Conditioned locomotion induced by unilateral intrastriatal administration of apomorphine: D(2) receptor activation is critical but not the expression of the unconditioned response

Pharmacological analysis of zebrafish lphn3.1 morphant larvae suggests that saturated dopaminergic signaling could underlie the ADHD-like locomotor hyperactivity

Polymorphisms in the gene coding for the adhesion G-protein coupled receptor LPHN3 are a risk factor for attention-deficit/hyperactivity disorder (ADHD). Transient down-regulation of latrophilin3.1 (lphn3.1), the zebrafish LPHN3 homologue, causes hyperactivity. Zebrafish injected with a lphn3.1-specific morpholino are hyperactive and display an impairment in dopaminergic neuron development. In the present study we used lphn3.1 morphants to further characterize the changes to dopaminergic signaling that trigger hyperactivity. We applied dopamine agonists (Apomorphine, Quinpirole, SKF-38393) and antagonists (Haloperidol, Eticlopride, SCH-23390) to Lphn3.1 morpholino-injected or control-injected animals. The percentage of change in locomotor activity was then determined at three different time periods (10-20 min, 30-40 min and 60-70 min). Our results show that drugs targeting dopamine receptors appear to elicit similar effects on locomotion in zebrafish larvae and mammals. In addition, we observed that lphn3.1 morphants have an overall hyposensitivity to dopamine agonists and antagonists compared to control fish. These results are compatible with a model whereby dopaminergic neurotransmission is saturated in lphn3.1 morphants.

Behavioral effects of dopamine receptor inactivation in the caudate-putamen of preweanling rats: role of the D2 receptor

RATIONALE

Inactivating dopamine (DA) receptors in the caudate-putamen (CPu) attenuates basal and DA agonist-induced behaviors of adult rats while paradoxically increasing the locomotor activity of preweanling rats.

OBJECTIVE

The purpose of this study was to determine (a) whether D1 or D2 receptor inactivation is responsible for the elevated locomotion shown by preweanling rats and (b) whether DA receptor inactivation produces a general state in which any locomotor-activating drug will cause a potentiated behavioral response.

METHODS

Dimethyl sulfoxide (DMSO) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was bilaterally infused into the CPu on postnatal day (PD) 17. In experiment 1, DA receptors were selectively protected from EEDQ-induced alkylation by pretreating rats with D1 and/or D2 antagonists. On PD 18, rats received bilateral microinjections of the DA agonist R(-)-propylnorapomorphine into the dorsal CPu, and locomotor activity was measured for 40 min. In subsequent experiments, the locomotion of DMSO- and EEDQ-pretreated rats was assessed after intraCPu infusions of the selective DA agonists SKF82958 and quinpirole, the partial agonist terguride, or after systemic administration of nonDAergic compounds.

RESULTS

Experiment 1 showed that EEDQ's ability to enhance the locomotor activity of preweanling rats was primarily due to the inactivation of D2 receptors. Consistent with this finding, only drugs that directly or indirectly stimulated D2 receptors produced a potentiated locomotor response in EEDQ-treated rats.

CONCLUSIONS

These results show that DA receptor inactivation causes dramatically different behavioral effects in preweanling and adult rats, thus providing additional evidence that the D2 receptor system is not functionally mature by the end of the preweanling period.

Dopamine receptor inactivation in the caudate-putamen differentially affects the behavior of preweanling and adult rats

The irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) has been used to study the ontogeny of dopamine (DA) receptor functioning in young and adult rats. Most notably, systemic administration of EEDQ blocks the DA agonist-induced behaviors of adult rats, while leaving the behavior of preweanling rats unaffected. The purpose of the present study was to: (a) determine whether the age-dependent actions of EEDQ involve receptors located in the dorsal caudate-putamen (CPu) and (b) confirm that EEDQ's behavioral effects result from the inactivation of DA receptors rather than some other receptor type. In Experiment 1, EEDQ or DMSO was bilaterally infused into the CPu on PD 17 or PD 84. After 24h, rats were given bilateral microinjections of the full DA agonist R(-)-propylnorapomorphine (NPA) or vehicle into the dorsal CPu and behavior was assessed for 40 min. In Experiment 2, preweanling rats were treated as just described, except that DA receptors were protected from EEDQ-induced alkylation by administering systemic injections of D1 (SCH23390) and D2 (sulpiride) receptor antagonists. As predicted, microinjecting EEDQ into the dorsal CPu attenuated the NPA-induced locomotor activity and stereotypy of adult rats. In contrast, rats given bilateral EEDQ infusions on PD 17 exhibited a potentiated locomotor response when treated with NPA. Experiment 2 showed that DA receptor inactivation was responsible for NPA's actions. A likely explanation for these results is that EEDQ inactivates a sizable percentage of DA receptors on PD 17, but leaves the remaining receptors in a supersensitive state. This receptor supersensitivity, which probably involves alterations in G protein coupling, could account for NPA-induced locomotor potentiation. It is likely that adult rats to not show a similar EEDQ-induced change in receptor dynamics or DA receptor inactivation was more complete in older animals and effectively eliminated the expression of DA agonist-induced behaviors.

