The effects of photic and nonphotic stimuli in the 5-HT7 receptor knockout mouse

β-arrestin biased signaling is not involved in the hypotensive actions of 5-HT7 receptor stimulation: use of Serodolin

The 5-hydroxytryptamine 7 receptor (5-HT7) is necessary for 5-HT to cause a concentration-dependent vascular relaxation and hypotension. 5-HT7 is recognized as having biased signaling, transduced through either Gs or β -arrestin. It is unknown whether 5-HT7 signals in a biased manner to cause vasorelaxation/hypotension. We used the recently described β-arrestin selective 5-HT7 receptor agonist serodolin to test the hypothesis that 5-HT7 activation does not cause vascular relaxation or hypotension via the β -arrestin pathway. Isolated abdominal aorta (no functional 5-HT7) and vena cava (functional 5-HT7) from male Sprague Dawley rats were used in isometric contractility studies. Serodolin (1 nM - 10 μM) did not change baseline tone of isolated tissues and did not relax the endothelin-1 (ET-1)-contracted vena cava or aorta. In the aorta, serodolin acted as a 5-HT2A receptor antagonist, evidenced by a rightward shift in 5-HT-induced concentration response curve [pEC50 5-HT [M]: Veh = 5.2 +/- 0.15; Ser (100 nM) = 4.49 +/- 0.08; p < 0.05]. In the vena cava, serodolin acted as a 5-HT7 receptor antagonist, shifting the concentration response curve to 5-HT left and upward (%10 μM NE contraction; Veh = 3.2 +/- 1.7; Ser (10 nM) = 58 +/- 11; p < 0.05) and blocking relaxation of pre-contracted tissue to the 5-HT1A/7 agonist 5-carboxamidotryptamine. In anesthetized rats, 5-HT or serodolin was infused at 5, 25 and 75 μg/kg/min, iv. Though 5-HT caused concentration-dependent depressor responses, serodolin caused an insignificant small depressor responses at all three infusion rates. With the final dose of serodolin on board, 5-HT was unable to reduce blood pressure. Collectively the data indicate that serodolin functions as a 5-HT7 antagonist with additional 5-HT2A blocking properties. 5-HT7 activation does not cause vascular relaxation or hypotension via the β -arrestin pathway.

Day time-restricted feeding shows differential synchronizing effects on age-related changes of serotonin metabolism in SCN and the pineal gland in male Wistar rats

The circadian timing system is synchronized by the environmental photic and non-photic signals. Light is the major cue that entrains the master circadian oscillator located in suprachiasmatic nucleus (SCN). With aging condition ocular light impairs because of the age-related deficiencies in the eye as a result the clock becomes less sensitive to light. In such case non-photic cues may play a major role in synchronizing the clock. Earlier studies have linked altered meal timings to induce many physiological changes including serotonin in different brain regions such as hypothalamus, brain stem and striatum. Much is not known about the effect of timed food restriction as a non-photic stimulus on serotonergic system in SCN under aging condition. We report here the synchronizing effects of time-restricted feeding (TRF) as a non-photic stimulus on serotonin and its related metabolites in the SCN and pineal gland of male Wistar rats upon aging. Under food restriction daily rhythmicity of serotonin 5-HT and 5-HTOH was abolished whereas NAS, 5-MIAA and NAT showed a significant decrease in their daily pulses upon food restriction in 3 months (m) old rats. Under forced day time feeding schedule the mean 24 h levels of serotonin have significantly decreased in 12 and 24 m old animals in SCN and pineal gland. Most of the serotonin metabolites in the SCN and pineal gland of 12 and 24 m old ad libitum fed group rats have shown rhythmicity. 5-HT, NAS, MEL and NAT have shown daily rhythm in the SCN of 12 and 24 m old rats whereas 5-MIAA and 5-MTOH did not show daily rhythm in both the age groups. The mean 24 h levels of 5-HTP, 5-HIAA, 5-MIAA, 5-MTOH, MEL and NAT were increased in the pineal gland of 12 and 24 months old rats. This work help demonstrate the role of TRF in synchronising age induced desynchronization in serotonin metabolome.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Towards selective CNS PET imaging of the 5-HT7 receptor system: Past, present and future

