Dibutyryl-cAMP increases functions of 5-hydroxytryptamine2 receptors, but not of beta 2-adrenergic receptors, in a clonal cell line of rat neurotumor RT4.

A peripheral nervous system cell line RT4-B, established by Imada and Sueoka (Dev. Biol., 66:97-108, 1978), was shown to respond to serotonin [5-hydroxytryptamine (5-HT)] and catecholamines. 5-HT induced a small and transient increase in cytosolic free Ca2+ concentration ([Ca2+]i) in the RT4-B cells. The increase was effectively blocked by 5-HT2 receptor antagonists (spiperone, ritanserin and mianserin), but not by a 5-HT3 receptor antagonist (MDL72222), or a alpha 1-adrenergic receptor antagonist (prazosin), indicating that RT4-B cells express 5-HT2 receptors. On the other hand, catecholamines increased cyclic AMP production by RT4-B. The order of potency for stimulating cyclic AMP synthesis was isoproterenol greater than epinephrine much greater than norepinephrine much greater than dopamine, and the stimulation was effectively inhibited by the nonselective beta-adrenergic receptor antagonist propranolol, but not by the beta 1-adrenergic receptor antagonist atenolol, suggesting that RT4-B cells express beta 2-adrenergic receptors. The differentiating agent N6,2'-O-dibutyryladenosine 3',5'-monophosphate (dibutyryl-cAMP) enhanced the 5-HT-induced [Ca2+]i increase, but not the catecholamine-induced cyclic AMP production. The increase in the 5-HT response paralleled the increase in the density of 5-HT2 receptors. n-Butyric acid (2 mM) and 8-bromoadenosine 3',5'-monophosphate (1 mM) also increased the 5-HT response, and the sum of these increases was nearly equal to that induced by dibutyryl-cAMP. These results indicate that RT4-B is a novel model cell line for the study of 5-HT2 and beta 2-adrenergic receptors and their second messenger responses and for the analysis of the mechanisms how 5-HT2 receptor gene expression is controlled.     

Serotonergic modulation of retinal input to the mouse suprachiasmatic nucleus mediated by 5-HT1B and 5-HT7 receptors

Serotonin (5-HT) and 5-HT receptor agonists can modify the response of the mammalian suprachiasmatic nucleus (SCN) to light. It remains uncertain which 5-HT receptor subtypes mediate these effects. The effects of 5-HT receptor activation on optic nerve-mediated input to SCN neurons were examined using whole-cell patch-clamp recordings in horizontal slices of ventral hypothalamus from the male mouse. The hypothesis that 5-HT reduces the effect of retinohypothalamic tract (RHT) input to the SCN by acting at 5-HT1B receptors was tested first. As previously described in the hamster, a mixed 5-HT(1A/1B) receptor agonist, 1-[3-(trifluoromethyl)phenyl]-piperazine hydrochloride (TFMPP), reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by selectively stimulating the optic nerve of wild-type mice. The agonist was negligibly effective in a 5-HT1B receptor knockout mouse, suggesting minimal contribution of 5-HT1A receptors to the TFMPP-induced reduction in the amplitude of the optic nerve-evoked EPSC. We next tested the hypothesis that 5-HT also reduces RHT input to the SCN via activation of 5-HT7 receptors. The mixed 5-HT(1A/7) receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), reduced the evoked EPSC amplitude in both wild-type and 5-HT1B receptor knockout mice. This effect of 8-OH-DPAT was minimally attenuated by the selective 5-HT1A receptor antagonist WAY 100635 but was reversibly and significantly reduced in the presence of ritanserin, a mixed 5-HT(2/7) receptor antagonist. Taken together with the authors' previous ultrastructural studies of 5-HT1B receptors in the mouse SCN, these results indicate that in the mouse, 5-HT reduces RHT input to the SCN by acting at 5-HT1B receptors located on RHT terminals. Moreover, activation of 5-HT7 receptors in the mouse SCN, but not 5-HT1A receptors, also results in a reduction in the amplitude of the optic nerve-evoked EPSC. The findings indicate that 5-HT may modulate RHT glutamatergic input to the SCN through 2 or more 5-HT receptors. The likely mechanism of altered RHT glutamatergic input to SCN neurons is an alteration of photic effects on the SCN circadian oscillator.     