Importance of D1 and D2 receptors in the dorsal caudate-putamen for the locomotor activity and stereotyped behaviors of preweanling rats

Dopaminergic compounds often affect the unlearned behaviors of preweanling and adult rats differently, although the brain regions underlying these age-dependent behavioral effects have not been specified. A candidate brain region is the dorsal caudate-putamen (CPu); thus, a goal of the present study was to determine whether D1 and D2 receptors in the dorsal CPu are capable of modulating the unlearned behaviors of preweanling rats. In Experiments 1 and 2, selective and nonselective dopamine agonists were bilaterally microinjected into the dorsal CPu on postnatal day (PD) 18 and both locomotor activity and stereotypy were measured. In Experiment 3, the functional coupling of D1 and D2 receptors was assessed by microinjecting the D1 agonist SKF-82958 and the D₂/D₃ agonist quinpirole either alone or in combination. In Experiments 4 and 5, quinpirole and the D1 receptor antagonist SCH-23390, or SKF-82958 and the D2 receptor antagonist raclopride, were co-administered into the dorsal CPu to further assess whether a functional D1 or D2 receptor system is necessary for the expression of quinpirole- or SKF-82958-induced behaviors. Results showed that selective stimulation of D1 or D2 receptors in the dorsal CPu increased both the locomotor activity and stereotypy of preweanling rats. Receptor coupling was evident on PD 18 because co-administration of a subthreshold dose of SKF-82958 and quinpirole produced more locomotor activity than either agonist alone. Lastly, the dopamine antagonist experiments showed that both D1 and D2 receptor systems must be functional for SKF-82958- or quinpirole-induced locomotor activity to be fully manifested. When the present data are compared to results from non-ontogenetic studies, it appears that pharmacological manipulation of D1 and D2 receptors in the dorsal CPu affects the behavior of preweanling and adult rats in a generally similar manner, although some important age-dependent differences are apparent. For example, D1 and/or D2 agonists preferentially induce locomotor activity, and not intense stereotypy, in younger animals.

Caffeine promotes dopamine D1 receptor-mediated body temperature, heart rate and behavioural responses to MDMA ('ecstasy')

RATIONALE

Caffeine exacerbates the acute toxicity of 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') in rats characterised by hyperthermia, tachycardia and lethality. Depletion of central catecholamine stores and dopamine D(1) receptor blockade have been reported to attenuate the ability of caffeine to exacerbate MDMA-induced hyperthermia.

OBJECTIVES

Here, we investigate whether dopamine D(1) and D(2) receptors mediate the effects of caffeine on MDMA-induced changes in body temperature, heart rate and locomotor activity.

METHODS

All parameters were recorded continuously in individually housed rats using bioradiotelemetry from 1 h prior to 4 h following caffeine (10 mg/kg, s.c.) and/or MDMA (10 mg/kg, s.c.) administration.

RESULTS

Co-administration of caffeine with MDMA provoked a switch from MDMA-induced hypothermia and bradycardia to hyperthermia and tachycardia without influencing MDMA-induced hyperlocomotion. Pre-treatment with a specific dopamine D(1/5) antagonist SCH 23390 (1 mg/kg) enhanced MDMA-induced hypothermia and blocked the ability of caffeine to provoke a switch from MDMA-induced hypothermia to hyperthermia. Furthermore, SCH 23390 blocked MDMA-induced hyperactivity and the ability of caffeine to promote a tachycardic response to MDMA. By contrast, pre-treatment with the selective D(2) antagonist, sulpiride (100 mg/kg) blocked MDMA-induced hypothermia, failed to influence the ability of caffeine to promote tachycardia whilst enhancing MDMA-induced hyperactivity.

CONCLUSIONS

Our results highlight the importance of dopamine D(1) and D(2) receptors in shaping the behavioural and physiological response to MDMA and suggest that the ability of caffeine to provoke MDMA-induced toxicity is associated with the promotion of dopamine D(1) over D(2) receptor-related responses.

Apomorphine-induced context-specific behavioural sensitization is prevented by the D1 antagonist SCH-23390 but potentiated and uncoupled from contextual cues by the D2 antagonist sulpiride

RATIONALE

In the study of behavioural sensitization induced by dopamine agonists, D1 and D2 receptors have a critical, but a puzzling role.

