Characterization of the Induced Neuropeptide Y–Like Immunoreactivity in Primary Sensory Neurons following Complete Median Nerve Transection

Functional Anatomy of the Sensory Nervous System: Updates From the Neuroscience Bench

The simple tripartite classification of sensory neurons as A-beta, A-delta, and C fibers fails to convey the complexity of the neurons that encode stimuli as diverse as the texture of a surface, the location of a pinprick, or the direction of hair movement as a breeze moves across the skin. It has also proven to be inadequate when investigating the molecular mechanisms underlying pain, which can encompass any combination of chemical, tactile, and thermal modalities. Beginning with a brief overview of visceral and sensory neuroanatomy, this review expands upon sensory innervation of the skin as a prime example of the heterogeneity and complexity of the somatosensory nervous system. Neuroscientists have characterized defining features of over 15 subtypes of sensory neurons that innervate the skin of the mouse. This has enabled the study of cell-specific mechanisms of pain, which suggests that diverse sensory neuron subtypes may have distinct susceptibilities to toxic injury and different roles in pathologic mechanisms underlying altered sensation. Leveraging this growing body of knowledge for preclinical trials and models of neurotoxicity can vastly improve our understanding of peripheral nervous system dysfunction, advancing the fields of toxicologic pathology and neuropathology alike.

Elevated galanin receptor type 2 primarily contributes to mechanical hypersensitivity after median nerve injury

In this study, we investigated temporal changes in galanin receptor type 2 (GalR2) expression in NF200-, galanin-, neuropeptide Y (NPY)-, and neuronal nitric oxide synthase (nNOS)-like immunoreactive (LI) dorsal root ganglion (DRG) neurons after median nerve chronic constriction injury (CCI), and the effects of GalR2 on c-Fos expression in the cuneate nucleus (CN). Double immunofluorescence labeling methods were used to appraise changes in GalR2 expression in NF200-LI, galanin-LI, NPY-LI, and nNOS-LI DRG neurons after CCI. The von Frey assay was used to assess the efficiency of intraplantar administration of saline, M871 (a GalR2 antagonist), or AR-M1896 (a GalR2 agonist) on neuropathic signs of rats with CCI. The effects of alterations in c-Fos expression were assessed in all treatments. The percentage of GalR2-LI neurons in lesioned DRGs increased and peaked at 1 week after CCI. We further detected that percentages of GalR2-LI neurons labeled for NF200, galanin, NPY, and nNOS significantly increased following CCI. Furthermore, M871 remarkably attenuated tactile allodynia, but the sensation was slightly aggravated by AR-M1896 after CCI. Consequentially, after electrical stimulation of the CCI-treated median nerve, the number of c-Fos-LI neurons in the cuneate nucleus (CN) was significantly reduced in the M871 group, whereas it increased in the AR-M1896 group. These results suggest that activation of GalR2, probably through NPY or nitric oxide, induces c-Fos expression in the CN and transmits mechanical allodynia sensations to the thalamus.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Novel expression pattern of neuropeptide Y immunoreactivity in the peripheral nervous system in a rat model of neuropathic pain

Background

Neuropeptide Y (NPY) has been implicated in the modulation of pain. Under normal conditions, NPY is found in interneurons in the dorsal horn of the spinal cord and in sympathetic postganglionic neurons but is absent from the cell bodies of sensory neurons. Following peripheral nerve injury NPY is dramatically upregulated in the sensory ganglia. How NPY expression is altered in the peripheral nervous system, distal to a site of nerve lesion, remains unknown. To address this question, NPY expression was investigated using immunohistochemistry at the level of the trigeminal ganglion, the mental nerve and in the skin of the lower lip in relation to markers of sensory and sympathetic fibers in a rat model of trigeminal neuropathic pain.

