Quadazocine Decreases Responding Reinforced by Ethanol, Sucrose, and Phencyclidine Fluid Deliveries in Rhesus Monkeys: Comparison to Naltrexone’S Effects

Psychopharmacology (1999) 144:316Ð322 © Springer-Verlag 1999

ORIGINAL INVESTIGATION

Keith L. Williams á Eric D. Pakarinen James H. Woods Quadazocine decreases responding reinforced by ethanol, sucrose, and phencyclidine fluid deliveries in rhesus monkeys: comparison to naltrexone’s effects

Received: 24 June 1998 / Accepted: 18 February 1999

Abstract Rationale: The endogenous opioid system experiments, and thus we cannot rule out rate-dependent may mediate the reinforcing effects of ethanol as well as effects of the antagonists. sweet-tasting solutions. For example, opioid antagonists, such as naltrexone, reduce ethanol- and sucrose-rein- Key words Opioid antagonists á Alcohol reinforcement á forced responding in rhesus monkeys. If these effects are Self-administration á Primates due to blockade of the µ-receptor, then an opioid antagonist such as quadazocine with a receptor selectivity pro- file similar to that of naltrexone should reduce respond- Introduction ing at doses correlated with its µ-selectivity. Objectives: To determine whether quadazocine would reduce re- The endogenous opioid system modulates alcohol drink- sponding for ethanol and sucrose at µ-selective doses, ing. In preclinical and clinical studies, opioid antagonists and whether quadazocine and naltrexone would reduce reduce alcohol drinking. For instance, we have previous- responding for a bitter-tasting drug solution such as ly shown that naltrexone (NTX) pretreatment in rhesus phencyclidine. Methods: Rhesus monkeys were given monkeys reduced oral ethanol self-administration access to ethanol, sucrose, or phencyclidine concurrently (Williams et al. 1998). In other studies using many dif- with water. Prior to the drinking sessions, quadazocine ferent animal species, NTX and other opioid antagonists (0.032Ð3.2 mg/kg) or saline was injected intramuscular- reduced oral ethanol self-administration (Levine and ly. During the phencyclidine experiment, naltrexone (0.1 Billington 1989). When ethanol was available concur- and 0.32 mg/kg) was also tested. Results: The highest rently with water, the opioid antagonist naloxone quadazocine doses (1 and 3.2 mg/kg) reduced ethanol selectively reduced ethanol drinking (De Witte 1984; and sucrose fluid deliveries without affecting the concur- Froehlich et al. 1990). Because opioid antagonists effec- rently available water. Quadazocine reduced the fluid de- tively reduced ethanol drinking in animals, clinical trials liveries of both phencyclidine and water when concur- tested the longer lasting opioid antagonists NTX and nal- rently available. Naltrexone reduced only phencyclidine mefene, in alcohol-dependent patients. NTX and nal- fluid deliveries. Conclusions: The opioid antagonist ef- mefene increased abstinence, decreased relapse rates, fect on oral-reinforced responding is not selective for and reduced alcohol craving (O’Malley et al.1992; ethanol or sweet-tasting solutions; responding for Volpicelli et al. 1992; Mason et al. 1994). phencyclidine was reduced as well. Quadazocine and The endogenous opioid system may mediate ingestive NTX may reduce responding by blocking the µ-receptor behaviors in general rather than specifically ethanol-re- because the relative potency of these antagonists to related behaviors (Levine et al. 1985). Although effective duce oral self-administration was similar to their relative in the clinical trials with alcoholics, opioid antagonists potency to produce withdrawal in morphine-dependent reduce the consumption of many solutions and foods. monkeys. However, water responding was low in these For example, naloxone pretreatment in rats reduced feeding as well as water and sucrose drinking (Holtzman K.L. Williams á J.H. Woods 1974; Maickel et al. 1977; Stapleton et al. 1979). Thus, Department of Psychology, University of Michigan, opioid antagonists may mediate reductions in alcohol Ann Arbor, MI 48109-0632, USA drinking through a mechanism that modulates a wide K.L. Williams (✉) á E.D. Pakarinen á J.H. Woods class of stimuli that support food or fluid consumption. Department of Pharmacology, 1301 MSRB III, The ability of opioid antagonists to reduce consump- University of Michigan, Ann Arbor, MI 48109-0632, USA e-mail: [email protected] tion depends upon activity at opioid receptors. The an- Fax: +1-734-764-7118 tagonist effects are stereoselective; in