The role of 5‐HT2A receptor and 5‐HT2A/5‐HT1A receptor interaction in the suppression of catalepsy

In the present study, the 5‐HT_2A and 5‐HT_1A receptors functional activity and 5‐HT_2A receptor gene expression were examined in the brain of ASC/Icg and congenic AKR.CBAD13Mit76C mouse strains (genetically predisposed to catalepsy) in comparison with the parental catalepsy‐resistant AKR/J and catalepsy‐prone CBA/Lac mouse strains. The significantly reduced 5‐HT_2A receptor functional activity along with decreased 5‐HT_2A receptor gene expression in the frontal cortex was found in all mice predisposed to catalepsy compared with catalepsy‐resistant AKR/J. 5‐HT_2A agonist DOI (0.5 and 1 mg/kg, i.p.) significantly reduced catalepsy in ASC/Icg and CBA/Lac, but not in AKR.CBAD13Mit76C mice. Essential increase in 5‐HT_1A receptor functional activity was shown in catalepsy‐prone mouse strains in comparison with catalepsy‐resistant AKR/J mice. However, in AKR.CBAD13Mit76C mice it was lower than in ASC/Icg and CBA/Lac mice. The inter‐relation between 5‐HT_2A and 5‐HT_1A receptors in the regulation of catalepsy was suggested. This suggestion was confirmed by prevention of DOI anticataleptic effect in ASC/Icg and CBA/Lac mice by pretreatment with 5‐HT_1A receptor antagonist p‐MPPI (3 mg/kg, i.p.). At the same time, the activation of 5‐HT_2A receptor led to the essential suppression of 5‐HT_1A receptor functional activity, indicating the opposite effect of 5‐HT_2A receptor on pre‐ and postsynaptic 5‐HT_1A receptors. Thus, 5‐HT_2A/5‐HT_1A receptor interaction in the mechanism of catalepsy suppression in mice was shown.     

Effect of activation of serotonin 5-HT1A-receptors on the immune response in mice of the ASC strain with depressive-like behavior

Selective activation of serotonin 5-HT(1A)-receptors produced different effects on immunological reactivity in mice of ASC strain with genetic predisposition to depressive-like behavior, and parental CBA and AKR strains displaying no depressive reactions. Administration of 5-HT(1A)-receptors agonist 8-OH-DPAT at low dose (0.1 mg/kg) affecting upon presynaptic receptors resulted in immunostimulation in CBA mice and did not change the immune response level in mice of ASC strain. Activation of postsynaptic 5-HT(1A)-receptors with higher dose of 8-OH-DPAT (1.0 mg/kg) caused immunosuppression in CBA and AKR strains while under the same conditions the immune response of ASC mice was increased. Decrease the immune reactions in ASC mice was observed only after application of 8-OHDPAT at dose of 5 mg/kg. The changes of functional activity of pre- and postsynaptic 5-HT(1A)-receptors under a high predisposition to depressive-like behavior providing different effects of this receptor activation on immune function are discussed.     

The role of the glycoprotein gp130 in serotonin mediator system in mouse brain

Glycoprotein gp130 is involved in signaling out of significant cytokine receptors as interleukin-6 (IL-6), leukemia inhibitory factor and ciliary neurotrophic factor, which play critical role in immunity, inflammation and neurogenesis. IL-6 and brain neurotransmitter serotonin are involved in the mechanism of depression. The aim of this work was to investigat the role of protein gp130 in the regulation of expression of genes, coding the key enzyme of serotonin synthesis--tryptophan hydroxylase 2 (TPH2), 5-HT-transporter, 5-HT(1A)- and 5-HT(2A)-receptors of serotonin. The study was carried out on adult mouse males of AKR and congenic AKR.CBA-D13Mit76 strains, created by transfer of the fragment of chromosome 13 containing the gene coding gp130 protein from CBA/Lac strain to the genome of AKR/J strain. Decreased expression of 5-HT(1A) - 5-HT(2A)-receptor genes in hippocampus midbrain and TPH2 gene in midbrain in AKR.CBA-D13Mit76 mice compared with AKR mice were shown. Activation of nonspecific immunity by bacterial endotoxin lipopolysaccharide (LPS) administration did not affect the genes expression in AKR mice, but increased 5-HT(2A)-receptor expression in midbrain and decreased 5-HT(1A)-receptor expression in cortex in AKR.CBA-D13Mit76 mice. The results indicate: 1) the participation of gp130 in the regulation of TPH2, 5-HT(1A)- and 5-HT(2A)-receptor genes and 2) association of this protein in the genetically determined sensitivity to LPS.     

