Molecular biology of 5-HT receptors

Synthesis and Structure-Activity Relationships of 2,5-Dimethoxy-4-Substituted Phenethylamines and the Discovery of CYB210010: A Potent, Orally Bioavailable and Long-Acting Serotonin 5-HT2 Receptor Agonist

Structure-activity studies of 4-substituted-2,5-dimethoxyphenethylamines led to the discovery of 2,5-dimethoxy-4-thiotrifluoromethylphenethylamines, including CYB210010, a potent and long-acting serotonin 5-HT2 receptor agonist. CYB210010 exhibited high agonist potency at 5-HT2A and 5-HT2C receptors, modest selectivity over 5-HT2B, 5-HT1A, 5-HT6, and adrenergic α2A receptors, and lacked activity at monoamine transporters and over 70 other proteins. CYB210010 (0.1-3 mg/kg) elicited a head-twitch response (HTR) and could be administered subchronically at threshold doses without behavioral tolerance. CYB210010 was orally bioavailable in three species, readily and preferentially crossed into the CNS, engaged frontal cortex 5-HT2A receptors, and increased the expression of genes involved in neuroplasticity in the frontal cortex. CYB210010 represents a new tool molecule for investigating the therapeutic potential of 5-HT2 receptor activation. In addition, several other compounds with high 5-HT2A receptor potency, yet with little or no HTR activity, were discovered, providing the groundwork for the development of nonpsychedelic 5-HT2A receptor ligands.

Antagonism of 5-HT2 receptors attenuates self-sustained firing of human motor units

5-HT2 receptors on motoneurones play a critical role in facilitating persistent inward currents (PICs). Although facilitation of PICs can enhance self-sustained firing after periods of excitation, the relationship between 5-HT2 receptor activity and self-sustained firing in human motor units (MUs) has not been resolved. MU activity was assessed from the tibialis anterior of 10 healthy adults (24.9 ± 2.8 years) during two contraction protocols. Both protocols featured steady-state isometric contractions with constant descending drive to the motoneurone pool. However, one protocol also included an additional phase of superimposed descending drive. Adding and then removing descending drive in the middle of steady-state contractions altered MU firing behaviour across the motor pool, where newly recruited units in the superimposed phase were unable to switch off (P = 0.0002), and units recruited prior to additional descending drive reduced their discharge rates (P < 0.0001, difference in estimated marginal means (∆) = 2.24 pulses/s). The 5-HT2 receptor antagonist, cyproheptadine, was then administered to determine whether changes in MU firing were mediated by serotonergic mechanisms. 5-HT2 receptor antagonism caused reductions in MU discharge rate (P < 0.001, ∆ = 1.65 pulses/s), recruitment threshold (P = 0.00112, ∆ = 1.09% maximal voluntary contraction) and self-sustained firing duration (P < 0.0001, ∆ = 1.77s) after the additional descending drive was removed in the middle of the steady-state contraction. These findings indicate that serotonergic neuromodulation plays a key role in facilitating discharge and self-sustained firing of human motoneurones, where adaptive changes in MU recruitment must occur to meet the demands of the contraction. KEY POINTS: Animal and cellular preparations indicate that somato-dendritic 5-HT2 receptors regulate the intrinsic excitability of motoneurones. 5-HT2 receptor antagonism reduces estimates of persistent inward currents in motoneurones, which contribute to self-sustained firing when synaptic inputs are reduced or removed. This human study employed a contraction task that slowly increased (and then removed) the additional descending drive in the middle of a steady-state contraction where marked self-sustained firing occurred when the descending drive was removed. 5-HT2 receptor antagonism caused widespread reductions in motor unit (MU) discharge rates during contractions, which was accompanied by reduced recruitment threshold and attenuation of self-sustained firing duration after the removal of the additional descending drive to motoneurones. These findings support the role that serotonergic neuromodulation is a key facilitator of MU discharge and self-sustained firing of human motoneurones, where adaptative changes in MU recruitment must occur to meet the demands of the contraction.

Comparison of Intravenous Granisetron and Ondansetron on Hemodynamics and Sensory Motor Block After Spinal Anaesthesia With Hyperbaric Bupivacaine in Patients Undergoing Elective Surgery: A Randomized Double-Blind Study

BACKGROUND

Bezold Jarisch reflex (BJR) is mediated by peripheral serotonin receptor 5-HT₃ type. BJR and sympathetic blockade are important causes of hypotension and bradycardia after spinal anaesthesia. Premedication with serotonin receptor antagonists has a role in the attenuation of hemodynamic disturbances.

AIM

To compare the effect of intravenous granisetron and ondansetron on the hemodynamic and sensory-motor block after spinal anaesthesia with hyperbaric bupivacaine in patients undergoing elective surgery.

METHODOLOGY

Ninety patients posted for elective surgery under spinal anaesthesia were randomly divided into three groups of 30 each. Group A patients received ondansetron 4mg, group B received granisetron 1mg, and group C received normal saline intravenously. Hemodynamic variables such as heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure, and peripheral oxygen saturation, were recorded at baseline and then 2 minutes intervals for 20 minutes and thereafter every 5 minutes till the end of the surgery. The onset and duration of sensory and motor block were recorded at baseline and then every 2 minutes till the complete block was achieved.

RESULT

No patient was excluded from our study. During the intergroup comparison, heart rate and mean arterial pressure remained stable in group A compared to groups B and C. Time to reach peak sensory block level T4 was faster in group A compared to group B and group C. The rate of sensory block regression to two segments (T4 to T6) and thereafter up to T10, T12, and S1 was faster in group B compared to groups A and C. The attainment of complete motor block, Modified Bromage Score (MBS)=4 was faster in group A compared to group B and group C. The rate of motor block regression to MBS=3 and MBS=0 was faster in group B compared to group A and group C.

CONCLUSION

Premedication with ondansetron 4mg and granisetron 1mg intravenously significantly reduces ephedrine use. Ondansetron provides better hemodynamic stability, earlier onset of the sensory and motor blocks as well as prolonged duration of sensory and motor blocks, and duration of analgesia compared to granisetron.

Blockade of 5-HT2 receptors suppresses motor unit firing and estimates of persistent inward currents during voluntary muscle contraction in humans

Serotonergic neuromodulation contributes to enhanced voluntary muscle activation. However, it is not known how the likely motoneurone receptor candidate (5-HT₂ ) influences the firing rate and activation threshold of motor units (MUs) in humans. The purpose of this study was to determine whether 5-HT₂ receptor activity contributes to human MU behaviour during voluntary ramped contractions of differing intensity. High-density surface EMG (HDsEMG) of the tibialis anterior was assessed during ramped isometric dorsiflexions at 10, 30, 50 and 70% of maximal voluntary contraction (MVC). MU characteristics were successfully extracted from HDsEMG of 11 young adults (four female) pre- and post-ingestion of 8 mg cyproheptadine or a placebo. Antagonism of 5-HT₂ receptors caused a reduction in MU discharge rate during steady-state muscle activation that was independent of the level of contraction intensity [P < 0.001; estimated mean difference (∆) = 1.06 pulses/s], in addition to an increase in MU derecruitment threshold (P < 0.013, ∆ = 1.23% MVC), without a change in force during MVC (P = 0.652). A reduction in estimates of persistent inward current amplitude was observed at 10% MVC (P < 0.001, ∆ = 0.99 Hz) and 30% MVC (P = 0.003, ∆ = 0.75 Hz) that aligned with 5-HT changes in MU firing behaviour attributable to 5-HT₂ antagonism. Overall, these findings indicate that 5-HT₂ receptor activity has a role in regulating the discharge rate in populations of spinal motoneurones when performing voluntary contractions. This study provides evidence of a direct link between MU discharge properties, persistent inward current activity and 5-HT₂ receptor activity in humans. KEY POINTS: Activation of 5-HT receptors on the soma and dendrites of motoneurones regulates their excitability. Previous work using chlorpromazine and cyproheptadine has demonstrated that the 5-HT₂ receptor regulates motoneurone activity in humans with chronic spinal cord injury and non-injured control subjects. It is not known how the 5-HT₂ receptor directly influences motor unit (MU) discharge and MU recruitment in larger populations of human motoneurones during voluntary contractions of differing intensity. Despite the absence of change in force during maximal voluntary dorsiflexions, 5-HT₂ receptor antagonism caused a reduction in MU discharge rate during submaximal steady-state muscle contraction, in addition to an increase in MU derecruitment threshold, irrespective of the submaximal contraction intensity. Reductions in estimates of persistent inward currents after 5-HT₂ receptor antagonism support the viewpoint that the 5-HT₂ receptor plays a crucial role in regulating motor activity, whereby a persistent inward current-based mechanism is involved in regulating the excitability of human motoneurones.

