Cannabidiol exhibits anxiolytic-like effects and antipsychotic-like effects in mice models

Cannabidiol (CBD), a non-psychoactive compound derived from the cannabis plant, has been confirmed to induce anxiolytic-like and antipsychotic-like effects. However, the exact mechanisms remain unclear. This study substantiated CBD's interaction with the 5-HT1A receptor (5-HT1AR) in vitro (CHO cells expressing human 5-HT1AR) and in vivo (rat lower lip retraction test, LLR test). We then assessed the impact of CBD in mice using the stress-induced hyperthermia (SIH) model and the phencyclidine (PCP)-induced negative symptoms of schizophrenia model, respectively. Concurrently, we investigated whether WAY-100635, a typical 5-HT1AR antagonist, could attenuate these effects. Furthermore, the neurotransmitter changes through high-performance liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) were studied. Results revealed that CBD exhibits selective 5-HT1AR agonists-mediated effects in the rat lower lip retraction test, aligning with the robust agonistic (EC50 = 1.75 μM) profile observed in CHO cells. CBD at 3 mg/kg significantly reduced SIH (ΔT), a response that WAY-100635 abolished. Chronic administration of CBD at 100 mg/kg mitigated the increase in PCP-induced immobility time in the forced swim test (FST) and tail suspension test (TST). Moreover, it induced significant alterations in gamma-aminobutyric acid (GABA) and norepinephrine (NE) levels within the hippocampus (HPC). Thus, we concluded that the 5-HT1AR mediates CBD's anxiolytic-like effects. Additionally, CBD's effects on the negative symptoms of schizophrenia may be linked to changes in GABA and NE levels in the hippocampus. These findings offer novel insights for advancing the exploration of CBD's anxiolytic-like and antipsychotic-like effects.

Serotonergic agonism and pharmacologically-induced adolescent stress cause operant-based learning deficits in mice

BACKGROUND

The interplay between environmental stress and genetic factors is thought to play an important role in the pathogenesis and maintenance of obsessive-compulsive disorder (OCD). However, the relative contribution of these causative antecedents in the manifestation of cognitive inflexibility-a phenotype often seen in obsessive-compulsive (OC)- spectrum disorders-is not fully understood.

METHOD

In this study, we treated mice with 50 mg/L corticosterone (CORT, a glucocorticoid stress hormone) in their drinking water during adolescence. In adulthood, we assessed anxiety-like behaviour and locomotor activity; along with operant-based discrimination and reversal learning. RU-24969, a selective serotonin receptor 5-HT1A/1B receptor agonist, was used as an acute pharmacological model of OC-like behaviour. RU-24969 (5 mg/kg) was administered prior to each reversal learning testing session.

RESULTS

We found that acute treatment with 5 mg/kg RU-24969 induced stereotyped hyperlocomotion in vehicle- and CORT-treated mice. Furthermore, pre-treatment with CORT in adolescence produced subtle anxiety-like behaviour in adult mice, and also resulted in an impairment to late-stage discrimination learning and alterations to reversal learning. Finally, acute treatment with 5 mg/kg RU-24969 caused an impairment to early-stage reversal learning.

CONCLUSION

Whilst we revealed dissociable detrimental effects of adolescent CORT treatment and acute 5-HT1A/1B receptor agonism on discrimination and reversal learning, respectively, we did not find evidence of additive deleterious effects of these two treatments. We therefore suggest that while disrupted serotonergic signalling is likely to be involved in the cognitive phenotype of OC-spectrum disorders, distinct neuropathological pathways may be at play in mediating the role of stress as an antecedent in OCD and related illnesses.

The 5-HT1A receptor agonist 8-OH-DPAT modulates motor/exploratory activity, recognition memory and dopamine transporter binding in the dorsal and ventral striatum

In the present studies, we assessed the effect of the 5-HT1A receptor (R) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on motor and exploratory behaviors, object and place recognition and dopamine transporter (DAT) and serotonin transporter (SERT) binding in the rat brain. In Experiment I, motor/exploratory behaviors were assessed in an open field after injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle for 30 min without previous habituation to the open field. In Experiment II, rats underwent a 5-min exploration trial in an open field with two identical objects. After injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle, rats underwent a 5-min test trial with one of the objects replaced by a novel one and the other object transferred to a novel place. Subsequently, N-o-fluoropropyl-2b-carbomethoxy-3b-(4-[123I]iodophenyl)-nortropane ([123I]FP-CIT; 11 ± 4 MBq) was injected into the tail vein. Regional radioactivity accumulations were determined post mortem with a well counter. In both experiments, 8-OH-DPAT dose-dependently increased ambulation and exploratory head-shoulder motility, whereas rearing was dose-dependently decreased. In the test rial of Experiment II, there were no effects of 8-OH-DPAT on overall activity, sitting and grooming. 8-OH-DPAT dose-dependently impaired recognition of object and place. 8-OH-DPAT (3 mg/kg) increased DAT binding in the dorsal striatum relative to both vehicle and 0.1 mg/kg 8-OH-DPAT. Furthermore, in the ventral striatum, DAT binding was decreased after 3 mg/kg 8-OH-DPAT relative to vehicle. Findings indicate that motor/exploratory behaviors, memory for object and place and regional dopamine function may be modulated by the 5-HT1AR. Since, after 8-OH-DPAT, rats exhibited more horizontal and less (exploratory) vertical motor activity, while overall activity was not different between groups, it may be inferred, that the observed impairment of object recognition was not related to a decrease of motor activity as such, but to a decrease of intrinsic motivation, attention and/or awareness, which are relevant accessories of learning. Furthermore, the present findings on 8-OH-DPAT action indicate associations not only between motor/exploratory behavior and the recognition of object and place but also between the respective parameters and the levels of available DA in dorsal and ventral striatum.

The excitatory effect of 5-HT1A and 5-HT2B receptors on the caudal neurosecretory system Dahlgren cells in olive flounder, Paralichthys olivaceus

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) plays an essential role in the regulation of neural activity via multiple receptors. Here, we investigated the functional role of serotoninergic input on the Dahlgren cell population in the caudal neurosecretory system (CNSS) of olive flounder. In this study, the effect of 5-HT on the firing activity of Dahlgren cells was explored in terms of changes in firing frequency and firing pattern using multicellular recording electrophysiology ex vivo, and the role of several 5-HT receptor subtypes in the regulation was determined. The results revealed that 5-HT increased the firing frequency in a concentration-dependent manner and altered the firing pattern of Dahlgren cells. The effect of 5-HT on the firing activity of Dahlgren cells was mediated through the 5-HT1A and 5-HT2B receptors, selective agonists of both receptors effectively increased the firing frequency of Dahlgren cells, and selective receptor antagonists could also effectively inhibit the increase in firing frequency caused by 5-HT. In addition, the mRNA levels of major signaling pathway-related genes, ion channels, and major secretion hormone genes were significantly upregulated in CNSS after treatment with 5-HT. These findings demonstrate that 5-HT acts as an excitatory neuromodulator on Dahlgren cells and enhances neuroendocrine activity in CNSS.

NLX-101, a 5-HT1A receptor-biased agonist, improves pattern separation and stimulates neuroplasticity in aged rats

5-HT_1A serotonin receptors may play a role in cognitive function changes related to advanced age. Here, we investigated the effects of acute and repeated treatment with NLX-101 (F15599), a postsynaptic 5-HT_1A receptor-biased agonist, and F13714, a presynaptic 5-HT_1A receptor-biased agonist on spatial object pattern separation (OPS) in aged (22-24 months) rats. Neuroplasticity markers including brain-derived neurotrophic factor, PSD95, synaptophysin, and doublecortin were evaluated in the hippocampus. Unlike younger rats, aged rats were incapable of discriminating any new position of the objects in the arena, reflecting the detrimental effect of aging on pattern separation. However, aged animals treated with NLX-101 showed a significant cognitive improvement in the OPS test, accompanied by increases in hippocampal brain-derived neurotrophic factor and PSD95 protein levels. In contrast, no improvement in OPS performance was observed when aged rats received F13714. Both F13714 and NLX-101 increased the number of newborn neurons in the hippocampi of aged rats. These findings provide a rationale for targeting post-synaptic 5-HT_1A as a treatment for cognitive deficits related to aging.

On the role of serotonin 5-HT(1A) receptor in autistic-like behavior: сross talk of 5-HT and BDNF systems

Autism spectrum disorders (ASDs) are some of the most common neurodevelopmental disorders; however, the mechanisms underlying ASDs are still poorly understood. Serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) are known as key players in brain and behavioral plasticity and interact with each other. 5-HT_1A receptor is a principal regulator of the brain 5-HT system, which modulates normal and pathological behavior. Here we investigated effects of adeno-associated-virus-based 5-HT_1A receptor overexpression in the hippocampus of BTBR mice (which are a model of autism) on various types of behavior and on the expression of 5-HT₇ receptor, proBDNF, mature BDNF, and BDNF receptors (TrkB and p75^NTR). The 5-HT_1A receptor overexpression in BTBR mice reduced stereotyped behavior in the marble-burying test and extended the time spent in the center in the open field test. Meanwhile, this overexpression failed to affect social behavior in the three-chambered test, immobility time in the tail suspension test, locomotor activity in the open field test, and associative learning within the "operant wall" paradigm. The 5-HT_1A receptor overexpression in the hippocampus raised hippocampal 5-HT₇ receptor mRNA and protein levels. Additionally, the 5-HT_1A receptor overexpression lowered both mRNA and protein levels of TrkB receptor but failed to affect proBDNF, mature BDNF, and p75^NTR receptor expression in the hippocampus of BTBR mice. Thus, obtained results suggest the involvement of the 5-HT and BDNF systems' interaction mediated by 5-HT_1A and TrkB receptors in the mechanisms underlying autistic-like behavior in BTBR mice.

