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Selected opioids modify intake of sweetened ethanol solution among female rats
Alcohol, Vol. 4, pp. 157-160. ©PergamonJournals Ltd., 1987. Printed in the U.S.A.
0741-8329/87 $3.00 + .00
Selected Opioids Modify Intake of Sweetened Ethanol Solution Among Female Rats STEPHANIE
A. CZIRR, CHRISTOPHER L. HUBBELL, WILLIAM J A Y M. F R A N K A N D L A R R Y D. R E I D 1
C. M I L A N O ,
Department o f Psychology, Rensselaer Polytechnic Institute, Troy, N Y 12180 R e c e i v e d 8 D e c e m b e r 1986 CZIRR, S. A., C. L. HUBBELL, W. C. MILANO, J. M. FRANK AND L. D. REID. Selected opioids modify intake of sweetened ethanol solution among female rats. ALCOHOL 4(3) 157-160, 1987.--Water-deprived female rats were given a daily, 1.5-hr opportunity to take either a sweetened ethanol solution or water. Across days, they increased their intake of ethanol solution and had stable intakes of about 2 g of pure ethanol/kg after 3 weeks. Morphine (I.0 mg/kg) alone, and in combination with diprenorphine (25/zg/kg), increased intake of ethanol solution among females similar to the increased intake seen with males under similar procedures. Fentanyl dose-relatedly increased intake of ethanol. The data strengthen the idea that one or more of the endogenous opioid systems, but not all, are involved with instances of "excessive" intake of alcoholic beverages. Ethanol intake
Female rats
Opioids
Diprenorphine
Fentanyl
Morphine
Ethanol Solution and Drugs
ON several occasions, it has been demonstrated that small doses of morphine (MOR) will increase rats' intake of ethanol solutions (ESs) [2, 7, 8, 16, 17]. This small dose MOR effect occurs under a variety of procedural variations including (a) housing conditions (grouped and individual), (b) deprivation conditions (deprived and not deprived of water), (c) day or night, and (d) the use of sweetened or unsweetened ESs [7]. Furthermore, the small dose MOR effect is persistent across many days of MOR injections [7,8]. The above mentioned studies all used male rats as the subjects. In the present study, therefore, we examined the effects of selected opioids on the intake of sweetened ES among females. Specifically, the effects of (a) MOR, (b) MOR in combination with diprenorphine (DIP), and (c) several doses of fentanyl (FEN) on ES intake were examined using female rats as subjects. F E N is a short acting, but highly potent, opioid analgesic [14]. DIP is an antagonist to many of M O R ' s effects including M O R ' s (and F E N ' s ) analgesia [14].
The ES was 5 g of sucrose and 12 g of pure ethanol (E) in 100 g of total fluid. MOR sulfate was administered at a dose of 1.0 mg/kg (measured as the salt). DIP hydrochloride was administered at a dose of 25/xg/kg (as base). F E N citrate was administered at doses of 5, 10, 20, and 40/zg/kg (as salt). Physiological saline, the vehicle of drugs, was administered as a placebo. Drugs and placebos w e r e administered by subcutaneous injections, in volumes of 1.0 ml/kg, 15 min before presentation of fluids.
Measures Fluids were presented in glass bottles equipped with ballpoint sipping tubes, which prevent substantial evaporation [13]. Bottles were weighed to the nearest 0.1 g before and after presentation of solutions. The resulting scores were corrected for spillage [7,13]. Each subject had daily measures of water and ES intake, as well as body weight. From these, further indices of rats' avidity toward E were derived, i.e., preference ratio (g of ES taken/total fluid intake) and g of pure E consumed per kg of body weight (g/kg). Since in these experiments, g of ES, preference ratio, and g/kg are highly correlated, only g/kg are reported.
METHOD
Subjects The subjects were 12 female, Sprague-Dawley rats weighing 150-200 g when purchased from Taconic Farms (Germantown, NY). They were individually housed in standard hanging cages in a windowless room maintained at 24°C. The colony room had 12 hr of artificial light each day beginning at 0900 hr. Food and water were always available prior to specific procedures.
