Modulation of saccharin preference by morphine and naloxone: inversion of drug effects as a function of saccharin concentration

The effect of quinine in two bottle choice procedures in C57BL6 mice: Opioid preference, somatic withdrawal, and pharmacokinetic outcomes

Previous reports assessing morphine effects in two bottle choice (TBC) paradigms often use taste adulterants such as sweeteners (e.g., saccharin) and/or bitterants (e.g., quinine) to demonstrate morphine preference with C57BL6 mice. The effect of these additional components on the morphine preference of C57BL6 remains poorly understood. Thus, we sought to elucidate the interrelationship of morphine and quinine in the TBC paradigm. As expected, when morphine was included in the opposite bottle from quinine, a preference for the morphine solution was observed. Conversely, when quinine was included in each bottle, or when fentanyl without quinine was used, no preference was observed. All opioid-drinking mice displayed withdrawal signs, and morphine was detectable in plasma and brain. When these results were compared to previous results via conversion to quinine preference scores, quinine was revealed to determine largely the measured morphine preference. Thus, quinine is effective to drive morphine consumption and engender dependence but may confound the ability to measure oral abuse liability of morphine. Together, these results suggest future TBC procedures should consider the effect of quinine upon measured preference for compounds in the opposite bottle, and that excessively high quinine concentrations appear to influence preference more so than the opposite solute when using C57BL6 mice. Alternative conditions to assess oral abuse liability may be necessary to complement and confirm results from TBC experiments utilizing morphine or other opioids.

Brain stimulation reward performance and sucrose maintained behaviors in alcohol-preferring and -nonpreferring rats

BACKGROUND

The relation between ethanol (EtOH) preference and sensitivity to brain stimulation reward (BSR) was examined under multiples schedules of reinforcement in the current study. For comparison, the study also examined the relation between EtOH preference and motivation for a sweet, palatable sucrose solution under similar schedules of reinforcement.

METHODS

To investigate sensitivity to BSR performance, alcohol-preferring and -nonpreferring rats were tested using the curve-shift (rate-frequency) paradigm under several intensity levels during a 20-min session. Animals were first trained under an optimal current intensity, which produced maximal responding (i.e., 100%) across a series of descending frequencies (i.e., 300-20 Hz). BSR was then evaluated at 100%, 75%, and 50% of the optimal current. The sensitivity of the curve-shift method was further evaluated under the animal's optimal current using the FR1, FR6, and FR12 schedules. To examine responding for the sucrose solution, a separate group of alcohol-preferring and -nonpreferring rats was initially stabilized on an FR1 schedule and then subsequently on FR6 and FR12 schedules.

RESULTS

The results demonstrated that reducing the reinforcing efficacy of BSR via reduction in current intensity/reinforcement schedule produced marked orderly rightward shifts in the rate-frequency curves relating responding to stimulation frequency in both rat lines. However, no differences were found between the lines with either manipulation. Specifically, both lines demonstrated orderly reductions in response rate and increases in BSR threshold parameters (i.e., half maximal frequency/responding, minimum and maximum frequencies). In contrast to BSR, genetic selection for EtOH preference was highly associated with responding for the sweet, palatable sucrose solution. The association was even more salient as the reinforcement schedule increased (i.e., reward cost).

CONCLUSION

The results demonstrate that responding for BSR is not associated with EtOH preference, insofar as alcohol-preferring and -nonpreferring rats respond similarly under an array of reinforcement schedules and current intensities. In contrast, genetic selection for EtOH preference is highly associated with responding for a palatable sucrose reward, and the relation increases as the reward cost for the sucrose increases. These findings suggest that similar/overlapping mechanisms of action regulate the reinforcing properties of EtOH and sucrose but that overlapping yet distinct neuronal mechanism may modulate the reward characteristics of BSR and EtOH preference.

