Elsevier

Neuropharmacology

Volume 54, Issue 8, June 2008, Pages 1208-1214
Neuropharmacology

Selective ablation of dorsal horn NK1 expressing cells reveals a modulation of spinal alpha2-adrenergic inhibition of dorsal horn neurones

https://doi.org/10.1016/j.neuropharm.2008.03.014Get rights and content

Abstract

Activity in descending systems from the brainstem modulates nociceptive transmission through the dorsal horn. Intrathecal injection of the neurotoxin saporin conjugated to SP (SP–SAP) into the lumbar spinal cord results in the selective ablation of NK1 receptor expressing (NK1+ve) neurones in the superficial dorsal horn (lamina I/III). Loss of these NK1+ve neurones attenuates excitability of deep dorsal horn neurones due to a disruption of both intrinsic spinal circuits and a spino-bulbo-spinal loop, which activates a descending excitatory drive, mediated through spinal 5HT3 receptors.

Descending inhibitory pathways also modulate spinal activity and hence control the level of nociceptive transmission relayed to higher centres. To ascertain the spinal origins of the major descending noradrenergic inhibitory pathway we studied the effects of a selective alpha2-adrenoceptor antagonist, atipamezole, on neuronal activity in animals pre-treated with SP–SAP. Intrathecal application of atipamezole dose dependently facilitated the mechanically evoked neuronal responses of deep dorsal horn neurones to low intensity von Frey hairs (5–15 g) and noxious thermal (45–50 °C) evoked responses in SAP control animals indicating a physiological alpha2-adrenoceptor control. This facilitatory effect of atipamezole was lost in the SP–SAP treated group. These data suggest that activity within noradrenergic pathways have a dependence on dorsal horn NK1+ve cells. Further, noradrenergic descending inhibition may in part be driven by lamina I/III (NK1+ve) cells, and mediated via spinal alpha2-adrenoceptor activation. Since the same neuronal population drives descending facilitation and inhibition, the reduced excitability of lamina V/VI WDR neurones seen after loss of these NK1+ve neurones indicates a dominant role of descending facilitation.

Introduction

Neurokinin 1 receptor expressing (NK1+ve) neurones, the primary target for Substance P (SP), are integral for the transmission of nociception and in the generation and maintenance of abnormal pain states (Mantyh and Hunt, 2004). Selective ablation of NK1+ve neurones in lamina I and/or III of the dorsal horn results in reductions in behavioural hypersensitive responses seen in inflammatory, neuropathic and spinal cord injury models of pain (Mantyh et al., 1997, Mantyh and Hunt, 2004, Nichols et al., 1999, Suzuki et al., 2005, Yezierski, 2005).

The majority of these NK1+ve cells are located in lamina I of the dorsal horn (Mantyh et al., 1997, Spike et al., 2003) and are the origin of a spino-bulbo-spinal loop, driving a descending excitatory pathway from the brainstem onto spinal neurones. This circuit allows full coding of polymodal peripheral inputs under normal conditions, as well being essential for the more persistent chemical evoked responses of deep dorsal horn neurones. Pharmacological block of spinal 5HT3 receptors mimics many of the effects of ablating spinal NK1+ve cells (Suzuki et al., 2002), suggesting that these neurones are essential for the facilitatory serotonergic influence from the brainstem (Suzuki et al., 2002). Furthermore, this loop contributes to some of the pathophysiological changes that follow nerve injury (Suzuki et al., 2004a, Suzuki et al., 2005) but is less important in inflammation (Rahman et al., 2004). Although loss of NK1+ve cells reduces diffuse noxious inhibitory controls (Suzuki et al., 2002), it is unknown whether noradrenergic inhibitory modulations change – one possibility would be that an increase in noradrenergic descending inhibition contributes to the observed reduced spinal excitability that follows ablation of superficial NK1+ve cells. This could be secondary to the loss of descending facilitations.

Descending noradrenergic pathways exert powerful inhibitory influences onto the spinal cord, primarily via spinal alpha2-adrenoceptors (Fields and Basbaum, 1994, Millan, 2002), which are present in high density within the superficial lamina of the dorsal horn both on postsynaptic dorsal horn cells and primary afferent terminals (Nicholas et al., 1993, Roudet et al., 1994). Activation of noradrenergic receptors with alpha2-adrenoceptor agonists results in antinociception in acute and persistent pain models (Millan, 2002). Furthermore, changes in descending noradrenergic input to the dorsal horn of the spinal cord have been demonstrated (Green et al., 1998, Martin et al., 1999, Ren and Dubner, 1996, Tsuruoka and Willis, 1996, Wei et al., 1999). However, these enhancements in the descending noradrenergic system were only observed in persistent pain models, predominantly chemical inflammation.

