Hypothalamic cerebrospinal fluid-contacting neurons project to the rostral agranular insular cortex: An immunofluorescence and ultrastructural analysis in the rat.

Cerebrospinal fluid-contacting neurons (CSF-cNS) are considered mechanoreceptors and chemoreceptors involved in detecting changes in CSF circulation. However, considering that recent data suggest that this type of cell could exert an active response when an external stimulus is sensed, identification of CSF-cNS may be relevant. In this regard, some data suggest that a neuronal connection exists between the ventral region of the hypothalamic paraventricular nucleus (PVN) and rostral agranular insular cortex (RAIC); indeed, a potential CSF-cNS is hypothesized. However, a detailed analysis of this connection has not been conducted. Thus, using neuronal tracers (Fluoro-Gold® (FG) and cholera toxin (ChT)) coupled with transmission electron microscopy and immunofluorescence assays against Fluoro-Gold®, oxytocin (OXT), vasopressin (AVP) and oxytocin receptors (OTR), we describe an oxytocinergic or vasopressinergic CSF-cNS between the PVN and RAIC. Our results showed that CSF-cNS along the PVN labelled with oxytocin and/or AVP were present in dendritic projections near the third ventricle. This CSF-cNS in the PVN seems to project to the RAIC. Inside the RAIC, ultrastructural analysis showed that axons immunopositive for oxytocin from the PVN sustained synaptic connections with neurons that expressed OTR. These findings show that the CSF-cNS from the PVN sends projections to the RAIC. To the best of our knowledge, the relevance of CSF-cNS has not been elucidated; however, we hypothesized that the activation of cells could concomitantly release neuropeptides (i.e., oxytocin and AVP) in the CSF and RAIC. Thus, further analysis of the impact of neuropeptides released into the third ventricle and RAIC is warranted.

The differential in vivo contribution of spinal α2A- and α2C-adrenoceptors in tonic and acute evoked nociception in the rat.

Spinal α2-adrenoceptor induces analgesia by neuronal inhibition of primary afferent fibers. This family receptor coupled to G i/o proteins can be subdivided into three functional subtypes: α2A, α2B, and α2C-adrenoceptors, and current evidence on spinal analgesia supports the relevance of α2A and seems to exclude the role of α2B, but the functional contribution of α2C-adrenoceptors remains elusive. The present study was designed to pharmacologically dissect the contribution of spinal α2-adrenoceptor subtypes modulating tonic or acute peripheral nociception. Using male Wistar rats, we analyzed the effect of spinal clonidine (a non-selective α2A/α2B/α2C-adrenoceptor agonist) and/or selective subtype α2-adrenoceptor antagonists on: 1) tonic nociception induced by subcutaneous formalin (flinching behavior) or 2) acute nociception induced by peripheral electrical stimulus in in vivo extracellular recordings of spinal dorsal horn second-order wide dynamic range (WDR) neurons. Clonidine inhibited the nocifensive behavior induced by formalin, an effect blocked by BRL 44408 (α2A-adrenoceptor antagonist) but not by imiloxan (α2B-adrenoceptor antagonist) or JP 1302 (α2C-adrenoceptor antagonist). Similarly, spinal BRL 44408 reversed the clonidine-induced inhibition of nociceptive WDR activity. Interestingly, spinal JP 1302 per se produced behavioral antinociception (an effect blocked by bicuculline, a preferent GABAA channel blocker), but no correlation was found with the electrophysiological experiments. These data imply that, at the spinal level, 1) presynaptic α2A-adrenoceptor activation produces antinociception during acute or tonic nociceptive stimuli; and 2) under tonic nociceptive (inflammatory) input, spinal α2C-adrenoceptors are pronociceptive, probably by the inactivation of GABAergic transmission. This result supports a differential role of α2A and α2C-adrenoceptors modulating nociception.

CLARITY with neuronal tracing and immunofluorescence to study the somatosensory system in rats.