Differential responding for brain stimulation reward and sucrose in high-alcohol-drinking (HAD) and low-alcohol-drinking (LAD) rats

BACKGROUND

This study examined the associations among selective breeding for alcohol preference, intake of sweet solutions, and responding for brain stimulation reward (BSR), a nonoral reinforcer, in alcohol-preferring high-alcohol-drinking (HAD)-1 and nonpreferring low-alcohol-drinking (LAD)-1 rats.

METHODS

Adult male HAD-1 and LAD-1 rats were trained to lever press for medial forebrain bundle stimulation. Current intensity was varied in separate sessions to generate a rate/intensity function. To further examine BSR responding, the animals responded for stimulation at 100 Hz and at a fixed current intensity on an FR1 schedule. In subsequent sessions, the schedule was increased to FR6 and then to FR12. To examine responding for the sucrose solution, we trained a separate group of HAD-1/LAD-1 rats to bar press for sucrose on an FR1 schedule. Similar to the BSR experiment, in following sessions, the schedule was increased to an FR6 and then to an FR12 schedule.

RESULTS

No significant differences were observed between the two rat lines across a range of current intensities. As the reinforcement schedule increased, HAD-1 rats exhibited a dramatic decrease in BSR responding, whereas the LAD-1 rats displayed a more protracted reduction. In contrast to BSR, marked elevations in responding were observed for sucrose as the schedule increased. However, in HAD-1 rats, response rates were similar on the FR6 and FR12 schedules, whereas LAD-1 rats showed a reduction in response rates from the FR6 to FR12 schedule. Furthermore, HAD-1 rats exhibited significantly more responses compared with LAD-1 rats across the three reinforcement schedules. An analysis of the response profile for the three reinforcement schedules suggested that few if any postreinforcement pauses were exhibited when the reinforcer was BSR compared with sucrose in both lines.

CONCLUSION

Medial forebrain bundle BSR is a powerful reinforcer in both HAD-1 and LAD-1 lines. However, BSR responding was not associated with selective breeding for alcohol preference. In contrast, selective breeding for alcohol preference was associated with sucrose consumption, especially as the amount of work increased. The lack of correspondence between BSR and sweet taste rewards in HAD-1 and LAD-1 lines may suggest important differences yet an overlapping brain reward mechanism in the control of motivated behaviors in these selected lines.

Chronic caffeine exposure in rats blocks a subsequent nicotine-conditioned taste avoidance in a one-bottle, but not a two-bottle test

Two experiments were conducted in order to investigate nicotine-conditioned taste avoidance (CTA) following chronic preexposure to caffeine. Rats were given daily intraperitoneal injections of caffeine anhydrous (0, 10, or 30 mg/kg) for 10 or 30 days. Training of the nicotine-CTA began after the last day of caffeine preexposure. On five separate occasions access to a saccharin solution was followed immediately by an injection of 1.2 mg/kg nicotine hydrogen tartrate salt or saline. Nicotine-CTA readily developed in saline-preexposed controls. That is, paired rats drank less saccharin solution than unpaired rats after repeated saccharin-nicotine pairings. A similar pattern of nicotine-CTA was found for rats preexposed to 30 mg/kg caffeine for 10 days. Following 10 days of preexposure to 10 mg/kg caffeine, however, CTA did not develop under standard testing conditions. Thirty days of caffeine preexposure did not affect the development of a nicotine-CTA even though the anorexic effects of caffeine were evident after exposure to 30 mg/kg for this duration. Thus, caffeine exposure appears to weaken acquisition or expression of the conditioned avoidance properties of nicotine. This effect is sensitive to the dose of caffeine and duration of preexposure. Importantly, the pattern of nicotine-CTA does not appear to be due to nonspecific effects of caffeine.

