The pyramidal neurons in the medial prefrontal cortex show decreased response to 5-hydroxytryptamine-3 receptor stimulation in a rodent model of Parkinson's disease

Dorsal raphe nucleus-hippocampus serotonergic circuit underlies the depressive and cognitive impairments in 5×FAD male mice

BACKGROUND

Depressive symptoms often occur in patients with Alzheimer's disease (AD) and exacerbate the pathogenesis of AD. However, the neural circuit mechanisms underlying the AD-associated depression remain unclear. The serotonergic system plays crucial roles in both AD and depression.

METHODS

We used a combination of in vivo trans-synaptic circuit-dissecting anatomical approaches, chemogenetic manipulations, optogenetic manipulations, pharmacological methods, behavioral testing, and electrophysiological recording to investigate dorsal raphe nucleus serotonergic circuit in AD-associated depression in AD mouse model.

RESULTS

We found that the activity of dorsal raphe nucleus serotonin neurons (DRN5-HT) and their projections to the dorsal hippocampal CA1 (dCA1) terminals (DRN5-HT-dCA1CaMKII) both decreased in brains of early 5×FAD mice. Chemogenetic or optogenetic activation of the DRN5-HT-dCA1CaMKII neural circuit attenuated the depressive symptoms and cognitive impairments in 5×FAD mice through serotonin receptor 1B (5-HT1BR) and 4 (5-HT4R). Pharmacological activation of 5-HT1BR or 5-HT4R attenuated the depressive symptoms and cognitive impairments in 5×FAD mice by regulating the DRN5-HT-dCA1CaMKII neural circuit to improve synaptic plasticity.

CONCLUSIONS

These findings provide a new mechanistic connection between depression and AD and provide potential pharmaceutical prevention targets for AD.

The Implication of 5-HT Receptor Family Members in Aggression, Depression and Suicide: Similarity and Difference

Being different multifactorial forms of psychopathology, aggression, depression and suicidal behavior, which is considered to be violent aggression directed against the self, have principal neurobiological links: preclinical and clinical evidence associates depression, aggression and suicidal behavior with dysregulation in central serotonergic (5-HT) neurotransmission. The implication of different types of 5-HT receptors in the genetic and epigenetic mechanisms of aggression, depression and suicidality has been well recognized. In this review, we consider and compare the orchestra of 5-HT receptors involved in these severe psychopathologies. Specifically, it concentrates on the role of 5-HT_1A, 5-HT_1B, 5-HT_2A, 5-HT_2B, 5-HT_2C, 5-HT₃ and 5-HT₇ receptors in the mechanisms underlying the predisposition to aggression, depression and suicidal behavior. The review provides converging lines of evidence that: (1) depression-related 5-HT receptors include those receptors with pro-depressive properties (5-HT_2A, 5-HT₃ and 5-HT₇) as well as those providing an antidepressant effect (5-HT_1A, 5-HT_1B, 5-HT_2C subtypes). (2) Aggression-related 5-HT receptors are identical to depression-related 5-HT receptors with the exception of 5-HT₇ receptors. Activation of 5-HT_1A, 5-HT_1B, 5-HT_2A, 5-HT_2C receptors attenuate aggressiveness, whereas agonists of 5-HT₃ intensify aggressive behavior.

Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

5-hydroxytryptamine receptor subtype 3 (5-HT₃R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT₃R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT₃R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT₃R research.

Involvement of prelimbic 5-HT7 receptors in the regulation of anxiety-like behaviors in hemiparkinsonian rats

OBJECTIVE

At present, little is known about the role of serotonin₇ (5-HT₇) receptor in anxiety, particularly in Parkinson's disease-related anxiety. Here, we tested whether 5-HT₇ receptors in the prelimbic (PrL) cortex are involved in the regulation of anxiety-like behaviors in sham-operated rats and rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle (MFB).

METHODS

The open field and elevated plus maze (EPM) tests were performed to study the influence of MFB lesion and intra-PrL injection of 5-HT₇ agonist AS19 (0.5, 1 or 2 μg/rat) and antagonist SB269970 (1.5, 3 or 6 μg/rat) on anxiety-like behaviors. Additionally, changes in monoamine levels in limbic and limbic-related brain regions were observed after intra-PrL injection of AS19 (2 μg/rat) and SB269970 (6 μg/rat).

