Descending inhibition selectively counteracts the capsaicin-induced facilitation of dorsal horn neurons activated by joint nociceptive afferents.

Previous studies from our laboratory showed that in the anesthetized cat, the intradermal injection of capsaicin in the hindpaw facilitated the intraspinal field potentials (IFPs) evoked by stimulation of the intermediate and high-threshold myelinated fibers in the posterior articular nerve (PAN). The capsaicin-induced facilitation was significantly reduced 3-4 h after the injection, despite the persistence of hindpaw inflammation. Although this effect was attributed to an incremented descending inhibition acting on the spinal pathways, it was not clear if it was set in operation once the capsaicin-induced effects exceeded a certain threshold, or if it was continuously operating to keep the increased neuronal activation within manageable limits. To evaluate the changes in descending inhibition, we now examined the effects of successive reversible spinal blocks on the amplitude of the PAN IFPs evoked at different times after the intradermal injection of capsaicin. We found that after capsaicin the PAN IFPs recorded in laminae III-V by activation of high-threshold nociceptive Aδ myelinated fibers increased gradually during successive reversible spinal blocks, while the IFPs evoked by intermediate and low threshold proprioceptive Aß afferents were only slightly affected. It is concluded that during the development of the central sensitization produced by capsaicin, there is a gradual increase of descending inhibition that tends to limit the nociceptive-induced facilitation, mainly by acting on the neuronal populations receiving inputs from the capsaicin-activated afferents without significantly affecting the information on joint angle transmitted by the low threshold afferents.

The effects of intraplantar and intrathecal botulinum toxin type B on tactile allodynia in mono and polyneuropathy in the mouse.

BACKGROUND: Mononeuropathies (MNs: nerve ligation) and polyneuropathies (PNs: cisplatin) produce unilateral and bilateral tactile allodynia, respectively. We examined the effects of intraplantar (IPLT) and intrathecal (IT) botulinum toxin B (BoNT-B) on this allodynia. METHODS: Mice (male c57Bl/6) were prepared with an L5 nerve ligation. Others received cisplatin (IP 2.3 mg/kg/d, every other day for 6 injections). Saline and BoNT-B were administered through the IPLT or IT route. We examined mechanical allodynia (von Frey hairs) before and at intervals after BoNT. As a control, we injected IPLT BoNT-B treated with dithiothreitol to cleave heavy chain from light chain. We measured motor function using acute thermal escape and sensorimotor tests. RESULTS: MN and PN mice showed a persistent ipsilateral and bilateral allodynia, respectively. IPLT BoNT-B resulted in an ipsilateral dorsal horn reduction in the synaptic protein target of BoNT-B (vesicle-associated membrane protein) and a long-lasting (up to approximately 17 days) reversal of allodynia in PN and MN models. The predominant effect after IPLT delivery was ipsilateral to IPLT BoNT. The effects of IPLT BoNT-B in MN mice were blocked by prior reduction of BoNT-B with dithiothreitol. IT BoNT-B in mice with PN resulted in a bilateral reversal of allodynia. With these dosing parameters, hind paw placing and stepping reflexes were unaltered, and there were no changes in thermal escape latencies. After cisplatin, dorsal root ganglions displayed increases in activation transcription factor 3, which were reduced by IT, but not IPLT BoNT-B. CONCLUSIONS: BoNT-B given IPLT and IT yields a long-lasting attenuation of the allodynia in mice displaying MN and PN allodynia.

Phosphorylation of the GluN1 subunit in dorsal horn neurons by remifentanil: a mechanism for opioid-induced hyperalgesia.

