Increased release of striatal dopamine after long-term treatment with methadone in rats: inhibition by agents which increase central 5-hydroxytryptamine transmission

Chronic opiate treatment enhances both cocaine-reinforced and cocaine-seeking behaviors following opiate withdrawal

After chronic exposure to psychostimulants or opiates, self-administration or conditioned place preference with either class is increased (sensitized). Cross-sensitization of conditioned place preference, i.e., enhancement of psychostimulant-induced preferences after exposure to opiates, has also been described, but increases in cocaine self-administration after morphine pretreatment have not been reported. The present study evaluated effects of chronic morphine treatment on cocaine reinforcement. Opiate dependence was established in Wistar rats by administration of morphine as a constant infusion that was gradually increased to a dose of 50mg/kg per day over a 1-week period. Immediately after discontinuation of chronic morphine treatment, animals were allowed to acquire cocaine self-administration under a simple fixed-ratio schedule (FR-1), and were subsequently advanced to a progressive ratio schedule. Acquisition of cocaine self-administration under the FR-1 did not differ in saline- and morphine-pretreated animals. For cocaine self-administration under a progressive ratio schedule measured at 5 or more days after the onset of opiate withdrawal, chronic pretreatment with morphine increased the number of ratios completed, augmented final response requirements, and produced a more stable pattern of cocaine self-administration. Responding was also increased in morphine-pretreated animals during an initial extinction session. These results show that chronic opiate treatment can enhance both cocaine-reinforced and cocaine-seeking behaviors following opiate withdrawal. A similar effect may occur in human patients who discontinue methadone or other forms of replacement therapy for opiate abuse, and may contribute to relapse involving use of cocaine or other psychostimulants.

Methadone maintenance reduces heroin- and cocaine-induced relapse without affecting stress-induced relapse in a rodent model of poly-drug use

Although it is well established that methadone can be an effective treatment for opiate addiction, it is not clear how methadone maintenance affects cocaine use and cravings in individuals who self-administer both opiates and cocaine. In our attempt to explore the effect of methadone maintenance on the effects of cocaine, we first assessed the locomotor stimulatory effects of cocaine in rats maintained on methadone (0, 10, 20, or 30 mg/kg/day, via osmotic minipumps). Chronic methadone elevated baseline locomotion in a dose-dependent manner and did not reduce the direct stimulatory effects of cocaine (5 mg/kg). We then investigated the effects of the highest methadone maintenance dose (30 mg/kg/day) on heroin and cocaine seeking in extinction, and when it was precipitated by exposure to heroin, cocaine, or foot-shock stress in rats trained to self-administer both drugs in the same experimental context (heroin 0.05 mg/kg/inf; cocaine 0.5 mg/kg/inf, eight 3-h sessions each). In tests of reinstatement, rats responded selectively on the appropriate drug-associated lever after priming injections of heroin (0.25 mg/kg) or cocaine (20 mg/kg). Methadone maintenance blocked both cocaine- and heroin-induced reinstatement, but not stress-induced reinstatement, which was not lever selective. These results suggest that although methadone maintenance may not reduce the direct stimulatory effects of cocaine, it has the potential to reduce both spontaneous and cocaine-primed cocaine-seeking behavior.

Central 5-HT(4) receptors and dopamine-dependent motor behaviors: searching for a functional role

In this study, we evaluated the role of central 5-HT(4) receptors in the control of motor behaviors related to change of nigrostriatal dopamine (DA) transmission, namely, stereotyped behavior and catalepsy in rats. Indeed, given that 5-HT(4) receptors indirectly modulate nigrostriatal DA neuron activity, we hypothesized that these receptors would regulate nigrostriatal DA transmission in the basal ganglia, and consequently, associated motor responses. Stereotypy was induced either by an acute administration of apomorphine (0.3 and 1.5 mg/kg sc), or by a single morphine administration (15 mg/kg sc) in chronically morphine-treated (15 mg/kg sc, twice daily for 10 days) rats. Catalepsy was induced by the typical neuroleptic haloperidol (HAL; 1 mg/kg sc). The selective 5-HT(4) antagonist, GR 125487 (1 mg/kg ip), modified neither apomorphine- nor morphine-induced stereotypy. HAL-induced catalepsy, while reduced by the systemic administration of the 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg sc), was insensitive to GR 125487, systemically (1, 3, 10 mg/kg ip) or locally (20 and 40 nmol/20 microl) administered into the third ventricle. Also, HAL-induced catalepsy was not affected by the selective 5-HT(4) antagonist GR 113808 (3 mg/kg ip). The obtained results indicate that 5-HT(4) receptor antagonism does not modulate motor behaviors related to change of striatal DA transmission.

Dopamine receptor subsensitivity in the substantia nigra after chronic morphine treatment in rats

Several classes of 5-HT and dopamine (DA) receptor binding sites, and the levels of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), DA and dihydroxyphenylacetic acid (DOPAC) were examined in various brain regions 24 h after a 10-day treatment with morphine (2 X 15 mg/kg s.c. daily) in adult rats. Quantitative autoradiography of receptor binding sites revealed only a significant reduction of [3H]SCH-23390 and [3H]spiperone binding to D-1 and D-2 sites, respectively, in the substantia nigra pars compacta of morphine-treated rats. 5-HT and 5-HIAA levels remained unchanged in the substantia nigra and striatum, but the nigral levels of DA and DOPAC and the levels of DOPAC in the striatum were reduced significantly by morphine treatment. Apomorphine, at a dose (0.05 mg/kg s.c.) known to stimulate DA autoreceptors, decreased nigral and striatal DOPAC levels in controls but not in morphine-treated rats. It is concluded that chronic morphine treatment probably induces a down regulation of nigral D-1 and D-2 binding sites and reduces the negative feed-back mechanisms triggered by DA autoreceptors.

Effects of agents increasing serotonin transmission on the increase of dopamine metabolism caused by morphine in the rat nucleus accumbens

The effect of subcutaneous injection of 10 mg/kg morphine on homovanillic acid (HVA) concentrations in the nucleus accumbens was studied in rats which had received d-fenfluramine, a serotonin releaser, or m-chlorophenylpiperazine, a serotonin agonist, before morphine. 2.5 mg/kg d-fenfluramine or m-chlorophenylpiperazine had no effect on HVA in the nucleus accumbens but significantly reduced the rise in HVA induced by morphine. None of the drugs modified the levels of morphine in the rat brain. The results suggest that agents increasing serotonin transmission inhibit the effect of morphine on dopamine neurons innervating the nucleus accumbens.