Receptor endocytosis and dendrite reshaping in spinal neurons after somatosensory stimulation

Noxious mechanical stimulation evokes the segmental release of opioid peptides that induce mu-opioid receptor internalization in the presence of peptidase inhibitors

The internalization of mu-opioid receptors (MORs) provides an ideal way to locate areas of opioid peptide release. We used this method to study opioid release in the spinal cord evoked by noxious stimuli in anesthetized rats. Previous studies have shown that opioids released in the spinal cord produce MOR internalization only when they are protected from peptidase degradation. Accordingly, rats were implanted with chronic intrathecal catheters that were used to inject a mixture of peptidase inhibitors (amastatin, captopril and phosphoramidon) onto the lumbar spinal cord. Five minutes later, a noxious stimulus was delivered to the paw. Lumbar spinal segments were double-stained with antibodies against MORs and neurokinin 1 receptors (NK1Rs) using immunofluorescence. Mechanical stimulation of the hindpaw consisted of repeated 10 s clamps with a hemostat for 10 min. In the ipsilateral dorsal horn, the stimulus produced abundant NK1R internalization in segments L3-L6, and a more modest but significant MOR internalization in segments L5 and L6. In the contralateral dorsal horn, NK1R was substantially lower and MOR internalization was negligible. The same mechanical stimulus applied to a forepaw did not produce NK1R or MOR internalization in the lumbar spinal cord. Thermal stimulation consisted of immersing a hindpaw in water at 52 degrees C for 2 min. It produced substantial NK1R internalization ipsilaterally in segment L6, but no MOR internalization. These results show that mechanical stimulation induces segmental opioid release, i.e., in the dorsal horn receiving the noxious signals and not in other spinal segments.

A population of large neurons in laminae III and IV of the rat spinal cord that have long dorsal dendrites and lack the neurokinin 1 receptor

The dorsal horn of the rat spinal cord contains a population of large neurons with cell bodies in laminae III or IV, that express the neurokinin 1 receptor (NK1r) and have long dorsal dendrites that branch extensively within the superficial laminae. In this study, we have identified a separate population of neurons that have similar dendritic morphology, but lack the NK1r. These cells also differ from the NK1r-expressing neurons in that they have significantly fewer contacts from substance P-containing axons and are not retrogradely labelled following injection of tracer into the caudal ventrolateral medulla. We also provide evidence that these cells do not belong to the postsynaptic dorsal column pathway or the spinothalamic tract. It is therefore likely that these cells do not have supraspinal projections. They may provide a route through which information transmitted by C fibres that lack neuropeptides is conveyed to deeper laminae. The present findings demonstrate the need for caution when attempting to classify neurons solely on the basis of somatodendritic morphology.

Calcitonin receptor-like receptor and receptor activity modifying protein 1 in the rat dorsal horn: localization in glutamatergic presynaptic terminals containing opioids and adrenergic alpha2C receptors

Calcitonin gene-related peptide (CGRP) is abundant in the central terminals of primary afferents. However, the function of CGRP receptors in the spinal cord remains unclear. CGRP receptors are heterodimers of calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1). We studied the localization of CRLR and RAMP1 in the rat dorsal horn using well-characterized antibodies against them, which labeled numerous puncta in laminae I-II. In addition, RAMP1 was found in cell bodies, forming patches at the cell surface. The CRLR- and RAMP1-immunoreactive puncta were further characterized using double and triple labeling. Colocalization was quantified in confocal stacks using Imaris software. CRLR did not colocalize with primary afferent markers, indicating that these puncta were not primary afferent terminals. CRLR- and RAMP1-immunoreactive puncta contained synaptophysin and vesicular glutamate transporter-2 (VGLUT2), showing that they were glutamatergic presynaptic terminals. Electron microscopic immunohistochemistry confirmed that CRLR immunoreactivity was present in axonal boutons that were not in synaptic glomeruli. Using tyramide signal amplification for double labeling with the CRLR and RAMP1 antibodies, we found some clear instances of colocalization of CRLR with RAMP1 in puncta, but their overall colocalization was low. In particular, CRLR was absent from RAMP1-containing cells. Many of the puncta stained for CRLR and RAMP1 were labeled by anti-opioid and anti-enkephalin antibodies. CRLR and, to a lesser extent, RAMP1 also colocalized with adrenergic alpha(2C) receptors. Triple label studies demonstrated three-way colocalization of CRLR-VGLUT2-synaptophysin, CRLR-VGLUT2-opioids, and CRLR-opioids-alpha(2C) receptors. In conclusion, CRLR is located in glutamatergic presynaptic terminals in the dorsal horn that contain alpha(2C) adrenergic receptors and opioids. Some of these terminals contain RAMP1, which may form CGRP receptors with CRLR, but in others CRLR may form other receptors, possibly by dimerizing with RAMP2 or RAMP3. These findings suggest that CGRP or adrenomedullin receptors modulate opioid release in the dorsal horn.

The effect of high cervical spinal cord stimulation on the expression of SP, NK-1 and TRPV1 mRNAs during cardiac ischemia in rat

Spinal cord stimulation (SCS) is used to reduce angina that accompanies cardiac ischemia, but little is known about the molecular mechanisms mediating this effect. We studied the expression of SP, neurokinin-1 (NK-1) receptor, and transient receptor potential vanilloid type 1 (TRPV1) mRNA in the rat spinal cord at thoracic 4 (T4), cervical 2 (C2) and caudal brain stem by RT-PCR during intermittent occlusion of the left anterior descending coronary artery (CoAO), during sustained SCS by itself at the C2 spinal segment, and during sustained SCS plus intermittent CoAO. Only SP mRNA was increased significantly in T4 and brainstem during CoAO, while SCS decreased the mRNA levels of SP, NK-1 and TRPV1 significantly in T4 and the brainstem. SCS attenuated the increase of SP and TRPV1 mRNA levels at T4 level induced by intermittent CoAO when the stimulation was applied prior to the initiation of the cardiac ischemia. These results support the role for SP as a putative neurotransmitter for the myocardial ischemia-sensitive afferent neuron signal to the spinal level. They suggest that modification of the ischemic cardiac nociceptive afferent signal by SCS involves a change in SP and TRPV1 expression.

The role of Transient Receptor Potential Vanilloid 1 (TRPV1) channels in pancreatitis

Premature activation of digestive enzymes within the pancreas which leads to autodigestion of the gland is an early step in the pathogenesis of pancreatitis. Pancreatic injury is followed by other manifestations of inflammation including plasma extravasation, edema, and neutrophil infiltration which constitute the features of pancreatitis. Recent studies indicate that neural innervation of the pancreas may play an important role in the initiation and maintenance of the inflammatory response to injury. The pancreas is innervated by vagal, sympathetic and parasympathetic neurons, as well as sensory neurons. Activation of pancreatic primary sensory neurons causes the release of inflammatory neuropeptides both in the spinal cord to signal pain and in the pancreas itself where they produce plasma extravasation and neutrophil infiltration. Recent studies indicate that primary sensory neurons of the pancreas express transient receptor potential V1 (TRPV1) channels whose activation induces pancreatic inflammation. Moreover, blockade of these TRP channels significantly ameliorates experimental pancreatitis. This review describes our current understanding of the role of TRPV1 channels in pancreatitis and illustrates how this mechanism might be used to direct future treatments of pancreatic diseases.

