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Pharmacology, Biochemistry and Behavior 116 (2014) 39–44
Contents lists available at ScienceDirect
Pharmacology, Biochemistry and Behavior
journal homepage: www.elsevier.com/locate/pharmbiochembeh
Conditioned taste avoidance induced by Δ9-tetrahydrocannabinol in the
Fischer (F344) and Lewis (LEW) rat strains
Alison G.P. Wakeford ⁎, Anthony L. Riley
Psychopharmacology Laboratory, Department of Psychology, American University, Washington, DC 20016, USA
a r t i c l e
i n f o
Article history:
Received 10 October 2013
Accepted 7 November 2013
Available online 15 November 2013
Keywords:
THC
Fischer–Lewis
Taste avoidance
Core body temperature
a b s t r a c t
Although Fischer (F344) and Lewis (LEW) rats differ in their sensitivity to the rewarding effects of Δ9tetrahydrocannabinol (THC), no data have been reported on differences in their sensitivity to the drug's aversive
effects, a limiting factor in drug use and abuse. Examining the degree of differences (if any) in such effects in these
strains may help further characterize possible genetic factors important to abuse vulnerability. Accordingly, the
aversive effects of THC (1–5.6 mg/kg; intraperitoneal) were examined in 32 F344 and 32 LEW subjects using
the conditioned taste avoidance (CTA) procedure. Thermoregulation was assessed following an acute injection
of THC (same as CTA groups) after a week washout period following the last trial. Subjects in both strains
displayed dose-dependent THC-induced taste avoidance, with no significant strain difference. THC induced
dose-dependent decreases in core body temperature in both strains. LEW subjects displayed lower core body
temperatures than F344 rats, although this effect was independent of THC and was likely stress related. These results were discussed in terms of the nature of THC-induced taste avoidance and the basis of strain differences in
the aversive effects of drugs of abuse.
© 2013 Elsevier Inc. All rights reserved.
1. Introduction
Drug use and abuse are thought to be a function of the balance of the
rewarding and aversive effects of drugs, with the rewarding effects
maintaining drug taking and the aversive effects limiting such behavior
(Gaiardi et al., 1991; Lynch and Carroll, 2001; Riley et al., 2009; see Riley,
2011 for a review). Understanding this balance and the factors that affect it may provide insight into abuse vulnerability. One such factor contributing to this balance is the genetic background of the population
under investigation (Crabbe, 2002; Cunningham et al., 2009). Two inbred genetic strains that have received considerable attention in this
context are the Fischer (F344) and Lewis (LEW) rats (Kosten and
Ambrosio, 2002; Riley et al., 2009; Davis and Riley, 2010), and a host
of compounds have been identified for which the strains differ in their
rewarding and aversive effects.
In relation to the rewarding effects of drugs, LEW rats consume ethanol at a higher rate (Suzuki et al., 1988), acquire intravenous selfadministration of cocaine faster (Kosten et al., 1997), reach higher
break points when reinforced with morphine at doses as low as
1.0 mg/kg (Sánchez-Cardoso et al., 2007) and under some conditions
display greater place preferences (Kosten et al., 1994; though see
Davis et al., 2007) than their F344 counterparts. In relation to drugs'
⁎ Corresponding author at: Psychopharmacology Laboratory, Department of
Psychology, American University 4400 Mass. Ave., NW, Washington, DC 20016, USA.
Tel.: +1 202 885 1721; fax: +1 202 885 1081.
E-mail addresses: alison.presley@gmail.com (A.G.P. Wakeford), alriley@american.edu
(A.L. Riley).
0091-3057/$ – see front matter © 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.pbb.2013.11.005
aversive effects, differences in the acquisition of drug-induced conditioned taste avoidance (CTA; Garcia and Ervin, 1968; Hunt and Amit,
1987; Revusky and Garcia, 1970; Riley and Tuck, 1985; Rozin and
Kalat, 1971; see Freeman and Riley, 2009 for a history of CTA) have
been reported with LEW rats displaying stronger cocaine and caffeineinduced CTAs than F344 rats and the F344 strain displaying significantly
greater avoidance induced by morphine, nicotine and alcohol (Riley
et al., 2009). Assessing the relative sensitivities of these two strains to
drugs of abuse (both their rewarding and aversive effects) may be important to understanding the genetic basis of (or contribution to)
abuse vulnerability (Crabbe, 2002; Cunningham et al., 2009; Riley
et al., 2009; Davis and Riley, 2010), especially when specific physiological and biological effects can be associated with the behavioral differences between the two strains (see Grabus et al., 2004).
