Diabetes attenuates the antidepressant-like effect mediated by the activation of 5-HT1A receptor in the mouse tail suspension test

AMP-activated protein kinase in the amygdala and hippocampus contributes to enhanced fear memory in diabetic mice

BACKGROUND AND PURPOSE

Diabetic patients have an increased risk of psychiatric disorders. Because hyperglycaemia increases L-lactate in the brain and L-lactate inhibits AMP-activated protein kinase (AMPK), this study investigated the role of L-lactate and AMPK in strengthened fear memory, a model for human psychiatric disorders, in diabetic mice.

EXPERIMENTAL APPROACH

The diabetic model was mice injected with streptozotocin. Fear memory was measured using the conditioned fear test with low (0.45 mA) or high (0.50 mA) foot shock to cause low and high freezing, respectively. Protein levels of AMPK and phosphorylated AMPK (pAMPK) were measured by western blotting and immunohistochemistry.

KEY RESULTS

At 0.45 mA, the AMPK inhibitor dorsomorphin increased freezing, which was inhibited by the AMPK activator acadesine. In contrast, at 0.50 mA, acadesine decreased freezing, which was inhibited by dorsomorphin. In diabetic mice, pAMPK was decreased in the amygdala and hippocampus. Diabetic mice showed increased freezing at 0.45 mA, which was inhibited by acadesine. In the amygdala and hippocampus, L-lactate was increased in diabetic mice and injection of L-lactate into non-diabetic mice increased freezing at 0.45 mA. In addition, L-lactate decreased pAMPK in the hippocampus, but not the amygdala, and increase in freezing induced by L-lactate was inhibited by acadesine. Dorsomorphin-induced increase in freezing was inhibited by the AMPA receptor antagonist NBQX.

CONCLUSIONS AND INTERPRETATION

In diabetic mice, L-lactate is increased in the amygdala and hippocampus, possibly through hyperglycaemia, which strengthens fear memory through inhibition of AMPK and activation of glutamatergic function.

Depression and type 2 diabetes: A causal relationship and mechanistic pathway

Depression is a mood disorder that may increase risk for the development of insulin resistance (IR) and type 2 diabetes (T2D), and vice versa. However, the mechanistic pathway linking depression and T2D is not fully elucidated. The aim of this narrative review, therefore, was to discuss the possible link between depression and T2D. The coexistence of T2D and depression is twice as great compared to the occurrence of either condition independently. Hyperglycaemia and dyslipidaemia promote the incidence of depression by enhancing inflammation and reducing brain serotonin (5-hydroxytryptamine [5HT]). Dysregulation of insulin signalling in T2D impairs brain 5HT signalling, leading to the development of depression. Furthermore, depression is associated with the development of hyperglycaemia and poor glycaemic control. Psychological stress and depression promote the development of T2D. In conclusion, T2D could be a potential risk factor for the development of depression through the induction of inflammatory reactions and oxidative stress that affect brain neurotransmission. In addition, chronic stress in depression may induce the development of T2D through dysregulation of the hypothalamic-pituitary-adrenal axis and increase circulating cortisol levels, which triggers IR and T2D.

Affective and Cognitive Impairments in Rodent Models of Diabetes

Diabetes and related acute and long-term complications have a profound impact on cognitive, emotional, and social behavior, suggesting that the central nervous system (CNS) is a crucial substrate for diabetic complications. When anxiety, depression, and cognitive deficits occur in diabetic patients, the symptoms and complications related to the disease worsen, contributing to lower quality of life while increasing health care costs and mortality. Experimental models of diabetes in rodents are a fundamental and valuable tool for improving our understanding of the mechanisms underlying the close and reciprocal link between diabetes and CNS alterations, including the development of affective and cognitive disorders. Such models must reproduce the different components of this pathological condition in humans and, therefore, must be associated with affective and cognitive behavioral alterations. Beyond tight glycemic control, there are currently no specific therapies for neuropsychiatric comorbidities associated with diabetes; animal models are, therefore, essential for the development of adequate therapies. To our knowledge, there is currently no review article that summarizes changes in affective and cognitive behavior in the most common models of diabetes in rodents. Therefore, in this review, we have reported the main evidence on the alterations of affective and cognitive behavior in the different models of diabetes in rodents, the main mechanisms underlying these comorbidities, and the applicable therapeutic strategy.

Activation of 5-HT1A Receptors Normalizes the Overexpression of Presynaptic 5-HT1A Receptors and Alleviates Diabetic Neuropathic Pain

Neuropathic pain is a well-documented phenomenon in experimental and clinical diabetes; however, current treatment is unsatisfactory. Serotoninergic-containing neurons are key components of the descending autoinhibitory pathway, and a decrease in their activity may contribute at least in part to diabetic neuropathic pain (DNP). A streptozotocin (STZ)-treated rat was used as a model for type 1 diabetes mellitus (T1DM). Pain transmission was evaluated using well-established nociceptive-based techniques, including the Hargreaves apparatus, cold plate and dynamic plantar aesthesiometer. Using qRT-PCR, Western blotting, immunohistochemistry, and HPLC-based techniques, we also measured in the central nervous system and peripheral nervous system of diabetic animals the expression and localization of 5-HT1A receptors (5-HT1AR), levels of key enzymes involved in the synthesis and degradation of tryptophan and 5-HT, including tryptophan hydroxylase-2 (Tph-2), tryptophan 2,3-dioxygenase (Tdo), indoleamine 2,3-dioxygenase 1 (Ido1) and Ido2. Moreover, spinal concentrations of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) and quinolinic acid (QA, a metabolite of tryptophan) were also quantified. Diabetic rats developed thermal hyperalgesia and cold/mechanical allodynia, and these behavioral abnormalities appear to be associated with the upregulation in the levels of expression of critical molecules related to the serotoninergic nervous system, including presynaptic 5-HT1AR and the enzymes Tph-2, Tdo, Ido1 and Ido2. Interestingly, the level of postsynaptic 5-HT1AR remains unaltered in STZ-induced T1DM. Chronic treatment of diabetic animals with 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), a selective 5-HT1AR agonist, downregulated the upregulation of neuronal presynaptic 5-HT1AR, increased spinal release of 5-HT (↑ 5-HIAA/5-HT) and reduced the concentration of QA, decreased mRNA expression of Tdo, Ido1 and Ido2, arrested neuronal degeneration and ameliorated pain-related behavior as exemplified by thermal hyperalgesia and cold/mechanical allodynia. These data show that 8-OH-DPAT alleviates DNP and other components of the serotoninergic system, including the ratio of 5-HIAA/5-HT and 5-HT1AR, and could be a useful therapeutic agent for managing DNP.

