Pharmacology, Biochemistry and Behavior 101 (2012) 181–186
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Pharmacology, Biochemistry and Behavior
journal homepage: www.elsevier.com/locate/pharmbiochembeh
Assessment of the aversive effects of peripheral mu opioid receptor agonism in
Fischer 344 and Lewis rats
Catherine M. Davis, Jennifer L. Cobuzzi ⁎, Anthony L. Riley
Psychopharmacology Laboratory, Department of Psychology, American University, Washington, D.C. 20016, United States
a r t i c l e
i n f o
Article history:
Received 9 October 2011
Received in revised form 22 December 2011
Accepted 3 January 2012
Available online 11 January 2012
Keywords:
F344
LEW
Morphine
Loperamide
CTA
Drug abuse
a b s t r a c t
The Fischer 344 (F344) and Lewis (LEW) inbred rat strains differ on a host of biochemical, neuroanatomical,
immunological and behavioral endpoints. One behavioral difference of interest is their differential reactivity
to the aversive effects of morphine as indexed by the conditioned taste aversion preparation (aversions
acquired by F344 rats are significantly greater than those acquired by the LEW strain). This differential effect
appears to be specific to opioids that work primarily on the mu opioid receptor. Given that morphine works
systemically, it is unknown whether these differential effects in F344 and LEW animals are centrally or peripherally mediated. To address this issue, the present study investigated the ability of the peripherally acting
mu preferring opioid agonist loperamide to induce differential taste aversions in F344 and LEW animals. Both
F344 and LEW animals acquired dose-dependent taste aversions to the loperamide-associated solution with
no difference between them. Additionally, control animals initially injected with vehicle during aversion
training with loperamide and subsequently conditioned with morphine displayed the typical aversive profile
to morphine (F344 > LEW). Although the basis for the present data is unknown, their relation to morphineinduced taste aversions and the role of the interaction of stimulus effects of drugs that produce differential
abuse liability were discussed.
© 2012 Elsevier Inc. All rights reserved.
1. Introduction
Previous research has demonstrated that the Fischer 344 (F344)
and Lewis (LEW) inbred rat strains differ on a host of biochemical,
neuroanatomical, immunological and behavioral endpoints. Initial
assessments with these strains focused on their differential stress
reactivity (Sternberg et al., 1992) and responsivity to immunological
challenges (Sun et al., 1999). More recently, the F344 and LEW strains
have been investigated relative to their differential reactivity to a variety of drugs of abuse (for a review of this issue, see Kosten and
Ambrosio, 2002; Riley et al., 2009; Riley, 2011). Although the initial
work with these animals assessed the rewarding effects of such
drugs, subsequent investigations analyzed their differential responsivity to the drugs' aversive properties. In one of the first assessments
of differences in the aversive effects of drugs in these strains,
Lancellotti et al. (2001) reported that the F344 strain rapidly acquired
a robust taste aversion to a morphine-associated solution. LEW rats,
⁎ Correspondence author at: Psychopharmacology Laboratory, Department of
Psychology, American University, 4400 Mass. Ave., NW Washington, DC 20016, United
States. Tel.: + 1 202 885 1721; fax: + 1 202 885 1081.
E-mail address: jc9378a@american.edu (J.L. Cobuzzi).
0091-3057/$ – see front matter © 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.pbb.2012.01.001
on the other hand, failed to acquire an aversion even at high doses
and with repeated conditioning trials (for subsequent assessments,
see Gomez-Serrano et al., 2009; for a review, see Riley, 2011). This
strain difference in morphine-induced taste aversion learning was
not likely due to a general difference in learning or in the processing
of gustatory stimuli between the strains, in that LEW animals display
stronger cocaine-induced taste aversions at low and intermediate
doses than their F344 counterparts (Glowa et al., 1994), an effect opposite to that seen with morphine (Lancellotti et al., 2001). Instead, it
is more likely that the differential acquisition of taste aversions in
response to morphine administration in the F344 and LEW animals
is a consequence of their differential sensitivity to its aversive effects
(for similar assessments with other compounds, see Desko et al.,
2011; Foynes and Riley, 2004; Glowa et al., 1994; Pescatore et al.,
2005; Roma et al., 2006; Vishwanath et al., 2011).
