Sevak RJ, Koek W, Owens WA, Galli A, Daws
LC, France CP
Depart. Pharmacology,
University Texas Health Science Center, San
Antonio USA
The high co-morbidity of eating disorders
and substance abuse suggests that nutritional
status can impact vulnerability to drug abuse.
These studies used rats to examine the effects
of food restriction on dopamine clearance in
striatum and on the behavioral effects of
amphetamine (locomotion, conditioned place
preference), the dopamine receptor agonist
quinpirole (yawning), and the dopamine receptor
antagonist raclopride (catalepsy).
Amphetamine increased locomotion and
produced conditioned place preference. Food
restriction reduced dopamine clearance, which
was restored by repeated treatment with
amphetamine or by free feeding. Food restriction
also decreased sensitivity to quinpirole-induced
yawning and raclopride-induced catalepsy; normal
sensitivity to both drugs was restored by free
feeding. The same amphetamine treatment that
normalized dopamine clearance, failed to restore
normal sensitivity to quinpirole or raclopride,
suggesting that in food-restricted rats the
activity of dopamine transporters and dopamine
receptors is differentially affected by pathways
that are stimulated by amphetamine.
These studies show that modest changes in
nutritional status markedly alter dopamine
neurotransmission and the behavioral effects of
direct-acting dopamine receptor drugs (agonist
and antagonist). These results underscore the
potential importance of nutritional status
(e.g., glucose and insulin) in modulating
dopamine neurotransmission and in so doing they
begin to establish a neurochemical link between
the high co-morbidity of eating disorders and
drug abuse.
1. Introduction
Nutritional status can affect dopamine
systems, the significance of which might be
found in the high co-morbidity of substance
abuse and eating disorders where concentrations
of glucose, insulin, and other molecules can
fluctuate markedly (e.g., Wolfe and Maisto,
2000). The dopamine transporter regulates
dopamine neurotransmission by high-affinity
reuptake of dopamine and it is a site of action
for some drugs, including amphetamine. Dopamine
uptake is reduced in striatal synaptosomes from
food-deprived rats, and treatment with insulin
restores uptake (Patterson et al., 1998).
Converging evidence suggests that insulin
enhances dopamine transporter activity (Carvelli
et al., 2002; Knusel et al., 1990), while
hypoinsulinemia (e.g., streptozotocin) reduces
dopamine transporter activity (Owens et al.,
2005; Sevak et al., 2007b).
Because food restriction also reduces
insulin (Carr, 1996), we hypothesized that food
restriction reduces in vivo dopamine transporter
activity in the striatum. Because the dopamine
transporter is a target for amphetamine,
understanding how amphetamine affects the
transporter is critical for understanding the
neurochemistry of stimulant abuse. Although
others have examined modulation of dopamine
transporter activity by amphetamine and cocaine
(Zahniser and Doolen, 2001; Kahlig and Galli,
2003), little is known about modulation of
dopamine transporter activity by amphetamine in
animals with altered nutritional status. Owens
et al. (2005) showed that reduced dopamine
transporter activity in streptozotocin-treated
rats was restored by amphetamine. Because food
restriction causes hypoinsulinemia (Carr, 1996)
and can reduce dopamine uptake (Patterson et
al., 1998), we hypothesized that amphetamine
restores the reduced dopamine transporter
activity during food restriction. In addition to
modulating dopamine transporter activity, food
restriction can modify behavioral effects of
drugs acting on this transporter; food
restriction enhances positive reinforcing
effects of amphetamine and cocaine, as indicated
by increased sensitivity in conditioned place
preference and self-administration studies
(Carroll et al., 1981; Stuber et al., 2002; Bell
et al., 1997).