Procedure After I week at the laboratory, the subjects began a schedule of 22.5 hr of water deprivation followed by concurrent
~Requests for reprints should be addressed to L. D. Reid.
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Figure I depicts the effects of the various doses of FEN on E intake. The two smallest doses of F E N had no reliable effect on subjects' intake of E (ps>0.05). The two highest doses reliably increased the rats' intake of E Ips<0.05). FEN, at the doses tested, had no reliable effect on water intake (all ps>0.05).
1.50 0 r~ Q)
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DISCUSSION
0.75 1:3TM
t:~ 0 . 5 0 cO I1)
O.25 0 0
5
io
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Fentc]nyl
25
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35
40
(microg/kg)
FIG. 1. The mean difference scores (drug minus placebo), across 4 doses of fentanyl (FEN), for grams of pure ethanol (E) taken per kilogram of body weight (glkg) by 12 female rats are depicted. The 5 and 10 ~g/kg doses had no reliable effect on intake of E, ps>0.05. The 20 ~g/kg dose of FEN reliably increased subjects' mean intake of E from 2.25 g/kg under placebo to 3.67 g/kg, t( 11)=4.06, p<0.002. The 40 ~g/kg dose of FEN also increased mean E intake from 2.31 g/kg under placebo to 3.77 g/kg, t(l 1)=3.65,/)<0.01. presentation of water and ES for 1.5 hr each day during the early part of the light cycle. Food was always available. This daily regimen continued for 39 days. Toward the end of these 39 days, an attempt was made to chart the rats' reproductive cycles by examining vaginal smears taken 0.5 hr before presenting bottles. Although studies using more sensitive measures might indicate a relationship between the female's reproductive cycle and E intake, we could discern no such relationship. All subjects received placebo injections on Day 40. On Day 41, all subjects received MOR injections. Subsequently, on Day 42, all subjects received 2 placebo injections. Concurrent injections of MOR and DIP were administered to all subjects on Day 43. The regimen continued for another 40 days, then all subjects received placebo injections, and on the following day, an injection of F E N . The regimen continued across the next 28 days with injections of a placebo followed by an injection of F E N occurring periodically until all doses of F E N were tested. RESULTS
As in previous studies using male rats (e.g., [7]), the females gradually increased their intake of ES across days, and after 3 weeks they were taking about 2 g/kg of E each day. They were healthy as indicated by their steady weight gain across days, an observation which is in concordance with previous studies with males (e.g., [7]). The data were assessed by way of Student's t-tests for dependent measures. Alone, MOR reliably decreased mean water intake from 13.7 g under placebo to 11.1 g, t (11) = 3.12, p<0.01. MOR increased mean E intake from 2.47 g/kg under placebo to 4.23 g/kg, t(11)=4.40, p<0.002. MOR and DIP given concurrently reliably increased mean water intake from 11.27 g under placebo to 14.71 g, t(11)=3.43, p<0.01. MOR plus DIP increased mean E intake from 1.94 g/kg under placebo to 2.54 g/kg, t(11)=2.31, p<0.05.
