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

Recent animal models of experimental nerve injury have proven useful in eva luating potential sympathetic involvement in neuropathic pain syndromes. We have employed a widely adopted unilateral L-5/L-6 spinal nerve ligation mo del to compare the development of mechanical allodynia with neurochemical c hanges both at the site of peripheral nerve injury and in the dorsal root g anglia (DRG). We have focused on the expression of neuropeptide Y (NPY), a well-studied regulatory peptide and phenotypic marker of sympathetic neuron s, and functionally related Y-receptor binding sites following nerve injury . In sympathetic neurons, NPY is colocalized and coreleased with norepineph rine (NE) at peripheral sites of action. Furthermore, NPY gene expression i s induced within the population of medium- and large-diameter DRG neurons o f the A beta-fiber class after experimental nerve injury. We therefore hypo thesized that concurrent alterations in NPY and NE expression by sympatheti c and sensory neurons may be a contributing factor to sympathetically-maint ained neuropathic conditions. Animals with unilateral L-5/L-6 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 si gnificant induction of preproneuropeptide Y-encoding (PPNPY) mRNA, as detec ted by in situ hybridization histochemistry (ISHH), occurred in populations of medium- and large-diameter DRG neurons ipsilateral to the site of injur y. Immunohistochemical analysis indicated a marked decline in the number of labeled sympathetic axons positive for tyrosine hydroxylase-like and NPY-l ike immunoreactivities (TH-LI and NPY-LI, respectively) proximal to the sit e of nerve injury and almost complete elimination of immunopositive fibers distal to the site of ligation. Whereas, the extent of colocalization of NP Y-LI to TH-LI-positive sympathetic axone in unaffected L-4 or L-5 nerve seg ments exceeded 80%, this figure declined to approximately 50% in regenerati ng axons of ligated spinal nerve L-5 The portion of NPY-LI that was not col ocalized to sympathetic TH-LI-positive fibers was most likely contributed b y regenerating sensory axons, consistent with marked de novo synthesis of N PY by DRG neurons. In end bulb axon terminals, i.e. morphological profiles characteristic of neuromas, NPY-LI-positive elements that were not colocali zed to TH-LI-positive sympathetic elements appeared to be spatially segrega ted from those of sympathetic origin with colocalized TH-LI and NPY-LI. Rec eptor autoradiography indicated that small- and medium-diameter DRG somata of the C-fiber class normally express both Y-1 and Y-2 receptor subtypes. T he pattern of the distribution of Y-receptor binding sites appeared to be r elatively unaffected by spinal nerve ligation. In contrast, there was a mar ked increase in the density of Y-2 receptor binding sites in the proximal s egment of ligated spinal nerve L-5, consistent with previously published da ta indicating differential transport of the Y-2 autoregulatory receptor sub type to nerve terminals. Induction of NPY gene expression in injured DRG ne urons is consistent with appearance of NPY-LI-positive end bulbs derived fr om regenerating sensory axons that are found in developing neuromas contain ing a relatively high density of transported prejunctional Y-2 receptors. N ewly established functional interactions of spatially segregated sensory- a nd sympathetically-derived end bulbs in developing neuromas may enhance neu ronal hyperexcitability engendered by aberrant electrical activity at the s ite of injury. Injury-related alterations in the regulatory activities of NPY released wit hin the DRG at somally-distributed Y-receptors may also contribute to the d evelopment and/or persistence of symptoms characteristic of sympathetically -maintained pain. Finally, at later times NPY-mediated modulation of NE rel ease from invading sympathetic axon terminals within the DRG may affect the extent of alpha(2) receptor-mediated neuronal hyperexcitability associated with neuropathic pain. (C) 1999 International Association for the Study of Pain. Published by Elsevier Science B.V.