In an attempt to isolate the specific opioid receptor on which morphine might be working to induce its behavioral effects within the
aversion preparation, Davis et al. (2009) examined the ability of
three opiate agonists with differential binding profiles, e.g., SNC80
(relative selectivity for the delta opioid receptor subtype), (−)U50,488H (relative selectivity for the kappa opioid receptor subtype)
and heroin (affinity for all three receptor subtypes with preference
for the mu opioid receptor) to induce aversions in the F344 and
LEW rat strains. Although all three drugs induced aversions, those induced by heroin were significantly stronger than those induced by
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C.M. Davis et al. / Pharmacology, Biochemistry and Behavior 101 (2012) 181–186
SNC80 and (−)-U50,488H. Further, the only drug for which F344 and
LEW animals differed was heroin, wherein F344 rats acquired
significantly stronger aversions than the LEW strain, an effect consistent with the prior work with morphine (Lancellotti et al., 2001).
Although both heroin and morphine act non-specifically on all three
receptor subtypes, the differential aversions with these drugs coupled
with the failure of either the delta or kappa agonists to induce straindependent aversions support the position that morphine's aversive
effects are likely mediated by its action at the mu opioid receptor
(alone or in combination with other opioid receptor subtypes; see
Davis et al., 2009) and suggest that differential action at this site mediates the observed strain differences with morphine.
Although these results suggest that the mu opioid receptor is involved in the behavioral differences between the F344 and LEW
strains in the acquisition of morphine-induced taste aversions, it is
not known whether this effect is centrally or peripherally mediated,
especially given that systemically administered morphine (and heroin) act at both sites (Davis et al., 2009). Interestingly, there is relatively little work investigating central vs. peripheral mediation of taste
aversion learning, and for the drugs that have been investigated,
the mechanism of action appears drug specific. For example, for THC
(Amit et al., 1977) and nicotine (Kumar et al., 1983; Shoaib and
Stolerman, 1995), taste aversions appear to be mediated centrally;
for ethanol (Crankshaw et al., 2003), lithium chloride (LiCl; Smith,
1980), acetaldehyde (Brown et al., 1978), fenfluramine (Lorden et
al., 1980), and fluoxetine (Lorden and Nunn, 1982), aversions appear
to be mediated via peripheral mechanisms. Interestingly, the work
with morphine in outbred rats is quite consistent on the peripheral
mediation of such aversions. For example, morphine administered
icv (Stapleton et al., 1979) and direct infusions of morphine into either the hippocampus or caudate fail to induce taste aversions (see
Amit et al., 1977). In more direct assessments of central vs. peripheral
mediation, Bechara et al. (1987) reported that methylnaltrexone, an
opioid antagonist that does not cross the blood brain barrier, blocked
the acquisition of both morphine-induced place and taste aversions
when morphine was given intraperitoneally (for a related report,
see also Martin et al., 1988). Morphine-induced taste aversions are
also blocked by the destruction of peripheral opiate receptors with
capsaicin. In a related report (see Bechara and van der Kooy, 1985),
vagotomy blocked the acquisition of both place and taste aversions
induced by systemic morphine, indicating that peripherally-located
opioid receptors are a necessary component of the aversive response
to morphine administration.
If morphine's effects are mediated peripherally, it would be
expected that peripherally acting, mu agonists would induce aversions. Further, it would be expected that if the differential effects of
morphine in the F344 and LEW strains are mediated by these receptors, such drugs would differentially induce aversions in the two
strains (F344 > LEW). These predictions were tested in the following
experiment in which rats from the F344 and LEW strains were
given a novel saccharin solution to drink and then injected with vehicle or various doses of the peripherally acting, mu agonist loperamide
(Awouters et al., 1983; DeHaven-Hudkins et al., 1999; Giagnoni et al.,
1982; Nozaki-Taguchi and Yaksh, 1999; Wüster and Herz, 1978).