Possible role of transcriptional regulation of 5-HT1A receptor in the midbrain on unadaptation to stress in mice

The ability to adapt to stress is an essential defensive function of a living body, and disturbance of this ability in the brain may contribute to the development of affective illness. Previously, we reported that mice exposed to unadaptable restraint stress show emotional abnormality. Moreover, this emotional abnormality was alleviated by chronic treatment with flesinoxan, a serotonin (5-HT)_1A receptor agonist. 5-HT_1A receptor expression is regulated by several transcription factors such as nuclear deformed epidermal autoregulatory factor (NUDR/Deaf-1) and five prime repressors under dual repression binding protein 1 (Freud-1). The present study was designed to investigate the expression levels of 5-HT_1A receptor and its transcription factors in the midbrain and hippocampus of stress-adaptive and -unadaptive mice. Mice were exposed to 14 days of repeated adaptable (1 h/day) or repeated unadaptable (4 h/day) restraint stress, or were left in their home cage (non-stressed groups). In a western blot analysis, a significant increase in the expression levels of 5HT_1A receptor protein were observed in the hippocampal membrane fraction in stress-adaptive mice. In contrast, the expression levels of 5-HT_1A receptor protein in stress-unadaptive mice were significantly increased in both cytoplasmic and membrane fraction of the midbrain. Furthermore, real-time PCR analysis revealed that, in the midbrain of stress-unadaptive mice, the expression levels of 5-HT_1A receptor mRNA and Freud-1 or NUDR mRNA were significantly increased and decreased, respectively. These results suggest that increased expression of 5-HT_1A receptor due to decrease in the expression of Freud-1 and NUDR in the midbrain may play a pivotal role in the emotional abnormality of stress-unadaptive mice.

The 5-HT1A receptor as a serotonergic target for neuroprotection in cerebral ischemia

Cerebral ischemia due to stroke or cardiac arrest greatly affects daily functioning and the quality of life of patients and has a high socioeconomic impact due to the surge in their prevalence. Advances in the identification of an effective pharmacotherapy to promote neuroprotection and recovery after a cerebral ischemic insult are, however, limited. The serotonin 1A (5-HT_1A) receptor has been implicated in the regulation of several brain functions, including mood, emotions, memory, and neuroplasticity, all of which are deleteriously affected by cerebral ischemia. This review focuses on the specific roles and mechanisms of 5-HT_1A receptors in neuroprotection in experimental models of cerebral ischemia. We present experimental evidence that 5-HT_1A receptor agonists can prevent neuronal damage and promote functional recovery induced by focal and transient global ischemia in rodents. However, indiscriminate activation of pre-and postsynaptic by non-biased 5-HT_1A receptor agonists may be a limiting factor in the anti-ischemic clinical efficacy of these compounds since 5-HT_1A receptors in different brain regions can mediate diverging or even contradictory responses. Current insights are presented into the 'biased' 5-HT_1A post-synaptic heteroreceptor agonist NLX-101 (also known as F15599), a compound that preferentially and potently stimulates postsynaptic cortical pyramidal neurons without inhibiting firing of serotoninergic neurons, as a potential strategy providing neuroprotection in cerebral ischemic conditions.

Biased agonism in drug discovery: Is there a future for biased 5-HT1A receptor agonists in the treatment of neuropsychiatric diseases?

Serotonin (5-HT) is one of the fundamental neurotransmitters that contribute to the information essential for an organism's normal, physiological function. Serotonin acts centrally and systemically. The 5-HT_1A receptor is the most widespread serotonin receptor, and participates in many brain-related disorders, including anxiety, depression, and cognitive impairments. The 5-HT_1A receptor can activate several different biochemical pathways and signals through both G protein-dependent and G protein-independent pathways. Preclinical experiments indicate that distinct signaling pathways in specific brain regions may be crucial for antidepressant-like, anxiolytic-like, and procognitive responses. Therefore, the development of new ligands that selectively target a particular signaling pathway(s) could open new possibilities for more effective and safer pharmacotherapy. This review discusses the current state of preclinical studies focusing on the concept of functional selectivity (biased agonism) regarding the 5-HT_1A receptor and its role in antidepressant-like, anxiolytic-like, and procognitive regulation. Such work highlights not only the differential effects of targeted autoreceptors, vs. heteroreceptors, but also the importance of targeting specific downstream intracellular signaling processes, thereby enhancing favorable over unfavorable signaling activation.

The serotonin 1A receptor modulates the social behaviour within groups of a cooperatively-breeding cichlid

The neurotransmitter serotonin (5-HT) reduces aggressive behaviour in a number of vertebrates, and the 5-HT_1A receptor is known to be involved in this regulation. However, the role of this receptor in the modulation of sociopositive behaviour remains largely unknown. Here we investigated the role of the 5-HT_1A receptor in the regulation of aggressive, submissive and affiliative behaviour in the cooperatively-breeding cichlid Neolamprologus pulcher. In two experiments, we performed intramuscular injections of a 5-HT_1A agonist (8-OH-DPAT) and antagonist (Way-100635) followed by recordings of social behaviour of injected fish within their social groups. We determined the concentrations and post-injection times when the drugs had the greatest effect on social behaviour. We recorded spontaneous social behaviour in both experiments. In the second experiment we also recorded behaviour after social groups received a territorial challenge by live presentations of either conspecifics or egg predators. The 5-HT_1A agonist caused an increase in aggression and a decrease in submission and affiliation, whereas the antagonist had the opposite effects. Thus, the 5-HT_1A receptor plays an important regulatory role not only for aggressive but also sociopositive behaviour.

Uncaria rhynchophylla ameliorates unpredictable chronic mild stress-induced depression in mice via activating 5-HT1A receptor: Insights from transcriptomics

BACKGROUND

Depression is a pervasive or persistent mental disorder that causes mood, cognitive and memory deficits. Uncaria rhynchophylla has been widely used to treat central nervous system diseases for a long history, although its efficacy and potential mechanism are still uncertain.

PURPOSE

The present study aimed to investigate anti-depression effect and potential mechanism of U. rhynchophylla extract (URE).

STUDY DESIGN AND METHODS

A mouse depression model was established using unpredictable chronic mild stress (UCMS). Effects of URE on depression-like behaviours, neurotransmitters, and neuroendocrine hormones were investigated in UCMS-induced mice. The potential target of URE was analyzed by transcriptomics and bioinformatics methods and validated by RT-PCR and Western blot. The agonistic effect on 5-HT1A receptor was assayed by dual-luciferase reporter system.

RESULTS

URE ameliorated depression-like behaviours, and modulated levels of neurotransmitters and neuroendocrine hormones, including 5-hydroxytryptamine (5-HT), 5-hydroxyindole acetic acid (5-HIAA), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), corticosterone (CORT), corticotropin-releasing hormone (CRH), and adrenocorticotropic hormone (ACTH), in UCMS-induced mice. Transcriptomics and bioinformatics results indicated that URE could regulate glutamatergic, cholinergic, serotonergic, and GABAergic systems, especially neuroactive ligand-receptor and cAMP signaling pathways, revealing that Htr1a encoding 5-HT1A receptor was a potential target of URE. The expression levels of downstream proteins of 5-HT1A signaling pathway 5-HT1A, CREB, BDNF, and PKA were increased in UCMS-induced mice after URE administration, and URE also displayed an agonistic effect against 5-HT1A receptor with an EC50 value of 17.42 μg/ml.

CONCLUSION

U. rhynchophylla ameliorated depression-like behaviours in UCMS-induced mice through activating 5-HT1A receptor.

Role of 5-HT1A receptor-mediated serotonergic transmission in the medial prefrontal cortex in acute restraint stress-induced augmentation of rewarding memory of cocaine in mice

Stress enhances cocaine craving. We recently reported that acute restraint stress increases cocaine conditioned place preference (CPP) in mice; however, the underlying mechanisms remain unclear. This study aimed to examine the role of serotonergic transmission in the medial prefrontal cortex (mPFC) in cocaine CPP enhancement by acute restraint stress, which increases extracellular serotonin (5-HT) levels in the mPFC. Intra-mPFC infusion of the selective serotonin reuptake inhibitor (S)-citalopram prior to the test session significantly increased the cocaine CPP score under non-stressed conditions. This is indicative of the substantial role of increased mPFC 5-HT levels in cocaine CPP enhancement. Moreover, intra-mPFC and systemic administration of the 5-HT_1A receptor antagonist WAY100635 immediately before restraint stress exposure significantly attenuated stress-induced cocaine CPP enhancement. Our findings suggest that enhanced serotonergic transmission via 5-HT_1A receptors in the mPFC is involved in acute stress-induced augmentation of rewarding memory of cocaine; moreover, the 5-HT_1A receptor could be a therapeutic target for stress-induced cocaine craving.

