Mice deficient for delta- and mu-opioid receptors exhibit opposing alterations of emotional responses.

The role of the opioid system in controlling pain, reward and addiction is well established, but its role in regulating other emotional responses is poorly documented in pharmacology. The mu-, delta- and kappa- opioid receptors (encoded by Oprm, Oprd1 and Oprk1, respectively) mediate the biological activity of opioids. We have generated Oprd1-deficient mice and compared the behavioural responses of mice lacking Oprd1, Oprm (ref. 6) and Oprk1 (ref. 7) in several models of anxiety and depression. Our data show no detectable phenotype in Oprk1-/- mutants, suggesting that kappa-receptors do not have a role in this aspect of opioid function; opposing phenotypes in Oprm-/- and Oprd1-/- mutants which contrasts with the classical notion of similar activities of mu- and delta-receptors; and consistent anxiogenic- and depressive-like responses in Oprd1-/- mice, indicating that delta-receptor activity contributes to improvement of mood states. We conclude that the Oprd1-encoded receptor, which has been proposed to be a promising target for the clinical management of pain, should also be considered in the treatment of drug addiction and other mood-related disorders.

Intracranial pressure waveform in patients with essential hypertension.

Background: There is a strong association between hypertension and cerebrovascular diseases, but most of the mechanistic bases to justify this correlation remains misunderstood. Objective: To evaluate intracranial pressure waveform in long-term essential hypertensive patients with a non-invasive device, brain4care (b4c). Methods: Cross-sectional study in patients with hypertension. Office blood pressure was measured with an automatic oscillometric device. Intracranial pressure evaluation was acquired through a strain sensor that could detect and monitor nanometric skull bone displacements for each cardiac cycle. Under normal physiological conditions, P1 is greater than P2, and the normal P2/P1 ratio is <1. Time to peak (TTP) is the measurement in seconds of the beginning of waveform inscription until P1 and normal values are <0.20 s. The cut-off points ≥1.2 and ≥0.25 s were used to define intracranial hypertension (ICHT). Results: 391 consecutive patients were evaluated (75% female, mean age 64.3 ± 12.0 years). Mean value of P2/P1 ratio was 1.18 ± 0.25 and TTP 0.18 ± 0.63 s The obtained P2/P1 ratios were divided in three categories according to results of previous studies of normalcy (<1.0), intracranial compliance disturbance (1.0-1.19) and ICHT (≥1.2). Normal intracranial pressure was observed in 21.7% of patients, intracranial compliance disturbance in 32.7% and intracranial hypertension in 45.6%. Females showed a higher prevalence of ICHT (50.3%). Conclusion: The prevalence of 45.6% intra-cranial hypertension in patients with long-term hypertension, particularly in women, and in those over 65 years old, emphasizes the importance of evaluate intracranial pressure behaviour in these patients and raise a question concerning the real ability of cerebral autoregulation and vascular barriers to protect the brain.

Hypertension evaluated in the public and private Brazilian health system hypertension in public and private service.

Introduction: Hypertension (HT) remains the leading cause of death worldwide. In Brazil it is estimated that 35% of the adult population has HT and that about 20% of these have blood pressure values within the targets recommended for the reduction of cardiovascular risk. There are some data that point to different control rates in patients treated by cardiologists in public and private referral center and this is an important point to be investigated and discussed. Objective: To compare sociodemographic characteristics, body mass index (BMI), antihypertensive (AH) drugs, blood pressure (BP) and control rate in public (PURC) and private (PRRC) referral centers. Methodology: A cross-sectional multicenter study that analyzed data from hypertensive patients assisted by the PURC (one in Midwest Region and other in Northeast region) and PRRC (same distribution). Variables analyzed: sex, age, BMI, classes, number of AH used and mean values of systolic and diastolic BP by office measurement and home blood pressure measurement (HBPM). Uncontrolled hypertension (HT) phenotypes and BP control rates were assessed. Descriptive statistics and χ2 tests or unpaired t-tests were performed. A significance level of p < 0.05 was considered. Results: A predominantly female (58.9%) sample of 2.956 patients and a higher prevalence of obesity in PURC (p < 0.001) and overweight in PRRC (p < 0.001). The mean AH used was 2.9 ± 1.5 for PURC and 1.4 ± 0.7 for PRRC (p < 0.001). Mean systolic and diastolic BP values were higher in PURC as were rates of uncontrolled HT of 67.8% and 47.6% (p < 0.001) by office measurement and 60.4% and 35.3% (p < 0.001) by HBPM in PURC and PRRC, respectively. Conclusion: Patients with HT had a higher prevalence of obesity in the PURC and used almost twice as many AH drugs. BP control rates are worse in the PURC, on average 15.3 mmHg and 12.1 mmHg higher than in the PRRC by office measurement.

Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice.

The function of the central cannabinoid receptor (CB1) was investigated by invalidating its gene. Mutant mice did not respond to cannabinoid drugs, demonstrating the exclusive role of the CB1 receptor in mediating analgesia, reinforcement, hypothermia, hypolocomotion, and hypotension. The acute effects of opiates were unaffected, but the reinforcing properties of morphine and the severity of the withdrawal syndrome were strongly reduced. These observations suggest that the CB1 receptor is involved in the motivational properties of opiates and in the development of physical dependence and extend the concept of an interconnected role of CB1 and opiate receptors in the brain areas mediating addictive behavior.

Neuroadaptive changes and behavioral effects after a sensitization regime of MDPV.

3,4-methylenedioxypyrovalerone (MDPV) is a synthetic cathinone with cocaine-like properties. In a previous work, we exposed adolescent mice to MDPV, finding sensitization to cocaine effects, and a higher vulnerability to cocaine abuse in adulthood. Here we sought to determine if such MDPV schedule induces additional behavioral-neuronal changes that could explain such results. After MDPV treatment (1.5 mg kg-1, twice daily, 7 days), mice were behaviorally tested. Also, we investigated protein changes in various brain regions. MDPV induced aggressiveness and anxiety, but also contributed to a faster habituation to the open field. This feature co-occurred with an induction of ΔFosB in the orbitofrontal cortex that was higher than its expression in the ventral striatum. Early after treatment, D2R:D1R ratio pointed to a preponderance of D1R but, upon withdrawal, the ratio recovered. Increased expression of Arc, CDK5 and TH, and decrease in DAT protein levels persisted longer after withdrawal, pointing to a neuroplastic lasting effect similar to that involved in cocaine addiction. The implication of the hyperdopaminergic condition in the MDPV-induced aggressiveness cannot be ruled out. We also found an initial oxidative effect of MDPV, without glial activation. Moreover, although initially the dopaminergic signal induced by MDPV resulted in increased ΔFosB, we did not observe any change in NFκB or GluA2 expression. Finally, the changes observed after MDPV treatment could not be explained according to the autoregulatory loop between ΔFosB and the epigenetic repressor G9a described for cocaine. This provides new knowledge about the neuroadaptive changes involved in the vulnerability to psychostimulant addiction.

Chronic cocaine treatment alters dendritic arborization in the adult motor cortex through a CB1 cannabinoid receptor-dependent mechanism.

The CB1 cannabinoid receptors modulate the addictive processes associated with different drugs of abuse, including psychostimulants. Mice lacking CB1 receptors exhibit an important attenuation of the reinforcing responses produced by cocaine in an operant self-administration paradigm. We have investigated the effect of chronic cocaine treatment on dendrite structure and spine density of the principal cortical neuron, the pyramidal neuron, in CB1 knockout mice and wild type littermates. Layer III pyramidal cells of the motor cortex were injected intracellularly in fixed cortical slices and their morphometric parameters analyzed. Under basal conditions, the field area of the dendritic arbors was more extensive and dendritic spine density was higher in wild type mice than in CB1 knockout mice. Chronic treatment of cocaine diminished the size and length of the basal dendrites and spine density on pyramidal cells from wild type mice. However, the total number of spines in the pyramidal cells of CB1 knockout mice augmented slightly following chronic cocaine treatment, although no changes in the morphology of the dendritic arbor were observed. Our data demonstrate that microanatomy and synaptic connectivity are affected by cocaine, the magnitude and nature of these changes depend on the presence of CB1 receptors.

