Sensitivity
to apomorphine-induced yawning and hypothermia
in rats eating standard or high-fat
chow
Baladi MG, Thomas YM, France CP.
Department of Pharmacology,
University of Texas Health Science Center.
San Antonio, TX,
USA.
Abstract
Feeding conditions modify sensitivity to
indirect and direct acting dopamine receptor
agonists as well as the development of
sensitization to these drugs.
This study examined whether feeding
condition affects acute sensitivity to
apomorphine-induced yawning or changes in
sensitivity that occur over repeated drug
administration. Quinpirole-induced yawning was
also evaluated to see whether sensitization to
apomorphine confers cross-sensitization to
quinpirole.
Drug-induced yawning was measured in
different groups of male Sprague Dawley rats (n
= 6/group) eating high (34.3%) fat or standard
(5.7% fat) chow.
Five weeks of eating high-fat chow rendered
otherwise drug-naïve rats more sensitive to
apomorphine- (0.01-1.0 mg/kg, i.p.) and
quinpirole- (0.0032-0.32 mg/kg, i.p.) induced
yawning, compared with rats eating standard
chow. In other rats, tested weekly with
apomorphine, sensitivity to apomorphine-induced
yawning increased (sensitization) similarly in
rats with free access to standard or high-fat
chow; conditioning to the testing environment
appeared to contribute to increased yawning in
both groups of rats. Food restriction decreased
sensitivity to apomorphine-induced yawning
across five weekly tests. Rats with free access
to standard or high-fat chow and sensitized to
apomorphine were cross-sensitized to
quinpirole-induced yawning. The hypothermic
effects of apomorphine and quinpirole were not
different regardless of drug history or feeding
condition.
Eating high-fat chow or restricting access
to food alters sensitivity to direct-acting
dopamine receptor agonists (apomorphine,
quinpirole), although the relative contribution
of drug history and dietary conditions to
sensitivity changes appears to vary among
agonists.
-Baladi
MG, Newman AH, France CP. Dopamine D3
receptors mediate the discriminative stimulus
effects of quinpirole in free-feeding rats. J
Pharmacol Exp Ther. 2010;332(1):308-315
-Baladi MG,
Newman AH, France CP. Influence of body
weight and type of chow on the sensitivity of
rats to the behavioral effects of the
direct-acting dopamine-receptor agonist
quinpirole.Psychopharmacology (Berl).
2011;217:573&endash;585
-Baladi
MG, Thomas YM, France CP. Sensitivity to
apomorphine-induced yawning and hypothermia in
rats eating standard or high-fat chow.
Psychopharmacology (Berl).
2012;222(1):27-36
-Collins GT, JM
Witkin et al Dopamine agonist-induced
yawning in rats: a dopamine d3 receptor mediated
behavior. J Pharmacol Exp Ther
2005;314(1):310-9.
-Collins GT, Newman
AH,Woods JH et al.Yawning and hypothermia in
rats: effects of dopamine D3 and D2 agonists and
antagonists. Psychopharmacology
(Berl).
2007;193(2):159-170
-Collins GT. et
al. Food restriction alters
pramipexole-induced yawning, hypothermia, and
locomotor activity in rats: Evidence for
sensitization of dopamine D2 receptor-mediated
effects. JEPT 2008;325:691-697
-Collins
GT et al. Narrowing in on compulsions:
dopamine receptor functions Exp Clin
Psychopharmacol 2008,16(4):498-502
-Collins
GT et al. Pro-erectile Effects of Dopamine
D2-like Agonists are Mediated by the D3 Receptor
in Rats and Mice JPEP 2009;329(1):210-217
-Collins GT,
Truong YN, et al. Behavioral sensitization
to cocaine in rats: evidence for temporal
differences in dopamine D(3) and D (2) receptor
sensitivity. Psychopharmacology (Berl).