Effects of naltrexone on the intake of ethanol and flavored solutions in rats

It has been suggested that the endogenous opioid system may mediate the intake of preferred fluids, perhaps through an attenuation of reinforcement properties causing a subsequent shift in palatability. The purpose of the present study was to investigate the effects of the nonspecific opiate antagonist naltrexone on the intake of 10% ethanol, 0.1% saccharin, 0.0006% quinine, 0.4% saccharin + 10% ethanol, and 0.4% saccharin + 0.04% quinine solutions. Fluid intake was measured in male Long-Evans and Wistar rats under 24-h continuous and 30-min limited-fluid-access drinking paradigms. All rats received injections of naltrexone hydrochloride (10 mg/kg, i.p.) for 5 days after baseline intake measures and were monitored for a further 5 days (after-treatment phase). Results indicated that naltrexone did not affect intake of any solution when fluids were available over 24 h. However, under limited-access conditions, naltrexone caused a decrease in the intake of all fluids except quinine in both rat strains. On the basis of these findings, it is possible that the effects of this dose of naltrexone were not due to any true conditioning effect on the reinforcement properties of ethanol, but perhaps to some nonspecific effect of the drug, such as an alteration in palatability or an attenuation of locomotor activity. As well, due to the inconsistent results in fluid intake across drinking paradigms, the present findings do not provide evidence for an effective role for opiate mediation in ethanol intake as well as any ethanol-sweet fluid intake interactions.

Saccharin effects on morphine-induced antinociception in the mouse formalin test

This study was performed to investigate the role of sweetness and taste sensations of the non-caloric sweetener saccharin on pain and morphine antinociception by the formalin test in mice. The formalin test was chosen because it measures the response to a long-lasting nociceptive stimulus and thus may closely resemble clinical pain. The total time (seconds) spent licking and biting the injected paw (indices of nociception) during periods of 0-5 min (early phase) and 10-30 min (late phase) were measured as an indicator of pain and inflammatory responses. A 12 days pretreatment of animals with saccharin (0.04%, 0.08%, 0.16%) produced complex effects on the action of morphine. All doses significantly potentiated the low dose (1.5 mgkg(-1)) of morphine-induced analgesia in the early phase significantly but antagonized the effect of morphine (3 mgkg(-1)). The effect of high doses of morphine (6-9 mgkg(-1)) was antagonized by the low dose of saccharin (0.04%), but the effect of morphine (6 mgkg(-1)) was potentiated with high concentrations of saccharin (0.08% and 0.16%). All doses of saccharin decreased the analgesic effect of morphine at a dose of 9 mgkg(-1). Analgesic effects of low doses of morphine (1.5-3 mgkg(-1)) were decreased by all doses of saccharin in the late phase. Different concentrations of saccharin also affected the antagonistic effect of naloxone (0.4 mgkg(-1)) on morphine-induced analgesia in both phases of the formalin test. The high dose of saccharin (0.16%) potentiated the effect of naloxone in the late phase. The results obtained suggest that sweet sensation is an important factor in mediating morphine analgesic properties. It is therefore inappropriate to use different concentrations of sweet saccharin solutions interchangeably.

Interaction between calcium channel blockers and sweetening agents on morphine-induced analgesia in mice by formalin test

1. Calcium is known to be an important ion in the modulation of nociception and inflammation. Previous research has shown that mice drinking sweet-tasting solutions such as sucrose, saccharin and aspartame exhibit significant changes in morphine-induced analgesia in both phases of the formalin test. 2. In this study, the role of calcium channel blockers on the effectivity of a 12-day regimen of different sweetening agents (sucrose 32%, saccharin 0.08% and aspartame 0.16%) on the alteration of the morphine response has been investigated. 3. Male albino mice weighing 20-27 g were used for experiments. Animals were given 12 days to adapt to dietary conditions. Animals were given morphine (1.5, 3, 6, 9 mg/kg) subcutaneously 30 min before observation. Nifedipine (5 mg/kg), verapamil (5 mg/kg) and diltiazem (10 mg/kg) were administered intraperitoneally 20 min before morphine injection. 4. Recording of the early phase started immediately and lasted for 10 min after formalin injection. Recording of the late response started 20 min after formalin injection and lasted for 10 min. 5. Calcium channel blockers potentiated the antinociceptive effects of sweetening agents and diminished the antagonistic effects of these compounds on morphine-induced analgesia in the early and late phases of the formalin test. 6. It is proposed that calcium has a role for the interactive effects of sweetening agents and morphine on pain sensitivity.

