Preferences for morphine in rats: validation of an experimental model of dependence

Enhancing translation: A need to leverage complex preclinical models of addictive drugs to accelerate substance use treatment options

Preclinical models of addictive drugs have been developed for decades to model aspects of the clinical experience in substance use disorders (SUDs). These include passive exposure as well as volitional intake models across addictive drugs and have been utilized to also measure withdrawal symptomatology and potential neurobehavioral mechanisms underlying relapse to drug seeking or taking. There are a number of Food and Drug Administration (FDA)-approved medications for SUDs, however, many demonstrate low clinical efficacy as well as potential sex differences, and we also note gaps in the continuum of care for certain aspects of clinical experiences in individuals who use drugs. In this review, we provide a comprehensive update on both frequently utilized and novel behavioral models of addiction with a focus on translational value to the clinical experience and highlight the need for preclinical research to follow epidemiological trends in drug use patterns to stay abreast of clinical treatment needs. We then note areas in which models could be improved to enhance the medications development pipeline through efforts to enhance translation of preclinical models. Next, we describe neuroscience efforts that can be leveraged to identify novel biological mechanisms to enhance medications development efforts for SUDs, focusing specifically on advances in brain transcriptomics approaches that can provide comprehensive screening and identification of novel targets. Together, the confluence of this review demonstrates the need for careful selection of behavioral models and methodological parameters that better approximate the clinical experience combined with cutting edge neuroscience techniques to advance the medications development pipeline for SUDs.

Effect of stress on opioid-seeking behavior: evidence from studies with rats

Studies concerned with the relation between exposure to stress and the behavioral effects of opioid agonists in animal models of drug use are reviewed. These studies, which primarily utilized male rats, indicate that under certain conditions short-term mild stressors increase self-administration of opioid drugs and reinstate herein-seeking behavior following a drug-free period. On the other hand, there is evidence that long-term chronic inescapable stressors and severe acute stressors reduce the reinforcing effects of morphine as measured by a conditioned place preference procedure and decrease the behavioral effects of other positive reinforcers. The results of the studies reviewed suggest that stressors are important modulators of opioid-taking behavior, especially during drug-free periods. The implications of these findings to the understanding of the neurobiology of relapse to opioid-seeking behavior and for strategies for medication development to prevent relapse to heroin are discussed.

Intravenous morphine self-administration by rats with low versus high saccharin preferences

An experiment was performed to determine the relationship between saccharin preference and the self-administration of morphine via the oral and intravenous routes. On the basis of voluntary intake of a saccharin solution by male rats, low and high preference groups were formed. Rats selected for high saccharin preference self-administered more morphine intravenously than rats selected for low preference. The two groups did not differ in oral morphine intake. The positive relationship between the intake of saccharin and intravenous morphine self-administration may be due to their mediation by a common mechanism. Measures of taste sensitivity or preference may be useful in identifying individuals at risk for drug abuse.

Voluntary oral morphine self-administration in rats: effect of haloperidol or ondansetron

Rats were exposed to increasing concentrations of morphine hydrochloride (up to 0.4 mg/ml) in 5% w/v sucrose solution as their sole source of drinking water. Physical dependence was established as determined by the precipitation of withdrawal behaviour following administration of 1 mg/kg IP naloxone hydrochloride on day 23. The choice between either a 5% w/v sucrose solution or a 5% w/v sucrose solution containing 0.4 mg/ml morphine hydrochloride 4 days following withdrawal resulted in rats being categorized into two groups based on their respective consumption of the morphine-containing solution. The amount of morphine solution voluntarily consumed by approximately half the rats were sufficiently high as to lead to a relapse into physical dependence to morphine. The high preference for morphine shown by these rats could not be attributed to the taste of the morphine solution. Naive rats or rats exposed to a 5% w/v sucrose solution for 23 days failed to consume significant quantities of the morphine-containing solution when provided with a choice. The administration of either an IM slow-release formulation of 70.5 mg/kg haloperidol decanoate (= 50 mg/kg haloperidol) or 10 micrograms/kg IP ondansetron hydrochloride daily did not alter morphine ingestion in the high morphine-preferring rats.

