Localization of striatal dopamine receptor function by central injection of an irreversible receptor antagonist

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.

Importance of D1 and D2 receptor stimulation for the induction and expression of cocaine-induced behavioral sensitization in preweanling rats

The behavioral manifestations of psychostimulant-induced sensitization vary markedly between young and adult rats, suggesting that the neural mechanisms mediating this phenomenon differ across ontogeny. In this project we examined the importance of D1 and D2 receptors for the induction and expression of cocaine-induced behavioral sensitization during the preweanling period. In the behavioral experiments, rats were injected with reversible D1 and/or D2 antagonists (SCH23390 and/or raclopride) or an irreversible receptor antagonist (EEDQ) either before cocaine administration on the pretreatment day (induction) or before cocaine challenge on the test day (expression). In the EEDQ experiments, receptor specificity was assessed by using selective dopamine antagonists to protect D1 and/or D2 receptors from inactivation. Receptor binding assays showed that EEDQ caused substantial reductions in dorsal striatal D1 and D2 binding sites, while SCH23390 and raclopride fully protected D1 and D2 receptors from EEDQ-induced alkylation. Behavioral results showed that neither D1 nor D2 receptor stimulation was necessary for the induction of cocaine sensitization in preweanling rats. EEDQ disrupted the sensitization process, suggesting that another receptor type sensitive to EEDQ alkylation was necessary for the induction process. Expression of the sensitized response was prevented by an acute injection of a D1 receptor antagonist. The pattern of DA antagonist-induced effects described for preweanling rats is, with few exceptions, similar to what is observed when the same drugs are administered to adult rats. Thus, it appears that maturational changes in D1 and D2 receptor systems are not responsible for ontogenetic differences in the behavioral manifestation of cocaine sensitization.

Pretreatment or Posttreatment with Aripiprazole Attenuates Methamphetamine-induced Stereotyped Behavior in Mice

Aripiprazole is a third-generation atypical antipsychotic and a dopamine D₂ receptor partial agonist. In the present study, we investigated whether a single administration of aripiprazole to mice, either as a pretreatment or as a posttreatment, would affect stereotypy induced by methamphetamine (METH). Pretreatment of male ICR mice with aripiprazole (1 or 10 mg/kg, i.p.) attenuated the incidence of METH-induced stereotypical behavior in a dose-dependent manner. Pretreatment of mice with 1 mg/kg aripiprazole produced an increase in the locomotor activity in mice treated with METH compared with mice treated with vehicle plus METH and with 10 mg/kg aripiprazole plus METH. This increase in locomotion is indicative of a rightward shift in the dose–response curve for METH, consistent with a shift in the type of stereotypical behavior observed from biting to sniffing. Aripiprazole posttreatment, after METH-induced stereotypical behavior, was fully expressed and also significantly attenuated overall stereotypy in an aripiprazole dose-dependent manner. These data suggest that the antagonism of METH effects by aripiprazole should be investigated as a potential treatment for acute METH overdose.

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.

Atropine reduces raclopride-induced muscle rigidity by acting in the ventral region of the striatum

Parkinson-like extrapyramidal motor side effects associated with the use of antipsychotic drugs, such as increased muscle rigidity, are thought to result from blockade of striatal dopamine D2 receptors. While anticholinergic medications (muscarinic receptor antagonists) ameliorate extrapyramidal side effects, the mechanisms underlying their effectiveness remain unclear. We investigated the site of action of atropine, a non-selective muscarinic receptor antagonist, in reducing increased muscle rigidity, assessed as increases in tonic electromyographic (EMG) activity, induced by the selective dopamine D2 receptor antagonist, raclopride. Atropine significantly reduced raclopride-induced EMG increases in rat hindlimb muscles, when injected into the ventral striatum, but not the dorsal striatum or the substantia nigra. Atropine's site of action was localised to a small area of muscarinic receptors within the ventral part of the striatum, using quantitative autoradiography. These findings provide new information about the regulation of motor control by muscarinic receptor antagonists and additional evidence about the functional heterogeneity of the striatum.

