Serotonin 1A, 1B, and 7 receptors of the rat medial nucleus accumbens differentially regulate feeding, water intake, and locomotor activity

Lateral habenula 5-HT1B receptors are involved in regulation of anxiety-like behaviors in parkinsonian rats

Although the output of the lateral habenula (LHb) controls the activity of midbrain dopaminergic and serotonergic systems, which are implicated in the pathophysiology of anxiety, it is not clear the role of LHb 5-HT1B receptors in regulation of anxiety-like behaviors, particularly in Parkinson's disease-related anxiety. In this study, unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta in rats induced anxiety-like behaviors, led to decreased normalized δ power and increased normalized θ power in the LHb, and decreased dopamine (DA) level in the prelimbic cortex (PrL) compared with sham rats. Down-regulation of LHb 5-HT1B receptors by RNA interference produced anxiety-like effects, decreased normalized δ power and increased normalized θ power in the LHb in both sham and lesioned rats. Further, intra-LHb injection of 5-HT1B receptor agonist CP93129 induced anxiolytic-like responses, increased normalized δ power and decreased normalized θ power in the LHb, and increased DA and serotonin (5-HT) release in the PrL; conversely, 5-HT1B receptor antagonist SB216641 produced anxiety-like effects, decreased normalized δ power and increased normalized θ power in the LHb, and decreased DA and 5-HT release in the PrL in sham and lesioned rats. Additionally, effects of CP93129 and SB216641 on the behaviors, normalized δ and θ power in the LHb, and DA and 5-HT release in the PrL were decreased in lesioned rats, which were consistent with down-regulation of LHb 5-HT1B receptors after DA depletion. Collectively, these findings suggest that 5-HT1B receptors in the LHb are involved in the regulation of anxiety-like behaviors.

The role of brain serotonin signaling in excessive alcohol consumption and withdrawal: A call for more research in females

Alcohol Use Disorder (AUD) is a leading cause of death and disability worldwide, but current treatments are insufficient in fully addressing the symptoms that often lead to relapses in alcohol consumption. The brain's serotonin system has been implicated in AUD for decades and is a major regulator of stress-related behaviors associated with increased alcohol consumption. This review will discuss the current literature on the association between neurobiological adaptations in serotonin systems and AUD in humans as well as the effectiveness of serotonin receptor manipulations on alcohol-related behaviors like consumption and withdrawal. We will further discuss how these findings in humans relate to findings in animal models, including a comparison of systemic pharmacological manipulations modulating alcohol consumption. We next provide a detailed overview of brain region-specific roles for serotonin and serotonin receptor signaling in alcohol-related behaviors in preclinical animal models, highlighting the complexity of forming a cohesive model of serotonin function in AUD and providing possible avenues for more effective therapeutic intervention. Throughout the review, we discuss what is known about sex differences in the sequelae of AUD and the role of serotonin in these sequelae. We stress a critical need for additional studies in women and female animals so that we may build a clearer path to elucidating sex-specific serotonergic mechanisms and develop better treatments.

5-HT1B receptors in the basolateral amygdaloid nucleus regulate anxiety-like behaviors through AC-PKA signal pathway in a rat model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is commonly accompanied with anxiety, multiple studies indicate that the basolateral amygdaloid nucleus (BLA) is closely related to modulation of anxiety and expresses serotonin_1B (5-HT_1B) receptors, however, effects of BLA 5-HT_1B receptors on anxiety-like behaviors are unclear, particularly in PD-related anxiety.

METHODS

The open-field and elevated plus maze tests were used to examine anxiety-like behaviors. In vivo electrophysiology and microdialysis were performed to observe the firing activity of BLA neurons and GABA, glutamate, dopamine (DA) and 5-HT release in the BLA, respectively. Western blotting was used to analyze protein expression of 5-HT_1B receptors, adenylate cyclase (AC) and phosphorylated protein kinase A at threonine 197 site (p-PKA-Thr197) in the BLA.

RESULTS

Intra-BLA injection of 5-HT_1B receptor agonist CP93129 produced anxiety-like effects and antagonist SB216641 induced anxiolytic-like responses in sham-operated and 6-hydroxydopamine-lesioned rats. Further, pretreatment with AC inhibitor SQ22536 and PKA inhibitor KT5720 blocked the behavioral effects of CP93129, respectively. Intra-BLA injection of CP93129 increased the firing rate of BLA glutamate neurons and decreased GABA/glutamate ratio and DA and 5-HT levels in the BLA of sham-operated and the lesioned rats, while SB216641 induced the opposite effects. Compared with sham-operated rats, effects of CP93129 and SB216641 on behaviors, electrophysiology and microdialysis were decreased in the lesioned rats, which were associated with decreased expression of 5-HT_1B receptors, AC and p-PKA-Thr197 in the BLA.

CONCLUSION

These findings suggest that 5-HT_1B receptor-AC-PKA signal pathway in the BLA is involved in the regulation of PD-related anxiety.