The "medicinal effect": a possible mechanism in the development of severe alcohol dependence

An examination of the change in attractiveness of a flavor subsequent to pairing with recuperation from malaise was undertaken. Forty-five subjects consumed a flavor after the rotation-induced motion sickness at two different intervals. Twenty-four consumed, on three separate conditioning trials, the paired flavor immediately as malaise began to decline (short-delay group), the remainder, when malaise had completely diminished (long-delay group). All subjects also consumed an unpaired flavor the morning after each session. Analysis of variance revealed that the paired flavor was rated as more highly attractive than the unpaired flavor, and this effect was only present in the short delay group. These results were taken as supporting the existence of conditioned flavor preferences in humans and interpreted as a possible learning mechanism in the development of severe alcohol dependence.

Effect of stress on oral fentanyl consumption in rats in an operant self-administration paradigm

The effect of intermittent footshock stress (0.8 mA; 0.2 s on; 40 s off on the average; for 10 min/day) on oral fentanyl (50 or 75 micrograms/ml) self-administration (SA) in operant chambers was examined in male rats. In Experiment 1, after 1 month of initiation of the fentanyl SA by partial water deprivation, animals were tested for lever-pressing for fentanyl (75 micrograms/ml) under fixed-ratio-4 (FR-4) and progressive-ratio (PR) schedules of reinforcement for 30 min/day in operant chambers. Exposure to footshock stress increased fentanyl SA under the FR-4 and PR schedules compared with a nonstress condition. When water was substituted for the drug, the operant behavior persisted before extinction. In Experiment 2, different rats were tested for lever-pressing for fentanyl (50 micrograms/ml) under FR-6 and PR schedules. This experiment further assessed the role of taste in the stress-induced fentanyl SA and examined the effect of increasing the schedule requirements (i.e., FR-3, 6, and 12) on lever-pressing for fentanyl. Exposure to footshock stress increased lever-pressing for oral fentanyl SA under the FR schedules of reinforcement. When a quinine solution (30 micrograms/ml), matched for bitter taste with the fentanyl solution, was substituted for the drug solution, an extinction of the drug-reinforced behavior occurred, indicating that the stress-induced oral fentanyl SA is not related to stress-induced changes in taste sensitivity. In both experiments, no significant stress effects were observed for water consumption in home cage and lever-pressing on the nonoperative lever.

Water deprivation-induced oral self-administration of cocaine in the Lewis rat: evidence for locomotor effects but not reinforcement

Oral cocaine self-administration was studied in water-deprived Lewis rats. Liquid was available to rats only during daily 90-min sessions, in chambers equipped with spouts that delivered precise volumes of liquid following completion of lever-press responses. Blocks of training and testing sessions were alternately carried out during which increasing cocaine concentrations were presented: 0.0, 0.0125, 0.025, 0.05, 0.1, 0.2, 0.282, and 0.4 mg/ml. Although high cocaine intakes (23.3-33.0 mg/kg) were obtained, neither avoidance nor preference for cocaine developed. Subsequently, fixed-ratio size was increased, and then distinctive stimulus lights were correlated with each liquid. One rat showed a preference for water following these changes, but two rats continued to show no preference. To determine if the amounts of cocaine self-administered had behavioral effects, locomotor activity tests were run immediately following self-administration sessions. Locomotor activity was substantially higher following cocaine self-administration than following water self-administration. These results demonstrate that the cocaine intakes reached under the present conditions did produce locomotor, but not reinforcing, effects.

