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

The protective barrier role of satellite glial cells in sensory ganglia

Neurons in sensory ganglia are wrapped completely by satellite glial cells (SGCs). One putative function of SGCs is to regulate the neuronal microenvironment, but this role has received only little attention. In this study we investigated whether the SGC envelope serves a barrier function and how SGCs may control the neuronal microenvironment. We studied this question on short-term (<24 h) cell cultures of dorsal root ganglia and trigeminal ganglia from adult mice, which contain neurons surrounded with SGCs, and neurons that are not. Using calcium imaging, we measured neuronal responses to molecules with established actions on sensory neurons. We found that neurons surrounded by SGCs had a smaller response to molecules such as adenosine triphosphate (ATP), glutamate, GABA, and bradykinin than neurons without glial cover. When we inhibited the activity of NTPDases, which hydrolyze the ATP, and also when we inhibited the glutamate and GABA transporters on SGCs, this difference in the neuronal response was no longer observed. We conclude that the SGC envelope does not hinder diffusional passage, but acts as a metabolic barrier that regulates the neuronal microenvironment, and can protect the neurons and modulate their activity.

Acute toxicity of cypermethrin on the juvenile of red claw crayfish Cherax quadricarinatus

In order to assess the toxicity of Cypermethrin (CYP), the 50% lethal concentration (LC₅₀) of CYP on the juvenile of Cherax quadricarinatus is assessed. Meanwhile, the transcription level and the content in the antioxidant and biotransformation enzymes in hepatopancreas and immune enzymes in the serum of C. quadricarinatus exposed to CYP (0.1, 1, 10 and 100 ng·L-1) for 96 h were analyzed to reveal the CYP toxicity and detoxification mechanism. 24, 48, 72, 96 h LC₅₀ were 1305.14, 424.52, 287.10 and 215.99 ng·L-1, respectively. There was no significant change of the content of enzymes at low concentration (0.16 ng·L-1). The fast increase of SOD and CAT content was observed at early stage (24 h), subsequent decreased at later stage of trail at medium concentration (0.32 and 0.63 ng·L-1). However, high concentration (1.25 ng·L-1) of CYP significantly inhibited SOD and CAT content. There was a significant increase in the level of MDA, PC and the content of GPx, EROD, CarE, GST at medium and high concentration after 72 h and 96 h exposure. The Na⁺-K⁺-ATPase, PO, ALK content decreased at medium and high concentration, especially at the 72-h and the 96-h exposure. The transcription was altered similarly to enzyme content, but the transcriptional response was generally more immediate than enzymatic response. Heat shock protein (hsp70) and multidrug resistance-associated protein 2 (abcc2) genes were up-regulated.

Intercellular communication in sensory ganglia by purinergic receptors and gap junctions: implications for chronic pain

Peripheral injury can cause abnormal activity in sensory neurons, which is a major factor in chronic pain. Recent work has shown that injury induces major changes not only in sensory neurons but also in the main type of glial cells in sensory ganglia-satellite glial cells (SGCs), and that interactions between sensory neurons and SGCs contribute to neuronal activity in pain models. The main functional changes observed in SGCs after injury are an increased gap junction-mediated coupling among these cells, and augmented sensitivity to ATP. There is evidence that the augmented gap junctions contribute to neuronal hyperexcitability in pain models, but the mechanism underlying this effect is not known. The changes in SGCs described above have been found following a wide range of injuries (both axotomy and inflammation) in somatic, orofacial and visceral regions, and therefore appear to be a general feature in chronic pain. We have found that in cultures of sensory ganglia calcium signals can spread from an SGC to neighboring cells by calcium waves, which are mediated by gap junctions and ATP acting on purinergic P2 receptors. A model is proposed to explain how augmented gap junctions and greater sensitivity to ATP can combine to produce enhanced calcium waves, which can lead to neuronal excitation. Thus this simple scheme can account for several major changes in sensory ganglia that are common to a great variety of pain models.

P2X7 receptors in satellite glial cells mediate high functional expression of P2X3 receptors in immature dorsal root ganglion neurons

Background

The purinergic P2X3 receptor (P2X3R) expressed in the dorsal root ganglion (DRG) sensory neuron and the P2X7 receptor (P2X7R) expressed in the surrounding satellite glial cell (SGC) are two major receptors participating in neuron-SGC communication in adult DRGs. Activation of P2X7Rs was found to tonically reduce the expression of P2X3Rs in DRGs, thus inhibiting the abnormal pain behaviors in adult rats. P2X receptors are also actively involved in sensory signaling in developing rodents. However, very little is known about the developmental change of P2X7Rs in DRGs and the interaction between P2X7Rs and P2X3Rs in those animals. We therefore examined the expression of P2X3Rs and P2X7Rs in postnatal rats and determined if P2X7R-P2X3R control exists in developing rats.

