Sensitization to apomorphine in pigeons: unaffected by latent inhibition but still due to classical conditioning

When administered apomorphine, pigeons exhibit protracted bouts of pecking behavior. This response is subject to sensitization, as it initially increases with repeated drug injections. The hypothesis is examined that the sensitization is due to a Pavlovian conditioning of the drug-induced pecking to the environment in which it first takes effect. In a first experiment, we attempted to suppress this conditioning by extensively pre-exposing the birds to the test environment and saline injections (latent inhibition procedure). As the experiment yielded undiminished sensitization, it cast doubt on the conditioning hypothesis. However, while inhibitory pretraining also proved ineffective in a second experiment, a shortening of response latencies specific to the environment in which the animals had first experienced the apomorphine effect supported the conditioning hypothesis. It is suggested that the absence of latent inhibition may be due to the interference of a context-dependent conditioning effect. A third experiment that examined the hypothesis that the reinforcing properties of apomorphine might be attributable to its well known anorectic properties. The results provided some support for this notion. At the same time, they also confirmed that apomorphine-induced pecking conditions reliably to environmental cues. These cues are then by themselves capable of provoking conditioned pecking.     

Sensitization to and conditioning with apomorphine in pigeons

Pigeons that repeatedly experienced the effect of apomorphine in the same environment showed an augmented behavioural response to the same drug dose as compared with controls that experienced the effect of the drug dose in differing environments. Sensitization, an increase in the behavioural response that is observed in pigeons when the same dose of apomorphine is repeatedly administered, may thus be mainly due to a conditioning of the drug response to incidental environmental cues. Apomorphine injections also induced place preferences. Pigeons that had experienced a particular environment under the influence of apomorphine subsequently favoured that environment to one they had experienced while under saline. This suggests that apomorphine administration has reinforcing properties for birds, much as it has for mammals.     

Subcutaneous injections of apomorphine, stimulus generalization and conditioning: serious pitfalls for the examiner using apomorphine as a tool.

This report shows that stimulus generalization occurs in rats conditioned by a single injection of apomorphine. The data suggest that apomorphine initially acts as an unconditioned stimulus (UCS) of an unconditioned response (UCR) that, in turn, produces stimuli which become conditioned stimuli (CS) of a conditioned response (CR) having a nature identical to that of the UCR. The study also shows that behaviour elicited by a subcutaneous injection of apomorphine depends on the part of the body selected for administration. The mentioned properties should be taken into account when apomorphine is used as a tool in studies on brain and behaviour.     

Sensitization to cocaine in pigeons: interaction with an operant contingency

A 2-part study with pigeons investigated the role of an explicit operant contingency in determining how cocaine interacts with locomotor activity. In Part 1, pigeons pecked on a fixed-ratio-20 schedule of food presentation. In Part 2, different pigeons were studied without opportunity to peck for food. After determination of cocaine's initial effects, pigeons were exposed to daily administrations of a locomotion-increasing dose of cocaine. Locomotor sensitization was evident in the pigeons of Part 2, and tolerance developed to cocaine's effects on key pecking in the pigeons of Part 1. Locomotor sensitization was generally not evident in the pigeons of Part 1. These results suggest that explicitly conditioned operant behavior may compete with behavior sensitized by prolonged exposure to cocaine.     

Behavioral response to apomorphine and its interaction with opiates in domestic pigeons

Domestic pigeons received peripheral injections of saline or the dopamine agonist apomorphine (AM) at doses of 0.025, 0.05, 0.1, 0.25, 0.5 or 1 mg and their behavior was studied for 30 min after these treatments. Given at a dose of 0.025 mg, AM decreased pecking, whereas doses ranging from 0.1 to 1 mg strongly stimulated this behavior. The frequency of headshaking was enhanced by the administration of each dose of AM; at the 3 higher doses, the drug also attenuated the frequency of preening. In another experiment, AM was administered 40 min after the injection of either naloxone (0.5, 1 or 4 mg), the opiate agonist levorphanol (0.25, 0.5 or 1 mg) or its dextroisomer, dextrorphan (0.25, 0.5 or 1 mg), while the birds were observed as before. No interaction between AM and either naloxone or dextrorphan was detected. By contrast, injection of each dose of levorphanol attenuated preening, and completely antagonized the stimulating effect of AM treatment on headshaking. At a dose of 1 mg, levorphanol also slightly decreased the frequency and increased the latency of occurrence of pecking. It is concluded that in pigeons, opiates modulate the behavioral response to apomorphine in a complex fashion.     

