Oxytocin modulates the effects of galanin in carrageenan-induced hyperalgesia in rats

A New Gal in Town: A Systematic Review of the Role of Galanin and Its Receptors in Experimental Pain

Galanin is a neuropeptide expressed in a small percentage of sensory neurons of the dorsal root ganglia and the superficial lamina of the dorsal horn of the spinal cord. In this work, we systematically reviewed the literature regarding the role of galanin and its receptors in nociception at the spinal and supraspinal levels, as well as in chronic pain conditions. The literature search was performed in PubMed, Web of Science, Scopus, ScienceDirect, OVID, TRIP, and EMBASE using "Galanin" AND "pain" as keywords. Of the 1379 papers that were retrieved in the initial search, we included a total of 141 papers in this review. Using the ARRIVE guidelines, we verified that 89.1% of the works were of good or moderate quality. Galanin shows a differential role in pain, depending on the pain state, site of action, and concentration. Under normal settings, galanin can modulate nociceptive processing through both a pro- and anti-nociceptive action, in a dose-dependent manner. This peptide also plays a key role in chronic pain conditions and its antinociceptive action at both a spinal and supraspinal level is enhanced, reducing animals' hypersensitivity to both mechanical and thermal stimulation. Our results highlight galanin and its receptors as potential therapeutic targets in pain conditions.

Oxytocin Is a Positive Allosteric Modulator of κ-Opioid Receptors but Not δ-Opioid Receptors in the G Protein Signaling Pathway

Oxytocin (OT) influences various physiological functions such as uterine contractions, maternal/social behavior, and analgesia. Opioid signaling pathways are involved in one of the analgesic mechanisms of OT. We previously showed that OT acts as a positive allosteric modulator (PAM) and enhances μ-opioid receptor (MOR) activity. In this study, which focused on other opioid receptor (OR) subtypes, we investigated whether OT influences opioid signaling pathways as a PAM for δ-OR (DOR) or κ-OR (KOR) using human embryonic kidney-293 cells expressing human DOR or KOR, respectively. The CellKey^TM results showed that OT enhanced impedance induced by endogenous/exogenous KOR agonists on KOR-expressing cells. OT did not affect DOR activity induced by endogenous/exogenous DOR agonists. OT potentiated the KOR agonist-induced Gi/o protein-mediated decrease in intracellular cAMP, but did not affect the increase in KOR internalization caused by the KOR agonists dynorphin A and (-)-U-50488 hydrochloride (U50488). OT did not bind to KOR orthosteric binding sites and did not affect the binding affinities of dynorphin A and U50488 for KOR. These results suggest that OT is a PAM of KOR and MOR and enhances G protein signaling without affecting β-arrestin signaling. Thus, OT has potential as a specific signaling-biased PAM of KOR.

Acute Mono-Arthritis Activates the Neurohypophysial System and Hypothalamo-Pituitary Adrenal Axis in Rats

Various types of acute/chronic nociceptive stimuli cause neuroendocrine responses such as activation of the hypothalamo-neurohypophysial [oxytocin (OXT) and arginine vasopressin (AVP)] system and hypothalamo-pituitary adrenal (HPA) axis. Chronic multiple-arthritis activates the OXT/AVP system, but the effects of acute mono-arthritis on the OXT/AVP system in the same animals has not been simultaneously evaluated. Further, AVP, not corticotropin-releasing hormone (CRH), predominantly activates the HPA axis in chronic multiple-arthritis, but the participation of AVP in HPA axis activation in acute mono-arthritis remains unknown. Therefore, we aimed to simultaneously evaluate the effects of acute mono-arthritis on the activity of the OXT/AVP system and the HPA axis. In the present study, we used an acute mono-arthritic model induced by intra-articular injection of carrageenan in a single knee joint of adult male Wistar rats. Acute mono-arthritis was confirmed by a significant increase in knee diameter in the carrageenan-injected knee and a significant decrease in the mechanical nociceptive threshold in the ipsilateral hind paw. Immunohistochemical analysis revealed that the number of Fos-immunoreactive (ir) cells in the ipsilateral lamina I-II of the dorsal horn was significantly increased, and the percentage of OXT-ir and AVP-ir neurons expressing Fos-ir in both sides of the supraoptic (SON) and paraventricular nuclei (PVN) was increased in acute mono-arthritic rats. in situ hybridization histochemistry revealed that levels of OXT mRNA and AVP hnRNA in the SON and PVN, CRH mRNA in the PVN, and proopiomelanocortin mRNA in the anterior pituitary were also significantly increased in acute mono-arthritic rats. Further, plasma OXT, AVP, and corticosterone levels were significantly increased in acute mono-arthritic rats. These results suggest that acute mono-arthritis activates ipsilateral nociceptive afferent pathways at the spinal level and causes simultaneous and integrative activation of the OXT/AVP system. In addition, the HPA axis is activated by both AVP and CRH in acute mono-arthritis with a distinct pattern compared to that in chronic multiple-arthritis.

Sex Difference of Angiotensin IV-, LVV-Hemorphin 7-, and Oxytocin-Induced Antiallodynia at the Spinal Level in Mice With Neuropathic Pain

BACKGROUND

We demonstrated previously that angiotensin IV (Ang IV) and LVV-hemorphin 7 (LVV-H7) act through the blockade of insulin-regulated aminopeptidase to decrease oxytocin degradation, thereby causing antihyperalgesia at the spinal level in rats. We determined that intrathecal oxytocin can induce significant antihyperalgesia in male rats with inflammation but not in female rats. Thus, we speculate that Ang IV, LVV-H7, and oxytocin can induce antiallodynia, which could be of great therapeutic potential. Because the antihyperalgesia by using these peptides was with sex difference, their possible antiallodynia was examined in male and female mice for comparison. We investigated whether Ang IV, LVV-H7, and oxytocin produce antiallodynia at the spinal level in mice and whether this antiallodynia differs between the sexes.

METHODS

Partial sciatic nerve ligation surgery was performed on adult male and female C57BL/6 mice from the same litter (25-30 g). The effects of intrathecal injections of Ang IV (25.8 nmol), LVV-H7 (27.2 nmol), and oxytocin (0.125 or 1.25 nmol) were assessed through the von Frey test 3 days after partial sciatic nerve ligation.

RESULTS

Intrathecal injection of Ang IV, LVV-H7, and oxytocin all produced a potent antiallodynia in male mice. However, these antiallodynia effects were either extremely weak or absent in female mice at the same dose.