A comparison of the effects of the opioid antagonists naltrexone, naltrindole, and beta-funaltrexamine on ethanol consumption in the rat

The effects of the universal opioid antagonist naltrexone were compared to the delta-selective opioid antagonist naltrindole and the mu-selective opioid antagonist beta-funaltrexamine on ethanol consumption in the absence of food or fluid deprivation using a limited access procedure in Wistar rats. Both naltrexone, at doses of 0.1, 0.25, 0.5, 1.0, 3.0, and 10 mg/kg, and beta-funaltrexamine, at doses of 5.0 and 20.0 mg/kg, significantly decreased consumption of a 6% ethanol solution compared to saline control groups. Naltrindole, at doses of 5.0 and 15.0 mg/kg, failed to significantly reduce ethanol consumption. In addition, the highest doses of naltrexone, which antagonize delta as well as mu-opioid receptors, did not differ significantly from the lowest doses in their ability to reduce ethanol consumption. These data suggest that ethanol consumption using the limited access paradigm in the outbred rat is modulated by mu rather than delta-opioid receptors. Although this is not consistent with other data showing that delta antagonists decrease ethanol consumption, it is suggested that these difference may be related to the alcohol-preferring rats used in those experiments.

Effects of sweetening agents on morphine-induced analgesia in mice by formalin test

1. There is evidence that sweet-tasting substances such as sucrose and saccharin can interact with endogenous opioid systems. Further evidence showed that feeding mice different concentrations of sucrose and saccharin alter the latency in the tail-flick test. 2. In the current study, the effects of a 12-day regimen of different sweetening agents [sucrose (32%), saccharin (0.08%) and aspartame (0.16%)] on morphine-induced analgesia with the formalin test were investigated. 3. Male albino mice (20-27 g) were used for the experiments. Animals were given 12 days to adapt to dietary conditions. Animals were first given saline or morphine subcutaneously (1.5, 3.0, 6.0, or 9.0 mg/kg) 30 min before the observation period. The recording of the early phase started immediately and lasted for 10 min. The recording of the late response started 20 min after formalin injection and lasted for 10 min. Statistical analysis was performed by using analysis of variance followed by Newman-Keuls test, and P < or = 0.05 was considered significant. 4. Sucrose and aspartame increased morphine analgesia in the early phase, but saccharin had no effect on the early phase. On the other hand, saccharin and sucrose decreased the effect of morphine in the late phase, but aspartame increased the effect of morphine-induced analgesia. 5. In conclusion, the present data provide further evidence for an important role for dietary variables in determining the effects of exogenous opioids on pain sensitivity.

Morphine enhances selection of both sucrose and ethanol in a two-bottle test

The influence of a low dose of morphine on the self-selection of alcohol and sucrose solutions is investigated. When given a choice between sucrose sweetened ethanol and plain water, rats show a significant preference for the sweetened ethanol. However, when given a choice between sweetened ethanol and sweetened water, rats increase consumption of sweetened water. These results suggest that the low-dose morphine enhancement of sweetened alcohol solutions is mediated by the reinforcing properties of sucrose not ethanol. However, when rats receive small doses of morphine and a choice between unsweetened ethanol and water, the rats increase consumption of ethanol. Therefore, a low dose of morphine enhances the self-selection of both sucrose and ethanol solutions. This provides additional confirmation that opioids may enhance the rewarding properties of a variety of appetite reinforcers.