Amygdala activity and amygdala-hippocampus connectivity: Metabolic diseases, dementia, and neuropsychiatric issues

With rapid aging of the population worldwide, the number of people with dementia is dramatically increasing. Some studies have emphasized that metabolic syndrome, which includes obesity and diabetes, leads to increased risks of dementia and cognitive decline. Factors such as insulin resistance, hyperglycemia, high blood pressure, dyslipidemia, and central obesity in metabolic syndrome are associated with synaptic failure, neuroinflammation, and imbalanced neurotransmitter levels, leading to the progression of dementia. Due to the positive correlation between diabetes and dementia, some studies have called it "type 3 diabetes". Recently, the number of patients with cognitive decline due to metabolic imbalances has considerably increased. In addition, recent studies have reported that neuropsychiatric issues such as anxiety, depressive behavior, and impaired attention are common factors in patients with metabolic disease and those with dementia. In the central nervous system (CNS), the amygdala is a central region that regulates emotional memory, mood disorders, anxiety, attention, and cognitive function. The connectivity of the amygdala with other brain regions, such as the hippocampus, and the activity of the amygdala contribute to diverse neuropathological and neuropsychiatric issues. Thus, this review summarizes the significant consequences of the critical roles of amygdala connectivity in both metabolic syndromes and dementia. Further studies on amygdala function in metabolic imbalance-related dementia are needed to treat neuropsychiatric problems in patients with this type of dementia.

Significance of Brain Glucose Hypometabolism, Altered Insulin Signal Transduction, and Insulin Resistance in Several Neurological Diseases

Several neurological diseases share pathological alterations, even though they differ in their etiology. Neuroinflammation, altered brain glucose metabolism, oxidative stress, mitochondrial dysfunction and amyloidosis are biological events found in those neurological disorders. Altered insulin-mediated signaling and brain glucose hypometabolism are characteristic signs observed in the brains of patients with certain neurological diseases, but also others such as type 2 diabetes mellitus and vascular diseases. Thus, significant reductions in insulin receptor autophosphorylation and Akt kinase activity, and increased GSK-3 activity and insulin resistance, have been reported in these neurological diseases as contributing to the decline in cognitive function. Supporting this relationship is the fact that nasal and hippocampal insulin administration has been found to improve cognitive function. Additionally, brain glucose hypometabolism precedes the unmistakable clinical manifestations of some of these diseases by years, which may become a useful early biomarker. Deficiencies in the major pathways of oxidative energy metabolism have been reported in patients with several of these neurological diseases, which supports the hypothesis of their metabolic background. This review remarks on the significance of insulin and brain glucose metabolism alterations as keystone common pathogenic substrates for certain neurological diseases, highlighting new potential targets.

Increase in brain l-lactate enhances fear memory in diabetic mice: Involvement of glutamate neurons

Previous reports suggest that diabetes mellitus is associated with psychiatric disorders, including depression and anxiety, but the mechanisms involved are unknown. We have reported that streptozotocin (STZ)-induced diabetic mice show enhancement of conditioned fear memory. To clarify the mechanisms through which diabetes affects conditioned fear memory, the present study investigated the role of l-lactate and glutamatergic function in enhancement of conditioned fear memory in diabetes. l-lactate levels in the amygdala and hippocampus, which are known to play important roles in fear memory, were significantly increased in STZ-induced diabetic mice. The glucose transporter (GLUT) 1 was significantly increased both in the amygdala and in the hippocampus. In contrast, GLUT3, the monocarboxylic acid transporter (MCT) 1 and MCT2 in the amygdala and hippocampus were not altered in STZ-induced diabetic mice. I.c.v. injection of l-lactate to non-diabetic mice significantly increased duration of freezing, whereas the MCT inhibitor 4-CIN significantly inhibited duration of freezing in STZ-induced diabetic mice. Injection of l-lactate significantly increased glutamate levels in the amygdala and hippocampus. Duration of freezing induced by l-lactate was significantly inhibited by the AMPA receptor antagonist NBQX. In addition, injection of NBQX into the amygdala and hippocampus significantly inhibited duration of freezing in STZ-induced diabetic mice. These results suggest that l-lactate levels are increased in the amygdala and hippocampus in diabetic mice, which may enhance fear memory though activation of glutamatergic function in the amygdala and hippocampus.

Neuropeptide Y and glutamatergic mechanisms in the amygdala and ventral hippocampus differentially mediate impaired social behavior in diabetic mice

Though patients with diabetes mellitus are reported to show deficits in social interaction, the mechanisms of these impairments are unclear. The present study investigated the role of AMPA and neuropeptide Y (NPY) receptors in the ventral hippocampus (vHC) and basolateral amygdala (BLA) in the social behavior of diabetic mice. In the three-chamber test, streptozotocin (STZ)-induced diabetic mice showed impairment in social novelty preference, but not in sociability. Injection of the AMPA receptor antagonist NBQX into vHC or BLA each restored social novelty preference in STZ-induced diabetic mice. NPY content in amygdala, but not in vHC, of STZ-induced diabetic mice was increased relative to non-diabetic mice. In STZ-induced diabetic mice, injection of the NPY Y₂ receptor antagonist BIIE 0246 into BLA restored social novelty preference, whereas injection of BIIE 0246 into vHC was without effect. Finally, in non-diabetic mice social novelty preference was impaired by the NPY Y₂ receptor agonist NPY 13-36 injected into BLA and restored by co-injection of NBQX. These results indicate that in diabetic mice glutamatergic function is enhanced in both vHC and BLA, which impairs social novelty preference through AMPA receptors. In addition, they indicate that NPYergic function in BLA, but not vHC, is enhanced in diabetic mice, which impairs social novelty preference through NPY Y₂ receptors.

The impact of insulin on the serotonergic system and consequences on diabetes-associated mood disorders

The idea that insulin could influence emotional behaviours has long been suggested. However, the underlying mechanisms have yet to be solved and there is no direct and clear-cut evidence demonstrating that such action involves brain serotonergic neurones. Indeed, initial arguments in favour of the association between insulin, serotonin and mood arise from clinical or animal studies showing that impaired insulin action in type 1 or type 2 diabetes causes anxiety- and depressive symptoms along with blunted plasma and brain serotonin levels. The present review synthesises the main mechanistic hypotheses that might explain the comorbidity between diabetes and depression. It also provides a state of knowledge of the direct and indirect experimental evidence that insulin modulates brain serotonergic neurones. Finally, it highlights the literature suggesting that antidiabetic drugs present antidepressant-like effects and, conversely, that serotonergic antidepressants impact glucose homeostasis. Overall, this review provides mechanistic insights into how insulin signalling alters serotonergic neurotransmission and related behaviours bringing new targets for therapeutic options.

