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Effect of spontaneous ingestion of ethanol on brain dopamine metabolism
Life Sciences, Vol. 44, pp. 281-287 Printed in the U.S.A.
Pergamon Press
EFFECT OF SP(RTANE0t~ I N ~ ' T I ( N OF ETHANOL ON BRAIN D(~AMINE ~-TABOT.T~
F. Fadda, E. Mosca, G. Colombo snd G.L. Gessa Department of Neurosciences, University of Cagliari, C~gliari, Italy
(Received in final form December i, 1988)
summary The effect of ethsnol, either administered by gavage or voluntarily ingested, on brain dopamine (DA) metabolism was s ~ l e d in alcohol-preferring and alcohol ncr~-preferring rats. In alcohol non-preferring rats ethanol administration (2 g/kg) increased 3,4-dih y d r o x y p ~ l a c e t i c acid (DOPAC) ~ h(m~v8~lillic acid (HVA) and ~ DA levels in the caudate nucleus and olfactory tubercle but was ineffective in the medial p r e f r o n t a l cortex. In alcohol-preferring rats e t b a ~ l effect was greater than in non-preferring srdmals and ethanol influenced DA metabolism also in the medial prefrontal cortex. The effect of voluntary ethanol ingestion was studied in alcohol-preferring rats trained to consume their daily fluid intake within 2 hrs. Voluntary ingestion of ethanol (3.1+0.7 g/kg in 1 hr) increased DA metabolites and rec~uced DA levels in the c ~ d a t e r~eleus, olfactory tubercle and medial prefrontal cortex. The results suggest that voluntary ethanol ingestion increases the release of DA from nigro-striatal and meso-li~bic DA neurons. Central dopamine (DA) containing neurons are considered to play 8n important role in the mediation of reward from various sources (i) including intra-cranial electric self-stimulation (2), drugs of abuse, such as
amphetamine ( 3 , 4 ) , cocaine
(5), morphine
( 6 , 7 ) , h e r o i n (8)
and even
natural
reinforcers, such as food and water (9,10). In particular, several lines of evidence suggest that dopaminergic neurons mediate the reinforcing properties of ethanol (11,12). Accordingly, Gessa et al. (13) have shown that ethanol activates the firing rate of DA c o n t a i n ~ neurons and that DA neurons in the ventral tegmental area (AIO), which innervate limbic areas and prefrontal cortex, are more sensitive to ethanol than DA neurcr~ located in the pars cc~pacta of the substsntia nigra (Ag), which innervate the basal ganglia (14). In agreement with these results, Imperato and Di Chiara (11,15), by using the transcerebral dialysis technique, in freely moving rats, found that 0024-3205/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc
282
DA Metabolism and Voluntary Ethanol Intake
Vol. 44, No. 4, 1989
ethanol stimulates DA release to a greater degree in the nucleus accumbens ths~ it does in the cmadate nucleus. An important tool for understanding the neurochemlcal correlates of alcoholism is offered by lines of rats, that preferentially drink e t h a ~ l solution rather than water or vice versa. By selective breeding we have produced two of such lines, one of which in a free choice condition, preferentially consumes iO~ ethanol solution and the other that almost totally avoids it. The purpose of this investigation was: a) to compare the effect of e ~ l administration on brain DA metabolism in rats selected for their preference or non preference for ethsnol, b) To clarify the effect of s p c n ~ ingestion of ethanol on brain DA metabolism in ethanol-preferring rats. Materials and methods Two lines of rats one alcohol preferring snd the other alcohol rg~-preferring, were developed in our laboratory through selective breeding from Wistar rats (Charles River, Ccmo, Italy) according to the method of Lumeng et al. (16). To distinguish our newly developed lines from those developed in Li's laboratory (17) at Indisnapolis we have termed our rats by the symbols sP and sNP, meaning Sardinian alcohol-preferring and non-preferring. In the present study we have utilized the male rats from the 13th generation. Starting from the 3Oth day of age, rats were i n d i v i ~ l ly housed in a room at 22 ° C and 60~ relative humidity, having reversed light-dark cycle with light phase from 20.00 to 8.00 and dim red light from 8.00 to 20.00 hours. Rats had free access to standard laboratory food, tap