2011;215(4):609-20
Introduction
Feeding conditions can impact sensitivity to
the behavioral effects of drugs acting directly
at dopamine receptors (Baladi and France 2009;
Can et al. 2001; Collins et al. 2008; Sevak et
al. 2008). For example, the dose-response curve
for dopamine receptor agonist (e.g.,
quinpirole)-mduced yawning is an inverted
U-shape with the ascending and descending limbs
of the curve being mediated by D3 and D2
receptors, respectively (Baladi et al. 2010;
Collins et al. 2005). In rats eating high-fat
chow, both limbs of the dose-response curve are
shifted leftward; however, antagonism of
quinpirole-induced yawning is not different
between rats eating high-fat or standard chow
(Baladi et al. 2011), indicating that the shifts
leftward in the former reflect increased
sensitivity at D3 (ascending limb) and D2
(descending limb) receptors (Baladi and France
2009; Baladi et al. 2010, 2011; Collins et al.
2005). Food restriction, on the other hand,
decreases sensitivity to, or in some cases
eliminates, dopamine receptor agonist-induced
yawning as evidenced by a flattening of the
inverted U-shaped dose-response curve; this
flattening is thought to reflect a selective
increase in sensitivity at D2 receptors (i.e.,
descending limb; Coffins et al. 2008; Sevak et
al. 2008).
Sensitivity to the behavioral effects of
drugs acting on dopamine systems sometimes
increases over repeated tests (i.e.,
sensitization), and this phenomenon can occur
with both direct- (e.g., apomorphine) and
indirect (e.g., amphetamine)-acting dopamine
receptor agonists (Kalivas and Weber 1988;
Mattingly et al. 1991; Robinson and Becker 1986;
Silverman 1991). Repeated treatment with drugs
that directly or indirectly stimulate dopamine
receptors can result in longlasting behavioral
effects in humans as well (e.g.,
methamphetaminepsychosis; Sato et al. 1983;
Strakowski et al. 1996). The development and
expression of sensitization (e.g., druginduced
locomotor activity) is influenced by a number of
factors including the particular drug, dose,
frequency of drug administration, and the
context in which drug is administered (Keller et
al. 2002; Pens and Zahniser 1989; Post et al.
1981; Stewart and Badiani 1993). For example,
after repeated pairings, the environment in
which drug is administered can itself elicit a
drug-like response. Conditioning of drug effects
to environmental stimuli often develops with
indirect-acting dopamine receptor agonists like
amphetamine and cocaine (Gold et al. 1988;
Martin-Iverson and Reimer 1994) and less often
with direct-acting dopamine receptor agonists
like apomorphine or quinpirole (Rowlett et al.
1991; Szechtman et al. 1993; Willner et al.
1992). The conditioning of drug effects to
environmental stimuli likely depends on the
specific behavior being examined (Einat and
Szechtman 1993), and it is unclear whether
conditioning develops to drug-induced
yawning.
Dietary conditions might impact not only
acute sensitivity to drugs but also changes in
sensitivity that occur over repeated drug
administration (i.e., sensitization). For
example, sensitization to the locomotor
stimulating effects of the indirect-acting
dopamine receptor agonist methamphetamine is
enhanced in rats eating high-fat chow (McGuire
et al. 2011). A number of studies have examined
druginduced sensitization using measures of
locomotion; however, the generality of those
data to other behavioral effects (e.g., yawning)
is largely unknown.
The current study extends previous results
obtained with the direct-acting dopamine
receptor agonist quinpirole by examining the
effects of food restriction and of eating
high-fat chow on a well-characterized behavioral
effect (yawning) of another direct-acting
dopamine receptor agonist, apomorphine, in
drug-naïve rats and in rats that were
tested repeatedly with apomorphine. The feeding
conditions used in the current study were
selected in order to maximize potential
differences in drug effects between groups and
were based on a previous study (Baladi et al.
2011). In the first experiment, rats with free
access to either standard or high-fat chow were
used to determine whether eating highfat chow
for 5 or 6 weeks alters sensitivity to
apomorphine- or quinpirole-induced yawning,
respectively. In a second experiment, rats with
free access to either standard or high-fat chow
or with restricted access to standard chow were
tested with apomorphine once per week for 5
weeks to examine whether sensitization develops
to apomorphineinduced yawning and whether
sensitization is influenced by feeding
condition. It is well established that eating
high-fat chow shifts the quinpirole yawning
dose-response curve to the left (Baladi et al.
