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

5-HT2A receptor- and M1 muscarinic acetylcholine receptor-mediated activation of Gαq/11 in postmortem dorsolateral prefrontal cortex of opiate addicts

BACKGROUND

Chronic exposure to opiates causes the development of tolerance and physical dependence as well as persistent brain neuroplasticity. Despite a wealth of postmortem human studies for opiate addicts, little direct information regarding the functional status of serotonergic and cholinergic receptor-mediated signaling pathways in the human brain of opiate addicts is yet available.

METHODS

Functional activation of Gα_q/11 proteins coupled to 5-HT_2A and M₁ type muscarinic acetylcholine receptor (mAChR) was assessed by using the method named [³⁵S]GTPγS binding/immunoprecipitation in frontal cortical membrane preparations from postmortem human brains obtained from opiate addicts and matched controls.

RESULTS

Concentration-response curves for 5-HT and carbachol in individual subjects were analyzed according to a nonlinear regression model, which generated the values of maximum percent increase (%E_max), negative logarithm of the half-maximal effect (pEC₅₀) and slope factor. As for 5-HT_2A receptor-mediated Gα_q/11 activation, the %E_max values were reduced significantly and the pEC₅₀ values were decreased significantly in opiate addicts as compared to the control group. Regarding carbachol-induced Gα_q/11 activation, no significant difference in %E_max or pEC₅₀ values was detected between the both groups, whereas the slope factor was increased significantly in opiate addicts as compared to the control group.

CONCLUSION

Our data demonstrate that the signaling pathways mediated by Gα_q/11 proteins coupled with 5-HT_2A receptors and M₁ mAChRs in prefrontal cortex are functionally altered in opiate addicts in comparison with control subjects. These alterations may underpin some aspects of addictive behavior to opiate as well as neuropsychological consequences or comorbid mental disorders associated with opioid use.

mRNA Levels of MAOA and 5-HT 2 A Receptor in Patients With Pathological Internet Use: Correlations With Comorbid Symptoms

Objective: Uncontrolled internet use may lead to the emergence of pathological internet use (PIU). PIU has become a global public health concern that can cause a range of psychotic symptoms, including anxiety, depression, and impulse control disorder. To date, we know very little about the principal biological factors related to PIU. Monoamine oxidase type A (MAOA) and serotonin (5-HT) 5-HT_2A receptor (5-HT_2AR) play critical roles in the development of behavioural and drug addictions. Thus, the aim of this study was to measure the relative expression of mRNA of MAOA and 5-HT_2AR in peripheral blood mononuclear cells (PBMCs) of patients with PIU and to determine the correlations between these biological indicators and the comorbid symptoms of patients with PIU. Methods: In this study, the mRNA of MAOA and 5-HT_2AR was detected using real-time PCR in PBMCs of the patients with PIU (n = 24) and healthy controls (HCs, n = 25). The relationship between the mRNA levels of MAOA and 5-HT_2AR and clinical symptoms in patients with PIU was further investigated. Results: MAOA mRNA in PBMCs was significantly upregulated in patients with PIU compared with that in HCs. mRNA levels of 5-HT_2AR were not found to differ significantly between HCs and patients with PIU. Correlation analyses further revealed a significant positive correlation between the relative expression of MAOA mRNA in PBMCs of patients with PIU and the Young's Internet Addiction Test and Self-Rating Depression Scale scores. Conclusion: The present study revealed upregulated expression of MAOA mRNA in patients with PIU and an association between the expression of MAOA mRNA and clinical symptoms of PIU, suggesting that the neurobiological changes may be similar between PIU and substance addiction. Additionally, this study demonstrated a potential association between comorbid symptoms and mRNA levels of MAOA.

Palonosetron and hydroxyzine pre-treatment reduces the objective signs of experimentally-induced acute opioid withdrawal in humans: a double-blinded, randomized, placebo-controlled crossover study

BACKGROUND

Treatments for reducing opioid withdrawal are limited and prone to problematic side effects. Laboratory studies, clinical observations, and limited human trial data suggest 5-HT3-receptor antagonists and antihistamines may be effective.

OBJECTIVES

This double-blind, crossover, placebo-controlled study employing an acute physical dependence model evaluated whether (i) treatment with a 5-HT3-receptor antagonist (palonosetron) would reduce opioid withdrawal symptoms, and (ii) co-administration of an antihistamine (hydroxyzine) would enhance any treatment effect.

METHODS

At timepoint T = 0, healthy (non-opioid dependent, non-substance abuser) male volunteers (N = 10) were pre-treated with either a) placebo, b) palonosetron IV (0.75 mg), or c) palonosetron IV (0.75 mg) and hydroxyzine PO (100 mg) in a crossover study design. This was followed at T = 30 by intravenous morphine (10 mg/70kg). At T = 165, 10 mg/70kg naloxone IV was given to precipitate opioid withdrawal. The objective opioid withdrawal score (OOWS) and subjective opioid withdrawal score (SOWS) were determined 5 and 15 minutes after naloxone administration (T = 170, 180, respectively). Baseline measurements were recorded at T = -30 and T = -15.

RESULTS

Comparison of average baseline OOWS scores with OOWS scores obtained 15 minutes after naloxone was significant (p = 0.0001). Scores from 15 minutes post-naloxone infusion showed significant differences in OOWS scores between treatment groups: placebo, 3.7 ± 2.4; palonosetron, 1.5 ± 0.97; and palonosetron with hydroxyzine, 0.2 ± 0.1333.