water and i0~ ethanol solution in water (w/v). Fluids were offered according to the two bottles free choice pro~ecka-e (18). Within 3 weeks of isolation, rats showed a stable baseline of ethanol or water consumption. Experiment a) Ethanol administration Two groups of 24 rats each, one sP and the other of sNP, were used. The animals were approximately 60 days old and consumed 7.6+0.6 and I. 2+0.3 g/kg of ethanol daily, respectively. Etb2nol was wit/xlra~ for 48 hrs and then half of the rats of each group received by gavage 2 g/kg e t h a ~ l as i0~ solution in water, whereas the other half received by gavage the corresponding volume of water. Experiment b) V o l m t a r y ethanol ingestion One group of 24 sP rats, of approximately 60 days of age, was habituated to drink its daily fluid in a restricted period of 2 hrs, by presenting the two bottles, one containing water and the other 10% ethsnol solution, at i0.00 and removing them at 12.00, during the dark phase. Food was always available at
Vol. 44, No. 4, 1989
DA Metabolism and Voluntary Ethanol Intake
283
libitum. Within 2 weeks of habituation srdmals reached a stable baseline intake of about 90 m_I/kg fluid and of 5.1+0.4 g/kg ethanol wihhin the two-hours period. On the 15th day of such--schedule animals were divided in two s u b s r ~ s : to the first one the two bottles were presented as usually, to the other only water was offered instead. Animals were sacrificed 60 min after fluid presentation, during ~i%ich time they had drank most of their daily fluid intake. For brain snalyses c ~ t e r~cleus, medial preflx~tal cortex (MPFC) and olfactory tubercle were dissected on ice and stored at -
analysis of variance.
Results Ethanol administration to sP and sNP rats. As fig. 1 shows, ethanol administration to sNP rats signiflcaltly increased IX)PAD snd H A levels both in the csudate nucleus and in the olfactory tubercle snd produced a modest decrease in DA content in these areas. In these rats ethanol failed to modify DA metabolism in the MPFC. As fig. 1 shows, ethanol influenced DA metabolism to a greater degree in sP than in sNP rats. In fact in sP rats, ethsnol increased [X3PAD by 80, 88 and 30 % in the caudate nucleus, olfactory tubercle and ~ F C , respectively. In the same areas, HVA concentration ~ s ~ by 64, 72 stud 25 ~, respectively. Vice versa, DA concentration was decreased by 30, 35 and 24%, respectively. Voluntary ethanol ingestion by sP rats. The effect of ethsmol, voluntarily by sP rats, on DA metabolism was studied. As specified in the methods, these animals were sacrificed one hour sLft~r presentation of the e ~ l solution. Within this period they ~ an average of 3.1+0.7 ~ of ethanol. Fig. 2 shows that s p o n ~ ethsnol ingestion increased DOPAC levels by 38, 75 and 25 % in the csudate nucleus, olfactory tubercle and MPFC respectively. In the sane areas, HVA content w~s increased by 30, 70 and 20 % , respectively. DA ccr~entratlon was decreased by 20, 28 and 12 ~, respectively.
284
DA Metabolism and Voluntary Ethanol Intake
Vol. 44, No. 4, 1989
l ]OA ~'/~DOPAC ~HVA
ON
200,
OT
MPFC
÷
÷
÷
< @
O
*
@
i50
÷
< W v:
rr
iO0
I.( 0
50
0 sNP
SP
sNP
sP
sNP
sP
Fig. i Effect of ethanol administration (2 g/kg) on DA, DOPAC and HVA concentraticr~ in different brain areas in alcohol-preferring (sP) and alcohol ncn-preferrlng (SNP) r a t s . E t h s ~ l (20~ s o l u t i o n i n w a t e r , w/v) o r w a t e r was g i v e n o r a l l y by garage. Rats were sacrificed 60 rain after treatment. Results are the mesh + S.E.M. of data of three independent experiments, in each of which 4 rats per group were used (N = 12). Values are expressed as per cent of values obtained from water-treated rats. Values of water-treated sP and sNP rats were not statistically different therefore they were pooled. Values for DA in the csm,date nucleus (CN), medial prefrontal cortex (MPFC) s~h olfactory tubercle (OT) were (~g/g wet tissue): 11,815 + 702, 81 + 4 and 7326 + 39, respectively. In the sane areas DOPAC concentrations were 1436+86 , 33 __+ 2 snd 1794 + 88; HVA osncentrations were 964 _+ 51, 25 + 4 and 582 + 4~, respectively. * P<0.OI with respect to water-treated rats; + P~O.05 with respect to sNP rats receiving ethanol.