830 nm laser irradiation induces varicosity formation, reduces mitochondrial membrane potential and blocks fast axonal flow in small and medium diameter rat dorsal root ganglion neurons: implications for the analgesic effects of 830 nm laser

We report the formation of 830 nm (cw) laser-induced, reversible axonal varicosities, using immunostaining with beta-tubulin, in small and medium diameter, TRPV-1 positive, cultured rat DRG neurons. Laser also induced a progressive and statistically significant decrease (p<0.005) in MMP in mitochondria in and between static axonal varicosities. In cell bodies of the neuron, the decrease in MMP was also statistically significant (p<0.05), but the decrease occurred more slowly. Importantly we also report for the first time that 830 nm (cw) laser blocked fast axonal flow, imaged in real time using confocal laser microscopy and JC-1 as mitotracker. Control neurons in parallel cultures remained unaffected with no varicosity formation and no change in MMP. Mitochondrial movement was continuous and measured along the axons at a rate of 0.8 microm/s (range 0.5-2 microm/s), consistent with fast axonal flow. Photoacceptors in the mitochondrial membrane absorb laser and mediate the transduction of laser energy into electrochemical changes, initiating a secondary cascade of intracellular events. In neurons, this results in a decrease in MMP with a concurrent decrease in available ATP required for nerve function, including maintenance of microtubules and molecular motors, dyneins and kinesins, responsible for fast axonal flow. Laser-induced neural blockade is a consequence of such changes and provide a mechanism for a neural basis of laser-induced pain relief. The repeated application of laser in a clinical setting modulates nociception and reduces pain. The application of laser therapy for chronic pain may provide a non-drug alternative for the management of chronic pain.

Activated somatostatin type 2 receptors traffic in vivo in central neurons from dendrites to the trans Golgi before recycling

Understanding the trafficking of G-protein-coupled receptors (GPCRs) is of particular importance, especially when modifications of the neurochemic environment occur as in pathological or therapeutic circumstances. In the central nervous system, although some GPCRs were reported to internalize in vivo, little is known about their trafficking downstream of the endocytic event. To address this issue, distribution and expression pattern of the major somatostatin receptor subtype, the somatostatin type 2 (sst2), was monitored in the hippocampus using immunofluorescence, autoradiographic and immunogold experiments from 10 minutes to 7 days after in vivo injection of the receptor agonist octreotide. We then analyzed whether postendocytic trafficking of the receptor was dependent upon integrity of the microtubule network using colchicine-injected animals. Together, our results suggest that upon agonist stimulation, dendritic receptors are retrogradely transported through a microtubule-dependent mechanism to a trans Golgi domain enriched in the t-SNARE syntaxin 6 and trans Golgi network 38 proteins, before recycling. Because we show that the exit rate from the trans Golgi apparatus back to the plasma membrane (hours) is slower than the entry rate (minutes), the neuronal postendocytic trafficking of sst2 receptor is likely to have functional consequences in several neurological diseases in which an increase in somatostatin release occurs.

Sensory neuron and substance P involvement in symptoms of a zymosan-induced rat model of acute bowel inflammation

Intestinal inflammation is a painful syndrome with multiple symptoms, including chronic pain. This study examined the possible role of sensory neurons and substance P in symptoms of an animal model of acute intestinal inflammation. The model was induced by injecting ethanol and zymosan into the colon of anesthetized male rats. Three hours later, sections of the colon were stained with hematoxylin and eosin. To determine the role of substance P, 5 mg/kg of the neurokinin-1 receptor (NK-1r) antagonist, CP-96,345, or 300 microg/kg of an antisense oligonucleotide targeted at NK-1r mRNA was administered. Spinal cord sections were examined for internalization of NK-1r, as an indicator of substance P release. Sections of colon revealed infiltration of inflammatory cells following ethanol and zymosan treatment. Plasma extravasation in rats given ethanol and zymosan was significantly greater than in controls given saline only (P<0.0001) or saline and ethanol (P<0.001). In ethanol- and zymosan-treated rats given CP-96,345, plasma extravasation was significantly less than in rats given ethanol and zymosan without the antagonist (P<0.0001). Administration of the antisense oligonucleotide also resulted in lower levels of plasma extravasation compared with controls (P<0.01). Internalization of the NK-1r was observed in neurons of lamina I in the T13-L2 and L6-S2 regions of the spinal cord, as well as in sympathetic preganglionic neurons at the L1 level. This internalization was observed in the absence of any other stimulus besides the inflammation itself. This study implicates substance P and its receptor, the NK-1r, in acute inflammation of the colon.

Ideas about pain, a historical view

The expression 'painful' can be used to describe both an embarrassing moment and a cut on the finger. An explanation for this dichotomy can be found in the convoluted history of ideas about pain. Whether pain is an independent sensation and the product of dedicated neural mechanisms continues to be a topic of debate. This overview concentrates on the issue of specificity together with other notable information regarding pain that has emerged since 1800.

Low-speed subcellular fractionation method for determining noxious stimulus-evoked spinal neurokinin-1 receptor internalization

Substance P release from nociceptive primary afferents activates post-synaptic neurokinin-1 (NK-1) receptors causing subsequent NK-1 receptor internalization. Fluorescent immunohistochemistry is typically used to quantify NK-1 receptor internalization, an indirect measure of substance P (SP) release. However, this technique entails several limitations that restrict its application. Using simple subcellular fractionation and immunoblotting methods, we demonstrate that intrathecal SP invokes a rapid and dose-dependent increase in dorsal horn cytoplasmic NK-1 receptors. We also show that hind paw compression and noxious thermal stimulation increase cytoplasmic NK-1 receptor, when compared to sham stimulations. Fluorescent immunohistochemistry confirmed that increases in cytoplasmic NK-1 corresponded with increased NK-1 receptor internalization. Herein, we report that low-speed centrifugation and Western immunoblotting provide NK-1 internalization results consistent with those obtained by more traditional methods. These data support previous findings demonstrating a role for spinal NK-1 receptors in nociceptive processing.

Introducing the External Link Model for Studying Spine Fixation and Misalignment

This is the first article in a series introducing a new animal model, the External Link Model that we propose will allow researchers to produce and study spine lesions with the cardinal biomechanical features of the chiropractic subluxation: fixation (hypomobility) and misalignment.

Upregulation of substance P in low-threshold myelinated afferents is not required for tactile allodynia in the chronic constriction injury and spinal nerve ligation models

It has been proposed that substance P and calcitonin gene-related peptide (CGRP) are upregulated in low-threshold myelinated primary afferents after certain types of nerve injury, and that release of substance P from these afferents contributes to the resulting tactile allodynia. To test this hypothesis, we looked for neuropeptides in Abeta primary afferent terminals in the ipsilateral gracile nucleus and spinal dorsal horn in three nerve injury models: sciatic nerve transection (SNT), spinal nerve ligation (SNL), and chronic constriction injury (CCI). We also looked for evidence of neurokinin 1 (NK1) receptor internalization in the dorsal horn after electrical stimulation of Abeta afferents. We found no evidence of either substance P or CGRP expression in injured Abeta terminals in the spinal cord in any of the models. Although substance P was not detected in terminals of injured afferents in the gracile nucleus, CGRP was expressed in between 32 and 68% of these terminals, with a significantly higher proportion in the SNL and CCI models, compared with SNT. In addition, we did not detect any Abeta-evoked NK1 receptor internalization in neurons from laminas I, III, or IV of the dorsal horn in the CCI or SNL models. These results do not support the proposal that substance P is present at significant levels in the terminals of injured Abeta primary afferents in neuropathic models. They also suggest that any release of substance P from injured Abeta afferents is unlikely to activate NK1 receptors in the dorsal horn or contribute to neuropathic pain.

