Characterization
of the dopamine receptor system in adult rhesus
monkeys exposed to cocaine throughout
gestation
Lindsey R. Hamilton & Paul W.
Czoty,
H. Donald Gage Michael A. Nader
It has been estimated that over 45,000
infants born each year have been prenatally
exposed to cocaine (National Pregnancy and
Health Survey 1996). Cocaine use during
pregnancy is associated with several physical
deficits including reduced body weight, body
length, and head circumference at birth (Nair
and Watson 1991). However, the effects of
cocaine use during pregnancy on postnatal
development and long-term neurobiological and
behavioral outcomes have been less thoroughly
investigated. The present study compared a
population of rhesus monkeys that were
prenatally exposed to cocaine throughout
gestation to control monkeys with nearly
identical pharmacological and experimental
histories (Morris et al. 1996, 1997). At the
start of the present study, these monkeys were
adults (13 years old), with minimal drug
exposure since birth (see Paule et al. 1996,
2000; Morris et al. 1996). Despite the
escalating cocaine intakes of the mothers and
the lower infant weights at birth (Morris et al.
1997), over the first 18 months, no differences
were observed between cocaine and control groups
with respect to postnatal growth (Morris et al.
1996). A particular advantage of using nonhuman
primates in prenatal cocaine exposure studies is
the relatively long gestational period. In
rhesus macaques, the average gestational period
is approximately 24 weeks (Silk et al. 1993).
Despite this advantage, there are no studies
involving prenatal cocaine exposure in rhesus
monkeys that have examined the consequences of
gestational drug exposure in adults. For the
present studies, the dopamine (DA)
neurotransmitter system was examined using
several in vivo measures. Within the DA system,
there are two superfamilies of DA receptors, the
D1-like receptors with two receptor subtypes D1
and D5 and D2-like receptors with D2, D3, and D4
receptor subtypes. Both D1- and D2-like
receptors have been shown to be affected by
chronic cocaine exposure in adult humans and
nonhuman primates (e.g., Moore et al. 1998a, b;
Martinez et al. 2004; Nader et al. 2002; Volkow
et al. 1999). As it relates to effects on the
fetus, elevation of extracellular monoamine
concentrations during development may lead to
alterations in receptor signaling mechanisms at
birth and perhaps throughout life. Since DA is
among the first neurochemical pathways to
develop in the fetal brain (reviewed in Bhide
2009), the long-lasting effects of cocaine
exposure on the dopaminergic system during this
crucial development stage are of particular
interest.
In the present study, DA D2-like receptor
availability was assessed using positron
emission tomography (PET) and the tracer
[18F]fluoroclebopride (FCP), which does
not differentiate between D2-like receptor
subtypes (Mach et al. 1996). In adult rhesus
monkeys, D2-like receptor availability has been
shown to decrease as a consequence of chronic
cocaine exposure (Nader et al. 2006). We
hypothesized that D2-like receptor availability
would be lower in adult monkeys who had been
exposed to cocaine throughout gestation. While
data suggest that D2-like receptors are reduced
due to cocaine exposure, post-mortem studies
found D3 receptors to be higher in cocaine
overdose victims compared with age-matched
controls (Staley and Mash 1996). Thus, we used
the D3/D2 agonist quinpirole and the
unconditioned behavior yawning to assess
D3 receptor function in vivo. Earlier work in
rodents has shown that the ascending limb of the
quinpirole-elicited yawning
dose&endash;response curve, including the peak
of the curve, is mediated by D3 receptors
(Collins et al. 2005). As it relates to D1
receptors, Jones et al. (2000) demonstrated that
prenatal cocaine exposure induced early
desensitization of DA D1-like receptors in fetal
rabbit anterior cingulate cortex and caudate
nucleus that occurred without alterations of the
receptor protein itself, suggesting that the
D1-like receptors become uncoupled from their
Gprotein (Lidow 1998; Jones et al. 2000).