Vol. 44, No. 4, 1989
DA Metabolism and Voluntary Ethanol Intake
285
V--IDA
DOPAC Hv^
200
UJ D J < >
J 0 II IZ 0 U LL 0
CN
MPFC
OT
~.50 o --
iO0
e
O i
,T
T
50
Fig. 2 Effect o f voluntary ethanol consumption on DA, DOPAC and HVA levels in different brain areas, sP rats were ~ t e d to drink their daily fluid, i.e. i0~ ethanol or water solution, in a restrdcted time interval of 2 hrs (see methods). They were sacrificed 60 rain after fluid presentation. Within this time period they consumed an average of 3.1+0.7 g e ~ i / k g b.w. Values are means + S.E.M. obtained from three experiments each of mbich comprised 4 animals per groLp (n = 12). Results are expressed as per cent of values obtained from sP rats ~hich drsnk water instead of ethsnol solution. In v~atem-drinking alimals DA values (ng/g wet tissue) in the csudate nucleus (C~), medial prefrontal cortex (MPFC) and olfactory tubercle (OT) were 11,602+--724, and 6925~_189, respectively. In the sane areas Df~AC values were 1506188_, 31+__2 and 1602_+34; HVA values were 856~4_9, 2_6~ and 732+_54, respectively. * P
286
DA Metabolism and Voluntary Ethanol Intake
Vol. 44, No. 4, 1989
Blood samples collected from trunk blood, 60 rain after treabBents indicated BACs of 195+16 and 219+14 mg ~6 in sP and sNP rats, respectively, receiving ethanol by g - ~ , and 2~i+56 mg % in sP rats after voluntary e~hanol consumption. Discussion The present results confirm previous observations bhat etJmr~l administration increases brain DA metabolism (12,21,22). However, the two major findings in the present work are that ethanol modifies brain DA metabolism to a greater degree in sP than in sNP rats and that voluntary ethanol ingestion is associated with a marked increase in DA metabolism in different brain areas. The different effect of e ~ l in sP and sNP rats does not depend on phazmacokinetic differences, being the BACs in the 2 lines of animals not statistically different. O n the other hand, the greater effectiveness of ethanol in sP rats might reflect either a genetic trait, possibly linked to ethanol preference, or it might be the consequence of the fact that sP rats had consumed ethanol chronically prior to the ethanol challenge (18). If the second b~sothesis is correct, our finding might imply that chronic ethanol consumption p ~ e s a sensitization of DA neurons to the action of ethanol. In order to clarify this problem we are presently studing the effect of ethanol on DA metabolism in rats of sP and sNP strains that never experienced ethalol. Voluntary ethsnol ingestion produced similar ~ in brsln DA metabolism as after the oral administration of the drug by garage. Since the dose voluntarily ingested by our rats was within the rsm~e 12%at may be by human alcoholics it is likely that our findings have clinical relevance. As to the mechanism by which ethanol increases DOPAC and HVA levels, it is likely that this effect is bhe consequence of e ~ l - l n d u c e d release of DA from nerve terminals 8rid of the ~ retrieval and metabolism of the amine (18). In agreement with such a proposition is the finding that DOPAC and EVA acoxm~lation was associated with a modest depletion of DA content, also suggesting that DA synthesis rate is insufTicient to cope with e t h s m o l - ~ release and degr~wiqtion of DA. Accordingly, chronic ethanol administration has been shov~ to produce tolerance to its stimulant effect on DA synthesis (18). The dose of ethanol administered to our rats was inapproprlate for cMmonstrating a preferential action on limbic areas versus the c a x ~ t e nucleus, but the dose was chosen to be similar to that rats obtain through voluntary ethsnol consumption.
Considering that d o p ~ r g i c ~urons seem to p l ~ an important role i n brain reward mechs~isms ( i ) , the f i n d i r ~ that voluntary ethanol consumption increases DA metabolism and that such effect persists after chronic ethanol consumption might be r e l e v ~ t i n e x p l a i n i ~ the blological basis of ethanol
depon0ence.
Vol. 44, No. 4, 1989
DA Metabolism and Voluntary Ethanol Intake
287
Acknowledgements This work was partially s%pported by C.T. Laboratories, ~ , The s~thors wish to thsnk Mr.P.SuffiottJ for his technical assistance.