Stimulation of nucleus accumbens 5-HT6 receptors increases both appetitive and consummatory motivation in an effort-based choice task

Activity of the serotonin 6(5-HT6) receptor impacts food intake and body weight in animal models and has also shown potential as a target for treatment of anhedonia, a symptom of major depressive disorder. The nucleus accumbens (NAc) is a key region involved in motivational processes and has been implicated in the neural mechanisms underlying anhedonia. Here, we assessed the potential role that 5-HT6 receptors in the NAc play in regulating motivation towards food. Rats received surgical implantation of guide cannulas above the NAc shell. On testing days, they were injected with either the selective 5-HT6 agonist EMD 386088 (at 0.0, 1.0, and 4.0 mg/0.5 mL/side) or the 5-HT6 antagonist SB 252585 (at 0, 1.0, and 4.0 µg/0.5 µL/side) prior to completing a 1-h long effort-based choice task. The task simultaneously examined the impact of NAc 5-HT6 receptor manipulation on appetitive motivation, measured as the breakpoint for earning a preferred sugar pellet in a progressive ratio task, and consummatory motivation, quantified as the grams of freely-available rat chow consumed during the session. Stimulation of NAc 5-HT receptors significantly increased both appetitive and consummatory motivation as assessed in this effort-based choice task. In contrast, 5-HT6 antagonism did not affect break point nor the consumption of the freely-available chow. These data suggest that 5-HT6 receptors are functional within the NAc, that their stimulation increases motivated behavior, and that they may therefore be a viable target for the treatment of anhedonia and disorders that inhibit motivational processes.

Serotonergic control of feeding microstructure in Drosophila

To survive, animals maintain energy homeostasis by seeking out food. Compared to freely feeding animals, food-deprived animals may choose different strategies to balance both energy and nutrition demands, per the metabolic state of the animal. Serotonin mediates internal states, modifies existing neural circuits, and regulates animal feeding behavior, including in humans and fruit flies. However, an in-depth study on the neuromodulatory effects of serotonin on feeding microstructure has been held back for several technical reasons. Firstly, most feeding assays lack the precision of manipulating neuronal activity only when animals start feeding, which does not separate neuronal effects on feeding from foraging and locomotion. Secondly, despite the availability of optogenetic tools, feeding in adult fruit flies has primarily been studied using thermogenetic systems, which are confounded with heat. Thirdly, most feeding assays have used food intake as a measurement, which has a low temporal resolution to dissect feeding at the microstructure level. To circumvent these problems, we utilized OptoPAD assay, which provides the precision of optogenetics to control neural activity contingent on the ongoing feeding behavior. We show that manipulating the serotonin circuit optogenetically affects multiple feeding parameters state-dependently. Food-deprived flies with optogenetically activated and suppressed serotonin systems feed with shorter and longer sip durations and longer and shorter inter-sip intervals, respectively. We further show that serotonin suppresses and enhances feeding via 5-HT1B and 5-HT7 receptors, respectively.

Selective serotonin receptor stimulation of the ventral tegmentum differentially affects appetitive motivation for sugar on a progressive ratio schedule of reinforcement

Serotonin signaling influences satiety and motivation through known actions in the hindbrain and hypothalamus. Recently, we reported that some classes of serotonin receptors also modulate food intake through actions in the ventral tegmentum and the nucleus accumbens. In the current experiments, we examined whether activation or blockade of individual serotonin receptor subtypes in the ventral tegmentum might also affect appetitive motivation for sugar pellets as assessed in a progressive ratio (PR) task. Separate groups of rats were tested following stimulation or blockade of ventral tegmental serotonin 1A, 1B, 2A, 2B, 2C, or 3 receptors. Rats within each group received multiple doses of a single drug across days; each test was separated by 72 h. Progressive ratio break point was significantly affected by stimulation of ventral tegmental serotonin 1A receptors with 8-OH-DPAT (0, 2, 4, 8 μg/side) or stimulation of serotonin 3 receptors with mCPBG (0, 10, & 20 μg/side). High doses of both agents tended to decrease break point. Additionally, stimulation of serotonin 2C receptors with RO60-0175 (at 0, 2, and 5 μg/side) reduced total lever presses and demonstrated a trend towards reducing break point. There were no effects of stimulating ventral tegmental serotonin 1B, 2A, or 2B receptors on break point; neither did antagonism of any of the serotonin receptor subtypes significantly affect performance. These data provide additional evidence that serotonergic signaling in the mesolimbic pathway affects motivated behavior, and demonstrate that a subset of serotonin receptors impact not only food consumption, but appetitive food-seeking as well.

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.

