Effect of arginine vasopressin in the nucleus raphe magnus on antinociception in the rat

Upregulation of the hypothalamo-neurohypophysial system and activation of vasopressin neurones attenuates hyperalgesia in a neuropathic pain model rat

Arginine vasopressin (AVP) is a hypothalamic neurosecretory hormone well known as an antidiuretic, and recently reported to be involved in pain modulation. The expression kinetics of AVP and its potential involvement in the descending pain modulation system (DPMS) in neuropathic pain (NP) remains unclear. We investigated AVP expression and its effects on mechanical and thermal nociceptive thresholds using a unilateral spinal nerve ligation (SNL) model. All rats with SNL developed NP. Intensities of enhanced green fluorescent protein (eGFP) in the supraoptic and paraventricular nuclei, median eminence, and posterior pituitary were significantly increased at 7 and 14 days post-SNL in AVP-eGFP rats. In situ hybridisation histochemistry revealed significantly increased AVP mRNA expression at 14 days post-SNL compared with the sham control group. The chemogenetic activation of AVP neurones significantly attenuated mechanical and thermal hyperalgesia with elevated plasma AVP concentration. These analgesic effects were suppressed by pre-administration with V1a receptor antagonist. AVP neurones increased the neuronal activity of serotonergic dorsal raphe, noradrenergic locus coeruleus, and inhibitory interneurones in the spinal dorsal horn. These results suggest that the hypothalamo-neurohypophysial system of AVP is upregulated in NP and activated endogenous AVP exerts analgesic effects via the V1a receptors. AVP neurones may activate the DPMS.

Intranasal Vasopressin Relieves Orthopedic Pain After Surgery

BACKGROUND

Orthopedic pain after surgery is very common and difficult to manage. Although intranasal arginine vasopressin (AVP) relieves headache (tension-type headache and migraine mostly), the effect of intranasal AVP on the orthopedic pain after surgery is unknown.

AIMS

This study investigated the effect of intranasal AVP on orthopedic pain after surgery in a randomized controlled trial with a double-blind design.

PARTICIPANTS

The study included 653 orthopedic patients and 661 health volunteers.

METHODS

Orthopedic pain was analyzed by the visual analogue scales (VAS) and AVP concentration was determined by radioimmunoassay.

RESULTS

(1) intranasal AVP decreased the VAS level in orthopedic patients 2-4 weeks after surgery in a dose-dependent manner; (2) the cerebrospinal fluid (CSF) AVP concentration in orthopedic patients after surgery was higher than that in the health volunteers (38.57 ± 6.11 pg/mL vs 11.74 ± 2.85 pg/mL, p < .01), but had no change in plasma (p > .05); (3) CSF AVP concentration increased significantly in orthopedic patients during 24 hours after the intranasal AVP (p < .05 or .01), which related with VAS level negatively (all p < .01); (4) during 24 hours, intranasal AVP did not influence not only plasma AVP concentration, but also blood pressure, heart rate, respiratory rate and body temperature in orthopedic patients.

COUCLUSIONS

The findings contribute valuable information that intranasal AVP can treat orthopedic pain after surgery, and AVP could be an option for pain relief by intranasal administration.

The role of peripheral vasopressin 1A and oxytocin receptors on the subcutaneous vasopressin antinociceptive effects

BACKGROUND

Vasopressin (AVP) seems to play a role as an antinociceptive neurohormone, but little is known about the peripheral site of action of its antinociceptive effects. Moreover, AVP can produce motor impairment that could be confused with behavioural antinociception. Finally, it is not clear which receptor is involved in the peripheral antinociceptive AVP effects.

METHODS

In anaesthetized rats with end-tidal CO₂ monitoring, extracellular unitary recordings were performed, measuring the evoked activity mediated by Aβ-, Aδ-, C-fibres and post-discharge. Behavioural nociception and motor impairment were evaluated under subcutaneous AVP (0.1-10 μg) using formalin and rotarod tests. Selective antagonists to vasopressin (V_1A R) or oxytocin receptors (OTR) were used. Additionally, vasopressin and oxytocin receptors were explored immunohistochemically in skin tissues.

RESULTS

Subcutaneous AVP (1 and 10 μg/paw) induced antinociception and a transitory reduction of the end-tidal CO₂ . The neuronal activity associated with Aδ- and C-fibre activation was diminished, but no effect was observed on Aβ-fibres. AVP also reduced paw flinches in the formalin test and a transitory locomotor impairment was also found. The AVP-induced antinociception was blocked by the selective antagonist to V_1A R (SR49059) or OTR (L368,899). Immunohistochemical evidence of skin VP and OT receptors is given.