Development and challenges in the discovery of 5-HT1A and 5-HT7 receptor ligands

Serotonin (5-hydroxytryptamine) is a small molecule that acts both in the central and peripheral nervous system as a neurotransmitter and a hormone, respectively. Serotonin is synthesized via a multi-stage pathway beginning with l-tryptophan, which is converted by an enzyme called tryptophan hydroxylase into L-5-Hydroxytryptophan. It is well-known for its significance in the control of mood, anxiety, depression, and insomnia as well as in normal human functions such as sleep, sexual activity, and appetite. Thus, for medical chemists and pharmaceutical firms, serotonin is one of the most desirable targets. Among the seven different classes of serotonin receptors, the 5-HT_1A was one of the first discovered serotonin receptors, and the 5-HT₇ was the last addition to the serotonin receptor family. Both the classes were thoroughly examined. 5-HT_1A neurotransmission-related dysfunctions are linked to many psychological conditions such as anxiety, depression, and movement disorders. 5-HT₇ is a member of the cell surface receptor GPCR superfamily and is regulated by the serotonin neurotransmitter. It has been the focus of intensive research efforts since its discovery, which was prompted by its presence in functionally important regions of the brain. The thalamus and hypothalamus have the highest 5-HT₇ receptor densities. They are also found in the hippocampus and cortex at higher densities. Thermoregulation, circadian rhythm, learning and memory, and sleep are all associated with the 5-HT₇ receptor. It is also suspected that this receptor may be involved in the control of mood, indicating that it may be a beneficial target for depression treatment. Several differently structured molecules such as aminotetralins, ergolines, arylpiperazines, indolylalkylamines, aporphines, and aryloxyalkyl-amines are known to bind to 5-HT_1A and 5-HT₇ receptor sites. In brain serotonin receptors 5-HT_1A and 5-HT₇ are strongly co-expressed in regions involved in depression. However, their functional interaction has not been identified. An overview of the 5-HT_1A and 5-HT₇ receptor ligands belonging to different chemical groups is mentioned in this review.

Dual pancreatic adrenergic and dopaminergic signaling as a therapeutic target of bromocriptine

Bromocriptine is approved as a diabetes therapy, yet its therapeutic mechanisms remain unclear. Though bromocriptine's actions have been mainly attributed to the stimulation of brain dopamine D2 receptors (D2R), bromocriptine also targets the pancreas. Here, we employ bromocriptine as a tool to elucidate the roles of catecholamine signaling in regulating pancreatic hormone secretion. In β-cells, bromocriptine acts on D2R and α2A-adrenergic receptor (α2A-AR) to reduce glucose-stimulated insulin secretion (GSIS). Moreover, in α-cells, bromocriptine acts via D2R to reduce glucagon secretion. α2A-AR activation by bromocriptine recruits an ensemble of G proteins with no β-arrestin2 recruitment. In contrast, D2R recruits G proteins and β-arrestin2 upon bromocriptine stimulation, demonstrating receptor-specific signaling. Docking studies reveal distinct bromocriptine binding to α2A-AR versus D2R, providing a structural basis for bromocriptine's dual actions on β-cell α2A-AR and D2R. Together, joint dopaminergic and adrenergic receptor actions on α-cell and β-cell hormone release provide a new therapeutic mechanism to improve dysglycemia.

Receptor Interaction Profiles of 4-Alkoxy-3,5-Dimethoxy-Phenethylamines (Mescaline Derivatives) and Related Amphetamines

3,4,5-Trimethoxyphenethylamine (mescaline) is a psychedelic alkaloid found in peyote cactus. Related 4-alkoxy-3,5-dimethoxy-substituted phenethylamines (scalines) and amphetamines (3C-scalines) are reported to induce similarly potent psychedelic effects and are therefore potential novel therapeutics for psychedelic-assisted therapy. Herein, several pharmacologically uninvestigated scalines and 3C-scalines were examined at key monoamine targets in vitro. Binding affinity at human serotonergic 5-HT_1A, 5-HT_2A, and 5-HT_2C, adrenergic α_1A and α_2A, and dopaminergic D₂ receptors, rat and mouse trace amine-associated receptor 1 (TAAR1), and human monoamine transporters were assessed using target specific transfected cells. Furthermore, activation of human 5-HT_2A and 5-HT_2B receptors, and TAAR1 was examined. Generally, scalines and 3C-scalines bound with weak to moderately high affinity to the 5-HT_2A receptor (K_i = 150–12,000 nM). 3C-scalines showed a marginal preference for the 5-HT_2A vs the 5-HT_2C and 5-HT_1A receptors whereas no preference was observed for the scalines. Extending the 4-alkoxy substituent increased 5-HT_2A and 5-HT_2C receptors binding affinities, and enhanced activation potency and efficacy at the 5-HT_2A but not at the 5-HT_2B receptor. Introduction of fluorinated 4-alkoxy substituents generally increased 5-HT_2A and 5-HT_2C receptors binding affinities and increased the activation potency and efficacy at the 5-HT_2A and 5-HT_2B receptors. Overall, no potent affinity was observed at non-serotonergic targets. As observed for other psychedelics, scalines and 3C-scalines interacted with the 5-HT_2A and 5-HT_2C receptors and bound with higher affinities (up to 63-fold and 34-fold increase, respectively) when compared to mescaline.

Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT₂ antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

Environmental influence on the preclinical evaluation of substance use disorder therapeutics

Substance use disorders (SUD) develop as a result of complex interactions between the environment, the subject, and the drug of abuse. Preclinical basic research investigating each of these tripartite components of SUD individually has resulted in advancements in our fundamental knowledge regarding the progression from drug abuse to SUD and severe drug addiction and the underlying behavioral and neurobiological mechanisms. How these complex interactions between the environment, the subject, and the drug of abuse impact the effectiveness of candidate or clinically used medications for SUD has not been as extensively investigated. The focus of this chapter will address the current state of our knowledge how these environmental, subject, and pharmacological variables have been shown to impact candidate or clinical SUD medication evaluation in preclinical research using drug self-administration procedures as the primary dependent measure. The results discussed in this chapter highlight the importance of considering environmental variables such as the schedule of reinforcement, concurrent availability of alternative nondrug reinforcers, and experimental housing conditions in the context of SUD therapeutic evaluation. The thesis of this chapter is that improved understanding of environmental variables in the context of SUD research will facilitate the utility of preclinical drug self-administration studies in the evaluation and development of candidate SUD therapeutics.

Cardiovascular serotonergic system: Evolution, receptors, transporter, and function

The serotonergic system, serotonin (5HT), serotonin transporter (SERT), and serotonin receptors (5HT-x), is an evolutionarily ancient system that has clear physiological advantages to all life forms from bacteria to humans. This review focuses on the role of platelet/plasma serotonin and the cardiovascular system with minor references to its significant neurotransmitter function. Platelets transport and store virtually all plasma serotonin in dense granules. Stored serotonin is released from activated platelets and can bind to serotonin receptors on platelets and cellular components of the vascular wall to augment aggregation and induce vasoconstriction or vasodilation. The vascular endothelium is critical to the maintenance of cardiovascular homeostasis. While there are numerous ligands, neurological components, and baroreceptors that effect vascular tone it is proposed that serotonin and nitric oxide (an endothelium relaxing factor) are major players in the regulation of systemic blood pressure. Signals not fully defined, to date, that direct serotonin binding to one of the 15 identified 5HT receptors versus the transporter, and the role platelet/plasma serotonin plays in regulating hypertension within the cardiovascular system remain important issues to better understand many diseases and to develop new drugs. Also, expanded research of these pathways in lower life-forms may serve as important model systems to further our understanding of the evolution and mechanisms of action of serotonin.

