P2X3 is expressed by DRG neurons that terminate in inner lamina II

P2X receptors: targets for novel analgesics?

The ability of adenosine 5'-triphosphate (ATP) to evoke acute pain has been known for many years, but its role in nociceptive signaling is only now becoming clear. ATP acts via P2X and P2Y receptors, and of particular importance here is the P2X(3) receptor. It is expressed selectively at high levels in nociceptive sensory neurons, where it forms functional receptors on its own and in combination with the P2X(2) receptor. Recent reports using gene knockout methods; antisense oligonucleotide and small, interfering RNA technologies; and a novel, selective P2X(3) antagonist, A-317491, show that P2X(3) receptors are involved in chronic inflammatory and neuropathic pain. The mRNA for other P2X subunits is also found in sensory neurons, and there is evidence for functional P2X(1/5) or P2X(2/6) heteromers in some of these. These data support the possibility that P2X receptors, particularly the P2X(3) subtype, could be targeted in the search for new, effective analgesics.

Trigeminal P2X3 receptor expression differs from dorsal root ganglion and is modulated by deep tissue inflammation

The distribution and modulation of the P2X(3) receptor was studied in trigeminal ganglion neurons to provide insight into the role of ATP in craniofacial sensory mechanisms. Binding to the d-galactose specific lectin IB4 was found in 73% of P2X(3)-positive neurons while only 16% of IB4 neurons expressed P2X(3). Neurons expressing P2X(3) alone were significantly larger than IB4-or IB4/P2X(3)-positive neurons. Investigation of target-specificity revealed that 22% of trigeminal ganglion muscle afferent neurons were positive for P2X(3) versus 16% of cutaneous afferent neurons. Muscle P2X(3) afferents were significantly smaller than the overall muscle afferent population while P2X(3) cutaneous afferent neurons were not. Presumptive heteromeric (P2X(2/3)) muscle afferent neurons were also identified and comprised 77% of the P2X(3) muscle afferent population. Muscle afferent neurons co-expressed P2X(3) with either calcitonin gene-related peptide (15%) or substance P (4%). The number of P2X(3)-positive muscle afferent neurons significantly increased one and four days following complete Freund's adjuvant-induced masseter muscle inflammation, but significantly decreased after 12 days. These results indicate that within trigeminal ganglia: (1) the P2X(3) receptor is expressed in both small and medium-sized neurons; (2) the P2X(3) receptor is not exclusively expressed in IB4 neurons; (3) P2X(3) is co-expressed with neuropeptides; (4) differences in the proportion of cutaneous versus muscle P2X(3) afferents are not apparent. Trigeminal P2X(3) neurons therefore differ markedly from dorsal root ganglion P2X(3) afferents. This study also shows that deep tissue inflammation modulates expression of the P2X(3) receptor and thus may warrant exploration as a target for therapeutic intervention.

The P2X3 antagonist P1, P5-di[inosine-5'] pentaphosphate binds to the desensitized state of the receptor in rat dorsal root ganglion neurons

P2X3 purinergic receptors are predominantly expressed in dorsal root ganglion (DRG) neurons and play an important role in pain sensation. P2X3-specific antagonists are currently being sought to ameliorate pain in several indications. Understanding how antagonists interact with the P2X3 receptor can aid in the discovery and development of P2X3-specific antagonists. We studied the activity of the noncompetitive antagonist P1, P5-di[inosine-5'] pentaphosphate (IP5I) at the P2X3 receptor, compared with the well studied competitive antagonist TNP-ATP, using a whole-cell voltage-clamp technique in dissociated rat DRG neurons. IP5I blocked alphabeta-methylene ATP (alphabeta-meATP)-evoked P2X3 responses in a concentration-dependent manner (IC50 = 0.6 +/- 0.1 microM). IP5I effectively inhibited P2X3 currents when pre-exposed to desensitized but not unbound receptors. Furthermore, IP5I equally blocked 1 and 10 microM alphabeta-meATP-evoked currents and had no effect on the desensitization rate constant of these currents. This supports the action of IP5I as a noncompetitive antagonist that interacts with the desensitized state of the P2X3 receptor. In contrast, TNP-ATP inhibited the current evoked by 1 microM alphabeta-meATP significantly more than the one evoked by 10 microM alphabeta-meATP. It also significantly slowed down the desensitization rate constant of the current. These results suggest that TNP-ATP acts as a competitive antagonist and competes with alphabeta-meATP at the P2X3 agonist binding site. These findings may help to explain why IP5I acts selectively at the fast-desensitizing P2X1 and P2X3 subtypes of the P2X purinoceptor, while having much less potency at slow-desensitizing P2X2 and P2X(2/3) subtypes that lack the fast desensitized conformational state.

P2X2 and P2X3 receptor expression in postnatal and adult rat urinary bladder and lumbosacral spinal cord

P2X receptors mediate the effects of ATP in micturition and nociception. During postnatal maturation, a spinobulbospinal reflex and voluntary voiding replace primitive voiding reflexes. This may involve changes in neuroactive compounds and receptors in bladder reflex pathways. We examined P2X2 and P2X3 receptors in bladder and spinal cord from postnatal (P0-P36, indicating number of days) and adult Wistar rats. Western blot of whole bladders for P2X2 and P2X3 expression was performed. Immunostaining for P2X2 and P2X3 receptors in urothelium and detrusor smooth muscle whole mounts and spinal cord sections was examined. Western blot demonstrated an age-dependent decrease (R(2) = 0.96, P Collapse

Key Words

• postnatal development
• micturition reflexes
• sacral parasympathetic nucleus
• dorsal commissure
• dorsal horn

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MESH Headings

• Aging/metabolism
• Animals
• Female
• Lumbosacral Region
• Male
• Rats
• Rats, Wistar
• Receptors, Purinergic P2/metabolism
• Receptors, Purinergic P2X2
• Receptors, Purinergic P2X3
• Spinal Cord/metabolism
• Tissue Distribution
• Urinary Bladder/metabolism

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Grants

• DK-060481 NIDDK NIH HHS
• NS-040796 NINDS NIH HHS
• R01 DK065989 NIDDK NIH HHS
• R56 DK060481 NIDDK NIH HHS
• R29 DK051369 NIDDK NIH HHS
• DK-065989 NIDDK NIH HHS
• R01 DK060481 NIDDK NIH HHS
• R01 NS040796 NINDS NIH HHS
• DK-051369 NIDDK NIH HHS
• R01 DK051369 NIDDK NIH HHS

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Affiliation(s)

Simon Studeny Department of Neurology, Anatomy and
Ali Torabi Department of Neurology, Anatomy and
Margaret A. Vizzard Department of Neurology, Anatomy and Neurobiology, University of Vermont, College of Medicine, Burlington, Vermont, VT 05405

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Different basal levels of CaMKII phosphorylated at Thr286/287 at nociceptive and low-threshold primary afferent synapses

Postsynaptic autophosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr286/287 is crucial for the induction of long-term potentiation at many glutamatergic synapses, and has also been implicated in the persistence of synaptic potentiation. However, the availability of CaMKII phosphorylated at Thr286/287 at individual glutamatergic synapses in vivo is unclear. We used post-embedding immunogold labelling to quantitatively analyse the ultrastructural localization of CaMKII phosphorylated at Thr286/287 (pCaMKII) at synapses formed by presumed nociceptive and low-threshold mechanosensitive primary afferent nerve endings in laminae I-IV of rat spinal cord. Immunogold labelling was enriched in the postsynaptic densities of such synapses, consistent with observations in pre-embedding immunoperoxidase-stained dorsal horn. Presynaptic axoplasm also exhibited sparse immunogold labelling, in peptidergic terminals partly associated with dense core vesicles. Analysis of single or serial pCaMKII-immunolabelled sections indicated that the large majority of synapses formed either by presumed peptidergic or non-peptidergic nociceptive primary afferent terminals in laminae I-II of the spinal cord, or by presumed low-threshold mechanosensitive primary afferent terminals in laminae IIi-IV, contained pCaMKII in their postsynaptic density. However, the postsynaptic levels of pCaMKII immunolabelling at low-threshold primary afferent synapses were only approximately 50% of those at nociceptive synapses. These results suggest that constitutively autophosphorylated CaMKII in the postsynaptic density is a common characteristic of glutamatergic synapses, thus potentially contributing to maintenance of synaptic efficacy. Furthermore, pCaMKII appears to be differentially regulated between high- and low-threshold primary afferent synapses, possibly reflecting different susceptibility to synaptic plasticity between these afferent pathways.

