2-amino-6,7-dihydroxytetrahydronaphthalene and the receptor-site preferred conformation of dopamine--a commentary

Interaction between substituted 1-[2-(diphenylmethoxy)ethyl] piperazines and dopamine receptors

Substituted 1-[2-(diphenylmethoxy)ethyl]piperazines were tested for their affinity to specific [3H]dopamine binding sites in membrane preparations from the corpus striatum of the rat. In particular, 4-(3-phenyl-2-prop(en)yl)- and 4-(3-phenyl-2-butyl)-substitution yielded compounds potent in displacing [3H]dopamine from its binding sites, with IC50-values in the order of 10 nM. There was a significant correlation between the IC50-values determined in this binding assay and the IC50-values obtained for the same compounds in a previous study on their potency to inhibit the uptake of dopamine in synaptosomal preparations of the striatum of the rat. Current insight in structural requirements for binding to dopamine receptors, as obtained mainly with rigid analogues of dopamine, gives no satisfactory explanation for the dopaminergic activity of the piperazine derivatives tested.

Conformationally restricted dopamine congeners--a molecular mechanics-based study

Potential energies of several systems containing the phenylethylamine fragment were computed by the QCFF/PI method and some of them also by the PCILO method. The systems considered are: octahydrobenzo [f]- and [g]-quinolines, aporphine, 1-phenylbenzazepine, 4-phenyltetrahydroisoquinoline, 3-phenylpiperidine and 9-aminodihydrophenanthrene. No common set of torsion angles defining the stereostructure around the amine head is apparent if only the lowest energy conformations are considered. Leaving out the orientation of the N-CH3 group, and considering also some higher energy conformations, two groups of systems may be formed, A and B, the members of which are congruent amongst themselves in parameters defining the spatial relations of the amine site and the catechol ring, and in the orientation of the N+-H bond. The stereostructure of the rigid representatives of group A is postulated to be the one required by the brain and cardioaccelerator nerve receptor. A corresponding postulate for the vascular bed DA receptors cannot be made since systems forming group B, the derivatives of which are active both on brain and vascular bed receptors, have more conformation space and, moreover, they carry additional potential anchoring groups.

Multiple dopamine receptors and behavior

The therapeutic effects of dopamine (DA) agonists and DA antagonists used in the treatment of schizophrenia (antipsychotics, DA antagonists), Huntington's chorea (DA antagonists) and Parkinson's disease (antiparkinsonian agents, DA agonists) have been thought to result largely from actions on DA receptors located in the striatum (caudate nucleus and putamen). Many of the classical drugs used to treat these disorders are known to have a high incidence of extrapyramidal side effects (EPS). However, a number of drugs, the atypical antipsychotics and antiparkinsonian agents, have been developed which have a low incidence of EPS. It has been of enormous interest to researchers and clinicians alike to determine what characteristics of the atypical antipsychotics and antiparkinsonian agents are responsible for their unique behavioral profile. Because all of the antipsychotics and antiparkinsonian agents act on DA receptors, much attention has focused on potential differences in the interactions of the atypical agents with DA receptors. An hypothesis that has been raised, due to the knowledge that there are multiple subtypes of DA receptors located in the striatum, is that the atypical agents could have their therapeutic actions as a result of an interaction with one specific subtype of DA receptor. This review emphasizes two major points: (1) it is unlikely that the atypical antipsychotics and antiparkinsonian agents interact with only one subtype of DA receptor, or have their therapeutic actions only through that receptor; (2) other pharmacological characteristics of these agents are more critically involved in their unique behavioral effects. The applicability of animal models to assess the pharmacological and behavioral profiles of these agents is discussed, and the relevance to the clinical profiles of these agents is emphasized.

Conformational preferences of dopamine analogues for inhibition of norepinephrine N-methyltransferase. Conformationally defined adrenergic agents

A series of analogues of dopamine (DA) with varying degrees of conformational flexibility have been examined as potential substrates or competitive inhibitors of the enzyme norepinephrine N-methyltransferase (NMT). A conformationally defined (rigid) analogue of the fully extended conformation of DA, 2-amino-6,7-dihydroxybenzonorbornene hydrobromide (3; 6,7-D2HX) proved to be a better substrate than the non-catechol parent 2-aminobenzonorbornene (4; 2HX). However, analogues 3 and 4 displayed equivalent competitive inhibitory activity toward phenylethanolamine (PEA). Neither 6,7-ADTN (5), a DA analogue in the 2-aminotetralin (2AT) system, nor 6,7-DTHIQ (7), a DA analogue in the tetrahydroisoquinoline (THIQ) system, showed substrate activity; 6,7-ADTN was a poorer competitive inhibitor than the parent 2AT but 6,7-DTHIQ was a better competitive inhibitor than its parent, THIQ (8). A tricyclic conformationally defined analogue 9 of 6,7-ADTN was devoid of either substrate or inhibitory activity. From these results it may be concluded that a fully extended side chain conformation is required for NMT substrate activity, and the better substrate activity for 6,7-D2HX compared to 4 is consistent with a proper catechol orientation for interaction with the norepinephrine (NE) binding site of NMT.