Since its discovery in 1993, the serotonin receptor subtype 7 (5-HT₇) has attracted significant attention as a potential drug target; due to its elucidated roles in conditions such as insomnia, schizophrenia, and more. Therefore, it is unsurprising that there has been relatively early efforts undertaken to develop a positron emission tomography (PET) imaging agent for said receptor system. PET can be clinically used to probe receptor systems in vivo, permitting information such as a drug's occupancy against this system to be investigated. This review focuses on the efforts towards the development of a 5-HT₇R selective PET CNS tracer over the last 20 years, critically reflecting on applied strategies and commonly employed chemical frameworks and suggests future considerations that are needed to successfully develop a PET tracer for this clinically relevant target. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Creation of the 5-hydroxytryptamine receptor 7 knockout rat as a tool for cardiovascular research

Using CRISPR-Cas9 technology, we created a 5-HT7 receptor global knockout (KO) rat, on a Sprague-Dawley background, for use in cardiovascular physiology studies focused on blood pressure regulation. A stable line carrying indels in exons 1 and 2 of the rat Htr7 locus was established and validated. Surprisingly, 5-HT7 receptor mRNA was still present in the KO rat. However, extensive cDNA and genomic sequencing of KO tissues confirmed an 11 bp deletion in exon 1 and 4 bp deletion in exon 2. The exon 1 deletion resulted in a frameshifted mRNA sequence coding for a nonfunctional protein. While the Htr1B locus was a potential off-target for the guide RNAs designed for exon 2 of Htr7, there were no off-target sequence changes at this locus in the originating founder. When the F2 generation of KO was compared with wild-type (WT) counterparts, neither the male nor female KO rats were different in body size, fat weights, or mass of organs (kidney, heart, and brain) important to blood pressure. Females were smaller in mass than their counterpart males. Clinical measures of plasma from nonfasted rats revealed largely similar values, comparing WT and KO, of glucose, blood urea nitrogen, creatinine, phosphate, calcium, and albumin to name a few. Loss of a functional 5-HT7 receptor was validated by the complete loss of relaxation to the 5-HT1/7 receptor agonist 5-carboxamidotryptamine in the isolated abdominal vena cava. This newly created 5-HT7 receptor KO rat will be of use to investigate the importance of the 5-HT7 receptor in blood pressure regulation.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

Chronic BMY7378 treatment alters behavioral circadian rhythms

The mammalian circadian clock is synchronized to the day : night cycle by light. Serotonin modulates the circadian effects of light, with agonists inhibiting response to light and antagonists enhancing responses to light. A special class of serotonergic compounds, the mixed 5-HT_1A agonist/antagonists, potentiates light-induced phase advances by up to 400% when administered acutely. In this study, we examine the effects of one of these mixed 5-HT_1A agonist/antagonists, BMY7378, when administered chronically. Thirty adult male hamsters were administered either vehicle or BMY7378 via surgically implanted osmotic mini pumps over a period of 28 days. In a light : dark cycle, chronic BMY7378 advanced the phase angle of entrainment, prolonged the duration of the active phase and attenuated the amplitude of the wheel-running rhythm during the early night. In constant darkness, chronic treatment with BMY7378 significantly attenuated light-induced phase advances, but had no significant effect on light-induced phase delays. Non-photic phase shifts to daytime administration of a 5-HT_1A/7 agonist were also attenuated by chronic BMY7378 treatment. qRT-PCR analysis revealed that chronic BMY7378 treatment upregulated mRNA for 5-HT_1A and 5-HT_1B receptors in the hypothalamus and downregulated mRNA for 5-HT_1A and monoamine oxidase-A in the brainstem. These results highlight adaptive changes of serotonin receptors in the brain to chronic treatment with BMY7378 and link such up- and downregulation to changes in important circadian parameters. Such long-term changes to the circadian system should be considered when patients are treated chronically with drugs that alter serotonergic function.

Circadian behavior of adult mice exposed to stress and fluoxetine during development

INTRODUCTION

Women of child-bearing age are the population at greatest risk for depression. The stress experienced during pregnancy and the associated antidepressant treatments can both affect fetal development. Fluoxetine (FLX) is among the most common antidepressants used by pregnant women. We have previously demonstrated that perinatal exposure to FLX can alter expression of circadian rhythms in adulthood. Here, we examine the combined effects of maternal stress during pregnancy and perinatal exposure to the antidepressant FLX on the circadian behavior of mice as adults.

