Locus coeruleus inhibition of vibrissal responses in the trigeminal subnucleus caudalis are reduced in a diabetic mouse model.

Diabetic neuropathy is the loss of sensory function beginning distally in the lower extremities, which is also characterized by pain and substantial morbidity. Furthermore, the locus coeruleus (LC) nucleus has been proposed to play an important role in descending pain control through the activation of α2-noradrenergic (NA) receptors in the spinal dorsal horn. We studied, on control and diabetic mice, the effect of electrical stimulation of the LC nucleus on the tactile responses in the caudalis division of the spinal trigeminal nucleus (Sp5C), which is involved in the relay of orofacial nociceptive information. Diabetes was induced in young adult C57BL/6J mice with one intraperitoneal injection of streptozotocin (50 mg/kg) daily for 5 days. The diabetic animals showed pain in the orofacial area because they had a decrease in the withdrawal threshold to the mechanical stimulation in the vibrissal pad. LC electrical stimulation induced the inhibition of vibrissal responses in the Sp5C neurons when applied at 50 and 100 ms before vibrissal stimulation in the control mice; however, the inhibition was reduced in the diabetic mice. These effects may be due to a reduction in the tyrosine hydroxylase positive (TH+) fibers in the Sp5C, as was observed in diabetic mice. LC-evoked inhibition was decreased by an intraperitoneal injection of the antagonist of the α2-NA receptors, yohimbine, indicating that it was due to the activation of α2-NA receptors. The decrease in the LC-evoked inhibition in the diabetic mice was partially recovered when clonidine, a non-selective α2-agonist, was injected intraperitoneally. These findings suggest that in diabetes, there is a reduction in the NA inputs from the LC in the Sp5C that may favor the development of chronic pain.

Metabolomics and biochemical alterations caused by pleiotrophin in the 6-hydroxydopamine mouse model of Parkinson's disease.

Pleiotrophin (PTN) is a cytokine involved in nerve tissue repair processes, neuroinflammation and neuronal survival. PTN expression levels are upregulated in the nigrostriatal pathway of Parkinson's Disease (PD) patients. We aimed to characterize the dopaminergic injury and glial responses in the nigrostriatal pathway of mice with transgenic Ptn overexpression in the brain (Ptn-Tg) after intrastriatal injection of the catecholaminergic toxic 6-hydroxydopamine (6-OHDA) at a low dose (5 µg). Ten days after surgery, the injection of 6-OHDA induced a significant decrease of the number of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra and of the striatal TH contents in Wild type (Wt) mice. In contrast, these effects of 6-OHDA were absent in Ptn-Tg mice. When the striatal Iba1 and GFAP immunoreactivity was studied, no statistical differences were found between vehicle-injected Wt and Ptn-Tg mice. Furthermore, 6-OHDA did not cause robust glial responses neither on Wt or Ptn-Tg mice 10 days after injections. In metabolomics studies, we detected interesting metabolites that significantly discriminate the more injured 6-OHDA-injected Wt striatum and the more protected 6-OHDA-injected Ptn-Tg striatum. Particularly, we detected groups of metabolites, mostly corresponding to phospholipids, whose trends were opposite in both groups. In summary, the data confirm lower 6-OHDA-induced decreases of TH contents in the nigrostriatal pathway of Ptn-Tg mice, suggesting a neuroprotective effect of brain PTN overexpression in this mouse model of PD. New lipid-related PD drug candidates emerge from this study and the data presented here support the increasingly recognized "lipid cascade" in PD.

Experience-dependent plasticity in early stations of sensory processing in mature brains: effects of environmental enrichment on dendrite measures in trigeminal nuclei.