CONCLUSIONS

Intrathecal Ang IV, LVV-H7, and oxytocin can all cause significant antiallodynia in male mice. The Ang IV-, LVV-H7-, and oxytocin-induced antiallodynia effects differed between the sexes at the spinal level in mice.

From Autism to Eating Disorders and More: The Role of Oxytocin in Neuropsychiatric Disorders

Oxytocin (oxy) is a pituitary neuropeptide hormone synthesized from the paraventricular and supraoptic nuclei within the hypothalamus. Like other neuropeptides, oxy can modulate a wide range of neurotransmitter and neuromodulator activities. Additionally, through the neurohypophysis, oxy is secreted into the systemic circulation to act as a hormone, thereby influencing several body functions. Oxy plays a pivotal role in parturition, milk let-down and maternal behavior and has been demonstrated to be important in the formation of pair bonding between mother and infants as well as in mating pairs. Furthermore, oxy has been proven to play a key role in the regulation of several behaviors associated with neuropsychiatric disorders, including social interactions, social memory response to social stimuli, decision-making in the context of social interactions, feeding behavior, emotional reactivity, etc. An increasing body of evidence suggests that deregulations of the oxytocinergic system might be involved in the pathophysiology of certain neuropsychiatric disorders such as autism, eating disorders, schizophrenia, mood, and anxiety disorders. The potential use of oxy in these mental health disorders is attracting growing interest since numerous beneficial properties are ascribed to this neuropeptide. The present manuscript will review the existing findings on the role played by oxy in a variety of distinct physiological and behavioral functions (Figure 1) and on its role and impact in different psychiatric disorders. The aim of this review is to highlight the need of further investigations on this target that might contribute to the development of novel more efficacious therapies. Figure 1

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Long-lasting spinal oxytocin analgesia is ensured by the stimulation of allopregnanolone synthesis which potentiates GABA(A) receptor-mediated synaptic inhibition

Hypothalamospinal control of spinal pain processing by oxytocin (OT) has received a lot of attention in recent years because of its potency to reduce pain symptoms in inflammatory and neuropathic conditions. However, cellular and molecular mechanisms underlying OT spinal antinociception are still poorly understood. In this study, we used biochemical, electrophysiological, and behavioral approaches to demonstrate that OT levels are elevated in the spinal cord of rats exhibiting pain symptoms, 24 h after the induction of inflammation with an intraplantar injection of λ-carrageenan. Using a selective OT receptor antagonist, we demonstrate that this elevated OT content is responsible for a tonic analgesia exerted on both mechanical and thermal modalities. This phenomenon appeared to be mediated by an OT receptor-mediated stimulation of neurosteroidogenesis, which leads to an increase in GABA(A) receptor-mediated synaptic inhibition in lamina II spinal cord neurons. We also provide evidence that this novel mechanism of OT-mediated spinal antinociception may be controlled by extracellular signal-related protein kinases, ERK1/2, after OT receptor activation. The oxytocinergic inhibitory control of spinal pain processing is emerging as an interesting target for future therapies since it recruits several molecular mechanisms, which are likely to exert a long-lasting analgesia through nongenomic and possibly genomic effects.

Central administration of oxytocin reduces hyperalgesia in mice: implication for cannabinoid and opioid systems

The neuropeptide oxytocin (OXT) contributes to the regulation of diverse cognitive and physiological functions including nociception. Indeed, OXT has been reported to be analgesic when administered directly into the brain, the spinal cord, or systemically. Although many authors have reported the analgesic effects of OXT, its mechanism has not been well elucidated. Recently, it has been also hypothesize that OXT, increasing intracellular concentration of calcium, could regulate the production of mediators, like endocannabinoids (eCB). It has been well documented that eCB are able to suppress pain pathways. The present study investigates the effect of OXT in paw carrageenan-induced pain. Intracerebroventricular (icv) administration of OXT, but neither intraperitoneal nor intraplantar route, induces an antihyperalgesic effect increasing paw withdrawal latency to mechanical or thermal stimuli. Our results clearly demonstrate that 3 and 6h following carrageenan challenge, central administration of OXT (30 ng/mouse) shows a significant antihyperalgesic activity. Moreover, for the first time, we demonstrate that CB1 receptor plays a key role in the antihyperalgesic effect of OXT. In fact our results show CB1 antagonist, but not the specific CB2 antagonist reduce OXT-induced antihyperalgesic effect. In addition, our data show that central OXT administration is able to reduce carrageenan-induced hyperalgesia but does not modify carrageenan-induced paw edema. Finally, using opioid antagonists we confirm an important role of opioid receptors. In conclusion, our experiments suggest that central administration of OXT reduces hyperalgesia induced by intraplantar injection of carrageenan, and this effect may work via cannabinoid and opioid systems.

Newborn Analgesia Mediated by Oxytocin during Delivery

The mechanisms controlling pain in newborns during delivery are poorly understood. We explored the hypothesis that oxytocin, an essential hormone for labor and a powerful neuromodulator, exerts analgesic actions on newborns during delivery. Using a thermal tail-flick assay, we report that pain sensitivity is two-fold lower in rat pups immediately after birth than 2 days later. Oxytocin receptor antagonists strongly enhanced pain sensitivity in newborn, but not in 2-day-old rats, whereas oxytocin reduced pain at both ages suggesting an endogenous analgesia by oxytocin during delivery. Similar analgesic effects of oxytocin, measured as attenuation of pain-vocalization induced by electrical whisker pad stimulation, were also observed in decerebrated newborns. Oxytocin reduced GABA-evoked calcium responses and depolarizing GABA driving force in isolated neonatal trigeminal neurons suggesting that oxytocin effects are mediated by alterations of intracellular chloride. Unlike GABA signaling, oxytocin did not affect responses mediated by P2X3 and TRPV1 receptors. In keeping with a GABAergic mechanism, reduction of intracellular chloride by the diuretic NKCC1 chloride co-transporter antagonist bumetanide mimicked the analgesic actions of oxytocin and its effects on GABA responses in nociceptive neurons. Therefore, endogenous oxytocin exerts an analgesic action in newborn pups that involves a reduction of the depolarizing action of GABA on nociceptive neurons. Therefore, the same hormone that triggers delivery also acts as a natural pain killer revealing a novel facet of the protective actions of oxytocin in the fetus at birth.

