The effects of intrathecal neuropeptide Y on the spinal nociceptive flexor reflex in rats with intact sciatic nerves and after peripheral axotomy

Targeting spinal neuropeptide Y1 receptor-expressing interneurons to alleviate chronic pain and itch

An accelerating basic science literature is providing key insights into the mechanisms by which spinal neuropeptide Y (NPY) inhibits chronic pain. A key target of pain inhibition is the G_i-coupled neuropeptide Y1 receptor (Y1). Y1 is located in key sites of pain transmission, including the peptidergic subpopulation of primary afferent neurons and a dense subpopulation of small, excitatory, glutamatergic/somatostatinergic interneurons (Y1-INs) that are densely expressed in the dorsal horn, particularly in superficial lamina I-II. Selective ablation of spinal Y1-INs with an NPY-conjugated saporin neurotoxin attenuates the development of peripheral nerve injury-induced mechanical and cold hypersensitivity. Conversely, conditional knockdown of NPY expression or intrathecal administration of Y1 antagonists reinstates hypersensitivity in models of chronic latent pain sensitization. These and other results indicate that spinal NPY release and the consequent inhibition of pain facilitatory Y1-INs represent an important mechanism of endogenous analgesia. This mechanism can be mimicked with exogenous pharmacological approaches (e.g. intrathecal administration of Y1 agonists) to inhibit mechanical and thermal hypersensitivity and spinal neuron activity in rodent models of neuropathic, inflammatory, and postoperative pain. Pharmacological activation of Y1 also inhibits mechanical- and histamine-induced itch. These immunohistochemical, pharmacological, and cell type-directed lesioning data, in combination with recent transcriptomic findings, point to Y1-INs as a promising therapeutic target for the development of spinally directed NPY-Y1 agonists to treat both chronic pain and itch.

Spinal activation of the NPY Y1 receptor reduces mechanical and cold allodynia in rats with chronic constriction injury

Neuropeptide tyrosine (NPY) and its associated receptors Y1R and Y2R have been previously implicated in the spinal modulation of neuropathic pain induced by total or partial sectioning of the sciatic nerve. However, their role in chronic constrictive injuries of the sciatic nerve has not yet been described. In the present study, we analyzed the consequences of pharmacological activation of spinal Y1R, by using the specific Y1R agonist Leu³¹Pro³⁴-NPY, in rats with chronic constriction injury (CCI). CCI and sham-injury rats were implanted with a permanent intrathecal catheter (at day 7 after injury), and their response to the administration of different doses (2.5, 5, 7, 10 or 20μg) of Leu³¹Pro³⁴-NPY (at a volume of 10μl) through the implanted catheter, recorded 14days after injury. Mechanical allodynia was tested by means of the up-and-down method, using von Frey filaments. Cold allodynia was tested by application of an acetone drop to the affected hindpaw. Intrathecal Leu³¹Pro³⁴-NPY induced an increase of mechanical thresholds in rats with CCI, starting at doses of 5μg and becoming stronger with higher doses. Intrathecal Leu³¹Pro³⁴ also resulted in reductions in the frequency of withdrawal to cold stimuli, although the effect was somewhat more moderate and mostly observed for doses of 7μg and higher. We thus show that spinal activation of the Y1R is able to reduce neuropathic pain due to a chronic constrictive injury and, together with other studies, support the use of a spinal Y1R agonist as a therapeutic agent against chronic pain induced by peripheral neuropathy.

Touch Receptor-Derived Sensory Information Alleviates Acute Pain Signaling and Fine-Tunes Nociceptive Reflex Coordination

Painful mechanical stimuli activate multiple peripheral sensory afferent subtypes simultaneously, including nociceptors and low-threshold mechanoreceptors (LTMRs). Using an optogenetic approach, we demonstrate that LTMRs do not solely serve as touch receptors but also play an important role in acute pain signaling. We show that selective activation of neuropeptide Y receptor-2-expressing (Npy2r) myelinated A-fiber nociceptors evokes abnormally exacerbated pain, which is alleviated by concurrent activation of LTMRs in a frequency-dependent manner. We further show that spatial summation of single action potentials from multiple NPY2R-positive afferents is sufficient to trigger nocifensive paw withdrawal, but additional simultaneous sensory input from LTMRs is required for normal well-coordinated execution of this reflex. Thus, our results show that combinatorial coding of noxious and tactile sensory input is required for normal acute mechanical pain signaling. Additionally, we established a causal link between precisely defined neural activity in functionally identified sensory neuron subpopulations and nocifensive behavior and pain.

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

Intrathecal delivery of glial cell line-derived neurotrophic factor (GDNF) reverses mechanical allodynia after 5th lumbar (L5) spinal nerve ligation (SNL). However, the molecular mechanism behind this process is not fully understood. Following sciatic nerve injury, primary afferent neurons in the injured dorsal root ganglion (DRG) begin to express neuropeptide Y (NPY) that is absent in normal DRG. The aim of the current study was to determine the relationship of this de novo expression of NPY and the anti-allodynic effect of GDNF. Following L5 SNL, 73% of neurons began to express NPY mRNA in the ipsilateral L5 DRG and robust NPY-immunoreactive fibers appeared in the ipsilateral GN where the touch-sense mediating A-fiber primary afferents from the hindpaw terminate. Seven-daylong intrathecal infusion of GDNF at the L5 DRG level, starting on day three when mechanical allodynia had fully developed, reversed once-established these changes. The GN neurons normally expressed NPY Y1 receptor, but not Y2, Y4, or Y5 receptors, and L5 SNL did not change the expression pattern. Bolus intracisternal injection of BIBP3226, a Y1 receptor antagonist, dose-dependently reversed mechanical allodynia. We demonstrated that GDNF reversed once-established mechanical allodynia as well as NPY induction in the touch-sense processing pathway. NPY could facilitate touch-sense processing by Y1 receptor in the gracile nucleus after peripheral nerve injury. GDNF may exert anti-allodynic effects through mitigation of this NPY up-regulation. The effectiveness of delayed treatment further indicates the therapeutic potential of GDNF on neuropathic pain.

Neuropeptide Y receptor-expressing dorsal horn neurons: role in nocifensive reflex and operant responses to aversive cold after CFA inflammation

The spinal Neuropeptide Y (NPY) system is a potential target for development of new pain therapeutics. NPY and two of its receptors (Y1 and Y2) are found in the superficial dorsal horn of the spinal cord, a key area of nociceptive gating and modulation. Lumbar intrathecal injection of (NPY) is antinociceptive, reducing hyper-reflexia to thermal and mechanical stimulation, particularly after nerve injury and inflammation. We have also shown that intrathecal injection of the targeted cytotoxin, Neuropeptide Y-sap (NPY-sap), is also antinociceptive, reducing nocifensive reflex responses to noxious heat and formalin. In the present study, we sought to determine the role of dorsal horn Y1R-expressing neurons in pain by destroying them with NPY-sap and testing the rats on three operant tasks. Lumbar intrathecal NPY-sap (1) reduced Complete Freund's Adjuvant (CFA)-induced hyper-reflexia on the 10°C cold plate, (2) reduced cold aversion on the thermal preference and escape tasks, (3) was analgesic to noxious heat on the escape task, (4) reduced the CFA-induced allodynia to cold temperatures experienced on the thermal preference, feeding interference, and escape tasks, and (5) did not inhibit or interfere with morphine analgesia.

Inhibition of SNL-induced upregulation of CGRP and NPY in the spinal cord and dorsal root ganglia by the 5-HT(2A) receptor antagonist ketanserin in rats

Our previous study has demonstrated that topical and systemic administration of the 5-HT(2A) receptor antagonist ketanserin attenuates neuropathic pain. To explore the mechanisms involved, we examined whether ketanserin reversed the plasticity changes associated with calcitonin gene-related peptides (CGRP) and neuropeptide Y (NPY) which may reflect distinct mechanisms: involvement and compensatory protection. Behavioral responses to thermal and tactile stimuli after spinal nerve ligation (SNL) at L5 demonstrated neuropathic pain and its attenuation in the vehicle- and ketanserin-treated groups, respectively. SNL surgery induced an increase in CGRP and NPY immunoreactivity (IR) in laminae I-II of the spinal cord. L5 SNL produced an expression of NPY-IR in large, medium and small diameter neurons in dorsal root ganglion (DRG) only at L5, but not adjacent L4 and L6. Daily injection of ketanserin (0.3 mg/kg, s.c.) for two weeks suppressed the increase in CGRP-IR and NPY-IR in the spinal cord or DRG. The present study demonstrated that: (1) the expression of CGRP was enhanced in the spinal dorsal horn and NPY was expressed in the DRG containing injured neurons, but not in the adjacent DRG containing intact neurons, following L5 SNL; (2) the maladaptive changes in CGRP and NPY expression in the spinal cord and DRG mediated the bioactivity of 5-HT/5-HT(2A) receptors in neuropathic pain and (3) the blockade of 5-HT(2A) receptors by ketanserin reversed the evoked upregulation of both CGRP and NPY in the spinal cord and DRG contributing to the inhibition of neuropathic pain.

