5-HT2A receptor levels increase in MPTP-lesioned macaques treated chronically with L-DOPA

Long-term L-3,4-dihydroxyphenylalanine (L-DOPA) treatment in Parkinson's disease (PD) is associated with motor complications such as dyskinesia. There are clear functional interactions between dopaminergic and serotonergic type 2A receptors (5-HT_2A)-mediated neurotransmission. Moreover, 5-HT_2A receptor antagonists can reduce L-DOPA-induced dyskinesia (LID). We hypothesized that enhanced 5-HT_2A-mediated neurotransmission may be involved in the genesis of L-DOPA-induced dyskinesia. Radioligand binding autoradiography, using [³H]-ketanserin, was performed to define 5-HT_2A receptor levels in brain tissue from macaques: 6 normal; 5 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned, parkinsonian macaques, without exposure to L-DOPA; 6 MPTP-lesioned, parkinsonian macaques, receiving a single administration of L-DOPA, and exhibiting no dyskinesia; and 6 MPTP-lesioned, parkinsonian, macaques chronically treated with L-DOPA, and exhibiting dyskinesia. 5-HT_2A receptor binding was increased in the caudate, putamen, and middle layers of the motor cortex in chronically L-DOPA-treated animals, by 50%, 50%, and 45% respectively, compared with normal macaques. 5-HT_2A binding was not significantly altered in parkinsonian, untreated, or parkinsonian, single treatment, nondyskinetic macaques, compared with normal. These data provide an anatomical basis for mechanisms to explain the efficacy of 5-HT_2A antagonists against dyskinesia.     

The serotonergic system in motor and non-motor manifestations of Parkinson’s disease

The understanding of Parkinson’s disease (PD) classically revolves around dopamine depletion within the striatum. However, PD is a multi-systemic disease in which extra-dopaminergic systems are affected. The serotonergic (5-HT) system is one of these and has been extensively studied in PD. Although the 5-HT system uses one transporter (SERT) and 14 receptor sub-types, most of the studies in PD have focussed on SERT and serotonergic type 1A and 2A receptors (5-HT_1A and 5-HT_2A). Post-mortem autoradiographic binding studies and in vivo imaging studies have suggested an involvement of the 5-HT system in PD-related anxiety, depression, psychosis and L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia. Pre-clinical and clinical pharmacological studies have shown that SERT blockade might effectively alleviate depression and dyskinesia and, more recently, might exert disease-modifying effects. Enhancing the physiological activity of 5-HT_1A receptors with 5-HT_1A agonists might alleviate anxiety, dyskinesia and tremor, although a deleterious effect on the anti-parkinsonian efficacy of L-DOPA may ultimately limit the use of this class of compounds. Enhanced 5-HT_2A-mediated neurotransmission has been associated with depression, dyskinesia, psychosis and tremor. The current article critically reviews studies assessing the SERT, as well as 5-HT_1A and 5-HT_2A receptors in idiopathic PD and animal models of PD, and discusses unmet challenges to effectively treat manifestations of PD using SERT antagonists, 5-HT_1A agonists and 5-HT_2A antagonists.     

Hypothalamic serotonin-1A receptor binding measured by PET predicts the plasma level of dehydroepiandrosterone sulfate in healthy women

Serotonin modulates the activity of the hypothalamic–pituitary–adrenal (HPA) axis particularly via the serotonin-1A receptor (5-HT_1A). Therefore, the rationale of this positron emission tomography (PET) study was to investigate the influence of the 5-HT_1A receptor distribution in the human brain on plasma levels of dehydroepiandrosterone sulfate (DHEAS) and cortisol in vivo. Eighteen healthy female were measured with PET and the selective 5-HT_1A receptor radioligand [carbonyl-¹¹C]WAY-100635. Nine a priori defined brain regions (hypothalamus, orbitofrontal cortex, amygdala, hippocampus, anterior and posterior cingulate cortices, dorsal raphe nucleus, retrosplenial cortex, and insula) and the cerebellum (reference region) were delineated on coregistered MR images. DHEAS and cortisol plasma levels were collected by blood sampling in the morning of the PET day. Linear regression analysis of DHEAS plasma level as dependent variable and hypothalamic 5-HT_1A receptor binding potential (BP) as independent variable showed a highly significant association (r = .691, p = .002). The hypothalamic 5-HT_1A BP predicted 47.7% of the variability in DHEAS plasma levels. Regressions were borderline significant (p < .01, Bonferroni corrected threshold <.0056) between 5-HT_1A BP in the anterior cingulate and orbitofrontal cortices and free cortisol levels. No significant associations between DHEAS or cortisol and the 5-HT_1A receptor BP in other investigated brain regions were found. In conclusion, the serotonergic system may influence the DHEAS plasma level by modulating CRH and ACTH release via hypothalamic 5-HT_1A receptors as reported for cortisol before. As disturbances of the HPA axis as well as changes of the 5-HT_1A receptor distribution have been reported in affective disorders, future studies should focus on these interactions.     