Genetic and pharmacological approaches to evaluate the interaction between the cannabinoid and cholinergic systems in cognitive processes.

BACKGROUND AND PURPOSE: The objective of this study was to investigate the possible interactions between the cannabinoid and cholinergic systems in memory and learning processes by using genetic and pharmacological approaches in two different behavioural models, the active avoidance and the object recognition test. EXPERIMENTAL APPROACH: The effects induced by nicotine, physostigmine and scopolamine were studied in CB(1) receptor knockout and wild-type mice in the active avoidance paradigm. In addition, the effects of pretreatment with the CB(1) receptor antagonist rimonabant were evaluated on the responses induced by nicotine in the active avoidance and the object recognition tasks in wild-type mice. KEY RESULTS: Nicotine (0.5 mg kg(-1) s.c.) did not modify the performance of CB(1) knockout and wild-type mice in this model, whereas scopolamine (0.5 mgkg(-1) i.p.) impaired the performance in both genotypes. Physostigmine (0.1 mg kg(-1) i.p.) increased the active avoidance performance in wild-type but not in CB(1) knockout mice. Rimonabant (0.3, 1, 3, and 10 mg kg(-1)) did not modify the performance in the active avoidance test, given alone or co-administered with nicotine. In contrast, nicotine enhanced the performance in the object recognition task but this response was attenuated by rimonabant co-administration. CONCLUSIONS AND IMPLICATIONS: The present findings revealed that the cognitive effects of nicotine and physostigmine were attenuated in the absence of CB(1) receptor activity. Scopolamine effects were independent from CB(1) receptors.

Exposure of adolescent mice to 3,4-methylenedioxypyrovalerone increases the psychostimulant, rewarding and reinforcing effects of cocaine in adulthood.

BACKGROUND AND PURPOSE: 3,4-Methylenedioxypyrovalerone (MDPV) is a synthetic cathinone with powerful psychostimulant effects. It selectively inhibits the dopamine transporter (DAT) and is 10-50-fold more potent as a DAT blocker than cocaine, suggesting a high abuse liability. The main objective of the present study was to assess the consequences of an early (adolescence) MDPV exposure on the psychostimulant, rewarding and reinforcing effects induced by cocaine in adult mice. EXPERIMENTAL APPROACH: Twenty-one days after MDPV pretreatment (1.5 mg·kg-1 , s.c., twice daily for 7 days), adult mice were tested with cocaine, using locomotor activity, conditioned place preference and self-administration (SA) paradigms. In parallel, dopamine D2 receptor density and the expression of c-Fos and ΔFosB in the striatum were determined. KEY RESULTS: MDPV treatment enhanced the psychostimulant and conditioning effects of cocaine. Acquisition of cocaine SA was unchanged in mice pretreated with MDPV, whereas the breaking point achieved under a progressive ratio programme and reinstatement after extinction were higher in this group of mice. MDPV decreased D2 receptor density but increased ΔFosB expression three-fold. As expected, acute cocaine increased c-Fos expression, but MDPV pretreatment negatively influenced its expression. ΔFosB accumulation declined during MDPV withdrawal, although it remained elevated in adult mice when tested for cocaine effects. CONCLUSION AND IMPLICATIONS: MDPV exposure during adolescence induced long-lasting adaptive changes related to enhanced responsiveness to cocaine in the adult mice that seems to lead to a higher vulnerability to cocaine abuse. This particular behaviour correlated with increased expression of ΔFosB.