One drug that has received little attention in these strains is the primary psychoactive ingredient in marijuana (Δ9-tetrahydrocannabinol; THC),
which is surprising given the widespread use of this compound in
humans (Johnston et al., in press). In several assessments of the rewarding effects of THC in the F344 and LEW strains, THC has been reported
to lower the threshold for electrical brain stimulation to a greater degree
in LEW rats relative to the F344 strain (Gardner et al., 1988; Lepore et al.,
1996), suggesting that THC, like a number of other drugs of abuse, is more
rewarding in LEW rats (Suzuki et al., 1988; Kosten et al., 1994, 1997).
Work assessing the aversive effects of THC (as indexed in the conditioned
taste avoidance preparation) is relatively limited in general (Elsmore and
Fletcher, 1972; Amit et al., 1977; Fischer and Vail, 1980; Parker and Gillies,
1995; Schramm-Sapyta et al., 2007), and no work exists comparing the
F344 and LEW strains in this preparation.
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Although relatively little work has been done on THC-induced taste
avoidance, interestingly the basis for its aversive effects is well documented. For example, as early as 2002, Ghozland et al reported that in
outbred rats the aversive effects of THC (as assessed with place conditioning) appeared to be mediated through activity at the kappa opioid
receptor (KOR). Specifically, they examined the ability of three strains
of mice with selective gene deletions of the mu, delta and kappa opioid
receptor subtypes, i.e., MOR, DOR and KO, respectively, to acquire a conditioned place aversion induced by an injection of 5 mg/kg THC
(Ghozland et al., 2002). Although mice with the MOR and DOR deletions
displayed a THC-induced place aversion, KOR knock-out mice did not. In
a related paper supporting a role of kappa activity in THC's aversive effects, Zimmer et al. (2001) reported that knock-out mice deficient in the
prodynorphin gene (coding for the endogenous kappa agonist
dynorphin) failed to show a THC-induced conditioned place aversion,
whereas intact animals acquired a place aversion at the same dose
(5 mg/kg). The effects seen in the prodynorphin deficient mice were
paralleled in mice treated with the kappa-specific antagonist norbinaltorphimine (NBI, 10 mg/kg sc) prior to an injection of the same
dose of THC. Finally, Cheng et al. (2004) reported that the ablation of
the downstream regulatory element antagonist modulator (DREAM)
of prodynorphin gene transcription potentiated THC-induced conditioned place aversion at a dose of 1 mg/kg, a dose that did not produce
any place conditioning in wild-type mice (Cheng et al., 2004). Given
that the genetic ablation of DREAM results in an overexpression of the
prodynorphin gene and increased activation of kappa opioid receptors,
the potentiation of THC-induced aversions is consistent with previous
research demonstrating kappa mediation of the aversive effects of THC.
The fact that THC's aversive effects appear to be mediated via kappa
opioid activity is interesting in light of recent work assessing the ability
of various opioids (heroin, SNC80 and U50,488H) to induce taste avoidance in the F344 and LEW strains (Davis et al., 2009). Specifically, Davis
and colleagues report that while each of the opioid compounds tested
induced taste avoidance in both strains, only heroin, the relatively mu
selective compound (Goldstein and Naidu, 1989), induced differential
avoidance (F344 N LEW). Strain differences were not evident for
compounds relatively selective for the delta (SNC80) and kappa
(U50,488H) opioid receptor subtypes. Given that direct action at the
KOR was not differentially aversive in the F344 and LEW strains, it
might be predicted that THC, which acts through kappa receptor activity
to induce its aversive effects, would induce taste avoidance in both
strains, but such avoidance would not be strain dependent. Such a finding would be quite different from that seen with other drugs of abuse for
which the two strains differ in the avoidance response (see above) and
may provide insight into the specific characteristics mediating straindependent avoidance learning. Accordingly, in the following experiment rats from both strains were given access to a novel saccharin solution followed by various doses of THC (1–5.6 mg/kg) in an assessment
of the ability of the two strains to acquire THC-induced taste avoidance.
Although kappa activity appears to be involved in THC's aversive effects, the specific mechanism underlying this mediation is not known.
Ghozland et al. (2002) suggested that these effects may be a function
of THC-induced hypothermia (mediated through kappa activity).