Activity of Mitragyna speciosa ("Kratom") Alkaloids at Serotonin Receptors

Kratom alkaloids have mostly been evaluated for their opioid activity but less at other targets that could contribute to their physiological effects. Here, we investigated the in vitro and in vivo activity of kratom alkaloids at serotonin receptors (5-HTRs). Paynantheine and speciogynine exhibited high affinity for 5-HT1ARs and 5-HT2BRs, unlike the principal kratom alkaloid mitragynine. Both alkaloids produced antinociceptive properties in rats via an opioid receptor-independent mechanism, and neither activated 5-HT2BRs in vitro. Paynantheine, speciogynine, and mitragynine induced lower lip retraction and antinociception in rats, effects blocked by a selective 5-HT1AR antagonist. In vitro functional assays revealed that the in vivo 5-HT1AR agonistic effects may be due to the metabolites 9-O-desmethylspeciogynine and 9-O-desmethylpaynantheine and not the parent compounds. Both metabolites did not activate 5-HT2BR, suggesting low inherent risk of causing valvulopathy. The 5-HT1AR agonism by kratom alkaloids may contribute to the mood-enhancing effects associated with kratom use.

Molecular Cloning and Functional Characterization of Three 5-HT Receptor Genes (HTR1B, HTR1E, and HTR1F) in Chickens

The serotonin (5-hydroxytryptamine, 5-HT) signaling system is involved in a variety of physiological functions, including the control of cognition, reward, learning, memory, and vasoconstriction in vertebrates. Contrary to the extensive studies in the mammalian system, little is known about the molecular characteristics of the avian serotonin signaling network. In this study, we cloned and characterized the full-length cDNA of three serotonin receptor genes (HTR1B, HTR1E and HTR1F) in chicken pituitaries. Synteny analyses indicated that HTR1B, HTR1E and HTR1F were highly conserved across vertebrates. Cell-based luciferase reporter assays showed that the three chicken HTRs were functional, capable of binding their natural ligands (5-HT) or selective agonists (CP94253, BRL54443, and LY344864) and inhibiting intracellular cAMP production in a dose-dependent manner. Moreover, activation of these receptors could stimulate the MAPK/ERK signaling cascade. Quantitative real-time PCR analyses revealed that HTR1B, HTR1E and HTR1F were primarily expressed in various brain regions and the pituitary. In cultured chicken pituitary cells, we found that LY344864 could significantly inhibit the secretion of PRL stimulated by vasoactive intestinal peptide (VIP) or forskolin, revealing that HTR1F might be involved in the release of prolactin in chicken. Our findings provide insights into the molecular mechanism and facilitate a better understanding of the serotonergic modulation via HTR1B, HTR1E and HTR1F in avian species.

Design, Synthesis, and Biological Evaluation of a Series of 5- and 7-Hydroxycoumarin Derivatives as 5-HT1A Serotonin Receptor Antagonists

We have designed and synthesized a series of 60 new 5- and 7-hydroxycoumarin derivatives bearing the piperazine moiety with the expected binding to 5-HT_1A and 5-HT_2A receptors. Molecular docking of all investigated compounds revealed subnanomolar estimates of 5-HT_1AR K_i for three ligands and 5-HT_2AR Ki for one ligand as well as numerous low nanomolar estimates of K_i for both receptors. Intrigued by these results we synthesized all 60 new derivatives using microwave-assisted protocols. We show that three new compounds show a relatively high antagonistic activity against the 5HT_1A receptor, although lower than the reference compound WAY-100635. These compounds also showed relatively low binding affinities to the 5-HT_2A receptor. We also provide a detailed structure-activity analysis of this series of compounds and compare it with previously obtained results for an exhaustive series of coumarin derivatives.

Human corticospinal-motoneuronal output is reduced with 5-HT2 receptor antagonism

Animal models indicate that serotonin (5-HT) release onto motoneurons facilitates motor output, particularly during strong motor activities. However, evidence for 5-HT effects during human movement are limited. This study examined how antagonism of the 5-HT₂ receptor, which is a 5-HT receptor that promotes motoneuron excitability, affects human movement. Ten healthy participants (24.2 ± 1.9 yr) ingested 8 mg of cyproheptadine (competitive 5-HT₂ antagonist) in a double-blinded, placebo-controlled, repeated-measures design. Transcranial magnetic stimulation (TMS) of the motor cortex was used to elicit motor evoked potentials (MEPs) from biceps brachii. First, stimulus-response curves (90%-160% active motor threshold) were obtained during very weak elbow flexions (10% of maximal). Second, to determine if 5-HT effects are scaled to the intensity of muscle contraction, TMS at a fixed intensity was applied during elbow flexions of 20%, 40%, 60%, 80%, and 100% of maximal. Cyproheptadine reduced the size of MEPs across the stimulus-response curves (P = 0.045). Notably, MEP amplitude was 22.3% smaller for the cyproheptadine condition for the strongest TMS intensity. In addition, cyproheptadine reduced maximal torque (P = 0.045), lengthened the biceps silent period during maximal elbow flexions (P = 0.037), and reduced superimposed twitch amplitude during moderate-intensity elbow flexions (P = 0.035). This study presents novel evidence that 5-HT₂ receptors influence corticospinal-motoneuronal output, which was particularly evident when a large number of descending inputs to motoneurons were active. Although it is likely that antagonism of 5-HT₂ receptors reduces motoneuron gain to ionotropic inputs, supraspinal mechanisms may have also contributed to the study findings.NEW & NOTEWORTHY Voluntary contractions and responses to magnetic stimulation of the motor cortex are dependent on serotonin activity in the central nervous system. 5-HT₂ antagonism decreased evoked potential size to high-intensity stimulation, and reduced torque and lengthened inhibitory silent periods during maximal contractions. We provide novel evidence that 5-HT₂ receptors are involved in muscle activation, where 5-HT effects are strongest when a large number of descending inputs activate motoneurons.

Interactions between the cannabinoid and the serotonergic systems in modulation of pain perception

The aim of our study was to evaluate the effects of cannabinoids and serotonergic system on nociception in intact rats and after heat stress. Cannabinoid receptor type 1 (CB1) and 5-hydroxytryptamine receptor (5НТ1А) agonists and antagonists have been administered according to different experimental designs (alone and in combinations) in intact male Wistar rats, as well in animals subjected to one hour of heat stress. Pain perception has been evaluated by Paw pressure test. Our results pointed out that cannabinoids and the serotonergic system interact in nociception in intact animals as well as after heat stress. Cannabinoids seemed to have less prominent role in such interaction in intact animals than after heat stress. The interplay between the two systems probably involves different mechanisms in intact animals and after heat stress with time-dependent effects. The interaction between the cannabinoid and the serotonergic systems exerts a modulating rather than mediating effect on h-SIA.

Risperidone and 5-HT2A Receptor Antagonists Attenuate and Reverse Cocaine-Induced Hyperthermia in Rats

BACKGROUND

Cocaine (benzoylmethylecgonine) is one of the most widely used illegal psychostimulant drugs worldwide, and mortality from acute intoxication is increasing. Suppressing hyperthermia is effective in reducing cocaine-related mortality, but a definitive therapy has not yet been found. In this study, we assessed the ability of risperidone to attenuate acute cocaine-induced hyperthermia and delineated the mechanism of its action.

METHODS

Rats were injected i.p. with saline, risperidone, ketanserin, ritanserin, haloperidol, or SCH 23 390 before and after injection of cocaine (30 mg/kg) or with WAY-00 635, SB 206 553, or sulpiride before cocaine injection; thereafter, the rectal temperature was measured every 30 minutes for up to 4 hours. In vivo microdialysis was used to reveal the effect of risperidone on cocaine-induced elevation of dopamine (DA), serotonin (5-HT), and noradrenaline concentrations in the anterior hypothalamus. For post-administration experiments, saline or risperidone (0.5 mg/kg) were injected into rats, and cocaine (30 mg/kg) was injected 15 minutes later. For every 30 minutes thereafter, DA, 5-HT, and noradrenaline levels were measured for up to 240 minutes after cocaine administration.