Findings

We immunostained DRGs of immature rats and found that P2X3Rs were expressed only in neurons and P2X7Rs were expressed only in SGCs. Western blot analyses indicated that P2X3R expression decreased while P2X7R expression increased with the age of rats. Electrophysiological studies showed that the number of DRG neurons responding to the stimulation of the P2XR agonist, α,β-meATP, was higher and the amplitudes of α,β-meATP-induced depolarizations were larger in immature DRG neurons. As a result, P2X3R-mediated flinching responses were much more pronounced in immature rats than those found in adult rats. When we reduced P2X7R expression with P2X7R-siRNA in postnatal and adult rats, P2X3R-mediated flinch responses were greatly enhanced in both rat populations.

Conclusions

These results show that the P2X7R expression increases as rats age. In addition, P2X7Rs in SGCs exert inhibitory control on the P2X3R expression and function in sensory neurons of immature rats, just as observed in adult rats. Regulation of P2X7R expression is likely an effective way to control P2X3R activity and manage pain relief in infants.

Altered ATP release and metabolism in dorsal root ganglia of neuropathic rats

Background

Adenosine 5'-triphosphate (ATP) has a ubiquitous role in metabolism and a major role in pain responses after tissue injury. We investigated the changes in basal and KCl-evoked ATP release from rat dorsal root ganglia (DRG) after peripheral neuropathy induction by unilateral sciatic nerve entrapment (SNE).

Results

After SNE, rats develop long-lasting decreases in ipsilateral hindpaw withdrawal thresholds to mechanical and thermal stimulation. At 15–21 days after neuropathy induction, excised ipsilateral L4-L5 DRG display significantly elevated basal extracellular ATP levels compared to contralateral or control (naive) DRG. However, KCl-evoked ATP release is no longer observed in ipsilateral DRG. We hypothesized that the differential SNE effects on basal and evoked ATP release could result from the conversion of extracellular ATP to adenosine with subsequent activation of adenosine A1 receptors (A1Rs) on DRG neurons. Adding the selective A1R agonist, 2-chloro-N⁶-cyclopentyladenosine (100 nM) significantly decreased basal and evoked ATP release in DRG from naïve rats, indicating functional A1R activation. In DRG ipsilateral to SNE, adding a selective A1R antagonist, 8-cyclopentyl-1,3-dipropylxanthine (30 nM), further increased basal ATP levels and relieved the blockade of KCl-evoked ATP release suggesting that increased A1R activation attenuates evoked ATP release in neurons ipsilateral to SNE. To determine if altered ATP release was a consequence of altered DRG metabolism we compared O₂ consumption between control and neuropathic DRG. DRG ipsilateral to SNE consumed O₂ at a higher rate than control or contralateral DRG.

Conclusion

These data suggest that peripheral nerve entrapment increases DRG metabolism and ATP release, which in turn is modulated by increased A1R activation.

Roles of peripheral P2X and P2Y receptors in the development of melittin-induced nociception and hypersensitivity

A recent report from our laboratory shows that subcutaneous (s.c.) injection of melittin could induce persistent spontaneous nociception (PSN) and primary thermal or mechanical hyperalgesia. However, the exact peripheral mechanisms underlying melittin-induced multiple pain-related behaviors remain unclear. In this study, behavioral tests combined with pharmacological manipulations were used to explore potential roles of local P2X and P2Y receptors in melittin-induced inflammatory pain and hyperalgesia. Post-treatment of the primary injury site with s.c. injection of A-317491 (a potent P2X(3)/P2X(2/3) receptor antagonist) and Reactive Blue 2 (a potent P2Y receptor antagonist) could significantly suppress the development of melittin-evoked PSN and hypersensitivity (thermal and mechanical). Our control experiments demonstrated that local administration of either antagonist into the contralateral hindpaw produced no significant effect on any kind of pain-associated behaviors. Taken together, these data indicate that activation of P2X and P2Y receptors might be essential to the maintenance of melittin-induced primary thermal and mechanical hyperalgesia as well as on-going pain.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

Association of the ecto-ATPase NTPDase2 with glial cells of the peripheral nervous system

Cellular signaling via extracellular nucleotides appears to play a major role in the functioning of the peripheral nervous system. Information regarding the functional characterization of nucleotide P2 receptors or their expression pattern has been accumulating rapidly; however, very little is known regarding the distribution of ecto-nucleotidases in the periphery. The extracellular level of nucleotides is controlled by ecto-nucleotidases, whereby the three membrane-bound members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family are of special functional importance. Using enzyme histochemistry and immunostaining, we demonstrate that NTPDase2 is associated with nonmyelinating Schwann cells of the rat sciatic nerve, whereas NTPDase1 is restricted to blood vessel walls. NTPDase2 immunoreactivity was detected from embryonic day E18 onward, suggesting that immature Schwann cells express the enzyme. With the onset of myelination, NTPDase2 immunoreactivity remained associated solely with nonmyelinating Schwann cells. NTPDase2 was absent from perisynaptic Schwann cells but was associated with fibroblasts covering the endplate at some distance. In addition, NTPDase2 immunoreactivity was associated with the satellite glial cells in dorsal root ganglia and sympathetic ganglia, and with the enteric glia surrounding the cell bodies of ganglionic neurons of the myenteric and the submucous plexus. In contrast to NTPDase1, NTPDase2 preferentially hydrolyzes nucleoside triphosphates over nucleoside diphosphates and thus can act either in inactivating or in producing P2 receptor ligands. Our results suggest that NTPDase2 plays an important role in the control of nucleotide-mediated activation of peripheral neurons or glia and in the dialogue between these two cell types.

