Lack of evidence for P2X-purinoceptor involvement in fast synaptic responses in intact sympathetic ganglia isolated from guinea-pigs

Developmental Changes in Electrophysiological Properties and Synaptic Transmission in Rat Intracardiac Ganglion Neurons

We charted postnatal changes in the intrinsic electrophysiological properties and synaptic responses of rat intrinsic cardiac ganglion (ICG) neurons. We developed a whole-mount ganglion preparation of the excised right atrial ganglion plexus. Using intracellular recordings and nerve stimulation we tested the hypothesis that substantial transformations in the intrinsic electrical characteristics and synaptic transmission accompany postnatal development. Membrane potential ( Em) did not change but time constant (τ) and cell capacitance increased with postnatal development. Accordingly, input resistance ( Rin) decreased but specific membrane resistance ( Rm) increased postnatally. Comparison of the somatic active membrane properties revealed significant changes in electrical phenotype. All neonatal neurons had somatic action potentials (APs) with small overshoots and small afterhyperpolarizations (AHPs). Adult neurons had somatic APs with large overshoots and large AHP amplitudes. The range of AHP duration was larger in adults than in neonates. The AP characteristics of juvenile neurons resembled those of adults, with the exception of AHP duration, which fell midway between neonate and adult values. Phasic, multiply adapting, and tonic evoked discharge activities were recorded from ICG neurons. Most neurons displayed phasic discharge at each developmental stage. All neurons received excitatory synaptic inputs from the vagus or interganglionic nerve trunk(s), the strength of which did not change significantly with postnatal age. The changes in the electrophysiological properties of the postganglionic neuron suggest that increased complexity of parasympathetic regulation of cardiac function accompanies postnatal development.

Modulation of voltage-dependent calcium channels by neurotransmitters and neuropeptides in parasympathetic submandibular ganglion neurons

The control of saliva secretion is mainly under parasympathetic control, although there also could be a sympathetic component. Sympathetic nerves are held to have a limited action in secretion in submandibular glands because, on electrical stimulation, only a very small increase to the normal background, basal secretion occurs. Parasympathetic stimulation, on the other hand, caused a good flow of saliva with moderate secretion of acinar mucin, plus an extensive secretion of granules from the granular tubules. The submandibular ganglion (SMG) is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. Neurotransmitters and neuropeptides acting via G-protein coupled receptors (GPCRs) change the electrical excitability of neurons. In these neurons, many neurotransmitters and neuropeptides modulate voltage-dependent calcium channels (VDCCs). The modulation is mediated by a family of GPCRs acting either directly through the membrane delimited G-proteins or through second messengers. However, the mechanism of modulation and the signal transduction pathway linked to an individual GPCRs depend on the animal species. This review reports how neurotransmitters and neuropeptides modulate VDCCs and how these modulatory actions are integrated in SMG systems. The action of neurotransmitters and neuropeptides on VDCCs may provide a mechanism for regulating SMG excitability and also provide a cellular mechanism of a variety of neuronal Ca(2+)-dependent processes.

Inhibition of N-type voltage-activated calcium channels in rat dorsal root ganglion neurons by P2Y receptors is a possible mechanism of ADP-induced analgesia

Patch-clamp recordings from small-diameter rat dorsal root ganglion (DRG) neurons maintained in culture demonstrated preferential inhibition by ATP of high-voltage-activated, but not low-voltage-activated, Ca2+ currents (I(Ca)). The rank order of agonist potency was UTP > ADP > ATP. ATP depressed the omega-conotoxin GVIA-sensitive N-type current only. Pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate tetraammonium, two P2Y1 receptor antagonists, almost abolished the ATP-induced inhibition. Both patch-clamp recordings and immunocytochemistry coupled with confocal laser microscopy indicated a colocalization of functional P2X3 and P2Y1 receptors on the same DRG neurons. Because the effect of ATP was inhibited by intracellular guanosine 5'-O-(2-thiodiphosphate) or by applying a strongly depolarizing prepulse, P2Y1 receptors appear to block I(Ca) by a pathway involving the betagamma subunit of a G(q/11) protein. Less efficient buffering of the intracellular Ca2+ concentration ([Ca2+]i) by reducing the intrapipette EGTA failed to interfere with the ATP effect. Fura-2 microfluorimetry suggested that ATP raised [Ca2+]i by a Galpha-mediated release from intracellular pools and simultaneously depressed the high external potassium concentration-induced increase of [Ca2+]i by inhibiting I(Ca) via Gbetagamma. Adenosine 5'-O-(2-thiodiphosphate) inhibited dorsal root-evoked polysynaptic population EPSPs in the hemisected rat spinal cord and prolonged the nociceptive threshold on intrathecal application in the tail-flick assay. These effects were not antagonized by PPADS. Hence, P2Y receptor activation by ADP, which is generated by enzymatic degradation of ATP, may decrease the release of glutamate from DRG terminals in the spinal cord and thereby partly counterbalance the algogenic effect of ATP.

