Depletion of dopamine binding sites and changes in dopamine and dihydroxyphenylacetic acid levels in 17- and 90-day-old rat striatum after irreversible receptor antagonism

Age-dependent effects of dopamine receptor inactivation on cocaine-induced behaviors in male rats: Evidence of dorsal striatal D2 receptor supersensitivity

N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which irreversibly inactivates dopamine (DA) receptors, causes pronounced age-dependent behavioral effects in rats. For example, EEDQ either augments or does not affect the DA agonist-induced locomotor activity of preweanling rats while attenuating the locomotion of adolescent and adult rats. The twofold purpose of this study was to determine whether EEDQ would: (a) potentiate or attenuate the cocaine-induced locomotor activity of preweanling, adolescent, and adult rats; and (b) alter the sensitivity of surviving D2 receptors. Rats were treated with vehicle or EEDQ (2.5 or 7.5 mg/kg) on postnatal day (PD) 17, PD 39, and PD 84. In the behavioral experiments, saline- or cocaine-induced locomotion was assessed 24 hr later. In the biochemical experiments, dorsal striatal samples were taken 24 hr after vehicle or EEDQ treatment and later assayed for NPA-stimulated GTPγS receptor binding, G protein-coupled receptor kinase 6 (GRK6), and β-arrestin-2 (ARRB2). GTPγS binding is a direct measure of ligand-induced G protein activation, while GRK6 and ARRB2 modulate the internalization and desensitization of D2 receptors. Results showed that EEDQ potentiated the locomotor activity of preweanling rats, while attenuating the locomotion of older rats. NPA-stimulated GTPγS binding was elevated in EEDQ-treated preweanling rats, relative to adults, indicating enhanced functional coupling between the G protein and receptor. EEDQ also reduced ARRB2 levels in all age groups, which is indicative of increased D2 receptor sensitivity. In sum, the present results support the hypothesis that D2 receptor supersensitivity is a critical factor mediating the locomotor potentiating effects of EEDQ in cocaine-treated preweanling rats.

Dopamine D1 receptor density in the mPFC responds to cognitive demands and receptor turnover contributes to general cognitive ability in mice

In both humans and mice, performance on tests of intelligence or general cognitive ability (GCA) is related to dopamine D1 receptor-mediated activity in the prelimbic cortex, and levels of DRD1 mRNA predict the GCA of mice. Here we assessed the turnover rate of D1 receptors as well as the expression level of the D1 chaperone protein (DRiP78) in the medial PPC (mPFC) of mice to determine whether rate of receptor turnover was associated with variations in the GCA of genetically heterogeneous mice. Following assessment of GCA (aggregate performance on four diverse learning tests) mice were administered an irreversible dopamine receptor antagonist (EEDQ), after which the density of new D1 receptors were quantified. GCA was positively correlated with both the rate of D1 receptor recovery and levels of DRiP78. Additionally, the density of D1 receptors was observed to increase within 60 min (or less) in response to intense demands on working memory, suggesting that a pool of immature receptors was available to accommodate high cognitive loads. These results provide evidence that innate general cognitive abilities are related to D1 receptor turnover rates in the prefrontal cortex, and that an intracellular pool of immature D1 receptors are available to accommodate cognitive demands.

Behavioral effects of dopamine receptor inactivation during the adolescent period: age-dependent changes in dorsal striatal D2(High) receptors

RATIONALE

Dopamine (DA) receptor inactivation produces opposing behavioral effects across ontogeny. For example, inactivating DA receptors in the dorsal striatum attenuates DA agonist-induced behaviors of adult rats, while potentiating the locomotor activity of preweanling rats.

OBJECTIVE

The purpose of this study was to determine if DA receptor inactivation potentiates the DA agonist-induced locomotor activity of adolescent rats and whether alterations in D2(High) receptors are responsible for this effect.