Context-dependent catalepsy intensification is due to classical conditioning and sensitization

Haloperidol-induced catalepsy represents a model of neuroleptic-induced Parkinsonism. Daily administration of haloperidol, followed by testing for catalepsy on a bar and grid, results in a day-to-day increase in catalepsy that is completely context dependent, resulting in a strong placebo effect and in a failure of expression after a change in context. The aim of this study was to analyse the associative learning process that underlies context dependency. Catalepsy intensification was induced by a daily threshold dose of 0.25 mg/kg haloperidol. Extinction training and retesting under haloperidol revealed that sensitization was composed of two components: a context-conditioning component, which can be extinguished, and a context-dependent sensitization component, which cannot be extinguished. Context dependency of catalepsy thus follows precisely the same rules as context dependency of psychostimulant-induced sensitization. Catalepsy sensitization is therefore due to conditioning and sensitization.     

Sensitization to amphetamine,but not phencyclidine,disrupts prepulse inhibition and latent inhibition

Rationale Schizophrenia has been linked to dysregulation of dopamine and glutamate transmitter systems. Attempts to model aspects of schizophrenia in animals have made use of treatments that primarily affect dopaminergic (e.g., amphetamine, Amp) and glutamatergic (e.g., phencyclidine, PCP) function. In addition to exerting short-term acute effects, these agents also induce long-term effects, as seen, for example, in neurochemical and behavioural sensitization.Objectives The goal of this work was to compare Amp- and PCP-sensitized states on two measures of information processing that are impaired in schizophrenia, prepulse inhibition (PPI) of the acoustic startle reflex and latent inhibition (LI).Methods Rats received injections of Amp, PCP or saline 3 days per week for 3 weeks. The Amp dose increased from 1 to 3 mg/kg, at the rate of 1 mg/kg each week. The PCP dose was 3 mg/kg throughout. After various periods of withdrawal rats were tested for PPI and LI.Results Repeated intermittent treatment with Amp or PCP resulted in augmented locomotor responses to challenge with each drug, providing an operational index that sensitization had occurred. Rats sensitized to Amp showed disrupted PPI when tested drug free at 3, 21 and 60 days of withdrawal. Amp-sensitized rats also showed abolition of the LI effect. Rats sensitized to PCP did not show deficits in any of these behaviours when tested drug free.Conclusions Because disrupted PPI and LI have both been reported in schizophrenic patients, these results suggest that the Amp-sensitized state may represent a useful model for investigating the neural bases of information processing deficits in schizophrenia.     

The effects of nicotine on conditioning,extinction, and reinstatement in humans

Nicotine has been shown to enhance the reinforcement and reward-responsiveness of non-nicotine stimuli. To determine whether nicotine enhances the strength of conditioning to context, undergraduate participants with varying levels of nicotine dependence were recruited for a two-day study and tested on a virtual reality (VR) conditioned place preference (CPP) paradigm. On day one, participants explored two virtual rooms where they received multiple pairings of M&M rewards in one room and no rewards in the other room, followed by a free-access test session with no rewards. On day two, participants received three test sessions to assess extinction. Subsequently, participants received M&Ms. in a novel context and were then tested for reinstatement. Prior to testing on each day, subjects were administered either nicotine (4 mg) or placebo lozenges, in a between-subjects, four-group, 2 × 2 design (nicotine or placebo on days 1 and 2). After conditioning on day one, only participants who received placebo exhibited a CPP by spending significantly more time in the room previously-paired with M&Ms. Contrary to our hypothesis, nicotine-treated participants did not display a significant CPP, and there were no significant differences between treatment groups. However, post hoc analysis indicated that in a subset of participants with greater nicotine dependence, the nicotine group displayed a CPP by rating the M&M-paired room as significantly more enjoyable than those who received placebo. Additionally, while neither treatment group showed significant place preferences during the first two extinction sessions on Day 2, individuals who received nicotine on Day 1 or placebo on Day 2 spent significantly more time in the M&M-paired room during the final extinction session. Finally, those who received nicotine on Day 2 exhibited significantly greater reinstatement compared to placebo-treated participants. These results partially support preclinical evidence that nicotine can affect learning, extinction, and reinstatement.     