Cerebral decreases in opioid receptor binding in patients with central neuropathic pain measured by [11 C]diprenorphine binding and PET

Central neuropathic pain (CNP) is pain resulting from damage to the central nervous system. Up till now, it has not been possible to identify a common lesion or pharmacological deficit in these patients. This preliminary study in a group of patients with CNP with predominantly post-stroke pain, demonstrates that there is significantly less opioid receptor binding in a number of cortical and sub-cortical structures that are mostly, but not exclusively, within the medial pain system in patients compared to age-matched pain-free controls. The reductions in opioid receptor binding within the medial system were observed mainly in the dorsolateral (Brodman area 10) and anterior cingulate (Brodman area 24 with some extension into area 23) and insula cortices and the thalamus. There were also reductions in the lateral pain system within the inferior parietal cortex (Brodman area 40). These changes in binding could not be accounted for by the cerebral lesions shown by CT or MRI, which were outside the areas of reduced binding and the human pain system. To our knowledge this is the first systematic demonstration of a reduction in opioid receptor-binding capacity in neurones within the human nociceptive system in patients with CNP. This may be a key common factor resulting in undamped nociceptor activity within some of the structures that are predominantly within the medial nociceptive system. If confirmed, these findings may explain why certain patients with CNP require high doses of synthetic opiates to achieve optimum analgesia. The findings also raise the possibility of new pharmacological approaches to treatment.

Attenuation of pain-related behavior evoked by injury through blockade of neuropeptide Y Y2 receptor

Neuropeptide Y (NPY) has an important but still insufficiently defined role in pain modulation. We therefore examined the ability of NPY to modulate experimentally induced neuropathic pain by injecting it directly into dorsal root ganglion (DRG) immediately following spinal nerve ligation (SNL) injury. We have found that this application exacerbates pain-related behavior induced by SNL in a modality-specific fashion. When saline was injected after SNL, the expected increase in hyperalgesia responses to needle stimulation was present on the 8th postoperative day. When we injected NPY, hyperalgesic responses were increased in a manner similar to the SNL/saline group. To characterize NPY action, specific Y1 and Y2 antagonists were also delivered directly to DRG, which revealed that behavioral actions of NPY were abolished by Y2 receptor antagonist. We tested whether NPY effects were the result of its role in immunity by immunohistochemical staining for glial fibrillary acidic protein, in order to identify activation of DRG satellite cells and dorsal horn astrocytes. Exacerbation of pain-related behavior following NPY injection was accompanied by astrocyte activation in ipsilateral dorsal horn and with satellite cells activation in the DRG proximal to injury. This activation was reduced following Y2 receptor antagonist application. These findings indicate an important link between pain-related behavior and neuroimmune activation by NPY through its Y2 receptor.

Behavioral and anatomical characterization of the bilateral sciatic nerve chronic constriction (bCCI) injury: correlation of anatomic changes and responses to cold stimuli

Background

Unilateral constrictive sciatic nerve injury (uCCI) is a common neuropathic pain model. However, the bilateral constrictive injury (bCCI) model is less well studied, and shows unique characteristics. In the present study, we sought to correlate effects of bCCI on nocifensive responses to cold and mechanical stimuli with selected dorsal horn anatomic markers. bCCI or sham ligation of both rat sciatic nerves were followed up to 90 days of behavioural testing. Additional rats sacrificed at 15, 30 and 90 days were used for anatomic analyses. Behavioural tests included hindpaw withdrawal responses to topical acetone, cold plate testing, an operant thermal preference task and hindpaw withdrawal thresholds to mechanical probing.

Results

All nocifensive responses to cold increased and remained enhanced for >45 days. Mechanical withdrawal thresholds decreased for 25 days only. Densitometric analyses of immunoperoxidase staining in the superficial dorsal horn at L4-5 revealed decreased cholecystokinin (CCK) staining at all times after bCCI, decreased mu opiate receptor (MOR) staining, maximal at 15 days, increased neuropeptide Y (NPY) staining only at days 15 and 30, and increased neurokinin-1 receptor (NK-1R) staining at all time points, maximal at 15 days. Correlation analyses at 45 days post-bCCI, were significant for individual rat nocifensive responses in each cold test and CCK and NK-1R, but not for MOR or NPY.

Conclusions

These results confirm the usefulness of cold testing in bCCI rats, a new approach using CCI to model neuropathic pain, and suggest a potential value of studying the roles of dorsal horn CCK and substance P in chronic neuropathic pain. Compared to human subjects with neuropathic pain, responses to cold stimuli in rats with bCCI may be a useful model of neuropathic pain.

Involvement of neuropeptide Y in the acute, chronic and withdrawal responses of morphine in nociception in neuropathic rats: behavioral and neuroanatomical correlates

Although morphine is a potent antinociceptive agent, its chronic use developed tolerance in neuropathic pain (NP). Furthermore, opioid antagonist naloxone attenuated the antinociceptive effect of neuropeptide Y (NPY). The present study investigated the role of NPY and NPY Y1/Y5 receptors in acute and chronic actions of morphine in neuropathic rats using thermal paw withdrawal test and immunocytochemistry. In acute study, intracerebroventricular (icv) administration of morphine, NPY or NPY Y1/Y5 receptors agonist [Leu(31),Pro(34)]-NPY produced antinociception, whereas selective NPY Y1 receptors antagonist BIBP3226 caused hyperalgesia. While NPY or [Leu(31),Pro(34)]-NPY potentiated, BIBP3226 attenuated morphine induced antinociception. Chronic icv infusion of morphine via osmotic minipumps developed tolerance to its antinociceptive effect, and produced hyperalgesia following withdrawal. However, co-administration of NPY or [Leu(31),Pro(34)]-NPY prevented the development of tolerance and withdrawal hyperalgesia. Sciatic nerve ligation resulted in significant increase in the NPY-immunoreactive (NPY-ir) fibers in ventrolateral periaqueductal gray (VLPAG) and locus coeruleus (LC); fibers in the dorsal part of dorsal raphe nucleus (DRD) did not respond. While chronic morphine treatment significantly reduced NPY-ir fibers in VLPAG and DRD, morphine withdrawal triggered significant augmentation in NPY-immunoreactivity in the VLPAG. NPY-immunoreactivity profile of LC remained unchanged in all the morphine treatment conditions. Furthermore, removal of sciatic nerve ligation reversed the effects of NP, increased pain threshold and restored NPY-ir fiber population in VLPAG. NPY, perhaps acting via Y1/Y5 receptors, might profoundly influence the processing of NP information and interact with the endogenous opioid system primarily within the framework of the VLPAG.

Involvement of Y5 receptors in neuropeptide Y agonist-induced analgesic-like effect in the rat hot plate test

Neuropeptide Y (NPY) induces analgesic-like effects after central administration across diverse pain models in rodents. In spinal pain models, previous studies indicate a prominent role for Y(1) receptors at mediating this effect of NPY. In supraspinal pain models like the hot plate test, the NPY receptors involved have not been thoroughly explored. By intracerebroventricular (i.c.v.) administration of selective NPY receptor ligands, the possible involvement of Y(5) receptors in analgesic-like mechanisms was investigated using the hot plate test in rats. Both NPY and selective Y(5) agonists induced analgesic-like effects as revealed by prolonged hot plate latencies. Further consistent with a role for Y(5) receptors, pretreatment with a selective Y(5) receptor antagonist blocked the Y(5) agonist-induced analgesic-like effect. The present study indicates involvement of Y(5) receptors probably at the supraspinal level in mediation of NPY agonist-induced analgesic-like effects in the hot plate test.

