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Pain is often associated with depression of behavior and mood, and relief of pain-related depression is a common goal of treatment. This study tested the hypothesis that pain-related behavioral depression is mediated by activation of endogenous κ-opioid systems and subsequent depression of mesolimbic dopamine release. Adult male Sprague-Dawley rats were implanted with electrodes targeting the medial forebrain bundle (for behavior studies of intracranial self-stimulation (ICSS)) or with cannulae for microdialysis measures of nucleus accumbens dopamine (NAc DA). Changes in ICSS and NAc DA were examined after treatment with a visceral noxious stimulus (intraperitoneal injection of dilute lactic acid) or an exogenous κ-agonist (U69593). Additional studies examined the sensitivity of acid and U69593 effects to blockade by two analgesics (the nonsteroidal antiinflammatory drug ketoprofen and the μ-opioid agonist morphine) or by the κ-antagonist norbinaltorphimine (norBNI). The effects of acid were also examined on mRNA expression for prodynorphin (PDYN) and κ-opioid receptors (KORs) in mesocorticolimbic brain regions. Both acid and U69593 depressed ICSS and extracellular levels of NAc DA. Pain-related acid effects were blocked by ketoprofen and morphine but not by norBNI. The U69593 effects were blocked by norBNI but not by ketoprofen, and were only attenuated by morphine. Acid did not significantly alter PDYN or KOR in NAc, but it produced a delayed increase in PDYN in prefrontal cortex. These results support a key role for the mesolimbic DA system, but a more nuanced role for endogenous κ-opioid systems, in mediating acute pain-related behavioral depression in rats. [ABSTRACT FROM AUTHOR]

Behavioral and pharmacotherapeutic approaches constitute two prominent strategies for treating cocaine dependence. This study investigated interactions between behavioral and pharmacological strategies in a preclinical model of cocaine vs food choice. Six rhesus monkeys, implanted with a chronic indwelling double-lumen venous catheter, initially responded under a concurrent schedule of food delivery (1-g pellets, fixed-ratio (FR) 100 schedule) and cocaine injections (0-0.1 mg/kg/injection, FR 10 schedule) during continuous 7-day treatment periods with saline or the agonist medication phenmetrazine (0.032-0.1 mg/kg/h). Subsequently, the FR response requirement for cocaine or food was varied (food, FR 100; cocaine, FR 1-100; cocaine, FR 10; food, FR 10-300), and effects of phenmetrazine on cocaine vs food choice were redetermined. Decreases in the cocaine FR or increases in the food FR resulted in leftward shifts in the cocaine choice dose-effect curve, whereas increases in the cocaine FR or decreases in the food FR resulted in rightward shifts in the cocaine choice dose-effect curve. The efficacy of phenmetrazine to decrease cocaine choice varied systematically as a function of the prevailing response requirements, such that phenmetrazine efficacy was greatest when cocaine choice was maintained by relatively low unit cocaine doses. These results suggest that efficacy of pharmacotherapies to modulate cocaine use can be influenced by behavioral contingencies of cocaine availability. Agonist medications may be most effective under contingencies that engender choice of relatively low cocaine doses. [ABSTRACT FROM AUTHOR]

Chronic drug use may lead to sufficient drug intake to produce dependence and the emergence of abstinence signs during withdrawal. Although withdrawal can increase the reinforcing effects of some drugs (eg opioids), the impact of withdrawal on the reinforcing effects of stimulants like cocaine is less clear. This study used a novel cocaine vs food choice procedure to examine the relative reinforcing strength of cocaine before, during, and after exposure to graded levels of extended cocaine access. Responding in four rhesus monkeys was maintained by cocaine (0–0.1 mg/kg/injection) and food delivery under a concurrent-choice schedule during daily 2-h sessions. Under baseline conditions, cocaine maintained a dose-dependent increase in cocaine choice. Subsequently, subjects were exposed to and withdrawn from periods of extended cocaine access, which was accomplished by implementing daily 21-h supplemental sessions of cocaine self-administration in addition to daily choice sessions. During supplemental sessions, cocaine (0.1 mg/kg/injection) was available under a fixed-ratio 10/time-out X schedule, and the duration of the time-out was varied from 30 to 7.5 min. Cocaine intake increased 10-fold to >11 mg/kg/day during exposure to supplemental sessions with the shortest post-injection time-out. However, parameters of cocaine choice were not significantly affected either during or after extended cocaine access. These results do not support the hypothesis that cocaine withdrawal increases the reinforcing strength of cocaine. This differs from results with the opioid agonist heroin and suggests that withdrawal may have different functions in the maintenance of opioid and stimulant abuse. [ABSTRACT FROM AUTHOR]

Although serotonin (5-HT) can interact with dopamine (DA) systems to modulate the subjective and reinforcing effects of psychostimulants such as cocaine and 3,4-methyldioxymethamphetamine (MDMA, ecstasy), the long-term effects of exposure to psychostimulants on brain 5-HT systems are not well characterized. The present study assessed 5-HT transporter (SERT) availability using positron emission tomography (PET) in rhesus monkeys with the SERT-specific radioligand [11C]3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile (DASB). SERT availability was assessed in regions of interest including the caudate nucleus, putamen, anterior cingulate cortex, and cerebellum. [11C]DASB distribution volume ratios (DVRs) were calculated using the cerebellum as the reference region. DVRs were calculated in control monkeys and in cocaine or MDMA self-administering monkeys approximately 24 h after the last self-administration (SA) session. SERT availability did not differ between monkeys with a history of MDMA SA and control monkeys in any region examined. In contrast, monkeys with a history of cocaine SA showed significantly higher levels of SERT availability in the caudate nucleus and putamen compared to control subjects. These results suggest that chronic SA of cocaine, but not MDMA, leads to alterations in serotonergic function in brain areas relevant to drug abuse. The higher level of SERT availability in cocaine-experienced monkeys may lead to a reduced inhibitory tone of 5-HT on the DA system, which may explain, in part, differences in the abuse liability between cocaine and MDMA.Neuropsychopharmacology (2008) 33, 219–225; doi:10.1038/sj.npp.1301420; published online 18 April 2007 [ABSTRACT FROM AUTHOR]