Stephen V. Mahler

Endocannabinoid Hedonic Hotspot for Sensory Pleasure: Anandamide in Nucleus Accumbens Shell Enhances ‘Liking’ of a Sweet Reward

Cannabinoid drugs such as Δ9-THC are euphoric and rewarding, and also stimulate food intake in humans and animals. Little is known about how naturally occurring endogenous brain cannabinoids mediate pleasure from food or other natural sensory rewards. The taste reactivity paradigm measures effects of brain manipulations on affective orofacial reactions to intraorally administered pleasant and unpleasant tastes. Here we tested if anandamide microinjection into medial nucleus accumbens shell enhances these affective reactions to sweet and bitter tastes in rats. Anandamide doubled the number of positive ‘liking’ reactions elicited by intraoral sucrose, without altering negative ‘disliking’ reactions to bitter quinine. Anandamide microinjections produced Fos plumes of approximately 0.02–1 mm3 volume. Plume-based maps, integrated with behavioral data, identified the medial shell of accumbens as the anatomical hotspot responsible for hedonic amplification. Anandamide produced especially intense hedonic enhancement in a roughly 1.6 mm3 ‘hedonic hotspot’ in dorsal medial shell, where anandamide also stimulated eating behavior. These results demonstrate that endocannabinoid signals within medial accumbens shell specifically amplify the positive hedonic impact of a natural reward (though identification of the receptor specificity of this effect will require future studies). Identification of an endocannabinoid hotspot for sensory pleasure gives insight into brain mechanisms of natural reward, and may be relevant to understanding the neural effects of cannabinoid drugs of abuse and therapeutic agents.

Which Cue to “Want?” Central Amygdala Opioid Activation Enhances and Focuses Incentive Salience on a Prepotent Reward Cue

The central nucleus of the amygdala (CeA) helps translate learning into motivation, and here, we show that opioid stimulation of CeA magnifies and focuses learned incentive salience onto a specific reward cue (pavlovian conditioned stimulus, or CS). This motivation enhancement makes that cue more attractive, noticeable, and liable to elicit appetitive and consummatory behaviors. To reveal the focusing of incentive salience, we exploited individual differences in an autoshaping paradigm in which a rat prefers to approach, nibble, and sniff one of two reward-associated stimuli (its prepotent stimulus). The individually prepotent cue is either a predictive CS+ that signals reward (8 s metal lever insertion) or instead the metal cup that delivers sucrose pellets (the reward source). Results indicated that CeA opioid activation by microinjection of the μ agonist DAMGO (0.1 μg) selectively and reversibly enhanced the attractiveness of whichever reward CS was that rats prepotent cue. CeA DAMGO microinjections made rats more vigorously approach their particular prepotent CS and to energetically sniff and nibble it in a nearly frenzied consummatory manner. Only the prepotent cue was enhanced as an incentive target, and alternative cues were not enhanced. Conversely, inactivation of CeA by muscimol microinjection (0.25 μg) suppressed approach, nibbles, and sniffs of the prepotent CS. Confirming modulation of incentive salience, unconditioned food intake was similarly increased by DAMGO microinjection and decreased by muscimol in CeA. We conclude that opioid neurotransmission in CeA helps determine which environmental stimuli become most “wanted,” and how “wanted” they become. This may powerfully guide reward-seeking behavior.

Behavioral and hormonal responses of men to brief interactions with women

This study tested for behavioral and hormonal reactions of young men to brief social encounters with potential mating partners. Male college students were randomly assigned to engage in a short conversation with either a young man (male condition) or a young woman (female condition). Participants provided saliva samples before and after the conversation, completed a battery of psychological measures after the interaction, and had their behavior rated by their conversation partners. Salivary testosterone (T) increased significantly over baseline levels in the female condition only, though differences between conditions were not significant. In addition, change in T was significantly correlated with the degree to which the female confederates thought the male participants were trying to impress them. These behavioral ratings, in turn, were correlated with the participants’ ratings of the female confederates as potential romantic partners. Results were generally consistent with the hypothesis that human males may exhibit a behavioral and endocrine courtship response that is similar to that observed in males of many nonhuman vertebrate species.

Designer receptors show role for ventral pallidum input to ventral tegmental area in cocaine seeking.