BACKGROUND: The CLARITY technique enables researchers to visualize different neuronal connections along the nervous system including the somatosensory system. NEW METHOD: The present work describes the antero-lateral and dorsal column pathways until the thalamic and cortical stations, as well as descending oxytocinergic and vasopressinergic innervations by means of combined CLARITY, neuronal tracing, and immunofluorescence techniques. We used male Sprague-Dawley rats of 13, 30, and 60 days. RESULTS: The main results are as follows: A) CLARITY is a reliable technique that can be combined with fluorescent neuronal tracers and immunofluorescence techniques without major procedure modifications; B) at spinal level, some primary afferent fibers were labeled by CGRP, as well as the presence of neuronal populations that simultaneously project to the gracile and ventral posterolateral thalamic nuclei; C) corticothalamic connections were visible when retrograde tracers were injected at thalamic level; D) oxytocin receptors were expressed in the spinal dorsal horn by GABAergic-positive neurons, reinforcing previous outcomes about the possible mechanism for oxytocin blocking the primary afferent sensory input. COMPARISON WITH EXISTING METHODS AND CONCLUSIONS: The CLARITY technique lets us observe in a transparent way the entire processed tissue compared with classical histological methods. CLARITY is a potentially useful tool to describe neuroanatomical structures and their neurochemical stratus.

Intrathecal Oxytocin Improves Spontaneous Behavior and Reduces Mechanical Hypersensitivity in a Rat Model of Postoperative Pain.

The first few days post-surgery, patients experience intense pain, hypersensitivity and consequently tend to have minor locomotor activity to avoid pain. Certainly, injury to peripheral tissues produces pain and increases sensitivity to painful (hyperalgesia) and non-painful (allodynia) stimuli. In this regard, preemptive pharmacological treatments to avoid or diminish pain after surgery are relevant. Recent data suggest that the neuropeptide oxytocin when given at spinal cord level could be a molecule with potential preemptive analgesic effects, but this hypothesis has not been properly tested. Using a validated postoperative pain model (i.e. plantar incision), we evaluated in male Wistar rats the potential preemptive antinociceptive effects of intrathecal oxytocin administration measuring tactile hypersensitivity (across 8 days) and spontaneous motor activity (across 3 days). Hypersensitivity was evaluated using von Frey filaments, whereas spontaneous activity (total distance, vertical activity episodes, and time spent in the center of the box) was assessed in real time using a semiautomated open-field system. Under these conditions, we found that animals pretreated with spinal oxytocin before plantar incision showed a diminution of hypersensitivity and an improvement of spontaneous behavior (particularly total distance and vertical activity episodes). This report provides a basis for addressing the therapeutic relevance of oxytocin as a potential preemptive analgesic molecule.

The Rostral Agranular Insular Cortex, a New Site of Oxytocin to Induce Antinociception.

The rostral agranular insular cortex (RAIC) is a relevant structure in nociception. Indeed, recruitment of GABAergic activity in RAIC promotes the disinhibition of the locus ceruleus, which in turn inhibits (by noradrenergic action) the peripheral nociceptive input at the spinal cord level. In this regard, at the cortical level, oxytocin can modulate the GABAergic transmission; consequently, an interaction modulating nociception could exist between oxytocin and GABA at RAIC. Here, we tested in male Wistar rats the effect of oxytocin microinjection into RAIC during an inflammatory (by subcutaneous peripheral injection of formalin) nociceptive input. Oxytocin microinjection produces a diminution of (1) flinches induced by formalin and (2) spontaneous firing of spinal wide dynamic range cells. The above antinociceptive effect was abolished by microinjection (at RAIC) of the following: (1) L-368899 (an oxytocin receptor [OTR] antagonist) or by (2) bicuculline (a preferent GABAA receptor blocker), suggesting a GABAergic activation induced by OTR. Since intrathecal injection of an α2A-adrenoceptor antagonist (BRL 44408) partially reversed the oxytocin effect, a descending noradrenergic antinociception is suggested. Further, injection of L-368899 per se induces a pronociceptive behavioral effect, suggesting a tonic endogenous oxytocin release during inflammatory nociceptive input. Accordingly, we found bilateral projections from the paraventricular nucleus of the hypothalamus (PVN) to RAIC. Some of the PVN-projecting cells are oxytocinergic and destinate GABAergic and OTR-expressing cells inside RAIC. Aside from the direct anatomic link between PVN and RAIC, our findings provide evidence about the role of oxytocinergic mechanisms modulating the pain process at the RAIC level.SIGNIFICANCE STATEMENT Oxytocin is a neuropeptide involved in several functions ranging from lactation to social attachment. Over the years, the role of this molecule in pain processing has emerged, showing that, at the spinal level, oxytocin blocks pain transmission. The present work suggests that oxytocin also modulates pain at the cortical insular level by favoring cortical GABAergic transmission and activating descending spinal noradrenergic mechanisms. Indeed, we show that the paraventricular hypothalamicnucleus sends direct oxytocinergic projections to the rostral agranular insular cortex on GABAergic and oxytocin receptor-expressing neurons. Together, our data support the notion that the oxytocinergic system could act as an orchestrator of pain modulation.

