New Topics in Vasopressin Receptors and Approach to Novel Drugs: Vasopressin and Pain Perception

Taka-aki Koshimizu1 and Gozoh Tsujimoto2,* 1Division of Molecular Pharmacology, Department of Pharmacology, Jichi Medical University, Tochigi, 329-0498, Japan 2Department of Genomic Drug Discovery Science, Kyoto University Graduate School of Pharmaceutical Sciences, Kyoto 606-8501, Japan

Received October 1, 2008; Accepted November 22, 2008

Abstract. Arginine vasopressin (AVP) activates three vasopressin receptors and it also has an agonistic activity on the oxytocin receptor. For an accurate description of the target receptor subtype(s) responsible for complex AVP and oxytocin actions, a careful evaluation of ligand specificity and receptor activities are required, particularly when these receptors are co-expressed in the central nervous system. Previous studies suggest that AVP plays a regulatory role in nociception through the direct activation of central vasopressin receptors and also through the receptors that reside in the peripheral tissues. Genetically altered rodent models, including the AVP-deficient mutant Brattleboro rat and gene knockout mice lacking an endogenous opioid peptide, advanced the understanding of the interactions between the pain perception process and AVP system. This report reviews previous findings in this important field and reconciles them with the findings of recent gene knockout/knockdown studies.

Keywords: arginine vasopressin, oxytocin, pain, nociception, V1a receptor, V1b receptor

Introduction sible for the complex AVP actions in the CNS. There are three types of G-protein–coupled vasopressin receptors, Neurohypophyseal hormones, such as arginine vaso- termed V1a, V1b, and V2 receptors (22); and a single pressin (AVP) and oxytocin (OT), have a wide range of OT-receptor gene has been identified (20). V1a, V1b, effects on the central nervous system (CNS), including and OT receptors activate Gq heterotrimeric GTP nociception, learning and memory process, social binding protein and V2 stimulates Gs protein (22). The recognition, central regulation of the cardiovascular main receptors for AVP in the brain could be presumably system, and stress response (1 – 13). These CNS func- of the V1 type (23); no V2-specific ligand binding or V2 tions of AVP are not directly related to their peripheral mRNA were detected in previous reports (24, 25), while roles as circulating hormones (14). Indeed, neurons other studies suggest that there is a V2-like receptor in containing AVP peptides extend to extrahypothalamic the CNS (3, 26). A relatively good correlation was structures, which are important for pain perception reported between the autoradiographic distribution of (15 – 19). OT, on the other hand, is present in several mRNA transcripts for the V1a and autoradiographic thalamic nuclei, the mesencephalic central gray nucleus, distribution reported for V1-specific binding sites (25, the substantia nigra, the locus coeruleus, the raphe 27). The V1b receptor transcript, on the other hand, is nucleus, the nucleus of the solitary tract, and the spinal localized in several brain regions, including the olfactory cord (20, 21). bulb, CA2 pyramidal neurons in the hippocampus, Because AVP is a principal agonist to all vasopressin supraoptic, suprachiasmatic and dorsomedial hypo- receptors and also has agonistic activity on OT receptors, thalamic nuclei, piriform and entorhinal cortices, sub- it has been difficult to define the target receptor respon- stantia nigra, and dorsal motor nucleus of the vagus (28 – 30). The functional interactions between AVP / *Corresponding author. [email protected] OT peptides and their receptor system have been Published online in J-STAGE shown to provide diverse opportunities to modulate the doi: 10.1254/jphs.08R18FM efficiency of sensory transmission (31). This review will