RESULTS

Risperidone, 5-HT2A receptor antagonists, and D1 receptor antagonistic drugs prevented and reversed cocaine-induced hyperthermia. In contrast, receptor antagonists for 5-HT1A, 5-HT2B/2C, and D2 did not alter cocaine-induced hyperthermia. Risperidone treatment further attenuated cocaine-induced elevation of DA.

CONCLUSIONS

Our results indicate that risperidone attenuates cocaine-induced hyperthermia primarily by blocking the activities of the 5-HT2A and D1 receptors and may be potentially useful for treating cocaine-induced acute hyperthermia in humans.

Serotonin 2A receptors are a stress response system: implications for post-traumatic stress disorder

Serotonin, one of the first neurotransmitters to be identified, is an evolutionarily old molecule that is highly conserved across the animal kingdom, and widely used throughout the brain. Despite this, ascribing a specific set of functions to brain serotonin and its receptors has been difficult and controversial. The 2A subtype of serotonin receptors (5-HT2A receptor) is the major excitatory serotonin receptor in the brain and has been linked to the effects of drugs that produce profound sensory and cognitive changes. Numerous studies have shown that this receptor is upregulated by a broad variety of stressors, and have related 5-HT2A receptor function to associative learning. This review proposes that stress, particularly stress related to danger and existential threats, increases the expression and function of 5-HT2A receptors. It is argued that this is a neurobiological adaptation to promote learning and avoidance of danger in the future. Upregulation of 5-HT2A receptors during stressful events forms associations that tune the brain to environmental cues that signal danger. It is speculated that life-threatening situations may activate this system and contribute to the symptoms associated with post-traumatic stress disorder (PTSD). 3,4-Methylenedioxymethamphetamine, which activates 5-HT2A receptors, has been successful in the treatment of PTSD and has recently achieved status as a breakthrough therapy. An argument is presented that 3,4-methylenedioxymethamphetamine may paradoxically act through these same 5-HT2A receptors to ameliorate the symptoms of PTSD. The central thematic contention is that a key role of serotonin may be to function as a stress detection and response system.

Receptor Interaction Profiles of 4-Alkoxy-Substituted 2,5-Dimethoxyphenethylamines and Related Amphetamines

Background: 2,4,5-Trimethoxyamphetamine (TMA-2) is a potent psychedelic compound. Structurally related 4-alkyloxy-substituted 2,5-dimethoxyamphetamines and phenethylamine congeners (2C-O derivatives) have been described but their pharmacology is mostly undefined. Therefore, we examined receptor binding and activation profiles of these derivatives at monoamine receptors and transporters.

Methods: Receptor binding affinities were determined at the serotonergic 5-HT_1A, 5-HT_2A, and 5-HT_2C receptors, trace amine-associated receptor 1 (TAAR1), adrenergic α₁ and α₂ receptors, dopaminergic D₂ receptor, and at monoamine transporters, using target-transfected cells. Additionally, activation of 5-HT_2A and 5-HT_2B receptors and TAAR1 was determined. Furthermore, we assessed monoamine transporter inhibition.

Results: Both the phenethylamine and amphetamine derivatives (K_i = 8–1700 nM and 61–4400 nM, respectively) bound with moderate to high affinities to the 5-HT_2A receptor with preference over the 5-HT_1A and 5-HT_2C receptors (5-HT_2A/5-HT_1A = 1.4–333 and 5-HT_2A/5-HT_2C = 2.1–14, respectively). Extending the 4-alkoxy-group generally increased binding affinities at 5-HT_2A and 5-HT_2C receptors but showed mixed effects in terms of activation potency and efficacy at these receptors. Introduction of a terminal fluorine atom into the 4-ethoxy substituent by trend decreased, and with progressive fluorination increased affinities at the 5-HT_2A and 5-HT_2C receptors. Little or no effect was observed at the 5-HT_1A receptor for any of the substances tested (K_i ≥ 2700 nM). Phenethylamines bound more strongly to the TAAR1 (K_i = 21–3300 nM) compared with their amphetamine analogs (K_i = 630–3100 nM).

Conclusion: As seen with earlier series investigated, the 4-alkyloxy-substituted 2,5-dimethoxyamphetamines and phenethylamines share some trends with the many other phenethylamine pharmacophore containing compounds, such as when increasing the size of the 4-substituent and increasing the lipophilicity, the affinities at the 5-HT_2A/C subtype also increase, and only weak 5-HT_2A/C subtype selectivities were achieved. At least from the binding data available (i.e., high affinity binding at the 5-HT_2A receptor) one may predict mainly psychedelic-like effects in humans, at least for some of the compound investigated herein.

Dual inhibitory action of trazodone on dorsal raphe serotonergic neurons through 5-HT1A receptor partial agonism and α1-adrenoceptor antagonism

Trazodone is an antidepressant drug with considerable affinity for 5-HT_1A receptors and α₁-adrenoceptors for which the drug is competitive agonist and antagonist, respectively. In this study, we used cell-attached or whole-cell patch-clamp recordings to characterize the effects of trazodone at somatodendritic 5-HT_1A receptors (5-HT_1AARs) and α₁-adrenoceptors of serotonergic neurons in rodent dorsal raphe slices. To reveal the effects of trazodone at α₁-adrenoceptors, the baseline firing of 5-HT neurons was facilitated by applying the selective α₁-adrenoceptor agonist phenylephrine at various concentrations. In the absence of phenylephrine, trazodone (1–10 μM) concentration-dependently silenced neurons through activation of 5-HT_1AARs. The effect was fully antagonized by the selective 5-HT_1A receptor antagonist Way-100635. With 5-HT_1A receptors blocked by Way-100635, trazodone (1–10 μM) concentration-dependently inhibited neuron firing facilitated by 1 μM phenylephrine. Parallel rightward shift of dose-response curves for trazodone recorded in higher phenylephrine concentrations (10–100 μM) indicated competitive antagonism at α₁-adrenoceptors. Both effects of trazodone were also observed in slices from Tph2^-/- mice that lack synthesis of brain serotonin, showing that the activation of 5-HT_1AARs was not mediated by endogenous serotonin. In whole-cell recordings, trazodone activated 5-HT_1AAR-coupled G protein-activated inwardly-rectifying (GIRK) channel conductance with weak partial agonist efficacy (~35%) compared to that of the full agonist 5-CT. Collectively our data show that trazodone, at concentrations relevant to its clinical effects, exerts weak partial agonism at 5-HT_1AARs and disfacilitation of firing through α₁-adrenoceptor antagonism. These two actions converge in inhibiting dorsal raphe serotonergic neuron activity, albeit with varying contribution depending on the intensity of α₁-adrenoceptor stimulation.

Potentiation of the glycine response by serotonin on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice

The lamina II, also called the substantia gelatinosa (SG), of the trigeminal subnucleus caudalis (Vc), is thought to play an essential role in the control of orofacial nociception. Glycine and serotonin (5-hydroxytryptamine, 5-HT) are the important neurotransmitters that have the individual parts on the modulation of nociceptive transmission. However, the electrophysiological effects of 5-HT on the glycine receptors on SG neurons of the Vc have not been well studied yet. For this reason, we applied the whole-cell patch clamp technique to explore the interaction of intracellular signal transduction between 5-HT and the glycine receptors on SG neurons of the Vc in mice. In nine of 13 neurons tested (69.2%), pretreatment with 5-HT potentiated glycine-induced current (I_Gly). Firstly, we examined with a 5-HT₁ receptor agonist (8-OH-DPAT, 5-HT_1/7 agonist, co-applied with SB-269970, 5-HT₇ antagonist) and antagonist (WAY-100635), but 5-HT₁ receptor agonist did not increase I_Gly and in the presence of 5-HT₁ antagonist, the potentiation of 5-HT on I_Gly still happened. However, an agonist (α-methyl-5-HT) and antagonist (ketanserin) of the 5-HT₂ receptor mimicked and inhibited the enhancing effect of 5-HT on I_Gly in the SG neurons, respectively. We also verified the role of the 5-HT₇ receptor by using a 5-HT₇ antagonist (SB-269970) but it also did not block the enhancement of 5-HT on I_Gly. Our study demonstrated that 5-HT facilitated I_Gly in the SG neurons of the Vc through the 5-HT₂ receptor. The interaction between 5-HT and glycine appears to have a significant role in modulating the transmission of the nociceptive pathway.

