Regulation of central noradrenergic activity by 5-HT3 receptors located in the locus coeruleus of the rat

The raphe nuclei are the early lesion site of gastric α-synuclein propagation to the substantia nigra

Parkinson's disease (PD) is a neurodegeneration disease with α-synuclein accumulated in the substantia nigra pars compacta (SNpc) and most of the dopaminergic neurons are lost in SNpc while patients are diagnosed with PD. Exploring the pathology at an early stage contributes to the development of the disease-modifying strategy. Although the "gut-brain" hypothesis is proposed to explain the underlying mechanism, where the earlier lesioned site in the brain of gastric α-synuclein and how α-synuclein further spreads are not fully understood. Here we report that caudal raphe nuclei (CRN) are the early lesion site of gastric α-synuclein propagating through the spinal cord, while locus coeruleus (LC) and substantia nigra pars compacta (SNpc) were further affected over a time frame of 7 months. Pathological α-synuclein propagation via CRN leads to neuron loss and disordered neuron activity, accompanied by abnormal motor and non-motor behavior. Potential neuron circuits are observed among CRN, LC, and SNpc, which contribute to the venerability of dopaminergic neurons in SNpc. These results show that CRN is the key region for the gastric α-synuclein spread to the midbrain. Our study provides valuable details for the "gut-brain" hypothesis and proposes a valuable PD model for future research on early PD intervention.

Human tau-overexpressing mice recapitulate brainstem involvement and neuropsychiatric features of early Alzheimer's disease

Alzheimer's disease (AD) poses an ever-increasing public health concern as the population ages, affecting more than 6 million Americans. AD patients present with mood and sleep changes in the prodromal stages that may be partly driven by loss of monoaminergic neurons in the brainstem, but a causal relationship has not been firmly established. This is due in part to a dearth of animal models that recapitulate early AD neuropathology and symptoms. The goal of the present study was to evaluate depressive and anxiety-like behaviors in a mouse model of AD that overexpresses human wild-type tau (htau) prior to the onset of cognitive impairments and assess these behavior changes in relationship to tau pathology, neuroinflammation, and monoaminergic dysregulation in the dorsal raphe nucleus (DRN) and locus coeruleus (LC). We observed depressive-like behaviors at 4 months in both sexes and hyperlocomotion in male htau mice. Deficits in social interaction persisted at 6 months and were accompanied by an increase in anxiety-like behavior in males. The behavioral changes at 4 months coincided with a lower density of serotonergic (5-HT) neurons, downregulation of 5-HT markers, reduced excitability of 5-HT neurons, and hyperphosphorylated tau in the DRN. Inflammatory markers were also upregulated in the DRN along with protein kinases and transglutaminase 2, which may promote tau phosphorylation and aggregation. Loss of 5-HT innervation to the entorhinal cortex and dentate gyrus of the hippocampus was also observed and may have contributed to depressive-like behaviors. There was also reduced expression of noradrenergic markers in the LC along with elevated phospho-tau expression, but this did not translate to a functional change in neuronal excitability. In total, these results suggest that tau pathology in brainstem monoaminergic nuclei and the resulting loss of serotonergic and/or noradrenergic drive may underpin depressive- and anxiety-like behaviors in the early stages of AD.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Noradrenergic Components of Locomotor Recovery Induced by Intraspinal Grafting of the Embryonic Brainstem in Adult Paraplegic Rats

Intraspinal grafting of serotonergic (5-HT) neurons was shown to restore plantar stepping in paraplegic rats. Here we asked whether neurons of other phenotypes contribute to the recovery. The experiments were performed on adult rats after spinal cord total transection. Grafts were injected into the sub-lesional spinal cord. Two months later, locomotor performance was tested with electromyographic recordings from hindlimb muscles. The role of noradrenergic (NA) innervation was investigated during locomotor performance of spinal grafted and non-grafted rats using intraperitoneal application of α₂ adrenergic receptor agonist (clonidine) or antagonist (yohimbine). Morphological analysis of the host spinal cords demonstrated the presence of tyrosine hydroxylase positive (NA) neurons in addition to 5-HT neurons. 5-HT fibers innervated caudal spinal cord areas in the dorsal and ventral horns, central canal, and intermediolateral zone, while the NA fiber distribution was limited to the central canal and intermediolateral zone. 5-HT and NA neurons were surrounded by each other’s axons. Locomotor abilities of the spinal grafted rats, but not in control spinal rats, were facilitated by yohimbine and suppressed by clonidine. Thus, noradrenergic innervation, in addition to 5-HT innervation, plays a potent role in hindlimb movement enhanced by intraspinal grafting of brainstem embryonic tissue in paraplegic rats.