Cannabinoid withdrawal syndrome is reduced in pre-proenkephalin knock-out mice.

The functional interactions between the endogenous cannabinoid and opioid systems were evaluated in pre-proenkephalin-deficient mice. Antinociception induced in the tail-immersion test by acute Delta9-tetrahydrocannabinol was reduced in mutant mice, whereas no difference between genotypes was observed in the effects induced on body temperature, locomotion, or ring catalepsy. During a chronic treatment with Delta9-tetrahydrocannabinol, the development of tolerance to the analgesic responses induced by this compound was slower in mice lacking enkephalin. In addition, cannabinoid withdrawal syndrome, precipitated in Delta9-tetrahydrocannabinol-dependent mice by the injection of SR141716A, was significantly attenuated in mutant mice. These results indicate that the endogenous enkephalinergic system is involved in the antinociceptive responses of Delta9-tetrahydrocannabinol and participates in the expression of cannabinoid abstinence.

Absence of delta -9-tetrahydrocannabinol dysphoric effects in dynorphin-deficient mice.

The involvement of dynorphin on Delta-9-tetrahydrocannabinol (THC) and morphine responses has been investigated by using mice with a targeted inactivation of the prodynorphin (Pdyn) gene. Dynorphin-deficient mice show specific changes in the behavioral effects of THC, including a reduction of spinal THC analgesia and the absence of THC-induced conditioned place aversion. In contrast, acute and chronic opioid effects were normal. The lack of negative motivational effects of THC in the absence of dynorphin demonstrates that this endogenous opioid peptide mediates the dysphoric effects of marijuana.

Functional interaction between opioid and cannabinoid receptors in drug self-administration.

The present study was designed to explore the relationship between the cannabinoid and opioid receptors in animal models of opioid-induced reinforcement. The acute administration of SR141716A, a selective central cannabinoid CB1 receptor antagonist, blocked heroin self-administration in rats, as well as morphine-induced place preference and morphine self-administration in mice. Morphine-dependent animals injected with SR141716A exhibited a partial opiate-like withdrawal syndrome that had limited consequences on operant responses for food and induced place aversion. These effects were associated with morphine-induced changes in the expression of CB1 receptor mRNA in specific nuclei of the reward circuit, including dorsal caudate putamen, nucleus accumbens, and septum. Additionally, the opioid antagonist naloxone precipitated a mild cannabinoid-like withdrawal syndrome in cannabinoid-dependent rats and blocked cannabinoid self-administration in mice. Neither SR141716A nor naloxone produced any intrinsic effect on these behavioral models. The present results show the existence of a cross-interaction between opioid and cannabinoid systems in behavioral responses related to addiction and open new strategies for the treatment of opiate dependence.

Analysis of the endocannabinoid system by using CB1 cannabinoid receptor knockout mice.

The endocannabinoid system has been involved in the control of several neurophysiological and behavioural responses. To date, three lines of CB1 knockout mice have been established independently in different laboratories. This chapter reviews the main results obtained with these lines of CB1 knockout mice in several physiological responses that have been previously related to the activity of the endocannabinoid system. Studies using CB1 knockout mice have demonstrated that this receptor participates in the control of several behavioural responses including locomotion, anxiety- and depressive-like states, cognitive functions such as memory and learning processes, cardiovascular responses and feeding. Furthermore, the CB1 cannabinoid receptor is involved in the control of pain by acting at peripheral, spinal and supraspinal levels. The involvement of the CB1 cannabinoid receptor in the behavioural and biochemical processes underlying drug addiction has also been investigated. These CB1 knockouts have provided new findings to clarify the interactions between cannabinoids and the other drugs of abuse such as opioids, psychostimulants, nicotine and ethanol. Recent studies have demonstrated that endocannabinoids can function as retrograde messengers, modulating the release of different neurotransmitters, including opioids, gamma-aminobutyric acid (GABA), and cholecystokinin (CCK), which could explain some of the responses observed after the stimulation of the CB1 cannabinoid receptor. This review provides an update of the apparently controversial data reported in the literature using the three different lines of CB1 knockout mice, which seem to be mainly due to the use of different experimental procedures rather than any constitutive alteration in these lines of knockouts.