2011); quinpirole was examined in the current
study to test whether sensitization to
apomorphine-induced yawning confers cross
sensitization to quinpirole-induced yawning.
Apomorphine- and quinpiroleinduced hypothermia
was examined to test whether another effect of
these drugs was similarly changed as a
consequence of repeated drug testing or eating
conditions.
Discussion
This study examined the effects of food
restriction and of eating high-fat chow on
yawning produced by the directacting dopamine
receptor agonist apomorphine in rats. These
results extend previous fmdmgs with other
directacting dopamine receptor agonists such as
pramipexole and quinpirole (Collins et al. 2008;
Sevak et al. 2008) in demonstrating that food
restriction also reduces or eliminates
apomorphine-induced yawning. Second, results in
rats that were tested just once with drug after
a period of eating high-fat or standard chow
show that eating high-fat chow is sufficient to
increase sensitivity to agonist-induced yawning
and not to agonist-induced hypothermia. Third,
once weekly apomorphine tests cause
sensitization to apomorphineinduced yawning and
not to apomorphine-induced hypothermia;
development of sensitization is not impacted by
eating high-fat chow and yawning increases
significantly over repeated testing even in the
absence of drug (i.e., after vehicle injection),
suggesting that conditioning contributes to
apparent increases in sensitivity to drug.
Finally, rats that are sensitized to
apomorphine-induced yawning are crosssensitized
to quinpirole-induced yawning.
Dopamine receptor agonist-induced yawning
has been used to study changes in dopamine
systems that occur under a variety of
conditions, and it is well established that the
ascending and descending limbs of the agonist
(e.g., quinpirole) dose-response curve are
mediated by dopamine D3 and D2 receptors,
respectively (Baladi et al. 2010; Collins et al.
2005). A shift downward in the apomorphine
doseresponse curve in rats with restricted
access to food, as shown in the current study
and by others, might be due to decreased
sensitivity at D3 receptors, increased
sensitivity at D2 receptors, or to both
decreased sensitivity at D3 and increased
sensitivity at D2 receptors. However, because a
D2 receptor antagonist restores agonist-induced
yawning in food-restricted rats (Collins et al.
2008), the effects of food restriction are most
likely due to increased D2 receptor sensitivity.
Rats that were tested just once with drug after
5 or 6 weeks of eating high-fat chow were
significantly more sensitive (i.e., both limbs
of the dose-response curve shifted leftward)
than rats eating standard chow to
apomorphine-induced and quinpirole-induced
yawning, respectively, indicating increased
sensitivity at D3 and D2 receptors. These
behavioral data parallel results from
neurochemistry studies showing that food
restriction decreases extracellular dopamine
concentration in the nucleus accumbens (Pothos
et al. 1995), increases dopamine D2 receptor
binding (Thanos et al. 2008), and increases
coupling between D2 receptors and G proteins
(Can 2002). Moreover, rats eating high-fat chow
have decreased extracellular dopamine in the
nucleus accumbens (Rada et al. 2010) and
increased D2 receptor binding (South and Huang
2008). Thus, feeding conditions alone (i.e., in
the absence of repeated drug treatment) can
significantly increase sensitivity to the
behavioral and neurochemical effects of
drugs.