Oxytocin prevents neuronal network pain-related changes on spinal cord dorsal horn in vitro.

Recently, oxytocin (OT) has been studied as a potential modulator of endogenous analgesia by acting upon pain circuits at the spinal cord and supraspinal levels. Yet the detailed action mechanisms of OT are still undetermined. The present study aimed to evaluate the action of OT in the spinal cord dorsal horn network under nociceptive-like conditions induced by the activation of the N-methyl-d-aspartate (NMDA) receptor and formalin injection, using calcium imaging techniques. Results demonstrate that the spontaneous Ca2+-dependent activity of the dorsal horn cells was scarce, and the coactivity of cells was mainly absent. When NMDA was applied, high rates of activity and coactivity occurred in the dorsal horn cells; these rates of high activity mimicked the activity dynamics evoked by a neuropathic pain condition. In addition, although OT treatment increased activity rates, it was also capable of disrupting the conformation of coordinated activity previously consolidated by NMDA treatment, without showing any effect by itself. Altogether, our results suggest that OT globally prevents the formation of coordinated patterns previously generated by nociceptive-like conditions on dorsal horn cells by NMDA application, which supports previous evidence showing that OT represents a potential therapeutic alternative for the treatment of chronic neuropathic pain.

Inhibition of nociceptive dural input to the trigeminocervical complex through oxytocinergic transmission.

Migraine is a complex brain disorder that involves abnormal activation of the trigeminocervical complex (TCC). Since an increase of oxytocin concentration has been found in cerebrospinal fluid in migrainous patients and intranasal oxytocin seems to relieve migrainous pain, some studies suggest that the hypothalamic neuropeptide oxytocin may play a role in migraine pathophysiology. However, it remains unknown whether oxytocin can interact with the trigeminovascular system at TCC level. The present study was designed to test the above hypothesis in a well-established electrophysiological model of migraine. Using anesthetized rats, we evaluated the effect of oxytocin on TCC neuronal activity in response to dural nociceptive trigeminovascular activation. We found that spinal oxytocin significantly reduced TCC neuronal firing evoked by meningeal electrical stimulation. Furthermore, pretreatment with L-368,899 (a selective oxytocin receptor antagonist, OTR) abolished the oxytocin-induced inhibition of trigeminovascular neuronal responses. This study provides the first direct evidence that oxytocin, probably by OTR activation at TCC level inhibited dural nociceptive-evoked action potential in this complex. Thus, targeting OTR at TCC could represent a new avenue to treat migraine.

Recurrent antinociception induced by intrathecal or peripheral oxytocin in a neuropathic pain rat model.