NPY and pain as seen from the histochemical side

The expression of neuropeptide tyrosine (NPY) and two of its receptors (Y1- and Y2Rs) in different types of rodent dorsal root ganglion (DRG) and spinal cord neurons, and their regulation by peripheral nerve injury, have suggested a role in neuropathic pain. Here we present the spinal NPYergic system from an immunohistochemical perspective based on recent studies using two specific antibodies recognizing the Y1- and Y2Rs, respectively, as well as on data from a study on a Y1R knock-out mouse. We have, for example, defined seven different neuron populations of Y1R-expressing neurons in the rat spinal cord, representing multiple targets for spinally released NPY. The differential distribution of NPY receptors probably explains both the pro- and antinociceptive effects of NPY previously reported in the literature. One system possibly responsible for antinociception is a group of Y1R-positive, presumably glutamatergic interneurons in the superficial dorsal horn laminae. We also discuss the possibility that NPY released within DRGs can act in a paracrine fashion on NPY receptors on adjacent neurons, perhaps contributing to the so-called cross excitation, a concept advanced by Devor, Amir and collaborators. Taken together with behavioral and electrophysiological results summarized by Smith et al. in this volume, histochemical analyses have advanced the knowledge on the role of NPY in pain processing.

Neuropeptide tyrosine and pain

Research during the past two decades supports a complex role for neuropeptide tyrosine (NPY) and two of its associated receptors, the Y1 receptor and the Y2 receptor, in the modulation of pain, in addition to regeneration and survival mechanisms at the spinal level. Thus, NPY has been shown to both cause and reduce pain, in addition to having biphasic effects. Recent research has focused on the distribution of the spinal NPY-mediated system. Here, we propose various possible scenarios for the role of NPY in pain processing, based on its actions at different sites (axon versus cell body), through different receptors (Y1 receptor versus Y2 receptor) and/or types of neuron (ganglion neurons and intraganglionic cross-excitation versus interneurons versus projection neurons).

Neuropeptide Y modulates effects of bradykinin and prostaglandin E2 on trigeminal nociceptors via activation of the Y1 and Y2 receptors

BACKGROUND AND PURPOSE

Although previous studies have demonstrated that neuropeptide Y (NPY) modulates nociceptors, the relative contributions of the Y1 and Y2 receptors are unknown. Therefore, we evaluated the effect of Y1 and Y2 receptor activation on nociceptors stimulated by bradykinin (BK) and prostaglandin E2 (PGE2).

EXPERIMENTAL APPROACH

Combined immunohistochemistry (IHC) with in situ hybridization (ISH) demonstrated that Y1- and Y2-receptors are collocated with bradykinin (2) (B2)-receptors in rat trigeminal ganglia (TG). The relative functions of the Y1 and Y2 receptors in modulating BK/PGE2-evoked CGRP release and increased intracellular calcium levels in cultured TG neurons were evaluated.

KEY RESULTS

The Y1 and Y2 receptors are co-expressed with B2 in TG neurons, suggesting the potential for direct NPY modulation of BK responses. Pretreatment with the Y1 agonist [Leu31,Pro34]-NPY, inhibited BK/PGE2-evoked CGRP release. Conversely, pretreatment with PYY(3-36), a Y2 agonist, increased BK/PGE2 evoked CGRP release. Treatment with NPY evoked an overall inhibitory effect, although of lesser magnitude. Similarly, [Leu31,Pro34]-NPY inhibited BK/PGE2-evoked increases in intracellular calcium levels whereas PYY(3-36) increased responses. NPY inhibition of BK/PGE2-evoked release of CGRP was reversed by the Y1 receptor antagonist, BIBO3304, and higher concentrations of BIBO3304 significantly facilitated CGRP release. The Y2 receptor antagonist, BIIE0246, enhanced the inhibitory NPY effects.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that NPY modulation of peptidergic neurons is due to net activation of inhibitory Y1 and excitatory Y2 receptor systems. The relative expression or activity of these opposing receptor systems may mediate dynamic responses to injury and pain.

Deletion of the neuropeptide Y Y1 receptor affects pain sensitivity, neuropeptide transport and expression, and dorsal root ganglion neuron numbers

Neuropeptide Y has been implicated in pain modulation and is substantially up-regulated in dorsal root ganglia after peripheral nerve injury. To identify the role of neuropeptide Y after axotomy, we investigated the behavioral and neurochemical phenotype of neuropeptide Y Y1 receptor knockout mice with focus on dorsal root ganglion neurons and spinal cord. Using a specific antibody Y1 receptor immunoreactivity was found in dorsal root ganglia and in dorsal horn neurons of wild-type, but not knockout mice. The Y1 receptor knockout mice exhibited a pronounced mechanical hypersensitivity. After sciatic nerve axotomy, the deletion of Y1 receptor protected knockout mice from the axotomy-induced loss of dorsal root ganglion neurons seen in wild-type mice. Lower levels of calcitonin gene-related peptide and substance P were identified by immunohistochemistry in dorsal root ganglia and dorsal horn of knockout mice, and the axotomy-induced down-regulation of both calcitonin gene-related peptide and substance P was accentuated in Y1 receptor knockout. However, the transcript levels for calcitonin gene-related peptide and substance P were significantly higher in knockout than in wild-type dorsal root ganglia ipsilateral to the axotomy, while more calcitonin gene-related peptide- and substance P-like immunoreactivity accumulated proximal and distal to a crush of the sciatic nerve. These results indicate that the deletion of the Y1 receptor causes increased release and compensatory increased synthesis of calcitonin gene-related peptide and substance P in dorsal root ganglion neurons. Together, these findings suggest that, after peripheral nerve injury, neuropeptide Y, via its Y1 receptor receptor, plays a key role in cell survival as well as in transport and synthesis of the excitatory dorsal horn messengers calcitonin gene-related peptide and substance P and thus may contribute to pain hypersensitivity.

The neuropeptide tyrosine Y1R is expressed in interneurons and projection neurons in the dorsal horn and area X of the rat spinal cord

The localization of the neuropeptide tyrosine Y1 receptor was studied with immunohistochemistry in parasagittal and transverse, free-floating sections of the rat lumbar spinal cord. At least seven distinct Y1 receptor-positive populations could tentatively be recognized: Type 1) abundant small, fusiform Y1 receptor-positive neurons in laminae I-II, producing a profuse neuropil; Type 2) Y1 receptor-positive projection neurons in lamina I; Type 3) small Y1 receptor-positive neurons in lamina III, similar to Type 1 neurons, but less densely packed; Type 4) a number of large, multipolar Y1 receptor-positive neurons in the border area between laminae III-IV, with dendrites projecting toward laminae I-II; Type 5) a considerable number of large, multipolar Y1 receptor-positive neurons in laminae V-VI; Type 6) many large Y1 receptor-positive neurons around the central canal (area X); and Type 7) a small number of large Y1 receptor-positive neurons in the medial aspect of the ventral horns (lamina VIII). Many of the neurons present in laminae V-VI and area X produce craniocaudal processes extending for several hundred micrometers. Retrograde tracing using cholera toxin B subunit injected at the 9th thoracic spinal cord level shows that several Type 5 neurons in laminae V-VI, and at least a few Type 2 in lamina I and Type 6 in area X have projections extending to the lower segments of the thoracic spinal cord (and perhaps to supraspinal levels). The present results define distinct subpopulations of neuropeptide tyrosine-sensitive neurons, localized in superficial and deep layers of the dorsal, in the ventral horns and in area X. The lamina II neurons express somatostatin [The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons. Eur J Neurosci 11:2211-2225] and are presumably glutamatergic [Todd AJ, Hughes DI, Polgar E, Nagy GG, Mackie M, Ottersen OP, Maxwell DJ (2003) The expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in neurochemically defined axonal populations in the rat spinal cord with emphasis on the dorsal horn. Eur J Neurosci 17:13-27], that is they are excitatory interneurons under a Y1 receptor-mediated inhibitory influence. The remaining Y1 receptor-positive spinal neurons need to be phenotyped, for example if the large Y1 receptor-positive laminae III-IV neurons (Type 5) are identical to the neurokinin (NK)1R-positive neurons previously shown to receive neuropeptide tyrosine positive dendritic contacts [Polgár E, Shehab SA, Watt C, Todd AJ (1999) GABAergic neurons that contain neuropeptide Y selectively target cells with the NK1 receptor in laminae III and IV of the rat spinal cord. J Neurosci 19:2637-2646]. If so, neuropeptide tyrosine could have an antinociceptive action not only via Y1 receptor-positive interneurons (Type 1) but also projection neurons. The present results show neuropeptide tyrosine-sensitive neuron populations virtually in all parts of the lumbar spinal cord, suggesting a role for neuropeptide tyrosine signaling in many spinal functions, including pain.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Effect of peripheral axotomy on pain-related behavior and dorsal root ganglion neurons excitability in NPY transgenic rats