33 34 T Koshimizu and G Tsujimoto discuss the roles of endogenous and external AVP in has a critical role in conditional pain perception. When nociception. Owing to limitations of space, this article AVP was injected into the central nucleus of the amygdala is not a comprehensive overview of the subject. For and the nociceptive jaw opening reflex monitored in discussions concerning the variety of neurophysiological freely moving rats, AVP showed an analgesic effect in roles of central AVP and OT, other previous reviews diagnostic electromyograms, which was inhibited by a should also be consulted (3, 14, 20, 21, 32 – 35). V1, but not V2, receptor antagonist (44). Other studies examined the effect of AVP in nociception by injecting it Effect of AVP in pain perception into the periaqueductal gray and observed an increase in pain threshold (26). AVP increased the endorphin and The pain perception process can be altered by a enkephalin concentration in liquid samples perfused variety of peptide and non-peptide neurotransmitters through the periaqueductal gray (45). Not only intra- (33). Using rodent withdrawal reflex tests, such as the cranial injection, but also intrathecal administration of tail flick test, or more complex behavioral tests, such as AVP showed an antinociceptive effect (46, 47). However, the hot plate test, in which supraspinal motivational another study reported that AVP failed to influence processing is involved, AVP and OT were shown to nociceptive thresholds or to modify the antinociceptive modify the nociception threshold induced by these action of morphine (48). Intrathecal AVP also produced noxious heat stimuli (33). In early reports, the anti- scratching bouts and suppression of hindbody motor nociceptive activity was observed after either the intra- function (47). ventricular or subcutaneous administration of lysine vasopressin (LVP) in the rat or after intraperitoneal Opioid-dependency of AVP-induced antinociception injection of AVP in mice (1, 36, 37). Des-glycinamide- LVP, a vasopressin analog with no apparent pressor or So far, evidence has been accumulating that AVP antidiuretic action, or des-amino-AVP, a vasopressin administered into the intracranial and, in some reports, analog with minimal pressor activity but greatly enhanced intrathecal spaces causes an increase in the pain antidiuretic activity, was also relatively ineffective (36, threshold. The next question to be answered is whether 37). Later, intracerebroventricular injections of AVP or the pain-inhibiting effect of AVP could be mediated by OT were shown to lead to antinociceptive effects in rats intrinsic opioid peptide–receptor systems. To clarify and in human cancer patients (38 – 40). this, a selective μ-opioid–receptor antagonist, naloxone, In addition to the capacity of administered AVP to or several opioid agonists, such as morphine and other show antinociception, studies on nociceptive condition peptide agonists, were used together with AVP. The of Brattleboro rats, which are deficient in mature vaso- series of results so far have been equivocal. Naloxone pressin, are informative concerning the role of AVP in inhibited AVP-induced analgesia in several studies (44, pain sensitivity. When examined using the flinch-jump 49, 50), but in other studies, the antinociceptive actions threshold test, Brattleboro rats were in a hyperalgesic of AVP were not mediated by opioids (1, 37, 38, 46). state and stress analgesia was impaired in comparison Apart from the intracranial regions, anterior and to controls (41). These analgesic deficits observed in posterior pituitary functions are closely related to AVP Brattleboro rats was vasopressin-dependent; LVP intro- and OT. Since AVP is secreted under stress conditions duced into the lateral cerebral ventricle, or subcuta- and several reports describe evidence of AVP-secreting neously at a high dose, induced an antinociceptive effect stimulation also causing secretion of opioid peptides. (42). One of other methods to inhibit AVP action in vivo In the posterior pituitary where AVP and OT neurons is the administration of antiserum against AVP directly terminate, the hypophysial nerve terminals contain into the cerebral ventricle or brain nucleus. Administra- enkephalin peptides together with AVP or OT in rat tion of antiserum to the cerebral ventricle resulted in a pituitary (51). Dynorphin immunoreactivity was also small reduction of tail-flick latency from 3.9 s in the localized in AVP and OT neurons (52). These reports vehicle adminstration to 3.3 s at a high radiant heat level suggested that AVP and opioid peptides could be (50°C), but it prolonged the latency at moderate (46°C) secreted from the posterior pituitary and in the CNS. heat (43). On the other hand, AVP acts as stimulator of adreno- In the CNS, AVP is released synaptically in the lateral corticotropin secretion in the anterior pituitary. The septum, hippocampus, amygdala, habenula, and several adrenocorticotropin peptide is processed from the pro- other brain structures (15). Therefore, the brain region(s) opiomelanocortin (POMC) gene. Although the POMC where AVP acts as antinociceptive neurotransmitter is gene can also produce β-endorphin in the CNS of the of large concern. The amygdala, an important region human and rat pituitary, a lack of an appreciable amount regulating emotional responses, such as anxiety or fear, of active β-endorphin has been reported in basal condi- AVP and Pain Perception 35

