ATP potentiates neurotransmission in the rat trigeminal subnucleus caudalis

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

NMDA and purinergic processes modulate neck muscle activity evoked by noxious stimulation of dura

BACKGROUND

Adenosine triphosphate (ATP) and glutamate are associated with some headache conditions, and purinergic (P2X) and glutamatergic N-methyl-D-aspartate (NMDA) receptor-related processes in the medulla can modulate the effects of trigeminal nociceptive afferent inputs into the brainstem on craniofacial sensorimotor circuits. This study aimed to test whether neck muscle activity can be induced in rats by noxious stimulation of the frontal dura or superior sagittal sinus that involves P2X or NMDA receptor-dependent mechanisms.

METHODS

While electromyographic activities of neck and craniofacial muscles were being recorded in anesthetized rats (n = 46), the inflammatory irritant mustard oil (0.2 µL, 20% MO) or vehicle (mineral oil) was topically applied to the dura or sinus, preceded by 10 µL of the ATP antagonist 2',3'-O-(2,4,6- trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP, 0.1 mmol/L; n = 8) or 2-amino-5-phosphonopentanoic acid (APV, 0.05 mmol/L; n = 7) or phosphate-buffered saline (PBS as vehicle control; n = 10).

RESULTS

Application of MO but not vehicle to the frontal dura significantly increased (P < .05) neck electromyographic activity whereas MO application to the superior sagittal sinus did not significantly increase neck electromyographic activity unless MO had previously been applied to the dura. Pre-treatment (i.t.) with TNP-ATP or APV but not vehicle control significantly reduced neck electromyographic activity evoked by MO application to the dura.

CONCLUSIONS

These data suggest that noxious stimulation of the frontal dura (but not superior sagittal sinus) may enhance neck muscle activity that is P2X and NMDA receptor-dependent. These effects may contribute to neck muscle stiffness that occurs in some headache conditions.

Dura-evoked neck muscle activity involves purinergic and N-methyl-D-aspartate receptor mechanisms

We have previously demonstrated that noxious stimulation of craniofacial tissues including the frontal dura reflexly evokes significant increases in neck muscle electromyographic (EMG) activity. The primary aim of this study was to determine whether purinergic receptor mechanisms may be involved in these EMG effects, and whether N-methyl-D-aspartate (NMDA) receptor processes modulate the purinergic mechanisms. Application of the P2X1, P2X3 and P2X2/3 receptor agonist α,β-methylene ATP (but not vehicle) to the dural surface evoked a significant (P<0.05) increase in ipsilateral neck EMG activity that could be suppressed by dural or intrathecal application of the selective P2X1, P2X3 and P2X2/3 receptor antagonist 2',3'-O-(2,4,6-trinitrophenyl) ATP (TNP-ATP) but not by vehicle; the intrathecal application of 2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist, also significantly reduced the neck EMG activity evoked by dural application of α,β-methylene ATP. These data suggest that purinergic receptor mechanisms contribute to the increased neck activity that can be reflexly evoked by noxious stimulation of the frontal dura, and that NMDA as well as purinergic receptor mechanisms in the medulla may modulate these purinergic-related effects.

Enzymatic conversion of ATP to adenosine contributes to ATP-induced inhibition of glutamate release in rat medullary dorsal horn neurons

Purine nucleotides, such as ATP and ADP, activate ionotropic P2X and metabotropic P2Y receptors to regulate neurotransmitter release in the peripheral as well as central nervous system. Here we report another type of ATP-induced presynaptic modulation of glutamate release in rat medullary dorsal horn neurons. Glutamatergic excitatory postsynaptic currents (EPSCs) induced by electrical stimulation of trigeminal tract were recorded from horizontal brain stem slices using a whole-cell patch clamp technique. ATP decreased the amplitude of glutamatergic EPSCs in a reversible and concentration dependent manner and increased the paired-pulse ratio. In addition, ATP reduced the frequency of miniature EPSCs without affecting the current amplitude, suggesting that ATP acts presynaptically to reduce the probability of glutamate release. The ATP-induced decrease in glutamatergic EPSCs was not affected by P2X and P2Y receptor antagonists, but was completely blocked by DPCPX, a selective adenosine A1 receptor antagonist. The ATP-induced decrease in glutamatergic EPSCs was also inhibited by an inhibitor of tissue nonspecific alkaline phosphatase but not by inhibitors of other enzymes such as ecto-nucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidases. The results suggest that exogenously applied purine nucleotides are rapidly converted to adenosine by specific enzymes, and subsequently act on presynaptic A1 receptors to inhibit glutamate release from primary afferent terminals. This type of modulation mediated by purine nucleotides may play an important role in regulating nociceptive transmission from orofacial tissues.