some experiments, 317 the inactive naloxone stereoisomer had no effect on Materials and methods consumption (Brown and Holtzman 1980; Kirkham and Cooper 1988). Furthermore, the rank order of the Experiment 1: ethanol self-administration capacity of different antagonists to suppress water drinking corresponded with the rank order of antagonist po- Subjects tency to precipitate morphine withdrawal (Brown and Subjects were adult rhesus monkeys (Macaca mulatta; weighing Holtzman 1980). Therefore, the antagonist’s interaction 5.8Ð10.7 kg) maintained at approximately 80% of their free-feed- with the µ-opioid receptors may mediate the reduction in ing weights. For experiment 1, two of the five male subjects previ- consumption. ously participated in a study where they received NTX prior to ethanol access. In all of the following experiments the “Guide for The opioid antagonists NTX and quadazocine have the Care and Use of Laboratory Animals” (NIH publication, been well characterized, in vivo and in vitro. For exam- vol. 25, number 28, revised 1996) was followed. ple, NTX blocked the analgesic effect of the µ-agonist alfentanil with greater potency than it blocked the anal- Apparatus gesic effect of the κ-agonists U69,593 and bremazocine (Ko et al. 1998). In monkeys for whom responding was The animal housing room was on a 12-h light/dark cycle. Lights were turned on at 7:00 a.m. and turned off at 7:00 p.m. The mon- reinforced by food delivery, the opioid antagonist quad- keys were housed in individual cages measuring 64 cm×72 cm× azocine blocked the rate-decreasing effects of the µ-ago- 85 cm high. The cages were attached as a unit that was two cages nists alfentanil and fentanyl with greater potency than it high and two cages wide. A fluid-delivery panel, similar to that blocked the rate-decreasing effects of the κ-agonists used in other studies (Meisch et al. 1975; Williams et al. 1998), U69,593 and EKC. Thus, in vivo, NTX and quadazocine was attached to one wall of each cage during daily sessions. Holes κ were cut in the cage wall so that two brass spouts on the fluid-de- more potently block µ-receptors than -receptors. An in livery panel protruded into the cage 50 cm from the floor. A stim- vitro experiment using monkey brain showed that both ulus light that could be illuminated red or green was located 3 cm NTX and quadazocine had greatest affinity for the µ-re- above each spout. The drinking solutions were contained in ceptor, then κ- and δ-receptors (Emmerson et al. 1994). 1000 ml plastic bottles attached to the back of the panel. Plastic tubing connected each bottle to the spout valve. The fluid contain- Even though the receptor affinity order is similar for ers were elevated so that the liquid was gravity-fed to the spout NTX and quadazocine, higher quadazocine doses are re- valve and delivery was controlled by a solenoid switch. Contact quired to produce the same effects as NTX. For instance, with either spout closed an electrical circuit (drinkometer) and a a 10-fold higher quadazocine dose was needed to precip- response was recorded. The stimulus light above the spout flashed when contact was made with the spout. When the reinforcement itate withdrawal and generalize to an NTX stimulus in schedule was completed, the solenoid was activated and 0.5 ml morphine-dependent monkeys (Valentino et al. 1983; fluid was delivered. Solutions were measured after the session us- France et al. 1990). Because morphine produces its ef- ing graduated cylinders to confirm delivery amounts and check for fects through the µ-receptor, we can predict that if NTX gross discrepancies between the deliveries calculated by the com- puters and the volume gone from the bottles. The experiments reduces oral-reinforced responding through µ-receptor were controlled and the data recorded using IBM PCjr microcom- antagonism, then quadazocine should produce similar ef- puters located in a room adjacent to the housing room. The same fects at 10-fold higher doses. apparatus was used for the sucrose and PCP experiments. The purpose of this study was to determine if quadazocine reduced ethanol- and sucrose-reinforced re- Procedure sponding and to determine if both quadazocine and NTX reduced responding reinforced by fluid deliveries of the Each experimental session lasted 3 h per day, during which the animal could respond and obtain either ethanol or concurrently avail- NMDA antagonist phencyclidine (PCP). Quadazocine able water. Drug (ethanol) was available under the red stimulus