METHODS

Mouse dams were exposed to chronic unpredictable stress (embryonic (E) day 7 to E18), FLX (E15 to postnatal day 12), a combination of both stress and FLX, or were left untreated. At 2 months of age, male offspring were placed in recording chambers and circadian organization of wheel running rhythms and phase shifts to photic and non-photic stimuli were assessed.

RESULTS

Mice exposed to prenatal stress (PS) had smaller light-induced phase delays. Mice exposed to perinatal FLX required more days to re-entrainment to an 8-h phase advance of their light-dark cycle. Mice subjected to either perinatal FLX or to PS had larger light-induced phase advances and smaller phase advances to 8-OH-DPAT. FLX treatment partially reversed the effect of PS on phase shifts to late-night light exposure and to 8-OH-DPAT.

CONCLUSIONS

Our results suggest that, in mice, perinatal exposure to either FLX, or PS, or their combination, leads to discernible, persistent changes in their circadian systems as adults.

Selective pharmacological blockade of the 5-HT7 receptor attenuates light and 8-OH-DPAT induced phase shifts of mouse circadian wheel running activity

Recent reports have illustrated a reciprocal relationship between circadian rhythm disruption and mood disorders. The 5-HT7 receptor may provide a crucial link between the two sides of this equation since the receptor plays a critical role in sleep, depression, and circadian rhythm regulation. To further define the role of the 5-HT7 receptor as a potential pharmacotherapy to correct circadian rhythm disruptions, the current study utilized the selective 5-HT7 antagonist JNJ-18038683 (10 mg/kg) in three different circadian paradigms. While JNJ-18038683 was ineffective at phase shifting the onset of wheel running activity in mice when administered at different circadian time (CT) points across the circadian cycle, pretreatment with JNJ-18038683 blocked non-photic phase advance (CT6) induced by the 5-HT1A/7 receptor agonist 8-OH-DPAT (3 mg/kg). Since light induced phase shifts in mammals are partially mediated via the modulation of the serotonergic system, we determined if JNJ-18038683 altered phase shifts induced by a light pulse at times known to phase delay (CT15) or advance (CT22) wheel running activity in free running mice. Light exposure resulted in a robust shift in the onset of activity in vehicle treated animals at both times tested. Administration of JNJ-18038683 significantly attenuated the light induced phase delay and completely blocked the phase advance. The current study demonstrates that pharmacological blockade of the 5-HT7 receptor by JNJ-18038683 blunts both non-photic and photic phase shifts of circadian wheel running activity in mice. These findings highlight the importance of the 5-HT7 receptor in modulating circadian rhythms. Due to the opposite modulating effects of light resetting between diurnal and nocturnal species, pharmacotherapy targeting the 5-HT7 receptor in conjunction with bright light therapy may prove therapeutically beneficial by correcting the desynchronization of internal rhythms observed in depressed individuals.

Targeting the Serotonin 5-HT7 Receptor in the Search for Treatments for CNS Disorders: Rationale and Progress to Date

The 5-HT7 (5-hydroxytryptamine 7, serotonin 7) receptor is one of the most recently identified members of the serotonin receptor family. Pharmacological tools, including selective antagonists and, more recently, agonists, along with 5-HT7 receptor (5-HT7R) knock-out mice have revealed the involvement of this receptor in central nervous system processes. Its well-established role in controlling body temperature and regulating sleep and circadian rhythms has implicated this receptor in mood disorders. Thus, the 5-HT7R has gained much attention as a possible target for the treatment of depression. Although preclinical data support the antidepressant-like actions of 5-HT7R antagonists, their clinical efficacy has not been yet established. Other evidence has implicated the 5-HT7R in learning and memory. Preclinical findings suggest that blockade of this receptor may be beneficial against schizophrenia-like cognitive deficits. Other possible indications include nociception, epilepsy, migraine, autism spectrum disorders, and Rett Syndrome. However, the question is whether the beneficial effects may be achieved by activation or blockade of 5-HT7Rs. Hence, this review briefly summarises the recent findings on the role of 5-HT7Rs and their ligands in CNS disorders.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

