Rapid reversal of denervation supersensitivity of dopamine D1 receptors by l-dopa or a novel dopamine D1 receptor agonist, A68930

Dopamine D1-like receptor blockade and stimulation decreases operant responding for nicotine and food in male and female rats

Dopamine has been implicated in the reinforcing effects of smoking. However, there remains a need for a better understanding of the effects of dopamine D1-like receptor agonists on nicotine intake and the role of sex differences in the effects of dopaminergic drugs on behavior. This work studied the effects of D1-like receptor stimulation and blockade on operant responding for nicotine and food and locomotor activity in male and female rats. The effects of the D1-like receptor antagonist SCH 23390 (0.003, 0.01, 0.03 mg/kg) and the D1-like receptor agonist A77636 (0.1, 0.3, 1 mg/kg) on responding for nicotine and food, and locomotor activity were investigated. The effects of SCH 23390 were investigated 15 min and 24 h after treatment, and the effects of the long-acting drug A77636 were investigated 15 min, 24 h, and 48 h after treatment. Operant responding for nicotine and food and locomotor activity were decreased immediately after treatment with SCH 23390. Treatment with SCH 23390 did not have any long-term effects. Operant responding for nicotine was still decreased 48 h after treatment with A77636, and food responding was decreased up to 24 h after treatment. Treatment with A77636 only decreased locomotor activity at the 48 h time point. There were no sex differences in the effects of SCH 23390 or A77636. In conclusion, the D1-like receptor antagonist SCH 23390 reduces nicotine intake and causes sedation in rats. Stimulation of D1-like receptors with A77636 decreases nicotine intake at time points that the drug does not cause sedation.

Novel Dopamine Therapeutics for Cognitive Deficits in Schizophrenia

Schizophrenia is characterized by profound cognitive deficits that are not alleviated by currently available medications. Many of these cognitive deficits involve dysfunction of the newly evolved, dorsolateral prefrontal cortex (dlPFC). The brains of patients with schizophrenia show evidence of dlPFC pyramidal cell dendritic atrophy, likely reductions in cortical dopamine, and possible changes in dopamine D1 receptors (D1R). It has been appreciated for decades that optimal levels of dopamine are essential for dlPFC working memory function, with many beneficial actions arising from D1R stimulation. D1R are concentrated on dendritic spines in the primate dlPFC, where their stimulation produces an inverted-U dose response on dlPFC neuronal firing and cognitive performance during working memory tasks. Research in both academia and the pharmaceutical industry has led to the development of selective D1 agonists, e.g., the first full D1 agonist, dihydrexidine, which at low doses improved working memory in monkeys. Dihydrexidine has begun to be tested in patients with schizophrenia or schizotypal disorder. Initial results are encouraging, but studies are limited by the pharmacokinetics of the drug. These data, however, have spurred efforts toward the discovery and development of improved or novel new compounds, including D1 agonists with better pharmacokinetics, functionally selective D1 ligands, and D1R positive allosteric modulators. One or several of these approaches should allow optimization of the beneficial effects of D1R stimulation in the dlPFC that can be translated into clinical practice.

The Discovery of Novel Selective D1 Dopaminergic Agonists: A-68930, A-77636, A-86929, and ABT-413

The novel selective D1 dopaminergic full agonists A-68930, A-77636 were discovered by the synthesis of molecules to probe the bioactive conformation of the partial agonist SKF-38393, by the use of this information to add D1 affinity and selectivity to a screening hit, and by traditional medicinal chemistry exploration of structure-activity relationships. The subsequent design of A-86929 and ABT-413 capitalized on these results, recently disclosed agonists, and traditional medicinal chemistry.

