Yawning
and locomotor behavior induced by dopamine
receptor agonists in mice and
rats
Su-Min Li, Gregory T. Collins, Noel M. Paul,
Peter Grundt, Amy H. Newman, Ming Xu, David K.
Grandy, James H. Woods, Jonathan L. Katz
Psychobiology Section,
National Institute on Drug Abuse, Intramural
Research Program, National Institutes of Health,
Baltimore, Maryland USA.
Dopaminergic (DA) agonist-induced yawning in
rats seems to be mediated by DA D3 receptors,
and low doses of several DA agonists decrease
locomotor activity, an effect attributed to
presynaptic D2 receptors. Effects of several DA
agonists on yawning and locomotor
activity were examined in rats and mice.
Yawning was reliably produced in rats,
and by the cholinergic agonist, physostigmine,
in both the species. However, DA agonists were
ineffective in producing yawning in
Swiss&endash;Webster or DA D2R and DA D3R
knockout or wild-type mice. The drugs
significantly decreased locomotor activity in
rats at one or two low doses, with activity
returning to control levels at higher doses. In
mice, the drugs decreased locomotion across a
1000&endash;10 000-fold range of doses, with
activity at control levels (U-91356A) or above
control levels
[(±)-7-hydroxy-2-dipropylaminotetralin
HBr, quinpirole] at the highest doses. Low
doses of agonists decreased locomotion in all
mice except the DA D2R knockout mice, but were
not antagonized by DA D2R or D3R antagonists
(L-741 626, BP 897, or PG01037). Yawning
does not provide a selective in-vivo indicator
of DA D3R agonist activity in mice. Decreases in
mouse locomotor activity by the DA agonists seem
to be mediated by D2 DA receptors.
Introduction
The dopaminergic (DA) D3R is classified as a
member of the D2-like family of dopamine
receptors based on its close sequence homology
to the DA D2R (Sokoloff et al., 1990). The
relatively restricted localization of the DA D3R
in mesolimbic brain areas has contributed to the
interest in this receptor, and it has been
suggested as a therapeutic target for various
psychiatric disorders (see reviews by Joyce and
Millan, 2005; Newman et al., 2005).
Despite a considerable amount of research on
the DA D3R, an understanding of the receptor and
the pharmacology of its ligands, particularly in
vivo, remains incomplete (Levant, 1997; Xu et
al, 1999). Moreover, the relative selectivity of
various putative D3R ligands can vary
substantially from one study to the next, and
depend on features of the assays, including
especially the radiolabel used (Levant,
1997).
In addition to the paucity of selective
agonist and antagonist ligands, studies of the
in-vivo pharmacology of putative DA D3R ligands
have been hindered by a lack of functional
assays that are uniquely and selectively
responsive to actions at DA D3 receptors.
Several in-vivo effects have been suggested as
functional consequences of DA D3R activity.
Reports of studies conducted, in rats show
biphasic dose-effect curves with decreases in
locomotor activity at low doses yielding to
stimulation of activity at higher doses. Several
investigators have suggested that the decreases
obtained at low doses of DA agonists are because
of actions at presynaptic DA receptors, whereas
others have suggested that the decreases in
activity were because of actions at the DA D3R
(Daly and Waddington, 1993; Gilbert and Cooper,
1995; Pugsley et al., 1995; Bristow et al.,
1996; Depoortere et al., 1996; Maj et al., 1999;
Rogóz and Skuza, 2001). Most of the
earlier reports on the effects of DA agonists on
locomotor activity in mice indicate only
dose-related decreases (Pugsley et al., 1995;
Geter-Douglass et al., 1997; Tirelli et al.,
1997; Xu et al., 1999; Boulay et al., 1999a,
1999b; Pritchard et al., 2003).