Nervous systems respond with structural changes to environmental changes even in adulthood. In recent years, experience-dependent structural plasticity was shown not to be restricted to the cerebral cortex, as it also occurs at subcortical and even peripheral levels. We have previously shown that two populations of trigeminal nuclei neurons, trigeminothalamic barrelette neurons of the principal nucleus (Pr5), and intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus (Sp5C) that project to Pr5 underwent morphometric and topological changes in their dendritic trees after a prolonged total or partial loss of afferent input from the vibrissae. Here we examined whether and what structural alterations could be elicited in the dendritic trees of the same cell populations in young adult rats after being exposed for 2 months to an enriched environment (EE), and how these changes evolved when animals were returned to standard housing for an additional 2 months. Neurons were retrogradely labeled with BDA delivered to, respectively, the ventral posteromedial thalamic nucleus or Pr5. Fully labeled cells were digitally reconstructed with Neurolucida and analyzed with NeuroExplorer. EE gave rise to increases in dendritic length, number of trees and branching nodes, spatial expansion of the trees, and dendritic spines, which were less pronounced in Sp5C than in Pr5 and differed between sides. In Pr5, these parameters returned, but only partially, to control values after EE withdrawal. These results underscore a ubiquity of experience-dependent changes that should not be overlooked when interpreting neuroplasticity and developing plasticity-based therapeutic strategies.

Morphological predictors for microsatellite instability in urothelial carcinoma.

INTRODUCTION: Microsatellite instability occurs due to a series of mutations in the DNA pairing error repair (Mismatch repair; MMR) genes, which can affect germ cells as occurs in Lynch syndrome, whose patients are at high risk of developing multiple cancers. The loss of MMR protein is commonly determined by immunohistochemical studies. Although the relation between microsatellite instability and urothelial carcinomas has been widely studied, its evaluation is not currently performed in the analysis of urothelial carcinomas. METHODS: In this study, the microsatellite status of 139 urothelial carcinomas was analyzed and their clinicopathological characteristics were evaluated. We identified that 10.3% (13 patients) of urothelial carcinomas had loss of MMR protein expression (9 MLH1; 5 MSH2; 2 PMS2; 2 PSH6; n = 139). RESULTS: Results suggest that these tumors occur more frequently in males, are more frequently located in the bladder or ureters, and present a high tumor grade with a papillary histological pattern that does not infiltrate the lamina propria or, in the case of infiltrating tumors, that grows into perivesical tissues. CONCLUSIONS: We identified patients with the aforementioned tumor characteristics as patients with a high probability of presenting loss of MMR protein expression, and consider that only these patients should undergo further immunohistochemical and molecular techniques for proper diagnosis. Therefore, we propose that the clinicopathological characteristics found in the present study could become possible markers to determine which cases should undergo additional tests.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain.

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Modulation of mechanosensory vibrissal responses in the trigeminocervical complex by stimulation of the greater occipital nerve in a rat model of trigeminal neuropathic pain.

BACKGROUND: Stimulation of the occipital or trigeminal nerves has been successfully used to treat chronic refractory neurovascular headaches such as migraine or cluster headache, and painful neuropathies. Convergence of trigeminal and occipital sensory afferents in the 'trigeminocervical complex' (TCC) from cutaneous, muscular, dural, and visceral sources is a key mechanism for the input-induced central sensitization that may underlie the altered nociception. Both excitatory (glutamatergic) and inhibitory (GABAergic and glycinergic) mechanisms are involved in modulating nociception in the spinal and medullary dorsal horn neurons, but the mechanisms by which nerve stimulation effects occur are unclear. This study was aimed at investigating the acute effects of electrical stimulation of the greater occipital nerve (GON) on the responses of neurons in the TCC to the mechanical stimulation of the vibrissal pad. METHODS: Adult male Wistar rats were used. Neuronal recordings were obtained in laminae II-IV in the TCC in control, sham and infraorbital chronic constriction injury (CCI-IoN) animals. The GON was isolated and electrically stimulated. Responses to the stimulation of vibrissae by brief air pulses were analyzed before and after GON stimulation. In order to understand the role of the neurotransmitters involved, specific receptor blockers of NMDA (AP-5), GABAA (bicuculline, Bic) and Glycine (strychnine, Str) were applied locally. RESULTS: GON stimulation produced a facilitation of the response to light facial mechanical stimuli in controls, and an inhibition in CCI-IoN cases. AP-5 reduced responses to GON and vibrissal stimulation and blocked the facilitation of GON on vibrissal responses found in controls. The application of Bic or Str significantly reduced the facilitatory effect of GON stimulation on the response to vibrissal stimulation in controls. However, the opposite effect was found when GABAergic or Glycinergic transmission was prevented in CCI-IoN cases. CONCLUSIONS: GON stimulation modulates the responses of TCC neurons to light mechanical input from the face in opposite directions in controls and under CCI-IoN. This modulation is mediated by GABAergic and Glycinergic mechanisms. These results will help to elucidate the neural mechanisms underlying the effectiveness of nerve stimulation in controlling painful craniofacial disorders, and may be instrumental in identifying new therapeutic targets for their prevention and treatment.