Activation of 5-HT1A postsynaptic receptors by NLX-101 results in functional recovery and an increase in neuroplasticity in mice with brain ischemia

Pharmacological interventions that selectively activate serotonin 5-hydroxytryptramine-1A (5-HT_1A) heteroreceptors may prevent or attenuate the consequences of brain ischemic episodes. The present study investigated whether the preferential 5-HT_1A postsynaptic receptor agonist NLX-101 (a.k.a. F15599) mitigates cognitive and emotional impairments and affects neuroplasticity in mice that are subjected to the bilateral common carotid artery occlusion (BCCAO) model of brain ischemia. The selective serotonin reuptake inhibitor escitalopram (Esc) was used for comparative purposes because it is able to decrease morbidity and improve recovery in stroke patients and ischemic rodents. Sham and BCCAO mice received daily doses of NLX-101 (0.32 mg/kg, i.p) or Esc (20 mg/kg, i.p) for 28 days. During this period, they were evaluated for locomotor activity, anxiety- and despair-related behaviors and hippocampus-dependent cognitive function, using the open field, elevated zero maze, forced swim test and object location test, respectivelly. The mice's brains were processed for biochemical and histological analyses. BCCAO mice exhibited high anxiety and despair-like behaviors and performed worse than controls in the cognitive assessment. BCCAO induced neuronal and dendritic spine loss and decreases in the protein levels of neuronal plasticity markers, including brain-derived neurotrophic factor (BDNF), synaptophysin (SYN), and postsynaptic density protein-95 (PSD-95), in prefrontal cortex (PFC) and hippocampus. NLX-101 and Esc attenuated cognitive impairments and despair-like behaviors in BCCAO mice. Only Esc decreased anxiety-like behaviors due to brain ischemia. Both NLX-101 and Esc blocked the increase in plasma corticosterone levels and, restored BDNF, SYN and PSD-95 protein levels in the hippocampus. Moreover, both compounds impacted positively dentritic remodeling in the hippocampus and PFC of ischemic mice. In the PFC, NLX-101 increased the BDNF protein levels, while Esc in turn, attenuated the decrease in the PSD-95 protein levels induced by BCCAO. The present results suggest that activation of post-synaptic 5-HT_1A receptors is the molecular mechanism for serotonergic protective effects in BCCAO. Moreover, post-synaptic biased agonists such as NLX-101 might constitute promising therapeutics for treatment of functional and neurodegenerative outcomes of brain ischemia.

5-HT1AR alleviates Aβ-induced cognitive decline and neuroinflammation through crosstalk with NF-κB pathway in mice

The 5-hydroxytryptamine (5-HT) receptor is significant for the regulation of mood and memory. However, the role of 5-HT1AR in β-Amyloid protein (Aβ)-induced cognitive decline, neuroinflammation and the possible mechanism remains elusive. Thus, we aimed to evaluate the effects of 5-HT1AR on Aβ-induced learning and memory decline and neuroinflammation in mice. Novel object recognition and Morris water maze tests were performed to observe learning and memory behavior in mice. Protein levels of Iba1, GFAP, MAP2, TNF-α, Tβ4, C-fos, IKK-β, IKB-α, NF-κBp65, phospho-NF-κBp65 in the hippocampus were examined by immunostaining or western blotting. Aβ1-42-treatment inducing learning and memory decline was shown in novel object recognition and Morris water maze tests; neuroinflammation shown in immunostaining. Our study found out that 5-HT1AR inhibitor WAY100635 showed significant improvement in Aβ-induced learning and memory decline. Moreover, WAY100635 decreases levels of Iba1, GFAP, and TNF-α in the hippocampus, which were related to neuroinflammation. While treatment with 5-HT1AR agonist 8-OH-DPAT or ERK inhibitor U0126 exerted no effects or even aggravated Aβ-induced learning and memory decline. In addition, WAY100635 could downregulate phospho-NF-κB in the hippocampus of Aβ1-42-injected mice. These results provide new insight into the mechanism, for 5-HT1AR in Aβ-induced cognitive impairments through crosstalk with the NF-κB signaling pathway. Our data indicated that WAY100635 was involved in the protective effects against neuroinflammation and improvement of learning and memory in Alzheimer's disease.

Synthesis and antidepressant-like activity of novel alkoxy-piperidine derivatives targeting SSRI/5-HT1A/5-HT7

A series of novel alkoxy-piperidine derivatives were synthesized and evaluated for their serotonin reuptake inhibitory and binding affinities for 5-HT_1A/5-HT₇ receptors. In vivo antidepressant activities of the selective compounds were explored using the forced swimming test (FST) and tail suspension test (TST) in mice. The results showed that compounds 7a (reuptake inhibition (RUI), IC₅₀ = 177 nM; 5-HT_1A, K_i = 12 nM; 5-HT₇, K_i = 25 nM) and 15g (RUI, IC₅₀ = 85 nM; 5-HT_1A, K_i = 17 nM; 5-HT₇, K_i = 35 nM) were potential antidepressant agents in animal behavioral models with high 5-HT_1A/5-HT₇ receptor affinities and moderate serotonin reuptake inhibition, and good metabolic stability in vitro.

Pharmacological MRI to investigate the functional selectivity of 5-HT1A receptor biased agonists

The emerging concept of "biased agonism" denotes the phenomenon whereby agonists can preferentially direct receptor signalling to specific intracellular responses among the different transduction pathways, thus potentially avoiding side effects and improving therapeutic effects. The aim of this study was to investigate biased agonism by using pharmacological magnetic resonance imaging (phMRI). The cerebral blood oxygen level dependent (BOLD) signal changes induced by increasing doses of two serotonin 5-HT_1A receptor biased agonists, NLX-112 and NLX-101, were mapped in anaesthetized rats. Although both compounds display high affinity, selectivity and agonist efficacy for 5-HT_1A receptors, NLX-101 is known to preferentially activate post-synaptic receptors, whereas NLX-112 targets both pre- and post-synaptic receptors. We used several doses of agonists in order to determine if the regional selectivity of NLX-101 was dose-dependent. NLX-112 and NLX-101 induced different positive and negative hemodynamic changes patterns at equal doses. Importantly, NLX-101 had no significant effect in regions expressing pre-synaptic receptors contrary to NLX-112. NLX-112 also produced higher BOLD changes than NLX-101 in the orbital cortex, the somatosensory cortex, and the magnocellular preoptic nuclei. In other regions such as the retrosplenial cortex and the dorsal thalamus, the drugs had similar effects. In terms of functional connectivity, NLX-112 induced more widespread changes than NLX-101. The present phMRI study demonstrates that two closely-related agonists display notable differences in their hemodynamic "fingerprints". These data support the concept of biased agonism at 5-HT_1A receptors and raise the prospect of identifying novel therapeutics which exhibit improved targeting of brain regions implicated in neuropsychiatric disorders. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Atypical but not typical antipsychotic drugs ameliorate phencyclidine-induced emotional memory impairments in mice

Schizophrenia is associated with cognitive impairments related to hypofunction in glutamatergic N-methyl-D-aspartate receptor (NMDAR) transmission. Phencyclidine (PCP), a non-competitive NMDAR antagonist, models schizophrenia-like behavioral symptoms including cognitive deficits in rodents. This study examined the effects of PCP on emotional memory function examined in the passive avoidance (PA) task in mice and the ability of typical and atypical antipsychotic drugs (APDs) to rectify the PCP-mediated impairment. Pre-training administration of PCP (0.5, 1, 2 or 3 mg/kg) dose-dependently interfered with memory consolidation in the PA task. In contrast, PCP was ineffective when administered after training, and immediately before the retention test indicating that NMDAR blockade interferes with memory encoding mechanisms. The typical APD haloperidol and the dopamine D_2/3 receptor antagonist raclopride failed to block the PCP-induced PA impairment suggesting a negligible role of D₂ receptors in the PCP impairment. In contrast, the memory impairment was blocked by the atypical APDs clozapine and olanzapine in a dose-dependent manner while risperidone was effective only at the highest dose tested (1 mg/kg). The PCP-induced impairment involves 5-HT_1A receptor mechanisms since the antagonist NAD-299 blocked the memory impairment caused by PCP and the ability of clozapine to attenuate the impairment by PCP. These results indicate that atypical but not typical APDs can ameliorate NMDAR-mediated memory impairments and support the view that atypical APDs such as clozapine can modulate glutamatergic memory dysfunctions through 5-HT_1A receptor mechanisms. These findings suggest that atypical APDs may improve cognitive impairments related to glutamatergic dysfunction relevant for emotional memories in schizophrenia.

PET Neuroimaging Reveals Serotonergic and Metabolic Dysfunctions in the Hippocampal Electrical Kindling Model of Epileptogenesis

Glucose metabolism and serotonergic neurotransmission have been reported to play an important role in epileptogenesis. We therefore aimed to use neuroimaging to evaluate potential alterations in serotonin 5-HT_1A receptor and glucose metabolism during epileptogenesis in the rat electrical kindling model. To achieve this goal, we performed positron emission tomography (PET) imaging in a rat epileptogenesis model triggered by electrical stimulation of the hippocampus using 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG), a radiolabeled analog of glucose, and 2'-methoxyphenyl-(N-2'-pyridinyl)-p-¹⁸F-fluoro-benzamidoethylpiperazine (¹⁸F-MPPF), a radiolabeled 5-HT_1A receptor ligand, to evaluate brain metabolism and 5-HT_1A receptor functionality. Since the 5-HT_1A receptor is also highly expressed in astrocytes, glial fibrillary acidic protein (GFAP) immunofluorescence was performed to detect astrogliosis arising from the kindling procedure once the study was finalized. Lastly, in vitro¹⁸F-MPPF autoradiography was performed to evaluate changes in 5HT_1A receptor expression. ¹⁸F-FDG PET showed reduction of glucose uptake in cortical structures, whereas ¹⁸F-MPPF PET revealed an enhancement of tracer binding potential (BP_ND) in key areas rich in 5-HT_1A receptor involved in epilepsy, including septum, hippocampus and entorhinal cortex of kindled animals compared to controls. However, in vitro 5-HT_1A receptor autoradiography showed no changes in densitometric signal in any brain region, suggesting that the augmentation in BP_ND found by PET could be caused by reduction of synaptic serotonin. Importantly, astroglial activation was detected in the hippocampus of kindled rats. Overall, electrical kindling induced hypometabolism, astrogliosis and serotonergic alterations in epilepsy-related regions. Furthermore, the present findings point to 5-HT_1A receptor as a valuable epileptogenesis biomarker candidate and a potential therapeutic target.