Italy.
References
i . R.A. WISE, I n : Theory i n P B y e h o p ~ o i o g y : S . J . Cooper e d . , pp. 103-122, A c ~ c Press, New York (1981). 2. G. FOL~T~7OS and R.A. WISE, Brain P e s . , 103, 377-380 (1980). 3. M. LE MOAL, L. STI/~/S and H. SIMON, Nature, 280, 156-158 (1979). 4. W.H. LYNESS, N.M. FRTRNLEand K.E. MOORE, Pharmacol. Bioohem. Behav., 11, 553-556 (1979). 5. D.C.S. ROBERTS and G.F. K00B, Pharmacol. Biochem. Behav, 17, 901-904
(1983). 6.' K. GYBLING and R.Y. WANG, Brain Res., 227, 119-127 (1983). 7. R.T. MATTHEWS and D.C. G~L%N, Neuroscience, Ii, 617-625 (1984). 8. M.A. BOZARTH 8rid R.A. WISE, Life Sci., 29, 1881-1886 (1981). 9. R.A. WISE, J. SPINDLER, H. DE WIT and G.J. GERBER, Science, 201,
262-264 (1978). i0. R.A. WISE, J. SPINDLER and L. LEGAULT, Can. J. Psyohol., 32, 77-85
(1978). ii. G. DI CHIARA and A. IMPERATO, Eur. J. Pharmaeol., 115, 131-132 (1985). 12. F. FADDA and G.L. GESSA, In: Progress in Alcohol Research, Parvez et al. eds., pp. 147-161, VNU Science Press, Utrecht (1985). 13. G.L. GESSA, F. ~ , M. OOLI//, L. VA/~IU and G. MER~J, Brain Res., 348, 2 0 1 - ~ 3 (1985). 14. G. MEREU, F. FADDA and G.L. GESSA, Brain Res., 292, 63-69 (1984). 15. A. IMPERAT0 and G. DI C}{IARA, J. Phaxmaeol. Exp. Ther., 239, 219-228
(1986). 16. L. ~ , T.D. ~ and T.K. LI, In: Alcohol and Aldehyde metabolizing system, Tnurma% Williamson, Drott and Chance eds., pp. 537-544, Ac~d~nic Press (1985). 17. T.K. LI, L. U24Ek~, W.J. Me BRIDE and J.M. MURPHY, In: Advances in Biomedical Alcohol Research, Lindros, Ylikahri and Kiiarmma eds., pp. 91-96, Pergamon Press (1987). 18. F. FADDA, A. ARGIOLAS, M.R. M~I.TS, G. SERRA and G.L. GESSA, Life SCi., 27, 979-986 (1980). 19. Z.L. ROSSETTI, G. M ~ X ] R 0 ~nd C.A. RIVAN0, Life SCi., 33, 2287-2387
(1983). 20. G. IX]RITZ and E.B. TRUITT, Q.J. Stud. Alcohol., 25, 498-510 (1964). 21. A. CARLSSON, T. ~ , T.H. ~ and B. ~LDEC~, P s y c h o logy, 30, 27-36 (1973). 22. F. KAROUM, R.J. WYATT and E. MAJCHROWICZ, Br. J. FnarBmeol., 56,
403-411 (1976).