METHODS

In the behavioral experiment, the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) or its vehicle (100 % dimethyl sulfoxide, DMSO) was bilaterally infused into the dorsal striatum on postnatal day (PD) 39. On PD 40, adolescent rats were given intrastriatal infusions of the DA agonist R(-)-propylnorapomorphine (NPA) or vehicle and locomotor activity was measured for 40 min. In the receptor binding experiment, rats received IP injections of EEDQ or DMSO (1:1 (v/v) in distilled water) on PD 17, PD 39, or PD 84. One day later, striatal samples were taken and subsequently assayed for D2-specific binding and D2(High) receptors using [(3)H]-domperidone.

RESULTS

Unlike what is observed during the preweanling period, EEDQ attenuated the NPA-induced locomotor activity of adolescent rats. EEDQ reduced D2 receptor levels in the dorsal striatum of all age groups while increasing the proportion of D2(High) receptors. Regardless of pretreatment condition (i.e., DMSO or EEDQ), preweanling rats had a greater percentage of D2(High) receptors than adolescent or adult rats.

CONCLUSIONS

DA receptor inactivation affects the behaviors of preweanling and older rats differently. The DA supersensitivity exhibited by EEDQ-treated preweanling rats may result from an excess of D2(High) receptors.

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.

Behavioral effects of the reversible dopamine antagonist flupenthixol are not potentiated by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline in the preweanling rat

In the preweanling rat, the irreversible dopamine (DA) receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) does not diminish behaviors induced by the nonselective DA agonist R(-)-propylnorapomorphine (NPA). To determine whether EEDQ was simply inactivating an insufficient percentage of DA receptor antagonist) and/or EEDQ. When given alone, flupenthixol (0.04, 0.1, and 0.4 mg/kg, intraperitoneally [IP]) produced a dose-dependent decrease in the behavioral effects induced by 1.0 mg/kg NPA. Unexpectedly, EEDQ (7.5 mg/kg, IP) did not potentiate flupenthixol's actions. This suggests that EEDQ's inability to block the NPA-induced behaviors of preweanling rats was not the result of an insufficient percentage of DA receptors being inactivated.

Ontogenetic effects of EEDQ on amphetamine-induced behaviors of rats: role of presynaptic processes

Previous research has shown that the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) affects dopamine (DA) synthesis and metabolism in both preweanling and adult rats. In the present study, we attempted to determine the behavioral relevance of EEDQ's presynaptic actions. To that end, 17- and 90-day-old rats were injected with either EEDQ (7.5 mg/kg, IP) or its vehicle 30 min after half the rats were pretreated with the selective DA antagonists SCH 23390 and sulpiride. (SCH 23390/sulpiride pretreatment was used to protect D1 and D2 receptors from EEDQ-induced inactivation.) The behavioral effects of amphetamine (0, 0.1, 0.3 or 1.0 mg/kg, IP) were then assessed 1, 2, 4, and 8 days after EEDQ treatment. Amphetamine-induced behaviors were used to assess EEDQ's presynaptic actions, because amphetamine does not directly bind to the DA receptor, but rather releases DA from the presynaptic terminal. Further, since half of the EEDQ-treated rats had a full complement of DA receptors (i.e., those rats pretreated with SCH 23390/sulpiride), EEDQ's actions in the presynaptic terminal could be dissociated from actions at pre- and postsynaptic receptors. In general, the results showed that EEDQ blocked most of the amphetamine-induced behaviors of both 17- and 90-day-old rats. Surprisingly, pretreatment with SCH 23390 and sulpiride only protected the amphetamine-induced behaviors of adult rats, but not the behaviors of 17-day-old rat pups. When considered together, these results suggest that EEDQ's presynaptic effects are not behaviorally relevant to the adult rat, but may be responsible for eliminating amphetamine-induced behaviors in the 17-day-old rat pup.