D1 receptor antagonist-induced long-term depression in the medial prefrontal cortex of rat, in vivo: an animal model of psychiatric hypofrontality

The objective of the following experiment was to induce a pathogenic hypofrontal condition by administering a dopamine-1 receptor (D(1)R) antagonist to rats. The pathophysiological effect of this manipulation upon glutamate-based long-term potentiation (LTP) in the medial prefrontal cortex (mPFC) was examined in vivo. Subjects were surgically implanted with stimulating electrodes into the corpus callosum and recording electrodes into the mPFC. High-frequency stimulation (HFS) was combined with the administration of the selective D(1)R family agonist A68930 hydrochloride (0.4 mg/kg/mL) and the selective D(1)R family antagonist SKF 83566 (0.15 mg/kg/mL). The administration of SKF 83566 hydrobromide prevented mPFC LTP, and resulted in HFS-induced long-term depression. This indicates that D(1)R activation is necessary for the induction of mPFC glutamate-based LTP. This is supported by our finding that the administration of A68930 hydrochloride combined with HFS induced LTP comparable with saline control levels, suggesting that D(1)R activation is necessary for the induction of baseline levels of mPFC LTP. Given that the mPFC governs executive behaviours that are subserved by LTP, such as working memory, these findings are relevant for the study of psychopathological conditions in which hypodopaminergic conditions exist in the mPFC and are correlated with psychiatric symptomotology, such as drug addiction and schizophrenia.

Levodopa/dopamine replacement strategies in Parkinson's disease--future directions

After 40 years, levodopa remains the most effective therapy for the treatment of PD. However, long-term therapy is complicated by motor fluctuations and dyskinesia that can represent a source of significant disability for some patients. Other medical therapies that are currently available for the treatment of PD primarily represent an attempt to prevent or treat motor complications. Surgical therapies improve motor complications in appropriate candidates, but do not provide antiparkinsonian benefits that are superior to levodopa, and are themselves associated with potentially serious side effects. Increasing information suggests that levodopa-induced motor complications relate to pulsatile, nonphysiologic dopamine replacement. A therapeutic strategy that could deliver levodopa/dopamine to the brain in a more continuous and physiologic manner might be expected to provide all of the benefits of standard levodopa with reduced motor complications. Such a levodopa formulation might replace all current dopaminergic antiparkinsonian medications and avoid the need for surgery in most PD patients. However, problems of continuous dopaminergic stimulation must be addressed and avoided, and the issue of nondopaminergic features remains to be addressed.

Pharmacokinetics and pharmacodynamics of levodopa

The pharmacokinetics and pharmacodynamics of levodopa are dominated by two features: the short plasma half-life of the drug and the portion of the antiparkinsonian response that parallels the plasma levodopa levels, the so-called short-duration response. These features are the basis of motor fluctuations that complicate long-term therapy with levodopa. Motor fluctuations will predictably improve with measures that prolong the elevations of plasma levodopa or prolong the efficacy of dopamine synthesized from exogenous levodopa. Because dyskinesia is closely linked to the short-duration response and conceivably part of the short-duration response, it is less clear that dyskinesia will be improved by therapeutic strategies that reduce motor fluctuations.

Motor effects of (-)-OSU6162 in primates with unilateral 6-hydroxydopamine lesions

The effects of the novel compound, (-)-OSU6162 ((S)-(-)-3-methylsulfonylphenyl-1-propylpiperidine), on rotational behavior induced by dopamine receptor agonists was investigated in common marmosets (Callithrix jacchus) with unilateral 6-hydroxydopamine lesions. (-)-OSU6162 per se displayed no effect on the animals' behavior. On the other hand, pretreatment with (-)-OSU6162 attenuated rotational behavior induced by apomorphine (apomorphini hydrochloridum), L-DOPA (3,4-dihydroxyphenylalanine), and the dopamine D2 receptor agonist, quinpirole (trans-(-)-4aR-4,4a, 5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolol[3,4-g]quinoline hydrochloride), without inducing motor impairment such as akinesia or dystonia. In addition, treatment with (-)-OSU6162 for 5 consecutive days almost completely abolished the rotational behavior provoked by apomorphine and produced a transient subsensitization of such apomorphine-induced effects after it was discontinued. Moreover, pretreatment with (-)-OSU6162 in two monkeys augmented the rotational behavior elicited by the dopamine D1 receptor agonists, SKF-81297 (R(+)-6-chloro-7,8,dihydroxy-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrobromide) and A-77636 ((-)-(1R, 3S)-3-adamantyl-1-(aminomethyl)-3,4-dihydro-5, 6-dihydroxy-1H-2-benzopyran hydrochloride). The findings indicate that (-)-OSU6162 can exert indirect state-dependent effects that differentially affect dopamine D1 and dopamine D2 receptor agonist-induced behavior.