The ventral pallidum is centrally positioned within mesocorticolimbic reward circuits, and its dense projection to the ventral tegmental area (VTA) regulates neuronal activity there. However, the ventral pallidum is a heterogeneous structure, and how this complexity affects its role within wider reward circuits is unclear. We found that projections to VTA from the rostral ventral pallidum (RVP), but not the caudal ventral pallidum (CVP), were robustly Fos activated during cue-induced reinstatement of cocaine seeking—a rat model of relapse in addiction. Moreover, designer receptor–mediated transient inactivation of RVP neurons, their terminals in VTA or functional connectivity between RVP and VTA dopamine neurons blocked the ability of drug-associated cues (but not a cocaine prime) to reinstate cocaine seeking. In contrast, CVP neuronal inhibition blocked cocaine-primed, but not cue-induced, reinstatement. This double dissociation in ventral pallidum subregional roles in drug seeking is likely to be important for understanding the mesocorticolimbic circuits underlying reward seeking and addiction.

Motivational activation: a unifying hypothesis of orexin/hypocretin function

Orexins (hypocretins) are two peptides (orexin A and B) produced from the pre-pro-orexin precursor and expressed in a limited region of dorsolateral hypothalamus. Orexins were originally thought to specifically mediate feeding and promote wakefulness, but it is now clear that they participate in a wide range of behavioral and physiological processes under select circumstances. Orexins primarily mediate behavior under situations of high motivational relevance, such as during physiological need states, exposure to threats or reward opportunities. We hypothesize that many behavioral functions of orexins (including regulation of sleep/wake cycling) reflect a fundamentally integrated function for orexins in translating motivational activation into organized suites of psychological and physiological processes supporting adaptive behaviors. We also discuss how numerous forms of neural heterogeneity modulate this function, allowing orexin neurons to organize diverse, adaptive responses in a variety of motivationally relevant situations. Thus, the involvement of orexins in diverse behaviors may reflect a common underlying function for this peptide system.

Multiple roles for orexin/hypocretin in addiction

Orexins/hypocretins are hypothalamic peptides involved in arousal and wakefulness, but also play a critical role in drug addiction and reward-related behaviors. Here, we review the roles played by orexins in a variety of animal models of drug addiction, emphasizing both commonalities and differences for orexins involvement in seeking of the major classes of abused drugs, as well as food. One common theme that emerges is an involvement of orexins in drug seeking triggered by external stimuli (e.g., cues, contexts or stressors). We also discuss the functional neuronal circuits in which orexins are embedded, and how these circuits mediate addiction-related behaviors, with particular focus on the role of orexin and glutamate interactions within the ventral tegmental area. Finally, we attempt to contextualize the role of orexins in reward by discussing ways in which these peptides, expressed in only a few thousand neurons in the brain, can have such wide-ranging effects on behavior.

Fos Activation of Selective Afferents to Ventral Tegmental Area during Cue-Induced Reinstatement of Cocaine Seeking in Rats

Ventral tegmental area (VTA) dopamine neurons are crucial for appetitive responses to Pavlovian cues, including cue-induced reinstatement of drug seeking. However, it is unknown which VTA inputs help activate these neurons, transducing stimuli into salient cues that drive drug-seeking behavior. Here we examined 56 VTA afferents from forebrain and midbrain that are Fos activated during cue-induced reinstatement. We injected the retrograde tracer cholera toxin β subunit (CTb) unilaterally into rostral or caudal VTA of male rats. All animals were trained to self-administer cocaine, then extinguished of this behavior. On a final test day, animals were exposed to response-contingent cocaine-associated cues, extinction conditions, a non-cocaine-predictive CS−, or a novel environment, and brains were processed to visualize CTb and Fos immunoreactivity to identify VTA afferents activated in relation to behaviors. VTA-projecting neurons in subregions of medial accumbens shell, ventral pallidum, elements of extended amygdala, and lateral septum (but not prefrontal cortex) were activated specifically during cue-induced cocaine seeking, and some of these were also activated proportionately to the degree of cocaine seeking. Surprisingly, though efferents from the lateral hypothalamic orexin field were also Fos activated during reinstatement, these were largely non-orexinergic. Also, VTA afferents from the rostromedial tegmental nucleus and lateral habenula were specifically activated during extinction and CS− tests, when cocaine was not expected. These findings point to a select set of subcortical nuclei which provide reinstatement-related inputs to VTA, translating conditioned stimuli into cocaine-seeking behavior.

