aminobutyric acid-A and glycine receptors in acutely dissociated spinal dorsal horn neurons of the rat

The spinal cord plays an important role in modulating anesthetic-induced suppression of nociceptive transmission. To gain some insight into the anesthetic mechanisms of propofol at the spinal level, we investigated the direct action of propofol and its modulation on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol induced Cl(-) currents (I(Cl)), which were sensitive to bicuculline and, to a lesser extent, to strychnine. The activation, desensitization, and deactivation of propofol-induced I(Cl) were slower than those of GABA- and glycine-induced I(Cl). In addition, this study revealed similar modulatory actions of propofol on GABA(A)R and GlyR. Propofol potentiated both GABA- and glycine-induced I(Cl) at small con-centrations and inhibited both GABA- and glycine-induced I(Cl) at large concentrations. The potentiation of propofol on I(Cl) was caused by slowing current desensitization and deactivation, whereas the inhibition actions might be involved in the cross-desensitization between GABA- and propofol-induced I(Cl) and the cross-inhibition between the GABA(A)R and GlyR. The results suggest that propofol facilitation of GABA(A)R and GlyR at the spinal level could contribute significantly to general anesthetic-induced analgesia and anesthesia.

IMPLICATIONS

The actions of propofol on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) were investigated in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol was found to potentiate the functions of GABA(A)R and GlyR at the spinal level, which might contribute to propofol-induced analgesia and anesthesia.

Focal stimulation of single GABAergic presynaptic boutons on the rat hippocampal neuron

Evoked inhibitory postsynaptic currents (eIPSCs) generated from a single GABAergic bouton were recorded and the functional properties were investigated. Native single boutons attached to mechanically dissociated rat hippocampal CA1 neurons, namely "synaptic bouton" preparation, were visualized with FM 1-43 dye and selectively stimulated by a glass pipette directed to a single bouton by focal stimulation. The GABAergic eIPSCs were elicited in like all-or-none fashion regarding both stimulus strength and pipette location, thus indicating that the eIPSCs result from the activation of a single bouton. The GABA release from the boutons was action potential dependent since eIPSCs were blocked in the presence of either voltage-dependent Na(+) or Ca(2+)channel blocker. Even in the presence of tetrodotoxin (TTX), eIPSCs could be elicited by additional application of a voltage-dependent K(+) channel blocker, 4-AP. The GABA release depended on external Ca(2+) concentration. Amplitude histogram of eIPSCs did not follow Poisson distribution or show discrete peaks. As a result, this new experimental approach using both focal stimulation and a synaptic bouton preparation allows for a detailed study of the native synaptic machinery in nerve terminals measuring smaller than 1 microm in size in the CNS.

Synaptic P2X receptors

Over the past two years, ATP has clearly been shown to act as a co-transmitter with GABA, glycine and probably glutamate in the central nervous system. Our understanding of the ATP-gated P2X receptors is progressing rapidly, and the pharmacology, stoichiometry and subunit combinations of heteropolymeric P2X channels has been substantially elucidated.