Preclinical support for the therapeutic potential of zolmitriptan as a treatment for cocaine use disorders

Serotonin 1B receptor (5-HT_1BR) agonists enhance cocaine intake in rats during daily self-administration but attenuate cocaine intake after prolonged abstinence. Here we investigated whether the less selective but clinically available 5-HT_1D/1BR agonist, zolmitriptan, produces similar effects. Male and free-cycling female Sprague-Dawley rats were trained to lever press for cocaine (0.75 mg/kg, i.v.) or sucrose (45 mg pellet) reinforcement until performance rates stabilized. Rats then received zolmitriptan (3.0, 5.6, and 10 mg/kg, s.c.) prior to testing for its effects on response and reinforcement rates. Under cocaine testing conditions, rats had access to the training dose for the first hour followed by a lower cocaine dose (0.075 mg/kg, i.v.) for the second hour. Zolmitriptan decreased cocaine intake at both cocaine doses and in both sexes even without a period of abstinence and without altering sucrose intake. A separate group of rats underwent identical training procedures and were tested for effects of the selective 5-HT_1B and 5-HT_1D receptor antagonists, SB224289 (3.2, 5.6, and 10 mg/kg, s.c.) and BRL15572 (0.3, 1.0, and 3.0 mg/kg, i.p.), respectively, alone or in combination with zolmitriptan (5.6 mg/kg, s.c.) under identical cocaine testing procedures as above. The zolmitriptan-induced decrease in cocaine intake was reversed by SB224289 and to a lesser extent by BRL15572, suggesting that the effects of zolmitriptan involve both 5-HT_1B and 5-HT_1D receptors. Neither zolmitriptan, SB224289, or BRL15572 altered locomotor activity at the doses effective for modulating cocaine intake. These findings suggest that zolmitriptan has potential for repurposing as a treatment for cocaine use disorders.

Increasing serotonin bioavailability in preweaned dairy calves impacts hematology, growth, and behavior

Peripheral serotonin has been shown to regulate important physiological functions such as energy homeostasis and immunity, particularly in rodent and humans, but its role is poorly understood in livestock species. Herein, we tested the safety and effectiveness of increasing serotonin bioavailability in preweaned dairy calves by oral supplementation of a serotonin precursor (5-hydroxytryptophan, 5-HTP) or a serotonin reuptake inhibitor (fluoxetine, FLX). Bull Holstein calves (21 ± 2 d old; N = 24) were fed milk replacer (8 L/d) supplemented with either saline as control (CON, 8 mL/d, n = 8), FLX (40 mg/d, approx. 0.8 mg/kg; n = 8), or 5-HTP (90 mg/d, approx. 1.8 mg/kg; n = 8) for 10 consecutive days in a complete randomized block design. Heart rate (HR), respiration rate, rectal temperature, and health scores were recorded daily. Hip height and body weight were measured at d 1, 5, and 10 relative to initiation of supplementation. Blood samples were collected once before the supplementation period (d 1), during the 10-d supplementation period (daily), and during a 14-d withdrawal period (d 2, 3, 4, 7, and 14 relative to initiation of withdrawal). Cerebrospinal fluid and muscle tissue were collected from a subset of calves (n = 12) that were euthanized after the 10-d supplementation or 14-d withdrawal period. Whole blood serotonin concentrations increased in 5-HTP calves and decreased in FLX calves compared with CON (P < 0.001), indicating that serotonin bioavailability was increased in both groups. Whole blood serotonin concentrations of 5-HTP and FLX calves returned to CON levels after 7 d of withdrawal. All calves grew and were considered healthy throughout the study. In fact, calves fed 5-HTP had higher average daily gain compared with CON (0.87 vs 0.66 ± 0.12 kg/d, P = 0.05). Calves fed FLX had lower HR (P = 0.02) and greater red blood cells and hemoglobin counts on d 10 of supplementation compared with CON (P < 0.01). After the 14-d withdrawal period, FLX was not detected in circulation of FLX calves, but was still present in the muscle tissue. Our results demonstrate that manipulation of the serotonin pathway by supplementing FLX or 5-HTP is a feasible and safe approach in preweaned dairy calves; however, it takes more than 14 d for FLX to be completely withdrawn from the body.

Serotonin receptor oligomerization regulates cAMP-based signaling

Protein-protein interaction is often investigated using quantitative molecular microscopy with Förster resonant energy transfer (FRET). Here, we combined 'linear unmixing FRET' (lux-FRET) with the simultaneous application of a FRET-based biosensor for cAMP to investigate the oligomerization between the 5-HT₇ receptor (5-HT₇R, also known as HTR7) and the 5-HT_1A receptor (5-HT_1AR, also known as HTR1A) and its importance for cAMP signaling. We found that the 5-HT₇R not only stimulates cAMP production, but also forms hetero-oligomers with 5-HT_1AR, which blocks the inhibitory effect of the latter. 5-HT₇R signaling, however, is not affected by this hetero-oligomerization. By modeling the kinetics of intracellular cAMP level changes in relation to the 5-HT₇R:5-HT_1AR stoichiometry, we were able to decipher the complex signaling characteristics of endogenous serotonin receptors in cultured hippocampal neurons. Our findings indicate that serotonergic signaling is not only modulated by the concentration of an individual receptor but also by its specific interaction with other receptors in endogenous systems. We conclude that the regulated ratio of serotonin receptors in immature and mature neurons may be critically involved in both the onset and response to treatments of psychiatric diseases, such as anxiety and depression.