was tested on ethanol-reinforced responding because light, and water was available under the green stimulus light. The some research suggests that endogenous opioid activity side positions of the solutions were alternated daily. The monkeys mediates ethanol’s reinforcing effects and that opioid an- were reinforced with 0.5 ml fluid for every four mouth contacts on the spout. Thus, the reinforcement schedule was a fixed ratio 4 or tagonists reduce ethanol consumption by blocking etha- FR4 schedule. The reinforcement schedule on each of the two nol-induced opioid activity (Ulm et al. 1995). Quadazo- spouts operated concurrently and independently such that the re- cine was tested on sucrose-reinforced responding be- sponses on one spout did not alter the number of responses re- cause sucrose is a non-drug reinforcer that maintains a quired on the opposite spout. The experimental sessions were con- large amount of responding. Quadazocine and NTX were ducted at different times of the day depending upon the cages in which the monkeys were housed. For example, monkeys in the tested on responding reinforced by PCP deliveries be- lower cages were in the first session which started at 7:00 a.m., cause PCP has quinine-like taste properties (Aspen and the monkeys in the upper cages were in the second session 1997) and, while orally reinforcing (Carroll 1982), it which started at 11:30 a.m. Just prior to the first session, the water lacks calories. We compared the quadazocine and NTX hoses were disconnected from the cages. After the second session was finished the water hoses were reconnected and the monkeys doses required to reduce oral-reinforced responding in were fed their daily ration of chow all at once. Sessions were con- order to ascertain whether NTX reduces oral-reinforced ducted 7 days a week. These same procedures were used for the responding through an opioid mechanism. In addition, a sucrose and PCP experiments. choice procedure was used because it eliminates the need The ethanol concentrations were 1% for two monkeys and 2% for the other three monkeys. These concentrations were previously for additional days or groups to control for solution ac- shown to maintain the greatest amount of behavior (Williams et al. cess and it provides a means to determine the reinforcing 1998). Due to the similarity, these data were pooled to calculate effect of the test solution. the average fluid deliveries and ethanol intake in g/kg. 318 Saline or quadazocine (0.32, 1, or 3.2 mg/kg) injections were Data analysis administered 30 min prior to the drinking sessions. This same pretreatment time was used in experiments 2 and 3. Each quadazo- Each monkey’s average fluid deliveries, intake in g/kg, and fluid cine dose was tested twice in each monkey in an ascending dose deliveries expressed as a percentage of non-injection baseline order. Some studies show enhanced sensitivity to the effects of were used to calculate the mean and SE of the mean for the group opioid antagonists after repeated intermittent antagonist treatments of monkeys. The data are presented as the mean and SE of the (Warren and Morse 1985). Therefore, each quadazocine injection mean for the group data. was given at least a week apart so that enhanced sensitivity did not The fluid delivery data for each antagonist pretreatment/solu- develop in these monkeys. In addition 2Ð4 saline pretreatment tion condition (i.e. quadazocine/ethanol, quadazocine/sucrose, days preceded the first antagonist injection day and were inter- etc.) were analyzed separately using two-way repeated measures spersed between other antagonist injection days. Non-injection analysis of variance (RM ANOVA). A one-way RM ANOVA was days prior to the first antagonist injection day and between other used to analyze the drug intake data (g/kg for ethanol or mg/kg for antagonist or saline injection days were considered “baseline” PCP). For the ethanol, sucrose, or PCP data expressed as a per- days. These days served as a control for the saline injection days. centage of non-injection baseline control, a one-way RM ANOVA Due to solubility problems, the largest quadazocine dose was giv- was applied for each antagonist pretreatment/solution condition. en in three 1-ml syringes. This injection regimen was applied to The water data were not included in this analysis because the wa- the sucrose and PCP studies as well. ter baselines were low and small absolute changes in water fluid deliveries appeared large when expressed as a percent of non-injection baseline. The NTX/ethanol and NTX/sucrose data were Experiment 2: sucrose self-administration taken from a previous study (Williams et al. 1998) and were presented here for comparison to quadazocine. When either the main Subjects effect of antagonist or the interaction effect of solution and antagonist was significant, post-hoc Dunnett’s tests were used to com- Subjects were three male and two female monkeys. Two of the pare the antagonist effects to saline control. five subjects also participated in experiment 1 and three subjects received repeated NTX prior to sucrose access in an earlier experiment. Drugs