P2X3 receptors and peripheral pain mechanisms

ATP released from damaged or inflamed tissues can act at P2X receptors expressed on primary afferent neurones. The resulting depolarization can initiate action potentials that are interpreted centrally as pain. P2X(3) subunits are found in a subset of small-diameter, primary afferent neurones, some of which are also sensitive to capsaicin. They can form homo-oligomeric channels, or they can assemble with P2X(2) subunits into hetero-oligomers. Studies with antagonists selective for P2X(3)-containing receptors, experiments with antisense oligonucleotides to reduce P2X(3) subunit levels, and behavioural testing of P2X(3) knock-out mice, all suggest a role for the P2X(2/3) receptor in the signalling of chronic inflammatory pain and some features of neuropathic pain. The availability of such tools and experimental approaches promises to accelerate our understanding of the other physiological roles for P2X receptors on primary afferent neurones.

Differential expression patterns of mRNAs for P2X receptor subunits in neurochemically characterized dorsal root ganglion neurons in the rat

The ionotropic purine receptors, P2X receptors, are composed of an assembly of multiple P2X subunits. At present, seven subunits have been cloned and named "P2X1-7." We examined the precise distribution of mRNAs for these subunits in the rat lumbar dorsal root ganglion (DRG) by in situ hybridization histochemistry (ISHH) using riboprobes and characterized their expression among some neuronal subpopulations by ISHH and immunohistochemistry. P2X1 was not expressed by DRG neurons. P2X2 mRNA was preferentially expressed by neurofilament (NF)-200 negative small-sized neurons expressing Ret, but not TrkA or TrkC mRNAs. P2X3 mRNA was mainly expressed by NF-200-negative neurons. Most P2X3-positive neurons had Ret mRNA, and about a half of them coexpressed TrkA and TRPV1 mRNAs. P2X4 was the most ubiquitous subunit, evenly distributing among all examined neuronal subpopulations. P2X5 and P2X6 were expressed by about half of the neurons, and most of these neurons were NF-200-positive. P2X7 mRNA-expressing neurons were quite rare. We further examined the coexpression of all pairs of P2X2-P2X6 mRNAs in DRG neurons and found that: 1) P2X4 was always present in combination with the other subunits. 2) All TrkC neurons had three subunits, P2X4, P2X5, and P2X6, and made up 32% of the total neurons. 3) 12.5% of the total neurons had both P2X2 and P2X3. 4) 12.9% of the neurons had both P2X3 and P2X5. We determined the neuronal subpopulation-specific distribution of P2X subunits in the DRG. These findings suggest possible combinations of subunits of native P2X receptor in DRG neurons.

Differential Thermosensitivity of Sensory Neurons in the Guinea Pig Trigeminal Ganglion

Intracellular recordings were employed to study the effects of temperature on membrane properties and excitability in sensory neurons of the intact guinea pig trigeminal ganglion (TG) maintained in vitro. Neurons were classified according to the shape and duration of the action potential into F (short-duration, fast spike) and S (long duration, slow spike with a “hump”) types. Most type F (33/34) neurons had axons with conduction velocities >1.5 m/s, while only 30% (6/23) of type S neurons reached these conduction speeds suggesting differences in myelination. Cooling reduced axonal conduction velocity and prolonged spike duration in both neuronal types. In F-type neurons with strong inward rectification. cooling also increased the excitability, augmenting the input resistance and reducing the current firing threshold. These effects were not observed in S-type neurons lacking inward rectification. In striking contrast to results obtained in cultured TG neurons, cooling or menthol did not induce firing in recordings from the acutely isolated ganglion. However, after application of submillimolar concentrations (100 μM) of the potassium channel blocker 4-aminopyridine (4-AP), 29% previously unresponsive neurons developed cold sensitivity. An additional 31% developed ongoing activity that was sensitive to temperature. Only neurons with strong inward rectification (mostly F-type) became thermosensitive. Cooling- and 4-AP–evoked firing were insensitive to intracellular application of 4-AP or somatic membrane hyperpolarization, suggesting that their action was most prominent at the level of the axon. The lack of excitatory actions of low temperature in the excised intact ganglion contrasts with the impulse discharges induced by cooling in trigeminal nerve terminals of the same species, suggesting a critical difference between cold-transduction mechanisms at the level of the nerve terminals and the soma.