Following this treatment, animals injected with vehicle during conditioning were subsequently given saccharin followed by morphine to
confirm the typical strain difference (where F344 subjects display
greater morphine-induced aversions than the LEW strain). Given
that drug use and abuse are thought to be a function of the balance
between the rewarding and aversive effects of a compound (Riley,
2011; Wise et al., 1976), understanding the factors that impact both
these properties may lead to a better understanding of the vulnerability to drug use and abuse. Genetic mediation of these affective properties is one such factor, and the F344 and LEW animals provide a
useful model to investigate genetic contributions to the relative
balance between the aversive and rewarding effects of drugs.
2. Material and methods
2.1. Subjects and apparatus
Subjects were 134 experimentally naive LEW (n = 67) and F344
(n = 67) male rats (purchased from Harlan Sprague Dawley, Indianapolis). Subjects were run in two replicates (n = 33 per strain, Replicate 1; n = 34 per strain, Replicate 2), and all groups for each strain
were represented in each replicate. Given that there were no differences between replicates in terms of the ability of loperamide [t
(132) = 1.092,p = 0.277] or morphine [t(132) = 0.178,p = 0.859] to
induce taste aversions, data from the two replicates were pooled for
analysis and presentation. At the start of the experiment, animals
were approximately 90 days of age and weighed approximately
300 g (LEW) and 250 g (F344). Animals were housed in individual
wire-mesh cages and maintained on a 12:12 h light/dark cycle (lights
on at 0800 h) and at an ambient temperature of 23 °C for the duration
of the experiment. Rat chow (Harlan Sprague–Dawley, Indianapolis,
Indiana) was provided ad libitum. All fluids were presented in 50ml Nalgene tubes affixed to the front of the cages. Procedures recommended by the Guide for the Care and Use of Laboratory Animals
(National Research Council, 1996) and the Institutional Animal Care
and Use Committee at American University were followed at all
times.
2.2. Drugs and solutions
Loperamide hydrochloride (Sigma) was dissolved in 10% dimethylsulfoxide (DMSO) and saline (5 mg/ml), and morphine sulfate (generously supplied by the National Institute on Drug Abuse) was dissolved
in saline (5 mg/ml). All drug doses were expressed as the salt form
and were administered subcutaneously (sc); doses used for loperamide
were selected from dose ranges previously reported (see Medvedev
et al., 1998; Shinoda et al., 2007). Sodium saccharin (Sigma) was prepared as a 1 g/l solution in tap water.
2.3. Procedure
2.3.1. Phase I: Habituation
After 23 2/3-h water deprivation, rats were given 20-min access to
water, beginning at 1000 h. This procedure was repeated daily until
all rats were approaching and drinking from the tube within 2 s of
its presentation. Once this criterion was reached, aversion conditioning began.
2.3.2. Phase II: Loperamide conditioning
On Day 1 of this phase, animals received 20-min access to a novel
saccharin solution during their daily fluid-access period. Immediately
following saccharin access, rats within each strain were assigned to
one of four groups such that consumption was comparable across
groups and injected with one of four doses of loperamide. Specifically,
subjects were injected with 0 mg/kg (vehicle; n = 16 per strain),
10 mg/kg (n = 17 per strain), 18 mg/kg (n = 17 per strain) or 32 mg/
kg (n = 17 per strain) of loperamide, yielding Groups F0, F10, F18,
F32, L0, L10, L18 and L32. For each group, the letter denotes the strain
of the animal and the number denotes the dose of loperamide administered. On Days 2–4 of this phase, all animals received 20-min access
to water during the fluid-access period. No injections were given
following this access. This cycle of one conditioning day followed by 3
water-recovery days was repeated for three full cycles. The fourth conditioning trial was treated as a one-bottle test of the aversion to
saccharin.
