www.elsevier.com/locate/brainres

Brain Research 1064

Research Report

A opioid receptor agonist DAMGO-induced suppression of

saccharin intake in Lewis and Fischer rats

Chuang Liu*, Patricia Sue Grigson

Department of Neural and Behavioral Sciences, H181, Pennsylvania State University College of Medicine,

500 University Drive, Hershey, PA 17033, USA

Accepted 3 October 2005

Available online 2 November 2005

Abstract

Rats suppress intake of a saccharin cue when paired with a drug of abuse such as morphine or cocaine. Relative to Lewis rats, Fischer rats

exhibit greater avoidance of a saccharin cue following saccharin–morphine pairings. The present study used the A agonist, [d-Ala2,N-

MePhe4,Gly-ol5]enkephalin (DAMGO), to test whether strain differences in sensitivity of the A receptor contribute to this effect. Water-

deprived Lewis and Fischer rats were given 5 min access to 0.15% saccharin followed by an icv injection of either DAMGO (0.5 microg/1

microl/rat) or an equal volume of saline. There were six taste–drug pairings occurring at 48 h intervals. The results showed that, relative to

the saline treated controls, all rats reduced intake of the saccharin cue following saccharin–DAMGO pairings. No differences occurred

between strains. These data suggest that greater morphine-induced suppression of saccharin intake by the Fischer rats is not likely mediated

by differences in sensitivity of the A receptor. Other mechanisms are implicated.

D 2005 Elsevier B.V. All rights reserved.

Theme: Neural basis of behavior

Topic: Drugs of abuse: opioids and others

Keywords: DAMGO; Opioid; Saccharin; Conditioned taste aversion; Reward comparison; Rat; Anticipatory contrast

1. Introduction

Rats suppress intake of a saccharin conditioned stimulus

(CS) when paired with an unconditioned stimulus (US) such

as morphine, cocaine, amphetamine, or heroin [3,4,9,15,22].

The suppressive effects of these drugs of abuse have long

been attributed to conditioned taste aversion (CTA) [25].

Drug-induced suppression of CS intake, however, differs

greatly from the suppressive effects of the aversive agent,

lithium chloride (LiCl) [29–33]. Indeed, recent evidence

suggests that rats suppress intake of saccharin CS following

taste–drug pairings because the value of the saccharin CS

pales as it comes to predict the availability of the rewarding

properties of the drug of abuse [11,12], much as it does

when it predicts the availability of a highly rewarding

0006-8993/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.brainres.2005.10.005

* Corresponding author. Fax: +1 717 531 6916.

E-mail address: chliu@psu.edu (C. Liu).

concentration of sucrose [6]. Thus, the suppressive effects of

a rewarding sucrose US, morphine, and/or cocaine, but not

LiCl, are affected by the nature of the CS [2,7,11], are

augmented by a history of chronic treatment with morphine

[16], and are eliminated by bilateral lesions of the gustatory

thalamus [14,35–37].

Another factor that dissociates the suppressive effects of

drugs of abuse from those of the aversive agent, LiCl, is

strain. Relative to Fischer 344 rats, Lewis rats show a

greater conditioned preference for a location that has been

paired with the injection of morphine or cocaine [17,19] and

they more readily self-administer cocaine, ethanol, and

opiates [1,8,20,23,40]. The reward comparison hypothesis,

then, predicts that Lewis rats would exhibit greater

avoidance of a saccharin cue when paired with the highly

preferred drug of abuse than would the less preferring

Fischer rats. In accordance, ‘‘reward-preferring’’ Lewis rats

have been found to more greatly avoid intake of a saccharin

(2005) 155 – 160

C. Liu, P.S. Grigson / Brain Research 1064 (2005) 155–160156

CS when paired with the subcutaneous administration of

cocaine [9,13]. Thus, when taken with the findings

described above, the data are consistent with the conclusion

that Lewis rats more greatly avoid intake of a saccharin cue

following a saccharin–cocaine pairing because they are

more sensitive than Fischer rats to the rewarding properties

of the drug.

Unlike the pattern obtained with cocaine, however,

Fischer rats actually show greater avoidance of the same

saccharin cue when paired with morphine than Lewis rats.

