Role of the Nitric Oxide Pathway in -Opioid-Induced Hypothermia in Rats

The effect of central and peripheral administration of a nitric oxide synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME), on the hypothermia induced by the selective -opioid receptor agonist trans-( )3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide methane sulfate (U50,488H) was studied in male Sprague-Dawley rats. In the first series of experiments, we examined the effect of subcutaneous (s.c.) administration of L-NAME on the hypothermia induced by s.c. injection of U50,488H. L-NAME, at a dose of 50 mg/kg s.c., had no influence on body temperature (Tb). Coadministration of L-NAME (50 mg/ kg, s.c.) with U50,488H (10 mg/kg, s.c.) blocked the hypothermia induced by U50,488H. In the second series of experiments, we investigated the effect of intracerebroventricular (i.c.v.) administration of L-NAME on the hypothermia induced by s.c. injection of U50,488H. L-NAME itself, given i.c.v. at a dose of 1 mg/rat, did not evoke any change in Tb. Administration of L-NAME (1 mg/rat, i.c.v.) caused a significant suppression of U50,488H hypothermia. The results indicate that either central or peripheral nitric oxide synthesis is required for the production of hypothermia induced by U50,488H. The endogenous opioid system serves several physiological functions, including a role in temperature regulation. Three distinct opioid receptors ( , , and ) have been identified. Opioid agonists have been investigated in terms of their ability to alter Tb (Clark et al., 1983; Geller et al., 1983, 1986), the response being dependent upon a number of factors including species, strain, dosage, route of administration, ambient temperature, and receptor selectivity (Adler et al., 1988). Previous results from this and other laboratories demonstrated that i.c.v. administration of selective -receptor agonists produced hyperthermia (Spencer et al., 1988; Handler et al., 1992; Adler and Geller, 1993), whereas -receptor agonists produced hypothermia (Adler et al., 1983, 1986; Spencer et al., 1988). Nitric oxide (NO), recently recognized as a prominent second messenger (Breder and Saper, 1996), is produced by the enzyme nitric-oxide synthase (NOS) that uses L-arginine to make L-citruline and the radical gas NO. NO has been found in peripheral and central neurons (Breder and Saper, 1996). Three different isoforms of NOS have been described (LopezFigueroa et al., 1998). Two are constitutive forms, endothelial and neuronal (Moncada et al., 1991), and the third is inducible (Lowenstein et al., 1992). Within the last few years, a number of studies have been conducted to investigate whether NO plays a role in temperature regulation, fever, and hypothermia. Some authors have suggested that NO has an antipyretic function (Moncada et al., 1991; Gourine, 1995), and some have shown that NO is involved in hypothermia (Branco et al., 1997; Steiner et al., 1998; Almeida and Branco, 2001). However, other articles provide evidence that the formation of NO participates in the development of a febrile response (Lin and Lin, 1996; Scammell et al., 1996; Roth et al., 1998; Benamar et al., 2000). It should be noted that these studies differed in strategies to assess the role of NO, species of experimental animal, pyrogen administered, and route of administration, as well as in inhibitors used. Although -receptor-agonist-induced hypothermia has been intensively investigated, little is known about the mechanisms by which -opioids might alter the Tb. The aim of the present study was to investigate the role of NO during the hypothermia induced by U50,488H, a -opioid receptor agonist, and to determine whether the effect of L-NAME is the result of its peripheral or its central action. Materials and Methods Animals. Male Sprague-Dawley rats (Zivic-Miller Laboratories, Pittsburgh, PA) weighing 250–300g were used in this study. They were housed two per cage for at least 1 week before surgery and were This work was supported by Grants DA00376 and DA13429 from the National Institute on Drug Abuse (Bethesda, MD). Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. DOI: 10.1124/jpet.102.036269. ABBREVIATIONS: Tb, body temperature; NO, nitric oxide; NOS, nitric-oxide synthase; U50,488H, trans-( )3,4-dichloro-N-methyl-N-(2-[1pyrrolidinyl]cyclohexyl)-benzeneacetamide methane sulfate; L-NAME, N-nitro-L-arginine methyl ester; POAH, preoptic anterior hypothalamus; CNS, central nervous system. 0022-3565/02/3031-375–378$7.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 303, No. 1 Copyright © 2002 by The American Society for Pharmacology and Experimental Therapeutics 36269/1013561 JPET 303:375–378, 2002 Printed in U.S.A. 375 at A PE T Jornals on O cber 7, 2017 jpet.asjournals.org D ow nladed from fed laboratory chow and water ad libitum. The ambient temperature was 22 2°C and a 12-h light/dark cycle was used. All experiments were started between 9:00 and 10:00 AM to minimize the effect of circadian variation in Tb. Implantation of Cannula and Transmitter. Rats were anesthetized with an intraperitoneal (i.p.) injection of a mixture of ketamine hydrochloride (100–150 mg/kg) and acepromazine maleate (0.2 mg/kg). Each animal was placed in a stereotaxic instrument. One week before the experiments began, a polyethylene cannula was implanted into the right lateral ventricle and secured to the cranium with dental acrylic according to standard procedures in our laboratory (Adams et al., 1993), and transmitters were implanted i.p. The animals were returned to individual cages in the environmental