Polymorphism in 5-HT receptors as the background of serotonin functional diversity

The review concentrates on the role of different types and subtypes of 5-HT receptors in physiological and behavioural effects of the brain neurotransmitter serotonin. Specifically it describes: 1) the effects of 5-HT1A and 5-HT1B receptors on aggressive behavior, sexual arousal, food and water consumption; 2) the data showing reciprocal effect of 5-HT2A, 5-HT2C receptor agonists; 3) interaction of 5-HT3 and 5-HT1A-receptors in 5-HT3-induced hypothermia. The review provides converging lines of evidence that: different types and subtypes of 5-HT receptors are involved in the regulation of various kinds of behavior as additive as well as opposite factors providing neuroplasticity, compensatory and adaptive mechanism.     

Serotonin decreases generation of dopaminergic neurons from mesencephalic precursors via serotonin type 7 and type 4 receptors

Inductive signals mediating the differentiation of neural precursors into serotonergic (5-HT) or dopaminergic neurons have not been clarified. We have recently shown that in cell aggregates obtained from rat mesencephalic precursors, reduction of serotonin levels induces a marked increase in generation of dopaminergic neurons. In the present study we treated rat neurospheres with antagonists of the main subtypes of 5-HT receptors, 5-HT transport inhibitors, or 5-HT receptor agonists, and studied the effects on generation of dopaminergic neurons. Cultures treated with Methiothepin (5-HT(1,2,5,6,7) receptor antagonist), the 5-HT(4) receptor antagonist GR113808;67:00-.or the 5-HT(7) receptor antagonist SB 269970 showed a significant increase in generation of dopaminergic cells. Treatment with the 5-HT(1B/1D) antagonist GR 127935, the 5-HT(2) antagonist Ritanserin, the 5-HT transporter inhibitor Fluoxetine, the dopamine and norepinephrine transport inhibitor GBR 12935, or with both inhibitors together, or 5-HT(4) or 5-HT(7) receptor agonists induced significant decreases in generation of dopaminergic cells. Cultures treated with WAY100635 (5-HT(1A) receptor antagonist), the 5-HT(3) receptor antagonist Ondasetron, or the 5-HT(6) receptor antagonist SB 258585 did not show any significant changes. Therefore, 5-HT(4) and 5-HT(7) receptors are involved in the observed serotonin-induced decrease in generation of dopaminergic neurons from proliferating neurospheres of mesencephalic precursors. 5-HT(4) and 5-HT(7) receptors were found in astrocytes and serotonergic cells using double immunolabeling and laser confocal microscopy, and the glial receptors appeared to play a major role.     

Different receptor subtypes are involved in the serotonin-induced modulation of epileptiform activity in rat frontal cortex in vitro.

The frontal cortex is innervated by serotonergic terminals from the raphe nuclei and it expresses diverse 5-HT receptor subtypes. We investigated the effects of 5-HT and different 5-HT receptor subtype-selective agonists on spontaneous discharges which had developed in rat cortical slices perfused with a Mg2+-free medium and the GABA(A) receptor antagonist picrotoxin. The frequency of synchronous discharges, recorded extracellularly in superficial layers (II/III) of the frontal cortex, was dose-dependently enhanced by 5-HT (2.5-40 microM). That excitatory effect was blocked by the 5-HT2 receptor selective antagonist ketanserin. The 5-HT2A/2C receptor-selective agonist DOI and the 5-HT4 receptor agonist zacopride also increased the frequency of spontaneous discharges. In the presence of ketanserin, 5-HT decreased the discharge rate; a similar effect was observed when the 5-HT1A receptor agonist 8-OH-DPAT or the 5-HT1B receptor agonist CGS-12066B was applied. The 5-HT3 receptor agonist m-CPBG was ineffective. In conclusion, 5-HT produces multiple effects on epileptiform activity in the frontal cortex via activation of various 5-HT receptor subtypes. The excitatory action of 5-HT, which predominates, is mediated mainly by 5-HT2 receptors. The inhibitory effects can be attributed to activation of 5-HT1A and 5-HT1B receptors.     

