This study was conducted to assess the involvement of N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA) receptor systems, located in specific limbic brain regions. in the discriminative stimulus effects of ethanol. Male Long-Evans rats were trained to discriminate between intraperitoneal (i.p.) injections of ethanol (1 g/kg) and saline on a two-lever drug discrimination task. The rats were then implanted with bilateral injector guides aimed at the nucleus accumbens core (AcbC), prelimbic cortex (PrLC), hippocampus area CA1 (CA1), or extended amygdala (i.e., at the border of the central and basolateral nuclei). Infusions of the non-competitive NMDA antagonist MK 801 in the AcbC or CA1 resulted in dose-dependent full substitution for i.p. ethanol. MK 801 infusion in the PrLC or amygdala failed to substitute for ethanol. Injection of the competitive NMDA antagonist CPP in the AcbC also failed to substitute for ethanol. Co-infusion of MK 801 in the hippocampus potentiated the effects of MK 801 in the AcbC, whereas NMDA infusion in the hippocampus attenuated the ability of MK 801 in the AcbC to substitute for ethanol. The direct GABA(A) agonist muscimol resulted in dose-dependent full substitution for i.p. ethanol when it was injected into the AcbC or amygdala, but failed to substitute when administered in the PrLC. Co-infusion of MK 801, but not CPP, potentiated the effects of muscimol in the AcbC. These results demonstrate that ethanol's discriminative stimulus function is mediated centrally by NMDA and GABA(A) receptors located in specific limbic brain regions. The data also suggest that the discriminative stimulus effects of ethanol are mediated by interactions between ionotropic GABA(A) and NMDA receptors in the nucleus accumbens, and by interactions among brain regions.

These results indicate that blockade of GABA(A) receptors in the amygdala significantly reduces the potency of the GABA(A) agonist in the nucleus accumbens. This suggests that the ethanol-like stimulus effects of GABA(A) receptor activation in the nucleus accumbens are modulated by GABA(A) receptor activity in the amygdala. These data support the hypothesis that the addictive stimulus properties of alcohol are mediated by GABAergic transmission in a neural circuit involving the amygdala and nucleus accumbens.

Rationale: The neurobiological systems that mediate the discriminative stimulus effects of self-administered drugs are largely unknown. The present study examined the discriminative stimulus effects of self-administered ethanol. Methods: Rats were trained to discriminate ethanol (1 g/kg, IP) from saline on a two-lever drug discrimination task with sucrose (10% w/v) reinforcement. Test sessions were conducted with ethanol (0 or 10% v/v) added to the sucrose reinforcement to determine if self-administered ethanol would interact with the discriminative stimulus effects of investigator-administered ethanol, or with the ethanol-like discriminative stimulus effects of the GABA A -positive modulator pentobarbital or the non-competitive NMDA antagonist MK-801. Results: During a saline test session, ethanol (10% v/v) was added to the sucrose reinforcement. Responding by all animals began accurately on the salineappropriate lever and then switched to the ethanol-appropriate lever after rats self-administered a mean dose of 1.2±0.14 g/kg ethanol. During cumulative self-administration trials, responding initially occurred on the saline lever and then switched to the ethanol-appropriate lever after ethanol (0.68±0.13 g/kg) was self-administered. Investigator-administered MK-801 (0.01-1.0 mg/kg, cumulative IP) and pentobarbital (0.3-10.0 mg/kg, cumulative IP) dose-dependently substituted for ethanol. When ethanol (10% v/v) was added to the sucrose reinforcer, MK-801 and pentobarbital dose-response curves were shifted significantly to the left. Conclusions: Self-administered ethanol substituted for and potentiated the stimulus effects of investigator-administered ethanol, suggesting that the discriminative stimulus effects of self-administered ethanol are similar to those produced by investigator-administered ethanol. Self-administered ethanol enhanced the ethanol-like discriminative stimulus effects of MK-801 and pentobarbital, which suggests that the discriminative stimulus effects of self-administered ethanol are mediated by NMDA and GABA A receptors.