NLX-101, a highly selective 5-HT1A receptor biased agonist, mediates antidepressant-like activity in rats via prefrontal cortex 5-HT1A receptors

NLX-101 (also known as F15599) exhibits nanomolar affinity, exceptional selectivity and biased agonist activation of serotonin 5-HT_1A receptors. Given systemically, it displays antidepressant-like activity in the rat forced swim test (FST), and preferentially activates 5-HT_1A post-synaptic heteroreceptors in the prefrontal cortex (PFC), a brain region involved in the control of mood. Here, we assessed the ability of NLX-101 to produce antidepressant-like activity in the FST following in-situ PFC unilateral microinjection. (+)8-OH-DPAT and F13714, two 5-HT_1A receptor agonists that do not display cortical biased agonism, were tested as comparators. NLX-101 decreased time spent in immobility in a bi-modal manner, with a first MED of 0.25 μg (immobility reduced from 160 to 80 s) but immobility returned to control levels at the next dose (1 μg). At higher doses, immobility decreased monotonically, with a second MED of 16 μg and a maximal effect (36 s) at 32 μg. (+)8-OH-DPAT and F13714 also diminished immobility but, unlike NLX-101, they did so in a unimodal manner, with MEDs of 1 and 4 μg, and maximal responses of 31 and 4 s, for (+)8-OH-DPAT and F13714, respectively. The effects of (+)8-OH-DPAT (16 μg) and of both active doses of NLX-101 (0.25 and 16 μg) were prevented by the 5-HT_1A receptor antagonist WAY-100,635 (0.63 mg/kg s.c.). In conclusion, activation of 5-HT_1A receptors in the PFC by NLX-101 produces robust antidepressant-like effects in the rat FST, with a distinctive bimodal dose-response pattern. These data suggest that NLX-101 may target specific 5-HT_1A receptor subpopulations in PFC, likely located on GABAergic and/or glutamatergic neurons.

Increase in neuropeptide Y activity impairs social behaviour in association with glutamatergic dysregulation in diabetic mice

BACKGROUND AND PURPOSE

Patients with diabetes mellitus are reported to show a raised prevalence of mental disorders, which may be reflected in impaired social interaction. However, the mechanisms underlying such impairment in diabetes are unknown.

EXPERIMENTAL APPROACH

The present study investigated whether social interaction is impaired in diabetic mice and whether central neuropeptide Y (NPY) and glutamatergic function are involved in such impairment.

KEY RESULTS

In the three-chamber test, social novelty preference, but not sociability, was impaired in streptozotocin (STZ)-induced diabetic mice. The mRNA level of NPY in the hypothalamus was increased in STZ-induced diabetic mice. Injection of the NPY Y₂ receptor agonist NPY 13-36 into naïve mice impaired social novelty preference, but not sociability, and this effect was inhibited by the Y₂ receptor antagonist BIIE 0246. BIIE 0246 also reversed the impairment of social novelty preference in STZ-induced diabetic mice. Similarly, injection of the AMPA receptor agonist AMPA into naïve mice impaired social novelty preference, but not sociability, and this effect was inhibited by the AMPA receptor antagonist NBQX. Impairment of social novelty preference induced by NPY 13-36 was inhibited by NBQX, whereas impairment of social novelty preference induced by AMPA was not inhibited by BIIE 0246. Finally, impairment of social novelty preference in STZ-induced diabetic mice was reversed by NBQX.

CONCLUSION AND IMPLICATIONS

These findings suggest that NPY neurons are activated in diabetic mice and that this may impair social novelty preference by promoting glutamatergic function through Y₂ receptors.

Switching from high-fat feeding (HFD) to regular diet improves metabolic and behavioral impairments in middle-aged female mice

Obesity represents a risk factor for metabolic syndrome and cardiovascular and psychiatric disorders. Excessive caloric intake, particularly in dietary fats, is an environmental factor that contributes to obesity development. Thus, the observation that switching from long-standing dietary obesity to standard diet (SD) can ameliorate the high-fat diet-induced metabolic, memory, and emotionality-related impairments are particularly important. Herein we investigated whether switching from the high-fat diet (HFD) to SD could improve the metabolic and behavioral impairments observed in middle-aged females C57Bl/6 mice. During twelve weeks, the animals received a high-fat diet (61 % fat) or SD diet. After 12-weeks, the HFD group's diet was switched to SD for an additional four weeks. It was observed a progressive deleterious effect of HFD in metabolic and behavioral parameters in mice. After four weeks of HFD-feeding, the animals showed glucose intolerance and increased locomotor activity. A subsequent increase in the body mass gain, hyperglycemia, and depressive-like behavior was observed after eight weeks, and memory impairments after twelve weeks. After replacing the HFD to SD, it was observed an improvement of metabolic (loss of body mass, normal plasma glucose levels, and glucose tolerance) and behavioral (absence of memory and emotional alterations) parameters. These results demonstrate the temporal development of metabolic and behavioral impairments following HFD in middle-age female mice and provide new evidence that these alterations can be improved by switching back the diet to SD.

Loss of Glutamate Decarboxylase 67 in Somatostatin-Expressing Neurons Leads to Anxiety-Like Behavior and Alteration in the Akt/GSK3β Signaling Pathway

Major depressive disorder (MDD) is a highly prevalent psychiatric disorder worldwide. Several lines of evidence suggest that the dysfunction of somatostatin (SOM) neurons is associated with the pathophysiology of MDD. Importantly, most SOM neurons are γ-aminobutyric acid (GABA) interneurons. However, whether the dysfunction of GABAergic neurotransmission from SOM neurons contributes to the pathophysiology of MDD remains elusive. To address this issue, we investigated the emotional behaviors and relevant molecular mechanism in mice lacking glutamate decarboxylase 67 (GAD67), an isoform of GABA-synthesizing enzyme, specifically in SOM neurons (SOM-GAD67 mice). The SOM-GAD67 mice exhibited anxiety-like behavior in the open-field test without an effect on locomotor activity. The SOM-GAD67 mice showed depression-like behavior in neither the forced swimming test nor the sucrose preference test. In addition, the ability to form contextual fear memory was normal in the SOM-GAD67 mice. Furthermore, the plasma corticosterone level was normal in the SOM-GAD67 mice both under baseline and stress conditions. The expression ratios of p-Akt^Ser473/Akt and p-GSK3β^Ser9/GSK3β were decreased in the frontal cortex of SOM-GAD67 mice. Taken together, these data suggest that the loss of GAD67 from SOM neurons may lead to the development of anxiety-like but not depression-like states mediated by modification of Akt/GSK3β activities.