CONCLUSIONS

Pretreatment with palonosetron significantly reduced many signs of experimentally-induced opioid withdrawal. Co-administration with hydroxyzine further reduced opioid withdrawal severity. These results suggest that 5-HT3 receptor antagonists, alone or in combination with an antihistamine, may be useful in the treatment of opioid withdrawal.

5-HT2A Serotonin Receptor Density in Adult Male Rats' Hippocampus after Morphine-based Conditioned Place Preference

INTRODUCTION

A close interaction exists between the brain opioid and serotonin (5-HT) neurotransmitter systems. Brain neurotransmitter 5-HT plays an important role in the regulation of reward-related processing. However, a few studies have investigated the potential role of 5-HT2A receptors in this behavior. Therefore, the aim of the present study was to assess the influence of morphine and Conditioned Place Preference (CPP) on the density of 5-HT2A receptor in neurons of rat hippocampal formation.

METHODS

Morphine (10 mg/kg, IP) was injected in male Wistar rats for 7 consecutive days (intervention group), but control rats received just normal saline (1 mL/kg, IP). We used a hotplate test of analgesia to assess induction of tolerance to analgesic effects of morphine on days 1 and 8 of injections. Later, two groups of rats were sacrificed one day after 7 days of injections, their whole brains removed, and the striatum and PFC immediately dissected. Then, the NR1 gene expression was examined with a semi-quantitative RT-PCR method.

RESULTS

Our data showed that the maximum response was obtained with 2.5 mg/kg of morphine. The density of 5-HT2A receptor in different areas of the hippocampus increased significantly at sham-morphine and CPP groups (P<0.05). On the other hand, the CPP groups had more 5-HT2A receptors than sham-morphine groups and also the sham-morphine groups had more 5-HT2A receptors than the control groups.

CONCLUSION

We concluded that the phenomenon of conditioned place preference induced by morphine can cause a significant increase in the number of serotonin 5-HT2A receptors in neurons of all areas of hippocampus.

The role of serotonin 5-HT2A receptors in memory and cognition

Serotonin 5-HT_2A receptors (5-HT_2ARs) are widely distributed in the central nervous system, especially in brain region essential for learning and cognition. In addition to endogenous 5-HT, several hallucinogens, antipsychotics, and antidepressants function by targeting 5-HT_2ARs. Preclinical studies show that 5-HT_2AR antagonists have antipsychotic and antidepressant properties, whereas agonist ligands possess cognition-enhancing and hallucinogenic properties. Abnormal 5-HT_2AR activity is associated with a number of psychiatric disorders and conditions, including depression, schizophrenia, and drug addiction. In addition to its traditional activity as a G protein-coupled receptor (GPCR), recent studies have defined novel operations of 5-HT_2ARs. Here we review progress in the (1) receptor anatomy and biology: distribution, signaling, polymerization and allosteric modulation; and (2) receptor functions: learning and memory, hallucination and spatial cognition, and mental disorders. Based on the recent progress in basic research on the 5-HT_2AR, it appears that post-training 5-HT_2AR activation enhances non-spatial memory consolidation, while pre-training 5-HT_2AR activation facilitates fear extinction. Further, the potential influence that 5-HT_2AR-elicited visual hallucinations may have on visual cue (i.e., landmark) guided spatial cognition is discussed. We conclude that the development of selective 5-HT_2AR modulators to target distinct signaling pathways and neural circuits represents a new possibility for treating emotional, neuropsychiatric, and neurodegenerative disorders.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Opiate-induced changes in brain adenosine levels and narcotic drug responses

We have very little information about the metabolomic changes that mediate neurobehavioral responses, including addiction. It was possible that opioid-induced metabolomic changes in brain could mediate some of the pharmacodynamic effects of opioids. To investigate this, opiate-induced brain metabolomic responses were profiled using a semi-targeted method in C57BL/6 and 129Sv1 mice, which exhibit extreme differences in their tendency to become opiate dependent. Escalating morphine doses (10-40 mg/kg) administered over a 4-day period selectively induced a twofold decrease (p<0.00005) in adenosine abundance in the brainstem of C57BL/6 mice, which exhibited symptoms of narcotic drug dependence; but did not decrease adenosine abundance in 129Sv1 mice, which do not exhibit symptoms of dependence. Based on this finding, the effect of adenosine on dependence was investigated in genetically engineered mice with alterations in adenosine tone in the brain and in pharmacologic experiments. Morphine withdrawal behaviors were significantly diminished (p<0.0004) in genetically engineered mice with reduced adenosine tone in the brainstem, and by treatment with an adenosine receptor(1) (A(1)) agonist (2-chloro-N6-cyclopentyladenosine, 0.5mg/kg) or an A(2a) receptor (A(2a)) antagonist (SCH 58261, 1mg/kg). These results indicate that adenosine homeostasis plays a crucial role in narcotic drug responses. Opiate-induced changes in brain adenosine levels may explain many important neurobehavioral features associated with opiate addiction and withdrawal.

Morphine or its withdrawal affects plasma malondialdehyde, vitamin E levels and absence or presence of abstinence signs in rats

Objectives

Various experimental observations show that morphine treatment generates reactive oxygen species, and that its discontinuation leads to signs of withdrawal. We therefore investigated plasma malondialdehyde and vitamin E levels under both conditions to verify the occurrence of any alterations in oxidative metabolism, and whether these are associated with behavioural changes.