Changes in 5-HT2A-mediated behavior and 5-HT2A- and 5-HT1A receptor binding and expression in conditional brain-derived neurotrophic factor knock-out mice

Changes in brain-derived neurotrophic factor (BDNF) expression have been implicated in the etiology of psychiatric disorders. To investigate pathological mechanisms elicited by perturbed BDNF signaling, we examined mutant mice with central depletion of BDNF (BDNF^2L/2LCk-cre). A severe impairment specific for the serotonin 2A receptor (5-HT_2AR) in prefrontal cortex was described previously in these mice. This is of much interest, as 5-HT_2ARs have been linked to neuropsychiatric disorders and anxiety-related behavior. Here we further characterized the serotonin receptor alterations triggered by BDNF depletion. 5-HT_2A ([³H]-MDL100907) and 5-HT_1A ([³H]-WAY100635) receptor autoradiography revealed site-specific alterations in BDNF mutant mice. They exhibited lower 5-HT_2A receptor binding in frontal cortex but increased binding in hippocampus. Additionally, 5-HT_1A receptor binding was decreased in hippocampus of BDNF mutants, but unchanged in frontal cortex. Molecular analysis indicated corresponding changes in 5-HT_2A and 5-HT_1A mRNA expression but normal 5-HT_2C content in these brain regions in BDNF^2L/2LCk-cre mice. We investigated whether the reduction in frontal 5-HT_2AR binding was reflected in reduced functional output in two 5-HT_2A-receptor mediated behavioral tests, the head-twitch response (HTR) and the ear-scratch response (ESR). BDNF^2L/2LCk-cre mutants treated with the 5-HT_2A receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) showed a clearly diminished ESR but no differences in HTR compared to wildtypes. These findings illustrate the context-dependent effects of deficient BDNF signaling on the 5-HT receptor system and 5-HT_2A-receptor functional output.     

Gender and the use of hormonal contraception in women are not associated with cerebral cortical 5-HT 2A receptor binding

Gender influences brain function including serotonergic neurotransmission, which may play a role in the well-known gender variations in vulnerability to mood and anxiety disorders. Even though hormonal replacement therapy in menopause is associated with globally increased cerebral 5-HT_2A receptor binding it is not clear if gender or use of hormonal contraception exhibits associations with 5-HT_2A receptor binding. We found no significant effect of gender on cortical 5-HT_2A receptor binding (P=0.15, n=132). When adjusting for the personality trait neuroticism, known to be positively correlated to frontolimbic 5-HT_2A receptor binding and to be more pronounced in women, again, the effect of gender was not significant (P=0.42, n=127). Also, the use of hormonal contraception (n=14) within the group of pre-menopausal women (total n=29) was not associated with cortical 5-HT_2A receptor binding (P=0.31). In conclusion, neither gender, nor the use of hormonal contraception in premenopausal women was associated with cortical 5-HT_2A receptor binding.     

Effect of chronic activation of 5-HT3 receptors on 5-HT3, 5-HT1A and 5-HT2A receptors functional activity and expression of key genes of the brain serotonin system

