1 Neuroscienze S.c.a r.l., Via Palabanda 9, I-09123 Cagliari,
2 Bernard B. Brodie Department of Neuroscience, University of Cagliari, S.S. 554, Km. 4.5, I-09042 Monserrato (CA) and
3 C.N.R. Institute of Neurogenetics and Neuropharmacology, S.S. 554, Km. 4.5, I-09042 Monserrato (CA), Italy
Received 20 April 2001; in revised form 13 July 2001; accepted 16 August 2001
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ABSTRACT |
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INTRODUCTION |
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The high degree of similarity between the pharmacological profiles of GHB and ethanol intrinsically supports the possibility that GHB, like ethanol, can be abused in humans and produce positive reinforcing properties in laboratory animals. Accordingly, an increasing number of clinical observations have reported that GHB is used as a recreational drug, because of its capability, as the dose is increased, to produce feelings of euphoria, disinhibition, anxiolysis, relaxation and hypnosis (Galloway et al., 2000), i.e. a constellation of effects often described as closely resembling those of ethanol (Galloway et al., 1997
). Furthermore, GHB has been reported to induce conditioned place preference (Martellotta et al., 1997
) and be orally and intravenously self-administered in rats and mice (Colombo et al., 1995a
; Martellotta et al., 1998
). Interestingly, Sardinian alcohol-preferring (sP) rats, selectively bred for high ethanol preference and consumption, self-administered larger quantities of, and exhibited higher preference for, a GHB solution, than their alcohol-avoiding counterpart [namely, Sardinian alcohol non-preferring (sNP) rats] (Colombo et al., 1998
), suggesting that, in this rat line, GHB may exert reinforcing properties similar to those of ethanol. Consistent with the postulated reciprocal substitutability of GHB and ethanol, acute administration of ethanol markedly reduces voluntary GHB intake in GHB-consuming sP rats (Colombo and Gessa, 2000
).
In the wake of these results, the present study was designed to compare the reinforcing properties of GHB and ethanol, measuring the propensity of sP rats to consume GHB and/or ethanol when both drugs are concurrently available. In order to evaluate the possible influence of a prior exposure to ethanol, the present study included two separate groups of sP rats, ethanol-naive and ethanol-experienced, at the time of GHB presentation.
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MATERIALS AND METHODS |
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Procedure
Initially (Phase 1), eight rats (termed ethanol-naive) had continuous access solely to water for 14 consecutive days. Conversely, nine rats (termed ethanol-experienced) were given ethanol and water under the two-bottle free-choice regimen with unlimited access for 24 h/day. Specifically, rats were offered two graduated bottles, containing 10% (v/v) ethanol, and water.
Subsequently, both ethanol-naive and ethanol-experienced rats were forced to consume 1% (w/v) GHB [sodium salt; Laboratorio Farmaceutico C.T., Sanremo (IM), Italy] dissolved in water, as the sole drinking fluid available, for 14 consecutive days (Phase 2). This initial no-choice phase proved necessary, because the unpleasant taste of the GHB solution would hamper rats in a free-choice regimen to discover the reinforcing properties of the drug.
Immediately after the forced drinking period, rats were moved to the GHB vs water free-choice regimen (Phase 3). Two graduated bottles, containing water and the GHB solution, respectively, were continuously offered to ethanol-naive and ethanol-experienced rats. The concentration of the GHB solution was increased every 7 days from 1 to 2, 3, 4 and 6% (w/v); no interruption was interposed between each 7-day period.
All rats were then offered a free choice between 10% (v/v) ethanol, a GHB solution at the preferred concentration (see below) and water with unlimited access for 14 consecutive days (Phase 4). The preferred concentration of GHB was calculated individually for each rat from the data recorded in the 16% preference tests (Phase 3). Specifically, each value was based on the concentration at which each individual rat consumed the highest amount of GHB (in mg/kg) over the 7-day period during which each concentration was presented.
During all phases, fluid intakes were monitored daily. Bottles were refilled every day with fresh solution and their position changed daily on a random basis.
Data analyses
Data on daily ethanol and GHB intakes in ethanol-naive and ethanol-experienced rats, in Phases 2 and 4, as well as for each 7-day period with a specific GHB concentration in Phase 3, were compared by means of a two-way analysis of variance (rat groups x days).
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RESULTS |
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During the 14-day no-choice period (Phase 2), daily GHB intake occurred at pharmacologically relevant doses (Colombo et al., 1995a) in both ethanol-naive and ethanol-experienced rats, averaging 755 ± 7 and 753 ± 11 mg/kg, respectively [F(1,195) = 0.0031, P = 0.9563].
