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mise à jour du
30 novembre 2009
Pharmacol Biochem Behav
Biochemical and behavioral effects of boldine
and glaucine on dopamine systems
Asencio M, Delaquerrière B, Cassels BK
Speisky H, Comoy E, Protais P
Departamento de Química, Facultad de Ciencias, Universidad de Chile, Santiago


The aporphine alkaloids boldine and glaucine have been reported to show "neuroleptic-like" actions in mice, suggesting that they may act as dopamine antagonists. We have found that in vitro boldine displaces specific striatal [3H]-SCH 23390 binding with IC50 = 0.4 microM and [3H]-raclopride binding with IC50 = 0.5 microM, while the affinities of glaucine at the same sites are an order of magnitude lower.
In vivo, however, 40 mg/kg boldine (i.p.) did not modify specific striatal [3H]-raclopride binding and only decreased [3H]-SCH 23390 binding by 25%. On the other hand, 40 mg/kg glaucine (i.p.) displaced both radioligands by about 50%. Behaviors (climbing, sniffing, grooming) elicited in mice by apomorphine (0.75 mg/kg s.c.) were not modified by boldine at doses up to 40 mg/kg (i.p.) but were almost completely abolished by 40 mg/kg glaucine (i.p.).
In the apomorphine-induced (0.1 mg/kg s.c.) rat yawning and penile erection model, boldine and glaucine appeared to be similarly effective, inhibiting both behaviors by more than 50% at 40 mg/kg (i.p.). Boldine and glaucine, injected i.p. at doses up to 40 mg/kg, were poor modifiers of dopamine metabolism in mouse and rat striatum.
These data suggest that boldine does not display effective central dopaminergic antagonist activities in vivo in spite of its good binding affinity at D1- and D2-like receptors, and that glaucine, although less effective in vitro, does appear to exhibit some antidopaminergic properties in vivo.

A large number of compounds based on the aporphine skeleton are known to exhibit dopaminergic activities. Some, like the prototypical apomorphine [( R )-aporphine-10,11-diol], are agonists, while others antagonize some of the actions of dopamine, with strong agonist activity apparently being associated with the ( R ) configuration at C-6a and the presence of a hydroxyl group at C-11 (9,10,21). Thus, ( R )-apomorphine elicits stereotypical behaviors such as sniffing, licking, gnawing (8,13), and climbing (22).
On the other hand, the natural 11-hydroxyaporphine ( S )-bulbocapnine has long been known to induce catalepsy (7), and is also able to antagonize stereotypies elicited by apomorphine or ( 1 )-amphetamine in rats (30). Bulbocapnine, corytuberine, boldine, and glaucine, all aporphine alkaloids that share the ( S ) configuration and thus have a twisted biphenyl skeleton enantiomeric with that of apomorphine and its dopamine agonist congeners (2), have been reported to show "neuroleptic-like" actions in mice upon subcutaneous injection, which would seem to suggest that they may be acting as central dopamine antagonists (35). Both bulbocapnine and corytuberine bear phenolic hydroxyl groups at C-11.
As neither boldine nor glaucine carry any substituent at C-11, however, this could be interpreted as an indication that a hydrogen-bonding group at this position is not a requirement for aporphine derivatives to exhibit reasonably high affinity (although perhaps not intrinsic efficacy) for dopamine receptors. In recent years the diphenolic boldine [( S )-1,10-dimethoxyaporphine- 2,9-diol] has attracted attention in relation to its potent antioxidative and cytoprotective properties (3,4,28).
This alkaloid is present in high concentrations in the bark of the Chilean boldo tree ( Peumus boldus Mol., Monimiaceae), which makes it an interesting candidate for development as a natural drug. Although it does not carry a hydroxyl group at C-11, its diphenolic structure does incorporate the meta -hydroxy-phenethylamine moiety of dopamine in two different restrained conformations, at least one of which might reasonably be assumed to interact strongly with the crucial anionic center and one of the characteristic serine residues of the active site(s) of one or more subtypes of dopamine receptors (11,29).
Glaucine [( S )-1,2,9,10-tetramethoxyaporphine], the nonphenolic dimethyl ether of boldine, is almost as potent as an antioxidant, and in fact, it seems likely that aporphines in general may exhibit this behavior at low micromolar concentrations (6). Nevertheless, its complete lack of hydroxyl groups would seem to indicate the possibility of some mode of binding, other than that postulated for agonists, to explain its hypothetical dopaminergic activity. If antioxidative activity were present at concentrations capable of eliciting behavioral changes, aporphines might be valuable as cytoprotective CNS drugs.
On the contrary, if no CNS activity were discernible at central neuroprotective concentrations, some of these substances might be interesting candidates for development as antioxidants per se. The ready availability of boldine and glaucine, their demonstrated strong antioxidative properties, as well as the published data suggestive of dopamine antagonist activity, led us to delve deeper into the interactions of these alkaloids with dopaminergic systems in order to further evaluate the potential of aporphines as drug leads.
