Bidirectional modulation of windup by NMDA receptors in the rat spinal trigeminal nucleus

Short-term plasticity in the spinal nociceptive system

ABSTRACT

Somatosensory information is delivered to neuronal networks of the dorsal horn (DH) of the spinal cord by the axons of primary afferent neurons that encode the intensity of peripheral sensory stimuli under the form of a code based on the frequency of action potential firing. The efficient processing of these messages within the DH involves frequency-tuned synapses, a phenomenon linked to their ability to display activity-dependent forms of short-term plasticity (STP). By affecting differently excitatory and inhibitory synaptic transmissions, these STP properties allow a powerful gain control in DH neuronal networks that may be critical for the integration of nociceptive messages before they are forwarded to the brain, where they may be ultimately interpreted as pain. Moreover, these STPs can be finely modulated by endogenous signaling molecules, such as neurosteroids, adenosine, or GABA. The STP properties of DH inhibitory synapses might also, at least in part, participate in the pain-relieving effect of nonpharmacological analgesic procedures, such as transcutaneous electrical nerve stimulation, electroacupuncture, or spinal cord stimulation. The properties of target-specific STP at inhibitory DH synapses and their possible contribution to electrical stimulation-induced reduction of hyperalgesic and allodynic states in chronic pain will be reviewed and discussed.

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Central sensitization phenomena after third molar surgery: A quantitative sensory testing study

BACKGROUND

Surgical removal of third molars may carry a risk of developing persistent orofacial pain, and central sensitization appears to play an important role in the transition from acute to chronic pain.

AIM

The aim of this study was to investigate sensitization (primarily central sensitization) after orofacial trauma using quantitative sensory testing (QST).

METHODS

A total of 32 healthy men (16 patients and 16 age-matched control subjects) underwent a battery of quantitative tests adapted to the trigeminal area at baseline and 2, 7, and 30 days following surgical removal of a lower impacted third molar.

RESULTS

Central sensitization for at least one week was indicated by significantly increased pain intensity evoked by intraoral repetitive pinprick and electrical stimulation (p<0.05) including facilitation of temporal summation mechanisms (p<0.05), extraoral repetitive electrical stimulation (p<0.001), significantly more frequent aftersensation in patients (p<0.001), extraoral hyperalgesia due to single pinprick stimulation (p<0.05) and larger pain areas due to intranasal stimulation (p<0.001). Peripheral sensitization was indicated by intraoral hyperalgesia due to single pinprick (p<0.05).

CONCLUSION

We found clear signs of sensitization of the trigeminal nociceptive system for at least one week after the surgery. Our results indicate that even a minor orofacial surgical procedure may be sufficient to evoke signs of both central and peripheral sensitization, which may play a role in the transition from acute to chronic pain in susceptible individuals.

Ionotropic glutamate receptors in spinal nociceptive processing

Glutamate is the predominant excitatory transmitter used by primary afferent synapses and intrinsic neurons in the spinal cord dorsal horn. Accordingly, ionotropic glutamate receptors mediate basal spinal transmission of sensory, including nociceptive, information that is relayed to supraspinal centers. However, it has become gradually more evident that these receptors are also crucially involved in short- and long-term plasticity of spinal nociceptive transmission, and that such plasticity have an important role in the pain hypersensitivity that may result from tissue or nerve injury. This review will cover recent findings on pre- and postsynaptic regulation of synaptic function by ionotropic glutamate receptors in the dorsal horn and how such mechanisms contribute to acute and chronic pain.

Glycine inhibitory dysfunction turns touch into pain through PKCgamma interneurons

Dynamic mechanical allodynia is a widespread and intractable symptom of neuropathic pain for which there is a lack of effective therapy. During tactile allodynia, activation of the sensory fibers which normally detect touch elicits pain. Here we provide a new behavioral investigation into the dynamic component of tactile allodynia that developed in rats after segmental removal of glycine inhibition. Using in vivo electrophysiological recordings, we show that in this condition innocuous mechanical stimuli could activate superficial dorsal horn nociceptive specific neurons. These neurons do not normally respond to touch. We anatomically show that the activation was mediated through a local circuit involving neurons expressing the gamma isoform of protein kinase C (PKCγ). Selective inhibition of PKCγ as well as selective blockade of glutamate NMDA receptors in the superficial dorsal horn prevented both activation of the circuit and allodynia. Thus, our data demonstrates that a normally inactive circuit in the dorsal horn can be recruited to convert touch into pain. It also provides evidence that glycine inhibitory dysfunction gates tactile input to nociceptive specific neurons through PKCγ-dependent activation of a local, excitatory, NMDA receptor-dependent, circuit. As a consequence of these findings, we suggest that pharmacological inhibition of PKCγ might provide a new tool for alleviating allodynia in the clinical setting.

