Streptozotocin (STZ)-induced diabetes can
modulate dopamine (DA) neurotransmission and,
thereby, modify the behavioral effects of drugs
acting on DA systems. Insulin replacement, and
in some conditions repeated treatment with
amphetamine, can partially restore sensitivity
of STZ-treated rats to dopaminergic drugs. The
present study sought to characterize the role of
insulin and amphetamine in modulating the
behavioral effects of drugs that selectively act
on D2/D3 receptors. In control rats, quinpirole
and quinelorane produced yawning while
raclopride and GHB produced catalepsy.
Raclopride antagonized quinpirole- and
quinelorane-induced yawning with similar
potency. STZ treatment increased blood glucose
concentration, decreased body weight, and
markedly reduced sensitivity to
quinpirole-induced yawning,
quinelorane-induced yawning, as well as
to raclopride-induced catalepsy, while enhancing
sensitivity to GHB-induced catalepsy. Repeated
treatment with amphetamine partially restored
sensitivity of STZ-treated rats to
amphetamine-stimulated locomotion and also
produced CPP, without affecting blood glucose
and body weight changes. However, amphetamine
treatment did not restore sensitivity to the
behavioral effects of quinpirole, raclopride or
GHB, suggesting differential regulation of DAT
activity and sensitivity of D2 receptors in
hypoinsulinemic rats. Insulin replacement in
STZ-treated rats normalized blood glucose and
body weight changes and fully restored
sensitivity to quinpirole-induced yawning
as well as to raclopride-induced catalepsy,
while reducing sensitivity to GHB-induced
catalepsy. Overall, these data indicate that
changes in insulin status markedly affect
sensitivity to the behavioral effects of
dopaminergic drugs. The results underscore the
importance of insulin in modulating DA
neurotransmission; these effects might be
especially relevant to understanding the
co-morbidity of eating disorders and substance
abuse.
INTRODUCTION
Several drugs of abuse (e.g., amphetamine,
cocaine) and some drugs that are used in the
clinic (e.g., haloperidol, bromocriptine) are
believed to act predominantly on dopamine (DA)
systems. Activity at DA D2 receptors can
modulate DA neurotransmission by affecting DA
synthesis, release, uptake, or neuronal activity
(Zahniser and Doolen, 2001). Importantly,
insulin has been shown to regulate DA signaling
in the brain (e.g., Figlewicz et al., 1994;
1996). Insulin can cross the blood-brain barrier
and act on receptors (i.e., insulin receptors,
insulin-like growth factor-1 receptors) that are
densely concentrated in the basal ganglia, a
region richly expressing D2/D3 receptors and DA
transporters (DAT) (Larson and Ariano 1995;
Ciliax et al., 1995; Schulingkamp et al., 2000;
Figlewicz et al., 2003). The close proximity of
insulin and DA systems appears to have
functional consequences. For example, rats with
decreased circulating insulin showed decreased
coupling of DA D2 receptors to Gi/o proteins
(Abbracchio et al., 1989) and reduced DAT
activity (Owens et al., 2005) in the striatum.
Food deprived (i.e., hypoinsulinemic) rats also
showed reduced DAT mRNA in the ventral tegmental
area/substantia nigra and decreased DAT activity
in the striatum (Patterson et al., 1998).
Moreover, drug (alloxan or streptozotocin
[STZ])-induced hypoinsulinemia can alter
(increase [Lozovsky et al., 1981; Trulson
and Himmel, 1983; Serri et al., 1985] or
decrease [Rowland et al., 1985]) the
density of striatal D2 receptors and impair D2
receptor-coupled signal-transduction (Abbracchio
et al., 1989). Finally, hypoinsulinemic rats
showed decreased synthesis (Kono and Takada,
1994), uptake (Owens et al., 2005), and turnover
(Kwok and Juorio, 1986; Lim et al., 1994) of DA
in the striatum. Thus, it is clear that changes
in plasma insulin and glucose can have profound
effects on DA neurotransmission.
Changes in insulin status also can modify
the behavioral effects of dopaminergic drugs.