Serotonin and motherhood: From molecules to mood

Emerging research points to a valuable role of the monoamine neurotransmitter, serotonin, in the display of maternal behaviors and reproduction-associated plasticity in the maternal brain. Serotonin is also implicated in the pathophysiology of numerous affective disorders and likely plays an important role in the pathophysiology of maternal mental illness. Therefore, the main goals of this review are to detail: (1) how the serotonin system of the female brain changes across pregnancy and postpartum; (2) the role of the central serotonergic system in maternal caregiving and maternal aggression; and (3) how the serotonin system and selective serotonin reuptake inhibitor medications (SSRIs) are involved in the treatment of maternal mental illness. Although there is much work to be done, studying the central serotonin system's multifaceted role in the maternal brain is vital to our understanding of the processes governing matrescence and the maintenance of motherhood.

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.

d-Fenfluramine and lorcaserin inhibit the binge-like feeding induced by μ-opioid receptor stimulation of the nucleus accumbens in the rat

Multiple laboratories have shown that the stimulation of μ-opioid receptors in the nucleus accumbens (NAcc) powerfully increases intake of palatable and high-fat diets. Separate studies have demonstrated that serotonin agonists advance satiety processes, and several serotonin-targeting agents have been prescribed to promote weight loss. However, it is unknown if serotonin signaling can modulate the increased feeding elicited by activation of NAcc μ-opioid receptors. These experiments assessed the effects of systemic treatments with the serotonin agonists d-fenfluramine and lorcaserin on the binge-like feeding induced by μ-opioid receptor stimulation of the NAcc in Sprague-Dawley rats. Consistent with previous reports, stimulation of NAcc μ-opioid receptors (with 0.025 μg/0.5 μl/side DAMGO) significantly increased consumption of high-fat vegetable shortening, and systemic treatment with d-fenfluramine and lorcaserin dose-dependently decreased intake. Interestingly, d-fenfluramine and lorcaserin reversed the binge-like feeding observed following stimulation of NAcc μ-opioid receptors. Both serotonergic drugs also attenuated the increases of ambulation observed following administration of DAMGO in the NAcc. These data demonstrate that serotonergic anorectics, in addition to their known role in advancing satiety processes during normal feeding, can also inhibit the binge-like feeding that is elicited by activation of μ-opioid receptors within the ventral striatum.

Molecular cloning and functional expression of the 5-HT7 receptor in Chinese mitten crab (Eriocheir sinensis)

Serotonin (5-HT) regulates numerous physiological functions and processes, such as light adaptation, food intake and ovarian maturation, and plays the role through 5-HT receptors. To our knowledge, this is the first study to isolate and characterize the serotonin receptor 7 (5-HT₇ receptor) cDNA encoded in Eriocheir sinensis, an economically important aquaculture species in China, by performing rapid-amplification of cDNA ends. The full-length of 5-HT₇ receptor gene cDNA is 2328 bp and encodes a polypeptide with 590 amino acids that are highly homologous with other crustaceans 5-HT₇ receptor genes. Analysis of the deduced amino acid sequence of the 5-HT₇, including 7 transmembrane domains and some common features of G protein-coupled receptors (GPCRs), indicated that 5-HT₇ receptor was a member of GPCRs family. A gene expression analysis of the 5-HT₇ receptor by RT-PCR revealed that the 5-HT₇ receptor transcripts were widely distributed in various tissues, in which high expression levels were observed in the cranial ganglia, thoracic ganglia and intestines. Further study about the effects of photoperiods on the 5-HT₇ expression in the tissues showed that a significantly increasing expression of the 5-HT₇ receptor was observed in the thoracic ganglia induced by constant light. In addition, in the eyestalks, the expression levels of 5-HT₇ mRNA in constant darkness and constant light were lower than control treatment. Then, the expression levels of the 5-HT₇ receptor in three feeding statuses displayed that there were significantly increasing expressions in the hepatopancreas and intestines after feeding, compared with before feeding and during the feeding period. Finally, the 5-HT₇ mRNA expression levels in stage III and stage IV were higher than the levels in stage I of ovarian development. Our experimental results showed that the 5-HT₇ receptor structurally belongs to GPCRs, and the thoracic ganglia and eyestalks are the important tissues of the 5-HT₇ receptor for light adaptation. The 5-HT₇ receptor may also be involved in the physiological regulation of the hepatopancreas and intestines after ingestion in E. sinensis. In addition, the 5-HT₇ receptor is involved in the process of ovarian maturation. The study provided a foundation for further research of light adaptation, digestive functions and ovarian maturation of the 5-HT₇ receptor in Decapoda.