Responses to sympathomimetics in rat sensory neurones after nerve transection

Noradrenaline activation of sensory somata that project in damaged peripheral nerves has been postulated to underlie sympathetically-mediated pain. Intracellular recordings from some neurones with myelinated axons in acutely isolated rat dorsal root ganglia showed small prolonged depolarizations to brief applications of 0.1-5 mM noradrenaline whether or not the spinal nerve had been transected. Similar responses were evoked to noradrenaline when phentolamine was present, and also to 1-5 mM catechol, but not 1 mM clonidine, implying the responses were not adrenoceptor-mediated. In extracellular recordings from similar preparations after sciatic transection, many spontaneously active myelinated dorsal root axons were excited by noradrenaline and other sympathomimetics. Silent axons in injured or control ganglia did not respond. Thus, non-specific depolarizations may activate neurones that are hyperexcitable after a lesion but activation of neuronal alpha-adrenoceptors by sympathetically-released noradrenaline seems unlikely.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Vanilloid receptors presynaptically modulate cranial visceral afferent synaptic transmission in nucleus tractus solitarius

Although the central terminals of cranial visceral afferents express vanilloid receptor 1 (VR1), little is known about their functional properties at this first synapse within the nucleus tractus solitarius (NTS). Here, we examined whether VR1 modulates afferent synaptic transmission. In horizontal brainstem slices, solitary tract (ST) activation evoked EPSCs. Monosynaptic EPSCs had low synaptic jitter (SD of latency to successive shocks) averaging 84.03 +/- 3.74 microsec (n = 72) and were completely blocked by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo[f]quinoxaline (NBQX). Sustained exposure to the VR1 agonist capsaicin (CAP; 100 nm) blocked ST EPSCs (CAP-sensitive) in some neurons but not others (CAP-resistant). CAP-sensitive EPSCs had longer latencies than CAP-resistant EPSCs (4.65 +/- 0.27 msec, n = 48 vs 3.53 +/- 0.28 msec, n = 24, respectively; p = 0.011), but they had similar jitter. CAP evoked two transient responses in CAP-sensitive neurons: a rapidly developing inward current (I(cap)) (108.1 +/- 22.9 pA; n = 21) and an increase in spontaneous synaptic activity. After 3-5 min in CAP, I(cap) subsided and ST EPSCs disappeared. NBQX completely blocked I(cap). The VR1 antagonist capsazepine (10-20 microm) attenuated CAP responses. Anatomically, second-order NTS neurons were identified by 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide transported from the cervical aortic depressor nerve (ADN) to stain central terminals. Neurons with fluorescent ADN contacts had CAP-sensitive EPSCs (n = 5) with latencies and jitter similar to those of unlabeled monosynaptic neurons. Thus, consistent with presynaptic VR1 localization, CAP selectively activates a subset of ST axons to release glutamate that acts on non-NMDA receptors. Because the CAP sensitivity of cranial afferents is exclusively associated with unmyelinated axons, VR1 identifies C-fiber afferent pathways within the brainstem.

ATP suppresses activity in slowly adapting but not rapidly adapting mechanoreceptors in toad skin

To test the possible role of ATP in transducing or modulating touch sensation, an isolated skin-nerve preparation from the toad, Bufo marinus, perfused on the inner side, was used to examine the effects of ATP on slowly adapting (SA) and rapidly adapting (RA) mechanoreceptors, identified by ramp and hold indentation of the skin. ATP (1-50 mM) did not generate impulse activity in either SA or RA mechanoreceptors nor increase their responses to mechanical stimulation. Instead, ATP (> or =5 mM) reversibly and selectively suppressed impulse discharge from SA mechanoreceptors. Our results indicate that while ATP may modulate the responses of specific touch receptors, it is not involved in transducing touch into nerve impulses.

ATP can enhance the proton-induced CGRP release through P2Y receptors and secondary PGE(2) release in isolated rat dura mater

Trigeminal afferent neurons express ionotropic P2X receptors for extracellular ATP which are known to be sensitive to low interstitial pH. Both conditions - ATP release and tissue acidosis - may occur in the dura following the ischemia phase of migraine attacks. Aim of this study was to investigate whether and how ATP and protons may cooperate in exciting meningeal afferents. After removal of the cerebral hemispheres hemisected scull cavities of adult Wistar rats were used as organ bath of their own lining, the dura mater. The dura was chemically stimulated and the amounts of immunoreactive calcitonin gene-related peptide (iCGRP) and prostaglandin E(2) (PGE(2)) released into incubation fluid were measured using enzyme immunoassays. Stimulation with ATP (10(-4) and 10(-3)M) augmented iPGE(2) release dose-dependently whereas iCGRP secretion was minimally enhanced only if the dura had previously been depleted of extracellular ATP using hexokinase. Acid buffer solutions (pH 5.9 and 5.4) resulted in pH-dependent increase of iCGRP release but reduced iPGE(2) release. Purines (ATP 10(-3)>UTP 10(-4)M>ATP 10(-4)M) and PGE(2) (10(-5)M) were found to facilitate the proton-induced increase in iCGRP release. The proton-reduction of PGE(2) release was overcome by adding ATP (10(-3)M). S(+)-flurbiprofen (10(-6)M) suppressed both the basal and stimulated iPGE(2) release and prevented the ATP(10(-4)M)-induced facilitation of the proton response. The facilitating effect of ATP was also blocked under suramin, a non-selective P2 antagonist, and under reactive blue, an non-selective P2Y-antagonist, but not under pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, a P2X-antagonist. The present results provide evidence that ATP has poor, if at all, direct excitatory effects on CGRP-containing trigeminal nerve endings in the isolated dura and its facilitatory action seems to depend on G-protein coupled P2Y receptors and secondary PGE(2) release. The UTP effect and the antagonist profile is indicative for the P2Y(2) receptor subtype.