Sex differences in the long-lasting consequences of adolescent ethanol exposure for the rewarding effects of cocaine in mice.

RATIONALE: The practice of binge drinking is very common among adolescents of both sexes. It can have long-term consequences with respect to drug consumption during adulthood, but knowledge on these effects in females is limited. OBJECTIVES: The long-lasting effects of intermittent exposure to ethanol (EtOH) during adolescence on different cocaine-elicited behaviours, including locomotor reactivity, conditioned place preference (CPP) and intravenous self-administration, were evaluated in male and female adult mice. It was hypothesized that an EtOH binge during adolescence would increase sensitivity to the effects of a sub-threshold dose of cocaine and has a differential impact on the drug's effects in males and females. METHODS: Adolescent OF1 mice (postnatal day (PND) 26) underwent a 2-week pre-treatment schedule consisting of 16 doses of EtOH (2.5 g/kg) or saline (twice daily administrations separated by a 4-h interval i.p.) administered on two consecutive days separated by an interval of 2 days. Three weeks later (PND > 60), we assessed locomotor activity responses induced by an acute injection of different doses of cocaine in experiment 1 and the rewarding effects of cocaine on the CPP (1 mg/kg) and intravenous self-administration (1 mg/kg/infusion) paradigms in experiment 2. RESULTS: Pre-exposure to EtOH during adolescence altered motor reactivity to cocaine in a dose- and sex-dependent manner, increased sensitivity to cocaine in CPP and enhanced self-administration in adult mice. CONCLUSIONS: The effects of intermittent exposure to ethanol during adolescence are evident in adulthood, during which greater sensitivity and intake of cocaine is observed and differ in each sex.

Antinociceptive response induced by mixed inhibitors of enkephalin catabolism in peripheral inflammation.

RB101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-ox-opropyl)-L-phenylalanine benzyl ester) is a recently developed full inhibitor of the enkephalin-catabolizing enzymes able to cross the blood-brain barrier, whereas RB38A ((R)-3-(N-hydroxycarboxamido-2-benzylpropanoyl)-L-phenylalanine) is as potent as RB101 but almost unable to enter the brain. In this study, we have investigated the effects of systemic administration of morphine, RB101 and RB38A on nociception induced by pressure on inflamed peripheral tissues. Antinociceptive test was performed between 4 and 5 days after injection into the rat left hindpaw of Freund's complete adjuvant to produce localized inflammation. Morphine (1, 2 and 4 mg/kg, i.v.) induced antinociception in inflamed paws at all the doses used, and only at the highest dose in non-inflamed paws. RB101 (10 and 20 mg/kg, i.v.) induced an antinociceptive response only in the inflamed paws. RB38A, also induced an antinociceptive effect in the inflamed paws, but only at the highest dose (20 mg/kg, i.v.). The responses induced by morphine and the inhibitors of enkephalin catabolism were antagonized by the systemic administration of naloxone (1 mg/kg) or methylnaloxonium (2 mg/kg) which acts essentially outside the brain. Central injection (i.c.v.) of methylnaloxonium (2 micrograms) blocked the effect of morphine only in non-inflamed paws, and slightly decreased the response induced by RB101 on inflamed paws. These results indicate that the endogenous opioid peptides, probably enkephalins, are important in the peripheral control of nociception from inflamed tissues.

Lack of CB1 cannabinoid receptors modifies nicotine behavioural responses, but not nicotine abstinence.