Dorsal horn NK1-expressing neurons control windup of downstream trigeminal nociceptive neurons

Windup is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons to repetitive stimulation of primary afferent nociceptive C-fibers. Superficial dorsal horn neurokinin 1 receptor (NK1R)-expressing neurons were recently shown to regulate sensitization of WDR nociceptive neurons through activation of a defined spino-bulbo-spinal loop. However, the windup of WDR nociceptive neurons was not regulated through this loop. In the present study, we sought to identify the alternative circuit activated by dorsal horn NK1Rs that mediates WDR neuron windup. As a model we used the rat spinal trigeminal nucleus, in which the subnucleus oralis (Sp5O) contains a pool of WDR neurons that receive their nociceptive C-input indirectly via interneurons located in the medullary dorsal horn (MDH). First, we found that intravenous injection of NK1R antagonists (SR140333 and RP67580) produced a reversible inhibition of Sp5O WDR neuron windup. Second, we anatomically identified in the MDH lamina III a subpopulation of NK1R-expressing local interneurons that relay nociceptive information from the MDH to downstream Sp5O neurons. Third, using microinjections of NK1R antagonists during in vivo electrophysiological recordings from Sp5O WDR neurons, we showed that WDR neuron windup depends on activation of NK1Rs located in the MDH laminae I-III. We conclude that, in contrast to central sensitization that is controlled by a spino-bulbo-spinal loop, Sp5O WDR neuron windup is regulated through a local circuit activated by MDH lamina III NK1Rs.

Elimination of neurokinin-1 receptor neurons in caudal nucleus reverses the effects of systemic bicuculline on c-Fos expression in rat trigeminal sensory nucleus: I. High intensity electrical stimulation of the trigeminal ganglion

Although neurokinin-1 receptor (NK-1)-bearing neurons are distributed in lamina I of the trigeminal caudal nucleus (Vc) and constitute major projection neurons, little is known about their fundamental role(s) in nociceptive processing. This study examines the effect of intra cisterna magna injection of substance P (SP) conjugated to saporin (SP-Sap; 5 microM, 5 microl) [with/without systemic administration of bicuculline] on c-Fos expression in the trigeminal sensory nucleus (TSN) induced 2 h after 10 min repetitive electrical stimulation of the trigeminal ganglion (TG) at high intensity (1.0 mA, 5 Hz, 5 ms) in the urethane-anesthetized rat. In the SP-Sap-treated rats, the numbers of NK-1-immunopositive neurons in laminae I and III of the Vc decreased compared with rats similarly pretreated with saline (Sal; 5 microl) or blank-saporin (Bl-Sap; 5 microM, 5 microl). In Sal- or Bl-Sap-treated controls, high intensity stimulation induced c-Fos expression in neurons throughout the full extent of ipsilateral superficial layers of the Vc (VcI/II), magnocellular zone of the Vc (VcIII/IV) and the dorsal or dorsomedial subdivisions of the rostral TSN above the obex (trigeminal principal, oral (Vo) and interpolar nuclei). Preadministration of bicuculline (2 mg/kg, i.p.) decreased the numbers of c-Fos-immunopositive neurons in the VcI/II, VcIII/IV and Vo in Sal- or Bl-Sap-treated controls. In contrast, high intensity stimulation induced less c-Fos-immunopositive neurons in the VcI/II and Vo of rats treated with SP-Sap compared with those in Sal- or Bl-Sap-treated controls. In SP-Sap-treated rats preadministered with bicuculline, the numbers of c-Fos-immunopositive neurons in the VcI/II and Vo were increased compared with the SP-Sap-treated rats preadministered with Sal. These results suggest that NK-1-immunopositive neurons in laminae I and III of Vc play a pivotal role in the nociceptive specific processing in the TSN through GABA(A) receptors.