Serotonin 5-HT1A Receptor-Mediated Reduction of Excitatory Synaptic Transmission in Layers II/III of the Parasubiculum

Serotonin (5-HT) has important effects on cognitive function within the hippocampal region where it modulates membrane potential and excitatory and inhibitory synaptic transmission. Here, we investigated how 5-HT modulates excitatory synaptic strength in layers II/III of the parasubiculum in rat brain slices. Bath-application of 1 or 10 μM 5-HT resulted in a strong, dose-dependent, and reversible reduction in the amplitude of field excitatory postsynaptic potentials (fEPSPs) recorded in the parasubiculum. The 5-HT reuptake blocker citalopram (10 μM) also reduced fEPSP amplitudes, indicating that 5-HT released within the slice inhibits synaptic transmission. The reduction of fEPSPs induced by 5-HT was blocked by the 5-HT_1A receptor blocker NAN-190 (10 μM), but not by the 5-HT₇ receptor blocker SB-269970 (10 μM). Moreover, the 5-HT_1A agonist 8-OH-DPAT induced a reduction of fEPSP amplitude similar to that induced by 5-HT. The reduction was prevented by the 5-HT_1A receptor blocker NAN-190. The reduction in fEPSPs induced by either 5-HT or by 8-OH-DPAT was accompanied by an increase in paired-pulse ratio, suggesting that it is due mainly to reduced glutamate release. Our data suggest that the effects of serotonin on cognitive function may depend in part upon a 5-HT_1A-mediated reduction of excitatory synaptic transmission in the parasubiculum. This may also affect synaptic processing in the entorhinal cortex, which receives the major output projection of the parasubiculum.

Intrahippocampal 5-HT1A receptor antagonist inhibits the improving effect of low-frequency stimulation on memory impairment in kindled rats

In addition to its anticonvulsant effect, low frequency stimulation (LFS) improves learning and memory in kindled animals. In the present study, the role of 5-HT_1A receptors in mediating LFS' improving effect on spatial learning and memory was investigated in amygdala-kindled rats. Amygdala kindling was conducted in a semi-rapid kindling stimulations (12 stimulations per day) in male Wistar rats. LFS (4 trains of 0.1 ms pulse duration at 1 Hz, 200 pulses, 50-150 μA, at 5 min intervals) was applied after termination of kindling stimulations. NAD-299 (a selective 5-HT_1A receptor antagonist; 2.5 and 5 μg/μl) was microinjected into the hippocampal CA1 before applying LFS. The Morris water maze, and novel object recognition tests were conducted after the last kindling stimulation. Hippocampal samples were also prepared, and 5-HT_1A receptor gene expression levels were assessed using quantitative RT-PCR. In kindled animals, LFS reduced impairments in spatial learning and memory in the Morris water maze and novel object recognition tests. Microinjection of NAD doses of 5 μg/μl reduced the effects of LFS on learning and memory. The gene expression level of 5-HT_1A receptors increased significantly in the hippocampus of amygdala-kindled rats. However, LFS applied after kindling stimulations inhibited this effect. It seems that activation of 5-HT_1A receptors in the CA1 field is necessary for LFS' improving effects on spatial learning and memory in kindled animals; although surprisingly, LFS application prevented the elevation in gene expression of 5-HT_1A receptors in kindled animals.

5-HT1A receptor agonist 8-OH-DPAT induces serotonergic behaviors in mice via interaction between PKCδ and p47phox

Serotonin syndrome is an adverse reaction due to increased serotonin (5-hydroxytryptophan: 5-HT) concentrations in the central nervous system (CNS). The full 5-HT_1A receptor (5-HT_1AR) agonist (±)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) has been recognized to elicit traditional serotonergic behaviors. Treatment with 8-OH-DPAT selectively increased PKCδ expression out of PKC isoforms and 5-HT turnover rate in the hypothalamus of wild-type mice. Treatment with 8-OH-DPAT resulted in oxidative burdens, co-immunoprecipitation of 5-HT_1AR and PKCδ, and phosphorylation and membrane translocation of p47phox. Importantly, p47phox also interacted with 5-HT_1AR or PKCδ in the presence of 8-OH-DPAT. Consistently, the interaction and oxidative burdens were attenuated by 5-HT_1AR antagonism (i.e., WAY100635), PKCδ inhibition (i.e., rottlerin and genetic depletion of PKCδ), or NADPH oxidase/p47phox inhibition (i.e., apocynin and genetic depletion of p47phox). However, WAY100635, apocynin, or rottlerin did not exhibit any additive effects against the protective effect by inhibition of PKCδ or p47phox. Furthermore, apocynin, rottlerin, or WAY100635 also significantly protected from pro-inflammatory/pro-apoptotic changes induced by 8-OH-DPAT. Therefore, we suggest that 8-OH-DPAT-induced serotonergic behaviors requires oxidative stress, pro-inflammatory, and pro-apoptotic changes, that PKCδ or p47phox mediates the serotonergic behaviors induced by 8-OH-DPAT, and that the inhibition of PKCδ-dependent p47phox activation is critical for protecting against serotonergic behaviors.

The role of 5-HT1A receptors of hippocampal CA1 region in anticonvulsant effects of low-frequency stimulation in amygdala kindled rats

Low frequency stimulation (LFS) has been proposed as a method in the treatment of epilepsy, but its anticonvulsant mechanism is still unknown. In the current study, the hippocampal CA1 region was microinjected with NAD-299 (a selective 5-HT_1A antagonist), and its role in mediating the inhibitory action of LFS on amygdala kindling was investigated. Male Wistar rats were kindled by amygdala stimulation in a semi-rapid kindling manner (12 stimulations per day). LFS (0.1 ms pulse duration at 1 Hz, 200 pulses, 50-150 μA) was applied at 5 min after termination of daily kindling stimulations. NAD (a selective 5-HT_1A antagonist) was microinjected into the CA1 region of the hippocampus at the doses of 2.5 and 5 μg/1 μl. An open field test was also run to determine the motor activity of animals in different experimental groups. The application of LFS following daily kindling stimulations reduced the behavioral seizure stages, afterdischarge duration, and stage 5 seizure duration and increased the latency to stage 4 seizure compared to the kindled group. However, microinjection of NAD at the doses of 5 μg/1 μl, but not 2.5 μg/1 μl, blocked the inhibitory effect of LFS on behavioral and electrophysiological parameters in kindled animals. It could be presumed that 5-HT_1A receptors in the CA1 area are involved in mediating the antiepileptic effects of LFS.

5-HT1A receptor-mediated activation of neuroendocrine responses and multiple protein kinase pathways in the peripubertal rat hypothalamus

Increasing evidence suggests that multiple factors can produce effects on the immature brain that are distinct and more long-lasting than those produced in adults. The hypothalamic paraventricular nucleus (PVN) is a region integral to the hypothalamic-pituitary-adrenal axis and is affected by anxiety, depression, and drugs used to treat these disorders, yet receptor signaling mechanisms operative in hypothalamus prior to maturation remain to be elucidated. In peripubertal male rats, systemic injection of the selective serotonin 1A (5-HT_1A) receptor agonist (+)8-OH-DPAT (0.2 mg/kg) markedly elevated plasma levels of oxytocin and adrenocorticotropic hormone (ACTH) at 5 and 15 min post-injection. The 5-HT_1A receptor selectivity was demonstrated by the ability of the 5-HT_1A receptor selective antagonist WAY100635 to completely block both oxytocin and ACTH responses at 5 min, with some recovery of the ACTH response at 15 min. At 15 min post-injection, (+)8-OH-DPAT also increased levels of phosphorylated extracellular signal-regulated kinase (pERK) and phosphorylated protein kinase B (pAkt) in the PVN. As previously observed in adults, (+)8-OH-DPAT reduced levels of pERK in hippocampus. WAY100635 also completely blocked (+)8-OH-DPAT-mediated elevations in hypothalamic pERK and pAkt and the reductions in hippocampal pERK, demonstrating 5-HT_1A receptor selectivity of both kinase responses. This study provides the first demonstration of functional 5-HT_1A receptor-mediated ERK and Akt signaling pathways in the immature hypothalamus, activated by a dose of (+)8-OH-DPAT that concomitantly stimulates neuroendocrine responses. This information is fundamental to identifying potential signaling pathways targeted by biased agonists in the development of safe and effective treatment strategies in children and adolescents.

Higher 5-HT1A autoreceptor binding as an endophenotype for major depressive disorder identified in high risk offspring - A pilot study

Higher serotonin-1A (5-HT1A) receptor binding potential (BPF) has been found in major depressive disorder (MDD) during and between major depressive episodes. We investigated whether higher 5-HT1A binding is a biologic trait transmitted to healthy high risk (HR) offspring of MDD probands. Data were collected contemporaneously from: nine HR, 30 depressed not-recently medicated (NRM) MDD, 18 remitted NRM MDD, 51 healthy volunteer (HV) subjects. Subjects underwent positron emission tomography (PET) using [11C]WAY100635 to quantify 5-HT1A BPF, estimated using metabolite, free fraction-corrected arterial input function and cerebellar white matter as reference region. Multivoxel pattern analyses (MVPA) of PET data evaluated group status classification of individuals. When tested across 13 regions of interest, an effect of diagnosis is found on BPF which remains significant after correction for sex, age, injected mass and dose: HR have higher BPF than HV (84.3% higher in midbrain raphe, 40.8% higher in hippocampus, mean BPF across all 13 brain regions is 49.9% ± 11.8% higher). Voxel-level BPF maps distinguish HR vs. HV. Elevated 5-HT1A BPF appears to be a familially transmitted trait abnormality. Future studies are needed to replicate this finding in a larger cohort and demonstrate the link to the familial transmission of mood disorders.

