Alterations in neuropeptide Y, tyrosine hydroxylase, and Y-receptor subtype distribution following spinal nerve injury to rats

Abstract

Recent animal models of experimental nerve injury have proven useful in evaluating potential sympathetic involvement in neuropathic pain syndromes. We have employed a widely adopted unilateral L5/L6 spinal nerve ligation model to compare the development of mechanical allodynia with neurochemical changes both at the site of peripheral nerve injury and in the dorsal root ganglia (DRG). We have focused on the expression of neuropeptide Y (NPY), a well-studied regulatory peptide and phenotypic marker of sympathetic neurons, and functionally related Y-receptor binding sites following nerve injury. In sympathetic neurons, NPY is colocalized and coreleased with norepinephrine (NE) at peripheral sites of action. Furthermore, NPY gene expression is induced within the population of medium- and large-diameter DRG neurons of the A beta-fiber class after experimental nerve injury. We therefore hypothesized that concurrent alterations in NPY and NE expression by sympathetic and sensory neurons may be a contributing factor to sympathetically-maintained neuropathic conditions. Animals with unilateral L5/L6 spinal nerve ligation developed mechanical allodynia of the hind paw ipsilateral to the site of injury that persisted until sacrifice at postoperative day 10. A significant induction of preproneuropeptide Y-encoding (PPNPY) mRNA, as detected by in situ hybridization histochemistry (ISHH), occurred in populations of medium- and large-diameter DRG neurons ipsilateral to the site of injury. Immunohistochemical analysis indicated a marked decline in the number of labeled sympathetic axons positive for tyrosine hydroxylase-like and NPY-like immunoreactivities (TH-LI and NPY-LI, respectively) proximal to the site of nerve injury and almost complete elimination of immunopositive fibers distal to the site of ligation. Whereas, the extent of colocalization of NPY-LI to TH-LI-positive sympathetic axons in unaffected L4 or L5 nerve segments exceeded 80%, this figure declined to approximately 50% in regenerating axons of ligated spinal nerve L5. The portion of NPY-LI that was not colocalized to sympathetic TH-LI-positive fibers was most likely contributed by regenerating sensory axons, consistent with marked de novo synthesis of NPY by DRG neurons. In end bulb axon terminals, i.e. morphological profiles characteristic of neuromas, NPY-LI-positive elements that were not colocalized to TH-LI-positive sympathetic elements appeared to be spatially segregated from those of sympathetic origin with colocalized TH-LI and NPY-LI. Receptor autoradiography indicated that small- and medium-diameter DRG somata of the C-fiber class normally express both Y1 and Y2 receptor subtypes. The pattern of the distribution of Y-receptor binding sites appeared to be relatively unaffected by spinal nerve ligation. In contrast, there was a marked increase in the density of Y2 receptor binding sites in the proximal segment of ligated spinal nerve L5, consistent with previously published data indicating differential transport of the Y2 autoregulatory receptor subtype...

Neuropeptide Y inhibits capsaicin-sensitive nociceptors via a Y1-receptor-mediated mechanism

“…The substantial upregulation of NPY expression and release after peripheral nerve injury or inflammation has been well established (Wakisaka et al, 1991(Wakisaka et al, , 1992Ji et al, 1994;Marchand et al, 1999;Mark et al, 1997;Sten Shi et al, 1999). Injury-induced activation of the NPY system may have important implications in the development of these persistent pain states (Ossipov et al, 2002).…”

“…A presynaptic site of action on central terminals of sensory nociceptors has not been considered due to the interpretation of data suggesting that the Y1-R was restricted to the somata of afferent neurons . However, more recently, the Y1-R has been reported to be localized at pre-synaptic sites as well (Pickel et al, 1998;Marchand et al, 1999;St. Pierre et al, 2000;Brumovsky et al, 2002).…”

Neuropathic pain and the electrophysiology and pharmacology of nerve injury

“…Smith, unpublished observations) and are in the process of further characterising this effect. From the available ligand binding data [Marchand et al, 1999;Landry et al, 2000], we suspect that this effect is mediated via Y-2 receptors and are examining this possibility with appropriate Y-1, Y-2, Y-4/5 agonists and antagonists. Because Y-2 agonists become more effective in suppressing I Ca,N in DRG cell bodies after axotomy [Abdulla and Smith, 1999a], we hypothesise that they will also become more effective in reducing transmitter release and attenuating epsc's [Landry et al, 2000].…”

“…We will next ask whether the peptide continues to be as effective in substantia gelatinosa neurons of rats that have undergone sciatic nerve axotomy. Although Y-2 receptors are upregulated in peripheral sensory neurons after axotomy [Marchand et al, 1999;Abdulla and Smith, 1999a;Landry et al, 2000], no data are yet available about changes in dorsal horn. If postsynaptic Y-2 receptors are functionally upregulated on lamina II neurons, this would reinforce the idea that Y-2 agonists may be of use in the management of neuropathic pain.…”

A potential anti-allodynic mechanism of GDNF following L5 spinal nerve ligation; Mitigation of NPY up-regulation in the touch sense pathway

“…Furthermore, these effects may differ between intact and nerve-injured spinal cords (Xu et al, 1999). These complications may be due to the fact that at least four types of NPY receptors (Y1, Y2, Y4, and Y5) are differentially expressed throughout the nervous system and that the expression of these receptors are differentially regulated by peripheral nerve injury (Hokfelt et al, 1998;Marchand et al, 1999). On the other hand, there is only one study that has examined the contribution of endogenous NPY to neuropathic pain behaviors.…”