The present study investigated whether PD
128907 and 7-OH-DPAT, described as preferential
dopamine (DA) D(3) receptor agonists, produce
hypolocomotion by acting at postsynaptic
dopaminergic receptors within the nucleus
accumbens. Bilateral infusion of PD 128907 (1.5
and 3 microg/0.5 microl) induced a
dose-dependent hypolocomotion, whereas its
enantiomer, PD 128908, was inactive. Local
infusion of 7-OH-DPAT and the preferential DA
autoreceptor agonist, B-HT 920, at the same dose
range also decreased spontaneous locomotion. In
addition, both drugs induced yawning with B-HT
920 producing the greatest effect. In the second
experiment, the ability of these agonists to
reduce the locomotor activity induced by
intra-accumbens injection of DA (10 microg/0.5
microl) was studied. Pretreatment with either PD
128907 or 7-OH-DPAT (3 microg) reduced
DA-induced hyperactivity. Local infusion of B-HT
920 (3 microg) failed to antagonise the
locomotor effects of DA. Altogether these
findings suggest that PD 128907 and 7-OH-DPAT
induce hypolocomotion by acting in part at
postsynaptic DA receptors. The possible role of
D(2) and/or D(3) receptors in the mediation of
these effects is discussed.
1. Introduction
Among the newly discovered dopamine (DA)
receptors, the D3 receptor subtype has been of
particular interest, in part because of its
distinctive pattern of localisation in the
brain. The D3 receptor is predominantly
expressed in limbic regions of the brain (e.g.;
nucleus accumbens,, island of Calleja, and
hippocampus) known to be associated with
cognitive and emotional functions [9,2I,23,}
This neuroanatomical distribution together with
its high affinity for antipsychotic drugs
suggests that D3 receptors may be important
targets for the development of therapeutic
treatments for schizophrenia and drug abuse
[32,36,37]. In the past few years,
several existing compounds were reported to
display a greater selectivity for D3 receptors
as compared to D2 receptors. For instance, both
PD 128907 and 7-OH-DPAT were characterised as
D3-preferring receptor agonists in both binding
and cellular studies, with the former compound
being the most selective [5,23,33,35].
However, given systematically, both PD 128907
and 7OH-DPAT were found to produce behavioural
changes (e.g., hypolocomotion and yawning)
comparable to those observed with other DA
receptor agonists such as apomorphine.
Interestingly, unlike apomorphine or other
related agonists, the sedative effects induced
by systemic administration of 7-OH-DPAT have
been reported to occur independently from
changes of DA release or synthesis [40].
Similarly, PD 128907 was also found to inhibit
amphetamine-induced hyperactivity at doses that
had no effect on the increase of extracellular
DA levels induced by amphetamine in the ventral
striatum [10]. On the basis of these
observations, it has been suggested that the
hypolocomotion induced by the PD 128907 and
7-OHDPAT may be mediated via D3 postsynaptic DA
receptors [10,40]. In line with these
findings, subsequent studies have shown that PD
128907 could also reduce stereotypies induced by
direct DA receptor agdu.ist, apomorphine, and
the NMDA receptor antagonist, MK 801.
Furthermore, the effects of PD 128907 on MK
801-induced stereotypies could be prevented by
coadministration of the selective D3 antagonist,
NGB 2900 [43].
Although there is strong evidence that PD
128907 and 7OH-DPAT suppress locomotion by
acting at postsynaptic DA receptors, there have
been few attempts to explore the neuroanatomical
substrate underlying such action. To address
this issue we studied the effects of direct
infusion of PD 128907 and 7-OH-DPAT to the
nucleus accumbens, a brain structure where D3
receptors are highly expressed [39]. For
comparison we also studied the DA D2 agonist,
BHT 920 [35]. This compound has been
described as a preferential presynaptic DA
autoreceptor agonist on the basis of behavioural
and neurochemical studies in intact animals
[6,28,34]. For instance, B-HT 920
produces hypolocomotion over a wide dose range.
Furthermore, unlike direct dopaminergic agonists
(e.g., apomorphine and lisuride), high doses of
B-HT 920 fail to increase locomotion, suggesting
that this compound has only weak agonistic
actions at postsynaptic D2 receptors
[3,28]. This apparent selectivity for
presynaptic autoreceptors has been related to
the difference of receptor reserves in
dopaminergic neurones and their target neurones
[14,25].