RESULTS

The MFB lesion induced anxiety-like behaviors compared to sham-operated rats. Intra-PrL injection of AS19 showed anxiolytic effects by the open field and EPM tests in two groups of rats, and administration of SB269970 showed anxiogenic responses. However, the doses producing these effects in the lesioned rats were higher than those in sham-operated rats. Neurochemical results showed that intra-PrL injection of AS19 increased dopamine, 5-HT and noradrenaline (NA) levels in the medial prefrontal cortex, ventral hippocampus and amygdala in two groups of rats, whereas SB269970 decreased 5-HT and NA levels in these brain regions.

DISCUSSION

5-HT₇ receptors in the PrL are involved in the regulation of anxiety-like behaviors, which is attributable to changes in dopamine, 5-HT and NA levels in the limbic and limbic-related brain regions after activation and blockade of 5-HT₇ receptors.

ABBREVIATIONS

6-OHDA: 6-hydroxydopamine; DMSO: dimethyl sulfoxide; DA: dopamine; EPM: elevated plus maze; MFB: medial forebrain bundlem; PFC: medial prefrontal cortex; NA: noradrenaline; PD: Pakinson's disease; PrL: prelimbic; 5-HT: serotonin; vHip: ventral hippocampus.

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.

Activation and blockade of serotonin7 receptors in the prelimbic cortex regulate depressive-like behaviors in a 6-hydroxydopamine-induced Parkinson's disease rat model

The role of serotonin7 (5-HT7) receptors in the regulation of depression is poorly understood, particularly in Parkinson's disease-associated depression. Here we examined whether 5-HT7 receptors in the prelimbic (PrL) sub-region of the ventral medial prefrontal cortex (mPFC) involve in the regulation of depressive-like behaviors in sham-operated rats and rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. The lesion induced depressive-like responses as measured by the sucrose preference and forced swim tests when compared to sham-operated rats. Intra-PrL injection of 5-HT7 receptor agonist AS19 (0.5, 1 and 2 μg/rat) increased sucrose consumption, and decreased immobility time in sham-operated and the lesioned rats, indicating the induction of antidepressant-like effects. Further, intra-PrL injection of 5-HT7 receptor antagonist SB269970 (1.5, 3 and 6 μg/rat) decreased sucrose consumption, and increased immobility time, indicating the induction of depressive-like responses. However, the doses producing these effects in the lesioned rats were higher than those in sham-operated rats. Neurochemical results showed that intra-PrL injection of AS19 (2 μg/rat) increased dopamine, 5-hydroxytryptamine (5-HT) and noradrenaline (NA) levels in the mPFC, habenula and ventral hippocampus (vHip) in sham-operated and the lesioned rats; whereas SB269970 (6 μg/rat) decreased 5-HT levels in the habenula and vHip, and the levels of NA in the mPFC, habenula and vHip in the two groups of rats. The results suggest that 5-HT7 receptors in the PrL play an important role in the regulation of these behaviors, which attribute to changes in monoamine levels in the limbic and limbic-related brain regions after activation and blockade of 5-HT7 receptors.

Neuronal activity (c-Fos) delineating interactions of the cerebral cortex and basal ganglia

The cerebral cortex and basal ganglia (BG) form a neural circuit that is disrupted in disorders such as Parkinson's disease. We found that neuronal activity (c-Fos) in the BG followed cortical activity, i.e., high in arousal state and low in sleep state. To determine if cortical activity is necessary for BG activity, we administered atropine to rats to induce a dissociative state resulting in slow-wave electroencephalography but hyperactive motor behaviors. Atropine blocked c-Fos expression in the cortex and BG, despite high c-Fos expression in the sub-cortical arousal neuronal groups and thalamus, indicating that cortical activity is required for BG activation. To identify which glutamate receptors in the BG that mediate cortical inputs, we injected ketamine [N-methyl-d-aspartate (NMDA) receptor antagonist] and 6-cyano-nitroquinoxaline-2, 3-dione (CNQX, a non-NMDA receptor antagonist). Systemic ketamine and CNQX administration revealed that NMDA receptors mediated subthalamic nucleus (STN) input to internal globus pallidus (GPi) and substantia nigra pars reticulata (SNr), while non-NMDA receptor mediated cortical input to the STN. Both types of glutamate receptors were involved in mediating cortical input to the striatum. Dorsal striatal (caudoputamen, CPu) dopamine depletion by 6-hydroxydopamine resulted in reduced activity of the CPu, globus pallidus externa (GPe), and STN but increased activity of the GPi, SNr, and putative layer V neurons in the motor cortex. Our results reveal that the cortical activity is necessary for BG activity and clarifies the pathways and properties of the BG-cortical network and their putative role in the pathophysiology of BG disorders.