ATP stimulates peripheral axons in human, rat and mouse--differential involvement of A(2B) adenosine and P2X purinergic receptors

Receptors for ATP have been reported on peripheral nerve terminals. It is a widespread assumption that the axonal membrane does not possess this kind of chemosensitivity, although P2X purinoceptors have been found in isolated rat vagus nerve. Therefore, in the present study, effects of ATP and analogues were tested on the excitability of unmyelinated axons in isolated rat sural nerve, mouse dorsal roots, and human sural nerve. Bath application of ATP to all three types of axonal preparations increased axonal excitability, but the underlying receptors appear to differ in the various preparations. In rat sural nerve, alpha,beta-adenosine-5'-methylene triphosphate produced the strongest excitation. This effect was blocked by pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid and indicates the presence of P2X receptors. In mouse dorsal roots, differences were found between fast and slow C-fibres. The latter responded to both P2X receptor and adenosine receptor agonists. In contrast, effects of ATP on faster-conducting C-fibres seem to be caused exclusively by effects of ATP on adenosine receptors. Application of ATP also excited C-fibres in fascicles of isolated human nerve. The pharmacological profile indicates activation of A(2B) adenosine receptors. However, we could not detect P2X receptors in this preparation with our techniques. These data show that the ATP sensitivity of sensory neurones is not restricted to their terminals. Activation of axonal purinergic receptors may contribute to the transduction of sensory, including nociceptive, stimuli.

Peripheral inflammation sensitizes P2X receptor-mediated responses in rat dorsal root ganglion neurons

ATP-gated P2X receptors in nociceptive sensory neurons participate in transmission of pain signals from the periphery to the spinal cord. To determine the role of P2X receptors under injurious conditions, we examined ATP-evoked responses in dorsal root ganglion (DRG) neurons isolated from rats with peripheral inflammation, induced by injections of complete Freund's adjuvant (CFA) into the hindpaw. Application of ATP induced both fast- and slow-inactivating currents in control and inflamed neurons. CFA treatment had no effect on the affinity of ATP for its receptors or receptor phenotypes. On the other hand, inflammation caused a twofold to threefold increase in both ATP-activated currents, altered the voltage dependence of P2X receptors, and enhanced the expression of P2X2 and P2X3 receptors. The increase in ATP responses gave rise to large depolarizations that exceeded the threshold of action potentials in inflamed DRG neurons. Thus, P2X receptor upregulation could account for neuronal hypersensitivity and contribute to abnormal pain responses associated with inflammatory injuries. These results suggest that P2X receptors are useful targets for inflammatory pain therapy.

Concurrent release of ATP and substance P within guinea pig trigeminal ganglia in vivo

Neurons within sensory ganglia have been proposed to communicate via non-synaptic release of a diffusible chemical messenger, but the identity of the chemical mediator(s) remains unknown [J. Neurosci. 16 (1996) 4733-4741]. The present study addressed the possibility of co-released ATP and substance P (SP) within sensory ganglia to further advance the hypothesis of non-synaptic communication between sensory neurons. Microdialysis probes inserted into trigeminal ganglia (TRGs) of anesthetized guinea pigs were perfused with artificial cerebrospinal fluid and the collected perfusate analyzed for ATP and SP content using the firefly luciferin-luciferase (L/L) assay and radioimmunoassay, respectively. Significant reversible increases in ATP and SP levels were observed after infusion of 100 mM KCl or 1 mM capsaicin. Ca(2+)-free ACSF produced an eightfold increase in ATP levels, interpreted as a decrease in activity of Ca(2+)-dependent ecto-nucleotidases that degrade ATP. In contrast, KCl-induced release of ATP in the presence of normal Ca(2+) was blocked by Cd(2+), a voltage-gated Ca(2+) channel blocker, illustrating Ca(2+)-dependence of evoked ATP release. Since ganglionic release of ATP could arise from several neuronal and non-neuronal sources we directly tested acutely dissociated TRG neuron somata for ATP release. Neuron-enriched dissociated TRG cells were plated onto glass tubes and tested for ATP release using the L/L assay. Robust ATP release was evoked with 5 microM capsaicin. These data suggest that ATP is released concurrently with SP from the somata of neurons within sensory ganglia.