Remifentanil (an ultra-short acting µ-opioid receptor agonist) use has been associated with acute opioid tolerance and hyperalgesia. Previous electrophysiological studies have shown that remifentanil elicits rapid and prolonged upregulation of N-methyl-D-aspartate receptor (NMDAR) currents. However, the effect of remifentanil on the levels of the GluN1 subunit of the NMDAR in dorsal horn neurons (DHNs) has not been reported. We investigated the effect of remifentanil, along with ketamine (NMDAR antagonist) and naloxone (µ-opioid receptor antagonist), on GluN1 mRNA levels and the amount of phosphorylated GluN1 in primary cultures of embryonic rat DHNs. DHNs were isolated from 18-19-day rat embryos and treated with remifentanil or vehicle for 1 h. GluN1 mRNA and protein levels, determined by real time reverse transcription polymerase chain reaction (RT-PCR) and Western blot, respectively, were significantly and persistently increased by remifentanil exposure compared with the control group (P ＜ 0.05). These results may partially account for the mechanism of remifentanil-induced hyperalgesia. This increase was prevented by ketamine (NMDAR antagonist) and naloxone (µ-opioid receptors antagonist), thus providing a potential therapeutic mechanism for the prevention of opioid-induced hyperalgesia.

Pannexin 1: a novel participant in neuropathic pain signaling in the rat spinal cord.

Pannexin 1 (panx1) is a large-pore membrane channel expressed in many tissues of mammals, including neurons and glial cells. Panx1 channels are highly permeable to calcium and adenosine triphosphatase (ATP); on the other hand, they can be opened by ATP and glutamate, two crucial molecules for acute and chronic pain signaling in the spinal cord dorsal horn, thus suggesting that panx1 could be a key component for the generation of central sensitization during persistent pain. In this study, we examined the effect of three panx1 blockers, namely, 10panx peptide, carbenoxolone, and probenecid, on C-reflex wind-up activity and mechanical nociceptive behavior in a spared nerve injury neuropathic rat model involving sural nerve transection. In addition, the expression of panx1 protein in the dorsal horn of the ipsilateral lumbar spinal cord was measured in sural nerve-transected and sham-operated control rats. Sural nerve transection resulted in a lower threshold for C-reflex activation by electric stimulation of the injured hindpaw, together with persistent mechanical hypersensitivity to pressure stimuli applied to the paw. Intrathecal administration of the panx1 blockers significantly depressed the spinal C-reflex wind-up activity in both neuropathic and sham control rats, and decreased mechanical hyperalgesia in neuropathic rats without affecting the nociceptive threshold in sham animals. Western blotting showed that panx1 was similarly expressed in the dorsal horn of lumbar spinal cord from neuropathic and sham rats. The present results constitute the first evidence that panx1 channels play a significant role in the mechanisms underlying central sensitization in neuropathic pain.

Nongenomic effect of levothyroxine on the synchronous electrical activity of the spinal dorsal horn in the rat.

Levothyroxine (T4) has a well-known effect on the central nervous system (CNS). This effect requires hours of latency by genetic pathway. We tested for short latency nongenomic effects of T4 superfusion on the spinal dorsal horn (DH) evaluating lumbar somatosensory evoked potentials in rats. T4 increased N and P wave amplitudes and N wave area under the curve, but reduced P wave duration and N-P interval, suggesting that T4 exerts both excitatory and synchronizing effects on DH interneurons in less than 300 s, thus, providing evidence of nongenomic effects of T4 on DH.

Prolactin fractions from lactating rats elicit effects upon sensory spinal cord cells of male rats.

Recently it has been suggested that the neurohormone prolactin (PRL) could act on the afferent nociceptive neurons. Indeed, PRL sensitizes transient receptor potential vanilloid 1 (TRPV1) channels present in nociceptive C-fibers and consequently reduces the pain threshold in a model of inflammatory pain. Accordingly, high plasma PRL levels in non-lactating females have been associated with several painful conditions (e.g. migraine). Paradoxically, an increase of PRL secretion during lactation induced a reduction in pain sensitivity. This difference could be attributed to the fact that PRL secreted from the adenopituitary (AP) is transformed into several molecular variants by the suckling stimulation. In order to test this hypothesis, the present study set out to investigate whether PRL from AP of suckled (S) or non-suckled (NS) lactating rats affects the activity of the male Wistar wide dynamic range (WDR) neurons. The WDR neurons are located in the dorsal horn of the spinal cord and receive input from the first-order neurons (Ab-, Ad- and C-fibers). Spinal administration of prolactin variant from NS rats (NS-PRL) or prolactin variant from S rats (S-PRL) had no effect on the neuronal activity of non-nociceptive Ab-fibers. However, the activities of nociceptive Ad-fibers and C-fibers were: (i) increased by NS-PRL and (ii) diminished by S-PRL. Either NS-PRL or S-PRL enhanced the post-discharge activity. Taken together, these results suggest that PRL from S or NS lactating rats could either facilitate or depress the nociceptive responses of spinal dorsal horn cells, depending on the physiological state of the rats.