The small dose MOR effect is seen with females as well as males, a finding concordant with other tests showing that small doses of MOR increase intake of E across a wide array of procedural variables [2, 7, 8, 16, 17]. FEN also increases intakes at remarkably small doses with the minimally effective dose being on the order of 15/zg/kg. As shown recently, MOR plus DIP increased intakes of E [16]. It seems that one or more, but not all, opioid systems can modulate consumption of E. There are a number of procedural variables inherent to any one experiment. Sometimes these arbitrarily chosen variables are critical, and conclusions drawn from a single experiment do not generalize. The small dose MOR effect, however, transcends a wide array of procedural variables including sex of subjects. A consequential variable, however, is the length of the testing session; it has to be long enough for rats to have further opportunity to drink beyond what they would under placebo conditions. For rats, in these kind of situations, 30 min is too short, and 1.5 to 2 hr seems optimal [8]. Also, rats have to be taking some ES at baseline or under placebo before a small dose of MOR will increment intake. With these caveats, it seems fair to conclude that small doses of MOR and a number of related opioids, such as methadone [12] and FEN (this paper), increase intake of ES among rats. Recently, an idea that might be called the opioiddeficiency hypothesis has been advanced [21]. The hypothesis states there is " . . . an inverse relationship between ethanol preference and endorphin stimulation. When endorphin activity is stimulated by exogenous or endogenous opiates, voluntary ethanol consumption decreases and when a deficiency in opiate stimulation occurs (i.e., opiate withdrawal) ethanol drinking increases" ([21], p. 841). Or stated another way, " . . . one of the reinforcing mechanisms in ethanol drinking involves compensation for relative deficiencies in endorphin stimulation" ([21], p. 846). This hypothesis was advanced in an article supposedly showing that naltrexone blocks post-shock increases in E consumption. (We say supposedly showing because there was no control group in the experimental design to assess the effects of just the opportunity to continuing drinking with no shock, therefore, there is no way to know if the increased drinking was related to a history of having shock.) The gist of the article (in any case) is that antagonists, such as naltrexone or naloxone, reduce E intake as has been shown in other experiments [1, 3, 7, 17, 18]. On the surface, it appears that the data of the article itself [21] contradicts the hypothesis put forward in the introduction and discussion of the article [21]. Naltrexone, by blocking opioid receptors (opioceptors), should be the functional equivalent of reducing opioid systems activity, i.e., produce a relative deficiency in opiodergic (endorphin) stimulation. It follows, therefore, that administering naltrexone, in blocking doses, should, according to one interpretation of the hypothesis, increase intakes of alcoholic beverages. The data with antagonists (naloxone or naltrexone) suggest quite the op-
OPIOIDS A N D A L C O H O L
159
posite: When there is a " d e f i c i e n c y " in activity usually instigated by occupation o f opioceptors, rats drink less. A basic assumption underlying the deficiency-hypothesis is that " . . . morphine administration suppresses voluntary ethanol drinking in r a t s . . . " ([21], p. 841). Large doses of MOR [19] or methadone [12], as well as other agonists [6], do decrease voluntary drinking of E. Smaller doses of MOR ([7, 8, 17], this paper, methadone [12] or F E N (this paper), however, increase intakes of ESs. Large doses also produce a number of behaviors incompatible with drinking (such as catatonia); so, it might be unrealistic to expect large doses to increment drinking, even though they may increase a preference for alcoholic beverage (i.e., they can reduce water drinking more than alcohol drinking) [2]. When, however, potentially depressing features of MOR are antagonized by DIP, MOR together with DIP increased intakes of ES ([16], this paper). Along the same lines, small doses, rather than large doses, are more apt to mimic physiological processes of endorphinergic systems. There is no reason to believe that any endogenous release of opioids would match the effectiveness of doses of MOR of up to 60 mg/kg used in some of the experiments [19] from which the conclusion [21] is drawn that MOR reduces ES intake. There is one other line of evidence that could support tlae deficiency hypothesis. It was concluded that withdrawal from the chronic effects of MOR enhances E intake. The evidence here, however, does not uniformly support the conclusion of the withdrawal state itself goading enhanced consumption. In a summary of relevant experiments of Ho et al. [6], for example, only some experiments