THC's effects on temperature have been well characterized (Schmeling
and Hosko, 1976, 1977; Fennessy and Taylor, 1978; Malone and
Taylor, 1998; Nava et al., 2000), and drug-induced hypothermia has
been suggested to mediate the aversive effects of several compounds,
including nicotine and alcohol (Cunningham et al., 1988, 1992; Rinker
et al., 2008, though see 2011). To address strain differences in THCinduced hypothermia and their possible relationship with THCinduced taste avoidance learning, the present study also examined the
effects of THC on core body temperature at doses effective in inducing
taste avoidance in the two strains. Specifically, following taste avoidance conditioning and a 1-week washout, animals of both strains were
injected with one of a number of doses of THC and assessed for changes
in core body temperature.
2. Materials and methods
2.1. Method
2.1.1. Apparatus
All 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 50 ml Nalgene tubes affixed to
the front of the cages. Animals were handled daily approximately one
week before the beginning of the study to reduce the effects of handling
stress during conditioning and testing.
2.1.2. Subjects
Subjects were 64 experimentally naïve F344 (n = 32) and LEW
(n = 32) male rats (purchased from Harlan Sprague–Dawley,
Indianapolis). At the start of the experiment, the animals were between
90 and 113 days of age and weighed approximately 250 to 350 g (F344)
and 300 to 400 g (LEW). Procedures recommended by the National
Research Council (1996) and the Committee on Guidelines for the
Care and Use of Animals in Neuroscience and Behavioral Research
(2003) were followed at all times. The protocol for the research was
reviewed and approved by the Institutional Animal Care and Use Committee at American University.
2.1.3. Drugs and solutions
Twenty mg of THC (National Institute of Drug Abuse, NIDA) was dissolved in a solution of 1 ml ethanol/1 ml Cremophor (Sigma)/18 ml saline to yield a concentration of 1 mg/ml THC solution. The vehicle was
also prepared as a 1 ml ethanol/1 ml Cremophor (Sigma)/18 ml saline
solution. All injections were given intraperitoneally (IP). Saccharin (sodium saccharin, Sigma) was prepared as a 1 g/l (0.1%) solution in tap
water.
2.1.4. Condition taste avoidance procedure
2.1.4.1. Habituation. Following adaptation to the laboratory during
which time subjects were maintained on ad libitum food and water, all
animals were water deprived for 232/3 h and then given 20-min access
to water. This restricted access was given daily (1000 h) until water
consumption stabilized, i.e., all rats approached and drank from the
tube within 2 s of its presentation and consumption did not vary by
more than 2 ml over 3 consecutive days with no consistent increase
or decrease in intake. Throughout the study, fluids were presented in
graduated 50 ml Nalgene tubes and measured to the nearest 0.5 ml by
subtracting the difference from the pre to post consumption values.
All animals were weighed and handled during this period to minimize
the effects of handling stress on the subsequent phases of the
experiment.
2.1.4.2. Conditioning. On Day 1 of this phase, animals were given 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 IP with one of four doses of THC. Specifically, subjects were injected with 0 mg/kg (vehicle; n = 8 per
strain), 1.0 mg/kg (n = 8 per strain), 3.2 mg/kg (n = 8 per strain) or
5.6 mg/kg (n = 8 per strain) of THC, yielding Groups F0, F1.0, F3.2,
F5.6, L0, L1.0, L3.2 and L5.6. For each group, the letter denotes the strain
of the animal and the number denotes the dose of THC administered.
Doses used were based on previous research that found dosedependent rewarding and aversive effects with a range of THC doses, although not necessarily within the same study. Drug volume was varied
across drug doses, with drug concentration held constant. The vehicle
groups (F0 and L0) received injections comparable in volume to the
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2.1.5. Core body temperature
2.1.6. Statistical analysis
Saccharin consumption over conditioning and on the final onebottle test was analyzed using a 2 × 4 × 5 mixed model ANOVA with
the between-subjects factors of Strain (F344 and LEW) and Dose (0,
1.0, 3.2 and 5.6 mg/kg) and the within-subjects factor of Trial (1–4;
final aversion test). When appropriate, Tukey's post-hoc analyses
were performed to assess differences between groups on any specific
trial. Similarly, THC-induced hypothermia was analyzed using a
2 × 4 × 6 mixed model ANOVA with the between subjects factor of
Strain (F344 and LEW) and Dose (0, 1.0, 3.2 and 5.6 mg/kg) and the
within-subjects factors of Interval (Baseline and 30, 60, 90, 120 and
150 post injection). When appropriate, Tukey's post-hoc analyses
were performed to assess differences between groups on any specific
trial. Finally, paired samples t-tests were used to compare core body
temperatures for each strain (collapsed across dose) from baseline to
each post injection interval. Statistical significance was set at α = 0.05
with the exception of the paired-samples t-tests where α = 0.01 due
to Bonferroni corrections.