Lancellotti et al. [21] demonstrated that low (10 mg/kg),

intermediate (32 mg/kg), and high (56 mg/kg) doses of

morphine induced stronger suppression of intake of a 0.1%

saccharin solution in Fischer rats compared with Lewis rats.

We have replicated this finding using sc, ip, and iv delivery

of morphine [43]. These data demonstrate that the strain

differences are not due to differences in sensitivity to

aversive aspects of the peripheral injection. The finding,

then, is firm: relative to Lewis rats, Fischer rats exhibit

greater avoidance of a saccharin cue when paired with

morphine. The question is why? In a simple analysis, the

reward comparison hypothesis would have to predict that,

while Fischer rats appear less sensitive to the rewarding

properties of opiates in most paradigms [1,8,17,23], they

must be more sensitive to the rewarding properties of opiates

in this paradigm. The rewarding properties of opiates are

mediated primarily by activation of A receptors [26,38].

Some data suggest that Fischer rats may, in fact, be more

sensitive to activation of the A opioid receptor than Lewis

rats. Fischer rats are reportedly more sensitive to pharmaco-

logical stimulation of A opioid receptors than Lewis rats and

Fig. 1. Mean (TSEM) intake (ml/5 min) of 0.15% saccharin in Fischer and Lewis

saline (1 microl/rat, n = 8) or DAMGO (0.5 microg/1 microl/rat, n = 9).

the affinity of radioligand binding to A opioid receptors was

higher in the cortex of Fischer rats [18]. Moreover, Fischer

rats also showed significantly higher levels of net DAMGO-

stimulated [35S]GTPgS binding in the striatum than Lewis

rats [39]. The present study, then, tested whether Fischer rats

would more greatly avoid intake of an otherwise palatable

saccharin cue after it had been paired with intracerebroven-

tricular (icv) administration of a highly selective A opioid

agonist, [d-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAMGO).

2. Results

2.1. Histology

The data presented are from subjects that had confirmed

injection sites within the lateral ventricle. All cannulae were

found to be appropriately placed.

2.2. The effect of DAMGO on intake of saccharin

Intake of the saccharin CS was greatly suppressed when

the CS was paired with DAMGO in both Fischer and Lewis

rats (Fig. 1). The results revealed a significant main effect of

drug, F(1,28) = 9.82, P < 0.01, indicating that the rats

injected with DAMGO consumed less saccharin than the

saline injected controls overall. The treatment � trials

interaction also was significant, F(5,140) = 8.01, P <

0.0001. Post hoc Newman–Keuls tests revealed that all rats

treated with DAMGO suppressed intake beginning with trial

4, Ps < 0.05. There was no significant difference in

rats following 6 pairings with the intracerebroventricular injection of either

C. Liu, P.S. Grigson / Brain Research 1064 (2005) 155–160 157

suppression of saccharin CS intake between Lewis and

Fischer rats. Neither the main effect of strain, F < 1, strain �treatment interaction, F < 1, strain � trials interaction,

F(5,140) = 1.36, P = 0.24, nor strain � treatment � trials

interaction, F < 1, was significant.

2.3. Individual differences

As has generally been the case with outbred Sprague–

Dawley rats [10], some of the Lewis and Fischer rats were

found to exhibit greater avoidance of the saccharin cue

following saccharin–DAMGO pairings than were others.

Consequently, the rats in the saccharin–DAMGO groups

were divided into two groups on the basis of the median

intake of saccharin on the final 2 trials. Five of the Fischer

rats were identified as large suppressers and 4 as small

suppressers; 5 of the Lewis rats were identified as large

suppressers, and 4 as small suppressers (Fig. 2). The data

were reanalyzed using a 2 � 3 � 6 ANOVA varying strain,

group (saline, DAMGO small suppressers, and DAMGO

large suppressers), and trials. The results of this analysis

revealed that the main effects of group F(2,26) = 25.25, P <

0.0001, and trials F(5,130) = 10.19, P < 0.0001, were

highly significant, as was the group � trials interaction,

F(10,130) = 9.66, P < 0.0001. Post hoc Newman–Keuls

tests of this interaction showed that, for the small

suppressers, intake of the saccharin CS did not differ from

that of the saline injected controls throughout testing, Ps >

0.05. For the large suppressers, on the other hand, intake of

the saccharin CS was significantly reduced relative to the

saline controls, beginning with the second trial, Ps < 0.01.

