Unilateral inactivation of dopamine receptors after intrastriatal injection of N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ): a novel rotational model to investigate dopamine receptor interactions

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.

Lead exposure and dorsomedial striatum mediation of fixed interval schedule-controlled behavior

Prior studies demonstrate a critical role for mesolimbic dopamine systems, particularly nucleus accumbens, in the mediation of fixed interval (FI) schedule-controlled behavior and an enhancement of nucleus accumbens dopamine activity as a mechanism of chronic postweaning lead (Pb)-induced increases in Fl response rates. Since dorsomedial striatum, like nucleus accumbens, receives limbic input, it could also conceivably contribute to Pb-related effects on FI performance. Therefore, changes in FI schedule-controlled behavior were examined following administration of dopamine or the non-specific irreversible dopamine antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) into dorsomedial striatum of rats exposed from weaning to 0, 50 or 500 ppm Pb acetate drinking solutions. The 500 ppm exposure increased baseline FI response rates relative to both 0 and 50 ppm. Intra-dorsomedial striatum EEDQ and dopamine had no effects when examined across all animals. However, both compounds produced rate-dependent effects, i.e. increases or decreases in rate in different subjects, depending upon baseline Fl overall rates. The rate-increasing effects of intra-dorsomedial striatum dopamine actually mimicked Pb effects, increasing Fl overall and run rates and shortening postreinforcement pause times. Further, Pb exposure modulated effects of dopamine and EEDQ in dorsomedial striatum. While these collective findings conceivably suggest dorsomedial striatum as another potential site through which postweaning Pb exposure influences FI performance, this possibility is not supported by other studies that show that chronic postweaning Pb alters dopamine binding sites and evoked dopamine release in nucleus accumbens but not in dorsomedial striatum even over a year exposure period. Thus, while both regions may play a role in mediating Fl performance under normal conditions, it appears that alterations in nucleus accumbens dopamine activity may be sufficient to induce chronic postweaning Pb-induced increases in FI response rates.

The critical role of nucleus accumbens dopamine systems in the mediation of fixed interval schedule-controlled operant behavior

Microinjections of the irreversible dopamine antagonist EEDQ (N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline), into nucleus accumbens but not into dorsal striatum, markedly decreased response rates on a fixed interval schedule of reinforcement. These preferential effects in nucleus accumbens could be prevented by selectively activating either D1 or D2 receptors. These data demonstrate a critical role for mesolimbic dopamine systems in the mediation of fixed interval schedule-controlled operant behavior.

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.

Separate and combined effects of dopamine D1 and D2 receptor agonists on key pecking in the developing chick

The separate and combined effects of dopamine D1 and D2 agonists on key-peck responding of young chicks for heat reinforcement were determined. In the first experiment, 1- and 4-day-old chicks (n = 96) were injected SC with either distilled water or 5 mg/kg reserpine 18 h before testing. Twenty minutes before a 24-trial autoshaping session, chicks were injected IP with either distilled water or 10 mg/kg SKF 38393 and 2 mg/kg quinpirole. Chicks receiving both dopamine agonists had enhanced key-peck responding in both pretreatment conditions. In Experiment 2, 1- and 4-day-old chicks (n = 192) of two strains received 5 mg/kg reserpine SC 18 h before testing. Twenty minutes before their autoshaping session these chicks were injected IP with either distilled water, 10 mg/kg SKF 38393, 2 mg/kg quinpirole, or 10 mg/kg SKF 38393 and 2 mg/kg quinpirole. After reserpine pretreatment, chicks of both strains responded on more trials when given both dopamine agonists compared with controls given either distilled water or single agonist treatment. No age or strain differences in key pecking were produced by the combined administration of these dopamine agonists. Therefore, functional coupling of the dopamine D1 and D2 receptors is found within 1 day after hatching in the domestic chick.