Injection of nerve growth factor into a low back muscle induces long-lasting latent hypersensitivity in rat dorsal horn neurons.

Little is known about the central mechanisms underlying the transition from local or regional to widespread pain in low back pain patients. The aim of the study was to find out if muscle input induced by injection of nerve growth factor (NGF) can be used as an animal model for studying spinal mechanisms involved in widespread myofascial low back pain. Electrophysiological recordings from rat dorsal horn neurons were made in vivo to study alterations in their responsiveness caused by 2 injections of NGF into the multifidus muscle at an interval of 5 days. NGF is known to be closely associated with many painful muscle disorders. The results demonstrate that the 2 NGF injections-but not a single one-caused a significant hyperexcitability of spinal neurons. Five days after the first NGF injection, the neurons were not significantly sensitized but were easier to sensitize by a second injection. The state of the neurons resembles nociceptive priming. Important findings were that the proportion of neurons having multiple receptive fields (RFs) in various tissues was significantly higher after 2 NGF injections, and new RFs appeared on the distal hind limb. The new RFs were located not in the skin but in deep tissues (muscles, thoracolumbar fascia). If similar changes occur in patients, the data might explain the diffuse nature and spread of myofascial low back pain.

Dorsal horn antinociception mediated by the paraventricular hypothalamic nucleus and locus coeruleous: a comparative study.

The noradrenergic descending projection originating in the locus coeruleous (LC), as well as the oxytocinergic descending projection originating in the paraventricular hypothalamic nucleus (PVN), plays a pivotal role in nociception. The mechanisms used by these two systems to modulate synaptic nociceptive transmission in the dorsal horn have been well studied independently. However, little is known about interactions between them. Here, it is shown that both PVN and LC electric stimulation inhibit A-delta, C-fiber, and postdischarge nociceptive neuronal responses in the same dorsal horn wide dynamic range neurons. Moreover, simultaneous stimulation of both the PVN and LC produces synergic inhibitory effects. In addition, LC electrolytic lesion or intrathecal administration of the alpha-2-adrenoceptor antagonist yohimbine (YOH) blocks the inhibitory effect produced by PVN stimulation in A-delta and the postdischarge, without affecting the inhibition of C-fiber responses. The results suggest that the PVN could inhibit dorsal horn nociceptive responses directly or indirectly by modulating the LC descending noradrenergic system.

Involvement of spinal cord BDNF in the generation and maintenance of chronic neuropathic pain in rats.

Brain-derived neurotrophic factor (BDNF) is involved in neuronal survival and synaptic plasticity of the central and peripheral nervous system. In chronic pain, plastic changes in dorsal horn neurons contribute to a phenomenon of hypersensitivity to pain sensation that is maintained over time, known as central sensitization. This process is accompanied by BDNF overexpression, but the role of BDNF in the generation and maintenance of the hyperalgesic phenomenon is still unclear. The present study was aimed to investigate if exogenous BDNF administered to the rat spinal cord, in addition to trigger pain, participates in the maintenance of the central sensitization process (i.e., pain persistence) and to determine if the pain generated is comparable to that observed in a neuropathic pain model. Results showed that a single intrathecal injection of 0.003 ng of BDNF was able to decrease the nociceptive threshold (Randall-Selitto test) in normal rats, for at least a 42-day period. Furthermore, the hyperalgesia generated was comparable to that observed in rats with a 42-day history of mononeuropathy. Increasing the dose or administering additional doses of BDNF resulted neither in additional effectiveness in reducing the pain threshold nor in the prolongation of the hyperalgesic effect, thus showing that central sensitization induced by BDNF is a dose-independent, all-or-none process. It is concluded that BDNF alone is sufficient for generating a long-lasting neural excitability change in the spinal cord via tyrosine kinase B receptor signaling, similar to that observed in chronic pain models such as neuropathy.