CONCLUSIONS

Subcutaneous AVP produces antinociception and behavioural analgesia. Both V1a and OTR participate in those effects. Our findings suggest that antinociception could be produced in a local manner using a novel vasopressin receptor located in cutaneous sensorial fibres. Additionally, subcutaneous AVP also produces important systemic effects such as respiratory and locomotor impairment.

SIGNIFICANCE

Our findings support that AVP produces peripheral antinociception and behavioural analgesia in a local manner; nevertheless, systemic effects are also presented. Additionally, this is the first detailed electrophysiological analysis of AVP antinociceptive action after subcutaneous administration. The results are reasonably explained by the demonstration of V_1A R and OTR in cutaneous fibres.

The role of arginine vasopressin in electroacupuncture treatment of primary sciatica in human

It has been implicated that electroacupuncture can relieve the symptoms of sciatica with the increase of pain threshold in human, and arginine vasopressin (AVP) in the brain rather than the spinal cord and blood circulation participates in antinociception. Our previous study has proven that AVP in the brain played a role in the process of electroacupuncture analgesia in rat. The goal of the present study was to investigate the role of AVP in electroacupuncture in treating primary sciatica in human. The results showed that (1) AVP concentration of cerebrospinal fluid (CSF) (7.5 ± 2.5 pg/ml), not plasma (13.2 ± 4.2 pg/ml) in primary sciatica patients was lower than that in health volunteers (16.1 ± 3.8 pg/ml and 12.3 ± 3.4 pg/ml), although the osmotic pressure in CSF and plasma did not change; (2) electroacupuncture of the bilateral "Zusanli" points (St. 36) for 60 min relieved the pain sensation in primary sciatica patients; (3) electroacupuncture increased the AVP level of CSF, not plasma in primary sciatica patients; and (4) there was the positive correlation between the effect of electroacupuncture relieving the pain and the AVP level of CSF in the primary sciatica patients. The data suggested that central AVP, not peripheral AVP might improve the effect of electroacupuncture treatment of primary sciatica in human, i.e., central AVP might take part in the electroacupuncture relieving the pain sensation in primary sciatica patients.

Effect of arginine vasopressin on the cortex edema in the ischemic stroke of Mongolian gerbils

Brain edema formation is one of the most important mechanisms of ischemia-evoked cerebral edema. It has been demonstrated that arginine vasopressin (AVP) receptors are involved in the pathophysiology of secondary brain damage after focal cerebral ischemia. In a well-characterized animal model of ischemic stroke of Mongolian gerbils, the present study was undertaken to clear the effect of AVP on cortex edema in cerebral ischemia. The results showed that (1) occluding the left carotid artery of Mongolian gerbils not only decreased the cortex specific gravity (cortex edema) but also increased AVP levels in the ipsilateral cortex (ischemic area) including left prefrontal lobe, left parietal lobe, left temporal lobe, left occipital lobe and left hippocampus for the first 6 hours, and did not change of the cortex specific gravity and AVP concentration in the right cortex (non-ischemic area); (2) there were many negative relationships between the specific gravity and AVP levels in the ischemic cortex; (3) intranasal AVP (50 ng or 200 ng), which could pass through the blood-brain barrier to the brain, aggravated the focal cortex edema, whereas intranasal AVP receptor antagonist-D(CH2)5Tyr(ET)DAVP (2 µg) mitigated the cortex edema in the ischemic area after occluding the left carotid artery of Mongolian gerbils; and (4) either intranasal AVP or AVP receptor antagonist did not evoke that edema in the non-ischemic cortex. The data indicated that AVP participated in the process of ischemia-evoked cortex edema, and the cerebral AVP receptor might serve as an important therapeutic target for the ischemia-evoked cortex edema.

