Repeated ethanol intoxication induces behavioral sensitization in the absence of a sensitized accumbens dopamine response in C57BL/6J and DBA/2J mice

Abstract

Repeated exposure to drugs of abuse results in an increased sensitivity to their behavioral effects, a phenomena referred to as behavioral sensitization. It has been suggested that the same neuroadaptations underlying behavioral sensitization contribute to the maintenance and reinstatement of addiction. Dysregulation of dopamine (DA) neurotransmission in the mesoaccumbens system is one neuroadaptation that is thought to lead to the compulsive drug-seeking that characterizes addiction. Evidence that sensitization to psychostimulants and opiates is associated with an enhancement of drug-evoked DA levels in the nucleus accumbens has also been obtained. Like other drugs of abuse, the acute administration of ethanol (ETOH) stimulates DA release in this brain region. Moreover, repeated ETOH experience results in an enhanced behavioral response to a subsequent ethanol challenge. Data regarding the influence of repeated ethanol intoxication and withdrawal upon mesoaccumbal DA neurotransmission is limited. Studies examining ETOH-evoked alterations in mesoaccumbal DA neurotransmission as a function of withdrawal duration are lacking. The present experiments quantified basal and ethanol-evoked DA levels 14 days and 24 h following the cessation of a repeated ETOH intoxication protocol, which results in sensitization to the locomotor activating effects of ethanol. Locomotor activity was assessed in parallel groups of animals. Studies were conducted in two mouse strains, C57BL/6J and DBA/2J, which differ in their behavioral responses to ETOH. The results indicate the development of transient tolerance to both ETOH-induced behavioral activation and evoked accumbens DA release at early withdrawal. Moreover, no enhanced DA response to a subsequent ETOH challenge could be demonstrated in ETOH experienced animals 2 weeks after withdrawal, in spite of the observation of clear behavioral sensitization at this time point. These results suggest that, at least in the case of ethanol, sensitization of the DA mesolimbic system may not be necessary for the development of behavioral sensitization.

Figure 1

Diagram showing the experimental timeline used in the present studies. Animals used for microdialysis studies went through surgery before exposure to the ethanol chambers. The animals used for the locomotor activity studies did not receive any surgery. All animals went through the ethanol vapor exposure regimen (or normal air for controls) and then they were divided into two groups. The first group was tested at 1 day of withdrawal and the second group was tested at 14 days of withdrawal (see details in the Materials and methods section).

Figure 2

Blood ETOH concentrations (BEC) after the locomotor activity test 1 h after ETOH 2 g/kg i.p. (a) or immediately after the last ETOH vapor exposure (b). Values are mean±SEM, *p<0.05 vs control, Student–Newman–Keuls. ^#p<0.05 vs the other genotype, Student's t-test. Number of animals per group was: Figure 1a, C57BL/6J (n = 20, 13, and 6) and DBA/2J (n = 20, 15, and 7) for control, 1- and 14-day withdrawal groups, respectively. Figure 1b, n = 19 and 28 for C57BL/6J and DBA/2J, respectively.

Figure 3

Time course of the effects of a saline and ETOH 2 g/kg i.p. challenge on locomotor activity in naïve or ETOH withdrawn (1 or 14 day) C57BL/6J or DBA/2J mice. Values are mean±SEM. Number of animals per group was: C57BL/6J (n = 20, 13, and 6) and DBA/2J (n = 24, 15, and 13) for control, 1 and 14 days withdrawal groups, respectively.

Figure 4

Total ambulatory activity during the 20 min following saline (a) or ETOH 2 g/kg i.p. (b) challenge in naïve or ETOH withdrawn (1 or 14 day) C57BL/6J and DBA/2J mice. Values are mean±SEM. *p<0.05 vs control, ETOH-naïve animals, Mann–Whitney's rank-sum test. Number of animals as in Figure 3.

Figure 5

Time course of the effects of an ETOH 2 g/kg i.p. challenge on dialysate DA levels in the nucleus accumbens of naäve or ETOH withdrawn (1 or 14 day) mice. Values are mean±SEM. Number of animals per group was: C57BL/6J (n = 21, 17, and 12) and DBA/2J (n = 20, 5, and 13) for control, 1 and 14-day withdrawal groups, respectively.

Figure 6

Basal dialysate DA levels (a) and total ETOH-evoked dopamine outflow relative to basal levels (b) after an ETOH 2 g/kg i.p. challenge o in naïve or ETOH withdrawn (1 or 14 day) C57BL/6J and DBA/2J mice. *p<0.05 vs control, ETOH -naïve animals, Student–Newman–Keuls test. Number of animals as in Figure 5.