The search for new ligands to treat neuropathic pain remains a challenge. Recently, oxytocin has emerged as an interesting molecule modulating nociception at central and peripheral levels, but no attempt has been made to evaluate the effect of recurrent oxytocin administration in neuropathic pain. Using male Wistar rats with spinal nerve ligation, we evaluated the effects of recurrent spinal (1 nmol; given by lumbar puncture) or peripheral (31 nmol; given by intraplantar injection in the ipsilateral paw to spinal nerve ligation) oxytocin administration on pain-like behavior in several nociceptive tests (tactile allodynia and thermal and mechanical hyperalgesia) on different days. Furthermore, we used an electrophysiological approach to analyze the effect of spinal 1 nmol oxytocin on the activity of spinal dorsal horn wide dynamic range cells. In neuropathic rats, spinal or peripheral oxytocin partially restored the nociceptive threshold measured with the von Frey filaments (tactile allodynia), Hargreaves (thermal hyperalgesia) and Randall-Selitto (mechanical hyperalgesia) tests for 12 days. These results agree with electrophysiological data showing that spinal oxytocin diminishes the neuronal firing of the WDR neurons evoked by peripheral stimulation. This effect was associated with a decline in the activity of primary afferent Aδ- and C-fibers. The above findings show that repeated spinal or peripheral oxytocin administration attenuates the pain-like behavior in a well-established model of neuropathic pain. This study provides a basis for addressing the therapeutic relevance of oxytocin in chronic pain conditions.

Peptidergic nature of nociception-related projections from the hypothalamic paraventricular nucleus to the dorsal horn of the spinal cord.

Hypothalamic paraventricular nucleus (PVN) projections to the spinal dorsal horn (SDH) are related to antinociception. Several neuropeptides from this nucleus could be released to the spinal cord after nociceptive stimuli. Indeed, it has been shown that enkephalins, oxytocin and vasopressin could be released at this level. Although the antinociceptive effects of these neuropeptides are well studied, little is known about the potential interaction between these molecules. In this study, we provide anatomical evidence of the interaction between oxytocin (OT), vasopressin (AVP), dynorphin (DYN) and enkephalin (ENK) along the PVN projections to the spinal dorsal horn at L3 level. A retrograde tracer (True Blue®) microinjected at L3 in the SDH and immunofluorescence with antibodies against OT, AVP, DYN and ENK were used. The experiments showed different levels of peptide immunoreactivity distribution along the rostro-caudal area of the PVN. A high percentage of co-localizations between two of the peptides (OT-AVP, OT-DYN, AVP-ENK, DYN-ENK) were present along the PVN. The following co-localizations occupied 4.76-9.62% of the total PVN area. PVN projections to the SDH at L3 level showed similar results. Our results show that different antinociceptive peptides may be interacting with each other to evoke PVN antinociceptive effects as part of the endogenous system of nociceptive modulation.

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55.

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB1 and CB2) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB1/CB2 receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration. Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain. Results: This work summarized novel data supporting that, besides cannabinoid CB1 and CB2 receptors, GPR18 and GPR55 may be useful for pain treatment. Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

Oxytocin inhibits the rat medullary dorsal horn Sp5c/C1 nociceptive transmission through OT but not V1A receptors.

The medullary dorsal horn (MDH or Sp5c/C1 region) plays a key role modulating the nociceptive input arriving from craniofacial structures. Some reports suggest that oxytocin could play a role modulating the nociceptive input at the MDH level, but no study has properly tested this hypothesis. Using an electrophysiological and pharmacological approach, the present study aimed to determine the effect of oxytocin on the nociceptive signaling in the MDH and the receptor involved. In sevoflurane, anesthetized rats, we performed electrophysiological unitary recordings of second order neurons at the MDH region responding to peripheral nociceptive-evoked responses of the first branch (V1; ophthalmic) of the trigeminal nerve. Under this condition, we constructed dose-response curves analyzing the effect of local spinal oxytocin (0.2-20 nmol) on MDH nociceptive neuronal firing. Furthermore, we tested the role of oxytocin receptors (OTR) or vasopressin V1A receptors (V1AR) involved in the oxytocin effects. Oxytocin dose-dependently inhibits the peripheral-evoked activity in nociceptive MDH neurotransmission. This inhibition is associated with a blockade of neuronal activity of Aδ- and C-fibers. Since this antinociception was abolished by pretreatment (in the MDH) with the potent and selective OTR antagonist (L-368,899; 20 nmol) and remained unaffected after the V1AR antagonist (SR49059; 20 nmol or 200 nmol), the role of OTR is implied. This electrophysiological study demonstrates that oxytocin inhibits the peripheral-evoked neuronal activity at MDH, through OTR activation. Thus, OTR may represent a new potential drug target to treat craniofacial nociceptive dysfunction in the MDH.

