Brain serotonin turnover and morphine tolerance-dependence induced by multiple injections in the rat

Central serotonergic neurons are differentially required for opioid analgesia but not for morphine tolerance or morphine reward

Opioids remain the most effective analgesics despite their potential adverse effects such as tolerance and addiction. Mechanisms underlying these opiate-mediated processes remain the subject of much debate. Here we describe opioid-induced behaviors of Lmx1b conditional knockout mice (Lmx1bf/f/p), which lack central serotonergic neurons, and we report that opioid analgesia is differentially dependent on the central serotonergic system. Analgesia induced by a kappa opioid receptor agonist administered at the supraspinal level was abolished in Lmx1bf/f/p mice compared with their wild-type littermates. Furthermore, compared with their wild-type littermates Lmx1bf/f/p mice exhibited significantly reduced analgesic effects of mu and delta opioid receptor agonists at both spinal and supraspinal sites. In contrast to the attenuation in opioid analgesia, Lmx1bf/f/p mice developed tolerance to morphine analgesia and displayed normal morphine reward behavior as measured by conditioned place preference. Our results provide genetic evidence supporting the view that the central serotonergic system is a key component of supraspinal pain modulatory circuitry mediating opioid analgesia. Furthermore, our data suggest that the mechanisms of morphine tolerance and morphine reward are independent of the central serotonergic system.

Morphine withdrawal alters anterior pituitary hormone secretion, brain endopeptidase activity and brain monoamine metabolism in the rat

Rats were made tolerant to morphine by a 5-day regimen with increasing doses. The time course of changes in serum anterior pituitary hormone levels, brain endo- and exopeptidase activity, levels of brain biogenic amines and body weight were studied during abrupt morphine withdrawal. Cold stimulated secretion of thyrotropin and the secretion of growth hormone were both decreased whereas that of prolactin was increased. In the hypothalamus both prolyl endopeptidase and dipeptidyl peptidase IV activities were concomitantly increased. The hypothalamic 5 hydroxyindole acetic acid levels were also increased. Changes in hormone secretion, peptidase activity and monoamine turnover had returned to baseline levels by 92 hr. Our results indicate that morphine withdrawal and the associated stress produce alterations in anterior pituitary thyrotropin and growth hormone secretion. Concomitant increases in hypothalamic prolyl endopeptidase and dipeptidyl peptidase activities may contribute to these changes.

Modification of the characteristics of dopamine transporter in brain regions and spinal cord of morphine tolerant and abstinent rats

The specific binding of [3H]GBR 12935 to crude synaptosomal membranes of brain regions and spinal cord of morphine tolerant and abstinent rats was investigated. Male Sprague-Dawley rats were implanted with 6 morphine pellets each containing 75 mg of morphine base during a 7-day period. Placebo pellet implanted rats served as controls. Rats sacrificed without removal of the pellet were considered tolerant whereas those from which pellets were removed 16 hr prior to sacrificing were labeled abstinent. The binding of [3H]GBR 12935 was initially determined at a 1 nM concentration in all brain regions and spinal cord, which was followed by the determination of Bmax and Kd values in the corpus striatum, a highly enriched region for the dopamine transporter. In morphine tolerant rats, the binding of [3H]GBR 12935 was increased in the hypothalamus (182%) but was decreased in the corpus striatum (34%) and spinal cord (30%). The decrease in binding in the corpus striatum was due to an increase in the Kd value of [3H]GBR 12935. However, during morphine withdrawal, the binding of [3H]GBR 12935 was still higher in the hypothalamus (255%) but was decreased in the hippocampus (53%). Thus, chronic administration of morphine results in changes in the dopamine transporter function in selected brain regions and the spinal cord, and these changes are dependent upon whether or not the animals are undergoing the abstinence syndrome.