The GABAergic Gudden's dorsal tegmental nucleus: A new relay for serotonergic regulation of sleep-wake behavior in the mouse

Serotonin (5-HT) neurons are involved in wake promotion and exert a strong inhibitory influence on rapid eye movement (REM) sleep. Such effects have been ascribed, at least in part to the action of 5-HT at post-synaptic 5-HT_1A receptors (5-HT_1AR) in the brainstem, a major wake/REM sleep regulatory center. However, the neuroanatomical substrate through which 5-HT_1AR influence sleep remains elusive. We therefore investigated whether a brainstem structure containing a high density of 5-HT_1AR mRNA, the GABAergic Gudden's dorsal tegmental nucleus (DTg), may contribute to 5-HT-mediated regulatory mechanisms of sleep-wake stages. We first found that bilateral lesions of the DTg promote wake at the expense of sleep. In addition, using local microinjections into the DTg in freely moving mice, we showed that local activation of 5-HT_1AR by the prototypical agonist 8-OH-DPAT enhances wake and reduces deeply REM sleep duration. The specific involvement of 5-HT_1AR in the latter effects was further demonstrated by ex vivo extracellular recordings showing that the selective 5-HT_1AR antagonist WAY 100635 prevented DTg neuron inhibition by 8-OH-DPAT. We next found that GABAergic neurons of the ventral DTg exclusively targets glutamatergic neurons of the lateral mammillary nucleus (LM) in the posterior hypothalamus by means of anterograde and retrograde tracing techniques using cre driver mouse lines and a modified rabies virus. Altogether, our findings strongly support the idea that 5-HT-driven enhancement of wake results from 5-HT_1AR-mediated inhibition of DTg GABAergic neurons that would in turn disinhibit glutamatergic neurons in the mammillary bodies. We therefore propose a Raphe→DTg→LM pathway as a novel regulatory circuit underlying 5-HT modulation of arousal.

Discovery of DSP-1053, a novel benzylpiperidine derivative with potent serotonin transporter inhibitory activity and partial 5-HT1A receptor agonistic activity

We have previously shown that SMP-304, a serotonin uptake inhibitor with weak 5-HT_1A partial agonistic activity, may act under high serotonin levels as a 5-HT_1A antagonist that improves the onset of paroxetine in the rat swimming test. However, SMP-304 is mostly metabolized by CYP2D6, indicating limited efficacy among individuals and increased side effects. To reduce CYP2D6 metabolic contribution and enhance SERT/5-HT_1A binding affinity, we carried out a series of substitutions at the bromine atom in the left part of the benzene ring of SMP-304 and replaced the right part of SMP-304 with a chroman-4-one. This optimization work led to the identification of the antidepressant candidate DSP-1053 as a potent SERT inhibitor with partial 5-HT_1A receptor agonistic activity. DSP-1053 showed low CYP2D6 metabolic contribution and a robust increase in serotonin levels in the rat frontal cortex.

Synthesis and biological evaluation of a series of novel pyridinecarboxamides as potential multi-receptor antipsychotic drugs

In previous study, a series of benzamides was identified as potent antipsychotic agents. As a continuation of the program to discover novel antipsychotics, herein we reported the evaluation of a series of pyridinecarboxamide derivatives. The most promising compound 7h not only held good activities on dopamine D₂, serotonin 5-HT_1A and 5-HT_2A receptors, but also exhibited low potency for α_1A, H₁ and 5-HT_2C receptors, indicating a low propensity of side effects like orthostatic hypotension and weight gain. Furthermore, 7h exhibited more potent antipsychotic-like effect than aripiprazole in behavioral studies. The preliminary results were promising enough for further research around this scaffold.

Synthesis and biological evaluation of a series of multi-target N-substituted cyclic imide derivatives with potential antipsychotic effect

In the present study, a series of multi-target N-substituted cyclic imide derivatives which possessed potent dopamine D₂, serotonin 5-HT_1A and 5-HT_2A receptors properties were synthesized and evaluated as potential antipsychotics. Among these compounds, (3aR,4R,7S,7aS)-2-(4-(4-(benzo[b]thiophen-4-yl)piperazin-1-yl)butyl)-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindole-1,3(2H)-dione hydrochloride (3d) held a promising pharmacological profile. 3d not only showed potent and balanced in vitro activities on D₂/5-HT_1A/5-HT_2A receptors, but also endowed with low to moderate activities on 5-HT_2C, H₁, α_1A, M₃ receptors and hERG channel, suggesting a low liability to induce side effects such as weight gain, orthostatic hypotension and QT prolongation. In animal behavioral studies, 3d reduced phencyclidine-induced hyperlocomotion with a high threshold for catalepsy induction. Compound 3d was selected as a potential antipsychotic candidate for further development.

The serotonin hallucinogen 5-MeO-DMT alters cortico-thalamic activity in freely moving mice: Regionally-selective involvement of 5-HT1A and 5-HT2A receptors

5-MeO-DMT is a natural hallucinogen acting as serotonin 5-HT_1A/5-HT_2A receptor agonist. Its ability to evoke hallucinations could be used to study the neurobiology of psychotic symptoms and to identify new treatment targets. Moreover, recent studies revealed the therapeutic potential of serotonin hallucinogens in treating mood and anxiety disorders. Our previous results in anesthetized animals show that 5-MeO-DMT alters cortical activity via 5-HT_1A and 5-HT_2A receptors. Here, we examined 5-MeO-DMT effects on oscillatory activity in prefrontal (PFC) and visual (V1) cortices, and in mediodorsal thalamus (MD) of freely-moving wild-type (WT) and 5-HT_2A-R knockout (KO2A) mice. We performed local field potential multi-recordings evaluating the power at different frequency bands and coherence between areas. We also examined the prevention of 5-MeO-DMT effects by the 5-HT_1A-R antagonist WAY-100635. 5-MeO-DMT affected oscillatory activity more in cortical than in thalamic areas. More marked effects were observed in delta power in V1 of KO2A mice. 5-MeO-DMT increased beta band coherence between all examined areas. In KO2A mice, WAY100635 prevented most of 5-MeO-DMT effects on oscillatory activity. The present results indicate that hallucinatory activity of 5-MeO-DMT is likely mediated by simultaneous alteration of prefrontal and visual activities. The prevention of these effects by WAY-100635 in KO2A mice supports the potential usefulness of 5-HT_1A receptor antagonists to treat visual hallucinations. 5-MeO-DMT effects on PFC theta activity and cortico-thalamic coherence may be related to its antidepressant activity. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

B2-kinin receptors in the dorsal periaqueductal gray are implicated in the panicolytic-like effect of opiorphin

Reported results have shown that the pentapeptide opiorphin inhibits oligopeptidases that degrade brain neuropeptides, and has analgesic and antidepressant effects in experimental animals, without either tolerance or dependency after chronic administration. In a previous study we showed that opiorphin has a panicolytic-like effect in the dorsal periaqueductal gray (dPAG) electrical stimulation test (EST), mediated by the μ-opioid receptor (MOR). This study further analyzes the mechanism of opiorphin panicolytic action, using the EST and drug injection inside the dPAG. The obtained results showed that blockade of the 5-HT_1A receptors with WAY-100635 did not change the escape-impairing effect of opiorphin, and combined injection of sub-effective doses of opiorphin and the 5-HT_1A-agonist 8-OH-DPAT did not have a significant anti-escape effect. In contrast, the anti-escape effect of opiorphin was antagonized by pretreatment with the kinin B2 receptor blocker HOE-140, and association of sub-effective doses of opiorphin and bradykinin caused a significant anti-escape effect. The anti-escape effect of bradykinin was not affected by previous administration of WAY-100635. Therefore, the anti-escape effect of opiorphin in the dPAG seems to be mediated by endogenous bradykinin, acting on kinin B2 receptors, which previous results have shown to interact synergistically with MOR in the dPAG to restrain escape in two animal models of panic. Chemical compounds: Opiorphin (PubChem CID: 25195667); WAY100635 maleate salt (PubChem CID: 11957721); 8-OH-DPAT hydrobromide (PubChem CID: 6917794); Bradykinin (PubChem CID: 439201); HOE-140 (Icatibant) (PubChem CID: 6918173).

Psychopharmacology of combined activation of the serotonin1A and σ1 receptors

The selective serotonin (5-HT) reuptake inhibitors (SSRIs) are generally used for the treatment of major depressive disorders, and the 5-HT1A and σ1 receptors are considered to be targets for treatment of psychiatric disorders. Some SSRIs such as fluvoxamine have agonistic activity towards for the σ1 receptor, but it is not known whether the effect on the receptor plays a key role in the pharmacological effects. We have recently demonstrated that fluvoxamine shows an anti-anhedonic effect in picrotoxin-induced model of anxiety/depression, while the SSRI paroxetine, which have little affinity for the σ1 receptor, does not. We also suggest that the anti-anhedonic effect of fluvoxamine is mediated by combined activation of the 5-HT1A and σ1 receptors and it is associated with activation of prefrontal dopaminergic system. In these studies, picrotoxin-treated mice and adrenalectomized/castrated mice were used as decreased GABAA receptor function and neurosteroid-deficient models, respectively. These findings suggest that the functional interaction between the 5-HT1A and σ1 receptors activates prefrontal dopaminergic system under the conditions of decreased brain GABAA receptor function and the neurochemical effect is linked to the behavioral effect. This review summarizes the pharmacological role of the 5-HT1A and σ1 receptors, focusing on the functional interaction between these receptors, and the role of prefrontal dopaminergic system in depressive-like behaviors.