Metformin Promotes Anxiolytic and Antidepressant-Like Responses in Insulin-Resistant Mice by Decreasing Circulating Branched-Chain Amino Acids

Epidemiological studies indicate that insulin resistance (IR), a hallmark of type 2 diabetes, is associated with an increased risk of major depression. Here, we demonstrated that male mice fed a high-fat diet (HFD) exhibited peripheral metabolic impairments reminiscent of IR accompanied by elevated circulating levels of branched-chain amino acids (BCAAs), whereas both parameters were normalized by chronic treatment with metformin (Met). Given the role of BCAAs in the regulation of tryptophan influx into the brain, we then explored the activity of the serotonin (5-HT) system. Our results indicated that HFD-fed mice displayed impairment in the electrical activity of dorsal raphe 5-HT neurons, attenuated hippocampal extracellular 5-HT concentrations and anxiety, one of the most visible and early symptoms of depression. On the contrary, Met stimulated 5-HT neurons excitability and 5-HT neurotransmission while hindering HFD-induced anxiety. Met also promoted antidepressant-like activities as observed with fluoxetine. In light of these data, we designed a modified HFD in which BCAA dietary supply was reduced by half. Deficiency in BCAAs failed to reverse HFD-induced metabolic impairments while producing antidepressant-like activity and enhancing the behavioral response to fluoxetine. Our results suggest that Met may act by decreasing circulating BCAAs levels to favor serotonergic neurotransmission in the hippocampus and promote antidepressant-like effects in mice fed an HFD. These findings also lead us to envision that a diet poor in BCAAs, provided either alone or as add-on therapy to conventional antidepressant drugs, could help to relieve depressive symptoms in patients with metabolic comorbidities.SIGNIFICANCE STATEMENT Insulin resistance in humans is associated with increased risk of anxiodepressive disorders. Such a relationship has been also found in rodents fed a high-fat diet (HFD). To determine whether insulin-sensitizing strategies induce anxiolytic- and/or antidepressant-like activities and to investigate the underlying mechanisms, we tested the effects of metformin, an oral antidiabetic drug, in mice fed an HFD. Metformin reduced levels of circulating branched-chain amino acids, which regulate tryptophan uptake within the brain. Moreover, metformin increased hippocampal serotonergic neurotransmission while promoting anxiolytic- and antidepressant-like effects. Moreover, a diet poor in these amino acids produced similar beneficial behavioral property. Collectively, these results suggest that metformin could be used as add-on therapy to a conventional antidepressant for the comorbidity between metabolic and mental disorders.

Novel Aryloxyethyl Derivatives of 1-(1-Benzoylpiperidin-4-yl)methanamine as the Extracellular Regulated Kinases 1/2 (ERK1/2) Phosphorylation-Preferring Serotonin 5-HT1A Receptor-Biased Agonists with Robust Antidepressant-like Activity

Novel 1-(1-benzoylpiperidin-4-yl)methanamine derivatives were designed as "biased agonists" of serotonin 5-HT_1A receptors. The compounds were tested in signal transduction assays (ERK1/2 phosphorylation, cAMP inhibition, Ca²⁺ mobilization, and β-arrestin recruitment) which identified ERK1/2 phosphorylation-preferring aryloxyethyl derivatives. The novel series showed high 5-HT_1A receptor affinity, >1000-fold selectivity versus noradrenergic α₁, dopamine D₂, serotonin 5-HT_2A, histamine H₁, and muscarinic M₁ receptors, and favorable druglike properties (CNS-MPO, Fsp³, LELP). The lead structure, (3-chloro-4-fluorophenyl)(4-fluoro-4-(((2-(pyridin-2-yloxy)ethyl)amino)methyl)piperidin-1-yl)methanone (17, NLX-204), displayed high selectivity in the SafetyScreen44 panel (including hERG channel), high solubility, metabolic stability, and Caco-2 penetration and did not block CYP3A4, CYP2D6 isoenzymes, or P-glycoprotein. Preliminary in vivo studies confirmed its promising pharmacokinetic profile. 17 also robustly stimulated ERK1/2 phosphorylation in rat cortex and showed highly potent (MED = 0.16 mg/kg) and efficacious antidepressant-like activity, totally eliminating immobility in the rat Porsolt test. These data suggest that the present 5-HT_1A receptor-biased agonists could constitute promising antidepressant drug candidates.

Diminished responses to monoaminergic antidepressants but not ketamine in a mouse model for neuropsychiatric lupus

BACKGROUND

A significant proportion of patients suffering from major depression fail to remit following treatment and develop treatment-resistant depression. Developing novel treatments requires animal models with good predictive validity. MRL/lpr mice, an established model of systemic lupus erythematosus, show depression-like behavior.

AIMS

We evaluated responses to classical antidepressants, and associated immunological and biochemical changes in MRL/lpr mice.

METHODS AND RESULTS

MRL/lpr mice showed increased immobility in the forced swim test, decreased wheel running and sucrose preference when compared with the controls, MRL/MpJ mice. In MRL/lpr mice, acute fluoxetine (30 mg/kg, intraperitoneally (i.p.)), imipramine (10 mg/kg, i.p.) or duloxetine (10 mg/kg, i.p.) did not decrease the immobility time in the Forced Swim Test. Interestingly, acute administration of combinations of olanzapine (0.03 mg/kg, subcutaneously)+fluoxetine (30 mg/kg, i.p.) or bupropion (10 mg/kg, i.p.)+fluoxetine (30 mg/kg, i.p.) retained efficacy. A single dose of ketamine but not three weeks of imipramine (10 mg/kg, i.p.) or escitalopram (5 mg/kg, i.p.) treatment in MRL/lpr mice restored sucrose preference. Further, we evaluated inflammatory, immune-mediated and neuronal mechanisms. In MRL/lpr mice, there was an increase in autoantibodies' titers, [3H]PK11195 binding and immune complex deposition. There was a significant infiltration of the brain by macrophages, neutrophils and T-lymphocytes. p11 mRNA expression was decreased in the prefrontal cortex. Further, there was an increase in the 5-HT_2aR expression, plasma corticosterone and indoleamine 2,3-dioxygenase activity.

CONCLUSION

In summary, the MRL/lpr mice could be a useful model for Treatment Resistant Depression associated with immune dysfunction with potential to expedite antidepressant drug discovery.

The potential benefit of combined versus monotherapy of coenzyme Q10 and fluoxetine on depressive-like behaviors and intermediates coupled to Gsk-3β in rats

As a part of the serotoninergic dysfunction implicated in neurobiology of depression, evidence has focused on serotonin (5-HT) receptors downstream signaling intermediates including glycogen synthase kinase-3β (GSK-3β), cAMP response element binding protein (CREB) and brain derived neurotrophic factor (BDNF). Our team previously reported that coenzyme Q10 (CoQ10) exerted antidepressant-like effect in rats exposed to chronic unpredictable mid stress (CUMS) via elevating serotonin levels. However, the effect of CoQ10 has not been elucidated in downstream signaling molecules mediating 5HT receptors' effect involved in depressive disorder hitherto. In the present study, we focused on 5-HT_1A and 5-HT_2A receptors (activation of 5-HT_1A receptor and inhibition of 5-HT_2A receptors reduce depressive like-behaviors). We investigated the role of these 5-HT receptors and their linked GSK-3β signaling intermediates as an underlying mechanism of CoQ10 as monotherapy or combined with fluoxetine, a selective serotonin reuptake inhibitor, to alleviate depressive-like phenotype. Effects of CoQ10 (100mg/kg/day) or/and fluoxetine (10mg/kg/day) were determined on 5-HT_1A, 5-HT_2A receptors mRNA expression, GSK-3β and phosphorylated (p)GSK-3β, CREB, pCREB and BDNF protein expression in rats subjected to CUMS for 6weeks. CUMS rats exhibited obvious depressive-like behaviors (anhedonia-like behavior, negative alterations in social interaction, open field and forced swimming tests) with increased corticosterone and adrenal glands weight, decreased hippocampal levels of pGSK-3β, pCREB and BDNF protein expressions. Additionally, they exhibited decreased hippocampal 5-HT_1A and increased 5-HT_2A receptor mRNA expression. CoQ10 or fluoxetine significantly attenuated the behavioral and neurochemical alterations in stressed rats with more significance with combined treatment. These findings imply that CoQ10 or/and fluoxetine attenuated CUMS-induced depressive-like behavior partly through modulating dysfunctional regulation of post-serotonergic receptor signaling pathway focusing on GSK-3β, CREB and BDNF.

