Joseph G. Cannon

Differentiation of the dopamine mechanisms mediating stereotyped behaviour and hyperactivity in the nucleus accumbens and caudate-putamen

A number of dopamine agonists were applied intracerebrally to the nucleus accumbens and caudate‐putamen of rat in an attempt to differentiate the dopamine mechanisms in these nuclei which mediate hyperactivity and stereotyped behaviour. The major effect of dopamine was to induce hyperactivity from the nucleus accumbens and stereotypy from the caudate‐putamen; N‐n‐propyl‐norapomorphine induced hyperactivity and stereotypy from the nucleus accumbens whilst apomorphine induced a marked stereotypy from the caudate‐putamen, modest stereotypy from the nucleus accumbens and no hyperactivity. In contrast to apomorphine, 2‐(NN‐dipropyl)amino‐5,6‐dihydroxy TN‡ induced a more marked stereotypy from the nucleus accumbens and, again, no hyperactivity. The major effect of 2‐(NN‐diethyl)amino‐5,6‐dihydroxy TN was to cause an intense hyperactivity from the nucleus accumbens and marked stereotypy from the caudate‐putamen whilst the primary amine, 2‐amino‐5,6‐dihydroxy TN induced hyperactivity and stereotypy from both areas. The marked hyperactivity and stereotyped responses were inhibited by haloperidol, but not by α‐ or β‐blockers. These data would indicate that there may be different dopamine mechanisms in the nucleus accumbens and caudate‐putamen for the mediation of hyperactivity and stereotyped behaviour.

Differential activation by some 2-aminotetralin derivatives of the receptor mechanisms in the nucleus accumbens of rat which mediate hyperactivity and stereotyped biting

A series of 2-aminotetralin derivatives were injected into the nucleus accumbens of rat to assess the nature of the dopamine mechanisms in this nucleus which modulate hyperactivity and stereotyped behaviour. It was shown that (1) Derivatives with either 5,6- or 6,7-dihydroxy substitutions were each able to induce hyperactivity and stereotyped behaviour, but substitutions in the 5,6-positions conferred greater potency throughout the series. This differential was emphasised by the continued activity of 2-amino-5,6-dihydroxytetralin in the absence of nialamide whilst the action of 2-amino-6,7-dihydroxytetralin was greatly reduced. (2) The hydroxyl functions in both the 5,6- and 6,7-series were essential for activity: dimethyoxy derivatives were inactive. (3) Generally, substitution of the nitrogen atom with one or two methyl groups, or with a butyl group, reduced or abolished activity. However, N-ethyl and N-propyl substitution markedly enhanced stereotypic potential in the 5,6-dihydroxy series (but not in the 6,7-series). The N-isopropyl derivative in the 5,6-series reflected the activity of the N-propyl compound but a further substitution of the N atom with the methyl group (N-isopropyl-N-methyl) greatly reduced the stereotypic potential without modification of the hyperactivity response. In contrast, N,N-dipropyl substitution abolished the hyperactivity response whilst increasing sterotypic potential. (4) alpha and beta-adrenoceptor blocking agents and alpha-methyl-p-tyrosine failed to reduce the hyperactivity induced by 2-amino-5,6-dihydroxytetralin or the stereotyped behaviour induced by 2-(N,N-dipropyl)-amino-5,6-dihydroxytetralin. Both behaviours were, however, very sensitive to blockade by haloperidol, indicating that both the hyperactivity and stereotyped responses are dopamine-dependent. It is concluded that the dopamine mechanisms in the nucleus accumbens which mediate/regulate hyperactivity and stereotyped behaviour are different. Further, it is suggested that the 2-aminotetralins may be valuable tools in studies designed to assess the topography of cerebral dopamine systems.

Effects of some derivatives of 2-aminotetralin on dopamine-sensitive adenylate cyclase and on the binding of [3H]haloperidol to neuro-leptic receptors in the rat striatum