In this study, we investigated the effects of atipamezole, a selective alpha2-adrenoceptor antagonist (Scheinin et al., 1988, Schwartz and Clark, 1998), on the evoked activity of deep dorsal horn spinal neurones following ablation of NK1+ve cells. The purpose was to determine the spinal neuronal origins of the system and to assess whether potential alterations in the alpha2-noradrenergic system contribute to the deficits in mechanical and thermal evoked responses of deep WDR neuronal activity seen after selective ablation of lamina I/III NK1+ve cells using SP–SAP (Suzuki et al., 2002, Suzuki et al., 2004b).

Section snippets

Methods

Sprague–Dawley rats were employed for this study (Central Biological Services, University College London, UK), and all experimental procedures were approved by the UK Home Office and followed the guidelines under the International Association for the Study of Pain (Zimmermann, 1983).

Results

Following intrathecal injection of SAP or SP–SAP, animals showed normal grooming behaviour and weight gain.

Discussion

Noradrenaline, acting at spinal alpha2-adrenoceptors, is one of the main sources for descending inhibitory modulation of spinal nociceptive processing, with a large body of evidence showing a reduction in pain behaviours and noxious evoked neuronal activity with alpha2 agonists in models of acute and persistent pain states (Millan, 2002). Indeed, clonidine has been used as a clinical analgesic (Eisenach et al., 1996). However, whereas agonist studies report on the consequences of activation of

Acknowledgements

This work was supported by the Wellcome Trust.

References (39)

  • R. Suzuki et al.

    Descending facilitatory control of mechanically evoked responses is enhanced in deep dorsal horn neurones following peripheral nerve injury

    Brain Res.

    (2004)
  • R. Suzuki et al.

    Spinal–supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin

    Pain

    (2005)
  • R. Suzuki et al.

    Bad news from the brain: descending 5-HT pathways that control spinal pain processing

    Trends Pharmacol. Sci.

    (2004)
  • M. Tsuruoka et al.

    Descending modulation from the region of the locus coeruleus on nociceptive sensitivity in a rat model of inflammatory hyperalgesia

    Brain Res.

    (1996)
  • C.E. Urch et al.

    In vivo single unit extracellular recordings from spinal cord neurones of rats. Brain Res

    Protoc.

    (2003)
  • M. Zimmermann

    Ethical guidelines for investigations of experimental pain in conscious animals

    Pain

    (1983)
  • A.A. Cameron et al.

    The efferent projections of the periaqueductal gray in the rat: a Phaseolus vulgaris-leucoagglutinin study. II. Descending projections

    J. Comp. Neurol.

    (1995)
  • J. Eisenach et al.

    α2-Adrenergic agonists for regional anesthesia – a clinical review of clonidine (1984–1995

    Anesthesiology

    (1996)
  • H. Fields et al.

    Central nervous system mechanisms of pain modulation

  • Cited by (20)

    • Identification of Novel Macropinocytosing Human Antibodies by Phage Display and High-Content Analysis

      2017, Methods in Enzymology
      Citation Excerpt :

      The assay is based on saporin (Stirpe et al., 1983), a plant toxin that is not internalized on its own but is internalized when conjugated to an internalizing antibody. Internalization of saporin inhibits protein synthesis of within the target cell, resulting in cell death (Rahman et al., 2008; Vago et al., 2005). The following procedure is based on biotin—labeling the macropinocytosing antibody, mixing with streptavidin-conjugated toxin to form the immunotoxin, and incubating the immunotoxin with target and control cells to assess cell-type-specific loss of viability.

    • Ablating spinal nk1-bearing neurons eliminates the development of pain and reduces spinal neuronal hyperexcitability and inflammation from mechanical joint injury in the rat

      2014, Journal of Pain
      Citation Excerpt :

      In fact, only 11 of the 205 neurons identified here were nociceptive specific (Fig 3C). Others have investigated the effects of SP-SAP only in WDR neurons and found reduced evoked responses in those neurons, suggesting that adaptations occur, at least partially, in WDR neurons.26,30 Of note, uninjected or blank-SAP sham groups were not included in the current study.

    • Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

      2013, Neuroscience
      Citation Excerpt :

      The ablation of superficial spinal dorsal horn neurons that possess NK-1Rs prevented the development of hyperalgesia (Mantyh et al., 1997; Nichols et al., 1999) and sensitization of remaining dorsal horn neurons (Khasabov et al., 2002; Suzuki et al., 2002) without altering withdrawal responses to acute stimuli. The majority of spinal NK-1R expressing neurons in the spinal cord send projections to the brain (Todd et al., 2005; Al-Khater et al., 2008; Al-Khater and Todd, 2009) and likely drive descending facilitation (Khasabov et al., 2002, 2005; Suzuki et al., 2002; Rahman et al., 2008). A greater understanding of the mechanisms that initiate and maintain descending facilitation of nociceptive transmission may lead to novel approaches to disrupt this circuitry and thereby reduce central sensitization and hyperalgesia.

    • Complex regional pain syndrome

      2022, Clinical Pain Management: A Practical Guide, Second Edition
    View all citing articles on Scopus
    View full text