Interactions between VTA orexin and glutamate in cue-induced reinstatement of cocaine seeking in rats

RationaleGlutamate and orexin/hypocretin systems are involved in Pavlovian cue-triggered drug seeking.ObjectivesHere, we asked whether orexin and glutamate interact within ventral tegmental area (VTA) to promote reinstatement of extinguished cocaine seeking in a rat self-administration paradigm.Methods/resultsWe first found that bilateral VTA microinjections of the orexin 1 receptor (OX1R) antagonist SB-334867 (SB) or a cocktail of the AMPA and NMDA glutamate receptor antagonists CNQX/AP-5 reduced reinstatement of cocaine seeking elicited by cues. In contrast, neither of these microinjections nor systemic SB reduced cocaine-primed reinstatement. Additionally, unilateral VTA OX1R blockade combined with contralateral VTA glutamate blockade attenuated cue-induced reinstatement, indicating that VTA orexin and glutamate are simultaneously necessary for cue-induced reinstatement. We further probed the receptor specificity of glutamate actions in VTA, finding that CNQX, but not AP-5, dose-dependently attenuated cue-induced reinstatement, indicating that AMPA but not NMDA receptor transmission is required for this type of cocaine seeking. Given the necessary roles of both OX1 and AMPA receptors in VTA for cue-induced cocaine seeking, we hypothesized that these signaling pathways interact during this behavior. We found that PEPA, a positive allosteric modulator of AMPA receptors, completely reversed the SB-induced attenuation of reinstatement behavior. Intra-VTA PEPA alone did not alter cue-induced reinstatement, indicating that potentiating AMPA activity with this drug specifically compensates for OX1R blockade, rather than simply inducing or enhancing reinstatement itself.ConclusionsThese findings show that cue-induced, but not cocaine-primed, reinstatement of cocaine seeking is dependent upon orexin and AMPA receptor interactions in VTA.

What and when to “want”? Amygdala-based focusing of incentive salience upon sugar and sex

RationaleAmygdala-related circuitry helps translate learned Pavlovian associations into appetitive and aversive motivation, especially upon subsequent encounters with cues.ObjectivesWe asked whether μ-opioid stimulation via microinjections of the specific agonist d-Ala2, N-MePhe4, Gly-ol)-enkephalin (DAMGO) in central nucleus of amygdala (CeA), or the adjacent basolateral amygdala (BLA) would magnify sucrose or sex “wanting”, guided by available cues.Materials and methodsCeA or BLA DAMGO enhancement of cue-triggered “wanting” was assessed using Pavlovian to instrumental transfer (PIT). Unconditioned food “wanting” was measured via intake, and male sexual “wanting” for an estrous female was measured in a sexual approach test. Sucrose hedonic taste “liking” was measured in a taste reactivity test.ResultsCeA (but not BLA) DAMGO increased the intensity of phasic peaks in instrumental sucrose seeking stimulated by Pavlovian cues over precue levels in PIT, while suppressing seeking at other moments. CeA DAMGO also enhanced food intake, as well as sexual approach and investigation of an estrous female by males. DAMGO “wanting” enhancements were localized to CeA, as indicated by “Fos plume”-based anatomical maps for DAMGO causation of behavioral effects. Despite increasing “wanting”, CeA DAMGO decreased the hedonic impact or “liking” for sucrose in a taste reactivity paradigm.ConclusionsCeA μ-opioid stimulation specifically enhances incentive salience, which is dynamically guided to food or sex by available cues.

Mecamylamine and Ethanol Preference in Healthy Volunteers

BACKGROUND Recent evidence suggests that some of the behavioral effects of alcohol may be mediated through actions on nicotinic acetylcholine receptors. Mecamylamine, a nicotinic acetylcholine receptor antagonist, reduces alcohol preference and consumption in alcohol-preferring rats, and in humans, mecamylamine dampens some of the subjective, or mood-altering, effects of alcohol. This experiment was designed to investigate the effects of mecamylamine on consumption of alcohol in healthy social drinkers. METHODS Healthy volunteers (12 men, 12 women) participated in a choice procedure in which they chose between an alcoholic beverage and money (low, medium, or high amounts) after pretreatment with mecamylamine (7.5 or 15 mg) or placebo. Outcome measures were the number of alcoholic beverages consumed and the subjective effects of alcohol. RESULTS Mecamylamine (15 mg) decreased blood alcohol levels (BALs) after a small fixed dose of alcohol (0.2 g/kg). Even when the lower BALs were taken into account, mecamylamine reduced ratings of stimulation after alcohol (Addiction Research Center Inventory A scale). Mecamylamine did not significantly reduce choice for alcohol versus money. However, there was a tendency for the drug to decrease alcohol choice among participants who reported the greatest stimulant-like effects from alcohol. CONCLUSION These results provide only limited support for the idea that nicotinic acetylcholine receptors are involved in the rewarding effects of alcohol.