Cannabis is the most widely consumed illicit drug and its consumption is currently associated with tobacco, which contains another psychoactive compound, namely nicotine. Interactions between cannabinoids and other drugs of abuse, such as opioids, have been previously reported. The aim of the present study was to evaluate the possible role of CB1 cannabinoid receptor in responses induced by acute and repeated nicotine administration by using knockout mice lacking the CB1 cannabinoid receptor and their wild-type littermates. Acute nicotine (0.5, 1, 3 and 6 mg/kg, sc) administration decreased locomotor activity and induced antinociceptive responses in the tail-immersion and the hot-plate test, in wild-type animals. The antinociceptive effects in the tail-immersion test were significantly enhanced in CB1 knockout mice. In wild-type mice nicotine (0.5 mg/kg, sc) produced a significant rewarding effect, as measured by a conditioned place preference paradigm. This response was absent in CB1 knockout mice. Finally, a model of mecamylamine-induced abstinence in chronic nicotine-treated mice (10 mg/kg/day, sc) was developed. Mecamylamine (1 and 2 mg/kg, sc) precipitated several somatic signs of nicotine withdrawal in wild-type dependent mice. However, no difference in the severity of nicotine withdrawal was observed in CB1 knockout mice. These results demonstrate that some acute effects and motivational responses elicited by nicotine can be modulated by the endogenous cannabinoid system and support the existence of a physiological interaction between these two systems.

Inhibition of morphine withdrawal by the association of RB 101, an inhibitor of enkephalin catabolism, and the CCKB antagonist PD-134,308.

1. The effects induced in rats on naloxone-precipitated morphine withdrawal syndrome by the new mixed inhibitor of enkephalin catabolism able to cross the blood-brain barrier RB 101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-ox-opropyl-L-phenylalanine benzyl ester) given alone or associated with the selective CCKB antagonist, PD-134,308, were investigated. 2. The systemic administration of RB 101 (5, 10 and 20 mg kg-1, i.v.) elicited a significant decrease in 8 of the 14 withdrawal signs evaluated. PD-134,308 (3 mg kg-1, i.p.) did not modify the expression of morphine abstinence when given alone, but induced a strong facilitation of RB 101 responses (12 of 14 withdrawal signs were decreased). This potentiation was particularly intense in peripherally mediated withdrawal signs. 3. In order to clarify the biochemical mechanisms implicated in these responses, the effects induced by the association of RB 101 and PD-134,308 on the occupation of brain opioid receptors by endogenous enkephalins were also investigated in mice. PD-134,308, as well as RB 101, inhibited [3H]-diprenorphine binding to opioid receptors. These results suggest that an increase in endogenous enkephalin levels induced by PD-134,308 could participate in the facilitation of RB 101 behavioural responses. 4. RB 101 has a promising potential role in the management of the opiate withdrawal syndrome. CCKB antagonists, such as PD-134,308 may be useful in potentiating this anti-withdrawal effect.

The attenuation of morphine-conditioned place preference following chronic mild stress is reversed by a CCKB receptor antagonist.

Chronic exposure to mild unpredictable stress has been found to abolish the acquisition of preference for a distinctive environment paired with morphine, whereas morphine induced conditioning place preference in non-stressed rats. Chronic treatment for 21 days with the tricyclic antidepressant imipramine reversed the motivational effects produced by chronic mild stress, and animals showed a place preference for the morphine-paired compartment. When the CCKB receptor antagonist PD-134,308 was co-administered with morphine in stressed animals during the conditioning period, the preference for the morphine-paired compartment was also re-established. The CCKB receptor antagonist given alone did not induce rewarding effects in this paradigm. These findings indicate that the administration of a CCKB receptor antagonist reversed the effects of chronic mild stress on opiate rewarding properties.

The CCKB antagonist PD-134,308 facilitates rewarding effects of endogenous enkephalins but does not induce place preference in rats.