Pergamon Press
EFFECT OF SP(RTANE0t~ I N ~ ' T I ( N OF ETHANOL ON BRAIN D(~AMINE ~-TABOT.T~
F. Fadda, E. Mosca, G. Colombo snd G.L. Gessa Department of Neurosciences, University of Cagliari, C~gliari, Italy
(Received in final form December i, 1988)
summary The effect of ethsnol, either administered by gavage or voluntarily ingested, on brain dopamine (DA) metabolism was s ~ l e d in alcohol-preferring and alcohol ncr~-preferring rats. In alcohol non-preferring rats ethanol administration (2 g/kg) increased 3,4-dih y d r o x y p ~ l a c e t i c acid (DOPAC) ~ h(m~v8~lillic acid (HVA) and ~ DA levels in the caudate nucleus and olfactory tubercle but was ineffective in the medial p r e f r o n t a l cortex. In alcohol-preferring rats e t b a ~ l effect was greater than in non-preferring srdmals and ethanol influenced DA metabolism also in the medial prefrontal cortex. The effect of voluntary ethanol ingestion was studied in alcohol-preferring rats trained to consume their daily fluid intake within 2 hrs. Voluntary ingestion of ethanol (3.1+0.7 g/kg in 1 hr) increased DA metabolites and rec~uced DA levels in the c ~ d a t e r~eleus, olfactory tubercle and medial prefrontal cortex. The results suggest that voluntary ethanol ingestion increases the release of DA from nigro-striatal and meso-li~bic DA neurons. Central dopamine (DA) containing neurons are considered to play 8n important role in the mediation of reward from various sources (i) including intra-cranial electric self-stimulation (2), drugs of abuse, such as
amphetamine ( 3 , 4 ) , cocaine
(5), morphine
( 6 , 7 ) , h e r o i n (8)
and even
natural
reinforcers, such as food and water (9,10). In particular, several lines of evidence suggest that dopaminergic neurons mediate the reinforcing properties of ethanol (11,12). Accordingly, Gessa et al. (13) have shown that ethanol activates the firing rate of DA c o n t a i n ~ neurons and that DA neurons in the ventral tegmental area (AIO), which innervate limbic areas and prefrontal cortex, are more sensitive to ethanol than DA neurcr~ located in the pars cc~pacta of the substsntia nigra (Ag), which innervate the basal ganglia (14). In agreement with these results, Imperato and Di Chiara (11,15), by using the transcerebral dialysis technique, in freely moving rats, found that 0024-3205/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc
282
DA Metabolism and Voluntary Ethanol Intake
Vol. 44, No. 4, 1989
ethanol stimulates DA release to a greater degree in the nucleus accumbens ths~ it does in the cmadate nucleus. An important tool for understanding the neurochemlcal correlates of alcoholism is offered by lines of rats, that preferentially drink e t h a ~ l solution rather than water or vice versa. By selective breeding we have produced two of such lines, one of which in a free choice condition, preferentially consumes iO~ ethanol solution and the other that almost totally avoids it. The purpose of this investigation was: a) to compare the effect of e ~ l administration on brain DA metabolism in rats selected for their preference or non preference for ethsnol, b) To clarify the effect of s p c n ~ ingestion of ethanol on brain DA metabolism in ethanol-preferring rats. Materials and methods Two lines of rats one alcohol preferring snd the other alcohol rg~-preferring, were developed in our laboratory through selective breeding from Wistar rats (Charles River, Ccmo, Italy) according to the method of Lumeng et al. (16). To distinguish our newly developed lines from those developed in Li's laboratory (17) at Indisnapolis we have termed our rats by the symbols sP and sNP, meaning Sardinian alcohol-preferring and non-preferring. In the present study we have utilized the male rats from the 13th generation. Starting from the 3Oth day of age, rats were i n d i v i ~ l ly housed in a room at 22 ° C and 60~ relative humidity, having reversed light-dark cycle with light phase from 20.00 to 8.00 and dim red light from 8.00 to 20.00 hours. Rats had free access to standard laboratory food, tap water and i0~ ethanol solution in water (w/v). Fluids were offered according to the two bottles free choice pro~ecka-e (18). Within 3 weeks of isolation, rats showed a stable baseline of ethanol or water consumption. Experiment a) Ethanol administration Two groups of 24 rats each, one sP and the other of sNP, were used. The animals were approximately 60 days old and consumed 7.6+0.6 and I. 2+0.3 g/kg of ethanol daily, respectively. Etb2nol was wit/xlra~ for 48 hrs and then half of the rats of each group received by gavage 2 g/kg e t h a ~ l as i0~ solution in water, whereas the other half received by gavage the corresponding volume of water. Experiment b) V o l m t a r y ethanol ingestion One group of 24 sP rats, of approximately 60 days of age, was habituated to drink its daily fluid in a restricted period of 2 hrs, by presenting the two bottles, one containing water and the other 10% ethsnol solution, at i0.00 and removing them at 12.00, during the dark phase. Food was always available at
Vol. 44, No. 4, 1989
DA Metabolism and Voluntary Ethanol Intake
283
libitum. Within 2 weeks of habituation srdmals reached a stable baseline intake of about 90 m_I/kg fluid and of 5.1+0.4 g/kg ethanol wihhin the two-hours period. On the 15th day of such--schedule animals were divided in two s u b s r ~ s : to the first one the two bottles were presented as usually, to the other only water was offered instead. Animals were sacrificed 60 min after fluid presentation, during ~i%ich time they had drank most of their daily fluid intake. For brain snalyses c ~ t e r~cleus, medial preflx~tal cortex (MPFC) and olfactory tubercle were dissected on ice and stored at -
analysis of variance.