The first part of the study was designed to
characterise the behavioural effects induced by
local infusion of PD 128907, 7-OH-DPAT, and B-HT
920 into the nucleus accumbens. To further
investigate the possibility that a population of
postsynaptic DA 1ceptàrs might mediate
suppression of locomotion, we have studied the
ability of these agonists to reduce the
hyperactivity induced by bilateral
intra-accumbens infusions of DA. Since the
hyperlocomotor activity induced by DA is
mediated by postsynaptic DA receptors,
suppression of DA's effect would suggest that
D3-preferring agonists act via postsynaptic
mechanisms to reduce locomotion.
4. Discussion
The present behavioural study shows that all
three DA agonists, PD 128907, 7-OH-DPAT, and
B-HT 920, injected into the nucleus accumbens
reduced spontaneous locomotor activity. However,
only 7-OH-DPAT and B-HT 920 increased the
incidence of yawning. More importantly, these
three agonists differentially suppressed the
locomotor hyperactivity induced by local
infusion of DA into the nucleus accumbens.
Whilst both PD 128907 and 7-OHDPAT markedly
reduced DA-induced locomotor hyperactivity,
local infusions of the putative DA autoreceptor
agonist, B-HT 920, failed to antagonise the
effects of DA. These findings indicate that the
nucleus accumbens is one of the brain areas
mediating yawning and hypolocomotion induced by
systemic administration of low doses of
dopaminergic agonists. Furthermore, they suggest
that a population of postsynaptic DA receptors
within the nucleus accumbens may be involved in
mediating the locomotor suppression induced by
PD 128907 and 7-OH-DPAT.
Previous studies have shown that low doses
of 7-OHDPAT injected into the nucleus accumbens
reduced the locomotor activity of rats tested in
a novel environment [13,19]. Our study
confirms these previous findings and
further demonstrates that local injection of
the D3-preferring agonist, PD 128907, as well as
the preferential DA autoreceptor agonist, B-HT
920, can also induce hypolocomotion.
Interestingly, we have found that the three
agonists tested showed a marked difference in
their ability to induce yawning. Indeed, a clear
increase in the incidence of yawning was seen
after both injections of 7-OH-DPAT and B-HT 920,
with the latter inducing the greatest effect.
However, microinjection of PD 128907 at the same
dose range failed to elicit yawning (but the
possibility exists that a higher dose might be
effective). Thus, in the yawning test 7-OH-DPAT
displayed a behavioural profile more comparable
to B-HT 920 than PD 128907. It is unlikely that
this behavioural difference between PD 128907
and 7-OHDPAT reflects differences in
bioavailability or intrinsic activity. In fact,
at the higher dose tested (3 rig) both drugs
induced a comparable reduction of locomotor
activity (during the first 30 mm, PD 128907
reduced locomotor activity by 39% and 7-OH-DPAT
by 42% relative to the control). The present
finding would suggest that the differential
effects of PD 128907 and 7-OH-DPAT on yawning
might be due to an action on distinct receptors.
Several binding and functional in vitro studies
have shown that 7-OH-DPAT has a modest
selectivity for D3 versus D2 receptors than PD
128907 [12,35]. It is therefore possible
that populations of presynaptic D2 receptors may
mediate yawning induced by 7-OH-DPAT. In support
of this suggestion the autoreceptor agonist B-HT
920, which had the greatest effect on yawning,
was shown to display a higher functional
selectivity toward D2 receptor subtypes
[35]. However, we cannot exclude the
possibility that 7-OH-DPAT and B-HT 920 may
induce yawning by acting on other receptors. For
instance, B-HT 920 has also high affinity for
a1-adrenergic receptors, which have been
implicated in the modulation of yawning
[18].
To further investigate the possibility that
a population of postsynaptic DA receptors may
mediate suppression of locomotion, we have
studied the ability of PD 128907, 7OH-DPAT, and
B-HT 920 to reduce the hyperactivity induced by
bilateral intra-accumbens infusion of DA. In
Iine with previous studies, local infusion of DA
into the nucleus accumbens caused a marked
locomotor hyperactivity in rats
[2,29,31,41]. Interestingly, both PD
128907 and 7OH-DPAT markedly reduced the
lcomotor activity induced by intra-accumbens
injection of IA; in contrast, the putative DA
autoreceptor agonist, B-HT 920, was ineffective.
