Abstract. Methylxanthines produce a quasi-morphine with- drawal syndrome (QMWS) in opiate naive rats. Addition- ally, methylxanthine-induced suppression of ...
Psychopharmacology
Psychopharmacology (1989) 98: 231-235
~ Springer-Verlag 1989
Modification of quasi-morphine withdrawal with serotonin agonists and antagonists: evidence for a role of serotonin in the expression of opiate withdrawal Mark S. Kleven* and Sheldon B. Sparber Department of Pharmacology, University of Minnesota Medical School, 435 Delaware Street S.E., Minneapolis, MN 55455, USA
Abstract. Methylxanthines produce a quasi-morphine withdrawal syndrome (QMWS) in opiate naive rats. Additionally, methylxanthine-induced suppression of conditioned behavior in rats is reversed by the alpha2 adrenergic agonist clonidine which also attenuates true opiate withdrawal and the QMWS. Therefore, the operant behavioral effects of 3-isobutyl-l-methylxanthine (IBMX) provide a model with which to study mechanisms involved in the expression of opiate withdrawal. In order to examine the role of serotonin (5-HT) in the rate-decreasing effects of IBMX on operant behavior, the 5-HT precursor 5-hydroxytryptophan, and 5-HT reuptake blocker fluoxetine were administered in combination with IBMX to rats performing a fixed-ratio 30 operant for food reinforcement. Both drugs failed to reverse the behavioral suppression caused by relatively low doses of IBMX, suggesting that elevated 5-HT neurotransmission contributes to, rather than attenuates, the QMWS. The relatively selective 5-HT2 antagonists mianserin and pirenperone blocked the [BMX-induced suppression, whereas the classic 5-HT antagonist methysergide had no effect. The results indicate that the operant behavioral effects of IBMX and possibly the QMWS may be mediated by serotonergic mechanisms. Key words: QMWS - Opiate withdrawal - IBMX - Serotonin - Rats
The opiate withdrawal syndrome consists of a variety of autonomic and behavioral signs including diarrhea, urination, and irritability, which are reduced by administration of opiates. Nonopiates, such as clonidine (Sparber and Meyer 1978; Dwoskin et al. 1983) and mianserin (Neal and Sparber 1986), also alleviate signs of opiate withdrawal, indicating that expression of withdrawal may be distinct from either the initial pharmacological actions of opiates or the ensuing adaptive process. One possible way to dissociate the adaptive process, called dependence, from the expression of withdrawal is to study the quasi-morphine withdrawal syndrome (QMWS) produced by methylxanthines alone or in combination with naloxone (Collier 1974). The QMWS consists of many of the familiar signs of naloxone S.B. Sparber * Present address: Department of Pharmacological and Physiological Sciences, The University of Chicago, Chicago, IL, USA Offprint requests to:
precipitated withdrawal (Frances et al. 1975; Collier et al. 1981), including jumping, ptosis, salivation, diarrhea, body shakes, and teeth chattering. Thus, it should be possible to study mechanisms which are involved only in the expression of certain signs of opiate dependence in the absence of opiates. Several neuropharmacological effects of methylxanthines are consistent with effects seen during opiate withdrawal. For example, administration of 3-isobutyl-l-methylxanthine (IBMX) to rats produces increases in central nervous system catecholamine activity similar to those obstowed during naloxone precipitated abstinence (Berkowitz etal. 1970; Reinhard etal. 1983; Galloway and Roth 1983 a, b). In addition, both the QMWS and operant behavioral suppression induced by IBMX are antagonized by clonidine (Grant and Redmond 1982; Kleven and Sparber 1987). However, 5-HT has been implicated in opiate dependence as well. It has been reported that serotonin precursors or agonists attenuate the opiate withdrawal syndrome in rats (Huidobro etal. 1963; Huang etal. 1978; Samanin et al. 1980; Contreras et al. 1980) and man (Bellini et al. 1982). However, Schwartz and Eidelberg (1970) reported that concurrent treatment with morphine and parachlorophenylalanine (pCPA) failed to alter nalorphine-precipitated ptosis, diarrhea, shakes, or hypothermia in rats. Further, Maruyama et al. (1971) reported that pretreatment of mice with pCPA and morphine failed to alter naloxoneinduced jumping, while the monoamine oxidase inhibitor pargyline enhanced this sign of withdrawal. These latter studies suggest that 5-HT may facilitate the development of morphine dependence. Thus, the involvement of 5-HT in the expression of opiate withdrawal signs is unclear. Methylxanthines also appear to increase 5-HT metabolism in the rat (Berkowitz and Spector 1971; Karasawa et al. 1976; Cardinali 1978; Reinhard et al. 1983). However, few studies have addressed the possibility that 5-HT may be involved in either the behavioral effects of methylxanthines or the QMWS. tn order to determine if an increase in 5-HT neurotransmission caused by IBMX might mediate the QMWS, the effects of IBMX on operant behavior were examined in combination with 5-HT antagonists and indirect agonists. If the QMWS is mediated by 5-HT, akin to the wet-dog shakes produced by the 5-HT precursor 5-hydroxytryptophan [5-HTP (Yap and Taylor 1983; Lucki et al. 1984)], increasing serotonergic neurotransmission by administration of either 5-HTP or the 5-HT reuptake blocker fluoxetine (FLUO) should enhance the operant behavior-
232 al suppression caused by IBMX. Conversely administration of 5-HT antagonists such as methysergide (METH), mianserin (MIA), or pirenperone (PIR) should block the behavioral effects of IBMX.