Involvement of ATP in noxious stimulus-evoked release of glutamate in rat medullary dorsal horn: a microdialysis study

Our electrophysiological studies have shown that both purinergic and glutamatergic receptors are involved in central sensitization of nociceptive neurons in the medullary dorsal horn (MDH). Here we assessed the effects of intrathecal administration of apyrase (a nucleotide degrading enzyme of endogenous adenosine 5-triphosphate [ATP]), a combination of apyrase and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, an adenosine A1 receptor antagonist), or 2,3-O-2,4,6-trinitrophenyl-adenosine triphosphate (TNP-ATP, a P2X1, P2X3, P2X2/3 receptor antagonist) on the release of glutamate in the rat MDH evoked by application of mustard oil (MO) to the molar tooth pulp. In vivo microdialysis was used to dialyse the MDH every 5 min, and included 3 basal samples, 6 samples after drug treatment and 12 samples following application of MO. Tooth pulp application of MO induced a significant increase in glutamate release in the MDH. Superfusion of apyrase or TNP-ATP alone significantly reduced the MO-induced glutamate release in the MDH, as compared to vehicle. Furthermore, the suppressive effects of apyrase on glutamate release were reduced by combining it with DPCPX. This study demonstrates that application of an inflammatory irritant to the tooth pulp induces glutamate release in the rat MDH in vivo that may be reduced by processes involving endogenous ATP and adenosine.

Activation of peripheral P2X receptors is sufficient to induce central sensitization in rat medullary dorsal horn nociceptive neurons

Central sensitization and purinergic receptor mechanisms have been implicated as important processes in acute and chronic pain conditions following injury or inflammation of peripheral tissues. This study has documented that application of the P2X(1,2/3,3) receptor agonist αβ-meATP (100mM) to the rat tooth pulp induces central sensitization in medullary dorsal horn nociceptive neurons that is reflected in significant increases in mechanoreceptive field size and responses to noxious stimuli and decreased mechanical activation threshold. Furthermore, these responses can be blocked by pulp application of the P2X(1,2/3,3) antagonist TNP-ATP and also attenuated by medullary application of TNP-ATP. These results suggest that activation of P2X(1,2/3,3) receptors in orofacial tissues plays a critical role in producing central sensitization in medullary dorsal horn nociceptive neurons.

Sumatriptan inhibits TRPV1 channels in trigeminal neurons

OBJECTIVE

To understand a possible role for transient potential receptor vanilloid 1 (TRPV1) ion channels in sumatriptan relief of pain mediated by trigeminal nociceptors.

BACKGROUND

TRPV1 channels are expressed in small nociceptive sensory neurons. In dorsal root ganglia, TRPV1-containing nociceptors mediate certain types of inflammatory pain. Neurogenic inflammation of cerebral dura and blood vessels in the trigeminal nociceptive system is thought to be important in migraine pain, but the ion channels important in transducing migraine pain are not known. Sumatriptan is an agent effective in treatment of migraine and cluster headache. We hypothesized that sumatriptan might modulate activity of TRPV1 channels found in the trigeminal nociceptive system.

METHODS

We used immunohistochemistry to detect the presence of TRPV1 channel protein, whole-cell recording in acutely dissociated trigeminal ganglia (TG) to detect functionality of TRPV1 channels, and whole-cell recording in trigeminal nucleus caudalis (TNC) to detect effects on release of neurotransmitters from trigeminal neurons onto second order sensory neurons. Effects specifically on TG neurons that project to cerebral dura were assessed by labeling dural nociceptors with DiI.

