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

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.

Morphine preference in individual rats after morphine ingestion

Individual differences in drug intake were investigated. Inbred Sprague-Dawley male rats were choice-tested after various periods of morphine ingestion. Nearly 10% of the rats showed more than 50% preference already after 4 days ingestion on 340 mg morphine/kg/day, while a further 10% had a mean preference less than 30% over 6 days of choice, even after as long as 38 days' treatment on this same dose. High morphine preference was stable for long choice periods. It was also found that a high morphine preference level in an individual rat persisted over several choice tests, even if the animals had been without morphine for several months. The alpha 2-agonist clonidine diminished high preference to the same extent as it diminished overall morphine preference. There were no differences in food intake, body weight gain, severity of abstinence reactions, morphine serum levels, taste sensitivity tested with quinine, or learning the choice test behaviour comparing extremely high and low morphine preference rats. Thus, two subgroups of high and low morphine-ingesting rats were identified in the Sprague-Dawley strain.

A method for computer control of morphine ingestion by rats

In this paper, we describe a method for controlling the administration of liquid diet and morphine to sixteen rats using a computer. Morphine ingestion treatment was performed with 6 feeding occasions per 24 hr, all experimental animals receiving similar drug doses. The amount of drug was individual and based on body weight at each feeding occasion. Control and experimental animals were kept under isocaloric conditions. Corrections of drug doses in order to compensate for changes in body weight were made every 24 hr. Sensors registered the exact time of complete drug and diet consumption and prevented overdistribution. Rats were administered 103 mg/kg b.wt. morphine during 24 hr. In another experiment rats were administered 191 mg/kg b.wt. morphine during 48 hr, and no weight loss or decrease in fluid diet intake was registered during the time of drug administration in either of the experiments. After exclusion of morphine from the fluid diet, the body weight loss was 6.1% and 8.3%, respectively, and the liquid diet intake decreased by 12.4 ml and 13.4 ml, respectively, compared with control animal intake. This demonstrates the induction of physical drug dependence. A major advantage of using computer-aided administration of morphine-admixed, fluid diet is the stepwise, small dose increments provided several times a day, resulting in higher drug dose per unit time when compared with ingestion procedures using one feeding occasion per day. The method enables rats to rapidly ingest large morphine doses under standardized conditions.

Effects of prenatal morphine treatment of rats on mortality, bodyweight and analgesic response in the offspring

A method is presented for the maternal ingestion of morphine in rats in order to investigate morphine's prenatal effects. Effects on the offspring of the treatment throughout pregnancy were compared with effects of treatment during specific periods of the pregnancy. Morphine treatment (25 mg/kg per day) throughout pregnancy resulted in 0% survival of the offspring. Morphine treatment up to gestational day 21 resulted in 65.4 +/- 7.7% survival of the offspring, and up to gestational day 17 in 31.8 +/- 13% survival. Reduction in the neonatal birth weight was maximal in animals treated with 25 mg/kg of morphine per day from gestational day 4 throughout pregnancy. The analgesic effect of morphine in the offspring was tested with the hot plate method. Male offspring treated with morphine from gestational day 1 to day 17 showed an elevated thermal response after morphine injections on day 80-90 post partum. These findings indicate a long-lasting, post-partum alteration in that part of the opiate system involved in the perception of noxious stimuli.

Do persistent morphine effects involve interactions with the genome?

Male rats were administered morphine in a liquid diet until five days prior to mating with drug naive nulliparous female rats which received no treatment during gestation. The birth weight of resulting litters was significantly reduced. The preconceptionally morphine-treated offspring showed a 34.8% +/- 17.1% mortality during the first 8 days compared with 0% in the control group and their weight gain profile was decreased as compared with controls. A persistent effect of paternally administered morphine was seen in 90-day-old male offspring. A possible way to further enlighten the underlying mechanisms is proposed.

A possible link between morphine and alcohol preferences in rat

Sprague-Dawley rats were intoxicated with alcohol or morphine, by ingestion in a fluid diet. Alcohol or morphine preference was studied in two-bottle choice tests. Preference increased up to a certain level with increasing time of intoxication at one dose level. Some 10% of the animals had an extremely high preference already after a short ingestion period. Another 10% showed an extremely low drug preference even after long intoxication. When rats were consecutively intoxicated with morphine and alcohol, or vice versa, followed by choice tests with the respective drug, the statistical correlation between alcohol and morphine preference was weak in the individual rats. Approximately 20% of the high morphine preference rats also had a high alcohol preference. Early postnatal morphine intoxication slightly increased morphine, but not alcohol preference in the physically dependent male adult rats. Thus, there seem to be different and specific mechanisms underlying the alcohol and opiate preference in rats, although some overlapping probably exists.

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)

Stimulation of brain-stem protein synthesis by morphine

Rats were intoxicated with morphine as intraperitoneal (i.p.) single doses, or for 4 days (final dose 130 mg/kg b.w.) or for 13 days (final dose 340 mg/kg b.w.) using an ingestion method where intoxicated and control rats received the same amount of calories and fluid. The intoxicated groups showed different degrees of physical dependence, demonstrated by variously expressed abstinence symptoms after withdrawal of the drug or after administration of the opiate receptor antagonist naloxone. Soluble protein synthesis was measured in vivo in brain stem by double labelling with 3H and 14C valine and followed over time in the various rat groups after i.p. morphine injection in different doses. Protein synthesis in astroglial-enriched primary cultures from brain stem and secretion of labelled protein to the serum free incubation medium was also evaluated after morphine treatment. There were dose- and time-dependent effects of morphine on brain stem protein synthesis with an initial decrease and a later increase, 1-3 hr after a single dose of morphine administration. Following a morphine single dose of 25 mg/kg b.w. the stimulation was more rapid in onset and more pronounced in rats with a higher degree of physical dependence. Specific protein fractions including one with a subunit M.W. of approx. 80,000 were identified by electrophoretic separation of labelled proteins. Some similar protein fractions increased in synthesis and were released to the serum-free incubation medium when separating astroglial primary culture proteins after morphine treatment. It might be that the biphasic changes in protein synthesis after morphine administration underlie adaptive phenomena such as tolerance/physical dependence development and that some of the identified proteins including proteins synthesized in astroglial cells and secreted to the incubation media participate in these processes.