Multiple Morphometric Assessment of Microglial Cells in Deafferented Spinal Trigeminal Nucleus.

Microglia (MG) are the first cells to react to the abnormal incoming signals that follow an injury of sensory nerves and play a critical role in the development and maintenance of neuropathic pain, a common sequel of nerve injuries. Here we present population data on cell number, soma size, and length of processes of MG in the caudal division of the spinal trigeminal nucleus (Sp5C) in control mice and at the peak of microgliosis (7 days) following unilateral transection of the infraorbital nerve (IoN). The study is performed combining several bias- and assumption-free imaging and stereological approaches with different immunolabeling procedures, with the objective of tackling some hard problems that often hinder proper execution of MG morphometric studies. Our approach may easily be applied to low-density MG populations, but also works, with limited biases, in territories where MG cell bodies and processes form dense meshworks. In controls, and contralaterally to the deafferented side, MG cell body size and shape and branching pattern matched well the descriptions of "resting" or "surveillant" MG described elsewhere, with only moderate intersubject variability. On the superficial laminae of the deafferented side, however, MG displayed on average larger somata and remarkable diversity in shape. The number of cells and the length of MG processes per mm3 increased 5 and 2.5 times, respectively, indicating a net 50% decrease in the mean length of processes per cell. By using specific immunolabeling and cell sorting of vascular macrophages, we found only a negligible fraction of these cells in Sp5C, with no differences between controls and deafferented animals, suggesting that blood-borne monocytes play at most a very limited role in the microgliosis occurring following sensory nerve deafferentation. In sum, here we present reliable morphometric data on MG in control and deafferented trigeminal nuclei using efficient methods that we propose may equally be applied to any morphometric population analysis of these cells under different physiological or pathological conditions.

The greater occipital nerve and its spinal and brainstem afferent projections: A stereological and tract-tracing study in the rat.

The neuromodulation of the greater occipital nerve (GON) has proved effective to treat chronic refractory neurovascular headaches, in particular migraine and cluster headache. Moreover, animal studies have shown convergence of cervical and trigeminal afferents on the same territories of the upper cervical and lower medullary dorsal horn (DH), the so-called trigeminocervical complex (TCC), and recent studies in rat models of migraine and craniofacial neuropathy have shown that GON block or stimulation alter nociceptive processing in TCC. The present study examines in detail the anatomy of GON and its central projections in the rat applying different tracers to the nerve and quantifying its ultrastructure, the ganglion neurons subserving GON, and their innervation territories in the spinal cord and brainstem. With considerable intersubject variability in size, GON contains on average 900 myelinated and 3,300 unmyelinated axons, more than 90% of which emerge from C2 ganglion neurons. Unmyelinated afferents from GON innervates exclusively laminae I-II of the lateral DH, mostly extending along segments C2-3 . Myelinated fibers distribute mainly in laminae I and III-V of the lateral DH between C1 and C6 and, with different terminal patterns, in medial parts of the DH at upper cervical segments, and ventrolateral rostral cuneate, paratrigeminal, and marginal part of the spinal caudal and interpolar nuclei. Sparse projections also appear in other locations nearby. These findings will help to better understand the bases of sensory convergence on spinomedullary systems, a critical pathophysiological factor for pain referral and spread in severe painful craniofacial disorders.