REVIEW: Oxytocin: Crossing the bridge between basic science and pharmacotherapy

Is oxytocin the hormone of happiness? Probably not. However, this small nine amino acid peptide is involved in a wide variety of physiological and pathological functions such as sexual activity, penile erection, ejaculation, pregnancy, uterus contraction, milk ejection, maternal behavior, osteoporosis, diabetes, cancer, social bonding, and stress, which makes oxytocin and its receptor potential candidates as targets for drug therapy. In this review, we address the issues of drug design and specificity and focus our discussion on recent findings on oxytocin and its heterotrimeric G protein‐coupled receptor OTR. In this regard, we will highlight the following topics: (i) the role of oxytocin in behavior and affectivity, (ii) the relationship between oxytocin and stress with emphasis on the hypothalamo–pituitary–adrenal axis, (iii) the involvement of oxytocin in pain regulation and nociception, (iv) the specific action mechanisms of oxytocin on intracellular Ca²⁺ in the hypothalamo neurohypophysial system (HNS) cell bodies, (v) newly generated transgenic rats tagged by a visible fluorescent protein to study the physiology of vasopressin and oxytocin, and (vi) the action of the neurohypophysial hormone outside the central nervous system, including the myometrium, heart and peripheral nervous system. As a short nine amino acid peptide, closely related to its partner peptide vasopressin, oxytocin appears to be ideal for the design of agonists and antagonists of its receptor. In addition, not only the hormone itself and its binding to OTR, but also its synthesis, storage and release can be endogenously and exogenously regulated to counteract pathophysiological states. Understanding the fundamental physiopharmacology of the effects of oxytocin is an important and necessary approach for developing a potential pharmacotherapy.

Headache endocrinological aspects

In this chapter we review the current understanding of how hormones, neurohormones, and neurotransmitters participate in the pain modulation of primary headaches. Stressful conditions and hormones intimately implicated in headache neurobiology are also discussed. With the recent progress in neuroimaging techniques and the development of animal models to study headache mechanisms, the physiopathology of several of the primary headaches is starting to be better understood. Various clinical characteristics of the primary headaches, such as pain, autonomic disturbances, and behavioral changes, are linked to hypothalamic brainstem activation and hormonal influence. Headache is greatly influenced by the circadian circle. Over the millennia the nervous system has evolved to meet changing environmental conditions, including the light-dark cycle, in order to ensure survival and reproduction. The main elements for synchronization between internal biological events and the external environment are the pineal gland and its main secretory product, melatonin. Melatonin is believed to be a significant element in migraine and in other headache disorders, which has implications for treatment. A potential therapeutic use of melatonin has been considered in several headache syndromes. In short, primary headaches are strongly influenced by physiological hormonal fluctuations, when nociceptive and non-nociceptive pathways are differentially activated to modulate the perception of pain.

Antinociceptive action of oxytocin involves inhibition of potassium channel currents in lamina II neurons of the rat spinal cord

Background

Growing evidence in the literature shows that oxytocin (OT) has a strong spinal anti-nociceptive action. Oxytocinergic axons originating from a subpopulation of paraventricular hypothalamic neurons establish synaptic contacts with lamina II interneurons but little is known about the functional role of OT with respect to neuronal firing and excitability.

Results

Using the patch-clamp technique, we have recorded lamina II interneurons in acute transverse lumbar spinal cord slices of rats (15 to 30 days old) and analyzed the OT effects on action potential firing ability. In the current clamp mode, we found that bath application of a selective OT-receptor agonist (TGOT) reduced firing in the majority of lamina II interneurons exhibiting a bursting firing profile, but never in those exhibiting a single spike discharge upon depolarization. Interestingly, OT-induced reduction in spike frequency and increase of firing threshold were often observed, leading to a conversion of the firing profile from repetitive and delayed profiles into phasic ones and sometimes further into single spike profile. The observed effects following OT-receptor activation were completely abolished when the OT-receptor agonist was co-applied with a selective OT-receptor antagonist. In current and voltage clamp modes, we show that these changes in firing are strongly controlled by voltage-gated potassium currents. More precisely, transient I_A currents and delayed-rectifier currents were reduced in amplitude and transient I_A current was predominantly inactivated after OT bath application.

Conclusion

This effect of OT on the firing profile of lamina II neurons is in good agreement with the antinociceptive and analgesic properties of OT described in vivo.

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

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.

Oxytocin-induced antinociception in the spinal cord is mediated by a subpopulation of glutamatergic neurons in lamina I-II which amplify GABAergic inhibition

BACKGROUND

Recent evidence suggests that oxytocin (OT), secreted in the superficial spinal cord dorsal horn by descending axons of paraventricular hypothalamic nucleus (PVN) neurons, produces antinociception and analgesia. The spinal mechanism of OT is, however, still unclear and requires further investigation. We have used patch clamp recording of lamina II neurons in spinal cord slices and immunocytochemistry in order to identify PVN-activated neurons in the superficial layers of the spinal cord and attempted to determine how this neuronal population may lead to OT-mediated antinociception.

RESULTS

We show that OT released during PVN stimulation specifically activates a subpopulation of lamina II glutamatergic interneurons which are localized in the most superficial layers of the dorsal horn of the spinal cord (lamina I-II). This OT-specific stimulation of glutamatergic neurons allows the recruitment of all GABAergic interneurons in lamina II which produces a generalized elevation of local inhibition, a phenomenon which might explain the reduction of incoming Adelta and C primary afferent-mediated sensory messages.

CONCLUSION

Our results obtained in lamina II of the spinal cord provide the first clear evidence of a specific local neuronal network that is activated by OT release to induce antinociception. This OT-specific pathway might represent a novel and interesting therapeutic target for the management of neuropathic and inflammatory pain.

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

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.

Why Does Social Exclusion Hurt? The Relationship Between Social and Physical Pain

The authors forward the hypothesis that social exclusion is experienced as painful because reactions to rejection are mediated by aspects of the physical pain system. The authors begin by presenting the theory that overlap between social and physical pain was an evolutionary development to aid social animals in responding to threats to inclusion. The authors then review evidence showing that humans demonstrate convergence between the 2 types of pain in thought, emotion, and behavior, and demonstrate, primarily through nonhuman animal research, that social and physical pain share common physiological mechanisms. Finally, the authors explore the implications of social pain theory for rejection-elicited aggression and physical pain disorders.

