William H. Riffee

Dopamine receptor sensitivity after chronic dopamine agonists - Striatal 3H-spiroperidol binding in mice after chronic administration of high doses of apomorphine, N-n-propylnorapomorphine and dextroamphetamine

Several previous reports have demonstrated that chronic administration of both directly and indirectly acting dopamine agonists produces a supersensitive behavioral response to challenge doses of dopamine agonists when compared to the responses induced by acute administration of these drugs. That is, a given dose of a dopamine agonist will produce a greater response after chronic dopamine agonist treatment than is observed upon acute administration of that dose. A similar behavioral phenomenon resulting from chronic administration of dopamine antagonists has been suggested to be due to an increase in the number of dopamine receptors present in relevant brain areas. The same hypothesis has been put forward for the hypersensitivity induced by chronic dopamine agonist administration. The present study was designed to investigate the effect of chronic administration of high doses of both direct and indirect dopamine agonists on the dopamine receptors labeled by 3H-spiroperidol. Groups of animals (CD-1 mice) were sacrificed 1, 3 and 5 days following the last chronic injection. Striatal tissue from these mice was incubated with 3H-spiroperidol and dopamine receptor binding evaluated. Affinity of the receptors for the ligand was unaltered by treatments. The receptors labeled by 3H-spiroperidol showed no significant differences in number following the chronic administration of high doses of apomorphine (30 mg/kg). The Bmax was significantly decreased at only one time period following chronic administration of dextroamphetamine (4 mg/kg); however, these was a dramatic 30% reduction in the Bmax in striatal tissue from those mice treated with N-n-propylnorapomorphine. These results suggest that the hypersensitive behavioral response in mice following chronic administration of direct and indirect acting dopamine agonists is not due to an increase in the number of dopamine receptors in the striatum which are labeled by 3H-spiroperidol.

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.

Strain differences in dopamine receptor function and the initiation of movement

The relationship between voluntary movement initiation (VMI) and caudate nucleus dopamine receptor dynamics was analyzed in two rat strains. Charles River CD/F F-344 (CR-CD/F) and Zivic-Miller CD (ZM-CD) rats (male, 125-150 g) were trained to rapidly release and reset a response lever to avoid electric shock. Whereas 86% of all CR-CD/Fs completed training, only 43% of the ZM-CDs were able to do so. Of those rats completing training, the CR-CD/Fs showed marginally higher avoidance percentage and significantly faster VMI latencies. Physiologically, the more behaviorally-successful CR-CD/Fs showed significantly higher affinity for binding than the trained ZM-CDs and the large group of ZM-CDs which could not be successfully trained. In contrast, the trained ZM-CDs showed significantly higher density of dopamine receptors Bmax than the ZM-CDs which failed to train and the trained CR-CD/Fs. The behavior-physiology continuum is summarized as follows: CR-CD/F Rats = highest affinity and lowest Bmax--rapid, highest percentage avoidance; Trained ZM-CD Rats = lowest affinity and highest Bmax--slower, high percentage avoidance; ZM-CD rats that failed training = intermediate affinity and Bmax--avoidance failure.

Effects of multiple pretreatment with apomorphine and amphetamine on amphetamine-induced locomotor activity and its inhibition by apomorphine

Mice were given a saline preinjection and habituation to the testing environment followed by injection of amphetamine (0.675–5.0 mg/kg IP) and apomorphine (AP, 15–80 μg/kg SC) 15 min later. AP produced a dose-dependent inhibition of the amphetamine-induced locomotor activity. A dose of 40 μg/kg AP increased approximately threefold the amphetamine dose required to induce the same increase in activity. Repeated administration of AP (30 mg/kg IP once daily for 14 days) resulted in an enhanced response (in the early portion of the time response) to amphetamine challenge, while the ability of subsequent microgram challenge doses of AP to reduce the response were unaffected. Similarly, repeated administration (twice-daily IP injections for 5 days) of amphetamine (5.0 mg/kg) resulted in an enhanced locomotor response to amphetamine challenge and no change in the ability of AP to inhibit the response. These results suggest that repeated administrations of dopamine agonists, although acting through different mechanisms (i.e., indirect versus direct), increase the initial release of neurotransmitter. However, the repeated administration of these agonists does not attenuate the ability of AP to inhibit the release of the neurotransmitter induced by amphetamine. The regulatory functions (i.e., presynaptic receptor control) of release appears to remain intact, but the level of neuronal activity has been increased.