Phosphorylation of ERK in neurokinin 1 receptor-expressing neurons in laminae III and IV of the rat spinal dorsal horn following noxious stimulation

BACKGROUND

There is a population of large neurons with cell bodies in laminae III and IV of the spinal dorsal horn which express the neurokinin 1 receptor (NK1r) and have dendrites that enter the superficial laminae. Although it has been shown that these are all projection neurons and that they are innervated by substance P-containing (nociceptive) primary afferents, we know little about their responses to noxious stimuli. In this study we have looked for phosphorylation of extracellular signal-regulated kinases (ERKs) in these neurons in response to different types of noxious stimulus applied to one hindlimb of anaesthetised rats. The stimuli were mechanical (repeated pinching), thermal (immersion in water at 52 degrees C) or chemical (injection of 2% formaldehyde).

RESULTS

Five minutes after each type of stimulus we observed numerous cells with phosphorylated ERK (pERK) in laminae I and IIo, together with scattered positive cells in deeper laminae. We found that virtually all of the lamina III/IV NK1r-immunoreactive neurons contained pERK after each of these stimuli and that in the great majority of cases there was internalisation of the NK1r on the dorsal dendrites of these cells. In addition, we also saw neurons in lamina III that were pERK-positive but lacked the NK1r, and these were particularly evident in animals that had had the pinch stimulus.

CONCLUSION

Our results demonstrate that lamina III/IV NK1r-immunoreactive neurons show receptor internalisation and ERK phosphorylation after mechanical, thermal or chemical noxious stimuli.

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 and kinin antagonists

Neuropeptides and kinins are important messengers in the nervous system and--on the basis of their anatomical localisation and the effects produced when the substances themselves are administered, to animals or to human subjects-a significant number of them have been suggested to have a role in pain and inflammation. Experiments in gene deletion (knock-out or null mutant) mice and parallel experiments with pharmacological receptor antagonists in a variety of species have strengthened the evidence that a number of peptides, notably substance P and calcitonin gene-related peptide (CGRP), and the kinins have a pathophysiological role in nociception. Clinical studies with non-peptide pharmacological antagonists are now in progress to determine if blocking the action of these peptides might have utility in the treatment of pain.

Capsaicin-evoked brain activation and central sensitization in anaesthetised rats: A functional magnetic resonance imaging study

Functional magnetic resonance imaging (fMRI) of blood oxygen level dependent (BOLD) haemodynamic responses was used to study the effects of the noxious substance capsaicin on whole brain activation in isofluorane anaesthetised rats. Rats (n=8) received intradermal injection of capsaicin (30 microg/5 microl), or topical cream (0.1%) capsaicin and BOLD responses were acquired for up to 120 min. Effects of capsaicin versus placebo cream treatment on the BOLD response to a 15 g mechanical stimulus applied adjacent to the site of cream application were also studied. Both injection and cream application of capsaicin activated brain areas involved in pain processing, including the thalamus and periaqueductal grey (PAG) (p<0.05, corrected for multiple comparisons). Capsaicin also produced increases in BOLD signal intensity in other regions that contribute to pain processing, such as the parabrachial nucleus and superior colliculus. Mechanical stimulation in capsaicin-treated rats, but not placebo-treated rats, induced a significant decrease in BOLD signal intensity in the PAG (p<0.001). These data demonstrate that the noxious substance capsaicin produces brain activation in the midbrain regions and reveals the importance of the PAG in central sensitization.

Neurokinin-1 and neurokinin-3 receptors in primate substantia nigra

Striatonigral axons co-release GABA and substance P (SP) at their target sites, but little is known about the action of SP at nigral level. Therefore, we studied immunohistochemically the cellular and subcellular localization of SP and its high affinity receptors neurokinin-1 (NK-1R) and neurokinin-3 (NK-3R) at nigral level in squirrel monkeys. Immunofluorescent studies revealed that, although SP+ fibers arborised more densely in the pars reticulata (SNr) than in the pars compacta (SNc), the two nigral divisions harbored numerous neurons expressing NK-1R and NK-3R. Confocal microscopic analyses showed that numerous SNr neurons and virtually all SNc dopaminergic neurons contained both NK-1R and NK-3R. At the electron microscope level, NK-1R and NK-3R were mainly associated with intracellular sites or located at extrasynaptic position on plasma membrane. A small proportion of SP+ boutons also showed NK-3R immunoreactivity. The distribution of NK-1R and NK-3R in SNr and SNc suggests that SP exerts its effect through postsynaptic receptors, as well as via presynaptic autoreceptors and heteroreceptors. These findings indicate that the excitatory peptide SP can modulate the inhibitory action of GABA at nigral level and suggest that the co-release of these two neuroactive substances should be taken into account when considering the functional organization of the basal ganglia.

Protease-activated receptor 2 sensitizes the transient receptor potential vanilloid 4 ion channel to cause mechanical hyperalgesia in mice

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR2) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR2-mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). Immunoreactive TRPV4 was coexpressed by rat dorsal root ganglia (DRG) neurons with PAR2, substance P (SP) and calcitonin gene-related peptide (CGRP), mediators of pain transmission. In PAR2-expressing cell lines that either naturally expressed TRPV4 (bronchial epithelial cells) or that were transfected to express TRPV4 (HEK cells), pretreatment with a PAR2 agonist enhanced Ca2+ and current responses to the TRPV4 agonists phorbol ester 4alpha-phorbol 12,13-didecanoate (4alphaPDD) and hypotonic solutions. PAR2-agonist similarly sensitized TRPV4 Ca2+ signals and currents in DRG neurons. Antagonists of phospholipase Cbeta and protein kinases A, C and D inhibited PAR2-induced sensitization of TRPV4 Ca2+ signals and currents. 4alphaPDD and hypotonic solutions stimulated SP and CGRP release from dorsal horn of rat spinal cord, and pretreatment with PAR2 agonist sensitized TRPV4-dependent peptide release. Intraplantar injection of PAR2 agonist caused mechanical hyperalgesia in mice and sensitized pain responses to the TRPV4 agonists 4alphaPDD and hypotonic solutions. Deletion of TRPV4 prevented PAR2 agonist-induced mechanical hyperalgesia and sensitization. This novel mechanism, by which PAR2 activates a second messenger to sensitize TRPV4-dependent release of nociceptive peptides and induce mechanical hyperalgesia, may underlie inflammatory hyperalgesia in diseases where proteases are activated and released.

Biology of Bone Cancer Pain

Bone cancer pain is a devastating manifestation of metastatic cancer. Unfortunately, current therapies can be ineffective, and when they are effective, the duration of the patient's survival typically exceeds the duration of pain relief. New, mechanistically based therapies are desperately needed. Study of experimental animal models has provided insight into the mechanisms that drive bone cancer pain and provides an opportunity for developing targeted therapies. Mechanisms that drive bone cancer pain include tumor-directed osteoclast-mediated osteolysis, tumor cells themselves, tumor-induced nerve injury, stimulation of transient receptor potential vanilloid type 1 ion channel, endothelin A, and host cell production of nerve growth factor. Current and future therapies include external beam radiation, osteoclast-targeted inhibiting agents, anti-inflammatory drugs, transient receptor potential vanilloid type 1 antagonists, and antibody therapies that target nerve growth factor or tumor angiogenesis. It is likely that a combination of these therapies will be superior to any one therapy alone.