Neurotransmitter uptake in various brain regions of chronically morphinized rats

Rats were long-term morphine-intoxicated by a fluid-diet model ensuring an equal nutrient intake in morphinized and control rats. Uptake of neurotransmitters and D-ala2-met5-enkephalinamide (Enk) was studied in the particulate fractions (obtained at 10,000 g) from well defined brain regions of long-term intoxicated and morphine withdrawn rats. In control animals the accumulation of [3H]glutamate and [3H]gamma-aminobutyric acid (GABA) reached the highest tissue/medium (T/M) ratio values, 30-120, in the regions studied while monoamine T/M values were 2-10. No active uptake of [3H]Enk could be demonstrated. Striatum showed the most evident modifications in neurotransmitter uptake. In this region the equilibrium T/M ratio for [3H]glutamate and [3H]GABA was significantly lower in intoxicated rats versus controls. Moreover, the T/M ratio for [3H]5-hydroxytryptamine (5-HT) was lower, while that for [3H]dopamine (DA) was higher in abstinent rats in comparison with the controls. [3H]glutamate and [3H]GABA uptakes were also significantly lower, respectively, in frontal cortex, hippocampus and brain stem in intoxicated rats, while [3H]5-HT uptake was significantly lower in hypothalamus after morphine withdrawal. The possible involvement of the endogenous opioid system in the etiology of the alterations is discussed.

Amino acid incorporation during morphine intoxication. I: Dose and time effects of morphine on protein synthesis in specific regions of the rat brain and in astroglia-enriched primary cultures

Uptake and incorporation of 3H-valine into soluble protein were studied in normal and in physically dependent rat brain, or in astroglial primary cultures at various times after the administration of morphine at different doses. There was an increased protein synthesis in striatum and brain stem of previously untreated rats 2-3 hr after low-dose morphine administration (10 mg/kg bw IP). The changes were completely reversed by naloxone (5 mg/kg bw, IP) administered 10 min prior to the morphine. During the first hour after IP administration of 25 mg morphine/kg bw there was a decrease in striatum and brain stem protein synthesis, followed by an increase in brain stem protein synthesis between 2 and 3 hr later. These changes were blocked by naloxone. In all brain regions studied 40 mg morphine/1 kg bw, IP inhibited protein synthesis, an effect partially reversed by naloxone (5 mg/kg bw, IP). Similar dose- and time-dependent effects were obtained from primary astroglia-enriched cultures from cerebral hemispheres, suggesting that low or moderate doses of morphine affect brain cell protein synthesis. At least one phase of increased synthesis was seen, but within a few hours of a high morphine dose the protein synthesis was decreased. After long-term morphine intoxication (13 days; final dose 340 mg/kg bw/day po) incorporation of 3H-valine in vivo into TCA-precipitable soluble proteins during 60 min decreased in the hypothalamus and in the occipital and entorhinal cortex, and was unchanged in other brain regions examined. An IP injection of morphine (10 or 25 mg/kg bw/day) to such animals resulted in a pronounced and rapidly occurring increase in protein synthesis in many brain regions. This was further demonstrated with gel electrophoresis in brain stem and hypothalamus, where higher 3H-labeling was seen in many protein bands. The labeling of one band from brain stem having approx 80,000 MW was especially pronounced. Incorporation of 3H-valine into membrane-bound proteins of brain stem changed in a similar way to the soluble proteins, but was not as pronounced.

Decreased morphine intake by opiate addicted rats administered zimelidine, a 5-HT uptake inhibitor

Zimelidine, a specific 5-HT uptake inhibitor, reduced peroral morphine consumption by morphine-addicted adult male and female Sprague-Dawley rats and old male rats in choice tests. The effect was dose dependent in male rats. Thus, the availability of central 5-HT appears to be important for the regulation of morphine preference in rat. The results are discussed in relation to recent literature where ethanol preference has been found to be attenuated by zimelidine. The results may provide insights into the complex cellular mechanisms underlying opiate addiction.

Macromolecular changes in brain stem of morphinized rats

Incorporation of [3H]valine into trichloroacetic acid-(TCA)-precipitable, water-soluble or membrane-bound material of whole brain and brain-stem did not differ significantly in morphine-intoxicated, morphine abstinent and control rats. The animals were intoxicated with morphine (final dose 340 mg/kg b.w.) for 15 days, using an ingestion method with no impairment of the caloric intake compared to controls. Abstinent rats were withdrawn from morphine for 2 days after 13 days of intoxication. Measurements of [3H]valine or [14C]valine incorporated into soluble or membrane-bound brain stem proteins failed to demonstrate any significant changes in specific protein bands from morphinized rats. Separation was achieved by polyacrylamide gel electrophoresis with or without sodium-dodecyl sulphate (SDS) or by isoelectric focusing. After immunoabsorption chromatography to remove those proteins antigenically similar to serum proteins, an increase in the staining intensity and in incorporation of [3H]valine into two protein bands (with isoelectric points (Ip:s) 5.75 and 7.7) was seen in brain stem from long-term morphine-intoxicated rats. The results show that macromolecular interactions are involved in long-term morphine actions.