Food restriction
alters pramipexole-induced yawning, hypothermia,
and locomotor activity in rats: Evidence for
sensitization of dopamine D2 receptor-mediated
effects
Gregory T. Collins, Diane M. Calinski, Amy
Hauck Newman, Peter Grundt and James H.
Woods
Department of Pharmacology
University of Michigan Medical School Ann
Arbor
Abstract:
Food restriction enhances sensitivity to the
reinforcing effects of a variety of drugs of
abuse including opiates, nicotine, and
psychostimulants. Food restriction has also been
shown to alter a variety of behavioral and
pharmacological responses to dopaminergic
agonists including an increased sensitivity to
the locomotor stimulatory effects of direct- and
indirect-dopamine agonists, elevated
extracellular dopamine levels in responses to
psychostimulants, as well as suppression of
agonist-induced yawning. Behavioral and
molecular studies suggests that augmented
dopaminergic responses observed in
food-restricted animals result from a
sensitization of the dopamine D2 receptor,
however, little is known about how food
restriction affects dopamine D3 receptor
function. The current studies were aimed at
better defining the effects of food restriction
on D2 and D3 receptor function by assessing the
capacity of pramipexole to induce yawning,
penile erection (PE), hypothermia, and locomotor
activity in free-fed and food-restricted rats.
Food restriction resulted in a suppression of
pramipexoleinduced yawning, a sensitized
hypothermic response, and an enhanced locomotor
response to pramipexole, effects that are
suggestive of an enhanced D2 receptor activity;
no effect on pramipexole-induced PE was
observed. Antagonist studies further supported a
food restrictioninduced enhancement of D2
receptor activity as the D2 antagonist,
L-741,626, recovered pramipexole-induced yawning
to free-fed levels, while yawning and PE were
suppressed following pretreatment with the D3
antagonist, PG01037. The results of the current
studies suggest that food restriction sensitized
rats to the D2-mediated effects of pramipexole
while having no effect on the D3-mediated
effects of pramipexole.
Introduction:
Food restriction affects the function of a
variety of neurotransmitter systems including
dopaminergic (Carlson et al., 1988; Carr et al.,
2003), serotonergic (Gur et al., 2003; Jahng et
al., 2007), and cholinergic (Persinger et al.,
2002) systems, and is known to alter the effects
of drugs with diverse mechanisms of action. For
instance, food restriction has been shown to
enhance the reinforcing properties of opiates
(Carroll et al., 1979), ethanol (Meisch and
Thompson, 1973), nicotine (Donny et al., 1998),
and psychostimulants (Carroll et al., 1981;
Macenski and Meisch, 1999), elevate
extracellular dopamine levels in the nucleus
accumbens core in response to psychostimulants
(Cadoni et al., 2003), and enhance the locomotor
stimulatory effects of both direct- (Carr et
al., 2001; 2003), and indirect-dopamine agonists
(Deroche et al., 1993; Cadoni et al., 2003). A
growing literature supports the notion that the
sensitized behavioral responses to D2/D3
agonists, such as quinpirole, observed in
foodrestricted rats result from an enhancement
of the functional coupling of Gi G-proteins to
D2 receptors, and not an increase in D2 receptor
expression (Pothos et al., 1995; Carr et al.,
2003). Alternatively, changes in D3 receptor
expression and/or function could also explain
the behavioral sensitivity observed in
food-restricted animals, however, little is
known about how food restriction affects D3
receptors.
For example, previous studies suggest that
the enhancement of quinpirole-induced locomotor
activity observed in food-restricted rats
results from an enhanced functional activity of
the D2 receptor (Carr et al., 2003). However,
this effect could also be explained by a
tolerance, or down-regulation of the D3 receptor
as the inhibition of locomotor activity by D2/D3
agonists has been hypothesized to be mediated by
the D3 receptor (Svensson et al., 1994).
Interpretation of changes in D2/D3
agonist-induced locomotor activity is further
complicated by the fact that D2like antagonists
often alter locomotor activity on their own. In
addition to their effects on locomotor activity,
D2/D3 agonists are known to possess a variety of
other behavioral effects including the induction
of yawning (Yamada et al., 1986), penile
erection (PE) (Melis et al., 1987), and
hypothermia (Faunt and Crocker., 1987). While
post-synaptic D2/D3 receptors within the
mesolimbic dopaminergic pathway are thought to
mediate the locomotor effects of D2-like
agonists (Levant, 1997), the induction of
yawning and PE by D2-like agonists is thought to
be mediated by postsynaptic D2-like receptors on
oxytocinergic neurons in the paraventricular
nucleus (Argiolas and Melis, 1998).