indicated enhanced consumption. Furthermore, when enhanced consumption was observed, it was after withdrawal symptoms had abated. Perhaps, some days after acute withdrawal symptoms, endogenous opioid systems are as apt to be hyperactive as hypoactive. So, it is not so clear that withdrawal from opioids itself is an effective goad to enhance E intake. The facts that (a) small doses of MOR, methadone, and F E N increase intake of ES, (b) naloxone decreases intake, and (c) the concept that small doses of MOR are more akin to the activity following endogenous release of opioids, all lead to just the opposite conclusion than the opioid-deficiency hypothesis: It is not an opioid deficiency that leads to more intake, but rather an opioid surfeit that leads to more intake (as long as a toxic surfeit is not achieved). New data, as
happens often, does lead to radical revisions of some hypotheses. The data are, however, compatible with the general theoretical framework of Stewart et al. [20]. Most extant distinctions among types of opioid receptors (opioceptors) are based on in vitro assays. Since such assays are frequently done in a medium deviating remarkably from fluids bathing receptors in living individuals, and since such assays will produce different results and conclusions as the ions in the medium vary [10,11], it is difficult to make inferences concerning what putative types of opioceptors might be involved in the small-dose effects of agonists at opioceptors in living animals [15]. Furthermore, the doses of antagonists used in the typical study of laboratory subjects given a chance to drink ES are probably too large to produce any selectivity in terms of blocking one type of opioceptor over another. Consequently, it is probably premature to speculate on the type of opioceptors involved in the effects of small doses of agonists. Nevertheless, there are some limited conclusions that can be drawn. Small doses of MOR, methadone, and F E N all increase intake of sweetened alcoholic beverages among rats ([7, 8, 12, 17], this paper), and each drug has been labelled a mu selective agonist. Ethylketocyclazocine, a reputedly kappa selective agonist, does not produce marked increments in intake of sweetened ES [9]. One might, therefore, lean toward the conclusion that the small dose opioid effect is a product of activity at mu opioceptors rather than kappa opioceptors. There are, however, reasons for not accepting that conclusion presently. One, none of the drugs used is particularly selective for one type of opioceptor, especially at doses used in vivo. Two, such a distinction does not consider other putative types of opioceptors that might be extant. Three, as mentioned, there is some confusion concerning what exactly a mu opioceptor is functionally in vivo. If, however, one defines mu opioceptors as those modulating analgesia [5], then we can conclude that the small dose MOR effect is not related to mu opioceptors since MOR plus DIP, an antagonist to opioid analgesia, increments intake of ESs ([16], this paper). ACKNOWLEDGEMENTS We thank Jean Bestle and Betty Osganian for their clerical assistance. The research was supported, in part, by grant AA06212 from the National Institute on Alcohol Abuse and Alcoholism.
REFERENCES
1. Altshuler, H. L., P. E. Phillips and D. A. Feinhandler. Alteration of ethanol self-administration by naltrexone. Life Sci 26: 679-688, 1980. 2. Beaman, C. M., G. A. Hunter, L. L. Dunn and L. D. Reid. Opioids, benzodiazepines and intake of ethanol. Alcohol 1: 39-42, 1984. 3. Critcher, E. C., C. I. Lin, J. Patel and R. D. Myers. Attenuation of alcohol drinking in tetrahydroisoquinoline-treated rats by morphine and naltrexone. Pharmacol Biochem Behav 18: 225229, 1983. 4. De Witte, P. Naloxone reduces alcohol intake in a free-choice procedure even when both drinking bottles contain saccharin sodium or quinine substances. Neuropsychobiology 12: 73-77, 1984. 5. Fang, F. G., H. L. Fields and N. M. Lee. Action at the mu receptor is sufficient to explain the supraspinal analgesic effect of opiates. J Pharrnacol Exp Ther 238: 1039-1044, 1986.
6. Ho, A. K. S., R. C. A. Chen and J. M. Morrison. Opiate-ethanol interaction studies. In: Alcohol and Opiates, edited by K. Blum. New York: Academic Press, 1977, pp. 189-202. 7. Hubbell, C. L., S. A. Czirr, G. A. Hunter, C. M. Beaman, N. C. LeCann and L. D. Reid. Consumption of ethanol solution is potentiated by morphine and attenuated by naloxone persistently across repeated daily administrations. Alcohol 3: 39-54, 1986. 8. Hubbell, C. L., S. A. Czirr and L. D. Reid. Persistence and specificity of small doses of morphine on intake of alcoholic beverages. Alcohol 4: 149-156, 1987. 9. Hunter, G. A., C. M. Beaman, L. L. Dunn and L. D. Reid. Selected opioids, ethanol and intake of ethanol. Alcohol 1: 43-46, 1984. 10. Morley, J. E. Endogenous opiate receptors: keys to the brain? Diagn Med, special issue, 1-7, 1982.
