Regulation of drug and water intake in rats dependent on morphine

Rats were made dependent on morphine by training them to drink solutions of the drug even when water was available as an alternative. Experiments were carried out to examine the reliability of the technique. Drinking was recorded in two ways, and it was found that there was close agreement between the numbers of laps (drinkometer counts) and the volumes of fluids removed from the drinking bottles. When dependent rats were allowed to partially satisfy their needs for morphine and/or water shortly before morphine: water choice trials, they made appropriate adjustments in their subsequent consummatory responses. However, the value of using quinine solutions to control for effects due to the bitter taste of morphine was more limited than has sometimes been supposed; the relative palatabilities of the two drugs seemed to vary across experiments. Considered as a whole, the results support the view that preferences for morphine in rats can provide a valid measure of their need for the drug.     

Oral intake of morphine in selectively bred rats

Oral intake of morphine was investigated in selectively bred strains of rats. It was possible to induce all the five genetic lines to consume morphine in distilled water. Withdrawal of morphine resulted in significant decrease in the body weights of all the genetic lines in both the experiments. Significant drug-preference behavior was found only in three genetic lines with a common characteristic of relatively high emotional reactivity as compared to the two lines not showing such a behavior.     

Changes in morphine self-administration and morphine dependence after lesions of the caudate nucleus in rats

Rats were trained to bar press for intravenous infusions of morphine sulfate during 1-hr daily test sessions. Small, centrally placed bilateral lesions of the caudate nucleus reduced rates of morphine self-administration to approximately one seventh of preoperative levels; postoperative rates were similar to preoperative rates when the postoperative unit infusion dose of morphine was one tenth of the preoperative dose. Caudate lesions also lowered the threshold dose at which morphine's rewarding property could be detected. Physical dependence was studied in other rats receiving a 3-day continuous infusion of morphine sulfate via implanted subcutaneous silicone reservoirs. Caudate lesions ameliorated withdrawal-induced weight loss and naloxone-induced wet dog shakes. Both the self-administration and dependence data are consistent with the idea that morphine blocks dopaminergic transmission in the striatum.     

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

Summary Rats were induced to administer morphine to themselves by drinking solutions of it in preference to water; this behaviour was found to be a valid model of morphine dependence. Previous passive medication with morphine was not necessary; initial aversions for the bitter morphine solutions were converted into preferences after the rats were repeatedly given only morphine solutions to drink in order to relieve thirst. The consumption of solutions of quinine which were initially equally aversive did not increase, suggesting that the repeated pairing of a bitter taste with relief of thirst did not account for the preferences for the morphine solutions. It appeared that the post-ingestional effects of morphine provided primary reinforcement for the rats; they were able to regulate their daily intake of the drug after being injected with varying doses of it and they lost weight abruptly during enforced abstinence. There was also evidence that the bitter taste of morphine had become a secondary reinforcer for rats with established preferences.The work was supported by a grant from the Medical Research Council and by grant MH-03313 from the U.S. Public Health Service.One of us (R. K.) held a Beit Memorial Research Fellowship.     

Development of morphine dependence in rats: Lack of effect of previous ingestion of other drugs

It has often been observed that periods of abuse of non-narcotic drugs precede the development of dependence on opiates, but this correlation does not necessarily imply that there is a simple causal relationship. We have previously described a procedure for inducing rats to drink solutions of morphine in preference to water; self-administration of the drug in this way seems to be a valid model of dependence. For the first 46 days of the experiment described here, solutions of alcohol, amylobarbitone, chlordiazepoxide, cocaine or dexamphetamine were made available to different groups of rats. Most of these drugs were ingested in substantial doses, had clear effects on behaviour and produced characteristic patterns of drinking over the repeated trials. However, there was no unequivocal evidence of dependence, and indeed the rats learned to reject the solutions of dexamphetamine. Ingestion of these drugs did not affect the eventual development of dependence when solutions of morphine were substituted at a later stage, although the avoidance of dexamphetamine seemed to temporarily transfer to morphine. Further studies using other methods and species are needed before inferences can be made about escalation to dependence on opioids in man.     

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.     

Intragastric self-administration of morphine as a measure of addiction

In experiments carried out on male albino rats it was found that instrumental conditioning reinforced with direct intragastric infusion of morphine could be used for the investigation of addiction.This investigation was supported by the Polish Academy of Sciences under Agreement No. 09.4.1.1.4.2 for research on Central regulation of alimentary activity.     

