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.
DISCUSSION
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.