Simultaneous measurement of biogenic amines and their metabolites in rat brain regions after acute administration of and abrupt withdrawal from butorphanol or morphine

Changes in levels of biogenic amines and metabolites were measured using high performance liquid chromatography fitted with an electrochemical detection in various rat brain regions after acute administration of and abrupt withdrawal from continuous intracerebroventricular infusion of butorphanol (a mu/delta/kappa mixed opioid receptor agonist) or morphine (a mu-opioid receptor agonist). A single dose of butorphanol (26 nmol/5 microliters) or morphine (26 nmol/5 microliters) increased levels of 3,4-dihydroxyphenylacetic acid in the striatum and limbic region and of homovanilic acid in the cortex, striatum, and limbic region. In animals which had been infused with butorphanol (26 nmol/microliters/hr) or morphine (26 nmol/microliters/hr) for 3 days, an increase in dopamine turnover was observed. The levels of 3,4-dihydroxyphenylacetic acid was decreased and that of homovanilic acid was increased in the striatum, limbic region, and midbrain immediately after termination of opioid infusion. Both dopamine metabolites (in these areas) were decreased at 2 and 6 hr after butorphanol or morphine withdrawal. Changes in norepinephrine, serotonin, and 5-hydroxyindoleacetic acid levels in some brain regions were observed in the morphine-, but not in butorphanol-dependent rats. These data suggest that the increase and the decrease in dopaminergic activity, but not noradrenergic and serotonergic neurons, in the some brain regions are closely associated with the production of antinociception of and the expression of withdrawal syndrome from butorphanol and morphine, respectively.

Evidence for the behavioral supersensitivity of dopamine D2 receptors without receptor up-regulation in morphine-abstinent rats

The effect of morphine tolerance-dependence and abstinence on the characteristics of dopamine D2 receptors in brain regions and spinal cord was determined in the rat. Male Sprague-Dawley rats were implanted s.c. under light ether anesthesia with 6 morphine pellets for a 7-day period, each containing 75 mg of morphine free base. Rats implanted with placebo pellets served as controls. This procedure resulted in the development of tolerance to morphine as evidenced by decreased analgesic response to a challenge dose of morphine. Similarly, the development of physical dependence was evidenced by a decrease in body weight and colonic temperature after morphine pellet removal (withdrawal). The binding characteristics (Bmax and Kd values) of [3H]spiroperidol to dopamine D2 receptors were determined in the tissues of morphine-tolerant and morphine-abstinent rats. In the tolerant rats, the pellets were left intact at the time of sacrificing, whereas, in the abstinent rats the pellets were removed 18 h prior to sacrificing. The binding of [3H]spiroperidol was determined in membranes prepared from brain regions (hypothalamus, hippocampus, cortex, pons and medulla, midbrain, corpus striatum and amygdala) and spinal cord of rats from various treatment groups. [3H]Spiroperidol bound to brain regions and spinal cord at a single high affinity site. The Bmax or the Kd values in brain regions and spinal cord of morphine-tolerant and -abstinent rats did not differ from their respective placebo controls. The behavioral responses to a selective dopamine D2 receptor agonist, 2-bromo-alpha-ergocryptine were also determine in the morphine-abstinent rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple opiate receptors of brain and spinal cord in opiate addiction

1. Chronic administration of opiates to rodents results in the development of tolerance to their pharmacological effects. Physical dependence also develops and is shown by the appearance of abstinence syndrome. 2. Opiates produce their effects by acting on three types of opiate receptors, namely mu, delta and kappa. The qualitative and quantitative aspects of the tolerance-dependence and abstinence symptoms observed after chronic administration of agonists acting at mu-, delta- and kappa-opiate receptors appear to differ. 3. Tolerance-dependence on mu-opiate agonists, such as morphine, is associated with down-regulation of mu-opiate receptors in spinal cord and specific areas of the brain but delta- and kappa-opiate receptors are unchanged. During abstinence from mu-opiate agonists, brain and spinal cord mu-, delta- and kappa-opiate receptors are unaffected. 4. Chronic administration of kappa-opiate agonists, such as U-50,488H, results in the development of tolerance to its pharmacological effects and a mild degree of physical dependence. Such changes are associated not only with alterations of delta and kappa opiate receptors in brain and spinal cord, but also primarily with a down-regulation of kappa-opiate receptors in spinal cord and specific brain regions. mu-Opiate receptors are unaffected. 5. Chronic administration of delta-opiate agonists results in down-regulation of brain delta-opiate receptors. 6. It is concluded that tolerance-dependence on mu-, delta- and kappa-opiate receptors is associated with down-regulation of their own type of receptors in the spinal and supraspinal areas. Abstinence, on the other hand, does not alter brain and spinal cord opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of U-50,488H, a kappa-opiate receptor agonist on tolerance to the analgesic and hyperthermic effects of morphine in the rat