Changes in the axon terminals of primary afferents from a single vibrissa in the rat trigeminal nuclei after active touch deprivation or exposure to an enriched environment.

Lasting modifications of sensory input induce structural and functional changes in the brain, but the involvement of primary sensory neurons in this plasticity has been practically ignored. Here, we examine qualitatively and quantitatively the central axonal terminations of a population of trigeminal ganglion neurons, whose peripheral axons innervate a single mystacial vibrissa. Vibrissa follicles are heavily innervated by myelinated and unmyelinated fibers that exit the follicle mainly through a single deep vibrissal nerve. We made intraneural injections of a mixture of cholera-toxin B (CTB) and isolectin B4, tracers for myelinated and unmyelinated fibers, respectively, in three groups of young adult rats: controls, animals subjected to chronic haptic touch deprivation by unilateral whisker trimming, and rats exposed for 2 months to environmental enrichment. The regional and laminar pattern of terminal arborizations in the trigeminal nuclei of the brain stem did not show gross changes after sensory input modification. However, there were significant and widespread increases in the number and size of CTB-labeled varicosities in the enriched condition, and a prominent expansion in both parameters in laminae III-IV of the caudal division of the spinal nucleus in the whisker trimming condition. No obvious changes were detected in IB4-labeled terminals in laminae I-II. These results show that a prolonged exposure to changes in sensory input without any neural damage is capable of inducing structural changes in terminals of primary afferents in mature animals, and highlight the importance of peripheral structures as the presumed earliest players in sensory experience-dependent plasticity.

Sensory Input-Dependent Changes in Glutamatergic Neurotransmission- Related Genes and Proteins in the Adult Rat Trigeminal Ganglion.

Experience-dependent plasticity induces lasting changes in the structure of synapses, dendrites, and axons at both molecular and anatomical levels. Whilst relatively well studied in the cortex, little is known about the molecular changes underlying experience-dependent plasticity at peripheral levels of the sensory pathways. Given the importance of glutamatergic neurotransmission in the somatosensory system and its involvement in plasticity, in the present study, we investigated gene and protein expression of glutamate receptor subunits and associated molecules in the trigeminal ganglion (TG) of young adult rats. Microarray analysis of naïve rat TG revealed significant differences in the expression of genes, coding for various glutamate receptor subunits and proteins involved in clustering and stabilization of AMPA receptors, between left and right ganglion. Long-term exposure to sensory-enriched environment increased this left-right asymmetry in gene expression. Conversely, unilateral whisker trimming on the right side almost eliminated the mentioned asymmetries. The above manipulations also induced side-specific changes in the protein levels of glutamate receptor subunits. Our results show that sustained changes in sensory input induce modifications in glutamatergic transmission-related gene expression in the TG, thus supporting a role for this early sensory-processing node in experience-dependent plasticity.

Trigeminal intersubnuclear neurons: morphometry and input-dependent structural plasticity in adult rats.

Intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus that project to the principal nucleus (Pr5) play an active role in shaping the receptive fields of other neurons, at different levels in the ascending sensory system that processes information originating from the vibrissae. By using retrograde labeling and digital reconstruction, we investigated the morphometry and topology of the dendritic trees of these neurons and the changes induced by long-term experience-dependent plasticity in adult male rats. Primary afferent input was either eliminated by transection of the right infraorbital nerve (IoN), or selectively altered by repeated whisker clipping on the right side. These neurons do not display asymmetries between sides in basic metric and topologic parameters (global number of trees, nodes, spines, or dendritic ends), although neurons on the left tend to have longer terminal segments. Ipsilaterally, both deafferentation (IoN transection) and deprivation (whisker trimming) reduced the density of spines, and the former also caused a global increase in total dendritic length and a relative increase in more complex arbors. Contralaterally, deafferentation reduced more complex dendritic trees, and caused a moderate decline in dendritic length and spatial reach, and a loss of spines in number and density. Deprivation caused a similar, but more profound, effect on spines. Our findings provide original quantitative descriptions of a scarcely known cell population, and show that denervation- or deprivation-derived plasticity is expressed not only by neurons at higher levels of the sensory pathways, but also by neurons in key subcortical circuits for sensory processing.