Involvement of oxytocin in spinal antinociception in rats with inflammation

The present study was conducted on rats with inflammation induced by subcutaneous injection of carrageenan into the left hindpaw. Intrathecal administration of oxytocin produced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats with inflammation. The antinociceptive effect of oxytocin was blocked by intrathecal administration of atosiban, a selective oxytocin antagonist, indicating that oxytocin receptor mediates oxytocin-induced antinociception in the spinal cord. The oxytocin-induced antinociceptive effect was attenuated by intrathecal administration of the opioid antagonist naloxone, suggesting an involvement of the endogenous opioid system in oxytocin-induced antinociception in the spinal cord of rats with inflammation. Furthermore, the antinociceptive effect of oxytocin was attenuated by intrathecal injections of the mu-receptor antagonist beta-funaltrexamine and the kappa-receptor antagonist nor-binaltorphimine, but not by the delta-receptor antagonist naltrindole, illustrating that mu- and kappa-receptors, but not delta-receptor, are involved in oxytocin-induced antinociception in the spinal cord of rats with inflammation. Moreover, intrathecal administration of atosiban alone induced a hyperalgesia in rats with inflammation, indicating that endogenous oxytocin is involved in the transmission and regulation of nociceptive information in the spinal cord of rats with inflammation. The present study showed that both exogenous and endogenous oxytocin displayed antinociception in the spinal cord in rats with inflammation, and mu- and kappa-receptors were involved in oxytocin-induced antinociception.

Repeated massage-like stimulation induces long-term effects on nociception: contribution of oxytocinergic mechanisms

Massage-like stroking induces acute antinociceptive effects that can be reversed by an oxytocin antagonist, indicating activation of oxytocin on endogenous pain controlling systems. We now demonstrate an increase in hindpaw withdrawal latencies (HWLs), in response to thermal and mechanical stimuli, which was present after six treatments of massage-like stroking every other day and which continued to increase through the remaining seven treatments. Repeated massage-like stroking also resulted in increased oxytocin-like immunoreactivity (oxytocin-LI) levels in plasma and periaquaductal grey matter (PAG). Furthermore, increases in HWLs were also present after injections of oxytocin into the PAG (0.1, 0.5 and 1.0 nmol). Intra-PAG oxytocin injection of 1 nmol followed by 1 or 20 nmol of naloxone attenuated the increments in HWL. Also, there was a dose-dependent attenuation of the oxytocin-induced antinociceptive effects following intra-PAG injection of the mu-opioid antagonist beta-funaltrexamine (beta-FNA) and the kappa-opioid antagonist nor-binaltorphimine (nor-BNI) but not the delta-antagonist naltrindole. The long-term antinociceptive effects of massage-like stroking may be attributed, at least partly, to the oxytocinergic system and its interaction with the opioid system, especially the mu- and the kappa-receptors in the PAG.

Hypothalamic paraventricular axons projecting to the female rat lumbosacral spinal cord contain oxytocin immunoreactivity

Oxytocin-containing axons project from the hypothalamic paraventricular nucleus to the neurohypophysis and thoracic spinal cord to ultimately influence uterine contractions and autonomic activity, respectively. Whether or not oxytocin-immunoreactive axons project to the female rat lumbosacral spinal cord to influence autonomic outflow to pelvic organs has not been investigated. Thus, the present study was designed to investigate the presence, distribution, and origin of oxytocin-immunoreactive axons in the female rat lumbosacral spinal cord. Immunohistochemistry, spinal cord transections, and axonal tracing with Fluorogold, True Blue, and pseudorabies virus were used. Oxytocin-immunoreactive nerve fibers were present in the L6/S1 segments of the spinal cord. Prominent varicose axons were evident throughout the dorsal horn, along the lateral and medial collateral pathways, in the dorsal intermediate gray area, around the central canal in lamina X, and throughout the sacral parasympathetic nucleus. Injection of retrograde tracer into the L6/S1 spinal cord labeled neurons in the hypothalamic paraventricular nucleus. Transection of the thoracic spinal cord eliminated oxytocin-immunoreactive nerve axons in the L6/S1 spinal cord. In addition, transection of the thoracic spinal cord eliminated transport of retrograde axonal tracer from the L6/S1 spinal cord to the paraventricular nucleus. Pseudorabies virus, a transneuronal retrograde tracer, injected into the uterus and cervix marked uterine-related preganglionic neuronal cell bodies in the sacral parasympathetic nucleus and uterine-related neurons in the hypothalamic paraventricular nucleus. Double immuno-labeling of viral-infected spinal cord sections showed oxytocin-immunoreactive axons closely associated with viral labeled uterine-related preganglionic cell bodies of the sacral parasympathetic nucleus. The results of this study revealed that oxytocin-immunoreactive neurons of the hypothalamic paraventricular nucleus project axons to the lumbosacral spinal cord to areas involved in sensory processing and parasympathetic outflow to the uterus.

Anti-nociceptive effect of neuropeptide Y in periaqueductal grey in rats with inflammation

Experimental inflammation was induced by subcutaneous injection of carrageenan into the left hindpaw of rats. Intra-periaqueductal grey (PAG) injection of 0.02 or 0.1 nmol of neuropeptide Y (NPY), but not 0.004 nmol, induced significant increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats with inflammation. Furthermore, the anti-nociceptive effect of NPY was blocked partly by following intra-PAG injection of the Y1 receptor antagonist NPY28-36. The results demonstrated that NPY plays an anti-nociceptive role in PAG in rats with inflammation, in which Y1 receptor is involved.

Effects of intrathecal galanin on nociceptive responses in rats with mononeuropathy

The present study was performed on rats with experimental mononeuropathy induced by left common sciatic nerve loose ligation. Unilateral sciatic nerve loose ligation induced decreases of the hindpaw withdrawal latency to the hot-plate test, cold-plate test and the Randall Selitto test. Sciatic nerve loose ligation induced hyperesponsiveness to touch at room temperature also. Intrathecal administration of either 3 or 6 nmol of galanin, but not 1 nmol, induced significant bilateral increases in hindpaw withdrawal latencies to the hot-plate test, cold-plate test and the Randall Selitto tests in rats with left mononeuropathy. The results indicate that galanin may play important roles in transmission of presumed nociceptive information in the spinal cord of mononeuropathic rats.