5-HT3 Receptor Antagonists in Neurologic and Neuropsychiatric Disorders: The Iceberg Still Lies beneath the Surface

5-HT₃ receptor antagonists, first introduced to the market in the mid-1980s, are proven efficient agents to counteract chemotherapy-induced emesis. Nonetheless, recent investigations have shed light on unappreciated dimensions of this class of compounds in conditions with an immunoinflammatory component as well as in neurologic and psychiatric disorders. The promising findings from multiple studies have unveiled several beneficial effects of these compounds in multiple sclerosis, stroke, Alzheimer disease, and Parkinson disease. Reports continue to uncover important roles for 5-HT₃ receptors in the physiopathology of neuropsychiatric disorders, including depression, anxiety, drug abuse, and schizophrenia. This review addresses the potential of 5-HT₃ receptor antagonists in neurology- and neuropsychiatry-related disorders. The broad therapeutic window and high compliance observed with these agents position them as suitable prototypes for the development of novel pharmacotherapeutics with higher efficacy and fewer adverse effects.

Chronic fluoxetine reverses the effects of chronic corticosterone treatment on α2-adrenoceptors in the rat frontal cortex but not locus coeruleus

Disruption of the hypothalamic-pituitary-adrenal axis is an established finding in patients with anxiety and/or depression. Chronic corticosterone administration in animals has been proposed as a model for the study of these stress-related disorders and the antidepressant action. Alterations of the central noradrenergic system and specifically of inhibitory α₂-adrenoceptors seem to be part of the pathophysiology of depression and contribute to the antidepressant activity. The present study evaluates in male rats the effect of chronic corticosterone treatment during 35 days (16-20 mg kg^-1 day^-1) on the sensitivity of α₂-adrenoceptors expressed in the somatodendritic and terminal noradrenergic areas locus coeruleus (LC) and prefrontal cortex (PFC), respectively. Further, the effect of chronic fluoxetine treatment (5 mg kg^-1, i.p., since the 15th day) on the sensitivity of α₂-adrenoceptors was examined under control conditions and in corticosterone-treated rats. The α₂-adrenoceptor functionality was analysed in vitro by agonist-mediated [³⁵S]GTPγS binding stimulation and in vivo through the modulation of noradrenaline (NA) release evaluated by dual-probe microdialysis. The concentration-effect curves of the [³⁵S]GTPγS binding stimulation by the agonist UK14304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) demonstrated a desensitization of cortical α₂-adrenoceptors induced by corticosterone (-logEC₅₀ = 6.7 ± 0.2 vs 8.2 ± 0.3 in controls) that was reverted by fluoxetine treatment (-logEC₅₀ = 7.5 ± 0.3). Local administration of the α₂-adrenoceptor antagonist RS79948 ((8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulfonyl)-6H-isoquino[2,1-g][1,6]naphthyridine) (0.1-100 μmol L^-1) into the LC induced a concentration-dependent NA increase in the PFC of the control group (E_max = 191 ± 30%) but non-significant effect was observed in corticosterone-treated rats (E_max = 133 ± 46%), reflecting a desensitization of α₂-adrenoceptors that control the firing of noradrenergic neurons. Fluoxetine treatment did not alter the corticosterone-induced desensitization in this area (E_max = 136 ± 19%). No effect of fluoxetine on α₂-adrenoceptor functionality was observed in control animals (E_max = 223 ± 30%). In PFC, the local administration of RS79948 increased NA in controls (E_max = 226 ± 27%) without effect in the corticosterone group (E_max = 115 ± 26%), suggesting a corticosterone-induced desensitization of terminal α₂-adrenoceptors. Fluoxetine administration prevented the desensitization induced by corticosterone in the PFC (E_max = 233 ± 33%) whereas desensitized α₂-adrenoceptors in control animals (E_max = -24 ± 10%). These data indicate that chronic corticosterone increases noradrenergic activity by acting at different α₂-adrenoceptor subpopulations. Treatment with the antidepressant fluoxetine seems to counteract these changes by acting mainly on presynaptic α₂-adrenoceptors expressed in terminal areas.