Intestinal toxicity of oral warfarin intake in rats

Though warfarin is extensively used in the prevention and treatment of thromboembolic processes in humans, adverse effects of warfarin therapy have been recognized. Intestinal hemorrhage is one of the hazards of anticoagulant therapy, but the mechanisms of warfarin toxicity are virtually unknown. In this work, the effects of 30 days oral warfarin (0.35 mg/l and 3.5 mg/l) intake on rat’s gut were examined. Both doses resulted in prolongation of prothrombin time. Systemic effects of higher warfarin dose (increases in plasma AST, proteinuria, hematuria, changes in peripheral blood hematological parameters) were seen. Warfarin intake resulted in histologically evident tissue damage, leukocyte infiltration and intestinal inflammation [increases in myeloperoxidase activity, malondialdehyde content, superoxide dismutase and catalase activity, proinflammatory cytokine (IFN-γ, IL-17) concentrations in intestinal homogenates]. In contrast, suppression of gut-draining mesenteric lymph node (MLN) cell activity [proliferation responsiveness, production of IFN-γ and IL-17 to T lymphocyte mitogen Concanavalin A stimulation] was noted. Inhibition of regulatory cytokine IL-10 production by MLN cells, suggests commitment of MLN to the suppression of all inflammatory activities and creation of the microenvironment which is non-permissive for induction of potentially harmful immune response. These novel findings indicate the need of staying alert for (adverse) effects of warfarin therapy.     

Effect of early and later colony housing on oral ingestion of morphine in rats

Male and female rats were raised from weaning either in isolation or in a large colony. At 65 days of age, halfe the rats in each environment were moved to the other. At 80 days, the animals were given continuous access to water and to a sequence of 7 solutions: 3 sweet or bitter-sweet control solutions and 4 different concentrations of morphine hydrochloride (MHCl) in 10% sucrose solution. Rats housed in the colony at the time of testing drank less MHCl solution than isolated rats, but no less of the control solutions. Colony-dwelling rats previously housed in isolation tended to drink more MHCl solution than those housed in the colony since weaning, but this effect reached statistical significance only at the lowest concentration of MHCl. These data were related to the hypothesis that colony rats avoid morphine because it interferes with complex, species-specific behavior.     

Acute morphine tolerance in new born and young rats

The development of tolerance after two consecutive doses of morphine administered at 4 h intervals was studied in 16, 25 and 38 day-old Sprague-Dawley rats. Tolerance was measured by the diminution of the rats' response to morphine on a hot plate (56 C). The ages selected correspond to clear stages of the brain barrier permeability to morphine. A loss of sensitivity to morphine was observed according to age. Acute tolerance developed at all ages tested, except when higher doses were employed in 25 and 38 day-old rats. When acute tolerance had developed it persisted for a long time. It is concluded that the development of acute tolerance to morphine analgesia does not depend on the stage of development of the rat.Partially supported by grants from the Gildemeister Foundation, Santiago, Chile, and from the Fondo de Investigaciones Científicas, Universidad Católica de Chile, Santiago, Chile     

Some altered responses in rats formerly dependent on morphine

This investigation examined whether or not physical dependence or other abnormalities were detectable 1–3months after withdrawal in dependent rats that had been treated with the morphine (maintenance dose of 100×2mg/kg/day,s.c.) for 7 weeks. When narcotic antagonists were administered on the 32nd day after withdrawal, nalorphine caused a dose-dependent increase in spontaneous locomotor activity and a complete inhibition of wet-dog shakes and the writhing syndrome. Naloxone was ineffective. A remarkable increase in spontaneous locomotor activity on the 67th day and a significant increase in body weight on the 69th and 92nd day after withdrawal occurred after an acute injection of morphine (10 mg/kg, s.c.). When morphine (10 mg/kg) was administered for 3 days from the 92nd day after withdrawal, withdrawal from morphine produced a significant decrease in body weight. When morphine (10 mg/kg) was administered for 3 days from the 102nd day after withdrawal, a levallorphan injection caused a significant decrease in spontaneous locomotor activity and an increase in the frequency of the diarrheal syndrome. These abnormal responses, not observed in the naive rats, suggest the remains of some behavioral and biochemical abnormalities 3 months after morphine withdrawal.Part of this work was presented at the 4th Symposium on Drug-Activity held in Nagasaki, Japan, 30–31st October 1975     

