Inhibition of dopamine receptors by endogenous amines: Binding to striatal receptors and pharmacological effects on locomotor activity

Article abstract of DOI:10.1016/S0960-894X(00)00326-7

Endogenous amine 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ) derivatives are synthesized, and their activity for dopaminergic systems are evaluated in vitro and in vivo by receptor binding assay and pharmacological tests. It is proposed that 1BnTIQ d

Full text of DOI:10.1016/S0960-894X(00)00326-7

Bioorganic & Medicinal Chemistry Letters 10 (2000) 1669±1671
Inhibition of Dopamine Receptors by Endogenous Amines:
Binding to Striatal Receptors and Pharmacological
E€ects on Locomotor Activity
Hiroshi Kawai,a,y Yaichiro Kotakeb and Shigeru Ohtab,*
aGraduate School of Pharmaceutical Sciences, University of Tokyo, Tokyo 113-0033, Japan
bInstitute of Pharmaceutical Sciences, Hiroshima University School of Medicine, Hiroshima 734-8551, Japan
Received 7 April 2000; accepted 22 May 2000
AbstractÐEndogenous amine 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ) derivatives are synthesized, and their activity for
dopaminergic systems are evaluated in vitro and in vivo by receptor binding assay and pharmacological tests. It is proposed that
1BnTIQ derivatives can act as endogenous dopaminergic antagonists. # 2000 Elsevier Science Ltd. All rights reserved.
1,2,3,4-Tetrahydroisoquinoline (TIQ) derivatives exist not
only in plants but also in several tissues in mammals.1
Several TIQ derivatives have been proposed to relate with
the pathogenesis of Parkinson's disease.2 1-Benzyl-1,2,3,4-
tetrahydroisoquinoline (1BnTIQ, 1), 1-(30,40-dihydroxy-
benzyl)-1,2,3,4-tetrahydroisoquinoline (30,40DHBnTIQ,
2) and 1-benzyl-6,7-dihydroxy-1,2,3,4-tetrahydroiso-
quinoline (6,7DHBnTIQ, 3) are endogenous amines and
the former two compounds can induce parkinsonism to
animals.2c e They contain a dopamine and/or phenethyl-
amine moiety in their structure, and they are structurally
similar to apomorphine, a well-known dopaminergic
ligand 3 (Fig. 1). We considered that these 1BnTIQ deri-
vatives could be endogenous aporphine analogues. Since
many compounds containing TIQ skeleton are known as
agonists and antagonists of dopamine receptors,3 1BnTIQ
derivatives also can be dopaminergic ligands. Pre-
viously, it was reported about the interaction of Par-
kinsonism-inducible TIQ derivatives and dopaminergic
systems that some of the TIQ derivatives can be taken up
into dopaminergic neurons via dopamine transporter,2f,4
1BnTIQ derivatives (1 and 2) can kill the cultured-
mesencephalic neurons (our unpublished data), and 6,7-
dihydroxy-1,2-dimethyl-1,2,3,4-tetrahydroisoquinoline
(N-methyl-salsolinol) can induce apoptosis to dopami-
nergic neuroblastoma cells.2e But it has not been reported
that whether Parkinsonism-inducible TIQ derivative can
be a ligand of dopamine receptors, and if so, whether
this interaction is important for its ability to induce
Parkinsonism. In this paper, we examine the binding
anity of endogenous 1BnTIQ derivatives (1±3) for
dopamine receptors, and discuss the structural require-
ments for these compounds to a€ect dopamine receptors
and the relationship to the pharmacological e€ects.
Compounds 1 and 2 were synthesized by modifying the
method of Gray et al. (1989).5a N-(b-Phenylethyl)pheny-
lacetamide or N-(b-phenylethyl)-3,4-dimethoxyphenyl-
acetamide was synthesized from b-phenylethylamine and
phenylacetyl chloride or 3,4-dimethoxyphenylacetylchlo-
ride. The product was re¯uxed with P2O5 in anhydrous
toluene, a€ording 3,4-dihydroisoquinoline derivatives.
The resultant was re¯uxed with NaBH4 in ethanol,
a€ording 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1Bn
TIQ, 1) or 1-(30,40-dimethoxybenzyl)-1,2,3,4-tetrahydro-
isoquinoline. Compound 1 was treated with HCl, and
recrystallized from ethanol-diethylether, a€ording col-
orless crystals (1 HCl, mp 165±166 ꢀC). 1-(30,40-Dime-
thoxybenzyl)-1,2,3,4-tetrahydroisoquinoline hydrochlo-
ride was demethylated with 47% hydrobromic acid. The
precipitates obtained after cooling were recrystallized
from ethanol-diethylether, a€ording 1-(30,40-dihydroxy-
benzyl)-1,2,3,4-tetrahydroisoquinoline hydrobromide (2
HBr, mp 208±211 ꢀC). Compound 3 was synthesized from
dopamine and phenylacetaldehyde by Pictet±Spengler
condensation.5b Structures were con®rmed by 1H NMR
spectrometry and elemental analysis.
*Corresponding author. Tel.: +81-82-257-5325; fax: +81-82-257-5329;
e-mail: ohta@pharm.hiroshima-u.ac.jp
yPresent address: Division of Biological Chemistry and Biologicals,
National Institute of Health Sciences, Tokyo 158-8501, Japan.
Binding anity of 1BnTIQ derivatives with dopamine
receptors were determined by measuring their ability to
displace 3H-labeled speci®c ligand from rat striatal
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00326-7