CNS location of uterine-related neurons revealed by trans-synaptic tracing with pseudorabies virus and their relation to estrogen receptor-immunoreactive neurons

Retrograde, transneuronal tracing with Bartha's strain of pseudorabies virus was used in rats to identify spinal cord, brainstem and hypothalamic loci of uterine-related neurons that could function in the regulation of uterine activity. Based on the premise that estrogen might influence such uterine-related neurons, the existence of estrogen receptors in neurons in these same loci was examined. Viral injections were made into the uterine cervix, body and cervical end of the uterine horns, and the rats allowed to survive for four to six days. After four days, mainly the spinal cord, medulla and pons contained virus-infected neurons. After longer survival times, progressively higher levels of the neuraxis contained viral-labeled neurons, so that by six days hypothalamic uterine-related neurons were identified. First-order virus-infected neurons were visualized by immunohistochemistry in the pelvic paracervical parasympathetic ganglia and in inferior mesenteric sympathetic ganglia. Preganglionic and putative interneurons were labeled in the lumbosacral spinal cord and thoracic spinal cord mainly in the lateral horn area (sacral parasympathetic nucleus and intermediolateral nucleus), lateral aspect of the dorsal horn, intermediate gray, lamina X and dorsal gray commissural area. In the brainstem, labeling was most evident and consistent in the nucleus tractus solitarius, ventrolateral medulla, raphe magnus and pallidus nuclei, parapyramidal area, A5 cell group, Barrington's nucleus of the pons and periaqueductal gray of the midbrain. In the hypothalamus, virus-infected neurons were most marked in the paraventricular nucleus, with fewer in the medial preoptic area and ventromedial hypothalamic nucleus. Estrogen receptor-immunoreactive neurons were most often present among the virus-labeled uterine-related neurons of the spinal cord, nucleus tractus solitarius, ventrolateral medulla, periaqueductal gray, medial preoptic area and ventromedial hypothalamic nucleus. These results identify a multisynaptic pathway of neurons whose eventual output is involved in uterine functions, whose distribution is similar to that revealed by pseudorabies virus tracing from other visceral organs, and which are often mixed among estrogen-responsive neurons.

Effects of calcitonin gene-related peptide-(8-37) on withdrawal responses in rats with inflammation

The present study was performed to explore the effect of subcutaneous injection of carrageenan into the rat plantar region on hindpaw edema formation and the latency of hindpaw withdrawal to presumed nociceptive stimulation. Subcutaneous injection of carrageenan into the left hindpaw induced a significant increase in the volume of the left hindpaw, leaving the right side unaffected. In addition, we found a bilateral decrease in hindpaw withdrawal latency to heat and mechanical, but not to cold stimulation. The decreased bilateral hindpaw withdrawal latency to heat stimulation lasted for 14 days after carrageenan injection. The decreased bilateral hindpaw withdrawal latency to mechanical stimulation lasted for 2 days after the injection, then reversed and increased from day 3 to 14. Intrathecal injection of either 10 nmol of calcitonin gene-related peptide 8-37 or 26.6 nmol of morphine induced significant bilateral increases in hindpaw withdrawal latency, which were more pronounced with the morphine. The results show that experimentally induced unilateral hindpaw inflammation induces a bilateral decrease in hindpaw withdrawal latencies to presumed nociceptive stimulation while the sensory systems for heat and mechanical stimulation were differently affected after carrageenan injection.

Oxytocin increases thresholds of colonic visceral perception in patients with irritable bowel syndrome

AIM

The effects of oxytocin on colonic perception of intraluminal distension were evaluated in 26 patients with irritable bowel syndrome (IBS), using a flaccid bag placed in the descending colon and connected to a computerised barostat.

METHOD

Symptomatic responses (first sensation and pain) were evaluated during isobaric distensions (4 mm Hg increments, five minute duration, five minute interval with return to zero pressure between each step), performed automatically by the barostat, during a continuous infusion of placebo or various doses of oxytocin (10, 20, 30, and 50 mU/min).

RESULTS

The distension pressure (mean (SD)) required to induce a first abdominal sensation was 17.3 (5.5) mm Hg on placebo, 19.9 (5.8) on oxytocin 10 mU/min (NS), 22.3 (6.0) mm Hg on oxytocin 20 mU/min (p < 0.01), 23.1 (6.6) mm Hg on oxytocin 30 mU/min (p < 0.01), and 24.0 (7.1) mm Hg on oxytocin 50 mU/min (p < 0.01). The distension pressure required to induce pain was 24.8 (6.3) mm Hg on placebo, 26.0 (5.8) on oxytocin 10 mU/min (NS), 33.3 (7.8) mm Hg on oxytocin 20 mU/min (p < 0.01), 34.2 (7.6) mm Hg on oxytocin 30 mU/min (p < 0.01), and 34.3 (7.9) mm Hg on oxytocin 50 mU/ min (p < 0.01). Compliance curves were not different after placebo and oxytocin injection at the different doses. Naloxone did not inhibit the effect of oxytocin. Oxytocin also did not alter somatic perception, characterised by the RIII reflex at the level of the biceps femori.

CONCLUSIONS

Oxytocin significantly increases thresholds for visceral perception in IBS patients at doses equal or to greater than 20 mU/min, possibly by acting at the level of visceral afferents.

Galanin receptor antagonists attenuate spinal antinociceptive effects of DAMGO, tramadol and non-opioid drugs in rats

The involvement of endogenous galanin to antinociception elicited by intrathecally (i.t.) or systemically administered drugs from different chemical and therapeutic classes was investigated using the rat Randall-Selitto or the rat tail-flick test, in the absence or presence of the i.t. administered galanin receptor antagonists galantide and M-35. Antinociception elicited by i.t. tramadol (24 micrograms), DAMGO (1 microgram), clonidine (48 micrograms), desipramine (6 micrograms) or fenfluramine (60 micrograms) was attenuated by i.t. galantide (2 micrograms); the attenuation reached significance at least at one time point. A partial antagonism by i.t. galantide was also observed against the antinociception of i.p. tramadol (10 mg/kg), i.v. clonidine (1 mg/kg), i.p. desipramine (1 mg/kg), or i.p. dipyrone (1000 mg/kg), but antinociception by i.p. fenfluramine (30 mg/kg) was not affected. Using M-35 (2 micrograms i.t.), the antinociception of i.t. tramadol or DAMGO was attenuated, but no inhibition was observed when clonidine, desipramine or fenfluramine were used i.t. If drugs were administered systemically, only antinociception of i.p. fenfluramine but not that of i.p. tramadol, or i.v. clonidine, or i.p. desipramine or i.p dipyrone was attenuated. In the rat tail flick test, co-injection of either 2 micrograms i.t. galantide or M-35 with i.t. tramadol (12 micrograms) almost abolished the antinociceptive effect, whereas the antinociception of systemically administered tramadol (4.6 mg/kg i.p.) was only partially attenuated by i.t. galantide and not affected by i.t. M-35. Binding studies in dorsal spinal cord tissue showed no affinity of galantide or M-35 to spinal mu-, or delta-, or kappa-opioid receptors and none of the other drugs interfered with the spinal galanin binding site. These data give further support of at least a partial galanin link in spinal processes of antinociception.