Direct blockade of inflammatory hypernociception by peripheral A1 adenosine receptors: involvement of the NO/cGMP/PKG/KATP signaling pathway.

Through activation of the A1 adenosine receptors (A1Rs) at both the central and peripheral level, adenosine produces antinociception in a wide range of tests. However, the mechanisms involved in the peripheral effect are still not fully understood. Therefore, the mechanisms by which peripheral activation of A1Rs reduces inflammatory hypernociception (a decrease in the nociceptive threshold) were addressed in the present study. Immunofluorescence of rat dorsal root ganglion revealed significant expression of A1Rs in primary sensory neurons associated with nociceptive pathways. Functionally, peripheral activation of A1Rs reduced inflammatory hypernociception because intraplantar (i.pl.) administration of an A1R antagonist (DPCPX) enhanced carrageenan-induced hypernociception. On the other hand, local (paw) administration of CPA (a selective A1R agonist) reversed mechanical hypernociception induced by carrageenan or by the directly acting hypernociceptive mediator prostaglandin E(2) (PGE(2)). Down-regulation of A1Rs expression in primary nociceptive neurons by intrathecal treatment with antisense oligodeoxinucleotides significantly reduced peripheral antinociceptive action of CPA. Direct blockade of PGE(2) inflammatory hypernociception by the activation of A1Rs depends on the nitric oxide/cGMP/Protein Kinase G/KATP signaling pathway because the peripheral antinociceptive effect of CPA was prevented by pretreatment with inhibitors of neuronal nitric oxide synthase (N-propyl-l-arginine), guanylyl cyclase (ODQ), and Protein Kinase G (KT5823) as well as with a KATP blocker (glibenclamide). However, this effect of CPA was not reduced by naloxone, excluding the participation of endogenous opioids. These results suggest that the peripheral activation of A1R plays a role in the regulation of inflammatory hypernociception by a mechanism that involves the NO/cGMP/PKG/KATP intracellular signaling pathway.

Oxytocin, but not vassopressin, modulates nociceptive responses in dorsal horn neurons.

Oxytocin (OT) and vasopressin (VP) are synthesized and secreted by the paraventricular hypothalamic nucleus (PVN), and both peptides have been implicated in the pain modulatory system. In the spinal cord, activation of OT-containing axons modulates nociceptive neuronal responses in dorsal horn neurons; however, it is not known whether the direct VPergic descending projection participates. Here, we show that both PVN electrical stimulation and topical application of OT in the vicinity of identified and recorded dorsal horn WDR selectively inhibit Adelta and C-fiber responses. In contrast, the topical administration of VP on the same neurons did not affect the nociceptive responses. In addition, the reduction in nociceptive responses caused by PVN stimulation or OT administration was blocked with a selective OT antagonist. The results suggest that the VP descending projection does not modulate the antinociceptive effects mediated by the PVN on dorsal horn neurons; instead, it is the hypothalamic-spinal OT projection that regulates nociceptive information.

Sildenafil and glyceryl trinitrate reduce tactile allodynia in streptozotocin-injected rats.

The possible antiallodynic effect of phosphodiesterase 5 inhibitor sildenafil and nitric oxide donor glyceryl trinitrate as well as the changes in phosphodiesterase 5A2 mRNA expression in dorsal root ganglion and spinal cord of allodynic diabetic rats was assessed. Diabetes was induced by streptozotocin (50mg/kg, i.p.) in male Wistar rats. Streptozotocin injection produced hyperlglycemia, polydipsia, polyphagia and polyuria as well as long-term tactile allodynia (12 weeks) and a reduction of phosphodiesterase 5A2 mRNA expression in spinal cord of diabetic rats. Systemic administration of sildenafil (1-5.6 mg/kg, i.p.) reduced tactile allodynia in a dose-dependent manner in diabetic rats. Likewise, glyceryl trinitrate patches (0.2mg/h) also reduced tactile allodynia in diabetic rats. Moreover, both drugs reversed streptozotocin-induced phosphodiesterase 5A2 mRNA expression reduction. Our results indicate that glyceryl trinitrate and sildenafil reduce tactile allodynia in diabetic rats suggesting that nitric oxide and cyclic GMP supply is an important step in their mechanism of action of these drugs in diabetic animals. Data suggest that nitric oxide donors (as glyceryl trinitrate) and drugs which increase cyclic GMP levels (as sildenafil) could have a role in the pharmacotherapy of tactile allodynia in diabetic patients.