OBJECTIVE

The objective of this study is to examine the effects of the D1 antagonist SCH-23390 and the D2 antagonist sulpiride given repeatedly alone or in combination with apomorphine upon apomorphine conditioning and sensitization.

METHODS

Apomorphine-induced (2.0 mg/kg) conditioning and sensitization were assessed following five paired/unpaired treatments. Sulpiride (10, 30 and 100 mg/kg) and SCH-23390 (0.01, 0.02 and 0.05 mg/kg) were administered alone or in combination with apomorphine. In experiment 1, the effect of 5 days of sulpiride and SCH-23390 treatments given alone were assessed on apomorphine reactivity. In experiment 2, sulpiride and SCH-23390 were co-administered with apomorphine for 5 days and subsequently, conditioning and sensitization tests were performed. In experiment 3, following five apomorphine treatment sessions, sulpiride and SCH-23390 were administered prior to the conditioning and sensitization tests.

RESULTS

SCH-23390 and sulpiride induced hyper-reactivity to apomorphine. SCH-23390 when given after the induction of apomorphine sensitization, blocked the expression of apomorphine sensitization. When given in combination with apomorphine, SCH-23390 blocked the apomorphine conditioning and sensitization, whereas low-dose sulpiride permitted conditioning and enhanced apomorphine sensitization and high-dose sulpiride blocked conditioning but permitted apomorphine sensitization. Both sulpiride doses transformed apomorphine sensitization from context-specific to context-independent sensitization.

CONCLUSION

The SCH-23390 findings are supportive of a critical role for D1 receptors in apomorphine effects whereas the sulpiride effects diminish the importance of conditioning and dopamine autoreceptor subsensitivity mechanisms in the mediation of apomorphine sensitization.

Maternal separation fails to render animals more susceptible to methamphetamine-induced conditioned place preference

The maternal separation (MS) paradigm is an animal model that has been successfully used to study the long term effects of child abuse and neglect. Experiments showed that animals subjected to trauma and stress early in life display behavioural, endocrinological and growth factor abnormalities at a later stage in life, results that mirrored clinical conditions. It is apparent that adverse events early in life may affect the development and maturation of the brain negatively. The purpose of the present study was to investigate whether the abnormal brain development occurring in separated animals would also enhance the development of a preference for psychostimulant drug usage. Rats were subjected to maternal deprivation and further exposed to methamphetamine-induced conditioned place preference (CPP) which primarily measures drug reward (ventral striatum) learning and memory. Apomorphine-induced locomotor activity was also assessed to investigate the effects of methamphetamine on the dorsal (primarily locomotor activity) striatal dopaminergic system. We found that four consecutive injections of methamphetamine resulted in CPP behaviour 24 h after the 4th injection. A further four injections yielded similar CPP results and this effect lasted for at least 7 days until the third CPP assessment. These animals also had decreased ACTH and corticosterone secretions, but the prolactin levels were increased. Prior exposure to maternal separation did not have any effect on the CPP test. The ACTH and corticosterone secretions were also similarly reduced. However maternal separation decreased the release of prolactin and this reduction was not evident in the separated group that received methamphetamine. There was no significant difference in the apomorphine-induced locomotor activity of normally reared animals whether they received methamphetamine or saline. Interestingly there was a significant difference in locomotor activity between the two groups of animals that were subjected to maternal deprivation. The separated animals that received methamphetamine displayed markedly reduced locomotor activity upon apomorphine administration when compared to those that were treated with saline. Taken together, we conclude that maternal deprivation differentially influences dorsal and ventral striatal regions implicating dopaminergic mechanisms.

Exacerbated mechanical allodynia in rats with depression-like behavior

Although a clinical connection between pain and depression has long been recognized, how these two conditions interact remains unclear. Here we report that both mechanical allodynia and depression-like behavior were significantly exacerbated after peripheral nerve injury in Wistar-Kyoto (WKY) rats, a genetic variation of Wistar rats with demonstrable depression-like behavior. Administration of melatonin into the anterior cingular cortex contralateral to peripheral nerve injury prevented the exacerbation of mechanical allodynia with a concurrent improvement of depression-like behavior in WKY rats. Moreover, there was a lower plasma melatonin concentration and a lower melatonin receptor expression in the anterior cingular cortex in WKY rats than in Wistar rats. These results suggest that there exists a reciprocal relationship between mechanical allodynia and depression-like behavior and the melatoninergic system in the anterior cingular cortex might play an important role in the interaction between pain and depression.

Food restriction markedly increases dopamine D2 receptor (D2R) in a rat model of obesity as assessed with in-vivo muPET imaging ([11C] raclopride) and in-vitro ([3H] spiperone) autoradiography

INTRODUCTION

Dopamine (DA) regulates food intake by modulating food reward and motivation but its involvement in obesity is much less understood. Recent evidence points to the involvement of leptin in the DA-related modulation of food intake. Here we assess DA D2 receptors (D2R) in a genetic rodent obesity model characterized by leptin-receptor deficiency and assess the influence of food restriction on these receptors.