The calcitonin gene-related peptide antagonist CGRP8-37 increases the latency to withdrawal responses bilaterally in rats with unilateral experimental mononeuropathy, an effect reversed by naloxone

The present study was performed in rats with experimental mononeuropathy after left common sciatic nerve constriction. A bilateral decrease in hindpaw withdrawal latency to thermal and mechanical stimulation was observed after unilateral ligation of the left common sciatic nerve; however, it was more pronounced on the lesioned side. Compared with sham-operated rats, the content of calcitonin gene-related peptide-like immunoreactivity was significantly decreased in the left dorsal horn of the spinal cord and left dorsal root ganglia in rats with mononeuropathy. Blocking the receptor of calcitonin gene-related peptide, by intrathecal injection of 5 or 10 nmol of calcitonin gene-related peptide (8-37), induced a significant bilateral increase in hindpaw withdrawal latency to both thermal and mechanical stimulation which, however, was significantly less pronounced in mononeuropathic rats than in intact rats. The effect of calcitonin gene-related peptide (8-37) was reversed by intrathecal administration of the opioid antagonist naloxone. The contribution of calcitonin gene-related peptide and its receptors to transmission of presumed nociceptive information appears to be reduced in the sciatic nerve constriction model. The decrease in reflex responsiveness induced by calcitonin gene-related peptide (8-37) was counteracted by naloxone, indicating that opioids control the net effect of excitation in the spinal cord circuitry induced by calcitonin gene-related peptide and possibly other co-released neurotransmitters.

Intrathecal CGRP8-37-induced bilateral increase in hindpaw withdrawal latency in rats with unilateral inflammation

1. Recent work in our laboratory has demonstrated that intrathecal administration of a selective antagonist of calcitonin gene-related peptide (CGRP), CGRP8-37, increased the hindpaw withdrawal latency (HWL) to thermal stimulation and hindpaw withdrawal threshold (HWT) to pressure in normal rats, and that these effects were more pronounced than in rats with mononeuropathy. 2. The present study was performed to investigate the effects of intrathecal administration of CGRP8-37 on the HWL and HWT in rats with unilateral hindpaw inflammation induced by subcutaneous injection of carrageenin. The effect of naloxone was also studied. 3. Subcutaneous injection of 0.1 ml of carrageenin into the plantar region of the left hindpaw induced a significant increase in the volume of the ipsilateral hindpaw (P < 0.001), and significant bilateral decreases of the HWL to thermal stimulation (ipsilateral: P < 0.001; contralateral: P < 0.01) and HWT to pressure (ipsilateral: P < 0.001; contralateral: P < 0.01). 4. Intrathecal administration of 10 nmol of CGRP8-37, but not of 1 or 5 nmol, induced a significant bilateral increase in the HWL and HWT in rats with experimentally induced inflammation (thermal test: P < 0.001; mechanical test: P < 0.001). 5. The effect of intrathecal administration of 10 nmol CGRP8-37 on HWL and HWT was significantly more pronounced in intact rats than in rats with experimentally induced inflammation (ipsilateral: P < 0.001; contralateral: P < 0.001). 6. The effect of CGRP8-37 on withdrawal responses in the inflamed paw was partly reversed by intrathecal injection of naloxone at a dose of 88 nmol in the thermal (ipsilateral: P < 0.01; contralateral: P = 0.14) and mechanical tests (ipsilateral: P < 0.05; contralateral: P = 0.60). 7. A significant bilateral increase in the concentration of CGRP-like immunoreactivity in the perfusate of both hindpaws was demonstrated 24 h after unilateral injection of carrageenin (ipsilateral: P < 0.001; contralateral: P < 0.05). There was also an increase in the amount of CGRP-like immunoreactivity in the cerebrospinal fluid (P < 0.001), but not in plasma (P = 0.75). 8. The present study demonstrates that acute experimentally-induced unilateral hindpaw inflammation, induces bilateral increases in the amount of CGRP-like immunoreactivity in hindpaw perfusates. Intrathecal administration of CGRP8-37 increased the HWL to thermal stimulation and HWT to pressure bilaterally. 9. The results indicate that CGRP plays a role in the transmission of presumed nociceptive information in the spinal cord of rats with experimentally induced inflammation. Furthermore, our findings suggest that opioids can modulate CGRP-related effects in the spinal cord.

Opioid antagonists naloxone, beta-funaltrexamine and naltrindole, but not nor-binaltorphimine, reverse the increased hindpaw withdrawal latency in rats induced by intrathecal administration of the calcitonin gene-related peptide antagonist CGRP8-37

We recently demonstrated that intrathecal administration of calcitonin gene-related peptide 8-37 (CGRP8-37), a selective antagonist of calcitonin gene-related peptide receptors, dose-dependently increased the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in intact rats, indicating a role for CGRP and its receptors in the transmission of presumed nociceptive information in the spinal cord. The present study was performed to explore the interaction between CGRP and opioids in the spinal cord of rats. The effects of naloxone, a non-selective opioid receptor antagonist, and three different selective opioid receptor antagonists on the increased latency to withdrawal response induced by intrathecal injection of CGRP8-37 were explored. Intrathecal administration of 10 nmol of CGRP8-37 induced a significant bilateral increase in hindpaw withdrawal latency to both thermal and mechanical stimulation. The effect was partly reversed by intrathecal injection of 4 or 8 micrograms of naloxone, 10 nmol of either the mu opioid receptor antagonist beta-funaltrexamine or the delta opioid receptor antagonist naltrindole, but not by 10 nmol of the kappa opioid receptor antagonist nor-binaltorphimine. These results indicate that mu and delta, but not kappa, opioid receptors are involved in the modulation of post-synaptic effects and/or release of CGRP and other neurotransmitters.