Serotonin 5-HT3 receptor antagonism potentiates the antidepressant activity of citalopram

Activation of serotonin 5-HT₃ receptor (5HT3R) in the locus coeruleus (LC), the principal somatodendritic noradrenergic area, decreases LC firing activity and noradrenaline (NA) release in prefrontal cortex (PFC). Blockade of 5HT3R in coadministration with selective serotonin reuptake inhibitors (SSRIs) has been proposed as a potential strategy to accelerate the onset of action of SSRIs. Dual-probe microdialysis in rats was used to evaluate the involvement of 5HT3R in the in vivo effect exerted by the SSRI citalopram on NA release. Besides, forced swimming test (FST) was carried out in mice to evaluate the antidepressant-like effect of citalopram in combination with a 5HT3R antagonist (Y25130). Systemic administration of the 5HT3R agonist SR57227 (10 mg/kg i.p.) increased NA in LC (Emax = 200 ± 27%) and PFC (Emax = 133 ± 2%). The increase in PFC was enhanced in local presence into LC of Y25130 (50 μM) (Emax = 296 ± 41%) suggesting an inhibitory function on NA release exerted by the activation of 5HT3R located in somatodendritic areas. Citalopram administration (10 mg/kg i.p.) increased NA in LC (Emax = 185 ± 11%) and decreased it in PFC (Emax = -35 ± 7%). Intra-LC (50 μM) or systemic co-administration of Y25130 (10 mg/kg i.p.) with citalopram (10 mg/kg i.p.) switched NA release in the PFC from an inhibition to a stimulatory effect. In mice FST, systemic coadministration of citalopram (2.5 mg/kg i.p.) and Y25130 (10 mg/kg i.p.) potentiated the decrease of immobility time through the increase of both swimming and climbing behaviours. These results suggest that the addition of a 5HT3R antagonist to SSRIs could represent a feasible strategy to improve antidepressant response.

Genetic analysis of the RIC3 gene in Han Chinese patients with Parkinson's disease

Parkinson's disease (PD) is the second-most common etiologically complex neurodegenerative disease. Genetic abnormalities are thought to play an important role in the development of PD. Recently, mutations in the resistance to inhibitors of cholinesterase 3 gene (RIC3) have been reported to cause autosomal-dominant PD in Indian population. To determine whether RIC3 gene coding variant(s) are associated with PD in Han Chinese population, the RIC3 gene coding region in 218 mainland Han Chinese patients with PD and the identified variants in 242 normal controls were examined using direct sequencing analysis. Four known single nucleotide variants (c.354C>A, p.L118L, rs10839976; c.389G>A, p.C130Y, rs55990541; c.403C>T, p.P135S, rs73411617; and c.1054G>A, p.D352N, rs11826236) were identified in the RIC3 gene coding region. No significant differences were observed in either genotypic or allelic distributions between the PD patients and the normal controls (all P>0.05) for these four variants. Haplotype analysis showed that the presence of haplotype A-G-C-G (rs10839976-rs55990541-rs73411617-rs11826236) was associated with a 0.764-fold decreased risk (P=0.049, OR=0.764, 95% CI=0.585-0.999) for PD, whereas the presence of haplotype C-A-C-A was associated with a 2.143-fold increased risk (P=0.039, OR=2.143, 95% CI=1.023-4.488) for PD. The findings indicate that four variants: rs10839976, rs55990541, rs73411617 and rs11826236 in the RIC3 gene coding region may play little or no role in the development of PD. Two RIC3 gene haplotypes of four variants: A-G-C-G, and C-A-C-A might relate to either protection against or increased susceptibility to PD in the Han Chinese population, respectively.

Effect of subchronic corticosterone administration on α2-adrenoceptor functionality in rat brain: an in vivo and in vitro study

RATIONALE

Noradrenergic system plays a critical role in the hypothalamic-pituitary-adrenal (HPA) axis regulation and the stress response. A dysregulated HPA axis may be indicative of an increased biological vulnerability for depression. In addition, a variety of studies have focused on specific alterations of α₂-adrenoceptors as a mechanism involved in the pathogenesis of mood disorders and antidepressant response.

OBJECTIVES

This study aimed to evaluate the effect of subchronic corticosterone administration on rat brain α₂-adrenoceptor functionality by in vitro [³⁵S]GTPγS binding stimulation assays and in vivo dual-probe microdialysis determination of extracellular noradrenaline concentrations.

RESULTS

Implantation of a time release corticosterone pellet during 14 days induced sustained changes in endocrine function. However, there were no differences in α₂-adrenoceptor agonist UK14304-induced stimulation of [³⁵S]GTPγS binding in prefrontal cortex (PFC) between corticosterone-treated and control rats. In the same way, the in vivo evaluation of α₂-adrenoceptor-mediated noradrenaline release responses to the α₂-adrenoceptor agonist clonidine local administration into the locus coeruleus (LC), and the PFC did not show differences between the groups.

CONCLUSIONS

The present results show that subchronic corticosterone administration does not induce changes on functionality of α₂-adrenoceptors neither in the LC nor in noradrenergic cortical terminal areas.

