Interaction between serotonin transporter and dopamine D2/D3 receptor radioligand measures is associated with harm avoidant symptoms in anorexia and bulimia nervosa

Neuronal ablation of p-Akt at Ser473 leads to altered 5-HT1A/2A receptor function

The serotonergic system regulates a wide range of behavior, including mood and impulsivity, and its dysregulation has been associated with mood disorders, autism spectrum disorder, and addiction. Diabetes is a risk factor for these conditions. Insulin resistance in the brain is specifically associated with susceptibility to psychostimulant abuse. Here, we examined whether phosphorylation of Akt, a key regulator of the insulin signaling pathway, controls serotonin (5-HT) signaling. To explore how impairment in Akt function regulates 5-HT homeostasis, we used a brain-specific rictor knockout (KO) mouse model of impaired neuronal phosphorylation of Akt at Ser473. Cortical 5-HT1A and 5-HT2A receptor binding was significantly elevated in rictor KO mice. Concomitant with this elevated receptor expression, the 5-HT1A receptor agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) led to an increased hypothermic response in rictor KO mice. The increased cortical 5-HT1A receptor density was associated with higher 5-HT1A receptor levels on the cortical cell surface. In contrast, rictor KO mice displayed significantly reduced head-twitch response (HTR) to the 5-HT2A/C agonist 2,5-dimethoxy-4-iodoamphetamine (DOI), with evidence of impaired 5-HT2A/C receptor signaling. In vitro, pharmacological inhibition of Akt significantly increased 5-HT1A receptor expression and attenuated DOI-induced 5-HT2A receptor signaling, thereby lending credence to the observed in vivo cross-talk between neuronal Akt signaling and 5-HT receptor regulation. These data reveal that defective central Akt function alters 5-HT signaling as well as 5-HT-associated behaviors, demonstrating a novel role for Akt in maintaining neuronal 5-HT receptor function.

Mapping neurotransmitter networks with PET: an example on serotonin and opioid systems

All functions of the human brain are consequences of altered activity of specific neural pathways and neurotransmitter systems. Although the knowledge of "system level" connectivity in the brain is increasing rapidly, we lack "molecular level" information on brain networks and connectivity patterns. We introduce novel voxel-based positron emission tomography (PET) methods for studying internal neurotransmitter network structure and intercorrelations of different neurotransmitter systems in the human brain. We chose serotonin transporter and μ-opioid receptor for this analysis because of their functional interaction at the cellular level and similar regional distribution in the brain. Twenty-one healthy subjects underwent two consecutive PET scans using [(11)C]MADAM, a serotonin transporter tracer, and [(11)C]carfentanil, a μ-opioid receptor tracer. First, voxel-by-voxel "intracorrelations" (hub and seed analyses) were used to study the internal structure of opioid and serotonin systems. Second, voxel-level opioid-serotonin intercorrelations (between neurotransmitters) were computed. Regional μ-opioid receptor binding potentials were uniformly correlated throughout the brain. However, our analyses revealed nonuniformity in the serotonin transporter intracorrelations and identified a highly connected local network (midbrain-striatum-thalamus-amygdala). Regionally specific intercorrelations between the opioid and serotonin tracers were found in anteromedial thalamus, amygdala, anterior cingulate cortex, dorsolateral prefrontal cortex, and left parietal cortex, i.e., in areas relevant for several neuropsychiatric disorders, especially affective disorders. This methodology enables in vivo mapping of connectivity patterns within and between neurotransmitter systems. Quantification of functional neurotransmitter balances may be a useful approach in etiological studies of neuropsychiatric disorders and also in drug development as a biomarker-based rationale for targeted modulation of neurotransmitter networks.

Convergent dysregulation of frontal cortical cognitive and reward systems in eating disorders

A substantive literature has drawn a compelling case for the functional involvement of mesolimbic/prefrontal cortical neural reward systems in normative control of eating and in the etiology and persistence of severe eating disorders that affect diverse human populations. Presently, we provide a short review that develops an equally compelling case for the importance of dysregulated frontal cortical cognitive neural networks acting in concert with regional reward systems in the regulation of complex eating behaviors and in the presentation of complex pathophysiological symptoms associated with major eating disorders. Our goal is to highlight working models of major eating disorders that incorporate complementary approaches to elucidate functionally interactive neural circuits defined by their regulatory neurochemical phenotypes. Importantly, we also review evidence-based linkages between widely studied psychiatric and neurodegenerative syndromes (e.g., autism spectrum disorders and Parkinson’s disease) and co-morbid eating disorders to elucidate basic mechanisms involving dopaminergic transmission and its regulation by endogenously expressed morphine in these same cortical regions.

Does a shared neurobiology for foods and drugs of abuse contribute to extremes of food ingestion in anorexia and bulimia nervosa?

Is starvation in anorexia nervosa (AN) or overeating in bulimia nervosa (BN) a form of addiction? Alternatively, why are individuals with BN more vulnerable and individuals with AN protected from substance abuse? Such questions have been generated by recent studies suggesting that there are overlapping neural circuits for foods and drugs of abuse. To determine whether a shared neurobiology contributes to eating disorders and substance abuse, this review focused on imaging studies that investigated response to tastes of food and tasks designed to characterize reward and behavioral inhibition in AN and BN. BN and those with substance abuse disorders may share dopamine D2 receptor-related vulnerabilities, and opposite findings may contribute to "protection" from substance abuse in AN. Moreover, imaging studies provide insights into executive corticostriatal processes related to extraordinary inhibition and self-control in AN and diminished inhibitory self-control in BN that may influence the rewarding aspect of palatable foods and likely other consummatory behaviors. AN and BN tend to have premorbid traits, such as perfectionism and anxiety that make them vulnerable to using extremes of food ingestion, which serve to reduce negative mood states. Dysregulation within and/or between limbic and executive corticostriatal circuits contributes to such symptoms. Limited data support the hypothesis that reward and inhibitory processes may contribute to symptoms in eating disorders and addictive disorders, but little is known about the molecular biology of such mechanisms in terms of shared or independent processes.

Nothing tastes as good as skinny feels: the neurobiology of anorexia nervosa

Individuals with anorexia nervosa (AN) engage in relentless restrictive eating and often become severely emaciated. Because there are no proven treatments, AN has high rates of relapse, chronicity, and death. Those with AN tend to have childhood temperament and personality traits, such as anxiety, obsessions, and perfectionism, which may reflect neurobiological risk factors for developing AN. Restricted eating may be a means of reducing negative mood caused by skewed interactions between serotonin aversive or inhibitory and dopamine reward systems. Brain imaging studies suggest that altered eating is a consequence of dysregulated reward and/or awareness of homeostatic needs, perhaps related to enhanced executive ability to inhibit incentive motivational drives. An understanding of the neurobiology of this disorder is likely to be important for developing more effective treatments.