Behavioral supersensitivity to apomorphine following chronic narcotic treatment in the guinea pig

Conditioning of rotational behavior after the administration of a single dose of apomorphine in rats with unilateral denervation of the dopaminergic nigrostriatal pathway: relevance to drug addiction

Our aim is to study the relationship of drug activation of the dopamine neurotransmission system and the conditioning of environmental stimuli present at the time of drug administration. We injected a single dose of apomorphine (0.05 mg/kg SC) in rats with the nigrostriatal dopamine pathways unilaterally denervated with 6-hydroxydopamine, which generates rotational behavior contralateral to the lesioned hemisphere. We observed rotational behavior without apomorphine administration when animals were reexposed at different time intervals to the same environment in which they performed turning behavior. The present findings show that this rotational behavior can be conditioned to environmental stimuli in a strong and long-lasting way. In light of the relationship between opioids and the dopaminergic system, similar conditioning could take place in the learning processes implicated in drug addiction.

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.

The persistence of hyperresponsiveness to apomorphine in rats following REM sleep deprivation and the influence of housing conditions

The hyperresponsiveness to apomorphine disappears after REM sleep deprivation of rats; the time course of the disappearance was investigated. When REM sleep-deprived (REMd) rats were caged individually during the recovery period the increase in ambulation and in stereotyped behaviour elicited by apomorphine subsided within 8-24 h; on the other hand, the exaggerated aggressiveness persisted up to 64 days. However, when the deprived rats were allowed to recovery in groups of 3 per cage the increase in aggressiveness subsided after 4 days. Since individual housing of non-deprived rats for 4--11 days increased the apomorphine-aggressiveness steadily, 'isolation' probably summed with REM deprivation in facilitating the effects of apomorphine on aggressive behaviour. The results also suggest that the dopaminergic systems implicated in aggressive behaviour are sensitized by REM deprivation for a longer period than those subjacent to ambulation and stereotype.

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.

Apomorphine-induced stereotypic cage climbing in mice as a model for studying changes in dopamine receptor sensitivity

We have previously confirmed in mice that apomorphine (APO) induces dopamine specific stereotypic cage climbing. Apparent changes in dopamine receptor sensitivity induced by chronic drug administration appear to be measurable by this technique. In the present experiments, murine stereotypic cage climbing was evauated as a model system for assessing the dopamine receptor supersensitivity induced by chronic administration of the potent butyrophenone neuroleptic spiroperidol. Spiroperidol induced a significantly enhanced response induced by APO (about a 7-fold increase) manifest by 48 hr (but not 24 hr) following cessation of the last chronic injection. Time-response analyses demonstrated that the action of test doses of APO (1.0 or 4.5 mg/kg, IP) was significantly prolonged in the chronic-spiroperidol animals relative to controls. The supersensitivity in the spiroperidol-treated animals lasted more than three weeks for each dose of the neuroleptic and the APO dose-response curve was shifted to the left in spiroperidol-treated animals. Results are discussed in terms of the utility of the model for establishing dose-response, time-course, and duration of effect data within the same group of animals.

Time course of dopaminergic hypersensitivity following chronic narcotic treatment

Guinea pigs were injected SC for 3 weeks with 3 different dosage schedules of morphine or methadone, or with saline. For 8 weeks thereafter they were challenged weekly with the dopamine agonist apomorphine. Hypersensitivity was manifested in more intense stereotypies, as compared to the saline group, by all morphine and methadone groups. Hypersensitivity persisted longer after the termination of methadone treatment (maximum of 8 weeks) than after morphine administration (maximum of 3 weeks). The degree of hypersensitivity, and its duration after treatment, was positively related to methadone dosage. In some groups a period of hyposensitivity was seen following hypersensitivity. These data are interpreted with reference to the hypothesized mechanism underlying the development of hypersensitivity, the different durations of action of morphine and methadone, and the retention of methadone in brain following treatment.

Sensitivity to apomorphine in the guinea pig as a function of age and body weight

Male albino guinea pigs aged 4--10 weeks were challenged with 0.1, 0.2, and 0.4 mg/kg apomorphine. Mean stereotypy scores rose signfiicantly as a function of age. Stereotypy scores were better correlated with age than with body weight, suggesting that CNS maturation, rather than weight-related factors, were responsible. Although age and body weight were correlated, there was enough variability to make bvody weight an unreliable indicator of age.