Transmission of signals through sympathetic ganglia--modulation, integration or simply distribution?

AIM

On structural grounds, synaptic transmission in sympathetic ganglia is potentially complex with extensive divergence and convergence between preganglionic and postganglionic neurones. In this review, the focus is on what constitutes a functional synapse in sympathetic ganglia and how intracellular recordings have enabled us to identify how the transmission process operates in vivo.

RESULTS

Only one or two suprathreshold or 'strong' inputs are involved in activating each postganglionic neurone. The functional significance of the subthreshold or 'weak' inputs remains obscure. The strong inputs, and sometimes the weak ones as well, respond in the same way during reflexes. The expansion of ineffective weak connections enables the rapid restoration of functional control after lesions that damage preganglionic neurones. These novel connections may generate erroneous reflex responses after spinal injury. Postganglionic discharge in vivo consists of the summed firing of the strong preganglionic inputs limited, at high preganglionic discharge rates, by the properties of the afterhyperpolarization.

CONCLUSION

Preganglionic signals are distributed widely through paravertebral ganglia with little modification.

Fast excitatory postsynaptic currents in neurons of the rabbit pelvic plexus

Fast excitatory postsynaptic currents have been recorded at 23-27 degrees C from rabbit pelvic plexus neurons by a two-electrode voltage-clamp technique. The synaptic current decay was bi-exponential with the fast and slow components characterized at -50 mV by mean time constants of 4.0 +/- 0.3 and 21.9 +/- 2.8 ms (n = 11), respectively. Both components contributed to the synaptic current approximately equally and reversed at -5 mV. Hexamethonium (10 microM) decreased the amplitude and decay time constant of both synaptic current components; this effect increased with hyperpolarization and is consistent with a channel-blocking action. At - 50 mV, mean rate constants of hexamethonium association with open ion channels of nicotinic acetylcholine receptors presumably mediating the fast and slow synaptic current components were (18.4 +/- 2.3) x 10(6) and (6.1 +/- 1.2) x 10(6) M(-1) s(-1) (n = 4), respectively. These data suggest that the fast excitatory postsynaptic current in rabbit pelvic plexus neurons is probably mediated by at least two different subtypes of nicotinic acetylcholine receptors. Hexamethonium blocks open ion channels of both subtypes with efficiency allowing to exclude an appreciable presence of homomeric alpha7 nicotinic acetylcholine receptors on the subsynaptic membrane.

Nicotinic acetylcholine receptors in autonomic ganglia

Although alpha3beta4 subunit combination is clearly prevalent in the nAChRs of autonomic ganglia neurons, the ganglia are strikingly different in the ratio of neurons containing each particular nAChR subunit, as found with immunohistochemical methods and from the analysis of the effects of nAChR subunit-specific antibodies on the ACh-induced membrane currents. In particular, the number of neurons containing alpha3, alpha4, alpha5 or alpha7 subunits is by about three times higher in sympathetic ganglia than in parasympathetic ganglia. This difference may explain why the parasympathetic and sympathetic ganglia markedly differ in their pharmacology. Still, alpha7 subunit makes the highest contribution to ACh-induced membrane current. No correlation between the physiological functions of the ganglia and subunit composition of their nAChRs has been found as yet. High permeability for Ca2+ should permit the nAChRs with alpha7 subunits to influence a variety of Ca2+-dependent events in autonomic neurons. As found with biochemical methods and site-directed mutagenesis, the ACh binding site is formed in the alpha/beta subunits interface by multiple loops containing cysteine, tyrosine and tryptophan amino residues as important for ACh binding. Likewise, both alpha and beta subunits are important for the effects of blocking agents on nAChRs. As found by electrophysiological methods, each neuron of sympathetic and parasympathetic ganglia, as a rule, possesses nAChRs of two groups, "fast" and "slow", with the mean duration of the burst of single channel openings ranging approximately from 5 to 10 and from 25 to 45 ms, respectively. These groups of channels differ from each other with their pharmacology. The burst-like activity of autonomic nAChRs channels is possible only if the disulfide bonds are left intact, otherwise only single openings of the channel are observed. The ionic channel of a nAChRs pentamer is formed by M2 transmembrane segments arranging glutamate, serine, threonine, leucine, and valine rings critical for channel conductance and ionic selectivity. In particular, the mutations V251T and E237A, and insertion of proline or alanine, convert a cation-selective channel into an anion-selective one. The open-channel blockers bind to the nAChR channel at the level where the channel diameter is nearly 12 A, both for "fast" and "slow" channel groups.