Repeated, intermittent treatment can also
increase sensitivity to the behavioral effects
of drugs, and this phenomenon (i.e.,
sensitization) has been studied extensively for
the locomotor stimulating effects of
indirect-acting dopamine receptor agonists
(e.g., cocaine). Other studies have examined
sensitization to the locomotor effects of
direct-acting dopamine receptor agonists like
apomorphine and quinpirole (Rowlett et al. 1991;
Voikar et al. 1999). Although sensitizalion
develops to the locomotor effects of
direct-acting dopamine receptor agonists, it is
less clear whether sensitization develops to
other behavioral effects of the same drugs (but
see Silverman 1991) and whether eating
conditions impact the development of
sensitization to direct-acting agonists as it
can for indirect-acting agonists (e.g., McGuire
et al. 2011). In the current study, once weekly
tests with apomorphine decreased the maximally
effective dose of apomorphine for producing
yawning, reflecting a leftward shift in the
yawning doseresponse curve and providing
evidence for sensitization to this effect of a
direct-acting dopamine receptor agonist. The
diminishing effect obtained with a dose of 0.32
mg/kg apomorphine across weekly tests with
apomorphine (i.e., this was the maximally effect
dose on study day 7 and almost no yawning
occurred with this dose on study day 35; Fig. 2)
suggests an increased sensitivity at dopamine D2
receptors (i.e., shift leftward in the
descending limb of the dose-response
curve).
Progressively across weekly tests, yawning
increased after administration of even small
doses of apomorphine; over the same time course,
progressively more yawning was observed after
the administration of vehicle. Further tests in
the absence of drug indicate that environmental
stimuli (i.e., cage type and injection
procedure) that were repeatedly paired with
drug-induced yawning came to elicit yawning in
the absence of drug. Yawning in the absence of
drug did not develop in food-restricted rats,
perhaps because apomorphine produced very little
yawning in those rats. This is consistent with
the view that environmental stimuli must be
paired explicitly with drug-induced effects
(e.g., yawning) in order for sensitization to
develop (to drugs) and possibly for conditioned
increases in behavior to emerge (Einat and
Szechtman 1993; Willner et al. 1992). For
example, rats exposed to a running wheel
following repeated quinpirole injections become
sensitized to the locomotor effects of
quinpirole when tested on a runway. However,
rats exposed to a locked running wheel following
repeated quinpirole injections do not display
sensitization to the locomotor effects of
quinpirole when tested on a runway (Willner et
al. 1992). Thus, the absence of drug-induced
sensitization and the lack of conditioned
yawning in food-restricted rats might be due to
the fact that apomorphine produced very little
yawning in those rats. Results of the current
study provide evidence that conditioning to
dopamine receptor agonists extends beyond
locomotor stimulating effects to yawning.
That sensitization develops to
apomorphine-induced yawning in the current
study, but not to quinpirole-induced yawning in
a previous study (Baladi et al. 2011), might be
due to the relative selectivity of these
compounds for dopamine receptor subtypes.
Apomorphine has similar affinities for dopamine
D3, D2, and Dl receptors (Andersen et al. 1985;
Sokoloff et al. 1990) while quinpirole is more
than 113-fold selective for D3 receptors over D2
receptors (Sokoloff et al. 1990) with negligible
affinity for Dl receptors. The neurobiological
mechanisms underlying sensitization remain
unclear, and there is debate regarding the
involvement of specific dopamine receptor
subtypes in the development of sensitization
(Mattingly et al. 1991; Voikar et al. 1999). In
addition, non-dopaminergic systems (e.g.,
serotonin, glutamate) might also play a role in
sensitization (Kuczenski and Segal 1989; Schenk
et al. 1993; Voikar et al. 1999; Wolf and
Jeziorski 1993). Nevertheless, several reports
suggest that stimulation of Dl receptors is
critical for the development of sensitization to
both direct and indirect-acting dopamine
receptor agonists (Braun and Chase 1988;
Criswell et al. 1989; Henry and White 1991;
Mattingly et al. 1991; Stewart and Vezina 1989).
Thus, sensitization to yawning induced by
apomorphine, but not quinpirole, might be due to
the relatively high affinity of apomorphine and
relatively low affinity of quinpirole for
dopamine Dl receptors.