Fig. 2. A depiction of the data from mean intake of saccharin in Fischer and Lewis

saline or DAMGO, with the saccharin–DAMGO rats divided into small suppres

This pattern of data did not differ as a function of strain.

Neither the main effect of strain, F < 1, the strain � group,

F(2,26) = 1.36, P = 0.27, nor the strain � group � trials

interaction, F < 1, was significant.

2.4. dH2O intake

Five minute morning and 1 h afternoon dH2O intake

were analyzed using 2 � 2 � 6 ANOVAs. The results

showed that 5 min and 1 h dH2O intakes were not affected

by strain, drug, trials, or any interaction thereof, Ps > 0.05.

3. Discussion

Relative to Fischer rats, Lewis rats are more sensitive to

the rewarding properties of morphine, cocaine, and ethanol

[1,8,20,23,40]. They also show greater suppression of intake

of a saccharin cue when paired with the administration of a

rewarding sucrose solution or cocaine [9,13]. This finding,

as discussed, is in keeping with the reward comparison

hypothesis which suggests that rats avoid intake of a

saccharin cue following taste–drug pairings because they

are anticipating the rewarding rather than the aversive

properties of the drug [11,12].

The usefulness of this model, however, depends upon the

generality of the finding and an opposite pattern has been

obtained when the same saccharin cue was paired with

morphine. Relative to Lewis rats, Fischer rats showed

greater avoidance of the saccharin cue following saccharin–

morphine pairings [21]. Given that morphine is a potent A

rats following 6 pairings with the intracerebroventricular injection of either

sers (n = 4) and large suppressers (n = 5).

C. Liu, P.S. Grigson / Brain Research 1064 (2005) 155–160158

agonist, and the A opioid receptor is thought to primarily

mediate the rewarding properties of morphine [1,17,23,41],

the present study tested whether Fischer rats exhibit greater

DAMGO-induced suppression of CS intake relative to

Lewis rats. The results showed that both Lewis and Fischer

rats readily avoided intake of the saccharin cue following

pairings with the icv administration of the selective Aagonist, DAMGO, and the magnitude of this effect did not

differ across the two strains. In addition, rats from both

strains exhibited similar individual differences whereby

approximately half of the rats from each strain greatly

avoided intake of the saccharin cue following saccharin–

DAMGO pairings (referred to as the large suppressers),

while the other half did not (referred to as the small

suppressers). This finding demonstrates that DAMGO-

induced suppression of CS intake does not differ between

the strains and, as such, that the strain difference previously

obtained with morphine-induced suppression of CS intake

[21,43] is not likely due to a difference in sensitivity of the Areceptor.

Morphine, however, does not only bind to A receptors,

but to y and n opioid receptors as well [24,38]. Fischer rats,

then, may exhibit greater avoidance of a saccharin cue

following saccharin–morphine pairings because they are

differentially sensitive to activation of either the y or the nopioid receptors. Such a hypothesis is feasible because yopioid receptors also contribute to the rewarding properties

of morphine, although to a much lesser degree [42]. An

alternative consideration is that Fischer rats exhibit greater

morphine-induced suppression of CS intake than Lewis rats

because Fischer rats are more sensitive to the aversive

properties of the drug. Evidence suggests that the aversive

properties of opiates are mediated by activation of n opioid

receptors [27,28]. Differential sensitivity to morphine, then,

might be mediated by strain differences in sensitivity to

activation of the n opioid receptors. Three sets of data

support this possibility. First, Cook et al. [5] showed that

Fischer rats are more sensitive than Lewis rats to opioids

that have activity predominately at n receptors. Second,

Wheeler et al. [43] showed that the selective n opioid

receptor agonist, spiradoline, exhibited far greater suppres-

sion of intake of the saccharin CS in Fischer than Lewis rats.