Results

At 2 and 6 weeks after chronic constriction injury (CCI) of the mental nerve, de novo expression of NPY was seen in the trigeminal ganglia, in axons in the mental nerve, and in fibers in the upper dermis of the skin. In lesioned animals, NPY immunoreactivity was expressed primarily by large diameter mental nerve sensory neurons retrogradely labelled with Fluorogold. Many axons transported this de novo NPY to the periphery as NPY-immunoreactive (IR) fibers were seen in the mental nerve both proximal and distal to the CCI. Some of these NPY-IR axons co-expressed Neurofilament 200 (NF200), a marker for myelinated sensory fibers, and occasionally colocalization was seen in their terminals in the skin. Peptidergic and non-peptidergic C fibers expressing calcitonin gene-related peptide (CGRP) or binding isolectin B4 (IB4), respectively, never expressed NPY. CCI caused a significant de novo sprouting of sympathetic fibers into the upper dermis of the skin, and most, but not all of these fibers, expressed NPY.

Conclusions

This is the first study to provide a comprehensive description of changes in NPY expression in the periphery after nerve injury. Novel expression of NPY in the skin comes mostly from sprouted sympathetic fibers. This information is fundamental in order to understand where endogenous NPY is expressed, and how it might be acting to modulate pain in the periphery.

Changes in GABA and GABA(B) receptor expressions are involved in neuropathy in the rat cuneate nucleus following median nerve transection

This study examined the relationship between changes in GABA transmission and behavioral abnormalities after median nerve transection. Following unilateral median nerve transection, the percentage of GABA-like immunoreactive neurons in the cuneate nucleus and that of GABA(B) receptor-like immunoreactive neurons in the dorsal root ganglion in the injured side decreased and reached a nadir at 4 weeks after median nerve transection. Four weeks after bilateral median nerve transection and intraperitoneal application with saline, baclofen (2 mg kg⁻¹), or phaclofen (2 mg kg⁻¹) before unilateral electrical stimulation of the injured median nerve, we investigated the level of neuropeptide Y release and c-Fos expression in the stimulated side of the cuneate nucleus. The neuropeptide Y release level and the number of c-Fos-like immunoreactive neurons in the baclofen group were significantly attenuated, whereas those in the phaclofen group had increased compared to the saline group. These findings indicate that median nerve transection reduces GABA transmission, promoting injury-induced neuropeptide Y release and consequently evoking c-Fos expression in cuneate nucleus neurons. Furthermore, this study used the CatWalk method to assess behavioral abnormalities in rats following median nerve transection. These abnormalities were reversed by baclofen treatment. Overall, the results suggest that baclofen treatment block neuropeptide Y release, subsequently lessening c-Fos expression in cuneate neurons and consequently attenuating neuropathic signal transmission to the thalamus.

Decreases of glycine receptor expression induced by median nerve injury in the rat cuneate nucleus contribute to NPY release and c-Fos expression

AIMS

This study aimed to investigate temporal changes in glycine and its receptor expressions in cuneate neurons after median nerve transection (MNT), and the effects of glycine on neuropeptide Y (NPY) release and c-Fos expression in the cuneate nucleus (CN).

MAIN METHODS

Immunohistochemistry methods were used to appraise changes of glycine- and GlyR-like immunoreactive (LI) neurons in the CN after MNT. The alterations in NPY and c-Fos expressions were used to assess the effects of saline, glycine or strychnine treatment. The CatWalk method was used to assess the efficiency of glycine treatment on the neuropathic signs of rats with MNT.

KEY FINDINGS

Approximately half of GlyR-LI neurons were fluorogold-labeled cuneothalamic projection neurons in the CN. Following MNT, the number of GlyR-LI neurons significantly decreased in the injured side of CN at 2 and 4 weeks, but the number of glycine-LI neurons remained unchanged. Four weeks after MNT given with electrical stimulation, strychnine significantly decreased the NPY reduction level in the stimulated side CN compared to that of the saline group. However, numbers of c-Fos-LI neurons in the glycine and strychnine groups were both significantly less than that in the saline group. But the paw print width and area in CatWalk analysis showed only a moderate recovery.

SIGNIFICANCE

We conjecture that glycine increases glycine-mediated postsynaptic inhibition of cuneate neurons, and also blocks GABAergic neurons containing GlyRs which mediate presynaptic inhibition causing temperate NPY release. Consequently, the compromise results showed a weak reduction in c-Fos expression and a slight amelioration of neuropathic behaviors.