Solutions were prepared by mixing the appropriate amounts of Procedure 95% w/v ethanol, sucrose (cane sugar), or PCP with tap water. Both NTX, provided by NIDA Research Technology Branch, and Sucrose concentration 100 g/l was previously determined to main- quadazocine, provided by Sterling-Winthrop, Collegeville, Pa., tain a high amount of responding. Quadazocine doses (0.032, 1, or USA, were dissolved in sterile water. 3.2 mg/kg) were tested twice in each monkey in an ascending dose order for three monkeys and descending dose order for two monkeys. Results

Experiment 3: PCP self-administration Experiment 1: ethanol self-administration Subjects All quadazocine doses decreased ethanol fluid deliveries Subjects were one female and four male monkeys. Two of the five when compared to the ethanol fluid deliveries after sa- subjects also participated in experiment 1 while three of the sub- line injection as shown in Fig. 1 [interaction effect, jects received repeated NTX prior to sucrose access in an earlier experiment. F(4,16)=10.76, P<0.001; Dunnett’s test q’=7.56, 7.27, and 6.66, P<0.05]. Water fluid deliveries decreased after Procedure

Previously, we observed that the monkeys received most of their fluid deliveries during the first 40 min of the session (Williams et al. 1998). Additionally, we determined that the fluid deliveries obtained during a 3-h session within subjects were similar to those obtained during a 2-h session. Therefore, 2-h sessions were used for the PCP experiments to save time. Three of the five subjects in the present experiment self-administered PCP without induction procedures. The other two monkeys required minimal induction procedures. For example, during the first five sessions of PCP access, the monkeys received only 0.125 mg/ml PCP. No water was available during the session and the monkeys were fed after the session. Thereafter, water was introduced as the alternative solution. A PCP preference developed immediately. PCP sessions were conducted for approximately 2 weeks to establish a stable baseline. The best compromise between high rates of responding and drug intake (mg/kg) was at 0.125 mg/ml PCP. This PCP concentration was self-administered by monkeys in other studies Fig. 1 The average number of fluid deliveries of ethanol (open (Carroll and Stotz 1984; Campbell et al. 1998). In each monkey, and solid bars) and concurrently available water (lined bars) after 1 mg/kg quadazocine was tested 4 times and 3.2 mg/kg quadazo- different pretreatments: non-injection baseline, saline, quadazocine was tested twice. All four tests with 1 mg/kg were completed cine doses: 0.32, 1, and 3.2 mg/kg. The bars represent the average prior to testing with 3.2 mg/kg. Both NTX doses, 0.1 mg/kg and with the SE (n=5 monkeys; two consuming 1% ethanol and three 0.32 mg/kg, were tested 4 times in each monkey. All four tests consuming 2% ethanol). *Indicates a significant difference with 0.1 mg/kg were completed prior to testing with 0.32 mg/kg. (P<0.05) from ethanol fluid deliveries after saline 319

Fig. 2 The average number of fluid deliveries of 100 g/l sucrose Fig. 3 The average number of fluid deliveries of 0.125 mg/ml (open and solid bars) and concurrently available water (lined phencyclidine (open and solid bars) and concurrently available bars) after different pretreatments: non-injection baseline, saline, water (lined bars) after different pretreatments: non-injection quadazocine doses: 0.032, 1, and 3.2 mg/kg. The bars represent baseline, saline, quadazocine 1 mg/kg, and quadazocine 3.2 the average with the SE (n=5 monkeys). *Indicates a significant mg/kg. The bars represent the average with the SE (n=5 mon- difference (P<0.05) from sucrose deliveries after saline keys). *Indicates total fluid deliveries (PCP and water) significantly different (P<0.05) from total fluid deliveries after saline quadazocine, but this