3. Results
3.1. Conditioned taste avoidance
THC induced dose-dependent taste avoidance in both F344 and LEW
rats, with no differences between the two strains. The 2 × 4 × 5 mixed
model ANOVA on saccharin consumption over conditioning and on the
final one-bottle test revealed a significant effect of Dose [F (3,
56) = 81.608, p b 0.001] and Trial [F (4, 224) = 95.075, p b 0.001], as
well as a significant Dose × Trial interaction [F (12, 224) = 17.703,
p b 0.001]. There was no effect of Strain [F (1, 56) = 0.051, p N 0.8]
nor was there a significant Strain × Dose [F (3, 56) = 0.593 p N 0.6],
Strain × Trial [F (4, 224) = 0.533, p N 0.7] or Strain × Dose × Trial
[F (12, 224) = 0.462, p N 0.9] interaction.
Fig. 1 illustrates the amount consumed by each strain at each dose of
THC over the four conditioning trials and on the final avoidance test. In
relation to the significant Dose × Trial interaction (collapsed across
F0.0
F1.0
F3.2
15
F5.6
10
5
2.1.5.1. Implantation. Immediately following the final one-bottle test,
temperature transponders (IPTT-300p; Bio Medic Data Systems,
Seaford, DE) were subcutaneously implanted into the animal longitudinally above the shoulders. All subjects were given a full week to recover
from the implantation procedure before temperature recording was initiated. During this period, they were observed daily to insure that there
was no migration of the transponder or other aversive reaction.
0
1
2
3
4
5
Trial
20
L0.0
LEW
L1.0
Consumption
(ml)
2.1.5.2. Recordings. At the outset of this assessment, subjects were
injected with the dose of drug (or vehicle) given during conditioning
(see above). Temperatures were assessed immediately prior to and 30,
60, 90, 120 and 150 min following each injection. The transponder
probe was waved approximately 2–4 inches away from the animal in
order to get an accurate reading of the temperature. Previous literature
has shown this temperature assessment to yield results comparable to
rectal thermometry (Quimby et al., 2009). This system provides an efficient and accurate assessment of temperature without causing excessive stress or discomfort to the animal (Elcock et al., 2001; Quimby
et al., 2009).
F344
20
Consumption
(ml)
highest drug dose groups (F5.6 and L5.6). 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 procedure of conditioning followed by 3 water-recovery days was repeated for four complete cycles. On the day following the last cycle, all subjects were
given 20-min access to saccharin in a final one-bottle avoidance test.
No injections followed this presentation.
41
L3.2
15
L5.6
10
5
0
1
2
3
4
5
Trial
Fig. 1. CTA results. Mean (+/− SEM) saccharin consumption (ml) of F344 (top) and LEW
(bottom) subjects injected with various doses of THC over repeated conditioning trials.
There was no significant Strain effect or interaction with Strain as a factor.
strain, not pictured), all drug-injected subjects drank significantly less
than subjects injected with vehicle by Trial 2 (all p's b 0.05). Further,
subjects injected with 5.6 mg/kg THC (F5.6 and L5.6) drank significantly
less than those injected with 1.0 mg/kg (F1.0 and L1.0; p b 0.05). These
effects were maintained on Trials 3, 4 and on the final avoidance test
(all p's b 0.05).
3.2. Core body temperature
THC induced changes in core body temperature over the six intervals
measured, but it did not induce changes differentially in the two strains.
However, strain differences were found across the six intervals when
collapsed across dose (not pictured). The 2 × 4 × 6 mixed model
ANOVA on core body temperature revealed a significant effect of Strain
[F (1, 55) = 57.504, p b 0.001], Dose [F (3, 55) = 7.904, p b 0.001] and
Interval [F (5, 275) = 32.779, p b 0.001], as well as significant Dose ×
Interval [F (15, 275) = 2.018, p b 0.05] and Strain × Interval [F (5,
275) = 12.069, p b 0.001] interactions. There was no significant
Strain × Dose [F (3, 55) = 0.420, p N 0.7] or Strain × Dose × Interval
[F (15, 275) = 1.151, p N 0.3] interaction.
