Guadalupe Martínez-Lorenzana

Oxytocin and electrical stimulation of the paraventricular hypothalamic nucleus produce antinociceptive effects that are reversed by an oxytocin antagonist

Abstract In recent years, oxytocin has been implicated in a wide diversity of functions. The role of oxytocin in analgesia and pain modulation represents an important new function of an endogenous system controlling sensorial information. The paraventricular (PV) nucleus of the hypothalamus is one of the most important sources of oxytocin, and it has a very well‐defined projection to the spinal cord. The location of this PV spinal cord projection correlates well with oxytocin binding sites at the dorsal horn of the spinal cord. In this work, we used rats with a chronic (46 days) sciatic loose ligature, an electrical stimulating electrode, and an intrathecal cannula, which reached the L4–L5 levels of the spinal cord. We compared the oxytocin effects with electrical stimulation of the PV and observed a significant reduction of the withdrawal responses to mechanical and cold stimulation applied to the ipsilateral and contralateral hind paws. An oxytocin antagonist administered intrathecally blocked the PV effects. Naloxone was also intrathecally injected 2 min before the PV stimulation, and we also observed a significant reduction of the withdrawal responses; however, this reduction was less pronounced. Our results support the hypothesis that oxytocin is part of the descending inhibitory control mechanisms having an important antinociceptive action. We cannot exclude a minor opiate participation in the OT action.

Paraventricular hypothalamic influences on spinal nociceptive processing.

Oxytocin properties have been studied in different experimental models in order to obtain evidence for its analgesic properties. The analgesic effect of an oxytocinergic pathway descending from the hypothalamus reaching the dorsal horn of the spinal cord has been studied. In anesthetized rats, we recorded single units at the L4-L5 spinal dorsal horn level and stimulated the peripheral receptive field. The evoked responses were classified according to their latencies in A-beta, A-delta, C fibers, and postdischarge. We used these responses to evaluate the effects of electrical stimulation of the paraventricular nucleus (PV) of the hypothalamus. We observed a selective blockage of A-delta and C fibers related to the duration of the train stimulus duration. Similar effects were observed when oxytocin (OT) was applied directly on the spinal cord. The effects of OT and of PV electrical stimulation were reversed in a dose-dependent manner by application of the specific OT antagonist (OTA). These effects were observed in cells with reduced wind-up and cells displaying a clear wind-up response to peripheral stimulation. Superficial and deeper cells in the dorsal spinal cord were involved. The recorded cells were marked by pontamine blue iontophoretic injection after each cell recording, and their histological locations were specified. In order to obtain a behavioral correlation, we used rats with a loose ligature of the sciatic nerve and a chronic intrathecal catheter reaching the L4-L5 spinal cord level. We tested the hyperalgesia and allodynia of these animals using von Frey filaments and the application of acetone to the hind paws. Our results show a significant reduction in the mechanical and thermal test after the administration of 15 microl of 10(-6) M OT. Our electrophysiological, pharmacological, and behavioral results point out a clear OT antialgesic effect. The results are discussed on the basis of a previous work showing an OT blockage of glutamate activation. The paraventricular hypothalamic descending OT pathway is proposed as an interesting mechanism producing analgesia.

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.

PVN electrical stimulation prolongs withdrawal latencies and releases oxytocin in cerebrospinal fluid, plasma, and spinal cord tissue in intact and neuropathic rats

Abstract We are studying an endogenous, oxytocinergic analgesia system to obtain more information about normal and pathological pain processes. In the recent years, this oxytocinergic system has been shown to be involved in normal and pathological pain suppression. The paraventricular nucleus (PVN) of the hypothalamus is an important source of brain oxytocin (OT). A descending pathway reaching the dorsal horn in the spinal cord was postulated to mediate analgesic effects at the spinal cord level. However, the oxytocin concentration during pain conditions and during PVN electrical stimulation needs to be determined. We designed experiments to measure the OT concentration in cerebrospinal fluid (CSF), plasma, and OT protein in lumbar spinal cord tissue in control and neuropathic rats. Sciatic loose ligature was used as the experimental method to produce neuropathic pain. The main findings were (1) Chronic pain experiments in animals showed that the stimulation of the anterior part of the PVN increased OT concentration and produced analgesia states, as measured by von Frey, cold, and heat plantar tests. (2) Differential effects were produced by electrical stimulation of the anterior or posterior regions of the PVN; electrical stimulation of the anterior part of the PVN enhanced the OT concentration in CSF and plasma, and it also increased OT protein concentrations in the spinal cord tissue; in contrast, the stimulation of the posterior part of the PVN only increased OT concentrations in CSF. These results suggest the participation of an endogenous analgesia system mediated by OT.

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.

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.

Paraventricular oxytocinergic hypothalamic prevention or interruption of long-term potentiation in dorsal horn nociceptive neurons: Electrophysiological and behavioral evidence

ABSTRACT Spinal long‐term potentiation (LTP) elicited by noxious stimulation enhances the responsiveness of dorsal horn nociceptive neurons to their normal input, and may represent a key mechanism of central sensitization by which acute pain could turn into a chronic pain state. This study investigated the electrophysiological and behavioral consequences of the interactions between LTP and descending oxytocinergic antinociceptive mechanisms mediated by the hypothalamic paraventricular nucleus (PVN). PVN stimulation or intrathecal oxytocin (OT) reduced or prevented the ability of spinal LTP to facilitate selectively nociceptive‐evoked responses of spinal wide dynamic range (WDR) neurons recorded in anesthetized rats. In a behavioral model developed to study the effects of spinal LTP on mechanical withdrawal thresholds in freely moving rats, the long‐lasting LTP‐mediated mechanical hyperalgesia was transiently interrupted or prevented by either PVN stimulation or intrathecal OT. LTP mediates long‐lasting pain hypersensitivity that is strongly modulated by endogenous hypothalamic oxytocinergic descending controls.