In order to clarify the physiologic role of NPY in sensory processing, we obtained intracellular recordings of DRG neurons from wild type (WT) and NPY overexpressing transgenic rats (NPY-TG) before and after injury. We investigated medium and large diameter DRG neurons since upregulation of NPY peptide following the nerve injury occurs primarily in those cells. Neurons were classified as Aalpha/beta and Adelta using conduction velocity and action potential duration. Prior to the injury, Aalpha/beta neurons of NPY-TG rats conducted more slowly and had a more brief AHP than similar cells from the WT group. Adelta neurons at baseline conducted faster in TG animals compared to WT. Ligation of the 5th lumbar spinal nerve (SNL) produced certain changes in Aalpha/beta cells that were evident only in the TG group. These include increased refractory period, increased input resistance, AHP prolongation and a depolarizing shift in threshold for AP initiation. The expected injury-induced CV slowing was not seen in NPY-TG Aalpha/beta cells. In the Adelta cell group, injury produced a depolarizing shift in the resting membrane potential, an increase in AP duration and decrease in AHP and refractory period duration only in WT rats, while NPY-TG cells lacked these injury-induced changes. Behavior tests showed diminished sensory response to nerve injury in NPY-TG rats, i.e., shorter duration of enhanced pain-related behavior and attenuation of contralateral effect. In conclusion, our observations suggest that NPY overexpression leads to reduced neuronal activity following nerve injury in a cell-specific manner.

Neuropeptide Y2 receptor protein is present in peptidergic and nonpeptidergic primary sensory neurons of the mouse

The localization of the neuropeptide tyrosine (NPY) Y2 receptor (Y2R) protein was studied in mouse dorsal root ganglia (DRGs) and spinal cord, by using a recently developed rabbit anti-Y2R antibody and a sensitive immunohistochemical method. Y2R-like immunoreactivity (-LI) was observed in about 10% of the small/medium-sized lumbar DRG neurons. Among these, about 44% were calcitonin gene-related peptide-immunoreactive, and about 38% bound isolectin B4. In the dorsal horn of the spinal cord, an intense Y2R-LI was seen in the most superficial layers, mostly restricted to laminae I-II. This immunoreactivity was completely abolished by dorsal rhizotomy. Y2R-L1 was also detected on the skin, more abundantly in hairy than glabrous skin. Specificity experiments showed complete disappearance of the Y2R-LI described above after incubation with antibody preadsorbed with the immunogenic peptide. Furthermore, Y2R-LI was also absent in a Y2R knockout mouse. These results demonstrate that the NPY Y2R is associated mainly with both peptidergic and nonpeptidergic small, presumably nociceptive, neurons projecting to the superficial layers of the dorsal horn. The results also support a role for this receptor and NPY in pain mechanisms.

Opioid-Like Actions of Neuropeptide Y in Rat Substantia Gelatinosa: Y1 Suppression of Inhibition and Y2 Suppression of Excitation

Neuropathic pain that results from injury to the peripheral or CNS responds poorly to opioid analgesics. Y1 and Y2 receptors for neuropeptide Y (NPY) may, however, serve as targets for analgesics that retain their effectiveness in neuropathic pain states. In substantia gelatinosa neurons in spinal cord slices from adult rats, we find that NPY acts via presynaptic Y2 receptors to attenuate excitatory postsynaptic currents (EPSCs) and predominantly on presynaptic Y1 receptors to attenuate glycinergic and GABAergic inhibitory postsynaptic currents (IPSCs). Because NPY attenuates the frequency of TTX-resistant miniature EPSCs and IPSCs, perturbation of the neurotransmitter release process contributes to its actions at both excitatory and inhibitory synapses. These effects, which are reminiscent of those produced by analgesic opioids, provide a cellular basis for previously documented spinal analgesic actions mediated via Y1 and Y2 receptors in neuropathic pain paradigms. They also underline the importance of suppression of inhibition in spinal analgesic mechanisms.

Effect of a graded single constriction of the rat sciatic nerve on pain behavior and expression of immunoreactive NPY and NPY Y1 receptor in DRG neurons and spinal cord

In the present study, the rat sciatic nerve was constricted to varying degrees using only one ligature with a very thin polyethylene sheath placed between nerve and ligature thread. Complete nerve transection was studied for comparison. With a 40-80% constriction of the nerve we observed allodynia to a similar extent as in the so-called Bennett model based on four loose ligatures. We also monitored changes in the expression of neuropeptide Y (NPY) and the NPY Y1 receptor (Y1R) in the lumbar 4-5 dorsal root ganglia (DRG) and dorsal horn and found upregulation of NPY and downregulation of the Y1R in DRG neurons after injury. These results indicate that similar peptide and receptor changes occur in this model as after axotomy and in other nerve injury models, although the immunohistochemical and behavioral changes seem to be dependent on the degree of constriction of the nerve. Thus, it seems relevant to monitor the degree of constriction when evaluating pain and other post-injury events. The possibility that some of the changes in NPY-ergic neurotransmission are related to the generation of allodynia is discussed; as well as the possibility to use this mononeuropathic model based on a single ligature nerve constriction (SLNC) as a complementary approach to other widely used pain models.

A time course analysis of the changes in neuropeptide Y immunoreactivity in the rat cuneate nucleus following median nerve transection

Using median nerve injury and immunocytochemical methods, we examined the temporal changes in neuropeptide Y (NPY) expression in the cuneate nucleus (CN) in rats following median nerve transection. Under normal circumstances, neuropeptide Y-immunoreactive (NPY-IR) fibers was not detectable in the CN. A few NPY-IR fibers were observed in the ipsilateral CN 5 days after the median nerve transection, and peaked at 4 weeks. Thereafter, they were gradually returned to nearly control level after 16 weeks. Quantitative evaluation showed that the mean percentage of area occupied by NPY-IR fibers in entire and three subregions of the CN at 4 weeks were significantly higher than that at other post-operated time points, respectively. The present ultrastructural observations in the middle region of CN showed that the significantly increased NPY immunoreactivity was confined only in the myelinated axons and terminals but not detected in the dendrites, somata, and glial cells. The NPY-IR terminals made axodendritic synaptic contacts with unlabeled elements. The present results indicate that the time course of the increase of NPY immunoreactivity is similar to c-Fos expression as described in a previous study. It is speculated that the increased NPY in the CN after axotomy may affect the excitability of postsynaptic cuneate neurons, however, the functional interaction between NPY and c-Fos-IR neurons needs to be further studied.

Selective mediation of nerve injury-induced tactile hypersensitivity by neuropeptide Y

Prevention of nerve injury-induced tactile, but not thermal, hypersensitivity is achieved by ipsilateral lesions of the dorsal columns or lidocaine microinjection into the nucleus gracilis (n. gracilis). These and other data support the possibility that tactile hyperresponsiveness after nerve injury may be selectively mediated by a low-threshold myelinated fiber pathway to the n. gracilis. Here we identify a transmitter that might selectively mediate such injury-induced tactile hypersensitivity. Neuropeptide Y (NPY), normally not detected in the dorsal root ganglion (DRG) or in the n. gracilis of rats, became markedly upregulated at both sites and in the spinal cord after spinal nerve injury. Injury-induced NPY-IR occurred predominately in large-diameter DRG cells, and the NPY-IR in the n. gracilis was blocked by dorsal rhizotomy or dorsal column lesion. NPY microinjection into the n. gracilis of uninjured rats elicited reversible tactile, but not thermal, hypersensitivity only in the ipsilateral hindpaw. Administration of anti-NPY antiserum, but not control serum or preabsorbed serum, into the n. gracilis ipsilateral to nerve injury reversed tactile, but not thermal, hypersensitivity. Similarly, microinjection of the NPY antagonists NPY(18-36) and (R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate, into the n. gracilis ipsilateral to the injury reversed tactile, but not thermal, hypersensitivity. Antagonist administration into the contralateral n. gracilis had no effect on injury-induced hypersensitivity. These data suggest the selective mediation of nerve injury-induced tactile hypersensitivity by upregulated NPY via large fiber input to n. gracilis. Selective reversal of injury-induced tactile allodynia by NPY receptor antagonists would have significant implications for human neuropathic conditions.

