Differential involvement of dopamine D1 receptors in morphine- and lithium-conditioned saccharin avoidance

A leptin-mediated central mechanism in analgesia-enhanced opioid reward in rats

Opioid analgesics are commonly used in chronic pain management despite a potential risk of rewarding. However, it remains unclear whether opioid analgesia would enhance the opioid rewarding effect thereby contributing to opioid rewarding. Utilizing a rat paradigm of conditioned place preference (CPP) combined with ankle monoarthritis as a condition of persistent nociception, we showed that analgesia induced by either morphine or the nonsteroid anti-inflammatory drug ibuprofen increased CPP scores in arthritic rats, suggesting that analgesia itself had a rewarding effect. However, arthritic rats exhibited a significantly higher CPP score in response to morphine than ibuprofen. Thus, the rewarding effect of morphine was enhanced in the presence of persistent nociception, producing a phenomenon of analgesia-enhanced opioid reward. At the cellular level, administration of morphine activated a cascade of leptin expression, glial activation, and dopamine receptor upregulation in the nucleus accumbens (NAc), while administration of ibuprofen decreased glial activation with no effect on leptin expression in the NAc. Furthermore, the morphine rewarding effect was blocked in leptin deficient ob/ob mice or by neutralizing leptin or interleukin-1β in the NAc without diminishing morphine analgesia. The data indicate that systemic opioid can activate a leptin-mediated central mechanism in the NAc that led to the enhanced opioid rewarding effect. These findings provide evidence for an interaction between opioid analgesia and opioid rewarding, which may have implications in clinical opioid dose escalation in chronic pain management.

Acquisition and expression of conditioned taste aversion differentially affects extracellular signal regulated kinase and glutamate receptor phosphorylation in rat prefrontal cortex and nucleus accumbens

Conditioned taste aversion (CTA) can be applied to study associative learning and its relevant underpinning molecular mechanisms in discrete brain regions. The present study examined, by immunohistochemistry and immunocytochemistry, the effects of acquisition and expression of lithium-induced CTA on activated Extracellular signal Regulated Kinase (p-ERK) in the prefrontal cortex (PFCx) and nucleus accumbens (Acb) of male Sprague-Dawley rats. The study also examined, by immunoblotting, whether acquisition and expression of lithium-induced CTA resulted in modified levels of phosphorylation of glutamate receptor subunits (NR1 and GluR1) and Thr³⁴- and Thr^{75-Dopamine-and-cAMP-Regulated} PhosphoProtein (DARPP-32). CTA acquisition was associated with an increase of p-ERK-positive neurons and phosphorylated NR1 receptor subunit (p-NR1) in the PFCx, whereas p-GluR1, p-Thr³⁴- and p-Thr⁷⁵-DARPP-32 levels were not changed in this brain region. CTA expression increased the number of p-ERK-positive neurons in the shell (AcbSh) and core (AcbC) but left unmodified p-NR1, p-GluR1, p-Thr³⁴- and p-Thr^75-DARPP-32 levels. Furthermore, post-embedding immunogold quantitative analysis in AcbSh revealed that CTA expression significantly increased nuclear p-ERK immunostaining as well as p-ERK-labeled axo-spinous contacts. Overall, these results indicate that ERK and NR1, but not GluR1 and DARPP-32, are differentially phosphorylated as a consequence of acquisition and expression of aversive associative learning. Moreover, these results confirm that CTA represents an useful approach to study the molecular basis of associative learning in rats and suggest the involvement of ERK cascade in learning-associated synaptic plasticity.