Atropine acts in the ventral striatum to reduce raclopride-induced catalepsy

While muscarinic receptor antagonists are used to reduce motor side effects associated with the use of antipsychotic drugs, their site of action remains unclear. The study investigated the site of action of the non-selective muscarinic receptor antagonist atropine on catalepsy induced by the selective dopamine D2 receptor antagonist, raclopride. Initially, catalepsy and striatal muscarinic receptor occupancy was assessed 2 h following subcutaneous injection of raclopride and either atropine or vehicle. Catalepsy was significantly reduced by doses of atropine that occupied more than 69% of muscarinic receptors. Next, atropine was injected bilaterally into the ventral striatum, which produced a significant reduction in catalepsy, while injections into the dorsal striatum and substantia nigra had no effect. The site of atropine's action was localised to a discrete area of the ventral striatum through the use of quantitative autoradiographic techniques. These findings provide further evidence for the importance of the ventral striatum in the expression of behaviours.

The regulation of motor control: an evaluation of the role of dopamine receptors in the substantia nigra

The importance of the nigrostriatal dopaminergic pathway in motor control is widely accepted and it is generally believed that the motor symptoms of Parkinson's disease result solely from reduced release of dopamine from terminals in the striatum. Over recent years there has been a growing body of evidence which suggests that dendritic dopamine release in the substantia nigra is of importance in the regulation of neuronal activity and behaviour. This evidence is reviewed together with a description of our recent findings that show nigral dopamine receptors are essential for the maintenance of normal muscle tone. It is concluded that current views of the basal ganglia circuitry involved in motor control need to be re-evaluated to take into account these recent reports. A scheme is suggested to explain how dopamine mechanisms in the substantia nigra regulate motor activity.

Effects of the kappa-opioid receptor agonist U-50,488 on morphine-induced place preference conditioning in the developing rat

The ability of the kappa-opioid receptor agonist trans-(+/-)- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate (U-50,488) to modulate morphine-induced reward was assessed in preweanling (10- and 17-day-old) and periadolescent (35-day-old) rats using the conditioned place preference paradigm. Conditioning and testing were conducted in a three compartment chamber, with each end compartment having its own distinct tactile and odor cues (almond or lemon). An abbreviated conditioned place preference procedure was used in which rats received two saline-odor pairings on the first conditioning day, and two saline- or morphine-odor pairings on the second day. In some experiments, rats were given U-50,488 (2-10 mg/kg, s.c.) 30 min prior to being conditioned with morphine (0.1-8 mg/kg, i.p.). On the third day, rats were allowed free access to the entire chamber for 900 s and compartment preferences were determined. Similar to adult rats, morphine (0.5 mg/kg) was consistently able to induce conditioned place preferences in the two preweanling age groups. This effect was attenuated by kappa-opioid receptor agonist pretreatment, as U-50,488 not only enhanced the locomotor activity of 10- and 17-day-old rats, but it blocked the morphine-induced place preference conditioning of these younger animals. In contrast, periadolescent (35-day-old) rats did not exhibit morphine-induced place preferences, nor did they show enhanced locomotor activity after U-50,488 treatment; however, using the same procedure, a different group of similarly aged rats showed conditioned preference produced by 20 mg/kg cocaine (i.p.). Therefore, these results suggest that reward processes are functionally mature in the preweanling rat (at least by 10 days of age), but that periadolescent rats are generally unresponsive to mu- and kappa-opioid drugs.

Changes in behavioral sensitivity to SKF-38393 and quinpirole following withdrawal from continuous cocaine administration in rats