For example, STZ-treated rats were less
sensitive to the effects of apomorphine (a
direct-acting DA agonist) and amphetamine (an
indirect-acting DA agonist) on locomotor
activity and also to the positive reinforcing
effects of amphetamine (Marshall, 1978; Rowland
et al., 1985; Galici et al., 2003). Moreover,
the cataleptic effects of the DA receptor
antagonist haloperidol were markedly reduced in
STZ-treated rats (Sevak et al., 2005). Food
restriction, a condition that can alter insulin
and glucose status, can enhance oral as well as
i.v. drug intake (Carroll et al., 1981; Carroll
and Stotz, 1983) and potentiate
amphetamine-induced hyperactivity (Campbell and
Fibiger, 1971). Thus, changes in plasma insulin
that modulate DA neurotransmission can also
modify the behavioral effects of drugs acting on
DA systems.
Despite a growing literature on the role of
insulin signaling in regulating DA
neurotransmission, little is known regarding the
effects of altered insulin status on the
behavioral effects of direct-acting DA D2/D3
receptor agonists and antagonists. It is well
established that direct-acting DA receptor
agonists can produce yawning (Kurashima
et al., 1995; Collins et al., 2005) and
direct-acting DA receptor antagonists can
produce catalepsy (Kanes et al., 1993; Sevak et
al., 2004). These two behavioral endpoints were
used to examine changes in sensitivity to the
behavioral actions of direct-acting DA drugs and
also in drug combination studies with raclopride
to confirm the role of DA receptors in the
yawning produced by quinpirole and
quinelorane. The pharmacological selectivity of
changes in sensitivity to the behavioral effects
of direct-acting DA drugs was examined by
comparing those effects to the effects obtained
with an indirect-acting DA agonist (amphetamine
on locomotion and conditioned place preference)
and to the effects obtained with a drug
(£^-hydroxybutyric acid [GHB]) that
exerts cataleptic effects through a non-DA
(GABAB receptors) mechanism (Carter et al.,
2005). It was hypothesized that decreased
circulating insulin (after STZ) would decrease
sensitivity to the behavioral effects of drugs
acting directly on DA receptors and that insulin
replacement would restore sensitivity to those
drugs. DAT activity can affect DA D2 receptor
function (Jones et al., 1999). For example,
activation of D2/D3 receptors by quinpirole can
reduce DA synthesis, release and neuronal
firing, and mice lacking DAT are less sensitive
to the effects of quinpirole (Jones et al.,
1999). Because repeated treatment with
amphetamine can normalize DAT activity in
STZ-treated rats (Owens et al., 2005), the
present study also examined whether repeated
treatment with amphetamine restores sensitivity
of STZ-treated rats to the behavioral effects of
drugs acting directly at DA receptors.
DISCUSSION
Data from this study show that changes in
circulating insulin and glucose can
markedlyimpact the behavioral effects of drugs
acting on DA receptors. The data also suggest
that activity of DAT and D2/D3 receptors are
differentially affected by
hypoinsulinemia(hyperglycemia) in rats, because
amphetamine treatment that restored DAT activity
in STZtreated rats (Owens et al., 2005) did not
restore sensitivity to the behavioral effects of
drugs acting on D2/D3 receptors. Quinpirole and
quinelorane are agonists at D2-like (i.e., D2,
D3, and D4) receptors, and each agonist shows
similar affinity for D2 and D3 receptors
(Kebabian et al., 1997).
In agreement with others (Kurashima et al.,
1995; Collins et al., 2005), the present results
show that quinpirole and quinelorane produced
yawning, with dose-response curves being
inverted Ushaped. Collins et al. (2005)
suggested that the biphasic nature of
dose-response curves for quinpirole and
quinelorane involve two distinct DA receptor
mechanisms: D3 receptors mediating the emergence
of yawning (i.e., the ascending portion
of the dose-response curve) and D2 receptors
mediating the disappearance of yawning
(i.e., the descending portion of the
doseresponse curve). Results of the current
study showed that raclopride shifted both the
ascending and the descending limbs of the
quinpirole and quinelorane dose-response curves
to the right in a parallel manner. Moreover, the
Schild regressions for the ascending and
descending portions of the dose-response curves
for quinpirole and quinelorane could be fitted
adequately with a single line with a common
slope and a common pA2 value, indicating that
raclopride antagonized the effects of quinpirole
and quinelorane with similar potency, consistent
with the involvement of the same D2/D3 receptors
in their effects.
Increased blood glucose concentration and
decreased body weight in STZ-treated rats
confirm that STZ eliminates insulin-secreting
pancreatic beta-islet cells (Galici et al.,
2003), resulting in hypoinsulinemia (Carr 1996).