Development of purinergic sensitivity in sensory neurons after peripheral nerve injury in the rat

Purinoceptors are present in the cell bodies as well as in both peripheral and central terminals of many sensory neurons, where they may play a role in sensory transmission, including pain. After peripheral nerve injury at the spinal nerve level, some axotomized afferent neurons develop ongoing discharges (ectopic discharges) that originate in the dorsal root ganglion (DRG). In the present study, we attempted to determine whether or not purinergic sensitivity develops in injured sensory neurons which display ectopic discharges, as well as in silent units. The L(4) and L(5) spinal nerves were ligated in Sprague-Dawley rats. Four to 21 days after the surgery, the DRGs with attached dorsal roots and spinal nerves were removed and ectopic discharges were recorded from teased dorsal root fascicles using an in vitro recording set-up. The results showed that 75.6 and 65.1% of the chronically axotomized DRG neurons displaying ectopic discharges enhanced their activity after application of adenosine 5'-triphosphate (ATP, 1 mM) or alpha,beta-methylene ATP (mATP, 100 microM), respectively. In addition, application of these purinoceptor agonists evoked activity in 7 of 28 axotomized DRG neurons, which did not show ongoing discharges. In contrast, only 1 of 34 DRG neurons acutely isolated from normal rats (no previous spinal nerve ligation) responded to either mATP or ATP. In most of the tested units, mATP-induced enhancement of ectopic discharges was blocked by non-specific P2X receptor antagonists, PPADS or suramin. The data from the present study suggest that purinergic sensitivity develops in DRG neurons after chronic axotomy and that this purinergic sensitivity is likely to be mediated by P2X purinoceptors. This acquired purinergic sensitivity may play an important functional role in the enhancement of ectopic discharges and exacerbation of pain upon sympathetic activation in the neuropathic pain state.

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.

Novel purinergic sensitivity develops in injured sensory axons following sciatic nerve transection in rat

Teased fibers were made from 153 spontaneous A afferents ending in sciatic nerve end neuromas of 3-14 days standing, 21 A afferents from intact sensory endings in the contralateral sciatic nerve, and 50 intact A afferents from the sciatic nerve in intact rats. Ninety-two percent of the injured fibers responded to adenosine 5'-triphosphate (ATP) (i.v.). However, few fibers from the contralateral nerve or nerves from intact animals responded to ATP. P2 receptor antagonist suramin or reactive blue 2 blocked the ATP-induced response in 76% of the fibers tested, whereas the P1 receptor antagonist aminophylline blocked the ATP-evoked effect in only 18% of the fibers tested. Sympathectomy did not affect the ATP-induced effects in injured axons. Close-arterial injection of ATP caused similar results as i.v. injection of ATP. The present study suggests that a novel purinergic sensitivity is developed at the injury site after sciatic nerve transection in rats, which may play a role in neuropathic pain under some conditions such as sympathetic activation.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Diadenosine pentaphosphate is more potent than ATP at P2X receptors in isolated rat vagus nerve

The effects of diadenosine pentaphosphate (Ap5A), diadenosine tetraphosphate (Ap4A), alpha,beta-methyleneATP (alpha,beta-meATP), and ATP were studied on the excitability of unmyelinated axons in isolated rat vagus nerve by means of a computerized threshold tracking technique. All purinergic compounds produced an increase in excitability, however, only the effects of alpha,beta-meATP and of Ap5A were strongly reduced by 2'- (or 3') -O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), a selective blocker for P2X1, P2X3, and heteromeric P2X2/3 receptors. The rank order of potency for TNP-ATP-sensitive excitation was determined as follows (30 microM each): alpha,beta-meATP >Ap5A >> Ap4A = ATP. These data suggest that Ap5A might be an important naturally occurring agonist for P2X receptors at the axonal membrane of unmyelinated, including nociceptive, nerve fibres.

Distribution of P2X purinoceptor clusters on individual rat dorsal root ganglion cells

The distribution of P2X receptors on the cell bodies of acutely dissociated rat dorsal root ganglion cells has been determined using immunohistochemistry. Only medium and small ganglion cells showed P2X receptor labelling, which took the form of receptor clusters 0. 2-0.5 microm in diameter. P2X(3) was the predominant cluster type although clusters of all other subtypes, P2X(1), P2X(2), P2X(4), P2X(5) and P2X(6) were observed. There was little evidence for colocalization of the different receptor subtypes pointing to very little heteropolymerization of P2X receptors in the ganglion.

ATP P2X receptors play little role in the maintenance of neuropathic hyperalgesia

There is good evidence that ATP receptors play a role in nociception in the periphery. We sought evidence that they contribute to neuropathic hyperalgesia. We carried out a partial ligation of the sciatic nerve in rats to induce nerve injury and neuropathic hyperalgesia. Intrathecal injection of suramin (a P2 purinoceptor antagonist) provided minor alleviation of thermal hyperalgesia, while PPADS (a selective P2X receptor antagonist) had no effect. Both suramin and PPADS caused abnormal behavior including aggressiveness and subsequent hyporeactivity and immobility. P2X receptors in the spinal cord do not appear to play a significant role in the maintenance of thermal hyperalgesia. However, P2X receptors may play an important role in the control of behaviour.