The postnatal 5-HT1A receptor regulates adult anxiety and depression differently via multiple molecules

Serotonin (5-HT) and the 5-HT_1A receptor during development are known to modulate anxiety and depression in later life. However, the brain mechanisms linking the postnatal 5-HT system and adult behavior remain unknown. Here, we examined the effects of pharmacological 5-HT_1A receptor activation during the postnatal period on anxiety and depression-like behavior in adult BALB/c male mice. To elucidate the underlying mechanisms, we measured mRNA expression of the 5-HT_1A receptor, brain-derived neurotrophic factor (BDNF), GABA_A receptor subunits, and AMPA receptor subunits in the medial prefrontal cortex (mPFC), amygdala, and hippocampus. Treatment with the selective 5-HT reuptake inhibitor (fluoxetine) and 5-HT_1A receptor agonist (8-OH-DPAT) during the postnatal period decreased anxiety-like behavior in adulthood, whereas only 8-OH-DPAT treatment increased depression-like behavior. Concomitantly with the behavioral effects, postnatal treatment with fluoxetine and 8-OH-DPAT decreased the mRNA expression of the GABA_A receptor α3 subunit in the mPFC and ventral hippocampus in adulthood, while 8-OH-DPAT, but not fluoxetine, decreased the mRNA expression of the 5-HT_1A receptor and BDNF in the mPFC and the GABA_A receptor α2 subunit in the mPFC and ventral hippocampus. On the basis of the correlative changes between behavior and mRNA expression, these results suggest that the GABA_A receptor α3 subunit in the mPFC and ventral hippocampus may regulate anxiety-like behavior. In contrast, depression-like behavior may be regulated by the 5-HT_1A receptor and BDNF in the mPFC and by the GABA_A receptor α2 subunit in the mPFC and ventral hippocampus. In summary, activation of the 5-HT_1A receptor during the postnatal period may reduce anxiety levels, but increase depression levels during adulthood via different multiple molecules in the mPFC and ventral hippocampus.

Mirtazapine exerts an anxiolytic-like effect through activation of the median raphe nucleus-dorsal hippocampal 5-HT pathway in contextual fear conditioning in rats

The functional role of serotonergic projections from the median raphe nucleus (MRN) to the dorsal hippocampus (DH) in anxiety remains understood poorly. The purpose of the present research was to examine the functional role of this pathway, using the contextual fear conditioning (CFC) model of anxiety. We show that intra-MRN microinjection of mirtazapine, a noradrenergic and specific serotonergic antidepressant, reduced freezing in CFC without affecting general motor activity dose-dependently, suggesting an anxiolytic-like effect. In addition, intra-MRN microinjection of mirtazapine dose-dependently increased extracellular concentrations of serotonin (5-HT) but not dopamine in the DH. Importantly, intra-DH pre-microinjection of WAY-100635, a 5-HT1A antagonist, significantly attenuated the effect of mirtazapine on freezing. These results, for the first time, suggest that activation of the MRN-DH 5-HT1A pathway exerts an anxiolytic-like effect in CFC. This is consistent with the literature that the hippocampus is essential for retrieval of contextual memory and that 5-HT1A receptor activation in the hippocampus primarily exerts an inhibitory effect on the neuronal activity.

Agonist and antagonist bind differently to 5-HT1A receptors during Alzheimer's disease: A post-mortem study with PET radiopharmaceuticals

PET imaging studies using 5-HT1A receptor radiotracers show a decreased density of this receptor in hippocampi of patients with Alzheimer's disease (AD) at advanced stages. However, current 5-HT1A receptor radiopharmaceuticals used in neuroimaging are antagonists, thought to bind to 5-HT1A receptors in different functional states (i.e., both the one which displays high affinity for agonists and is thought to mediate receptor activation, as well as the state which has low affinity for agonists). Comparing the PET imaging obtained using an agonist radiotracer, which binds selectively to functional receptors, with the PET imaging obtained using an antagonist radiotracer would therefore provide original information on 5-HT1A receptor impairment during AD. Quantitative autoradiography using [(18)F]F13640 and [(18)F]MPPF, a 5-HT1A agonist and antagonist, respectively, was measured in hippocampi of patients with AD (n = 25, at different Braak stages) and control subjects (n = 9). The neuronal density was measured in the same tissues by NeuN immunohistochemistry. The specific binding of both radiotracers was determined by addition of WAY-100635, a selective 5-HT1A receptor antagonist. The autoradiography distribution of both 5-HT1A PET radiotracers varied across hippocampus regions. The highest binding density was in the pyramidal layer of CA1. Incubation with Gpp(NH)p, a non-hydrolysable analogue of GTP, reduced significantly [(18)F]F13640 binding in hippocampal regions, confirming its preferential interaction with G-coupled receptors, and slightly increased [(18)F]MPPF binding. In the CA1 subfield, [(18)F]F13640 binding was significantly decreased at Braak stages I/II (-19%), Braak stages III/IV (-23%), and Braak stages V/VI (-36%) versus control. In contrast, [(18)F]MPPF binding was statistically reduced only at the most advanced Braak stages V/VI compared to control (-33%). Since [(18)F]F13640 and [(18)F]MPPF can be used in vivo in humans, this neuropharmacological paradigm supports testing the concept of functional imaging using agonist radiopharmaceuticals in future clinical studies.

5-HT1A receptor gene silencers Freud-1 and Freud-2 are differently expressed in the brain of rats with genetically determined high level of fear-induced aggression or its absence

Serotonin 5-HT1A receptor is known to play a crucial role in the mechanisms of genetically defined aggression. In its turn, 5-HT1A receptor functional state is under control of multiple factors. Among others, transcriptional factors Freud-1 and Freud-2 are known to be involved in the repression of 5-HT1A receptor gene expression. However, implication of these factors in the regulation of behavior is unclear. Here, we investigated the expression of 5-HT1A receptor and silencers Freud-1 and Freud-2 in the brain of rats selectively bred for 85 generations for either high level of fear-induced aggression or its absence. It was shown that Freud-1 and Freud-2 levels were different in aggressive and nonaggressive animals. Freud-1 protein level was decreased in the hippocampus, whereas Freud-2 protein level was increased in the frontal cortex of highly aggressive rats. There no differences in 5-HT1A receptor gene expression were found in the brains of highly aggressive and nonaggressive rats. However, 5-HT1A receptor protein level was decreased in the midbrain and increased in the hippocampus of highly aggressive rats. These data showed the involvement of Freud-1 and Freud-2 in the regulation of genetically defined fear-induced aggression. However, these silencers do not affect transcription of the 5-HT1A receptor gene in the investigated rats. Our data indicate the implication of posttranscriptional rather than transcriptional regulation of 5-HT1A receptor functional state in the mechanisms of genetically determined aggressive behavior. On the other hand, the implication of other transcriptional regulators for 5-HT1A receptor gene in the mechanisms of genetically defined aggression could be suggested.

Verbal memory and 5-HT1A receptors in healthy volunteers--A PET study with [carbonyl-(11)C]WAY-100635

The serotonin 5-HT1A receptor is a putative drug development target in disorders with cognitive and in particular memory deficits. However, previous human positron emission tomography (PET) studies on 5-HT1A receptor binding and memory functions have yielded discrepant results. We explored the association between verbal memory and 5-HT1A receptor binding in 24 healthy subjects (14 male, 10 female, aged 18-41 years). The cognitive tests included the Wechsler Memory Scale-Revised (WMS-R), Wechsler Adult Intelligence Scale-Revised (WAIS-R) and Wisconsin Card Sorting Test (WCST). 5-HT1A receptor binding was measured with PET and the radioligand [carbonyl-(11)C]WAY-100635, which was quantified with the gold standard method based on kinetic modeling using arterial blood samples. We found that global 5-HT1A receptor binding was positively correlated with measures of verbal memory, such that subjects who had higher receptor binding tended to have better verbal memory than subjects who had lower receptor binding. Regional analyses suggested significant correlations in multiple neocortical brain regions and the raphe nuclei. We did not find significant correlations between 5-HT1A receptor binding and executive functions as measured with WCST. We conclude that neocortical as well as raphe 5-HT1A receptors are involved in verbal memory function in man.

5-Hydroxytryptamine 1A receptors in the dorsomedial hypothalamus connected to dorsal raphe nucleus inputs modulate defensive behaviours and mediate innate fear-induced antinociception

The dorsal raphe nucleus (DRN) is an important brainstem source of 5-hydroxytryptamine (5-HT), and 5-HT plays a key role in the regulation of panic attacks. The aim of the present study was to determine whether 5-HT1A receptor-containing neurons in the medial hypothalamus (MH) receive neural projections from DRN and to then determine the role of this neural substrate in defensive responses. The neurotracer biotinylated dextran amine (BDA) was iontophoretically microinjected into the DRN, and immunohistochemical approaches were then used to identify 5HT1A receptor-labelled neurons in the MH. Moreover, the effects of pre-treatment of the dorsomedial hypothalamus (DMH) with 8-OH-DPAT and WAY-100635, a 5-HT1A receptor agonist and antagonist, respectively, followed by local microinjections of bicuculline, a GABAA receptor antagonist, were investigated. We found that there are many projections from the DRN to the perifornical lateral hypothalamus (PeFLH) but also to DMH and ventromedial (VMH) nuclei, reaching 5HT1A receptor-labelled perikarya. DMH GABAA receptor blockade elicited defensive responses that were followed by antinociception. DMH treatment with 8-OH-DPAT decreased escape responses, which strongly suggests that the 5-HT1A receptor modulates the defensive responses. However, DMH treatment with WAY-100635 failed to alter bicuculline-induced defensive responses, suggesting that 5-HT exerts a phasic influence on 5-HT1A DMH neurons. The activation of the inhibitory 5-HT1A receptor had no effect on antinociception. However, blockade of the 5-HT1A receptor decreased fear-induced antinociception. The present data suggest that the ascending pathways from the DRN to the DMH modulate panic-like defensive behaviours and mediate antinociceptive phenomenon by recruiting 5-HT1A receptor in the MH.