The effects of withdrawal from continuous administration of cocaine on spontaneous locomotor activity and behavioral sensitivity to SKF-38393 and quinpirole were examined in rats. Subdermal minipumps that delivered either saline or 20 mg/kg/day cocaine hydrochloride were implanted for 14 days. Spontaneous locomotor activity, SKF-38393-induced (10 mg/kg, SC) grooming and tongue protrusions, and quinpirole-induced locomotor activity and stereotypy (0.32 and 1.0 mg/kg, SC) were examined either 4-5 h or 7 days after removal of the minipumps. Animals withdrawn from cocaine for 4 h exhibited a decrease in spontaneous locomotor activity relative to saline-pretreated controls, whereas animals withdrawn for 7 days did not differ from controls. Animals withdrawn from cocaine for 4 h did not differ from controls in their sensitivity to SKF-38393, whereas animals withdrawn from cocaine for 7 days exhibited an increase in SKF-38393-induced tongue protrusions relative to controls. In contrast, animals withdrawn from cocaine for 4 h exhibited a decrease in quinpirole-induced locomotion, whereas animals withdrawn for 7 days did not differ from controls. There were no differences in sensitivity to quinpirole-induced stereotypy relative to controls at either withdrawal period. These findings suggest that an increased sensitivity of D1-like receptors emerges within 7 days during the course of withdrawal from continuous cocaine administration, whereas a change in sensitivity of D2-like receptors may occur early during withdrawal but normalizes within 7 days.

A new view of the role of dopamine receptors in the regulation of muscle tone

1. Clinical and experimental studies support the view that nigrostriatal dopaminergic neurons are involved in the maintenance of muscle tone. 2. The effects of inactivating dopamine receptors in the rat substantia nigra on muscle tone, assessed as tonic electromyographic (EMG) activity, was investigated. 3. Dopamine receptors were inactivated by injections of the irreversible dopamine receptor antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) into the pars reticulata of the substantia nigra. 4. Significant increases in EMG activity were observed from 2 to 24 h after EEDQ injection and were associated with a loss of nigral dopamine receptors. 5. Increases in EMG activity were found only when both D1 and D2 receptors were inactivated. 6. Inactivation of alpha-adrenoceptors, 5HT2c and muscarinic receptors had no effect on EMG activity. 7. Injections of apomorphine did not reduce the increased EMG activity associated with nigral dopamine receptor loss. 8. The results are consistent with the hypothesis that dopamine receptors in the substantia nigra of the rat play an important role in the regulation of muscle tone.

Expression of stereotyped behaviour requires stimulation of nigral D1 dopamine receptors

Dopamine receptors in the pars reticulata of the substantia nigra were inactivated following bilateral injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Loss of D1, but not D2, receptors was associated with a significant decrease in the incidence of stereotyped head-down sniffing elicited by the mixed D1/D2 receptor agonist, apomorphine. The results support the hypothesis that D1 receptors in the substantia nigra are necessary for the expression of dopamine mediated stereotyped behaviour.

Dopamine receptors in the substantia nigra are involved in the regulation of muscle tone

The aim of the present study was to localize the dopamine receptors involved in the regulation of muscle tone. A strategy was used whereby the effects on muscle tone of injecting the irreversible dopamine receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in discrete brain regions were assessed. Increases in muscle tone were measured as changes in electromyographic activity of the gastrocnemius and tibialis muscles of conscious, unrestrained rats. No increases in muscle tone were found after injections of EEDQ into the anterior and posterior striatum, which produced marked reductions in dopamine receptor concentration. The effects of muscle tone of injecting EEDQ into the substantia nigra pars reticulata were also assessed. Large increases in muscle tone were observed associated with inactivation of either D1 or D2 dopamine receptors in the substantia nigra. The increased muscle tone was not reduced by subcutaneous administration of apomorphine, despite the presence of a normal population of striatal dopamine receptors. These findings provide evidence that dopamine receptors in the substantia nigra play an important role in the regulation of muscle tone. Further, they challenge the hypothesis that the muscle rigidity of Parkinson disease results primarily from loss of striatal dopamine receptor stimulation.