P2X purinoceptor-mediated excitation of trigeminal lingual nerve terminals in an in vitro intra-arterially perfused rat tongue preparation

A novel in vitro intra-arterially perfused adult rat tongue-nerve preparation was used to explore the possible actions of P2X purinoceptor agonists (ATP and alpha,beta-methylene ATP (alpha, beta-meATP)) on sensory nerve terminals innervating the rat tongue. We made whole-nerve recordings of the trigeminal branch of the lingual nerve (LN), which conducts general sensory information (pain, temperature, touch, etc.), and the chorda tympani (CT), which conducts taste information. Changes in LN and CT activity following intra-arterial application of P2X agonists were compared. In seven preparations, bolus close-arterial injection of ATP (30-3000 microM, 0.1 ml) or alpha,beta-meATP (10-300 microM, 0.1 ml) induced a rapid (< 1 s after injection), dose-related increase in LN activity that decayed within a few seconds. The minimal concentration of ATP (100 microM) required to elicit a response was about 10-fold higher than that of alpha,beta-meATP (10 microM). Bolus injection of ATP or alpha,beta-meATP induced a moderate decrease in firing frequency in three of seven CT preparations. LN responses to P2X agonists showed signs of rapid desensitisation with the peak frequency of discharge being smaller when the agonists were applied at short intervals. Suramin (200 microM) or PPADS (200 microM) applied by intra-arterial perfusion each antagonised the rapid increase in LN activity following application of alpha,beta-meATP (100 microM). Capsaicin (10 microM, 0.1 ml, n = 5 preparations) was injected intra-arterially to desensitise nociceptive fibres. This was found to block (n = 2) or greatly reduce (n = 3) the excitatory effects of alpha,beta-meATP (100 microM, 0.1 ml) on LN activity, implying that only capsaicin-sensitive nociceptive fibres in LN were responsive to P2X agonists. In contrast to the consistent excitatory responses in LN activity following fast application of P2X agonists as bolus, a variable and moderate change in discharge rate of LN and no change in CT activity (n = 5) was observed after applying ATP (100-300 microM, n = 21) or alpha,beta-meATP (100-300 microM, n = 14) by intra-arterial perfusion. The variable responses in LN activity to slow perfusion in contrast to close-arterial bolus injection are consistent with activation of the rapidly desensitising P2X3 receptors. In summary, ATP and alpha,beta-meATP preferentially activate general sensory afferent fibres (LN) but not taste fibres (CT). We suggest that the increase in whole-nerve activity of LN following application of P2X agonists represents activation of nociceptive fibres which possess P2X3 receptors. Our data indicate that ATP and P2X3 receptors may play a role in nociception, rather than taste sensation in the tongue.

Acute effects of capsaicin on gastrointestinal vagal afferents

Capsaicin is an important tool for investigation of thin afferent fibres, but its acute effects on subtypes of vagal afferent endings are unknown. In the gastrointestinal tract, these subtypes are: muscle endings (thought to be purely tension sensitive), mucosal endings (sensitive to stroking and chemical stimuli) and endings in the oesophagus with both properties. Acute capsaicin sensitivity was investigated in ferrets using in vivo and in vitro methods. Single-fibre activity was recorded from 63 vagal afferents: 12 Adelta-fibres, 15 C-fibres and 36 unclassified fibres with endings in the oesophagus (n=42), stomach (n=19) and duodenum (n=2). Responses to capsaicin occurred independently of motility changes and were therefore due to direct activation of the receptor ending. In the oesophagus in vivo, two of 10 tension receptors and one of one mucosal receptor responded to intraluminal application of 3.25 mM capsaicin. In the stomach and duodenum, five of 14 tension receptors and two of four mucosal receptors responded to close-systemic (32-164 nmol) capsaicin. In an in vitro gastro-oesophageal preparation, three of five tension, four of 21 mucosal and two of eight tension/mucosal receptors responded to topical application of 1mM capsaicin. Occurrence of responses was therefore unrelated to location of endings and isolation of tissue. Responsiveness was also unrelated to conduction velocity. Capsaicin caused desensitization of responses to further capsaicin application in 37% of afferents. It additionally caused cross-desensitization to mechanical stimuli, which was also seen in afferents that did not respond directly to capsaicin. In conclusion, capsaicin acutely activates all subtypes of gut vagal afferents in vivo and in vitro, although responsiveness is restricted to 30% of fibres and follows no specific pattern. Acute desensitization may be induced with or without a response.