Chronic citalopram administration desensitizes prefrontal cortex but not somatodendritic α2-adrenoceptors in rat brain

Selective serotonin reuptake inhibitors (SSRIs) regulate brain noradrenergic neurotransmission both at somatodendritic and nerve terminal areas. Previous studies have demonstrated that noradrenaline (NA) reuptake inhibitors are able to desensitize α₂-adrenoceptor-mediated responses. The present study was undertaken to elucidate the effects of repeated treatment with the SSRI citalopram on the α₂-adrenoceptor sensitivity in locus coeruleus (LC) and prefrontal cortex (PFC), by using in vivo microdialysis and electrophysiological techniques, and in vitro stimulation of [³⁵S]GTPγS binding autoradiography. Repeated, but not acute, treatment with citalopram (5 mg/kg, i.p., 14 days) increased extracellular NA concentration selectively in PFC. The α₂-adrenoceptor agonist clonidine (0.3 mg/kg, i.p.), administered to saline-treated animals (1 ml/kg i.p., 14 days) induced NA decrease in LC (E_max = -44 ± 4%; p < 0.001) and in PFC (E_max = -61 ± 5%, p < 0.001). In citalopram chronically-treated rats, clonidine administration exerted a lower decrease of NA (E_max = -25 ± 7%; p < 0.001) in PFC whereas the effect in LC was not different to controls (E_max = -36 ± 4%). Clonidine administration (0.625-20 μg/kg, i.v.) evoked a dose-dependent decrease of the firing activity of LC noradrenergic neurons in both citalopram- (ED₅₀ = 3.2 ± 0.4 μg/kg) and saline-treated groups (ED₅₀ = 2.6 ± 0.5 μg/kg). No significant differences between groups were found in ED₅₀ values. The α₂-adrenoceptor agonist UK14304 stimulated specific [³⁵S]GTPγS binding in brain sections containing LC (144 ± 14%) and PFC (194 ± 32%) of saline-treated animals. In citalopram-treated animals, this increase did not differ from controls in LC (146 ± 22%) but was lower in PFC (141 ± 8%; p < 0.05). Taken together, long-term citalopram treatment induces a desensitization of α₂-adrenoceptors acting as axon terminal autoreceptors in PFC without changes in somatodendritic α₂-adrenoceptor sensitivity.

Ca(2+) in the dorsal raphe nucleus promotes wakefulness via endogenous sleep-wake regulating pathway in the rats

Serotonergic neurons in the dorsal raphe nucleus (DRN) are involved in the control of sleep-wake states. Our previous studies have indicated that calcium (Ca(2+)) modulation in the DRN plays an important role in rapid-eye-movement sleep (REMS) and non-REMS (NREMS) regulation during pentobarbital hypnosis. The present study investigated the effects of Ca(2+) in the DRN on sleep-wake regulation and the related neuronal mechanism in freely moving rats. Our results showed that microinjection of CaCl2 (25 or 50 nmol) in the DRN promoted wakefulness and suppressed NREMS including slow wave sleep and REMS in freely moving rats. Application of CaCl2 (25 or 50 nmol) in the DRN significantly increased serotonin in the DRN and hypothalamus, and noradrenaline in the locus coeruleus and hypothalamus. Immunohistochemistry study indicated that application of CaCl2 (25 or 50 nmol) in the DRN significantly increased c-Fos expression ratio in wake-promoting neurons including serotonergic neurons in the DRN, noradrenergic neurons in the locus coeruleus, and orxinergic neurons in the perifornical nucleus, but decreased c-Fos expression ratio of GABAergic sleep-promoting neurons in the ventrolateral preoptic nucleus. These results suggest that Ca(2+) in the DRN exert arousal effects via up-regulating serotonergic functions in the endogenous sleep-wake regulating pathways.

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.

Are 5-HT3 antagonists effective in obsessive-compulsive disorder? A systematic review of literature

OBJECTIVE

The purpose of this literature database search-based review was to critically consider and evaluate the findings of literature focusing on efficacy and safety of 5-HT3 antagonists in the treatment of obsessive-compulsive disorder (OCD), so as to test whether preclinical data match clinical therapeutic trials.

DESIGN

The PubMed database has been searched for papers on 5-HT3 antagonists and OCD in humans and for animal models of OCD and 5-HT3 receptors.

RESULTS

Of the clinically tested 5-HT3 receptor antagonists, ondansetron has been used to treat OCD in five therapeutic studies, whereas granisetron only in one recent trial. Both showed some efficacy in open studies and superiority to placebo in double-blind studies, along with fair safety. No animal OCD model directly implicated 5-HT3 receptors.