Third, Wheeler et al. [43] showed that bilateral lesions of the

gustatory thalamus, which are known in Sprague–Dawley

rats to disrupt the suppressive effects of a rewarding sucrose

solution and morphine, but not those of the aversive agent,

LiCl [14,37], selectively disrupt morphine-induced suppres-

sion of CS intake in Lewis, but not Fischer, rats. Morphine-

induced suppression of CS intake in the Fischer rats, then,

may be due to strain differences in sensitivity to the n, ratherthan the A receptor opioid receptor.

In summary, published data show that Fischer rats

exhibit greater avoidance of a saccharin cue following

saccharin–morphine pairings. Morphine binds to A, y, andn receptors, and evidence shows that the rewarding pro-

perties of the drug are mediated primarily by activation of

the A receptor. The results of the present report, however,

demonstrate that Lewis and Fischer rats exhibit equivalent

avoidance of the saccharin cue when paired with the icv

administration of the highly selective A agonist, DAMGO.

Augmented morphine-induced suppression of CS intake in

the Fischer rats, then, is not likely due to differential

sensitivity of the A receptors. Although y receptors also

mediate the rewarding properties of morphine, independent

data show that Fischer rats are, in fact, more sensitive to

activation of the n receptors than are Lewis rats [5]. Thus,

while morphine also has relatively low affinity for n opioid

receptors [24], Fischer rats may exhibit greater morphine-

induced suppression because they are more sensitive to the

aversive, n mediated, properties of the drug. This hypoth-

esis is readily testable. Finally, whether the hypothesis

proves out, or not, it is abundantly clear that the reduction

in CS intake following pairings with a drug of abuse (or

any US for that matter) cannot be assumed, out of hand, to

reflect either appetitive or aversive conditioning. Additional

tests are required to determine whether the effect is due to

appetitive or aversive conditioning and the multitude of

factors that might mediate appetitive (e.g., hedonic value,

calories, relief from pain, or fear) or aversive (e.g., pain,

stress, fear, or illness) conditioning.

4. Experimental procedure

4.1. Animals

The subjects were 17 male Lewis rats and 17 male

Fischer rats (Harlan, Indianapolis, IN) weighing between

251 and 308 g at the start of the experiment. All animals

were housed individually in stainless steel hanging cages in

a temperature-controlled (21 -C) animal care facility with a

12:12 h light:dark cycle (lights on at 7:00 a.m.). The rats

were maintained with free access to rodent diet and water,

except where otherwise noted. To minimize nonspecific

stress, rats were handled daily before the start of the

experiment. All procedures pertaining to the use of animals

were carried out in strict accordance with institutional and

NIH guidelines.

4.2. Surgery

Rats were anesthetized with ketamine (Fort Dodge

Animal Health, Fort Dodge, IA) 70 mg/kg and xylazine

(Phoenix Scientific Inc., St. Joseph, MO) 14 mg/kg im. Rats

were then secured in a Kopf stereotaxic frame (David Kopf

Instruments, Tujunga, CA). A cut was made down the

midline and the skull was exposed. Permanent 22-gauge

stainless-steel guide cannulae (Plastics-One, Roanoke, VA)

were stereotactically placed in the left lateral cerebral

ventricle at 1.8 mm lateral and 0.9 mm posterior to bregma

and implanted 3.7 mm below the dura [34]. The cannula

was cemented to the skull using dental acrylic (Lang Dental

C. Liu, P.S. Grigson / Brain Research 1064 (2005) 155–160 159

MFG, Co., Inc., Wheeling, IL) anchored with four stainless

steel screws (Plastics One, Inc., Roanoke, VA). After

surgery, 28-gauge stainless steel wire stylets (Plastics One,

Inc., Roanoke, VA) were placed in the guide cannulae to

prevent occlusion. The subjects were given at least 7 days to

recover prior to the start of behavioral testing.

4.3. Apparatus

The rats were trained in their home cages. The water or

saccharin solution was delivered using inverted Nalgene

graduated cylinders with silicone stoppers and stainless steel

spouts. The cylinders were attached to the front of the home

cage using two springs and fluid intake was recorded to the

nearest 0.5 ml.