Relations between dose magnitude, subject sensitivity, and the development of tolerance to cocaine-induced behavioral disruptions in pigeons

Keypecking by 12 pigeons, maintained by a fixed-ratio 30 schedule of food presentation, was decreased in rate by acute pre-session injections of cocaine in a dose-dependent manner, with larger doses producing more disruption. A constant dose of cocaine was then injected prior to every session for 40 days. Some subjects received a relatively small dose, some received a medium-sized dose, and others received a large dose. Subsequently, dose-effects were reassessed via once-weekly probe injections, with every other session continuing to be preceded by injection of the daily dose of cocaine. Then a different dose of cocaine was administered daily for 40 more days, after which the dose-effect function was redetermined in like manner. In general, tolerance to cocaine-induced response-rate reductions was most likely to develop when (a) the repeatedly-administered dose of cocaine was relatively small (even without acute effect on keypecking) and (b) the subject's keypecking was disrupted by smaller doses of cocaine in the initial dose-effect assessment. Tolerance was generally observed as a shift in the dose-effect function that, in several cases, could be eliminated by increasing the magnitude of the daily administered dose. In addition, every subject's rate of keypecking following saline injections was lowered after daily exposure to cocaine. These results (a) are partially consistent with the reinforcement-loss account of tolerance to cocaine-induced behavioral disruptions, and (b) support previous observations of withdrawal symptoms following cessation of extended exposure to cocaine.     

Disruption of selective attention by apomorphine,but not amphetamine,in the mongolian gerbil

To test the hypothesis that apomorphine, but not amphetamine, disrupts selective attention to a novel stimulus, gerbils were exposed to a novel object for one 60-s trial following an injection of 0, 1, 3, or 6 mg/kg d-amphetamine base, or 0, 0.1, 0.3, 1, 3, or 10 mg/kg apomorphine HClSC. They were tested the next day for habituation to the stimulus. As a control, half of each group of gerbils were injected but not exposed to the object on day 1. All non-exposed gerbils and all exposed gerbils that received amphetamine showed a decrement in investigation, indicative of habituation, on day 2. Furthermore, a gradient of responding during dishabituation was obtained from gerbils given d-amphetamine (1 mg/kg) which was dependent on the distance a novel object was moved, indicating a perception of location as occurs in normal gerbils. In contrast, those exposed gerbils that received 1 mg/kg or more of apomorphine did not show habituation on day 2. That the disruption of habituation by apomorphine was due to a failure of input rather than of retrieving the information was demonstrated in an experiment in which two groups of gerbils were habituated to a novel object prior to injection with apomorphine (1 mg/kg) or saline. Both groups continued to show habituation on subsequent trials and increased responding when the object was moved. Thus, the motor capabilities necessary for investigation were functional. When gerbils that received apomorphine were pretreated with the dopamine receptor blocker pimozide, habituation occurred on day 2, suggesting that the disruption of habituation was mediated by dopamine. On the other hand, the depressant effect of large doses of apomorphine on initial investigation was not blocked completely by pimozide.     

Endothelium-derived hyperpolarizing factor, but not nitric oxide or prostacyclin release, is resistant to menadione-induced oxidative stress in the bovine coronary artery

The interaction of superoxide anions (O₂ ^–) generated by menadione with the synthesis and/or action of nitric oxide (NO), prostacyclin (PGI₂) and endothelium-derived hyperpolarizing factor (EDHF) was investigated in segments of the left anterior descending coronary artery (LAD) isolated from bovine hearts. EDHF and NO release were monitored by superfusion bioassay in segments pre-constricted with the thromboxane mimetic, U46619, PGI₂ release in addition by enzyme immunoassay for 6-keto-prostaglandin F_1α, and generation of O₂ ^– by lucigenin-enhanced chemiluminescence. Bradykinin (1–1,000 pmol) elicited a prominent, endothelium-dependent, relaxant response, 50–60% of which was insensitive to combined blockade of cyclooxygenase with diclofenac (1 μM) and NO synthase with N ^G-nitro-l-arginine (50 μM). This diclofenac/N ^G-nitro-l-arginine-insensitive relaxant response was completely abrogated in the presence of tetrabutylammonium (3 mM), a non-selective inhibitor of Ca²⁺-dependent K⁺ channels, or when the segments were pre-constricted with potassium chloride (60 mM) instead of U46619, and thus most likely mediated by EDHF. Despite causing a two- to fourfold increase in the concentration of O₂ ^– in or at the vessel wall, menadione (30 μM) did not affect the diclofenac/N ^G-nitro-l-arginine-insensitive relaxant response to bradykinin. When administered in the absence of N ^G-nitro-l-arginine, however, menadione significantly inhibited the relaxant response to bradykinin, presumably by attenuating the NO-mediated relaxation. Menadione also abolished the bradykinin-stimulated release of PGI₂ from luminally perfused segments of the LAD. This effect was more pronounced in the absence of N ^G-nitro-l-arginine, indicating that PGI₂ release in this preparation may be more sensitive to inhibition by peroxynitrite, the reaction product of NO and O₂ ^–, than to O₂ ^– alone. These findings suggest that, in contrast to NO and PGI₂, the release, and presumably also the mechanism of action, of EDHF in the bovine LAD is resistant to an increase in the local concentration of O₂ ^– or peroxynitrite which is thought to play an important role in coronary heart disease. Received: 27 August 1998 / Accepted: 18 November 1998     