Heat hyperalgesia induced by endoneurial nerve growth factor and the expression of substance P in primary sensory neurons.

Endoneurial nerve growth factor (30 ng) produced significant heat hyperalgesia in rats on postinjection days 3 and 5. The percentage of neuron profiles expressing the sensory neuropeptide substance P in the dorsal root ganglion (DRG), and the density and distribution of substance P immunoreactivity at the DRG and the dorsal horn remained essentially unchanged throughout the 10 days of study. NGF increased pain scores in the second phase of the formalin test on postinjection day 3, but not on days 5 and 10. Our results indicate that the observed heat hyperalgesia is not dependent on NGF-induced changes in SP content and release from primary sensory neurons.

Hypothalamospinal oxytocinergic antinociception is mediated by GABAergic and opiate neurons that reduce A-delta and C fiber primary afferent excitation of spinal cord cells.

Recent results implicate a new original mechanism involving oxytocin (OT), as a mediator via descending fibers of the paraventricular hypothalamic nucleus (PVN), in antinociception and analgesia. In rats electrical stimulation of the PVN or topical application of OT selectively inhibits A-delta and C fiber responses in superficial dorsal horn neurons, and this inhibition is reversed by a selective OT antagonist. However, little is known about the mechanisms and the spinal elements participating in this phenomenon. Here we show that topical application of bicuculline blocks the effects produced by PVN electrical stimulation or OT application. PVN electrical stimulation also activates a subpopulation of neurons in lamina II. These PVN-On cells are responsible for the amplification of local GABAergic inhibition. This result reinforces the suggestion that a supraspinal descending control of pain processing uses a specific neuronal pathway in the spinal cord in order to produce antinociception involving a GABAergic interneuron. Moreover, the topical administration of naloxone or a mu-opiate receptor antagonist beta-funaltrexamine only partially blocks the inhibitory effects produced by OT application or PVN electrical stimulation. Thus, this OT mechanism only involves opiate participation to a minor extent. The OT-specific, endogenous descending pathway represents an interesting mechanism to resolve chronic pain problems in special the neuropathic pain.

Expression of nitric oxide synthase isoforms in the dorsal horn of monoarthritic rats: effects of competitive and uncompetitive N-methyl-D-aspartate antagonists.

Chronic pain is associated with N-methyl-D-aspartate (NMDA) receptor activation and downstream production of nitric oxide, which has a pivotal role in multisynaptic local circuit nociceptive processing in the spinal cord. The formation of nitric oxide is catalyzed by three major nitric oxide synthase (NOS) isoforms (neuronal, nNOS; inducible, iNOS; endothelial, eNOS), which are increased in the spinal cord of rodents subjected to some tonic and chronic forms of experimental pain. Despite the important role of NOS in spinal cord nociceptive transmission, there have been no studies exploring the effect of NMDA receptor blockade on NOS expression in the dorsal horn during chronic pain. Furthermore, NOS isoforms have not been fully characterized in the dorsal horn of animals subjected to arthritic pain. The aim of this work was therefore to study the expression of nNOS, iNOS and eNOS in the dorsal horns of monoarthritic rats, and the modifications in NOS expression induced by pharmacological blockade of spinal cord NMDA receptors. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the right tibio-tarsal joint. At week 4, monoarthritic rats were given either the competitive NMDA antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or the uncompetitive NMDA antagonist ketamine. After 6 and 24 hours, animals were killed and posterior quadrants of the lumbar spinal cord were dissected. Sample tissues were homogenized and subjected to immunoblotting with anti-nNOS, anti-iNOS or anti-eNOS monoclonal antibodies. The nNOS isoform, but not the iNOS and eNOS isoforms, were detected in the dorsal horns of control rats. Monoarthritis increased the expression of nNOS, iNOS and eNOS in the dorsal horns ipsilateral and contralateral to the inflamed hindpaw. Intrathecal administration of CPP and ketamine reduced nNOS expression in monoarthritic rats but increased the expression of iNOS and eNOS. Results suggest that blockade of spinal cord NMDA receptors produces complex regulatory changes in the expression of NOS isoforms in monoarthritic rats that may be relevant for nitridergic neuronal/glial mechanisms involved in the pathophysiology of monoarthritis and in the pharmacological response to drugs interacting with NMDA receptors.