Dopamine D1 receptor desensitization profile in MPTP-lesioned primates

The motor effects of dopamine D1 receptor activation and the optimal way to stimulate these receptors were studied in a primate model of parkinsonism induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), using 2 selective full dopamine D1 receptor agonists: A-77636 ([1 R,3S] 3-(1'-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1 H-2-benzopyran hydrochloride), and SKF 82958 (6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrobromide). A-77636 was administered to one group of primed monkeys (N = 4) previously treated with levodopa and other dopamine receptor agonists, while SKF 82958 was given to another group of drug-naive monkeys (N = 3). These drugs have different durations of efficacy, lasting > 20 h and approximately 1 h, respectively, and were administered once daily (A-77636) or thrice daily (SKF 82958) for 7 days. Both drugs demonstrated excellent antiparkinsonian efficacy and locomotor stimulation. However, a rapid, functionally important, homologous (selective for D1 receptor agonists) desensitization process took place as early as on the second day with the longer-acting drug and a dose escalation of A-77636 failed to restore the initial benefit. Thrice daily dosing at a 4-h interval with the short-acting agent SKF 82958 maintained the maximal antiparkinsonian response but some shortening in the duration of response was observed after several days. These behavioral results show that dopamine D1 receptors are susceptible to desensitization after prolonged occupancy and can be desensitized profoundly and independently of dopamine D2 receptors in vivo in this model. Potent dopamine D1 receptor agonists with an intermediate half-life may prove to be better adjuncts in the treatment of Parkinson's disease. Clinical entities with pathologically enhanced dopamine D1 receptor-linked neural transmission might eventually also benefit from such desensitization.

Dyskinesias and tolerance induced by chronic treatment with a D1 agonist administered in pulsatile or continuous mode do not correlate with changes of putaminal D1 receptors in drug-naive MPTP monkeys

Nine monkeys (Macaca fascicularis) were rendered parkinsonian after intravenous administration of the toxin MPTP. Three of these animals received pulsatile administration of the D1 receptor agonist SKF 82958 (1 mg/kg, three times daily) while three were treated by continuous infusion via an osmotic mini-pump with SKF 82958 (at an equivalent amount daily) for 29 days. Untreated MPTP as well as healthy control animals were also studied. Relief of parkinsonian symptoms was observed in the three animals of the pulsatile group. However, dyskinesia occurred in two monkeys which had striatal dopamine depletion of > 99% compared to the non-dyskinetic animal slightly less denervated (94%). Monkeys receiving continuous SKF 82958 showed no anti-parkinsonian effect and no dyskinesia. All monkeys from the pulsatile and continuous group had measurable amount of plasma SKF 82958 as assayed by HPLC with electrochemical detection. In the putamen of all SKF 82958-treated monkeys, Bmax of D1 receptors labeled with [3H]SCH 23390 were increased versus untreated MPTP-monkeys with no change in Kd. In contrast, a decrease D1 receptor density was observed in the nucleus accumbens of untreated MPTP monkeys versus controls and this was not corrected with either pulsatile or continuous SKF 82958 treatments. D2 receptor density measured with [3H]spiperone binding was increased in the posterior putamen of SKF 82958-treated monkeys whereas no change was observed in the accumbens compared to control animals. Hence, tolerance with the continuous administration of a D1 agonist is not associated with a decrease of putaminal D1 or D2 receptor densities and dyskinesia could not be specifically associated with an increase of putaminal D1 receptors.

On-off effects of dopamine receptor agonists in the hemi-parkinsonian rat

Rats lesioned in one substantia nigra were treated daily with bromocriptine, SKF-77434 or cocaine and their rotational (circling) behavior was quantified. Within animal fluctuations in response to the direct-acting agonists were remarkable. In consecutive daily sessions individual animals completed from zero to hundreds or thousands of rotations. Every rat was totally unresponsive to drug treatment at least once in 14 days, with a response failure rate > or = 30% for both 1.0 mg/kg bromocriptine and 0.25 mg/kg SKF-77434. When the bromocriptine dose was increased from 1 to 2.5 to 5 mg/kg the response stabilized in some animals, and overall failure rate declined. In the SKF-77434-treated rats the interval between administrations was manipulated. The response failure rate did not decrease when drug was given every third day, but decreased significantly upon weekly administration. Rats treated repeatedly with the indirect agonist cocaine exhibited no response failures indicating that failures in response to the direct agonists were unlikely to be due to procedural artifact. The results suggest the possibility that large changes in responsivity to direct dopamine receptor agonists are a characteristic of their interaction with the denervated striatum. This approach may prove useful for study of the on-off effects that typically develop in parkinsonism and for identification of drugs with lesser propensity to induce such effects.

