Etonitazene as a reinforcer for rats: Increased etonitazene-reinforced behavior due to food deprivation

Etonitazene-reinforced performance of rats was increased by food deprivation and decreased by food satiation. These changes were not due to general increases in either activity or liquid intake.These data were reported to the Committee on Problems of Drug Dependence (1978)     

The failure of cocaine to serve as an orally self-administered reinforcer in Lewis rats

The aim of this study was to develop a procedure to establish orally delivered cocaine as a reinforcer for rats. Several procedures that have proven reliable in other studies were instituted. (1) Food-induced drinking: food was presented to engender high rates of drinking, then cocaine solutions replaced water, and finally the food was removed. Peak drug intakes ranged from 9.4 to 13.8mg/kg. (2) Ethanol-fade: ethanol was established as a reinforcer, cocaine was gradually added to the ethanol, and the ethanol was gradually removed. Peak cocaine intakes ranged from 3.6 to 5.2mg/kg. (3) Modified food-induced drinking: food was presented, but was followed by a timeout period, allowing digestion to progress prior to cocaine drinking. Peak drug intakes ranged from 12.9 to 18.6mg/kg. However, cocaine was not established as a reinforcer with any of these methods. High cocaine doses may be necessary, but are not sufficient, to establish oral cocaine self-administration in rats.     

Etonitazene as a reinforcer: Oral intake of etonitazene by rhesus monkeys

Drinking of etonitazene HCl was studied in three rhesus monkeys during daily 3-h sessions. As the drug concentration was increased, the number of liquid deliveries decreased, and etonitazene intake (g/kg body weight) increased. As fixed-ratio (FR) requirements were increased, rate of responding increased, and liquid deliveries slightly decreased. When water was substituted for the drug, there was a large increase in responding for several sessions, followed by a slow decline to low rates. When etonitazene was reintroduced, responding abruptly increased to previous drug levels. These data suggest that etonitazene can serve as a positive reinforcer when taken orally by rhesus monkeys.     

Spontaneous withdrawal in opiate-dependent Fischer 344, Lewis and Sprague-Dawley rats

The Lewis (LEW) and Fischer 344 (F344) inbred rat strains react differentially to acute morphine administration for a variety of behavioral and neurochemical measures. Investigations into effects of chronic morphine are less common, and investigations assessing dependence have been limited to those utilizing antagonist-precipitated withdrawal. The present experiment extended these assessments by examining spontaneous withdrawal in the LEW and F344 strains. In this preparation, males of the LEW, F344 and the outbred Sprague-Dawley (SD) strain were made dependent on morphine. Following this, opiate administration was terminated and animals were examined for spontaneous withdrawal by the acquisition of a withdrawal-associated taste aversion, changes in body weight loss and the display of several behaviors characteristic of opiate withdrawal. Although all morphine-treated subjects decreased body weight and avoided consumption of the withdrawal-associated solution, indicating successful induction of dependence, no difference between the strains emerged in these indices of withdrawal severity. The only strain difference to appear in the behavioral indicators of withdrawal was with diarrhea (LEW > F344). That the strains differ in acute reactivity to opioids, but not in the overall severity of withdrawal, was discussed in relation to the need to examine the relationship between neurochemical and behavioral data in a variety of neural systems and behavioral endpoints.     

Orally delivered cocaine as a reinforcer for rhesus monkeys

Orally delivered cocaine was established as a reinforcer for six rhesus monkeys. Cocaine and its vehicle, water, were available from separate spouts under independent concurrent fixed-ratio schedules. The positions of cocaine and water were reversed between spouts from session to session. Cocaine consistently maintained higher response rates than water. Cocaine concentration was systematically varied for three of the six monkeys tested, and cocaine intake (mg of drug/kg of body wt.) increased with increases in drug concentration.     