RESULTS

Immunohistochemistry demonstrated that TRPV1 channels are present in cerebral dura, in trigeminal ganglion, and in the TNC. Capsaicin, a TRPV1 agonist, produced depolarization and repetitive action potential firing in current clamp recordings, and large inward currents in voltage clamp recordings from acutely dissociated TG neurons, demonstrating that TRPV1 channels are functional in trigeminal neurons. Capsaicin increased spontaneous excitatory postsynaptic currents in neurons of layer II in TNC slices, showing that these channels have a physiological effect on central synaptic transmission. Sumatriptan (10 µM), a selective antimigraine drug, inhibited TRPV1-mediated inward currents in TG and capsaicin-elicited spontaneous excitatory postsynaptic currents in TNC slices. The same effects of capsaicin and sumatriptan were found in acutely dissociated DiI-labeled TG neurons innervating cerebral dura.

CONCLUSION

Our results build on previous work indicating that TRPV1 channels in trigeminal nociceptors play a role in craniofacial pain. Our findings that TRPV1 is inhibited by the specific antimigraine drug sumatriptan, and that TRPV1 channels are functional in neurons projecting to cerebral dura suggests a specific role for these channels in migraine or cluster headache.

Chronic intermittent hypoxia augments sympatho-excitatory response to ATP but not to L-glutamate in the RVLM of rats

The development of sympathetic overactivity and hypertension in rats submitted to chronic intermittent hypoxia (CIH) involve alterations in the central mechanisms controlling respiratory and autonomic functions. Herein, we assessed whether CIH alters glutamatergic and/or purinergic signaling in the ventrolateral medulla (VLM), a region that encompasses the pre-sympathetic neurons and respiratory neurons of the ventral respiratory column. Groups of juvenile rats were exposed for 10 days to CIH (6% O(2) for 40s, every 9min, 8h/day) or normoxia (controls). Following treatment, in situ working heart-brainstem preparations were performed to record simultaneously respiratory and sympathetic motor outputs. In separate CIH and control groups, the VLM was dissected for western-blot analyses of ionotropic glutamatergic and P2 receptors. l-glutamate microinjections (1, 3 or 10mM) into VLM of control (n=6) and CIH groups (n=10) produced similar increases of sympathetic and abdominal activities associated with phrenic nerve inhibition; immunoreactive NMDAR1 and GluR2/3 densities at the VLM were also alike between groups (n=4). In contrast, VLM microinjections of ATP (1, 10 or 50mM) evoked larger sympatho-excitatory responses in CIH (n=8) than in control rats (n=7, P<0.05) whilst the abdominal increase and phrenic nerve inhibition were of comparable magnitudes. The immunoreactive densities of P2X3 and P2X4 receptors, but not P2X1 and P2Y2, were 20% higher in VLM of CIH (n=8; P<0.05) than controls (n=8). Altogether, our findings suggest that CIH augments purinergic signaling in the VLM, supporting the concept that nucleotides play a role in the dynamic central control of the sympathetic autonomic function.

Neuronal soma-satellite glial cell interactions in sensory ganglia and the participation of purinergic receptors

It has been known for some time that the somata of neurons in sensory ganglia respond to electrical or chemical stimulation and release transmitters in a Ca2+-dependent manner. The function of the somatic release has not been well delineated. A unique characteristic of the ganglia is that each neuronal soma is tightly enwrapped by satellite glial cells (SGCs). The somatic membrane of a sensory neuron rarely makes synaptic contact with another neuron. As a result, the influence of somatic release on the activity of adjacent neurons is likely to be indirect and/or slow. Recent studies of neuron-SGC interactions have demonstrated that ATP released from the somata of dorsal root ganglion neurons activates SGCs. They in turn exert complex excitatory and inhibitory modulation of neuronal activity. Thus, SGCs are actively involved in the processing of afferent information. In this review, we summarize our understanding of bidirectional communication between neuronal somata and SGCs in sensory ganglia and its possible role in afferent signaling under normal and injurious conditions. The participation of purinergic receptors is emphasized because of their dominant roles in the communication.