Differential phosphoproteome of the striatum from pleiotrophin knockout and midkine knockout mice treated with amphetamine: correlations with amphetamine-induced neurotoxicity.

The neurotrophic factors pleiotrophin (PTN) and midkine (MK) have been shown to modulate amphetamine-induced neurotoxicity. Accordingly, PTN-/- and MK-/- mice show an increased vulnerability to amphetamine-induced neurotoxic effects. In an effort to uncover new pharmacological targets to prevent amphetamine neurotoxic effects, we have now used a proteomic approach to study protein phosphorylation, in which we combined phosphoprotein enrichment, by immobilized metal affinity chromatography (IMAC), with two-dimensional gel electrophoresis and mass spectrometry, in order to identify the phosphoproteins regulated in the striatum of PTN-/-, MK-/- and wild type (WT) mice treated with amphetamine. We identified 13 differentially expressed phosphoproteins that are judged to be relevant in the neuroprotective roles of PTN and MK against amphetamine-induced neurotoxicity. It is very interesting to note that 4 of these phosphoproteins, annexin A7 (ANXA7), COP9 signalosome subunit 5 (COPS5), aldehyde dehydrogenase family 1 member A1 (ALDH1A1) and creatine kinase U-type (CKMT1), are known to be involved in Parkinson's disease, a result of significant importance since PTN and MK have been also demonstrated to limit Parkinson's disease (PD) progress and have been suggested to be among the important genetic factors possibly preventing the development of PD in methamphetamine abusers. The data identify phosphoproteins differentially regulated by amphetamine treatment and/or the presence of endogenous PTN/MK which may be relevant mediators of PTN/MK neuroprotective effects against amphetamine-induced neurotoxicity. The data support further studies to validate the phosphoproteins here identified as possible new pharmacological targets to prevent amphetamine neurotoxic effects.

Maintenance of amphetamine-induced place preference does not correlate with astrocytosis.

Astrocytosis, a process in which astrocytes undergo proliferation and enhancement of glial fibrillary acidic protein (GFAP) expression, has been suggested to play important roles in the maintenance of dependence to amphetamine and its derivatives. It was previously shown that mice with genetic deletion of pleiotrophin (PTN), a neurotrophic factor upregulated in different brain areas after administration of amphetamine, show a longer lasting amphetamine-induced conditioned place preference (CPP) when compared to wild type mice. In this work, we aimed to pursue the possibility of a different astrocytic response induced by amphetamine in PTN-/- and PTN+/+ mice, which could underlie the higher vulnerability of PTN-/- mice to maintain amphetamine CPP. In confirmation of previous studies, we found that PTN-/- mice significantly maintained amphetamine (3mg/kg)-induced CPP 5 days after the last drug administration compared to PTN+/+ mice. Interestingly, the number of astrocytes in nucleus accumbens (NAcc), cingulate cortex (CG) and caudate putamen (CPu) did not differ between mice that maintained and did not maintain amphetamine-induced CPP independently of the genotype considered. However, we found that PTN-/- mice showed significantly decreased numbers of astrocytes in CG and CPu compared to PTN+/+ mice independently of whether they maintained amphetamine-induced CPP 5 days after the last drug administration or not. The data demonstrate that maintenance of amphetamine-induced CPP depends on the endogenous expression of PTN. The data tend to discard a correlation between activated astrocytes and maintenance of amphetamine conditioning effects and suggest PTN as a potential modulator of activation of astrocytes after amphetamine treatment.

Periadolescent amphetamine treatment causes transient cognitive disruptions and long-term changes in hippocampal LTP depending on the endogenous expression of pleiotrophin.