However, little is known about the
relationship between dopamine transporter
activity and behavioral effects of drugs acting
on the transporter during altered nutritional
status. Dopamine transporters are highly
concentrated in striatum (Ciliax et al., 1995;
Fuxe et al., 1985; Cass and Gerhardt, 1995) and
converging lines of evidence implicate striatal
dopamine systems in the effects of abused drugs
(Belin and Everitt, 2008; Everitt and Robbins,
2005; McCann et al., 2008). Experiment 1
examined the effects of food restriction on
amphetamine-induced locomotion and conditioned
place preference; the same rats were used to
evaluate whether dopamine transporter activity
was altered by food restriction or by
amphetamine.
Despite a growing literature on the
relationship between nutritional status and
behavioral effects of indirect-acting dopamine
agonists (e.g., amphetamine), little is known
about food restriction and behavioral effects of
direct-acting dopamine receptor drugs.
Experiment 2 used catalepsy by the dopamine
receptor antagonist raclopride and yawning by
the direct-acting dopamine receptor agonist
quinpirole, as indices of dopamine receptor
sensitivity in food-restricted rats.
Streptozotocin and food restriction can reduce
circulating insulin (Carr, 1996) and
streptozotocin decreases sensitivity to
catalepsy produced by haloperidol (Sevak et al.,
2005); thus, we hypothesized that food
restriction reduces sensitivity to the
behavioral effects of dopamine receptor agonists
and antagonists. Since dopamine transporter
activity co-varies with dopamine (D2) receptor
function (Jones et al., 1999; Dickinson et al.,
1999), and repeated treatment with amphetamine
can normalize the reduced transporter activity
that occurs after streptozotocin treatment
(Owens et al., 2005; Sevak et al., 2007b), the
last experiment (3) examined whether the same
amphetamine treatment (i.e., dose, route, and
frequency of administration) that restores
transporter activity in food-restricted rats
also restores sensitivity to the behavioral
effects of direct-acting dopamine drugs.
4. Discussion
These studies show that modest food
restriction markedly reduces DA clearance and
sensitivity to drugs acting at dopamine
receptors. Amphetamine normalizes dopamine
clearance in food-restricted rats without
normalizing sensitivity to the behavioral
effects of direct-acting dopamine receptor
drugs, suggesting that dopamine transporter
activity and dopamine receptor sensitivity are
regulated differentially by pathways stimulated
by amphetamine. Given the important role of
dopamine in motivated behavior and of
insulin-related pathways in the effects of drugs
of abuse (e.g., Russo et al., 2007), these data
suggest that nutritional status could be a
critical determinant of vulnerability to drug
abuse and that the same neurochemical substrates
might contribute to the high comorbidity of
eating disorders and substance abuse.
Food restriction decreased dopamine uptake,
consistent with previous studies showing reduced
dopamine transporter activity during food
restriction (Zhen et al., 2006) or after
experimentally-induced hypoinsulinemia
(Patterson et al., 1998; Owens et al., 2005).
This reduced transporter activity is not due to
a loss of transporter, because binding of a
high-affinity dopamine transporter ligand
([125I] RTI-121) does not change in the
striatum or the nucleus accumbens of
food-deprived rats (Patterson et al., 1998).
RTI-121 can bind to cell surface or
intracellular dopamine transporters and the
activity of the transporter can be regulated by
trafficking between the plasma membrane and
intracellular regions (Zahniser and Doolen,
2001). Thus, food restriction might decrease
dopamine transporter number at the cell surface,
thereby reducing transport capacity.
Although blood glucose was unchanged in this
study, insulin levels are known to vary across
feeding conditions. For example, Carr (1996)
reported that 14 days of 10 g/day of food (same
as used in this study) significantly reduced
plasma insulin. That decreased insulin might
contribute to reduced dopamine transporter
activity is supported by Patterson et al. (1998)
who showed that dopamine uptake was reduced in
striatal synaptosomes from fasted rats, and that
addition of a physiological concentration of
insulin normalized dopamine uptake. More
recently, in vivo dopamine transporter activity
was reduced in the striatum of rats made
hypoinsulinemic by streptozotocin (Owens et al.,
2005). Together with the observation that
insulin can enhance dopamine transporter
activity (Carvelli et al., 2002; Knusel et al.,
1990), these data suggest that reduced insulin
could underlie reduced dopamine clearance in
food-restricted rats. However, food restriction
can increase plasma corticosterone and decrease
plasma leptin (Carr, 1996; Havel et al., 1998);
although corticosterone directly affects
striatal dopamine transporter activity,
long-term elevations in corticosterone are
correlated with increased dopamine uptake
(Copeland et al., 2005).