The rat corticospinal system is functionally and anatomically segregated.

The descending corticospinal (CS) projection has been considered a key element for motor control, which results from direct and indirect modulation of spinal cord pre-motor interneurons in the intermediate gray matter of the spinal cord, which, in turn, influences motoneurons in the ventral horn. The CS tract (CST) is also involved in a selective and complex modulation of sensory information in the dorsal horn. However, little is known about the spinal network engaged by the CST and the organization of CS projections that may encode different cortical outputs to the spinal cord. This study addresses the issue of whether the CS system exerts parallel control on different spinal networks, which together participate in sensorimotor integration. Here, we show that in the adult rat, two different and partially intermingled CS neurons in the sensorimotor cortex activate, with different time latencies, distinct spinal cord neurons located in the dorsal horn and intermediate zone of the same segment. The fact that different populations of CS neurons project in a segregated manner suggests that CST is composed of subsystems controlling different spinal cord circuits that modulate motor outputs and sensory inputs in a coordinated manner.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception.

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10-5-1×10-4nmol) or 5-HT (1×10-3-1×10-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10-5nmol) plus 5-HT (1×10-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]OVT (oxytocin receptor antagonist; 2×10-2nmol); or (ii) methiothepin (a non-specific 5-HT1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

Intracisternal injection of palmitoylethanolamide inhibits the peripheral nociceptive evoked responses of dorsal horn wide dynamic range neurons.

Endogenous palmitoylethanolamide (PEA) has a key role in pain modulation. Central or peripheral PEA can reduce nociceptive behavior, but no study has yet reported a descending inhibitory effect on the neuronal nociceptive activity of Aδ- and C-fibers. This study shows that intracisternal PEA inhibits the peripheral nociceptive responses of dorsal horn wide dynamic range cells (i.e., inhibition of Aδ- and C-fibers), an effect blocked by spinal methiothepin. These results suggest that a descending analgesic mechanism mediated by the serotonergic system could be activated by central PEA.

The multitarget drug approach in migraine treatment: the new challenge to conquer.

Migraine is a complex neurovascular disorder where a complex and interrelated neuronal spinal, supraspinal and central mechanisms are involved. Although we have greatly advanced in the knowledge of the main pathways involved in this disorder, the current drugs used are not effective in all patients, suggesting that the key mechanism related to headache relief remains elusive. In this context, the multi-target drug approach or network pharmacology emerges as a new step in the development of innovative migraine pharmacotherapy. The design, discovery, and development of new drugs that reach several (instead of unique) specific targets (functional selectivity) involved in the migraine pathophysiology is essential to progress in the migraine treatment and open a new field of study about the main pathways and targets that could synergistically improve the migraine management.

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.

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.

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.

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.

Paraventricular hypothalamic nucleus stimulation modulates nociceptive responses in dorsal horn wide dynamic range neurons.

Effects of different parameters of hypothalamic paraventricular nucleus (PVN) electrical stimulation on somatic responses, in dorsal horn neurons were examined. In anaesthetized rats, single-unit extracellular recordings were made from dorsal horn lumbar segments, which receive afferent input from the toe and hind paw regions. We compared the neuronal responses evoked by electrical stimulation of the receptive field (RF) with the responses preceded by ipsilateral PVN stimulation. Only the responses corresponding to Adelta and C-fiber activation were inhibited when PVN stimulation was delivered. Fast-evoked responses corresponding to Abeta fibers were not modified. The magnitude of inhibition depends on the intensity and duration of the PVN stimulation train and gradually decreases as the time interval between the PVN and RF stimulations increases. The results indicate that PVN modulates nociceptive, but not non-nociceptive neuronal responses at the spinal cord level, and this modulation depends on the parameters of the stimulus utilized to activate PVN neurons.