Pre- and postsynaptic actions of ATP on neurotransmission in rat submandibular ganglia

The pre- and postsynaptic actions of exogenously applied ATP were investigated in intact and dissociated parasympathetic neurones of rat submandibular ganglia. Nerve-evoked excitatory postsynaptic potentials (EPSPs) were not inhibited by the purinergic receptor antagonists, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), or the desensitising agonist, alpha,beta-methylene ATP. In contrast, EPSPs were abolished by the nicotinic acetylcholine receptor antagonists, hexamethonium and mecamylamine. Focal application of ATP (100 microM) had no effect on membrane potential of the postsynaptic neurone or on the amplitude of spontaneous EPSPs. Taken together, these results suggest the absence of functional purinergic (P2) receptors on the postganglionic neurone in situ. In contrast, focally applied ATP (100 microM) reversibly inhibited nerve-evoked EPSPs. Similarly, bath application of the non-hydrolysable analogue of ATP, ATP gamma S, reversibly depressed EPSPs amplitude. The inhibitory effects of ATP and ATP gamma S on nerve-evoked transmitter release were antagonised by bath application of either PPADS or suramin, suggesting ATP activates a presynaptic P2 purinoceptor to inhibit acetylcholine release from preganglionic nerves in the submandibular ganglia. In acutely dissociated postganglionic neurones from rat submandibular ganglia, focal application of ATP (100 microM) evoked an inward current and subsequent excitatory response and action potential firing, which was reversibly inhibited by PPADS (10 microM). The expression of P2X purinoceptors in wholemount and dissociated submandibular ganglion neurones was examined using polyclonal antibodies raised against the extracellular domain of six P2X purinoceptor subtypes (P2X(1-6)). In intact wholemount preparations, only the P2X(5) purinoceptor subtype was found to be expressed in the submandibular ganglion neurones and no P2X immunoreactivity was detected in the nerve fibres innervating the ganglion. Surprisingly, in dissociated submandibular ganglion neurones, high levels of P2X(2) and P2X(4) purinoceptors immunoreactivity were found on the cell surface. This increase in expression of P2X(2) and P2X(4) purinoceptors in dissociated submandibular neurones could explain the increased responsiveness of the neurones to exogenous ATP. We conclude that disruption of ganglionic transmission in vivo by either nerve damage or synaptic blockade may up-regulate P2X expression or availability and alter neuronal excitability.

P2X receptors in peripheral neurons

P2X receptors are a family of ligand-gated ion channels, activated by extracellular ATP. The seven subunits cloned (P2X1-7) can assemble to form homomeric and heteromeric receptors. Peripheral neurons of neural crest origin (e.g. those in dorsal root, trigeminal, sympathetic and enteric ganglia) and placodal origin (e.g. those in nodose and petrosal ganglia) express mRNAs for multiple P2X subunits. In this review, we summarize the molecular biological, electrophysiological and immunohistochemical evidence for P2X receptor subunits in sensory, sympathetic, parasympathetic, pelvic and myenteric neurons and adrenomedullary chromaffin cells. We consider the pharmacological properties of these native P2X receptors and their physiological roles. The responses of peripheral neurons to ATP show considerable heterogeneity between cells in the same ganglia, between ganglia and between species. Nevertheless, these responses can all be accounted for by the presence of P2X2 and P2X3 subunits, giving rise to varying proportions of homomeric and heteromeric receptors. While dorsal root ganglion neurons express predominantly P2X3 and rat sympathetic neurons express mainly P2X2 receptors, nodose and guinea-pig sympathetic neurons express mixed populations of P2X2 and heteromeric P2X2/3 receptors. P2X receptors are important for synaptic transmission in enteric ganglia, although their roles in sympathetic and parasympathetic ganglia are less clear. Their presence on sensory neurons is essential for some processes including detection of filling of the urinary bladder. The regulation of P2X receptor expression in development and in pathological conditions, along with the interactions between purinergic and other signalling systems, may reveal further physiological roles for P2X receptors in autonomic and sensory ganglia.