Enhanced thermal avoidance in mice lacking the ATP receptor P2X3

P2X3 is an ATP-gated cation channel subtype expressed by a subpopulation of primary sensory neurons. In vivo spinal cord recordings in mice lacking P2X3 (P2X3-/-) have suggested that this protein may be important for the coding of peripheral warm stimuli. To explore this possibility more thoroughly, we examined behavioral and electrophysiological responses to thermal stimuli in P2X3-/- mice. As previously reported, recording from the spinal cord dorsal horn of anesthetized P2X3-/- mice revealed a blunted response of wide dynamic range neurons to hind paw heating. When placed in a thermal gradient, however, P2X3-/- mice exhibited an unexpectedly enhanced avoidance of both hot and cold temperatures, relative to controls. In the tail immersion test, mutant mice exhibited shorter withdrawal latencies at temperatures above and below thermoneutrality. Consistent with these changes, P2X3-/- mice exhibited enhanced induction of spinal cord c-FOS following hind paw heating to 45 degrees C. Thus, gain- and loss-of-function thermosensory phenotypes coexist in P2X3-/- mice. No changes in thermal preference were observed in wild-type mice injected subcutaneously with the P2X3 antagonist, A317491 or intrathecally with the P2X3 and P2X1 antagonist TNP-ATP. The reason for this apparent discrepancy is unclear, but we cannot exclude the possibility that compensatory events contribute, at least in part, to the P2X3-/- phenotype. Regardless, this study illustrates the utility of thermal preference assays as part of a comprehensive approach to the analysis of mouse thermosensation.

Localization of P2X and P2Y Receptors in Dorsal Root Ganglia of the Cat

The distribution of P2X and P2Y receptor subtypes in upper lumbosacral cat dorsal root ganglia (DRG) has been investigated using immunohistochemistry. Intensity of immunoreactivity for six P2X receptors (P2X5 receptors were immuno-negative) and the three P2Y receptors examined in cat DRG was in the order of P2Y2 = P2Y4>P2X3>P2X2 = P2X7>P2X6>P2X1 = P2X4>P2Y1. P2X3, P2Y2, and P2Y4 receptor polyclonal antibodies stained 33.8%, 35.3%, and 47.6% of DRG neurons, respectively. Most P2Y2, P2X1, P2X3, P2X4, and P2X6 receptor staining was detected in small- and medium-diameter neurons. However, P2Y4, P2X2, and P2X7 staining was present in large- and small-diameter neurons. Double-labeling immunohistochemistry showed that 90.8%, 32.1%, and 2.4% of P2X3 receptor-positive neurons coexpressed IB4, CGRP, and NF200, respectively; whereas 67.4%, 41.3%, and 39.1% of P2Y4 receptor-positive neurons coexpressed IB4, CGRP, and NF200, respectively. A total of 18.8%, 16.6%, and 63.5% of P2Y2 receptor-positive neurons also stained for IB4, CGRP, and NF200, respectively. Only 30% of DRG neurons in cat were P2X3-immunoreactive compared with 90% in rat and in mouse. A further difference was the low expression of P2Y1 receptors in cat DRG neurons compared with more than 80% of the neurons in rat. Many small-diameter neurons were NF200-positive in cat, again differing from rat and mouse.

TRPV channels as thermosensory receptors in epithelial cells

Temperature-sensitive transient receptor potential vanilloid (TRPV) ion channels are critical contributors to normal pain and temperature sensation and therefore represent attractive targets for pain therapy. When these channels were first discovered, most attention was focused on their potential contributions to direct thermal activation of peripheral sensory neurons. However, recent anatomical, physiological, and behavioral studies have provided evidence that TRPV channels expressed in skin epithelial cells may also contribute to thermosensation in vitro and in vivo. Here, we review these studies and speculate on possible communication mechanisms from cutaneous epithelial cells to sensory neurons.

Differential chemosensory function and receptor expression of splanchnic and pelvic colonic afferents in mice

Lumbar splanchnic (LSN) and sacral pelvic (PN) nerves convey different mechanosensory information from the colon to the spinal cord. Here we determined whether these pathways also differ in their chemosensitivity and receptor expression. Using an in vitro mouse colon preparation, individual primary afferents were tested with selective P2X and transient receptor potential vanilloid receptor 1 (TRPV1) receptor ligands. Afferent cell bodies in thoracolumbar and lumbosacral dorsal root ganglia (DRG) were retrogradely labelled from the colon and analysed for P2X3- and TRPV1-like immunoreactivity (LI). Forty per cent of LSN afferents responded to alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP; 1 mm), an effect that was concentration dependent and reversed by the P2X antagonist pyridoxyl5-phosphate 6-azophenyl-2',4'-disulphonic acid (PPADS) (100 microm). Significantly fewer PN afferents (7%) responded to alpha,beta-meATP. Correspondingly, 36% of colonic thoracolumbar DRG neurones exhibited P2X3-LI compared with only 19% of colonic lumbosacral neurones. Capsaicin (3 microm) excited 61% of LSN afferents and 47% of PN afferents; 82% of thoracolumbar and 50% of lumbosacral colonic DRG neurones displayed TRPV1-LI. Mechanically insensitive afferents were recruited by alpha,beta-meATP or capsaicin, and were almost exclusive to the LSN. Capsaicin-responsive LSN afferents displayed marked mechanical desensitization after responding to capsaicin, which did not occur in capsaicin-responsive PN afferents. Therefore, colonic LSN and PN pathways differ in their chemosensitivity to known noxious stimuli and their corresponding receptor expression. As these pathways relay information that may relate to symptoms in functional gastrointestinal disease, these results may have implications for the efficacy of therapies targeting receptor modulation.

P2X receptors in the rat uterine cervix, lumbosacral dorsal root ganglia, and spinal cord during pregnancy

ATP, an intracellular energy source, is released from cells during tissue stress, damage, or inflammation. The P2X subtype of the ATP receptor is expressed in rat dorsal root ganglion (DRG) cells, spinal cord dorsal horn, and axons in peripheral tissues. ATP binding to P2X receptors on nociceptors generates signals that can be interpreted as pain from damaged tissue. We have hypothesized that tissue stress or damage in the uterine cervix during late pregnancy and parturition can lead to ATP release and sensory signaling via P2X receptors. Consequently, we have examined sensory pathways from the cervix in nonpregnant and pregnant rats for the presence of purinoceptors. Antiserum against the P2X3-receptor subtype showed P2X3- receptor immunoreactivity in axon-like structures of the cervix, in small and medium-sized neurons in the L6/S1 DRG, and in lamina II of the L6/S1 spinal cord segments. Retrograde tracing confirmed the projections of axons of P2X3-receptor-immunoreactive DRG neurons to the cervix. Some P2X3-receptor-positive DRG neurons also expressed estrogen receptor-alpha immunoreactivity and expressed the phosphorylated form of cyclic AMP response-element-binding protein at parturition. Western blots showed a trend toward increases of P2X3-receptor protein between pregnancy (day 10) and parturition (day 22-23) in the cervix, but no significant changes in the DRG or spinal cord. Since serum estrogen rises over pregnancy, estrogen may influence purinoceptors in these DRG neurons. We suggest that receptors responsive to ATP are expressed in uterine cervical afferent nerves that transmit sensory information to the spinal cord at parturition.

Endogenous ATP involvement in mustard-oil-induced central sensitization in trigeminal subnucleus caudalis (medullary dorsal horn)

Central sensitization represents a sustained hypersensitive state of dorsal horn nociceptive neurons that can be evoked by peripheral inflammation or injury to nerves and tissues. It reflects neuroplastic changes such as increases in neuronal spontaneous activity, receptive field size, and responses to suprathreshold stimuli and a decrease in activation threshold. We recently demonstrated that purinergic receptor mechanisms in trigeminal subnucleus caudalis (Vc; medullary dorsal horn) are also involved in the initiation and maintenance of central sensitization in brain stem nociceptive neurons of trigeminal subnucleus oralis. The aim of the present study was to investigate whether endogenous ATP is involved in the development of central sensitization in Vc itself. The experiments were carried out on urethan/alpha-chloralose anesthetized and immobilized rats. Single neurons were recorded and identified as nociceptive-specific (NS) in the deep laminae of Vc. During continuous saline superfusion (0.6 ml/h it) over the caudal medulla, Vc neuronal central sensitization was readily induced by mustard oil application to the tooth pulp. However, this mustard-oil-induced central sensitization could be completely blocked by continuous intrathecal superfusion of the wide-spectrum P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2, 4-disulphonic acid tetra-sodium (33-100 microM) and by apyrase (an ectonucleotidase enzyme, 30 units/ml). Superfusion of the selective P2X1, P2X3 and P2X(2/3) receptor antagonist 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (6-638 microM) partially blocked the Vc central sensitization. The two P2X receptor antagonists did not significantly affect the baseline nociceptive properties of the Vc neurons. These findings implicate endogenous ATP as an important mediator contributing to the development of central sensitization in nociceptive neurons of the deep laminae of the dorsal horn.

Re-organization of P2X3 receptor localization on epidermal nerve fibers in a murine model of cancer pain

To determine whether ATP and P2X3 receptors contribute to bone-cancer pain in a mouse model, immunohistochemical techniques were used to identify whether changes in the labeling of P2X3 receptors on epidermal nerve fibers (ENFs) occurred during tumor development. C3H mice were injected with osteolytic fibrosarcoma cells in and around the calcaneus bone. These mice exhibited mechanical hyperalgesia by day 10 post-implantation as assessed using von Frey monofilaments. Biopsies of the plantar skin overlying the tumor were obtained at days 10, 14, and 18 post-implantation. Confocal images were analyzed for the number of PGP 9.5, P2X3, and CGRP immunoreactive (ir) ENFs. The overall ENF population (PGP-ir) decreased progressively over time, whereas the subsets of P2X3-ir fibers demonstrated a modest increase and CGRP-ir nerve fibers remained fairly constant. Importantly, the proportion of CGRP-ir fibers that labeled for P2X3 increased from approximately 6% in control animals to nearly 30% at day 14 following tumor cell implantation. These studies demonstrate increased expression of P2X3 receptors on CGRP-ir ENFs during tumor growth and suggest a role for ATP in cancer-related pain.