4.4. Procedure

Ten days before the start of the experiment, the rats were

placed on a water-deprivation schedule in which they

received 5 min access to water each morning and 1 h each

afternoon. Once intake stabilized (10 days), the subjects

were matched on the basis of mean 5 min water intake

during the final 2 days of baseline and divided into two

groups: saccharin–DAMGO (9 Lewis; 9 Fischer); saccha-

rin–saline (8 Lewis; 8 Fischer). During testing, all subjects

received 5 min access to 0.15% saccharin presented at room

temperature. Five minutes later, half of the rats were infused

icv with either 0.5 microg/1microl/rat DAMGO or an equal

volume of saline vehicle. The injections were made using a

28-gauge injector cannula (Plastics One, Inc., Roanoke, VA)

that extended 1.0 mm below the tip of the guide cannula

which was connected to 10 ml capacity glass Hamilton

syringes (Hamilton Co., Reno, NV) with polyethylene

tubing. Infusion delivery time was slightly more than 1

min. Following the infusion, the injector was left in place

for about 30 s to allow for diffusion of the test article. The

injector was then removed and the stylet was placed into the

guide cannula. There was one such taste–drug pairing a day,

occurring every other day, for a total of six trials. All

subjects received 5 min access to distilled water each

morning between conditioning trials and 1 h access to water

each afternoon to rehydrate.

4.5. Drugs

Sodium saccharin and DAMGO were obtained from the

Sigma Chemical Co., St. Louis, MO. Saccharin was

prepared in distilled water 24 h ahead of testing and pre-

sented at room temperature. DAMGO was dissolved in

sterile 0.9% saline immediately before testing.

4.6. Histology

At the end of the experiment, the animals were anes-

thetized with an overdose of pentobarbital sodium (100 mg/

kg, ip) and transcardially perfused with isotonic saline and

then with 10% formalin. The brains were removed, stored in

10% formalin, and then transferred to 30% sucrose + 10%

formalin for 24 h before the brains were sliced. Serial

sections of the brains were cut (50 Am sections) using a

freezing microtome and stained with cresyl Lecht violet.

The location of the injection site was determined under light

microscopic examination.

4.7. Data analysis

All data are presented as group means with the standard

error of the mean (SEM). Saccharin intake was analyzed

using a 2 � 2 � 6 mixed factorial analysis of variance

(ANOVA) varying strain (Fischer, Lewis), treatment (saline,

DAMGO), and trials. The drug-treated rats were then

divided into small and large suppressors using a median

split. The resultant data were analyzed using a 2 � 3 � 6

ANOVA varying strain (Fischer, Lewis), group (saline,

DAMGO small suppressers, DAMGO large suppressers),

and trials. When appropriate, post hoc analyses were

conducted using the Newman–Keuls test with the a level

set at 0.05.

Acknowledgments

This research was supported by U.S. Public Health

Service Grants DA 09815 and DA 12473 from the National

Institute on Drug Abuse. We thank Dr. Michael E. Smith for

his technical assistance. Part of the work was presented at

the annual meeting of the Society for Neuroscience in 2003.

References

[1] E. Ambrosio, S. Goldberg, G. Elmer, Behavior genetic investigation of

the relationship between spontaneous locomotor activity and the

acquisition of morphine self-administration behavior, Behav. Pharma-

col. 6 (1995) 229–237.

[2] R.A. Bevins, T.A. Delzer, M.T. Bardo, Second-order conditioning

detects unexpressed morphine-induced salt aversion, Anim. Learn.

Behav. 24 (1996) 221–229.

[3] H. Cappell, A.E. LeBlanc, Conditioned aversion to saccharin by single

administrations of mescaline and d-amphetamine, Psychopharmacolo-

gia 22 (1971) 352–356.

[4] H. Cappell, A.E. LeBlanc, Parametric investigations of the effects of

prior exposure to amphetamine and morphine on conditioned

gustatory aversion, Psychopharmacology 51 (1977) 265–271.

[5] C.D. Cook, A.C. Barrett, E.L. Roach, J.R. Bowman, M.J. Picker, Sex-

related differences in the antinociceptive effects of opioids: importance

of rat genotype, nociceptive stimulus intensity, and efficacy at the Aopioid receptor, Psychopharmacology 150 (2000) 430–442.

[6] C.F. Flaherty, S. Checke, Anticipation of incentive gain, Anim. Learn.

Behav. 10 (1982) 177–182.

