Uncoupling the dopamine D1-D2 receptor complex: a novel target for antidepressant treatment

Characterization of rice endosperm-derived antidepressant-like peptide (REAP): An orally active novel tridecapeptide derived from rice protein

It is ideal to ingest bioactive substances from daily foods to stay healthy. Rice is the staple food for almost half of the human population. We found that an orally administered enzymatic digest of rice endosperm protein exhibits antidepressant-like effects in the tail suspension test (TST) using mice. A comprehensive peptide analysis of the digest using liquid chromatography-tandem mass spectrometry was performed, and a tridecapeptide QQFLPEGQSQSQK, detected in the digest, was chemosynthesized. Oral administration of the tridecapeptide exhibited antidepressant-like effects at a low dose comparable to classical antidepressant in the TST. This also exhibited anti-depressant-like effect in the forced swim test. We named it rice endosperm-derived antidepressant-like peptide (REAP). Intriguingly, intraperitoneal administration had no effect. Orally administered REAP(8-13) but not REAP(1-7) exhibited antidepressant-like activity, suggesting that the C-terminal structure is important for the antidepressant-like effect. We confirmed the presence of REAP, corresponding to rice glutelin type B4(130-142) and B5(130-142), in the digest. The effects of REAP were blocked by both dopamine D1 and D2 antagonists. These results suggest that it exerts its antidepressant-like activity through activation of the dopamine system. Taken together, oral administration of a novel tridecapeptide exhibited antidepressant-like effects via the dopamine system. This is the first report of a rice-derived peptide that exhibits antidepressant-like effects.

Dysregulation of Striatal Dopamine Receptor Binding in Suicide

Inconsistent evidence implicates disruptions of striatal dopaminergic indices in suicide and major depression. To determine whether there are alterations in the striatal dopamine system in suicide, we conducted a quantitative autoradiographic survey of dopamine transporter (DAT; [³H]mazindol), D1 receptor ([³H]SCH23390), and D2 receptor ([³H]sulpiride) binding in the dorsal striatum postmortem from matched suicides and controls. Axis I and axis II psychiatric diagnosis, recent treatment history, and early life adversity (ELA) were determined by psychological autopsy. Mean DAT, D2, and D1 receptor binding did not differ in suicide. However, there was a positive correlation between D1 and D2 receptor binding in the dorsal striatum of control subjects (R²=0.31, p<0.05) that was not present in suicides (R²=0.00, p=0.97). In suicides and controls with reported ELA, there was no correlation between striatal DAT and D1 receptor binding (R²=0.07, p=0.33), although DAT and D1 receptor binding was positively correlated in subjects with no report of ELA (R²=0.32, p<0.05). After controlling for age, there were no significant ELA-related mean differences. Binding of D1 receptors and DAT throughout the striatum correlated negatively with age (D1 receptor: R²=0.12, p<0.05; DAT: R²=0.36, p<0.001). There appears to be an imbalance in dopaminergic receptor and transporter expression related to suicide that differs from that associated with ELA or age.

Do certain signal transduction mechanisms explain the comorbidity of epilepsy and mood disorders?

It is well known that mood disorders are highly prevalent in patients with epilepsy. Although several studies have aimed to characterize alterations in different types of receptors associated with both disturbances, there is a lack of studies focused on identifying the causes of this comorbidity. Here, we described some changes at the biochemical level involving serotonin, dopamine, and γ-aminobutyric acid (GABA) receptors as well as signal transduction mechanisms that may explain the coexistence of both epilepsy and mood disorders. Finally, the identification of common pathophysiological mechanisms associated with receptor-receptor interaction (heterodimers) could allow designing new strategies for treatment of patients with epilepsy and comorbid mood disorders.

Intranasal delivery of a peptide with antidepressant-like effect

A critical issue in drug development is developing effective, noninvasive delivery routes to the central nervous system (CNS). Major depressive disorder (MDD) is an illness associated with significant morbidity. Even with multiple antidepressant trials, 10-15% of patients continue to experience persistent depressive symptoms. We previously developed an interfering peptide that has antidepressant-like effects in rats when injected directly into the brain. To be clinically viable, it must demonstrate efficacy via a noninvasive administration route. We report here that the interfering peptide designed to disrupt the interaction between the D1 and D2 dopamine receptors can be delivered to relevant brain areas using the Pressurized Olfactory Device (POD), a novel intranasal delivery system developed by Impel NeuroPharma. We validate this delivery method by demonstrating that, at doses ⩾1.67 nmol/g, the D1-D2 interfering peptide has a significant antidepressant-like effect comparable to that of imipramine in the forced swimming test (FST), a common test for antidepressant efficacy. The antidepressant-like effect of the interfering peptide can be detected for 2 h after intranasal administration. Furthermore, we show that the interfering peptide disrupts the D1-D2 interaction and it can be detected in the prefrontal cortex after intranasal administration. This study provides strong preclinical support for intranasal administration of the D1-D2 interfering peptide as a new treatment option for patients suffering from MDD.