1. The effect of intraperitoneal injections of U-50,488H, a kappa-opiate receptor agonist, on the development of tolerance to the analgesic and hyperthermic effects of morphine was determined in male Sprague-Dawley rats. 2. Tolerance was induced by implantation of four morphine pellets during a 3-day period (4/3 schedule) or six morphine pellets during a 7-day period (6/7 schedule). 3. Administration of U-50,488H (25 mg/kg, twice a day for 3 days in 4/3 schedule) or (5, 10 and 20 mg/kg twice a day for 7 days in 6/7 schedule) did not affect the development of tolerance to the pharmacological actions of morphine. 4. It is concluded that activation of kappa-opiate receptors does not modify the development of tolerance to morphine in the rat.

Down-regulation of hypothalamic 5-HT1A receptors in morphine-abstinent rats

The effects of morphine tolerance-dependence and abstinence on 5-HT1A receptors in brain regions and spinal cord of the rat were determined. Tolerance to and physical dependence on morphine was induced in male Sprague-Dawley rats by implanting six morphine pellets (each containing 75 mg of morphine free base) during a seven day period. Two groups of rats were used for binding studies. In one group the pellets were left intact (tolerant-dependent) and in the other they were removed (abstinent). Rats were killed, and spinal cords and brains were excised. Brain was dissected into seven regions (amygdala, hippocampus, hypothalamus, striatum, midbrain, pons + medulla and cortex). 5-HT1A receptors were characterized by using [3H]8-hydroxy-di-n-propylaminotetralin [( 3H]DPAT) as the ligand and unlabelled 5-HT to determine the non-specific binding. In morphine and placebo tolerant-dependent rats the binding of [3H]DPAT to 5-HT1A receptors in brain regions and spinal cord did not differ. The Bmax value of [3H]DPAT in the hypothalamus of morphine abstinent rats was decreased by 61.9%. No change in Bmax value was observed in other brain regions and spinal cord. The Kd values were unaffected. Subcutaneous administration of DPAT produced hypothermia in rats from which pellets had been removed. The intensity of DPAT-induced hypothermic response was greater in morphine abstinent rats as compared to placebo abstinent rats. Since DPAT is believed to have a major action on the presynaptic 5-HT neurons, it is concluded that in morphine abstinent rats 5-HT1A receptors are down-regulated in hypothalamus, but in morphine tolerant-dependent rats they are unaffected.

Brain and spinal cord 5-HT2 receptors of morphine-tolerant-dependent and -abstinent rats

The effects of morphine tolerance-dependence and abstinence on 5-HT2 receptors in brain and spinal cord of the rat were determined. Tolerance to and physical dependence on morphine was induced in male Sprague-Dawley rats by implanting six morphine pellets (each containing 75 mg of morphine free base) during a seven day period. Two groups of rats were used for binding studies. In one group the pellets were left intact and in the other they were removed. The rats were killed and spinal cords and brains were excised and dissected into six regions (amygdala, hippocampus, hypothalamus, striatum, midbrain and pons + medulla). 5-HT2 receptors were characterized by using [3H]spiperone as the ligand and unlabelled ketanserin to determine the non-specific binding. In morphine and placebo abstinent rats the binding of [3H]spiperone to 5-HT2 receptors in brain regions and spinal cord did not differ. The Bmax values of [3H]spiperone to bind to membranes prepared from non-abstinent morphine-dependent rats were increased in amygdala (78.0%), midbrain (65.0%) and pons + medulla (92.0%). The Kd values were unaffected. It is concluded that in morphine-tolerant-dependent rats 5-HT2 receptors are up-regulated in amygdala, midbrain and pons + medulla, but in morphine-abstinent rats they are unaffected.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents: II

This survey continues a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1979, but data from many earlier papers are also tabulated.

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.