Axotomy of single fluorescent nerve fibers in developing mammalian spinal cord by photoconversion of diaminobenzidine

A technique has been developed for cutting single nerve fibers in mammalian spinal cord. In the presence of diaminobenzidine (DAB), a laser microbeam was applied to carbocyanine (Dil) stained sensory fibers in cultured spinal cords of the newly born opossum Monodelphis domestica. Digital images of fluorescent fibers were acquired with an intensified video CCD-camera coupled to an image processor. Laser illumination of two spots on a fiber in the presence of 3 mg/ml DAB cut it, so that following DAB wash out, Dil fluorescence did not return after the intermediate segment was bleached. In contrast, when a similar procedure was carried out without DAB, fluorescence of the bleached segment was recovered within minutes in darkness, by dye diffusion from adjacent regions of the uncut fiber. After exposure to DAB, through-conduction of compound action potentials continued in undamaged fibers. The DAB reaction product remained as a dark precipitate, helping to localize the lesion sites. By illuminating a continuous series of spots it was possible to cut whole nerve roots. Fluorescent fibers extended across the cut segment 24 h later. With minor modifications, the procedure described here allows a precise lesioning of single fibers within an intact nervous system.

Antihyperalgesic effects of intrathecal neuropeptide Y during inflammation are mediated by Y1 receptors

Inflammation induces an up-regulation of neuropeptide tyrosine (NPY) and its receptors in the dorsal horn, suggesting an important role in nociceptive transmission. Our initial studies revealed that NPY dose-dependently increased hotplate response latency, and to a lesser degree, thermal paw withdrawal latency (PWL); these effects occurred at doses that affect neither motor coordination (as assessed by the rotarod test) nor paw skin temperature. We next evaluated the behavioral effects of intrathecal administration of NPY and NPY antagonists with the aim of assessing the contribution of NPY to correlates of persistent nociception associated with the unilateral plantar injection of carrageenan or complete Freund's adjuvant (CFA). NPY robustly and dose-dependently increased PWL on the side ipsilateral to carrageenan injection, with only a small effect on the contralateral side. Similarly, NPY (30 microg) produced a large and long-lasting increase in PWL on the side ipsilateral to CFA injection (140% change), with only a small effect on the contralateral side (25% change). The ipsilateral effect of NPY was completely inhibited with the potent Y1 antagonist, BIBO 3304 (3 microg), but not the Y2 antagonist, BIIE 0246. When administered alone, BIBO 3304 (but not BIIE 0246) slightly decreased thermal PWL on the side ipsilateral (25% change), but not contralateral, to CFA injection; this suggests that inflammation strengthens inhibitory NPY tone. We conclude that spinal Y1 receptors contribute to the inhibitory effects of NPY on thermal hypersensitivity in the awake rat. Further studies are necessary to determine whether enhanced release of NPY and Y1-mediated inhibition of spinal nociceptive transmission ultimately results in a compensatory, adaptive inhibition of thermal hypersensitivity in the setting of inflammation.

Neuropeptide Y in trigeminal ganglion following chronic constriction injury of the rat infraorbital nerve: is there correlation to somatosensory parameters?

The aim of this study was to investigate neuropeptide Y (NPY) levels in trigeminal ganglia following infraorbital nerve injury. Two experimental procedures were performed in three groups of rats: a unilateral chronic constriction injury (CCI) to the infraorbital nerve (n=13), nerve manipulation without CCI (n=13) and unoperated controls (n=8). All rats underwent baseline and regular assessment of mechanical withdrawal threshold (Von Frey) and reaction to pin prick as well as free behavior evaluations. CCI to the infraorbital nerve induced significant hyperalgesia and allodynia within 9-12 days. At 6 days seven rats were euthanized and trigeminal ganglia harvested for immunocytochemical (ICC) studies. The study was ended at 14 days when all rats were euthanized and their ganglia harvested for ICC and radioimmunoassay (RIA) studies. An increase in NPY levels was seen in the ipsilateral ganglia of manipulated and CCI rats at 6 days, when rats displayed no pain-related behavior. At 14 days, ICC and RIA both detected significant increases in NPY levels in the ipsilateral ganglia of CCI and manipulated rats but not in unoperated controls. The possible roles of NPY in pain modulation and nerve injury are discussed in light of these findings.

Reduced antinociception and plasma extravasation in mice lacking a neuropeptide Y receptor

Neuropeptide Y (NPY) is believed to exert antinociceptive actions by inhibiting the release of substance P and other 'pain neurotransmitters' in the spinal cord dorsal horn. However, the physiological significance and potential therapeutic value of NPY remain obscure. It is also unclear which receptor subtype(s) are involved. To identify a possible physiological role for the NPY Y1 receptor in pain transmission, we generated NPY Y1 receptor null mutant (Y1-/-) mice by homologous recombination techniques. Here we show that Y1-/- mice develop hyperalgesia to acute thermal, cutaneous and visceral chemical pain, and exhibit mechanical hypersensitivity. Neuropathic pain is increased, and the mice show a complete absence of the pharmacological analgesic effects of NPY. In the periphery, Y1 receptor activation is sufficient and required for substance P release and the subsequent development of neurogenic inflammation and plasma leakage. We conclude that the Y1 receptor is required for central physiological and pharmacological NPY-induced analgesia and that its activation is both sufficient and required for the release of substance P and initiation of neurogenic inflammation.

Immunohistochemical localization of the neuropeptide Y Y1 receptor in rat central nervous system

The diverse effects of neuropeptide Y (NPY) are mediated through interaction with G-protein coupled receptors. Pharmacological analysis suggests the Y1 receptor mediates several of NPY's central and peripheral actions. We sought to determine the distribution of Y1 protein throughout the rat central nervous system by means of indirect immunofluorescence using the tyramide signal amplification method and a novel, amino terminally-directed Y1 antisera. This antisera was verified as specific for Y1 by solution-phase competition ELISA, Western blot and in situ blocking experiments. High concentrations of Y1 immunoreactivity were found in the claustrum, piriform cortex (superficial layer), arcuate hypothalamic nucleus, interpeduncular nucleus, paratrigeminal nucleus, and lamina II of the spinal trigeminal nucleus and entire spinal cord. Moderate levels of Y1 immunoreactivity were found the in the main olfactory bulb, dorsomedial part of suprachiasmatic nucleus, paraventricular hypothalamic nucleus, ventral nucleus of lateral lemniscus, pontine nuclei, mesencephalic trigeminal nucleus, external cuneate nucleus, area postrema, and nucleus tractus solitarius. Low levels of Y1 immunostaining were distributed widely throughout layers II-III of the cerebral cortex (i.e., orbital, cingulate, frontal, parietal, insular, and temporal regions), nucleus accumbens core, amygdalohippocampal and amygdalopiriform areas, dentate gyrus, CA1 and CA2 fields of hippocampus, principal and oral divisions of the spinal trigeminal nucleus, islands of Calleja and presubiculum. These findings are discussed with reference to previously reported receptor autoradiography, immunohistochemistry and mRNA analyses to further support the role of Y1 in NPY-mediated biology.

K(+)-Evoked [(3)H]D-aspartate release in rat spinal cord synaptosomes: modulation by neuropeptide Y and calcium channel antagonists

This study was conducted to investigate mechanisms regulating the release of [(3)H]D-aspartate (or endogenous glutamate) in the rat spinal cord. Presynaptic modulation of glutamate release was studied in superfused synaptosomes depolarized with 20 mM KCl. Calcium-channel antagonists, omega-conotoxin GVIA (omega-CgTx GVIA; N-type), nifedipine (L-type), and omega-conotoxin MVIIC (omega-CmTx MVIIC; P/Q type), were used to characterize the voltage-operated Ca(2+) channels (VOCCs) involved in this release. Nifedipine had no significant effect on the K(+)-evoked release of [(3)H]D-aspartate, but the omega-conotoxins GVIA and MVIIC produced dose-dependent inhibitory effects that were additive. The most substantial reduction (54.30% +/- 4.40%) was seen with omega-CgTx GVIA, indicating that N-type channels play a major role in the release of glutamate in this tissue. We investigated the effects of neuropeptide Y (NPY), NPY(13-36), and [Leu(31)][Pro(34)]NPY on Ca(2+)-dependent, K(+)-evoked [(3)H]D-aspartate release. NPY and NPY(13-36) equipotently inhibited the release of glutamate in a concentration-dependent manner. The half-maximal response was observed at about 12 nM; maximal inhibition of 44.22% +/- 4.60% was achieved with 0.3 microM. The selective GABA(B) agonist (-)baclofen inhibited K(+)-evoked [(3)H]D-aspartate release from superfused spinal cord synaptosomes by 50.00% +/- 4.80% at 10 microM. When NPY(13-36) and (-)baclofen were used together at maximal doses, their release-inhibiting effects were not additive. In addition, neither of the agonists was able to enhance the inhibition produced by pretreating the synaptosomes with the selective inhibitor of N-type VOCCs omega-CgTx GVIA. These results are consistent with the hypothesis that presynaptic Y(2)-like and GABA(B) receptors regulate glutamate release by blocking Ca(2+) currents through N-type VOCCs. Characterization of the receptors that can inhibit the release of glutamate may provide useful information for treatment of conditions characterized by excessive glutamatergic transmission in the spinal cord.