Like insulin, leptin can induce
translocation of Glut4 to the plasma membrane
and activate glucose uptake in a PI 3-kinase
dependent manner (Benomar et al., 2006). PI
3-kinase pathways, which can be activated by
insulin, increase dopamine uptake and recruit
dopamine transporters to the plasma membrane
(Carvelli et al., 2002). However, it is not
clear whether leptin has a similar effect on
dopamine transporters through a PI-3 kinase
mechanism. Decreased food intake can result in
decreased water intake (Oatley and Tonge, 1969),
potentially resulting in dehydration. Moreover,
decreased circulating insulin can promote
dehydration in brain which can be reversed by
insulin replacement (Haraldseth et al., 1997).
Thus, it is possible that dehydration altered
diffusion of pressure-ejected dopamine. Free
access to food or administration of amphetamine
(that restored dopamine clearance in
streptozotocin-treated rats [Owens et al.,
2005; Sevak et al., 2007b]) fully restored
dopamine clearance in food-restricted rats.
Although insulin is decreased by streptozotocin
as well as by food restriction (Carr, 1996;
Patterson et al., 1998), restoration of normal
dopamine transporter activity by amphetamine
suggests that noninsulin dependent mechanisms
can dramatically alter (in this case restore)
dopamine transporter activity.
Amphetamine and insulin can modulate
dopamine D2 receptor density and activity in the
brain (Amano et al., 2003; Seeman et al., 2002).
Activation of dopamine D2 receptors can enhance
dopamine transporter activity (Cass and
Gerhardt, 1994) and the dopamine D2 receptor
antagonist raclopride can prevent
amphetamineinduced restoration of dopamine
clearance in streptozotocin-treated rats (Sevak
et al., 2007b); thus, dopamine D2 receptors
appear to indirectly mediate, at least in part,
amphetamine-induced restoration of dopamine
transporter activity in food-restricted rats. A
primary site of action for amphetamine is the
dopamine transporter (e.g., Giros et al., 1996)
yet despite reduced transporter activity, food
restriction did not affect either spontaneous
locomotion or amphetamine-stimulated locomotion
(see also Stuber et al., 2002). The
locomotor-stimulating effect of the first
amphetamine injection in foodrestricted rats
suggests that mechanisms other than or in
addition to the dopamine transporter contribute
to this behavioral effect of amphetamine.
Other components of dopaminergic systems
could be modified by food restriction or other
monoaminergic systems might play a role.
Amphetamine binds to norepinephrine transporters
(Rothman et al., 2001), resulting in increased
neurotransmitter release (Han and Gu, 2006;
Rothman and Baumann, 2003) that is correlated
with locomotion and reward-related effects
(Drouin et al., 2002; Hilber et al., 2005;
Kuczenski and Segal, 2001). Dopamine transporter
mRNA is decreased by streptozotocin whereas
norepinephrine transporter mRNA is increased by
streptozotocin (Figlewicz et al., 1996). Thus,
norepinephrine transporters could provide a
compensatory mechanism for regulating dopamine
and other neurotransmitters under conditions
where dopamine transporter activity is reduced.
Food restriction can enhance the effects of some
drugs: decreased threshold for electrical brain
stimulation after amphetamine is enhanced by
food restriction (Carr, 1996); limited access to
food can increase self-administration of
amphetamine and other drugs (Carroll et al.,
1981); and cocaine-induced conditioned place
preference is enhanced with food
restriction.