The effects of acute levodopa withdrawal on motor performance and dopaminergic receptor sensitivity in patients with Parkinson's disease

The effects of acute levodopa withdrawal were studied in nine patients with levodopa related on-off oscillations. One patient withdrew from the study due to off period confusion and hallucinations. A marked deterioration in motor disability occurred in all patients following overnight withdrawal of levodopa and a further mild delayed deterioration was present over a mean withdrawal period of 44 hours. Patients with more severe disease were able to tolerate levodopa withdrawal for a shorter period of time than those with milder disease severity. The minimum therapeutic dose of subcutaneous apomorphine needed to produce a similar improvement in patients' mobility, before and after several days of drug withdrawal, did not differ, thus providing no clinical evidence for alterations in striatal dopamine receptor sensitivity after acute levodopa withdrawal.

Differential effects of chronic dopamine D1 and D2 receptor agonists on rotational behavior and dopamine receptor binding

The effects of chronic continuous and intermittent administration of the dopamine D1 receptor agonist SKF 38393 or the D2 receptor agonist quinpirole on rotational behavior and dopamine receptor binding were examined in rats with a unilateral 6-hydroxydopamine lesion of the nigrostriatal pathway. Continuous and intermittent SKF 38393 both decreased the rotational response to subsequent challenge with SKF 38393. Intermittent SKF 38393 increased quinpirole rotation, while continuous SKF 38393 had no effect. Continuous administration of quinpirole did not affect rotation elicited by either SKF 38393 or quinpirole. Intermittent quinpirole, however, increased both SKF 38393- and quinpirole-induced rotation. Autoradiographic techniques were used to measure D1 receptor binding in striatum and substantia nigra pars reticulata and D2 receptor binding in striatum and nucleus accumbens. Intermittent SKF 38393 reduced D1 receptor Bmax and increased D1 Kd in the striatum, while both continuous and intermittent treatment with the D1 agonist decreased D1 binding in the substantia nigra pars reticulata. Intermittent quinpirole decreased D1 receptor Kd in striatum, and continuous quinpirole reduced D1 binding slightly in substantia nigra pars reticulata. Striatal D2 receptor binding was unaffected by treatment with either SKF 38393 or quinpirole. Intermittent SKF 38393 and continuous quinpirole both reversed the lesioned-induced elevation in D2 binding in the nucleus accumbens, while intermittent quinpirole decreased D2 binding in the accumbens on both the intact and denervated sides. Thus, the effects of chronic treatment with D1 and D2 agonists on behavioral responses to D1 and D2 receptor stimulation differed considerably and were dependent on the treatment regimen employed.(ABSTRACT TRUNCATED AT 250 WORDS)

Continuous treatment with the D2 dopamine receptor agonist quinpirole decreases D2 dopamine receptors, D2 dopamine receptor messenger RNA and proenkephalin messenger RNA, and increases mu opioid receptors in mouse striatum

Dopamine-mediated behaviors and certain biochemical and molecular events associated with these behaviors were examined following continuous infusion of the D1 dopamine agonist SKF38393 or the D2 dopamine agonist quinpirole into mice for six days. SKF38393 produced a transient grooming behavior while quinpirole initially induced stereotypy, which was followed by an increased locomotor behavior. Continuous infusion of quinpirole caused a significant down-regulation of striatal D2 dopamine receptors without significantly changing the density of D1 receptors. This was accompanied by a decrease in the level of D2 receptor messenger RNA in striatum as measured by Northern analysis. The down-regulation of dopamine receptors was selective for D2 dopamine receptors, since treatment with SKF38393 had no significant effects on either D1 or D2 dopamine receptors, nor did it alter the messenger RNAs for the D1 and D2 receptors. Continuous treatment with quinpirole resulted in a significant increase in striatal mu opioid receptor levels without significant changing delta opioid receptors. This treatment also induced a significant decrease in proenkephalin messenger RNA in striatum. Taken together, these results suggest that the down-regulation of D2 dopamine receptor and D2 receptor messenger RNA is the result of the persistent stimulation of D2 receptors and that the up-regulation of mu opioid receptors may be a compensatory response to a decreased biosynthesis of enkephalin. They suggest further that the biochemical and molecular changes that take place in dopaminergic and enkephalinergic systems following continuous treatment with dopamine agonists may underlie the mechanisms by which certain dopamine-mediated behaviors occur.