Suppressing effects of neuroactive peptides on the inward current caused by achatin-I, an Achatina endogenous peptide

1. Modulatory effects of the four molluscan neuroactive peptides. FMRFamide (Phe-Met-Arg-Phe-NH2), APGW-amide (Ala-Pro-Gly-Trp-NH2), oxytocin and [SER2]-Mytilus inhibitory peptide ([SER2]-MIP) (Gly-Ser-Pro-Met-Phe-Val-NH2) were examined on the inward current (Iin) caused by achatin-I (Gly-D-Phe-Ala-Asp), which has been isolated from the Achatina ganglia. 2. Two Achatina giant neurone types, v-RCDN (ventral-right cerebral distinct neurone) and PON (periodically oscillating neurone), were used. Achatin-I was applied locally to the neurone tested by brief pneumatic pressure ejection, and the other molluscan neuroactive peptides were perfused around the ganglia. 3. FMRFamide, perfused at 3 microM, suppressed markedly the Iin elicited by the achatin-I of both v-RCDN and PON. APGW-amide at 3 microM also suppressed the Iin of v-RCDN, but did not affect that of PON. Oxytocin at 1 microM suppressed the Iin of PON, but did not affect that of v-RCDN. [Ser2]-MIP at 3 microM did not affect the Iin of v-RCDN. 4. The dose-response curves of FMRFamide, APGW-amide and oxytocin, indicated that their respective suppressive effects on the Iin of achatin-I were dose-dependent, and that APGW-amide was slightly more potent than the other peptides. The dose (pressure duration)-response curves of achatin-I (1 kg/cm2, 10(-3) M, 5 min interval), obtained by varying the duration of the achatin-I pressure ejection, were measured in the presence and absence of each of the three peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of pretreatment with tachykinin antagonists and galanin on the development of spinal cord hyperexcitability following sciatic nerve section in the rat

The effects of acute section of the sciatic nerve on the excitability of the flexor reflex was examined in decerebrate, spinalized, unanaesthetized rats. In control experiments without drugs, the excitability of the flexor reflex was dramatically increased in two phases following axotomy. An early intense, brief reflex hyperexcitability was followed by a less intense, prolonged period of facilitation. The selective NK1 tachykinin receptor antagonist CP-96,345 injected intrathecally at lower (1.2-2.4 nmol) and higher (12 nmol) doses blocked both components of spinal sensitization. The selective NK2 tachykinin receptor antagonist Men 10376 at a dose of 2.4 nmol also reduced both response components, as did the same dose of the inhibitory neuropeptide galanin. Thus, antagonists of excitatory neuropeptides released during and after nerve section, such as substance P and neurokinin A, can block the spinal response to peripheral nerve injury. Furthermore, the inhibitory neuropeptide galanin also reduced spinal cord sensitization.

The oxytocin antagonist 1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin reverses the increase in the withdrawal response latency to thermal, but not mechanical nociceptive stimuli following oxytocin administration or massage-like stroking in rats

In this study the effect of exogenous oxytocin and of massage-like stroking on the withdrawal latency responses to heat and mechanical nociceptive stimulation were investigated in rats. A hot-plate test and the Randall-Selitto test were used to assess the withdrawal responses. Exogenous oxytocin (0.1-1 mg/kg) and stroking (a low frequency mechanical stimulation) significantly increased the withdrawal latencies in response to mechanical and to thermal nociceptive stimuli. The effect of oxytocin and of stroking in the hot-plate test was reversed by the oxytocin antagonist (1-deamino-2-D-Tyr-(Oet)-4-Thr-8-Orn-oxytocin) directed against the uterine receptor. In contrast, the antagonist did not affect the prolonged response latency in the mechanical nociceptive stimulation test following either exogenous oxytocin or stroking. These results support the view that (1) oxytocin administration affects directly or indirectly nociceptive related behaviour in response to heat stimulation, and (2) massage-like stroking may have an anti-nociceptive effect via activation of oxytocinergic mechanisms. Since the response to mechanical stimulation was not blocked by the antagonist the mechanisms mediating the withdrawal latency to heat and mechanical stimulation could be different.

Ultrastructural studies on peptides in the dorsal horn of the rat spinal cord--II. Co-existence of galanin with other peptides in local neurons

Using light microscopic immunoperoxidase and immunofluorescence histochemistry, double-staining methodology, and electron microscopic pre-embedding and post-embedding immunocytochemistry, we studied galanin-immunoreactive neurons in the superficial dorsal horn of the rat spinal cord. Co-existence of galanin with other neuropeptides was also analysed. The lumbar 4 and 5 segments of normal rats and after rhizotomy or spinal cord transection were studied. Galanin-positive local neurons in lamina II were often islet cells and could be classified as type A, which had abundant electron-dense cytoplasm containing many large dense-core vesicles, and type B, which had electron-lucent cytoplasm with only a few large dense-core vesicles. Galanin-positive and -negative peripheral afferent terminals made synaptic contact mostly with galanin-negative dendrites and cell bodies, but also with type B galanin cell bodies and with galanin-positive dendrites of unidentified type. Galanin-immunoreactive terminals from local neurons could also be classified into two types. Type alpha terminals were most common; they contained densely packed synaptic vesicles and many large dense-core vesicles, were strongly immunostained and most frequently made synaptic contact with galanin-negative dendrites. Type beta terminals contained loosely packed synaptic vesicles and a few large dense-core vesicles, and were weakly immunostained. Axosomatic synaptic contact were sometimes found between type beta terminals and type B galanin-positive cell bodies, but were most often associated with galanin-negative dendrites. Double immunostaining showed that galanin-like immunoreactivity co-localized mainly with enkephalin-like, but sometimes also with neuropeptide Y-like immunoreactivity in some local neurons in lamina II. Galanin-like and substance P-like immunoreactivities were identified in the same neurons in deeper layers of the dorsal horn. Coexistence of these neuropeptides and neurotensin with galanin was demonstrated not only in terminals in lamina II but also in large dense-core vesicles, as revealed by post-embedding immunocytochemistry. These results show that galanin-immunoreactive neurons in lamina II receive inputs directly from primary afferents and frequently make synaptic contacts with other intrinsic neurons. Galanin in the superficial dorsal horn may be released both from primary afferents and local neurons to modulate sensory processing in many different ways, including interacting with enkephalin, neuropeptide Y, neurotensin and substance P released from the same and/or other local neurons.