Importantly, D1- like receptor alterations in
rabbits and rodents prenatally exposed to
cocaine have been shown to persist into
adolescence and adulthood (Bayer et al. 2000;
Stanwood and Levitt 2007). Therefore, in the
present study, D1-like receptor function was
investigated by assessing the ability of the
high-efficacy agonist SKF 81297 to elicit eye
blinking (Jutkiewicz and Bergman 2004). For
these studies, there was a near-equal
distribution of male and female monkeys, so the
effects of prenatal cocaine exposure and sex
were factors in all analyses.
Discussion
The purpose of the present studies was to
determine if there were long-term alterations in
dopamine function in adult monkeys that were
exposed to cocaine in utero. Ten monkeys (male
and female) prenatally exposed to cocaine were
compared to ten age-matched control monkeys who
had nearly identical postnatal experimental
histories. There were no differences between
groups in D1-like receptor function, as assessed
by SKF 81297- elicited eye blinks, or in D2-like
receptor availability as determined with PET
imaging. In contrast, the D3/D2 receptor agonist
quinpirole elicited significantly more yawns in
monkeys prenatally exposed to cocaine compared
with control monkeys. Furthermore, a significant
correlation was observed between maximal daily
gestational dose of cocaine and peak effects of
quinpirole. These findings suggest long-lasting
effects of prenatal cocaine exposure on DA D3
receptor function.
Accumulating evidence suggests that chronic
cocaine exposure can produce significant
reductions in DA D2-like receptor availability
in adult humans and animals (e.g., Volkow et al.
1999; Martinez et al. 2004; Nader et al. 2002,
2006). However, earlier work suggested that the
effects of chronic cocaine on fetal DA receptor
densities may be different from those observed
in adults. For example, Fang et al. (1997)
observed significantly higher levels of D2-like
receptor densities in the fetal monkey striatum
following gestational cocaine exposure. Data
from the present PET imaging study suggest that
any changes in D2-like receptor availability
that may have occurred in utero or in the
developing brain have recovered in adulthood.
Compared with the Fang et al. (1997) rhesus
monkey study, the present study involved longer
in utero treatments (approximately 6 months),
full-term pregnancy, and 13 years of abstinence.
Future longitudinal PET imaging experiments
conducted at multiple points during a monkey's
lifespan following in utero cocaine exposure
would directly address the time course of
recovery.
No significant sex differences were observed
in D2-like receptor availability in any of the
regions of interest. This is consistent with the
lack of sex differences seen in striatal D2/D3
receptor binding using [18F]-fallypride
in adolescent rhesus monkeys (Christian et al.
2009) and with previous reports of women and men
showing equivalent D2-like receptor availability
(Farde et al. 1995; Pohjalainen et al. 1998;
Munro et al. 2006). However, it has been
suggested that female sex hormones may enhance
presynaptic dopamine turnover (Laakso et al.
2002), and the radiotracer used in this
experiment (FCP) is sensitive to fluctuations in
menstrual cycle phase (Czoty et al. 2009). In
addition, sex differences have been reported in
a study using [11C] raclopride and PET
in healthy men and women of ages ranging from
19&endash;82 years old (Pohjalainen et al.
1998). Therefore, it remains possible that
differences in D2-like receptor availability in
males and females may have been observed at
earlier time points or may yet be seen as these
monkeys age.
The PET radiotracer used in the present
study does not differentiate between D2, D3, and
D4 subtypes of the D2- like receptor
superfamily. Thus, it is conceivable that
prenatal cocaine exposure could have long-term
effects on subtypes of this superfamily which
would be obscured by opposite adaptations in
another subtype. For example, in vitro receptor
autoradiography studies have shown lower D2-like
receptor densities (e.g., Moore et al. 1998b;
Nader et al. 2002) and higher D3 receptor
densities (e.g., Staley and Mash 1996) in
cocaine-exposed individuals compared with
age-matched controls. To determine if there were
differences in D3 receptor function, the D3/D2
receptor agonist quinpirole was used to examine
the sensitivity of behavior related to this
subtype in both groups of monkeys and as a
function of sex. Collins et al. (2005, 2007)
have shown that the ascending limb of the
quinpirole dose&endash; response curve is
mediated by D3 receptors while the descending
limb is mediated by D2 receptors. Based on
previous experiments in rhesus monkeys (Martelle
et al. 2007), the dose range of quinpirole
administered in the present study is situated on
the ascending limb of the dose&endash; response
curve and therefore is thought to assess
primarily D3 receptor function. The greater
ability of quinpirole to elicit yawning
in the prenatally cocaine-exposed monkeys is
similar to results from Moody et al. (1992), who
demonstrated that rat pups exposed to cocaine
throughout gestation exhibited a
supersensitivity to the stimulating effects of
quinpirole with respect to behaviors such as
forward locomotion, rearing, and directed oral
movements compared with control pups.