Effect of intranasal arginine vasopressin on human headache

Arginine vasopressin (AVP), a nonapeptide hormone of posterior pituitary, reaches the central nervous system from systemic blood circulation with a difficulty because of the blood-brain barrier (BBB). The interest has been expressed in the use of the nasal route for delivery of AVP to the brain directly, exploiting the olfactory pathway. Our previous study has demonstrated that AVP in the brain rather than the spinal cord and blood circulation plays an important role in rat pain modulation. For understanding the role of AVP on pain modulation in human, the communication tried to investigate the effect of intranasal AVP on human headache. The results showed that (1) AVP concentration in both plasma and cerebrospinal fluid (CSF) increased significantly in headache patients, who related with the headache level; (2) there was a positive relationship between plasma and CSF AVP concentration in headache patients; and (3) intranasal AVP could relieve the human headache in a dose-dependent manner. The data suggested that intranasal AVP, which was delivered to the brain through olfactory region, could treat human headache and AVP might be a potential drug of pain relief by intranasal administration.

Arginine vasopressin in hypothalamic paraventricular nucleus is transferred to the caudate nucleus to participate in pain modulation

Arginine vasopressin (AVP), which is synthesized and secreted in the hypothalamic paraventricular nucleus (PVN), is the most important bioactive substance in the pain modulation. Our pervious study had shown that AVP plays an important role in pain modulation in caudate nucleus (CdN). The experiment was designed to investigate the source of AVP in CdN by the nucleus push-pull perfusion and radioimmunoassay. The results showed that: (1) pain stimulation increased the AVP concentration in the CdN perfusion liquid, (2) PVN decreased the effect of pain stimulation which was stronger in both sides than in one side of PVN cauterization; and (3) L-glutamate sodium would excited the PVN neurons by the PVN microinjection that could increase the AVP concentration in the CdN perfusion liquid. The data suggested that AVP in the CdN might come from the PVN in the pain process, i.e., AVP in the PVN might be transferred to the CdN to participate in the pain modulation.

Norepinephrine plays an important role in antinociceptive modulation of hypothalamic paraventricular nucleus in the rat

Our previous study has proven that hypothalamic paraventricular nucleus (PVN) plays a role in antinociception. The effects of studied classical neurotransmitter on PVN antinociceptive modulation were investigated in the rat. The results showed: (1) Pain stimulation increased norepinephrine (NE), but not epinephrine, dopamine (DA), 3,4-dihydroxyphenylacetic acid (DA metabolic product), homovanilic acid (DA metabolic product), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HT metabolic product), acetycholine (Ach), choline (Ach metabolic product), gamma-aminobutyric acid (GABA), and L-glutamate acid concentrations in the PVN perfusion liquid; (2) PVN stimulation with L-glutamate sodium, which excited local neurons only, did not influence the concentrations of the studied classical neurotransmitter and metabolic product in the PVN perfusion liquid; (3) Microinjection of NE, epinephrine, or L-glutamate sodium into the PVN elevated pain threshold, and local administration of GABA decreased pain threshold in a dose-dependent manner, but PVN administration of Ach, DA, or 5-HT did not change pain threshold; (4) Microinjection of phentolamine (alpha-receptor antagonist) or MK801 [NMDA-receptor antagonist] into the PVN reduced pain threshold, and local administration of bicuculline (GABA-receptor antagonist) raised pain threshold, but PVN administration of propranolol (beta-receptor antagonist), atropine (Muscarinic cholinergic receptor antagonist), 6-OH gallamine (Nicotinic cholinergic receptor antagonist), fluperidol (DA-receptor antagonist), or cyproheptadine (5-HT-receptor antagonist) did not alter pain threshold. The data suggested that endogenous NE, not epinephrine, 5-HT, Ach, GABA, and L-glutamate acid played an important role in the PVN antinociceptive modulation.

Arginine vasopressin induces rat caudate nucleus releasing acetylcholine to participate in pain modulation

A lot of studies have pointed that acetylcholine (Ach), a classic neurotransmitter can regulate pain process in the caudate nucleus (CdN). Our previous report has proven that arginine vasopressin (AVP) effects on pain modulation in the CdN. The communication was designed to investigate the interaction between AVP and Ach in the rat CdN during the pain process. AVP concentration was determined by radioimmunoassay (RIA) and Ach concentration by high performance liquid chromatography (HPLC). The results showed that pain stimulation increased both AVP and Ach concentrations in the CdN perfusion liquid; AVP increased Ach concentration in the CdN perfusion liquid, while AVP receptor antagonist including d(CH(2))(5)Tyr(Me)AVP (V(1) receptor antagonist) and d(CH(2))(5)[D-Ile(2), Ile(4), Ala-NH(2)(9)]AVP (V(2) receptor antagonist) decreased Ach concentration in the CdN perfusion liquid. The data indicated that the analgesic effect of AVP might be involved in the Ach system in the CdN.