Among serotonin (5-HT) receptors, the 5-HT₃ receptor is the only ligand-gated ion-channel. Little is known about the interaction between the 5-HT₃ receptor and other 5-HT receptors and influence of 5-HT₃ chronic activation on other 5-HT receptors and the expression of key genes of 5-HT system. Chronic activation of 5-HT₃ receptor with intracerebroventricularly administrated selective agonist 1-(3-chlorophenyl)biguanide hydrochloride (m-CPBG) (14 days, 40 nmol, i.c.v.) produced significant desensitization of 5-HT₃ and 5-HT_1A receptors. The hypothermic responses produced by acute administration of selective agonist of 5-HT₃ receptor (m-CPBG, 40 nmol, i.c.v.) or selective agonist of 5-HT_1A receptor (8-hydroxy-2-(di-n-propylamino)tetralin) (8-OH-DPAT, 1 mg/kg, i.p.) was significantly lower in m-CPBG treated mice compared with the mice of control groups. Chronic m-CPBG administration failed to induce any significant change in the 5-HT_2A receptor functional activity and in the expression of the gene encoding 5-HT_2A receptor. Chronic activation of 5-HT₃ receptor produced no considerable effect on the expression on 5-HT₃, 5-HT_1A, and 5-HT transporter (5-HTT) and tryptophan hydroxylase-2 (TPH-2) genes – the key genes of brain 5-HT system, in the midbrain, frontal cortex and hippocampus. In conclusion, chronic activation of ionotropic 5-HT₃ receptor produced significant desensitization of 5-HT₃ and postsynaptic 5-HT_1A receptors but caused no considerable changes in the expression of key genes of the brain 5-HT system.     

Central 5-HT3 receptor-induced hypothermia in mice: Interstrain differences and comparison with hypothermia mediated via 5-HT1A receptor

The selective agonist of serotonin 5-HT₃ receptor 1-(3-chlorophenyl)biguanide hydrochloride (m-CPBG) administered intracerebroventricularly (40, 80 or 160 nmol) produced long-lasting dose-dependent hypothermic response in AKR/2J mice. m-CPBG (160 nmol i.c.v.) induced profound hypothermia (delta t = −4 °C) that lasted up to 7 h. m-CPBG (40 nmol i.c.v.)-induced hypothermia was attenuated by 5-HT₃ receptor antagonist ondansetron pretreatment. At the same time, intraperitoneal administration of m-CPBG in a wide range of doses (0.5, 1.0, 5.0 or 10.0 mg/kg) did not affect the body temperature. These findings indicate: (1) the implication of central, rather than peripheral 5-HT₃ receptor in the thermoregulation; (2) the inability of m-CPBG to cross blood–brain barrier in mice. The comparison of brain 5-HT₃-induced hypothermic reaction in six inbred mouse strains (DBA/2J, CBA/Lac, C57BL/6, BALB/c, ICR, AKR/J) was performed and two highly sensitive to m-CPBG strains (CBA/Lac and C57BL/6) were found. In the same six mouse strains the functional activity of 5-HT_1A receptor was studied. The comparison of hypothermic reactions produced by 5-HT_1A receptor agonist 8-OH-DPAT (1.0 mg/kg i.p.) and m-CPBG revealed significant correlation between 5-HT₃ and 5-HT_1A-induced hypothermia in five out of six investigated mouse strains. 5-HT_1A receptor antagonist p-MPPI pretreatment (1 mg/kg i.p.) diminished hypothermia produced by centrally administered m-CPBG (40 nmol i.c.v.). The data suggest the cross-talk between 5-HT_1A and 5-HT₃ receptors in the mechanism of 5-HT-related hypothermia.     

The role of residues in binding loop A in desflurane and propofol modulation of recombinant 5-HT3A receptor

5-Hydroxytryptamine type 3 (5-HT₃) receptor is modulated by general anesthetics and regarded as a possible site of anesthetic adverse action. Although two amino acids located in transmembrane (TM) 2 and TM3 of LGICs were reported as critical for allosteric modulation by anesthetics and alcohols, other residues could regulate anesthetic modulation. Earlier studies identified the role of glutamate 129 and phenylalanine 130 in the non-TM extracellular region in the agonist binding and coupling in the 5-HT_3A receptor. We investigated whether these non-TM amino acids are involved in desflurane and propofol modulation of the 5-HT_3A receptor in mutant 5-HT_3A receptors (mutants) expressed in Xenopus laevis oocytes. E129D and F130Y mutants were functionally expressed but E129Y and F130S mutants were not gated by serotonin. The wild type and F130Y mutants demonstrated positive modulation by desflurane at 6 and 12 vol.%. In contrast, E129D mutants were inhibited by desflurane in a concentration dependent manner. Propofol (1–100 μM) demonstrated depression of the currents in all receptors examined. These findings suggest the role of non-TM residues in the extracellular domain in the anesthetic modulation of the 5-HT_3A receptor.     