Oncostatin M in the development of the nervous system

Oncostatin M (OSM) is a member of the interleukin-6 family of cytokines. Of these cytokines, OSM is closely related structually, genetically and functionally to leukemia inhibitory factor. However, OSM-specific biological activities have been reported in hematopoiesis and liver development. Recently, we have demonstrated OSM-specific activities in the nervous systems. In the adult central nervous system (CNS), OSM receptor (OSMR) beta was observed in meningeal cells of pia mater, epithelial cells of the choroid plexus and olfactory astrocyte-like glia surrounding the glomeruli of the olfactory bulb. In the CNS of neonatal mice, OSMRbeta was also expressed in the ventral subnucleus of the hypoglossal nucleus, but disappeared at post-natal day (P) 14. In contrast with the CNS, OSMRbeta was strongly expressed in small-sized non-peptidergic neurons of the dorsal root ganglia (DRG) and trigeminal ganglia (TG). Interestingly, all OSMRbeta-positive neurons in these ganglia also expressed both TRPV1 (a vanilloid receptor) and P2X3 (a purinergic receptor). In OSM-deficient mice, TRPV1/P2X3/OSMRbeta triple-positive neurons were significantly decreased. Consistent with such histological findings, OSM-deficient mice exhibited a reduction in responses to various stimuli, including mechanical and thermal stimuli. These findings suggest an important role for OSM in the development of a subset of nociceptive neurons.

Isolectin B4 binding neurons are not present in the rat knee joint

Small-diameter sensory neurons are key contributors in joint pain and have been implicated in the pathogenesis of rheumatoid arthritis (RA). Small-diameter sensory neurons can be separated into at least two distinct populations, which include isolectin B4 (IB4)-binding and tyrosine receptor kinase (trk) A-expressing. While trkA-expressing neurons have been identified in the rat knee joint there are no data, we are aware of, to suggest that IB4-binding neurons are also present. We aimed to determine whether or not there exists a population of IB4-binding neurons in the rat knee joint. Retrograde nerve tracing with fluoro-gold (FG) was used to identify the complete population of knee joint afferents in the lumbar dorsal root ganglia (DRG) L3 and L4 of female Wistar rats. IB4 conjugated to fluorescein isothiocyanate (FITC) was used to identify the cell bodies of IB4-binding neurons in the DRG. Of 1096 FG-labeled cell bodies in the DRG of knee joint injected animals (n=4), none were double labeled with FITC. Injection of FG into skin over the medial aspect of the rat knee (n=3) showed 48% of these cutaneous afferents in L3 and L4 DRG were double-labeled with FG and FITC. A complete absence of IB4-binding neurons in the rat knee joint makes it unlikely that this predominantly cutaneous, IB4-binding population of afferent neurons could have any significant influence in chronic inflammatory joint disease. This suggests that trkA-expressing neurons are the sole population of small-diameter sensory neurons in the knee joint and implies a significant role for these afferents in the progression of RA.

Histochemical localisation of a galactose-containing glycoconjugate expressed by sensory neurones innervating different peripheral tissues in the rat

The plant lectin Bandeiraea simplicifolia I-isolectin B4 (BSI-B4) identifies a galactose-containing, membrane-associated glycoconjugate expressed by a discrete subpopulation of unmyelinated primary sensory neurones in the rat. We have previously suggested that BSI-B4 selectively binds to primary sensory neurones that innervate the skin. However, in that study, the tracer diamidino yellow was applied to the cut ends of peripheral nerves to identify neurones innervating particular target tissues. In this study, we have avoided axotomy by retrogradely labelling primary sensory neurones from peripheral tissues using the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbacyanine perchlorate (DiI). DiI was injected into the plantar skin, gastrocnemius muscle, and pyloric region of the stomach in rats. Corresponding ganglia were sectioned, incubated in BSI-B4 conjugated to fluorescein isothiocyanate, and examined with a fluorescence microscope. DiI-labelled cells were identified by red fluorescence within the cytoplasm, whereas cells binding BSI-B4 displayed green fluorescence associated with the plasma membrane and Golgi apparatus. Quantitative analysis revealed that 36.2% of cutaneous neurones, 7.6% of muscle neurones, and 6.8% of visceral neurones expressed the BSI-B4-binding site, indicating that a small but significant proportion of small-diameter primary sensory neurones innervating muscle and viscera also express BSI-B4-binding sites.

Amitriptyline produces multiple influences on the peripheral enhancement of nociception by P2X receptors

Peripherally administered amitriptyline exhibits potential to be a locally active analgesic, while ATP augments peripheral nociception by interacting with P2X(3) receptors on sensory afferents. The present study examined the effects of amitriptyline on flinching and biting/licking behaviours and thermal hyperalgesia produced by alphabeta-methylene-ATP (alphabeta-MeATP), a ligand for P2X(3) receptors, following intraplantar administration into the hindpaw of rats. Coadministration of low doses of amitriptyline (up to 100 nmol) with alphabeta-MeATP augmented thermal hyperalgesia and flinching behaviours. The most active dose of amitriptyline (100 nmol) had no intrinsic effect. Augmentation of alphabeta-MeATP actions appears to be due to increased tissue levels of biogenic amines resulting from inhibition of uptake, as phentolamine (alpha(1)/alpha(2)-adrenergic receptor antagonist) and methysergide (5-hydroxytryptamine or 5-HT(1)/5-HT(2) receptor antagonist) inhibit the augmented flinching produced by alphabeta-MeATP/amitriptyline. When noradrenaline and 5-HT were coadministered with alphabeta-MeATP (both increase the effect of alphabeta-MeATP), amitriptyline had no effect on flinching produced by alphabeta-MeATP/noradrenaline but inhibited flinching produced by alphabeta-MeATP/5-HT. In the presence of low concentrations of formalin (0.5%, 1%; which also increase the effect alphabeta-MeATP), amitriptyline inhibited augmented behaviours. Higher doses of amitriptyline (300-1000 nmol) increased thermal thresholds, suppressed thermal hyperalgesia produced by alphabeta-MeATP, and inhibited flinching produced by alphabeta-MeATP. Collectively, these results indicate that amitriptyline produces complex influences on peripheral pain signaling by P2X receptors. Lower doses augment nociception by alphabeta-MeATP (probably by inhibiting noradrenaline and 5-HT uptake) but inhibit alphabeta-MeATP responses in the presence of inflammatory mediators (perhaps reflecting receptor blocking properties); higher doses uniformly inhibit nociception by alphabeta-MeATP (perhaps reflecting local anesthetic properties).

A P2X receptor-mediated nociceptive afferent pathway to lamina I of the spinal cord

Of the six lamina regions in the dorsal horn of the spinal cord, lamina I is a major sensory region involved in nociceptive transmission under both physiological and pathological conditions. While P2X receptors have been shown to be involved in nociception, it remains unknown if P2X receptors are involved in nociceptive transmission to lamina I neurons. Using rat spinal cord slice preparations and patch-clamp recordings, we have demonstrated that the excitatory synaptic transmission between primary afferent fibers and lamina I neurons is significantly affected by ATP and alpha,beta-methylene-ATP. The synaptic effects of them include the increases of the frequency of both miniature excitatory postsynaptic currents (mEPSCs) and spontaneous EPSCs (sEPSCs), and decreases of evoked EPSCs (eEPSCs). These effects were blocked by pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS, 10 microM) and suramin (30 microM). In the neurons for which ATP and alpha,beta-methylene-ATP had effects on mEPSCs, sEPSCs and eEPSCs, capsaicin produced similar synaptic effects. Our results indicate that P2X receptors are expressed on many afferent fibers that directly synapse to lamina I neurons. Furthermore, these P2X receptor-expressing afferent fibers are capsaicin-sensitive nociceptive afferents. Thus, this study reveals a P2X receptor-mediated nociceptive afferent pathway to lamina I of the spinal cord and provides a new insight into the nociceptive functions of P2X receptors.