An autoradiographic study of the differential effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on striatal and extrastriatal D-1 and D-2 dopamine receptors in the rat

The effect of in vivo administration of the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on striatal and extrastriatal D-1 and D-2 dopamine (DA) receptors was investigated in the rat. N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline treatment reduced specific [3H]SCH 23390 (7-chloro-8-hydroxy-2,3,4,5-tetrahydro-3-methyl-1-phenyl-1H-3-benzaze pin e) binding to D-1 DA receptors in the striatum (42-46% of saline-treated controls), entopeduncular nucleus (20%) and substantia nigra pars reticulata (23%). Similarly, specific [3H]spiperone binding to D-2 DA receptors was decreased in the striatum (28-37% of saline-treated controls). However, [3H]spiperone binding in the substantia nigra pars compacta (67%) was much less affected. In vivo pretreatment with the D-1 DA antagonist SCH 23390 selectively and dose dependently protected [3H]SCH 23390 binding against the effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline in the striatal/extrastriatal regions. Pretreatment with the D-2 DA antagonist raclopride or the D-2 DA agonist quinpirole selectively protected [3H]spiperone binding. In contrast, pretreatment with the D-1 DA agonist SKF 38393 (7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) not only protected [3H]-SCH 23390 binding but at very high doses protected striatal [3H]spiperone binding. The differential alkylating effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline on striatal vs extrastriatal D-1 and D-2 DA receptors may be related to their post- (striatal DA receptors) and pre-synaptic (extrastriatal DA receptors) localizations, respectively. The present results further demonstrate that in vivo, SCH 23390 and raclopride/quinpirole retain their D-1 and D-2 DA receptor selectivity.

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.

Inhibition of substantia nigra dopamine cell firing by R(-)-N-n-propylnorapomorphine: electrophysiological and autoradiographic studies after regional inactivation of dopamine receptors by microinjection of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline

The irreversible receptor inactivator, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), was injected into rat striatum or substantia nigra to study potential contributions of dopamine receptors in each area to the inhibition of substantia nigra (A9) dopamine cell firing by i.v. R(-)-N-n-propylnorapomorphine (NPA), a dopamine agonist. Extracellular, single unit recording studies showed that the numbers of active dopamine cells, basal firing rates and responses to i.v. R(-)-NPA were unchanged a day after striatal EEDQ injections, despite significant losses of striatal D1 and D2 receptors (confirmed by autoradiography). These results indicate that striatal receptors do not control the basal activity of A9 neurons, nor do they mediate inhibitions of firing by R(-)-NPA. Microinjections of EEDQ into substantia nigra, however, inactivated 75-78% of nigral D1 and D2 receptors and reduced the number of active dopamine cells and slightly increased firing rates. Moreover, dose-response curves to R(-)-NPA were shifted 10-fold to the right and the maximum inhibitory response was depressed. Furchgott analysis of the dose-response curves yielded a steep occupancy-response curve with maximum (> 95%) inhibition of firing at only 24% receptor occupation (i.e., 76% reserve). Thus, the substantial (approximately 70%) receptor reserve previously shown to exist for inhibition of dopamine cell firing by i.v. R(-)-NPA20,21 appears to be intrinsic to the nigra. To assess contributions of nigral D1 and D2 receptors to this response, selective inactivation of each receptor subtype was achieved (confirmed autoradiographically) by treating rats with SCH 23390 (4 mg/kg) or S(-)eticlopride (2 mg/kg), respectively, 30 min before intranigral EEDQ. Selective D2, but not D1, receptor inactivation produced rightward shifts and depressed the maximum of the R(-)-NPA dose-response curve in a manner like that observed after non-selective inactivation of nigral dopamine receptors. Unexpectedly, pretreatment with SCH 23390 (to protect D1 receptors) also produced a modest rightward shift in the R(-)-NPA dose-response curve, suggesting a slight role for D1 receptors in this response. These results indicate that inhibition of A9 dopamine cell firing by i.v. R(-)-NPA is mediated by dopamine receptors located in substantia nigra, but not striatum and confirm the predominant role of nigral D2 receptors.