Behavioral facilitation following chronic administration of N-n-propylnorapomorphine

Chronic administration of drugs which interfere with normal neurotransmission within animal nervous tissue (e.g. neurotransmitter receptor antagonists) is known to result in the development of behavioral supersensitivity. During recent years, evidence has been presented which indicates that neurotransmitter receptor agonists also produce behavioral supersensitivity. This study shows that, using stereotypic cageclimbing behavior in mice, chronic administration of apomorphine, and N-n-propylnorapomorphine (two direct-acting dopamine agonists) and d-amphetamine (an indirect dopamine agonist) produced an enhanced behavioral response to a test dose of apomorphine 4, 8 and 12 days after cessation of chronic drug injections.

Behavioral sensitization following subchronic apomorphine treatment — possible neurochemical basis

Subchronic treatment with the dopamine agonist apomorphine produces a sensitization to the stereotypic effects of subsequent apomorphine challenge. The present study investigated the effects of this subchronic treatment on apomorphine induced stereotypic behavior and striatal dopamine synthesis, release, metabolism, and D2 receptor binding. The pretreatment, which enhanced the behavioral response to apomorphine challenge, also elevated basal dopamine synthesis and metabolism, but left the ability of a challenge dose of apomorphine to inhibit dopamine synthesis and metabolism unaltered. Thus, ongoing dopamine synthesis and extracellular levels of metabolites would be higher following apomorphine challenge in animals treated subchronically with the agonist. In contrast, neither synaptosomal dopamine release in response to depolarizing stimuli nor the density of D2 dopamine receptors was altered by the treatment. Overall, the results suggest that, while we did not find evidence of autoreceptor desensitization per se, apomorphine treatment may result in enhanced extracellular dopamine levels following dopamine agonist challenge to provide a greater stimulation of an intact dopamine receptor system.

Modification of drug-induced behavioral arousal by preinjection routines in mice.

Drug-induced changes in behavioral arousal in mice can be altered by environment (home cage vs. a novel environment), injection routines, and handling. This study reveals that these factors can conceal behavioral arousal (locomotor activity, repetitive movements, and rearing) caused by administration of dextroamphetamine and apomorphine, as detected by an electromagnetic sensor. The concealment was discovered by the use of a saline preinjection of both control (a second saline injection) and experimental (drug-treated) animals one hour before the second injection. This procedure resulted in substantially less behavioral arousal in the control groups following the second saline injection. Therefore, with saline preinjection, the differences in behavioral arousal between control and experimental groups of mice were maximized, statistical variance minimized, and an orderly time-response relationship observed. By contrast, those groups receiving no saline preinjection showed relatively few statistically significant differences between control and drug-treatment, and larger statistical variations were observed. It was also found that the home-cage environment was necessary for the behavioral arousal to be detected. A novel environment, even with saline preinjection, introduced a variable that in itself caused differences in behavioral arousal induced by a low dose of dextroamphetamine (an actual reduction), but not by apomorphine (i.e., no difference among no preinjection, familiar environment and pre-injection, novel environment).