Spinal P2X receptor modulates reflex pressor response to activation of muscle afferents

Static contraction of skeletal muscle evokes increases in blood pressure and heart rate. Previous studies suggested that the dorsal horn of the spinal cord is the first synaptic site responsible for those cardiovascular responses. In this study, we examined the role of ATP-sensitive P2X receptors in the cardiovascular responses to contraction by microdialyzing the P2X receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) into the L7 level of the dorsal horn of nine anesthetized cats. Contraction was elicited by electrical stimulation of the L7 and S1 ventral roots. Blockade of P2X receptor attenuated the contraction induced-pressor response [change in mean arterial pressure (delta MAP): 16 +/- 4 mmHg after 10 mM PPADS vs. 42 +/- 8 mmHg in control; P < 0.05]. In addition, the pressor response to muscle stretch was also blunted by PPADS (delta MAP: 27 +/- 5 mmHg after PPADS vs. 49 +/- 8 mmHg in control; P < 0.05). Finally, activation of P2X receptor by microdialyzing 0.5 mM alpha,beta-methylene into the dorsal horn significantly augmented the pressor response to contraction. This effect was antagonized by prior PPADS dialysis. These data demonstrate that blockade of P2X receptors in the dorsal horn attenuates the pressor response to activation of muscle afferents and that stimulation of P2X receptors enhances the reflex response, indicating that P2X receptors play a role in mediating the muscle pressor reflex at the first synaptic site of this reflex.

Altered primary afferent anatomy and reduced thermal sensitivity in mice lacking galectin-1

The transmission of nociceptive information occurs along non-myelinated, or thinly myelinated, primary afferent axons. These axons are generally classified as peptidergic (CGRP-expressing) or non-peptidergic (IB4-binding), although there is a sub-population that is both CGRP-positive and IB4-binding. During neuronal development and following injury, trophic factors and their respective receptors regulate their survival and repair. Recent reports also show that the carbohydrate-binding protein galectin-1 (Gal1), which is expressed by nociceptive primary afferent neurons during development and into adulthood, is involved in axonal pathfinding and regeneration. Here we characterize anatomical differences in dorsal root ganglia (DRG) of Gal1 homozygous null mutant mice (Gal1(-/-)), as well as behavioural differences in tests of nociception. Gal1(-/-) mice have a significantly reduced proportion of IB4-binding DRG neurons, an increased proportion of NF200-immunoreactive DRG neurons, increased depth of central terminals of IB4-binding and CGRP-immunoreactive axons in the dorsal horn, and a reduced number of Fos-positive second order neurons following thermal (cold or hot) stimulation. While there is no difference in the total number of axons in the dorsal root of Gal1(-/-) mice, there are an increased number of myelinated axons, suggesting that in the absence of Gal1, neurons that are normally destined to become IB4-binding instead become NF200-expressing. In addition, mice lacking Gal1 have a decreased sensitivity to noxious thermal stimuli. We conclude that Gal1 is involved in nociceptive neuronal development and that the lack of this protein results in anatomical and functional deficits in adulthood.

Changes in P2X3 purinoceptors in sensory ganglia of the mouse during embryonic and postnatal development

The expression of the P2X(3) nucleotide receptor in embryonic day 14-18, postnatal day 1-14 and adult mouse sensory ganglia was examined using immunohistochemistry. Nearly all sensory neurons in dorsal root ganglia, trigeminal ganglia and nodose ganglia in embryos at embryonic day 14 expressed P2X(3) receptors, but after birth there was a gradual decline to about 50% of neurons showing positive immunostaining for P2X(3). In embryos there were only small neurons, while from postnatal day 7 both large and small neurons were present. Isolectin B(4) (IB(4))-positive neurons in dorsal, trigeminal and nodose ganglia did not appear until birth, but the numbers increased to about 50% by postnatal day 14 when a high proportion of IB(4)-positive neurons were also positively labelled for the P2X(3) receptor. About 10% of neurons in dorsal, trigeminal and nodose ganglia were positive for calcitonin gene-related peptide in embryos, nearly all of which stained for P2X(3) receptors. This increased postnatally to about 35-40% in adults, although only a few colocalised with P2X(3) receptors. Neurofilament 200 was expressed in about 50% of neurons in trigeminal ganglia in the embryo, and this level persisted postnatally. All neurofilament 200-positive neurons stained for P2X(3) in embryonic dorsal root ganglia, trigeminal ganglia and nodose ganglia, but by adulthood this was significantly reduced. The neurons that were positive for calbindin in embryonic dorsal, trigeminal and nodose ganglia showed colocalisation with P2X(3) receptors, but few showed colocalisation postnatally.

P2Y receptor expression on astrocytes in the nucleus accumbens of rats

The expression of purinoceptor (P2)Y-subtypes on astrocytes in vivo under physiological conditions and after stab wound injury was investigated. Reverse transcriptase-polymerase chain reaction with specific primers for the receptor-subtypes P2Y1,2,4,6,12 in tissue extracts of the nucleus accumbens of untreated rats revealed the presence of all P2Y receptor mRNAs investigated. Double immunofluorescence visualized with laser scanning microscopy indicated the expression of the P2Y1,4 receptors on glial fibrillary acidic protein (GFAP)-labeled astrocytes under physiological conditions. After stab wound injury the additional expression of the P2Y2 and P2Y6 receptors, and an up-regulation of the P2Y1,4 receptor-labeling on astrocytic cell bodies and/or processes was observed. Astrocytes of cortical, in contrast to accumbal areas exhibited P2Y1,2,4,6 receptor-immunoreactivity (IR) under control conditions, which was up-regulated after stab would injury. Labeling for the P2Y12 receptor was not observed on GFAP-positive cortical and accumbal astrocytes under any of the conditions used. For the first time, the co-localization of different P2 receptor-subtypes (e.g. P2Y1 and P2X3) on the same astrocyte was shown immunocytochemically. The up-regulation of P2Y1 receptor-IR on astrocytes and non-glial cells after mechanical injury could be facilitated by microinfusion of the P2Y1,12,13 receptor agonist adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS). Proliferative changes after ADPbetaS-microinjection were characterized by means of double-staining with antibodies against GFAP and 5-bromo-2'-deoxyuridine. The non-selective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, the P2Y1 receptor antagonist N6-methyl-2'-deoxyadenosine 3',5'-bisphosphate and the P2Y1 receptor-antibody itself inhibited the agonist-induced effects. The data indicate the region-specific presence of P2Y receptors on astrocytes in vivo and their up-regulation after injury as well as the co-localization of P2X and P2Y receptor-subtypes on the same astrocyte. The dominant role of P2Y1 receptors in proliferation and the additional stimulation of non-P2Y1 receptors has been demonstrated in vivo suggesting the involvement of this receptor-type in the gliotic response under physiological and pathological conditions.

Peripheral P2X receptors and nociception: interactions with biogenic amine systems

ATP is implicated in peripheral nociception following activation of P2X, and particularly P2X(3) receptors. The present study examined interactions between alphabeta-methylene-ATP (a P2X(3) receptor ligand) and 5-hydroxytryptamine (5-HT), noradrenaline (NA) and histamine, following local administration into the hindpaw, on spontaneous pain behaviors and thermal hyperalgesia in Sprague-Dawley rats. The interaction with NA was further explored using systemic 6-hydroxydopamine (6-OHDA) and locally administered indomethacin. alphabeta-methylene-ATP produced no spontaneous pain behaviors. Coadministration of 5-HT with alphabeta-methylene-ATP mildly augmented flinching behaviors, while histamine had no such effect. Coadministration of NA with alphabeta-methylene-ATP produced a pronounced expression of flinching and biting/licking behaviors. alphabeta-Methylene-ATP, given alone, produced thermal hyperalgesia, and this was markedly augmented by both 5-HT and NA, but not histamine. 6-OHDA (neurotoxin for sympathetic neurons) and indomethacin (cyclooxygenase inhibitor) reduced the augmenting effect of NA on alphabeta-methylene-ATP-induced thermal hyperalgesia, but had no effect on spontaneous pain behaviors produced by the alphabeta-methylene-ATP/NA combination. Effects of alphabeta-methylene-ATP, NA and their combination were also examined in Long Evans and Wistar rats. In both strains, alphabeta-methylene-ATP and NA both individually led to significant intrinsic flinching behaviors, and the effect of their combination was even more pronounced than in Sprague-Dawley rats. These results provide evidence for: (a) a strong enhancement by NA and 5-HT of nociception produced by peripheral P2X receptors in Sprague-Dawley rats, (b) an indirect action of NA, via sympathetic efferents and prostanoids, with thermal hyperalgesia, and (c) a greater expression of spontaneous pain behaviors with alphabeta-methylene-ATP and NA alone, and with their combination, in Wistar and Long Evans rats compared to Sprague-Dawley rats.

Involvement of locus coeruleus noradrenergic neurons in supraspinal antinociception by alpha,beta-methylene-ATP in rats

We reported previously that intracerebroventricular (i.c.v.) administration of P2X-receptor agonists produced antinociception and the effect was attenuated by i.c.v. pretreatment with beta(2)-adrenergic receptor antagonists. The present study examined the involvement of noradrenergic neurons arising from the locus coeruleus (LC) in the supraspinal antinociception by the P2X-receptor agonist alpha,beta-methylene-ATP in rats. We found that pretreatment with DSP-4 (50 mg/kg, i.p.), which is a neurotoxin to selectively disrupt noradrenergic neurons arising from the LC, significantly attenuated the antinociception by i.c.v. administration of alpha,beta-methylene-ATP (10 nmol/rat). Microinjection of alpha,beta-methylene-ATP (0.1 and 1 nmol/side) into the bilateral LC significantly elevated the nociceptive threshold more potently than the i.c.v. administration at a dose of 10 nmol/rat. The antinociception by intra-LC injection of alpha,beta-methylene-ATP (1 nmol/side) was significantly attenuated by co-injection of pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (1 nmol/side), a non-selective P2X-receptor antagonist. These results suggest that noradrenergic neurons arising from the LC are involved in the supraspinal antinociception by alpha,beta-methylene-ATP through P2X receptors in the LC.