Morphine enhancement of sucrose palatability: analysis by the taste reactivity test

The ability of morphine to modify sucrose palatability was assessed by the taste reactivity test. In Experiment 1, rats were injected with morphine (0.0, 0.5, 2.0, and 10.0 mg/kg, subcutaneously), 30 min before receiving a 10-min intraoral infusion of 2% or 20% sucrose solution. A dose of 2.0 mg/kg morphine enhanced ingestive reactions elicited by both concentrations of sucrose solution. In Experiment 2, the interval between morphine pretreatment and the taste reactivity test was manipulated. Rats given 2.0 mg/kg morphine 30 or 120 min before testing displayed enhanced ingestive reactions elicited by 20% sucrose solution during the first 5 min of a 10-min test. The results support the hypothesis that morphine enhances the hedonic assessment of sucrose solution.

Morphine-induced modification of quinine palatability: effects of multiple morphine-quinine trials

Morphine pretreatment attenuates aversive taste reactions elicited by quinine solution when assessed by the taste reactivity test. To determine whether this effect changes across trials, rats were administered morphine (2 mg/kg, subcutaneously) 30 min before a 5-min intraoral infusion of quinine solution (0.05%) on each of eight trials. Neither tolerance nor sensitization developed to morphine-induced attenuation of quinine aversiveness; morphine suppressed quinine-elicited aversive reactions on each trial. In addition, when tested in the absence of morphine, rats displayed a reduced aversion to quinine, suggesting that quinine became conditionally less aversive following previous pairings with morphine.

Effects of morphine injection into the parabrachial area on saccharin preference: modulation by lateral hypothalamic neurons

The aim of the present study was to analyze the effects of morphine injected into the second relay station of the gustatory input pathways, the parabrachial area, on preference for saccharin over water. This study was carried out using both rats whose lateral hypothalamic neurons had been lesioned by ibotenic acid and sham-lesioned rats. As already shown, an 0.3 mM solution of the sweetener, which was clearly preferred over water by the sham-lesioned animals, was neutral for the lesioned rats. The injection of 50 ng of morphine into each parabrachial area transformed this neutral response of the lesioned rats to a clear preference for the sweetener, whereas the preference of sham-lesioned rats for the same solution was converted to an aversive response. Likewise, with a more palatable solution of saccharin (2.5 mM), the injection of 50 ng of morphine decreased the preference of the nonlesioned rats but increased the preference of the lesioned animals. Using the 2.5 mM solution of saccharin, the intraparabrachial injection of higher doses of morphine (100 and 500 ng) did not greatly modify the preference for the sweetener but induced a significant decrease in total fluid intake that was still observed 11 h after the injection of the opiate. These results are discussed: the morphine-induced aversion observed in the nonlesioned rats could be explained either by a specific influence on certain opioid receptors in the parabrachial area or, more probably, by the stimulation of pathways involved in taste or visceral aversive processes and relaying in the parabrachial area.(ABSTRACT TRUNCATED AT 250 WORDS)

Consumption of sweet, salty, sour, and bitter solutions by selectively bred alcohol-preferring and alcohol-nonpreferring lines of rats

To determine whether selective breeding for high or low oral ethanol consumption is associated with different preferences for nonpharmacological solutions with various flavors, the oral intake of a range of concentrations of sucrose (0.5-64.0 g/100 ml), NaCl (0.025-3.2 g/100 ml), citric acid (0.008-2.048 g/liter), and sucrose octaacetate (0.002-0.512 g/liter) was studied in alcohol-preferring (P) and alcohol-nonpreferring (NP) rats. Separate groups of 7-8 rats from each line were tested for consumption of each of the four flavors. The flavored solutions were presented continuously with water and food always available, and the concentrations were doubled every 48 hr. Although rats from both lines showed a strong preference for the sucrose solutions, P rats consumed greater amounts than NP rats [F(7,98) = 5.57, p < 0.001]. Rats of the P line drank less of the NaCl solutions than NP rats [F(7,98) = 3.88, p < 0.001], but the effect was not as robust as the line differences seen with sucrose. The P and NP rats did not differ in citric acid or sucrose octaacetate intake at any of the concentrations tested. Selective breeding for high oral ethanol preference appears to be positively associated with consumption of sweet solutions and negatively associated with intake of salty solutions.