Amphetamine treatment during adolescence causes long-term cognitive deficits in rats. Pleiotrophin (PTN) is a cytokine with important roles in the modulation of synaptic plasticity, whose levels of expression are significantly regulated by amphetamine administration. To test the possibility that the long-term consequences of periadolescent amphetamine treatment cross species and, furthermore, to test the hypothesis that PTN could be one of the factors involved in the adult cognitive deficits observed after periadolescent amphetamine administrations, we comparatively studied the long-term consequences of periadolescent amphetamine treatment (3 mg/kg intraperitoneal, daily during 10 days) in normal wild-type (PTN+/+) and in PTN genetically deficient (PTN-/-) mice. Within the first week after cessation of treatment, significant deficits in the passive avoidance and Y-maze tests were only observed in amphetamine-pretreated PTN-/- mice. However, 13 and 26 days after the last administration, we did not find significant differences in Y-maze between amphetamine- and saline-pretreated PTN-/- mice. In addition, we did not find any genotype- or treatment-related anxiogenic- or depressive-like behaviour in adult mice. Furthermore, we observed a significantly enhanced long-term potentiation (LTP) in CA1 hippocampal slices from saline-pretreated PTN-/- mice compared with saline-pretreated PTN+/+ mice. Interestingly, amphetamine pre-treatment during adolescence significantly enhanced LTP in adult PTN+/+ mice but did not cause any effect in PTN-/- mice, suggesting LTP mechanisms saturation in naïve PTN-/- mice. The data demonstrate that periadolescent amphetamine treatment causes transient cognitive deficits and long-term alterations of hippocampal LTP depending on the endogenous expression of PTN.

The heparin binding growth factors midkine and pleiotrophin regulate the antinociceptive effects of morphine through α(2)-adrenergic independent mechanisms.

Genetic deletion of pleiotrophin (PTN) impairs spinal nociceptive transmission suggesting that this heparin binding growth factor could play roles in acute pain processing. Despite the high functional redundancy between PTN and midkine (MK), the only other member of this family of growth factors, we now demonstrate that genetic inactivation of MK does not alter acute nociceptive transmission since pain responses of female MK genetically deficient (MK-/-) and wild type (WT+/+) mice were found to be similar in the hot-plate and tail-immersion tests. It has also been shown that morphine administration significantly regulates MK levels within the brain, suggesting that MK could play a role in morphine-induced antinociceptive effects. To test this hypothesis, we have now studied morphine-induced antinociceptive effects in female MK-/- and WT+/+ mice. We did not find differences among genotypes using different doses of morphine (2.5, 5 and 10 mg/kg) in the hot-plate test. In contrast, we found that morphine significantly delayed pain responses in MK-/- mice compared to WT+/+ mice in the tail-immersion test. In confirmation of previous results from our group, we also found significantly enhanced morphine-induced antinociceptive effects in PTN-/- mice in the tail-immersion test. In addition, we now demonstrate that enhanced morphine analgesic effects in PTN-/- and MK-/- mice are not caused by a different contribution of descending noradrenergic inhibitory pathways since the α(2)-adrenergic antagonist yohimbine failed to alter morphine-induced analgesia in all genotypes. The data demonstrate that MK is an endogenous modulator of morphine antinociceptive effects, identify significant differences between PTN and MK in the control of pain processing at the spinal level, and support the hypothesis that inhibitors of the PTN/MK signaling pathway could potentiate opioid analgesia which may be relevant in opioid-refractory pain cases.

Uncovering new pharmacological targets to treat neuropathic pain by understanding how the organism reacts to nerve injury.

The neuropathic pain syndrome is complex. Current drugs to treat neuropathic pain, including anticonvulsivants and antidepressants, fail in up to 40-50% of the patients, while in the rest of them total alleviation is not normally achieved. Increased research advances in the neurobiology of neuropathic pain have not translated in more successful pharmacological treatments by the moment, but recent progress in the experimental methods available for this purpose could result in significant advances in the short term. One rational possibility for the pharmaceutical development of new drugs, including target identification, drug design and evaluation studies, could be to focus on mimicking what organism does to limit nerve damage or to enhance the regeneration of injured axons. Following this strategy, neurotrophic factors such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have been postulated as potential pharmacological targets to treat neuropathic pain. In addition, during the last few years, strong scientific evidences point to novel neurotrophic factors, such as pleiotrophin (PTN), as important factors to limit neuropathic pain development because of their remodeling and angiogenic actions in the injured area. This review focuses on recent research advances identifying new pharmacological targets in the treatment of the cause, not only the symptoms, of neuropathic pain.