Antidepressant-like effects of lycii radicis cortex and betaine in the forced swimming test in rats

The purpose of the present study was to examine the effect of Lycii Radicis Cortex (LRC) and betaine (BT) on immobility and neurochemical change in the forced swimming test (FST) in the rat. LRC, BT or fluoxentine was administered intraperitoneally to Sprague-Dawley rats three times (1, 5 and 23.5 h) before the FST. To investigate antidepressant-like effect, serotonin (5-HT) and norepinephrine (NE) were examined in the hippocampus and hypothalamus of rats. LRC (100 mg/kg) and BT (30, 100 mg/kg) significantly decreased the immobility time in the FST. LRC (100 mg/kg) significantly increased both 5-HT and NE levels in the hypothalamus of rats exposed to FST. BT (100 mg/kg) significantly increased 5-HT levels in the hypothalamus and hippocampus of rats. Taken together, these results demonstrated that improvement in the behavioral changes after LRC and BT administration may be mediated by elevation of 5-HT level in the hypothalamus and hippocampus, indicating a possible antidepressant-like activity. The present results suggest that the efficacy of LRC and BT in an animal model of depression may provide anti-depressant effects in human, which remains to be determined.

Elevated dopamine D2 receptor in prefrontal cortex of CUMS rats is associated with downregulated cAMP-independent signaling pathway

Because depression is associated with significant morbidity and functional disability, it is important to reveal the mechanism of action. A variety of studies have suggested the involvement of dopaminergic receptors in the pathophysiological mechanism of non-stress-associated depression-like behavior in rodents. Nevertheless, controversy exists about whether chronic stress acts on dopaminergic receptors in the prefrontal cortex. Thus, we investigated the level of dopamine D2 receptors (DRD2) and the possible mechanisms involved in a chronic unpredictable mild stress (CUMS) rat model of depression. The results showed CUMS-induced, depression-like symptoms in the rat, characterized by reduced sucrose consumption and body mass, and increased duration of immobility in a forced swimming test. Moreover, chronic stress upregulated the expression of DRD2 but downregulated protein kinase A (PKA), transcription factor cAMP response element binding protein (CREB), and phospho-CREB (p-CREB) in the prefrontal cortex, as demonstrated by Western blot. Notably, in the rat model of depression, decreased cyclic adenine monophosphate (cAMP) levels and PKA activity were present at the same time, which is consistent with clinical findings in depressed patients. Our findings suggested that dopaminergic system dysfunction could play a central role in stress-related disorders such as depression.

Understanding resilience

Resilience is the ability to adapt successfully in the face of stress and adversity. Stressful life events, trauma, and chronic adversity can have a substantial impact on brain function and structure, and can result in the development of posttraumatic stress disorder (PTSD), depression and other psychiatric disorders. However, most individuals do not develop such illnesses after experiencing stressful life events, and are thus thought to be resilient. Resilience as successful adaptation relies on effective responses to environmental challenges and ultimate resistance to the deleterious effects of stress, therefore a greater understanding of the factors that promote such effects is of great relevance. This review focuses on recent findings regarding genetic, epigenetic, developmental, psychosocial, and neurochemical factors that are considered essential contributors to the development of resilience. Neural circuits and pathways involved in mediating resilience are also discussed. The growing understanding of resilience factors will hopefully lead to the development of new pharmacological and psychological interventions for enhancing resilience and mitigating the untoward consequences.

Heterodimerization with the prostacyclin receptor triggers thromboxane receptor relocation to lipid rafts

OBJECTIVE

Prostacyclin and thromboxane mediate opposing cardiovascular actions through receptors termed IP and TP, respectively. When dimerized with IP, the TP shifts to IP-like function. IP localizes to cholesterol-enriched membrane rafts, but TP and IPTP heterodimer localization is not defined. We examined these receptors' membrane localization and the role of rafts in receptor function.

METHODS AND RESULTS

Microdomain distribution of IP, TP, and IPTP heterodimers was examined in COS-7 cells by measuring energy transfer from renilla luciferase-fused receptors to fluorescently labeled rafts. IP raft association was confirmed. TP was raft excluded, but redistributed to rafts upon dimerization with IP. Signaling of the IP and IPTP heterodimer, but not TP alone, was suppressed after raft disruption by cholesterol depletion. Cholesterol enrichment also selectively suppressed IP and IPTP function. Native IP and IPTP signaling in smooth muscle cells and macrophages were similarly sensitive to cholesterol manipulation, whereas macrophages from hypercholesterolemic mice displayed suppressed IP and IPTP function.

CONCLUSIONS

IP and TP function within distinct microdomains. Raft incorporation of TP in the IPTP heterodimer likely facilitates its signaling shift. We speculate that changes in IP and IPTP signaling after perturbation of membrane cholesterol may contribute to cardiovascular disease associated with hypercholesterolemia.