Opiate receptors, food intake and obesity

The present studies tested the effect of acute and chronic administration of naloxone on food intake of lean and genetically obese (ob/ob) mice. Acute administration of naloxone, a drug which blocks opiate receptors, produced a greater reduction of food intake in obese (ob/ob) mice than in the lean littermates. For chronic experiments with naloxone, the daily feeding period was shortened to eight hours and two injections of naloxone were given four hours apart. With this procedure of scheduled-feeding the food intake of both lean and obese mice was depressed during the first hour after injecting naloxone. However, beginning on the second day of treatment, the lean mice began to eat more food than the untreated controls during the eight hour feeding period. Food consumption by lean mice reached values 140 to 200% above the control levels between the fourth and sixth day. In the obese mice the rise in food intake was more gradual and did not reach 200% of the control value until the sixth day. Body weight changes reflected the changes in food intake. In contrast to naloxone, chronic treatment with morphine lowered food intake and blocked the stimulatory effect of naloxone. Our findings suggest that endogenous opioids may play a role in signalling satiety and in regulating long-term energy balance.

The relationship between morphine analgesia and the activity of bulbo-spinal serotonergic system as studied by tolerance phenomenon

The effect of various doses of acute morphine on both analgesia and 5-hydroxytryptamine (5-HT) synthesis in the brain and the spinal cord has been studied in rats rendered tolerant by chronic administration of the analgesic. In morphine-tolerant rats, the incorporation of tritiated-L-tryptophan (TRP) in the brain and the spinal cord was higher than in non-tolerant rats, but there was no significant difference in the synthesis rate of the newly formed 5-HT between the two groups. An acute dose of morphine (10 mg/kg) which induced a powerful analgesia and a large increase in 5-HT synthesis in non-tolerant rats, did not produce analgesia nor changes in 5-HT synthesis in tolerant rats. Higher acute doses of morphine which restored analgesia in tolerant rats, induced a discrete increase in [3H]TRP incorporation and a marked increase in 5-HT synthesis in the spinal cord of these animals. The same doses significantly increased [3H]TRP incorporation in the forebrain but did not modify 5-HT synthesis. These results show that tolerance to morphine is associated with a decrease in the effects of the drug on 5-HT synthesis in the spinal cord and the brain and tend further support to the hypothesis that an enhancement of 5-HT synthesis in the spinal cord, induced independently of modifications of the availability of TRP, is associated with the analgesic effect of morphine.

The role of liver tryptophan pyrrolase in the opposite effects of chronic administration and subsequent withdrawal of drugs of dependence on rat brain tryptophan metabolism

1. Chronic administration of morphine, nicotine or phenobarbitone has previously been shown to inhibit rat liver tryptophan pyrrolase activity by increasing hepatic [NADPH], whereas subsequent withdrawal enhances pyrrolase activity by a hormonal-type mechanism. 2. It is now shown that this enhancement is associated with an increase in the concentration of serum corticosterone. 3. Chronic administration of the above drugs enhances, whereas subsequent withdrawal inhibits, brain 5-hydroxytryptamine synthesis. Under both conditions, tryptophan availability to the brain is altered in the appropriate direction. 4. The chronic drug-induced enhancement of brain tryptophan metabolism is reversed by phenazine methosulphate, whereas the withdrawal-induced inhibition is prevented by nicotinamide. 5. The chronic morphine-induced changes in liver [NADPH], pyrrolase activity, tryptophan availability to the brain and brain 5-hydroxytryptamine synthesis are all reversed by the opiate antagonist naloxone. 6. It is suggested that the opposite effects on brain tryptophan metabolism of chronic administration and subsequent withdrawal of the above drugs of dependence are mediated by the changes in liver tryptophan pyrrolase activity. 6. Similar conclusions based on similar findings have previously been made in relation to chronic administration and subsequent withdrawal of ethanol. These findings with all four drugs are briefly discussed in relation to previous work and the mechanism(s) of drug dependence.

Neurotransmitter turnover in rat striatum is correlated with morphine self-administration

Drugs of abuse probably exert their reinforcing effects through 'reward' pathways in the central nervous system (CNS). Neuronal systems mediating opiate reinforcement have been investigated using pharmacological and electrolytic lesion procedures. Drugs that interfere with catecholaminergic and cholinergic neuronal activity decrease intravenous (i.v.) morphine self-administration in monkeys and rats. Electrolytic lesion procedures in rats have demonstrated that the medial forebrain bundle and caudate nucleus are important in maintaining i.v. morphine self-administration. We have now carried out a direct investigation of striatal (caudate nucleus, putamen and globus pallidus) neuronal systems. We show here that striatal catecholaminergic systems are important in mediating opiate reinforcement, and present direct evidence for the involvement of neurotransmitter systems in morphine reward.