Fig. 2 illustrates the core body temperature for each strain at each
dose of THC over the six measurement periods. In relation to the significant Dose × Interval interaction (collapsed across strain, not pictured),
subjects injected with 5.6 mg/kg (F5.6 and L5.6) displayed significantly
lower core temperatures than those injected with 1.0 (F1.0 and L1.0) at
60 min (p b 0.05). From 90 min to 150 min post injection, subjects
injected with 5.6 had significantly lower core temperatures than those
given vehicle (F0 and L0) and 1.0 (all p's b 0.05). No other comparisons
were significant. In relation to the significant Strain × Interval interaction (collapsed across dose, not pictured), LEW subjects displayed significantly lower core temperatures than F344 subjects from 30 min
through 150 min post injection (all p's b 0.001). Further, paired
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A.G.P. Wakeford, A.L. Riley / Pharmacology, Biochemistry and Behavior 116 (2014) 39–44
F344
Temperature
(Degrees Celsius)
39
F0.0
F1.0
38
F3.2
F5.6
37
36
Pre
30
60
90
120
150
Time Point
39
Temperature
(Degrees Celsius)
LEW
L0.0
L1.0
38
L3.2
L5.6
37
36
Pre
30
60
90
120
150
Time Point
Fig. 2. Temperature assessment. Mean (+/− SEM) core body temperature (C) of F344
(top) and LEW (bottom) vehicle- and drug-treated groups taken immediately prior to injection as well as 30, 60, 90, 120 and 150 min post injection. There was no significant
Strain × Dose × Interval interaction.
samples t-tests for F344 subjects revealed that core body temperatures
differed from baseline at 30 [t(31) = 14.98, p = 0.000], 60
[t(31) = 10.80, p = 0.000], 90 [t(31) = 7.75, p = 0.000], 120
[t(31) = 7.21, p = 0.000] and 150 [t(31) = 5.56, p = 0.000] min
post injection. LEW subjects never differed significantly from baseline
[all t's(30) N 2.51, p's N 0.018], although the comparison from baseline
to 30 min did approach significance (p = 0.018).
4. Discussion
As previously reported, the F344 and LEW rat strains differ significantly in the acquisition of taste avoidance induced by a variety of
drugs of abuse (Glowa et al., 1994; Lancellotti et al., 2001; Pescatore
et al., 2005; Roma et al., 2006; Vishwanath et al., 2011; see Riley et al.,
2009 for a review). Such behavioral differences are drug dependent
and thought to reflect differential sensitivities to the drugs tested. The
present experiment extended these analyses by examining the ability
of THC to induce avoidance in these strains. As described, THCinduced dose-dependent taste avoidance in both strains with the degree of avoidance directly related to the dose of the drug. Interestingly,
these effects were similar between the two strains i.e., there were no
differences between F344 and LEW subjects in the rate at which the
avoidance was acquired or the degree of the suppression.
The absence of strain differences with THC contrasts sharply with
the often reported strain differences in taste avoidance reported with
other drugs of abuse (see Riley, 2011). As described, with other assessments of taste avoidance induced by drugs of abuse the two strains differ, although the direction of the differences is drug specific. For
example, cocaine, caffeine and naloxone (in non-opiate dependent animals) induce significantly stronger avoidance in LEW rats (Glowa et al.,
1994; Vishwanath et al., 2011; Desko et al., 2012 respectively) whereas
morphine, nicotine and alcohol induce stronger avoidance in the F344
strain (Lancellotti et al., 2001; Pescatore et al., 2005; Roma et al., 2006
respectively). Although inconsistent with these earlier assessments,
the present results are predicted based on prior work on the neurochemical basis of THC-induced avoidance. As noted above, THCinduced taste avoidance appears to be mediated by kappa opioid
activity. Specifically, manipulations that potentiate or block kappa opioid activity in outbred rats have been reported to increase and decrease,
respectively, the aversive effects of THC as assessed with place conditioning (Zimmer et al., 2001; Ghozland et al., 2002; Cheng et al.,
2004). The fact that kappa activity mediates the aversive effects of
THC is important in the context of work with kappa-induced taste
avoidance in these two rat strains. Specifically, Davis et al., (2009) reported that while the F344 and LEW rat strains do differ in avoidance induced by heroin, the two strains do not display differential avoidance
induced by the selective kappa agonist, U50,488H. Thus, the failure to
see strain differences in THC-induced avoidance is consistent with the
effects of kappa agonists in these two strains and implicates this system
in THC's aversive effects.