Yawning in rodents has long been
associated with DA activity (e.g. Mogilnicka and
Klimek, 1977; Holmgren and Urbá-Holmgren,
1980; Yamada and Furukawa, 1980). As noted by
Collins et al. (2005), early hypotheses
attributed yawning to various DA
mechanisms, and more recently the biphasic
dose-effect curve has been suggested to be
because of a DA D3R mediated stimulation of
yawning, accompanied by a DA D2R
inhibition of the effect. Drugs with
preferential activity at the DA D3R should
produce a biphasic dose-effect curve (Kostrzewa
and Brus, 1991; Levant, 1997). Indeed, studies
of several DA agonists, including; PD 128907
((S)-(+)-(4aR, 10bR)-3,4,4a,
10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxa
zin-9-ol HCl), quinelorane, and
(±)-7-hydroxy-2-dipropy-laminotetralin HBr
(7-OH-DPAT) produced significant, dose-dependent
increases in yawning with maximum
stimulation at an intermediate dose, and
decreases from this maximum at the highest
doses. Several antagonists with reported
selectivity for the DA D3R, including U-99,194A,
SB 277011A, PG01037, and less so nafadotride,
shifted the ascending limb of the biphasic
dose-effect curve rightward, without the effects
on the descending limb. In contrast, the
nonselective antagonists, haloperidol, and
raclopride, shifted the entire biphasic
dose-effect curve rightward, whereas the DA D2R
preferring antagonist, L-741626, selectively
shifted the descending limb of the biphasic
dose-effect curve (Collins et al., 2005; Sevak
et al., 2007).
These studies were initiated to further
explore the respective roles of dopamine
receptor subtypes in the yawning induced
by DA D3R preferring agonists. It was
hypothesized that the biphasic dose-effect curve
for the agonists would be absent, or
substantially modified, in mice with a genetic
deletion (knockout) of the DA D2 receptor (DA
D2R KO) compared with wild-type (DA D2R WT)
mice. In the initial stages of the study it was
determined that, in contrast to what was
obtained in rats, yawning was not
produced by DA D3R preferring agonists in mice,
including 7-OH-DPAT, PD 128907, quinpirole,
quinelorane, and the nonselective agonist,
apomorphine. However, casual observations
indicated that several of the DA D3R preferring
agonists decreased locomotor activity which was
then studied fully to ensure that the absence of
yawning was not because of the lack of
sufficient dosage. The decreases in activity
became of interest because they were reversed at
higher doses with several agonists, and because
they occurred over a profound range of doses.
The range of doses was much greater than that
for the decreases in activity in rats, and
indeed much greater than the range of doses over
which most behaviorally active drugs have their
effects. In this study, we focused on the
decrease in activity using pharmacological tools
and mutant mouse lines to characterize its
mechanism.
Discussion
In this study the DA agonists, 7-OH-DPAT,
and PD 128907, produced dose-related increases
in yawning in rats at low to intermediate
doses, and less of an increase at the highest
doses, as in earlier studies (Collins et al,
2005, 2007). In addition, low doses of the drugs
produced a decrease in locomotor activity that
resolved at higher doses. In contrast,
yawning was not produced in mice by any
doses of several DA agonists, but was reliably
produced by physostigmine. The effects of
physostigmine indicate that the yawning
response can be induced in mice, but does not
appear to be activated by a variety of drugs
that induce yawning in rats through DA
D3R mechanisms.
Collins et al. (2005) suggested that the
inhibition of yawning that appears at
higher doses of DA agonists and contributes to
the descending limb of the biphasic dose-effect
curve appears to be mediated by DA D2R agonist
actions. In their study, the DA D2R preferring
antagonist L-741626, selectively antagonized the
descending limb of the PD 128907 and quinelorane
dose-effect curves. This effect was also
obtained without appreciable change in the
ascending limb of the dose-effect curve for a
variety of other DA D3R-preferring agonists,
consistent with these two dopamine receptors
producing opposing effects on yawning
(Collins et al, 2007).
Thus, the possibility exists that the
absence of yawning in mice is because of
prepotent DA D2R mediated inhibitory effects
that preclude the expression of an otherwise DA
D3R mediated stimulation of yawning.
We examined the hypothesis that prepotent DA
D2R-mediated effects interfered with the
expression of a DA D3R-mediated induction of
yawning by examining mice with a genetic
deletion of the DA D2R, and by administering the
putative DA D2R selective antagonist, L-741626.
According to the hypothesis, eliminating actions
either by genetic deletion or by pharmacological
blockade of the receptor would be expected to
show a full expression of yawning
behavior in the mouse.
However, neither DA D2R KO nor WT mice
showed any yawning after administration
of either 7-OH-DPAT or PD 128907. In addition,
casual observations during studies of
interactions of L-741626 and the agonists on
locomotor activity did not show instances of
yawning. Furthermore, the lack of
difference between DA D2R WT and KO mice in the
effects of physostigmine indicates the absence
of a generalized DA D2R-mediated inhibitory
effect on yawning.
Thus, the absence of yawning appears
unrelated to the relative potencies of DA
D2R-mediated and D3R-mediated actions in the
mouse and suggests differences between rats and
mice with regard to the pharmacological actions
of DA D3R agonists.