Arginine vasopressin in hypothalamic paraventricular nucleus is transferred to the nucleus raphe magnus to participate in pain modulation

Hypothalamic paraventricular nucleus (PVN) is one of the main sources of arginine vasopressin (AVP) synthesis and secretion. AVP is the most important bioactive substance in PVN regulating pain process. Our previous study has pointed that pain stimulation induced AVP increase in the nucleus raphe magnus (NRM), which plays a role in pain modulation. The present study was designed to investigate the source of AVP in the rat NRM during pain process using the methods of nucleus push-pull perfusion and radioimmunoassay. The results showed that pain stimulation increased the AVP concentration in the NRM perfusion liquid, PVN cauterization inhibited the role that pain stimulation induced the increase of AVP concentration in the NRM perfusion liquid, and PVN microinjection of L-glutamate sodium, which excited the PVN neurons, could increase the AVP concentration in the NRM perfusion liquid. The data suggested that AVP in the PVN might be transferred to the NRM to participate in pain modulation.

Norepinephrine regulates arginine vasopressin secretion in hypothalamic paraventricular nucleus relating with pain modulation

Our previous study has pointed that arginine vasopressin (AVP) and norepinephrine (NA) are two most important bioactive substances that play a role in hypothalamic paraventricular nucleus (PVN) regulating pain process. The communication was designed to investigate the interaction between AVP and NA in the rat PVN during the pain process. We used the potassium iontophoresis inducing tail-flick to test the pain threshold, PVN push-pull perfusion to collect the samples, high performance chromatography (HPLC) to determine the NA concentration and radioimmunoassay (RIA) to measure the AVP concentration. The results showed that (1) pain stimulation increased both NA and AVP concentrations in the PVN perfusion liquid; (2) PVN administration of l-glutamate sodium increased AVP, not NA concentration in the PVN perfusion liquid; (3) AVP or d(CH(2))(5)Tyr(Et)DAVP (AVP-receptor antagonist) neither changed pain threshold, nor influenced NA concentration in the PVN perfusion liquid; (4) Microinjection of NA into PVN could increase pain threshold in a dose-dependent manner, while PVN administration with phentolamine (alpha-receptor antagonist), not propranolol (beta-receptor antagonist) decreased pain threshold; (5) Administration of NA increased AVP concentration, while phentolamine, not propranolol decreased AVP concentration in the PVN perfusion liquid. These data suggested that it is through alpha-receptor rather than beta-receptor, NA induced PVN secretion of AVP that was delivered to the related brain regions to participate in pain modulation.

Arginine vasopressin antinociception in the rat nucleus raphe magnus is involved in the endogenous opiate peptide and serotonin system

Arginine vasopressin (AVP) in the nucleus raphe magnus (NRM) has been implicated in antinociception. This communication was designed to investigate which neuropeptide and neurotransmitter are involved in AVP antinociception in the rat NRM. The results showed that (1) in the NRM perfuse liquid, pain stimulation could increase the concentrations of AVP, leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), beta-endorphin (beta-Ep), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), but not change the concentrations of dynorphinA(1-13) (DynA(1-13)), oxytocin, achetylcholine, choline, gamma-aminobutyric acid, glutamate, dopamine, 3,4-dihydroxyphenylacetic acid, homovanilic acid, norepinephrine and epinephrine; (2) in the NRM perfuse liquid, AVP increased the concentrations of L-Ek, M-Ek, beta-Ep, DynA(1-13), 5-HT and 5-HIAA, but did not change the concentrations of oxytocin and the other studied neurotransmitters; (3) AVP antinociception in the NRM was attenuated by cypoheptadine (a 5-HT-receptor antagonist) or naloxone (an opiate receptor antagonist), but was not influenced by the other studied receptor antagonists. The data suggested that AVP antinociception in the NRM might be involved in endogenous opiate peptide and 5-HT system.

