Selective ablation of dorsal horn NK1 expressing cells reveals a modulation of spinal alpha2-adrenergic inhibition of dorsal horn neurones
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)
- et al.
Alpha2-adrenoceptor antagonists enhance responses of dorsal horn neurones to formalin induced inflammation
Eur. J. Pharmacol.
(1998) - et al.
Setting the tone: superficial dorsal horn projection neurons regulate pain sensitivity
Trends Neurosci.
(2004) - et al.
Differential effects of neurotoxic destruction of descending noradrenergic pathways on acute and persistent nociceptive processing
Pain
(1999) Descending control of pain
Prog. Neurobiol.
(2002)- et al.
An investigation of neurones that possess the alpha 2C-adrenergic receptor in the rat dorsal horn
Neuroscience
(2002) - et al.
Superficial NK1 expressing spinal dorsal horn neurones modulate inhibitory neurotransmission mediated by spinal GABA(A) receptors
Neurosci. Lett.
(2007) - et al.
Descending serotonergic facilitation mediated through rat spinal 5HT3 receptors is unaltered following carrageenan inflammation
Neurosci. Lett.
(2004) - et al.
Behavioural and neurochemical effects of antipamezole, a novel alpha 2-adrenoceptor antagonist
Eur. J. Pharmacol.
(1988) - et al.
Spatial summation of heat pain within and across dermatomes in fibromyalgia patients and pain-free subjects
Pain
(2004) - et al.
The antinociceptive actions of dexmedetomidine on dorsal horn neuronal responses in the anaesthetized rat
Eur. J. Pharmacol.
(1992)
Descending facilitatory control of mechanically evoked responses is enhanced in deep dorsal horn neurones following peripheral nerve injury
Brain Res.
Spinal–supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin
Pain
Bad news from the brain: descending 5-HT pathways that control spinal pain processing
Trends Pharmacol. Sci.
Descending modulation from the region of the locus coeruleus on nociceptive sensitivity in a rat model of inflammatory hyperalgesia
Brain Res.
In vivo single unit extracellular recordings from spinal cord neurones of rats. Brain Res
Protoc.
Ethical guidelines for investigations of experimental pain in conscious animals
Pain
The efferent projections of the periaqueductal gray in the rat: a Phaseolus vulgaris-leucoagglutinin study. II. Descending projections
J. Comp. Neurol.
α2-Adrenergic agonists for regional anesthesia – a clinical review of clonidine (1984–1995
Anesthesiology
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 EnzymologyCitation 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 PainCitation 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, NeuroscienceCitation 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 EditionWhat goes up must come down: insights from studies on descending controls acting on spinal pain processing
2020, Journal of Neural Transmission