Serotonin-2C receptor involved serotonin-induced Ca²⁺ mobilisations in neuronal progenitors and neurons in rat suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN), the central circadian pacemaker in mammals, undergoes serotonergic regulation, but the underlying mechanisms remain obscure. Here, we generated a subclone of an SCN progenitor cell line expressing Ca(2+) sensors (SCN2.2YC) and compared its 5-HT receptor signalling with that of rat SCN neurons in brain slices. SCN2.2YC cells expressed 5-HT1A/2A/2B/2C, but not 5A/7, while all six subtypes were expressed in SCN tissues. High K(+) or 5-HT increased cytosolic Ca(2+) in SCN2.2YC cells. The 5-HT responses were inhibited by ritanserin and SB-221284, but resistant to WAY-100635 and RS-127445, suggesting predominant involvement of 5-HT2C for Ca(2+) mobilisations. Consistently, Ca(2+) imaging and voltage-clamp electrophysiology using rat SCN slices demonstrated post-synaptic 5-HT2C expression. Because 5-HT2C expression was postnatally increased in the SCN and 5-HT-induced Ca(2+) mobilisations were amplified in differentiated SCN2.2YC cells and developed SCN neurons, we suggest that this signalling development occurs in accordance with central clock maturations.

Photic and nonphotic responses of the circadian clock in serotonin-deficient Pet-1 knockout mice

The neurotransmitter serotonin plays an important role in the regulation of the circadian clock. To gain further insight into the mechanisms by which serotonin regulates rhythmicity, the authors investigated photic and nonphotic effects on the circadian clock in Pet-1 knockout mice. In these mice, the serotonergic system suffers a developmental loss of 70% of serotonin neurons, with the remaining neurons being deficient in serotonergic function as well. Pet-1 knockout mice show significantly decreased phase delays of the circadian clock in response to light pulses in the early night; however, this difference was not reflected in a difference in the expression of Fos protein in the suprachiasmatic nucleus. There were no genotypic differences detected in the phase-shifting response to injection of the 5-HT1A/7 (serotonin 1A and 7) agonist 8-OH-DPAT ((±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide); however, there were small but significant differences in the phase-shifting responses to cages between genotypes and sexes. Several different patterns of wheel-running activity were observed in knockout mice that differed from those in wild-type mice, suggesting that normal serotonergic function is necessary for the proper consolidation of nocturnal activity. Overall, these data are consistent with other pharmacological and genetic studies demonstrating a significant role for serotonin in circadian clock function.

The effects of perinatal fluoxetine treatment on the circadian system of the adult mouse

RATIONAL

Depression is prevalent among women of childbearing age and is frequently treated with selective serotonin reuptake inhibitors (SSRIs). As some SSRIs, such as fluoxetine (Flx), can cross the placenta, it is possible that the neurodevelopment of the fetus may be affected, leading to altered behavior in adulthood.

OBJECTIVES

In this study, we examined the effects of perinatal Flx exposure on the subsequent expression of circadian rhythms in adult mice.

METHODS

Dams were treated with 25 mg/kg/day Flx in their drinking water from embryonic day 15 to postnatal day 12. Circadian organization of wheel running rhythms and phase shifts to photic and non-photic stimuli were assessed in the offspring starting at 6 weeks of age.

RESULTS

We found that perinatal Flx exposure led to larger light-induced phase advances (1.19 ± 0.51 vs. 0.55 ± 0.25 h), smaller phase advances to the serotonin agonist 8-OH-DPAT during the mid-subjective day (0.44 ± 0.15 vs. 0.70 ± 0.17 h), and a shorter free-running period in constant darkness (23.47 ± 0.13 vs. 23.64 ± 0.13 h).

CONCLUSIONS

These results suggest that perinatal exposure to SSRIs may have consequences for the functioning of the circadian system later in life.

