Time course of dopaminergic hypersensitivity following chronic narcotic treatment

Brain-stimulation reward, morphine-induced oral stereotypy, and sensitization: implications for abuse

Early professionals believed that it was unlikely that anyone but the emotionally unstable received pleasure and became addicted to narcotic drugs. This position was well entrenched and influenced much of the thinking well into the latter half of the last century. Although the discovery of a brain reward system was made early in the 1950's it was not until 20 years later that this discovery was applied to the study of the mechanisms involved in the rewarding aspects of abused substances. Along this vein results will be shown in which opiate antagonist block the dopamine agonist activation of the brain reward system as well as the corollary. Sensitization of the reward system suggests that a driving force for drug use is 'liking' and not just 'wanting.' Basal changes in cerebral metabolic rates of glucose are seen in the presence of cues associated with morphine experience, giving evidence for the role of drug related cues in craving. Finally we asked the question 'Do old rats have as much fun as young rats?'

Rats sensitized to morphine are resistant to the behavioral effects of an unavoidable stress

In agreement with the results of other authors, rats sensitized to morphine and challenged with 5 mg/kg of morphine after 7 days of wash-out showed intense stereotyped movements, the expression of which was selectively antagonized by SCH 23390. Sensitized rats were exposed to an unavoidable stress (which consistently produces an escape deficit in control animals) after 3, 7 and 21 days of morphine wash-out. Twenty-four hours after the unavoidable stress, animals were tested for their capacity to escape and their performance was compared to that of control-stressed and naive rats. Morphine sensitization completely prevented the development of escape deficit. This protective effect was similar to that induced by a chronic imipramine treatment and, like the effect of imipramine, it was antagonized by the administration of SCH 23390 before the unavoidable stress. However, it was not affected by the administration of naloxone. Moreover, when rats presenting a clear-cut escape deficit, induced by a 10-day treatment with SKF 38393, were exposed to the morphine sensitization protocol, a complete recovery of their capacity to avoid a noxious stimulus was observed. Finally, the down-regulation of both the number of D(1)-dopamine receptors and of the coupled adenylyl cyclase activity in the pre-frontal cortex (PFC) produced by long-term SKF 38393 administration was reverted by the superimposed morphine sensitization. Thus, the condition of morphine sensitization appears to share several common effects with chronic imipramine treatment.

Behavioural and neurochemical sensitization of morphine-withdrawn rats to quinpirole

The sensitivity of dopamine D2-like receptors in morphine-withdrawn rats was studied using the selective agonist quinpirole. Morphine was administered twice daily increasing the daily dose from 20 to 50 mg/kg during 7 days. Twenty-four hours after the last morphine administration the rats were given quinpirole (0.01-1 mg/kg) and their behavior was assessed. Withdrawal from repeated morphine treatment enhanced yawning behavior and penile erections induced by small doses (0.01-0.1 mg/kg) as well as the intensity of stereotypy induced by a large dose (1.0 mg/kg) of quinpirole. In the morphine-withdrawn rats the dose of 1 mg/kg of quinpirole caused less yawning than in the control rats, whereas the number of erections induced by this dose was enhanced as compared with the control animals. In the control rats, the striatal and limbic concentrations of dopamine metabolites, 3,4-dihydroxphenylacetic acid (DOPAC), and homovanillic acid (HVA), were not clearly affected by the smallest dose of quinpirole. However, the small dose of quinpirole (0.01 mg/kg) significantly reduced the levels of DOPAC and HVA in the striatum and limbic forebrain of the rats withdrawn from morphine either for 24 or 48 h. These findings indicate that withdrawal from repeated morphine treatment enhances the sensitivity of dopamine D2-like receptors.

Role of dopamine receptors in the dual effect of naloxone on quinpirole-induced yawning in morphine pretreated rats

The present study was undertaken to determine the state of sensitivity of dopamine D2/D3 receptors involved in the mediation of yawning behaviour at various times following acute morphine administration to rats. Morphine (3.0 mg/kg, s.c.) induced a biphasic effect on locomotor activity: an initial inhibitory phase lasting for about 30 min was after about an hour followed by a phase of locomotor activation lasting for about 60 min. Dopamine D2/D3 receptor agonist quinpirole (0.01-0.1 mg/kg, s.c.) induced yawning behaviour in rats. Morphine given at 15 or 60 min before (inhibitory phase) inhibited the yawning response to quinpirole (0.1 mg/kg) but not when given at 90 or 120 min before (stimulatory phase). Naloxone (1.0 mg/kg) given 10 min before quinpirole restored yawning inhibited by morphine pretreatment during the inhibitory phase (15-60 min after morphine). However, during the morphine-induced stimulatory phase naloxone strongly inhibited the yawning response to quinpirole. D1 receptor antagonist SCH 23390 [R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepin-7-ol hemimaleate] at 0.01 mg/kg did not affect quinpirole-induced yawning or its inhibition by morphine. However, in rats which received morphine 90 min prior to testing yawning, SCH 23390 enhanced quinpirole-induced yawning behaviour as compared with morphine- or saline-pretreated animals. The data obtained in the present study indicate that morphine pretreatment initially induces a lack of responsiveness of the D2/D3 receptors mediating yawning behaviour and subsequently increases their sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

High-dose naloxone in tardive dyskinesia

Tardive dyskinesia (TD) is thought to result from nigrostriatal dopaminergic supersensitivity secondary to prolonged neuroleptic exposure. Preclinical studies have demonstrated that the opiate antagonist naloxone can acutely reverse a haloperidol-induced hyperdopaminergic state. In a trial of high-dose naloxone, 20 patients with TD received i.v. naloxone (20 mg, 40 mg, and placebo) under double-blind conditions. At baseline and at regular postdrug intervals, patients were evaluated using a battery of motor, clinical, and neuropsychological measures to study effects on neurological, behavioral, and cognitive functions. There was a significant improvement in involuntary movements at 30 min postnaloxone, together with improvement in clinical ratings at that time point, as well as some cognitive changes. The implications of these findings for the putative functional relationship between dopaminergic and enkephalinergic systems in the nigrostriatal area are discussed.