Methods

We investigated the effects of morphine or morphine plus naloxone on plasma malondialdehyde, vitamin E levels and withdrawal signs such as jumping, wet dog shakes and faecal excretion in rats. Furthermore, isopropylnoradrenaline was injected in rabbits to verify its effects on plasma malondialdehyde levels.

Key findings

Morphine treatment increased free malondialdehyde and decreased vitamin E levels. The elevation in malondialdehyde levels were exacerbated by the abrupt removal of morphine by naloxone, which also led to the appearance of withdrawal signs. The increased malondialdehyde values can be attributed to the interactions of reactive oxygen species with unsaturated fatty acids, and the lowered levels of vitamin E to its interactions with reactive oxygen species.

Conclusions

A connection seems to exist between altered peroxide status and withdrawal signs in abstinent animals.

From mouse to man: the 5-HT3 receptor modulates physical dependence on opioid narcotics

OBJECTIVES

Addiction to opioid narcotics represents a major public health challenge. Animal models of one component of addiction, physical dependence, show this trait to be highly heritable. The analysis of opioid dependence using contemporary in-silico techniques offers an approach to discover novel treatments for dependence and addiction.

METHODS

In these experiments, opioid withdrawal behavior in 18 inbred strains of mice was assessed. Mice were treated for 4 days with escalating doses of morphine before the administration of naloxone allowing the quantification of opioid dependence. After haplotypic analysis, experiments were designed to evaluate the top gene candidate as a modulator of physical dependence. Behavioral studies as well as measurements of gene expression on the mRNA and protein levels were completed. Finally, a human model of opioid dependence was used to quantify the effects of the 5-HT3 antagonist ondansetron on signs and symptoms of withdrawal.

RESULTS

The Htr3a gene corresponding to the 5-HT3 receptor emerged as the leading candidate. Pharmacological studies using the selective 5-HT3 antagonist ondansetron supported the link in mice. Morphine strongly regulated the expression of the Htr3a gene in various central nervous system regions including the amygdala, dorsal raphe, and periaqueductal gray nuclei, which have been linked to opioid dependence in previous studies. Using an acute morphine administration model, the role of 5-HT3 in controlling the objective signs of withdrawal in humans was confirmed.

CONCLUSION

These studies show the power of in-silico genetic mapping, and reveal a novel target for treating an important component of opioid addiction.

On the mechanism of tolerance to morphine-induced Straub tail reaction in mice

The effect of 5-HT and opioid receptor antagonists on morphine-induced Straub tail was studied in mice. Straub tail behavior was induced by subcutaneous administration of different doses (20, 30, and 40 mg/kg) of morphine hydrochloride to mice. The effect of morphine was dose-dependent. Maximum response was obtained with 40 mg/kg of the drug. The response induced by morphine (20 and 40 mg/kg) was decreased by different doses of intraperitoneal injection of naloxone (1 and 2 mg/kg) or methysergide, mianserin, and ritanserin (1 and 2 mg/kg). The effect of morphine (40 mg/kg) was also reduced by intracerebroventricular injection of naloxone (0.4-0.8 microg/animal) or mianserin (2 and 4 microg/animal). Different groups of mice received one daily dose (50 mg/kg sc) of morphine sulfate for 3 days to develop tolerance to morphine. The Straub tail reaction induced by morphine hydrochloride (40 mg/kg) was tested on the fourth day. Naloxone injection (1 and 2 mg/kg ip) on Day 3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose of morphine hydrochloride), decreased tolerance induced to morphine. Methysergide, mianserin, or ritanserin (intraperitoneal) on Days 2 and 3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose of morphine hydrochloride), also decreased tolerance induced to morphine. Intracerebroventricular injection of either naloxone or mianserin also reduced tolerance to morphine. It is concluded that 5-HT(2) and opioid receptor mechanisms are involved in morphine-induced Straub tail reaction and tolerance induced to morphine also may be mediated through these receptors.

Effects of ondansetron administration on opioid withdrawal syndrome observed in rats

This study tested whether a 5-HT3 receptor antagonist could reverse the signs of precipitated opioid withdrawal. Rats were treated with either saline or morphine for 4 days. After the four days, half of the rats in each group received naloxone and half received saline. Each animal also received one of four doses of ondansetron (0, 1, 2 and 4 mg/kg i.p.). Administration of ondansetron to rats receiving naloxone after chronic morphine decreased the intensity of withdrawal signs such as increased defecation, jumping and wet-dog shakes, elevated the nociceptive threshold values which were decreased by precipitated withdrawal, but produced no change in urination, rectal temperature or salivation. The effects exhibited by ondansetron administration may be explained through interference of its 5-HT3 receptor antagonist activity with serotoninergic mechanisms involved in the regulation of these withdrawal symptoms. The use of this drug is thus suggested as a possible treatment of opioid withdrawal signs in heroin addicts.