Effects of clomipramine on self-control choice in Lewis and Fischer 344 rats

Rates of delay discounting (impulsive choice) have been shown to vary among individuals, particularly people who abuse drugs relative to those who do not, but factors that may contribute to these differences have not been identified. To explore a role for possible genetic and neurochemical determinants, Lewis (n = 8) and Fischer 344 (n = 8) rats were allowed to choose between one food pellet delivered immediately and three food pellets delivered after increasing delays. The delays to the large reinforcer (0, 10, 20, 40, 60 s) were increased across five blocks of trials in daily experimental sessions. For both groups of rats, choice for the larger reinforcer decreased as the delay to presentation increased. However, the Lewis rats were more likely to choose the smaller, immediate reinforcer earlier in the session, i.e., at shorter large-reinforcer delays, than the Fisher 344 rats. This difference in choice was statistically significant. Repeated administration of 3.0 mg/kg, i.p. clomipramine (mean of last five sessions) did not significantly alter choice, relative to baseline, for either strain. The present findings suggest that differences in delay discounting/impulsive choice may involve genetic, e.g., neurochemical, differences.     

Lobeline does not serve as a reinforcer in rats

Abstract Rationale. Previous results demonstrated that pretreatment with lobeline attenuates d-methamphetamine self-administration in rats. Objective. The present experiments determined if lobeline serves as a reinforcer, if it decreases d-methamphetamine-induced reinstatement of d-methamphetamine self-administration, and if it activates the mesolimbic and nigrostriatal dopamine (DA) pathways in Sprague-Dawley male rats. Methods. The ability of intravenous (IV) lobeline (0.015–0.15 mg/kg per infusion) to engender responding and the ability of lobeline (0.015 and 0.05 mg/kg per infusion) to substitute for d-methamphetamine was determined using the self-administration paradigm. Experiments were also performed to determine if lobeline (1.0 and 3.0 mg/kg) reinstates responding for d-methamphetamine or alters the ability of d-methamphetamine (1.0 mg/kg per infusion) to reinstate responding following extinction. The effect of lobeline (3.0 mg/kg) or d-methamphetamine (1.0 and 3.0 mg/kg) on DA and dihydroxyphenylacetic acid (DOPAC) levels in the nucleus accumbens and striatum were also determined. Results. Lobeline was not self-administered and did not substitute for d-methamphetamine. Also, lobeline did not reinstate responding for d-methamphetamine following extinction nor did it alter d-methamphetamine-induced reinstatement. Furthermore, lobeline did not alter DA or DOPAC levels in the either the nucleus accumbens or striatum. Conclusions. Taken together, the present results indicate that lobeline decreases d-methamphetamine self-administration by decreasing reward, not by acting as a substitute reinforcer. Electronic Publication     

Captopril as an antioxidant in lead-exposed Fischer 344 rats

Recent studies suggest that lead induces oxidative stress in various tissues. Captopril ([2S]-1-[3-mercapto-2-methylpropionyl]-L-proline), an angiotensin-converting enzyme inhibitor, is a well-known antihypertensive agent and is also believed to function as an antioxidant. In the present study the antioxidant effect of captopril on lead-induced oxidative stress was studied in Fischer 344 rats. Their liver, brain and kidneys were assayed for glutathione (GSH), glutathione disulfide (GSSG), malondialdehyde concentrations, and catalase activities. The results from animals treated with captopril were compared to results of control and lead-exposed non-treated groups. The captopril-treated samples showed higher GSH:GSSG ratios in the liver, brain and kidneys, as well as slightly decreased malondialdehyde concentrations. The catalase activity was not significantly affected.     

Different patterns of pharmacological reinstatement of cocaine-seeking behavior between Fischer 344 and Lewis rats

Rationale Fischer 344 (F344) and Lewis (LEW) rats differ in cocaine self-administration behaviors. Whether or not these inbred strains of rats differ in pharmacological reinstatement of cocaine-seeking behavior is unknown.Objectives The purpose of the present study was to determine if inbred strains of rats demonstrate differences in alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and cocaine-induced reinstatement of previously extinguished cocaine-seeking behavior.Methods F344 and LEW rats received indwelling jugular catheters, bilateral guide cannula aimed at the nucleus accumbens core, and were trained to lever press for 0.5 mg/kg intravenous cocaine during 2-h self-administration sessions. Following 14 sessions, rats underwent extinction sessions, where previously reinforced lever pressing resulted in no programmed consequences. Just prior to beginning extinction session 7, rats received an intracranial infusion of saline. Lever pressing was not reinforced. During subsequent extinction sessions, rats received AMPA injections (0.2, 0.4, or 0.6 nM). Dosing order was determined by a within-subject Latin square design. At least two extinction sessions separated each AMPA session. Rats then underwent cocaine-induced reinstatement test sessions (lever pressing was not reinforced). Rats received passive intravenous cocaine (0.0, 0.5, 1.0, or 2.0 mg/kg) after being placed in the experimental chamber. At least two extinction sessions separated each cocaine-prime session, and subjects were tested at each dose according to a within-subjects Latin square design.Results LEW rats demonstrated blunted maximal responding to AMPA-induced reinstatement and heightened sensitivity to cocaine-induced reinstatement compared with F344 rats.Conclusions This study demonstrates that inbred strains differ in pharmacological reinstatement of cocaine-seeking behavior.     