Recently, D3-selective antagonists have been
shown to produce selective rightward shifts of
the ascending limbs, while D2-selective
antagonists shifted only the descending limbs of
the dose-response curves for D2-like
agonistinduced yawning and PE (Collins et al.,
2005; 2007; submitted) suggesting that the
induction of yawning and PE by D2/D3 agonists is
mediated by a selective activation of the D3
receptor while the inhibition of yawning and PE
observed at higher doses is mediated by agonist
activity at the D2 receptor. D2 receptors have
also been reported to mediate the hypothermic
effects of D2-like (Boulay et al., 1999;
Chaperon et al., 2003; Collins et al., 2007).
Interestingly, food restriction has been shown
to suppress apomorphine-induced yawning (Nasello
et al., 1995), an effect that is suggestive of a
decrease in D3 receptor expression and/or
function. However, based on the findings that
yawning is differentially mediated by the D3
(induction) and D2 (inhibition) receptors, the
suppression of D2/D3 agonist-induced yawning
observed during food restriction could also
result from an enhanced or sensitized D2
response.
The present studies were aimed at
determining the effects of food restriction on
D2 and D3 receptor function in rats. Thus, the
capacity of pramipexole to induce yawning, PE,
hypothermia, and locomotor activity was first
assessed in free-fed rats, assessed in the same
rats following 10 days of food restriction, and
then reassessed following 7 days of free
feeding. Additionally, antagonists selective for
the D2 and D3 receptors were assessed for their
capacity to alter the induction of yawning and
FE in both free-fed and food-restricted rats to
determine whether changes in D2 and/or D3
receptor function and/or sensitivity could be
observed. Finally, as yawning can be induced by
a variety of mechanisms, the capacity of the
cholinesterase inhibitor, physostigmine, and the
5-HT2 receptor agonist, TFMPP, to induce yawning
was assessed in free-fed and food-restricted
rats. Results from the study of the effects of
food restriction on the behavioral effects of
pramipexole alone, and in combination with
antagonists suggest that food restriction
effectively sensitized rats to the D2-mediated
effects of pramipexole while not altering the
function and/or sensitivity of the D3
receptor.
Food restriction has been shown to enhance
and/or sensitize the D2-mediated behavioral and
molecular effects of dopaminergic agonists
(Deroche et al., 1993; Candoni et al., 2003;
Carr et al., 2001; 2003), however, the effects
of food restriction on the function and/or
sensitivity of D3 receptors is not well
understood. The current studies were aimed at
characterizing the effects of food restriction
on the induction of putative D3- (yawning and
PE), and D2-mediated (hypothermia and locomotor
activity) effects by the D3-preferring agonist,
pramipexole (90-fold selective for D3 over D2
receptors in vitro; Millan et al., 2002). Food
restriction differentially affected the
D3-mediated effects of pramipexole, suppressing
pramipexole-induced yawning while not altering
pramipexole-induced PE. Food restriction had
similar effects on the D2mediated effects of
pramipexole, enhancing and/or sensitizing rats
to the hypothermic and locomotor stimulatory
effects of pramipexole. While food restriction
altered both D2- and D3mediated behavioral
effects of pramipexole, convergent evidence from
the effects of pramipexole alone, and in
combination with D2- and D3-selective
antagonists suggests that food restriction
sensitized rats to the D2-mediated effects of
pramipexole while not altering the function
and/or sensitivity of D3 receptors.