Putative dopamine agonists from a series of 2-aminotetralin derivatives were assessed for their ability to stimulate dopamine-sensitive adenylate cyclase and to inhibit the binding of [3H]haloperidol to neuroleptic receptors in the rat striatum. Comparisons were made to the ability of these agents to stimulate motor function on intrastriatal injection (stereotyped biting/hyperactivity) or peripheral administration (stereotyped biting). Of the two primary amines, 2-amino-5,6-dihydroxytetralin and 2-amino-6,7-dihydroxytetralin, the 6,7-dihydroxy compound was found to be 20 times more potent than the corresponding 5,6 derivative to stimulate adenylate cyclase. The activities of a series of mono- and di-alkylated substitutes of the two primary amines (methyl, ethyl, propyl, butyl) were determined. Of particular interest, and in contrast with the primary amines, 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin was shown to be twice as potent as the corresponding 6,7-derivative. Of the compounds which were shown to stimulate adenylate cyclase, little difference was found in their activities. In the second assay procedure, inhibition of [3H]haloperidol binding, 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin was the most active compound studied (a hundred times activity of dopamine). Similarly to observations with adenylate cyclase, the 6,7- derivative was the more active of the primary amines (times ten) whilst the 5,6- derivative was the more active of the N,N-dipropylcompounds (times three). General comparisons could be made between the biochemical findings and behavioural observations: those agents most active to induce dopamine-like motor effects on subcutaneous or intrastriatal injection were generally effective to stimulate adenylate cyclase and to inhibit [3H]haloperidol binding, whilst compounds inactive behaviourally were also inactive in vitro [2-(amino)-5,6-dimethoxytetralin and 2-(N, N-dimethyl)amino-5,6-dimethoxytetralin]. However, an absolute correlation between in vivo and in vitro potency could not be found. The most important observation of the present studies is the potency of 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin in the [3H]haloperidol binding assay which indicates that this 2-aminotetralin derivative may be a useful tool in future studies on dopamine receptors.

Dopamine agonists: structure-activity relationships

In the several years since Blaschko [1] first suggested that dopamine might be a neurotransmitter substance, many chemical compounds have been found to possess dopamine-like actions. This review surveys classes of structures for which putative dopaminergic agonism has been reported, and cites structure-activity correlations. Some aspects of metabolism of the agents are addressed, which seem to have relevance to structure-activity considerations and to strategy of drug design.

A comparison of dopamine agonist action to inhibit locomotor activity and to induce stereotyped behaviour in the mouse.

51 purported dopamine agonists from the phenylethylamine, tetralin, octahydrobenzo(f)- and (g)quinoline, benzocycloheptene, aporphine and ergoline series were tested in the mouse for ability to cause motor inhibition at low doses and stereotyped responding (motor facilitation) at higher doses. Motor inhibition was characterised either by a freezing akinesia (spiroperidol sensitive) or by sedation (resistant to spiroperidol). Agents potent to induce the freezing response could, if the dose was raised sufficiently (at least 10 fold), cause stereotypy. Within all series tested N-n-propyl substitution generally conferred greatest selectivity of motor inhibitory action. Radioligand binding assays using [3H]ADTN as ligand and rat striatal tissue showed correlations between abilities to associate with the dopamine receptor and to cause motor inhibition or facilitation, but discrepancies were apparent, particularly within the tetralin series. It is concluded that whilst there exists clear potency differences to inhibit locomotor activity and to induce stereotyped behaviour, it is difficult to demonstrate unequivocally an absolute selectivity of dopamine agonist action for the motor inhibitory dopamine system.

Congeners of the .beta. conformer of dopamine derived from cis- and trans-octahydrobenzo[f]quinoline and trans-octahydrobenzo[g]quinoline

The so-called beta conformer of dopamine has been proposed to be involved in agonist--receptor interactions at several sites in the dopaminergic nervous system. Further to evaluate this proposal, rigid congeners of the beta conformer derived from linearly and angularly annelated octahydrobenzoquinolines have been synthesized. Certain N-alkylated trans-angularly annelated systems exhibited unusually potent and highly selective dopamine-like effects in an assay on a cardioaccelerator nerve preparation in the cat, but these compounds were inactive in a variety of assays for CNS effects. These compounds present a clear separation of CNS effects from some potent peripheral effects.

Characteristics of [3H]hemicholinium-3 binding to rat striatal membranes: evidence for negative cooperative site-site interactions.