Hyperglycemic properties of serotonin receptor antagonists

Several serotonin (5-HT) receptor antagonists with varying specificities for the 5-HT receptor types, were studied with regard to their effects on blood glucose levels in mice. The non-selective antagonists, metergoline and methysergide, proved to be hyperglycemic at doses commonly used to antagonize 5-HT receptors. In contrast, ritanserin (a 5-HT2 and 5-HT1c antagonist) and MDL 72222 (a 5-HT3 antagonist) were effective only at doses which surpassed the dose range considered to be selective for their respective receptors. The results suggest that 5-HT systems play a role in maintaining glucose homeostasis and that 5-HT1 receptors may be particularly important in this function. Furthermore, the inherent hyperglycemic properties of non-selective serotonin antagonists described here, are pertinent to studies using these agents to investigate glucose metabolism.     

An analysis of the 5-hydroxytryptamine (serotonin) receptor subtypes of central neurones of Helix aspersa.

1. Intracellular recordings were made from identified neurones in the central nervous system of Helix aspersa. Two types of cell were used, those excited by 5-hydroxytryptamine (5-HT) and acetylcholine and those inhibited by 5-HT and dopamine. The actions of a range of 5-HT agonists and antagonists were tested for their ability to interact with 5-HT receptors. 2. 5-Carboxyamidotryptamine, alpha-methyl-5-HT and N-methyl-5-HT were active on cells excited by 5-HT, with similar potencies to 5-HT. Only 5-carboxyamidotryptamine and 5-methoxytryptamine were equiactive with 5-HT on cells inhibited by 5-HT. Most of the non-indole analogues were inactive or very weak agonists on both receptors. 3. MDL 72222 was the most active antagonist tested against 5-HT excitation, showing some selectivity for 5-HT over acetylcholine. Cinanserin and ketanserin also showed selectivity for 5-HT over acetylcholine. 4. Tryptamine was inhibitory on both cell types and was a potent antagonist of 5-HT excitation, showing selectivity for 5-HT over acetylcholine. 5. It is concluded that the 5-HT excitatory receptor recognizes the indole nucleus with substitution on position 5, save for 5-fluorotryptamine which was inhibitory. It does not appear that these 5-HT receptors can be classified in terms of the vertebrate subtypes of 5-HT receptor. However, it should be noted that only two receptor subtypes located on a small number of neurones were studied in these experiments and other 5-HT receptor suptypes may be located on other groups of neurones and peripheral tissues. These receptors may recognize other 5-HT receptor ligands including non-indoles.     

Serotonergic regulation of blood glucose levels in the crayfish, Procambarus clarkii: site of action and receptor characterization

The present study investigated the site of action of 5-hydroxytryptamine (5-HT) and pharmacologically characterized the receptors involved in regulating blood glucose levels in the crayfish, Procambarus clarkii. Injection of 5-HT into intact animals increased glucose levels in a dose-dependent manner. In contrast, 5-HT failed to elicit a hyperglycemic response in eyestalk-ablated animals. Effects of several 5-HT receptor agonists and antagonists were examined. 5-CT, oxymetazoline (both 5-HT(1) receptor agonists) and alpha-methyl-5-HT (a 5-HT(2) receptor agonist), but not 1-phenylbiguanide, m-CPBG (both 5-HT(3) receptor agonists), or RS 67333 (a 5-HT(4) receptor agonist), induced hyperglycemic responses in a dose-dependent manner. In addition, 8-OH-DPAT (a 5-HT(1A) receptor agonist), L-694,247 (a 5-HT(1B/1D) receptor agonist), and DOI (a 5-HT(2A) receptor agonist) were effective in significantly increasing the glucose levels, whereas both BW 723C86 (a 5-HT(2B) receptor agonist) and m-CPP (a 5-HT(2C) receptor agonist) were ineffective. Finally, ketanserin (a 5-HT(2A) receptor antagonist), but not p-MPPF (a 5-HT(1A) receptor antagonist), GR 55562 (a 5-HT(1B/1D) receptor antagonist), SB 206553 (a 5-HT(2B/2C) receptor antagonist), or tropisetron (a 5-HT(3) receptor antagonist), was able to block 5-HT-induced hyperglycemia. The combined results support the hypothesis that 5-HT exerts its hyperglycemic effect by enhancing the release of hyperglycemic factor(s) from the eyestalks, and suggest that 5 HT-induced hyperglycemia is mediated by 5-HT(1)- and 5-HT(2)-like receptors.     