Immobilization stress-induced oral opioid self-administration and withdrawal in rats: role of conditioning factors and the effect of stress on "relapse" to opioid drugs

The effect of 15 min/day of immobilization (IM) stress on oral self-administration (SA) of morphine (0.5 mg/ml) or fentanyl (25 micrograms/ml) and withdrawal was examined in rats. In addition, the role of conditioning factors in these effects was assessed. For each drug, four groups of subjects were exposed for 50 days to IM stress prior to the drug SA period [Paired-Stress (P-S) groups], to IM stress prior to the drug SA period on half of the days and after the drug SA period on the rest of the days [Partial Paired-Stress (PP-S) groups], to IM stress several hours after the drug SA period [Unpaired-Stress (UP-S) groups], or to no IM stress [Control (C) groups]. The P-S and PP-S groups increased their drug SA during choice days in which both the opioid solution and water were available, and tended to manifest a more severe withdrawal syndrome after a naloxone challenge compared with the UP-S and C groups. Reinstatement of the opioid SA under conditions of paired-stress or no stress was further examined after 3 weeks without exposure to either stress or drugs. The paired stress animals had higher levels of drug SA and manifested a more severe withdrawal syndrome than those tested without stress. These results indicate that the learned association between exposure to stress and the drug availability may mediate, in part, the stress-induced enhancement of opioid SA and withdrawal effects.

No psychological dependence after oral administration of morphine to rats

Rats subjected to forced oral self-administration of morphine solutions without or in combination with two daily i.p. injections of morphine preferred drinking water when this was offered in addition to morphine solutions. The daily intake of morphine during the terminal phase of self-administration of morphine was 50-80 mg/kg (oral application alone) or 270 mg/kg (oral and i.p. application). Morphine treated animals showed withdrawal symptoms on administration of naloxone 1 mg/kg i.p. during the period of self-administration, but not when they had started drinking exclusively water. The tail-flick test revealed no tolerance during prolonged treatment with morphine. The results indicate that no psychological dependence developed when morphine was applied orally and regularly.

Effect of stress on oral morphine and fentanyl self-administration in rats

The effect of immobilization stress (15 min/day) or no stress on oral morphine (0.25-0.5 mg/ml) or fentanyl (5-20 micrograms/ml) self-administration was examined in rats. Animals had access to a morphine or fentanyl solution for 4 days, followed by a single-choice day of access to the opioid solution and a separate water bottle. This 5-day cycle was repeated five times for 7 h/day in home cages. Morphine consumption and preference were assessed for an additional 30 days (i.e., six more cycles) in a subgroup of subjects. Plasma corticosterone levels in the stress groups indicated that the stress manipulation was effective. Over the course of the experiment, animals in the stress groups significantly increased their preference for the opioid solutions during choice days compared to nonstress controls. Morphine preference after 55 days was twice as high in the stress group (70% morphine/30% water) in comparison to controls (34% morphine/66% water). These results indicate that stress increases oral opioid self-administration in rats. Future directions and the implications of this work are discussed.

Is there a genetic control of morphine preference in rat?

Morphine preference was tested in two-bottle, voluntary-choice situations on physically dependent Sprague-Dawley rats. The animals ingested morphine which was dissolved in a fluid diet. Choice tests were performed under similar experimental conditions as the ingestions. Approximately 10% of the physically dependent rats voluntarily preferred large amounts of morphine already after a short treatment. The preference level was found to correlate with the animals' requirement for the drug. There was a gradual increase in morphine preference in F1 and F2 offspring of extremely high morphine preference rats. In F3 (of such extremely high morphine preference rats) up to 65% died shortly after birth. The surviving rats showed a low morphine preference after ingestion when adult. Our result of increasing preference over two generations and death of a large number of rats in the third generation with a low morphine preference of the surviving rats was seen also in F4-F8. The data suggest that high morphine preference is under genetic control.

Self-administration of morphine contingent on heart rate in the rat

The tendency to associate a given response-reinforcement combination reflects the adaptive significance of the association. Biologically relevant reinforcement can be much more effective in modifying certain responses. For example, treatments that result in various types of illness readily condition aversions to novel flavors, but electric shock is relatively ineffective. While opioid self-administration contingent on lever pressing has been extensively studied, the potential for opioids to reinforce visceral responses remains to be determined. An approach to reinforcing changes in heart rate with drug infusions is described. Methods to control for unconditioned drug effects include reversing the direction of change in heart rate required for infusions and addition of a yoked control subject. In several instances, rats exposed to .1 mg/kg infusions of morphine sulfate contingent on tachycardia showed trends for elevated heart rate, with increased locomotor and grooming activity preceding infusions. Increases in heart rate were most pronounced during daytime, normally inactive periods.