160 I 1. Morley, J. E., A. S. Levine, G. K. Lim and M. T. Lowy. Opioid modulation of appetite. Neurosci Biobehav Rev 7: 281-305, 1983. 12. Mudar, P. J., N. C. LeCann, S. A. Czirr, C. L. Hubbell and L. D. Reid. Methadone, pentobarbital, pimozide and ethanolintake. Alcohol 3: 303-308, 1986. 13. Myers, W. D., K. T. Ng, S. Marzuki, R. D. Myers and G. Singer. Alteration of alcohol drinking in the rat by peripherally self-administered acetaldehyde. Alcohol 1: 229-236, 1984. 14. Pollerberg, G. E., T. Costa, G. T. Shearman, A. Herz and L. D. Reid. Opioid antinociception and positive reinforcement are mediated by different types of opioid receptors. Life Sci 33: 1549-1559, 1983. 15. Reid, L. D. Endogenous opioid peptides and regulation of drinking and feeding. Am J Clin Nutr 42:1099-1132, 1985. 16. Reid, L. D., S. A. Czirr, C. C. Bensinger, C. L. Hubbell and A. J. Volanth. Morphine and diprenorphine together potentiate intake of alcoholic beverages. Alcohol 4: 161-168, 1987.
CZIRR ET AL. 17. Reid, L. D. and G. A. Hunter. Morphine and naloxone modulate intake of ethanol. Alcohol 1: 33-37, 1984. 18. Samson, H. H. and T. F. Doyle. Oral ethanol selfadministration in the rat: Effect of naloxone. Pharmacol Biochem Behav 22: 91-99, 1985. 19. Sinclair, J. D., J. Adkins and S. Walker. Morphine-induced suppression of voluntary alcohol drinking in rats. Nature 246: 425-427, 1973. 20. Stewart, J., H. de Wit and R. Eikelboom. Role of unconditioned and conditioned drug effects in the self-administration of opiates and stimulants. Psychol Rev 91: 251-268, 1984. 21. Volpicelli, J. R., M. A. Davis and J. E. Olgin. Naltrexone blocks the post-shock increase of ethanol consumption. Life Sci 38: 841-847, 1986.
0741-8329/87 $3.00 + .00
Selected Opioids Modify Intake of Sweetened Ethanol Solution Among Female Rats STEPHANIE
A. CZIRR, CHRISTOPHER L. HUBBELL, WILLIAM J A Y M. F R A N K A N D L A R R Y D. R E I D 1
C. M I L A N O ,
Department o f Psychology, Rensselaer Polytechnic Institute, Troy, N Y 12180 R e c e i v e d 8 D e c e m b e r 1986 CZIRR, S. A., C. L. HUBBELL, W. C. MILANO, J. M. FRANK AND L. D. REID. Selected opioids modify intake of sweetened ethanol solution among female rats. ALCOHOL 4(3) 157-160, 1987.--Water-deprived female rats were given a daily, 1.5-hr opportunity to take either a sweetened ethanol solution or water. Across days, they increased their intake of ethanol solution and had stable intakes of about 2 g of pure ethanol/kg after 3 weeks. Morphine (I.0 mg/kg) alone, and in combination with diprenorphine (25/zg/kg), increased intake of ethanol solution among females similar to the increased intake seen with males under similar procedures. Fentanyl dose-relatedly increased intake of ethanol. The data strengthen the idea that one or more of the endogenous opioid systems, but not all, are involved with instances of "excessive" intake of alcoholic beverages. Ethanol intake
Female rats
Opioids
Diprenorphine
Fentanyl
Morphine
Ethanol Solution and Drugs
ON several occasions, it has been demonstrated that small doses of morphine (MOR) will increase rats' intake of ethanol solutions (ESs) [2, 7, 8, 16, 17]. This small dose MOR effect occurs under a variety of procedural variations including (a) housing conditions (grouped and individual), (b) deprivation conditions (deprived and not deprived of water), (c) day or night, and (d) the use of sweetened or unsweetened ESs [7]. Furthermore, the small dose MOR effect is persistent across many days of MOR injections [7,8]. The above mentioned studies all used male rats as the subjects. In the present study, therefore, we examined the effects of selected opioids on the intake of sweetened ES among females. Specifically, the effects of (a) MOR, (b) MOR in combination with diprenorphine (DIP), and (c) several doses of fentanyl (FEN) on ES intake were examined using female rats as subjects. F E N is a short acting, but highly potent, opioid analgesic [14]. DIP is an antagonist to many of M O R ' s effects including M O R ' s (and F E N ' s ) analgesia [14].