Quantitative evaluation of opioid withdrawal signs in rats repeatedly treated with morphine and injected with naloxone, in the absence or presence of the antiabstinence agent clonidine

An opioid withdrawal syndrome was induced in rats by repeated morphine administration and final naloxone injection. The withdrawal causes alteration of several physiological signs. The aim of the study was to describe a quantitative opioid abstinence syndrome to validate the methodology by utilizing clonidine, a well-known antiwithdrawal agent, and propose the procedure for the screening of antiabstinence drugs. In particular, rats were treated with saline, morphine, naloxone, morphine and naloxone and four doses of clonidine (0, 0.04, 0.1, and 0.25 mg/kg orally). In rats repeatedly exposed to morphine and then injected with naloxone, signs like excretion of feces and urine, salivation, behavioral jumping and wet dog shakes, rectal temperature, and pain threshold have been observed. Consequently, the objective symptoms observed in morphine plus naloxone-treated animals have been taken as markers of opioid withdrawal. These factors have been quantitatively measured and grouped to form a standardized procedure of opioid abstinence syndrome. In addition, it is possible to observe that the antiabstinence drug clonidine exerted effects on modified physiological signs appearing in morphine-dependent rats treated with naloxone, like fecal excretion, levels of rectal temperature, latency times, salivation as well as jumping behavior. The effects exerted by clonidine in this procedure and in other methods are compared and appear to be similar. In addition, comparative observations referring to both the previous methods and the present procedure related to the type of signs studied, the modality of evaluation, and suppressive activity exerted by the antiwithdrawal agent, clonidine, are performed: the greater accuracy of the proposed method becomes apparent. Thus, this experimental model, validated by the antiabstinence agent clonidine, is proposed as a useful screen for drugs affecting opioid withdrawal syndrome.

Effects exerted by otilonium bromide administration on precipitated opioid withdrawal syndrome in rats

An opioid withdrawal syndrome was induced in rats by repeated morphine administration and final naloxone injection. The withdrawal causes alteration of several physiological signs. The aim of the study was to prevent the altered physiological profiles by utilising otilonium bromide. Morphine was administered in three daily i.p. injections for 4 days at doses of 9, 16 and 25 mg/kg (1st day), 25, 25 and 50 mg/kg (2nd day), 50, 50 and 50 mg/kg (3rd day) and 50, 50 and 100 mg/kg (4th day). Naloxone was injected (30 mg/kg) i.p. 180 min after the last morphine injection. Otilonium bromide was administered orally at 0, 2, 4 and 8 mg/kg, 120 min before the naloxone administration. Signs like faecal and urine excretion, rectal temperature and pain threshold levels, salivation, jumping and wet dog shakes were affected in different ways. Notably the administration of otilonium bromide in rats receiving morphine together with naloxone decreased the intensity of certain withdrawal symptoms, such as excretion of faeces, wet dog shake behaviour, and elevated the nociceptive threshold values. The effects exhibited by otilonium bromide administration may be explained through its calcium antagonist activity interfering with a mechanism involved in the regulation of these previously mentioned withdrawal symptoms. The use of this drug is thus suggested as a possible control of some acute opioid withdrawal signs in heroin addicts.

Quantitative opioid withdrawal signs in rats: effects exerted by clothiapine administration

An opioid withdrawal syndrome, which causes alteration of several physiological signs, was induced in rats by repeated morphine administration and final naloxone injection. The aim of this study was prevention of the altered physiological profiles by utilising clothiapine, which is capable of affecting fecal and urinary excretion, rectal temperature, pain threshold levels and salivatory behaviour. Morphine was administered in three daily intraperitoneal (ip) injections for 4 days at doses of 9, 16 and 25 mg/kg (d 1), 25, 25 and 50 mg/kg (d 2), 50, 50 and 50 mg/kg (d 3) and 50, 50 and 100 mg/kg (d 4). Naloxone was injected (30 mg/kg) ip 180 min after the last morphine injection. Clothiapine was administered orally 0.7, 2 and 6 mg/kg 2 hours before the naloxone administration. Signs such as fecal and urine excretion, rectal temperature and latency times to thermal stimulus salivation, jumping and wet dog shakes were affected in different ways by morphine, naloxone, clothiapine and combination of them. Notably the administration of clothiapine in rats receiving morphine and naloxone decreased the intensity of certain withdrawal symptoms, such as altered excretion of feces, temperature values, salivation, jumping and wet dog shakes behaviour, and elevated the nociceptive threshold values. The effects exhibited by clothiapine administration may be explained through its antimuscarinic, antiadrenergic and antidopaminergic activities interfering with the mechanisms involved in the regulation of these previously mentioned withdrawal symptoms. The use of this drug is thus suggested as a possible control of the acute phase of opioid withdrawal in heroin addicts.

Differential effects of intrahypothalamic administration of opioids on food intake in naive and tolerant rats

We investigated the effects of intrahypothalamic administrations of the opioid agonists morphine (MOR) and ketocyclazocine (KCZ) and antagonists naltrexone (NALTX) and Mr2266 on food intake (FI) during light and dark phases of the diurnal cycle, after acute or chronic administration in rats. Acute intralateral hypothalamic (LH) administration of MOR or KCZ (1 microgram/rat) enhanced FI during dark and light phases, respectively, whereas intraventromedial hypothalamic (VMH) injections resulted in moderate hyperphagia during dark phases by both mu and kappa agonists. The receptor specificity was evident from blockade of the responses to MOR or KCZ by the respective antagonists NALTX and Mr2266. After repeated administrations of MOR and KCZ, FI responses to the test dose of these agonists injected in LH were modulated in opposite directions. However, the adaptative changes in FI after intra-VMH injection of KCZ were similar to those seen with MOR. These results are discussed in light of a differential opioid receptor involvement and their possible functional interactions within the hypothalamus during food intake.