Additionally, when all monkeys prenatally
exposed to cocaine were used in the analysis, we
found that D3 receptor sensitivity correlated
with the maximum daily dose of cocaine each
individual monkey received in utero. Taken
together, the present results provide evidence
for long-term neuropharmacological consequences
of prenatal cocaine exposure on D3 receptor
function under conditions in which no difference
in D2-like receptors was observed using PET
imaging. The combination of effects lead to
interesting hypotheses regarding differential
sensitivity to the reinforcing effects of
cocaine. For example, because PET imaging
studies in monkeys have shown a relationship
between D2-like receptor availability and
cocaine reinforcement (see Nader et al. 2008),
the PET imaging data would suggest no
differences between prenatally cocaine-exposed
and control monkeys in vulnerability to cocaine
reinforcement. However, D3 receptor sensitivity
has been associated with impulsivity (e.g., Dodd
et al. 2005; Sokoloff et al. 2006), which would
suggest differential sensitivity of
cocaine-exposed monkeys compared with controls
in acquisition of cocaine self-administration.
Additional behavioral studies in these monkeys,
including assessing the reinforcing effects of
cocaine, will provide important information as
to the long-term consequences of prenatal
cocaine exposure and the role of D2-like
receptor subtypes in these behavioral
outcomes.
In an effort to more fully characterize DA
receptor activity in vivo, functional studies of
the D1 receptor were also undertaken in these
same monkeys. D1-like receptor densities have
previously been shown to be affected by chronic
cocaine exposure in adult monkeys (Moore et al.
1998b) and not necessarily in a manner similar
to the effects of cocaine on D2-like receptors
(Nader et al. 2002). Fang et al. (1997) reported
that cocaine treatment from gestational day 22
to 70 resulted in significant increases in
D1-like receptor densities in day 70 fetal
monkey striatum. In rodent and rabbit models,
several studies suggest that prenatal cocaine
exposure uncoupled the D1 receptor from its
G-protein resulting in an attenuation of D1
receptor signaling (Friedman et al. 1996; Wang
et al. 1995; Lidow 1998; Jones et al. 2000;
Unterwald et al. 2003). However, there are no
data assessing D1-like receptor function in
adults who had been prenatally exposed to
cocaine. In the present study, no differences in
potency or effects of SKF 81297-elicited eye
blinks were observed in adult monkeys prenatally
exposed to cocaine versus controls. Because it
has been argued that this unconditioned behavior
is a sensitive measure of D1 signaling
(Jutkiewicz and Bergman 2004), these data
suggest that any functional differences in
D1-like receptor sensitivity observed in
prenatally cocaine-exposed animals shortly after
birth are no longer apparent in these animals as
adults.
It should be noted that under other
conditions in socially housed monkeys, SKF
81297-elicited eye blinking did not
differentiate monkeys based on social rank
(Czoty et al. 2004), even though differences in
sensitivity to cocaine reinforcement were
observed (Czoty et al. 2005). It remains
possible that other functional measures of
D1-like receptor activity (e.g., drug
discrimination or drug self-administration) may
yield differential sensitivity due to prenatal
cocaine exposure. The present findings are also
the first to note sex differences in sensitivity
to the D1-like agonist effects elicited by SKF
81297. It is important to note that D3 receptor
function (quinpirole-elicited yawning)
was also differentially affected by sex. The
present findings add to a growing body of
evidence for sex differences in the behavioral
effects of drugs. Taken together, these findings
indicate that prenatal cocaine exposure can have
long-lasting effects on DA receptor function and
that males and females are equally sensitive to
these perturbations.