Impaired tolerance to the effects of oral amphetamine intake in rats with frontal cortex ablations

When offered a solution of d-amphetamine sulphate (0.025 mg/ml) in place of water, normal rats initially drank more drug solution per day than previously consumed water. The drug solution quickly became aversive to normal rats as daily intake decreased. Tolerance to the anorexic effect of d-amphetamine paralleled the decrease in daily drug intake. Rats with bilateral lesions of frontal cortex initially consumed as much drug solution and as much food as normal rats. Although frontal rats' daily intake of drug solution also decreased, an aversion never developed. Tolerance to d-amphetamine's anorexic effect took much longer to occur in frontal rats. The results indicated possible roles for both cumulative drug effects and conditioning factors in the response to chronic d-amphetamine use. A possible mechanism by which frontal cortical lesions interfered with chronic changes was suggested.This research was supported by NIMH grant 1 R01 MH-21554-01 to S. D. Glick. The author thanks S. Greenstein and M. Angerola for indispensable technical assistance.     

Differential effects of morphine on food and water intake in food deprived and freely-feeding rats

In two experiments the effects of a range of doses of morphine (1, 3, 10 and 30 mg/kg) on the food and water consumption of rats were studied. The results of the first experiment showed that in 24 h food-deprived rats, morphine reduced levels of food and water intake. The duration of these actions was dependent upon dose, with only the highest dose (30 mg/kg) producing any effect persisting for longer than 4 h. In contrast a second experiment showed that morphine increased levels of food and water intake in non-deprived animals. The effect on food intake was most apparent when measurements were taken at 2 h and 4 h after drug administration, while water intake remained above control levels for over 6 h. This study shows that the actions of morphine on ingestion of food and water are affected by food deprivation, and the results are consistent with the hypothesised role of endogenous opiates in the mediation of such behaviour.     

EEG correlates of morphine challenge in post-addict rats

Adult female Sprague-Dawley rats were prepared with chronically implanted cortical and temporal muscle electrodes and i.v. cannulae. Morphine injections of 10 mg/kg were given i.p. to naive rats and to post-addict rats at successive intervals following morphine withdrawal. These intervals extended in a few cases up to one year. In the naive rat, high voltage EEG slow bursts associated with stuporous behavior appeared almost immediately after injection and prevailed for 60–90 min. This phase was followed by continuous EEG and behavioral arousal for another period of 60–90 min, after which sleep appeared. In contrast, morphine challenge to post-addict rats was followed by an EEG and behavioral arousal for as long as 180 min, the degree of which was less pronounced at the longer intervals following withdrawal. The EEG and behavioral arousal of the post-addict rat in response to morphine challenge is reminiscent of similar responses in human postaddicts already reported and may further emphasize the pharmacodynamic factors in morphine addiction.Supported by NIMH Grant MH-16693; a preliminary report on this work appeared in Fed. Proc. 30, 277, (1971).NIMH Postdoctoral Fellow.     

Intraventricular self-administration of morphine in naive laboratory rats

Male Wistar rats implanted with cannulae aimed at the left lateral cerebral ventricle were individually maintained in Skinner boxes for 11 consecutive days. Animals were neither predependent on morphine nor shaped to press the operant lever. Experimental animals (n=7) obtained intraventricular infusions of a 1% morphine HCl solution (2 l per 5-s infusion) for each lever press while control animals (n=7) received only the vehicle. Four animals were yoked to experimental animals and received equivalent but non-contingent morphine HCl infusions. The mean number of lever presses per day for the experimental group was significantly higher than for the vehicle control or yoked control groups suggesting that naive rats will work for the positive reinforcing properties of morphine when it is infused centrally.     

Butorphanol precipitates abstinence in morphine dependent rats

Butorphanol precipitated a withdrawal syndrome in rats receiving continuous intracerebroventricular infusions of morphine for three days. However, the potency of butorphanol to induce defecation, urination, teeth chattering and escape behavior was one to two orders of magnitude less on a molar basis than that of naloxone. Wet shake behavior, a prominent feature of naloxone precipitated withdrawal, was absent in morphine dependent animals challenged with butorphanol. These data indicate qualitative as well as quantitative differences in the withdrawal syndromes induced by butorphanol and naloxone.     