Results Ethanol administration to sP and sNP rats. As fig. 1 shows, ethanol administration to sNP rats signiflcaltly increased IX)PAD snd H A levels both in the csudate nucleus and in the olfactory tubercle snd produced a modest decrease in DA content in these areas. In these rats ethanol failed to modify DA metabolism in the MPFC. As fig. 1 shows, ethanol influenced DA metabolism to a greater degree in sP than in sNP rats. In fact in sP rats, ethsnol increased [X3PAD by 80, 88 and 30 % in the caudate nucleus, olfactory tubercle and ~ F C , respectively. In the same areas, HVA concentration ~ s ~ by 64, 72 stud 25 ~, respectively. Vice versa, DA concentration was decreased by 30, 35 and 24%, respectively. Voluntary ethanol ingestion by sP rats. The effect of ethsmol, voluntarily by sP rats, on DA metabolism was studied. As specified in the methods, these animals were sacrificed one hour sLft~r presentation of the e ~ l solution. Within this period they ~ an average of 3.1+0.7 ~ of ethanol. Fig. 2 shows that s p o n ~ ethsnol ingestion increased DOPAC levels by 38, 75 and 25 % in the csudate nucleus, olfactory tubercle and MPFC respectively. In the sane areas, HVA content w~s increased by 30, 70 and 20 % , respectively. DA ccr~entratlon was decreased by 20, 28 and 12 ~, respectively.
284
DA Metabolism and Voluntary Ethanol Intake
Vol. 44, No. 4, 1989
l ]OA ~'/~DOPAC ~HVA
ON
200,
OT
MPFC
÷
÷
÷
< @
O
*
@
i50
÷
< W v:
rr
iO0
I.( 0
50
0 sNP
SP
sNP
sP
sNP
sP
Fig. i Effect of ethanol administration (2 g/kg) on DA, DOPAC and HVA concentraticr~ in different brain areas in alcohol-preferring (sP) and alcohol ncn-preferrlng (SNP) r a t s . E t h s ~ l (20~ s o l u t i o n i n w a t e r , w/v) o r w a t e r was g i v e n o r a l l y by garage. Rats were sacrificed 60 rain after treatment. Results are the mesh + S.E.M. of data of three independent experiments, in each of which 4 rats per group were used (N = 12). Values are expressed as per cent of values obtained from water-treated rats. Values of water-treated sP and sNP rats were not statistically different therefore they were pooled. Values for DA in the csm,date nucleus (CN), medial prefrontal cortex (MPFC) s~h olfactory tubercle (OT) were (~g/g wet tissue): 11,815 + 702, 81 + 4 and 7326 + 39, respectively. In the sane areas DOPAC concentrations were 1436+86 , 33 __+ 2 snd 1794 + 88; HVA osncentrations were 964 _+ 51, 25 + 4 and 582 + 4~, respectively. * P<0.OI with respect to water-treated rats; + P~O.05 with respect to sNP rats receiving ethanol.