Ultrastructural analysis of the central terminals of primary sensory neurones labelled by transganglionic transport of bandeiraea simplicifolia I-isolectin B4

In this study the ultrastructural appearance of primary sensory neurones labelled by the injection of the plant lectin Bandeiraea simplicifolia I-isolectin B(4) (BSI-B(4)) into a peripheral nerve has been examined in the rat. Electron microscopy of the somata of retrogradely labelled neurones showed the lectin to be associated with the inner surface of cytoplasmic vesicles, supporting the premise that the uptake of BSI-B(4) into sensory neurones is by the process of receptor-mediated endocytosis. Light and electron microscopic analysis of the spinal cord revealed transganglionically transported lectin in unmyelinated axons in the dorsolateral funiculus and axon terminals concentrated mainly within lamina II of the dorsal horn. Detailed analysis of 1377 of these axon terminals revealed that the majority were glomerular in shape and surrounded by up to 14 other unlabelled profiles. These findings suggest that primary sensory neurones which transganglionically transport BSI-B(4) have a synaptic ultrastructure similar to that which has been previously reported for unmyelinated primary sensory neurones. Moreover, it appears that the axon terminals of these neurones are subjected to extensive modulation. Examination of the vesicle content of lectin labelled axon terminals revealed that the majority contained small agranular vesicles while large granular vesicles were observed only occasionally. These findings support the suggestion that the populations of neurones expressing binding sites for BSI-B(4) are fairly distinct from those containing neuroactive peptides. In conclusion, the results of the current study suggest that the lectin BSI-B(4) can be used as a histological marker for a subpopulation of small diameter primary sensory neurones and provide further evidence for the potential of this lectin as a useful tool in the study of pain.

Identification of negative residues in the P2X3 ATP receptor ectodomain as structural determinants for desensitization and the Ca2+-sensing modulatory sites

On nociceptive neurons, one important mechanism to generate pain signals is the activation of P2X(3) receptors, which are membrane proteins gated by extracellular ATP. In the presence of the agonist, P2X(3) receptors rapidly desensitize and then recover slowly. One unique property of P2X(3) receptors is the recovery acceleration by extracellular Ca(2+) that can play the role of the gain-setter of receptor function only when P2X(3) receptors are desensitized. To study negatively charged sites potentially responsible for this action of Ca(2+), we mutated 15 non-conserved aspartate or glutamate residues in the P2X(3) receptor ectodomain with alanine and expressed such mutated receptors in human embryonic kidney cells studied with patch clamping. Unlike most mutants, D266A (P2X(3) receptor numbering) desensitized very slowly, indicating that this residue is important for generating desensitization. Recovery appeared structurally distinct from desensitization because E111A and D266A had a much faster recovery and D220A and D289A had a much slower one despite their standard desensitization. Furthermore, E161A, E187A, or E270A mutants showed lessened sensitivity to the action of extracellular Ca(2+), suggesting that these determinants were important for the effect of this cation on desensitization recovery. This study is the first report identifying several negative residues in the P2X(3) receptor ectodomain differentially contributing to the general process of receptor desensitization. At least one residue was important to enable the development of rapid desensitization, whereas others controlled recovery from it or the facilitating action of Ca(2+). Thus, these findings outline diverse potential molecular targets to modulate P2X(3) receptor function in relation to its functional state.

Activation of thin-fiber muscle afferents by a P2X agonist in cats

The responses of group III and IV triceps surae muscle afferents to intra-arterial injection of alpha,beta-methylene ATP (50 microg/kg) was examined in decerebrate cats. We found that this P2X(3) agonist stimulated only three of 18 group III afferents but 7 of 9 group IV afferents (P < 0.004). The three group III afferents stimulated by alpha,beta-methylene ATP conducted impulses below 4 m/s. Pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, a P2-receptor antagonist, prevented the stimulation of these afferents by alpha,beta-methylene ATP. We conclude that P2X(3) agonists stimulate only the slowest conducting group III muscle afferents as well as group IV afferents.

Purinergic and vanilloid receptor activation releases glutamate from separate cranial afferent terminals in nucleus tractus solitarius

Vanilloid (VR1) and purinergic (P2X) receptors are found in cranial afferent neurons in nodose ganglia and their central terminations within the solitary tract nucleus (NTS), but little is known about their function. We mechanically dissociated dorsomedial NTS neurons to preserve attached native synapses and tested for VR1 and P2X function primarily in spindle-shaped neurons resembling intact second-order neurons. All neurons (n = 95) exhibited spontaneous glutamate (EPSCs) and GABA (IPSCs)-mediated synaptic currents. VR1 agonist capsaicin (CAP; 100 nm) reversibly increased EPSC frequency, effects blocked by capsazepine. ATP (100 microm) increased EPSC frequency, actions blocked by P2X antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS; 20 microm). In all CAP-resistant neurons, P2X agonist alphabeta-methylene-ATP (alphabeta-m-ATP) increased EPSC frequency. Neither CAP nor alphabeta-m-ATP altered EPSC amplitudes, kinetics, or holding currents. Thus, activation of VR1 and P2X receptors selectively facilitated presynaptic glutamate release on different NTS neurons. PPADS and 2',3'-O-(2,4,6-trinitrophenyl)-ATP blocked alphabeta-m-ATP responses, but P2X1-selective antagonist NF023 (8,8'-[carbonylbis (imino-3,1-phenylene carbonylimino)]bis-1,3,5-naphthalenetrisulfonic acid) did not. The pharmacological profile and transient kinetics of ATP responses are consistent with P2X3 homomeric receptors. TTX and Cd(2+) did not eliminate agonist-evoked EPSC frequency increases, suggesting that voltage-gated sodium and calcium channels are not required. In nodose ganglia, CAP but not alphabeta-m-ATP evoked inward currents in slow conducting neurons and the converse pattern in myelinated, rapidly conducting neurons (n = 14). Together, results are consistent with segregation of glutamatergic terminals into either P2X sensitive or VR1 sensitive that correspondingly identify myelinated and unmyelinated afferent pathways at the NTS.

Purinergic component of mechanosensory transduction is increased in a rat model of colitis

ATP contributes to mechanosensory transduction in the rat colorectum. P2X3 receptors are present on dorsal root ganglia (DRG) neurons that supply this area of the gut. Previous studies have shown an increased role for ATP in inflamed tissues. We aimed to investigate whether an increased purinergic component exists during mechanosensory transduction in a rat model of colitis. An in vitro rat colorectal preparation was used to investigate whether distension increased ATP release and to evaluate the role of purinergic antagonists in distension-evoked sensory discharges in the pelvic nerve in normal and colitis preparations. DRG neuron purinoceptors were also studied. Distension-evoked responses in the colitis model were attenuated to a significantly greater extent by 2',3'-O-trinitrophenyl-ATP and pyridoxyl 5-phosphate 6-azophenyl-2',4'-disulfonic acid. Inflammation caused augmented distension-evoked sensory nerve excitation after application of ATP and alpha,beta-methylene ATP. Single-fiber analysis confirmed that mean firing per unit was increased. Distension-evoked increases in ATP release from epithelial cells were substantially higher. The number of DRG neurons responding to ATP and the number of those staining for the P2X3 receptor, particularly those containing calcitonin gene-related peptide, were increased. Adenosine, after ectoenzymatic breakdown of ATP, is involved to a lesser degree in the longer-lasting distension-evoked sensory discharge, suggesting reduced ATPase activity. It was therefore concluded that ATP has an enhanced role in mechanosensory transduction in the inflamed rat colorectum. The underlying mechanisms appear to involve increased distension-evoked release of ATP as well as an increase in the number of DRG neurons supplying the colorectum expressing P2X3 receptors, especially those containing calcitonin gene-related peptide.

High voltage-activated Ca2+ channel currents in isolectin B4-positive and -negative small dorsal root ganglion neurons of rats

Voltage-gated Ca(2+) channels in the primary sensory neurons are important for neurotransmitter release and regulation of nociceptive transmission. Although multiple classes of Ca(2+) channels are expressed in the dorsal root ganglion (DRG) neurons, little is known about the difference in the specific channel subtypes among the different types of DRG neurons. In this study, we determined the possible difference in high voltage-activated Ca(2+) channel currents between isolectin B(4) (IB(4))-positive and IB(4)-negative small-sized (15-30 microm) DRG neurons. Rat DRG neurons were acutely isolated and labeled with IB(4) conjugated to a fluorescent dye. Whole-cell patch clamp recordings of barium currents flowing through calcium channels were performed on neurons with and without IB(4). The peak current density of voltage-gated Ca(2+) currents was not significantly different between IB(4)-positive and IB(4)-negative neurons. Also, both nimodipine and omega-agatoxin IVA produced similar inhibitory effects on Ca(2+) currents in these two types of neurons. However, block of N-type Ca(2+) channels with omega-conotoxin GVIA produced a significantly greater reduction of Ca(2+) currents in IB(4)-positive than IB(4)-negative neurons. Furthermore, the IB(4)-positive neurons had a significantly smaller residual Ca(2+) currents than IB(4)-negative neurons. These data suggest that a higher density of N-type Ca(2+) channels is present in IB(4)-positive than IB(4)-negative small-sized DRG neurons. This differential expression of the subtypes of high voltage-activated Ca(2+) channels may contribute to the different function of these two classes of nociceptive neurons.