Depressed concentrations of oxytocin and cortisol in children with recurrent abdominal pain of non-organic origin

The purpose of the present study was to measure plasma concentrations of oxytocin, cortisol and prolactin in children with recurrent abdominal pain of non-organic origin (RAP). Forty children with RAP and 34 controls, matched for age and sex, participated in the study. A blood sample was collected after an overnight fast in association with clinical examinations. Oxytocin, cortisol and prolactin levels were determined by radioimmunoassay (RIA). Oxytocin and cortisol concentrations in the children with RAP were found to be significantly reduced compared with those of the controls (approximately 24 versus 63 pmol/l for oxytocin and 160 versus 300 nmol/l for cortisol, respectively). The low oxytocin and cortisol levels persisted at a second examination 3 months later. No significant differences in the prolactin levels were observed between RAP and controls.

The calcitonin gene-related peptide antagonist CGRP8-37 increases the latency to withdrawal responses in rats

The present study explored the effects of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in rats. (1) Intrathecal injection of 10 nmol of CGRP had no effects on the latency to hindpaw withdrawal; intrathecal injection of 5 nmol of substance P (SP) decreased the latency to both withdrawal responses. (2) Intrathecal administration of 5 nmol or 10 nmol of CGRP8-37, but not 1 nmol, induced a significant increase in hindpaw withdrawal latency. (3) Intrathecal administration of CGRP8-37 not only reversed the SP-induced decrease in latency to both withdrawal responses but also mediated a significant increase in response latency compared to basal levels. The demonstrated results suggest that intrathecal administration of CGRP8-37 has a possible antinociceptive effect, and CGRP receptors in the spinal cord may be involved.

Is systemically administered oxytocin an analgesic in rats?

The effect of systemically administered oxytocin and a specific oxytocin antagonist, 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin, on heat pain sensitivity was examined in rats. Intraperitoneal (i.p.) oxytocin at 1 mg/kg, but not at 0.1 and 0.3 mg/kg, significantly increased response latencies on the hot-plate test. However, the rats displayed clear signs of sedation, motor impairment and vasoconstriction after 1 mg/kg oxytocin. Skin temperature on the plantar surface of the hind paws was also significantly decreased by this dose of oxytocin. The oxytocin antagonist (1 mg/kg i.p.) did not influence response latency. Since increased response latency was not the only behavioral effect of oxytocin, we conducted electrophysiological experiments to examine the effect of systemic oxytocin on the nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats. Oxytocin at 0.1 mg/kg i.p. did not influence flexor reflex magnitude, mean blood pressure or heart rate. Oxytocin at 0.3 and 1 mg/kg caused a gradual increase in blood pressure with stronger effect observed with 1 mg/kg. Neither 0.3 nor 1 mg/kg oxytocin significantly influenced the flexor reflex magnitude and heart rate. We thus conclude that systemic oxytocin did not produce analgesia in rats and the observed increase in response latency in the hot-plate test may result from the sedative and vasoconstrictive effects of this peptide. Furthermore, since the oxytocin antagonist did not significantly alter response latency on the hot-plate test, it is unlikely that endogenous oxytocin exerts a tonic effect on the pain threshold in rats.

Anti-nociceptive effects of oxytocin in rats and mice

The existence of neural opioid-mediated networks that are specific for the modulation of nociception is well established. Parallel non-opioid pathways exist, but their underlying physiology is little known. We now report that oxytocin administered intraperitoneally to rats, and intraperitoneally or intracisternally to mice has an anti-nociceptive effect, which is related to the activation of descending anti-nociceptive pathways. This anti-nociceptive effect can be reversed by an oxytocin antagonist but not by the opioid antagonist naloxone. The anti-nociceptive effect of oxytocin is not directly dependent on the activation of serotonergic pathways or to changes in temperature. Our data indicate that the oxytocinergic system has a modulatory function on nociception.

Role of efferent and afferent vagal nerve activity during reproduction: integrating function of oxytocin on metabolism and behaviour

The vagal nerves convey information between the brain and the gastrointestinal tract in both afferent and efferent direction. The efferent vagal nerves regulate gastrointestinal secretory and motor function and also the activity in the endocrine system of the gut. Gastrointestinal function and anabolic metabolism is optimized during gestation and lactation as a means of saving energy for this calorie-demanding process. This enhanced activity seems to involve central oxytocinergic transmission which increases efferent vagal nerve activity as reflected by altered levels of gastrointestinal hormones in plasma. In parallel a behaviour pattern characterized by sedation is induced. In addition, vagal afferents transfer information from the gut to the brain, e.g. as to the presence or absence of calories in the gastrointestinal tract. It is well known that satiety and sedation following a meal is in part caused by an activation of vagal afferents in response to food intake. Cholecystokinin released from the intestine is one of the factors that triggers the afferent vagal nerve activity. Also reproductive "interactive behaviours" such as milk ejection and maternal behaviour can be induced by CCK via afferent activation of vagal nerves. In contrast, when no food reaches the gut, milk production and ejection is blocked. Parts of these effects are vagally mediated, since they can be observed in vagotomized rats receiving full amounts of calories.

The antinociceptive effect of non-noxious sensory stimulation is mediated partly through oxytocinergic mechanisms

The objective of the present study was to investigate whether oxytocinergic mechanisms may contribute to the antinociceptive effect of non-noxious, sensory stimulation. To test this hypothesis, oxytocin levels in plasma and cerebrospinal fluid (CSF) were measured in control rats as well as in rats exposed for 30 min to electro-acupuncture (2 Hz), thermal stimulation (40 degrees C) or vibration (100 Hz). All modes of stimulation induced significant elevations of oxytocin levels in plasma and/or in CSF, 30 or 90 min after the end of stimulation. Secondly, the antinociceptive effects of these treatments were investigated in the tail-flick test with and without prior administration of the oxytocin antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin (1 mg kg-1 i.p.). All three modes of stimulation caused a significant delay of the tail-flick latency to the same degree as that caused by injection of oxytocin 1 mg kg-1 i.p. (electro-acupuncture P < 0.01, thermal stimulation and vibration P < 0.05). In all cases, the delay was reversed by administration of the oxytocin antagonist (1 mg kg-1 i.p.). These findings suggest that analgesic effects induced by non-noxious sensory stimulation may, in part, be mediated through activation of oxytocinergic mechanisms.