A behavioral mechanistic investigation of the role of 5-HT1A receptors in the mediation of rat maternal behavior

Previous work suggests that 5-HT_1A receptors play a special role in rodent maternal aggression, but not in other aspects of maternal care (e.g. pup retrieval and nest building). The present study re-assessed the basic effects of 5-HT_1A activation or blockade on various maternal responses in postpartum female rats. We also examined the possible behavioral mechanisms underlying the maternal effects of 5-HT_1A. Sprague-Dawley mother rats were injected with a 5-HT_1A agonist 8-OH-DPAT (0.1, 0.5 or 1.0 mg/kg, sc), a 5-HT_1A antagonist WAY-101405 (0.1, 0.5 or 1.0 mg/kg, sc) or 0.9% saline solution on postpartum days 3, 5, and 7. Maternal behavior was tested 30 min before, 30 min, 120 min, and 240 min after the injection. Acute and repeated 8-OH-DPAT treatment significantly disrupted pup retrieval, pup licking, nursing, and nest building in a dose-dependent fashion, whereas WAY-101405 had no effect at the tested doses. The 5-HT_1A receptor specificity of 8-OH-DPAT's action was confirmed as its maternal disruption effect was reversed by pretreatment of WAY-100635 (a highly selective 5-HT_1A receptor antagonist). Subsequent pup preference test found that 8-OH-DPAT did not decrease the pup preference over a novel object, thus no inhibition on maternal motivation or maternal affect. The pup separation test and pup retrieval on an elevated plus maze test also failed to find any motivational and motor impairment effect with 8-OH-DPAT. However, 8-OH-DPAT at the maternal disruptive dose did disrupt the prepulse inhibition (a measure of attentional function) of acoustic startle response and enhanced the basal startle response. These findings suggest that stimulation of 5-HT_1A receptors by 8-OH-DPAT impairs maternal care by partially interfering with the attentional processing or basal anxiety. More work is needed to further delineate the psychological and neuronal mechanisms underlying the maternal disruptive effect of 5-HT_1A receptor activation.

Tryptophan overloading activates brain regions involved with cognition, mood and anxiety

Tryptophan is the only precursor of serotonin and mediates serotonergic activity in the brain. Previous studies have shown that the administration of tryptophan or tryptophan depletion significantly alters cognition, mood and anxiety. Nevertheless, the neurobiological alterations that follow these changes have not yet been fully investigated. The aim of this study was to verify the effects of a tryptophan-enriched diet on immunoreactivity to Fos-protein in the rat brain. Sixteen male Wistar rats were distributed into two groups that either received standard chow diet or a tryptophan-enriched diet for a period of thirty days. On the morning of the 31st day, animals were euthanized and subsequently analyzed for Fos-immunoreactivity (Fos-ir) in the dorsal and median raphe nuclei and in regions that receive serotonin innervation from these two brain areas. Treatment with a tryptophan-enriched diet increased Fos-ir in the prefrontal cortex, nucleus accumbens, paraventricular hypothalamus, arcuate and ventromedial hypothalamus, dorsolateral and dorsomedial periaqueductal grey and dorsal and median raphe nucleus. These observations suggest that the physiological and behavioral alterations that follow the administration of tryptophan are associated with the activation of brain regions that regulate cognition and mood/anxiety-related responses.

Contrasting effects of 5-HT3 receptor stimulation of the nucleus accumbens or ventral tegmentum on food intake in the rat

Although serotonin (5-HT) signaling is known to regulate food intake and energy homeostasis, the roles of the 5-HT₃ receptor in feeding processes have been elusive. 5-HT₃ receptors are found throughout mesolimbic circuitry that promote feeding not only in response to hunger, but also to the palatable and rewarding properties of food. These experiments examined if stimulation or blockade of the 5-HT₃ receptor of the nucleus accumbens (NAcc) or ventral tegmentum affected food intake in the rat in response to hunger or the presence of a palatable diet. Rats (N=6-9/group) received bilateral injections of the 5-HT₃ agonist m-chlorophenylbiguanide hydrochloride (mCPBG; at 0.0, 10.0, or 20.0μg/0.5μl/side) or the 5-HT₃ antagonist ondansetron hydrochloride (at 0.0, 1.0, 2.0, or 5.0μg/0.5μl/side) into either the NAcc or the ventral tegmentum. NAcc 5-HT₃ receptor stimulation significantly increased 2-h food intake in food-deprived animals offered rat chow and in a separate group of unrestricted rats offered a sweetened fat diet. In contrast to the feeding increase seen with NAcc treatments, stimulation of 5-HT₃ receptors of the ventral tegmentum significantly reduced food and water intake in food-restricted animals; reductions of intake in non-restricted rats offered the palatable diet did not approach significance. Blockade of the 5-HT₃ receptor had no effect on feeding in either brain region. These data support a functional role for serotonergic signaling in the mesolimbic pathway on motivated behavior, and demonstrate that 5-HT₃ receptors differentially modulate food consumption in a region-dependent manner.