5HT3 receptors: Target for new antidepressant drugs

5HT3 receptors (5HT3Rs) have long been identified as a potential target for antidepressants. Several studies have reported that antagonism of 5HT3Rs produces antidepressant-like effects. However, the exact role of 5HT3Rs and the mode of antidepressant action of 5HT3R antagonists still remain a mystery. Here, we provide a comprehensive overview of 5HT3Rs: (a) regional and subcellular distribution of 5HT3Rs in discrete brain regions, (b) preclinical and clinical evidence supporting the antidepressant effect of 5HT3R antagonists, and (c) neurochemical, biological and neurocellular signaling pathways associated with the antidepressant action of 5HT3R antagonists. 5HT3Rs located on the serotonergic and other neurotransmitter interneuronal projections control their release and affect mood and emotional behavior; however, new evidence suggests that apart from modulating the neurotransmitter functions, 5HT3R antagonists have protective effects in the pathogenic events including hypothalamic-pituitary-adrenal-axis hyperactivity, brain oxidative stress and impaired neuronal plasticity, pointing to hereby unknown and novel mechanisms of their antidepressant action. Nonetheless, further investigations are warranted to establish the exact role of 5HT3Rs in depression and antidepressant action of 5HT3R antagonists.

Diabetes-associated depression: the serotonergic system as a novel multifunctional target

Diabetes associated depression is a largely understudied field which nonetheless carries a significant disease burden. The very low therapeutic efficacy of the existing conventional drugs with poor outcome may be, in part, due to uncertainty of the mechanism involved that clearly explains the existing comorbidity. The main purpose of this review was to address the sophisticated mechanisms of this comorbidity with a view of developing potential novel targets with higher efficacy and specificity. Data were collected from database searches including PubMed, references from relevant English language research/review articles and other official publications. Articles from 1990 to 2013 were included, and a broad search term criteria were followed for data mining so that relevant information was not missed out. Some of the search terms used included; diabetes-induced depression, diabetes and serotonin, hypothalamic-pituitary-adrenal (HPA) axis and diabetes and glucocorticoids in diabetes. Neuropathologically, depletion of brain monoaminergic activity specifically the serotonin (5-hydroxytryptamine [5-HT]) system, due to chronically persisting diabetic state may lead to the mood and behavioral complications that further add on worsening the quality life years. The 5-HT system through multifunctional tasks regulates neurogenesis and plasticity and by complex receptor mechanism controls the emotional and behavioral activity. Persisting hyperglycemia leads to impaired neurogenesis, decreased synaptic plasticity, undesired neuro-anatomical alterations, neurochemical deficits, and reduced neurotransmitter activity. The neurotrophic factors and secondary messenger functions affected at molecular and genetic levels indicate the impact of diabetes-mediated dysregulation on neuronal circuits. HPA activity, glycogen synthase kinase 3, and insulin signaling controls were also found to be hampered, interlinked to 5-HT system following diabetic progression.

High-fat diet-induced metabolic disorders impairs 5-HT function and anxiety-like behavior in mice

BACKGROUND AND PURPOSE

The link between type 2 diabetes mellitus (T2DM) and depression is bidirectional. However, the possibility that metabolic disorders may elicit anxiogenic-like/depressive-like symptoms or alter the efficacy of antidepressant drugs remains poorly documented. This study explored the influence of T2DM on emotionality and proposed a therapeutic strategy that might be used in depressed diabetic patients.

EXPERIMENTAL APPROACH

Mice were fed a high-fat diet (HFD) and subjected to a full comprehensive metabolic and behavioural analysis to establish correlations between metabolic and psychiatric disorders. In vivo intra-hippocampal microdialysis was also applied to propose a mechanism underpinning the phenotype of mice fed the HFD. Finally, we tested whether chronic administration of the selective 5-HT reuptake inhibitor escitalopram or HFD withdrawal could reverse HFD-induced metabolic and behavioural anomalies.

KEY RESULTS

The increased body weight, hyperglycaemia and impaired glucose tolerance in response to HFD were correlated with anxiogenic-like/depressive-like symptoms. Moreover, this phenotype was associated with decreased extracellular 5-HT levels in the hippocampus which may result from increased sensitivity of the dorsal raphe 5-HT1A autoreceptor. Interestingly, the beneficial effect of prolonged administration of escitalopram was abolished in HFD-fed mice. On the contrary, HFD withdrawal completely reversed metabolic impairments and positively changed symptoms of anxiety, although some behavioural anomalies persisted.

CONCLUSIONS AND IMPLICATIONS

Our data provide clear-cut evidence that both pathologies are finely correlated and associated with impaired 5-HT mediated neurotransmission in the hippocampus. Further experiments are warranted to define the most adequate strategy for the treatment of such co-morbidity.

LINKED ARTICLES

This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

A novel 5HT3 antagonist 4i (N-(3-chloro-2-methylphenyl)quinoxalin-2-carboxamide) prevents diabetes-induced depressive phenotypes in mice: Modulation of serotonergic system

Despite the presence of a multitudinous pharmacotherapy, diabetes-induced depressive disorder remains undertreated. Evidence suggests that brain serotonergic deficits are associated with depressive-like behavior in diabetes and that 5HT3 receptor (5HT3R) antagonists have serotonergic facilitatory effects. This study examined the effects of a novel 5HT3R antagonist, 4i (N-(3-chloro-2-methylphenyl)quinoxalin-2-carboxamide), in diabetes-induced depressive phenotypes. Experimentally, (1) to evaluate the effects of 4i, mice with 8-weeks of diabetes (induced by streptozotocin, 200mg/kg, i.p.) were treated with vehicle, 4i (0.5 and 1mg/kg/day, i.p.), fluoxetine (10mg/kg/day, i.p.) for 4-weeks and subjected to neurobehavioral assays, followed by biochemical estimation of serotonin levels in midbrain, prefrontal-cortex and cerebellum. (2) To evaluate the role of 5HT3R in the postulated effect of 4i, diabetic mice were given 4i (1mg/kg/day, i.p.) after 1h of 1-(m-chlorophenyl)-biguanide (mCPBG, a 5HT3R agonist, 10mg/kg/day, i.p.) treatment and subjected to the same protocol. The results showed that diabetic mice exhibited a significant behavioral deficit, including depression-like behavior in forced swim test, anxiety-like in open field test and sociability deficits in social interaction test, along with a significant decrease in serotonin level in these brain regions. 4i (1mg/kg), similar to fluoxetine, prevented these behavioral abnormalities and normalized brain serotonin levels. 4i (0.5mg/kg) ameliorated only diabetes-induced depressive-like behavior and serotonin deficits, but not anxiety-like effects. mCPBG blunted 4i-mediated behavioral response and increase in brain serotonin levels. Altogether, this study suggests that 4i prevents diabetes-induced depressive phenotypes in mice, which may involve antagonism of 5HT3Rs and increase in serotonin levels in discrete brain regions.