Orally delivered methadone as a reinforcer in rhesus monkeys

Methadone usually is taken orally for drug abuse treatment in humans but oral methadone self-administration by laboratory animals has not been investigated extensively. The present study examines acquisition and maintenance of oral methadone maintained responding in four adult male rhesus monkeys. Drug solution was available from one liquid delivery system and water from a second system during daily 3-h sessions. Locations of liquids were reversed each session, and liquid (0.65 ml per delivery) was delivered according to a fixed-ratio reinforcement schedule. Initially a test for the reinforcing effects of 0.00625–0.4 mg/ml methadone solutions was carried out but a consistent preference for drug over water was not seen. To establish methadone as a reinforcer, a fading procedure was used in which responding was first maintained by solutions of methadone (0.00625–0.4 mg/ml) combined with ethanol (0.0325–2.0% w/v). Subsequently, the concentration of the ethanol in the combination was gradually reduced to zero. Methadone-maintained responding (0.4 mg/ml) persisted when ethanol was no longer present. To confirm that the drug was serving as a reinforcer, the dose was varied: (a) by changing the volume delivered while the concentration was held constant and (b) by changing the concentration of the methadone while the volume per delivery was held constant. Over a wide range of doses, deliveries of methadone solution usually exceeded deliveries of concurrently available water. Orderly relationships were observed among methadone dose, response rate, and drug intake. The study of oral self-administration of opioid drugs by nonhuman primates may be a useful strategy for the development and evaluation of new drug substitution or replacement therapies.     

Differences in extinction responding and reinstatement of methamphetamine-seeking behavior between Fischer 344 and Lewis rats

Fischer 344 (F344) and Lewis (LEW) rats differ in a number of self-administration behaviors. Whether or not these strains differ in methamphetamine-primed reinstatement of extinguished responding is unknown. F344 and LEW rats were trained to self-administer intravenous (i.v.) methamphetamine (0.06 mg/kg) during daily 2-h limited access sessions for 14 days. Following methamphetamine self-administration, subjects underwent a minimum of 6 extinction sessions where responding on the previously active lever resulted in no programmed consequences. Following extinction sessions, we evaluated strain and dose dependency of methamphetamine-primed (0.06, 0.12, or 0.24 mg/kg/i.v.) reinstatement of responding. All subjects received each dose once. Dosing order was determined by utilizing a within-subjects Latin square design. We found partial strain differences in daily methamphetamine self-administration. In addition, F344 rats responded significantly more during the first extinction session compared LEW rats. Last, the LEW rats demonstrated a heightened propensity to reinstate responding following methamphetamine priming injections compared to F344 rats. Our results suggest that genetic background influences differences in methamphetamine-seeking behaviors in rats.     

Metabolism and disposition of moxonidine in Fischer 344 rats.

The metabolism and disposition of moxonidine (4-chloro-5-(imidazolidin-2-ylidenimino)-6-methoxy-2-methylp yrimidine ), a potent central-acting antihypertensive agent, were investigated in F344 rats. After an i.v. or oral administration of 0.3 mg/kg of [(14)C]moxonidine, the maximum plasma concentrations of moxonidine were determined to be 146.0 and 4.0 ng/ml, respectively, and the elimination half-lives were 0.9 and 1.1 h, respectively. The oral bioavailability of moxonidine was determined to be 5.1%. The metabolic and elimination profiles of moxonidine were determined after an oral administration of 5 mg/kg of [(14)C]moxonidine. More than fifteen phase I and phase II metabolites of moxonidine were identified in the different biological matrices (urine, plasma, and bile). Oxidative metabolism of moxonidine leads to the formation of hydroxymethyl moxonidine and a carboxylic acid metabolite as the major metabolites. Several GSH conjugates, cysteinylglycine conjugates, cysteine conjugates, and a glucuronide conjugate were also identified in rat bile samples. The radiocarbon was eliminated primarily by urinary excretion in rats, with 59.5% of total radioactivity recovered in the urine and 38.4% recovered in the feces within 120 h. In bile duct-cannulated rats, about 39.7% of the radiolabeled dose was excreted in the urine, 32.6% excreted in the bile, and approximately 2% remained in the feces. The results from a quantitative whole body autoradiography study indicate that radiocarbon associated with [(14)C]moxonidine and/or its metabolites was widely distributed to tissues, with the highest levels of radioactivity observed in the kidney and liver. In summary, moxonidine is well absorbed, extensively metabolized, widely distributed into tissues, and rapidly eliminated in rats after oral administration.     

