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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/authorsrights Author's personal copy 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. Author's personal copy 40 A.G.P. Wakeford, A.L. Riley / Pharmacology, Biochemistry and Behavior 116 (2014) 39–44 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 Author's personal copy A.G.P. Wakeford, A.L. Riley / Pharmacology, Biochemistry and Behavior 116 (2014) 39–44 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 Author's personal copy 42 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 Author's personal copy A.G.P. Wakeford, A.L. Riley / Pharmacology, Biochemistry and Behavior 116 (2014) 39–44 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. 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