Implication of 5-HT2A receptors in the genetic mechanisms of the brain 5-HT system autoregulation

Brain serotonin (5-HT) system has been implicated in pathophysiology of anxiety, depression, drug addiction, and schizophrenia. 5-HT2A receptor is involved in the mechanisms of stress-induced psychopathology and impulsive behavior. Here, we investigated the role of 5-HT2A receptor in the autoregulation of the brain 5-HT system. The chronic treatment with agonist of 5-HT2A receptor DOI (1.0 mg/kg, i.p./14 days) produced considerable decrease of 5-HT2A receptor-mediated "head-twitches" in AKR/J mice indicating desensitization of 5-HT2A receptors. Chronic DOI treatment failed to alter 5-HT2A receptor gene expression in the midbrain, hippocampus and frontal cortex. At the same time, the increase in the expression of the gene encoding key enzyme of 5-HT synthesis, tryptophan hydroxylase 2 (TPH2), the increase in TPH2 activity and 5-HT levels and decreased expression of serotonin transporter (5-HTT) gene was found in the midbrain of DOI-treated mice. The results provide new evidence of receptor-gene cross-talk in the brain 5-HT system and the implication of 5-HT2A receptor in the autoregulation of the brain 5-HT system.     

Involvement of striatum serotonergic system in the expression of genetically defined catalepsy

The activity of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase, and specific binding of [3H]ketanserin to 5-HT2A receptors and [3H]8-OH-DPAT to 5-HT1A receptors in the striatum of genetically predisposed to catalepsy rats and mice have been studied. The activity of tryptophan hydroxylase in the striatum of rats bred for many generations for predisposition to catalepsy was higher than in nonselected rats. Mice of highly susceptible to pinch-induced catalepsy CBA strain also differed from noncataleptic AKR and C57BL mouse strains by higher activity of tryptophan hydroxylase in striatum. Inhibition of tryptophan hydroxylase with p-chlorophenylalanine or p-chloromethamphetamine significantly decreased immobility time in genetically predisposed to catalepsy rats and mice. A decrease in the [3H]ketanserin specific binding in the striatum of cataleptic rats and CBA mice was found indicating a decrease in 5-HT2A receptor density. A decrease in [3H]8-OH-DPAT binding in striatum of cataleptic rats but not in CBA mice was shown. These results indicate that serotonergic system of striatum is involved in the expression of hereditary catalepsy and suggest that hereditary catalepsy may result from genetic changes in the regulation of serotonin metabolism and reception in striatum.     

Involvement of 5-HT<Subscript>2A</Subscript> receptors in genetic mechanisms of autoregulation of brain 5-HT system

The brain serotonin (5-HT) system has been implicated in the pathophysiology of anxiety, depression, drug addiction, and schizophrenia. 5-HT_2A receptors are involved in the mechanisms of stressinduced psychopathology and impulsive behavior. In this work, we investigated the role of 5-HT_2A receptors in the autoregulation of the brain 5-HT system. Chronic treatment with DOI, a 5-HT_2A receptor agonist (1.0 mg/kg, i.p./14 days), produced a considerable decrease in the number of 5-HT_2A receptor-mediated head twitches in AKR/J mice, indicating the desensitization of 5-HT_2A receptors. Chronic DOI treatment did not affect the expression of the 5-HT_2A receptor gene in the midbrain, hippocampus and frontal cortex. At the same time, an increase in the expression of the gene encoding a key enzyme of 5-HT synthesis, tryptophan hydroxylase-2 (TPH-2), accompanied with an increase in TPH-2 activity and 5-HT levels, and decreased expression of the serotonin transporter (5-HTT) gene were observed in the midbrain of DOI-treated mice. These results provide new evidence of receptor-gene cross-talk in the brain 5-HT system and implication 5-HT_2A receptors in the autoregulation of the brain 5-HT system.     

