Hypocretin-1 and -2 (Hcrt-1 and Hcrt-2), also referred to as orexin-A and -B, are neuropeptides synthesized by a few thousand neurons in the lateral hypothalamus. Hypocretin-containing neurons project throughout the brain, with a prominent input to basal forebrain structures involved in motivation, reward, and stress. However, the role of hypocretins in addiction-related behaviors remains largely unexplored. Here we show that intracerebroventricular infusions of Hcrt-1 lead to a dose-related reinstatement of cocaine seeking without altering cocaine intake in rats. Hcrt-1 also dramatically elevates intracranial self-stimulation thresholds, indicating that, unlike treatments with reinforcing properties such as cocaine, Hcrt-1 negatively regulates the activity of brain reward circuitries. Hypocretin-induced reinstatement of cocaine seeking was prevented by blockade of noradrenergic and corticotropinreleasing factor systems, suggesting that Hcrt-1 reinstated drug seeking through induction of a stress-like state. Consistent with this interpretation, the selective Hcrt-1 receptor antagonist SB-334867 blocked footshock-induced reinstatement of previously extinguished cocaine-seeking behavior. These findings reveal a previously unidentified role for hypocretins in driving drug seeking through activation of stress pathways in the brain.D rug addiction is characterized by relapse to drug-taking behavior during periods of abstinence. Identification of brain mechanisms responsible for vulnerability to relapse is crucial for the development of effective treatments for drug addiction (1). Hypocretin-1 and -2 (Hcrt-1 and Hcrt-2), recently discovered lateral hypothalamic (LH) neuropeptides (2, 3), regulate a wide variety of physiological processes such as feeding, energy metabolism (4), and the maintenance of arousal (5, 6). Compelling evidence also indicates that Hcrt neurons in the LH receive inputs from diverse sensory and limbic systems and drive hyperarousal through modulation of stress responses (7,8) and adaptive behavior associated with energy metabolism (9). Mutant mice deficient in Hcrt fail to respond to fasting with increased activity and wakefulness (10) and display diminished signs of precipitated opiate withdrawal (11). Further, leptin, which hyperpolarizes Hcrt neurons in mice (10), attenuates fasting-induced heroin-seeking behavior in rats (12). These observations suggest a role for LH Hcrt neurons in reward seeking (13)(14)(15). Consistent with this hypothesis, c-Fos activation of LH Hcrt neurons was recently correlated with preference, in rats, for an environment repeatedly paired with food and drug rewards (16). Importantly, however, the mechanisms by which Hcrt systems may reinstate drug-seeking behaviors remain largely unexplored. Here, we show that the Hcrt-1 peptide reinstates previously extinguished cocaine-seeking behavior and induces a long-lasting brain reward deficit. Further, we demonstrate that antagonism of Hcrt-1 receptors prevents footshockinduced reinstatement of cocaine-seeking behavior in ra...

The escalation of drug intake observed with extended access is produced at multiple doses of methamphetamine. The rapidity of escalation depends on the dose. Ultimately, all doses in the dose-response study engendered self-administration of the same amount of total drug in a 6-h session in the extended-access group. Results suggest that the rapidity of escalation is dependent on dose and has an upper limit of intake over a period of 21 days.

The present study characterized nicotine intake, circadian patterns of food and water intake, precipitated somatic signs of withdrawal, and extinction of nicotine-seeking behavior in rats with 23-h access to intravenous self-administration (IVSA). Separate groups of animals were allowed access to nicotine IVSA (0.015, n ϭ 9; 0.03, n ϭ 14; 0.06, n ϭ 16; mg/kg/0.1 ml infusion/s; fixed ratio 1) and trained to nosepoke for food and water 23 h/day for 40 consecutive days. Somatic signs of nicotine withdrawal were examined following saline or mecamylamine administration (1.5 mg/kg i.p.), and extinction of nicotine-seeking behavior was assessed. A dose-dependent decrease in lever responding and an increase in nicotine intake were observed, with the highest nicotine dose producing the lowest amount of lever responding and the highest amount of nicotine intake. Nicotine acutely reduced diurnal and nocturnal food intake, producing smaller and fewer meals, and an increased rate of eating. Differences in rate of nicotine intake between the light and dark phase decreased significantly, especially in rats receiving higher unit nicotine doses (0.03 and 0.06 mg/kg), along with long-term decreases in the circadian profile and amplitude of feeding. Mecamylamine precipitated robust withdrawal signs, the magnitude of which was positively correlated with the total amount of self-administered nicotine. Extinction of nicotine-seeking behavior was observed and was facilitated by removal of nicotine-associated cues. The results demonstrate that rats will self-administer nicotine to the point of producing dependence, as measured by somatic signs, resistance to extinction, and measures of food intake.To more closely model tobacco use in humans, recent studies have examined extended access to nicotine intravenous self-administration (IVSA) in rats. For example, female rats display increased nicotine IVSA during the active phase of the light cycle during 3 weeks of continuous nicotine access (Cox et al., 1984). These rats also display a compensatory increase in nicotine IVSA when the dose is lowered (0.03 to 0.003 mg/kg) and a decrease in nicotine-seeking behavior when nicotine is replaced with saline. Moreover, male rats display nicotine IVSA in extended access models (6 -23 h) using low nicotine doses (0.00375 mg/kg/injection), and the level of nicotine intake approximates that of human smokers (Valentine et al., 1997;Paterson and Markou, 2004;Kenny and Markou, 2006). The 23-h access model of nicotine IVSA seems to be sensitive to genetic differences, since nicotine intake is more quickly acquired and persistently maintained in Lewis versus Holtzman and Fisher strains of male rats (Brower et al., 2002). Furthermore, the 23-h model of nicotine IVSA is sensitive to passive nicotine administration. Nicotine intake decreased following implantation of a minipump that delivers doses of nicotine that are equal to, or higher than, peak levels associated with simulated nicotine intake (LeSage et al., 2002). In addition, nicotine intake in ...