The ES was 5 g of sucrose and 12 g of pure ethanol (E) in 100 g of total fluid. MOR sulfate was administered at a dose of 1.0 mg/kg (measured as the salt). DIP hydrochloride was administered at a dose of 25/xg/kg (as base). F E N citrate was administered at doses of 5, 10, 20, and 40/zg/kg (as salt). Physiological saline, the vehicle of drugs, was administered as a placebo. Drugs and placebos w e r e administered by subcutaneous injections, in volumes of 1.0 ml/kg, 15 min before presentation of fluids.
Measures Fluids were presented in glass bottles equipped with ballpoint sipping tubes, which prevent substantial evaporation [13]. Bottles were weighed to the nearest 0.1 g before and after presentation of solutions. The resulting scores were corrected for spillage [7,13]. Each subject had daily measures of water and ES intake, as well as body weight. From these, further indices of rats' avidity toward E were derived, i.e., preference ratio (g of ES taken/total fluid intake) and g of pure E consumed per kg of body weight (g/kg). Since in these experiments, g of ES, preference ratio, and g/kg are highly correlated, only g/kg are reported.
METHOD
Subjects The subjects were 12 female, Sprague-Dawley rats weighing 150-200 g when purchased from Taconic Farms (Germantown, NY). They were individually housed in standard hanging cages in a windowless room maintained at 24°C. The colony room had 12 hr of artificial light each day beginning at 0900 hr. Food and water were always available prior to specific procedures.
Procedure After I week at the laboratory, the subjects began a schedule of 22.5 hr of water deprivation followed by concurrent
~Requests for reprints should be addressed to L. D. Reid.
157
CZIRR ET' AL.
158
Figure I depicts the effects of the various doses of FEN on E intake. The two smallest doses of F E N had no reliable effect on subjects' intake of E (ps>0.05). The two highest doses reliably increased the rats' intake of E Ips<0.05). FEN, at the doses tested, had no reliable effect on water intake (all ps>0.05).
1.50 0 r~ Q)
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DISCUSSION
0.75 1:3TM
t:~ 0 . 5 0 cO I1)
O.25 0 0
5
io
15
2o
Fentc]nyl
25
I
I
I
30
35
40
(microg/kg)
FIG. 1. The mean difference scores (drug minus placebo), across 4 doses of fentanyl (FEN), for grams of pure ethanol (E) taken per kilogram of body weight (glkg) by 12 female rats are depicted. The 5 and 10 ~g/kg doses had no reliable effect on intake of E, ps>0.05. The 20 ~g/kg dose of FEN reliably increased subjects' mean intake of E from 2.25 g/kg under placebo to 3.67 g/kg, t( 11)=4.06, p<0.002. The 40 ~g/kg dose of FEN also increased mean E intake from 2.31 g/kg under placebo to 3.77 g/kg, t(l 1)=3.65,/)<0.01. presentation of water and ES for 1.5 hr each day during the early part of the light cycle. Food was always available. This daily regimen continued for 39 days. Toward the end of these 39 days, an attempt was made to chart the rats' reproductive cycles by examining vaginal smears taken 0.5 hr before presenting bottles. Although studies using more sensitive measures might indicate a relationship between the female's reproductive cycle and E intake, we could discern no such relationship. All subjects received placebo injections on Day 40. On Day 41, all subjects received MOR injections. Subsequently, on Day 42, all subjects received 2 placebo injections. Concurrent injections of MOR and DIP were administered to all subjects on Day 43. The regimen continued for another 40 days, then all subjects received placebo injections, and on the following day, an injection of F E N . The regimen continued across the next 28 days with injections of a placebo followed by an injection of F E N occurring periodically until all doses of F E N were tested. RESULTS
As in previous studies using male rats (e.g., [7]), the females gradually increased their intake of ES across days, and after 3 weeks they were taking about 2 g/kg of E each day. They were healthy as indicated by their steady weight gain across days, an observation which is in concordance with previous studies with males (e.g., [7]). The data were assessed by way of Student's t-tests for dependent measures. Alone, MOR reliably decreased mean water intake from 13.7 g under placebo to 11.1 g, t (11) = 3.12, p<0.01. MOR increased mean E intake from 2.47 g/kg under placebo to 4.23 g/kg, t(11)=4.40, p<0.002. MOR and DIP given concurrently reliably increased mean water intake from 11.27 g under placebo to 14.71 g, t(11)=3.43, p<0.01. MOR plus DIP increased mean E intake from 1.94 g/kg under placebo to 2.54 g/kg, t(11)=2.31, p<0.05.