1670
H. Kawai et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1669±1671
that there are two major binding sites in the dopamine
receptor, which interact with N and catechol, especially
m-OH, of ligand molecules.3,7 Appropriate orientation
of N and OH are required for high-anity binding.
Although the anity of 1BnTIQ derivatives are weaker
than that of the known dopaminergic ligands such as
aporphines, we speculate that 3 meets this requirement,
that is, 6,7-diOH group contributes to increase the
binding anity with dopamine receptors, although 30,40-
diOH group had little e€ect. Since 2 does not have 6,7di-
OH, its anity is equipotent to 1 and is lower than 3.
6,7-Dihydroxy-1,2,3,4-tetrahydroisoquinoline (norsalsoli-
nol), which is structurally similar to 3 but lacks the benzyl
moiety at the 1-position, had weaker anity than 3 (data
not shown), suggesting that 1-benzyl moiety seems to
contribute to the binding with receptors by hydrophobic
interaction.
Figure 1. Structures of 1BnTIQ, 30,40DHBnTIQ, 6,7DHBnTIQ and
aporphine derivatives.
synaptosomes.6 [3H]-SCH23390 (1.5 nM), [3H]-YM
09151-2 (0.5 nM), and [3H]-GBR12935 (1.0 nM) were
We conclude that OH (catechol) in TIQ moiety, not in 1-
benzyl moiety, N atom and benzyl functions are impor-
tant for the binding of 1BnTIQ derivatives to dopamine
receptors. These functions in 3 are preferably located so
that it can strongly bind to the dopamine receptors.
3
used as H-labeled ligands for dopamine receptors (D1
and D2 subtype), and dopamine transporter, respectively
(Fig. 2). Nonspeci®c binding was determined as that
obtained from assay in the presence of 10 or 100 mM
unlabeled ligands (SCH23390, sulpiride, and GBR12909),
and was subtracted, respectively. 1BnTIQ derivatives
7
The binding anity of 1BnTIQ derivatives to the dopa-
mine transporter was also measured (Table 1). All of
them can bind to dopamine transporter with Ki values of
can bind to dopamine receptors with Ki values of 10
±
4
10 M (Table 1). The order of binding anity was
3>152, and all three compounds prefer D2 than D1.
Both 2 and 3 are dihydroxylated derivatives of 1 and
structurally resemble (Fig. 1), but only 3 had higher
anity than 1 and 2.
4
10 5±10 M. Compound 2 had slightly higher anity
than compound 3. This is consistent with our previous
®nding that 2 and 3 are substrates of the dopamine
transporter and the anity is 2>3.2f
The structure±activity relationship studies using dopamine
and dopamine-containing dopaminergic ligand indicate
The dopamine receptors (D1 and D2) and dopamine
transporter would recognize 1BnTIQ derivatives in di€er-
ent manner. Dopamine transporter prefers 2 as substrate,
while D1 and D2 prefer 3. It is considered that both
dopamine receptors and transporter recognize dopamine
moiety of DHBnTIQ, but the favorable conformation
of dopamine moiety should be di€erent. DHBnTIQ
bind to dopamine transporter with the extended con-
formation of dopamine, but bind to dopamine receptors
with compact conformation of dopamine, probably
because it is better for hydrophobic interaction by 1-
benzyl moiety.
In order to examine the e€ect of DHBnTIQs (2, 3) in
vivo, an open ®eld test was carried out. Each DHBnTIQ
(100±250 mg/kg) was administered intraperitoneally to
male C57BL mice (7 weeks), and locomotor activity was
measured by means of the open ®eld test, 5, 15, 25, 35 min
after administration. The modi®ed Hall's method and
apparatus were used for open ®eld test.8 The apparatus
consisted of a bucket (28 cm diameter and 15 cm deep),
the bottom of which was divided into 19 areas, with a
light (60 W) at 50 cm above the bottom. A mouse was
placed in the center of the ®eld, and its ambulation and
rearing were measured for 2 min as parameters of loco-
motor activity. Both DHBnTIQs dose-dependently
reduced ambulation and rearing scores, and 3 showed
larger e€ect than 2 (Fig. 3). After the open ®eld test was
completed, the induction of catalepsy was evaluated.
The mouse was placed with forelimbs on a wire 5 cm
Figure 2. Structures of the 3H-labeled compounds used in binding assay.
Table 1. Binding anities of 1BnTIQ derivatives for dopamine recep-
tors. Ki values are shown in mMa
D1
D2
Dopamine transporter
1
2
3
248.80
174.84
9.27
3.57
8.79
0.64
36.62
21.49
57.55
aKi values were determined by using the relationship, Ki ˆ IC50
=
ꢁ1 ‡ ‰LŠ=KD†, where [L] is concentration of the radioligands used in
the assay, and KD is dissociation constant. KD=1.3, 0.2 and 5.0 nM,
respectively.