Effects of intrahypothalamic morphine and its interactions with oxytocin and vasopressin during food intake in rats

The effects of intra-hypothalamic microinjections of morphine (MOR) into the ventromedial (VMH) and lateral (LH) nucleus and its interactions with oxytocin (OXY) and vasopressin (AVP) were evaluated during food intake FI) in rats. Acute MOR clearly enhanced FI during the dark phase (6-24 h) of the diurnal cycle, while no appreciable changes in the light phase (0-6 h) FI was seen after intra-VMH or LH injections. Long term MOR administration into both VMH and LH attenuated the enhancements in FI seen after acute MOR during the dark phase, while FI was enhanced during the light phase. Pretreatment with OXY or AVP clearly antagonized such tolerance or reverse tolerance/sensitization development to the orexic effect of MOR. These results are discussed in the light of complex peptidergic interactions in the hypothalamus during ingestive behavior.

Down-regulation of N-methyl-D-aspartate (NMDA) receptors of brain regions and spinal cord of rats treated chronically with morphine

1. The effects of morphine tolerance and abstinence on the characteristics of N-methyl-D-aspartate (NMDA) receptors, labeled with [3H]MK-801, were determined in the brain regions and spinal cord of the rat. 2. Male Sprague-Dawley rats were rendered tolerant to and physically dependent on morphine by subcutaneous implantation of six morphine pellets during a 7-day period. In tolerant (non-abstinent) rats, the pellets were left intact at the time of sacrificing, whereas in the abstinent rats the pellets were removed 16 hr prior to sacrificing. 3. The binding of [3H]MK-801, an NMDA receptor antagonist, to membranes prepared from spinal cord and brain regions (cortex, striatum, amygdala, hippocampus, hypothalamus, midbrain and pons-medulla) was determined using 5 nM concentration of the ligand in the presence of 30 microM glycine and 50 microM of glutamate. 4. In non-abstinent morphine tolerant rats, the binding of [3H]MK-801 was decreased by 40 and 33% in the midbrain and spinal cord, respectively, in comparison with their placebo controls. In morphine abstinent rats, the binding of [3H]MK-801 was decreased by 42, 29 and 50% in hypothalamus, midbrain and spinal cord, respectively, in comparison with their placebo controls. The binding of [3H]MK-801 to other brain regions and spinal cord of morphine tolerant and abstinent rats did not differ from their respective placebo controls. 5. Thus, these studies demonstrate, for the first time, that in the presence of glutamate and glycine, NMDA receptors of selected brain regions and spinal cord are down-regulated in rats treated chronically with morphine.

Effect of morphine tolerance and abstinence on the binding of [3H]MK-801 to brain regions and spinal cord of the rat

The effect of chronic administration of morphine to rats on the N-methyl-D-aspartate (NMDA) receptors labeled with [3H]MK-801, a non-competitive antagonist, was determined in brain regions and spinal cord. Male Sprague-Dawley rats were rendered tolerant to and physically dependent on morphine by subcutaneous implantation of 6 morphine pellets during a 7-day period. Each pellet contained 75 mg of morphine free base. Animals serving as controls were similarly implanted with placebo pellets. This procedure resulted in the development of a high degree of tolerance and physical dependence on morphine. Two sets of rats were used. In one, the pellets were left intact at the time of sacrifice (tolerant) and in the other the pellets were removed 16 h prior to sacrificing (abstinent). The binding constants, Bmax and Kd values of [3H]MK-801 were determined in cortex, hippocampus, hypothalamus, corpus striatum, midbrain and spinal cord. In the absence of glycine and glutamate, [3H]MK-801 bound to tissue membranes at a single high affinity site. The Bmax and Kd values of [3H]MK-801 were not altered in any of the tissues of the morphine abstinent rats. The Bmax value of [3H]MK-801 was significantly decreased in cerebral cortex of morphine tolerant rats as compared to their placebo controls but the Kd values did not change. In other brain regions and spinal cord of morphine tolerant rats and their placebo controls, the Bmax and Kd values of [3H]MK 801 did not differ.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of naltrexone pellet implantation on morphine tolerance and physical dependence in the rat

1. The effect of naltrexone pellets containing either 10 or 30 mg of naltrexone base on the development of tolerance and physical dependence on morphine was assessed in male Sprague-Dawley rats. Tolerance-dependence on morphine was induced by s.c. implantation of six morphine pellets, each containing 75 mg morphine base for 7 days. 2. Naltrexone pellet implantation blocked the development of tolerance to the analgesic and hyperthermic effects of morphine. Similarly, naltrexone pellet implantation reversed morphine withdrawal-induced body weight loss. The effect of pellets containing 10 and 30 mg naltrexone did not differ. 3. The effect of naltrexone (10 mg) pellet implantation on various signs of naltrexone-precipitated withdrawal such as body weight loss, hypothermia and increases in urinary and fecal output was investigated. Naltrexone pellet implantation did not alter the naltrexone-precipitated withdrawal-induced body weight loss. Concurrent naltrexone pellet implantation blocked the naltrexone-precipitated withdrawal-induced hypothermia, increased fecal and urinary output in morphine-dependent rats. 4. These results indicate that a single pellet of 10 mg of naltrexone can effectively block morphine tolerance and physical dependence in the rat. Such a procedure may be useful in studying biochemical, endocrinological and immunological mechanisms involved in opioid addiction processes.