The small dose MOR effect is seen with females as well as males, a finding concordant with other tests showing that small doses of MOR increase intake of E across a wide array of procedural variables [2, 7, 8, 16, 17]. FEN also increases intakes at remarkably small doses with the minimally effective dose being on the order of 15/zg/kg. As shown recently, MOR plus DIP increased intakes of E [16]. It seems that one or more, but not all, opioid systems can modulate consumption of E. There are a number of procedural variables inherent to any one experiment. Sometimes these arbitrarily chosen variables are critical, and conclusions drawn from a single experiment do not generalize. The small dose MOR effect, however, transcends a wide array of procedural variables including sex of subjects. A consequential variable, however, is the length of the testing session; it has to be long enough for rats to have further opportunity to drink beyond what they would under placebo conditions. For rats, in these kind of situations, 30 min is too short, and 1.5 to 2 hr seems optimal [8]. Also, rats have to be taking some ES at baseline or under placebo before a small dose of MOR will increment intake. With these caveats, it seems fair to conclude that small doses of MOR and a number of related opioids, such as methadone [12] and FEN (this paper), increase intake of ES among rats. Recently, an idea that might be called the opioiddeficiency hypothesis has been advanced [21]. The hypothesis states there is " . . . an inverse relationship between ethanol preference and endorphin stimulation. When endorphin activity is stimulated by exogenous or endogenous opiates, voluntary ethanol consumption decreases and when a deficiency in opiate stimulation occurs (i.e., opiate withdrawal) ethanol drinking increases" ([21], p. 841). Or stated another way, " . . . one of the reinforcing mechanisms in ethanol drinking involves compensation for relative deficiencies in endorphin stimulation" ([21], p. 846). This hypothesis was advanced in an article supposedly showing that naltrexone blocks post-shock increases in E consumption. (We say supposedly showing because there was no control group in the experimental design to assess the effects of just the opportunity to continuing drinking with no shock, therefore, there is no way to know if the increased drinking was related to a history of having shock.) The gist of the article (in any case) is that antagonists, such as naltrexone or naloxone, reduce E intake as has been shown in other experiments [1, 3, 7, 17, 18]. On the surface, it appears that the data of the article itself [21] contradicts the hypothesis put forward in the introduction and discussion of the article [21]. Naltrexone, by blocking opioid receptors (opioceptors), should be the functional equivalent of reducing opioid systems activity, i.e., produce a relative deficiency in opiodergic (endorphin) stimulation. It follows, therefore, that administering naltrexone, in blocking doses, should, according to one interpretation of the hypothesis, increase intakes of alcoholic beverages. The data with antagonists (naloxone or naltrexone) suggest quite the op-
OPIOIDS A N D A L C O H O L
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posite: When there is a " d e f i c i e n c y " in activity usually instigated by occupation o f opioceptors, rats drink less. A basic assumption underlying the deficiency-hypothesis is that " . . . morphine administration suppresses voluntary ethanol drinking in r a t s . . . " ([21], p. 841). Large doses of MOR [19] or methadone [12], as well as other agonists [6], do decrease voluntary drinking of E. Smaller doses of MOR ([7, 8, 17], this paper, methadone [12] or F E N (this paper), however, increase intakes of ESs. Large doses also produce a number of behaviors incompatible with drinking (such as catatonia); so, it might be unrealistic to expect large doses to increment drinking, even though they may increase a preference for alcoholic beverage (i.e., they can reduce water drinking more than alcohol drinking) [2]. When, however, potentially depressing features of MOR are