5-HT activates ERK MAP kinase in cultured-human peripheral blood mononuclear cells via 5-HT1A receptors

In the present work, we tested the hypothesis that serotonin (5-hydroxytryptamine = 5-HT) might activate the extracellular signal-regulated kinase (ERK) pathway in human peripheral blood mononuclear cells (PBMC). PBMC were maintained in culture for 72 hrs at 37 degrees C prior to the addition of 5-HT. Our results showed an increase in ERK activation by 5-HT with a peak effect at 30 min and maximal stimulation with 5-HT at 1microM. This activation of ERK did not occur in adherent monocytes suggesting that the effect was on lymphocytes. In addition, p38 MAP kinase was not activated under these conditions. The effect of 5-HT on ERK activation appeared to be mediated through the activation of 5-HT1A receptors since similar results were obtained with R-+-8-hydroxy-DPAT, a selective 5-HT1A receptor agonist and WAY100635, a selective 5-HT1A receptor antagonist, reversed the 5-HT and the R-+-8-hydroxy-DPAT effects. Results from Western blot analysis confirmed the presence of 5-HT1A receptors on the PBMC. A 5-HT2A antagonist, ketanserin, and a 5-HT transport inhibitor, fluoxetine, both failed to block the activation of ERK by 5-HT. Our results indicate that 5-HT activates ERK, but not p38, MAP kinase of human PBMC via a 5-HT1A receptor.     

An analysis of the 5-hydroxytryptamine (serotonin) receptor subtypes of central neurones of Helix aspersa

1. Intracellular recordings were made from identified neurones in the central nervous system of Helix aspersa. Two types of cell were used, those excited by 5-hydroxytryptamine (5-HT) and acetylcholine and those inhibited by 5-HT and dopamine. The actions of a range of 5-HT agonists and antagonists were tested for their ability to interact with 5-HT receptors.2. 5-Carboxyamidotryptamine, α-methyl-5-HT and N-methyl-5-HT were active on cells excited by 5-HT, with similar potencies to 5-HT. Only 5-carboxyamidotryptamine and 5-methoxytryptamine were equiactive with 5-HT on cells inhibited by 5-HT. Most of the non-indole analogues were inactive or very weak agonists on both receptors.3. MDL 72222 was the most active antagonist tested against 5-HT excitation, showing some selectivity for 5-HT over acetylcholine. Cinanserin and ketanserin also showed selectivity for 5-HT over acetylcholine.4. Tryptamine was inhibitory on both cell types and was a potent antagonist of 5-HT excitation, showing selectivity for 5-HT over acetylcholine.5. It is concluded that the 5-HT excitatory receptor recognizes the indole nucleus with substitution on position 5, save for 5-fluorotryptamine which was inhibitory. It does not appear that these 5-HT receptors can be classified in terms of the vertebrate subtypes of 5-HT receptor. However, it should be noted that only two receptor subtypes located on a small number of neurones were studied in these experiments and other 5-HT receptor subtypes may be located on other groups of neurones and peripheral tissues. These receptors may recognize other 5-HT receptor ligands including non-indoles.     

Activation of 5-HT1A receptors in medullary raphé disrupts sleep and decreases shivering during cooling in the conscious piglet

