Serotonin (5-HT) controls affective and motivational aspects of palatable food and drug reward and the 5-HT2C receptor (5-HT2CR) has emerged as a key regulator in this regard. We have evaluated the efficacy of a selective 5-HT2CR agonist, WAY 163909, in cocaine and sucrose self-administration and reinstatement assays employing parallel experimental designs in free-fed rats. WAY 163909 dose-dependently reduced the reinforcing efficacy of cocaine (ID50=1.19 mg/kg) and sucrose (ID50=0.7 mg/kg) as well as reinstatement (ID50=0.5 mg/kg) elicited by exposure to cocaine-associated contextual cues, but not sucrose-associated contextual cues. The ID50 of WAY 163909 predicted to decrease the reinforcing efficacy of cocaine or sucrose as well as reinstatement upon exposure to cocaine-associated cues was ~5–12-fold lower than that predicted to suppress horizontal ambulation (ID50 = 5.89 mg/kg) and ~2-5-fold lower than that predicted to suppress vertical activity (ID50= 2.3 mg/kg). Thus, selective stimulation of the 5-HT2CR decreases the reinforcing efficacy of cocaine and sucrose in freely-fed rats, but differentially alters the incentive-salience value of cocaine- vs. sucrose-associated cues at doses that do not impair locomotor activity. Future research is needed to tease apart the precise contribution of 5-HT2CR neurocircuitry in reward and motivation and the learning and memory processes that carry the encoding for associations between environmental cues and consumption of rewarding stimuli. A more complete preclinical evaluation of these questions will ultimately allow educated proof-of-concept trials to test the efficacy of selective 5-HT2CR agonists as adjunctive therapy in chronic health maladies including obesity, eating disorders and drug addiction.

Relapse to cocaine dependence, even after extended abstinence, involves a number of liability factors including impulsivity (predisposition toward rapid, unplanned reactions to stimuli without regard to negative consequences) and cue reactivity (sensitivity to cues associated with cocaine-taking which can promote cocaine-seeking). These factors have been mechanistically linked to serotonin (5-hydroxytryptamine, 5-HT) signaling through the 5-HT 2A receptor (5-HT 2A R) and 5-HT 2C R; either a selective 5-HT 2A R antagonist or a 5-HT 2C R agonist suppresses impulsivity and cocaine-seeking in preclinical models. We conducted proof-of-concept analyses to evaluate whether a combination of 5-HT 2A R antagonist plus 5-HT 2C R agonist would have synergistic effects over these liability factors for relapse as measured in a 1-choice serial reaction time task and cocaine self-administration/reinstatement assay. Combined administration of a dose of the selective 5-HT 2A R antagonist M100907 plus the 5-HT 2C R agonist WAY163909, each ineffective alone, synergistically suppressed cocaine-induced hyperactivity, inherent and cocaine-evoked impulsive action, as well as cue-and cocaine-primed reinstatement of cocaine-seeking behavior. The identification of synergism between a 5-HT 2A R antagonist plus a 5-HT 2C R agonist to attenuate these factors important in relapse indicates the promise of a bifunctional ligand as an anti-addiction pharmacotherapeutic, setting the stage to develop new ligands with improved efficacy, potency, selectivity, and in vivo profiles over the individual molecules. KEYWORDS: 1-Choice serial reaction time task, 5-HT 2A receptor, 5-HT 2C receptor, cocaine, cue reactivity, impulsivity, self-administration, serotonin D ependence on the psychostimulant cocaine is marked by problematic vulnerability to relapse even years into abstinence. 1,2 Two important liability factors that contribute to relapse are impulsivity (predisposition toward rapid, unplanned reactions to stimuli without regard to negative consequences) 3 and cue reactivity (sensitivity to cues associated with cocaine-taking that can promote cocaine-seeking). 4,5 Impulsivity and cue reactivity appear to be interrelated in human cocaine users, 6 and new pharmacotherapeutic strategies that effectively diminish both are likely to enhance abstinence in the highly vulnerable population of cocaine-dependent individuals.The underlying neurobiology of these liability factors includes a regulatory role for serotonin (5-HT; 5-hydroxytryptamine) neurotransmission. The actions of 5-HT in neurons are transduced by at least 14 subtypes of 5-HT receptors (5-HT X Rs) which are presently grouped into seven families (5-HT 1 R−5-HT 7 R) according to their structural and functional characteristics. 7−9 Selective blockade of the 5-HT 2A R 10−12 or activation of the 5-HT 2C R 10,13,14 consistently reduces impulsivity as measured by premature responses assessed in the 1-or 5-choice serial reaction time (1-or 5-CSRT) task, a preclinical Special Issue: Celebratin...

Serotonin (5-hydroxytryptamine; 5-HT) signaling through the 5-HT2C receptor (5-HT2CR) is essential in normal physiology, whereas aberrant 5-HT2CR function is thought to contribute to the pathogenesis of multiple neural disorders. The 5-HT2CR interacts with specific protein partners, but the impact of such interactions on 5-HT2CR function is poorly understood. Here, we report convergent cellular and behavioral data that the interaction between the 5-HT2CR and protein phosphatase and tensin homolog (PTEN) serves as a regulatory mechanism to control 5-HT2CR-mediated biology but not that of the closely homologous 5-HT2AR. A peptide derived from the third intracellular loop of the human 5-HT2CR [3L4F (third loop, fourth fragment)] disrupted the association, allosterically augmented 5-HT2CR-mediated signaling in live cells, and acted as a positive allosteric modulator in rats in vivo. We identified the critical residues within an 8 aa fragment of the 3L4F peptide that maintained efficacy (within the picomolar range) in live cells similar to that of the 3L4F peptide. Last, molecular modeling identified key structural features and potential interaction sites of the active 3L4F peptides against PTEN. These compelling data demonstrate the specificity and importance of this protein assembly in cellular events and behaviors mediated by 5-HT2CR signaling and provide a chemical guidepost to the future development of drug-like peptide or small-molecule inhibitors as neuroprobes to study 5-HT2CR allostery and therapeutics for 5-HT2CR-mediated disorders.

Intraerythrocytic malaria parasites use host hemoglobin as a major nutrient source. Aspartic proteases (plasmepsins) and cysteine proteases (falcipains) function in the early steps of the hemoglobin degradation pathway. There is extensive functional redundancy within and between these protease families. Plasmepsins are synthesized as integral membrane proenzymes that are activated by cleavage from the membrane. This cleavage is mediated by a maturase activity whose identity has been elusive. We have used a combination of cell biology, chemical biology, and enzymology approaches to analyze this processing event. These studies reveal that plasmepsin processing occurs primarily via the falcipains; however, if falcipain activity is blocked, autoprocessing can take place, serving as an alternate activation system. These results establish a further level of redundancy between the protease families involved in Plasmodium hemoglobin degradation.