The lack of effect of B-HT 920 suggests that (1)
the suppression of DA-induced locomotor
hyperactivity is not due to a potential
nonspecific effect resulting from combined
injection of the drugs and (2) that the
D3-preferring agonists reduce DA effects by
acting at postsynaptic rather than presynaptic
receptors. Consistent with this finding,
systemic administration of PD 128907 was found
to inhibit amphetamine-induced locomotor
hyperactivitiin rats at doses that had no effect
on the increase of extracellular DA level
induced by amphetamine in the ventral striatum
[10]. More importantly, PD 128907 also
reduced the hyperactivity induced by the D1
receptor agonist SKF 81297, as well as
stereotypies induced by the direct DA receptor
agonist apomorphine, and the NMDA receptor
antagonist MK 801 [27,43]. Furthermore,
the effects of PD 128907 on MK 801induced
stereotypies could be prevented by
coadministration of selective D3 antagonist NGB
2900 [43]. Given that PD 128907
possesses a higher affinity for the D3 versus D2
receptors, it is conceivable that its
suppressive effect on DAinduced hyperactivity
may be due to the stimulation of postsynaptic D3
receptors. In line with this suggestion, It was
recently shown that the ontogeny of motor
inhibition induced by low doses of PD 128907
coincides in time with the developmental
expression of DA D3 receptors within the nucleus
accumbens [15,36]. Further evidence for
the role of D3 receptors in the inhibition of
locomotor activity is also provided by several
studies using more selective strategies such as
the antisense knockdown approach. Indeed,
intracerebroventricular infusion of an antisense
oligodeoxynucleotide directed against D3
receptor subtypes mRNA reduced D3 receptor
density in the nucleus accumbens and increased
spontaneous locomotor activity in rat
[11,45]. Furthermore, D3 antisense
oligodeoxynucleotide treatment increased
locomotor activity induced by apomorphine in
DA-depleted rats [26], thus revealing
the role of postsynaptic D3 receptors in the
inhibition of locomotor activity. It is
important to stress that these effects were
observed with different sequences of D3
antisense oligodeoxynucleotides, thus confirming
the selectivity of the antisense knockout
approach. Moreover, we found that the D2
antisense oligodeoxynucleotide induces an
opposing effect on locomotion [45].
Altogether, these findings strongly suggest that
D2 and D3 receptors regulate the expression of
locomotor behaviour in rat in an opposing
direction.
It should be noted, however, that recent
data from gene knockout studies have questioned
the role of D3 receptors in the suppression of
locomotion. While some authors have reported
increased locomotion in mice lacking the D3
receptor gene [1,44], others failed to
observe these effects [8]. Furthermore,
only mice lacking D2, but not D3 receptor genes
were found to be nonresponsive to the.
hypolocomotion induced by 7-OH-DPAT and PD
128907, thus suggesting that these compounds may
be rather acting on D2 receptors to inhibit
locomotion [7]. As recently demonstrated
by Levin [22], D3 receptors are
differentially expressed in certain brain
regions in mice and rats, and it is thus
possible that they may also differentially
regulate certain behaviours in these two
species. It should be also stressed that there
are clear phenotypic differences between D3
mutant mice generated in different laboratories
[1,8,38,44], and this seems also the
case for D2 mutant mice [4,7,17]. These
discrepancies strongly suggest that the
behavioural phenotypes of the mutants are not
due only to the loss of the targeted receptors
but also to several other factors including
developmental adaptations and genetic background
[17]. As such, it cannot be
excluded
that these factors might also influence drug
responses [16,24], and thus contribute
to the discrepant results obtained in rats and
mice.
Finally, it was recently shown that
7-OH-DPAT and PD 128907 act as partial agonist
at both D2 and D3 receptors [42], but
see Ref. [35]. It is therefore possible
that these ligands might behave as D2 receptor
agonists or antagonists depending on the level
of the dopaminergic tone. For instance, the
suppression of spontaneous locomotion may be due
to the stimulation of presynaptic D213
receptors, whereas the blockade of DA-induced
locomotor hyperactivity may be related to an
antagonism of postsynaptic D2 receptors.
However, this hypothesis seems unlikely,
especially for PD 128907. Firstly, in functional
assays unlike 7OH-DPAT, PD 128907 was found to
behave as a partial agonist that lacks
antagonist properties [42]. Furthermore,
pretreatment with the selective D3 receptor
antagonist NGB 2900 was shown to prevent PD
128907-induced blockade of stereotypies evoked
by MK 801 [43], which strongly suggests
that PD 128907 acts via stimulation of D3
receptors.
In conclusion, our findings confirm previous
studies showing that 7-OH-DPAT and PD 128907
reduce spontaneous and DA agonist-induced
locomotion. Furthermore, they suggest that this
suppression of locomotion can be mediated via at
least two independent mechanisms within the
nucleus accumbens: (1) a presynaptic mechanism
involving inhibition of DA release, probably via
stimulation of D2 receptor subtypes
[20], and (2) a postsynaptic mechanism
involving activation of D2-like receptors.
Further studies using intra-accumbens infusions
of highly selective D2 and D3 receptor ligands
are clearly needed to confirm whether the
postsynaptic receptors are of D2 or D3 subtype.