Corticotrophin-releasing factor, vasopressin and pro-opiomelanocortin mRNA responses to stress and opiates in the rat

1. Rats underwent either: (1) acute or chronic morphine or naloxone administration; (2) simple morphine withdrawal or naloxone-precipitated withdrawal in morphine-dependent animals; or (3) stress from I.P. administration of hypertonic saline. 2. Quantitative in situ hybridization histochemistry was performed using synthetic oligonucleotide probes for corticotrophin-releasing factor (CRF), vasopressin, pro-opiomelanocortin (POMC), dynorphin, enkephalin and oxytocin mRNAs. The paraventricular and supraoptic nuclei were examined in all studies and the arcuate nucleus and pituitary gland in the acute withdrawal study. 3. Neither acute nor chronic morphine administration altered either (a) hypothalamic parvocellular or magnocellular CRF mRNA, or (b) anterior pituitary or pars intermedia POMC mRNA. 4. Naloxone-precipitated morphine withdrawal resulted in a marked increase in parvocellular (but not magnocellular) CRF mRNA within 4 h and levels remained elevated through 24 h. There was no change in arcuate nucleus or pars intermedia POMC mRNA, but in the anterior pituitary there was a delayed increase, significant at 24 h. 5. Simple morphine withdrawal without the use of naloxone did not result in any change in CRF mRNA but there were increases in magnocellular vasopressin and dynorphin mRNA, presumably related to decreased water intake. 6. Intraperitoneal hypertonic saline stress also resulted in a marked accumulation of both parvocellular CRF and vasopressin mRNA without any concomitant change in magnocellular vasopressin mRNA. Increased translation of CRF mRNA was also evidenced by increased immunoreactive CRF detected by immunocytochemistry.

The concentration of morphine in serum of rats made dependent using a drug-admixed food method

The present study reports on the induction of physical dependence in rats using morphine-admixed food and addresses the question of the resulting concentration of morphine in serum. The stability of morphine in food is good, since no decrease in concentration could be observed. The concentration of morphine in serum during the experiment was measured using a radioimmunoassay technique. A correlation was found between the food intake during a 7-hour period and the concentration of morphine in the serum at the end of that period, both for a 1 g/kg and a 2 g/kg batch of morphine-admixed food. The concentration of morphine in serum was also found to be dose-related during a period of 6-23 days when the rats were fed for a prolonged period. After long-term administration of 1 g/kg morphine in food a steady-state level of about 0.5 mg/l serum was obtained. Similarly with 2 g/kg morphine in food a steady-state level of 0.8-1.1 mg/l serum was reached. After withdrawal of morphine the serum concentration of morphine dropped to 0.1 mg/l within 24 hours and to below the detection limit within 48 hours. During the withdrawal period sharp drops were noted in body weight (20%) and food intake (50%) after one day.

Are astroglial cells involved in morphine tolerance?

Morphine gives rise to a cascade of events in the nervous system affecting, among others, neurotransmitter metabolism. Tolerance develops for various effects shortly after administration of the drug. Also, physical dependence develops and can be demonstrated by precipitation of withdrawal reactions. Biochemical events in nervous tissue have been extensively studied during morphine treatment. This overview will focus upon brain protein metabolism since macromolecular events might be of importance for development of long-term effects, such as tolerance and physical dependence. Both dose- and time-dependent changes in brain protein synthesis and the syntheses of specific proteins have been demonstrated after morphine treatment, although methodological considerations are important. Different experimental models (animal and tissue culture models) are presented. It might be interesting to note that astroglial protein synthesis and the secretion of proteins to the extracellular medium are both changed after morphine treatment, these having been evaluated in astroglial enriched primary cultures and in brain tissue slices. The possibility is suggested that proteins released from astroglial cells participate in the communication with other cells, including via synaptic regions, and that such communication might of significance in modifying the synaptic membranes during morphine intoxication.