Rats eating standard chow and sensitized to
apomorphineinduced yawning were cross-sensitized
to quinpirole-induced yawning. That is, both
limbs (ascending D3 -receptor mediated and
descending D2 receptor-mediated) of the
quinpirole yawning dose-response curve were
shifted to the left in rats eating standard chow
and tested once weekly with apomorphine,
compared with rats eating standard chow that
were tested only once with apomorphine (Fig 5,
upper panel, circles; Table 1). However, in rats
eating high-fat chow, cross sensitization from
apomorphine to quinpirole was only evident in
the descending limb (D2 receptor-mediated) of
the quinpirole dose-response curve. That is, the
descending limb of the quinpirole dose-response
curve was shifted leftward in rats eating
high-fat chow and tested weekly with apomorphine
as compared with rats eating high-fat chow and
tested just once with apomorphine (Fig. 5, upper
panel, squares; Table 1). Thus, rats eating
high-fat or standard chow show similar, but not
identical, cross sensitization from apomorphine
to quinpirole. The extent to which the ascending
limb (D3 receptormediated) of the quinpirole
dose-response curve can be shifted leftward,
either by dietary or pharmacological
manipulation, remains to be determined; however,
it is clear that the same receptors (D3 and D2)
mediate quinpirole-induced yawning in rats
eating standard or high-fat chow (Baladi et al.
2011).
Both of the direct-acting dopamine receptor
agonists produced hypothermia in all rats.
Moreover, the hypothermic effects of apomorphine
and quinpirole were similar among groups of rats
with different drug and diet histories.
Direct-acting dopamine receptor agonist-induced
hypothermia is thought to be mediated
predominantly by D2 receptors because those
effects are antagonized by D2, and not D3,
receptor antagonists (Baladi et al. 2010;
Chaperon et al. 2003; Collins et al. 2007).
Because neither repeated drug treatment nor
eating high-fat chow alter the hypothermic
effects of apomorphine or quinpirole, drug- and
diet-induced changes in yawning produced by
these drugs are likely due to pharmacodynamic
and not pharmacokinetic mechanisms. A similar
dissociation between hypothermia and other
effects has been reported for another dopamine
receptor agonist in cocaine-sensitized rats
(Collins et al. 2011) and for cannabinoid
receptor agonists in rats eating high-fat chow
(Wiley et al. 2011). Differential effects of
feeding condition on dopamine agonist-induced
yawning and hypothermia might reflect different
populations of dopamine receptors mediating
these effects [the paraventricular nucleus
of the hypothalamus mediating yawning (Argiolas
and Melis 1998) and the anterior
hypothalamus/preoptic area mediating hypothermia
(Lin et al. 1982)] and/or other compensatory
mechanisms regulating body temperature.
In summary, this study extends previous
results regarding the effects of food
restriction and eating high-fat chow on
sensitivity to dopamine receptor agonist-induced
yawning (Baladi et al. 2011; Collins et al.
2008; Sevak et al. 2008). Food restriction
decreased and eventually eliminated
apomorphine-induced yawning. Eating high-fat
chow for 5 weeks, in the absence of any other
treatment, was sufficient to increase the
sensitivity of rats to apomorphineinduced
yawning. On the other hand, repeated weekly
testing with apomorphine increased the
sensitivity of all rats to apomorphine-induced
yawning regardless of the type of chow they ate,
a finding that is in contrast to what is
observed after repeated testing with quinpirole
(i.e., shift left in the dose-response curve for
animals eating high-fat chow and not for those
eating standard chow). Overall, these results
are similar but not identical to the increased
sensitivity that develops to quinpirole-induced
yawning in rats eating high-fat chow and tested
weekly with quinpirole. These results also
provide some of the first evidence showing
conditioning to drug-induced yawning, an effect
that might mask diet-induced differences in the
development of sensitization to
apomorphine-induced yawning. Although many
factors can impact sensitivity to the behavioral
effects of drugs (Dafiiy and Yang 2006),
together with a growing body of literature,
these results clearly support the view that
eating conditions modify the behavioral effects
of drugs acting on dopamine systems in a manner
that might be relevant to understanding
individual differences in response to
recreational as well as therapeutic drug use
(Carroll et al. 1981; Goodwin et al. 1987).
