Glutamatergic transmission in opiate and alcohol dependence

Abstract

Both the nucleus accumbens (NAcc) and central amygdala (CeA) are thought to play roles in tolerance to, and dependence on, abused drugs. Although our past studies in rat brain slices suggested a role for NMDA receptors (NMDARs) in NAcc neurons in the effects of acute and chronic opiate treatment, the cellular and molecular mechanisms remained unclear. Therefore, we examined the effects of morphine dependence on electrophysiological properties of NMDARs in freshly isolated NAcc neurons and on expression of mRNA coding for NR2A-C subunits using single-cell RT-PCR. Chronic morphine did not alter the affinity for NMDAR agonists glutamate, homoquinolinate, or NMDA, but decreased the affinity of the coagonist glycine. Chronic morphine altered the NMDAR inhibition by two NMDAR antagonists, 7-Cl-kynurenate and ifenprodil, but not that by d-APV or Mg2+. Chronic morphine accelerated the NMDA current desensitization rate in NAcc neurons. In single-cell RT-PCR, chronic morphine predominantly reduced the number of neurons expressing multiple NR2 subunits. Ethanol also alters NMDARs. We found that low ethanol concentrations (IC50 = 13 mM) inhibited NMDA currents and NMDA-EPSPs in most NAcc neurons in a slice preparation. NAcc neurons from ethanol-dependent rats showed enhanced NMDA sensitivity. In CeA neurons, acute ethanol decreased (by 10-25%) non-NMDA- and NMDA-EPSPs in most neurons. In CeA neurons from ethanol-dependent rats, acute ethanol decreased the non-NMDA-EPSPs to the same extent as in naïve rats, but inhibited (by 30-40%) NMDA-EPSPs significantly more than in controls, suggesting sensitization to ethanol. Preliminary studies with microdialysis and real-time PCR analysis support this idea: local ethanol administration in vivo had no effect on glutamate release, but chronic ethanol nearly tripled the expression of NR2B subunits (the most ethanol sensitive) in CeA. These combined findings suggest that changes in glutamatergic transmission in NAcc and CeA may underlie the neuroadaptions that lead to opiate and ethanol dependence.