Endogenous opiate peptides in the spinal cord are involved in the analgesia of hypothalamic paraventricular nucleus in the rat

Many studies have shown that hypothalamic paraventricular nucleus (PVN) plays a role in pain process, and endogenous opiate peptide system in the spinal cord is involved in nociception. This communication was designed to study the relationship between PVN and endogenous opiate system in the spinal cord in the rat. The results showed that in both the thoracic and the lumber spinal cord, microinjection of 100 ng L-glutamate sodium into PVN could increase leucine-enkephalin (L-Ek), beta-endorphin (beta-Ep), dynorphinA(1-13) (DynA(1-13)) concentrations and PVN cauterization decreased L-Ek and beta-Ep concentrations. Pretreatment of the spinal cord with 5 microg naloxone, an opiate receptor antagonist could partly reverse the analgesia induced by microinjection of 100 ng L-glutamate sodium into PVN. The data suggested that PVN analgesia might be involved in the endogenous opiate peptide system in the spinal cord independently.

Effect of arginine vasopressin on acupuncture analgesia in the rat

Arginine vasopressin (AVP) has been proven to be involved in the process of pain regulation. This communication was designed to investigate the effect of AVP on acupuncture analgesia in the rat model. The results showed that intraventricular injection (icv) of AVP could enhance acupuncture analgesia in a dose-dependent manner, whereas icv of anti-AVP serum decreased acupuncture analgesia. However, neither intrathecal (ith) nor intravenous injection (iv) of AVP or anti-AVP serum could influence acupuncture analgesia. Electrical acupuncture of "Zusanli" points (St. 36) decreased AVP concentration in the hypothalamic paraventricular nucleus (PVN), and increased AVP concentration in the hypothalamic supraoptic nucleus (SON), periaqueductial gray (PAG), caudate nucleus (CdN) and raphe magnus nucleus (RMN), but did not change AVP concentration in the pituitary, spinal cord and plasma. The effect of AVP on acupuncture analgesia was partly reversed by pretreatment with naloxone, an opiate receptor antagonist. These data suggested that AVP in the brain played a role in the process of acupuncture analgesia in combination with the endogenous opiate peptide system.

Effect of oxytocin on acupuncture analgesia in the rat

Oxytocin has been demonstrated to be involved in pain modulation. Acupuncture analgesia is a very useful clinical tool for pain relief, which has over 2500-year history in China. The present study investigated the role of oxytocin in acupuncture analgesia in the rat through oxytocin administration and measurement. Central administration of oxytocin (intraventricular injection or intrathecal injection) enhanced acupuncture analgesia, while central administration of anti-oxytocin serum weakened acupuncture analgesia in a dose-dependent manner. However, intravenous injection of oxytocin or anti-oxytocin serum did not influence acupuncture analgesia. Electrical acupuncture of "Zusanli" (St. 36) reduced oxytocin concentration in the hypothalamic supraoptic nucleus, and elevated oxytocin concentration in the hypothalamic suprachiasmatic nucleus, hypothalamic ventromedial nucleus, thalamic ventral nucleus, periaqueductal gray, raphe magnus nucleus, caudate nucleus, thoracic spinal cord and lumbar spinal cord, but did not alter oxytocin concentration in the hypothalamic paraventricular nucleus, anterior pituitary, posterior pituitary and plasma. The data suggested that oxytocin in central nervous system rather than in peripheral organs is involved in acupuncture analgesia.

Arginine vasopressin induces periaqueductal gray release of enkephalin and endorphin relating to pain modulation in the rat

Previous study has proven that microinjection of arginine vasopressin (AVP) into periaqueductal gray (PAG) raises the pain threshold, in which the antinociceptive effect of AVP can be reversed by PAG pretreatment with V2 rather than V1 or opiate receptor antagonist. The present work investigated the AVP effect on endogenous opiate peptides, oxytocin (OXT) and classical neurotransmitters in the rat PAG. The results showed that AVP elevated the concentrations of leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek) and beta-endorphin (beta-Ep), but did not change the concentrations of dynorphinA(1-13) (DynA(1-13)), OXT, classical neurotransmitters including achetylcholine (Ach), choline (Ch), serotonin (5-HT), gamma-aminobutyric acid (GABA), glutamate (Glu), dopamine (DA), norepinephrine (NE) and epinephrine (E), and their metabolic products in PAG perfusion liquid. Pain stimulation increased the concentrations of AVP, L-EK, M-Ek, beta-Ep, 5-HT and 5-HIAA (5-HT metabolic product), but did not influence the concentrations of DynA(1-13), OXT, the other classical neurotransmitters and their metabolic products. PAG pretreatment with naloxone - an opiate receptor antagonist completely attenuated the pain threshold increase induced by PAG administration of AVP, but local pretreatment of OXT or classical neurotransmitter receptor antagonist did not influence the pain threshold increase induced by PAG administration of AVP. The data suggested that AVP in PAG could induce the local release of enkephalin and endorphin rather than dynophin, OXT and classical neurotransmitters to participate in pain modulation.