Effects of repeated dopamine D1 receptor stimulation on rotation and c-fos expression

Acute injections of the full dopamine D1 receptor agonist A-77636 ((1R,3S)-3-(1'adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1 H-2-benzopyran hydrochloride) induce ipsilateral Fos-like immunoreactivity and contralateral rotation into rats bearing unilateral dopamine depleting lesions. However, following repeated treatment, A-77636 fails to induce either striatal c-fos or rotation. Our results suggest that striatal c-fos activation mirrors behavioral indices of dopamine D1 receptor sensitivity.

Sensitization, response fluctuation and long-term effect of SKF-82958 and bromocriptine in the hemi-parkinsonian rat

Rats with a unilateral 6-hydroxydopamine lesion of substantia nigra were treated with the dopamine agonists SKF-82958 (D1 receptor selective) or bromocriptine (D2 receptor-selective) and their circling response recorded. Both of the compounds induced an acute episode of rotation directed away from the lesioned side. Consecutive daily treatments with either compound usually resulted in a significantly increased average response (sensitization) over a 3- to 6-day treatment period. But nearly all animals treated with low doses of either SKF-82958 or bromocriptine exhibited one or more days when they were totally unresponsive to drug treatment. Response fluctuations thus were not exclusively associated with D1 or D2 receptor agonist treatment. When subsequently tested, undrugged, in the drug-associated environment, 2, 4 and 10 weeks after their last drug treatment, rats that had previously been treated with SKF-82958 exhibited rapid contralateral rotation while rats that had previously been treated with bromocriptine showed no such undrugged rotation. This result is consistent with previous findings that the D1 receptor agonist, SKF-38393, but not the D2 receptor agonist, quinpirole, had long-term behavioral effect in nigral rats, and suggests that persistent motor consequences of limited treatment with dopamine receptor agonists are D1 receptor-related.

A-77636: a potent and selective dopamine D1 receptor agonist with antiparkinsonian activity in marmosets

A-77636, ((1R,3S) 3-(1'-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1H-2-benz opyran hydrochloride), is a selective dopamine D1 receptor agonist. In a battery of receptor binding assays, A-77636 shows the highest affinity (pKi = 7.40 +/- 0.09; Ki = 39.8 nM) for the dopamine D1 receptor. A-77636 is an agonist at the dopamine D1 receptors in the fish retina (pEC50 = 8.13; EC50 = 1.1 nM; intrinsic activity = 102% of dopamine) and the rat caudate-putamen (pEC50 = 8.97; intrinsic activity = 134% of dopamine). The compound is functionally inactive at dopamine D2 receptors (EC50 > 10 microM). In rats with unilateral 6-OHDA (6-hydroxydopamine) lesions of the nigro-striatal dopaminergic pathway, A-77636 elicits prolonged (> 20 h) contralateral turning that is blocked by SCH 23390, a D1 receptor antagonist, but not by haloperidol at doses selective for the dopamine D2 receptor. Higher doses of A-77636 produce forelimb clonus in rats and mice. When tested in marmosets treated with MPTP to induce a parkinsonian-like state, A-77636 increases locomotor activity and decreases the severity of the parkinsonian-like symptoms: the compound is active after either subcutaneous or oral administration. A-77641, the optical antipode of A-77636, has a lower affinity towards the dopamine D1 receptor (pKi = 5.14, Ki = 7200 nM), is less potent as a dopamine D1 receptor agonist (pEC50 = 5.65; EC50 = 2200 nM), fails to elicit turning in the 6-OHDA-lesioned rat, and lacks antiparkinsonian efficacy in the MPTP-treated marmoset.(ABSTRACT TRUNCATED AT 250 WORDS)