Neuropeptide- and tyrosine hydroxylase-immunoreactive nerve fibers in painful Morton's neuromas

We examined the expression of three neuropeptides that have been implicated in nociceptive transmission, and the sympathetic nerve fiber marker tyrosine hydroxylase, in 11 painful human Morton's neuromas, using immunohistochemistry. Antibodies against the neural markers RT97 and PGP 9.5 were used to map the general nerve fiber organization of the neuromas. Four specimens of normal human peripheral nerves were used as controls. Substance P, calcitonin gene-related peptide, and neuropeptide Y immunoreactivities were more pronounced in neuroma tissue than in control nerves. Neuropeptide immunofluorescence was seen both in larger nerve fiber trunks and in masses of disorganized axon profiles dispersed in loose connective tissue. Tyrosine hydroxylase immunoreactivity was present at varying levels of expression in neuroma nerve fiber trunks, in connective tissue nerve fiber bundles, and around some blood vessels. Our findings suggest that neuropeptides are involved in the response to injury in Morton's neuromas and that they could play a role in initiation or modulation of pain. In addition, pain from Morton's neuromas could be influenced by sympathetic nerve fibers.

Partial sciatic nerve ligation induced more dramatic increase of neuropeptide Y immunoreactive axonal fibers in the gracile nucleus of middle-aged rats than in young adult rats

Neuropeptide changes in primary sensory neurons caused by partial nerve injury are likely involved in the development of neuropathic pain. In this study, using immunocytochemistry, we examined neuropeptide Y (NPY) expression in lumbar dorsal root ganglion (DRG) cells of young adult (2-3 months old) and middle-aged (8-10 months old) rats 4 weeks after partial sciatic nerve ligation (PSNL). Significantly higher NPY immunoreactivity was induced in the injured side DRG neurons, the dorsal horn and the gracile nuclei in middle-aged rats than in young rats. Using combined fluorescent dye tracing and NPY immunostaining, we found in middle-aged rats that 46% injured DRG neurons projected to the gracile nucleus and 45% of injured neurons were also NPY-IR, whereas 42% spared DRG neurons projected to the gracile nucleus and 18% of spared neurons were also NPY-IR. Thus PSNL induces NPY up-regulation in spared as well as injured DRG neurons, both contribute to the increased NPY immunoreactivity in the gracile nucleus in the middle-aged rats. The more dramatic increase of NPY in DRG neurons of middle-aged rats after PSNL shows that the responses to partial nerve injury are age-dependent, that suggests a possible relevance to the higher incidence of neuropathic pain in human middle age.

Effect of axotomy on expression of NPY, galanin, and NPY Y1 and Y2 receptors in dorsal root ganglia and the superior cervical ganglion studied with double-labeling in situ hybridization and immunohistochemistry

Using double-labeling techniques for both in situ hybridization and immunohistochemistry some peptides and peptide receptors were studied quantitatively in a sensory and a sympathetic ganglion after axotomy. In the lumbar 5 dorsal root ganglion (DRG) normally no neuropeptide Y- and only a few galanin-positive cell bodies are seen. Following complete transection of the sciatic nerve around 60% of all neuropeptide Y (NPY) neuron profiles (NPs) were galanin positive (+) and 33-44% of all galanin NPs were NPY(+). A good agreement between immunohistochemistry and in situ hybridization was observed for NPY and galanin. NPY Y1- and Y2-receptor (R) mRNAs were found in around 40% of all NPY mRNA(+) NPs, and more than half of the Y1-R mRNA(+) NPs and two-thirds of the Y2-R mRNA(+) NPs were NPY(+). In addition, more than one-third of the galanin mRNA-containing NPs showed colocalization with NPY receptor mRNAs and up to 70% of the Y2-R mRNA(+) NPs also expressed galanin mRNA. In the control superior cervical ganglion (SCG) 10% of the NPY(+) NPs were Y2-R mRNA(+), and 85% of the Y2-R(+) NPs were NPY mRNA(+), and the corresponding percentages after axotomy were around 35 and 45%, respectively. Following axotomy of the carotid nerves around half of all NPY(+) NPs were galanin(+), and conversely around 50% of all galanin NPs were NPY(+) at the mRNA level, whereas much lower percentages (15 and 9%, respectively) were observed with immunohistochemistry. These results demonstrate that double-labeling procedures are valid tools to quantitatively evaluate coexistence situations in sensory and sympathetic ganglia, showing a high degree of coexistence for NPY and galanin in axotomized neurons both in the lumbar 5 DRG and in the SCG. However, the immunohistochemical analysis in the SCG demonstrated much lower numbers of peptide-positive neurons than seen with in situ hybridization, suggesting that the latter technique is more sensitive. The fact that a considerable number of neurons express NPY together with Y1- and/or Y2-Rs indicates that both receptors may act as autoreceptors, the Y1-R presumably at the level of the cell body and the Y2-R on nerve terminals in the dorsal horn and/or the periphery. The present results also show that in both sensory and sympathetic neurons there is a strong upregulation of the Y2-R after nerve injury, suggesting a possible role in trophic and regenerative events.

Regulation of galanin and neuropeptide Y in dorsal root ganglia and dorsal horn in rat mononeuropathic models: possible relation to tactile hypersensitivity

The expression of galanin and neuropeptide Y in rat lumbar 5 (L5) dorsal root ganglia and dorsal horn (L4-5) was studied after four types of peripheral nerve injury using immunohistochemistry and in situ hybridization. The possible correlation between these two peptides and tactile allodynia-like behaviour was analysed as well. The models employed were the Gazelius (photochemical lesion) and Seltzer and Bennett (constriction lesions) models, as well as complete sciatic nerve transection (axotomy). Two weeks after surgery, the Gazelius model rats more frequently displayed a greater tactile allodynia than the rats from the Seltzer and Bennett models. Tactile allodynia was not observed in any of the axotomized rats. A marked increase in the number of galanin-immunoreactive and galanin messenger RNA-positive neuron profiles was observed in ipsilateral dorsal root ganglia in all types of models. The increase in allodynic rats (Gazelius, Seltzer and Bennett models) was less pronounced than that after axotomy. In addition, in the Bennett model the number of galanin-immunoreactive neurons was significantly lower in allodynic rats as compared to non-allodynic rats, and the same tendency, but less obvious was found in the Seltzer model. Furthermore, an increase in galanin-immunoreactive fibres was found in the superficial laminae of the ipsilateral dorsal horn in all lesion models, especially in lamina II. A dramatic increase in the number of neuropeptide Y and neuropeptide Y messenger RNA-positive neuron profiles was also found in the ipsilateral dorsal root ganglia in all models, but no significant difference was found in peptide levels between allodynic and non-allodynic rats in any of the models. The present results suggest that the levels of endogenous galanin may play a role in whether or not allodynia develops in the Bennett model.