Similar to previous reports in free-fed rats
(Collins et al., 2005; 2007; submitted),
pramipexole induced yawning and PE over low
doses with inhibition of both behaviors
occurring at higher doses that also corresponded
to the induction of hypothermia, suggestive of a
selective activation of D3 receptors at low
doses, and a concomitant D2 receptor activation
at higher doses. Food restriction affected
pramipexole-induced yawning, locomotor activity,
and hypothermia, but did not alter
pramipexole-induced PE. While the enhanced
and/or sensitized locomotor stimulatory and
hypothermic effects of pramipexole suggest that
food restriction enhanced the function and/or
sensitivity of D2 receptors in the mesolimbic
pathway (Ouagazzal and Creese, 2000) and
anterior hypothalamus/preoptic area (Lin et al.,
1982), respectively, the effects of food
restriction on pramipexole-induced yawning and
PE are less clear. Previous studies (Melis et
al., 1987; Collins et al., submitted) suggest
that D2-like agonist-induced yawning and PE are
similarly mediated by D3 (induction) and D2
(inhibition) receptors within the
paraventricular nucleus of the hypothalamus, yet
food restriction differentially affected
pramipexole-induced yawing and PE, suppressing
yawning while not affecting the induction of PE.
While it is possible that these effects
represent a decreased function and/or
sensitivity of only some D3 receptors, the
effects of food restriction on pramipexole's
D2-mediated effects, as well as a comparison of
the effects of D3- and D2-selective antagonists
on pramipexole-induced yawning and PE suggest
that the food restriction-induced suppression of
yawning resulted from changes in the function
and/or sensitivity of D2, but not D3
receptors.
Unlike the hypothermic effects of D2-like
agonists which have been shown to be mediated by
D2, but not D3 receptors (Boulay et al., 1999;
Chaperon et al., 2003; Collins et al., 2007),
the induction of yawning by D2-like agonists has
been shown to be mediated by the D3 receptor,
with the subsequent inhibition of yawning
resulting from a concomitant D2 receptor
activation (Collins et al., 2005; 2007;
submitted). Therefore, although decreases in D3
receptor function could explain the suppressed
yawning response in food-restricted rats,
increases in D2 receptor function and/or
sensitivity would also be expected to suppress
pramipexole-induced yawning. Support for the
notion that food restriction-induced changes in
D2, but not D3 receptor function and/or
sensitivity was provided by the effects of the
D3-selective, PGO1 037 (-133-fold selective for
D3 over D2 receptors in vitro; Grundt et al.,
2005; 2007), and D2-selective, L-741,626 (13fold
selective for D2 over D3 receptors in vitro;
Millan et al., 2000) antagonists on
pramipexoleinduced yawning.
Similar to previous reports (Collins et al.,
2005; 2007; submitted), pretreatment with the
D3selective antagonist, PGO1 037, inhibited
pramipexole-induced yawning and PE in both the
freefed and food-restricted conditions,
regardless of whether the responses were
affected by food restriction. These data not
only support a role for the D3 receptor in the
induction of PE by pramipexole, but also suggest
that food restriction does not alter, at least
some of, the D3mediated behavioral effects of
pramipexole. Likewise, the D2-selective
antagonist, L-741,626, had similar effects in
both free-fed and food-restricted rats,
reversing the inhibition of yawning and PE
observed at higher doses while not altering
their induction at lower doses of pramipexole.
However, while L-741,626 increased the low
levels of yawning observed at higher doses in
both free-fed and food-restricted rats, this
effect was observed at a lower dose of
pramipexole in the food-restricted (0.1 mg/kg)
compared to free-fed condition (0.32 mg/kg),
suggestive of a leftward shift in the
D2-mediated effects of pramipexole when food was
restricted. Moreover, comparison of the effects
of L-741,626 on pramipexole-induced yawning in
food-restricted and free-fed rats suggests that
the D2-selective antagonist was not only
effective at reversing the D2-mediated
inhibition of yawning in both conditions, but
also that it was capable of unmasking
pramipexole's D3-mediated effects, effectively
restoring the food restricted yawning
dose-response curve to that of free fed levels.
When taken together with the enhanced
hypothermic and locomotor stimulatory effects of
pramipexole, these data strongly suggest that
food restriction enhanced the function and/or
sensitivity of D2 receptors in mesolimbic
(locomotor activity) and hypothalamic
(hypothermia and yawning) brain regions, while
not altering the function and/or sensitivity of
D3 receptors.
