Mu opioid receptor modulation of somatodendritic dopamine overflow: GABAergic and glutamatergic mechanisms.

Mu opioid receptor (MOR) regulation of somatodendritic dopamine neurotransmission in the ventral tegmental area (VTA) was investigated using conventional microdialysis in freely moving rats and mice. Reverse dialysis of the MOR agonist DAMGO (50 and 100 microm) into the VTA of rats produced a concentration-dependent increase in dialysate dopamine concentrations. Basal dopamine overflow in the VTA was unaltered in mice lacking the MOR gene. However, basal gamma-aminobutyric acid (GABA) overflow in these animals was significantly increased, whereas glutamate overflow was decreased. Intra-VTA perfusion of DAMGO into wild-type (WT) mice increased dopamine overflow. GABA concentrations were decreased, whereas glutamate concentrations in the VTA were unaltered. Consistent with the loss of MOR, no effect of DAMGO was observed in MOR knockout (KO) mice. These data provide the first direct demonstration of tonically active MOR systems in the VTA that regulate basal glutamatergic and GABAergic neurotransmission in this region. We hypothesize that increased GABAergic neurotransmission following constitutive deletion of MOR is due to the elimination of a tonic inhibitory influence of MOR on GABAergic neurons in the VTA, whereas decreased glutamatergic neurotransmission in MOR KO mice is a consequence of intensified GABA tone on glutamatergic neurons and/or terminals. As a consequence, somatodendritic dopamine release is unaltered. Furthermore, MOR KO mice do not exhibit the positive correlation between basal dopamine levels and the glutamate/GABA ratio observed in WT mice. Together, our findings indicate a critical role of VTA MOR in maintaining an intricate balance between excitatory and inhibitory inputs to dopaminergic neurons.

Augmentation of morphine-induced sensitization but reduction in morphine tolerance and reward in delta-opioid receptor knockout mice.

Studies in experimental animals have shown that individuals exhibiting enhanced sensitivity to the locomotor-activating and rewarding properties of drugs of abuse are at increased risk for the development of compulsive drug-seeking behavior. The purpose of the present study was to assess the effect of constitutive deletion of delta-opioid receptors (DOPr) on the rewarding properties of morphine as well as on the development of sensitization and tolerance to the locomotor-activating effects of morphine. Locomotor activity testing revealed that mice lacking DOPr exhibit an augmentation of context-dependent sensitization following repeated, alternate injections of morphine (20 mg/kg; s.c.; 5 days). In contrast, the development of tolerance to the locomotor-activating effects of morphine following chronic morphine administration (morphine pellet: 25 mg: 3 days) is reduced relative to WT mice. The conditioned rewarding effects of morphine were reduced significantly in DOPrKO mice as compared to WT controls. Similar findings were obtained in response to pharmacological inactivation of DOPr in WT mice, indicating that observed effects are not due to developmental adaptations that occur as a consequence of constitutive deletion of DOPr. Together, these findings indicate that the endogenous DOPr system is recruited in response to both repeated and chronic morphine administration and that this recruitment serves an essential function in the development of tolerance, behavioral sensitization, and the conditioning of opiate reward. Importantly, they demonstrate that DOPr has a distinct role in the development of each of these drug-induced adaptations. The anti-rewarding and tolerance-reducing properties of DOPr antagonists may offer new opportunities for the treatment and prevention of opioid dependence as well as for the development of effective analgesics with reduced abuse liability.

Targeting endogenous mu- and delta-opioid receptor systems for the treatment of drug addiction.

Drug addiction is a chronic, relapsing disorder that is characterized by a compulsion to take drug regardless of the adverse consequences that may ensue. Although the involvement of mesoaccumbal dopamine neurons in the initiation of drug abuse is well-established, neuroadaptations within the limbic cortical- striatopallidal circuit that occur as a consequence of repeated drug use are thought to lead to the behavioral dysregulation that characterizes addiction. Opioid receptors and their endogenous ligands are enriched in brain regions comprising this system and are, thus, strategically located to modulate neurotransmission therein. This article will review data suggesting an important role of mu-opioid receptor (MOPr) and delta opioid receptor (DOPr) systems in mediating the rewarding effects of several classes of abused drugs and that aberrant activity of these opioid systems may not only contribute to the behavioral dysregulation that characterizes addiction but to individual differences in addiction vulnerability.