Circadian entrainment and its role in depression: a mechanistic review

The natural rotation of the earth generates an environmental day-night cycle that repeats every 24 h. This daily transition from dawn to dusk provides one of the most important time cues to which the majority of organisms synchronise their activity. Under these conditions, natural light, a photic stimulus, provides the principal entraining cue. In mammals, an endogenous circadian pacemaker located within the suprachiasmatic nucleus (SCN) of the hypothalamus acts as a coordinating centre to align physiological activity with the environmental light-dark cycle. However, the SCN also receives regulatory input from a number of behavioural, non-photic, cues such as physical activity, social interactions and feeding routines. The unique ability of the SCN to integrate both photic and non-photic cues allows it to generate a rhythm that is tailored to the individual and entrained to the environment. Here, we review the key neurotransmitter systems involved in both photic and non-photic transmission to the SCN and their interactions that assist in generating an entrained output rhythm. We also consider the impact on health of a desynchronised circadian system with a focus on depressive affective disorders and current therapies aimed at manipulating the relationship between photic and non-photic SCN regulators.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Role of the 5-HT7 receptor in the central nervous system: from current status to future perspectives

Pharmacological and genetic tools targeting the 5-hydroxytryptamine (5-HT)7 receptor in preclinical animal models have implicated this receptor in diverse (patho)physiological processes of the central nervous system (CNS). Some data obtained with 5-HT7 receptor knockout mice, selective antagonists, and, to a lesser extent, agonists, however, are quite contradictory. In this review, we not only discuss in detail the role of the 5-HT7 receptor in the CNS but also propose some hypothetical models, which could explain the observed inconsistencies. These models are based on two novel concepts within the field of G protein-coupled receptors (GPCR), namely biphasic signaling and G protein-independent signaling, which both have been shown to be mediated by GPCR dimerization. This led us to suggest that the 5-HT7 receptor could reside in different dimeric contexts and initiate different signaling pathways, depending on the neuronal circuitry and/or brain region. In conclusion, we highlight GPCR dimerization and G protein-independent signaling as two promising future directions in 5-HT7 receptor research, which ultimately might lead to the development of more efficient dimer- and/or pathway-specific therapeutics.

Serotonin 5-HT7 receptor agents: Structure-activity relationships and potential therapeutic applications in central nervous system disorders

Since its discovery in the 1940s in serum, the mammalian intestinal mucosa, and in the central nervous system, serotonin (5-HT) has been shown to be involved in virtually all cognitive and behavioral human functions, and alterations in its neurochemistry have been implicated in the etiology of a plethora of neuropsychiatric disorders. The cloning of 5-HT receptor subtypes has been of importance in enabling them to be classified as specific protein molecules encoded by specific genes. The 5-HT(7) receptor is the most recently classified member of the serotonin receptor family. Since its identification, it has been the subject of intense research efforts driven by its presence in functionally relevant regions of the brain. The availability of some selective antagonists and agonists, in combination with genetically modified mice lacking the 5-HT(7) receptor, has allowed for a better understanding of the pathophysiological role of this receptor. This paper reviews data on localization and pharmacological properties of the 5-HT(7) receptor, and summarizes the results of structure-activity relationship studies aimed at the discovery of selective 5-HT(7) receptor ligands. Additionally, an overview of the potential therapeutic applications of 5-HT(7) receptor agonists and antagonists in central nervous system disorders is presented.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

5-HT(7) receptor modulators: a medicinal chemistry survey of recent patent literature (2004 - 2009)

IMPORTANCE OF THE FIELD

The 5-HT(7) receptors are discretely localized within the CNS (thalamus, hypothalamus, limbic and cortical regions). The 5-HT(7) receptors are also present in smooth muscle cells from blood vessels and have been reported in gastrointestinal tract as well as in rat lumbar dorsal root and sympathetic ganglia. The 5-HT(7) receptors have been implicated in depression, disorders related to circadian rhythms, pain and migraine. Thus, there is a great interest in developing potent and selective 5-HT(7) receptor modulators.

AREAS COVERED IN THIS REVIEW

This review article highlights the research advances published in the patent literature between January 2004 and December 2009, giving emphasis to the medicinal chemist's standpoint.

WHAT THE READER WILL GAIN

Readers will rapidly gain an overview of the various 5-HT(7) receptor modulators reported in the patent literature in the past 6 years. Furthermore, the readers will learn which structure type can interact with 5-HT(7) receptor and also the different companies that are the main players in the field.

TAKE HOME MESSAGE

Although no 5-HT(7) modulator has entered clinical trials, the development and future use of different agonists and antagonists suitable for use in vivo seem very promising.