2.3.3. Phase III: Morphine conditioning
Following 3 water-recovery days, animals injected with vehicle
during Phase II, i.e., Groups F0 and L0, were given 20-min access to
C.M. Davis et al. / Pharmacology, Biochemistry and Behavior 101 (2012) 181–186
saccharin. Immediately following saccharin access, rats within each
strain were assigned to one of two groups such that consumption
was comparable between groups and injected with either vehicle or
morphine (5 mg/kg), yielding Groups FV-V, FV-M, LV-V and LV-M
(n's = 8 per group). For each group, the first two letters denote the
strain of the animal and the treatment during Phase II; the third letter
denotes the injection given following saccharin in Phase III. On the
3 days following morphine conditioning, all subjects were given 20min access to water. No injections followed these water-recovery sessions. This procedure of conditioning followed by 3 water-recovery
days was repeated for four additional cycles. On the day following
the last cycle of Phase III, all subjects were given 20-min access to
saccharin in a one-bottle aversion test.
2.4. Statistical analysis
An Independent Sample's t-test revealed that LEW and F344 rats
differed in consumption on the initial saccharin exposure at the outset of Phase II [t(132) = 6.692, p b 0.001]. Consequently, each strain
was evaluated independently. For each strain, differences in mean
saccharin consumption during loperamide conditioning (Phase II)
was analyzed using a 4 × 4 mixed analysis of variance (ANOVA)
with a between-subjects factor of Dose (0, 10, 18, 32 mg/kg) and a
within-subjects factor of Trials (1–4). Given that saccharin consumption at the outset of Phase III did not differ between F344 and LEW
animals [t(132) = 0.636, p = 0.526], saccharin consumption during
morphine treatment (Phase III) was analyzed using a 2 × 2 × 5 mixed
ANOVA with between-subjects factors of Strain (F344; LEW) and
Drug (VEH; MOR) and a within-subjects factor of Trials (1–5).
Where appropriate, one-way ANOVAs followed by Tukey's post-hoc
tests were performed on each trial to analyze differences in saccharin
consumption between groups; significance was assessed at α ≤ 0.05.
3. Results
3.1. Phase II: Loperamide conditioning
F344: The Trial × Dose mixed ANOVA on mean saccharin consumption over conditioning (Phase II) revealed significant effects of
Trial [F(3,189) = 163.219, p b 0.001] and Dose [F(3,63) = 97.907,
p b 0.001] as well as a significant Trial × Dose interaction [F(9,189) =
32.968, p b 0.001]. In relation to this interaction, a subsequent oneway ANOVA revealed significant differences between doses of loperamide on Trials 2, 3 and 4 (all p's b 0.001). Tukey's post-hoc analysis
indicated that on Trials 2, 3 and 4, F344 animals injected with 10,
18 and 32 mg/kg loperamide consumed significantly less saccharin
than F344 animals injected with vehicle (p's b 0.001) and on Trials 2
and 3 F344 animals injected with 32 mg/kg loperamide consumed
significantly less saccharin than F344 animals injected with 10 mg/
kg loperamide (p's b 0.05; see Fig. 1, Panel A).
LEW: The Trial × Dose mixed ANOVA on mean saccharin consumption revealed significant main effects of Trial [F(3,189) = 217.021,
p b 0.001] and Dose [F(3,63) = 833.65, p b 0.001] as well as a significant Trial × Dose [F(9,189) = 27.193, p b 0.001] interaction. In relation
to this interaction, a subsequent one-way ANOVA revealed significant
differences between doses of loperamide on Trials 2, 3 and 4 (all
p's b 0.001). Tukey's post-hoc analysis indicated that on Trials 2, 3
and 4 LEW animals injected with 10, 18 and 32 mg/kg loperamide
consumed significantly less saccharin than LEW animals injected with
vehicle (p's b 0.001). Further, on Trial 3 LEW animals injected with
32 mg/kg loperamide consumed significantly less saccharin than LEW
animals injected with 10 or 18 mg/kg loperamide (p's b 0.05) and on
Trial 4 LEW animals injected with 32 mg/kg loperamide consumed significantly less saccharin than LEW animals injected with 10 mg/kg
loperamide (pb 0.05; see Fig. 1, Panel B).