Localization of dopamine receptor subtypes occupied by intra-accumbens antagonists that reverse cocaine-induced locomotion

The purpose of this study was to examine whether blockade of either dopamine D1-like or D2-like receptors by selective antagonist administration into the nucleus accumbens (NAc) is sufficient to reverse cocaine-induced locomotion, and to develop a new technique that enables the population of receptors occupied by the antagonists to be quantified. Locomotor activity was assessed in rats that had received bilateral intra-accumbens injections of the D1-selective antagonist SCH-23390 (0-3.0 micrograms/0.5 microliters/side) or the D2/D3-selective antagonist sulpiride (0-1.0 micrograms/0.5 microliters/side), followed 15 min later by injections of saline or cocaine (15 mg/kg, i.p.). To assess receptor occupancy by the antagonists, 105 min prior to sacrifice the rats received intra-accumbens injections of the antagonist, followed 15 min later by an injection of the non-selective irreversible antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ; 10 mg/kg, i.p.). Receptors were labeled with [3H]SCH-23390 or [3H]sulpiride in sections containing the NAc, and the autoradiograms allowed quantitation of receptors occupied (i.e. protected from EEDQ) by the antagonist given in vivo. Only a dose of 3 micrograms/side SCH-23390 reversed cocaine-induced locomotion, whereas a dose of 0.5 microgram/side did not alter cocaine-induced locomotion despite occupying the same amount of [3H]SCH-23390 binding sites in the NAc. Intermediate doses of 0.1 and 0.3 microgram/side sulpiride reversed cocaine-induced locomotion, and also occupied the greatest number of [3H]sulpiride binding sites in the NAc. The results suggest that blockade of D2-like, but not D1-like, receptors in the NAc is sufficient to reverse cocaine-induced locomotion, and also demonstrate the importance of quantifying receptors occupied by drugs administered intracranially.

Anatomical localization of SKF-38393-induced behaviors in rats using the irreversible monoamine receptor antagonist EEDQ

This study was designed to localize the population of dopamine D1-like receptors involved in grooming and oral movements elicited by systemic administration of the D1-selective agonist SKF-38393. Receptors in specific dopamine terminal regions were inactivated by intracranial injection of the nonselective irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The effect of these injections on behaviors induced by systemic administration of SKF-38393 (10 mg/kg) was measured 48 hours later. The specific populations of D1-like receptors inactivated by the EEDQ injections were identified as a loss of 3H-SCH-23390 binding in a given region using quantitative autoradiography. EEDQ (1.5 micrograms/microliters/side) injected into the nucleus accumbens (NAc) did not alter SKF-38393-induced behaviors. Similarly, injection of EEDQ into the medial caudate-putamen (CPu) failed to alter these behaviors. In contrast, EEDQ (0.15-1.5 micrograms/microliters/side) injected into the lateral CPu decreased both SKF-38393-induced grooming and oral movements, with complete blockade of grooming observed at the highest dose. To determine whether this effect of EEDQ was due to inactivation of D1-like receptors, separate groups of animals were pretreated with SCH-23390 (3 mg/kg, S.C.) 15 min prior to injection with EEDQ. Pretreatment with SCH-23390 prevented the disruption of SKF-38393-induced behaviors, as well as the loss of 3H-SCH-23390-labeled binding sites observed after injection of EEDQ into the lateral CPu. EEDQ injections that produced disruption of SKF-38393-induced behaviors were associated with a greater loss of binding in the lateral CPu relative to other regions examined including the NAc, medial CPu, and globus pallidus. Furthermore, EEDQ injections that produced the greatest loss of 3H-SCH-23390 binding in the latter three regions did not disrupt SKF-38393-induced behavior. These results demonstrate that stimulation of D1-like receptors in the lateral CPu is necessary for behaviors induced by systemic administration of SKF-38393. The results also demonstrate the utility of this "receptor lesion" technique to localize receptor-mediated 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.

Dopamine agonist-mediated inhibition of acetylcholine release in rat striatum is modified by thyroid hormone status

K(+)-evoked acetyl[3H]choline ([3H]ACh) release was inhibited in a concentration-dependent manner by apomorphine and the D2 agonist quinpirole in striatal slices prepared from euthyroid and hypothyroid rats. However, there was a significant increase in the maximum inhibition observed with both agonists in the hypothyroid compared with the euthyroid group, which paralleled the increased D2 agonist sensitivity reported for stereotyped behavior. The D2 antagonist raclopride decreased, and the D1 antagonist SCH 23390 increased, the inhibition of [3H]ACh release by apomorphine, confirming an inhibitory role for D2 receptors and an opposing role for D1 receptors. Because there is no difference in D1 or D2 receptor concentration between the euthyroid and hypothyroid groups, it is suggested that thyroid hormone modulation of D2 receptor sensitivity affects a receptor-mediated event. Following intrastriatal injection of pertussis toxin (PTX), apomorphine no longer inhibited [3H]ACh release. In fact, increased [3H]-ACh release was observed, an effect reduced by SCH 23390, providing evidence that D1 receptors enhance [3H]-ACh release, and confirming that a PTX-sensitive G protein mediates the D2 response. As it has been reported that thyroid hormones modulate G protein expression, this mechanism may underlie their effect on dopamine agonist-mediated inhibition of ACh.