The common carboxyl-terminal region of novel tachykinin peptides contributes to induce desensitization in scratching behavior of rats

Some novel tachykinin peptides exhibiting homology with known members of the tachykinin family have been recently reported; however, little is known about the function of these peptides. Repeated intrathecal administration of substance P (SP) causes desensitization by binding SP to neurokinin 1 (NK1) receptor. Thus, to clarify the characteristics of the receptors involved in these novel peptides, we investigated whether desensitization is induced by intrathecal administration of these peptides in rats since desensitization is induced by binding these peptides to the receptor. Intrathecal administration of 10(-3) M hemokinin-1 (HK-1) and 10(-3) M decapeptide common in the carboxyl-terminal region of endokinin A and endokinin B (EKA/B) as well as SP evoked scratching behavior. When each peptide was administered twice with an interval of 15 min, remarkable desensitization of scratching behavior was produced. Furthermore, the first administration of EKA/B or SP produced clear cross-desensitization to SP, EKA/B and HK-1, whereas the first administration of HK-1 demonstrated weak cross-desensitization to EKA/B and SP. These results suggest that EKA/B and SP may bind to both the NK1 receptor and HK-1-preferred receptor, and HK-1 may preferentially bind to its preferred receptor.

Receptor recycling mediates plasma membrane recovery of dopamine D1 receptors in dendrites and axons after agonist-induced endocytosis in primary cultures of striatal neurons

The pharmacological stimulation of G-protein-coupled receptor induces receptor internalization. Receptor's fate after the step of internalization remains poorly characterized despite its incidence on the neuronal responsiveness. In this context, we studied the dopamine (DA) D1 receptor (D1R) trafficking in a model of striatal neuronal culture that endogenously express the D1R. We first characterized by immunohistochemistry the spatial distribution of the compartments involved in the endocytic pathways and then the D1R trafficking in dendrites and axons. In dendrites, immunohistochemical analysis showed that acute stimulation by the D1R agonist SKF 82958 (1 microM) induces an internalization of D1R in early endosomes labeled with Alexa-488-conjugated transferrin. We show that, 20 min after removal of the agonist, the D1R immunolabeling pattern returns to the basal state in dendrites and in axons. Recovery was unaffected by cycloheximide (70 microM) but was prevented by monensin (100 microM) that inhibits endosomal acidification and receptor recycling. These data suggest that dendritic and axonal D1Rs are internalized after agonist stimulation and targeted to the recycling pathway demonstrating that the machinery involved in GPCR endocytosis and recycling is functional both in dendrites and in axons. Temporal characteristics observed for the recovery of D1R density to the basal state and those observed for the resensitization process strongly suggest that D1R recycling supports the receptor resensitization.

Local disruption of the celiac ganglion inhibits substance P release and ameliorates caerulein-induced pancreatitis in rats

Primary sensory neurons of the C and Adelta subtypes express the vanilloid capsaicin receptor TRPV1 and contain proinflammatory peptides such as substance P (SP) that mediate neurogenic inflammation. Pancreatic injury stimulates these neurons causing the release of SP in the pancreas resulting in pancreatic edema and neutrophil infiltration that contributes to pancreatitis. Axons of primary sensory neurons innervating the pancreas course through the celiac ganglion. We hypothesized that disruption of the celiac ganglion by surgical excision or inhibition of C and Adelta fibers through blockade of TRPV1 would reduce the severity of experimental pancreatitis by inhibiting neurogenic inflammation. Resiniferatoxin (RTX) is a specific TRPV1 agonist that, in high doses, selectively destroys C and Adelta fibers. Sprague-Dawley rats underwent surgical ganglionectomy or application of 10 microg RTX (vs. vehicle alone) to the celiac ganglion. One week later, pancreatitis was induced by six hourly intraperitoneal injections of caerulein (50 microg/kg). The severity of pancreatitis was assessed by serum amylase, pancreatic edema, and pancreatic myeloperoxidase (MPO) activity. SP receptor (neurokinin-1 receptor, NK-1R) internalization in acinar cells, used as an index of endogenous SP release, was assessed by immunocytochemical quantification of NK-1R endocytosis. Caerulein administration caused significant increases in pancreatic edema, serum amylase, MPO activity, and NK-1R internalization. RTX treatment and ganglionectomy significantly reduced pancreatic edema by 46% (P < 0.001) and NK-1R internalization by 80% and 51% (P < 0.001 and P < 0.05, respectively). RTX administration also significantly reduced MPO activity by 47% (P < 0.05). Neither treatment affected serum amylase, consistent with a direct effect of caerulein. These results demonstrate that disruption of or local application of RTX to the celiac ganglion inhibits SP release in the pancreas and reduces the severity of acute secretagogue-induced pancreatitis. It is possible that selectively disrupting TRPV1-bearing neurons could be used to reduce pancreatitis severity.

Cellular and subcellular localization of neurokinin-1 and neurokinin-3 receptors in primate globus pallidus

The primate globus pallidus receives massive innervations from GABAergic striatal neurons that co-release the neuropeptide substance P (SP). To expand our knowledge regarding SP interaction at pallidal level, we used single and double antigen retrieval methods to study the cellular and subcellular localization of SP and its high-affinity receptors neurokinin-1 (NK-1R) and neurokinin-3 (NK-3R) in the globus pallidus of the squirrel monkey (Saimiri sciureus). At the light microscopic level, a large number of neurons and fibers located in both the external (GPe) and internal (GPi) segments of the globus pallidus expressed NK-1R or NK-3R immunoreactivity. At the electron microscopic level, both NK-1R and NK-3R were mainly associated with intracellular sites or located at extrasynaptic positions on the plasma membrane. Presynaptic axon terminals forming symmetric and asymmetric synapses occasionally contained NK-1R and NK-3R. Neurokinin receptors were also observed in a proportion of SP-immunoreactive axon terminals, but these terminals preferentially expressed NK-3R. The pattern of distribution of NK-1R and NK-3R in GPe and GPi indicates that SP effects at pallidal level are mediated through postsynaptic receptor as well as presynaptic autoreceptors and heteroreceptors. These morphological data suggest that, either alone or in conjunction with GABA, SP could have a wide range of effects at pallidal level. This neuroactive peptide may influence in a significant manner the integration and treatment of neural information that flows through the basal ganglia.

The role of neurokinin 1 receptors in the maintenance of visceral hyperalgesia induced by repeated stress in rats

BACKGROUND & AIMS

The neurokinin 1 receptors (NK(1)Rs) and substance P (SP) have been implicated in the stress and/or pain pathways involved in chronic pain conditions. Here we examined the participation of NK(1)Rs in sustained visceral hyperalgesia observed in rats exposed to chronic psychological stress.

METHODS

Male Wistar rats were exposed to daily 1-hour water avoidance stress (WA) or sham WA for 10 consecutive days. We tested intraperitoneal or intrathecal injection of the NK(1)R antagonist SR140333 on the visceromotor reflex to colorectal distention in both groups at day 11. Real-time reverse-transcription polymerase chain reaction, Western blot, and immunohistochemistry were used to assess the expression of NK(1)Rs and/or SP in samples of colon, spinal cord, and dorsal root ganglia.

RESULTS

Both intraperitoneal and intrathecal SR140333 injection diminished the enhanced visceromotor reflex to colorectal distention at day 11 in stressed rats but did not affect the response in control animals. Real-time polymerase chain reaction and Western blotting demonstrated stress-induced up-regulation of spinal NK(1)Rs. Immunohistochemistry showed an increased number of NK(1)R-expressing neurons in the laminae I of the dorsal horn in stressed rats. The expression of NK(1)Rs was decreased in colon from stressed rats compared with control. The expression of SP gene precursor in dorsal root ganglia was unchanged in stressed rats compared with controls.

CONCLUSIONS

Stress-induced increased NK(1)R expression on spinal neurons and the inhibitory effect of intrathecal NK(1)R antagonist on visceral hyperalgesia support the key contribution of spinal NK(1)Rs in the molecular pathways involved in the maintenance of visceral hyperalgesia observed after chronic WA.