The effect of intrathecal selective agonists of Y1 and Y2 neuropeptide Y receptors on the flexor reflex in normal and axotomized rats

We have examined the effects of intrathecal (i.t.) administration of [Leu31,Pro34]-neuropeptide Y (NPY) or NPY-(13-36), selective agonists of NPY Y1 or Y2 receptors, respectively, on the excitability of the flexor reflex in normal rats and after unilateral transection of the sciatic nerve. In rats with intact and sectioned sciatic nerves, i.t. [Leu31,Pro34]-NPY induced a similar biphasic effect on the flexor reflex with facilitation at low doses and facilitation followed by depression at high doses. In contrast, i.t. NPY-(13-36) only facilitated the flexor reflex in normal rats, and at high dose it caused ongoing discharges in the electromyogram. NPY-(13-36) caused dose-dependent depression of the flexor reflex in rats after sciatic nerve transection, in addition to its facilitatory effect. Topical application of [Leu31,Pro34]-NPY or NPY-(13-36) caused a moderate and brief reduction in spinal cord blood flow. No difference was noted between the vasoconstrictive effect of [Leu31,Pro34]-NPY and NPY-(13-36). It is suggested that activation of Y1 receptors may be primarily responsible for the reflex depressive effect of i.t. neuropeptide Y in rats with intact sciatic nerves, whereas both Y1 and Y2 receptors may be involved in mediating the depressive effect of NPY after axotomy.

The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons

Using indirect immunofluorescence, neuropeptide Y Y1 receptor (Y1 receptor)-like immunoreactivity (LI) was localized close to the plasmalemma of small neurons in lumbar dorsal root ganglia (DRGs) and neurons in the inner lamina II of the lumbar spinal cord of the rat. Using confocal microscopy, colocalization of Y1 receptor-LI and transferrin receptor-LI, a marker for endosomes and coated vesicles, was observed in dot-like structures along the plasmalemma. Under the electron microscope, Y1 receptor-LI was localized in coated vesicles and endosomes, in the membrane of tubular cisternae, sometimes connected to multivesicular bodies, and in the plasmalemma. These complex distribution patterns may reflect receptor turnover and internalization processes. In the lamina II of the spinal dorsal horn, Y1 receptor-LI was localized in the plasmalemma of neurons without any apparent association with paramembrane structures, as described above for the DRG neurons. Many dendrites were Y1 receptor-positive, and some of them made synaptic contacts with unstained axonal terminals. In general, Y1 receptor-LI was localized in the membrane outside the postsynaptic density. Double-immunofluorescence staining showed that most Y1 receptor-immunoreactive neurons in lamina II contained somatostatin-LI. Both in DRG and dorsal horn neurons, the Y1 receptor thus seems to represent a postjunctional/postsynaptic receptor.

Neuropeptides, nitric oxide synthase and GAP-43 in B4-binding and RT97 immunoreactive primary sensory neurons: normal distribution pattern and changes after peripheral nerve transection and aging

We have here sought to cross-correlate the expression of immunoreactivities for several neuropeptides, nitric oxide synthase (NOS) and the growth associated protein GAP-43 in subpopulations of dorsal root ganglion (DRG) neurons tagged by the selective markers isolectin B4 and the neurofilament antibody RT97, selective for, respectively, subpopulations of small and large DRG neurons. By use of double- and triple-labeling immunohistochemistry, non-manipulated and sciatic nerve transected young adult rats as well as aged (30-months-old) rats were examined using a confocal microscope equipped with enhanced spectral separation. In young adult rats, the DRG neuron profiles could be divided into three subpopulations (B4 binding (B4+) approximately 50%; RT97-immunoreactive (RT97+) approximately 35%; B4-/RT97- approximately 15%). Calcitonin gene-related peptide (CGRP) is expressed in all three subpopulations. Galanin message-associated peptide (GMAP) colocalize with CGRP (100%) but is not expressed in RT97+ profiles. NOS is present in the RT97- subpopulations and frequently colocalize with CGRP (92%). GAP-43 is expressed in all three DRG subpopulations and colocalize with CGRP (88%), GMAP (38%) and/or NOS (22%). Only very small differences were seen among the young adult rats, implicating that the size of respective subpopulation as well as the expression pattern for neuropeptides, NOS and GAP-43 are fairly stable. Sciatic nerve transection reduced B4-binding but not RT97-like immunoreactivity. Distinct changes in the expression of neuropeptides, NOS and GAP-43 were evident in the DRG subpopulations and, furthermore, the regulatory changes were very similar among the lesioned animals. The relative size of the DRG subpopulations was unaffected by aging, while the expression of neuropeptides was altered showing similarities with the changes induced by axotomy in young adult rats.

GABAergic neurons that contain neuropeptide Y selectively target cells with the neurokinin 1 receptor in laminae III and IV of the rat spinal cord

Neuropeptide Y (NPY) is contained in a population of GABAergic interneurons in the spinal dorsal horn and, when administered intrathecally, can produce analgesia. We previously identified a strong monosynaptic link between substance P-containing primary afferents and cells in lamina III or IV with the neurokinin 1 (NK1) receptor. Because some of these cells belong to the spinothalamic tract, they are likely to have an important role in pain mechanisms. In this study, we used confocal microscopy to examine the input to lamina III/IV NK1 receptor-immunoreactive neurons from NPY-containing axons. All of the cells studied received a dense innervation from NPY-immunoreactive axons, and electron microscopy revealed that synapses were often present at points of contact. Most NPY-immunoreactive boutons were also GABAergic, which supports the suggestion that they are derived from local neurons. The association between NPY-containing axons and NK1 receptor-immunoreactive neurons was specific, because postsynaptic dorsal column neurons (which were located in laminae III-V but did not possess NK1 receptors) and lamina I neurons with the NK1 receptor received significantly fewer contacts from NPY-immunoreactive axons. In addition, the NK1 receptor-immunoreactive lamina III/IV cells received few contacts from nitric oxide synthase-containing axons (which belong to a different population of GABAergic dorsal horn neurons). The NPY-containing axons appeared to be targeted to the NK1 receptor-immunoreactive neurons themselves rather than to their associated substance P-immunoreactive inputs. The dense innervation of these cells by NPY-containing axons suggests that they may possess receptors for NPY and that activation of these receptors may contribute to NPY-mediated analgesia.

The effect of intrathecal neuropeptide Y on the flexor reflex in rats after carrageenan-induced inflammation

We examined the effects of intrathecal (i.t.) administration of neuropeptide Y (NPY) on the excitability of the flexor reflex in normal rats and 24 h after inflammation induced by subcutaneous carrageenan. In normal rats, i.t. NPY at low doses (10 and 100 ng) caused a brief facilitation of the flexor reflex with no subsequent depression. At higher doses (1 and 10 microg), the effect of NPY was mainly inhibitory, causing substantial and usually prolonged depression of the flexor reflex. At 24 h after the injection of carrageenan, when inflammation was at its peak, the magnitude of the reflex was increased and discharge duration became prolonged. I.t. NPY produced similar pattern of dose-dependent facilitatory and depressive effects on the flexor reflex. The facilitatory effect of i.t. NPY, particularly for the higher doses, was significantly enhanced in inflamed rats compared to normals. In contrast, the depressive effect of high doses of i.t. NPY was unchanged. These data suggest that the changes in levels of NPY and NPY receptors in the spinal cord known to occur after inflammation, are associated with an increased excitatory effect of this peptide.

The release of immunoreactive neuropeptide Y in the spinal cord of the anaesthetized rat and cat

The release of immunoreactive (ir-) neuropeptide Y (NYP) was studied in the anaesthetized rat and cat by means of microprobes bearing immobilized antibodies to the C terminus of NPY. An extensive basal release of ir-NYP was detected throughout the dorsal and upper ventral horn of the rat. This spontaneous release was not significantly altered by sectioning the spinal cord at the thoraco-lumbar junction nor by electrical stimulation of peripheral nerves. Since NPY is virtually absent in primary afferents it is probable that spontaneous release within the spinal cord comes from active NPY-containing intrinsic spinal neurones. In the spinal cat spontaneous release of ir-NPY was detected in the mid-dorsal horn and this was unaltered by peripheral noxious thermal or noxious mechanical stimuli. As in the rat, release from intrinsic spinal neurones is most probable. The extensive spontaneous release of ir-NPY in both species suggests a widespread role in spinal cord function.