Fig. 1. The co-expression of MOR1 and DOR, but not κ (KOR), opioid receptors and OT oxytocin receptor transcripts in a human neuroblastoma cell line. Real time PCR analysis detected MOR1 and DOR (A) and OT receptor (B) transcripts as the predominant opioid receptors and vasopressin/OT receptor, respectively, in human neuroblastoma cells. C: The specific OT-receptor agonist 2+ TG stimulated [Ca ]i mobilization and desensitized the corresponding receptor, resulting in no Ca2+ response upon subsequent 2+ AVP (1 μM) application. The TG-induced [Ca ]i mobilization was inhibited by pretreatment of the cells with 2 μM Mpomeovt, a selective OT-receptor antagonist (D), but Mpomeovt failed to inhibit the angiotensin II–stimulated Ca2+ response. Mpomeovt: 1-deamino-2-O-methyl-tyrosyl-8-ornithine-1-(β-mercapto-(β,β- cyclopentamethylene)propionic acid)oxytocin; TG, [Thr4,Gly7] oxytocin. tions and under corticotropin-releasing stimuli, such as the molecular mechanism for the enhanced nociceptive AVP administration or insulin-induced hypoglycemia effect produced by OT. The neuroblastoma cells express (53). Therefore, in mice the pituitary gland is one of the both the μ-opioid receptor (MOR1) and δ-opioid major sources of peripherally circulating β-endorphin, receptor (DOR), in addition to the OT receptor (Fig. 1). but its role in analgesia remains uncertain (54). Previous OT receptor–Gq coupling and Ca2+ responses are evoked studies have shown that hypophysectomy may in fact by application of a specific OT-receptor agonist, which enhance most types of opioid analgesia (55). Further- more, the lack of stress-induced analgesia documented in β-endorphin–knockout mice may be due to the loss of central β-endorphin rather than loss of pituitary-derived β-endorphin (54).

OT and analgesia

Physiological stimulations that induce a large increase in OT concentrations in the blood, such as parturition and vaginal dilation, are known to raise the pain threshold (21). The analgesic effect of this type of OT action is not a morphine-sensitive process and peripheral injections of OT have no analgesic effect (21). OT, however, attenuates the development of tolerance to the analgesic action of morphine (56). Lesions of the Fig. 2. The inhibition of cyclicAMP production by OT, AVP, and hypothalamic paraventricular nucleus deplete endo- co-application of AVP and morphine. A: The inhibitory effects of OT genous immunoreactive AVP and OT from the rat spinal and AVP against forskolin-induced cyclic AMP production. The cells were stimulated at the indicated concentrations of OT or AVP for cord, but fail to modify the nociceptive thresholds (48). 5 min at ambient temperature and reactions were terminated by Therefore, complex interactions are suggested between heating cells at 100°C for 5 min. Cyclic AMP concentrations of the the central OT systems and opioid analgesia. cellular extracts were examined by an enzyme-linked immunosorbent Using a human neuroblastoma cell line the relation- assay. B: The co-applications of morphine and AVP enhanced inhibition of adenylate cyclase activities. In the presence of 1 μM AVP, the ship of opioid- and OT-dependent cellular signaling inhibition of forskolin-stimulated adenylate cyclase by morphine was was recently examined in our laboratory to determine significantly enhanced. *P<0.05. 36 T Koshimizu and G Tsujimoto are inhibited by an antagonist, Mpomeovt (Fig. 1: C and Neurosci. 2004;7:1048–1054. D). 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