Transient receptor potential vanilloid 1, calcitonin gene-related peptide, and substance P mediate nociception in acute pancreatitis

The mechanism of pancreatitis-induced pain is unknown. In other tissues, inflammation activates transient receptor potential vanilloid 1 (TRPV1) on sensory nerves to liberate CGRP and substance P (SP) in peripheral tissues and the dorsal horn to cause neurogenic inflammation and pain, respectively. We evaluated the contribution of TRPV1, CGRP, and SP to pancreatic pain in rats. TRPV1, CGRP, and SP were coexpressed in nerve fibers of the pancreas. Injection of the TRPV1 agonist capsaicin into the pancreatic duct induced endocytosis of the neurokinin 1 receptor in spinal neurons in the dorsal horn (T10), indicative of SP release upon stimulation of pancreatic sensory nerves. Induction of necrotizing pancreatitis by treatment with L-arginine caused a 12-fold increase in the number of spinal neurons expressing the proto-oncogene c-fos in laminae I and II of L1, suggesting activation of nociceptive pathways. L-arginine also caused a threefold increase in spontaneous abdominal contractions detected by electromyography, suggestive of referred pain. Systemic administration of the TRPV1 antagonist capsazepine inhibited c-fos expression by 2.5-fold and abdominal contractions by 4-fold. Intrathecal, but not systemic, administration of antagonists of CGRP (CGRP(8-37)) and SP (SR140333) receptors attenuated c-fos expression in spinal neurons by twofold. Thus necrotizing pancreatitis activates TRPV1 on pancreatic sensory nerves to release SP and CGRP in the dorsal horn, resulting in nociception. Antagonism of TRPV1, SP, and CGRP receptors may suppress pancreatitis pain.

Disinhibition opens the gate to pathological pain signaling in superficial neurokinin 1 receptor-expressing neurons in rat spinal cord

Blockade of local spinal cord inhibition mimics the behavioral hypersensitivity that manifests in chronic pain states. This suggests that there is a pathway capable of mediating allodynia/hyperalgesia that exists but is normally under strong inhibitory control. Lamina I and III neurokinin 1 (NK1) receptor expressing (NK1R+) dorsal horn neurons, many of which are projection neurons, are required for the development of this hypersensitivity and are therefore likely to be a component of this proposed pathway. To investigate, whole-cell patch-clamp recordings were made from lamina I and III NK1R+ neurons in the spinal cord slice preparation with attached dorsal root. Excitatory postsynaptic currents were recorded in response to electrical stimulation of the dorsal root. Lamina I NK1R+ neurons were shown to receive high-threshold (Adelta/C fiber) monosynaptic input, whereas lamina III NK1R+ neurons received low-threshold (Abeta fiber) monosynaptic input. In contrast, lamina I neurons lacking NK1 receptor (NK1R-) received polysynaptic A fiber input. Blockade of local GABAergic and glycinergic inhibition with bicuculline (10 microm) and strychnine (300 nm), respectively, revealed significant A fiber input to lamina I NK1R+ neurons that was predominantly Abeta fiber mediated. This novel A fiber input was polysynaptic in nature and required NMDA receptor activity to be functional. In lamina I NK1R- and lamina III NK1R+ neurons, disinhibition enhanced control-evoked responses, and this was also NMDA receptor dependent. These disinhibition-induced changes, in particular the novel polysynaptic low-threshold input onto lamina I NK1R+ neurons, may be an underlying component of the hypersensitivity present in chronic pain states.

Localization of soluble guanylyl cyclase in the superficial dorsal horn

Nitric oxide (NO) has been implicated in pain processing at the spinal level, but the mechanisms mediating its effects remain unclear. In the present work, we studied the organization of the major downstream effector of NO, soluble guanylyl cyclase (sGC), in the superficial dorsal horn of rat. Almost all neurokinin 1 (NK1) receptor-positive neurons in lamina I (a major source of ascending projections) were strongly immunopositive for sGC. Many local circuit neurons in laminae I-II also stained for sGC, but less intensely. Numerous fibers, presumably of unmyelinated primary afferent (C fiber) origin, stained for calcitonin gene-related peptide or isolectin B4, but none of these was immunopositive for sGC. These data, along with immunoelectron microscopy results, imply that unmyelinated primary afferent fibers terminating in the superficial dorsal horn lack sGC. Double labeling showed that neuronal nitric oxide synthase (nNOS) seldom colocalized with sGC, but nNOS-positive structures were frequently closely apposed to sGC-positive structures, suggesting that in the superficial dorsal horn NO acts mainly in a paracrine manner. Our data suggest that the NK1 receptor-positive projection neurons in lamina I are a major target of NO released in superficial dorsal horn. NO may also influence local circuit neurons, but it does not act on unmyelinated primary afferent terminals via sGC.

Rat/mouse hemokinin-1, a mammalian tachykinin peptide, markedly potentiates the antinociceptive effects of morphine administered at the peripheral and supraspinal level

Rat/mouse hemokinin 1 (r/m HK-1) is a mammalian tachykinin peptide whose biological functions are not fully understood. Our recent report showed that i.c.v. administration of r/m HK-1 could produce dose- and time-related antinociceptive effect at nanomole concentration, and naloxone significantly antagonized this effect. Thus, we provide indirect evidence favoring a role of NK1 supraspinal receptors in the inhibitory control of descending pain pathways, a role that seems to partially involve the activation of the endogenous opioid systems. Based on this report, the present study was conducted to further investigate the direct functional interaction between supraspinal tachykinin (r/m HK-1) and opioid systems. The results demonstrate that i.c.v. administration of r/m HK-1 (5 nmol/kg) could significantly potentiate the antinociceptive effects of morphine which was injected at peripheral and supraspinal level. These antinociceptive effects were blocked by prior treatment with the classical opioid receptors antagonist naloxone, indicating that the potentiated analgesic response is mediated by opioid-responsive neurons. Consistent with previous biochemical data, a likely mechanism underlying the peptide-mediated enhancement of opioid analgesia may center on the ability of r/m HK-1 to release endogenous opioid peptides. We suggest that there may be a cascade amplification mechanism in pain modulation when the two agents were co-administrated. The synergistic analgesic relationship of morphine and r/m HK-1 established here supports the hypothesis that supraspinal tachykinin and peripheral and central opioid systems have a direct functional interaction in the modulation of local nociceptive responses.

Basic science of pain

The origin of the theory that the transmission of pain is through a single channel from the skin to the brain can be traced to the philosopher and scientist René Descartes. This simplified scheme of the reflex was the beginning of the development of the modern doctrine of reflexes. Unfortunately, Descartes' reflex theory directed both the study and treatment of pain for more than 330 years. It is still described in physiology and neuroscience textbooks as fact rather than theory. The gate control theory proposed by Melzack and Wall in 1965 rejuvenated the field of pain study and led to further investigation into the phenomena of spinal sensitization and central nervous system plasticity, which are the potential pathophysiologic correlates of chronic pain. The processing of pain takes place in an integrated matrix throughout the neuroaxis and occurs on at least three levels-at peripheral, spinal, and supraspinal sites. Basic strategies of pain control monopolize on this concept of integration by attenuation or blockade of pain through intervention at the periphery, by activation of inhibitory processes that gate pain at the spinal cord and brain, and by interference with the perception of pain. This article discusses each level of pain modulation and reviews the mechanisms of action of opioids and potential new analgesics. A brief description of animal models frames a discussion about recent advances regarding the role of glial cells and central nervous system neuroimmune activation and innate immunity in the etiology of chronic pain states. Future investigation into the discovery and development of novel, nonopioid drug therapy may provide needed options for the millions of patients who suffer from chronic pain syndromes, including syndromes in which the pain originates from peripheral nerve, nerve root, spinal cord, bone, muscle, and disc.