Across the range of doses that were
examined, several of the DA agonists produced a
decrease in locomotor activity in mice and rats.
In rats, the decreases in activity were obtained
over a restricted range of doses. In particular
with 7-OH-DPAT a significant effect at 0.03
mg/kg was not obtained at three-fold higher or
three-fold lower doses. In contrast, the
relation of this effect to dose in mice was
remarkable in that decreases were obtained
across a relatively wide range of doses, from
1000-fold to 10 000-fold with 7-OH-DPAT and
quinpirole. There were substantial differences
among the agonists with respect to effects at
the higher doses in mice. For PD 128907, none of
the higher doses returned activity to control
levels, and with U-91356A, the highest dose
studied only returned activity to control
levels. The variability across drugs in the
effects of the highest doses, and the remarkably
wide range of doses over which the decreases in
locomotor activity were obtained, prompted the
present focus on the pharmacology of the
decreases in activity. The observation that
some, but not all DA agonists produced
hyperactivity at the highest doses tested tempts
an interpretation of differences among the drugs
with respect to high-dose toxicity, however,
that suggestion requires further
investigation.
A low-dose inhibition of locomotor activity
that is resolved at higher doses has been
reported in the past (e.g. Daly and Waddington,
1993; Pugsley et al., 1995; Bristow et al, 1996;
Maj et al, 1999), and has been variously
attributed to presynaptic DA D2R activity (e.g.
Millan et al, 2004) and actions mediated by the
DA D3R (e.g. Gilbert and Cooper, 1995; Bristow
et al, 1996; Shafer and Levant, 1998; Maj et al,
1999). These studies with DA D2R KO and WT mice
suggest that the effect is mediated, at least in
part, by the DA D2R, as it was obtained in the
DA D2R WT but not KO mice. In addition,
decreases in locomotor activity were obtained in
both DA D3R WT and KO mice. The DA D2R actions
mediated by both presynaptic or postsynaptic DA
D2R requires additional studies.
None of the antagonists studied were
effective in blocking the agonist-induced
decreases in locomotor activity obtained at the
low to intermediate doses. Indeed L-741626,
rather than antagonizing, added to the locomotor
decreasing effects of 7-OH-DPAT, quinpirole, and
PD 128907. The lack of antagonism may not be
surprising given the multitude of
pharmacological agents that can decrease
locomotor activity, and by implication the
presumed mechanisms that may contribute to the
effect on locomotor activity. Studies of
antagonist effects in mutant mice may elucidate
some of these mechanisms. Nonetheless, an
antagonist with sufficient selectivity such as
what has been reported at least among dopamine
receptor subtypes (e.g. Grundt et al, 2007),
should have blocked the effects of its
respective agonist, if the agonist was
sufficiently selective in producing its effects.
Despite significant effects of genotype in the
effects of L-741626 on DA D2R mutant mice, the
differences in sensitivity to the effects of
L-741626 itself were relatively small in the two
lines of mice. That finding, along with the
general lack of antagonism of the effects of the
agonists underscores the extant need for more
selective pharmacological tools, both agonists
and antagonists, to study
dopaminergically-mediated behavioral effects
better.
These studies document significant
differences in the pharmacology of DA agonists
in mice and rats.
The most pronounced of these differences is
the absence of yawning induced by the
agonists in the mouse. Although yawning
can be induced through other mechanisms in the
mouse, it appears that yawning will not provide
an in-vivo indication of DA D3R activation in
the mouse. PD 128907 was different from the
other DA agonists in that the decreases in
locomotor activity were not resolved at high
doses of the drug. Mechanisms contributing to
the differences in the pattern of dose-effects
on locomotor activity are not presently clear
though it seems likely that these differences
reflect differences in potencies for effects
contributing to the decreases and the increases
at higher doses.
In summary, the DA D3R preferring
agonists, 7-OH-DPAT, and PD 128907, produced a
biphasic stimulation of yawning in rats
and a biphasic inhibition of locomotor activity.
In contrast, these effects were different in
mice; yawning was not produced by DA
agonists, and the decreases in locomotor
activity were characteristically obtained over a
relatively broad range of doses. The decreases
in locomotor activity appear to be because of
the actions of the drugs at the DA D2R. These
results are not inconsistent with earlier
results indicating opposing modulation of
yawning by the DA D3R and D2R in rats,
though further studies on the role of the DA D3R
in mice are clearly necessary.
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