Modulation of fast synaptic transmission by presynaptic ligand-gated cation channels

There is now considerable evidence demonstrating that ligand-gated cation channels (i.e., P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors), in addition to mediating fast excitatory neurotransmission, may be located presynaptically on nerve terminals in the peripheral and central nervous systems where they function to modulate neurotransmitter release. This modulation can be facilitation, inhibition or both. In this article, we first outline the multiple mechanisms by which activation of presynaptic ligand-gated cation channels can modulate spontaneous and evoked neurotransmitter release, before reviewing in detail published electrophysiological studies of presynaptic P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors.

Are there functional P2X receptors on cell bodies in intact dorsal root ganglia of rats?

P2X purinoceptors have been suggested to participate in transduction of painful stimuli in nociceptive neurons. In the current experiments, ATP (1-10 mM), alpha,beta-methylene-ATP (10-30 microM) and capsaicin (10 nM-1 microM) were applied to neurons impaled with high resistance microelectrodes in rat dorsal root ganglia (L4 and L5) isolated in vitro together with the sciatic nerve and dorsal roots. The agonists were either bath applied or focally applied using a picospritzer. GABA (100 microM) and 40-80 mM K+ solutions gave brisk responses when applied by either technique. Only three of 22 neurons with slowly conducting axons (C cells) showed evidence of P2X-purinoceptor-mediated responses. Only two of 13 cells which responded to capsaicin (putative nociceptors), and none of 29 cells with rapidly conducting axons (A cells), responded to the purinergic agonists. When acutely dissociated dorsal root ganglion cells were studied using patch-clamp techniques, all but four of 30 cells of all sizes responded with an inward current to either ATP or alpha,beta-methylene-ATP (both 100 microM). Our data suggest that few sensory cell bodies in intact dorsal root ganglia express functional purinoceptors.

Modulation by adenine nucleotides of epileptiform activity in the CA3 region of rat hippocampal slices

1. Hippocampal slices (450 microm) generate epileptiform bursts of an interictal nature when perfused with a zero magnesium medium containing 4-aminopyridine (50 microM). The effect of adenine nucleotides on this activity was investigated. 2. ATP and adenosine depressed this epileptiform activity in a concentration-dependent manner, with both purines being equipotent at concentrations above 10 microM. 3. Adenosine deaminase 0.2 u ml(-1), a concentration that annuls the effect of adenosine (50 microM), did not significantly alter the depression of activity caused by ATP (50 microM). 4. 8-Cyclopentyl-1,3-dimethylxanthine (CPT), an A1 receptor antagonist, enhanced the discharge rate significantly and inhibited the depressant effect of both ATP and adenosine such that the net effect of ATP or adenosine plus CPT was excitatory. 5. Several ATP analogues were also tested: alpha, beta-methyleneATP (alpha, beta-meATP), 2-methylthioATP (2-meSATP) and uridine triphosphate (UTP). Only alpha, beta-meATP (10 microM) produced an increase in the frequency of spontaneous activity which suggests a lack of involvement of P2Y or P2U receptors. 6. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), P2 receptor antagonists, failed to inhibit the depression produced by ATP (50 microM). The excitatory effect of alpha, beta-meATP (10 microM) was inhibited by suramin (50 microM) and PPADS (5 microM). 7. ATP therefore depresses epileptiform activity in this model in a manner which is not consistent with the activation of known P1 or P2 receptors, suggesting the involvement of a xanthine-sensitive nucleotide receptor. The results are also indicative of an excitatory P2X receptor existing in the hippocampal CA3 region.

Qualitative analysis of membrane currents in glial cells from normal and gliotic tissue in situ: down-regulation of Na+ current and lack of P2 purinergic responses