Vol. 44, No. 4, 1989
DA Metabolism and Voluntary Ethanol Intake
285
V--IDA
DOPAC Hv^
200
UJ D J < >
J 0 II IZ 0 U LL 0
CN
MPFC
OT
~.50 o --
iO0
e
O i
,T
T
50
Fig. 2 Effect o f voluntary ethanol consumption on DA, DOPAC and HVA levels in different brain areas, sP rats were ~ t e d to drink their daily fluid, i.e. i0~ ethanol or water solution, in a restrdcted time interval of 2 hrs (see methods). They were sacrificed 60 rain after fluid presentation. Within this time period they consumed an average of 3.1+0.7 g e ~ i / k g b.w. Values are means + S.E.M. obtained from three experiments each of mbich comprised 4 animals per groLp (n = 12). Results are expressed as per cent of values obtained from sP rats ~hich drsnk water instead of ethsnol solution. In v~atem-drinking alimals DA values (ng/g wet tissue) in the csudate nucleus (C~), medial prefrontal cortex (MPFC) and olfactory tubercle (OT) were 11,602+--724, and 6925~_189, respectively. In the sane areas Df~AC values were 1506188_, 31+__2 and 1602_+34; HVA values were 856~4_9, 2_6~ and 732+_54, respectively. * P
286
DA Metabolism and Voluntary Ethanol Intake
Vol. 44, No. 4, 1989
Blood samples collected from trunk blood, 60 rain after treabBents indicated BACs of 195+16 and 219+14 mg ~6 in sP and sNP rats, respectively, receiving ethanol by g - ~ , and 2~i+56 mg % in sP rats after voluntary e~hanol consumption. Discussion The present results confirm previous observations bhat etJmr~l administration increases brain DA metabolism (12,21,22). However, the two major findings in the present work are that ethanol modifies brain DA metabolism to a greater degree in sP than in sNP rats and that voluntary ethanol ingestion is associated with a marked increase in DA metabolism in different brain areas. The different effect of e ~ l in sP and sNP rats does not depend on phazmacokinetic differences, being the BACs in the 2 lines of animals not statistically different. O n the other hand, the greater effectiveness of ethanol in sP rats might reflect either a genetic trait, possibly linked to ethanol preference, or it might be the consequence of the fact that sP rats had consumed ethanol chronically prior to the ethanol challenge (18). If the second b~sothesis is correct, our finding might imply that chronic ethanol consumption p ~ e s a sensitization of DA neurons to the action of ethanol. In order to clarify this problem we are presently studing the effect of ethanol on DA metabolism in rats of sP and sNP strains that never experienced ethalol. Voluntary ethsnol ingestion produced similar ~ in brsln DA metabolism as after the oral administration of the drug by garage. Since the dose voluntarily ingested by our rats was within the rsm~e 12%at may be by human alcoholics it is likely that our findings have clinical relevance. As to the mechanism by which ethanol increases DOPAC and HVA levels, it is likely that this effect is bhe consequence of e ~ l - l n d u c e d release of DA from nerve terminals 8rid of the ~ retrieval and metabolism of the amine (18). In agreement with such a proposition is the finding that DOPAC and EVA acoxm~lation was associated with a modest depletion of DA content, also suggesting that DA synthesis rate is insufTicient to cope with e t h s m o l - ~ release and degr~wiqtion of DA. Accordingly, chronic ethanol administration has been shov~ to produce tolerance to its stimulant effect on DA synthesis (18). The dose of ethanol administered to our rats was inapproprlate for cMmonstrating a preferential action on limbic areas versus the c a x ~ t e nucleus, but the dose was chosen to be similar to that rats obtain through voluntary ethsnol consumption.
Considering that d o p ~ r g i c ~urons seem to p l ~ an important role i n brain reward mechs~isms ( i ) , the f i n d i r ~ that voluntary ethanol consumption increases DA metabolism and that such effect persists after chronic ethanol consumption might be r e l e v ~ t i n e x p l a i n i ~ the blological basis of ethanol
depon0ence.
Vol. 44, No. 4, 1989
DA Metabolism and Voluntary Ethanol Intake
287
Acknowledgements This work was partially s%pported by C.T. Laboratories, ~ , The s~thors wish to thsnk Mr.P.SuffiottJ for his technical assistance.
Italy.
References
i . R.A. WISE, I n : Theory i n P B y e h o p ~ o i o g y : S . J . Cooper e d . , pp. 103-122, A c ~ c Press, New York (1981). 2. G. FOL~T~7OS and R.A. WISE, Brain P e s . , 103, 377-380 (1980). 3. M. LE MOAL, L. STI/~/S and H. SIMON, Nature, 280, 156-158 (1979). 4. W.H. LYNESS, N.M. FRTRNLEand K.E. MOORE, Pharmacol. Bioohem. Behav., 11, 553-556 (1979). 5. D.C.S. ROBERTS and G.F. K00B, Pharmacol. Biochem. Behav, 17, 901-904
(1983). 6.' K. GYBLING and R.Y. WANG, Brain Res., 227, 119-127 (1983). 7. R.T. MATTHEWS and D.C. G~L%N, Neuroscience, Ii, 617-625 (1984). 8. M.A. BOZARTH 8rid R.A. WISE, Life Sci., 29, 1881-1886 (1981). 9. R.A. WISE, J. SPINDLER, H. DE WIT and G.J. GERBER, Science, 201,
262-264 (1978). i0. R.A. WISE, J. SPINDLER and L. LEGAULT, Can. J. Psyohol., 32, 77-85
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