The P2Y agonist UTP activates cutaneous afferent fibers

The majority of adenosine triphosphate (ATP)-induced nociceptive transduction and pain has been attributed to ionotropic P2X3 receptors. Metabotropic P2Y receptors, some of which bind pyrimidines as well as purines, have received little attention. Here we have examined the ability of P2Y receptor signaling to evoke action potential firing in functionally identified afferent fibers using the skin nerve preparation from adult mouse. The P2Y2/P2Y4 ligand UTP activated sustained action potential firing in 54% of C fibers in a concentration-dependent manner. The effect was specific for P2Y2/P2Y4 receptors, as the P2Y6 ligand UDP never activated C fibers. In comparison to C fibers, few thinly myelinated A-mechanoreceptors (AM) (12%) were activated by UTP. The majority (70-80%) of the UTP-sensitive C and Adelta fibers responded to the algogen capsaicin with a barrage of action potentials, whereas the UTP-insensitive fibers were largely unresponsive to capsaicin. Furthermore, 86% of the UTP-sensitive C fibers and 100% of the UTP-sensitive AM fibers also responded to the P2X agonist alpha,beta-methylene ATP, indicating that P2Y and P2X receptors are widely co-expressed. Surprisingly, a significant proportion (20-40%) of low threshold slowly and rapidly adapting Abeta fibers were also activated by UTP and alpha,beta-methylene ATP. These data indicate that P2Y receptors on the terminals of capsaicin-sensitive cutaneous sensory neurons effectively evoke nociceptive transmission, and support the hypothesis that UTP may be an endogenous nociceptive messenger. Furthermore, P2Y signaling may contribute to mechanotransduction in low threshold Abeta fibers under normal or pathological conditions.

Effects of isolectin B4-conjugated saporin, a targeting cytotoxin, on bladder overactivity induced by bladder irritation

In order to clarify the functional role of the isolectin B4 (IB4)-binding afferent pathway in the micturition reflex, we investigated the effects on bladder activity of intrathecal application of the IB4-saporin conjugate, a targeting cytotoxin that destroys neurons binding IB4. In rats, IB4-saporin (2.5 micro m) or vehicle was administered through an intrathecal catheter implanted at the level of the L6-S1 spinal cord. Three weeks after IB4-saporin administration, cystometry in conscious animals revealed a reduction in bladder overactive responses induced by intravesical capsaicin or ATP infusion without affecting normal voiding function. In histochemical studies, double staining for IB4 and saporin was detected in L6 dorsal root ganglia (DRG) neurons 2 days after the treatment. Three weeks after the treatment, the area in lamina II of the L6 spinal cord stained with IB4 was significantly reduced compared with the area stained in control rats. The staining in the L1 spinal cord was not affected. The percentage of neurons in the L6 DRG intensely labeled with IB4 was also reduced in IB4-saporin-treated rats. These results indicate that intrathecal treatment with the IB4-saporin conjugate at the level of L6-S1 spinal cord, which reduces IB4 afferent nerve terminal staining in lamina II of the L6 spinal cord as well as the number of IB4-binding neurons in L6 DRG, suppressed bladder overactivity induced by bladder irritation without affecting normal micturition. Thus targeting IB4-binding, non-peptidergic afferent pathways sensitive to capsaicin and adenosine 5'-triphosphate may be an effective treatment for overactivity and/or pain responses in the bladder.

Primary Afferent Stimulation Differentially Potentiates Excitatory and Inhibitory Inputs to Spinal Lamina II Outer and Inner Neurons

Spinal lamina II (substantia gelatinosa) neurons play an important role in processing of nociceptive information from primary afferent nerves. Anatomical studies suggest that neurons in the outer (lamina IIo) and inner (lamina IIi) zone of lamina II receive distinct afferent inputs. The functional significance of this preferential afferent termination in lamina II remains unclear. In this study, we examined the differential synaptic inputs to neurons in lamina IIo and IIi in response to primary afferent stimulation. Whole cell voltage-clamp recordings were performed on neurons in lamina IIo and IIi of the rat spinal cord slice under visual guidance. Capsaicin (1 μM) significantly increased the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in all 27 lamina IIo neurons and significantly increased the amplitude of mEPSCs in 12 of 27 lamina IIo neurons. However, capsaicin only significantly increased the frequency of mEPSCs in 9 of 22 (40.9%) lamina IIi neurons and increased the amplitude of mEPSCs in 6 of these 9 neurons. Furthermore, the peak amplitude of EPSCs, evoked by electrical stimulation of the attached dorsal root, in 40 lamina IIo neurons was significantly greater than that [160.5 ± 16.7 vs. 87.0 ± 10.4 (SE) pA] in 37 lamina IIi neurons. On the other hand, the peak amplitude of evoked inhibitory postsynaptic currents (IPSCs) in 40 lamina IIo neurons was significantly smaller than that (103.1 ± 11.6 vs. 258.4 ± 24.4 pA) in 37 lamina IIi neurons. In addition, the peak amplitudes of both EPSCs and IPSCs, evoked by direct stimulation of lamina II, were similar in lamina IIo and IIi neurons. This study provides new information that stimulation of primary afferents differentially potentiates synaptic inputs to neurons in lamina IIo and IIi. The quantitative difference in excitatory and inhibitory synaptic inputs to lamina IIo and IIi neurons may be important for integration of sensory information from primary afferent nerves.

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.

The co-expression of P2X3 receptor with VR1 and VRL-1 in the rat trigeminal ganglion

The co-expression of P2X3 receptor with the vanilloid receptor subtype I (VR1) and vanilloid receptor 1-like receptor (VRL-1) was examined in the rat trigeminal ganglion (TG) by a double immunofluorescence method. P2X3 receptor-immunoreactive (ir) neurons were predominantly small to medium-sized (range=93.8-1844.4 microm(2), mean+/-S.D.=503.8+/-286.5 microm(2)); 35% and 9% of P2X3 receptor-ir TG neurons were immunoreactive for VR1 and VRL-1, respectively. Small and medium-sized P2X3 receptor-ir neurons contained VR1-immunoreactivity (ir), whereas medium-sized and large P2X3 receptor-ir neurons showed VRL-1-ir. The retrograde tracing and immunohistochemical methods revealed that 30% of the TG neurons retrogradely labeled from the facial skin and tooth pulp exhibited P2X3 receptor-ir. The co-expression of P2X3 receptor and VR1 was detected in 16% of cutaneous TG neurons and 6% of tooth pulp neurons. On the other hand, the co-expression of P2X3 receptor and VRL-1 was common in tooth pulp neurons (23%) and rare in cutaneous TG neurons (8%). In the tooth pulp, 95% of P2X3 receptor-ir TG neurons contained VRL-1-ir. The present study indicates that P2X3 receptor-ir TG neurons, which co-express VR-ir, are abundant in the facial skin. The tooth pulp is probably innervated by TG neurons, which contain both P2X3-and VRL-1-ir.

Effect of tetramethylpyrazine on acute nociception mediated by signaling of P2X receptor activation in rat

Tetramethylpyrazine (TMP) has been used in traditional Chinese medicine as an analgesic for dysmenorrhea. In the present study, we try to investigate the effects of TMP on acute nociception mediated by P2X receptor activation of rat hindpaw and the membrane depolarization of rat dorsal root ganglion (DRG) neurons induced by P2X receptor agonists. The subcutaneous administration of TMP (0.1-10 mmol) into rat hindpaw in a dose-dependent manner decreased acute paw flinching responses mediated by adenosine 5'-triphosphate (ATP, 1000 nmol) or alpha,beta-methylene ATP (alpha,beta-meATP, 600 nmol). The subcutaneous administration of TMP (5 or 10 mmol) into rat hindpaw inhibited significantly the first phase of nociceptive behaviors induced by 5% formalin and attenuated slightly the second phase of nociceptive behaviors induced by 5% formalin. The subcutaneous administration of TMP (10 mmol) into rat hindpaw reduced the nociceptive responses induced by alpha,beta-meATP (200 nmol) co-injected with Prostaglandin E2 (PGE2), 5 micromol). The membrane depolarization induced by ATP (200 micromol) or alpha,beta-meATP (50 micromol) in DRG neurons was inhibited by TMP (300 micromol). The data suggest that the antinociceptive effect of TMP is involved in blocking the signaling of P2X3 receptor activation in rat.