A Strain × Dose univariate ANOVA on saccharin consumption during the one-bottle aversion test (Trial 4) of Phase II revealed a significant main effect of Dose [F(3,126) = 269.794, p b 0.001] and no other
significant effects or interactions. Tukey's post-hoc analysis revealed
that animals injected with 10, 18 and 32 mg/kg loperamide consumed
significantly less saccharin than vehicle-injected animals (p'sb 0.001)
and animals injected with 32 mg/kg loperamide consumed significantly
less saccharin than animals injected with 10 mg/kg loperamide
(pb 0.01). There was no significant effect of Strain (alone or in combination with Dose).
3.2. Phase III: Morphine conditioning
The Strain × Drug × Trial mixed ANOVA on mean saccharin consumption over morphine conditioning (Phase III) revealed significant
main effects of Trial [F(4,112) = 8.618, p b 0.001], Strain [F(1,28) =
27.816, p b 0.001] and Drug [F(1,28) = 94.208, p b 0.001], as well as
significant Trial × Strain [F(4,112) = 10.311, p b 0.001], Trial× Drug [F
(4,112) = 51.493, p b 0.001], Strain × Drug [F(1,28) = 32.35, p b 0.001]
and Trial× Strain × Drug [F(4,112)= 18.689, p b 0.001] interactions. In
relation to this final interaction, a subsequent one-way ANOVA indicated significant group differences on Trials 2–5 (all p's b 0.001).
Tukey's post hoc analysis revealed that on Trials 2–5 F344 animals
injected with morphine consumed significantly less saccharin than
F344 animals injected with vehicle (all p's b 0.001; see Fig. 2, Panel
A), indicative of the formation of a robust taste aversion to morphine
in F344 animals. LEW animals injected with morphine consumed significantly less saccharin than LEW animals injected with vehicle only
on Trials 4 and 5 (both p's b 0.05; see Fig. 2, Panel B). On Trials 2–5,
F344 animals injected with morphine consumed significantly less
saccharin than comparably treated LEW animals (all p's b 0.005),
F344
Mean Saccharin
Consumption (ml)
15
LEW
A
15
*
183
*
B
*
*
10
*
*
10
#
5
V
10
18
32
#
0
1
2
3
Trial
4
5
#
^
0
1
2
3
4
Trial
Fig. 1. Mean (± SEM) saccharin consumption throughout Phase II: CTA Conditioning, by strain. Panel A: (F344) *F344 animals injected with 32, 18 and 10 mg/kg loperamide consumed significantly less saccharin than F344 animals injected with vehicle. #F344 animals injected with 32 mg/kg loperamide consumed significantly less saccharin than F344
animals injected with 10 mg/kg loperamide. Panel B: (LEW) *LEW animals injected with 32, 18 and 10 mg/kg loperamide consumed significantly less saccharin than LEW animals
injected with vehicle. #LEW animals injected with 32 mg/kg loperamide consumed significantly less saccharin than LEW animals injected with 10 or 18 mg/kg loperamide. ^LEW
animals injected with 32 mg/kg loperamide consumed significantly less saccharin than LEW animals injected with 10 mg/kg loperamide.