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.

Quantitative electromyographic changes following modification of central dopaminergic transmission

Muscle tone was assessed by measurement of quantitative electromyographic (EMG) activity recorded from electrodes chronically implanted in the gastrocnemius and anterior tibialis muscles of conscious, unrestrained rats. Following treatment with reserpine or denervation with 6-hydroxydopamine a significant increase in EMG activity of both muscles was observed, confirming the utility of the method for studying the effects of changes in central dopaminergic transmission on muscle tone.

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.

Grafts in the treatment of Parkinson's disease: animal models

Long-term treatment of parkinsonian patients with L-DOPA leads to a loss of efficacy over time and the appearance of important side effects such as dyskinesias. Grafts of chromaffin cells of the adrenal medulla or fetal ventral mesencephalic neurons bring behavioral improvement in animal models of Parkinson's disease. These improvements are likely to be related to the secretion of dopamine by the grafted cells and/or to the reinnervation of the host tissue. In addition, a leak in the blood-brain barrier may allow peripheral catecholamines to gain access to the brain. Lack of clear effects of grafts in parkinsonian patients may be due to their poor survival in the human brain. Improvement of grafting techniques as well as the addition of neurotrophic factors to grafts may help increase their survival and improve behavioral effects. Recently, genetic techniques have allowed the creation of genetically modified cell lines which can produce L-DOPA and these cells may be grafted in the brain. Interestingly, these cell lines may be encapsulated in permselective membranes which can protect them from immunological rejection and avoid the uncontrolled cell growth of these mitotically active cells. Grafting techniques seem to be an interesting alternative to treat parkinsonian patients. Improvement of grafting procedures may help increase survival of grafts and thus enhance behavioral improvements. Moreover, genetic modification of well-known tumor cell lines or patient's own cells such as astrocytes may help avoid the low availability as well as ethical and immunological problems linked to the use of fetal human tissue.

Localisation of striatal muscarinic receptors involved in dopamine receptor-mediated behavioural responses

Stereotyped sniffing responses to the direct dopamine agonist, apomorphine, were assessed following intrastriatal injection of the alkylating derivative of oxotremorine, BR 401. Subsequently, the localisation and extent of muscarinic receptor alkylation after injection into various striatal sites were assessed by quantitative autoradiography. The results of these experiments provide evidence that a regional subset of striatal muscarinic receptors is involved in apomorphine-induced stereotyped sniffing. In addition, these receptors are localised in a similar area to that previously shown to contain the dopamine D2 receptors responsible for mediating apomorphine-induced sniffing. Thus, striatal muscarinic receptors involved in dopamine-agonist induced behaviours share a close anatomical association with the dopamine receptors at which the agonist acts.

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

Irreversible antagonism of dopamine (DA) receptors by the alkylating compound N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) results in a depression of DA agonist mediated behaviors in adult, but not preweanling rats. DA D1 and D2 receptors, dihydroxyphenylacetic acid (DOPAC) and DA levels were assessed in 17- and 90-day-old rats in order to identify a neurochemical mechanism for this difference. EEDQ caused a depletion in D1 and D2 receptors in both young and adult rats, with the depletion in adults being relatively greater. In both age groups, EEDQ caused an unexpected decrease in DA levels which returned to normal levels by 30 days post-injection. In addition, DOPAC levels of adult rats, but not rat pups, were elevated after EEDQ treatment. The depletion of endogenous DA levels must be considered when interpreting the effects of EEDQ. Age-dependent differences in DA metabolism may be important for understanding the ontogenetic differences in EEDQ's behavioral effects.