[7] C.F. Flaherty, J. Turovsky, K.L. Krauss, Relative hedonic value

modulates anticipatory contrast, Physiol. Behav. 55 (1994)

1047–1054.

[8] F.R. George, S.R. Goldberg, Genetic approaches to the analysis of

addiction processes, Trends Pharmacol. Sci. 10 (1989) 78–83.

C. Liu, P.S. Grigson / Brain Research 1064 (2005) 155–160160

[9] J.R. Glowa, A.E. Shaw, A.L. Riley, Cocaine-induced conditioned taste

aversions: comparisons between effects in LEW/N and F344/N rat

strains, Psychopharmacology 114 (1994) 229–232.

[10] F. Gomez, N.A. Leo, P.S. Grigson, Morphine-induced suppression of

saccharin intake is correlated with elevated corticosterone levels, Brain

Res. 863 (2000) 52–58.

[11] P.S. Grigson, Conditioned taste aversions and drugs of abuse: a

reinterpretation, Behav. Neurosci. 111 (1997) 129–136.

[12] P.S. Grigson, Drugs of abuse and reward comparison: a brief review,

Appetite 35 (2000) 89–91.

[13] P.S. Grigson, C.S. Freet, The suppressive effects of sucrose and

cocaine, but not lithium chloride, are greater in Lewis than in Fischer

rats: evidence for the reward comparison hypothesis, Behav. Neurosci.

114 (2000) 353–363.

[14] P.S. Grigson, P. Lyuboslavsky, D. Tanase, Bilateral lesions of the

gustatory thalamus disrupt morphine- but not LiCl-induced intake

suppression in rats: evidence against the conditioned taste aversion

hypothesis, Brain Res. 858 (2000) 327–337.

[15] P.S. Grigson, R.C. Twining, R.M. Carelli, Heroin-induced suppression

of saccharin intake in water-deprived and free-feeding rats, Pharmacol.

Biochem. Behav. 66 (2000) 603–608.

[16] P.S. Grigson, R.A. Wheeler, D.S. Wheeler, S.M. Ballard, Chronic

morphine treatment exaggerates the suppressive effects of sucrose and

cocaine, but not lithium chloride, on saccharin intake in Sprague–

Dawley rats, Behav. Neurosci. 115 (2001) 403–416.

[17] X. Guitart, D. Beitner-Johnson, D.W. Marby, T.A. Kosten, E.J.

Nestler, Fischer and Lewis rat strains differ in basal levels of

niurofilament proteins and their regulation by chronic morphine in

the mesolimbic dopamine system, Synapse 12 (1992) 242–253.

[18] G. Herradon, L. Morales, L.F. Alguacil, Differences of A-opioidreceptors between Lewis and F344 rats, Life Sci. 73 (2003)

1537–1542.

[19] T.A. Kosten, M.J.D. Miserendino, S. Chi, E.J. Nestler, Fischer and

Lewis rat strains show differential cocaine effects in conditioned

place preference and behavioral sensitization but not in locomotor

activity or conditioned taste aversion, J. Pharmacol. Exp. Ther. 269

(1994) 137–144.

[20] T.A. Kosten, M.J.D. Miserendino, C.N. Haile, J.L. DeCaprio, P.I.

Jatlow, E.J. Nestler, Acquisition and maintenance of intravenous

cocaine self-administration in Lewis and Fischer inbred rat strains,

Brain Res. 778 (1997) 418–429.

[21] D. Lancellotti, B.M. Bayer, J.R. Glowa, R.A. Houghtling, A.L. Riley,

Morphine-induced conditioned taste aversions in the LEW/N and

F344/N rat strains, Pharmacol. Biochem. Behav. 68 (2001) 603–610.

[22] J. Le Magnen, Peripheral and systemic actions of food in caloric

regulation of intake, Ann. N. Y. Acad. Sci. 157 (1969) 1126–1157.

[23] S. Martin, J. Manzanares, J. Corchero, C. Garcia-Lecumberri, J.A.

Crespo, J.A. Fuentes, E. Ambrosio, Differential basal proenkephalin

gene expression in dorsal striatum and nucleus accumbens, and

vulnerability to morphine self-administration in Fischer 344 and

Lewis rats, Brain Res. 821 (1999) 350–355.