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Subjects and apparatus. Male Long-Evans rats (Blue Spruce Farms, Altamont, NY) weighing 400-450 g (free-feeding) were used. Rats were individually housed in hanging, stainless steel cages in a room maintained at 21 + 1~ C temperature and 40-50% relative humidity with water available ad lib. Behavioral sessions took place in standard operant chambers (Model no. 143-22; BRS/LVE, Beltsville, MD), enclosed in custom built insulated environmental cubicles equipped with closed-circuit video cameras (Sparber 1980). Chambers were equipped with a lever which could be depressed by a mass of 20-25 g. A house light and stimulus light above the lever were illuminated during the session when responses were reinforced. Food pellets weighing 45 mg (formulation T101, BioServ, Frenchtown, NJ) were dispensed upon completion of 30 lever-press responses (FR 30). Behavior sessions were controlled by a custom made microcomputer interface for TRS-80| Color Computers (Tandy Radio Shack, Fort Worth, TX). When responding was stable (defined as a coefficient of variation of less than 10% during three consecutive sessions) rats were exposed to saline and vehicle injections (1 or 2 ml/kg) prior to the behavioral sessions. This procedure allowed the use of previous sessions, when saline or drug vehicle was administered, to be used for comparison with drug sessions. The behavioral sessions were divided into two or more epochs. After the first 10-min epoch, the chamber lights were extinguished and reinforcers no longer delivered for 1 min. At this time the animal was removed from the chamber, injected with drug or vehicle, and placed back into the chamber for the remaining 30-min epoch. During control days, vehicle was administered 30 rain before the start of the session and immediately after the first 10-rain epoch ended. The initial 10-rain epoch therefore permitted evaluation of the effects of IBMX alone or in combination with the test drug.
Agonist studies. Rats were assigned to one of six groups (n = 4-5/group) administered combinations of IBMX and 5-HTP. Rats were administered one of two doses of IBMX (2.5 or 5.0 mg/kg) 10 rain after the start of the behavioral sessions. Ten minutes later, rats from both groups were removed from the chamber and given one of three doses of 5-HTP (0, 5.0, or 15 mg/kg) and returned to the chamber for the remaining 20-rain epoch. At least 1 week after the completion of this study, rats were randomly reassigned to one of three groups (n =4-5/group) administered 0, 5, or 10 mg fluoxetine/kg 30 min prior to an operant session. These rats then received either 0 or 2.5 mg IBMX/kg 10 rain after the start of the operant session.
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IBMX (mg/kg) Fig. 1. Effect of 5-hydroxytryptophan (5-HTP) in combination with IBMX. IBMX (2.5 or 5.0 mg/kg) was administered 10 rain after the start of a 40-rain FR 30 operant session, and 5-HTP (0, 5, or 15 mg/kg) was administered 20 rain after the start of the session. Data (n = 4-5/group) are expressed as a percentage of average control sessions (n=8) when vehicle (0.9% saline, distilled water, or 2% Pluronic F68) was administered, The pre-5-HTP epoch lasted 10 rain and the post-5-HTP epoch lasted 20 min. Groups did not differ in response rates during the 10-rain epoch prior to the administration of IBMX (data not shown). * P