Conditioned saccharin avoidance induced by infusion of amphetamine in the nucleus accumbens shell and morphine in the ventral tegmental area: behavioral and biochemical study

Drugs of abuse possess the seemingly paradoxical property of conditioning rats to avoid from drinking a saccharin solution that had been predictively paired with their systemic administration (conditioned saccharin avoidance, CSA). CSA is dependent upon an intact dopamine (DA) transmission but the locus, central or peripheral, and eventually the brain area from which this effect originates and its relationship with the rewarding properties of the drug is debated. In order to clarify this issue we tested the ability of amphetamine and morphine to induce CSA after infusion at the same dose-range and in the same areas from which these drugs induce conditioned place preference (CPP). Drugs were infused intracerebrally immediately after saccharin drinking in two acquisition trials and CSA was tested on a two bottle saccharin/water choice. Amphetamine (10 and 20 μg/0.5 μl) induced CSA after infusion in the NAc shell but was ineffective in the NAc core. Morphine (0.5 and 1 μg/0.5 μl) induced CSA from the VTA at both doses tested. Amphetamine (20 μg/0.5 μl) and morphine (1 μg/0.5 μl) failed to induce CSA after infusion 1.2mm dorsal the NAc shell and the VTA respectively. Finally, morphine (1 μg/0.5 μl), infused in the VTA, elicited a selective increase in dialysate DA in the NAc shell. These results indicate that drugs of abuse induce CSA from the same intracerebral sites and at the same doses at which they induce CPP. These observations are consistent with the existence of a strong relationship between CSA and drug reward related to their ability to stimulate DA transmission in the NAc shell.

The effects of haloperidol on cocaine-induced conditioned taste aversions

Although the mechanism underlying the rewarding effects of cocaine has been well characterized, little is known about the mechanism underlying its aversive effects. Several reports have indicated a possible role of dopamine (DA) in the aversive effects; however, several procedural issues limit any conclusions regarding its specific role. In order to investigate a possible dopaminergic role in cocaine-induced CTAs using procedures that circumvented these possible issues, the present series of investigations assessed the aversive effects of the DA antagonist haloperidol alone (Experiment 1) and in combination with cocaine (Experiment 2). Haloperidol, at doses that were determined to be non-aversive, yet behaviorally active in a locomotor assessment, attenuated cocaine-induced taste aversions, suggesting that cocaine's aversive effects are mediated in part by dopaminergic activity. These findings were discussed in consideration with other evidence implicating DA and other neurotransmitter systems in cocaine-induced CTAs.

The D2/D3-receptor antagonist tiapride impairs concurrent but not sequential taste aversion learning

Taste aversion learning (TAL) is a learning modality in which the animals reject a gustatory stimulus associated with the administration of noxious visceral substances. This learning can be established by concurrent or sequential procedures that involve different anatomical and functional mechanisms and may constitute distinct learning modalities. The dopaminergic system has been related to various learning processes and goal-directed behaviours. The present study examined the effect of the administration of tiapride, a D(2)/D(3) dopaminergic antagonist, on concurrent and sequential TAL. Results obtained showed that pre-treatment with tiapride blocks the acquisition of concurrent TAL but does not affect sequential TAL, including reversal learning tasks. These results demonstrate the involvement of the D(2)/D(3) dopaminergic receptors in the former but not the latter learning process. The dopaminergic system appears to participate in concurrent TAL, an "implicit" learning modality, but not in sequential TAL, which is considered a relational/explicit acquisition process.

Paradoxical simultaneous occurrence of amphetamine-induced conditioned taste aversion and conditioned place preference with the same single drug injection: a new "pre- and post-association" experimental paradigm

The paradoxical phenomenon of co-existing physically aversive and psychologically rewarding effects of drugs is a crucial issue for drug addiction. The present study employed a new experimental paradigm to test whether the rewarding and aversive properties of amphetamine (AMPH) can exist simultaneously. Rats were given a 15 min period of exposure to saccharin injected with 0.15M NaCl or 1.5mg/kg AMPH and then were confined to one compartment of a test box for 30 min. After three paired and unpaired cycles, the aversive and rewarding effects were assessed. A reduction in consumption of the paired flavored solution provided evidence of avoidance while preference for the AMPH injection context provided evidence of rewarding effects. The present findings demonstrate that the development of AMPH-induced rewarding and aversive effects depends on the particular behavioral conditions and support both the task-dependent drug effects hypothesis and the reward comparison hypothesis. The formation of associations with stimuli that comes before (pre) vs. after (post) the unconditioned stimulus and the role of the dopaminergic system in such associations are discussed.