Inhibition by spinal mu- and delta-opioid agonists of afferent-evoked substance P release

Opioid mu- and delta-receptors are present on the central terminals of primary afferents, where they are thought to inhibit neurotransmitter release. This mechanism may mediate analgesia produced by spinal opiates; however, when they used neurokinin 1 receptor (NK1R) internalization as an indicator of substance P release, Trafton et al. (1999) noted that this evoked internalization was altered only modestly by morphine delivered intrathecally at spinal cord segment S1-S2. We reexamined this issue by studying the effect of opiates on NK1R internalization in spinal cord slices and in vivo. In slices, NK1R internalization evoked by dorsal root stimulation at C-fiber intensity was abolished by the mu agonist [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) (1 microM) and decreased by the delta agonist [D-Phe2,5]-enkephalin (DPDPE) (1 microM). In vivo, hindpaw compression induced NK1R internalization in ipsilateral laminas I-II. This evoked internalization was significantly reduced by morphine (60 nmol), DAMGO (1 nmol), and DPDPE (100 nmol), but not by the kappa agonist trans-(1S,2S)-3,4-dichloro-N-mathyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride (200 nmol), delivered at spinal cord segment L2 using intrathecal catheters. These doses of the mu and delta agonists were equi-analgesic as measured by a thermal escape test. Lower doses neither produced analgesia nor inhibited NK1R internalization. In contrast, morphine delivered by percutaneous injections at S1-S2 had only a modest effect on thermal escape, even at higher doses. Morphine decreased NK1R internalization after systemic delivery, but at a dose greater than that necessary to produce equivalent analgesia. All effects were reversed by naloxone. These results indicate that lumbar opiates inhibit noxious stimuli-induced neurotransmitter release from primary afferents at doses that are confirmed behaviorally as analgesic.

Safety Evaluation of Intrathecal Substance P-Saporin, a Targeted Neurotoxin, in Dogs

Intrathecal (IT) substance P-Saporin (SP-SAP), a 33-kDa-targeted neurotoxin, produces selective destruction of superficial neurokinin 1 receptor (NK1r)-bearing cells in the spinal dorsal horn. In rats, SP-SAP prevents the formation of hyperalgesia and can reverse established neuropathic pain behavior in rodents. To determine the safety of this therapeutic modality in a large animal model, beagles received bolus IT lumbar injections of vehicle, SP-SAP (1.5, 15, 45, or 150 microg), or a nontargeted preparation of saporin (SAP, 150 microg) for immunohistological analysis of spinal cords. Doses of 15 microg SP-SAP and above produced a significant and equivalent loss of NK1r-bearing cells and dendrites in lumbar laminae II and I compared to vehicle- or SAP-treated animals. Cervical regions in all animals displayed no loss of NK1r immunoreactivity as compared to controls. Total numbers of neurons in the lumbar dorsal horn or alpha-motor neurons in the ventral horn demonstrated no significant changes. No increases in the astrocytic marker glial fibrillary acidic protein were noted following treatment with SP-SAP, suggesting a lack of generalized neurotoxicity. Additional dogs received doses of 1.5-150 microg SP-SAP or SAP and were sacrificed after 28 or 90 days to assess behavioral and physiological parameters. Although some acute motor signs were observed with both SP-SAP and SAP, no long-lasting significant events were noted in any of these animals. These data indicate no adverse toxicity at doses up to 10 times those necessary for producing loss of superficial NK1r-bearing neurons in a large animal model.

Advances in understanding bone cancer pain

Experimental animal models of bone cancer pain have emerged and findings have provided a unique glimpse into unraveling the mechanism that drives this debilitating condition. Key contributors to the generation and maintenance of bone cancer pain are tumor-induced osteolysis, tumor itself, and production of nociceptive mediators in the bone-tumor microenvironment.

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.

Morphology and neurokinin 1 receptor expression of spinothalamic lamina I neurons in the rat spinal cord

Distinct morphological types of spinothalamic tract (STT) lamina I (LI) neurons have been identified in the cat and monkey spinal dorsal horn. Because these morphological types appear to differ in functional properties and receptor expression, we examined their distribution in the rat to test how their identification relates to earlier classification schemes. LI STT cells were retrogradely labeled with cholera toxin subunit b (CTb). Three types were recognized on the basis of cell body shape and proximal dendrites in the horizontal plane: fusiform, multipolar, and pyramidal. The relative distribution of these types was: 43, 26, and 28%, respectively, similar to that observed in the cat and monkey. 3D reconstructions were used to view each cell in all three major projection planes: horizontal, parasagittal, and transverse. Most LI STT neurons appeared fusiform in the parasagittal plane even though they belonged to different types based on their appearance in the horizontal plane, except in the most lateral portion of the dorsal horn, where LI curves ventrally. The proportion of STT neurons within LI was quantified by using the optical dissector method. To label all LI neurons, we used an anti-neuron-specific nuclear protein (NeuN) antibody. We found that approximately 9% of LI neurons projected to the thalamus. We also investigated neurokinin 1 receptor (NK-1r) expression in LI STT neurons. As in the monkey, most pyramidal STT neurons did not express NK-1r. These results provide further evidence that distinct morphological types of neurons differ in phenotype but not in their projection pattern.

Secondary hyperalgesia in the monoarthritic rat is mediated by GABAB and NK1 receptors of spinal dorsal horn neurons: A behavior and c-fos study

Secondary hyperalgesia in the monoarthritic rat is accompanied by a decrease in nociceptive activation of spinal neurons expressing GABA(B) receptors and by the opposite effect in the cells expressing neurokinin 1 (NK1)-receptors. In order to ascertain the relative role of each receptor, the effects of intrathecal administration of SP-saporin (SP-SAP), baclofen or both were evaluated, using a model of secondary hyperalgesia that consists of mechanical stimulation of the hindlimb skin close to an inflamed joint. Four days after the induction of monoarthritis by intraarticular injection of Complete Freund's Adjuvant (CFA), a cannula was implanted at T(13)-L(1) level and 10 microl of saline or SP-SAP (10(-6) M) were intrathecally (i.t.) injected. Fourteen days after CFA-injection, half of the animals from each group received i.t. injections of 10 microl saline and the remainder were injected with the same volume of baclofen (1 microg). Ten minutes later, the animals were behaviorally evaluated by the von Frey test or submitted to noxious mechanical stimulation to analyze c-fos expression. The von Frey thresholds increased after the treatments, but more pronouncedly after baclofen or SP-SAP plus baclofen. In segments L(2)-L(3), the spinal area that receives input from the stimulated skin close to the inflamed joint, the numbers of Fos-immunoreactive neurons were reduced after the three treatments both in the superficial and deep dorsal horn. In segments T(13)-L(1), the numbers of Fos-immunoreactive neurons were significantly reduced after treatment with SP-SAP plus baclofen in both dorsal horn regions, and in the deep dorsal horn after baclofen treatment. We conclude that both GABA(B) and NK1 receptors of spinal dorsal horn neurons participate in secondary hyperalgesia in the monoarthritic rat, although the decrease in GABA inhibition appears to play a more important role than the increase in SP-mediated effects.