Intrathecal neuropeptide Y exacerbates nerve injury-induced mechanical hyperalgesia

In normal animals, spinal administration of neuropeptide Y induces analgesia to thermal stimuli, but has no effect on mechanical thresholds. Recent anatomical studies, however, have shown that following nerve injury there is an altered expression of neuropeptide Y and its receptors. The aim of this behavioural study, therefore, is to examine the effect of intrathecal administration of neuropeptide Y, its agonists and an antagonist on mechanical nociceptive thresholds in rats with partial injury to the sciatic nerve. Test agents were administered for 14 days via osmotic pumps (0.5 microliter/day) attached to intrathecal catheters and the nociceptive flexion reflex was quantified using an Ugo Basile Analgesymeter. Partial injury to the sciatic nerve, in animals treated intrathecally with saline, induces a significant decrease in mechanical threshold as compared to the sham operated, contralateral paw. The nerve injury-induced hyperalgesia is exacerbated by 2 microM neuropeptide Y and by 2 microM [Leu31,Pro34]-neuropeptide Y, a Y1 receptor agonist. The Y2 receptor agonist, N-acetyl-[Leu28,Leu31]-neuropeptide Y24-36 (2 microM), had no effect on the nerve injury-induced hyperalgesia. The putative neuropeptide Y antagonist, alpha-trinositol (10 microM), significantly attenuated the nerve injury-induced hyperalgesia. This study suggests that neuropeptide Y may contribute to nerve injury-induced mechanical hyperalgesia via the Y1 receptor and provides further insight into the possible mechanisms underlying nerve injury-induced hyperalgesia to mechanical stimuli.

Expression and regulation of the neuropeptide Y Y2 receptor in sensory and autonomic ganglia

The Y2 subtype of neuropeptide tyrosine (NPY) receptors (Y2R) and some neuropeptides have been studied with in situ hybridization in sensory and autonomic neurons of rat and monkey. Between 10% and 20% of the lumbar dorsal root ganglion (DRG) neuron profiles (NPs) contain Y2R mRNA in the rat and monkey. In rat DRGs Y2R mRNA is expressed in calcitonin gene-related peptide (CGRP)-positive, medium-sized, and large neurons, that is in a complementary fashion to the Y1R that is located in small CGRP neurons. In monkey DRGs Y2R mRNA is expressed mainly in small neurons. Peripheral axotomy up-regulates the Y2R in small and large DRG neurons in both species. Y2R and NPY mRNAs are colocalized in many large neurons in axotomized rat DRGs. Y2R mRNA is expressed in 50% of the NPs in the nodose ganglion with a modest increase after axotomy. Y2R mRNA is detected in a few NPs in normal rat superior cervical ganglia, with a marked increase after transection of the carotid nerves. No Y2R mRNA-positive, but many (approximately 30%) weakly Y1R mRNA-positive NPs were found in the sphenopalatine ganglion. Finally, Y2R mRNA levels increase in rat spinal motoneurons after axotomy. Thus, under normal circumstances NPY may act on Y1 and Y2Rs expressed, respectively, in small and large CGRP-positive DRG neurons in the rat. Y2R may be an important receptor in the viscero-sensory neurons. Y2Rs may be particularly important after axotomy serving as presynaptic and/or autoreceptors on rat DRG, superior cervical ganglion, and nodose ganglion neurons and as presynaptic receptors in monkey DRG neurons.

Regulation of VIP and other neuropeptides by c-Jun in sensory neurons: implications for the neuropeptide response to axotomy

Peripheral axotomy of adult rat sensory neurons causes induction of the transcription factor c-Jun and increased expression of the neuropeptides vasoactive intestinal polypeptide (VIP), galanin and neuropeptide Y. To determine whether VIP induction is dependent on transcriptional regulation by c-Jun, we exploited the fact that c-Jun and VIP are also induced in cultured sensory neurons. We blocked c-Jun synthesis by microinjecting antisense oligonucleotides and found that VIP expression, determined by quantitative immunofluorescence, was specifically reduced. Blockade of c-June expression also resulted in reduced neuropeptide Y expression but left galanin, substance P and calcitonin gene-related peptide unaffected. Since in vitro electrophoretic mobility shift assays showed that a nominal cyclic AMP responsive element (CRE) associated with the rat VIP gene could bind c-Jun-containing transcription factor complexes, we next investigated whether VIP expression in sensory neurons might depend on transcription factor binding to the CRE. When a DNA plasmid containing multiple copies of the CRE was injected into newly cultured sensory neurons to sequester transcription factors binding the endogenous CRE, there was a selective reduction in VIP expression. VIP induction in sensory neurons therefore probably results from transcriptional activation by c-Jun acting in combination with other factor(s), possibly acting through the CRE. These results show that c-Jun can regulate transcription of other genes affected by axotomy and imply that it could be a key regulator of the neuronal axotomy response.

Antinociceptive effects of neuropeptide Y and related peptides in mice

This study compares the antinociceptive and orexigenic activities of NPY and analogs after intracerebroventricular administration in mice. NPY had an antinociceptive action in the mouse writhing test which was not affected by prior treatment with naltrexone, yohimbine, idazoxan or reserpine. A detailed examination revealed that NPY (0.023-0.7 nmol), PYY (0.007-0.07 nmol), NPY2-36 (0.023-0.23 nmol) and the Y1 agonist [Leu31, Pro34]-NPY (0.07-0.7 nmol) all produced a dose-dependent and complete suppression of acetic acid-induced writhing. In contrast, the Y2 agonist, NPY13-36, had little or no antinociceptive effect. As shown by their ED50 values, the relative potency of the peptides was PYY > NPY2-36 > or = NPY > [Leu31, Pro34]-NPY > > NPY13-36, suggesting that a Y1 rather than a Y2 or Y3 receptor subtype was implicated in the antinociceptive action. Thereafter, all peptides were assessed for their effects on food intake. With respect to dose and peptide specificity, the hyperphagic effects of NPY and related peptides paralleled those on nociception, suggesting a common receptor mechanism. However, a purported NPY antagonist, [D-Trp32]-NPY, attenuated NPY's effect on feeding yet this same peptide elicited a dose-dependent inhibition of acetic acid-induced writhing, suggesting some molecular distinction between antinociception and stimulation of food intake.

Intrathecal pituitary adenylate cyclase activating polypeptide facilitates the spinal nociceptive flexor reflex in the rat

We have examined the effects of intrathecal (i.t.) pituitary adenylate cyclase activating polypeptide on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanaesthetized rats. The flexor reflex was elicited by electrical stimulation applied subcutaneously to the sural nerve innervation area and recorded as electromyogram activity from ipsilateral hamstring muscles. Pituitary adenylate cyclase activating polypeptide(l-27) was administered over a wide dose range (10 ng to 10 mu g) and elicited a dose-dependent facilitation of the flexor reflex and did not depress the reflex at any dose. Furthermore, pituitary adenylate cyclase activating polypeptide did not inhibit the facilitation of the flexor reflex induced by repetitive stimulation of C-fibres. It is concluded that pituitary adenylate cyclase activating polypeptide had an excitatory effect on spinal cord function which may indicate a role for this peptide in nociceptive transmission and modulation. Moreover, in contrast to previous studies, we found no evidence suggesting that pituitary adenylate cyclase activating polypeptide exerts antinociceptive action at spinal level.

Effect on nociception of intracerebroventricular administration of low doses of neuropeptide Y in mice

The present study shows further evidence about the implication of neuropeptide Y (NPY) in nociception. The effect of NPY (1-36), when intracerebroventricularly administered, has been studied using two physical models of acute pain (hot plate test and electrical tail stimulation) and the formalin test. The animal response to these three pain models has been shown to be integrated at different levels in the CNS. A decrease in pain threshold was exhibited in both the hot plate test (10, 30, 60, 120 and 480 pmol of NPY i.c.v.) and the electrical tail simulation test (10, 30 and 60 pmol of NPY i.c.v.), while in the formalin test (10, 30, 60 and 120 pmol of NPY icv) the licking response decreased in phase I but not in phase 2. In these three tests NPY showed hyperalgesic or analgesic effects when administered at low doses, while at high doses it failed to induce any effect. Results show that the effect of NPY on nociception is clearly test-dependent and is only observed at low doses.

Exogenous NT-3 mitigates the transganglionic neuropeptide Y response to sciatic nerve injury

Peripheral sciatic nerve transection injury produces a marked increase in immunoreactive NPY in rat lumbar DRG and, as a 'transganglionic' response, in their central axonal projections to the dorsal spinal cord and gracile nuclei. Local application of neurotrophin-3 (NT-3) using implanted silicone chambers applied to the proximal transected sciatic nerve stump significantly diminished the transganglionic NPY response in the gracile nucleus and spinal cord but did not affect the CGRP transganglionic response. Conversely, local application of NGF affected the transganglionic CGRP response to axotomy but did not substantially affect transganglionic gracile NPY upregulation.