Morphine dependence and protracted abstinence: regional alterations in CNS radioligand binding

Rats (Fisher F-344) were given free access to a 10% sucrose solution containing 0.5 mg/ml morphine sulfate (controls received the sucrose vehicle only) as their sole source of fluid. Daily morphine intake averaged 119 +/- 21 mg/kg, an amount sufficient to induce physical dependence. After 18 days on this regimen, the control and dependent subjects were sacrificed. A protracted abstinence group was weaned from morphine by reducing its concentration in the vehicle by 20% over the next 5 days, followed by a 5-week drug-free period before sacrifice concurrent with the other groups. These subjects showed no signs of an abstinence syndrome. Binding assays for alpha-2 adrenergic sites (3H-clonidine), beta-1/beta-2 adrenergic sites (3H-dihydroalprenolol), and dopaminergic (D2)/serotonergic (5-HT2) sites (3H-spiroperidol) were performed on tissue from frontal cortex, hippocampus, striatum, and brainstem. No alterations in 3H-clonidine or 3H-dihydroalprenolol binding were observed in dependence or protracted abstinence, suggesting that noradrenergic systems are well-regulated both during dependence and in protracted abstinence. 3H-spiroperidol binding was significantly elevated in the striatum (D2 sites) and hippocampus (5-HT2 sites) during dependence. Hippocampal 3H-spiroperidol binding returned to control levels in protracted abstinence, reflecting a morphine-induced change in 5-HT2 binding sites which had normalized by 5 weeks post-drug. Striatal 3H-spiroperidol binding was significantly decreased below control levels after withdrawal, suggesting that alterations of D2 sites in this structure may play a role in protracted abstinence.

Cyclo(Leu-Gly) has opposite effects on D-2 dopamine receptors in different brain areas

Cyclo(Leu-Gly) (cLG), a diketopiperazine analog of Pro-Leu-Gly-NH2 (MIF), affects a number of physiological and behavioral responses to the endogenous neurotransmitter, dopamine (DA). In the present series of experiments, the effect of in vivo administration of cLG (8 mg/kg) was investigated five days following subcutaneous administration. It was found that cLG administration of cLG (8 mg/kg) was investigated five days following subcutaneous administration. It was found that cLG administration caused a supersensitive behavioral response, measured by increased stereotypic sniffing, to the DA agonist, apomorphine (APO). At the same time, an increase was found in the affinity for dopamine (DA), as measured by dopamine inhibition of 3H-spiroperidol binding to D-2 DA receptors in striatum (nigro-striatal DA tract). In contrast, the same peptide treatment caused a subsensitive physiological response to APO-induced hypothermia, concomitant with a decrease in affinity for dopamine, as measured by DA inhibition of 3H-spiroperidol binding to D-2 DA receptors in hypothalamus (incerto-hypothalamic DA tract). These results suggest that a single neuromodulatory agent, the peptide cLG, can elicit diametrically opposite effects on D-2 DA receptors and on the corresponding physiological endpoints in two different brain areas.

Interaction of opiates with dopamine receptors: receptor binding and behavioral assays

We examined the hypothesis that opiates act as dopamine (DA) receptor-blocking agents thereby inducing a compensatory increase in DA receptor density during chronic administration, and that increased receptor density could account for the behavioral hypersensitivity to DA agonists seen after treatment with opiates. Morphine and methadone did not inhibit the specific binding of 3H-spiroperidol to DA receptors in vitro, nor did they decrease affinity or apparent receptor density in the striatum when administered acutely in vivo in behaviorally effective doses. In contrast, neuroleptics had the expected inhibitory effect in both these experiments. Stereotypy and locomotion in response to apomorphine were measured before and after a 3-week treatment with saline or methadone. About half the methadone-treated rats showed significant increases over predrug baselines in stereotypy or locomotion, as did a few saline-treated animals. However, in those animals showing enhanced stereotypy or locomotion, DA receptor density was not elevated in striatum or mesolimbic areas respectively. These results indicate that opiates do not act as antagonists at DA receptor sites, and that changes in DA receptor density cannot account for opiate-induced behavioral hypersensitivity.

Stereotypy and striatal dopamine receptor changes with 1-alpha-acetylmethadol (LAAM)

Narcotics like morphine and methadone produce effects on striatal dopaminergic mechanisms as evidenced by changes in a stereotyped behavior induced by apomorphine and binding or radioactive ligands to dopamine (DA) receptors. LAAM may replace methadone in the treatment of narcotic addiction. We investigated the effect of LAAM on apomorphine induced stereotypy and the binding of 3H-spiroperidol to dopamine receptors isolated from mouse and guinea pig striata. Chronic LAAM treatment produced a behavioral supersensitivity to apomorphine. The biochemical expression of the behavioral effect was a significant increase in the number of dopamine receptor binding sits in mouse and guinea pig striata. These effects can be due to either a direct chronic blockade or postsynaptic dopamine receptors or stimulation of a presynaptic receptor which acts to reduce the amount of dopamine released during nerve activity. Work is in progress to determine the mechanism of this effect.