When GHB was offered in free choice with water (Phase 3), all rats showed days of preference for the GHB solution over water and alternate periods of high daily intake of GHB with temporarily self-imposed avoidance of the GHB solution. Daily GHB intake did not differ significantly between ethanol-naive and ethanol-experienced rats during any of the 7-day periods with a specific concentration of GHB [1%: F(1,90) = 0.9733, P = 0.2792; 2%: F(1,90) = 0.1879, P = 0.6709; 3%: F(1,90) = 0.0033, P = 0.9546; 4%: F(1,90) = 0.0026, P = 0.9570; 6%: F(1,90) = 0.0063, P = 0.9376]. The highest average daily GHB intake was recorded at the concentration of 2% in one ethanol-naive rat, 3% in one ethanol-naive rat, and 4% in six ethanol-naive rats, and in all the ethanol-experienced rats. When data from the preferred concentration of each rat were examined, daily GHB intake averaged 462 ± 29 and 390 ± 24 mg/kg in ethanol-naive and ethanol-experienced rats, respectively, with no significant group differences [F(1,90) = 1.3032, P = 0.2715].
In Phase 4 (when ethanol, GHB and water were concurrently presented), ethanol intake in ethanol-experienced rats immediately resumed to the intake levels monitored in Phase 1 (Fig. 1, lower panel). In ethanol-naive rats, average ethanol intake was >4 g/kg from the first day of exposure and rose to the 5.56 g/kg range over the first 7 days (Fig. 1
, upper panel). Daily ethanol intake did not diverge significantly between ethanol-naive and ethanol-experienced rats [F(1,195) = 2.3063, P = 0.1496]. Co-presentation of ethanol resulted in an immediate decline, by
75%, in daily GHB intake in both ethanol-naive and ethanol-experienced rats; indeed, over the 14 days of Phase 4, GHB intake averaged 126 ± 14 and 93 ± 10 mg/kg in ethanol-naive and ethanol-experienced rats, respectively (Fig. 1
), with no significant group differences [F(1,195) = 1.3633, P = 0.2612]. Only two and three rats in ethanol-naive and ethanol-experienced groups respectively, consumed occasionally amounts of GHB higher than the average consumption recorded at the preferred concentration.
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DISCUSSION |
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The possible influence of taste (pre-ingestive) factors on the outcome of the present study cannot at present be completely ruled out. Although both ethanol and GHB intakes in sP rats are presumably driven by the psychopharmacological effects of the 2 drugs, appropriate studies using the intragastric self-administration are needed to elucidate this point.
In some contrast with the guiding hypothesis of the present work, i.e. the reciprocal substitutability of GHB and ethanol (Colombo and Gessa, 2000), intake of GHB did not result in a proportional decline in ethanol consumption. Indeed, in the few rats exhibiting a pharmacologically relevant intake of GHB (>500 mg/kg/day), daily ethanol intake was not affected by the concomitant high intake of GHB. For example, rat no. 20 from the ethanol-naive group on day 13 of Phase 4 consumed the usual amount of ethanol (5.5 g/kg/day), although it also self-administered a dose of GHB (1182 mg/kg/day) expected to produce robust neuropharmacological effects. Apparently, the central effects of ethanol that maintain ethanol drinking behaviour in sP rats are not closely reproduced by those elicited by GHB.
The negligible intake of GHB monitored in sP rats having concurrent access to ethanol may reproduce the low abuse liability of GHB observed in human alcoholics. Indeed, in some contrast with the increasing number of episodes of recreational use of GHB outside a therapeutic protocol (Galloway et al., 2000), self-directed intake of GHB appears to be a limited phenomenon among alcoholics: the percentage of alcoholics increasing voluntarily the dose of GHB over the recommended amount varied between 10 and 15 in the two studies addressing this issue (Addolorato et al., 1996
; Gallimberti et al., 2000
). The majority of these episodes were interpreted as an attempt to adjust the therapeutic dose, rather than the search for certain psychotropic effects of the drug; accordingly, no cases of abuse have been reported after a proper fractioning of the daily dose (Addolorato et al., 1998
), and only a single case of shift from alcohol dependence to GHB dependence has been described (Addolorato et al., 1999b
). These observations are in agreement with the favourable safety profile of GHB as a pharmacotherapy for alcoholism (Beghè and Carpanini, 2000
).
In conclusion, the results of the present study demonstrate that voluntary ethanol intake surmounts GHB intake in sP rats that had previously consumed voluntarily relevant doses of GHB. In the few rats showing a high intake of GHB, ethanol intake was not concomitantly altered, suggesting that the postulated reciprocal substitutability of the two drugs does not completely include, at least in sP rats, the reinforcing properties.
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ACKNOWLEDGEMENTS |
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FOOTNOTES |
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