[24] M. Minami, Molecular pharmacology of opioid receptors, Nippon

Yakurigaku Zasshi 123 (2004) 95–104.

[25] M. Nachman, D. Lester, J. Magnen, Alcohol aversion in the rat:

behavioral assessment of noxious drug effects, Science 168 (1970)

1244–1246.

[26] M. Narita, M. Funada, T. Suzuki, Regulations of opioid dependence

by opioid receptor types, Pharmacol. Ther. 89 (2001) 1–15.

[27] E.J. Nestler, Molecular basis of long-term plasticity underlying

addiction, Nat. Rev., Neurosci. 2 (2001) 119–128.

[28] Z.Z. Pan, A-opposing actions of the n-opioid receptor, Trends

Pharmacol. Sci. 19 (1998) 94–98.

[29] L.A. Parker, Behavioral conditioned responses across multiple con-

ditioning/testing trials elicited by lithium- and amphetamine-paired

flavors, Behav. Neural Biol. 41 (1984) 190–199.

[30] L.A. Parker, Positively reinforcing drugs may produce a different kind

of CTA than drugs which are not positively reinforcing, Learn. Motiv.

19 (1988) 207–220.

[31] L.A. Parker, Taste reactivity responses elicited by reinforcing drugs: a

dose– response analysis, Behav. Neurosci. 105 (1991) 955–964.

[32] L.A. Parker, Taste reactivity responses elicited by cocaine-, phency-

clidine-, and methamphetamine-paired sucrose solutions, Behav.

Neurosci. 107 (1993) 118–129.

[33] L.A. Parker, Rewarding drugs produce taste avoidance, but not taste

aversion, Neurosci. Biobehav. Rev. 19 (1995) 143–151.

[34] G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates, 2nd

edR, Academic Press, San Diego, 1986.

[35] S. Reilly, The role of the gustatory thalamus in taste-guided behavior,

Neurosci. Biobehav. Rev. 22 (1998) 883–901.

[36] G. Scalera, P.S. Grigson, R. Norgren, Gustatory functions, sodium

appetite, and conditioned taste aversion survive excitotoxic lesions of

the thalamic taste area, Behav. Neurosci. 111 (1997) 633–645.

[37] P.L. Schroy, R.A. Wheeler, C. Davidson, G. Scalera, R.C. Twining,

P.S. Grigson, Role of gustatory thalamus in anticipation and

comparison of rewards over time in rats, Am. J. Physiol.: Regul.,

Integr. Comp. Physiol. 288 (2005) R966–R980.

[38] D.W. Self, L. Stein, Receptor subtypes in opioid and stimulant reward,

Pharmacol. Toxicol. 70 (1992) 87–94.

[39] D.E. Selley, J.T. Herbert, D. Morgan, C.D. Cook, M.J. Picker, L.J.

Sim-Selley, Effect of strain and sex on A opioid receptor-mediated G-

protein activation in rat brain, Brain Res. Bull. 60 (2003) 201–208.

[40] T. Suzuki, F.R. George, R.A. Meisch, Differential establishment

and maintenance of oral ethanol reinforced behavior in Lewis and

Fischer 344 inbred rat strains, J. Pharmacol. Exp. Ther. 245 (1988)

164–170.

[41] T. Suzuki, K. Otani, Y. Koike, M. Misawa, Genetic differences in

preferences for morphine and codeine in Lewis and Fischer 344 inbred

rat strains, Jpn. J. Pharmacol. 47 (1988) 425–431.

[42] T. Suzuki, H. Ikeda, M. Tsuji, M. Misawa, M. Narita, H. Nagase,

Antisense oligodeoxynucleotide to y opioid receptors attenuates

morphine dependence in mice, Life Sci. 61 (1997) PL165–PL170.

[43] R.A. Wheeler, C. Liu, K.W. Sanchez, N. Sublette, E. Leuenberger, P.S.

Grigson, Fischer 344 rats are more sensitive to the aversive n-mediated effects of opioids than Lewis rats, Abstracts of Society for

the Study of Ingestive Behavior: Annual Meeting 2002, Santa Cruz,

Appetite, 39, 2002, p. 106.