Local cerebral glucose utilization after relapse in ethanol drinking in alcohol-preferring (P) rats

Abstract

The [¹⁴C]-2-deoxyglucose ([¹⁴C]-2-DG) quantitative autoradiographic technique was used to determine rates of local cerebral glucose utilization (LCGU) in discrete brain regions of adult, male alcohol-preferring (P) rats after 2 weeks of ethanol deprivation (E-D), 3 and 14 days of ethanol-relapse drinking (E-R3; E-R14), and in P rats, which were chronically drinking ethanol (E-C) for 8 weeks during daily 3-h scheduled-access sessions to 15% (vol./vol.) ethanol and water, or were ethanol-naive (E-N). The hypothesis to be tested was that ethanol-relapse drinking is initiated to restore changes in functional activity back to their chronic ethanol–exposed state. The LCGU rates were measured 1 h before the scheduled-access session. Mean ethanol intake did not differ among the groups. The LCGU rates were decreased in 41 of 57 regions or subregions examined in E-C rats compared with findings for E-N rats, including subregions of the cerebral cortex, hippocampus, and structures in the mesocorticolimbic and nigrostriatal systems. After 2 weeks of deprivation, LCGU values tended to return toward control values (E-N) in most CNS regions and did not differ significantly from control values in 12 regions or subregions (e.g., parts of the limbic system, cerebral cortex). Compared with LCGU values that recovered toward control levels in the E-D group, ethanol-relapse drinking was associated with decreased LCGU values in (1) 4 of 10 limbic structures, (2) all 6 cerebral cortical regions, (3) 1 of 4 basal ganglia regions, (4) 2 of 7 hippocampus subregions, and (5) 1 of 6 thalamic nuclei. The present results indicate that ethanol-relapse drinking was associated with reduced LCGU rates in most CNS regions that recovered toward control values and suggested to us that ethanol-relapse drinking may be initiated to restore neuronal function to its prior chronic ethanol–exposure state.

Introduction

Few studies have examined the neural substrates associated with ethanol-relapse drinking or the reinstatement of ethanol-seeking behavior after an imposed abstinence period. It has been reported that opioid and serotonergic mechanisms may have a role in mediating ethanol- and stress-induced reinstatement of ethanol-seeking behavior, respectively (Le et al., 1999). It has also been shown that environmental cue–induced reinstatement of responding for ethanol could be attenuated by naltrexone, suggesting to these investigators that opioid systems are involved in the motivating effects of ethanol-associated stimuli, which may precipitate relapse (Katner et al., 1999). In addition, Katner and Weiss (1999) demonstrated that increased nucleus accumbens dopamine efflux might be associated with the anticipation of ethanol access after forced ethanol abstinence. Finally, Rodd-Henricks et al. (2000) have shown that serotonin-3 (5-HT₃) receptor antagonists reduce ethanol intake during acquisition and maintenance of ethanol consumption, but not ethanol-relapse drinking on the first day of reexposure, supporting the suggestion that 5-HT₃ receptors may have a unique role in ethanol-relapse drinking.

Other evidence that neuronal changes are occurring over periods of prolonged ethanol deprivation was found by using the [¹⁴C]-2-deoxyglucose ([¹⁴C]-2-DG) quantitative autoradiographic technique (Smith et al., 2001b). Alcohol-preferring (P) rats, consuming ethanol under daily 4-h limited- access, free-choice conditions for 8 weeks, had significantly lower local cerebral glucose utilization (LCGU) rates in most regions examined, compared with findings for ethanol-naive animals, supporting the suggestion that prolonged ethanol drinking results in reduced basal neuronal activity in this rat line. Furthermore, in the P line of rats that consumed ethanol under the same conditions but were subsequently ethanol deprived for 2 weeks, LCGU rates in different brain regions (1) remained at the same reduced level as that seen in the chronic ethanol–drinking group, (2) partially recovered toward control levels, or (3) completely recovered to basal (ethanol-naive) levels. In several limbic regions, no recovery of LCGU rates was observed after the deprivation interval, whereas partial recovery to ethanol-naive LCGU levels was observed in the nucleus accumbens core (ACB-C), the nucleus accumbens shell (ACB-Sh), and the lateral hypothalamus (LH). Complete recovery was shown in the central amygdaloid nuclei (CeA) and basolateral amygdaloid nuclei (BLA) (Smith et al., 2001b). These results indicate that multiple neuronal alterations are occurring during the deprivation interval and that these changes may result in imbalanced interactions among and within several neurotransmitter systems, some of which may contribute to ethanol-relapse drinking.