Chronic treatment with SCH 23390 increases the production rate of dopamine D1 receptors in the nigro-striatal system of the rat

The effects of chronic treatment with the selective dopamine D1 receptor antagonist, SCH 23390, on the steady-state densities and turnover rates of these receptors were investigated in the striatum and substantia nigra of the rat. To this aim, we assessed the repopulation kinetics of [3H]SCH 23390 binding sites after irreversible inactivation of dopamine D1 receptors induced by a single dose of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 10 mg/kg s.c.) in rats chronically treated with SCH 23390. The receptor repopulation was analyzed using a theoretical model that assumes a constant rate of receptor production and a first-order receptor degradation rate. The repeated administration of SCH 23390 (0.05 mg/kg s.c., thrice daily for 21 days) enhanced the steady-state density of dopamine D1 receptors in the striatum (+30%) and substantia nigra (+24%). This treatment also increased the production rates of dopamine D1 receptors in the striatum (+44%) and substantia nigra (+54%). By contrast, the rate constants of dopamine D1 receptor degradation were unchanged in both brain areas. These results suggest that the up-regulation of dopamine D1 receptors induced by chronic treatment with SCH 23390 is determined by modifications in the processes that control the rate of receptor production but not of receptor degradation.

Behavioral indices of moderate nigro-striatal 6-hydroxydopamine lesion: a preclinical Parkinson's model

Asymmetries in turning and scanning were investigated in rats with different degrees of neostriatal dopamine depletion after unilateral injections of 6-hydroxydopamine into the substantia nigra. Animals with severe lesions, i.e., residual dopamine levels of < 20%, spontaneously turned ipsiversive and showed more scanning behavior with the side ipsilateral to the lesion. These asymmetries were reversed by the dopamine receptor agonist apomorphine. Animals with less severe dopamine depletion, i.e., residual dopamine levels of 20-65%, did not show an asymmetry in spontaneous turning, but an ipsilateral asymmetry in scanning was still observed, indicating a greater sensitivity of this measure for moderate striatal dopamine depletions. Furthermore, in animals with residual dopamine levels of 45-65%, the dopamine receptor agonist apomorphine did not lead to a behavioral reversal as with severe lesions, but induced ipsilateral scanning and ipsiversive turning. These ipsiversive asymmetries are discussed in relation to asymmetries in self-regulatory mechanisms of the nigro-striatal dopamine system, such as dopamine autoreceptors controlling the release of this transmitter. Dopamine receptor-stimulated behavioral asymmetry in animals with moderate depletions of dopamine is suggested as a preclinical model to study mechanisms affected in the early state of Parkinson's disease.

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.

Rate of recovery of D1 and D2 dopaminergic receptors in young vs. adult rat striatal tissue following alkylation with ethoxycarbonyl-ethoxy-dihydroquinoline (EEDQ)

The rate of recovery of D1 and D2 receptor binding sites in rat striatal tissue labeled with [3H]SCH-23390 or [3H]YM-09151-2 was followed daily after irreversible blockade of these receptors with the alkylating agent ethoxycarbonyl-ethoxy-dihydroquinoline (EEDQ). These rates were significantly higher in young post-weaning rats than in young adults (14% vs. 7% per day). The findings suggest that rates of synthesis of new D1 and D2 receptor proteins may be increased during a phase of neurodevelopment in the first postnatal month when the accumulation of both receptor types, especially of D1 receptors, is about maximal.

Age-related changes in the turnover rates of D1-dopamine receptors in the retina and in distinct areas of the rat brain

The steady-state density and the turnover rates of D1-dopamine receptors were investigated in the striatum, nucleus accumbens, substantia nigra, and retina of adult (3-month-old) and aged (23-month-old) rats. The turnover rates were measured by monitoring the repopulation kinetics of D1-dopamine receptors labeled with [3H]-SCH 23390 after the irreversible inactivation induced by a single dose of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 10 mg/kg, s.c.). In all the neural tissues examined, the repopulation of D1 dopamine receptors could be adequately described by a theoretical model that assumes a constant rate of receptor production (i.e. zero order) and a rate of degradation that is dependent on the receptor density at any time (i.e. first order). The results obtained indicate that the reduction in the density of D1-dopamine receptors in the striatum, nucleus accumbens and substantia nigra of aged rats is the result of a larger decrease in the receptor production rate (-44 to -60%) than in the receptor degradation rate (-21 to -46%). By contrast, the production rate of D1-dopamine receptors in the retina of aged rats remains unchanged, whilst the degradation rate is reduced by 25%. This results in an age-related increase in the density of D1-dopamine receptors in the rat retina.