A systematic investigation of the differential roles for ventral tegmentum serotonin 1- and 2-type receptors on food intake in the rat

Central serotonin (5-HT) pathways are known to influence feeding and other ingestive behaviors. Although the ventral tegmentum is important for promoting the seeking and consumption of food and drugs of abuse, the roles of 5-HT receptor subtypes in this region on food intake have yet to be comprehensively examined. In these experiments, food restricted rats were given 2-h access to rat chow; separate groups of non-restricted animals had similar access to a sweetened fat diet. Feeding and locomotor activity were monitored following ventral tegmentum stimulation or blockade of 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, or 5-HT2C receptors. 5-HT1A receptor stimulation transiently inhibited rearing behavior and chow intake in food-restricted rats, and had a biphasic effect on non-restricted rats offered the palatable diet. 5-HT1B receptor agonism transiently inhibited feeding in restricted animals, but did not affect intake of non-restricted rats. In contrast, 5-HT1B receptor antagonism decreased palatable feeding. Although stimulation of ventral tegmental 5-HT2B receptors with BW723C86 did not affect hunger-driven food intake, it significantly affected palatable feeding, with a trend for an increasing intake at 2.0µg/side but not at 5.0µg/side. Antagonism of the same receptor modestly but significantly inhibited feeding of the palatable diet at 5.0µg/side ketanserin. Neither stimulation nor blockade of 5-HT2A or 5-HT2C receptors caused prolonged effects on intake or locomotion. These data suggest that serotonin's effects on feeding within the ventral tegmentum depend upon the specific receptor targeted, as well as whether intake is motivated by food restriction or the palatable nature of the offered diet.

Functional regulation of PI3K-associated signaling in the accumbens by binge alcohol drinking in male but not female mice

It is well established that binge alcohol consumption produces alterations in Group 1 metabotropic glutamate receptors (mGlus) and related signaling cascades in the nucleus accumbens (NAC) of adult male mice, but female and adolescent mice have not been examined. Thus, the first set of studies determined whether repeated binge alcohol consumption produced similar alterations in protein and mRNA levels of Group 1 mGlu-associated signaling molecules in the NAC of male and female adult and adolescent mice. The adult (9 weeks) and adolescent (4 weeks) C57BL/6J mice were exposed to 7 binge alcohol sessions every 3rd day while controls drank water. Repeated binge alcohol consumption produced sexually divergent changes in protein levels and mRNA expression for Group 1 mGlus and downstream signaling molecules in the NAC, but there was no effect of age. Binge alcohol intake decreased mGlu5 levels in females, whereas it decreased indices of phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), 4E-binding protein 1, and p70 ribosomal protein S6 kinase in males. Expression of genes encoding mGlu1, mGlu5, the NR2A subunit of the NMDA receptor, and Homer2 were all decreased by binge alcohol consumption in males, while females were relatively resistant (only phosphoinositide-dependent protein kinase 1 was decreased). The functional implication of these differences was investigated in a separate study by inhibiting mTOR in the NAC (via infusions of rapamycin) before binge drinking sessions. Rapamycin (50 and 100 ng/side) significantly decreased binge alcohol consumption in males, while consumption in females was unaffected. Altogether these results highlight that mTOR signaling in the NAC was necessary to maintain binge alcohol consumption only in male mice and that binge drinking recruits sexually divergent signaling cascades downstream of PI3K and presumably, Group 1 mGlus. Importantly, these findings emphasize that sex should be considered in the development of potential pharmacotherapeutic targets.

Idalopirdine - a small molecule antagonist of 5-HT6 with therapeutic potential against obesity

5HT6 receptor antagonists offer the potential for safe and effective drugs against obesity, because they can reduce weight without causing serious side effects in the cardiovascular system. Also, their anorexic effect is associated with reduced food intake via an enhancement of satiety. In the present study we investigated the anorexic effect of idalopirdine (LuAE58054) in a model of obesity induced by high-fat diet. To induce obesity in rats, the animals were treated with feed with a fat content of 40 %. Body weight was controlled and the amount of food and water consumed was determined. The influence of the test compound on the lipid profile and glucose level was measured, as well as locomotor activity in home cages on the 20th day of the treatment. LuAE58054, at 5 mg kg(-1)/day i.p., was significantly anorectic in this model of obesity. Animals treated with LuAE58054 weighed 8 and 9.2 % less than the control obese animals on the 12th and 21st days, respectively. It significantly reduced food intake and the amount of peritoneal fat in animals, and reduced the level of triglycerides in plasma. LuAE58054 did not have a statistically significant effect on the spontaneous activity of diet-induced obese rats. The present study clearly demonstrates the effectiveness of LuAE58054 in reducing body weight. This compound is in phase III of clinical trials for the treatment of cognitive deficits associated with Alzheimer's disease and schizophrenia. It is a 5HT6 receptor antagonist and is, therefore, free of those unacceptable side effects that preclude chronic use of anti-obesity drugs with other mechanisms of action. The search for an effective and safe anti-obesity drug is essential for an increasingly obese population; therefore, the anorectic action of LuAE58054 is important and there is a need for more research in this direction.