Because the dose-response curve for
amphetamineinduced conditioned place preference
is inverted-U shaped (Stuber et al., 2002), an
apparent decrease in the effectiveness of
amphetamine in food-restricted rats could
reflect an increase (i.e., shift leftward) or a
decrease (shift rightward) in sensitivity. A
single dose of amphetamine was used in this
study specifically to test whether the treatment
regimen that is known to restore dopamine
transporter activity in streptozotocin-treated
rats also restores transporter activity and
sensitivity to direct-acting dopamine receptor
agonists and antagonist in food-restricted rats.
One striking result from these studies is that
food restriction significantly decreased
quinpirole-induced yawning and
raclopride-induced catalepsy, consistent with a
recent study with other dopamine receptor
agonists (Collins et al., 2008).
Similar to results obtained with food
restriction, streptozotocin-treated rats were
less sensitive to catalepsy produced by the
dopamine receptor antagonist haloperidol (Sevak
et al., 2005). Decreased sensitivity to
agonist-induced yawning and antagonist-induced
catalepsy in streptozotocin-treated rats were
reversed by insulin (Sevak et al., 2007a),
providing support for the view that reduced
circulating insulin might contribute to a
decreased sensitivity to the behavioral effects
of direct-acting dopamine receptor drugs.
Dopamine D2 receptor density in the caudate
putamen was not increased by food restriction
(Pothos et al., 1995), although the
concentration-response curve for
quinpirolestimulated [35S]GTP?S binding
was slightly, but significantly, increased (Carr
et al. ,2003), and the locomotor-stimulating
effects of quinpirole were enhanced (Carr et
al., 2001).
These data suggest that increased
sensitivity to dopamine receptor drugs could
result from increased dopamine receptor
signaling. Results from the present study fail
to confirm earlier reports showing increased
sensitivity of food-restricted rats to the
locomotor-stimulating effects of indirect-acting
dopamine agonists (amphetamine); however, the
current study clearly shows that the behavioral
effects of direct-acting dopamine agonists and
antagonists are markedly attenuated by food
restriction. The biphasic nature of the
dose-response curve for quinpirole-induced
locomotion (Frantz and Van Hartesveldt, 1999)
and quinpirole-induced yawning might contribute
to apparent inconsistencies in the literature
regarding the effect of food restriction on the
behavioral actions or direct-acting dopamine
receptor drugs. The responses to direct-acting
dopamine receptor agonists and antagonists were
both attenuated by food restriction and restored
by free feeding, clearly demonstrating an effect
of nutritional status on dopaminergic
systems.
There is a complex interplay between
dopamine transporter activity and dopamine
receptor function. Deletion of dopamine
transporters decreased dopamine D2 receptor gene
expression (Fauchey et al., 2000) and function
(Jones et al., 1999) and either blockade or
deletion of dopamine D2 receptors reduced
dopamine transporter activity (Cass and
Gerhardt, 1994; Dickinson et al., 1999).
Amphetamine restored dopamine transporter
activity in food-restricted rats without
restoring sensitivity to drugs acting directly
at dopamine receptors, suggesting that dopamine
transporter activity and dopamine D2 receptor
sensitivity do not always co-vary. In summary,
modest changes in food intake can profoundly
affect dopamine transporter activity and
sensitivity to the behavioral effects of
direct-acting dopaminergic drugs. Several
mechanisms could underlie the effects of food
restriction on dopamine transporter activity and
receptors, including reduced plasma insulin and
leptin, increased plasma corticosterone, as well
as compensatory changes in dopamine or other
neurotransmitter systems.
Elucidating the mechanisms that contribute
to altered transporter activity and behavioral
responsiveness as a consequence of altered
feeding conditions may help identify biological
risk factors contributing to the high
co-morbidity of substance abuse and eating
disorders.
-Sevak RJ,
Koek W, Galli A, France CP Insulin
replacement restores the behavioral effects of
quinpirole and raclopride in
streptozotocin-treated rats. J Pharmacol Exp
Ther. 2007;320(3):1216-1223