CONCLUSIONS

Overall, results indicate some utility, but the available literature is too scanty to allow for valid conclusions to be drawn. The mismatch between animal models of obsessive-compulsive disorder and clinical data with 5-HT3 antagonists needs more clinical data to ensure that it is not an artefact.

Selective serotonin reuptake inhibition modulates response inhibition in Parkinson's disease

Impulsivity is common in Parkinson's disease even in the absence of impulse control disorders. It is likely to be multifactorial, including a dopaminergic 'overdose' and structural changes in the frontostriatal circuits for motor control. In addition, we proposed that changes in serotonergic projections to the forebrain also contribute to response inhibition in Parkinson's disease, based on preclinical animal and human studies. We therefore examined whether the selective serotonin reuptake inhibitor citalopram improves response inhibition, in terms of both behaviour and the efficiency of underlying neural mechanisms. This multimodal magnetic resonance imaging study used a double-blind randomized placebo-controlled crossover design with an integrated Stop-Signal and NoGo paradigm. Twenty-one patients with idiopathic Parkinson's disease (46-76 years old, 11 male, Hoehn and Yahr stage 1.5-3) received 30 mg citalopram or placebo in addition to their usual dopaminergic medication in two separate sessions. Twenty matched healthy control subjects (54-74 years old, 12 male) were tested without medication. The effects of disease and drug on behavioural performance and regional brain activity were analysed using general linear models. In addition, anatomical connectivity was examined using diffusion tensor imaging and tract-based spatial statistics. We confirmed that Parkinson's disease caused impairment in response inhibition, with longer Stop-Signal Reaction Time and more NoGo errors under placebo compared with controls, without affecting Go reaction times. This was associated with less stop-specific activation in the right inferior frontal cortex, but no significant difference in NoGo-related activation. Although there was no beneficial main effect of citalopram, it reduced Stop-Signal Reaction Time and NoGo errors, and enhanced inferior frontal activation, in patients with relatively more severe disease (higher Unified Parkinson's Disease Rating Scale motor score). The behavioural effect correlated with the citalopram-induced enhancement of prefrontal activation and the strength of preserved structural connectivity between the frontal and striatal regions. In conclusion, the behavioural effect of citalopram on response inhibition depends on individual differences in prefrontal cortical activation and frontostriatal connectivity. The correlation between disease severity and the effect of citalopram on response inhibition may be due to the progressive loss of forebrain serotonergic projections. These results contribute to a broader understanding of the critical roles of serotonin in regulating cognitive and behavioural control, as well as new strategies for patient stratification in clinical trials of serotonergic treatments in Parkinson's disease.

Involvement of serotonin 5-HT3 receptors in the modulation of noradrenergic transmission by serotonin reuptake inhibitors: a microdialysis study in rat brain

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs), in addition to being able to enhance serotonergic neurotransmission, are able to modulate other brain systems involved in depression.

OBJECTIVES

This study evaluates the neurochemical effect of the SSRI citalopram on brain noradrenergic activity and the serotonin receptor involved in this effect.

METHODS

Dual-probe microdialysis in the locus coeruleus (LC) and prefrontal cortex (PFC) was performed in freely awake rats.

RESULTS

Systemic citalopram (10 mg/kg, i.p.) increased noradrenaline (NA) in the LC (E max = 141 ± 13%) and simultaneously decreased NA in the PFC (Emax = -46 ± 7%). In the local presence into the LC of the α2-adrenoceptor antagonist RS79948 (1 μM), systemic citalopram increased NA in the LC (Emax = 157 ± 25%) and PFC (Emax = 175 ± 24%). Local citalopram (0.1-100 μM) into the LC induced NA increase in the LC (Emax = 210 ± 25%) and decrease in the PFC (Emax = -38 ± 9%). Local LC citalopram effect was abolished by LC presence of the 5-HT3 receptor antagonist MDL72222 (1 μM) but not the 5-HT1/2 receptor antagonist methiothepin (1 μM). Systemic citalopram in the LC presence of MDL72222 did not modify NA in the LC but increased NA in the PFC (Emax = 158 ± 26%). Local citalopram into the PFC enhanced NA (Emax = 376 ± 18%) in the area, which was prevented by MDL72222.

CONCLUSIONS

The SSRI citalopram modulates central noradrenergic neurotransmission by activation, through endogenous serotonin, of 5-HT3 receptors expressed in the somatodendritic (LC) and terminal (PFC) areas, which subsequently promote an enhancement of local NA. Therefore, 5-HT3 receptors and somatodendritic α2-adrenoceptors in the LC play an important role in the global effect of SSRIs.