effect was not significant. Between as ataxia and altered aggression in some monkeys. The monkeys, the average ethanol fluid deliveries varied concurrently available water maintained more fluid de- from 394 to 1487. Over the 3-h session, the average eth- liveries than during the ethanol or sucrose experiments. anol intake during baseline was 0.71±0.21 g/kg and the When quadazocine was given prior to PCP access, average ethanol intake after saline was 0.70±0.18 g/kg. shown in Fig. 3, the interaction effect of quadazocine We did not observe behavioral signs of intoxication at and solution was not significant (P=0.056). Therefore, these intake levels. All quadazocine doses reduced etha- post-hoc comparisons were conducted for the main effect nol intake to approximately 0.4 g/kg which was signifi- of quadazocine, which was significant [F(3,12)=4.82, cantly different from the intake after saline [F(4,16)=15, P<0.05]. The results indicated that the total fluid deliver- P<0.001; Dunnett’s test q’=4.85, 5.05, and 4.75, ies of both PCP and water were reduced compared to the P<0.05]. total fluid deliveries after saline injection [Dunnett’s test q’= 2.75 and 3.38, P<0.05]. Thus, neither PCP nor water was selectively reduced. Although 1 mg/kg quadazocine Experiment 2: sucrose self-administration decreased PCP intake to 3.84±1.16 mg/kg and 3.2 mg/kg quadazocine decreased PCP intake to 2.78±1.50 mg/kg, Quadazocine, 1 and 3.2 mg/kg, reduced sucrose fluid these intakes were not significantly different from the deliveries when compared to the sucrose fluid deliveries PCP intake after saline (P=0.11). after saline injection as shown in Fig. 2 [interaction ef- Both NTX doses, 0.1 and 0.32 mg/kg, reduced PCP fect, F(4,16)=3.92, P<0.05; Dunnett’s test q’=4.45 and fluid deliveries [interaction effect, F(3,12)=4.89, 3.66, P<0.05]. The reduction in sucrose fluid deliveries P<0.05; Dunnett’s test q’=4.48 and 5.05, P<0.05] with- after 1 mg/kg quadazocine was similar to that after out affecting water fluid deliveries, as shown in Fig. 4. 3.2 mg/kg quadazocine. The water fluid deliveries were The monkeys consumed 5.62±0.9 mg/kg PCP at base- almost zero under all conditions. Although the average line, 5.37±1.31 mg/kg after saline pretreatment, data do not suggest a dose-related effect, quadazocine 2.71±0.62 mg/kg after 0.1 mg/kg NTX, and 2.51± dose-dependently reduced sucrose fluid deliveries in 0.62 mg/kg after 0.32 mg/kg NTX. Both NTX doses sig- three of the five monkeys. After 0.032 mg/kg quadazo- nificantly reduced PCP intake when compared to PCP cine, which was 10 times less than the lowest dose test- intake after saline injection [F(3,12)=11.8, P<0.0001; ed on ethanol drinking monkeys, sucrose fluid deliveries Dunnett’s test q’=3.86 and 4.16, P<0.05]. remained similar to those during baseline and after sa- The effects of quadazocine and NTX on the fluid de- line. liveries of ethanol, sucrose, and PCP are expressed as percentage of non-injection baseline control in Fig. 5. Because some subjects were used in multiple antago- Experiment 3: PCP self-administration nist/solution conditions and others were not, we could only make statistical comparisons within each antagonist When PCP was available concurrently with water, the pretreatment/solution condition. Quadazocine, 1 and monkeys consumed 4.54±1.2 mg/kg PCP at baseline and 3.2 mg/kg, reduced ethanol [F(3,12)=9.52, P<0.01; Dun- 5.22±1.41 mg/kg after saline pretreatment. These intake nett’s test q’=4.32 and 4.62, P<0.05], sucrose levels produced behavioral effects of intoxication such [F(3,12)=6.23, P<0.01; Dunnett’s test q’=3.36 and 3.41, 320 Discussion

The results showed that quadazocine reduced ethanol and sucrose fluid deliveries without affecting the concurrently available water. Although quadazocine failed to selectively reduce PCP fluid deliveries, Fig. 5 indicates that the significant reduction of total fluid deliveries (PCP and water) was primarily due to reduced PCP fluid deliveries. In addition, NTX reduced PCP fluid deliveries without affecting water. When the antagonist effects were compared across all solutions as a percentage of non-injection baseline control, the effects of NTX and quadazocine were similar. Fig. 4 The average number of fluid deliveries of 0.125 mg/ml The endogenous opioid system may modulate etha- phencyclidine (open and solid bars) and concurrently available water (lined bars) after different pretreatments: non-injection nol-, sucrose-, and PCP-reinforced responding. Some