Effects of irreversible dopamine receptor inactivation on locomotor activity and grooming in the 17- and 90-day-old rat

Ontogenetic differences in the behavioral recovery of R(-)-propylnorapomorphine (NPA) treated rats were assessed following irreversible DA receptor antagonism by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). In the first two experiments, 17- and 90-day-old rats were given EEDQ (7.5 or 15.0 mg/kg, IP) or vehicle after half the rats were initially treated with the selective DA D-1 and D-2 antagonists SCH 23390 and sulpiride. (The sulpiride/SCH 23390 treatment protects DA receptors from EEDQ-induced inactivation.) NPA's (0.1 or 1.0 mg/kg) effects on locomotor activity and grooming were assessed 1, 2, 4 and 8 days after the EEDQ pretreatment. In a third experiment, the effects of habituating the 17- and 90-day-old rats to the testing chamber were assessed 1, 2 and 4 days after EEDQ pretreatment. In this experiment, some groups received successive treatments of saline or NPA prior to behavioral testing. To assess the possible effects of drug-sensitization other groups received saline on days 1 and 2 and NPA on day 4. In 90-day-old rats, EEDQ eliminated, for up to 4 days, the ability of NPA to enhance locomotor activity and depress grooming. Prior treatment with DA antagonist drugs was sufficient to protect DA receptors from EEDQ-induced inactivation, since these groups exhibited normal behavioral responses after challenge with NPA. In contrast, EEDQ did not eliminate, and may have enhanced, NPA's effect on the locomotor activity and grooming of 17-day-old rat pups. Habituating the rats to the testing chamber decreased the locomotor activity of the mature rats, but not the 17-day-old rat pups.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional changes of striatal dopamine receptors following denervation by 6-hydroxydopamine and fetal mesencephalic grafts in the rat

Young adult female rats received a 6-hydroxydopamine lesion in the left substantia nigra and, 3 weeks later, some of them were grafted with a cell suspension from the ventral mesencephalon of rat embryos (14-15 days old). Six months after transplantation, some grafted rats, following injection of amphetamine, had switched to turning only toward the intact side (type 1), whereas others turned toward the intact side only during the first half of the test (type 2). Levels of dopamine, dihydroxyphenylacetic acid and homovanillic acid were, respectively, 2%, 15% and 35% of the intact side in the denervated striatum of 6-hydroxydopamine rats. Dopamine concentrations were restored to 13% and 10% of the intact side in the grafted striatum of type 1 and type 2 animals, respectively. Levels of homovanillic acid were unchanged following grafts whereas those of dihydroxyphenylacetic acid increased by 209% and 247% in the grafted striatum of type 1 and type 2 animals, respectively. The ratios of dihydroxyphenylacetic acid/dopamine as well as homovanillic acid/dopamine were low in the intact striatum whereas they increased in the denervated striatum with or without graft. The tyrosine hydroxylase immunoreactivity decreased by about 80% in the denervated striatum of 6-hydroxydopamine rats. In type 1 rats, tyrosine hydroxylase immunoreactivity revealed that the graft was localized in the dorsomedial part of the denervated striatum, whereas in type 2 animals, it was also in the medial striatum but it overlapped the dorsal and ventral parts of it equally. D1 as well as D2 dopamine receptors were measured throughout the striatum (9.0-7.6 rostral-caudal coordinates), by autoradiography, using [3H]SCH 23390 (D1 antagonist) and [3H]spiperone (D2 antagonist) binding. Supersensitive D2 receptors were normalized in the dorso- and ventromedial parts of the grafted striatum. D2 receptor density was higher in type 2 than in type 1 rats, more specifically at 8.6-8.2 rostral-caudal coordinates, where the graft was. D1 receptor supersensitivity was modest compared to D2 receptors in the striatum of 6-hydroxydopamine rats and decreased following grafts. DA receptors changes in the striatum, following fetal mesencephalic grafts, may explain the behavioral recovery seen in grafted rats.