Dopamine uptake during fast-phase endogenous dopamine release from mouse striatal synaptosomes

[3H]Dopamine (DA) uptake and endogenous DA release were measured in mouse striatal P2 synaptosomes over a 1 30-s period in the presence and absence of KCl depolarization. Approximately 70% of the total DA release over the 30-s period occurred within the first 5 s of depolarization. [3H]DA uptake over this same period was less than 5% of the corresponding release value. The magnitudes and rates of uptake were moderately inhibited by 30 mM KCl depolarization. These decreases were not fully explained by the reduction in sodium present during depolarization or by the dilution of labelled DA by released endogenous DA. Non-linear regression analysis of the uptake data revealed fast (less than 1 s) and slow (greater than 1 s) components of uptake which were differentially affected by KCl depolarization. The results of this study indicate the presence of a rapid, multi-component synaptosomal DA uptake process which appears to play a small role in regulating in vitro endogenous DA release during the initial seconds of KCl depolarization. If synaptosomal uptake reflects the in vivo situation, presynaptic neuronal uptake may be more important in the long-term regulation of transmitter release by maintaining intracellular dopamine storage pools which are linked to synthesis and release processes.

Inhibition of R-(−)-Apomorphine-induced Stereotypic Cage-climbing Behavior in Mice by S-(+)-Apomorphine

Apomorphine exists in both R-(−)- and S-(+)-enantiomeric forms. Only R-(−)-apomorphine (R-(−)-APO) has previously been shown to have agonistic dopaminergic activity. In this study, the ability of S-(+)-apomorphine (S-(+)-APO) to antagonize R-(−)-APO-induced stereotypic cage-climbing behavior in mice was investigated. Initial studies using a racemic mixture showed a depression of the cage-climbing behavior relative to that expected if S-(+)-APO were merely inactive. In subsequent experiments, S-(+)-APO was administered alone or one minute prior to the injection of R-(−)-APO and dose- and time-response analyses were carried out. These studies demonstrated that S-(+)-APO possessed no agonistic activity at 30 and 50 mg/kg compared to R-(−)-APO which showed an ED 50 of 4.3 mg/kg in the cage-climb model. S-(+)-APO did however, act as an antagonist of R-(−)-APO-induced stereotypic activity. Pretreatment of mice with S-(+)-APO (30 mg/kg) caused an orderly shift to the right in the dose response curve for R-(−)-APO (5 to 20 mg/kg). Furthermore, a dose-related inhibition of cage-climbing was observed when S-(+)-APO (2.5 to 60 mg/kg) pretreated mice were administered subsequently R-(−)-APO (5 mg/kg). From these experiments, an antagonist ED 50 of 7.7 mg/kg was calculated for S-(+)-APO. The S-(+)-isomer of apomorphine, thought heretofore to be pharmacologically inactive, possesses significant dopamine antagonistic activity.

Apomorphine fails to inhibit cocaine-induced behavioral hypersensitivity

The subchronic administration of cocaine will induce a behavioral sensitization to challenge doses of the drug administered several days after cessation of treatment. This sensitization is similar behaviorally to that observed for other stimulants such as amphetamine. Similarities and differences in the sensitization induced by cocaine and amphetamine (which are though to have different mechanisms of actions although common behavioral outcomes) have not been thoroughly studied. The purpose of the present experiment was to examine the effects of these two drugs on basic horizontal locomotion and changes occurring subsequent to their subchronic administration in mice. Cocaine and amphetamine were administered acutely in various doses to compare time and dose responses in the behavioral paradigm used (infrared detection of horizontal locomotion). Subsequently, cocaine (10 mg/kg) or amphetamine (2.5 mg/kg) were administered twice a day for 5 days and the animals challenged 3 days after the last treatment with the same doses received subchronically. Two other groups of mice received the same subchronic treatment and in addition were administered 80 micrograms/kg apomorphine (5 to 15 min after each dose of the stimulant) and then tested for their response to challenge doses of the stimulants 72 hours after the last pretreatment dose. Acutely, cocaine produced a maximum locomotor activity that was significantly lower than that of amphetamine and the former had a much shorter duration of action than the latter. After subchronic administration, both stimulants induced sensitization, however, apomorphine inhibited the sensitization induced by amphetamine but failed to do so in the cocaine-treated animals. Possible mechanisms for these differences are discussed.