The juxtacapsular bed nucleus of the stria terminalis (jcBNST) is activated in response to basolateral amygdala (BLA) inputs through the stria terminalis and projects back to the anterior BLA and to the central nucleus of the amygdala. Here we show a form of long-term potentiation of the intrinsic excitability (LTP-IE) of jcBNST neurons in response to high-frequency stimulation of the stria terminalis. This LTP-IE, which was characterized by a decrease in the firing threshold and increased temporal fidelity of firing, was impaired during protracted withdrawal from self-administration of alcohol, cocaine, and heroin. Such impairment was graded and was more pronounced in rats that self-administered amounts of the drugs sufficient to maintain dependence. Dysregulation of the corticotropin-releasing factor (CRF) system has been implicated in manifestation of protracted withdrawal from dependent drug use. Administration of the selective corticotropin-releasing factor receptor 1 (CRF 1 ) antagonist R121919 [2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7-dipropylamino-pyrazolo[1,5-a]pyrimidine)], but not of the CRF 2 antagonist astressin 2 -B, normalized jcBNST LTP-IE in animals with a history of alcohol dependence; repeated, but not acute, administration of CRF itself produced a decreased jcBNST LTP-IE. Thus, changes in the intrinsic properties of jcBNST neurons mediated by chronic activation of the CRF system may contribute to the persistent emotional dysregulation associated with protracted withdrawal.

Previous studies showed that an extended 6-h session duration produced an increasing rate of cocaine self-administration in rats. The present study further investigated the effect of dose and session duration on cocaine self-administration. Eight groups of rats (4 doses ϫ 2 session durations) self-administered one of four cocaine doses (0.25, 0.5, 1, and 2 mg/kg/ injection) in either 1-or 6-h sessions under a fixed-ratio schedule. In another experiment, two other groups of rats selfadministered 0.5 mg/kg/injection of cocaine in either 3-or 12-h sessions. Cocaine self-administration increased at all doses in 6-h sessions but not in 1-h sessions. Cocaine intake (milligram/ kilogram) reached an asymptote earlier at a higher dose, but the rate of responding increased faster when the dose was lower.

The data suggest that hypersensitivity of the CRF system occurs with extended access to cocaine self-administration and that this altered CRF system may contribute to the increased motivation to self-administer cocaine that develops during psychostimulant dependence.

The effects of chronic desmethylimipramine (DMI) treatment on measures of incentive motivation for cocaine were assessed in order to investigate the predictive validity of the extinction/reinstatement model of drug craving. Rats were trained to respond for cocaine infusions (0.75 mg/kg per 0.1 ml i.v.) or received yoked-saline infusions during daily 3-h sessions. A light and tone were presented with the infusions. Following self-administration training, each group received daily injections of either saline or DMI (10 mg/kg, i.p.) for 21 days of withdrawal from the self-administration regimen. On days 12-21 of withdrawal, rats were allowed to respond in the absence of cocaine reinforcement (extinction phase). After reaching an extinction criterion of no responses for 1 h, the cocaine-paired stimuli were repeatedly presented to reinstate responding (reinstatement phase). In the control group, DMI treatment did not alter responding during either test phase, but increased the response latency during the extinction phase. In contrast, DMI treatment in the cocaine group decreased responding and increased the response latency during both test phases, and decreased the extinction latency during the extinction phase. Overall, the effects of DMI were consistent with a reduction of incentive motivation for cocaine, lending support for the predictive validity of the extinction/reinstatement model of drug craving.

The effects of systemic (0-1.0 mg/kg) or intraaccumbens (0-1.0 microg/side) administration of SCH-23390 on cocaine-induced (0 or 4.2 mg/kg, i.v.) locomotion, sniffing, and conditioned place preference (CPP) were investigated in rats. After behavioral testing was completed, animals were injected with their respective dose of SCH-23390 into the nucleus accumbens (NAc), followed by a systemic injection of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Receptors occupied by intraaccumbens SCH-23390, and therefore protected from EEDQ-induced inactivation, were then quantified from autoradiograms of sections labeled with 3H-SCH-23390. Systemic administration of 0.5 and 1.0 mg/kg SCH-23390 reversed cocaine-induced locomotion, sniffing, and CPP, suggesting that stimulation of D1-like receptors is necessary for these behavioral changes. Intraaccumbens administration of 1.0 microg/side SCH-23390 reversed cocaine-CPP, and this dose occupied D1-like receptors primarily in the rostral pole of the NAc. Intraaccumbens administration of 0.5 microg/side SCH-23390 reversed cocaine-induced locomotion. However, this dose occupied a similar number of D1-like receptors in the NAc as a lower and behaviorally ineffective dose of 0.1 microg/side, but occupied more receptors in the caudate-putamen relative to both the 0.1 and 1.0 microg/side doses. These findings suggest that stimulation of D1-like receptors in the NAc is necessary for cocaine-CPP, but not for cocaine-induced locomotion.