Aspects of abstinence after morphine ingestion

Sprague-Dawley male rats were intoxicated with morphine, using an ingestion method where exposed and control rats received equivalent amounts of calories and nutrients. The degree of physical dependence on morphine was demonstrated by studying and quantifying abstinence symptoms after withdrawal or after administration of opiate antagonists. The aims of the study were (1) to further enlighten the specificity and validity of the intoxication method concerning physical dependence, and (2) to determine whether some of the abstinence signs might be of value to facilitate quantitation of the degree of physical dependence on morphine, with diet and fluid intake being maintained under control. Withdrawn rats showed a decreased fluid diet intake and a body weight loss, the latter partly due to anorexia. Other mild abstinence signs were irritation, tremor and some motor excitation. The body weight loss during the first day of morphine withdrawal was proportional to the accumulated drug dose (between 25 and 300 mg morphine PO/kg b.wt.). However, prolonged morphine treatment on one dose (340 mg/kg b.wt.) did not reinforce the body weight changes caused by morphine withdrawal. The succeeding weight gain some days after morphine withdrawal was not entirely dependent on the amount of fluid diet intake. Methadone was shown to partially block the decrease in diet intake and the weight loss seen during morphine withdrawal. The naloxone-precipitated withdrawal symptoms were motor excitation, cholinergic signs, body weight loss, diarrhoea and decreased diet intake. The weight loss 2 hr after naloxone administration to long-term intoxicated rats was proportional to the naloxone dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphine and clonidine oral self-administration: a study in morphine dependent or abstinent rats

The ability of morphine and of clonidine to support self-administration has been evaluated in morphine dependent or abstinent rats, using an orally reinforced operant technique (F.R. 20). It was found that: Morphine drinking can function as a reinforcer of an operant response both in dependent and abstinent rats. The bitter taste of morphine becomes a secondary reinforcer for rats usually working for the alkaloid. Clonidine supports oral self-administration in morphine abstinent, but not in morphine dependent rats. The data are in line with clinical findings and give a further evidence that oral self administration in rats can be an useful model for the analysis of drug dependence.

Morphine preference in rats previously morphine dependent

Morphine preference and tendency to relapse to morphine tolerance and dependence were studied in rats which were previously made morphine dependent. Tolerance to, and physical dependence on, morphine were initially produced by administration of increasing concentrations of morphine sulphate in 5% sucrose solution for 3 weeks. A test for drinking preference was performed 4 days after the rats had been successfully detoxified and showed no significant signs of morphine dependence. It was found that, while control animals drank only negligible amounts of morphine solution, previously morphine-dependent rats consumed significantly larger volumes of morphine solution and had recurrence of morphine tolerance and dependence. The present findings show that chronic administration of morphine in drinking fluid produces tolerance and physical dependence as well as addiction in rats; the latter definition is exemplified by these animals having a high tendency to relapse after successful drug withdrawal.

Repeated withdrawal from barbital as a drive for 'drug taking behavior' in rats

Investigations were performed to determine whether the pharmacodynamic effect of barbital, the development of tolerance to or the physical dependence on the hypnotic are responsible for drug-taking behavior. Three groups of male rats, untreated, tolerant to and physically-dependent on barbital, were given free choice between 0.5% barbital solution and tap water. Drug-taking behavior was estimated according to specified criteria. Initially naive rats rejected an unsweetened barbital solution. Tolerant rats also refused the hypnotic, even after they had experienced abstinence symptoms only once. However, tolerant rats that repeatedly underwent withdrawal after an intake of more than 400 mg/kg/day of barbital did show drug taking behavior. Therefore, several experiences with pronounced abstinence symptoms seem to be necessary for initiating and sustaining barbital drug taking behavior in rats.