The role of the glycoprotein gp130 in the serotonin mediator system in the mouse brain

Glycoprotein gp130 is involved in signal transduction from the receptors of such important cytokines as interleukin-6 (IL-6), leukemia inhibitory factor, and ciliary neurotrophic factor, which play a critical role in immunity, inflammation, and neurogenesis. Both IL-6 and the brain neurotransmitter serotonin are involved in the mechanism of depression. The aim of this work was to investigate the role of gp130 in regulating the gene expression of the tryptophan hydroxylase 2 (TPH2), the key enzyme of the serotonin synthesis, as well as of the 5-HT transporter and the 5-HT_1A and 5-HT_2A receptors. The study was carried out on adult male mice of the congenic strains AKR and AKR.CBA-D13Mit76; the latter was created by transferring a gp130-containing fragment of chromosome 13 from the CBA/Lac strain into the AKR/J genome. The expression of 5-HT_1A and 5-HT_2A receptor genes in the hippocampus and midbrain and of the TPH2 gene in the midbrain was decreased in AKR.CBA-D13Mit76 mice in comparison to AKR mice. Activation of nonspecific immunity by administration of a bacterial endotoxin lipopolysaccharide did not affect the gene expression in AKR mice but increased the 5-HT_2A receptor expression in the midbrain and decreased the 5-HT_1A receptor expression in the cortex in AKR.CBA-D13Mit76 mice. These results suggest that gp130 is involved in the regulation of TPH2, 5-HT_1A and 5-HT_2A receptor genes and is associated with the genetically determined sensitivity to lipopolysaccharides.     

Selective breeding for catalepsy changes the distribution of microsatellite D13Mit76 alleles linked to the 5-HT serotonin receptor gene in mice

Catalepsy (pronounced motor inhibition) is a natural defensive reaction against predator. Recently, the quantitative trait locus for catalepsy was mapped on mouse chromosome 13 near the 5-HT(1A) serotonin receptor gene. Here, the linkage between catalepsy and the 5-HT(1A) receptor gene was verified using breeding experiment. Selective breeding for high predisposition to catalepsy was started from backcross BC[CBA x (CBA x AKR)] generation between catalepsy-prone (CBA) and catalepsy-resistant (AKR) mouse strains. CBA and AKR strains also differed in the 5-HT(1A) receptor functional activity. A rapid increase of cataleptic percentage from 21.2% in the backcrosses to 71% in the third generation of selective breeding (S3) was shown. The fragment of chromosome 13 including the 5-HT(1A) receptor gene was marked with D13Mit76 microsatellite. Breeding for catalepsy increased the concentration of CBA-derived and decreased the concentration of AKR-derived alleles of microsatellite D13Mit76 in the S1 and S2. All mice of the S9 and S12 were homozygous for CBA-derived allele of D13Mit76 marker. Mice of the S12 showed CBA-like receptor activity. These findings indicate that selective breeding for behavior can involve selection of polymorphic variants of the 5-HT(1A) receptor gene.     

Effect of hypothyroidism on 5-HT1A-, 5-HT2A-receptors and serotonin transporter in the rat brain

Effects of thyroid hormone deficiency on 5-HT1A receptors, 5-HT2A receptors and serotonin transporter in the brain were studied in thyroidectomised Wistar rats receiving an iodine-free diet and receiving 15 micrograms/kg of thyroxine for 21 days. Binding of 3H-8-OH-DPAT to 5-HT1A receptors and 3H-cytalopram to serotonin transporter were unchanged in hypothyroid rats as compared to the control. 3H-ketanserin binding to 5-HT2A receptors was significantly decreased in the frontal cortex in hypothyroid rats. The cortical 3H-ketanserin binding in thyroidectomised rats was normalised after thyroxine replacement. The data suggest that the decrease in the cortical 5-HT2A receptors is the main consequence of impairing effect of hypothyroidism on serotonin neurotransmission.     

