Joachim Mierau

Pramipexole binding and activation of cloned and expressed dopamine D2, D3 and D4 receptors

Pramipexole (SND 919; 2-amino-4,5,6,7-tetrahydro-6-propylamino-benzthiazole-dihydrochlor ide) is a potent dopamine autoreceptor agonist. We have carried out an analysis of the binding affinities of dopamine D2L, D2S, D3, and D4 receptors for pramipexole using both [3H]pramipexole and [3H]spiperone as radioligands at cloned and heterologously expressed receptors. Studies were carried out using rat and human D2L, D2S and D3 receptors with equivalent results. When the binding of pramipexole to the high affinity, guanine nucleotide-sensitive state of each receptor was analyzed, pramipexole is most selective for D3 compared to D2 and D4 receptors. These results indicate a 5-fold selectivity of pramipexole for D3 receptors, while quinpirole and bromocriptine are non-selective or more D2/D4 receptor selective. Two measurements of receptor activation for dopamine D2, D3, and D4 receptors also show that pramipexole is most potent for activation of D3 receptors. The dopamine D3 receptor selectivity of pramipexole may explain the previously described properties of this drug, including its potent autoreceptor preference.

Biochemical and pharmacological studies on pramipexole, a potent and selective dopamine D2 receptor agonist

Pramipexole (SND 919; 2-amino-4,5,6,7-tetrahydro-6-propyl-amino-benzthiazole- dihydrochloride) was tested for its agonistic activity at pre- and postsynaptic dopamine (DA) receptors. L-Dihydroxyphenylalanine (L-dopa) accumulation in the rat striatum and limbic system and the alpha-methyltyrosine-induced reduction of DA were inhibited. Both effects were fully antagonized by haloperidol but not by the selective DA D1 receptor antagonist SCH 23390. Pramipexole decreased the levels of DA metabolites dose dependently, whereas striatal DA levels remained unchanged. In mice, pramipexole (0.001-1 mg/kg s.c.) reduced exploratory locomotor activity. In rats with unilateral striatal lesions, only weak ipsilateral rotation was produced by pramipexole at the highest dose. However, in rats with unilateral lesions of the medial forebrain bundle, pramipexole potently induced contralateral circling (ED50 0.026 mg/kg s.c.). In the N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model, pramipexole also had potent stimulatory effects. Finally, in haloperidol-sensitized monkeys, the substance did not elicit dyskinesia/dystonia when given alone, but rather inhibited those symptoms which had been induced by haloperidol (ED50 0.116 mg/kg i.m.). It is concluded that pramipexole has therapeutic potential for schizophrenic patients, as a result of its autoreceptor agonistic effects and its weak effects at normosensitive postsynaptic DA receptors. Furthermore, its potent stimulatory effects in DA-depleted animals suggest a possible use in the treatment of Parkinsons disease.

The dopamine agonist pramipexole scavenges hydroxyl free radicals induced by striatal application of 6-hydroxydopamine in rats: an in vivo microdialysis study.

Hydroxyl free radical production seems to play an important role in the pathogenesis of Parkinsons disease. In the present study, we investigated the dopamine agonists pramipexole and pergolide as well as the nitrone compound S-PBN (N-tert-butyl-alpha-(2-sulfophenyl)nitrone) to reduce hydroxyl radical formation. Microdialysis experiments were carried out in non-anaesthetized Wistar rats. Salicylate was incorporated into the perfusion fluid to measure indirectly hydroxyl radicals indicated by 2,3-dihydroxybenzoic acid (2,3-DHBA). Local perfusion with 0.2 or 2 nmol/2 microl/min 6-hydroxydopamine (6-OHDA) via the microdialysis probe significantly increased 2,3-DHBA levels 14-fold and 47-fold, respectively. Systemic application of either pergolide (0.05 mg/kg) or pramipexole (1 mg/kg) failed to significantly reduce 6-OHDA-induced hydroxyl radical production. In contrast, a 40 min pretreatment with pramipexole (2 and 10 nmol/2 microl/min via the probe) before onset of 6-OHDA perfusion, significantly attenuated 2, 3-DHBA levels compared with vehicle controls. S-PBN pretreatment (2 nmol/2 microl/min) was not effective to reduce 2,3-DHBA levels. In conclusion, pramipexole was able to reduce hydroxyl radical levels induced by 6-OHDA in vivo after local application. This property of pramipexole may be beneficial under conditions of enhanced hydroxyl radical formation in parkinsonian brains and may add to its well known dopamine D(2)-like receptor agonistic effects.