Brain signaling systems in the Type 2 diabetes and metabolic syndrome: promising target to treat and prevent these diseases

The changes in the brain signaling systems play an important role in etiology and pathogenesis of Type 2 diabetes mellitus (T2DM) and metabolic syndrome (MS), being a possible cause of these diseases. Therefore, their restoration at the early stages of T2DM and MS can be regarded as a promising way to treat and prevent these diseases and their complications. The data on the functional state of the brain signaling systems regulated by insulin, IGF-1, leptin, dopamine, serotonin, melanocortins and glucagon-like peptide-1, in T2DM and MS, are analyzed. The pharmacological approaches to restoration of these systems and improvement of insulin sensitivity, energy expenditure, lipid metabolism, and to prevent diabetic complications are discussed.

Cholinergic and serotonergic modulations differentially affect large-scale functional networks in the mouse brain

Resting-state functional MRI (rsfMRI) is a widely implemented technique used to investigate large-scale topology in the human brain during health and disease. Studies in mice provide additional advantages, including the possibility to flexibly modulate the brain by pharmacological or genetic manipulations in combination with high-throughput functional connectivity (FC) investigations. Pharmacological modulations that target specific neurotransmitter systems, partly mimicking the effect of pathological events, could allow discriminating the effect of specific systems on functional network disruptions. The current study investigated the effect of cholinergic and serotonergic antagonists on large-scale brain networks in mice. The cholinergic system is involved in cognitive functions and is impaired in, e.g., Alzheimer's disease, while the serotonergic system is involved in emotional and introspective functions and is impaired in, e.g., Alzheimer's disease, depression and autism. Specific interest goes to the default-mode-network (DMN), which is studied extensively in humans and is affected in many neurological disorders. The results show that both cholinergic and serotonergic antagonists impaired the mouse DMN-like network similarly, except that cholinergic modulation additionally affected the retrosplenial cortex. This suggests that both neurotransmitter systems are involved in maintaining integrity of FC within the DMN-like network in mice. Cholinergic and serotonergic modulations also affected other functional networks, however, serotonergic modulation impaired the frontal and thalamus networks more extensively. In conclusion, this study demonstrates the utility of pharmacological rsfMRI in animal models to provide insights into the role of specific neurotransmitter systems on functional networks in neurological disorders.

Serotonergic Systems in the Pathophysiology of Ethanol Dependence: Relevance to Clinical Alcoholism

Alcoholism is a progressive brain disorder that is marked by increased sensitivity to the positive and negative reinforcing properties of ethanol, compulsive and habitual use despite negative consequences, and chronic relapse to alcohol drinking despite repeated attempts to reduce intake or abstain from alcohol. Emerging evidence from preclinical and clinical studies implicates serotonin (5-hydroxytryptamine; 5-HT) systems in the pathophysiology of alcohol dependence, suggesting that drugs targeting 5-HT systems may have utility in the treatment of alcohol use disorders. In this Review, we discuss the role of 5-HT systems in alcohol dependence with a focus on 5-HT interactions with neural circuits that govern all three stages of the addiction cycle. We attempt to clarify how 5-HT influences circuit function at these different stages with the goal of identifying neural targets for pharmacological treatment of this debilitating disorder.

Serotonin receptor expression is dynamic in the liver during the transition period in Holstein dairy cows

Nonneuronal serotonin (5-HT) participates in glucose metabolism, but little is known regarding the actions of 5-HT in the liver during the transition period in dairy cattle. Here, we explore circulating patterns of 5-HT and characterize the hepatic 5-HT receptor and glucose transporter profiles around calving in multiparous Holstein dairy cows (n = 6, average lactation = 4 ± 1.9). Concentrations of serum 5-HT decreased on day -3 compared with -5 and -7 precalving (167.7 ± 80 vs 1511.1 ± 602 ng/mL). 5-HT nadir was on day -1 precalving and remained low postcalving (481.4 ± 49 ng/mL). Plasma glucose concentrations decreased precalving (P = 0.008) and were positively correlated with 5-HT during the precalving period (r = 0.55, P = 0.043). On day 1, postcalving hepatic messenger RNA expression of 5-HT1D, 2B, 3C, 6, and 7 receptors were decreased compared with day -7 (P < 0.048). The 5-HT3A and 5-HT3B decreased on day 7. The 5-HT2A increased on days 1 and 7 compared with -7 (P < 0.05). The 5-HT1F and 5-HT1A receptors were increased 2.5- and 3.8-fold on day 7, respectively, compared with days -7 and 1 (P < 0.046). The 5-HT5A was not detected, and 5-HT4 was detected on days -7 and 1 only. Expression of Glut-2,-5 and SGLT1 were decreased on days 1 and 7 compared with -7 (P < 0.05), whereas Glut-1 was increased on day 7 compared with -7 (P < 0.05). These results indicate that 5-HT could be important for liver glucose homeostasis possibly through receptor mediated signaling at specific times. Additional research is needed to further explore the functional role of these receptors in the liver during the transition from pregnancy to lactation.

