Elsevier

Peptides

Volume 29, Issue 7, July 2008, Pages 1183-1190
Peptides

Neuropeptide FF receptors antagonist, RF9, attenuates opioid-evoked hypothermia in mice

https://doi.org/10.1016/j.peptides.2008.02.016Get rights and content

Abstract

The present study used the endpoint of hypothermia to investigate opioid and neuropeptide FF (NPFF) interactions in conscious animals. Both opioid and NPFF systems played important roles in thermoregulation, which suggested a link between opioid receptors and NPFF receptors in the production of hypothermia. Therefore, we designed a study to investigate the relationship between opioid and NPFF in control of thermoregulation in mice. The selective NPFF receptors antagonist RF9 (30 nmol) injected into the third ventricle failed to induce significant effect, but it completely antagonized the hypothermia of NPFF (45 nmol) after cerebral administration in mice. In addition, RF9 (30 nmol) co-injected i.c.v. in the third ventricle reduced the hypothermia induced by morphine (5 nmol,) or nociceptin/orphanin FQ (N/OFQ) (2 nmol). Neither the classical opioid receptors antagonist naloxone (10 nmol) nor NOP receptor antagonist [Nphe1]NC(1-13)NH2 (7.5 nmol) reduced the hypothermia induced by the central injection of NPFF at dose of 45 nmol. Co-injected with a low dose of NPFF (5 nmol), the hypothermia of morphine (5 nmol) or N/OFQ (2 nmol) was not modified. These results suggest that NPFF receptors activation is required for opioid to produce hypothermia. In contrast, NPFF-induced hypothermia is mainly mediated by its own receptors, independent of opioid receptors in the mouse brain. This interaction, quantitated in the present study, is the first evidence that NPFF receptors mediate opioid-induced hypothermia in conscious animals.

Introduction

Opioid receptors are currently classified as classical (MOP, DOP and KOP receptors) and nonclassical (NOP receptor) [40]. Opioid receptors were involved in the physiological control of numerous functions of the central nervous system, including thermoregulation. It has long been recognized that opioid such as morphine could produce a range of effects on body temperature in a number of species including man [1], [37], [41]. The effect on body temperature of morphine, which acted on the classical opioid receptors, were biphasic in mice, with low doses producing hyperthermia and higher doses resulting in hypothermia [3], [12], [32], [44]. The hypothermia induced by morphine could be antagonized by the opioid receptor antagonists naloxone (classical opioid receptors antagonist), naloxonazine (MOP receptor antagonist), BNTX (DOP receptor antagonist) and DIPPA (KOP receptor antagonist), which provided further evidence for the involvement of classical opioid receptors [3], [33]. N-nitro-l-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, enhanced the hypothermic effects of morphine in mice [38]. In addition, NOP receptor is the fourth member of the opioid receptor family [20]. Nociceptin/orphanin FQ (N/OFQ) isolated from the mammal brain, a 17-amino-acid peptide, was an endogenous ligand of the NOP receptor [19], [29]. N/OFQ decreased body temperature in rats, which was reduced by NOP receptor antisense oligonucleotides treatment in rats [5], [14], [28], [39], [42].

Neuropeptide FF (NPFF, FLFQPQRF-NH2) belongs to a family of amidated neuropeptides related to the molluscan cardioexcitatory neuropeptide FMRFamide (FMRFa) [25]. NPFF immunoreactivity was localized in different CNS sites involved in temperature regulation [2], [7]. In thermoregulatory study, injection into the third ventricle of NPFF or 1DMe (a stable NPFF analog) elicited a marked decrease in basal rectal temperature [6], [8], [10], [11], [27]. L-NAME markedly potentiated hypothermia induced by 1DMe injected in the mouse brain [45]. The previous studies indicated that NPFF might act as a modulator of endogenous opioid functions [22], [23], [24], [34], [35], [43]. At the cellular level, NPFF receptors induced anti-opioid actions [34]. In vivo, the studies of NPFF were mostly focused on the pro- and anti-opioid effects on morphine antinociception or morphine tolerance and dependence [16], [17], [18], [26]. However, the possible interactions between NPFF and opioid agonists in the mediation of hypothermic effects are not very clear and need more investigations.

Both opioid and NPFF systems were involved in thermoregulation of the rodents. NO participated, in the same manner, in hypothermia evoked by NPFF and morphine [38], [45]. Therefore, these data suggested a link between opioid and NPFF systems in body temperature regulation. In the present study, we used the endpoint of hypothermia to investigate opioid and NPFF systems interactions in conscious mice.

Section snippets

Animals

Male Kunming strain mice were obtained from the Experimental Animal Center of Lanzhou University. All animals were cared for and experiments were carried out in accordance with the European Community guidelines for the use of experimental animals (86/609/EEC). All the protocols in this study were approved by the Ethics Committee of Lanzhou University, China.

Chemicals

NPFF, RF9, N/OFQ and [Nphe1]NC(1-13)NH2 were synthesized on a solid-phase support following the previous report [9]. Peptides were prepared

Morphine–NPFF interaction

As shown in Fig. 1, Fig. 2, central injection of morphine (5 nmol) evoked the decrease in body temperature (AUC: −108.2 ± 7.8; P < 0.001, vs. saline), which was blocked by co-injection of naloxone (10 nmol) (P < 0.01, vs. 5 nmol morphine group) (Fig. 2A). At a dose of 45 nmol, NPFF produced significant hypothermia following injection into the third ventricle (AUC: −87.9 ± 16.1; P < 0.01, vs. saline) (Fig. 1, Fig. 2). The hypothermic effects of NPFF (45 nmol) were fully prevented by co-injection of RF9 (30 

Discussion

The major finding of the present study was that the NPFF receptors antagonist RF9 attenuated the hypothermia caused by two different opioid agonists, morphine and N/OFQ. In contrast, the hypothermia evoked by the central administration of NPFF was not affected by opioid antagonists.

The previous studies indicated that NPFF might act as a modulator of opioid functions [22], [23], [24], [34], [35], [43]. In rodents, NPFF receptors agonists exhibited either anti-opioid activities or potentiate

Acknowledgements

This study was supported by the grants from the National Natural Science Foundation of China (Nos. 20525206, 20772052 and 20621091), the Specialized Research Fund for the Doctoral Program in Higher Education Institutions (No. 20060730017), and the Chang Jiang Program of the Ministry of Education of China.

References (45)

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