Differential influence of selective 5-HT5A vs 5-HT1A, 5-HT1B, or 5-HT2C receptor blockade upon light-induced phase shifts in circadian activity rhythms: interaction studies with citalopram

Though serotonergic mechanisms modulate circadian rhythms, roles of individual serotonin (5-HT) receptors remain uncertain since data are lacking for antagonists. Herein, both the 5-HT(5A) receptor antagonist, A843277 (10 mg/kg), and the 5-HT(1B) antagonist, SB224289 (1 mg/kg), inhibited light-induced phase advances in hamster circadian wheel-running rhythms. Conversely, though 5-HT(1A) and 5-HT(7) receptors are likewise implicated in circadian scheduling, their blockade by WAY100635 (0.5 mg/kg) and SB269970 (1 mg/kg), respectively, was ineffective. Since actions of 5-HT reuptake inhibitors are modified by antagonists, we evaluated their influence on suppression of phase advances by citalopram (10 mg/kg). Its action was potentiated by WAY100635 and the 5-HT(2C) antagonist, SB242084 (1 mg/kg), but not by A842377, SB224289, SB269970, and antagonists at 5-HT(2A) (MDL100907) and 5-HT(6) (SB399885) receptors. In conclusion, this is the first in vivo evidence for an influence of 5-HT(5A) receptors upon circadian rhythms, but no single class of 5-HT receptor mediates their control by citalopram.

New light on the serotonergic paradox in the rat circadian system

The main mammalian circadian clock, localized in the suprachiasmatic nuclei can be synchronized not only with light, but also with serotonergic activation. Serotonergic agonists and serotonin reuptake inhibitors (e.g., fluoxetine) have a non-photic influence (shifting effects during daytime and attenuation of photic resetting during nighttime) on hamsters' and mice' main clock. Surprisingly, in rats serotonergic modulation of the clock shows essentially photic-like features in vivo (shifting effects during nighttime). To delineate this apparent paradox, we analyzed the effects of fluoxetine and serotonin agonists on rats' clock. First, fluoxetine induced behavioral phase-advances associated with down-regulated expression of the clock genes Per1 and Rorbeta and up-regulated expression of Rev-erbalpha during daytime. Moreover, fluoxetine produced an attenuation of light-induced phase-advances in association with altered expression of Per1, Per2 and Rorbeta during nighttime. Second, we showed that 5-HT(1A) receptors -maybe with co-activation of 5-HT(7) receptors- were implicated in non-photic effects on the main clock. By contrast, 5-HT(3) and 5-HT(2C) receptors were involved in photic-like effects and, for 5-HT(2C) subtype only, in potentiation of photic resetting. Thus this study demonstrates that as for other nocturnal rodents, a global activation of the serotonergic system induces non-photic effects in the rats' clock during daytime and nighttime.

Altered photic and non-photic phase shifts in 5-HT(1A) receptor knockout mice

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) is thought to be modulated by 5-HT. 5-HT is though to inhibit photic phase shifts by inhibiting the release of glutamate from retinal terminals, as well as by decreasing the responsiveness of retinorecipient cells in the SCN. Furthermore, there is also evidence that 5-HT may underlie, in part, non-photic phase shifts of the circadian system. Understanding the mechanism by which 5-HT accomplishes these goals is complicated by the wide variety of 5-HT receptors found in the SCN, the heterogeneous organization of both the circadian clock and the location of 5-HT receptors, and by a lack of sufficiently selective pharmacological agents for the 5-HT receptors of interest. Genetically modified animals engineered to lack a specific 5-HT receptor present an alternative avenue of investigation to understand how 5-HT regulates the circadian system. Here we examine behavioral and molecular responses to both photic and non-photic stimuli in mice lacking the 5-HT(1A) receptor. When compared with wild-type controls, these mice exhibit larger phase advances to a short late-night light pulse and larger delays to long 12 h light pulses that span the whole subjective night. Fos and mPer1 expression in the retinorecipient SCN is significantly attenuated following late-night light pulses in the 5-HT(1A) knockout animals. Finally, non-photic phase shifts to (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) are lost in the knockout animals, while attenuation of the phase shift to the long light pulse due to rebound activity following a wheel lock is unaffected. These findings suggest that the 5-HT(1A) receptor plays an inhibitory role in behavioral phase shifts, a facilitatory role in light-induced gene expression, a necessary role in phase shifts to 8-OH-DPAT, and is not necessary for activity-induced phase advances that oppose photic phase shifts to long light pulses.