Isolation versus grouped housing in rats: differential effects of low doses of heroin in the place preference paradigm

Male Long Evans rats were reared from weaning (21-23 days) either in isolation or in groups of four for 40 days. Animals were then individually introduced to a testing apparatus consisting of two distinct chambers. A modified place preference paradigm was used consisting of 3 phases: (1) An habituation phase (4 days) during which rats were allowed free access to the entire test apparatus for 15 min. periods daily; (2) A conditioning phase (4 days) during which rats were confined to their non-preferred side for 15 minutes each day immediately following subcutaneous injection of 0, 20, 40 and 80 micrograms/kg of heroin HCl; (3) A test phase (1 day) during which rats were again allowed free access to the testing chamber following injection of vehicle. The difference in time spent on the conditioned side during habituation and test periods was determined. The group-reared rats showed similar effects for all doses of heroin whereas the same magnitude of drug effect was attained only at the highest dose used in the isolated rats. This differential sensitivity to heroin in the place preference paradigm is discussed in terms of the modification of behavioral effects of opiates by environmental influences.

A new ingestion method for long-term morphine intoxication in rat

A new method for long-term morphine intoxication in rat was developed. It was designed to deal with the nutritional imbalance and body weight loss that generally occurs using conventional techniques for morphine treatment. The morphine is administered in a nutritionally complete diet. Also pair-feeding is used to deal with intoxicated rats that do not eat the same amount of food as controls. The technique was validated during the study of different intoxication conditions, using different initial doses, dose increments and final doses. An initial dose of 25 mg morphine/kg b.w., raised exponentially up to 340 mg/kg b.w. in 8 days, made the rats dependent, as tested by withdrawal signs, precipitated by excluding morphine from the diet, or by administration of antagonists. A final dose of up to 715 mg morphine/kg b.w. was reached in 13 days without decreased food intake. However, initial doses of 340 or 715 mg/kg led to impaired weight gain and diet consumption. Plasma morphine levels of 3 micrograms/ml serum were reached on a dose of 340 mg/kg b.w. Also, preference for morphine diet over control diet was evaluated by choice tests. The technique is simple, time-saving and inexpensive, allowing the treatment of numerous animals for long periods under standardized intoxication conditions. No animals get ill or die.

Production of tolerance and physical dependence in the rat by simple administration of morphine in drinking water

1 Rats are capable of consuming solutions of morphine sulphate in drinking water ad libitum in the absence of taste-masking chemicals and without the need for scheduled provision or prior parenteral administration of the drug. 2 The success of this method depends on the initial provision of a 0.1 mg/ml solution of morphine sulphate. 3 When the drug concentration is increased to 0.4 mg/ml, the rats achieve an average daily intake of 50 mg/kg body wt. each. 4 Daily intake of morphine may be increased by at least about three fold by increasing the drug concentration to 1.2 mg/ml. 5 Oral morphine administration causes only a moderate loss in body weight. 6 Rats whose daily intake of the drug is 50 mg/kg exhibit tolerance to the analgesic action of morphine and show a drastic loss in body weight at 24 h after withdrawal and most of the behavioural symptoms of the naloxone-precipitated withdrawal syndrome. 7 It is suggested that this simple method of morphine administration is suitable for further biochemical and behavioural studies of the actions of the drug.

Alcohol and taste-mediated learning

Taste-mediated learning is relevant to the alcohol consumption patterns of animals. This review concludes that taste aversion learning has thus far prevented development of an animal model of alcoholism. The presence of a taste cue, lack of control over alcohol administration, and high alcohol concentrations or dosages all facilitate the development of alcohol aversions. There is little evidence that taste preference learning is involved in the development of alcohol dependence. Data from taste-mediated learning research with animals are consistent with drinking patterns of human alcoholics.