Interestingly, the effects seen here with THC (and with those reported earlier with U50,488H) parallel those with a number of other drugs
for which taste avoidance does not differ between the two strains.
Such drugs include loperamide (Davis et al., 2012), LiCl (Foynes and
Riley, 2004) and naloxone in opiate-dependent animals (Stephens and
Riley, 2009; see also Cobuzzi and Riley, 2011). The question becomes
if there are similarities among these compounds that account for the
similar behavioral effects. One similarity among THC, LiCl and naloxone
(but not yet tested with U50,488H or loperamide) is the fact that each
compound induces aversive taste reactivity in orofacial preparations in
which an intraoral infusion of saccharin is paired with a drug injection.
In such designs, THC, LiCl and naloxone induce gaping, chin rubbing and
paw pushing, responses that are commonly used as indicators of disgust
(Grill and Norgren, 1978; for a review, see Parker et al., 2009). Further,
they do so at doses that support taste avoidance learning for which the
two strains do not differ (for a review, see Parker, 2003).
In contrast, drugs that do not induce rejection responses in the
orofacial design generally induce strain-dependent taste avoidance.
Some drugs display dose-dependent effects in the taste reactivity preparation. For example, low doses of nicotine (i.e., 0.2 – 0.8 mg/kg) fail to
induce rejection responses, whereas such effects are evident at higher
doses (1.2 – 2 mg/kg) (Parker, 1991). The doses that fail to induce rejection responses fall within the range of doses for which strain differences
in taste avoidance have been reported. It remains to be seen if strain differences in avoidance learning are reported at these higher doses. In this
context, it would be important to assess lower doses of THC in terms of
its ability to induce strain-dependent taste avoidance in the LEW and
F344 strains. Like nicotine, THC also produces biphasic effects in the
taste reactivity preparation such that high doses induce taste reactivity,
while low doses do not (Parker, 2003). The doses used here that failed to
produce strain-dependent avoidance fall within the range that induces
rejection.
Parker and colleagues (Parker, 2003; Parker et al., 2009) have argued
that displays of taste rejection in the taste reactivity design index conditioned nausea produced by drugs that are emetic in nature. Given that
the compounds that produce taste rejection in the taste reactivity design do not differentially induce taste avoidance in the F344 and LEW
rat strains, it is tempting to argue that it is this characteristic (emesis)
of the drugs that mediates their ability to induce taste avoidance. The
fact that the strains do not differ with these compounds could be used
to argue that the strains are not differentially sensitive to such emetic effects. While plausible, there are several caveats to this position. First,
THC has generally been considered an anti-emetic (Sallan et al., 1975;
Abrahamov et al., 1995) and recent research shows it can even block
the effects of classical emetics in the taste reactivity preparation
(Limebeer and Parker, 1999; Limebeer et al., 2006). It is important to
note here, however, that such anti-emetic effects are generally seen at
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doses lower than those reported here or those that induce rejection
(Limebeer and Parker, 1999; for a review, see Parker et al., 2009). Secondly, the basis for taste avoidance learning simply is not known and,
in fact, is likely multifaceted and drug dependent. Parker argues that
emesis is not a necessary (but possibly a sufficient) condition for the
conditioning of taste avoidance and that any disruption in normal homeostasis is sufficient to produce a fear response that in turn is sufficient to condition an avoidance of the drug-paired taste (Parker, 2003).