The interaction between cholecystokinin and endogenous opioid systems on rewarding responses was examined. Motivational effects induced by peripheral administration of a complete inhibitor of enkephalin catabolism, RB 101 or the CCKB antagonist PD-134,308, and by both compounds in combination were evaluated in the conditioned place preference test in rats. RB 101 (5, 10, 20, 40 and 80 mg/kg, IP, and 20 mg/kg, IV) given alone produced a bell-shaped dose-effect function. A significant increase of the preference for the drug-associated compartment was only observed at doses of 10 and 20 mg/kg (IP). The effect observed with morphine was stronger, and all the doses used of this compound (1.25, 2.5 and 5 mg/kg, SC) were found to be active. These results suggest that the inhibitor of enkephalin catabolism has weak rewarding properties. Pretreatment with the CCKB antagonist PD-134,308 (0.1, 0.3, 1 and 3 mg/kg, IP) alone failed to produce a reliable aversion or preference on the paradigm studied. When PD-134,308 (0.3 mg/kg, IP) was coadministered with a subthreshold dose of morphine (0.6 mg/kg, SC) or RB 101 (5 mg/kg, IP), a conditioned place preference was observed, indicating that the CCKB antagonist facilitated the motivational responses induced by endogenous enkephalins as compared to morphine. This suggests that endogenous cholecystokinin, acting through CCKB receptors, modulates the rewarding effects of endogenous enkephalins.

Influence of different benzodiazepines on the experimental morphine abstinence syndrome.

The abuse of benzodiazepines by narcotic addicts has been well documented. However, the pharmacological basis of this abuse is not clear. In this study the effects of different benzodiazepines (flunitrazepam: 0.5, 1 and 2 mg/kg; nitrazepam: 0.5, 1, 2.5, 5 and 10 mg/kg; diazepam: 0.5, 1, 2.5, 5 and 10 mg/kg; chlordiazepoxide: 0.5, 1, 2.5, 5 and 10 mg/kg; and triazolam: 0.5, 1 and 2 mg/kg) on the morphine withdrawal syndrome in mice have been compared. Drugs were administered 30 min before naloxone-induced withdrawal. All benzodiazepines tested induced similar changes in some of the signs of morphine abstinence: decreased jumping behavior and increased wet dog shake frequency. Jumping behavior was particularly decreased by triazolam and wet dog shakes were mainly increased by flunitrazepam and nitrazepam. Forepaw treading was reduced by flunitrazepam, diazepam and triazolam, increased by nitrazepam and not changed by chlordiazepoxide. Tremor was effectively reduced by diazepam but less reliably by the other benzodiazepines. Teeth chattering was reduced by flunitrazepam and diazepam. These results indicate that benzodiazepines can interfere with the expression of the morphine withdrawal syndrome.

Similar involvement of several brain areas in the antinociception of endogenous and exogenous opioids.

The complete inhibitor of the enkephalin degrading enzymes, RB 101, N-{(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithio]-1- oxopropyl}-L-phenylalanine benzyl ester, which crosses the blood-brain barrier, induced antinociceptive effects similar to those of exogenous opiates. The almost complete absence of tolerance and dependence after chronic administration of RB 101 is therefore due to limited stimulation of opioid receptors by 'protected' endogenous enkephalins. In order to clarify the mechanisms involved in these response, we have investigated the participation of several brain structures in the antinociceptive effects induced by systemic administration of morphine or RB 101. Rats were implanted with bilateral cannulae into the ventro-basal thalamus, central amygdala and periaqueductal gray matter, or with a cannula into the raphe magnus nucleus. The antinociceptive responses induced by systemic morphine or RB 101 were measured by using the tail-electrical stimulation test, where three different thresholds were determined: motor response, vocalization and vocalization post-discharge. The ability of the opioid receptor antagonist methylnaloxonium to block these antinociceptive responses was evaluated after local injection into the different brain structures. The blockade of morphine- and RB 101-induced antinociception was similar, and was stronger when methylnaloxonium was injected into the periaqueductal gray matter and raphe magnus nucleus than when it was injected into the ventro-basal thalamus and amygdala. These results suggest that brain structures related to the control of pain seem to be the same for the antinociception induced by exogenous opiates and endogenous opioids.