Abstract: The characteristics of [3H]hemicholinium‐3 ([3H]HC‐3) interactions with rat striatal membranes were investigated. Under the described assay conditions, [3H]‐HC‐3 binds with a saturable population of membrane binding sites having the following regional distribution: striatum « hippocampus ≧ cerebral cortex > cerebellum. The specific binding of [3H]HC‐3 showed an obligatory requirement for NaCl; other halide salts of sodium or KCl failed to substitute for NaCl. The Scatchard transformation of saturation isotherm data generated a curvilinear plot with high‐and low‐affinity components of binding. The dissociation of [3H]HC‐3 at infinite dilution was also multiexponential. The dissociation could, however, be accelerated if unla‐beled HC‐3 was included in the diluting buffer, and this increase in dissociation appeared to be dependent on the concentrations of unlabeled HC‐3 used, with the maximal increase demonstrable at 100 nM The dissociation was also dependent on the fractional saturation of binding sites with labeled HC‐3, such that, at higher fractional saturation of binding sites, the overall dissociation was faster and the difference in the dissociation observed between „dilution only” and „dilution + unlabeled HC‐3” was reduced. This occupancy‐dependent change in dissociation could also be influenced by temperature and pH. Based on the results of these kinetic studies, the steady‐state [3H]HC‐3 binding data were analyzed for a homogeneous population of binding sites undergoing site‐site interactions of the negative cooperative type. Such an analysis yielded a KD of 9.3 nM for the high‐affinity state and a KD of 22.8 nM for the low‐affinity state of binding sites, with a Bmax of 434 fmol/ mg of protein. Competitive binding studies showed that unlabeled HC‐3 was most potent in displacing [3H]HC‐3, followed by choline. Other drugs known to have little influence on the synaptosomal sodium‐dependent high‐affinity choline uptake system (SDHACU) had no significant effect on [3H]HC‐3 binding sites. Similarities in ionic dependencies, regional distributions, and pharmacological selectivi‐ties of [3H]HC‐3 binding with synaptosomal SDHACU suggest that [3H]HC‐3 selectively labels SDHACU sites located on presynaptic cholinergic neurons in rat CNS. We suggest that the two affinity states of [3H]HC‐3 binding sjtes represent the different „functional” states of the SDHACU system. The binding of HC‐3 (or choline) with the high‐affinity state of the binding sites induces negative cooperative site‐site interactions among the binding sites, resulting in the formation of a low‐affinity binding state. Because the affinities of HC‐3 and choline for this low‐affinity state of the [3H]HC‐3 binding sites correspond to the affinities of these agents for the SDHACU system, we also suggest that the low‐affinity binding state represents the „functional” form for SDHACU.

Dopaminergic effects of non-hydroxylated rigid analogs of apomorphine☆

A series of rigid analogs of apomorphine lacking aromatic hydroxyl substitutents were evaluated for dopaminergic properties. Three compounds, N-methyl-N-propyl-2-aminotetralin (Me-Pr-2-AT), N-N-dipropyl-2-aminotetralin (Di-pr-2-AT) and N,N-dipropyl-2-aminoindane (Di-Pr-2-AI) induced emesis in dogs, contralateral circling in unilaterally lesioned rats, and inhibited prolactin secretion. The induced circling responses, however, were attenuated by prior treatment with alpha-methyl-p-tyrosine methyl ester (AMPTME) and the compounds were weak inhibitors of 3-H-dopamine binding in calf caudate homogenates. The possibility that these agents may be metabolically activated in vivo is discussed.

Bulbocapnine's ability to antagonize the adrenergic inhibitory action of dopamine and analogs

Dopamine in the presence of cocaine, 5,6-dihydroxy-2-dimethylaminotetralin (M-7), apomorphine and N,N-dimethyldopamine produced dose dependent inhibition on the heart rate increase due to stimulation of the right cardioaccelerator postganglionic nerves. Inhibitory actions of these compounds were antagonized by bulbocapnine. Also bulbocapnine antagonized the blood pressure increase due to N,N-dimethyldopamine but did not antagonize the pressor response of dopamine.

Methylpiperidine analog of hemicholinium-3: A selective, high affinity non-competitive inhibitor of sodium dependent choline uptake system

The potency of hemicholinium-3 (HC-3) and its analogs to inhibit sodium dependent high affinity choline uptake were evaluated in rat striatal synaptosomal preparation. Hemicholinium-3 inhibited sodium dependent high affinity choline uptake (IC50 = 18 nM) while the half molecule of HC-3, HC-15, was inactive. The order of potency for choline uptake inhibition of piperidine substituted HC-3 molecule was as follows: 4-methylpiperidine (A-5 and CA-5) much greater than HC-3 much greater than unsubstituted piperidines (CA-1 and A-1) much greater than 2- or 3-methylpiperidine (A-2 and A-3) and 4-hydroxypiperidine (A-7). The tertiary amine derivative of 4-methylpiperidine substituted HC-3 (A-4) was nearly 10-fold less potent than its corresponding quaternary derivative (A-5). Choline uptake was inhibited competitively by HC-3 and non-competitively by A-5. The inhibition of choline uptake by A-5 was readily reversible by washing. A-5 did not inhibit the uptake of dopamine and gamma-aminobutyric acid. These findings suggest that the N-methyl,4-methylpiperidine analog of HC-3 (A-5) is the most potent of all known inhibitors of sodium dependent high affinity choline uptake and that the inhibition of choline uptake by this compound is mediated through a mechanism distinct from a simple competitive one.