Responding for oral ethanol after naloxone treatment by alcohol-preferring AA rats

The present study examined the effect of a relatively nonselective opioid antagonist, naloxone, on lever pressing for oral ethanol by the alcohol-preferring AA rats. The AAs, housed continually in operant chambers with free access to food and water, learned to respond for 10% oral ethanol during daily 60-min alcohol access periods indicated by a stimulus light. The rats developed stable ethanol responding, resulting in mean ethanol intakes of 1.2 g/kg/60 min and measurable blood alcohol levels. In the first experiment, single systemic injections of naloxone (0.05-2.5 mg/kg) had no effect on the initial rate of responding; dose-dependent decreases were observed later during the alcohol access. The second experiment examined the effects of repeated injections of 0.5 and 2.5 mg/kg naloxone on 5 consecutive days. Naloxone suppressed responding dose-relatedly over the treatment days. In contrast to the effects of single injections, repeated injections with 2.5 mg/kg naloxone produced progressive decreases within the first minutes of access. The results suggest that naloxone may attenuate the reinforcing actions of ethanol.

Saccharin intake predicts ethanol intake in genetically heterogeneous rats as well as different rat strains

Saccharin and ethanol intakes were measured in seven strains of rats known to differ in their preferences for ethanol: The Fawn-Hooded (FH), alcohol-preferring (P) and Maudsley Reactive rats have been reported to drink ethanol voluntarily, whereas the alcohol-nonpreferring, Maudsley Nonreactive and Flinders Line (FSL and FRL) rats do not. Saccharin and ethanol intakes were highly correlated (r = +0.61) over all strains, with the FH rats drinking the most of both solutions. Correlation coefficients between pairs of drinking versus nondrinking rat strains were even higher. In a second experiment, genetically heterogeneous F2 progeny from cross-breeding the ethanol-preferring FH rats with the ethanol-nonpreferring Flinders Resistant Line (FRL) rats were studied. The results indicated a high positive correlation between saccharin and ethanol intakes (+0.65). These findings suggest that the association between saccharin and ethanol intakes previously reported in rat strains with different preferences for ethanol may have a similar genetic basis.

Hypothalamic melanin-concentrating hormone and alpha-neoendorphin-immunoreactive neurons project to the medial part of the rat parabrachial area

Neurons in the middle and posterior parts of the lateral hypothalamus project to the parabrachial area, and in particular to the gustatory relay-station located in the medial part of this area. In the present study we have examined some of the neuropeptide immunoreactivities of the lateral hypothalamus neurons that project to the gustatory region of the parabrachial area. By coupling retrograde transport and immunohistochemistry, we found that 50-60% of medial parabrachial area-projecting cells located in the juxta-capsular region of the posterior lateral hypothalamus are labeled by rat melanin-concentrating hormone antiserum, while 28% of the retrogradely labeled neurons located in the perifornical lateral hypothalamus are visualized with alpha-neoendorphin antiserum. Moreover, a large number of terminals distributed throughout the parabrachial nucleus are immunoreactive to melanin-concentrating hormone or alpha-neoendorphin antisera. These immunoreactivities are not co-localized within the same lateral hypothalamic neurons. The potential role of these peptidergic projections in the reward mechanisms elicited in the medial parabrachial area and in the control of palatability is discussed.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Ibotenic acid lesion of the hypothalamic paraventricular nucleus produces weight gain but modifies neither preference nor aversion for saccharin