Dynorphin and the pathophysiology of drug addiction.

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Paradoxical effects of prodynorphin gene deletion on basal and cocaine-evoked dopaminergic neurotransmission in the nucleus accumbens.

Quantitative and conventional microdialysis were used to investigate the effects of constitutive deletion of the prodynorphin gene on basal dopamine (DA) dynamics in the nucleus accumbens (NAc) and the responsiveness of DA neurons to an acute cocaine challenge. Saline- and cocaine-evoked locomotor activity were also assessed. Quantitative microdialysis revealed that basal extracellular DA levels were decreased, while the DA extraction fraction, an indirect measure of DA uptake, was unchanged in dynorphin (DYN) knockout (KO) mice. The ability of cocaine to increase NAc DA levels was reduced in KO. Similarly, cocaine-evoked locomotor activity was decreased in KO. The selective kappa opioid receptor agonist U-69593 decreased NAc dialysate DA levels in wildtype mice and this effect was enhanced in KO. Administration of the selective kappa opioid receptor (KOPr) antagonist nor-binaltorphimine to KO mice attenuated the decrease in cocaine-induced DA levels. However, it was ineffective in altering the decreased locomotor response to cocaine. These studies demonstrate that constitutive deletion of prodynorphin is associated with a reduction of extracellular NAc DA levels and a decreased responsiveness to acute cocaine. Data regarding the effects of U-69593 and nor-binaltorphimine in KO suggest that the kappa opioid receptor is up-regulated as a consequence of prodynorphin gene deletion and that this adaptation underlies the decrease in basal DA dynamics and cocaine-evoked DA levels observed in DYN KO mice. These findings suggest that the phenotype of DYN KO mice is not solely due to loss of endogenous opioid peptide but also reflects developmental compensations that occur at the level of the opioid receptor.

Contrasting effects of mu opioid receptor and delta opioid receptor deletion upon the behavioral and neurochemical effects of cocaine.

Conventional brain microdialysis was used to assess basal and cocaine-induced dopamine (DA) levels in the nucleus accumbens of wildtype (WT) C57BL/6J mice and mice with constitutive deletion of ether mu- or delta-opioid receptors (MOR or DOR knockout [KO], respectively). Locomotor activity was assessed in these same animals. Basal locomotor activity of DOR KO was elevated relative to MOR KO, but did not differ from that of WT mice. DOR mice, but not WT or MOR KO, exhibited a significant increase in activity in response to an injection of saline. The acute administration of cocaine produced a dose-related increase in locomotor activity in the three genotypes. The locomotor activating effects of a low dose (10 mg/kg) of cocaine were enhanced in DOR KO mice whereas the locomotor activating effects of both a low and higher (20 mg/kg) dose of cocaine were reduced in MOR KO animals. Microdialysis studies revealed no difference between genotypes in basal DA levels. Acute administration of cocaine, but not saline, increased DA levels in WT and KO animals. Paradoxically, however, the magnitude of this effect was smaller in DOR KO as compared with that in either WT or MOR KO. These data indicate that constitutive deletion of either MOR or DOR results in contrasting effects upon responsiveness to cocaine, which is consistent with the distinct phenotypes previously described for these mutants.

Severe brain hypothermia as a factor underlying behavioral immobility during cold-water forced swim.

Behavioral immobility during forced swim is usually considered a consequence of inescapable stress, and is used to screen antidepressant drugs. However, immobility in this test may also result from inhibition of neural functions because of brain hypothermia due to body cooling. To explore this possibility, we measured brain temperature dynamics during a 10-min forced swim in cold (25 degrees C) and warm (37 degrees C) water and correlated brain temperatures with behavioral changes. Cold water forced swim resulted in significant brain hypothermia (-6-7 degrees C) and immobility, while no immobility was observed during warm water forced swim, when brain temperature transiently increased (0.5 degrees C) then decreased below baseline in the post-swim period. These data suggest that immobility, which rapidly develops during forced swim in cold water, may result from dramatic inhibition of neural functions because of severe brain hypothermia.

Changes in basal and cocaine-evoked extracellular dopamine uptake and release in the rat nucleus accumbens during early abstinence from cocaine: quantitative determination under transient conditions.