Serotonergic Psychedelics: Experimental Approaches for Assessing Mechanisms of Action

Recent, well-controlled - albeit small-scale - clinical trials show that serotonergic psychedelics, including psilocybin and lysergic acid diethylamide, possess great promise for treating psychiatric disorders, including treatment-resistant depression. Additionally, fresh results from a deluge of clinical neuroimaging studies are unveiling the dynamic effects of serotonergic psychedelics on functional activity within, and connectivity across, discrete neural systems. These observations have led to testable hypotheses regarding neural processing mechanisms that contribute to psychedelic effects and therapeutic benefits. Despite these advances and a plethora of preclinical and clinical observations supporting a central role for brain serotonin 5-HT2A receptors in producing serotonergic psychedelic effects, lingering and new questions about mechanisms abound. These chiefly pertain to molecular neuropharmacology. This chapter is devoted to illuminating and discussing such questions in the context of preclinical experimental approaches for studying mechanisms of action of serotonergic psychedelics, classic and new.

A novel analytical method of TFMPP and mCPP in fluids of drug addicts using LLE-GC/NPD

In recent years, drug-abuse problem is growing by leaps and bounds all over the world. The master minds spearheading its proliferation among the youth are difficult to identify, so drug-abuse case has become a hard nut to crack even with the help of best international experts in forensic science and criminology. Because most nations have tightened their controls on traditional drugs, the younger generation is now hooked onto new-type drugs: 1-(3- trifluoromethylphenyl) piperazine (TFMPP), 1-(3-chlorophenyl) piperazine (mCPP) and other new piperazine-drugs, acting as hallucinogens like ‘ecstasy’, are being consumed by vulnerable masses all over the world. However, only few research studies have focused on developing highly effective detection methods for TFMPP and mCPP in biological fluids; the number of detection methods for these new-type drugs is almost nil in China. Therefore, it is difficult to detect and prevent drug abuse cases related to piperazine drugs in China. There is an urgent need to develop some simple, fast, and reliable methods for detecting piperazine-drugs in vulnerable masses. Thus, the development of novel detection methods with high sensitivity and selectivity is a difficult task for the officials working in the department of forensic science in China. In this work, a new method was developed for the detection of piperazine derivatives: it was performed under the various specific conditions required for conducting chromatography and mass spectrometry analysis. With this novel method, TFMPP and mCPP was successfully detected with high accuracy in various biological samples. By comparing the purification effect of different solid-phase extraction columns for TFMPP and mCPP in biological fluids (urine and blood), we confirmed the validity of the novel method. In addition, this method has good linear relationship and a low detection line when GC/MS was performed for detecting TFMPP, mCPP in the biological fluids (urine and blood). It is a simple, reproducible method that is highly specific in the detection of piperazine-drugs. Thus, it is indeed a reliable method in forensic science.

Contractile Effects of Serotonin (5-HT) in the Rat Cauda Epididymis: Expression and Functional Characterization of 5-HT Receptors

Serotonin [5-hydroxytryptamine (5-HT)] exerts multiple central and peripheral functions. High concentrations of 5-HT have been found in the epididymis, a ductal organ that plays pivotal roles in sperm transport and maturation. The contraction of the epididymal smooth muscle is essential for sperm transport and emission during ejaculation. The contributions of the epididymal 5-HT system to these events are poorly understood. Here, we assessed the contractile function of 5-HT in the rat cauda epididymis (CE), pharmacologically targeting the receptor(s) and the reuptake mechanism involved in this system. Segments of CE duct from adult Wistar rats were set up in an organ bath system for isometric tension recordings, and concentration-response curves to 5-HT and norepinephrine were obtained. 5-HT elicited concentration-dependent contractions of the CE duct (pEC₅₀ = 6.5 ± 0.1) that were potentiated with high potency by the norepinephrine transporter (NET) inhibitor desipramine and with low potency by the highly selective serotonin transporter inhibitor paroxetine, indicating that the NET is the major mediator of 5-HT reuptake in vitro. CE contractions to 5-HT were antagonized by the α ₁-adrenoceptor (α ₁-AR) antagonist prazosin (pA ₂ ≅ 8.9), 5-HT_2A/2C antagonists ketanserin (pA ₂ ≅ 9.4) and fluoxetine (pA ₂ ≅ 7.4), and 5-HT_1A ligands WAY 100635 (pA ₂ ≅ 8.9) and buspirone (pA ₂ ≅ 7.3). Reverse transcriptase polymerase chain reaction analysis demonstrated that 5-HT_1A and 5-HT_2A transcripts are highly abundant in the cauda epididymis, whereas 5-HT_2C transcript was not found. Altogether, our results reveal that contractions of the CE duct to 5-HT encompasses at least activation of α ₁-ARs and 5-HT_1A and 5-HT_2A receptors, providing new insights into the roles of 5-HT on the epididymal function.

5-HT1D receptors inhibit the monosynaptic stretch reflex by modulating C-fiber activity

The monosynaptic stretch reflex (MSR) plays an important role in feedback control of movement and posture but can also lead to unstable oscillations associated with tremor and clonus, especially when increased with spinal cord injury (SCI). To control the MSR and clonus after SCI, we examined how serotonin regulates the MSR in the sacrocaudal spinal cord of rats with and without a chronic spinal transection. In chronic spinal rats, numerous 5-HT receptor agonists, including zolmitriptan, methylergonovine, and 5-HT, inhibited the MSR with a potency highly correlated to their binding affinity to 5-HT_1D receptors and not other 5-HT receptors. Selective 5-HT_1D receptor antagonists blocked this agonist-induced inhibition, although antagonists alone had no action, indicating a lack of endogenous or constitutive receptor activity. In normal uninjured rats, the MSR was likewise inhibited by 5-HT, but at much higher doses, indicating a supersensitivity after SCI. This supersensitivity resulted from the loss of the serotonin transporter SERT with spinal transection, because normal and injured rats were equally sensitive to 5-HT after SERT was blocked or to agonists not transported by SERT (zolmitriptan). Immunolabeling revealed that the 5-HT_1D receptor was confined to superficial lamina of the dorsal horn, colocalized with CGRP-positive C-fibers, and eliminated by dorsal rhizotomy. 5-HT_1D receptor labeling was not found on large proprioceptive afferents or α-motoneurons of the MSR. Thus serotonergic inhibition of the MSR acts indirectly by modulating C-fiber activity, opening up new possibilities for modulating reflex function and clonus via pain-related pathways. NEW & NOTEWORTHY Brain stem-derived serotonin potently inhibits afferent transmission in the monosynaptic stretch reflex. We show that serotonin produces this inhibition exclusively via 5-HT_1D receptors, and yet these receptors are paradoxically mostly confined to C-fibers. This suggests that serotonin acts by gating of C-fiber activity, which in turn modulates afferent transmission to motoneurons. We also show that the classic supersensitivity to 5-HT after spinal cord injury results from a loss of SERT, and not 5-HT_1D receptor plasticity.

Molecular Imaging in Huntington's Disease

Huntington's disease (HD) is a rare monogenic neurodegenerative disorder caused by a trinucleotide CAG repeat expansion in the huntingtin gene resulting in the formation of intranuclear inclusions of mutated huntingtin. The accumulation of mutated huntingtin leads to loss of GABAergic medium spiny neurons (MSNs); subsequently resulting in the development of chorea, cognitive dysfunction and psychiatric symptoms. Premanifest HD gene expansion carriers, provide a unique cohort to examine very early molecular changes, occurring before the development of overt symptoms, to elucidate disease pathophysiology and identify reliable biomarkers of HD progression. Positron emission tomography (PET) is a non-invasive molecular imaging technique allowing the evaluation of specific molecular targets in vivo. Selective PET radioligands provide invaluable tools to investigate the role of the dopaminergic system, brain metabolism, microglial activation, phosphodiesterase 10A, and cannabinoid, GABA, adenosine and opioid receptors in HD. PET has been employed to monitor disease progression aiming to identify a reliable biomarker to predict phenoconversion from premanifest to manifest HD.