Genetic dysfunction of serotonin 2A receptor hampers response to antidepressant drugs: A translational approach

Pharmacological studies have yielded valuable insights into the role of the serotonin 2A (5-HT2A) receptor in major depressive disorder (MDD) and antidepressant drugs (ADs) response. However, it is still unknown whether genetic variants in the HTR2A gene affect the therapeutic outcome of ADs and the mechanism underlying the regulation of such response remains poorly described. In this context, a translational human-mouse study offers a unique opportunity to address the possibility that variations in the HTR2A gene may represent a relevant marker to predict the efficacy of ADs. In a first part of this study, we investigated in depressed patients the effect of three HTR2A single nucleotide polymorphisms (SNPs), selected for their potential functional consequences on 5-HT2A receptor (rs6313, rs6314 and rs7333412), on response and remission rates after 3 months of antidepressant treatments. We also explored the consequences of the constitutive genetic inactivation of the 5-HT2A receptor (i.e. in 5-HT2A(-/-) mice) on the activity of acute and prolonged administration of SSRIs. Our clinical data indicate that GG patients for the rs7333412 SNP were less prone to respond to ADs than AA/AG patients. In the preclinical study, we demonstrated that the 5-HT2A receptor exerts an inhibitory influence on the neuronal activity of the serotonergic system after acute administration of SSRIs. However, while the chronic administration of the SSRIs escitalopram or fluoxetine elicited a progressive increased in the firing rate of 5-HT neurons in 5-HT2A(+/+) mice, it failed to do so in 5-HT2A(-/-) mutants. These electrophysiological impairments were associated with a decreased ability of the chronic administration of fluoxetine to stimulate hippocampal plasticity and to produce antidepressant-like activities. Genetic loss of the 5-HT2A receptor compromised the activity of chronic treatment with SSRIs, making this receptor a putative marker to predict ADs response.

Cannabidiol induces rapid-acting antidepressant-like effects and enhances cortical 5-HT/glutamate neurotransmission: role of 5-HT1A receptors

Cannabidiol (CBD), the main non-psychotomimetic component of marihuana, exhibits anxiolytic-like properties in many behavioural tests, although its potential for treating major depression has been poorly explored. Moreover, the mechanism of action of CBD remains unclear. Herein, we have evaluated the effects of CBD following acute and chronic administration in the olfactory bulbectomy mouse model of depression (OBX), and investigated the underlying mechanism. For this purpose, we conducted behavioural (open field and sucrose preference tests) and neurochemical (microdialysis and autoradiography of 5-HT1A receptor functionality) studies following treatment with CBD. We also assayed the pharmacological antagonism of the effects of CBD to dissect out the mechanism of action. Our results demonstrate that CBD exerts fast and maintained antidepressant-like effects as evidenced by the reversal of the OBX-induced hyperactivity and anhedonia. In vivo microdialysis revealed that the administration of CBD significantly enhanced serotonin and glutamate levels in vmPFCx in a different manner depending on the emotional state and the duration of the treatment. The potentiating effect upon neurotransmitters levels occurring immediately after the first injection of CBD might underlie the fast antidepressant-like actions in OBX mice. Both antidepressant-like effect and enhanced cortical 5-HT/glutamate neurotransmission induced by CBD were prevented by 5-HT1A receptor blockade. Moreover, adaptive changes in pre- and post-synaptic 5-HT1A receptor functionality were also found after chronic CBD. In conclusion, our findings indicate that CBD could represent a novel fast antidepressant drug, via enhancing both serotonergic and glutamate cortical signalling through a 5-HT1A receptor-dependent mechanism.

TREK1 channel blockade induces an antidepressant-like response synergizing with 5-HT1A receptor signaling

Current antidepressants often remain the inadequate efficacy for many depressive patients, which warrant the necessary endeavor to develop the new molecules and targets for treating depression. Recently, the two-pore domain potassium channel TREK1 has been implicated in mood regulation and TREK-1 antagonists could be the promising antidepressant. This study has screened a TREK1 blocker (SID1900) with a satisfactory blood-brain barrier permeation and bioavailability. Electrophysiological research has shown that SID1900 and the previously reported TREK1 blocker (spadin) efficiently blocked TREK-1 current in HEK293 cells and specifically blocked two-pore domain potassium channels in primary-cultured rat hippocampal neurons. SID1900 and spadin induced a significant antidepressant-like response in the rat model of chronic unpredictable mild stress (CUMS). Both two TREK1 blockers substantially increased the firing rate of 5-HT-ergic neurons in the dorsal raphe nuclei (DRN) and PFC of CUMS rats. SID1900 and spadin significantly up-regulated the expression of PKA-pCREB-BDNF signaling in DRN, hippocampus and PFC of CUMS rats, which were enhanced and reversed by a 5-HTR1A agonist (8-OH-DPAT) and antagonist (WAY100635) respectively. The present findings suggested that TREK1 channel blockers posses the substantial antidepressant-like effect and have the potential synergistic effect with 5-HT1A receptor activation through the common CREB-BDNF signal transduction.

Presynaptic Serotoninergic Regulation of Emotional Processing: A Multimodal Brain Imaging Study

BACKGROUND

The amygdala is a central node in the brain network that processes aversive emotions and is extensively innervated by dorsal raphe nucleus (DRN) serotonin (5-hydroxytryptamine [5-HT]) neurons. Alterations in DRN 5-HT1A receptor availability cause phenotypes characterized by fearful behavior in preclinical models. However, it is unknown whether 5-HT1A receptor availability is linked specifically to the processing of aversive emotions in humans or whether it modulates connectivity in brain networks involved in emotion processing. To answer this question, we investigated the relationship between DRN 5-HT1A receptor availability and amygdala reactivity to aversive emotion and functional connectivity within the amygdala-cortical network.

METHODS

We studied 15 healthy human participants who underwent positron emission tomography scanning with [(11)C]CUMI-101, a 5-HT1A partial agonist radioligand, and functional magnetic resonance imaging of brain responses during an incidental emotion processing task including happy, fearful, and neutral faces. Regional estimates of 5-HT1A receptor binding potential (nondisplaceable) were obtained by calculating total volumes of distribution for presynaptic DRN and amygdala. Connectivity between the amygdala and corticolimbic areas was assessed using psychophysiologic interaction analysis with the amygdala as the seed region.

RESULTS

Analysis of the fear versus neutral contrast revealed a significant negative correlation between amygdala response and DRN binding potential (nondisplaceable) (r = -.87, p < .001). Availability of DRN 5-HT1A receptors positively correlated with amygdala connectivity with middle frontal gyrus, anterior cingulate cortex, bilateral precuneus, and left supramarginal gyrus for fearful (relative to neutral) faces.

CONCLUSIONS

Our data show that DRN 5-HT1A receptor availability is linked specifically to the processing of aversive emotions in the amygdala and the modulation of amygdala-cortical connectivity.

Potentiation of neurite outgrowth by brexpiprazole, a novel serotonin-dopamine activity modulator: a role for serotonin 5-HT1A and 5-HT2A receptors

Brexpiprazole, a novel atypical antipsychotic drug, is currently being tested in clinical trials for treatment of psychiatric disorders, such as schizophrenia and major depressive disorder. The drug is known to act through a combination of partial agonistic activity at 5-hydroxytryptamine (5-HT)1A, and dopamine D2 receptors, and antagonistic activity at 5-HT2A receptors. Accumulating evidence suggests that antipsychotic drugs act by promoting neurite outgrowth. In this study, we examined whether brexpiprazole affected neurite outgrowth in cell culture. We found that brexpiprazole significantly potentiated nerve growth factor (NGF)-induced neurite outgrowth in PC12 cells, in a concentration dependent manner. The selective 5-HT1A receptor antagonist, WAY-100,635, was able to block the effects of brexpiprazole on neurite outgrowth, unlike the selective dopamine D2 receptor antagonist, raclopride. Furthermore, the selective 5-HT2A receptor antagonist M100907, but not DOI (5-HT2A receptor agonist), significantly potentiated NGF-induced neurite outgrowth. Moreover, xestospongin C and 2-aminoethoxydiphenyl borate (2-APB), both specific inhibitors of inositol 1,4,5-triphosphate (IP3) receptors, significantly blocked the effects of brexpiprazole. These findings suggest that brexpiprazole-induced neurite outgrowth is mediated through 5-HT1A and 5-HT2A receptors, and subsequent Ca(2+) signaling via IP3 receptors.

Activation of 5-HT₁A receptors in the medial subdivision of the central nucleus of the amygdala produces anxiolytic effects in a rat model of Parkinson's disease

Although the medial subdivision of the central nucleus of the amygdala (CeM) and serotonin-1A (5-HT1A) receptors are involved in the regulation of anxiety, their roles in Parkinson's disease (PD)-associated anxiety are still unknown. Here we assessed the importance of CeM 5-HT1A receptors for anxiety in rats with unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle (MFB). The lesion induced anxiety-like behaviors, increased the firing rate and burst-firing pattern of CeM γ-aminobutyric acid (GABA) neurons, as well as decreased dopamine (DA) levels in the striatum, medial prefrontal cortex (mPFC), amygdala and ventral part of hippocampus (vHip). Intra-CeM injection of the selective 5-HT1A receptor agonist 8-OH-DPAT produced anxiolytic effects in the lesioned rats, and decreased the firing rate of CeM GABAergic neurons in two groups of rats. Compared to sham-operated rats, the duration of the inhibitory effect on the firing rate of GABAergic neurons was shortened in the lesioned rats. The injection increased DA levels in the mPFC and amygdala in two groups of rats and the vHip in the lesioned rats, and increased 5-HT level in the lesioned rats, whereas it decreased NA levels in the mPFC in two groups of rats and the vHip in the lesioned rats. Moreover, the mean density of 5-HT1A receptor and GABA double-labeled neurons in the CeM was reduced after the lesioning. These results suggest that activation of CeM 5-HT1A receptor produces anxiolytic effects in the 6-OHDA-lesioned rats, which involves decreased firing rate of the GABAergic neurons, and changed monoamine levels in the limbic and limbic-related brain regions.