Orally delivered methadone as a reinforcer: Effects of the opioid antagonist naloxone.

The effects of the opioid antagonist naloxone were studied with three monkeys under a mutually exclusive fixed-interval 15 s (FI 15 s) schedule of reinforcement. Under this schedule, at the end of each interval, the monkey could obtain one liquid delivery from either the spout that delivered methadone (0.8 mg/ml) or the spout that delivered vehicle (deionized water). Naloxone doses from 0.0125 to 0.2 mg/kg (IM daily 10 min prior to the session) were studied in an ascending then descending order. In the ascending series, low naloxone doses produced increases of methadone deliveries in the first hour of the session for three monkeys and increases over the entire 3-h session for two of the three monkeys. At higher doses naloxone decreased methadone deliveries in all three monkeys. Naloxone's effects were usually greater during the descending dose series than during the ascending series. These findings suggest that a history of naloxone injections is one determinant of response to the drug. Vehicle maintained responding was generally low and not changed by naloxone in a systematic way. The time course of methadone deliveries showed that naloxone's effects were greatest in the first hour of the session and were a direct function of dose. These experiments demonstrate that oral methadone reinforced behavior is sensitive to naloxone pretreatment and that the effects of naloxone are a direct function of dose.     

Rapid establishment of ethanol as a reinforcer for rats

Daily 6-h sessions were run during which each lever press by rats produced brief access to water, or to 8?/0 (W/V) ethanol on experimental days. Food pellets were presented noncontingently on a 1 min fixed-time schedule during the last, 4 h of each session. A stable baseline of water responding developed, characterized by little or no responding during the first 2 h followed by high rates and schedule-induced polydipsic drinking during the last 4 h. Following the development of a stable water baseline, 8% (W/V) ethanol was substituted for water on alternate days. After one previous session with ethanol, rats' responding for ethanol during the first 2 h of a session substantially exceed water baseline rates, indicating that ethanol had been established as a reinforcer. Subsequently, when food pellet presentations were discontinued, and the ethanol concentration was increased from 8 to 16 to 32% (W/V), ethanol intake persisted at values exceeding water control levels; these results confirm that ethanol was functioning as a reinforcer.     

Assessment of the aversive effects of peripheral mu opioid receptor agonism in Fischer 344 and Lewis rats

The Fischer 344 (F344) and Lewis (LEW) inbred rat strains differ on a host of biochemical, neuroanatomical, immunological and behavioral endpoints. One behavioral difference of interest is their differential reactivity to the aversive effects of morphine as indexed by the conditioned taste aversion preparation (aversions acquired by F344 rats are significantly greater than those acquired by the LEW strain). This differential effect appears to be specific to opioids that work primarily on the mu opioid receptor. Given that morphine works systemically, it is unknown whether these differential effects in F344 and LEW animals are centrally or peripherally mediated. To address this issue, the present study investigated the ability of the peripherally acting mu preferring opioid agonist loperamide to induce differential taste aversions in F344 and LEW animals. Both F344 and LEW animals acquired dose-dependent taste aversions to the loperamide-associated solution with no difference between them. Additionally, control animals initially injected with vehicle during aversion training with loperamide and subsequently conditioned with morphine displayed the typical aversive profile to morphine (F344 > LEW). Although the basis for the present data is unknown, their relation to morphine-induced taste aversions and the role of the interaction of stimulus effects of drugs that produce differential abuse liability were discussed.     