Differential turnover rates of D1 dopamine receptors in the retina and in distinct areas of the rat brain

The present study was designed to examine the steady-state density and the turnover rates of D1 dopamine (DA) receptors in the striatum, nucleus accumbens, substantia nigra, and retina of the rat. The turnover rates were measured by monitoring the repopulation kinetics of D1 DA receptors labelled with [3H]SCH 23390 after the irreversible inactivation induced by a single dose of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (10 mg/kg s.c.). The repopulation of D1 DA receptors could be described adequately in all the neural tissues investigated by a theoretical model that assumes a constant rate of receptor production (i.e., zero order) and a rate of degradation that is dependent on the receptor density at any time (i.e., first order). The quantitative analysis of the experimental data using this theoretical model revealed significant regional differences in the rates of receptor production and degradation. Thus, the receptor production rates determined in the nucleus accumbens and striatum (8.03 and 9.96 fmol/mg of protein/h, respectively) were four- to sixfold larger than those measured in the substantia nigra (1.80 fmol/mg of protein/h) and retina (1.50 fmol/mg of protein/h). On the other hand, the receptor degradation rates in the striatum, nucleus accumbens, and retina (0.0093 h-1, 0.0110 h-1, and 0.0123 h-1, respectively) were 2.6-3.5-fold larger than the receptor degradation rate in the substantia nigra (0.0035 h-1).

Interactions between dopamine D1 and D2 receptors in the model of thyrotropin-releasing hormone (TRH)-induced behaviour in rabbits

The interactions between dopamine D1 and D2 receptors in the model of thyrotropin-releasing hormone (TRH)-induced behavioural excitation were assessed in adult male rabbits. As we had previously observed with the dopamine D1 receptor antagonist SCH 23390 (0.01 mg/kg i.v.), sulpiride (12 mg/kg i.v.), a dopamine D2 receptor antagonist, also significantly antagonized the scratching behaviour elicited by TRH (100 micrograms/10 microliters i.c.v.). Sulpiride (6 and 12 mg/kg i.v.) also induced marked grooming. SKF 38393 (10 mg/kg i.v.), a dopamine D1 receptor agonist, did not modify the TRH-induced scratching. However, LY 17155 (0.5 mg/kg i.v.), a dopamine D2 receptor agonist, significantly increased it. The potentiating effects of LY 171555 were completely antagonized by SCH 23390. These results demonstrate that a concomitant activation of both dopamine D1 and D2 receptors is required for the expression of TRH-induced scratching. They also suggest that scratching behaviour may be a D2-dependent, D1-enabled response. The involvement of dopamine D1 and D2 receptors on grooming behaviour is also discussed.

Selective unilateral inactivation of striatal D1 and D2 dopamine receptor subtypes by EEDQ: turning behavior elicited by D2 dopamine receptor agonists

The unilateral intrastriatal injection of the irreversible dopamine (DA) receptor blocker N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) induces a marked decrease in the density of D1 (-48%) and D2 (-51%) DA receptors available for binding to [3H]SCH 23390 and [3H]raclopride, respectively. A challenge dose of the D2 agonist LY 171555 (1 mg/kg, i.p., 24 h after EEDQ) causes intensive ipsiversive circling behavior, whereas the selective D1 agonist SKF 38393 (20 mg/kg, i.p., 24 h after EEDQ) is unable to induce rotations. The density of D1 and D2 DA receptors returns to basal levels by 7 days after the intrastriatal infusion of EEDQ. This biochemical recovery is associated with a progressive decrease in the number of rotations elicited by a challenge dose of LY 171555, suggesting that EEDQ does not cause any relevant neuronal damage. A selective inactivation of striatal D1 or D2 DA receptors can be obtained by injecting EEDQ 30 min after the administration of the D2 antagonist raclopride (20 mg/kg, i.p.) or of the D1 antagonist SCH 23390 (2 mg/kg, s.c.), respectively. The intensity of the circling behavior induced by LY 171555 24 h after EEDQ in animals with a selective inactivation of D2 DA receptors is similar to that found in rats in which both D1 and D2 DA receptors have been inactivated. In contrast, LY 171555 does not cause rotations when the density of D1 DA receptors is selectively decreased by EEDQ in rats pretreated with raclopride.(ABSTRACT TRUNCATED AT 250 WORDS)