184
C.M. Davis et al. / Pharmacology, Biochemistry and Behavior 101 (2012) 181–186
Mean Saccharin
Consumption (ml)
15
A
10
*
F344
*
LEW
*
*
15
B
*
*
4
5
10
#
#
#
5
5
#
V-V
V-M
0
1
2
3
4
5
Trial
0
1
2
3
Trial
Fig. 2. Mean (± SEM) saccharin consumption throughout Phase III: Morphine Conditioning, by strain. Panel A: F344 animals injected with vehicle in Phase II and undergoing morphine conditioning. *F344 animals injected with morphine consumed significantly less saccharin than F344 animals injected with vehicle. #F344 animals injected with morphine
consumed significantly less saccharin than LEW animals injected with morphine (see Panel B). Panel B: LEW animals injected with vehicle in Phase II and undergoing morphine
conditioning. *LEW animals injected with morphine consumed significantly less saccharin than LEW animals injected with vehicle.
indicative of strain differences in the acquisition of the morphineinduced aversion. Vehicle-injected animals did not differ in saccharin consumption during this phase (all p's > 0.05).
4. Discussion
The F344 and LEW inbred rat strains display differential acquisition and expression of taste aversions in response to morphine (and
heroin) administration (Davis et al., 2009; Lancellotti et al., 2001). It
is unknown whether this differential response is mediated centrally
or peripherally given that systemic morphine (and heroin) works at
both sites (Davis et al., 2009). To test a possible peripheral mediation
of aversions in the two strains, the present experiment examined the
ability of the peripherally acting, mu preferring agonist loperamide to
produce aversions in F344 and LEW rats. In this assessment, F344 and
LEW animals developed comparable dose-dependent aversions to the
loperamide-associated solution, i.e., loperamide induced aversions
that were not strain dependent. These results are in marked contrast
to those with systemically-acting opioid agonists, e.g., morphine,
where the F344 strain displays greater aversions than the LEW strain.
Interestingly, animals initially injected with vehicle during loperamide conditioning and subsequently conditioned with morphine displayed a profile similar to that typically reported with morphine. As
described, F344 animals displayed a robust taste aversion relative to
LEW animals and their own vehicle-controls, where LEW animals
displayed a weak and late-onset taste aversion to the morphineassociated solution. The differences reported here with morphine
are unlikely a function of differential latent inhibition in the two
strains, given that they parallel those seen in animals without such
a history (Lancellotti et al., 2001). It is important to note, however,
that latent inhibition has not been examined in these two strains
and any role that it might have in animals with familiarization to
the taste CS is not known (although the highly correlated prepulse
inhibition response does not differ between the two strains under
baseline conditions; see Varty and Geyer, 1998).
Given that loperamide is a peripherally-acting opioid with limited
access to the CNS (Awouters et al., 1983; DeHaven-Hudkins et al.,
1999; Giagnoni et al., 1982; Nozaki-Taguchi and Yaksh, 1999; Wüster
and Herz, 1978), aversions induced by loperamide are likely mediated
by peripherally-located opioid receptors. The fact that loperamide can
induce such aversions by its action at these peripheral sites is consistent with other assessments of opiate-induced aversions in outbred
animals in which peripheral sites of action have been implicated.
For example, differential manipulations that result in the destruction
of peripherally located opiate receptors (Bechara and van der Kooy,
1985; Bechara et al., 1987) result in the loss of morphine-induced
place and taste aversions. Additionally, the administration of opiate
antagonists, such as methylnaltrexone that do not readily cross
the blood brain barrier, block morphine-induced place and taste
aversions (Bechara et al., 1987). Further, administration of morphine
into the hippocampus and caudate putamen (Amit et al., 1977) or via
an intracerebroventricular (icv) route (Hunt et al., 1983) fails to
induce taste aversions (though see Liu and Grigson, 2005 for evidence
of aversions induced by icv DAMGO).