P2X and NMDA receptor involvement in temporomandibular joint-evoked reflex activity in rat jaw muscles

We have previously shown that injection of the excitatory amino glutamate into the rat temporomandibular joint (TMJ) evokes reflex activity in both anterior digastric (DIG) and masseter (MASS) muscles that can be attenuated by prior TMJ injection of an N-methyl-d-aspartate (NMDA) receptor antagonist. The aim of the present study was to test if jaw muscle activity could also be evoked by P2X receptor agonist injection into the rat TMJ region and if the reflex activity could be modulated by TMJ injection of P2X receptor antagonist or NMDA receptor antagonist. The selective P2X subtype agonist alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-me ATP) and vehicle (phosphate-buffered saline) or the selective P2X antagonist, 2'-(or-3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) or the selective NMDA antagonist (+/-)-d-2-amino-5-phosphonovalerate(APV) were injected into the rat TMJ region. Electromyographic (EMG) reflex activity was recorded in both DIG and MASS muscles. Compared with the baseline EMG activity, alpha,beta-me-ATP injection into the TMJ (but not its systemic administration) following pre-injection of the vehicle significantly increased the magnitude and the duration of ipsilateral DIG and MASS EMG activity in a dose-dependent manner. The alpha,beta-me-ATP-evoked responses could be antagonized by pre-injection of TNP-ATP into the same TMJ site but contralateral TMJ injection of TNP-ATP proved ineffective. Furthermore, the alpha,beta-me-ATP-evoked responses could also be antagonized by APV injected into the same TMJ site but not by its systemic injection. These results indicate the interaction of peripheral purinergic as well as glutamatergic receptor mechanisms in the processing of TMJ nociceptive afferent inputs that evoke reflex activity in jaw muscles.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Electrophysiological and morphological properties of neurons in the substantia gelatinosa of the mouse trigeminal subnucleus caudalis

The excitability of the second order neurons within the trigeminal subnucleus caudalis underlies pain perception and processing in migraine and trigeminal neuralgia. These neurons were studied with whole-cell patch-clamp technique in slices from mouse brain stem. Electrical and morphological characteristics of 56 neurons were determined. Four categories were distinguished from electrophysiological properties: tonic (39%), phasic (34%), delayed (16%) and single spiking (11%). These categories did not show distinct morphological properties. Neurons had tetrodotoxin-sensitive sodium currents that activated and inactivated within milliseconds. They also showed a high voltage-activated, slowly inactivating calcium current: up to half of this current was blocked by omega-conotoxin GVIA (1microM) and omega-agatoxin IVA (100-300 nM), but it was not affected by nifedipine (10microM). Exogenously applied capsaicin (1microM) and alphabetamethylene-5'-adenosine triphosphate (100microM) elicited large amplitude, spontaneous excitatory postsynaptic currents that were blocked by capsazepine (10microM) and 5-[(3-phenoxybenzyl)-(1,2,3,4-tetrahydro-naphthalen-1-yl)-carbamoyl]-benzene-1,2,4-tricarboxylic acid (A-317491: 10microM), respectively. Thus, neurons of the mouse trigeminal subnucleus caudalis substantia gelatinosa exhibit N-type and P/Q-type voltage-gated calcium channels, and receive presynaptic afferents that express TRPV1 and P2X(2/3) receptors. These results suggest possible therapeutic interventions, and serve as a basis for the characterization of cellular changes that may underlie trigeminal neuropathic pain.

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

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

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Localization of P2X2 and P2X3 receptors in rat trigeminal ganglion neurons

Purine receptors have been implicated in central neurotransmission from nociceptive primary afferent neurons, and ATP-mediated currents in sensory neurons have been shown to be mediated by both P2X3 and P2X2/3 receptors. The aim of the present study was to quantitatively examine the distribution of P2X2 and P2X3 receptors in primary afferent cell bodies in the rat trigeminal ganglion, including those innervating the dura. In order to determine the classes of neurons that express these receptor subtypes, purine receptor immunoreactivity was examined for colocalization with markers of myelinated (neurofilament 200; NF200) or mostly unmyelinated, non-peptidergic fibers (Bandeiraea simplicifolia isolectin B4; IB4). Forty percent of P2X2 and 64% of P2X3 receptor-expressing cells were IB4 positive, and 33% of P2X2 and 31% of P2X3 receptor-expressing cells were NF200 positive. Approximately 40% of cells expressing P2X2 receptors also expressed P2X3 receptors and vice versa. Trigeminal ganglion neurons innervating the dura mater were retrogradely labeled and 52% of these neurons expressed either P2X2 or P2X3 or both receptors. These results are consistent with electrophysiological findings that P2X receptors exist on the central terminals of trigeminal afferent neurons, and provide evidence that afferents supplying the dura express both receptors. In addition, the data suggest specific differences exist in P2X receptor expression between the spinal and trigeminal nociceptive systems.