The influence of a GABA transaminase inhibitor, gamma-vinyl GABA, on voluntary morphine consumption in the rat

Morphine-dependent and control rats in an oral free-choice protocol were treated with gamma-vinyl GABA (GVG), 60, 120 and 240 mg/kg IP, for 3 days over three successive periods. Morphine dependence was assessed with naltrexone. Fluid intake of morphine-dependent rats was reduced during GVG treatment and the proportion of fluid taken as morphine was also decreased, but not sufficiently to induce withdrawal signs. Intake by controls was only affected by 240 mg/kg GVG, when body weight decreased. The attentuation of morphine consumption by GVG treatment may, within the limitations of this protocol, interfere with the positive reinforcing properties of morphine.

Characteristics of chronic self-administration of morphine by dogs

Each of five dogs that had been trained to chronically self-administer IV morphine was tested with changes in the morphine dose from the baseline dose (1 mg/kg/infusion) to 0.125, 0.5, or 2 mg/kg/infusion, and with passive administration of the usual daily morphine intake while either continuing the morphine self-administration at the baseline dose or changing the self-administered solution to saline. Each treatment lasted 5 weeks. Results indicated that there is a significant negative regression of response on dose, and chronic self-administration of morphine is not entirely accounted for by a need to avoid abstinence or to obtain a direct drug effect. A third element, which may be an acquired need to obtain a response-contingent drug effect, is necessary to account for the chronic self-administration of morphine by the dog.

Pre-exposure effects of morphine, diazepam and delta 9-THC on the formation of conditioned taste aversions

Prior to taste aversion conditioning with morphine, diazepam or delta 9-tetrahydrocannabinol (delta 9-THC), rats received pre-exposures to the vehicle or one of the three drugs. Morphine pre-exposures blocked the aversion normally induced by morphine, but not by delta 9-THC or diazepam. Diazepam pre-exposures attenuated both the morphine- and diazepam-induced taste aversions to a significantly greater degree than the taste aversion induced by delta 9-THC. As a result of delta 9-THC pre-exposures, the aversions induced by diazepam and delta 9-THC were attenuated as well as the morphine-induced aversion, which was the most greatly attenuated. These results demonstrate that pre-exposure effects are not necessarily bi-directional and, moreover, they are inconsistent with current hypotheses which attempt to account for the attenuating effect of drug pre-exposures on taste-aversion conditioning.

Development of dependence on levorphanol in rats by oral intake of the drug -- the influence of taste on drinking behaviour in rats physically dependent on levorphanol

Oral administration of levorphanol solution induces physical dependence in rats within a few days, as demonstrated by abstinence symptoms such as loss of body weight, sensitivity to touch and inversion of locomotor activity after withdrawal from the drug. In order to examine whether the physically dependent rats show an active drug-seeking behaviour they were given successively free choice between sweetened levorphanol solution (LSa) and two alternative drinking liquids -- sweetened tap water (WSa) and unadulterated water (W). In the case of LSa and W the rats chose LSa, but they preferred WSa to LSa. Another group of rats made dependent on unsweetened levorphanol solution (L) had the choice between L and W. They rejected L immediately.

Failure to find an effect of catheterization on oral morphine consumption in rats

Oral morphine consumption of 10 catheterized rats and 10 non-operated controls was compared to test the hypothesis that stress associated with catheterization might increase opiate consumption by rats. No significant differences were found. An alternate interpretation of oral morphine consumption studies is suggested.

Rewarding and aversive effects of morphine: temporal and pharmacological properties

To assess morphine-induced location preferences and flavor aversions, rats were administered morphine sulfate (10 mg/kg, IP) either immediately before (Experiment 1) or immediately after (Experiment 2) confinement for 20 min in one side of a shuttlebox with access to a flavored solution. On control trails the rats were administered saline and confined for 20 min on the opposite side with a differently flavored solution. In subsequent choice tests, it was found that morphine injections before confinement produced a preference for the side associated with morphine and indifference to the flavors, whereas morphine injections after confinement produced an aversion to the flavor paired with morphine and indifference to the sides. Experiments 3 and 4, using a procedure similar to that of Experiment 1, showed that naloxone (1 mg/kg, IP) blocked the morphine-induced side preference, although given alone it was without effect in this test.