We showed recently that bilateral ibotenic acid lesions of the lateral hypothalamus (LH) produced three main behavioral disturbances in the rat, i.e., an increase in the gustatory preference and aversion thresholds for saccharin, permanent body weight and water intake deficits, and an alteration of morphine-induced modulation of taste. The two first results could suggest that the modification of the gustatory thresholds and the ingestive deficits are closely interrelated. Given this situation, we hypothesized that, conversely, a brain lesion known to induce obesity and hyperdipsia would therefore decrease the gustatory preference and aversion thresholds for saccharin. In order to test this hypothesis we analyzed the effects of the bilateral lesion of the hypothalamic paraventricular nucleus (PVH) by injection of ibotenic acid (2 micrograms in each side) on saccharin preference. The main results are as follows: 1) The neurotoxin selectively destroyed parvicellular neurons while the magnocellular cells were spared. 2) In comparison to the normal daily gain in body weight of the sham-lesioned animals, the lesioned rats showed an enhanced weight gain that became significant from the third day after the surgery up until the day of sacrifice, 37 days later. 3) In contrast to electrolytic lesions of the PVH, the ibotenic acid lesions of this nucleus did not induce hyperdipsia. 4) Preference and aversion thresholds for saccharin were not significantly modified by the lesion. 5) Whereas low doses of morphine suppressed the preference for saccharin in sham-lesioned rats when the concentration of the sweetener solution was at the threshold value, this suppressive effect was not observed in PVH-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of morphine and naloxone on ethanol- and sucrose-reinforced responding in nondeprived rats

In the following series of experiments, effects of morphine (0.1, 0.3, 1.0, 3.0, and 10.0 mg/kg) and naloxone (0.1, 0.3, and 1.0 mg/kg) were assessed in nondeprived rats trained to leverpress with 10% ethanol, sweetened ethanol, or 5% sucrose and water as the reinforcers. Morphine, at doses of 0.1, 0.3, and 1.0 mg/kg had little effect on responding with ethanol or sweetened ethanol available on a fixed ratio 4 (FR4) schedule of reinforcement, but at the 3.0 mg/kg dose, morphine suppressed responding to near zero. Similar results were obtained when 10% ethanol and water were available on a concurrent FR4 FR4 schedule of reinforcement. When 5% sucrose and water were available concurrently, morphine suppressed responding at 3.0 and 10 mg/kg. Naloxone (0.1, 0.3, and 1.0 mg/kg) decreased responding for ethanol, sweetened ethanol, and sucrose solutions in a dose-dependent manner. Naloxone decreased total number of responses/session by shortening the duration of responding without affecting momentary rate. Overall, the data suggest that the endogenous opioid system plays a role in the ability of ethanol to reinforce operant behavior. However, this role does not appear to be specific to ethanol because similar results were observed with sucrose reinforcement. Failure to find enhanced ethanol intakes following morphine injections in the operant situation suggests that the method used to measure ethanol self-administration makes a difference in assessing the effects of drugs on ethanol intake.

Taste preferences in rat lines selected for low and high alcohol consumption

Alcohol-avoiding (ANA), alcohol-preferring (AA), and control Wistar rats were tested sequentially for their initial preferences for single concentration solutions of quinine, saccharin, salt, and citric acid, and then for an ascending series of saccharin concentrations. A similar study was subsequently conducted with the alcohol-nonpreferring (NP) and alcohol-preferring (P) rat lines. Both lines developed for low alcohol consumption drank much less saccharin than their respective lines developed for high alcohol intake when tested with the single concentration and with the ascending series. The ANAs also generally drank less of the bitter, salty, and sour solutions than the AAs or Wistars but little difference was found between the NPs and Ps with the other tastes. The curve relating saccharin consumption to concentration reached a maximum at about the same concentrations for AAs, Wistars, NPs, and Ps but for the ANAs, was shifted to the left. The results support a close relationship between the genetic factors influencing alcohol and saccharin intake in both line pairs. This relationship is probably not caused by saccharin tasting like alcohol to a rat, because other results indicate that the NPs do not have more negative reactions initially to the taste of alcohol, but it might be related to similar mechanisms mediating the reinforcement from sweet tastes and from systemic alcohol.