Effects of naltrexone on the binding of [3H]D-Ala2, MePhe4, Gly-ol5-enkephalin to brain regions and spinal cord and pharmacological responses to morphine in the rat

1. The effects of naltrexone pellet implantation and removal on the analgesic and hypothermic effects of morphine and the binding of 3H-D-Ala2, MePhe4, Gly-ol5-enkephalin (DAMGO) to mu-opiate receptors in rat brain regions and spinal cord were determined. 2. Male Sprague-Dawley rats were implanted subcutaneously with a pellet containing 10 mg of naltrexone for 7 days. Placebo pellet implanted rats served as controls. The pellets were removed on day 8, and the analgesic and hyperthermic effects were determined in the rat 24 hr later. Morphine produced a dose-dependent analgesic and hyperthermic responses in rats implanted with placebo pellets. Enhanced analgesic and hyperthermic responses to morphine were produced in rats implanted with naltrexone pellets. 3. The binding constants (Bmax and Kd values) of [3H]DAMGO in regions of the brain (amygdala, hypothalamus, striatum, midbrain, hippocampus, pons + medulla and cortex), and spinal cord of rats with naltrexone pellet left intact or removed were determined. The Bmax values of [3H]DAMGO were increased in all brain regions and spinal cord of rats in which the naltrexone pellets were left in place or removed prior to sacrificing. However, the Kd values of [3H]DAMGO were unaffected by naltrexone treatment. 4. It is concluded that enhanced analgesic and hyperthermic response to morphine is produced in rats implanted with naltrexone pellets and such alterations in the pharmacological responses are due to up-regulation of mu-opiate receptors in all the brain regions and spinal cord. Additionally whether the pellets were left intact (receptors blocked) or removed (receptors not blocked), the mu-opiate receptors were up-regulated in spinal cord and all the regions of the brain.

Evidence for the role of nitric oxide in kappa-opiate tolerance in mice

The hypothesis that inhibition of nitric oxide (NO) synthase with subsequent decrease in the production of NO might attenuate the development of kappa-opiate tolerance was examined. Concurrent treatment of NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA) (2-8 mg/kg, i.p.) along with U-50,488H (25 mg/kg, i.p.) twice daily for 4 days dose-dependently attenuated the development of tolerance to the analgesic and hypothermic effects of U-50,488H (25 mg/kg, i.p.). L-NMMA by itself did not modify the analgesic and hypothermic effects of acute administration of U-50,488H. A potential role for NO in the development of kappa-opiate tolerance is suggested.

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)

The effect of U-50,488H tolerance-dependence and abstinence on the levels of dynorphin (1-13) in brain regions, spinal cord, pituitary gland and peripheral tissues of the rat

Male Sprague-Dawley rats were rendered tolerant to and physically dependent on U-50,488H, a kappa-opiate agonist, by injecting 25 mg/kg of the drug intraperitoneally twice a day for 4 days. Two sets of rats were used. Rats labeled as tolerant-dependent were injected with U-50,488H (25 mg/kg) 1 h before sacrificing on day 5, whereas the abstinent rats were sacrificed on day 5 without the injection of U-50,488H. Of all the tissues on day 5 without the injection of U-50,488H. Of all the tissues examined, the pituitary gland had the highest level of dynorphin (1-13), whereas the heart had the lowest level. The levels of dynorphin (1-13) increased in the hypothalamus, hippocampus and pons/medulla of U-50,488H tolerant-dependent rats, whereas in abstinent rats the levels of dynorphin (1-13) were elevated only in the midbrain. The levels of dynorphin (1-13) in the pituitary gland of U-50,488H tolerant-dependent or abstinent rats were unchanged. In peripheral tissues, the levels of dynorphin (1-13) in the heart of U-50,488H tolerant-dependent rats were increased. In the abstinent rats they were elevated in the adrenals, spleen, and the heart but were decreased in the kidneys. Compared to morphine tolerant-dependent and abstinent rats, significant differences in the levels of dynorphin (1-13) in tissues of 50,488H tolerant-dependent and abstinent rats were observed and may explain many pharmacological differences in the mu- and kappa-opiate induced tolerance-dependence and abstinence processes.

Abstinence from U-50,488H, a kappa-opiate receptor agonist, decreases the binding of [3H]DPAT to 5-HT1A receptors in the hypothalamus of the rat

The effect of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzene-acetamide methane sulfonate (U-50,488H), a kappa-opiate agonist-induced tolerance and abstinence on 5-HT1A receptors was determined in regions of the brain and spinal cord of the rat. The administration of U-50,488H (25 mg/kg, i.p., twice daily) to male Sprague-Dawley rats for 4 days resulted in the development of almost complete tolerance to its analgesic and hypothermic effects. On day 5, the animals were divided into tolerant and abstinent groups and sacrificed. The brain and spinal cord were excised from all groups of rats and the brain was dissected into 6 regions, namely, amygdala, hypothalamus, striatum, midbrain, pons+medulla and cortex. The 5-HT1A receptors were characterized by using [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]DPAT) as the ligand. The binding constants (Bmax and Kd values) of [3H]DPAT in regions of the brain and spinal cord of rats tolerant to U-50,488H and vehicle did not differ. However, the Bmax value of [3H]DPAT in the hypothalamus of U-50,488H-abstinent rats was decreased but the Kd value did not change. In the other regions of the brain and spinal cord of U-50,488H-abstinent rats, the Bmax and Kd values of [3H]DPAT were unaffected. Subcutaneous administration of DPAT produced hypothermic response in vehicle- and U-50,488H-treated rats. The intensity of this effect was more marked in U-50,488H-abstinent group. It is concluded that 5-HT1A receptors are down-regulated in the hypothalamus of U-50,488H-abstinent rats but the hypothermic response to 5-HT1A agonist is intensified.