The effects of morphine treatment and morphine withdrawal on the dynorphin and enkephalin systems in sprague-dawley rats

The effect of morphine tolerance and withdrawal on prodynorphin peptides was studied in relevant brain areas and in the pituitary gland of male Sprague-Dawley rats, and compared with effects on the proenkephalin-derived peptide Met-enkephalin. After 8 days of morphine injections (twice daily), dynorphin A and B levels increased in the nucleus accumbens and dynorphin A levels increased also in the striatum. Morphine treatment increased striatal Met-enkephalin. Leu-enkephalinArg⁶ levels were reduced in the ventral tegmental area (VTA). Morphine-treated rats had very low Leu-enkephalinArg⁶ levels in the hippocampus as compared to saline control rats. Comparison of the relative amounts of dynorphin peptides and the shorter prodynorphin-derived peptides, Leu-enkephalinArg⁶ and Leu-enkephalin, revealed a relative increase in dynorphin peptides versus shorter fragments in the nucleus accumbens, VTA and hippocampus. Morphine-tolerant rats had lower levels of dynorphin A in both lobes of the pituitary gland, whereas hypothalamic dynorphin levels were unaffected by morphine. Leu-enkephalinArg⁶ levels were reduced in the hypothalamus, but not changed in the pituitary gland. Naloxone-precipitated withdrawal accentuated the increase in dynorphin A and B levels in the accumbens and dynorphin A levels in the striatum, while inducing an increase in enkephalin levels in the accumbens and Met-enkephalin in the VTA. In the hippocampus, Leu-enkephalinArg⁶ levels remained low in the withdrawal state. The low dynorphin levels in the anterior part of the pituitary gland were reversed by naloxone, whereas the low dynorphin A levels in the neurointer-mediate lobe were even lower in the withdrawal state. In conclusion, morphine tolerance and withdrawal affected prodynorphin-derived peptides in areas related to central reward mechanisms, and in the pituitary gland. The dynorphin peptides and the LeuenkephalinArg⁶ fragment were not affected similarly, indicating an effect also on metabolic interconversion.     

Some correlates of marihuana self-administration in man: A study of titration of intake as a function of drug potency

Previous research has suggested that psychosocial rather than strictly pharmacological factors may play a dominant role in the subjective response to socially relevant (i.e., low) doses of marihuana. The insensitivity of experienced marihuana users to pharmacological factors was studied in an ad lib self-administration situation. Subjects tested repeatedly with varying drug potencies were asked to smoke a sufficient amount to achieve a predefined subjective state of intoxication. Several indices of amount consumed suggested that effective titration of intake did not occur. Rather, subjects consumed more total THC the greater the potency of the material. Doses which produced a subjectively optimal high had significant behavioral and physiological effects. The results are consistent with the hypothesis that psychosocial variables may be more significant than pharmacological variables in determining the recreational intake of marihuana.     

Enhanced analgesic response to morphine in adult rats exposed to morphine prenatally

Morphine exposure during development has been shown to produce fetal tolerance to morphine as measured by spontaneous activity only if a particular injection schedule is used. The present study was undertaken to compare the morphine-induced analgesic response in adult offspring of rats which had been injected during the last half of gestation on schedules known to produce fetal tolerance (5 mg/kg morphine at 6 hour intervals) versus a schedule known not to produce fetal tolerance (10 mg/kg morphine at 12 hour intervals). At 30 days postnatally the offspring of animals injected on these 2 schedules show no changes in their responsiveness to the analgetic effect of morphine as determined in the hot-plate test. The present study shows that adult offspring of mothers injected with 20 mg/kg/day of morphine in four divided doses on days 12-20 of gestation have an enhanced analgetic response to morphine in the tail-flick test. In contrast, offspring of mothers injected during the same period of gestation with 20 mg/kg/day of morphine in two divided doses respond to the analgetic effect of morphine in the same manner as the offspring of saline-treated mothers. These results show that the schedule for prenatal morphine administration can play a role in the behavioral effects of morphine in adulthood.     

Reduction of the antinociceptive effect of 5-hydroxytryptophan in morphine tolerant rats

The effect of 5-hydroxytryptophan on pain threshold was studied in rats both tolerant and nontolerant to the analgesic action of morphine as assessed using a procedure of electrical stimulation. The compound elevated pain threshold and exhibited an additive effect with morphine analgesia in nontolerant rats. A marked reduction of the antinociceptive action of the serotonin precursor as well as absence of the additive effect with morphine was observed in rats tolerant to the analgesic. These results are discussed in terms of the possible mechanism of action of serotonin on morphine effects.