What remains to be explained is what the present results mean in
relation to the reported differences between the F344 and LEW
strains in morphine-induced taste aversions (Lancellotti et al., 2001;
see also Davis et al., 2009). As noted, morphine-induced aversions
are significantly greater in the F344 strain relative to LEW rats (see
also present results with morphine). Given that peripherally-acting
loperamide induces aversions that are not strain-dependent suggests
one of two possibilities in relation to the effects with morphine. First,
morphine may act on different subtypes (and/or varieties) of peripheral opiate receptors (Pasternak, 2004; Wolozin and Pasternak, 1981)
than does loperamide and these different sites of peripheral action
may mediate the differential effects produced by the two drugs (i.e.,
strain differences with morphine; no strain differences with loperamide). A second possibility is that morphine may be acting centrally
to produce its strain-dependent effects.
In relation to the possible differences between morphine and
loperamide in their peripheral activity, Shannon and Lutz (2002)
reported that 10 times more naloxone (a mu-preferring opioid antagonist) was required to antagonize the analgesic effects of loperamide
than morphine when both compounds were administered subcutaneously. This suggests that loperamide and morphine work on separate
subsets of opiate receptors in the peripheral nervous system or that
they possess differential binding affinity to their preferred receptor
type that might lead to different aversive profiles. Others argue,
however, that loperamide and morphine work on the same receptor
subtype (Awouters et al., 1983; Mackerer et al., 1976; Manara and
Bianchetti, 1985), so it is unknown to what degree, if any, differential
binding in the periphery mediates the reported differences in aversion learning in response to morphine and loperamide. Further, if it
were the case that these compounds work on different opiate receptors to produce their effects, it would also have to be demonstrated
that LEW and F344 rats differ in binding to morphine at such peripheral sites to account for the differential behavioral effects with
morphine. In the absence of direct assessments matching the parameters under which the behavioral differences have been reported,
the possibility of such differential peripheral action between the
F344 and LEW rats remains speculative.
An alternative explanation for the differential mediation of aversions in the F344 and LEW strains with morphine is associated with
its central activity following systemic administration. In this context,
morphine may be acting centrally on some subset of receptors, e.g.,
kappa opioid receptors, to induce taste aversions and the strains differ in kappa opioid receptor activation. Although possible, Davis et
al. (2009) have recently reported that when the kappa agonist
C.M. Davis et al. / Pharmacology, Biochemistry and Behavior 101 (2012) 181–186
U50,488H was used to induce taste aversions, aversions were acquired in both the F344 and LEW strains, but there was no
strain difference. Another possibility involving central mediation concerns morphine's rewarding effects which are generally associated
with its central action (Bechara and van der Kooy, 1985; Koob and
Nestler, 1997). It is possible that morphine's central activity modulates any aversive effects produced by its peripheral action. Accordingly, this position argues that the peripheral aversive effects
produced by morphine are similar in the LEW and F344 strain but
the central, rewarding effects differ. Given that the LEW strain has
been reported to display greater opiate-induced conditioned place
preferences and more robust opiate self-administration (Ambrosio
et al., 1995; Martin et al., 1999; Sánchez-Cardoso et al., 2007), these
animals are generally assumed to display greater sensitivity to the
opiate's rewarding effects. This greater rewarding effect may be
impacting morphine's perceived aversiveness, reducing its ability
to induce taste aversions. Given that loperamide acts peripherally
due to either its restriction by the blood–brain barrier (DeHavenHudkins et al., 1999; Giagnoni et al., 1982; Nozaki-Taguchi and
Yaksh, 1999; Stahl et al., 1977) or its accumulation in peripheral
tissues and subsequent rapid metabolism (Awouters et al., 1983;
Wüster and Herz, 1978), its ability to activate central receptors is
minimal, if at all, and aversions are comparable for the two strains.
Although this interaction of central and peripheral activity is possible,
in the absence of any direct test of central opiate modulation of peripheral aversions, this, too, must remain speculative (see also Liu
and Grigson, 2005). The position suggests that drugs have multiple
stimulus effects that may interact in such a way to impact their perceived effect which in turn may impact use and abuse vulnerability
(Riley, 2011; Stolerman and D'Mello, 1981; Verendeev and Riley,
2011; Wise et al., 1976).