Dopamine sensitivity in rats selectively bred for increases in cholinergic function

Because of the extensive literature demonstrating an interaction between cholinergic and dopaminergic systems, the Flinders Sensitive (FSL) and Flinders Resistant (FRL) Lines of rats, selectively bred for differences in cholinergic function, were tested for differences in dopamine sensitivity. Large differences in sensitivity to dopamine agonists were detected, but the direction depended upon the function: The FSL rats were supersensitive to the hypothermic effects of dopamine agonists, but were subsensitive to the stereotypy-inducing effects. Measurement of dopamine receptors by either standard binding techniques or autoradiography failed to demonstrate any receptor differences in the FSL and FRL rats. Behavioural studies with dopamine antagonists were less clear-cut, but suggested that the FSL rats might be more sensitive to their catalepsy-inducing effects. These findings indicate that the changes in dopamine sensitivity which accompany cholinergic supersensitivity are function-dependent, but are not associated with parallel changes in dopamine receptor concentration.

The hypothyroid rat as a model of increased sensitivity to dopamine receptor agonists

Control and hypothyroid rats were challenged with a range of doses (0.5-4 mumol/kg) of either the nonselective dopamine agonist, apomorphine, or the selective D2 receptor agonist. LY 171555, and their stereotyped head-down sniffing (SHDS) responses measured. The dose-response curves for both agonists were shifted to the left in the hypothyroid rats compared to water-treated controls. Increasing doses of the selective D2 antagonist, raclopride, caused a parallel shift to the right in the LY 171555-induced SHDS dose-response curve. Schild analysis revealed a decreased sensitivity to raclopride in the hypothyroid animals. The selective D1 antagonist SCH 23390 was observed to decrease the maximal response elicited by LY 171555 in a dose-dependent manner and the hypothyroid rats were more sensitive to this effect. It was concluded that hypothyroid rats showed an apparent increased sensitivity to D2 receptor agonists and a decreased sensitivity to D2 antagonists. In addition, the facilitation effect of the D1 receptor on the D2 receptor appeared less tightly coupled in the hypothyroid rats.

Effects of inactivation of D1 dopamine receptors on stereotypic and thermic responses to quinpirole (LY 171555)

The purpose of the present study was to investigate reports that the expression of dopamine agonist-induced behaviours is dependent upon the ratio of D1 to D2 receptor activation. Selective inactivation of the D1 dopamine receptor was achieved using the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) while D2 receptors were maintained at control levels by the use of a D2 antagonist, raclopride. Stereotypic and hypothermic responses to quinpirole (LY 171555) were assessed and related to striatal D1 and D2 receptor concentrations. Results showed that the incidence of stereotyped sniffing behaviour induced by LY 171555 was reduced in parallel with D1 receptor loss and sniffing behaviour was abolished at low D1 receptor (less than 40% of control) concentrations. Hypothermic responses to LY 171555 were unaffected by D1 receptor loss. These findings suggest that activation of D1 receptors is a critical component of stereotypic, but not hypothermic, responses to LY 171555, the magnitude of the sniffing response being positively associated with D1 receptor concentration.

Pretreatment with an irreversible muscarinic agonist affects responses to apomorphine

The main aim of the present study was to investigate if responses to the direct dopamine agonist, apomorphine, could be modified by changes in the activity of cholinergic neurones. A novel approach was adopted in which these responses were assessed following reduction of muscarinic receptor concentration (mAChR) in the brain (assessed from [3H] QNB binding) by the alkylating derivative of oxotremorine, N-[4-(2-chloroethylmethylamino)]-2-pyrrolidone (BM 123). Stereotyped responses elicited by apomorphine were significantly reduced when QNB binding was 12% and 50% of control values. No changes in [3H] spiperone binding were found. There was significant hypothermia in the group with 12% QNB binding sites which was significantly increased by apomorphine. Body temperature returned to normal when QNB binding was 50% of control values. There was a significant decrease in activity when QNB sites were reduced to 12% of normal and vertical activity was still significantly reduced at 50% QNB binding, though horizontal activity was not then different from controls. These data are consistent with the hypothesis that changes in the function of mAChR modify responses elicited by dopamine receptor stimulation in both the striatum and other brain regions.