Activation of 5-HT1A receptors in the medullary raphé decreases sympathetically mediated brown adipose tissue (BAT) thermogenesis and peripheral vasoconstriction when previously activated with leptin, LPS, prostaglandins, or cooling. It is not known whether shivering is also modulated by medullary raphé 5-HT1A receptors. We previously showed in conscious piglets that activation of 5-HT1A receptors with (+/-)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the raphé that contains substantial numbers of 5-HT neurons, eliminates rapid eye movement (REM) sleep and decreases shivering in a cold environment, but does not attenuate peripheral vasoconstriction. Hoffman JM, Brown JW, Sirlin EA, Benoit AM, Gill WH, Harris MB, Darnall RA. Am J Physiol Regul Integr Comp Physiol 293: R518-R527, 2007. We hypothesized that, during cooling, activation of 5-HT1A receptors in the medullary raphé would also eliminate REM sleep and, in contrast to activation of 5-HT1A receptors in the PGCL, would attenuate both shivering and peripheral vasoconstriction. In a continuously cool environment, dialysis of 8-OH-DPAT into the medullary raphé resulted in alternating brief periods of non-REM sleep and wakefulness and eliminated REM sleep, as observed when 8-OH-DPAT is dialyzed into the PGCL. Moreover, both shivering and peripheral vasoconstriction were significantly attenuated after 8-OH-DPAT dialysis into the medullary raphé. The effects of 8-OH-DPAT were prevented after dialysis of the selective 5-HT1A receptor antagonist WAY-100635. We conclude that, during cooling, exogenous activation of 5-HT1A receptors in the medullary raphé decreases both shivering and peripheral vasoconstriction. Our data are consistent with the hypothesis that neurons expressing 5-HT1A receptors in the medullary raphé facilitate spinal motor circuits involved in shivering, as well as sympathetic stimulation of other thermoregulatory effector mechanisms.     

Vagal CCK and 5-HT(3) receptors are unlikely to mediate LPS or IL-1beta-induced fever

Previous studies suggested that peripheral immune mediators may involve intermediates acting on the vagus nerve, such as CCK or serotonin (5-HT). We have therefore investigated a possible role for vagal CCK-A and 5-HT(3) receptors in the febrile response after intraperitoneal human recombinant interleukin-1beta (IL-1beta) or lipopolysaccharide (LPS). Unanesthetized, adult male rats instrumented with abdominal thermistors were given intraperitoneal CCK-8 sulfate (100 or 150 microgram/kg) or 2-methyl-5-hydroxytryptamine maleate (4 mg/kg). In other experiments, rats were treated with either antagonists to the 5-HT(3) receptor (ondansetron HCl; 100 microgram/kg) or the CCK-A receptor (L-364,718, 100 or 200 microgram/kg) in combination with LPS or IL-1beta. CCK administration caused a short-lived hypothermia, but interference with the action of endogenous CCK at CCK-A receptors was without effect on IL-1beta- or LPS-induced fever. Neither activation of 5-HT(3) receptors nor blockade of 5-HT(3) receptors affected body temperature or LPS fever. Taken together, our data support the idea that vagal afferents responsive to pyrogenic cytokines may be different from those responsive to CCK or 5-HT.     

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.     

Locomotor and peripheral effects of sibutramine modulated by 5-HT2 receptors

Sibutramine has been described as an anti-obesity drug with the ability to inhibit serotonin (5-HT), noradrenaline, and dopamine re-uptake, but without affinity to histamine and muscarinic receptors. On the other hand, cyproheptadine antagonizes serotonin 5-HT(2A), 5-HT(2B), and 5-HT(2C), histamine H1, and muscarinic (M) receptors. There are many reports concerning the influence of sibutramine on central serotoninergic pathways. In this study, we suggest that peripheral pathways may also be involved in the serotoninergic effects of sibutramine. In vivo experiments were undertaken to investigate the serotoninergic effects of sibutramine on body mass, the glycogen concentration in the diaphragm of rats, and locomotor behaviour. Rats were submitted to oral treatment with sibutramine, cyproheptadine, or sibutramine applied in combination with cyproheptadine, for a period of 2 months to investigate the 5-HT2 effects of sibutramine on these parameters. As the results demonstrated, the lower increase in body mass and the increased glycogen levels in the diaphragm muscle of rats treated with sibutramine seem to be modulated by 5-HT2 receptors, since these effects were completely antagonized by cyproheptadine in the group treated with the 2 drugs co-applied. Furthermore, the behavioural results also suggest that mechanisms modulated by 5-HT2 receptors are involved in the increase of locomotion in the rats treated with sibutramine, since the effect did not occur in the rats treated with sibutramine co-applied with the 5-HT2 receptor antagonist, cyproheptadine. The results suggest that sibutramine modifies energy-related parameters such as body mass, diaphragm glycogen, and locomotor behaviour in rats via 5-HT2 serotoninergic pathways.     

Design and synthesis of 2-[4-[4-(m-(ethylsulfonamido)-phenyl) piperazin-1-yl]butyl]-1,3-dioxoperhydropyrrolo[1,2-c]imidazole (EF-7412) using neural networks. A selective derivative with mixed 5-HT1A/D2 antagonist properties.