Functional Responses to the Chronic Activation of 5-HT<Subscript>1A</Subscript> Receptors in Mice with Genetic Predisposition to Catalepsy

The effects of chronic 5-HT_1A receptor activation on the behavior, functional activity of 5-HT_1A receptors, and expression of key genes of the brain 5-HT system were studied in mice of the catalepsy-prone CBA strain and the catalepsy-resistant C57BL/6 strain. Chronic treatment with 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (1.0 mg/kg i.p., 14 days) led to a significant decrease in the hypothermic response to acute administration of 8-OH-DPAT in CBA and C57BL/6 mice, which indicates the desensitization of 5-HT_1A receptors in both strains. Pretreatment with the 5-HT⁷ receptor agonist SB 269970 did not affect the hypothermic response to the acute administration of 8-OH-DPAT, which suggests an independent functional response of 5-HT_1A receptors. The treatment did not induce any changes in the behavior in the open field paradigm in CBA mice, but significantly increased the total path, the time spent in the center, and the number of rearings in C57BL/6 mice, which indicates the enhancement of locomotor and exploratory activity in C57BL/6 mice. The chronic activation of 5-HT_1A receptor downregulated 5-HT_1A gene expression, as well as the expression of the gene that encodes tryptophan hydroxylase 2, a key enzyme of 5-HT biosynthesis, in the midbrain and the expression of the gene that encodes the 5-HT_2A receptor in the frontal cortex of CBA, but not C57BL/6 mice. The obtained data provide a new evidence on the receptor–gene cross talk in the brain 5-HT system that may underlie the loss of pharmacological efficacy of 5-HT_1A receptor agonists. In turn, the loss of the behavioral response and compensatory alterations in key genes of the brain 5- HT system in CBA mice suggests that catalepsy-prone and -resistant genotypes demonstrate different sensibility to the effects of drugs.     

Endogenous 5-HT2B receptor activation regulates neonatal respiratory activity in vitro

An implication of 5-HT(2B) receptors in central nervous system has not yet been clearly elucidated. We studied the role of different 5-HT(2) receptor subtypes in the medullary breathing center, the pre-B?tzinger complex, and on hypoglossal motoneurons in rhythmically active transversal slice preparations of neonatal rats and mice. Local microinjection of 5-HT(2) receptor agonists revealed tonic excitation of hypoglossal motoneurons. Excitatory effects of the 5-HT(2B) receptor agonist BW723C86 could be blocked by bath application of LY272015, a highly selective 5-HT(2B) receptor antagonist. Excitatory effects of the 5-HT(2A/B/C) receptor agonist alpha-methyl 5-HT could be blocked by the preferential 5-HT(2A) receptor antagonist ketanserin. Therefore, 5-HT-induced excitation of hypoglossal motoneurons is mediated by convergent activation of 5-HT(2A) and 5-HT(2B) receptors. Local microinjection of BW723C86 in the pre-B?tzinger complex increased respiratory frequency. Bath application of LY272015 blocked respiratory activity, whereas ketanserin had no effect. Therefore, endogenous 5-HT appears to support tonic action on respiratory rhythm generation via 5-HT(2B) receptors. In preparations of 5-HT(2B) receptor-deficient mice, respiratory activity appeared unaltered. Whereas BW723C86 and LY272015 had no effects, bath application of ketanserin disturbed and blocked rhythmic activity. This demonstrates a stimulatory role of endogenous 5-HT(2B) receptor activation at the pre-B?tzinger complex and hypoglossal motoneurons that can be taken up by 5-HT(2A) receptors in the absence of 5-HT(2B) receptors. The presence of functional 5-HT(2B) receptors in the neonatal medullary breathing center indicates a potential convergent regulatory role of 5-HT(2B) and -(2A) receptors on the central respiratory network.     