GABA-mediated oxytocinergic inhibition in dorsal horn neurons by hypothalamic paraventricular nucleus stimulation.

In anaesthetized rats, we tested whether the unit activity of dorsal horn neurons that receive nociceptive input is modulated by electrical stimulation of the hypothalamic paraventricular nucleus (PVN). An electrophysiological mapping of dorsal horn neurons at L3-L4 let us choose cells responding to a receptive field located in the toes region of the left hindpaw. Dorsal horn neurons were classified according to their response properties to peripheral stimulation. Wide Dynamic Range (WDR) cells responding to electrical stimulation of the peripheral receptive field and presenting synaptic input of Adelta, Abeta, and C-fibers were studied. Suspected interneurons that are typically silent and lack peripheral receptive field responses were also analyzed. PVN electrical stimulation inhibits Adelta (-55.0+/-10.2%), C-fiber (-73.1+/-6.7%), and post-discharge (-75.0+/-8.9%) peripheral activation in WDR cells, and silent interneurons were activated. So, this last type of interneuron was called a PVN-ON cell. In WDR cells, the inhibition of peripheral responses caused by PVN stimulation was blocked by intrathecal administration of a specific oxytocin antagonist or bicuculline. However, PVN-ON cell activation was blocked by the same specific oxytocin antagonist, but not by bicuculline. Our results suggest that PVN stimulation inhibits nociceptive peripheral-evoked responses in WDR neurons by a descending oxytocinergic pathway mediated by GABAergic PVN-ON cells. We discuss our observation that the PVN electrical stimulation selectively inhibits Adelta and C-fiber activity without affecting Abeta fibers. We conclude that Adelta and C-fibers receive a presynaptic inhibition mediated by GABA.

The C-terminus of murine S100A9 protein inhibits hyperalgesia induced by the agonist peptide of protease-activated receptor 2 (PAR2).

BACKGROUND AND PURPOSE: S100A9 protein induces anti-nociception in rodents, in different experimental models of inflammatory pain. Herein, we investigated the effects of a fragment of the C-terminus of S100A9 (mS100A9p), on the hyperalgesia induced by serine proteases, through the activation of protease-activated receptor-2 (PAR2). EXPERIMENTAL APPROACH: Mechanical and thermal hyperalgesia induced by PAR2 agonists (SLIGRL-NH2 and trypsin) was measured in rats submitted to the paw pressure or plantar tests, and Egr-1 expression was determined by immunohistochemistry in rat spinal cord dorsal horn. Calcium flux in human embryonic kidney cells (HEK), which naturally express PAR2, in Kirsten virus-transformed kidney cells, transfected (KNRK-PAR2) or not (KNRK) with PAR2, and in mouse dorsal root ganglia neurons (DRG) was measured by fluorimetric methods. KEY RESULTS: mS100A9p inhibited mechanical hyperalgesia induced by trypsin, without modifying its enzymatic activity. Mechanical and thermal hyperalgesia induced by SLIGRL-NH2 were inhibited by mS100A9p. SLIGRL-NH2 enhanced Egr-1 expression, a marker of nociceptor activation, and this effect was inhibited by concomitant treatment with mS100A9p. mS100A9p inhibited calcium mobilization in DRG neurons in response to the PAR2 agonists trypsin and SLIGRL-NH2, but also in response to capsaicin and bradykinin, suggesting a direct effect of mS100A9 on sensory neurons. No effect on the calcium flux induced by trypsin or SLIGRL in HEK cells or KNRK-PAR2 cells was observed. CONCLUSIONS AND IMPLICATIONS: These data demonstrate that mS100A9p interferes with mechanisms involved in nociception and hyperalgesia and modulates, possibly directly on sensory neurons, the PAR2-induced nociceptive signal.