A test series of 32 phenylpiperazines III with affinity for 5-HT1A and alpha1 receptors was subjected to QSAR analysis using artificial neural networks (ANNs), in order to get insight into the structural requirements that are responsible for 5-HT1A/alpha1 selectivity. Good models and predictive power were obtained for 5-HT1A and alpha1 receptors. A comparison of these models gives information for the design of the new ligand EF-7412 (5-HT1A:Ki(nM)= 27; alpha1: Ki(nM) > 1000). This derivative displayed affinity for dopamine D2 receptor (Ki = 22 nM) and is selective for all other receptor examined (5-HT2A, 5-HT3, 5-HT4 and Bz). EF-7412 acts an antagonist in vivo in pre- and postsynaptic 5-HT1A receptor sites and as an antagonist in dopamine D2 receptor.     

The 5-HT(1A) receptor agonist 8-OH-DPAT prevents prefrontocortical glutamate and serotonin release in response to blockade of cortical NMDA receptors

We studied the role of 5-HT(1A) receptors in controlling the release of glutamate (GLU) in the medial prefrontal cortex (mPFC) of conscious rats with the in vivo microdialysis technique. The effect of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin infused in the prefrontal cortex was examined under basal conditions and on the rise of extracellular GLU (+106%) induced by co-infusion of the competitive N-methyl-d-aspartate receptor antagonist 3-[(R)-2-carboxypiperazin-4yl]-propyl-1-phosphonic acid (CPP). 8-OH-DPAT (0.3 and 3 microm) had no effect on basal extracellular GLU, but the higher concentration completely abolished the rise of extracellular GLU induced by CPP. CPP also increased extracellular serotonin (5-HT) in the mPFC (+50%) and this effect was antagonized by 3 microm 8-OH-DPAT which, by itself, had no effect on basal 5-HT release. The effects of 8-OH-DPAT on extracellular GLU and 5-HT were reversed by the 5-HT(1A) receptor antagonist WAY100 635 (100 microm), indicating a selective involvement of 5-HT(1A) receptors. WAY100 635 had no effect by itself. These results show that the stimulation of cortical 5-HT(1A) receptors prevents the CPP-evoked rise of extracellular GLU and 5-HT and suggest that these effects may contribute to the ability of intracortical 8-OH-DPAT to counteract cognitive deficits caused by the blockade of NMDA receptors.     

The 5-hydroxytryptamine stimulated formation of inositol phosphate is inhibited in platelets from alcoholics

The accumulation of inositol monophosphate (IP1) was measured after stimulation of 5-hydroxytryptamine2 (5-HT2) receptors on platelets from alcoholics and healthy controls. In controls, 5-HT induced a dose-dependent response with an EC50 = 2 x 10(-6) M and a maximal response at 10(-5) M. Ritanserin, a selective 5-HT2 antagonist, markedly reduced the accumulation. The IP1 formation after stimulation by 10(-5) M 5-HT was significantly impaired in platelets from alcoholics as compared to controls. This study indicates that the 5-HT2 receptor function is inhibited in alcoholics. It also illustrates the possibility of using IP1 formation in peripheral cells as a mean of studying receptor function in disease.     

5-Hydroxytryptamine affects rat migrating myoelectric complexes through different receptor subtypes: evidence from 5-hydroxytryptophan administration

The effect of the serotonin precursor 5-hydroxytryptophan (5-HTP) on jejunal migrating myoelectric complexes (MMCs) was investigated in conscious rats. Subcutaneous administration of low doses of 5-HTP (1-2 mg/kg) shortened the period between migrating complexes, whereas high doses of the compound (4-8 mg/kg) disrupted the MMC pattern. The serotonin (5-HT2) antagonist methysergide (8 mg/kg s.c.) did not alter basal MMC, neither did it prevent the effect of a low dose of 5-HTP; conversely, it antagonized the disruption due to the high dose. The 5-HT3 antagonist ICS 205-930 (30 micrograms/kg s.c.) decreased MMC frequency; administration of 2 mg/kg 5-HTP following ICS 205-930 brought the frequency of myoelectric complexes back to basal values. Both effects of 5-HTP were prevented by the decarboxylase inhibitor benserazide (85 mg/kg i.p.), which per se caused a transient inhibition of spiking activity. The results suggest that rat MMCs can be influenced in a composite fashion by progressively increasing concentrations of 5-HT, which in turn activate different receptor subtypes. A peripheral neuronal receptor, probably belonging to the 5-HT3 subclass, mediates the increase in MMC frequency observed after low doses of 5-HTP; higher levels of serotonin activate 5-HT2 receptors, causing disruption of cycling activity. Additionally, 5-HT3 receptors, but not 5-HT2, appear to be relevant for the regulation of the MMC pattern by the endogenous amine.     