Arginine vasopressin is an important regulator in antinociceptive modulation of hypothalamic paraventricular nucleus in the rat

Our previous study has proven that hypothalamic paraventricular nucleus (PVN) stimulation increases pain threshold and PVN cauterization decreases pain threshold. The studied neuropeptides in PVN were investigated to involve to pain modulation in the rat. The results showed that (1) intraventricular injection (icv) of anti-arginine vasopressin (AVP) serum completely reversed pain threshold increase induced by l-glutamate sodium (Glu) injection into the PVN, and local administration (icv) of anti-leucine-enkephalin (L-Ek) serum or anti-beta-endorphin (beta-Ep) serum partly attenuated pain threshold increase induced by Glu injection into the PVN, but pre-treatment of anti-oxytocin (OXT), dynorphinA(1-13) (DynA(1-13)), cholecystokinin-like peptide (CCK), neurotensin (NT), corticotrophin-releasing hormone (CRH), adrenocorticotrophin (ACTH), somatostatin (SST), prolactin-releasing hormone (PRH), angiotensinII (AngII), vasoactive intestinal polypeptide (VIP), melanotropin-releasing hormone (MRH), thyrotropin-releasing hormone (TRH), substance P (SP) or growth hormone-releasing hormone (GHRH) serum (icv) did not influence the analgesic effect of PVN administration with Glu; (2) PVN stimulation with Glu elevated the concentrations of AVP, OXT, CCK, NT, CRH, SST, PRH and DynA(1-13) in PVN perfusion liquid, and could not change the concentrations of L-Ek, beta-Ep, AngII, ACTH, VIP, MRH, TRH, SP and GHRH in PVN perfusion liquid; (3) Pain stimulation increased the concentrations of AVP, L-Ek, beta-Ep, DynA(1-13), CRH and ACTH in PVN perfusion liquid, and did not alter the concentrations of OXT, CCK, NT, SST, PRH, AngII, VIP, MRH, TRH, SP and GHRH in PVN perfusion liquid. The data suggested that AVP played a more important role than the other studied peptides (OXT, L-Ek, beta-Ep, DynA(1-13), CCK, NT, CRH, ACTH, SST, PRH, AngII, VIP, MRH, TRH, SP and GHRH) in PVN antinociceptive progress.

Central oxytocin enhances antinociception in the rat

The study aimed to investigate the effect of oxytocin on antinociception in the rat. The pain threshold was elevated by oxytocin following intraventricular (icv) or intrathecal injection (ith), and reduced by anti-oxytocin serum (icv or ith). But the pain threshold was not altered by intravenous injection (iv) of oxytocin or anti-oxytocin serum. Pain stimulation induced oxytocin concentration decrease in the hypothalamic supraoptic nucleus, and increase in the locus coeruleus, raphe magnus nucleus, caudate nucleus and spinal cord, but no change in the hypothalamic paraventricular nucleus and plasma. The results indicated that central, not peripheral oxytocin could enhance antinociception.

Arginine vasopressin in periaqueductal gray, which relates to antinociception, comes from hypothalamic paraventricular nucleus in the rat

Hypothalamic paraventricular nucleus (PVN) is a major source of arginine vasopressin (AVP). Our previous work has proven that: (1) pain stimulation enhances PVN synthesis and secretion of AVP; (2) AVP in periaqueductal gray (PAG) plays a role in antinociception; (3) pain stimulation increases AVP concentration in PAG tissue. The present study was to investigate AVP source in PAG during pain modulation of the rat. The results showed that: (1) pain stimulation elevated AVP concentration in both PVN and PAG perfusion liquid, in which the peak of AVP concentration in PVN perfusion liquid occurred earlier than that in PAG perfusion liquid; (2) PVN cauterization weakened pain stimulation-induced PAG releasing AVP, in which the inhibitive effect of bilateral PVN cauterization showed stronger than that of unilateral PVN cauterization; (3) microinjection of l-glutamate sodium into PVN, which excited local neurons, increased AVP concentration in PAG perfusion liquid in a dose-dependent manner. The data suggest that AVP in PAG, which relates with pain modulation, comes from PVN.