A serotonin receptor (Cg5-HTR-1) mediating immune response in oyster Crassostrea gigas

Serotonin receptors, including ligand-gated ion channel (LGICs) and G protein-coupled receptors (GPCR), play vital roles in modulating physiological processes and immunoreaction. In the present study, a homologue of serotonin (5-HT) receptor was identified from oyster Crassostrea gigas (designated Cg5-HTR-1). Its open reading frame (ORF) was of 1239 bp, encoding a polypeptide of 412 amino acids with a seven transmembrane region. Cg5-HTR-1 shared high similarity with the 5-HTRs from other animals. The cAMP contents in HEK293T cells decreased significantly after Cg5-HTR-1 transfection and 5-HT incubation (p < .05), while blocking Cg5-HTR-1 with specific receptor antagonist reversed this downtrend. The intracellular Ca²⁺ concentrations increased significantly (p < .05) after cell transfection and 5-HT incubation, and the antagonist treatment also arrested this process. Cg5-HTR-1 transcripts were widely distributed in various tissues, with the highest level in hepatopancreas and lowest level in mantle and gill. The mRNA expression of Cg5-HTR-1 in hemocyte increased significantly after lipopolysaccharide (LPS) stimulation and reached the peak level (6.47-fold, p < .05) at 6 h post treatment. The inhibition of Cg5-HTR-1 significantly reduced the expression of tumor necrosis factor (TNF) mRNA in hemocyte, down-regulated the superoxide dismutase (SOD) activity in serum, and induced the apoptosis of hemocyte (p < .05). These results suggested that Cg5-HTR-1 was a novel member of 5-HT₁ receptor family and it mediated serotonergic immunomodulation on both cellular and humoral immune responses.

Role of tandospirone, a 5-HT1A receptor partial agonist, in the treatment of central nervous system disorders and the underlying mechanisms

5-hydroxytryptamine (5-HT, serotonin) is an important neurotransmitter in the modulation of the cognitive, behavioral and psychological functions in animals and humans. Among the fourteen subtypes of 5-HT receptor, 5-HT1A receptor has been extensively studied. Tandospirone, an azapirone derivative with strong and selective agonist effect on 5-HT1A receptor, has been used for the treatment of anxiety disorders especially generalized anxiety disorder for decades. Recently, tandospirone showed the efficacy in relieving the syndromes of social anxiety disorder and post-traumatic stress disorder as well as in potentiating the effect of antidepressants in the treatment of depression in both preclinical and clinical studies. More impressively, the beneficial effect of tandospirone has been revealed on improvement of motor dysfunction of Parkinson's disease and cognitive deficits of schizophrenia either in monotherapy or in combination with other drugs. This review discusses the superiority of tandospirone in the treatment of the disorders and associated mechanisms in central nervous system from the literature.

Pericytes impair capillary blood flow and motor function after chronic spinal cord injury

Blood vessels in the central nervous system (CNS) are controlled by neuronal activity. For example, widespread vessel constriction (vessel tone) is induced by brainstem neurons that release the monoamines serotonin and noradrenaline, and local vessel dilation is induced by glutamatergic neuron activity. Here we examined how vessel tone adapts to the loss of neuron-derived monoamines after spinal cord injury (SCI) in rats. We find that, months after the imposition of SCI, the spinal cord below the site of injury is in a chronic state of hypoxia owing to paradoxical excess activity of monoamine receptors (5-HT₁) on pericytes, despite the absence of monoamines. This monoamine-receptor activity causes pericytes to locally constrict capillaries, which reduces blood flow to ischemic levels. Receptor activation in the absence of monoamines results from the production of trace amines (such as tryptamine) by pericytes that ectopically express the enzyme aromatic L-amino acid decarboxylase (AADC), which synthesizes trace amines directly from dietary amino acids (such as tryptophan). Inhibition of monoamine receptors or of AADC, or even an increase in inhaled oxygen, produces substantial relief from hypoxia and improves motoneuron and locomotor function after SCI.

The antihypertensive drug pindolol attenuates long-term but not short-term binge-like ethanol consumption in mice

Alcohol dependence is a debilitating disorder with current therapies displaying limited efficacy and/or compliance. Consequently, there is a critical need for improved pharmacotherapeutic strategies to manage alcohol use disorders (AUDs). Previous studies have shown that the development of alcohol dependence involves repeated cycles of binge-like ethanol intake and abstinence. Therefore, we used a model of binge-ethanol consumption (drinking-in-the-dark) in mice to test the effects of compounds known to modify the activity of neurotransmitters implicated in alcohol addiction. From this, we have identified the FDA-approved antihypertensive drug pindolol, as a potential candidate for the management of AUDs. We show that the efficacy of pindolol to reduce ethanol consumption is enhanced following long-term (12 weeks) binge-ethanol intake, compared with short-term (4 weeks) intake. Furthermore, pindolol had no effect on locomotor activity or consumption of the natural reward sucrose. Because pindolol acts as a dual beta-adrenergic antagonist and 5-HT_1A/1B partial agonist, we examined its effect on spontaneous synaptic activity in the basolateral amygdala (BLA), a brain region densely innervated by serotonin and norepinephrine-containing fibres. Pindolol increased spontaneous excitatory post-synaptic current frequency of BLA principal neurons from long-term ethanol-consuming mice but not naïve mice. Additionally, this effect was blocked by the 5-HT_1A/1B receptor antagonist methiothepin, suggesting that altered serotonergic activity in the BLA may contribute to the efficacy of pindolol to reduce ethanol intake following long-term exposure. Although further mechanistic investigations are required, this study demonstrates the potential of pindolol as a new treatment option for AUDs that can be fast-tracked into human clinical studies.

The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens

Classic hallucinogens share pharmacology as serotonin 5-HT_2A, 5-HT_2B, and 5-HT_2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT_2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT_2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT_2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancing inhibitory activity via intrinsic mechanisms. Together with experiments that show that addictive drugs, such as cocaine, potentiate Kv1.x channels, thereby suppressing NAc shell GABAergic activity, this hypothesis provides a mechanism by which classic hallucinogen-mediated stimulation of 5-HT_2C receptors could thwart addiction. It also provides a potential reason for the non-addictive nature of classic hallucinogens.

Marine cyanobacteria-derived serotonin receptor 2C active fraction induces psychoactive behavioral effects in mice

CONTEXT

Marine cyanobacteria offer a robust resource for natural products drug discovery due to the secondary metabolites they produce.

OBJECTIVE

To identify novel cyanobacterial compounds that exhibit CNS psychoactive effects.

MATERIALS AND METHODS

Cyanobacteria were collected from Las Perlas Archipelago, Panama and subjected to dichloromethane/methanol extraction and fractionation by column chromatography before being screened for affinity against a panel of CNS targets. A 50:50 ethyl acetate:methanol fraction of one cyanobacterial extract (2064H) was subjected to HPLC and the major peak was isolated (2064H3). At a dose of 20 μg per animal, 2064H and 2064H3 were tested in mice using behavioral assays that included the forced swim, open field and formalin tests.

RESULTS

2064H was shown to bind to the serotonin 2C (5-HT_2C) receptor, a known target for depression and pain treatment. 2064H showed 59.6% inhibition of binding of [³H]-mesulergine with an IC₅₀ value of 179 ng/mL and did not show inhibition of binding greater than 45% with any other receptors tested. Both 2064H and 2064H3 decreased immobility time in the first minute of the tail suspension test. 2064H increased time, distance and number of entries in the center region in the first half of the open field test. 2064H increased overall nocifensive behaviors in the formalin test.

DISCUSSION AND CONCLUSION

Overall, manipulating the 5-HT_2C receptor with these receptor-specific ligands derived from cyanobacteria altered pain, depression and anxiety-like behaviors, illustrating the importance of this receptor in affective behaviors. These results demonstrate the potential of cyanobacteria as a source for CNS active compounds.