antagonized by DIP, MOR together with DIP increased intakes of ES ([16], this paper). Along the same lines, small doses, rather than large doses, are more apt to mimic physiological processes of endorphinergic systems. There is no reason to believe that any endogenous release of opioids would match the effectiveness of doses of MOR of up to 60 mg/kg used in some of the experiments [19] from which the conclusion [21] is drawn that MOR reduces ES intake. There is one other line of evidence that could support tlae deficiency hypothesis. It was concluded that withdrawal from the chronic effects of MOR enhances E intake. The evidence here, however, does not uniformly support the conclusion of the withdrawal state itself goading enhanced consumption. In a summary of relevant experiments of Ho et al. [6], for example, only some experiments indicated enhanced consumption. Furthermore, when enhanced consumption was observed, it was after withdrawal symptoms had abated. Perhaps, some days after acute withdrawal symptoms, endogenous opioid systems are as apt to be hyperactive as hypoactive. So, it is not so clear that withdrawal from opioids itself is an effective goad to enhance E intake. The facts that (a) small doses of MOR, methadone, and F E N increase intake of ES, (b) naloxone decreases intake, and (c) the concept that small doses of MOR are more akin to the activity following endogenous release of opioids, all lead to just the opposite conclusion than the opioid-deficiency hypothesis: It is not an opioid deficiency that leads to more intake, but rather an opioid surfeit that leads to more intake (as long as a toxic surfeit is not achieved). New data, as
happens often, does lead to radical revisions of some hypotheses. The data are, however, compatible with the general theoretical framework of Stewart et al. [20]. Most extant distinctions among types of opioid receptors (opioceptors) are based on in vitro assays. Since such assays are frequently done in a medium deviating remarkably from fluids bathing receptors in living individuals, and since such assays will produce different results and conclusions as the ions in the medium vary [10,11], it is difficult to make inferences concerning what putative types of opioceptors might be involved in the small-dose effects of agonists at opioceptors in living animals [15]. Furthermore, the doses of antagonists used in the typical study of laboratory subjects given a chance to drink ES are probably too large to produce any selectivity in terms of blocking one type of opioceptor over another. Consequently, it is probably premature to speculate on the type of opioceptors involved in the effects of small doses of agonists. Nevertheless, there are some limited conclusions that can be drawn. Small doses of MOR, methadone, and F E N all increase intake of sweetened alcoholic beverages among rats ([7, 8, 12, 17], this paper), and each drug has been labelled a mu selective agonist. Ethylketocyclazocine, a reputedly kappa selective agonist, does not produce marked increments in intake of sweetened ES [9]. One might, therefore, lean toward the conclusion that the small dose opioid effect is a product of activity at mu opioceptors rather than kappa opioceptors. There are, however, reasons for not accepting that conclusion presently. One, none of the drugs used is particularly selective for one type of opioceptor, especially at doses used in vivo. Two, such a distinction does not consider other putative types of opioceptors that might be extant. Three, as mentioned, there is some confusion concerning what exactly a mu opioceptor is functionally in vivo. If, however, one defines mu opioceptors as those modulating analgesia [5], then we can conclude that the small dose MOR effect is not related to mu opioceptors since MOR plus DIP, an antagonist to opioid analgesia, increments intake of ESs ([16], this paper). ACKNOWLEDGEMENTS We thank Jean Bestle and Betty Osganian for their clerical assistance. The research was supported, in part, by grant AA06212 from the National Institute on Alcohol Abuse and Alcoholism.
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