Behavioral and neurochemical effects of alpha-lipoic Acid in the model of Parkinson's disease induced by unilateral stereotaxic injection of 6-ohda in rat

This study aimed to investigate behavioral and neurochemical effects of α -lipoic acid (100 mg/kg or 200 mg/kg) alone or associated with L-DOPA using an animal model of Parkinson's disease induced by stereotaxic injection of 6-hydroxydopamine (6-OHDA) in rat striatum. Motor behavior was assessed by monitoring body rotations induced by apomorphine, open field test and cylinder test. Oxidative stress was accessed by determination of lipid peroxidation using the TBARS method, concentration of nitrite and evaluation of catalase activity. α -Lipoic acid decreased body rotations induced by apomorphine, as well as caused an improvement in motor performance by increasing locomotor activity in the open field test and use of contralateral paw (in the opposite side of the lesion produced by 6-OHDA) at cylinder test. α -lipoic acid showed antioxidant effects, decreasing lipid peroxidation and nitrite levels and interacting with antioxidant system by decreasing of endogenous catalase activity. Therefore, α -lipoic acid prevented the damage induced by 6-OHDA or by chronic use of L-DOPA in dopaminergic neurons, suggesting that α -lipoic could be a new therapeutic target for Parkinson's disease prevention and treatment.

The rapid recovery of 5-HT cell firing induced by the antidepressant vortioxetine involves 5-HT(3) receptor antagonism

The therapeutic effect of current antidepressant drugs appears after several weeks of treatment and a significant number of patients do not respond to treatment. Here, we report the effects of the multi-modal antidepressant vortioxetine (Lu AA21004), a 5-HT(3) and 5-HT(7) receptor antagonist, 5-HT(1B) receptor partial agonist, 5-HT(1A) receptor agonist and 5-HT transporter (SERT) inhibitor, on rat 5-HT neurotransmission. Using in vivo electrophysiological recordings in the dorsal raphe nucleus of anaesthetized rats, we assessed the acute and subchronic effects of vortioxetine and/or the selective 5-HT(3) receptor agonist, SR57227 or the selective 5-HT(1A) receptor agonist flesinoxan, on 5-HT neuronal firing activity. Using ex-vivo autoradiography, we correlated SERT occupancy and presumed 5-HT firing activity. The selective serotonin reuptake inhibitor, fluoxetine, was used as comparator. Importantly, the recovery of 5-HT neuronal firing was achieved after 1 d with vortioxetine and 14 d with fluoxetine. SR57227 delayed this recovery. In contrast, vortioxetine failed to alter the reducing action of 3 d treatment of flesinoxan. Acute dosing of vortioxetine inhibited neuronal firing activity more potently than fluoxetine. SR57227 prevented the suppressant effect of vortioxetine, but not of fluoxetine. In contrast, flesinoxan failed to modify the suppressant effect of vortioxetine acutely administered. Differently to fluoxetine, vortioxetine suppressed neuronal firing without saturating occupancy at the SERT. Vortioxetine produced a markedly faster recovery of 5-HT neuronal firing than fluoxetine. This is at least partly due to 5-HT(3) receptor antagonism of vortioxetine in association with its reduced SERT occupancy.

Serotonin and molecular neuroimaging in humans using PET

The serotonergic system is one of the most important modulatory neurotransmitter systems in the human brain. It plays a central role in major physiological processes and is implicated in a number of psychiatric disorders. Along with the dopaminergic system, it is also one of the phylogenetically oldest human neurotransmitter systems and one of the most diverse, with 14 different receptors identified up to this day, many of whose function remains to be understood. The system's functioning is even more diverse than the number of its receptors, since each is implicated in a number of different processes. This review aims at illustrating the distribution and summarizing the main functions of the serotonin (5-hydroxytryptamin, 5-HT) receptors as well as the serotonin transporter (SERT, 5-HTT), the vesicular monoamine transporter 2, monoamine oxidase type A and 5-HT synthesis in the human brain. Recent advances in in vivo quantification of these different receptors and enzymes that are part of the serotonergic system using positron emission tomography are described.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.