Distribution of antinociceptive adenosine A1 receptors in the spinal cord dorsal horn, and relationship to primary afferents and neuronal subpopulations

Adenosine can reduce pain and allodynia in animals and man, probably via spinal adenosine A1 receptors. In the present study, we investigate the distribution of the adenosine A1 receptor in the rat spinal cord dorsal horn using immunohistochemistry, in situ hybridization, radioligand binding, and confocal microscopy. In the lumbar cord dorsal horn, dense immunoreactivity was seen in the inner part of lamina II. This was unaltered by dorsal root section or thoracic cord hemisection. Confocal microscopy of the dorsal horn revealed close anatomical relationships but no or only minor overlap between A1 receptors and immunoreactivity for markers associated with primary afferent central endings: calcitonin gene-related peptide, or isolectin B4, or with neuronal subpopulations: mu-opioid receptor, neuronal nitric oxide synthase, met-enkephalin, parvalbumin, or protein kinase Cgamma, or with glial cells: glial fibrillary acidic protein. A few adenosine A1 receptor positive structures were double-labeled with alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionic acid glutamate receptor subunits 1 and 2/3. The results indicate that most of the adenosine A1 receptors in the dorsal horn are located in inner lamina II postsynaptic neuronal cell bodies and processes whose functional and neurochemical identity is so far unknown. Many adenosine A1 receptor positive structures are in close contact with isolectin B4 positive C-fiber primary afferents and/or postsynaptic structures containing components of importance for the modulation of nociceptive information.

Agonist-dependence of recovery from desensitization of P2X(3) receptors provides a novel and sensitive approach for their rapid up or downregulation

1. Fast-desensitizing P2X(3) receptors of nociceptive dorsol root ganglion (DRG) neurons are thought to mediate pain sensation. Since P2X(3) receptor efficiency is powerfully modulated by desensitization, its underlying properties were studied with patch-clamp recording. 2. On rat cultured DRG neurons, 2 s application of ATP (EC(50)=1.52 microm), ADP (EC(50)=1.1 microm) or alpha,beta-meATP (EC(50)=1.78 microm) produced similar inward currents that fully desensitized, at the same rate, back to baseline. Recovery from desensitization was much slower after ATP and ADP than after alpha,beta-meATP and, in all cases, it had sigmoidal time course. 3. By alternating the application of ATP and alpha,beta-meATP, we observed complete cross-desensitization indicating that these agonists activated the same receptors. This notion was confirmed by the similar antagonism induced by 2', 3'-O-(2,4,6,trinitrophenyl)-adenosine triphosphate (TNP-ATP). 4. Recovery from desensitization elicited by ATP was unexpectedly shaped by transient application of alpha,beta-methylene-adenosine triphosphate (alpha,beta-meATP), and vice versa. Thus, short-lasting, full desensitization produced by alpha,beta-meATP protected receptors from long-lasting desensitization induced by subsequent ATP applications. ATP and ADP had similar properties of recovery from desensitization. 5. Low nm concentrations of alpha,beta-meATP (unable to evoke membrane currents) could speed up recovery from ATP-induced desensitization, while low nm concentrations of ATP enhanced it. Ambient ATP levels were found to be in the pm range (52+/-3 pm). 6. The phenomenon of cross-desensitization and protection was reproduced by rP2X(3) receptors expressed by rat osteoblastic cell 17/2.8 or human embryonic kidney cell 293 cells, indicating P2X(3) receptor specificity. 7. It is suggested that transient application of an agonist that generates rapid recovery from desensitization, is a novel, powerful tool to modulate P2X(3) receptor responsiveness to the natural agonist ATP.

Transgenic mice possessing increased numbers of nociceptors do not exhibit increased behavioral sensitivity in models of inflammatory and neuropathic pain

At least two classes of neciceptors can be distinguished based on their growth factor requirements: glial cell-line derived neurotrophic factor (GDNF)- and nerve growth factor (NGF)-dependent primary afferent neurons. Based on numerous anatomical and biochemical differences, GDNF- and NGF-dependent neurons have been proposed to be involved in the development of different types of persistent pain. To examine this hypothesis we used two lines of transgenic mice that contained a supernormal number of either NGF- or GDNF-dependent neurons (referred to as NGF-OE and GDNF-OE mice, respectively). These mice were tested in a model of inflammatory pain (induced by injection of complete Freund's adjuvant) and neuropathic pain (using a spinal nerve ligation protocol). Contrary to expectations, neither line of transgenic mice became more hyperalgesic following induction of persistent pain. In fact, NGF-OE mice recovered more rapidly and became hypoalgesic despite extensive paw swelling in the inflammatory pain model. In the neuropathic pain model, only wildtype mice became hyperalgesic. Real-time PCR analysis showed that the NGF-OE and GDNF-OE mice exhibited changes in neuronal-specific mRNAs in the dorsal root ganglia but not the spinal cord dorsal horn. These results indicate that increasing the number of nociceptors results in potent compensatory mechanisms that may begin with changes in the sensory neurons themselves.

Characterization of the primary spinal afferent innervation of the mouse colon using retrograde labelling

Visceral pain is the most common form of pain produced by disease and is thus of interest in the study of gastrointestinal (GI) complaints such as irritable bowel syndrome, in which sensory signals perceived as GI pain travel in extrinsic afferent neurones with cell bodies in the dorsal root ganglia (DRG). The DRG from which the primary spinal afferent innervation of the mouse descending colon arises are not well defined. This study has combined retrograde labelling and immunohistochemistry to identify and characterize these neurones. Small to medium-sized retrogradely labelled cell bodies were found in the DRG at levels T8-L1 and L6-S1. Calcitonin gene-related peptide (CGRP)- and P2X3-like immunoreactivity (LI) was seen in 81 and 32%, respectively, of retrogradely labelled cells, and 20% bound the Griffonia simplicifolia-derived isolectin IB4. CGRP-LI and IB4 were co-localized in 22% of retrogradely labelled cells, whilst P2X3-LI and IB4 were co-localized in 7% (vs 34% seen in the whole DRG population). Eighty-two per cent of retrogradely labelled cells exhibited vanilloid receptor 1-like immunoreactivity (VR1-LI). These data suggest that mouse colonic spinal primary afferent neurones are mostly peptidergic CGRP-containing, VR1-LI, C fibre afferents. In contrast to the general DRG population, a subset of neurones exist that are P2X3 receptor-LI but do not bind IB4.

Glial cell line-derived neurotrophic factor increases calcitonin gene-related peptide immunoreactivity in sensory and motoneurons in vivo

Calcitonin gene-related peptide (CGRP) is expressed at high levels in roughly 50% of spinal sensory neurons and plays a role in peripheral vasodilation as well as nociceptive signalling in the spinal cord. Spinal motoneurons express low levels of CGRP; motoneuronal CGRP is thought to be involved in end-plate plasticity and to have trophic effects on target muscle cells. As both sensory and motoneurons express receptors for glial cell line-derived neurotrophic factor (GDNF) we sought to determine whether CGRP was regulated by GDNF. Rats were treated intrathecally for 1-3 weeks with recombinant human GDNF or nerve growth factor (NGF) (12 microg/day) and dorsal root ganglia and spinal cords were stained for CGRP. The GDNF treatment not only increased CGRP immunoreactivity in both sensory and motoneurons but also resulted in hypertrophy of both populations. By combined in situ hybridization and immunohistochemistry we found that, in the dorsal root ganglia, CGRP was up-regulated specifically in neurons expressing GDNF but not NGF receptors following GDNF treatment. Despite the increase in CGRP in GDNF-treated rats, there was no increase in thermal or mechanical pain sensitivity, while NGF-treated animals showed significant decreases in pain thresholds. In motoneurons, GDNF increased the overall intensity of CGRP immunoreactivity but did not increase the number of immunopositive cells. As GDNF has been shown to promote the regeneration of both sensory and motor axons, and as CGRP appears to be involved in motoneuronal plasticity, we reason that at least some of the regenerative effects of GDNF are mediated through CGRP up-regulation.

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.

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.

Do central terminals of intact myelinated primary afferents sprout into the superficial dorsal horn of rat spinal cord after injury to a neighboring peripheral nerve?

In order to investigate whether normal myelinated primary afferent axons sprout into the territories of adjacent injured peripheral nerve fibers in the superficial dorsal horn of the spinal cord, adult rats underwent either sectioning of the saphenous or femoral nerves on one side, or else unilateral denervation of the skin of the posterior thigh. Two weeks later cholera toxin B subunit (CTb), which is normally transported selectively by myelinated somatic primary afferents, was injected into the ipsilateral (intact) sciatic nerve. The relationship between CTb, vasoactive intestinal peptide (VIP), and binding of Bandeiraea simplicifolia isolectin B4 (IB4) was then examined in the ipsilateral dorsal horn of the second to fifth lumbar spinal segments (L2-L5). Sectioning of the femoral or saphenous nerves resulted in a reduction of IB4 binding in laminae I-II in the medial third of the dorsal horn of L2, L3, and the upper part of L4. VIP-immunoreactivity was upregulated in exactly the same regions in which IB4-binding was reduced. These correspond to the areas that were previously innervated by unmyelinated afferents in the sectioned nerves. CTb-labeling was detected in regions known to receive input from myelinated sciatic afferents: lamina I and a band extending from the inner part of lamina II (IIi) to lamina V in the L3-5 segments, and the deepest part of the dorsal horn in L2. Importantly, no CTb-labeling was detected in the outer part of lamina II (IIo) in the denervated areas. Sectioning of branches of the posterior cutaneous nerve of the thigh resulted in a reduction of IB4-binding and upregulation of VIP-immunoreactivity in the lateral part of the superficial dorsal horn of caudal L4 and L5. Again, CTb-immunoreactivity showed the normal sciatic pattern in L4-L5, with no labeling detected in lamina IIo in the denervated region. These results do not support the suggestion that the central terminals of intact myelinated afferents sprout into regions of lamina II occupied by adjacent nerves that have been axotomized peripherally.