Interestingly, dopaminergic, cholinergic,
and serotonergic systems within the
corticostriatal and hypothalamic regions have
been implicated in a variety of aspects of
feeding behavior including, motor control,
motivation to obtain food, food intake, and
satiation (e.g., Leibowitz and Alexander, 1998;
Kelley et al., 2005). Thus, food
restriction-induced increases in the function
and/or sensitivity of mesolimbic and/or
hypothalamic D2 receptors may be beneficial for
several reasons. First, increased D2 receptor
activity within the nucleus accumbens may serve
to increase the motivational aspects of food or
the orientation towards food-related stimuli
(e.g., Robinson and Berridge, 1993; Kelley et
al., 2005), while changes in D2 receptor
activity affecting the integration of accumbal
and hypothalamic dopamine systems may also alter
motor control, food intake, and feeding duration
(Kelley et al., 2005; Meguid et al., 2000).
Moreover, dopaminergic neurons within the
hypothalamus are known to interact with other
neurotransmitters and neurohormones (i.e.,
serotonin and orexin) and thus changes in the
function and/or sensitivity of hypothalamic D2
receptors may indirectly influence a variety of
behaviors including arousal, food preference
(i.e., carbohydrate vs. protein / palatable vs.
nonpalatable), and satiety (Leibowitz et al.,
1990; Meguid et al., 2000; Isaac and Berridge,
2003; Alberto et al., 2006; Palmiter et al.,
2007).
Unlike the effects of food restriction on
pramipexole-induced behaviors which generally
returned to baseline levels following 7 days of
re-feeding, decrements in physostigmine- and
TFMPPinduced yawning were still evident
following 7 days of unrestricted access to food
suggesting a prolonged effect of food
restriction on cholinergic and serotonergic
function. Interestingly, as both cholinergic and
serotonergic systems have been strongly
implicated in satiety mechanisms (e.g.,
Leibowitz et al., 1990; Meguid et al., 2000;
Kelley et al., 2005), it is possible that a
persistent decrease in cholinergic and
serotonergic function may allow for increased
levels of food intake once food is available.
Thus, while these studies were not primarily
aimed at the effects of food restriction on
cholinergic and serotonergic function they do
suggest that food restriction induced a
prolonged decrease in cholinergic and
serotonergic receptor function and/or
sensitivity.
To summarize, evidence was provided
in support of the notion that food restriction
sensitized rats to the D2-mediated effects of
pramipexole while not altering their sensitivity
to the D3mediated effects of pramipexole. Food
restriction suppressed pramipexole-induced
yawning while resulting in a sensitization
and/or enhancement of the hypothermic and
locomotor stimulatory effects of pramipexole;
all of which suggest an increased function
and/or sensitivity of the D2 receptor. This
notion is further supported by the finding that
the effects of food restriction on
pramipexole-induced yawning were reversed by the
D2 antagonist, L-741,626, and when combined with
the finding that food restriction did not alter
pramipexole-induced PE, these data strongly
suggest that food restriction altered the D2-,
but not D3-mediated effects of pramipexole.
Importantly, while food restriction suppressed
dopaminergic-, cholinergic-, and
serotonergic-mediated behaviors, differences in
the duration of these effects were observed and
may be reflective of differential roles for
dopamine, acetylcholine, and serotonin in
feeding behaviors. For instance, while food
restriction-induced changes in dopaminergic
function may serve to increase the motivation to
obtain food when food is unavailable,
sensitization of D2 receptors would serve little
purpose once food is readily available.
Conversely, prolonged decreases in cholinergic
and serotonergic sensitivity may allow for a
sustained increase in meal frequency and size
following extended periods of food deprivation.
Moreover, while food restriction altered a
variety of D2-mediated behaviors, food
restriction failed to alter the proerectile
effects of pramipexole suggesting that food
restriction-induced changes in D2 receptors may
serve a more general purpose to increase arousal
and/or enhance dopamine-mediated reward (or
prediction of reward), while allowing for other
behaviors (reproduction) to be maintained. In
conclusion, these studies suggest that food
restriction enhanced the function and/or
sensitivity of D2 receptors while having no
effect on the function and/or sensitivity of D3
receptors.
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Based on studies in the
rat, Sokoloff et al. have made the valuable
suggestion that the D3 receptor is a
particularly important target for antipsychotics
in the mesolimbic DA system. These study in the
human demonstrates that the distribution of D3
receptors and D3 mRNA-bearing neurons is
consistent with relative segregation of the D3
subtype to the limbic striatum as well as it
primary and secondary targets and many sources
of its afferents.