re- baseline, saline, naltrexone 0.1 mg/kg, and naltrexone 0.32 mg/kg. searchers postulate that increased opioid activity modu- The bars represent the average with the SE (n=5 monkeys). *Indi- lates ethanol reward and that NTX may reduce ethanol cates a significant difference (P<0.05) from phencyclidine fluid drinking by blocking the reward associated with the eth- deliveries after saline anol-induced opioid activity (Volpicelli et al. 1992; Gianoulakis and de Waele 1994). If NTX blocks the en- P<0.05], and PCP [F(2,8)=6.48, P<0.05; Dunnett’s test dogenous opioids released by ethanol, then increasing q’=2.64 and 3.44, P<0.05] to a similar extent. Although ethanol dose should overcome the effects of the opioid 0.032 mg/kg quadazocine was ineffective when tested antagonist. However, our data do not fully support this against sucrose, 0.32 mg/kg quadazocine reduced etha- hypothesis. In a previous paper (Williams et al. 1998), nol to the same degree that 1 and 3.2 mg/kg quadazocine we showed that the NTX effect on oral and intravenous reduced ethanol. NTX, 0.32 mg/kg, reduced ethanol ethanol self-administration was not surmountable by in- [F(3,12)=6.47, P<0.01; Dunnett’s test q’=4.19, P<0.05], creasing the available ethanol concentration or dose. In sucrose [F(2,8)=32.7, P<0.001; Dunnett’s test q’=8.04, addition, the present study shows that the opioid antago- P<0.05], and PCP [F(2,8)=10.2, P<0.01; Dunnett’s test nist quadazocine reduced ethanol-, sucrose-, and PCP-re- q’=4.31, P<0.05]. NTX, 0.1 mg/kg, reduced only su- inforced responding. Furthermore, NTX doses that re- crose and PCP [Dunnett’s test q’=3.24 and 3.37, duced ethanol-reinforced responding also reduced su- P<0.05]. With all solutions, NTX appeared to produce crose- and PCP-reinforced responding. If the ethanol- dose-related effects. Overall, the quadazocine-induced opioid hypothesis is correct, then our data suggest that reductions were not as dose-related as the NTX-induced ethanol, sucrose, and PCP must increase opioid activity reductions. to the same degree. If the endogenous opioid activity af-

Fig. 5 The average number of fluid deliveries of ethanol (left panels), sucrose (middle panels), and phencyclidine (right panels), expressed as a percentage of non-injection baseline control after saline or quadazocine pretreatment (upper panels) and after saline or naltrexone pretreatment (lower panels). *Indicates a significant difference (P<0.05) from the saline for that particular antagonist/solution condition 321 ter drinking sucrose or PCP was less than that after The present experiments also have some other limita- drinking ethanol, we might expect much smaller quad- tions. The quadazocine dose-range was limited and small- azocine or NTX doses to reduce sucrose- and PCP-rein- er quadazocine doses should have been used. Because forced responding. smaller doses were not tested and the dose-effect curves Opioid receptors appear to mediate the antagonist-in- appear flat, we simply do not know if smaller quadazo- duced reduction in oral-reinforced responding. Although cine doses would decrease oral-reinforced responding. the opioid antagonist doses used were quite large, our Another potential limitation is that quadazocine and NTX data indicate that the antagonist interaction with opioid were tested at ethanol and sucrose concentrations where receptors mediates the reduction of oral-reinforced re- responding for the concurrently available water was low. sponding. Earlier studies showed that a 10-fold higher This problem raises the question of whether the antago- quadazocine dose was needed to precipitate withdrawal nist effects were rate-dependent. In preliminary experi- or generalize to a NTX stimulus in morphine-dependent ments (Williams and Woods 1998b), we examined the ef- monkeys (Valentino et al. 1983; France et al. 1990). fect of NTX at ethanol concentrations where ethanol Since morphine acts primarily through µ-receptors, we maintained more, equal to, or less fluid deliveries than postulated that if NTX reduces oral-reinforced respond- the concurrently available water. NTX reduced the fluid ing through the µ-receptor, then quadazocine would pro- deliveries of the fluid that maintained the greatest behav- duce similar effects at a 10-fold higher dose than NTX. ior whether it was ethanol or water. Although opioid an- In our study, fluid deliveries were reduced after 1Ð3.2 tagonists reduce responding reinforced by ethanol, su- mg/kg quadazocine and 0.1Ð0.32 mg/kg NTX. These re- crose, and PCP, it