Interaction with vesicle luminal protachykinin regulates surface expression of delta-opioid receptors and opioid analgesia

Opioid and tachykinin systems are involved in modulation of pain transmission in the spinal cord. Regulation of surface opioid receptors on nociceptive afferents is critical for opioid analgesia. Plasma-membrane insertion of delta-opioid receptors (DORs) is induced by stimulus-triggered exocytosis of DOR-containing large dense-core vesicles (LDCVs), but how DORs become sorted into the regulated secretory pathway is unknown. Here we report that direct interaction between protachykinin and DOR is responsible for sorting of DORs into LDCVs, allowing stimulus-induced surface insertion of DORs and DOR-mediated spinal analgesia. This interaction is mediated by the substance P domain of protachykinin and the third luminal domain of DOR. Furthermore, deletion of the preprotachykinin A gene reduced stimulus-induced surface insertion of DORs and abolished DOR-mediated spinal analgesia and morphine tolerance. Thus, protachykinin is essential for modulation of the sensitivity of nociceptive afferents to opioids, and the opioid and tachykinin systems are directly linked by protachykinin/DOR interaction.

Rho GTPases, dendritic structure, and mental retardation

A consistent feature of neurons in patients with mental retardation is abnormal dendritic structure and/or alterations in dendritic spine morphology. Deficits in the regulation of the dendritic cytoskeleton affect both the structure and function of dendrites and synapses and are believed to underlie mental retardation in some instances. In support of this, there is good evidence that alterations in signaling pathways involving the Rho family of small GTPases, key regulators of the actin and microtubule cytoskeletons, contribute to both syndromic and nonsyndromic mental retardation disorders. Because the Rho GTPases have been shown to play increasingly well-defined roles in determining dendrite and dendritic spine development and morphology, Rho signaling has been suggested to be important for normal cognition. The purpose of this review is to summarize recent data on the Rho GTPases pertaining to dendrite and dendritic spine morphogenesis, as well as to highlight their involvement in mental retardation resulting from a variety of genetic mutations within regulators and effectors of these molecules.

Somatostatin receptor 2 is activated in cortical neurons and contributes to neurodegeneration after focal ischemia

Somatostatin receptor 2 (SSTR2) mediates neuromodulatory signals of somatostatin and cortistatin in the cerebral cortex. Recently, SSTR2 has been shown to enhance conserved death ligand- and mitochondria-mediated apoptotic pathways in non-neuronal cells. Whether somatostatin receptors are activated in cerebrocortical neurons and contribute to neurodegeneration after experimental focal ischemia was unknown until now. Here we examined internalization of SSTR2 in a rat model of middle cerebral artery occlusion (MCAO) by confocal microscopy. At 3 and 6 hr after MCAO, SSTR2 was internalized excessively in cerebrocortical neurons adjacent to the infarct, which was prevented by intracerebroventricular application of the SSTR2-selective antagonist BIM-23627. SSTR2 internalization was associated with a transient depletion of somatostatin from axonal terminals and increased expression of SSTR2 mRNA. The initial loss of somatostatin was followed by an increase in somatostatin mRNA levels, whereas cortistatin mRNA expression was decreased. In SSTR2-deficient mice with lacZ under the control of the SSTR2 promoter, MCAO-induced upregulation of SSTR2 gene expression was less pronounced than in wild types. SSTR2-deficient mice exhibited a 40% reduction of infarct size after permanent distal MCAO and a 63% reduction after transient proximal MCAO. In summary, we provide direct evidence for activation of SSTR2 by an endogenous ligand after focal ischemia. Activation of functional SSTR2 receptors contributes to increased SSTR2 gene expression and postischemic neurodegeneration.

Ionotropic and metabotropic receptors, protein kinase A, protein kinase C, and Src contribute to C-fiber-induced ERK activation and cAMP response element-binding protein phosphorylation in dorsal horn neurons, leading to central sensitization

Molecular mechanisms underlying C-fiber stimulation-induced ERK (extracellular signal-regulated kinase) activation in dorsal horn neurons and its contribution to central sensitization have been investigated. In adult rat spinal slice preparations, activation of C-fiber primary afferents by a brief exposure of capsaicin produces an eightfold to 10-fold increase in ERK phosphorylation (pERK) in superficial dorsal horn neurons. The pERK induction is reduced by blockade of NMDA, AMPA/kainate, group I metabotropic glutamate receptor, neurokinin-1, and tyrosine receptor kinase receptors. The ERK activation produced by capsaicin is totally suppressed by inhibition of either protein kinase A (PKA) or PKC. PKA or PKC activators either alone or more effectively together induce pERK in superficial dorsal horn neurons. Inhibition of calcium calmodulin-dependent kinase (CaMK) has no effect, but pERK is reduced by inhibition of the tyrosine kinase Src. The induction of cAMP response element binding protein phosphorylation (pCREB) in spinal cord slices in response to C-fiber stimulation is suppressed by preventing ERK activation with the MAP kinase kinase inhibitor 2-(2-diamino-3-methoxyphenyl-4H-1-benzopyran-4-one (PD98059) and by PKA, PKC, and CaMK inhibitors. Similar signaling contributes to pERK induction after electrical stimulation of dorsal root C-fibers. Intraplantar injection of capsaicin in an intact animal increases expression of pCREB, c-Fos, and prodynorphin in the superficial dorsal horn, changes that are prevented by intrathecal injection of PD98059. Intrathecal PD98059 also attenuates capsaicin-induced secondary mechanical allodynia, a pain behavior reflecting hypersensitivity of dorsal horn neurons (central sensitization). We postulate that activation of ionotropic and metabotropic receptors by C-fiber nociceptor afferents activates ERK via both PKA and PKC, and that this contributes to central sensitization through post-translational and CREB-mediated transcriptional regulation in dorsal horn neurons.

Mechanisms of antinociception of spinal galanin: how does galanin inhibit spinal sensitization?

Galanin by a spinal action has been shown to have an antihyperalgesic action. Thus, in rats with lumbar intrathecal (IT) catheters, the thermal hyperalgesia evoked by carrageenan paw injection was blocked by IT delivery of galanin(1-29) (Gal(1-29)) and galanin(2-11) (Gal(2-11)) with the rank order of activity being Gal(1-29)>Gal(2-11). We sought to determine whether this spinal action reflects an effect upon afferent transmitter release, e.g., substance P (SP), and/or on secondary neurons, e.g., signaling postsynaptic to neurokinin 1 (NK1) receptor activation. To address the question on afferent release, we investigated the effect of IT administration of galanin on tissue injury-induced spinal NK1 internalization (an indicator of SP release). Noxious stimulation (paw compression) produced an increase in NK1 internalization in dorsal horn lamina I. IT pretreatment of rats with Gal(1-29) and Gal(2-11) significantly attenuated the evoked NK1 internalization, with the rank order of activity being Gal(1-29)>Gal(2-11)>saline. To address the question of postsynaptic action, we examined the effects of IT galanin upon IT SP-induced thermal hyperalgesia and spinal PGE2 release. Application of SP (30 nmol) directly to spinal cord led to a decrease in thermal thresholds and a profound increase in PGE(2) concentration in spinal dialysates. Both phenomena were reversed by Gal(1-29) and Gal(2-11) (10nmol, IT). These findings suggest that the antihyperalgesic effect of spinal galanin is due to its action on sites both presynaptic (inhibition of SP release) and postsynaptic (blockade of SP-evoked hyperalgesia and PGE2 production) to the primary afferents.

The role of leukotriene B4 in Clostridium difficile toxin A-induced ileitis in rats

BACKGROUND & AIMS

Clostridium difficile toxin A is a potent intestinal inflammatory agent that has been shown to act at least partially by neurogenic mechanisms involving activation of the transient receptor potential vanilloid 1 (TRPV1) (capsaicin) receptor. We tested the hypothesis that leukotriene B4 (LTB4) mediates the effects of toxin A via activation of the TRPV1 receptor.

METHODS

Isolated rat ileal segments were pretreated with pharmacologic agents before intraluminal injection of toxin A or LTB4. After 3 hours, the treated segments were removed and inflammation was assessed by luminal fluid accumulation, myeloperoxidase activity, and histology.