METHODS

We compared D2R levels between Zucker Obese (fa/fa) and Lean (Fa/Fa) rats at 1 and 4 months of age and in two different feeding conditions (restricted and unrestricted food access) using in-vivo muPET imaging ([11C] raclopride, which is a method sensitive to competition with endogenous DA) and in-vitro ([3H] spiperone washed to ensure no competition with endogenous DA) autoradiography (ARG).

RESULTS

Both ARG and muPET showed that D2R were higher at 1 month than at 4 months of age and that food restricted animals had higher D2R than unrestricted animals. However there were significant differences in the results obtained at 4 months between ARG and muPET. ARG showed that at 1 month and at 4 months unrestricted lean rats (Le U) had significantly higher D2R binding than obese unrestricted rats (Ob U) but showed no differences between restricted obese (Ob R) and restricted lean rats (Le R). It also showed that D2R decline between 1 and 4 months of age was significantly attenuated in food restricted rats [both obese and lean]. In contrast, muPET showed that at 4 months of age, Ob U showed greater D2R availability than Le U rats but like ARG showed no differences between Ob R and Le R rats.

CONCLUSION

The lower D2R binding in Ob U than Le U rats observed with ARG most likely reflects decreases in striatal D2 receptors levels whereas the increased availability observed with muPET is likely to reflect reduced DA release (resulting in decreased competition with endogenous DA). Lack of a significant difference between Ob R and Le R suggests that the differences in dopamine activity and D2R levels between Ob and Le Zucker rats are modulated by access to food. The ARG finding of an attenuation of the age-related loss of D2R binding corroborates previous studies of the salutary effects of food restriction in the aging process. Because [11C] raclopride is sensitive to competition with endogenous DA, the higher D2R binding in obese rats with raclopride despite the lower D2R levels shown with spiperone could reflect lower extracellular DA in the Ob rats and merits further investigation.

Behavioral sensitization produced by a single administration of apomorphine: Implications for the role of Pavlovian conditioning in the mediation of context-specific sensitization

The present study examined the minimal number of exposures to the D1/D2 agonist apomorphine capable of producing behavioral sensitization. Rats received one (experiment 1) or two administrations on two successive days (experiment 2) of apomorphine (0.5 and 2.0 mg/kg) paired or unpaired to an open-field environment. After 2 days of drug withdrawal, the rats received a challenge injection with the same dose of apomorphine (sensitization test) and locomotion, rearing and sniffing were measured. The results of the first experiment showed that locomotor sensitization occurred after a single acute exposure to apomorphine and that 0.5 and 2.0 mg/kg treatments were equally effective. This sensitization effect was context-specific and was limited to locomotion. The second experiment revealed a differential dose effect on the sensitization test. Two treatments with 2.0 mg/kg potentiated locomotor sensitization as compared with a single treatment but two treatments with 0.5 mg/kg did not increase the sensitization effect more than the single 0.5 mg/kg treatment. This result indicates an interaction between drug dose and frequency of drug treatment for the induction of apomorphine locomotor sensitization. In that the sensitization effects are considered to be a core contributor to psychostimulant addiction, the present findings are of importance to understanding addiction because they indicate that sensitization processes can be initiated with a single drug experience and amplified with exposure to higher drug dosage levels.

Comparing nucleus accumbens and dorsal striatal dopamine responses to self-administered cocaine in naïve rats

Dopamine (DA) responses in the nucleus accumbens (NAcc) and dorsal striatum (DS) are commonly associated with different aspects of cocaine effects. Enhanced NAcc DA has been most convincingly linked with the positive reinforcing effects of cocaine, while DS DA is thought to mediate cocaine-induced motoric effects. Though several studies have shown NAcc DA enhancement following cocaine self-administration, very little work has examined the effects of cocaine self-administration on DS DA. In this study, DA levels in the NAcc and DS, and locomotor responses to a single self-administered cocaine injection (1.5mg/kg) were assessed in operant-trained, drug-naïve Sprague-Dawley rats. Locomotor activity, NAcc and DS DA levels increased significantly over baseline activity immediately after cocaine injection. However, while basal and cocaine-stimulated NAcc DA concentrations (nM) were significantly greater than DS DA levels, the magnitude of response was statistically comparable between brain regions. These findings indicate that, though both the NAcc and DS are importantly involved in the dopaminergic response to self-administered cocaine in drug-naïve rats, basal DA differences in dialysis data are obscured by statistical conversions to baseline percentages.