Identification of oxytocin receptor in the dorsal horn and nociceptive dorsal root ganglion neurons

Oxytocin (OT) secreted by the hypothalamo-spinal projection exerts antinociceptive effects in the dorsal horn. Electrophysiological evidence indicates that OT could exert these effects by activating OT receptors (OTR) directly on dorsal horn neurons and/or primary nociceptive afferents in the dorsal root ganglia (DRG). However, little is known about the identity of the dorsal horn and DRG neurons that express the OTR. In the dorsal horn, we found that the OTR is expressed principally in neurons cell bodies. However, neither spino-thalamic dorsal horn neurons projecting to the contralateral thalamic ventral posterolateral nucleus (VPL) and posterior nuclear group (Po) nor GABaergic dorsal horn neurons express the OTR. The OTR is not expressed in skin nociceptive terminals or in dorsal horn nociceptive fibers. In the DRG, however, the OTR is expressed predominantly in non-peptidergic C-fiber cell bodies, but not in peptidergic or mechanoreceptor afferents or in skin nociceptive terminals. Our results suggest that the antinociceptive effects of OT are mediated by direct activation of dorsal horn neurons and peripheral actions on nociceptive, non-peptidergic C-afferents in the DRG.

Branched oxytocinergic innervations from the paraventricular hypothalamic nuclei to superficial layers in the spinal cord

The paraventricular nucleus (PVN) of the hypothalamus is an interesting structure with diverse functions due to its different neuronal populations, neurotransmitters, and projections to other central nervous system structures. The PVN is a primary source of oxytocin (OT) in the central nervous system. In fact, a direct PVN projection to the spinal cord has been demonstrated by retrograde and anterograde tracers, and more than the 50% of this projection is oxytocinergic. This OT descending projection is proposed to be an endogenous system that controls the nociceptive information arriving at the spinal cord. However, we have no information about the specific organization of the OT descending innervations to the different spinal cord segments. The aim of the present study was to determine whether the projecting PVN neurons arrive at cervical regions and then continue to lumbar regions. That is, we sought to establish if the OT projecting cells have a topic or a diffuse projection in order to obtain histological data to support the endogenous OT diffuse mechanism of analgesia described elsewhere. With this purpose in mind we combined the OT immunohistochemistry technique with retrograde neuronal tracers in the spinal cord. We applied Diamidino Yellow (DY) for the superficial dorsal horn cervical segments and True Blue (TB) for the lumbar segments. Data were collected from eight rats with well-placed injections. We only used the animals in which the tracer deposits were confined to superficial layers I and II of the dorsal horn. A mainly ipsilateral projection was observed, but stained neurons were also observed in the contralateral PVN. A large fraction of the stained PVN cells was doubled labeled but some were single labeled. Combining the retrograde tracer techniques and the OT detection procedure, we observed triple-labeled neurons. The present results demonstrate that PVN neurons send collaterals at least to the superficial cervical and lumbar segments of the dorsal horn of the spinal cord. Moreover, some of these stained cells use OT as a neurotransmitter. These results are of great relevance since they demonstrate that the PVN plays an important role in the somatosensorial system, and they support anatomic evidence of an endogenous mechanism involved in analgesia. Finally, we also describe median raphe nucleus double-labeled cells (DY+TB) signaling diffuse descending projections for this largely studied nucleus that are involved in endogenous analgesia.

Paraventricular hypothalamic oxytocinergic cells responding to noxious stimulation and projecting to the spinal dorsal horn represent a homeostatic analgesic mechanism

The participation of the hypothalamic paraventricular nucleus (PVN) in an endogenous central mechanism of analgesia has been observed using rats in various experimental procedures including electrophysiological and behavioral tests. However, little is known about the PVN neuronal responses to noxious stimulation. The only data available indicate a c‐fos increase after noxious visceral stimulations. Our electrophysiological recordings of single PVN cells showed that, out of 223 cells, 79 responded to noxious mechanical and/or thermal stimuli, and another 10 responsive cells were found in the Reuniers thalamic nucleus. These cells responded only to noxious stimuli mainly in the ipsilateral hind limb but we also observed cells responding to stimulation of both hind limbs and also the tail. Mechanical stimulation was most effective but some cells could respond to both mechanical and thermal noxious stimuli. Some of the responding PVN cells were identified by antidromic stimulation in the ipsilateral lumbar dorsal horn spinal cord. Finally, in order to document the nature of the neurotransmitter and the projection to the spinal cord of the PVN cells that responded to noxious stimulation, we used a juxtacellular approach to record and stain some neurons and found them to be oxytocinergic by immunofluorescence procedures. The PVN cells activated by noxious stimuli may suppress the peripheral incoming afferent A‐delta and C fibers, completing a circuit involved in diffuse endogenous analgesia. This mechanism strongly suggests that the PVN participates in a homeostatic mechanism involved in pain and analgesia.