Independent of the mechanism underlying the differential expression of morphine-induced taste aversions in F344 and LEW animals
(F344 > LEW), it is important to note that there are other compounds
for which the strains display differential aversions. Specifically, F344
animals display greater nicotine- and ethanol-induced aversions
than LEW rats (Pescatore et al., 2005; Roma et al., 2006), while LEW
animals display stronger cocaine- and caffeine-induced aversions
than F344 rats (Glowa et al., 1994; Vishwanath et al., 2011). Given
that these compounds come from very different drug classes and
that the direction of the differences in the acquisition of taste aversions in the two strains is drug dependent, it is unlikely that any
common neurochemical or neuroanatomical pathway mediates the
aversions or the differences between the two strains. Although the
basis for the differences remains unknown, further examination of
other compounds in their ability to induce aversions in the two
strains may provide a more thorough characterization of the basis
for the reported strain differences.
The focus of the present work was to identify the central or peripheral mediation of the aversive effects of morphine in the F344
and LEW strains. This emphasis makes the assumption that the avoidance of tastes previously paired with morphine (or any drug) is a
function of the drug's aversive effects. It should be noted, however,
that there are other interpretations of this behavioral avoidance (for
a history of aversion learning, see Freeman and Riley, 2009). One particular position argues that the avoidance of drug-associated tastes
does not reflect anything about the aversive effects of the drug at
all, but instead reflects the drug's rewarding properties. This position,
the reward comparison hypothesis, argues that the reduction in the
intake of the drug-paired taste is a function of the acquired associated
between the taste and the rewarding effects of the drug such that
when presented with the taste on subsequent exposures the rat
anticipates the drug. Given that the taste pales in comparison to
the drug, it is avoided (Grigson, 1997; Grigson et al., 2009). This hypothesis predicts that there should be a direct relationship between
the ability of a drug to suppress consumption and produce their
185
rewarding effects, given that avoidance is a function of the drug's
rewarding effects. Interestingly, data in support of this position is
limited and a number of investigations have demonstrated that the
rewarding and suppressive effects of drugs are dissociable (see
Gaiardi et al., 1991; Martin et al., 1988; Simpson and Riley, 2005).
Most recently, Verendeev and Riley (2011) performed a direct test
of the reward comparison hypothesis by investigating individual subject differences in sensitivity to the rewarding and aversive effects of
both morphine and amphetamine. Specifically, they ran a concurrent
CTA/CPP preparation in which animals were given access to a novel
saccharin solution, injected with morphine (or amphetamine) and
then placed in one compartment of a place preference chamber.
Under these conditions, taste aversions and place preferences were
acquired, but there was no relationship between the two, i.e., individual animals that displayed strong taste aversions were just as likely to
show weak or strong place preferences, and vice versa (see also
Turenne et al., 1996). Taken together, these data provide evidence
that the subjective effects produced by drugs of abuse can in fact be
dissociated and the rewarding effects of drugs are not likely mediating the reduced consumption of the taste in the conditioned taste
aversion preparation.
The present assessment is the first of its kind to address the issue
of whether the strain difference observed in morphine-induced taste
aversions might be mediated via central or peripheral mechanisms.
This assessment was made by examining if the peripherally acting
mu agonist loperamide differentially induced aversions in the two
strains. As reported, the F344 and LEW rat strains do not differ in
their acquisition of aversions induced by the peripherally acting,
mu agonist loperamide, despite the fact that they display the typical
aversive profile (F344 > LEW) in response to the systemic opioid morphine. Although the basis for the differential reactivity of these strains
to morphine is not known, these results suggest a central mediation
of the strain difference in morphine-induced aversions, possibly
involving the drug's rewarding effects.
Acknowledgments
This research was supported by a grant from the Mellon Foundation to A.L.R. The Mellon Foundation had no further role in the
study design, data collection, analysis and interpretation, the writing
of the report, or the decision to submit the manuscript for publication.
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