Validation of a High Throughput Scintillation Proximity Assay for 5-HydroxytryptaminelE Receptor Binding Activity

Membranes prepared from stable transfected cells expressing the human gene encoding a functional 5-hydroxytryptaminelE (5HT1E) receptor were used to investigate high-affinity [3H]serotonin ([3H]5-HT) binding with scintillation proximity assay (SPA) technology. In this nonseparation format, membranes are captured by WGA-coated SPA fluoromicrospheres for detection of receptor-bound radioligand. Total binding observed was approximately 2000 cpm compared to a nonspecific signal of 100 cpm determined in the presence of 10,uM unlabeled 5-HT. Non-proximity effects (NPE) for the radiolabel and SPA beads averaged less than 100 cpm. Saturation binding analysis yielded an equilibrium dissociation constant (Kd) of 5.38 ± 0.43 nM and a maximum binding capacity (Bmax) of 6.42 + 0.15 pmol/mg protein. Competition with unlabeled serotonergic compounds demonstrated a specificity of the assay with rank potencies (5-HT > a-Me-5-HT > 2-Me-5-HT > 5-CT) similar to those observed using traditional fitration techniques. The variability of the assay and the stability of all reagents were investigated to validate the assay for extended use throughout a typical high throughput screen operation lasting several months.

Morphine-induced Straub tail reaction in mice treated with serotonergic compounds

Constitutively active 5-HT₂ receptors have been suggested to contribute to motoneuronal excitability, muscle spasms and spasticity. Accordingly, 5-HT_2C receptor inverse agonists have been demonstrated in pilot experiments to reduce spasticity in animal model of spasticity and patients with spinal cord injuries. Thus, 5-HT_2C receptor inverse agonists may represent a novel class of anti-spasticity agents justifying a search for compounds with robust 5-HT_2C receptor inverse agonist activity either among the existing medications or via a dedicated drug discovery program. Morphine-induced Straub tail response in mice is regarded as a model of transient spasticity that may be suitable for supporting such drug discovery efforts. Subcutaneous injection of morphine (10-60mg/kg) induced a dose-dependent Straub tail reaction in male Swiss mice with maximum response obtained 15-30min after the morphine administration. When given prior to morphine, 5-HT_2B/2C receptor inverse agonists cyproheptadine (1-10mg/kg, i.p.) and SB206553 (0.3-3mg/kg, i.p.) diminished Straub tail reaction dose-dependently without affecting spontaneous locomotor activity. In contrast, 5-HT_2B/2C receptor antagonist methysergide (1-5.6mg/kg, i.p.) and 5-HT_2C receptor antagonist SB242084 (1-5.6mg/kg, i.p.) as well as 5-HT_2A receptor inverse agonist pimavanserin (1-10mg/kg, i.p.) had no appreciable effects on Straub tail response. Taken together, the findings indicate that constitutive activity of 5-HT_2B/2C receptor may be involved in the mechanisms of morphine-induced spasticity. Thus, morphine-induced Straub tail response may be evaluated further as a candidate higher throughput test to identify 5-HT_2C receptor inverse agonists with anti-spasticity effects in vivo.

Modulatory effect of the 5-HT1A agonist buspirone and the mixed non-hallucinogenic 5-HT1A/2A agonist ergotamine on psilocybin-induced psychedelic experience

The mixed serotonin (5-HT) 1A/2A/2B/2C/6/7 receptor agonist psilocybin dose-dependently induces an altered state of consciousness (ASC) that is characterized by changes in sensory perception, mood, thought, and the sense of self. The psychological effects of psilocybin are primarily mediated by 5-HT2A receptor activation. However, accumulating evidence suggests that 5-HT1A or an interaction between 5-HT1A and 5-HT2A receptors may contribute to the overall effects of psilocybin. Therefore, we used a double-blind, counterbalanced, within-subject design to investigate the modulatory effects of the partial 5-HT1A agonist buspirone (20mg p.o.) and the non-hallucinogenic 5-HT2A/1A agonist ergotamine (3mg p.o.) on psilocybin-induced (170 µg/kg p.o.) psychological effects in two groups (n=19, n=17) of healthy human subjects. Psychological effects were assessed using the Altered State of Consciousness (5D-ASC) rating scale. Buspirone significantly reduced the 5D-ASC main scale score for Visionary Restructuralization (VR) (p<0.001), which was mostly driven by a reduction of the VR item cluster scores for elementary and complex visual hallucinations. Further, buspirone also reduced the main scale score for Oceanic Boundlessness (OB) including derealisation and depersonalisation phenomena at a trend level (p=0.062), whereas ergotamine did not show any effects on the psilocybin-induced 5D-ASC main scale scores. The present finding demonstrates that buspirone exerts inhibitory effects on psilocybin-induced effects, presumably via 5-HT1A receptor activation, an interaction between 5-HT1A and 5-HT2A receptors, or both. The data suggest that the modulation of 5-HT1A receptor activity may be a useful target in the treatment of visual hallucinations in different psychiatric and neurological diseases.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

The conformational space of the neurotransmitter serotonin: how the rotation of a hydroxyl group changes all

Serotonin shows a conformer-dependent competition of two polar groups to establish a hydrogen bond with the same H-atom.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Identification of Glycyrrhiza as the rikkunshito constituent with the highest antagonistic potential on heterologously expressed 5-HT3A receptors due to the action of flavonoids

The traditional Japanese phytomedicine rikkunshito is traditionally used for the treatment of gastrointestinal motility disorders, cachexia and nausea. These effects indicate 5-HT₃ receptor antagonism, due to the involvement of these receptors in such pathophysiological processes. E.g., setrons, specific 5-HT₃ receptor antagonists are the strongest antiemetics, developed so far. Therefore, the antagonistic effects of the eight rikkunshito constituents at heterologously expressed 5-HT_3Areceptors were analyzed using the two-electrode voltage-clamp technique. The results indicate that tinctures from Aurantii, Ginseng, Zingiberis, Atractylodis and Glycyrrhiza inhibited the 5-HT_3A receptor response, whereas the tinctures of Poria cocos, Jujubae and Pinellia exhibited no effect. Surprisingly, the strongest antagonism was found for Glycyrrhiza, whereas the Zingiberis tincture, which is considered to be primarily responsible for the effect of rikkunshito, exhibited the weakest antagonism of 5-HT_3A receptors. Rikkunshito contains various vanilloids, ginsenosides and flavonoids, a portion of which show an antagonistic effect on 5-HT₃ receptors. A screening of the established ingredients of the active rikkunshito constituents and related substances lead to the identification of new antagonists within the class of flavonoids. The flavonoids (-)-liquiritigenin, glabridin and licochalcone A from Glycyrrhiza species were found to be the most effective inhibitors of the 5-HT-induced currents in the screening. The flavonoids (-)-liquiritigenin and hesperetin from Aurantii inhibited the receptor response in a non-competitive manner, whereas glabridin and licochalcone A exhibited a potential competitive antagonism. Furthermore, licochalcone A acts as a partial antagonist of 5-HT_3A receptors. Thus, this study reveals new 5-HT_3A receptor antagonists with the aid of increasing the comprehension of the complex effects of rikkunshito.

5-hydroxytryptamine medications for the treatment of obesity

The central 5-hydroxytryptamine (5-HT; serotonin) system represents a fundamental component of the brain's control of energy homeostasis. Medications targeting the 5-HT pathway have been at the forefront of obesity treatment for the past 15 years. Pharmacological agents targeting 5-HT receptors (5-HTR), in combination with genetic models of 5-HTR manipulation, have uncovered a role for specific 5-HTRs in energy balance and reveal the 5-HT2 C R as the principal 5-HTR mediating this homeostatic process. Capitalising on this neurophysiological machinery, 5-HT2 C R agonists improve obesity and glycaemic control in patient populations. The underlying therapeutic mechanism has been probed using model systems and appears to be achieved primarily through 5-HT2 C R modulation of the brain melanocortin circuit via activation of pro-opiomelanocortin neurones signalling at melanocortin4 receptors. Thus, 5-HT2 C R agonists offer a means to improve obesity and type 2 diabetes, which are conditions that now represent global challenges to human health.