Actions of oxytocin and interactions with glutamate on spontaneous and evoked dorsal spinal cord neuronal activities.

Among the numerous pain control mechanisms that have been proposed, those acting at the spinal cord have been broadly studied, but little is known about how neuropeptides originating in supraspinal structures may relate to pain and analgesic mechanisms. Oxytocin (OT), in addition to its well known hormonal action, produces neuronal effects in various regions of the central nervous system. Indeed, some parvocellular neurons in the hypothalamic paraventricular nucleus (PVN) are oxytocinergic and project to the caudal part of the brain and the spinal cord. Moreover, the rat spinal cord shows a good overlap between the oxytocinergic hypothalamo-spinal neuron projections and the distribution of OT binding sites. However, the physiological significance of these binding sites is largely unknown. Extracellular unit activity of spinal cord neurons was recorded at the T13-L1 levels in male rats anesthetized with halotane. Somatic stimulation was applied to the inner and outer thigh of the ipsilateral hindpaw, and glutamate (GLU) and OT were locally delivered by pressure using pipettes coupled to recording electrodes. Our results show that spinal cord neurons, mainly located in the dorsal horn, in the intermediolateral cell column (IML) and in the intermediomedial gray matter (IMM), respond to the application of OT (71.5%) with activation (48%) or inhibition (52%). In some cases, opposite OT effects were observed during simultaneous recordings of two cells, suggesting OT activation of an inhibitory interneuron followed by the inhibition of the second recorded neuron. Increases in neuronal firing rate produced by GLU could be blocked by prior OT application. Finally, OT could reduce or partially block the responses to tactile and nociceptive somatic stimulation. We found that spinal cord neurons are sensitive to OT indicating that OT binding sites are functionally active. OT effects suggest the activation of inhibitory interneurons acting on a second order projecting cells to modulate afferent tactile and nociceptive information.

Nitric oxide modulates spontaneous cord dorsum potentials in the cat spinal cord.

A previous study has shown that lumbar spontaneous cord dorsum potentials (CDPs) are produced by background activity of a neuronal ensemble located in the dorsal horn. Here, the effects produced by intravenous application of the nitric oxide synthase inhibitor L-N(G)-nitro arginine (L-NOARG, 100 microg/kg) and of the nitric oxide donor 3-morpholinosydnonimine hydrochloride (SIN-1, 500 microg/kg) on spontaneous CDPs were examined. Experiments were performed on pentobarbitally anesthetized, paralyzed and spinalized cats. The amplitude of spontaneous CDPs increased after L-NOARG, however, decreased after SIN-1. These observations suggest that electrical activity of dorsal horn neurones generating spontaneous CDPs is dependent on nitric oxide production.

Evidence for down-regulation of GAP-43 mRNA in Wobbler mouse spinal motoneurons by corticosterone and a 21-aminosteroid.

Expression of the growth-associated protein GAP-43 is increased in the spinal cord of ALS patients and Wobbler (wr) mice, murine models of the disease. In this work we examined if expression of GAP-43 mRNA in control and wr mice was sensitive to steroid treatment. A group of control and wr mice received s.c. a 50 mg pellet of the natural hormone corticosterone (CORT) or the antioxidant 21-aminosteroid U-74389F during 4 days. Basal levels of GAP-43 mRNA were 10-fold elevated in ventral horn motoneurons of untreated wr mice, compared to the low levels in controls. The high expression of GAP-43 mRNA in wr was attenuated by treatment with CORT (41%, p < 0.001) and U-74389F (36%, p < 0.001). Although specific GAP-43 mRNA labelling was present in some neurons around the central canal, its cellular expression was similar in controls and wr. Also, steroid treatment was ineffective in neurons around the central canal. Other regions of the spinal cord (i.e., dorsal horn neurons) expressed GAP-43 mRNA slightly above background levels. It is possible that attenuation of GAP-43 expression due to the natural hormone and the antioxidant steroid resulted from reversal of motoneuron degeneration or aberrant sprouting. Therefore, steroid therapy may be of value to prevent denervation and/or muscular atrophy in this animal model.