Age-dependent differences in the rate of recovery of striatal dopamine D1 and D2 receptors after inactivation with EEDQ

Recovery of striatal dopamine D1 and D2 binding sites in 10-, 16-, and 39-day-old rats was measured 1, 2, 4, and 8 days after irreversible antagonism with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Ontogenetic and EEDQ-induced changes in D1 and D2 binding sites were determined by Scatchard analyses using six concentrations of either [3H]SCH 23390 or [3H]spiperone. Twenty-four hours after EEDQ (7.5 mg/kg) treatment, a significant depletion of both dopamine D1 and D2 receptors was found for all age groups; however, the magnitude of the depletion was greater in 39-day-old rats than in the two preweanling age groups. Both 16- and 39-day-old rats showed significant recovery of dopamine D1 and D2 receptors by the eighth day after EEDQ treatment, but the 16-day-old rats showed a faster recovery of dopamine D1 receptors than did the 39-day-olds. Unexpectedly, 10-day-old rats did not show any evidence of receptor recovery, as the percent control values for these animals did not change across the 8-day recovery period. Pretreatment with the dopamine D1 receptor antagonist SCH 23390 and the dopamine D2 receptor antagonist sulpiride was sufficient to protect dopamine D1 and D2 receptors from EEDQ-induced inactivation. Protein values and receptor affinity (pKd values) were not affected by EEDQ treatment at any of the ages tested. Therefore, these results indicate that the rate of dopamine receptor repopulation varies across ontogeny, with 10-day-old rats exhibiting slower recovery than older rat pups or postweanling rats.

Ontogenetic differences in the effects of EEDQ on dopamine-mediated behaviors

Previous results suggest that 17-day-old rat pups may have substantial reserves of both D1 and D2 receptors. To assess this possibility, the behavioral effects of a nonselective dopamine (DA) agonist, R-propylnorapomorphine (NPA), were measured in 11- and 17-day-old rat pups previously treated with the irreversible DA receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Rat pups were treated with EEDQ (7.5 mg/kg) either alone or in combination with the D1 and D2 antagonists, SCH 23390 (1.0 mg/kg) and sulpiride (100 mg/kg), respectively. (The SCH 23390 and sulpiride were used to protect dopamine receptors from EEDQ-induced inactivation.) NPA's effects on stereotyped sniffing and locomotor activity were then assessed 1, 2, and 4 days after EEDQ pretreatment. Results showed that NPA (0.01, 0.1, 1.0, or 5.0 mg/kg) produced a dose-dependent increase in the stereotyped sniffing of both aged rats. Unexpectedly, however, EEDQ did not disrupt the NPA-induced stereotyped sniffing of either the 11- or 17-day-old rat pups. Thus a behavior (i.e., stereotyped sniffing) that requires the activation of a large complement of DA receptors was not sensitive to the receptor-depleting actions of EEDQ. Moreover, the behaviors of 11-day-old rats, which have fewer DA receptors than older pups or adults, were also not susceptible to the effects of EEDQ. When taken together, these results suggest that EEDQ's inability to block the agonist-induced behaviors of preweanling rat pups cannot be explained by ontogenetic changes in DA receptor reserves.

Behavioral effects of selective and nonselective dopamine agonists on young rats after irreversible antagonism of D1 and/or D2 receptors

In general, preweanling and adult rats respond similarly when challenged with competitive dopamine (DA) agonists or antagonists. In contrast, results using a noncompetitive antagonist suggest that the D1 and D2 receptor systems of preweanling and adult rats differ in some critical way. To further assess this phenomenon, the behavioral effects of irreversible receptor blockade were assessed across 8 days in NPA (a nonselective DA agonist), quinpirole (a D2 agonist), or SKF 38393 (a D1 agonist) treated 17-day-old rat pups. The irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) did not block the locomotor activity and rearing of NPA- or quinpirole-treated rat pups, nor did EEDQ reduce SKF 38393-induced grooming. Moreover, pretreatment with EEDQ appeared to potentiate the normal increases in locomotor activity and rearing produced by NPA, but only when D2 receptors were not protected by a previous injection of sulpiride (a D2 antagonist). Taken together, these results are consistent with the presence of large reserves of D1 and D2 receptors in the preweanling rat pup.