Behavioral sensitization following a single apomorphine pretreatment--selective effects on the dopamine release process.

Once daily subchronic pretreatments with the dopamine (DA) agonist apomorphine (APO) increase striatal DA synthesis and metabolism. Such changes imply that adaptations to APO do not dissipate completely within 24 h. In the present report we evaluated the effects of a single APO treatment 24 h prior to euthanasia on behavior and on striatal DA synthesis, metabolism, release and receptor binding. The single APO pretreatment reduced DA release from striatal synaptosomes. In contrast, striatal DA synthesis, metabolism, and the high-affinity binding of DA to the D2 receptor were unaltered 24 h after agonist pretreatment. At this time the stereotypic response to a subsequent APO challenge was enhanced. This adaptive pattern is different from that observed 60 min following an acute APO pretreatment, when high-affinity D2 binding is reduced. The pattern 24 h following a single APO pretreatment is also different from that observed following subchronic agonist dosing, when stereotypic behavior is enhanced, while basal DA synthesis and metabolism are increased.

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.

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.

Prevention of amphetamine-induced behavioral hypersensitivity by concomitant treatment with microgram doses of apomorphine.

Behavioral hypersensitivity induced by subchronic administration of amphetamine in mice was inhibited by concomitant administration of apomorphine in microgram doses. Acutely, amphetamine followed in 15 min by apomorphine, produces a normal rise in locomotor activity subsequent to the action of amphetamine and an inhibition of that activity by the modulatory action of apomorphine. We suggest that apomorphine acts at a presynaptic neuronal site to interrupt the process(es) responsible for the development of stimulant-induced sensitization.

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.

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 micrograms/kg SC) 15 min later. AP produced a dose-dependent inhibition of the amphetamine-induced locomotor activity. A dose of 40 micrograms/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.

Effects of ascorbate on a dopaminergic response: apomorphine-induced modification of pentylenetetrazol-induced seizures in mice.

The threshold for pentylenetetrazol (PTZ)-induced clonic seizures is below control levels 15 min after administration of apomorphine (APO) but above them 60 min after APO administration. Pretreatment with ascorbic acid (10 mg/kg) or the presence of ascorbic acid in the APO solution (1 mg/ml) inhibited the early (15 min) decrease in seizure threshold caused by 60 mg/kg APO and reversed the increase in seizure threshold 60 min after a 50 mg/kg APO challenge. Ascorbate co-administration concomitant with APO also counteracted the increase in seizure threshold 60 min after 70/kg APO. Results suggest a potential model for modulation of a seizure system through dietary supplementation.

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, there 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: bioavailability and effect on stereotyped cage climbing in mice.

Plasma levels fo apomorphine and its conjugates were studied following intravenous, intraperitoneal, and oral administrations to mice. Following hydrolysis, apomorphine and its conjugates were assayed by high-performance liquid chromatography. The absolute bioavailability of apomorphine was 4%. A significant first-pass effect due to extensive conjugation in the liver was postulated based on calculated bioavailabilities and comparison of plasma levels of apomorphine and its conjugates following oral, intraperitoneal, and intravenous administrations. Apomorphine-induced stereotypical cage climbing in mice was investigated following administration of apomorphine by the three routes. Analysis of time-course data obtained from the cage-climbing experiments indicated an absolute bioavailability of 16% for apomorphine.

Effects of apomorphine and piribedil on pentylenetetrazol-induced seizures in mice.

Based on previous work examining the effects of dextroamphetamine on pentylenetetrazol (PTZ)-induced clonic seizure threshold, the objective of the present study was to determine the effects of two other dopamine agonists, apomorphine (AP) and piribedil, on PTZ seizures. TD50 and LD50 values for CD-1 mice were determined initially for the two drugs. Subsequently, dose- and time-response analyses established that AP decreased PTZ seizure threshold 15 min after administraton, but increased the threshold at 60 min. Piribedil elevated the seizure threshold, but like AP, did not exhibit a clear dose-response relationship. Subsequent blocker studies with phentolamine, (-)sotalol, pimozide, and atropine suggested the possible neurotransmitter systems involved in the modulation of the PTZ-induced seizures by AP and piribedil. Pimozide blocked the changes in seizure threshold induced by both drugs. Atropine also decreased the AP-induced increase in threshold at 60 min. The pattern of inhibition of seizure threshold changes induced by the neurotransmitter blockers suggested that piribedil blocked seizures by means of indirect actions on several neurotransmitters.

Stability of apomorphine in solutions containing ascorbic acid and bisulfite and effects of antioxidants on apomorphine-induced cage climbing and hypothermia in mice.

Ascorbic acid (100 mg/ml) and sodium bisulfite (0.5 and 20 mg/ml) prevented more than 10% oxidation of apomorphine hydrochloride in water maintained at room temperature over 1-3 days. Refrigeration at 5 degrees prevented oxidation of apomorphine hydrochloride in aqueous solutions for 1 week. Neither ascorbic acid nor sodium bisulfite affected murine stereotyped cage climbing or hypothermia induced by apaomorphine.

Polydimethylsiloxane pellets for sustained delivery of morphine in mice.

A new dosage form was designed whereby a polymeric silicone elastomer provided sustained delivery of morphine to mice over 11 days. These pellets, which can be made easily and inexpensively with a standard tablet mold, gradually released morphine sulfate into the implanted mice. Maximal morphine-induced physical dependence, measured by jumping during naloxone-induced withdrawal, was observed 3--5 days after implantation. At this time, slightly less than 50% of the morphine sulfate had been released. Drug release continued through Day 11 and was accompanied by a physical dependence of decreased magnitude compared to that observed on Day 3 or 5.

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.

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 cage-climbing 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.

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.

Hypothermic effects of N-n-propylnorapomorphine in mice: antagonism by neurotransmitter receptor blockers.

Six receptor blockers were compared in mice for their ability to alter hypothermia induced by the dopamine agonist N-n-propylnorapomorphine (NPA). The dopamine antagonist haloperidol inhibited NPA-induced hypothermia whereas phentolamine, propranolol, sotalol, atropine, cyproheptadine, or naloxone (alpha-adrenergic, beta-adrenergic, muscarinic cholinergic, serotonergic, and opiate antagonists, respectively) failed to inhibit it. The degree of antagonism of the hypothermic effect of NPA induced by haloperidol pretreatment suggests that the hypothermic response may serve as a useful model of specific dopaminergic activity of aporphines. Hypothermia induced by NPA and other aporphines may supplement the measure of aporphine-induced stereotypic cage-climbing previously reported to be specifically dopaminergic.

Plasma levels of apomorphine following intravenous, intraperitoneal and oral administration to mice and rats.

Clinical use of the potent dopaminergic partial-agonist apomorphine (APO) in a wide variety of neuropsychiatric disorders is hampered by a lack of data concerning tissue/plasma levels following various routes of administration. In the present experiments, plasma levels were assessed at various times up to 4 hours after APO administration IV, IP, and PO to mice and rats. Plasma levels of total radioactivity after PO administration of [3H]-APO were 50 to 65% of those seen after IV administration, but brain levels were almost undetectable after PO administration. Organic solvent-extractable concentrations of tritium-labelled material after IV and IP administration of [3H]-APO to mice were significantly lower than the levels of total radioactivity, while after PO administration, these concentrations were minimal. Similar results were observed in rats following IV and PO administration of [3H]-APO.