An important finding of this study is that STZ
decreased quinpirole-and quinelorane-induced
yawning as well as raclopride-induced
catalepsy. Reduced sensitivity of STZ-treated
rats to the behavioral effects of drugs acting
on D2/D3 receptors parallels changes that can
occur in DA receptors (e.g., receptor density
and signaling)in hypoinsulinemic rats (Lozovsky
et al., 1981, Rowland et al., 1985; Abbracchio
et al., 1989).
To the extent that activity at different DA
receptors accounts for the ascending (D3) and
descending (D2) portions of the dose-response
curve for yawning (Collins et al., 2005),
the near absence of yawning observed in
STZ-treated rats could indicate a decreased
sensitivity of D3 receptors to quinpirole and
quinelorane, an increased sensitivity of D2
receptors to quinpirole and quinelorane, or to
changes in sensitivity at both receptor types.
What ever the underlyingmechanism(s), insulin
replacement restored quinpirole-induced
yawning, raclopride-induced catalepsy,
blood glucose concentration, and body weight in
STZ-treated rats. Thus, attenuation of the
behavioral effects of drugs acting on D2/D3
receptors in STZ treated rats appears to be a
confirmation of marked changes in dopaminergic
systems that can occur under conditions where
insulin and glucose concentrations are
perturbed. The pharmacologic selectivity of
decreased sensitivity to the behavioral effects
of drugs acting on D2/D3 receptors was evident
by the finding that STZ-treated rats were more
sensitive to the cataleptic effect of GHB, a
drug that does not act at DA receptors. The
mechanism by which GHB induces catalepsy is not
known, although emerging evidence suggests that
this effect involves agonist activity at GABAB
receptors (e.g., Carter et al., 2005). Evidence
that different mechanisms contribute to the
cataleptic effects of raclopride and GHB was
provided by an earlier study in which the same
dose of the N-methyl-D-aspartate (glutamate)
receptor antagonist dizocilpine attenuated
catalepsy produced by the D2 receptor antagonist
haloperidol, while enhancing catalepsy produced
by GHB (Sevak et al., 2004). Thus, changes in
insulin and glucose status do not affect all
drugs in a similar manner, although it remains
to be determined whether the effects of drugs
acting on other receptors and neurochemical
systems (e.g., other monoamines) also change as
a function of insulin and glucose status.
Repeated treatment with amphetamine can
normalize DAT activity in STZ-treated rats
(Owens et al., 2005) and under some conditions
DAT activity co-varies with changes in D2
receptor function (Jones et al., 1999; Fauchey
et al., 2000). While amphetamine increased
locomotion and produced CPP in control and in
STZ-treated rats, the same treatment that
restored DAT activity (Owens et al., 2005)
failed to restore sensitivity to
quinpirole-induced yawning or
raclopride-induced catalepsy in STZ-treated
rats. That amphetamine treatment did not restore
sensitivity of STZ-treated rats to drugs acting
directly at DA receptors indicates that DAT
activity and sensitivity of DA receptors are
differentially affected by altered insulin
status and by amphetamine treatment.
In summary, this study shows that
changes in insulin and glucose status affect
sensitivity of rats to the behavioral effects of
drugs acting directly at D2/D3 receptors. It
remains to be seen whether these changes reflect
altered sensitivity at D2, D3 or both D2 and D3
receptors. STZ treatment can also affect
norepinephrine neurotransmission (e.g.,
Figlewicz et al., 1996), and amphetamine can
modulate norepinephrine uptake (e.g., Kuczenski
and Segal, 2001); thus, mechanisms in addition
to DA might underlie these differences in
sensitivity to amphetamine and other drugs
observed in STZ-treated rats. Ongoing studies
are evaluating whether less dramatic changes in
glucose and insulin status (e.g., produced by
modest food restriction) also 24 modify
sensitivity to indirect-acting and direct-acting
DA receptoragonists. Several reports indicate
that eating disorders, where plasma insulin
levels markedly fluctuate, show high comorbidity
with substance abuse (Holderness et al., 1994;
Krahn 1991). Because dopaminergic mechanisms are
presumed to account for the positive reinforcing
effects of many drugs of abuse and insulin can
regulate DA neurotransmission, understanding the
functional relationships among insulin status,
glucose status, and the behavioral effects of
dopaminergic drugs could facilitate the
development of treatments for substance abuse
and eatingdisorders.
-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