RESULTS

LTB4 caused ileitis similar to that caused by toxin A and antagonism of TRPV1 receptors but not LTB4 receptors inhibited LTB4-induced inflammation. LTB4 also stimulated TRPV1-mediated substance P release and pretreatment with a specific substance P-receptor antagonist blocked LTB4-induced substance P action and ileitis. Inhibition of the LTB4 biosynthetic enzyme 5-lipoxygenase inhibited toxin A-induced increases in ileal LTB4 levels and toxin A- but not LTB4-induced ileitis.

CONCLUSIONS

LTB4 mediates the inflammatory effects of toxin A via activation of TRPV1 receptors.

Fos induction in lamina I projection neurons in response to noxious thermal stimuli

Lamina I of the spinal cord contains many projection neurons: the majority of these are activated by noxious stimulation, although some respond to other stimuli, such as innocuous cooling. In the rat, approximately 80% of lamina I projection neurons express the neurokinin 1 (NK1) receptor, on which substance P acts. Lamina I neurons can be classified into three main morphological classes: pyramidal, fusiform and multipolar cells. It has been reported that in the cat, pyramidal cells respond to innocuous cooling, and whilst both fusiform and multipolar cells are activated by noxious mechanical and heat stimuli, only cells in the latter group respond to noxious cold [Nat Neurosci 1 (1998) 218]. However, we have previously shown that NK1 receptor-immunoreactive projection neurons belonging to each morphological class are equally likely to up-regulate the transcription factor Fos after noxious chemical stimulation, and that the density of innervation by substance P-containing (nociceptive) afferents is similar for cells of each type [J Neurosci 22 (2002) 4103]. This suggests that the morphological-physiological correlation that has been reported in the cat may not apply in the rat. We have tested this further by examining Fos expression in lamina I spinoparabrachial neurons in the rat after application of noxious heat or noxious cold stimuli under general anesthesia. Following noxious heat, 57-69% of NK1 receptor-immunoreactive spinoparabrachial neurons expressed Fos, and the proportion did not differ significantly between morphological groups. However, after noxious cold stimulation Fos was present in 63% of multipolar neurons, but only 19-26% of fusiform or pyramidal cells. These results suggest that although most NK1 receptor-expressing spinoparabrachial neurons are activated by noxious stimuli, responsiveness to noxious cold is significantly more common in those of the multipolar type. There therefore appears to be a correlation between morphology and function for lamina I projection neurons in the rat.

Postnatal changes in the spatial distributions of substance P and neurokinin-1 receptor in the trigeminal subnucleus caudalis of mice

Nociceptive afferent signals from the orofacial area are transmitted to the trigeminal subnucleus caudalis (Vc) through the release of glutamate and/or substance P (SP). Although nociceptive transmission and/or modulating mechanisms are known to develop during the postnatal period, the specific developmental changes in nociception and/or modulation remain unclear. The present study examined postnatal changes in the spatial relationship between SP and its receptor, the NK1 receptor (NK1R), in the mouse Vc by immunohistochemistry and quantitative analysis. The medulla was removed from C57BL/6N mice (1, 2, 4, and 8 weeks of age) after perfusion and fixation, and cut horizontally at a thickness of 40 mum. The relative densities of SP- and NK1R-immunoreactive areas and their changes with age were assessed statistically. One- and 2-week-old mice showed relatively high densities of SP-positive structures in the marginal layer (Mar) and the deep part of the magnocellular layer (Mag). The SP distribution in the superficial Vc remained unchanged, but the density in the deep Mag gradually decreased with age, resulting in a complete loss after postnatal week 4. The NK1R-immunoreactivity exhibited a similar distribution pattern to that of SP, but the pattern remained unchanged during the postnatal period. Double-immunofluorescence staining for SP and NK1R demonstrated only moderate direct contact of SP-positive structures with NK1R in the superficial area. These separate distributions and the postnatal changes in SP and NK1R suggest the possibility of another nociceptive afferent transmission mechanism, that is, volume transmission, in the Vc other than synapse-mediated transmission.

Rat trigeminal lamina I neurons that project to thalamic or parabrachial nuclei contain the mu-opioid receptor

Ligands of the mu-opioid receptor are known to inhibit nociceptive transmission in the dorsal horn, yet the cellular site(s) of action for this inhibition remain to be fully elucidated. Neurons located in lamina I of the dorsal horn are involved in distinct aspects of nociceptive transmission. Neurons projecting to the thalamus are thought to be involved in sensory-discriminative aspects of pain perception, while neurons projecting to the parabrachial nucleus are thought to be important for emotional and/or autonomic responses to noxious stimuli. The present study examined these two populations of lamina I projection neurons in the trigeminal dorsal horn to determine if the mu-opioid receptor protein (MOR1) is differentially located in these populations of neurons. Lamina I projection neurons were identified using the retrograde tracer FluoroGold (FGold). FGold was injected into either the contralateral thalamus (ventral posterolateral (VPM)/ventral posterolateral (VPL) thalamic region) or into the ipsilateral parabrachial nuclei. The distribution of MOR1 in these neurons was determined using immunocytochemistry. The distribution of MOR1-ir within these two populations of lamina I projection neurons was examined by both confocal and electron microscopy. We found that both populations of projection neurons contained MOR1. Immunogold analyses revealed the presence of MOR1-ir at membrane sites and within the cytoplasm of these neurons. Cytoplasmic receptor labeling may represent sites of synthesis, recycling or reserve populations of receptors. MOR1 was primarily found in the somata and proximal dendrites of projection neurons. In addition, these neurons rarely received synaptic input from MOR1-containing axon terminals. These results indicate that lamina I neurons in trigeminal dorsal horn that project to the thalamic and parabrachial nuclei contain MOR1 and are likely sites of action for MOR ligands that modulate sensory and/or autonomic aspects of pain transmission in the trigeminal dorsal horn.

Chronic stress decreases the number of parvalbumin-immunoreactive interneurons in the hippocampus: prevention by treatment with a substance P receptor (NK1) antagonist

Previous studies have demonstrated that stress may affect the hippocampal GABAergic system. Here, we examined whether long-term psychosocial stress influenced the number of parvalbumin-containing GABAergic cells, known to provide the most powerful inhibitory input to the perisomatic region of principal cells. Adult male tree shrews were submitted to 5 weeks of stress, after which immunocytochemical and quantitative stereological techniques were used to estimate the total number of hippocampal parvalbumin-immunoreactive (PV-IR) neurons. Stress significantly decreased the number of PV-IR cells in the dentate gyrus (DG) (-33%), CA2 (-28%), and CA3 (-29%), whereas the CA1 was not affected. Additionally, we examined whether antidepressant treatment offered protection from this stress-induced effect. We administered fluoxetine (15 mg/kg per day) and SLV-323 (20 mg/kg per day), a novel neurokinin 1 receptor (NK1R) antagonist, because the NK1R has been proposed as a possible target for novel antidepressant therapies. Animals were subjected to a 7-day period of psychosocial stress before the onset of daily oral administration of the drugs, with stress continued throughout the 28-day treatment period. NK1R antagonist administration completely prevented the stress-induced reduction of the number of PV-IR interneurons, whereas fluoxetine attenuated this decrement in the DG, without affecting the CA2 and CA3. The effect of stress on interneuron numbers may reflect real cell loss; alternatively, parvalbumin concentration is diminished in the neurons, which might indicate a compensatory attempt. In either case, antidepressant treatment offered protection from the effect of stress and appears to modulate the hippocampal GABAergic system. Furthermore, the NK1R antagonist SLV-323 showed neurobiological efficacy similar to that of fluoxetine.