Serotonergic modulation of murine fundic tone

Fundic tone is maintained through a balance of excitatory and inhibitory input to fundic smooth muscle. The aim of this study was to determine the role of serotonin (5-HT) and 5-HT receptors in modulating murine fundic tone. Muscle strips were prepared from the murine fundus. Intracellular recordings were made from circular smooth muscle cells, and the effects of 5-HT on tone and excitatory and inhibitory junction potentials evoked by electrical field stimulation (EFS) were determined. 5-HT induced a concentration-dependent contraction and smooth muscle depolarization that was tetrodotoxin resistant. The 5-HT(1B/D) receptor antagonists GR-127935 and BRL-155172 significantly inhibited 5-HT-induced contractions. The 5-HT(1B/D) agonist sumatriptan contracted murine fundic muscle. The 5-HT(1A) receptor agonist buspirone relaxed fundic smooth muscle, and the relaxation was inhibited by WAY-100135 but not by N(omega)-nitro-l-arginine or tetrodotoxin. 5-HT enhanced both the excitatory and inhibitory responses to EFS. The 5-HT(3) receptor antagonist MDL-72222 partly inhibited both the excitatory and inhibitory response elicited by EFS, whereas the 5-HT(4) receptor antagonist GR-113808 partly inhibited the EFS-evoked inhibitory response. The 5-HT reuptake inhibitor fluoxetine contracted smooth muscle strips, a contraction that was partially inhibited by GR-127935 and abolished by tetrodotoxin. In conclusion, the data suggest that 5-HT modulates murine fundic contractile activity through several different receptor subtypes. Sustained release of 5-HT maintains fundic tone through postjunctional 5-HT(1B/D) receptors. 5-HT(3) receptors modulate excitatory neural input to murine fundic smooth muscle, and both 5-HT(3) and 5-HT(4) receptors modulate inhibitory neural input to murine fundic smooth muscle.     

Decreased plasma ghrelin contributes to anorexia following novelty stress

We hypothesized that anorexia induced by novelty stress caused by exposure to a novel environment may be due to activation of corticotropin-releasing factor (CRF) and subsequently mediated by decreasing peripheral ghrelin concentration via serotonin (5-HT) and melanocortin-4 receptors (MC4R). Each mouse was transferred from group-housed cages to individual cages to establish the novelty stress. We observed the effect of changes in feeding behavior in a novel environment using the method of transferring group-housed mice to individual cages. We investigated the effect of an intracerebroventricular injection of antagonists/agonists of CRF1/2 receptors (CRF1/2Rs), 5-HT(1B)/(2C) receptors (5-HT(1B)/(2C)R), and MC4R to clarify the role of each receptor on the decrease in food intake. Plasma ghrelin levels were also measured. The novelty stress caused a reduction in food intake that was abolished by administering a CRF1R antagonist. Three hours after the novelty stress, appetite reduction was associated with reduced levels of neuropeptide Y/agouti-related peptide mRNA, increased levels of proopiomelanocortin mRNA in the hypothalamus, and a decrease in plasma ghrelin level. Administering a CRF1R antagonist, a 5-HT(1B)/(2C)R antagonist, an MC4R antagonist, exogenous ghrelin, and an enhancer of ghrelin secretion, rikkunshito, resolved the reduction in food intake 3 h after the novelty stress by enhancing circulating ghrelin concentrations. We showed that anorexia during a novelty stress is a process in which CRF1R is activated at the early stage of appetite loss and is subsequently activated by a 5-HT(1B)/(2C)R and MC4R stimulus, leading to decreased peripheral ghrelin concentrations.