Imipramine and citalopram reverse corticosterone-induced alterations in the effects of the activation of 5-HT(1A) and 5-HT(2) receptors in rat frontal cortex.

Using extracellular recording we studied changes in the reactivity of rat frontal cortical slices to the 5-HT(1A), 5-HT(2) and 5-HT(4) receptor agonists, (+/-)-2-dipropyloamino-8-hydroxy-1,2,3,4-tetrahydronaphtalene hydrobromide (8-OH-DPAT), (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and zacopride, respectively, induced by an earlier treatment of animals with corticosterone lasting 1 or 3 weeks. Spontaneous bursting activity was recorded in ex vivo slices incubated in a medium devoid of Mg(2+) ions and containing picrotoxin (30 microM). Repetitive, but not single, corticosterone administration resulted in an attenuation of the effect of the activation of 5-HT(1A) receptors and in an enhancement of the effect related to 5-HT(2) receptors. The effect of 5-HT(4) receptor activation remained unchanged. In separate two sets of experiments rats were treated with corticosterone for 3 weeks and additionally with imipramine or citalopram, beginning on the eighth day of corticosterone administration. In the corticosterone plus imipramine as well as corticosterone plus citalopram groups the effects of 8-OH-DPAT and DOI were not different from control indicating that corticosterone-induced functional modifications in the reactivity of 5-HT(1A) and 5-HT(2) receptors were reversed by antidepressant treatments.     

Brain region-specific alterations of 5-HT2A and 5-HT2C receptors in serotonin transporter knockout mice

The aim of the present studies was to determine the effects of reduced or absent serotonin (5-HT) transporters (5-HTTs) on 5-HT2A and 5-HT2C receptors. The density of 5-HT2C receptors was significantly increased in the amygdala and choroid plexus of 5-HTT knockout mice. On the other hand, the density of 5-HT2A receptors was significantly increased in the hypothalamus and septum, but reduced in the striatum, of 5-HTT knockout mice. However, 5-HT2A mRNA was not changed in any brain region measured. 5-HT2C mRNA was significantly reduced in the choroid plexus and lateral habenula nucleus of these mice. The function of 5-HT2A receptors was evaluated by hormonal responses to (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Oxytocin, but not adrenocorticotrophic hormone or corticosterone, responses to DOI were significantly greater in 5-HTT knockout mice. In addition, Gq and G11 proteins were not significantly changed in any brain region measured. The present results suggest that the constitutive alteration in the function of 5-HTTs changes the density of 5-HT2A and 5-HT2C receptors in a brain region-specific manner. These changes may not be mediated by alterations in their gene expression or in the level of Gq/11 proteins. The alterations in these receptors may be related to the altered behaviors of 5-HTT knockout mice.     

Curcumin prevents corticosterone-induced neurotoxicity and abnormalities of neuroplasticity via 5-HT receptor pathway

Curcumin, a major active component of Curcuma longa, possesses antioxidant and neuroprotective activities. The present study explores the mechanisms underlying the neuroprotective effect of curcumin against corticosterone and its relation to 5-hydroxy tryptamine (5-HT) receptors. Exposure of cortical neurons to corticosterone results in decreased mRNA levels for three 5-HT receptor subtypes, 5-HT(1A), 5-HT(2A) and 5-HT(4), but 5-HT(1B,) 5-HT(2B), 5-HT(2C), 5-HT(6) and 5-HT(7) receptors remain unchanged. Pre-treatment with curcumin reversed this effect on mRNA for the 5-HT(1A) and 5-HT(4) receptors, but not for the 5-HT(2A) receptor. Moreover, curcumin exerted a neuroprotective effect against corticosterone-induced neuronal death. This observed effect of curcumin was partially blocked by either 5-HT(1A) receptor antagonist p-MPPI or 5-HT(4) receptor antagonist RS 39604 alone; whereas, the simultaneous application of both antagonists completely reversed the effect. Curcumin was also found to regulate corticosterone-induced morphological changes such as increases in soma size, dendritic branching and dendritic spine density, as well as elevate synaptophysin expression in cortical neurons. p-MPPI and RS 39604 reversed the effect of curcumin-induced change in neuronal morphology and synaptophysin expression of corticosterone-treated neurons. In addition, an increase in cyclic adenosine monophosphate (cAMP) level was observed after curcumin treatment, which was further prevented by RS 39604, but not by p-MPPI. However, curcumin-induced elevation in protein kinase A activity and phosphorylation of cAMP response element-binding protein levels were inhibited by both p-MPPI and RS 39604. These findings suggest that the neuroprotection and modulation of neuroplasticity exhibited by curcumin might be mediated, at least in part, via the 5-HT receptor-cAMP-PKA-CREB signal pathway.     