Effects of amyloid-β peptides on the serotoninergic 5-HT1A receptors in the rat hippocampus

A recent [¹⁸F]MPPF-positron emission tomography study has highlighted an overexpression of 5-HT_1A receptors in the hippocampus of patients with mild cognitive impairment compared to a decrease in those with Alzheimer's disease (AD) [Truchot, L., Costes, S.N., Zimmer, L., Laurent, B., Le Bars, D., Thomas-Antérion, C., Croisile, B., Mercier, B., Hermier, M., Vighetto, A., Krolak-Salmon, P., 2007. Up-regulation of hippocampal serotonin metabolism in mild cognitive impairment. Neurology 69 (10), 1012-1017]. We used in vivo and in vitro neuroimaging to evaluate the longitudinal effects of injecting amyloid-β (Aβ) peptides (1-40) into the dorsal hippocampus of rats. In vivo microPET imaging showed no significant change in [¹⁸F]MPPF binding in the dorsal hippocampus over time, perhaps due to spatial resolution. However, in vitro autoradiography with [¹⁸F]MPPF (which is antagonist) displayed a transient increase in 5-HT_1A receptor density 7 days after Aβ injection, whereas [¹⁸F]F15599 (a radiolabelled 5-HT_1A agonist) binding was unchanged suggesting that the overexpressed 5-HT_1A receptors were in a non-functional state. Complementary histology revealed a loss of glutamatergic neurons and an intense astroglial reaction at the injection site. Although a neurogenesis process cannot be excluded, we propose that Aβ injection leads to a transient astroglial overexpression of 5-HT_1A receptors in compensation for the local neuronal loss. Exploration of the functional consequences of these serotoninergic modifications during the neurodegenerative process may have an impact on therapeutics targeting 5-HT_1A receptors in AD.     

Impact of COMT genotype on serotonin-1A receptor binding investigated with PET

Alterations of the inhibitory serotonin-1A receptor (5-HT_1A) constitute a solid finding in neuropsychiatric research, particularly in the field of mood and anxiety disorders. Manifold factors influencing the density of this receptor have been identified, e.g., steroid hormones, sunlight exposure and genetic variants of serotonin-related genes. Given the close interactions between serotonergic and dopaminergic neurotransmission, we investigated whether a common single-nucleotide-polymorphism of the catechol-O-methyltransferase (COMT) gene (VAL158MET or rs4680) coding for a key enzyme of the dopamine network that is associated with the pathogenesis of mood disorders and antidepressant treatment response, directly affects 5-HT_1A receptor binding potential. Fifty-two healthy individuals (38 female, mean age ± standard deviation = 40.48 ± 14.87) were measured via positron emission tomography using the radioligand [carbonyl-¹¹C]WAY-100635. Genotyping for rs4680 was performed using DNA isolated from whole blood with the MassARRAY platform of the software SEQUENOM^®. Whole brain voxel-wise ANOVA resulted in a main effect of genotype on 5-HT_1A binding. Compared to A carriers (AA + AG) of rs4680, homozygote G subjects showed higher 5-HT_1A binding potential in the posterior cingulate cortex (F _(2,49) = 17.7, p = 0.05, FWE corrected), the orbitofrontal cortex, the anterior cingulate cortex, the insula, the amygdala and the hippocampus (voxel-level: p < 0.01 uncorrected, t > 2.4; cluster-level: p < 0.05 FWE corrected). In light of the frequently reported alterations of 5-HT_1A binding in anxiety and mood disorders, this study proposes a potential implication of the COMT genotype, more specifically the VAL158MET polymorphism, via modulation of the serotonergic neurotransmission.     

Selective decline of 5-HT1A receptor binding sites in rat cortex, hippocampus and cholinergic basal forebrain nuclei during aging

The effect of aging on 5-HT_1A receptor binding in several forebrain areas associated with the basal forebrain cholinergic system was investigated in rats of 3-, 24- and 30-months-old by receptor autoradiography and biochemical binding assay using [³H]8-OH-DPAT as a ligand. Autoradiographic measurements demonstrated a marked region-specific decline of ligand binding in: (i) regions of the basal forebrain cholinergic cell groups, i.e. the medial septum, diagonal band nuclei and magnocellular nucleus basalis, (ii) the frontal and parietal neocortex and (iii) the dentate gyrus of the hippocampus. No change or only a slight decrease of the 5-HT_1A receptor density was found in other areas investigated: the CA1 and CA3 sectors of hippocampus, the cingular and perirhinal cerebral cortex and the lateral septum. The autoradiographic findings were substantiated by the biochemical binding assay, which revealed a comparable loss of 5-HT_1A receptor in the hippocampus and neocortex at the age of 30 months. The results clearly show that with increasing age the decrement of 5-HT_1A receptor binding in the rat forebrain is remarkably region-selective and particularly affects the cholinergic cell groups that innervate cortex and hippocampus. This phenomenon appears to be especially significant in relation to the neuronal substrates underlying the age-related alterations of mood and cognition.     