Environmentally relevant concentrations of citalopram partially inhibit feeding in the three-spine stickleback (Gasterosteus aculeatus)

Selective Serotonin Re-uptake Inhibitors (SSRI) are mood-altering, psychotropic drugs commonly used in the treatment of depression and other psychological illnesses. Many of them are poorly degraded in sewage treatment plants and enter the environment unaltered. In laboratory studies, they have been demonstrated to affect a wide range of behaviours in aquatic organisms. In this study we investigated the effect of a three-week exposure to 0.15 and 1.5 μg/l of the SSRI citalopram dissolved in the ambient water on the feeding behaviour in three-spine stickleback (Gasterosteus aculeatus). Feeding, measured as the number of attacks performed on a piece of frozen bloodworms during a 10-min period, was reduced by 30-40% in fish exposed to both 0.15 and 1.5 μg/l citalopram. The effects of the environmentally relevant concentration 0.15 μg/l on feeding, an important fitness characteristic, suggests that the ecological significance of environmental SSRI exposure may be pronounced.

The Effect of Long-Term Intranasal Serotonin Treatment on Metabolic Parameters and Hormonal Signaling in Rats with High-Fat Diet/Low-Dose Streptozotocin-Induced Type 2 Diabetes

In the last years the treatment of type 2 diabetes mellitus (DM2) was carried out using regulators of the brain signaling systems. In DM2 the level of the brain serotonin is reduced. So far, the effect of the increase of the brain serotonin level on DM2-induced metabolic and hormonal abnormalities has been studied scarcely. The present work was undertaken with the aim of filling this gap. DM2 was induced in male rats by 150-day high-fat diet and the treatment with low dose of streptozotocin (25 mg/kg) on the 70th day of experiment. From the 90th day, diabetic rats received for two months intranasal serotonin (IS) at a daily dose of 20 μg/rat. The IS treatment of diabetic rats decreased the body weight, and improved glucose tolerance, insulin-induced glucose utilization, and lipid metabolism. Besides, it restored hormonal regulation of adenylyl cyclase (AC) activity in the hypothalamus and normalized AC stimulation by β-adrenergic agonists in the myocardium. In nondiabetic rats the same treatment induced metabolic and hormonal alterations, some of which were similar to those in DM2 but expressed to a lesser extent. In conclusion, the elevation of the brain serotonin level may be regarded as an effective approach to treat DM2 and its complications.

5-HT7 receptors as modulators of neuronal excitability, synaptic transmission and plasticity: physiological role and possible implications in autism spectrum disorders

Serotonin type 7 receptors (5-HT7) are expressed in several brain areas, regulate brain development, synaptic transmission and plasticity, and therefore are involved in various brain functions such as learning and memory. A number of studies suggest that 5-HT7 receptors could be potential pharmacotherapeutic target for cognitive disorders. Several abnormalities of serotonergic system have been described in patients with autism spectrum disorder (ASD), including abnormal activity of 5-HT transporter, altered blood and brain 5-HT levels, reduced 5-HT synthesis and altered expression of 5-HT receptors in the brain. A specific role for 5-HT7 receptors in ASD has not yet been demonstrated but some evidence implicates their possible involvement. We have recently shown that 5-HT7 receptor activation rescues hippocampal synaptic plasticity in a mouse model of Fragile X Syndrome, a monogenic cause of autism. Several other studies have shown that 5-HT7 receptors modulate behavioral flexibility, exploratory behavior, mood disorders and epilepsy, which include core and co-morbid symptoms of ASD. These findings further suggest an involvement of 5-HT7 receptors in ASD. Here, we review the physiological roles of 5-HT7 receptors and their implications in Fragile X Syndrome and other ASD.

The effects of nucleus accumbens μ-opioid and adenosine 2A receptor stimulation and blockade on instrumental learning

Prior research has shown that glutamate and dopamine receptors in the nucleus accumbens (NAcc) core are critical for the learning of an instrumental response for food reinforcement. It has also been demonstrated that μ-opioid and adenosine A2A receptors within the NAcc impact feeding and motivational processes. In these experiments, we examined the potential roles of NAcc μ-opioid and A2A receptors on instrumental learning and performance. Sprague-Dawley rats were food restricted and trained to lever press following daily intra-accumbens injections of the A2A receptor agonist CGS 21680 (at 0.0, 6.0, or 24.0ng/side), the A2A antagonist pro-drug MSX-3 (at 0.0, 1.0, or 3.0μg/side), the μ-opioid agonist DAMGO (at 0.0, 0.025, or 0.025μg/side), or the opioid receptor antagonist naltrexone (at 0.0, 2.0 or 20.0μg/side). After five days, rats continued training without drug injections until lever pressing rates stabilized, and were then tested with a final drug test to assess potential performance effects. Stimulation, but not inhibition, of NAcc adenosine A2A receptors depressed lever pressing during learning and performance tests, but did not impact lever pressing on non-drug days. Both μ-opioid receptor stimulation and blockade inhibited learning of the lever-press response, though only naltrexone treatment caused impairments in lever-pressing after the task had been learned. The effect of A2A receptor stimulation on learning and performance were consistent with known effects of adenosine on effort-related processes, whereas the pattern of lever presses, magazine approaches, and pellet consumption following opioid receptor manipulations suggested that their effects may have been driven by drug-induced shifts in the incentive value of the sugar reinforcer.