Lack of physical dependence in mice after repeated systemic administration of the mixed inhibitor prodrug of enkephalin-degrading enzymes, RB101

Development of physical dependence is observed after treatment with opioid agonists, but not after chronic i.c.v. administration of mixed inhibitors of enkephalin-degrading enzymes. The aim of this study was to investigate further this promising result of repeated administration of the systemically active mixed inhibitor prodrug RB101, N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopro pyl]- L-phenylalanine benzyl ester. In a comparative study, the naloxone-evoked withdrawal syndrome was quantified in mice chronically treated with i.p. administered morphine or RB101 (6 and 160 mg/kg, respectively) for 5 days, twice daily. After administration of naloxone (5 mg/kg s.c.) on the sixth day, large behavioral changes (jumps, paw shakes, wet-dog shakes, tremor, teeth chattering) and body weight losses occurred in the morphine-treated mice. In contrast, no significant behavioral signs of physical dependence, or body weight changes were observed in the RB101-treated mice. The difference between morphine and RB101 could be partially due to a very low tonic release of enkephalins in the locus coeruleus, a brain region critically involved in the development of physical dependence. These results confirm the potential of mixed inhibitors of enkephalin-degrading enzymes as new non-addictive analgesics.

Studies on the possible role of pharmacokinetics in the development of tolerance to morphine in the rat

1. The possible role of pharmacokinetics of morphine in the development of tolerance to the analgesic and hyperthermic effects of morphine was studied in the rat. 2. Male Sprague-Dawley rats were made tolerant to morphine by implanting 6 morphine pellets each containing 75 mg of morphine base for 7 days. The assessment of the degree of tolerance to morphine and pharmacokinetic parameters were done 72 hr after pellet removal. 3. Tolerance developed to both the analgesic and hyperthermic effects of morphine as evidenced by decreased responses to morphine in morphine pellet implanted rats compared with placebo pellet implanted rats. 4. The pharmacokinetic parameters, AUC0-->infinity, Cmax, t1/2, k, MRT, Vss and Clt were determined after injecting 5 and 10 mg/kg doses of morphine intravenously to placebo and morphine pellet implanted rats and using a highly sensitive and specific RIA method to quantitate serum levels of morphine. For a 5 mg/kg dose of morphine, the AUC0-->infinity and t1/2 in morphine pellet implanted rats were significantly higher than in placebo pellet implanted rats, but the k value was lower. The other pharmacokinetic parameters for morphine in the two treatment groups did not differ. For 10 mg/kg dose, the only change was an increase in the MRT in morphine tolerant rats when compared to nontolerant rats. 5. The results establish that the development of tolerance to the analgesic and hyperthermic effects of morphine is not related to pharmacokinetics of morphine in serum but may be related to modification of receptor systems in the central nervous system.

Differences in the binding of [3H][D-Ser2,Thr6]leucine-enkephalin and [3H][D-Pen2,D-Pen5]enkephalin to brain membranes of morphine tolerant-dependent rats

The effect of morphine tolerance-dependence and abstinence on the characteristics of delta-opiate receptors was determined in male Sprague-Dawley rats. Two ligands used for characterizing the receptors were [3H][D-Ser2,Thr6]leucine-enkephalin ([3H]DSTLE) and [3H][D-Pen2,D-Pen5]enkephalin ([3H]DPDPE). Rats were implanted s.c. under light ether anesthesia with six morphine pellets (each containing 75 mg of morphine free base). Rats which served as controls were implanted similarly with placebo pellets. Two sets of rats were used. In one group of rats, the pellets were left intact (tolerant-dependent) at the time of sacrificing and in the other the pellets had been removed 18 h earlier (abstinent). The spinal cord and brain regions (amygdala, hippocampus, hypothalamus, corpus striatum, mid-brain, pons and medulla and cortex) were dissected for binding studies. The binding of [3H]DSTLE to membranes of cerebral cortex of morphine-tolerant-dependent rats was decreased in comparison to control rats, and was due to a decrease in Bmax rather than Kd value. The binding of [3H]DSTLE to other brain regions or spinal cord of morphine-tolerant-dependent and abstinent rats did not differ from their respective controls. On the other hand, the binding of [3H]DPDPE was unaffected in any brain region or the spinal cord of morphine-tolerant-dependent and abstinent rats when compared to their controls. The decrease in binding of [3H]DSTLE to cortical membranes of morphine-tolerant-dependent rats amounted to 15%. Since DSTLE also binds to mu-opiate receptors, which have earlier been shown to be decreased in cortex of morphine-tolerant-dependent rats, and the binding of a more selective delta-opiate ligand [3H]DPDPE was unaffected, it is concluded that central delta-opiate receptors do not play a role in the development of morphine-induced tolerance-dependence or abstinence processes in the rat.