Despite an abundance of studies on mechanisms of behavioral sensitization, considerable uncertainty exists as to whether alterations in dopamine neurotransmission underlie the exacerbated behavioral effects of cocaine observed during the early stages of abstinence. One of the factors contributing to the uncertainty and controversy may be the limitations in utilized measurement techniques (mostly conventional microdialysis). The techniques of quantitative microdialysis under transient conditions and rotating disk electrode voltammetry were used to characterize basal dopamine dynamics as well as time-related changes in extracellular dopamine concentrations and dopamine uptake that occur in response to an acute drug challenge in control animals and animals with previous history of cocaine. Basal extracellular dopamine concentrations were unaltered on abstinence day 3 from repeated cocaine administration (5 days, 20 mg/kg, i.p.). The extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly lower in cocaine-sensitized animals relative to controls. These two facts, taken together, suggest that basal dopamine release is depressed in cocaine-sensitized animals on abstinence day 3. At the same time, a cocaine challenge decreased the extraction fraction and increased the extracellular dopamine concentration in both experimental groups. The magnitude of the increase in extracellular dopamine concentration was greater in cocaine-sensitized animals, while the ability of cocaine to decrease the extraction fraction was unaltered, suggesting that the increase in extracellular dopamine concentration reflects an increase in drug-evoked dopamine release. Moreover, cocaine-pretreated rats demonstrated greater depolarization-induced dopamine release and the ability of dopamine D(2) receptor agonist, quinpirole, to inhibit release was decreased in these animals. These data demonstrate that a cocaine treatment regimen resulting in behavioral sensitization is associated with a reduction in basal dopamine release, an enhancement in both cocaine and K(+)-evoked dopamine release, and a subsensitivity of dopamine D(2) autoreceptors that regulate dopamine release in the nucleus accumbens.

Modulation of the behavioral and neurochemical effects of psychostimulants by kappa-opioid receptor systems.

The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often long-lasting alterations in behavior and brain chemistry. It has been suggested that these consequences of drug use play a critical role in drug craving and relapse to addiction. This article reviews the effects of psychostimulant administration on dopaminergic and excitatory amino acid neurotransmission in brain regions comprising the brain's motive circuit and provides evidence that the activation of endogenous kappa-opioid receptor systems in these regions opposes the behavioral and neurochemical consequences of repeated drug use. The role of this opioid system in mediating alterations in mood and affect that occur during abstinence from repeated psychostimulant use are also discussed.

Kappa-opioid receptor activation prevents alterations in mesocortical dopamine neurotransmission that occur during abstinence from cocaine.

In vivo microdialysis was used to characterize basal dopamine dynamics and cocaine-evoked dopamine levels in the medial prefrontal cortex of male Sprague-Dawley rats that had previously received once daily injections of cocaine (days 1-5; 20mg/kg, i.p.) in combination with the selective kappa-opioid receptor agonist U-69593 (days 3-5; 0.32mg/kg, s.c.) or its vehicle. The influence of these treatments on [3H]dopamine uptake in medial prefrontal cortex synaptosomes was also determined. Three days following the cessation of drug treatment, animals with prior history of cocaine administration exhibited enhanced psychomotor stimulation in response to a subsequent cocaine challenge. This effect was not apparent in animals that had previously received the cocaine treatment regimen in combination with the kappa-opioid receptor agonist U-69593. Cocaine challenge increased prefrontal dopamine levels in all pretreatment groups, but cocaine-pre-exposed animals had lower cocaine-evoked dopamine levels and higher basal in vivo extraction fraction, indicative of an increase in basal dopamine uptake relative to controls. Pretreatment with U-69593 prevented these effects of cocaine. Measurement of [3H]dopamine uptake in synaptosomes revealed a significant increase in uptake three days after the cessation of cocaine treatment. No increase in uptake was observed in animals that had received the cocaine treatment regimen in combination with U-69593. These results demonstrate that the early phase of abstinence from cocaine is associated with marked alterations in medial prefrontal cortex dopamine neurotransmission and that these neuroadaptations are prevented by the activation of kappa-opioid receptors. Furthermore, they raise the possibility that mesocortical dopamine neurons may be an important neural substrate upon which kappa-opioid agonists act to prevent the development of cocaine-induced behavioral sensitization.