The effect of housing and gender on preference for morphine-sucrose solutions in rats

To determine whether opiate consumption is affected by laboratory housing, individually caged and colony rats were given a choice between water and progressively more palatable morphine-sucrose solutions. The isolated rats drank significantly more of the opiate solution, and females drank significantly more than males. In the experimental phase during which morphine-sucrose solution consumption was greatest, the isolated females drank five times as much, and the isolated males sixteen times as much morphine (mg/kg) as the colony females and males respectively.

Preparation and evaluation of a sustained morphine delivery system in rats

A new drug delivery system to induce physical dependence to morphine in rats is described. The device consists of a silicone polymer containing a water soluble "carrier" material, sodium alginate, which swells on contact with moisture to release the drug. The silicone or silastic pellets formulated to contain morphine sulfate are very easily prepared and the advantages over existing methods to induce physical dependence to morphine are discussed. In addition, a comparison of the percent of drug released and withdrawal intensities in rats was made with a silastic-morphine sulfate pellet, silastic-morphine base pellet and a microcrystalline cellulose-morphine base pellet.

Preference for morphine and drug-seeking behavior in morphine dependent rats

We have already reported that morphine pretreated rats prefered morphine-admixed food during choice trials with the two cup method. In the present work, we utilized both the five and two cup methods and observed the preference for morphine in rats forcedly pretreated with the drug and the increasing rate of preference for the drug in rats where the feeding time was limited. In morphine pretreated rats, preference rate for morphine was 61.2 +/- 3.0% with the five cup method and 61.8 +/- 3.3% with the two cup method during the choice trials. In rats that were limitedly treated with morphine, each preference rate for morphine during choice trials rapidly increased in the five cup method, i.e. 5.5 leads to 14.4 leads to 31.7 leads to 43.6 leads to 61.2%, and the preference rate for morphine stabilized at approximately 60 percent. Findings with the two cup method were similar. After the preference for morphine was stabilized at the 60 percent level, morphine was given subcutaneously and it was found that the preference rate was dependent on the dose injected. When the drug-admixed concentration was changed from 1 mg/g food to 0.5 and 2 mg/g food, the preference rate changed in parallel with the concentration. When the number of food cups containing morphine-admixed food was changed from 1/5 to 2/5, 3/5 and 4/5 food cup, the preference rate was not effected. These studies clearly demonstrate drug-seeking behavior in rats. In the process of preference for morphine, morphine treatment enhances spontaneous intake of morphine-admixed food.

Pituitary-adrenal axis and oral morphine consumption in rats

Removal of the pituitary gland in rats leads to suppression of oral morphine and quinine intake behavior. Experiments measuring oral intake of solutions containing graded concentrations of morphine or quinine, revealed that the detection acuity for bitter taste is changed in hypophysectomized (hypox) animals. Treatment of these rats with ACTH 1--24 restored oral morphine intake towards that on intact rats. Morphine consumption in hypox rats was not affected by administration of ACTH 4--10 or ACTH 11--24, but was normalized by treatment with corticosterone. Adrenalectomy also diminished oral morphine intake. It is concluded that hypophysectomized animals refuse a morphine solution because their threshold for bitter taste quality is altered, presumably due to a diminished release of corticosteroids.

The effect of housing and gender on morphine self-administration in rats

To determine the effect of housing conditions on morphine self-administration, rats isolated in standard laboratory cages and rats living socially in a large open box (8.8 m2) were given morphine in solution (0.5 mg/ml) as their only source of fluid for 57 days. They were then exposed to a series of 3-day cycles previously shown by Nichols et al. (1956) to increase self-administration of morphine in caged rats. On morphine/water choice days late in the period of forced consumption, between the Nichols cycles, and during a subsequent period of abstinence, the isolated rats drank significantly more morphine solution than the social rats, and the females drank significantly more morphine solution than the males. During the four choice days in the Nichols Cycle Period the isolated rats increased their consumption, but the socially housed animals decreased theirs.