It has been suggested that the aversive effects of THC may be a function of its ability to induce hypothermia (presumably mediated through
kappa activity (Ghozland et al., 2002). To address the possible relationship between THC-induced hypothermia and THC's aversive effects, the
present study also examined the effects of THC on core body temperature at doses demonstrated to induce taste avoidance. Specifically, following avoidance conditioning and a 1-week washout, animals were
injected with one of a number of doses of THC and assessed for changes
in core body temperature for up to 150 min post injection. As described,
THC induced dose-dependent temperature changes in both strains. Specifically, animals injected with the highest dose of THC (5.6 mg/kg)
displayed significantly lower core body temperature than controls and
subjects injected with 1 mg/kg. It is important to note that these differences do not reflect a hypothermic effect of THC. That is, collapsed
across strain, THC did not decrease temperature below baseline
(i.e., temperature prior to the injection) for either the F344 or LEW
strain. It is unknown why THC did not induce hypothermia in these
two strains given that such effects have been well documented in
other assessments. One possibility for the absence of hypothermia in
the present assessment is that animals became tolerant to this effect
as a result of their experience with THC during conditioning. Ghozland
et al. (2002) have demonstrated that although intravenous THC induces
significant hypothermia at 20 mg/kg in mice, this effect is no longer evident after 3–4 injections, i.e., tolerance developed to THC's hypothermic effect (see also Hutcheson et al., 1998; Pertwee et al., 1993). The
difficulty in assessing whether or not tolerance to THC's hypothermic effects plays a role in the present results is that the majority of data on this
issue have been produced in studies assessing larger doses of THC,
employing different routes of administration and/or using more frequent THC administration. Thus, it is not known to what extent (if
any) the prior experience with THC during conditioning in the present
experiment affected its ability to induce hypothermic effects.
Although the two strains differed in core temperature with LEW
subjects displaying significantly lower temperature than the F344
strain, there was no significant interaction between Strain and Dose,
suggesting that these strain differences were independent of THC.
That the two strains differed even when administered vehicle argues
that the differences between the strains may have been a function of differential reactivity to the injection itself, an effect consistent with the
previously reported stress reactivity of the F344 strain relative to its
LEW counterpart (Dhabhar et al., 1993; Kosten and Ambrosio, 2002).
Specifically, F344 and LEW rats have differential hypothalamic–
pituitary–adrenal (HPA) axis function, with F344 rats having increased
plasma corticotropin (ACTH) and corticosterone which may explain
the possible stress effect seen in the current study (Sternberg et al.,
1989; Dhabhar et al., 1993; Kosten and Ambrosio, 2002).
The assessment of THC-induced changes in body temperature was
made to see if doses that produced taste avoidance induced changes
in core temperature and if there was any relationship between these
two effects in the F344 and LEW strains. As noted, there were dosedependent effects in both taste avoidance and temperature; however,
these effects appeared unrelated. As described, for both strains avoidance was evident at doses that did not significantly affect body temperature (e.g., see 1 mg/kg; Figs. 1 and 2). Further, Pearson's correlations
made on the amount consumed on the final conditioning test and the
peak temperature revealed no significant relationships at any dose
and for any strain (all p's N 0.117). Although changes in core temperature have been reported to be related to strength of taste avoidance
43
with ethanol and nicotine, the present results are consistent with
other assessments in which such relationships have not been found
(see Merluzzi et al., in press for recent work with MDPV) and argue
that the relationship between temperature and the aversive effects of
drugs may be drug dependent.
5. Conclusion
Accounting for the similarity of THC-induced taste avoidance in the
F344 and LEW strains is important in that the failure to see such a difference (with THC or any of the other compounds for which differences
have not been reported) suggests that the two strains are comparably
sensitive to the aversive effects of the drugs that condition such an
avoidance. Further assessments with compounds with emetic properties and/or drugs that reliably elicit rejection responses within the
taste reactivity procedure should provide additional support for this position. What such an explanation does not do, however, is provide any
specific insight into the basis for the differences reported for drugs
such as morphine, cocaine, caffeine, alcohol and nicotine for which the
two strains reliably differ in their conditioned taste avoidance. What is
clear is that the differences that have been reported do not reflect any
general differences in the sensory processing of taste, learning and
memory or a general blunting or sensitization to drugs (given that not
all drugs report strain differences and when differences are reported
they are often in opposite directions). Further examination of other
drugs of abuse, both their ability to induce differential taste avoidance
and taste reactivity as well as their specific mechanisms of action, may
provide insight into the basis of strain differences and the genetic contributions to drug sensitivity and abuse vulnerability.
Acknowledgments
This work was supported in part by a grant from the Mellon Foundation to Anthony L. Riley. The authors wish to thank Cheryl Limebeer at
the University of Guelph, Guelph, Ontario, Canada for her technical assistance in cannabinoid drug preparation. Requests for reprints should
be sent to Alison G. P. Wakeford, Psychopharmacology Laboratory, Department of Psychology, American University, Washington, DC 20016
(or alison.presley@gmail.com).
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