Previous research findings have shown that the acute administration of ethanol produces anatomically distinct changes in rates of LCGU that are dependent on time, dose, and route of administration Lyons et al. 1998, Porrino et al. 1998b, Williams-Hemby & Porrino 1994, Williams-Hemby & Porrino 1997, Williams-Hemby et al. 1996. The intraperitoneal injection of a 1-g/kg dose of ethanol given 10 min before [¹⁴C]-2-DG administration produced a 10%–30% reduction in LCGU rates in 17 of 54 regions examined (Williams-Hemby & Porrino, 1994). In contrast, the intragastric (i.g.) administration of a 1-g/kg dose of ethanol given 3 min before [¹⁴C]-2-DG injection produced an increase of about 15% in LCGU rates in several limbic regions (Williams-Hemby & Porrino, 1997). In another study, rats orally self-administering ethanol for 70 days under a schedule-induced polydipsia paradigm consumed an average dose of ethanol of 1.5 g/kg during the drinking session (Williams-Hemby et al., 1996). The LCGU rates were measured immediately after the 50-min drinking session, and reduced rates were observed in a few limbic regions.

Overall, results of these studies indicate that both acute ethanol administration (i.e., a 1-g/kg dose) and ethanol drinking can produce alterations in LCGU rates in several CNS regions. However, the effects of ethanol-relapse drinking on LCGU rates in rats have not been examined. Because the effects of ethanol exposure span many brain regions and multiple neurotransmitter systems Koob et al. 1998, McBride & Li 1998, it would be important to examine the entire CNS to identify accurately the effects of relapse drinking. The [¹⁴C]-2-DG method is used to determine LCGU rates throughout the CNS, and because energy metabolism is tied closely to functional neuronal activity (Sokoloff, 1981), this method can be used to identify the neural substrates of the effects of ethanol exposure.

It is also important to use animal models that exhibit voluntary oral self-administration of ethanol [for review, see McBride & Li (1998)]. The selectively bred alcohol-preferring P line of rats satisfies the perceived criteria for an animal model of alcoholism Cicero 1979, McMillen 1997, and these rats will voluntarily consume unadulterated ethanol without the use of conditioning paradigms. The P line of rats will voluntarily consume 5–8 g of ethanol per kilogram of body weight per day, attain blood ethanol concentrations of 50–200 mg% in free-choice drinking paradigms (Murphy et al., 1986), and maintain ethanol consumption in the presence of other palatable solutions (Lankford et al., 1991). It has been shown that ethanol has CNS-reinforcing actions in the P line of rats because they will self-administer ethanol intragastrically Murphy et al. 1988, Waller et al. 1984 and directly into the ventral tegmental area (VTA; Gatto et al., 1994). The P line of rats will also work by bar-pressing to obtain ethanol orally when food and water are available ad libitum (Murphy et al., 1989).

The objectives of the present study were to determine the neural substrates of the effects of chronic ethanol drinking, prolonged ethanol deprivation, and short- and long-term ethanol-relapse drinking in the P line of rats with the use of the [¹⁴C]-2-DG whole-brain imaging technique. Although the effects of chronic ethanol intake (i.e., 8 weeks of scheduled-access ethanol drinking) and subsequent ethanol deprivation in the P line of rats were determined in a previous study (Smith et al., 2001b), they were reassessed in the present study to control for environmental or procedural differences that may exist between the two experiments and, therefore, to make a direct comparison with the ethanol- relapse drinking groups. The hypothesis to be tested was that ethanol-relapse drinking is initiated to restore alterations in functional activity back to their prior chronic ethanol–exposed state.