LP-211 is a brain penetrant selective agonist for the serotonin 5-HT7 receptor

We have determined the pharmacological profile of the new serotonin 5-HT₇ receptor agonist N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (LP-211). Radioligand binding assays were performed on a panel of 5-HT receptor subtypes. The compound was also evaluated in vivo by examining its effect on body temperature regulation in mice lacking the 5-HT₇ receptor (5-HT₇^−/−) and their 5-HT₇^+/+ sibling controls. Disposition studies were performed in mice of both genotypes. It was found that LP-211 was brain penetrant and underwent metabolic degradation to 1-(2-diphenyl)piperazine (RA-7). In vitro binding assays revealed that RA-7 possessed higher 5-HT₇ receptor affinity than LP-211 and a better selectivity profile over a panel of 5-HT receptor subtypes. In vivo it was demonstrated that LP-211, and to a lesser degree RA-7, induced hypothermia in 5-HT₇^+/+ but not in 5-HT₇^−/− mice. Our results suggest that LP-211 can be used as a 5-HT₇ receptor agonist in vivo.     

Receptor-genes cross-talk: effect of chronic 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin treatment on the expression of key genes in brain serotonin system and on behavior

Dysfunction in brain serotonin (5-HT) system has been implicated in the psychopathology of anxiety, depression, drug addiction, and schizophrenia. The 5-HT_1A receptors play a central role in the control of 5-HTergic neurotransmission. There are some scarce data showing cross-regulation between 5-HT receptors. Here, we investigated whether interaction exists between 5-HT_1A receptor and genes encoding key members in brain 5-HT system. Chronic treatment with selective agonist of 5-HT_1A receptor 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (1.0 mg/kg i.p., 14 days) produced considerable decrease in hypothermic response to acute administration of 8-OH-DPAT in CBA/Lac mice indicating desensitization of 5-HT_1A receptors. The decrease in 5-HT_1A gene expression as well as decrease in the expression of gene encoding key enzyme in 5-HT synthesis, tryptophan hydroxylase-2 (TPH-2) in the midbrain, and the expression of the gene encoding 5-HT_2A receptor in the frontal cortex was shown. There were no significant changes in 5-HT transporter mRNA level in the midbrain. Despite considerable decrease in the expression of the genes encoding tryptophan hydroxylase-2, 5-HT_1A and 5-HT_2A receptors, chronic 8-OH-DPAT treatment failed to produce significant changes in 5-HT_1A-linked behavior (intermale aggression, open-field behavior, light-dark box, and pinch-induced catalepsy), suggesting compensatory and adaptive effect of genes suppression. The obtained data on the effect of 8-OH-DPAT-induced desensitization of 5-HT_1A receptors on 5-HT_1A, 5-HT_2A and TPH-2 gene expression demonstrated the role of 5-HT_1A receptor as indirect regulator of gene expression. The results provide the first evidence of receptor-key genes interaction in brain 5-HT system and may have profound implications in understanding the functioning of the brain neurotransmitter systems.     

Loss of serotonin 2A receptors exceeds loss of serotonergic projections in early Alzheimer's disease: a combined [C]DASB and [F]altanserin-PET study

In patients with Alzheimer's disease (AD), postmortem and imaging studies have revealed early and prominent reductions in cerebral serotonin 2A (5-HT_2A) receptors. To establish if this was due to a selective disease process of the serotonin system, we investigated the cerebral 5-HT_2A receptor and the serotonin transporter binding, the latter as a measure of serotonergic projections and neurons. Twelve patients with AD (average Mini Mental State Examination [MMSE]: 24) and 11 healthy age-matched subjects underwent positron emission tomography (PET) scanning with [¹⁸F]altanserin and [¹¹C]N,N-Dimethyl-2-(2-amino-4-cyanopheylthio)benzylamine ([¹¹C]DASB). Overall [¹⁸F]altanserin binding was markedly reduced in AD by 28%-39% (p = 0.02), whereas the reductions in [¹¹C]DASB binding were less prominent and mostly insignificant, except for a marked reduction of 33% in mesial temporal cortex (p = .0005). No change in [¹¹C]DASB binding was found in the midbrain. We conclude that the prominent reduction in neocortical 5-HT_2A receptor binding in early AD is not caused by a primary loss of serotonergic neurons or their projections.     