Improvement of dizocilpine-induced social recognition deficits in mice by brexpiprazole, a novel serotonin-dopamine activity modulator

Cognitive impairment, including impaired social cognition, is largely responsible for the deterioration in social life suffered by patients with psychiatric disorders, such as schizophrenia and major depressive disorder (MDD). Brexpiprazole (7-{4-[4-(1-benzothiophen-4-yl)piperazin-1-yl]butoxy}quinolin-2(1H)-one), a novel serotonin-dopamine activity modulator, was developed to offer efficacious and tolerable therapy for different psychiatric disorders, including schizophrenia and adjunctive treatment of MDD. In this study, we investigated whether brexpiprazole could improve social recognition deficits (one of social cognition deficits) in mice, after administration of the N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine). Dosing with dizocilpine (0.1mg/kg) induced significant impairment of social recognition in mice. Brexpiprazole (0.01, 0.03, 0.1mg/kg, p.o.) significantly ameliorated dizocilpine-induced social recognition deficits, without sedation or a reduction of exploratory behavior. In addition, brexpiprazole alone had no effect on social recognition in untreated control mice. By contrast, neither risperidone (0.03mg/kg, p.o.) nor olanzapine (0.03mg/kg, p.o.) altered dizocilpine-induced social recognition deficits. Finally, the effect of brexpiprazole on dizocilpine-induced social recognition deficits was antagonized by WAY-100,635, a selective serotonin 5-HT1A antagonist. These results suggest that brexpiprazole could improve dizocilpine-induced social recognition deficits via 5-HT1A receptor activation in mice. Therefore, brexpiprazole may confer a beneficial effect on social cognition deficits in patients with psychiatric disorders.

Interactions between serotonin and glutamate-nitric oxide pathways in zebrafish scototaxis

NMDA receptors have been implicated in the acute response to stress, possibly mediated the nitric oxide pathway; serotonin has also been implicated in these responses, and has recently been shown to modulate the nitric oxide pathway via 5-HT1 and 5-HT2 receptors. In this work, we compare the effects of NMDA and a 5-HT1A receptor ligands on light/dark preference in adult zebrafish, and investigate whether nitric oxide mediates the effects of such drugs. The noncompetitive NMDA receptor antagonist MK-801 decreased dark preference (scototaxis), while NMDA increased it; the effects of NMDA were completely blocked by pretreatment with the nitric oxide synthase (NOS) antagonist L-NAME. SNP, a nitric oxide donor, produced a bell-shaped dose-response profile on scototaxis. Treatment with 5-HTP increased scototaxis, an effect which was potentiated by pre-treatment with NMDA, but not MK-801, and partially blocked by L-NAME. The 5-HT1A receptor antagonist WAY 100,635 decreased scototaxis, an effect which was completely blocked by L-NAME. These results suggest that tonic NOS inhibition is an important downstream effector of 5-HT1A receptors in the regulation of dark preference behavior in zebrafish, and that NOS is also under phasic independent control by NMDA receptors.

The role of activation of the 5-HT1A receptor and adenylate cyclase in the antidepressant-like effect of YL-0919, a dual 5-HT1A agonist and selective serotonin reuptake inhibitor

This study aimed to explore the possible mechanisms underlying the antidepressant-like effect of YL-0919, a novel antidepressant candidate with dual activity as a 5-HT1A receptor agonist and a selective serotonin reuptake inhibitor. The animal models commonly used to evaluate potential antidepressants, i.e., tail suspension (TST) in mice and forced swimming test (FST) in mice were used to evaluate the antidepressant effect of YL-0919. The activity of adenylate cyclase (AC) on the synaptic membrane was determined by the homogeneous time-resolved fluorescence resonance energy transfer (TR-FRET) immunoassay. The results indicated that YL-0919 (1.25-2.5mg/kg, i.g.) significantly decreased the immobility time in both the tail suspension test and the forced swim test in a dose-dependent manner, demonstrating the antidepressant-like effect of YL-0919. Furthermore, this effect was completely antagonized by the co-administration of WAY-100635 (0.3mg/kg, s.c.), a 5-HT1A selective antagonist. YL-0919 (10(-9)-10(-5)mol/L) was also shown to activate AC in vitro in a dose-dependent manner in synaptic membranes extracted from the rat prefrontal cortex, and this effect (10(-7)-10(-5)mol/L) was antagonized by WAY-100635 (10(-7)mol/L). Finally, the antidepressant-like effect of YL-0919 (2.5mg/kg, i.g.) was also blocked by the co-administration of H-89 (3 μg/site, i.c.v.), a protein kinase A (PKA) selective inhibitor. These results indicate that the activation of 5-HT1A receptors and the subsequent activation of the AC-cAMP-PKA signaling pathway in the frontal cortex play a critical role in the antidepressant-like effect of YL-0919.

Platelet 5-HT(1A) receptor correlates with major depressive disorder in drug-free patients

The platelet serotonergic system has potential biomarker utility for major depressive disorder (MDD). In the present study, platelet expression of 5-HT1A receptors and serotonin transporter (SERT) proteins, and serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were quantified in 53 patients with MDD and 22 unaffected controls. All were drug-free, non-smokers and had no other psychiatric and cardiovascular comorbidity. The severity of depression symptoms was evaluated using the 17-item Hamilton Depression Rating Scale (HAMD-17) and the Self-rating Depression Scale (SDS). Patients with MDD had significantly higher expression of platelet 5-HT1A receptors but significantly lower contents of platelet 5-HT, platelet-poor plasma (PPP) 5-HT and PPP 5-HIAA compared to healthy controls, and this was correlated with the severity of depression. SERT expression did not differ between the two groups. Correlation analysis confirmed a strong, inverse relationship between the 5-HT1A receptor expression and the 5-HT and 5-HIAA levels. Thus overexpression of platelet 5-HT1A receptors and reduced 5-HT tone may function as a peripheral marker of depression.

(6aR)-11-amino-N-propyl-noraporphine, a new dopamine D2 and serotonin 5-HT1A dual agonist, elicits potent antiparkinsonian action and attenuates levodopa-induced dyskinesia in a 6-OHDA-lesioned rat model of Parkinson's disease

Parkinson's disease (PD) drug therapy remains a challenge. Dual modulation of dopamine and 5-HT receptors has emerged as a promising approach in anti-PD drug development. Taking advantage of the newly discovered aporphine analogue(s), (6aR)-11-amino-N-propyl-noraporphine (SOMCL-171), which exhibited dual D2/5-HT1A receptor agonistic activity, we studied the effects of the compound on levodopa-induced dyskinesia (LID) in a PD animal model. The results demonstrated that SOMCL-171 elicited a potent anti-PD effect in a 6-OHDA-lesioned rat model. Chronic use of SOMCL-171 reduced LID without compromising the antiparkinsonian efficacy. Furthermore, we found that the antidyskinesia effect of SOMCL-171 is associated with its 5-HT1A agonistic activity and the up-regulation of the striatal 5-HT1A receptor. The present data indicated that chronic SOMCL-171 alone produced potent antiparkinsonian effects with weak dyskinesia, compared with that of levodopa. In addition, chronic SOMCL-171 application attenuated the development of levodopa-induced LID at no expense to the antiparkinsonian efficacy. Taken together, our data suggested that dual modulation of D2/5-HT1A receptors may provide a novel approach for drug development in PD and LID.

Adaptive changes in serotonin metabolism preserve normal behavior in mice with reduced TPH2 activity

Polymorphisms in the TPH2 gene coding for the serotonin synthesizing enzyme in the brain are considered as risk factors associated with depression and anxiety in humans. However, whether a certain variation in the TPH2 gene leads to decreased brain serotonin production and development of psychological abnormalities remains unresolved. We generated a new mouse model, carrying one Tph2-null allele and one Tph21473G-allele, coding for a hypoactive form of the enzyme. We tested these mice along with C57BL/6 mice (Tph2C/C), congenic C57BL/6 mice homozygous for the Tph21473G-allele (Tph2G/G), and heterozygous Tph2-deficient mice (Tph2C/-) for anxiety- and depression-like behavior, and evaluated brain serotonin metabolism and 5-HT1AR signaling by high-performance liquid chromatography and quantitative autoradiography, respectively. Progressive reduction in TPH2 activity had no effect on emotional behavior, and only slightly affected brain serotonin levels. However, serotonin degradation rate was drastically decreased in mice with reduced TPH2 activity, thereby compensating for the lowered rate of serotonin production in these mice. In addition, the hypothermic response to the 5-HT1AR agonist, 8-OH-DPAT, was attenuated in mice with reduced serotonin production. In contrast, 5-HT1A autoreceptor density and G-protein coupling were not changed in mice with gradual decrease in central serotonin. Taken together, these data suggest that in conditions of reduced serotonin production lowered serotonin degradation rate contributes to the maintenance of brain serotonin at levels sufficient for adequate behavior responses. These findings reveal that decreased TPH2 activity cannot be considered a reliable predisposition factor for impaired emotional behavior.

A hippocampal nicotinic acetylcholine alpha 7-containing receptor complex is linked to memory retrieval in the multiple-T-maze in C57BL/6j mice

The link between the cholinergic and serotonergic system in cognitive function is well-documented. There is, however, limited information on spatial memory and this formed the rationale to carry out a study with the aim to show a specific link between nicotinic and serotonergic receptor complexes rather than the corresponding subunits, to spatial memory retrieval in a land maze. A total of 46 mice were used and divided into two groups, trained and untrained (yoked) in the multiple-T-Maze (MTM) and following training during the first four days, probe trials for memory retrieval were performed on days 8, 16 and 30. Six hours following scarification, hippocampi were taken for the analysis of native receptor complex levels using blue-native gels followed by immunoblotting with specific antibodies. 5-HT1A-, 5-HT7-, nAChα4- and nACh-α7-containing receptor complexes were observed and were paralleling memory retrievals and receptor complex levels were shown to be significantly different between trained and yoked animals. Only levels of a nicotinic acetylcholine α7 receptor-containing complex at an apparent molecular weight of approximately 480kDa were shown to be linked to memory retrieval on day 8 but not to retrievals on days 16 and 30 when memory extinction has taken place. Correlation between nAChα4-, 5-HT1A- and 5-HT7-containing receptors and latencies on day 16 may point to a probable link in extinction mechanisms. A series of the abovementioned receptor complexes were correlating among each other probably indicating a serotonergic/cholinergic network paralleling spatial memory formation.