On the preferred rotameric conformation for dopamine agonist action: an illusory quest

Putative dopamine agonists from the 2-aminotetrahydronaphthalene and trans-octahydrobenzo (f) and (g) quinoline series were shown to inhibit the spontaneous locomotor activity of mice. Marked potency differences were observed between the alpha-and beta-rotameric conformations, compounds having the alpha-rotameric conformation having the greater potency. Thus, 2-di-n-propylmino-5,6-dihydroxytetrahydronaphthalene was 339 times more potent than the 6, 7-dihydroxy isomer, and 2-n-propylamino-5,6-dihydroxy-compound was respectively 79 times and 179 times more than 6,7-hydroxy-and 7,8-dihydroxycompounds. trans-7,8-dihydroxy-1-n-propyl-1,2,3,4,a,9,10,10b-octahydrobenzo(f)quinoline was 11 times more potent than the beta-rotamer, the 6,7-dihydroxy compound, and within the trans-octahydrobenzo(g)quinoline series the alpha-rotameric N-propyl derivative was 467 times more potent than the beta-rotamer, and the alpha-rotameric greater than N-H analogue was 46 fold more potent than the beta-rotamer. Thus, the alpha-rotamer appears the more potent in causing the present functional dopaminergic change. The dopaminergic nature of the response was indicated by its sensitivity to spiroperidol but not to yohimbine or prazosin. The possibility that a difference in behavioural potency between the alpha- and beta-rotamers may reflect a differential metabolism by catechol-O-methyl transferase was assessed by administration of different agonists after pyrogallol pretreatment. This potentiated the activity of 2-di-n-propylamino-6,7-dihydroxytetrahydro-naphthalene but not that of the 5,6-dihydroxy analogue. However, changes in the effects of N-propyl derivatives of trans-octahydrobenzo (f) and (g)quinoline were not marked and, in all experiments, pyrogallol treatment failed by orders of magnitude to shift the dose-response curves of the beta-rotamers to indicate a comparable potency to the rotameric forms.

Neurochemical and behavioural profiles of five dopamine analogues

The dopaminergic actions of five hydroxylated dopamine analogues have been examined for: i) Ability to induce stereotypy, ii) Effects upon dopamine metabolism, iii) Ability to antagonise the rise in striatal dopamine caused by gammabutyrolactone. With the exception of the resorcinol derivative 2-(3,5-dihydroxyphenyl)-N,N-dipropylethylamine, all of the compounds tested exhibited dopamine-like actions, and similarities were found in the induction of stereotypy and in the reduction of dopamine metabolism. For example, 2-(3-hydroxyphenyl)-N,N-dipropylethylamine had a short duration of action as far as reducing dopamine metabolism and inducing stereotypy were concerned. On the other hand, 2-(3-hydroxyphenyl)-N-n-propyl-N-phenyl-ethyl-ethylamine (e) and also 5-hydroxy-2-(N-n-propyl-N-phenylethyl)-aminotetralin had a long duration of agonist-like effects upon both parameters, the aminotetralin derivative being the more potent of the two. Thus, in going from the simple dopamine-like structure to the aminotetralin compound there has been an increase in dopamine agonist-like activity. The differences in dopamine agonist potency of the drugs used are discussed in relation to the structure of these compounds, and are compared with the potencies or related compounds. Also, the potencies of the compounds under investigation upon presynaptic dopamine receptors (using the gammabutyrolactone model as a test system) were investigated, and the ester, 2-(3-benzoyloxyphenyl)-N-n-propyl-N-phenylethyl-ethylamine was the most potent. This ester, which is probably converted to (e) in the brain, also had a long duration of action in the stereotypy and dopamine metabolism tests. The results suggest that certain of the compounds might be useful leads for the design of dopamine agonists of possible clinical use.

A theoretical conformational study of substituted O-anisamides as models of a class of dopamine antagonists

The quantum mechanical PCILO method has been used to investigate the conformational behaviour of N-(2-aminoethyl)- and N-(2-dimethylaminoethyl)-o-anisamide, two model molecules of substituted benzamides. The molecules are shown to have only limited conformational freedom due to the presence of two intramolecular hydrogen bonds acting as conformational locks. The molecules in their preferred conformation are characterized by a distance between the centre of the aromatic ring and the nitrogen atom of almost 6 A, i.e. almost 1 A longer than in the fully extended dopamine conformers. Some implications at the receptor level of this topographical dissimilarity are discussed.