Serotonin 5-HT2 and 5-HT1A-like receptors differentially modulate aggressive behaviors in Drosophila melanogaster

Aggressive behavior is widespread throughout the animal kingdom, and is a complex social behavior influenced by both genetics and environment. Animals typically fight over resources that include food, territory, and sexual partners. Of all the neurotransmitters, serotonin (5-HT) has been the most implicated in modulating aggressive behaviors in mammalian systems. In the fruit fly, Drosophila melanogaster, the involvement of 5-HT itself in aggressive behaviors has been recently established, however, the underlying mechanisms have largely remained elusive. Here we describe the influence of different 5-HT receptor subtypes on aggressive behaviors in Drosophila. Drosophila express homologs of three mammalian 5-HT receptors: the 5-HT_1A, 5-HT₂, and 5-HT₇ receptors. Significantly, these receptors mediate important behaviors in mammalian systems ranging from feeding, aggression, and sleep, to cognition. To examine the role of the 5-HT₂Dro receptor, we utilized the selective 5-HT₂ receptor agonist (R)-1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI), and the 5-HT₂ receptor antagonist, ketanserin. To examine the role of 5-HT_1A-like receptors we used the 5-HT_1A receptor agonist 8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT), and the 5-HT_1A receptor antagonist WAY100635. We find that activation of 5-HT₂ receptors with (R)-DOI appears to decrease overall aggression, whereas activation of 5-HT_1A-like receptors with 8-OH-DPAT increases overall aggression. Furthermore, the different 5-HT receptor circuitries appear to mediate different aspects of aggression: 5-HT₂ receptor manipulation primarily alters lunging and boxing, whereas 5-HT_1A-like receptor manipulation primarily affects wing threats and fencing. Elucidating the effects of serotonergic systems on aggression in the fly is a significant advancement not only in establishing the fly as a system to study aggression, but as a system relevant to elucidating molecular mechanisms underlying aggression in mammals, including humans.     

Cardiovascular and respiratory reflexes of the gulf toadfish (Opsanus beta) during acute hypoxia

The acute cardiovascular and respiratory responses of the gulf toadfish, Opsanus beta, to acute hypoxia or exposure to the O₂ chemoreceptor stimulant, sodium cyanide (NaCN) were characterized and the role of serotonin type 2 (5-HT₂) receptors in mediating these responses was investigated. Toadfish responded to hypoxia or NaCN exposure with a decrease in heart rate (fH) and an increase in breathing amplitude (V_AMP) but no change in breathing frequency (fR). The bradycardia appeared to be mediated to some extent by 5-HT₂ receptors, as methysergide, a non-selective 5-HT_1/2 receptor antagonist, and ketanserin, a 5-HT₂ receptor antagonist, attenuated the response. Injection of α-methyl-5-HT, a 5-HT₂ agonist, also resulted in bradycardia that was inhibited by ketanserin, lending further support for 5-HT₂ receptor involvement, possibly 5-HT_2A or 5-HT_2C, in the regulation of fH. External NaCN exposure resulted in a significant decrease in caudal arterial blood pressure (P_CA) that was attenuated by methysergide. In contrast, injection with α-methyl-5-HT resulted in a substantial increase in P_CA that was not affected by ketanserin, suggesting the possible involvement of 5-HT_2B or 5-HT_2C receptors. These data are the first to suggest a unique distribution of 5-HT_2B/2C receptors may be involved in mediating vasoconstriction of the systemic vasculature of toadfish. These data also provide mechanistic support for why pulsatile urea excretion, believed to be regulated by 5-HT via the toadfish 5-HT_2A receptor, is not triggered by hypoxia or external chemoreceptor activation.     

Role of various types of serotonin receptors in regulation of drinking behavior and salt appetite in vasopressin-deficient brattleboro rats

Serotonin 5-HT_1A receptor agonist 8-OH DPAT suppressed drinking behavior in Brattleboro and Wistar rats. 5-HT_1B agonist CGS-12066A and 5-HT_2A antagonist ketanserin did not affect drinking behavior in Brattleboro rats; 5-HT₃ antagonist ondansetron suppressed water consumption and 5-HT_1A agonist stimulated salt appetite in Brattleboro, but not in Wistar rats. Presumably, vasopressin regulates thirst and salt appetite by modulating sensitivity/density of various types of 5-HT receptors.