Sensitization to cocaine is inhibited after intra-accumbal GR103691 or rimonabant, but it is enhanced after co-infusion indicating functional interaction between accumbens D3 and CB1 receptors

Rationale

Dopamine D₃ receptors and cannabinoid CB₁ receptors are both expressed in the nucleus accumbens, and they have been involved in motor sensitization to cocaine. The objectives were: (1) to study the effects of blockade of these receptors on sensitization to repeated cocaine, by using GR103691, D₃ receptor blocker, and rimonabant, CB₁ receptor ligand, and (2) to discern if both receptors interact by co-infusing them.

Materials and methods

Cocaine (10?mg/kg) was injected daily for 3?days (induction phase) and later on day?8 (expression phase), and locomotor activity was measured during 2?h after cocaine. GR103691 and rimonabant were bilaterally injected (0.5???l volume of each infusion) in the nucleus accumbens through cannulae (GR103691, 0, 4.85, and 9.7???g/??l; rimonabant, 0, 0.5, and 1.5???g/??l), before cocaine, during either induction or expression phases of sensitization.

Results

The findings indicated that sensitizing effects of cocaine were abolished after D₃ receptor blocking during both induction and expression phases, as well as rimonabant infusion during the expression (not induction) phase. A functional interaction between both receptors was also observed, because if GR103691 was injected during induction and rimonabant during expression, sensitizing effects of cocaine were observed to be normal or further enhanced.

Conclusion

Dopamine D₃ receptors within the nucleus accumbens are critical for the development and consolidation of sensitization, and cannabinoid CB₁ receptors are critical for the expression of sensitization. Co-blockade of D₃ and CB₁ receptors exert opposite effects to blockade of these receptors separately, revealing the existence of a functional interaction between them.     

MLH1-deficient tumor cells are resistant to lipoplatin, but retain sensitivity to lipoxal

Lipoplatin, currently under phase III evaluation, is a novel liposomal cisplatin formulation highly effective against cancers. Lipoplatin has eliminated or reduced the systemic toxicity frequently seen for cisplatin. The objective of the present study was to determine whether the cytotoxic effect of lipoplatin is dependent on the functional integrity of DNA mismatch repair (MMR), a post-replicative DNA repair machinery implicated in cell cycle control and apoptosis. Clonogenic data revealed a significant (P<0.05) 2-fold resistance to lipoplatin of HCT116 human colorectal adenocarcinoma cells lacking MLH1, one of five proteins crucial to MMR function, as compared to MLH1-expressing HCT116 cells. In addition, MLH1-deficient cells were at least 3-fold less susceptible to apoptosis (DNA fragmentation) than MLH1-proficient cells. However, proteolytic processing of caspase-3, caspase-7 and poly(ADP-ribose)polymerase-1 following lipoplatin treatment was comparable in MLH1-deficient cells and -proficient cells. Furthermore, MLH1-deficient cells retained the ability to attenuate cell cycle progression past the G2/M checkpoint following lipoplatin treatment. In conclusion, our results indicate that the lipoplatin-sensitive phenotype of MLH1-proficient cells correlated with increased apoptosis which may occur via caspase-independent pathways. They also suggest that the integrity of MMR function is a relevant determinant accounting for the cytotoxicity of lipoplatin. However, this does not seem to apply to lipoxal, a novel liposomal formulation of oxaliplatin, because MLH1-deficient cells were as sensitive to lipoxal as MLH1-proficient cells.     