Ondansetron, a 5HT3 receptor antagonist reverses depression and anxiety-like behavior in streptozotocin-induced diabetic mice: possible implication of serotonergic system

Increased prevalence and high comorbidity of depression-like mood disorders and diabetes have prompted investigation of new targets and potential contributing agents. There is considerable evidence supporting the inconsistent clinical efficacy and persistent undesirable effects of existing antidepressant therapy for depression associated with diabetes. Therefore, the present study was aimed at investigating the effect of ondansetron, a selective 5HT3 receptor antagonist in attenuating depression and anxiety-like behavior comorbid with diabetes. Experimentally, Swiss albino mice were rendered diabetic by a single intraperitoneal (i.p.) injection of streptozotocin (STZ, 200 mg/kg). After 8 weeks, diabetic mice received a single dose of vehicle/ondansetron (0.5 and 1 mg/kg, p.o.)/fluoxetine (the positive control, 10 mg/kg p.o.) for 28 days. Thereafter, behavioral studies were conducted to test depression-like behavior using forced swim test (FST) and anxiety-like deficits using hole-board and light-dark tests, followed by biochemical estimation of serotonin content in discrete brain regions. The results demonstrated that, STZ-induced diabetic mice exhibited increased duration of immobility and decreased swimming behavior in FST, reduced exploratory behavior during hole-board test and increased aversion to brightly illuminated light area in light-dark test as compared to non-diabetic mice, while ondansetron (similar to fluoxetine) treatment significantly reversed the same. Biochemical assay revealed that ondansetron administration attenuated diabetes-induced neurochemical impairment of serotonin function, indicated by elevated serotonin levels in discrete brain regions of diabetic mice. Collectively, the data indicate that ondansetron may reverse depression and anxiety-like behavioral deficits associated with diabetes in mice and modulation of serotonergic activity may be a key mechanism of the compound.

Restricted access to standard or high fat chow alters sensitivity of rats to the 5-HT(2A/2C) receptor agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane

Feeding conditions can impact sensitivity to drugs acting on dopamine receptors; less is known about the impact of feeding conditions on the effects of drugs acting on serotonin (5-HT) receptors. This study examined the effects of feeding conditions on sensitivity to the direct-acting 5-HT(2A/2C) receptor agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM; 0.1-3.2 mg/kg) and the direct-acting dopamine D3/D2 receptor agonist quinpirole (0.0032-0.32 mg/kg). Male Sprague-Dawley rats had free access (11 weeks), followed by restricted access (6 weeks), to high fat (34.3%, n=8) or standard (5.7% fat; n=7) chow. Rats eating high fat chow became insulin resistant and gained more weight than rats eating standard chow. Free access to high fat chow did not alter sensitivity to DOM-induced head twitch but increased sensitivity to quinpirole-induced yawning. Restricting access to high fat or standard chow shifted the DOM-induced head twitch dose-response curve to the right and shifted the quinpirole-induced yawning dose-response curve downward in both groups of rats. Some drugs of abuse and many therapeutic drugs act on 5-HT and dopamine systems; these results show that feeding conditions impact sensitivity to drugs acting on these systems, thereby possibly affecting vulnerability to abuse, as well as the therapeutic effectiveness of drugs.

The antidepressant-like action of mGlu5 receptor antagonist, MTEP, in the tail suspension test in mice is serotonin dependent

RATIONALE

Numerous studies indicate the potential antidepressant actions of several mGlu5 receptor antagonists, including 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP). The explanation for the mechanism of these effects might be a key step in finding new antidepressant drugs (AD).

OBJECTIVES

The aim of the present study was to investigate the possible role of the serotonergic system in the antidepressant-like activity of MTEP in the tail suspension test (TST) in C57BL/6J mice, using selected antagonists of serotonergic receptors and by applying two different methods of serotonin (5-HT) depletion.

RESULTS

The results of our studies showed that the mGlu5 receptor antagonist, MTEP, similar to the fluoxetine used as reference AD, did not induce antidepressant-like effects in mice pretreated with tryptophan hydroxylase inhibitor, parachlorophenylalanine. On the other hand, MTEP worked as a potential AD in the TST in mice fed on a tryptophan-free (TRP-free) diet for 3 weeks. However, fluoxetine, which was used as a reference control was also active in this experiment, suggesting that a TRP-free diet was not sufficiently effective in reducing the 5-HT level. Furthermore, we showed that the 5HT2A/2C antagonist, ritanserin, yet not the 5-HT1A antagonist, WAY100635, 5HT1B antagonist, SB224289 or 5HT4 antagonist, GR125487, reversed the antidepressant-like effects of MTEP in the TST. Finally, a sub-effective dose ofMTEP coadministered with a sub-effective dose of citalopram induced an antidepressant-like effect in the TST in mice.

CONCLUSION

The results of our studies suggest the involvement of serotonergic system activation in the antidepressant-like effects of the mGlu5 antagonist, MTEP, in the TST in mice.

Sensitivity of hippocampal 5-HT1A receptors to mild stress in BDNF-deficient mice

Serotonin 1A (5-HT(1A)) receptors in brain play an important role in cognitive and integrative functions, as well as emotional states. Decreased brain-derived neurotrophic factor (BDNF) expression and/or function, particularly in hippocampus, are implicated in the pathophysiology of stress-related disorders such as major depression. BDNF(+/-) mice are more vulnerable to stress than wild-type mice, exhibiting behavioural despair after mild handling stress. We examined the effect of mild handling stress on 5-HT(1A) receptor function, as measured by 8-OH-DPAT stimulated [(35)S]GTPγS binding, in BDNF(+/-) mice and mice with a forebrain-specific reduction in BDNF (embryonic BDNF inducible knockout mice). Our data show a remarkable sensitivity of hippocampal 5-HT1A receptors to mild stress and a deficiency in BDNF. Other 5-HT(1A) receptor populations, specifically in frontal cortex and dorsal raphe, were resistant to the combined detrimental effects of mild stress and reductions in BDNF expression. Decreases in hippocampal 5-HT(1A) receptor function induced by mild stress in BDNF-deficient mice were prevented by administration of the selective serotonin reuptake inhibitor fluoxetine, which increased activation of TrkB, the high affinity receptor for BDNF, in wild-type and BDNF(+/-) mice. In hippocampal cultures, BDNF increased the capacity of 5-HT(1A) receptors to activate G proteins, an effect eliminated by the knockout of TrkB, confirming TrkB activation increases 5-HT(1A) receptor function. The mechanisms underlying the sensitivity of hippocampal 5-HT(1A) receptors to mild stress and decreased BDNF expression remain to be elucidated and may have important implications for the emotional and cognitive impairments associated with stress-related mental illness.