The effects of thyrotropin-releasing hormone on the central nervous system responses to chronic morphine administration

The effects of thyrotropin-releasing hormone (TRH) on abrupt and naloxone-precipitated abstinence symptoms were determined in male Swiss-Webster mice rendered dependent on morphine by SC implantation of morphine pellets. Intracerebral (IC) administration of TRH inhibited the hypothermic response observed during abrupt (removal of morphine pellets) and naloxone (0.1 mg/kg SC) precipitated withdrawal. IC injection of TRH also inhibited the naloxone-precipitated withdrawal jumping response as evidenced by increases in the dose of naloxone required to elicit the response. The effects of TRH on the development of morphine dependence were also investigated. A single SC injection of TRH (4-16 mg/kg) did not modify development of morphine dependence. Administration of TRH prior to and during morphine pellet implantation inhibited the development of dependence as evidenced by inhibition in the development of abrupt and naloxone-induced withdrawal hypothermia. Even though the hypothermic response was blocked, multiple SC administration of TRH failed to modify naloxone-induced stereotyped jumping response. These studies indicate that TRH administration can modify central nervous system responses to chronic morphine treatment and that separate sites may initiate withdrawal jumping behavior and affect temperature regulation during abrupt and antagonist-induced abstinence.

Increased weight gain as a morphine withdrawal response in rats

Adult male Wistar rats injected daily with 20 or 200 mg/kg morphine-SO4 for 35 days suffered a dose-dependent weight loss over the first 3 days of morphine withdrawal. However, during the next 28 days they gained weight more rapidly than controls, the rates being related to the previous morphine dosage. These findings were replicated in Sprague-Dawley rats treated for 26 days with 60 mg/kg morphine. Food-restricted controls suffering weight losses equal to those of the morphine-treated or morphine-withdrawn groups did not subsequently gain weight as rapidly as the latter groups. Therefore the rapid post-withdrawal weight gain may be a true adaptive response to the weight suppressing effects of morphine. Also, comparisons of weight changes during treatment in the two experiments indicated possible strain differences for tolerance to morphine's direct weight-reducing effect.

The synthesis rate and turnover time of 5-hydroxy-tryptamine in brains of rats treated chronically with morphine

1. Four schedules of subcutaneous pellet implantation were used to induce tolerance to and physical dependence on morphine in Sprague-Dawley rats. 2. The schedules included implantation of four morphine pellets (each containing 75 mg of morphine free base) during a 3 day period (schedule 1); six pellets during 3 days (schedule 2); six pellets during 7 days (schedule 3) and ten pellets during a 10 day period (schedule 4). 3. A high degree of tolerance and dependence on morphine, comparable to that induced in mouse by implantation of a single morphine pellet for 3 days, was produced with schedule 4. 4. Brain 5-hydroxytryptamine (5-HT) turnover rates as measured by rate of accumulation of 5-HT after monoamine oxidase inhibition by pargyline were not different in rats rendered tolerant to and dependent on morphine according to schedules 1 to 4 when compared with corresponding placebo pellet-implanted rats. 5. The turnover rates of 5-HT in brain of morphine-and placebo pellet-implanted rats (schedule 4) from which the pellets had been removed for 24 h were also similar. 6. It is concluded that tolerance to, and physical dependence upon morphine in the rat is not associated with changes in brain 5-HT dynamics.

The effects of naltrexone on the development of physical dependence on morphine

A single i.p. injection of naltrexone (20 mg/kg) partially inhibited the development of physical dependence upon morphine in mice rendered dependent on morphine by implantation of a pellet containing 75 mg of morphine free base for three days. This was evidenced by an increase in the dose of naloxone (ED50) required to precipitate withdrawal jumping response. The increase in naloxone ED50 was much more pronounced when naltrexone was given prior to and during the course of pellet implantation. Inhibition was also observed when naltrexone was administered one day after the morphine pellet implantation, i.e., after some dependence had already developed. Naltrexone administration prior to and during the development of dependence also inhibited, but only partially, the loss of body weight and hypothermic response observed during abrupt withdrawal of morphine in morphine-dependent mice. The inhibitory effect of naltrexone on morphine dependence development was not associated with changes in brain morphine concentration.