Neuronal disinhibition in the trigeminal nucleus caudalis in a model of chronic neuropathic pain.

The mechanisms underlying neuropathic facial pain syndromes are incompletely understood. We used a unilateral chronic constriction injury of the rat infraorbital nerve (CCI-IoN) as a facial neuropathic model. Pain-related behavior of the CCI-IoN animals was tested at 8, 15 and 26 days after surgery (dps). The response threshold to mechanical stimulation with von Frey hairs on the injured side was reduced at 15 and 26 dps, indicating the presence of allodynia. We performed unitary recordings in the caudalis division of the spinal trigeminal nucleus (Sp5C) at 8 or 26 dps, and examined spontaneous activity and responses to mechanical and thermal stimulation of the vibrissal pad. Neurons were identified as wide dynamic range (WDR) or low-threshold mechanoreceptive (LTM) according to their response to tactile and/or noxious stimulation. Following CCI-IoN, WDR neurons, but not LTM neurons, increased their spontaneous activity at 8 and 26 dps, and both types of Sp5C neurons increased their responses to tactile stimuli. In addition, the on-off tactile response in neurons recorded after CCI-IoN was followed by afterdischarges that were not observed in control cases. Compared with controls, the response inhibition observed during paired-pulse stimulation was reduced after CCI-IoN. Immunohistochemical studies showed an overall decrease in GAD65 immunoreactivity in Sp5C at 26 dps, most marked in laminae I and II, suggesting that following CCI-IoN the inhibitory circuits in the sensory trigeminal nuclei are depressed. Consequently, our results strongly suggest that disinhibition of Sp5C neurons plays a relevant role in the appearance of allodynia after CCI-IoN.

Effects of removal of dietary polyunsaturated fatty acids on plasma extravasation and mechanical allodynia in a trigeminal neuropathic pain model.

BACKGROUND: Neuropathic pain (NP) is partially mediated by neuroinflammatory mechanisms, and also modulates local neurogenic inflammation. Dietary lipids, in particular the total amount and relative proportions of polyunsaturated fatty acids (PUFAs) of the omega-3 and omega-6 families, have been reported to modify the threshold for thermal and mechanical allodynia in the partial sciatic nerve ligation model of NP in rats. The effects of dietary lipids on other popular NP models, such as the chronic constriction injury (CCI), have not yet been examined. It is also unknown whether dietary PUFAs exert any effect on the capsaicin (CAP)-induced neurogenic inflammation under control or NP conditions. In this study we investigated these interrelated phenomena in the trigeminal territory, which has been much less explored, and for which not all data derived from limb nerves can be directly applied. RESULTS: We studied the effects of a CCI of the infraorbital nerve (IoN) on the development of mechanical allodynia and CAP-induced plasma extravasation in rats fed either a regular diet (RD), or a modified diet (MD) with much lower total content and omega-3:omega-6 ratio of PUFAs. In rats kept on MD, mechanical allodynia following CCI-IoN was more pronounced and developed earlier. Extravasation was substantially increased in naive rats fed MD, and displayed differential diet-depending changes one and four weeks after CCI-IoN. When compared with basal levels (in naive and/or sham cases), the net effect of CCI-IoN on ipsilateral extravasation was a reduction in the MD group, but an increase in the RD group, effectively neutralizing the original intergroup differences. CONCLUSION: In summary, PUFA intake reduces CAP-induced neurogenic plasma extravasation in the trigeminal territory, and their removal significantly alters the mechanical allodynia and the plasma extravasation that result from a unilateral CCI-IoN. It is likely that this "protective" effect of dietary lipids is temporary. Also, the presence of contralateral effects of CCI-IoN precludes using the contralateral side as control.