P2X purinoceptor-induced sensitization of ferret vagal mechanoreceptors in oesophageal inflammation

1. Using an in vitro single unit recording technique we studied the changes in mechanical and chemical sensitivity of vagal afferent fibres in acute oesophagitis, with particular attention to inflammatory products such as purines. 2. Histologically verified oesophagitis was induced by oesophageal perfusion of 1 mg ml-1 pepsin in 150 mM HCl in anaesthetized ferrets for 30 min on two consecutive days. Controls were infused with 154 mM NaCl. 3. The number of action potentials evoked in oesophageal mucosal afferents by mucosal stroking with calibrated von Frey hairs (10-1000 mg) was stimulus dependent. In oesophagitis responsiveness was reduced across the range of stimuli compared with controls. 4. Topical application of the P2X purinoceptor agonist alphabeta-methylene ATP had no direct excitatory effect on afferents. In oesophagitis, but not in controls, there was a significant increase in responses to stroking with von Frey hairs during superfusion with alphabeta-methylene ATP (1 microM). 5. Mucosal afferents responded directly to one or more chemical stimuli: 26 % (5/19 afferents) responded in controls, and 47 % (7/15 afferents) in oesophagitis. There were no differences in responsiveness to bradykinin (1 microM), prostaglandin E2 (100 microM), 5-hydroxytryptamine (100 microM), capsaicin (1 mM) or hydrochloric acid (150 mM) between control and oesophagitis groups. 6. We conclude that a sensitizing effect of a P2X purinoceptor agonist on mechanosensory function is induced in oesophagitis. This effect is offset by a decrease in basal mechanosensitivity.

Effects of spinally administered P2X receptor agonists and antagonists on the responses of dorsal horn neurones recorded in normal, carrageenan-inflamed and neuropathic rats

1. The function and role of P2X receptors in the spinal transmission of nociception was investigated using the selective P2X receptor agonists, alpha,beta-methylene ATP (alpha,beta-me ATP) and beta, gamma-methylene-L-ATP (beta,gamma-me-L-ATP) and the P2X receptor antagonists pyridoxal-phosphate-6-azophenyl-2',4'-disulphonate (PPADS) and suramin. 2. Intrathecal administration of 5 and 50 microg of beta,gamma-me-L-ATP produced a significant facilitation of the C-fibre evoked response and a tendency towards increased excitability of the post-discharge, but not Abeta-fibre evoked response of dorsal horn neurones recorded in normal animals. Administration of similar doses of alpha,beta-me ATP did not produce an overall change in the response of the neuronal population. 3. Peripheral administration of 20 microg of these agonists into the paw of the rat evoked firing in the dorsal horn neurones. 4. Intrathecal administration of the antagonists, suramin (50 and 500 microg) and PPADS (5, 50 and 500 microg), to normal animals and to animals with a model of neuropathy induced by spinal nerve ligation did not alter the evoked neuronal responses. In contrast, intrathecal administration of 500 microg of suramin to animals 3 h after the induction of carrageenan inflammation produced a significant inhibition of the C-fibre evoked response of the neurones. Similar inhibitions were also seen following high doses of intrathecal PPADS, although this did not reach significance. 5. These results suggest that spinal P2X receptors may play a role in the modulation of spinal nociceptive transmission following the development of inflammation, but that these receptors play at most a minor role in spinal nociceptive processing in normal and neuropathic animals.

Ectopic purinergic sensitivity develops at sites of chronic nerve constriction injury in rat

The possibility that ectopic purinergic sensitivity develops following peripheral nerve injury was investigated in chronic constriction injury (CCI). Spontaneous firing of A-fibers originated from the injury site or from sensory endings of afferents in the contralateral sciatic nerve. ATP injected intravenously excited most of the injured fibers whereas none of the contralateral afferents responded to ATP. The ATP-induced effect was blocked by the P2 receptor antagonist reactive blue 2, but not the P1 receptor antagonist aminophylline. Neither the alpha-adrenoreceptor antagonist phentolamine nor the cyclooxygenase inhibitor indomethacin attenuated the ATP-evoked effect. We conclude that a novel ectopic purinergic sensitivity mediated by P2 receptors develops at sites of the CCI of nerves in the rat, which may contribute to neuropathic pain.

The effects of inflammation and inflammatory mediators on nociceptive behaviour induced by ATP analogues in the rat

1. We have studied the behavioural effects of intraplantar injections of adenosine 5'-triphosphate (ATP) and related compounds in freely moving rats and investigated whether these nociceptive effects are augmented in the presence of inflammatory mediators. 2. We find that in normal animals ATP and analogues produce dose-dependent nocifensive behaviour (seen as bursts of elevation of the treated hindpaw), and localized thermal hyperalgesia. The rank order of potency was: alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-methylene ATP) > 2-methylthioadenosine triphosphate (2-methylthio ATP) > ATP. After neonatal treatment with capsaicin, to destroy small calibre primary sensory neurones, nocifensive behaviour was largely absent. 3. The effects of ATP analogues were assessed in three models of peripheral sensitization: 2 h after dilute intraplantar carrageenan (0.25% w v(-1)); 24 h after irradiation of the hindpaw with ultraviolet (U.V.) B; immediately following prostaglandin E2 (PGE2) treatment. In all models the effect of alpha,beta-methylene ATP was greatly augmented. After carrageenan, significant hindpaw-lifting behaviour activity was induced by injection of only 0.05 nmol of alpha,beta-methylene ATP, some 100 times less than necessary in normal skin. 4. Our data suggest that it is much more likely that endogenous levels of ATP will reach levels capable of exciting nociceptors in inflamed versus normal skin. Our data also suggest the involvement of P2X3 receptor subunits in ATP-induced nociception.