The repeated use of opiates, stimulants and other drugs of abuse produces enduring changes in behavior and brain function that contribute to the compulsive drug-seeking that characterizes addiction. Tolerance occurs as a consequence of continued drug administration and is a limiting factor in the therapeutic use of opiates for the treatment of persistent pain. The long-term goal of these studies is to identify effective targets for the treatment of drug addiction and pain, as well as biological factors that contribute to individual differences in sensitivity to drugs of abuse.? Opioid peptides and their receptors are enriched in brains regions that subserve incentive motivation, habit learning and the perception of pain. Using ligands selective for the various opioid receptor types, we have shown that tonically active mu-, and delta-opioid receptor (KOPr, DOPr) systems regulate dopamine(DA), GABA and glutamatergic transmission in regions comprising the mesocorticolimbic system. KOPr are selectively located on cell bodies of ventral tegmental DA neurons that project to the medial prefrontal cortex and their activation inhibits somatodendritic and terminal DA release. In contrast, KOPr located on DA nerve terminals in the nucleus accumbens inhibit mesoaccumbal DA transmission by affecting uptake and release processes. Our studies have provided the first demonstration that DOPr regulate glutamatergic and GABAergic neurotransmission in the ventral tegmental area. Furthermore, they indicate that DOPr modulate somatodendritic and terminal DA release by regulating excitatory and inhibitory inputs to DA neurons. DOPr located on GABA interneurons inhibit GABA release, thereby, increasing DA transmission. In contrast, tonically active DOPr residing on glutamate terminals inhibit glutamate neurotransmission and decrease DA neuronal activity. ? Our previous studies led us to hypothesize that drugs of abuse increase the activity of KOPr systems and that this increase is a homeostatic mechanism that opposes alterations in behavior and brain function that occur as a consequence of repeated drug use. Consistent with this hypothesis, pharmacological blockade or genetic deletion of KOPr enhances the responsiveness of an individual to the behavioral and neurochemical effects produced by acute administration of either cocaine or ethanol. Subsequent self-administration studies have shown that blockade of this opioid receptor system increases ethanol-evoked DA release in the nucleus accumbens. However, no alteration in responding or consumption is seen indicating differential involvement of KOPr in modulating the neurochemical and reinforcing effects ethanol. Using self-administration and reward-devaluation techniques, on-going studies are examining the role of KOPr in modulating the compulsive seeking and taking behavior produced by drugs of abuse and the neurochemical basis of these effects. These studies have been facilitated by the Section's development of capillary electrophoresis laser fluorescence detection techniques which permit quantification of rapid changes in neurotransmitter release that occur in response to a specific behavior.? Progress has been made in identifying the intracellular mechanisms by which KOPr agonists regulate the DA transporter. Using live cell imaging techniques and heterologus expression systems, we have shown that KOPr activation increases DA transporter function. Inhibition of ERK, in contrast to p38 MAPK attenuates this effect suggesting a critical role of this signaling cascade in mediating KOPr/DA transporter interactions. ? Repeated drug use leads to the development of tolerance and dependence. These adaptive responses have been implicated in the escalation of drug taking observed in human addicts and limit the use of opiates for the treatment of pain. Our behavioral studies have revealed a critical role of DOPr in the development of opiate dependence. Systemically administered DOPr antagonists attenuate the somatic and affective components of morphine withdrawal in rodents and similar effects are observed in response to selective blockade of DOPr in the ventral tegmental area. Inhibition of the GABA transporter-1 attenuates the expression of morphine dependence suggesting an involvement of both GABA and DOPr in the development and expression of opiate dependence. Evidence that induction of DOPr underlies the development of morphine tolerance has been obtained.
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