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.

Bladder and cutaneous sensory neurons of the rat express different functional P2X receptors

The expression and functional responses of P2X receptors in bladder and cutaneous sensory neurons of adult rats and mice have been studied using immunohistochemistry and patch clamp techniques. Cell bodies of bladder pelvic afferents were identified in L6 and S1 dorsal root ganglia (DRG), following Fast Blue injection into the muscle wall of the urinary bladder. Similarly, cutaneous sensory neurons were identified in L3 and L4 DRG, following Fast Blue injection into the saphenous nerve innervating the skin. Bladder sensory neurons contained only weak to moderate P2X(3)-immunoreactivity (IR), in contrast to strong P2X(3)-IR observed in a sub-population of cutaneous afferents. Whole-cell patch-clamp recordings revealed that approximately 90% of bladder afferent neurons responded to alpha beta-methylene ATP (alpha beta meATP) and ATP (30 microM) with persistent currents, which were inhibited by 2',3'-O-trinitrophenyl-ATP (TNP-ATP) (0.3 microM) to 6.4+/-1.9% and 8.0+/-2.6% of control, respectively (n=8). The remaining bladder sensory neurons demonstrated biphasic, transient or no response to P2X agonists. In contrast, only 24% of cutaneous afferent neurons gave persistent currents to alpha beta meATP (30 microM), with 66% of cells giving transient or biphasic currents and the remaining 10% being non-responsive. Our results suggest that, in contrast to DRG neurons in general, bladder sensory neurons projecting via pelvic nerves express predominantly P2X(2/3) heteromeric receptors, which are likely to mediate the important roles of ATP as a signaling molecule of urinary bladder filling and nociception.

P2X7 receptors in rat brain: presence in synaptic terminals and granule cells

ATP stimulates [Ca2+]i increases in midbrain synaptosomes via specific ionotropic receptors (P2X receptors). Previous studies have demonstrated the implication of P2X3 subunits in these responses, but additional P2X subunits must be involved. In the present study, ATP and BzATP proved to be able to induce intrasynaptosomal calcium transients in the midbrain synaptosomes, their effects being potentiated when assayed in a Mg2+-free medium. Indeed, BzATP was shown to be more potent than ATP, and their effects could be inhibited by PPADS and KN-62, but not by suramin. This activity profile is consistent with the presence of functional P2X7 receptors in the midbrain terminals. The existence of presynaptic responses to selective P2X7 agonists could be confirmed by means of a microfluorimetric technique allowing [Ca2+]i measurements in single synaptic terminals. Additionally, the P2X7 receptor protein could be identified in the midbrain synaptosomes and in axodendritic prolongations of cerebellar granule cells by immunochemical staining.

Purinergic mechanisms contribute to mechanosensory transduction in the rat colorectum

BACKGROUND & AIMS

Adenosine 5'-triphosphate plays a role in peripheral sensory mechanisms and, in particular, mechanosensory transduction in the urinary system. P2X(3) receptors are selectively expressed on small-diameter sensory neurons in the dorsal root ganglia; sensory neurons from dorsal root ganglia L1 and S1 supply the colorectum. This study investigated whether purinergic signaling contributes to mechanosensory transduction in the rat colorectum.

METHODS

A novel in vitro rat colorectal preparation was used to elucidate whether adenosine 5'-triphosphate is released from the mucosa in response to distention and to evaluate whether it contributes to sensory nerve discharge during distention.

RESULTS

P2X(3) receptor immunostaining was present on subpopulations of neurons in L1 and S1 dorsal root ganglia, which supply the rat colorectum. Distention of the colorectum led to pressure-dependent increases in adenosine 5'-triphosphate release from colorectal epithelial cells and also evoked pelvic nerve excitation, which was mimicked by application of adenosine 5'-triphosphate and alpha,beta-methylene adenosine 5'-triphosphate. The sensory nerve discharges evoked by distention were potentiated by alpha,beta-methylene adenosine 5'-triphosphate and ARL-67156, an adenosine triphosphatase inhibitor, and were attenuated by the selective P2X(1), P2X(3), and P2X(2/3) antagonist 2',3'-O-trinitrophenyl-adenosine 5'-triphosphate and by the nonselective P2 antagonists pyridoxyl 5-phosphate 6-azophenyl-2',4'-disulfonic acid and suramin. Adenosine, after ectoenzymatic breakdown of adenosine 5'-triphosphate, seems to be involved in the longer-lasting distention-evoked sensory discharge. Single-fiber analysis showed that high-threshold fibers were particularly affected by alpha,beta-methylene adenosine 5'-triphosphate, suggesting a correlation between purinergic activation and nociceptive stimuli.

CONCLUSIONS

Adenosine 5'-triphosphate contributes to mechanosensory transduction in the rat colorectum, and this is probably associated with pain.

ATP analogues with modified phosphate chains and their selectivity for rat P2X2 and P2X2/3 receptors

1. Heteromeric P2X2/3 receptors are much more sensitive than homomeric P2X2 receptors to alphabeta-methylene-ATP, and this ATP analogue is widely used to discriminate the two receptors on sensory neurons and other cells. 2. We sought to determine the structural basis for this selectivity by synthesising ADP and ATP analogues in which the alphabeta and/or betagamma oxygen atoms were replaced by other moieties (including -CH2-, -CHF-, -CHCl-, -CHBr-, -CF2-, -CCl2-, -CBr2-, -CHSO3-, -CHPO3-, -CFPO3-, -CClPO3-, -CH2-CH2-, C triple bond C, -NH-, -CHCOOH-). 3. We tested their actions as agonists or antagonists by whole-cell recording from human embryonic kidney cells expressing P2X2 subunits alone (homomeric P2X2 receptors), or cells expressing both P2X2 and P2X3 subunits, in which the current through heteromeric P2X2/3 receptors was isolated. 4. ADP analogues had no agonist or antagonist effect at either P2X2 or P2X2/3 receptors. All the ATP analogues tested were without agonist or antagonist activity at homomeric P2X2 receptors, except betagamma-difluoromethylene-ATP, which was a weak agonist. 5. At P2X2/3 receptors, betagamma-imido-ATP, betagamma-methylene-ATP, and betagamma-acetylene-ATP were weak agonists, whereas alphabeta,betagamma- and betagamma,gammadelta-bismethylene-AP4 were potent full agonists. betagamma-Carboxymethylene-ATP and betagamma-chlorophosphonomethylene-ATP were weak antagonists at P2X2/3 receptors (IC50 about 10 microm). 6. The results indicate (a). that the homomeric P2X2 receptor presents very stringent structural requirements with respect to its activation by ATP; (b). that the heteromeric P2X2/3 receptor is much more tolerant of alphabeta and betagamma substitution; and (c). that a P2X2/3-selective antagonist can be obtained by introduction of additional negativity at the betagamma-methylene.

Dehydroepiandrosterone potentiates native ionotropic ATP receptors containing the P2X2 subunit in rat sensory neurones

We have studied the modulatory effect of dehydroepiandrosterone (DHEA), the most abundant neurosteroid produced by glial cells and neurones, on membrane currents induced by the activation of ionotropic ATP (P2X) receptors in neonatal rat dorsal root ganglion neurones. ATP (1 microM) induced three types of currents/responses termed F (fast and transient), S (slowly desensitizing) and M (mixed, sum of F- and S-type responses). DHEA (10 nM to 100 microM) concentration-dependently increased the amplitude of plateau-like currents of S- and M-type responses evoked by submaximal (1 microM) but not saturating (100 microM or 1 mM) concentrations of ATP. Alphabeta-methylene ATP (alphabetame-ATP, 5 microM) also evoked F-, S- and M-type responses, the plateau phases of which were potentiated by lowering external pH (6.3) and by ivermectin (IVM, 3 microM), indicating the presence heteromeric P2X2-containing receptors and possibly of functional native P2X4/6 receptors. There was a strict correlation between the potentiating effects of low pH and DHEA on alphabetame-ATP responses but not between that of IVM and DHEA, suggesting that DHEA selectively modulated P2X2-containing receptors. DHEA also potentiated putative homomeric P2X2 receptor responses recorded in the continuous presence of 1 microM 2'-(or 3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP). Our results constitute the first demonstration of a fast potentiation of P2X receptors by a neurosteroid and suggest that DHEA could be an endogenous modulator of P2X2-containing receptors thereby contributing to the facilitation of the detection and/or the transmission of nociceptive messages, particularly under conditions of inflammatory pain where the P2X receptor signalling pathway appears to be upregulated.