To date, the electrophysiological properties of glial cells located in reactive scar tissue are unknown. To address this issue two subtypes of hippocampal glial cells, located in thin vital slices of normal or gliotic brain tissue, were analysed for their voltage controlled ion channels using the patch-clamp technique. Reactive gliosis was induced in adult rats by a single peritoneal injection of kainic acid. The intensity of the following seizures was rated ascending from 1 to 6. Rats which exhibited seizures of level 3 or higher showed, within three days, a marked loss of pyramidal cells (60% in CA1 and CA3) and an increase in the density of glial fibrillary acidic protein immunostaining, representing an apparent increase in the number and size of astrocytes in all layers of the hippocampal CA1 subfield. Reactive and normal astrocytes of one subtype, electrophysiologically characterized by time-independent potassium currents, did not significantly differ in membrane potential and potassium conductivity. Glutamine synthetase-positive, but mostly glial fibrillary acidic protein-negative, glial cells (presumably representing immature astrocytes) were also included in this study. This subtype of glial cells showed several voltage- and time-dependent potassium currents and, under control conditions, tetrodotoxin-sensitive voltage-gated Na+ channels, which were almost completely lost after reactive gliosis. Another part of this study focuses on the sensitivity of reactive and control glial cells for extracellular ATP. Several in vitro studies suggest that P2 purinergic receptors on glial cells could trigger the induction of reactive gliosis. In contrast to results described on cultured astrocytes, we found in situ that hippocampal glial cells were not sensitive to ATP or stable P2 receptor agonists in control or in gliotic brain slices. In summary, the presence of at least two different subtypes of hippocampal astrocytes was demonstrated for control as well as for gliotic brain tissue. A dramatic down-regulation of tetrodotoxin-sensitive sodium channels in one subpopulation of reactive astrocytes was shown. This result supports the hypothesis that the presence of active neurons could be required to maintain glial voltage-gated sodium channels. Furthermore, we conclude that there is no longtime expression of P2 purinoceptors on hippocampal astrocytes in situ, and therefore the involvement of astrocytic ATP receptors in the genesis of reactive gliosis is unlikely.

Receptors controlling transmitter release from sympathetic neurons in vitro

Primary cultures of postganglionic sympathetic neurons were established more than 30 years ago. More recently, these cultures have been used to characterize various neurotransmitter receptors that govern sympathetic transmitter release. These receptors may be categorized into at least three groups: (1) receptors which evoke transmitter release: (2) receptors which facilitate; (3) receptors which inhibit, depolarization-evoked release. Group (1) comprises nicotinic and muscarinic acetylcholine receptors, P2X purinoceptors and pyrimidinoceptors. Group (2) currently harbours beta-adrenoceptors, P2 purinoceptors, receptors for PACAP and VIP, as well as prostanoid EP1 receptors. In group (3), muscarinic cholinoceptors, alpha 2- and beta-adrenoceptors, P2 purinoceptors, and receptors for the neuropeptides NPY, somatostatin (SRIF1) and LHRH, as well as opioid (delta and kappa) receptors can be found. Receptors which regulate transmitter release from neurons in cell culture may be located either at the somatodendritic region or at the sites of exocytosis, i.e. the presynaptic specializations of axons. Most of the receptors that evoke release are located at the soma. There ionotropic receptors cause depolarizations to generate action potentials which then trigger Ca(2+)-dependent exocytosis at axon terminals. The signalling mechanisms of metabotropic receptors which evoke release still remain to be identified. Receptors which facilitate depolarization-evoked release appear to be located preferentially at presynaptic sites and presumably act via an increase in cyclic AMP. Receptors which inhibit stimulation evoked release are also presynaptic origin and most commonly rely on a G protein-mediated blockade of voltage-gated Ca2+ channels. Results obtained with primary cell cultures of postganglionic sympathetic neurons have now supplemented previous data about neurotransmitter receptors involved in the regulation of ganglionic as well as sympatho-effector transmission. In the future, this technique may prove useful to identify yet unrecognized receptors which control the output of the sympathetic nervous system and to elucidate underlying signalling mechanisms.

P2-purinoceptors on postganglionic sympathetic neurones

1. Postganglionic sympathetic neurones possess both excitatory and inhibitory P2-purinoceptors. 2. The mechanisms of action of excitatory P2-purinoceptors have recently been studied on cultured sympathetic neurones of the rat. The receptors mediate fast increases in intracellular Ca2+ levels and a release of noradrenaline. They are likely to belong to the neuronal types of P2X-purinoceptors and to be located on the sympathetic nerve cell bodies or their dendrites. 3. Inhibitory P2-purinoceptors have been shown to operate at sympathetic axon terminals in isolated tissues. Adenine nucleotides decreased the stimulation-evoked release of noradrenaline by activation of these receptors. The receptors are likely to belong to the group of G-protein-coupled P2Y-purinoceptors. They mediate a negative feedback in which co-transmitter ATP inhibits subsequent sympathetic transmitter release.