Streptozotocin-induced insulin deficiency leads to development of behavioral deficits in rats

Diabetes mellitus is one of the most common serious metabolic disorders in humans that develops due to diminished production of insulin (type I) or resistance to its effect (type II and gestational). The present study was designed to determine the neuropsychological deficits produced following streptozotocin-induced diabetes in rats. Rats were made diabetic by the intra-peritoneal administration of 60 mg/kg streptozotocin (STZ) which induces type-1 diabetes by the destruction "β-cells" of pancreas. Body weight, food and water intake was monitored daily. Open field test (OFT) model, forced swim test (FST) and Morris water maze (MWM) model were performed for the evaluation of ambulation, depression-like symptoms and memory effects, respectively. After 10 days of diabetes induction the exploratory activity of rats was monitored by OFT while depression-like symptoms and memory effects in rats were analyzed by FST and MWM. Results showed that there was no significant effect of STZ-induced diabetes on body weight but food and water intake of STZ-induced diabetic rats was significantly increased. Exploratory activity was significantly decreased and short-term and long-term memory was significantly impaired while the depression-like symptoms was significantly increased in STZ diabetic rats. Thus, it may be suggested that STZ-induced diabetes alters the brain functions and may play an important role in the pathophysiology of certain behavioral deficits like depression, impaired learning and memory functions related to diabetes. This finding may be of relevance in the pathophysiology and in the clinical picture, which could be related to an altered brain serotonin metabolism and neurotransmission and may possibly be related to neuropsychiatric disorders in diabetic patients.

Eating high fat chow increases the sensitivity of rats to 8-OH-DPAT-induced lower lip retraction

Eating high fat food can alter sensitivity to drugs acting on dopamine systems; this study examined whether eating high fat food alters sensitivity to a drug acting on serotonin (5-HT) systems. Sensitivity to (+)-8-hydroxy-2-(dipropylamino) tetralin hydrobromide (8-OH-DPAT; 5-HT1A receptor agonist)-induced lower lip retraction was examined in separate groups (n=8-9) of rats with free access to standard (5.7% fat) or high fat (34.3% fat) chow; sensitivity to quinpirole (dopamine D3/D2 receptor agonist)-induced yawning was also examined. Rats eating high fat chow gained more body weight than rats eating standard chow and, after 6 weeks of eating high fat chow, they were more sensitive to 8-OH-DPAT (0.01-0.1 mg/kg)-induced lower lip retraction and quinpirole (0.0032-0.32 mg/kg)-induced yawning. These changes were not reversed when rats that previously ate high fat chow were switched to eating standard chow and sensitivity to 8-OH-DPAT and quinpirole increased when rats that previously ate standard chow ate high fat chow. These data extend previous results showing changes in sensitivity to drugs acting on dopamine systems in animals eating high fat chow to a drug acting at 5-HT1A receptors and they provide support for the notion that eating certain foods impacts sensitivity to drugs acting on monoamine systems.

Brain insulin dysregulation: implication for neurological and neuropsychiatric disorders

Arduous efforts have been made in the last three decades to elucidate the role of insulin in the brain. A growing number of evidences show that insulin is involved in several physiological function of the brain such as food intake and weight control, reproduction, learning and memory, neuromodulation and neuroprotection. In addition, it is now clear that insulin and insulin disturbances particularly diabetes mellitus may contribute or in some cases play the main role in development and progression of neurodegenerative and neuropsychiatric disorders. Focusing on the molecular mechanisms, this review summarizes the recent findings on the involvement of insulin dysfunction in neurological disorders like Alzheimer's disease, Parkinson's disease and Huntington's disease and also mental disorders like depression and psychosis sharing features of neuroinflammation and neurodegeneration.

Depressive phenotypes evoked by experimental diabetes are reversed by insulin

Clinical studies suggest a bidirectional relationship between diabetes and depression, where diabetes may increase risk for depressive symptoms and depression may increase risk for diabetes. Preclinical models examining the effects of diabetes on brain and behavior can provide insights to the pathophysiology underlying this relationship. The current study comprehensively examined, in C57BL/6 mice, the development of depressive phenotypes evoked by diabetes induced by streptozotocin (STZ) and determined if insulin treatment was able to reverse the diabetes-related changes on brain and affective behavior. Since anxiety is often comorbid with mood disturbances, behavioral tests for both anxiety and depression were administered. Possible physiological correlates of behavioral changes, including hippocampal cell proliferation, brain derived neurotrophic factor, and plasma corticosterone, were also measured. STZ-induced diabetes resulted in increased immobility in the tail suspension test, increased intracranial self-stimulation thresholds, decreased hippocampal cell proliferation, and increased corticosterone levels. Insulin treatment, on the other hand, reduced hyperglycemia, reversed the behavioral effects, and returned hippocampal cell proliferation and corticosterone to levels comparable to the control group. Anxiety-related behaviors were unaffected. This study showed that experimental diabetes in the mouse produced depressive phenotypes that were reversed by insulin therapy. Changes in reward-related behaviors and hippocampal cell proliferation may be useful markers to identify therapeutic interventions for comorbid diabetes and depression.

5-HT1A receptor-regulated signal transduction pathways in brain

Serotonin is an influential monoamine neurotransmitter that signals through a number of receptors to modulate brain function. Among different serotonin receptors, the serotonin 1A (5-HT1A) receptors have been tied to a variety of physiological and pathological processes, notably in anxiety, mood, and cognition. 5-HT1A receptors couple not only to the classical inhibitory G protein-regulated signaling pathway, but also to signaling pathways traditionally regulated by growth factors. Despite the importance of 5-HT1A receptors in brain function, little is known about how these signaling mechanisms link 5-HT1A receptors to regulation of brain physiology and behavior. Following a brief summary of the known physiological and behavioral effects of 5-HT1A receptors, this article will review the signaling pathways regulated by 5-HT1A receptors, and discuss the potential implication of these signaling pathways in 5-HT1A receptor-regulated physiological processes and behaviors.

On the mechanism of the antidepressant-like action of group II mGlu receptor antagonist, MGS0039

RATIONALE

Several studies have suggested that modulation of the glutamatergic system could be a new, efficient way to achieve antidepressant activity. Behavioral data showed that group II mGlu receptor antagonists (i.e., (1R, 2R, 3R, 5R, 6R)-2-amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039) and (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xan th-9-yl) propanoic acid (LY341495)) elicited antidepressant activity in several animal models of depression in rats and/or mice. Although the antidepressant-like activity of MGS0039 and LY341495 is well documented, the mechanism of the antidepressant action of these compounds is still not clear.