H. Kawai et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1669±1671
1671
dopamine transporter seems important for the chronic
e€ect (Parkinsonism), whereas the inhibition of dopamine
receptors causes the acute e€ect (reduction of locomotor
activity) of DHBnTIQs.
In conclusion, 1BnTIQ and DHBnTIQs, especially
6,7DHBnTIQ 3, show anity for dopamine receptors,
and DHBnTIQs can induce a remarkable reduction of
locomotor activity in vivo. Because they are endogenous
amines, we speculate that they act as physiological
modulators of the dopaminergic systems.
Acknowledgements
This work was supported by a Grant-in-Aid for Scienti®c
Research on Priority Areas from the Ministry of Educa-
tion, Science, Sports, and Culture, Japan.
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Figure 3. E€ect of DHBnTIQ derivatives on locomotor activity.
Results of the open ®eld test of 2-treated mice (ambulation (A), rear-
ing (C)) and 3-treated mice (ambulation (B), rearing (D)) are shown.
Doses: ~, 100 mg/kg; *, 150 mg/kg; &, 200 mg/kg; ^, 250 mg/kg;
&, control (saline treated). Each plot represents the mean Æ SEM
(mean+SEM or SEM in some cases) of 6 experiments.
above the ¯oor, and if it retained that unnatural posture
for over 3 min, it was evaluated as cataleptic. Compound
3 treated mice (2:6 of 150 mg/kg and 3:6 of 200 mg/kg
treated group) exhibited catalepsy. Both DHBnTIQs can
reduce locomotion in mice and 3 is more e€ective than 2.
Locomotive activity is regulated by dopamine receptors,
especially D2, and D2 antagonists can reduce locomotor
activities and induce catalepsy.9 DHBnTIQs are ligands
for dopamine receptors (Table 1), and from the behavioral
point of view, they can reduce locomotion (Fig. 3), there-
fore they may be the antagonists of dopamine receptors.
Compound 3 could block the D2 receptor so eciently
(Table 1) that it induced catalepsy at high doses.
In our previous report, we showed that 2 could induce
parkinsonism in mice, and proposed its mechanism of
action that 2 is accumulated in dopaminergic neurons
by dopamine transporter and thereafter it inhibits the
mitochondrial respiration, which lead to cell death and
parkinsonism.2f,10 Compound 3 showed no chronic
e€ect,2f in spite of its relatively potent activity to inhibit
dopamine receptors (Table 1) and to reduce locomotor
activity acutely in mice (Fig. 3). If inhibition of dopamine
receptors is associated with parkinsonism, 3 should have
more potent activity to induce parkinsonism than 2. We
think that dopamine receptor antagonism can lead to
acute reduction of locomotor activity, but does not
contribute to the pathogenesis of Parkinson's disease.
The acute and chronic toxicity of DHBnTIQs may be
associated with di€erent biochemical activity of these
compounds. Accumulation in dopaminergic neurons by