5-HT1B receptor knockout mice exhibit an enhanced response to constant light

Serotonin (5-HT) can act presynaptically at 5-HT1B receptors on retinal terminals in the suprachiasmatic nucleus (SCN) to inhibit glutamate release, thereby modulating the effects of light on circadian behavior. 5-HT1B receptor agonists (1) inhibit light-induced phase shifts of circadian activity rhythms, (2) attenuate light-induced Fos expression in the SCN, and (3) reduce the amplitude of optic nerve-evoked excitatory postsynaptic currents in SCN neurons in vitro. To determine whether functional disruption of the 5-HT1B presynaptic receptors would result in an amplified response of the SCN to light, the period (tau) of the circadian rhythm of wheel-running activity was estimated under several different conditions in 5-HT1B receptor knockout (KO) mice and genetically matched wild-type animals. Under constant light (LL) conditions, the tau of 5-HT1B receptor KO mice was significantly greater than the tau of wild-type mice. A quantitative analysis of the wheel-running activity revealed no differences between wild-type and KO mice in either total activity or the temporal distribution of activity under LL conditions, suggesting that the observed increase in tau was not a function of reduced activity. Under constant dark conditions, the period of the circadian rhythm of wheel-running activity of wild-type and 5-HT1B receptor KO mice was similar. In addition, no differences were noted between wild-type and 5-HT1B receptor KO mice in the rate of reentrainment to a 6 h phase advance in the 12:12 light:dark cycle or in phase shifts in response to a 10 min light pulse presented at circadian time 16. The enhanced response of the SCN circadian clock of the 5-HT1B receptor KO mice to LL conditions is consistent with the hypothesis that the endogenous activation of 5-HT1B presynaptic receptors modulates circadian behavior by attenuating photic input to the SCN.     

Novel class of arylpiperazines containing N-acylated amino acids: their synthesis, 5-HT1A, 5-HT2A receptor affinity, and in vivo pharmacological evaluation

Novel arylpiperazines with N-acylated amino acids, selected on the basis of a preliminary screening of two libraries previously synthesized on SynPhase Lanterns, were prepared in solution and their affinity for 5-HT(1A), 5-HT(2A), and D(2) receptors was evaluated. The compounds bearing (3-acylamino)pyrrolidine-2,5-dione (19-26) and N-acylprolinamide (29-34) moieties showed high affinity for 5-HT(1A) (K(i)=3-47 nM), high-to-low for 5-HT(2A) (K(i)=4.2-990 nM), and low for D(2) receptors (K(i)=0.77-21.19 microM). All the new o-methoxy derivatives of (3-acylamino)pyrrolidine-2,5-diones tested in vivo revealed agonistic activity at postsynaptic 5-HT(1A) receptors, while m-chloro derivatives were classified as antagonists of these sites; similar relations were observed for o-methoxy (29) and m-chlorophenylpiperazine derivatives of N-acylprolinamides. The reported results show that the amino acid-derived terminal fragment modified the in vivo functional profile. Finally, the selected compounds 19 and 20, a 5-HT(1A) partial agonist and a full agonist, respectively, and 26, a mixed 5-HT(1A)/5-HT(2A) antagonist, were evaluated in preclinical animal models of depression and anxiety. The project allowed selecting the lead compound 20 which exhibited an anxiolytic-like effect in the four-plate test in mice and revealed distinct antidepressant-like effects in the forced swimming and tail suspension tests in mice.