Effects of acute and chronic morphine on food intake in rats: modulation by oxytocin and vasopressin

The effects of acute and chronic morphine administration and the interaction with oxytocin and vasopressin on food intake response were investigated at various intervals during a 24-h schedule in rats. Acute morphine (5 mg/kg, IP) produced a generalized hyperphagic effect in both light (0-6 h) and dark (6-24 h) phases, the most marked effects being at 0-1 h, 1-3 h and 6-24 h. Chronic morphine (7 days) in an escalating dose schedule (5-35 mg/kg/day) produced (a) an enhancement of the hyperphagic effect in the light phase and (b) an attenuation of the food intake response during the dark phase. Neither oxytocin nor vasopressin had any significant influence on food intake, per se, after either acute or chronic administrations. However, both OXY and AVP reduced the hyperphagic response to acute morphine throughout the 24-h observation period. Further, on chronic administration, both neurohypophyseal peptides blocked the enhancements of morphine-induced hyperphagia (reverse tolerance) during light phase, whereas only vasopressin was effective in attenuating the reduction of hyperphagia (tolerance) during dark phase. These results are discussed in light of complex opiate-oxytocin/vasopressin interactions in the regulation of food intake.

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)

Tolerance-dependence and serum elimination of morphine in rats implanted with morphine pellets

1. In order to determine whether the degree of tolerance to and physical dependence on morphine induced by pellet implantation procedure in the rat depends on the dose used and the kinetic parameters, the effect of implantation of different number of pellets on tolerance-dependence and elimination kinetics of morphine from serum was determined. 2. Male Sprague-Dawley rats were implanted subcutaneously with pellets. Each pellet contained 75 mg of morphine free base. Three schedules of implantation were used. They included 2 pellets during a 3-day period (2/3), 4 pellets during a 3-day period (4/3) and 6 pellets during a 7-day period (6/7). Placebo pellets which did not contain morphine were implanted in rats which served as controls. 3. The degree of tolerance to and physical dependence on morphine increased as the number of morphine pellets implanted increased. 4. In separate groups of rats implanted with pellets, elimination kinetics of morphine was studied using radioimmunoassay. The kinetic parameters were: area under serum morphine concentration time curve (AUC0----infinity), serum concentration of morphine extrapolated to time zero (Cmax), half-life (t1/2), elimination rate constant (k), mean residence time (MRT) and total body clearance (Clt). 5. The AUC0----infinity and Cmax increased in proportion to the number of pellets implanted. The t1/2, k, MRT and Clt values for 2/3 and 4/3 schedules did not differ, but for 6/7 schedule were significantly different from the other two schedules. The degree of tolerance to and physical dependence on morphine was directly related to the AUC0----infinity and Cmax. The longer t1/2 and MRT and lower Clt and k values in 6/7 schedule may reflect a saturation of glucuronic acid transferase, the main enzyme metabolizing morphine in the liver, and may account for the greater degree of tolerance and physical dependence.

Down-regulation of brain and spinal cord mu-opiate receptors in morphine tolerant-dependent rats

The effect of chronic administration of morphine and its withdrawal on the characteristics of mu-opiate receptors was determined in male Sprague-Dawley rats. The ligand used for characterizing the receptors was [3H][D-Ala2,MePhe4,Gly5-ol]enkephalin ([3H]DAMGO). Rats were implanted s.c. under light ether anesthesia with six morphine pellets (each containing 75 mg of morphine free base). Rats serving as controls were implanted similarly with placebo pellets. Two sets of animals were used. In one group of rats, the pellets were left intact (tolerant-dependent) at the time of killing and in the other the pellets had been removed 18 h earlier (abstinent). The spinal cord and brain regions (amygdala, hippocampus, hypothalamus, corpus striatum, midbrain, pons and medulla, and cortex) were dissected. In morphine-abstinent rats, the binding of ligands of mu-opiate receptors to membranes of spinal cord and brain regions did not change. In non-abstinent morphine-tolerant-dependent rats, the binding of [3H]DAMGO to membranes of spinal cord, pons and medulla, and cerebral cortex was found to be decreased. These changes were due to decreases in the Bmax values rather than Kd values for the binding of [3H]DAMGO. The results clearly indicate that morphine-induced tolerance-dependence in the rat is associated with changes in the selected brain regions and spinal cord with mu-opiate receptors being down-regulated in spinal cord, pons and medulla, and cerebral cortex. It is concluded that tolerance to morphine in the rats may be due to down-regulation of central mu-opiate receptors. However, mu-opiate receptors are unaffected in morphine abstinence.

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.

Effect of thyrotropin releasing hormone on the development of tolerance to the analgesic and hyperthermic actions of morphine in the rat

The effect of thyrotropin releasing hormone (TRH) on the development of tolerance to the analgesic and hyperthermic actions of morphine was determined in male Sprague-Dawley rats. The tolerance to morphine was induced by subcutaneous implantation of 6 morphine pellets, each containing 75 mg of morphine free base. Rats serving as controls were implanted with 6 placebo pellets. Subcutaneous injections of TRH (1, 3 or 10 mg/kg) twice a day for 7 days inhibited the development of tolerance to the analgesic, but not to the hyperthermic action of morphine. Doses lower than 1 mg/kg or greater than 10 mg/kg administered twice a day for 7 days did not modify tolerance development. The inability of higher doses of TRH to inhibit tolerance to morphine may be related to rapid down-regulation of TRH receptors. The results suggest that in appropriate doses, TRH can selectively inhibit the development of tolerance to the analgesic action of morphine.