Responses to apomorphine,amphetamine, and neuroleptics in schizophrenic subjects

Twenty-one schizophrenic subjects, who had been neuroleptic-free, were tested for responsiveness to dopaminergic agonists: Apomorphine emesis threshold was determined and change in psychopathology after 0.5 mg/kg d-amphetamine orally was rated. The subjects' subsequent response to neuroleptic treatment were also determined. Sensitivity to apomorphine emesis was also determined in a nonschizophrenic control group. Apomorphine emesis threshold was not significantly different in the schizophrenic and control groups. Correlations were done between baseline psychopathology, apomorphine sensitivity, and changes in psychopathology after amphetamine and after neuroleptic treatment. On the Brief Psychiatric Rating Scale (BPRS), baseline psychopathology correlated with improvement after neuroleptics and, on the clinical global impressions (CGI), increase of psychopathology after amphetamine also correlated with improvement after neuroleptic treatment. An inverse correlation was found between several indices of sensitivity to amphetamine (psychopathology change) and emetic sensitivity to apomorphine. An examination of individual subjects' responses to amphetamine and, subsequently, neuroleptics, suggested that in the absence of significant clinical change after amphetamine a brisk therapeutic response to neuroleptics was rare:     

SCH 23390, a selective dopamine D1 antagonist, activates dopamine neurons but fails to prevent their inhibition by apomorphine

SCH 23390, a rather selective D1 receptor blocker, activates the firing rate of dopamine (DA) neurons in the substantia nigra (SN-DA neurons) in rats, similarly to haloperidol (a D1-D2 receptor antagonist) and sulpiride (a selective D2 receptor blocker). These neuroleptics produce no additional increase over the maximal activation produced by SCH 23390. Unlike haloperidol or sulpiride, SCH 23390 fails to prevent the inhibition by apomorphine of SN-DA neurons, a DA autoreceptor-mediated effect. It is suggested that the doses of SCH 23390 that stimulate DA neurons block D2 in addition to D1 receptors, or that D1 blockade results in the functional inactivation of a specific population of D2 receptors as well. The failure of SCH 23390 to block the apomorphine effect indicates that DA autoreceptors can be pharmacologically differentiated form postsynaptic DA receptors.     

Individual differences in behavior following amphetamine,GBR-12909, or apomorphine but not SKF-38393 or quinpirole

Subjects that respond more to a novel environment show a greater locomotor response to drugs of abuse such as cocaine and amphetamine. The current study was performed to examine differences between high (HR) and low (LR) responding rats to a novel environment following administration of amphetamine, a selective dopamine uptake blocker (GBR-12909), a nonselective dopamine agonist (apomorphine), and selective dopamine D₁ and D₂/D₃ agonists. A behavioral checklist and a rating scale were used to determine the behavioral arousal caused by administration of amphetamine (0, 0.5, 2.0, and 8.0 mg/kg), GBR-12909 (0, 1.25, 5.0, and 20.0 mg/kg), apomorphine (0, 0.1, 0.3, and 1 mg/kg), SKF 38393 (0, 2.5, 10, and 40 mg/kg), or quinpirole (0, 0.05, 0.5, and 5.0 mg/kg). The five drugs produced behavioral activation profiles distinct from each other. Following amphetamine administration, both HR and LR subjects showed dose dependent increases in behavioral arousal. The behaviors primarily affected were sniffing, locomotor activity, rearing, and oral activity. HR rats showed a greater overall behavioral response to amphetamine administration compared with LR rats and there were differences in specific behaviors between the two groups. Following GBR-12909 administration, all subjects showed dose dependent increases in sniffing, locomotor activity, and rearing. Differences between HR and LR were observed in sniffing, locomotor activity, and rearing behaviors. HR and LR both showed dose dependent increases in behavior following apomorphine administration. HR showed greater behavioral activation after apomorphine than LR. SKF 38393 produced pronounced increases in the amount of sniffing, grooming, and intense grooming, in addition to increasing the overall behavioral rating of all subjects, while quinpirole produced increases in sniffing, locomotor activity, and oral movements. However, the behavioral effects of SKF 38393 and quinpirole did not differ between HR and LR. These results suggest that activation of the dopamine system but probably not only one type of dopamine receptor is sufficient to produce behavioral differences between high and low responding subjects.