Evidence for a specific effect of BHT 920, an azepine derivative, on tyrosine hydroxylase in the dopaminergic system of the rat

The effect of BHT 920, a putative presynaptic dopamine receptor agonist, on tyrosine hydroxylase was investigated in rats. The activity of the high affinity (BH4) form of striatal tyrosine hydroxylase was investigated dose-dependent manner in rats treated with BHT 920. This effect was pronounced in the dopaminergic system and was not observed to the same extent in the adrenal medulla. In vitro, BHT 920 had no effect upon striatal tyrosine hydroxylase activity. BHT 920 also did not affect either striatal adenylate cyclase activity or the extent of its stimulation by dopamine. The results concerning tyrosine hydroxylase were complemented by measurements of dopamine and DOPA in the striatal and the limbic system. The reduction in DOPA accumulation and in the high affinity form of tyrosine hydroxylase activity elicited by BHT 920 could be blocked by haloperidol, suggesting that BHT 920 may interact with the D2 dopamine receptor although a functional antagonism could not be ruled out. The present results suggest that BHT 920 may exert a specific effect upon tyrosine hydroxylase in dopaminergic nervous tissue which is not mediated by alpha 2-adrenoceptors.

WAL 2014 FU (Talsaclidine) : A preferentially neuron activating muscarinic agonist for the treatment of Alzheimer's disease

The functional selectivity of WAL 2014 FU with regard to stimulation of the neuronal muscarinic M1 receptor subtype in vitro and in vivo is shown in different receptor preparations, isolated organ models, whole animal testing, and finally humans. From receptor binding experiments in membrane preparations from rat tissues and from Chinese hamster ovary cells expressing human muscarinic receptor subtypes, it can be delineated that the ratio between M1 and M2 (hm1 and hm2) is shifted in favour of the M1 receptor affinity, when compared to several classic muscarinic agonists such as carbachol, arecoline, and oxotremorine. The intermediate GTP‐shift of 7.5 for WAL 2014 FU in a M2 muscarinic receptor preparation (rat heart) indicates only partial agonistic activity at this subtype, carbachol (e.g., shows a shift of 51). Moreover, the ratio from agonist to antagonist receptor binding comparing the affinities using [3H]cis‐methyldioxolane and [3H]N‐methylscopolamine as radioligands, suggests only a partial agonist behaviour at M2 receptors, too.

Dopamine Autoreceptor Agonists: Computational Studies, Synthesis and Biological Investigations

Based on molecular modeling studies on the electronic properties of dopamine and dopamine agorusts the pyraz.o10(1,5-alpyridine structure has been evaluated as a catechol surrogate. The azaergoline analogue 6, including this moiety has been synthesized via 1,3-dipolar cycloaddition, anionic ring closure and electrophilic amination. The target compounds (6) exhibit DA autoreceptor agonistic activity. Presynaptic dopamine receptors serve an inhibitory feed back function on dopaminergic neurotransmissi0n.l Functionally, stimulation of the D-2 autoreceptor inhibits dopamine (DA) synthesis, release and DA neuronal firing. Since schizophrenia is associated with hyperactivity in the mesolimbic system (A 10 system) application of selective DA autoreceptor agonists is a very promising approach to a new class of neuroleptic drugs. * These atypical neuroleptics may offer a more subtle neurotransmitter regulation than the classical neuroleptics with D-2 antagonistic properties. As a consequence, considerable efforts have been dedicated to the design and synthesis of DA autoreceptor agonists without appreciable activity at the postsynaptic DA receptors resulting in compounds as (-)-3-PPP,3 EMD 2344g4 BHT 920 (talipexole) 5 and SND 919 (pramipexole).6 Since it is assumed that the binding sites of the postsynaptic D-2 receptor and its presynaptic analogues are very similar,7 it seemed to be a valuable strategy to modify the structure of known D-2 receptor agonists. We have previously reported that the (S)-enantiomer of the aminotetrahydroindolizine 1 strongly reduces DA synthesis, causes sedation in mice and reveals strong and selective affinity to the D-2 receptor, labeled with the selective autoreceptor agonist SND 919.* On the other hand the regioisomer 2 exhibits only a low order of dopaminergic effects. Thus, structural manipulation of the aromatic moiety seems to be very promising but also very sensitive. 1 and 2 are structurally related to the DA agonistic isoindole derivative 3, which is assumed to be the pharmacophoric core-structure of the ergolines (4).9 Based on these observations, we here report on molecular design, synthesis and tricyclic ergolines 5.9 pharmacological investigations of analogues of the DA active

Novel HO-DPAT (hydroxy-2-dipropylaminotetralin) isomers: Stereoselective synthesis and receptor binding studies

Abstract The synthesis of the novel HO-DPAT isomers 2 and 3 from the β-amino acid 4 is reported. 2 showed strong and selective affiity to the dopamine autoreceptor, labelled with [ 3 H]-pramipexole.