Pre-emptive treatment of lidocaine attenuates neuropathic pain and reduces pain-related biochemical markers in the rat cuneate nucleus in median nerve chronic constriction injury model

This study investigates the effects of lidocaine pre-emptive treatment on neuropathic pain behavior, injury discharges of nerves, neuropeptide Y (NPY) and c-Fos expression in the cuneate nucleus (CN) after median nerve chronic constriction injury (CCI). Behavior tests demonstrated that the pre-emptive lidocaine treatment dose dependently delayed and attenuated the development of mechanical allodynia within a 28-day period. Electrophysiological recording was used to examine the changes in injury discharges of the nerves. An increase in frequency of injury discharges was observed and peaked at postelectrical stimulation stage in the presaline group, which was suppressed by lidocaine pre-emptive treatment in a dose-dependent manner. Lidocaine pretreatment also reduced the number of injury-induced NPY-like immunoreactive (NPY-LI) fibers and c-Fos-LI neurons within the CN in a dose-dependent manner. Furthermore, the mean number of c-Fos-LI neurons in the CN was significantly correlated to the NPY reduction level and the sign of mechanical allodynia following CCI.

Nitric oxide implicates c-Fos expression in the cuneate nucleus following electrical stimulation of the transected median nerve

In this study, we investigated whether nitric oxide (NO) modulated injury-induced neuropeptide Y (NPY) releasing and c-Fos expression in the cuneate nucleus (CN) after median nerve transection (MNT). We first examined the temporal changes of neuronal nitric oxide synthase (nNOS) expression in the dorsal root ganglion (DRG) and CN after MNT. Following MNT, the amounts of nNOS-like immunoreactive (nNOS-LI) neurons in the DRG and CN significantly increased as compared with those of the sham-operated rats. Furthermore, 4 weeks after MNT, the increases of nNOS-LI neurons in the DRG and CN were attenuated by pre-emptive lidocaine treatment in a dose-dependent manner. Finally, 4 weeks after MNT, pre-stimulation administration of L-NAME (N (ω)-Nitro-L: -arginine methyl ester) or 7-NI (7-nitroindazole) suppressed the amount of NPY release from the stimulated terminals and thus attenuated c-Fos expression in the CN. Our data implied that NO would modulate neuronal activity in the DRG and CN both after MNT.

Topography and time course of changes in spinal neuropeptide Y immunoreactivity after spared nerve injury

We used a new computer-assisted method to precisely localize and efficiently quantify increases in neuropeptide Y immunoreactivity (NPY-ir) along the mediolateral axis of the L4 dorsal horn (DH) following transection of either the tibial and common peroneal nerves (thus sparing the sural branch, spared nerve injury (SNI)), the tibial nerve, or the common peroneal and sural nerves. Two weeks after SNI, NPY-ir increased within the tibial and peroneal innervation territories; however, NPY-ir in the central-lateral region (innervated by the spared sural nerve) was indistinguishable from that of sham. Conversely, transection of the sural and common peroneal nerves induced an increase in NPY-ir in the central-lateral region, while leaving the medial region (innervated by the tibial nerve) unaffected. All nerve injuries increased NPY-ir in dorsal root ganglia (DRG) and nucleus gracilis (NG). By 24 weeks, both NPY-ir upregulation in the DH and hyper-responsivity to cold and noxious mechanical stimuli had resolved. Conversely, NPY-ir in DRG and NG, and hypersensitivity to non-noxious static mechanical stimuli, did not resolve within 24 weeks. Over this time course, the average cross-sectional area of NPY-immunoreactive DRG neurons increased by 151 mum(2). We conclude that the upregulation of NPY after SNI is restricted to medial zones of the DH, and therefore cannot act directly upon synapses within the more lateral (sural) zones to control sural nerve hypersensitivity. Instead, we suggest that NPY in the medial DH tonically inhibits hypersensitivity by interrupting mechanisms of central sensitization and integration of sensory signals at the spinal and supraspinal levels.