Distribution of serotonin 5-HT1A-binding sites in the brainstem and the hypothalamus, and their roles in 5-HT-induced sleep and ingestive behaviors in rock pigeons (Columba livia)

Serotonin 1A receptors (5-HT1ARs), which are widely distributed in the mammalian brain, participate in cognitive and emotional functions. In birds, 5-HT1ARs are expressed in prosencephalic areas involved in visual and cognitive functions. Diverse evidence supports 5-HT1AR-mediated 5-HT-induced ingestive and sleep behaviors in birds. Here, we describe the distribution of 5-HT1ARs in the hypothalamus and brainstem of birds, analyze their potential roles in sleep and ingestive behaviors, and attempt to determine the involvement of auto-/hetero-5-HT1ARs in these behaviors. In 6 pigeons, the anatomical distribution of [(3)H]8-OH-DPAT binding in the rostral brainstem and hypothalamus was examined. Ingestive/sleep behaviors were recorded (1h) in 16 pigeons pretreated with MM77 (a heterosynaptic 5-HT1AR antagonist; 23 or 69 nmol) for 20 min, followed by intracerebroventricular ICV injection of 5-HT (N:8; 150 nmol), 8-OH-DPAT (DPAT, a 5-HT1A,7R agonist, 30 nmol N:8) or vehicle. 5-HT- and DPAT-induced sleep and ingestive behaviors, brainstem 5-HT neuronal density and brain 5-HT content were examined in 12 pigeons, pretreated by ICV with the 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (N:6/group). The distribution of brainstem and diencephalic c-Fos immunoreactivity after ICV injection of 5-HT, DPAT or vehicle (N:5/group) into birds provided with or denied access to water is also described. 5-HT1ARs are concentrated in the brainstem 5-HTergic areas and throughout the periventricular hypothalamus, preoptic nuclei and circumventricular organs. 5-HT and DPAT produced a complex c-Fos expression pattern in the 5-HT1AR-enriched preoptic hypothalamus and the circumventricular organs, which are related to drinking and sleep regulation, but modestly affected c-Fos expression in 5-HTergic neurons. The 5-HT-induced ingestivebehaviors and the 5-HT- and DPAT-induced sleep behaviors were reduced by MM77 pretreatment. 5,7-DHT increased sleep per se, decreased tryptophan hydroxylase expression in the raphe nuclei and decreased prosencephalic 5-HT release but failed to affect 5-HT- or DPAT-induced drinking or sleep behavior. 5-HT- and DPAT-induced ingestive and sleep behaviors in pigeons appear to be mediated by heterosynaptic and/or non-somatodendritic presynaptic 5-HT1ARs localized to periventricular diencephalic circuits.

Influence of Altered Hormonal Status on Platelet 5-HT and MAO-B Activity in Cigarette Smokers

The present study was designed to understand the cigarette smoking-induced alterations in hormones and the resulting changes in platelet serotonin (5-hydroxytryptamine, 5-HT) and monoamine oxidase (MAO-B) activity in chronic smokers. Human male volunteers aged 35 ± 8 years, were divided into two groups, namely controls and smokers (12 ± 2 cigarettes per day for 7-10 years). Results showed that cigarette smoking significantly (p < 0.05) elevated plasma triiodothyronine (T3), cortisol and testosterone levels with significant (p < 0.05) reduction in plasma tryptophan and thyroxin (T4). Moreover, smokers showed reduced platelet 5-HT levels and MAO-B activity. In smokers, plasma cortisol was negatively correlated with tryptophan (r = -0.386), platelet MAO-B (r = -0.264), and 5-HT (r = -0.671), and positively correlated with testosterone (r = 0.428). However, testosterone was negatively correlated with platelet MAO-B (r = -0.315), and 5-HT (r = -.419) in smokers. Further, smokers plasma T3 levels were negatively correlated with platelet MAO-B (r = -0.398), and 5-HT (r = -0.541), whereas T4 levels were positively correlated with platelet MAO-B (r = 0.369), and 5-HT (r = 0.454). In conclusion, our study showed that altered testosterone and cortisol levels may aggravate behavior, mood disturbances and symptoms of depression by decreasing platelet 5-HT and MAO-B activity in smokers.

BF-1--a novel selective 5-HT2B receptor antagonist blocking neurogenic dural plasma protein extravasation in guinea pigs

Serotonin 5-HT2B receptor antagonists have been proposed as migraine prophylactic drugs, but previously available 5-HT2B receptor antagonists displayed multiple monoaminergic side effects and had to be withdrawn from the market. Here, we set out to identify a novel antagonist with high affinity and selectivity towards 5-HT2B receptors. To test the affinity of new compounds towards various receptors, we generated a broad series of cells functionally coupling human monoaminergic receptors to luciferase. Using the cell lines we revealed pimethixene (1-methyl-4-(9H-thioxanthen-9-ylidene)piperidine) as highly potent, albeit non-selective 5-HT2B receptor antagonist and optimized its chemical structure to create highly potent and selective 5-HT2B receptor antagonists. We selected the methoxythioxanthene BF-1 for further analysis. In comparison to pimethixene, it lacked high affinities to 5-HT1A, 5-HT2A, 5-HT2C, histamine H1, dopamine D1 and D2 as well as muscarinic M1 and M2 receptors. BF-1 was tested as potential migraine prophylactic drug by blocking meta-chlorophenylpiperazine, (mCPP) or BW723C86 (5-((thiophen-2-yl)methoxy)-α-methyltryptamine) induced neurogenic dural plasma protein extravasation in a guinea pig model that may resemble a migraine attack. BF-1 was significantly more potent in this assay compared to the well know non-selective 5-HT2B antagonists, methysergide ((6aR,9R)-N-[(2S)-1-Hydroxybutan-2-yl]-4,7-dimethyl-6,6a,8,9-tetrahydroindolo[4,3-fg]quinoline-9-carboxamide) or pizotifen (4-(1-methyl-4-piperidylidine)-9,10-dihydro-4H-benzo-[4,5]cyclohepta[1,2]-thiophene). Therefore, we propose BF-1 as a new compound that may be developed for prophylactic migraine treatment without the typical monoaminergic side effects.

Interplay between serotonin 5-HT1A and 5-HT7 receptors in depressive disorders

Serotonin (5-hydroxytryptamine or 5-HT) is an important neurotransmitter regulating a wide range of physiological and pathological functions via activation of heterogeneously expressed 5-HT receptors. Besides the important role of 5-HT receptors in the pathogenesis of depressive disorders and in their clinical medications, underlying mechanisms are far from being completely understood. This review focuses on possible cross talk between two serotonin receptors, 5-HT1A and the 5-HT7 . Although these receptors are highly co-expressed in brain regions implicated in depression, and most agonists developed for the 5-HT1A or 5-HT7 receptors have cross-reactivity, their functional interaction has not been yet established. It has been recently shown that 5-HT1A and 5-HT7 receptors form homo- and heterodimers both in vitro and in vivo. From the functional point of view, heterodimerization has been shown to play an important role in regulation of receptor-mediated signaling and internalization, suggesting the implication of heterodimerization in the development and maintenance of depression. Interaction between these receptors is also of clinical interest, because both receptors represent an important pharmacological target for the treatment of depression and anxiety.

Valvular Heart Disease in Pergolide-Treated Parkinson's Disease

BACKGROUND

A 61-year-old woman with Parkinson's disease, receiving pergolide 1.75 mg four times daily, was admitted with progressive dyspnea.

METHODS

Investigations revealed mitral and aortic regurgitation. She underwent surgical mitral replacement and aortic repair, but had a post-operative course characterized by repeated bouts of congestive heart failure.

RESULTS

Severe tricuspid valve (TV) regurgitation developed within one month after the TV was reported on echocardiography to be relatively normal. Subsequent discontinuation of pergolide was associated with symptomatic improvement.

CONCLUSIONS

This case illustrates the severity and rapidity with which cardiac valvular abnormalities can develop in patients receiving pergolide.