OBJECTIVES

The aim of the present study was to specify the role of the serotonergic system in the mechanism of the antidepressant-like activity of group II mGlu receptor ligands by using the tail suspension test (TST) in mice; the role of AMPA receptors was also investigated. Furthermore, the possible antidepressant-like action of MGS0039 using the olfactory bulbectomy (OB) model of depression in rats was investigated.

RESULTS

The results of the TST studies showed that antidepressant-like action of group II mGlu receptor antagonists does not depend on serotonergic system activation. However, the AMPA receptor seems to play a key role in the antidepressant-like action of these compounds. Moreover, we have shown that repeated administration of MGS0039 attenuated OB-related deficits, confirming antidepressant-like activity of the tested compound.

CONCLUSIONS

The results suggest that the blockade of group II mGlu receptors may be effective in the treatment of depression. Moreover, we have found that the mechanism of action of group II mGlu receptor antagonists differs from that of typical antidepressants, such as SSRIs.

Food restriction and streptozotocin differentially modify sensitivity to the hypothermic effects of direct- and indirect-acting serotonin receptor agonists in rats

Food restriction and experimentally-induced diabetes (streptozotocin) can modify serotonin (5-HT) neurotransmission and sensitivity to drugs acting on 5-HT systems. This study examined the effects of food restriction and streptozotocin on the hypothermic effects of the 5-HT(1A) receptor agonist (+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT), the 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-methylamphetamine hydrochloride (DOM), the 5-HT releaser fenfluramine, and the selective 5-HT reuptake inhibitor (SSRI) fluoxetine. All four drugs significantly decreased body temperature in free feeding rats. Limiting rats to 10 g/day of food for 7 days decreased body weight and sensitivity to 8-OH-DPAT induced hypothermia, without affecting sensitivity to DOM, fenfluramine, or fluoxetine induced hypothermia. Subsequently, 7 days of free feeding restored body weight and sensitivity to 8-OH-DPAT. Sensitivity to all drugs was significantly decreased 7 days after 50 mg/kg streptozotocin; subsequently, 10 days of insulin replacement restored sensitivity to all drugs. These results extend to body temperature the observation that food restriction and experimentally-induced diabetes differentially modify sensitivity to drugs acting on 5-HT systems and they further suggest that the clinical response to therapeutic drugs acting on 5-HT systems might be impacted by nutritional and insulin status.

Potentiation of the antidepressant-like effect of fluoxetine by aripiprazole in the mouse tail suspension test

We examined the effect of the novel atypical antipsychotic drug aripiprazole alone or in combination with the selective serotonin reuptake inhibitor fluoxetine in the mouse tail suspension test. We also investigated the effect of aripiprazole on glucose metabolism. Combined treatment with aripiprazole and a sub-effective dose of fluoxetine significantly decreased the duration of immobility in the tail suspension test. Aripiprazole by itself did not affect the duration of immobility. While olanzapine significantly increased blood glucose level in the glucose tolerance test, aripiprazole did not affect glucose metabolism. We suggest that aripiprazole augments the antidepressant-like effect of fluoxetine without affecting glucose metabolism.

Depression, diabetes and metabolic-nutritional factors in elderly Hispanics

OBJECTIVE

To examine the relationship of depression to metabolic and nutritional risk factors in older Hispanics.

DESIGN

Crossectional study.

SETTING

Subjects were part of a community-based, cognitive evaluation project that examined 301 subjects in the Eastern San Fernando Valley of Southern California.

PARTICIPANTS

Two elderly Hispanic groups: 53 clinically depressed, with memory complaints but not demented subjects, and 33 generally healthy, cognitively asymptomatic subjects.

MEASUREMENTS

The results of functional and nutritional questionnaires, a medical and neurological examination, 12-hour fasting clinical laboratory tests, MRI or CT scans, and neuropsychological testing.

RESULTS

Both groups were nearly identical along socio-demographic variables. However, the depressed group differed significantly from the general healthy group not only in percent of diabetics (38% vs.18%), but in the amount of poorly controlled diabetes, and the depressed group consumed about half the amount of fish that the generally healthy group did.

CONCLUSIONS

This study suggests that factors such as poorly controlled diabetes combined with low consumption of foods high in omega-3 fatty acid content such as sea fish may be associated with an increased risk of developing depression in late life. These factors may be socio-economically and culturally influenced and are therefore amenable to modification.

Food restriction and streptozotocin treatment decrease 5-HT1A and 5-HT2A receptor-mediated behavioral effects in rats

Food restriction and hypoinsulinemia can affect the synthesis, turnover, and receptor function of serotonin (5-HT) in brain. This study explored the effects of food restriction and streptozotocin treatment on behavioral effects related to 5-HT1A (+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) and 5-HT2A [(+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)] receptor activation. Lower lip retraction and flat body posture (8-OH-DPAT) and head twitching (DOI) were measured in rats during free feeding, food restriction, after treatment with streptozotocin, and finally after insulin replacement. 8-OH-DPAT induced lower lip retraction and flat body posture whereas DOI induced head twitching. One week of food restriction (10 g/day) decreased 8-OH-DPAT-induced lower lip retraction, 8-OH-DPAT-induced flat body posture, and DOI-induced head twitching. Subsequently, 1 week of free access to food restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. Finally, 1 week after streptozotocin, 8-OH-DPAT-induced flat body posture and DOI-induced head twitching were markedly reduced whereas 8-OH-DPAT-induced lower lip retraction was unchanged. One week of insulin replacement restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. These results show that modest food restriction or experimentally induced diabetes can profoundly affect sensitivity to drugs acting at 5-HT1A or 5-HT2A receptors; these results could be relevant to understanding the comorbidity of depression and diabetes.

Pleiotropic impact of constitutive fosB inactivation on nicotine-induced behavioral alterations and stress-related traits in mice

Multiple genes are thought to influence both susceptibility to nicotine dependence and its comorbid behavioral traits in humans. However, which specific genes contribute to this pleiotropic effect is poorly understood. Previous rodent studies have shown that many addictive substances and stressful stimuli increase the expression of the transcription factor FosB in limbic and associated regions and that the protein products of fosB contribute to certain behavioral effects of cocaine and morphine. However, the role of this gene in nicotine-regulated behaviors and dependence-related behavioral traits is unknown. We tested the hypothesis that a constitutive level of FosB affects nicotine-regulated behaviors and comorbid behavioral traits using constitutive fosB knockout (KO) mice. Following repeated or prolonged nicotine administration, but not a single acute administration, KO mice were impaired in conditioned place preference, oral nicotine intake and motor suppression. In wild-type mice, repeated nicotine injections, but not a single acute injection, increased the expression of FosB and its truncated variant DeltaFosB in the targets but not at the origins of the mesolimbic and nigrostriatal dopamine pathways; no detectable level of FosB/DeltaFosB was found in KO mice. In tasks designed to assess behavioral traits, KO mice exhibited more pronounced behavioral abnormalities when stress levels were high than when they were minimized. Our results suggest that the constitutive absence of fosB has a pleiotropic influence on the behavioral effects of repeated or prolonged nicotine administration and on stress-related behavioral traits in mice.