In vitro binding data indicate that boldine and glaucine (Fig. 5) are able to recognize striatal dopamine D1 and D2 binding sites labeled respectively with [3H]-SCH 23390 and [3H]-raclopride (12,15). Although boldine is about 10 times more potent than glaucine, each of the two products appears to be equally active at dopamine D1 and D2 binding sites. The affinities of boldine and glaucine for D1 and D2 dopamine receptors appear consistent with previous studies on aporphine derivatives (25), but are rather low compared to those of classical dopamine antagonists (18).
Several of our results indicate that in vivo, glaucine acts as a weak but effective dopamine antagonist at D1 and D2 receptors, especially at a dose of 40 mk/kg (IP): 1) it partially inhibits in mice the in vivo striatal binding of [3H]-SCH 23390 and [3H]-raclopride, and completely inhibits the binding of [3H]- raclopride in olfactory tubercles; 2) like classical dopamine antagonists acting at dopamine D2 receptors (17), and like chlorpromazine, it increases in mice and rats the release of dopamine in striatum, as evidenced by the increased levels of dopamine metabolites and HVA/DA ratio; 3) it antagonizes apomorphine-induced climbing and sniffing in mice, two behaviors produced by the simultaneous stimulation of dopamine D1 and D2 receptors (31,33); 4) it does not allow, in apomorphine- treated mice, the reappearance of grooming, a behavior induced by the isolated stimulation of dopamine D1 receptors (20,32); and 5) in rats treated with a low dose of apomorphine, it antagonizes yawning and penile erection resulting from the stimulation of dopamine D2 receptors in the paraventricular nucleus of the hypothalamus (19,20).
Compared to glaucine, boldine administered IP is able 1) to more weakly inhibit striatal [3H]-SCH 23390 binding in mice, with no effect on [3H]-raclopride binding; and 2) to increase the levels of dopamine metabolites in mice less than glaucine, and to an even lesser extent in rats. Nevertheless, both boldine and glaucine are able to decrease D2 receptordependent apomorphine-induced yawning and penile erections in rats. Because the in vitro binding studies indicate that boldine displays a 10-fold greater affinity for dopamine receptors than does glaucine, these data suggest that boldine 1) does not easily cross the blood&endash;brain barrier, or 2) is metabolized and/or excreted extensively before it can reach the brain, or 3) does not act as an antagonist at some dopamine receptor subtypes. In the understanding that the changes observed in striatal dopamine metabolite levels are a consequence of postsynaptic dopamine receptor blockade, and considering that boldine is about 10 times more potent than glaucine in in vitro binding studies, it may be postulated that at similar doses more glaucine than boldine is made available to the brain.
Conversely, the same potency ratio and the fact that boldine and glaucine inhibit yawning and penile erections to similar extents may be construed to indicate that boldine reaches the D2 receptors of the paraventricular nucleus of the hypothalamus at effective concentrations about 10 times lower than those attained by glaucine in the same area. Preliminary data from our group (27) indicate that boldine is rapidly cleared from plasma in rats. Comparison of the administration pathway-associated effects of boldine on apomorphine- induced behaviors indicates that at 40 mg/kg, boldine is completely ineffective as an inhibitor of apomorphineinduced climbing and sniffing when administered IP, but partially effective to inhibit these behaviors when administered SC. Therefore, although boldine may act as an antagonist at dopamine receptors, its poor access, at least to certain regions of the CNS, added to its very short plasma half-life, do not allow this property to be easily revealed in some in vivo experiments.
As an alternative, it might be suggested that boldine could act as a partial agonist at dopamine receptors. Indeed, in the present in vitro binding studies, the displacement of [3H]-raclopride is best fitted using a two-site model as described for agonists (14). Furthermore, the contention that boldine is poorly effective in inhibiting in vivo [3H]-SCH 23390 and [3H]-raclopride binding is consistent with this hypothesis, because dopamine agonists are known to be able to inhibit in vivo binding at dopamine receptors only at very high doses. For instance, apomorphine doses of 3 mg/kg are necessary to achieve 50% inhibition of the in vivo binding at D1 and D2 dopamine receptors, while doses lower than 1 mg/kg are sufficient for the induction of obvious and long-lasting behaviors (1,22,23,31,34). Taken together, these data suggest that glaucine and especially boldine display weak in vivo antidopaminergic activities compared to classical dopamine antagonists (18,22,23,31,34). Nevertheless, insofar as the lack of potency of boldine may be due in part to its unfavorable pharmacokinetics, appropriate prodrugs might overcome this limitation.
On the other hand, because in vivo results indicate that these aporphine alkaloids are able to reach the brain at concentrations approaching those at which they exhibit antioxidative activity, they might be valuable leads for the development of cytoprotective drugs for use in stroke or trauma.