is difficult to determine how the antag- sults support our hypothesis. Because the potency of onists produce these effects. Some research suggests that quadazocine relative to NTX matches that previously opioid antagonists reduce palatability (Cooper and Turk- studied with morphine withdrawal, we conclude that opi- ish 1989; Giraudo et al. 1993). In the present study, etha- oid receptors, probably of the µ-subtype, mediate antago- nol and sucrose may be palatable while the PCP has taste nist-induced reductions in oral-reinforced responding. qualities similar to quinine (Aspen 1997). The fact that This conclusion differs from that in a previous study PCP fluid deliveries were reduced by antagonist pretreat- (Williams and Woods 1998a). In that study, we conclud- ment suggests that the palatability explanation fails to ex- ed that the µ-receptor did not mediate the antagonist-in- plain fully the antagonist effects. In addition, because duced reduction in oral-reinforced responding because PCP has no caloric content, the antagonist effects are an irreversible µ-antagonist that was highly effective in probably not mediated by a mechanism governing caloric other assays failed to affect ethanol-reinforced respond- intake. Also, the antagonist effects are unlikely to be re- ing. However, antagonist availability and solubility is- lated to the pharmacological properties of the solutions sues prevented us from testing higher doses. because responding for the non-drug sucrose solution was Enhanced sensitivity to opioid antagonists may be a reduced. Another alternative is that opioid antagonists confounding factor in our experiments. In previous ex- produce nausea or aversive effects that reduce consum- periments with squirrel monkeys responding for food, re- matory behaviors. Opioid antagonists have been shown to peated NTX administration enhanced the monkeys’ sen- produce conditioned aversions in rodents (Mucha and sitivity to the rate-decreasing effects of NTX (Warren Walker 1987; Parker and Rennie 1992) and at least one and Morse 1985). In addition, rats with enhanced NTX clinical trial with alcohol-dependent subjects reported sensitivity showed cross-sensitivity to other opioid an- nausea as a side-effect of NTX (O’Malley 1992). tagonists (Schindler et al. 1993). This enhanced sensitiv- Although this paper suggests that opioid receptors ity is observed in animals responding for food, but not in (perhaps µ-receptors) mediate the effects of opioid an- animals responding to avoid a shock (France and Morse tagonists on oral-reinforced behaviors, some points re- 1989; Warren and Morse 1989). Animals may develop main to be clarified. For example, much larger antago- enhanced sensitivity to NTX because the NTX-induced nist doses are required to reduce consumption than to interoceptive state may become conditionally associated block an exogenous opioid effect such as analgesia. At with the food or taste stimulus over repeated testing. We these larger doses, NTX and quadazocine block µ-, κ-, tried to avoid this sensitivity issue in our monkeys by and δ-opioid receptors (Negus et al. 1993; Ko et al. pretreating with antagonists only once per week and giv- 1998). The necessary use of large antagonist doses to re- ing many saline injections between antagonist pretreat- duce oral-reinforced responding may indicate that opioid ments. In spite of our precautions, enhanced sensitivity receptors are not the sole mediators of the antagonist ef- or conditioned association may account for the reduced fects on reinforced responding. In addition, it is not un- ethanol-reinforced responding after 0.32 mg/kg quadazo- derstood why the NTX effect on oral- and intravenous- cine. In the quadazocine/ethanol experiment, two of the reinforced responding is not surmountable by increasing five monkeys had previously been exposed to repeated the ethanol concentration or dose (Williams et al. 1998). intermittent injections of NTX prior to ethanol or su- Also, the role of enhanced sensitivity and conditioning in crose access. Because many of these animals received the antagonist effect on oral-reinforced responding re- repeated NTX injections prior to ever receiving quadazo- mains unexplained. It is not clear how these factors in- cine, enhanced sensitivity may have contributed to quad- teract or whether they have significance in clinically azocine’s effect on reinforced responding. treating alcoholism with opioid antagonists. 322

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