Effects of acute and chronic administration of diazepam on delay discounting in Lewis and Fischer 344 rats

Impulsive choice is often examined using a delay-discounting procedure, where there is a choice between two reinforcers of different magnitudes presented at varying delays. Individual discounting rates can be influenced by many factors including strain differences and drug effects. Lewis (LEW) and Fischer 344 (F344) rats have behavioral and neurochemical differences relevant to delay discounting and were used to examine effects of acute and chronic administration of diazepam on impulsive choice. Consistent with the previous literature, larger-reinforcer choice decreased as a function of increasing delays for all rats, and steeper discounting functions were obtained for LEW relative to F344 rats. Acute and chronic administration of diazepam resulted in differential effects between rat strains and sometimes between subjects within the same rat strain. Overall, larger-reinforcer choice remained unchanged across multiple phases of the experiment for LEW rats. For F344 rats, larger-reinforcer choice increased following the acute administration of smaller doses of diazepam and decreased following the acute administration of the largest dose tested. Decreases in larger-reinforcer choice occurred for F344 rats during chronic and postchronic administration and persisted throughout a nondrug return-to-baseline phase. These results suggest potential directions for future investigation of environmental, genetic, and neurochemical variables involved in delay discounting.     

Subchronic inhalation study of 1-hexene in Fischer 344 rats.

The objective of this study was to evaluate the toxicity of 1-hexene following repeated inhalation exposures in male and female Fischer 344 rats. Groups of 40 male and 40 female rats were exposed for 6 hours per day, 5 days per week, over a 13-week period. Treatment groups consisted of air-exposed control (0 ppm) and three test groups of 300, 1000, and 3000 ppm 1-hexene. During the treatment period, the rats were observed daily for clinical signs of toxicity; body weights and neuromuscular coordination [females only] were measured at 7-day intervals. After 7 weeks of exposure and at the end of the treatment period, the rats were subject to macroscopic and microscopic pathology, clinical chemistry, hematology, urinalysis, and sperm counts. No mortalities were observed during the course of the study. No clinical signs of toxicity attributable to 1-hexene exposure were observed. Female rats exposed to 3000 ppm had significantly lower body weights compared to control rats from exposure day 5 persisting throughout the treatment period. Male rats exposed to 3000 ppm had slightly but not statistically significant lower body weights in comparison to controls. Male rats exhibited slightly increased absolute and relative testicular weights, and female rats had slightly decreased absolute [but not relative] liver and kidney weights, at 3000 ppm. There were no gross or microscopic morphological findings attributed to treatment. Exposure to 1-hexene did not affect neuromuscular coordination in females as determined using the Rotarod, nor sperm counts in male rats. Several statistically significant effects in hematology, clinical chemistry, and urinalysis evaluations were observed, but were either of small magnitude or did not correlate with histopathological findings, and thus did not appear to be of biological significance. In summary, the no-adverse-effect-level for this study was determined to be 1000 ppm, based on decreased weight gain in female rats, and on slight organ weight changes in both sexes at 3000 ppm.     

In vivo kinetics of trichloroacetate in male Fischer 344 rats.

Trichloroacetate (TCA) is a toxicologically important metabolite of the industrial solvents trichloroethylene and tetrachloroethylene, and a by-product of the chlorination of drinking water. Tissue disposition and elimination of 14C-TCA were investigated in male Fischer 344 rats injected iv with 6.1, 61, or 306 micromol TCA/kg body weight. Blood and tissues were collected at various time points up to 24 h. No metabolites were observed in plasma, urine, or tissue extracts. Overall TCA kinetics in tissues were similar at all doses. Based on similar terminal elimination rate constants, tissues could be divided into three classes: plasma, RBC, muscle, and fat; kidney and skin; and liver, small intestine, and large intestine. Nonextractable radiolabel, assumed to be biologically incorporated metabolites in both liver and plasma, increased with time, peaking at 6-9 h postinjection. The fraction of the initial dose excreted in the urine at 24 h increased from 67% to 84% as the dose increased, whereas fecal excretion decreased from 7% to 4%. The cumulative elimination of TCA as CO2 at 24 h decreased from 12% to 8% of the total dose. Two important kinetic processes were identified: a) hepatic intracellular concentrations of TCA were significantly greater than free plasma concentrations, indicating concentrative transport at the hepatic sinusoidal plasma membrane, and b) TCA appears to be reabsorbed from urine postfiltration at the glomerulus, either in the renal tubules or in the bladder. These processes have an impact on the effective tissue dosimetry in liver and kidney and may play an important role in TCA toxicity.