Effects of hypergravic fields on serotonergic neuromodulation in the rat hippocampus

The effects of 7 day exposure to 2G fields on serotonergic modulation at two synapses on a hippocampal pathway were examined by recording dentate gyrus and CA1 pyramidal cell layer electrical activity. Serotonin decreased the amplitude of the population spike (synchronous action potentials in hundreds of neurons) in both the dentate gyrus and CA1 regions of rats exposed to 2G fields for 7 days. The inhibition, averaging 26 +/- 4% (mean +/- SEM) in the dentate gyrus and 80 +/- 5% in the CA1 region, was not significantly different from inhibitory responses observed in 1G controls. The 5-HT1A agonist 8-OH-DPAT mimicked this inhibition in the dentate and CA1 regions of 1G rats. 8-OH-DPAT responses were not affected by exposure to 2G fields. We conclude that the hippocampus contains surplus 5-HT receptors so that decreases in receptor density reported in receptor binding studies do not result in a decrease in modulatory capability. A model to account for the physiological pathway that relates gravitational field strength to 5-HT receptor density without changing the effectiveness of 5-HT neuromodulation is discussed.     

Quantitative analysis of glutamatergic and GABAergic neurons expressing 5-HT(2A) receptors in human and monkey prefrontal cortex

5-hydroxytryptamine (5-HT) or serotonin 2A receptors play an important role in modulation of prefrontal cortex (PFC) activity and have been implicated in the physiopathology of psychiatric disorders. There is no quantitative information on the percentage of glutamatergic and GABAergic cells that express 5-HT(2A) receptors in human and monkey PFC. We have used double in situ hybridization to quantify the mRNA co-localization of 5-HT(2A) receptor with the glutamatergic transporter vesicular glutamate transporter 1, and with the GABAergic marker glutamic acid decarboxylase 65/67 and in parvalbumin and calbindin GABAergic cell populations. Our results show that nearly every glutamatergic cell (86-100%) in layers II-V expressed 5-HT(2A) receptor mRNA in both species. This percentage was lower in layer VI (13-31%). In contrast, not all the GABAergic interneurons (13-46%) expressed 5-HT(2A) receptor mRNA. This receptor was expressed in 45-69% of parvalbumin and in 61-87% of calbindin positive cells. These results indicate that, while the majority of glutamatergic neurons can be sensitive to 5-HT action via 5-HT(2A) receptors, this modulation occurs only in a limited population of GABAergic interneurons and provides new neuroanatomical information about the role played by serotonin through 5-HT(2A) receptors in the PFC and on the sites of action for drugs such as antipsychotics and antidepressants used in treatment of psychiatric disorders.     

Serotonin-induced inhibition of locomotor rhythm of the rat isolated spinal cord is mediated by the 5-HT1 receptor class.

The neurotransmitter serotonin (5-HT) induces rhythmic motor patterns (fictive locomotion) of the neonatal rat spinal cord in vitro; this is a useful experimental model to study the generation of a motor programme at exclusively spinal level. Nevertheless, 5-HT slows down the fictive locomotion typically elicited by activation of NMDA glutamate receptors, suggesting a complex action of this monoamine. By means of electrophysiological recordings from multiple ventral roots we demonstrated that the decrease caused by 5-HT in NMDA-induced periodicity was dose-dependent, enhanced after pharmacological blocking of 5-HT2 excitatory receptors, and imitated by pharmacological agonists of the 5-HT1 receptor family. Selective blockers of the 5-HT1A or 5-HT1B/D receptor classes, either alone or in combination, largely (but not completely) attenuated this inhibitory action of 5-HT. It is concluded that the principal inhibitory action of 5-HT on the spinal locomotor network was mediated by certain subtypes of the 5-HT1 receptor class, which tends to oppose the 5-HT2 receptor-mediated excitation of the same network.     

Simultaneous effects of the platelet 5-HT2 and alpha 2-adrenergic receptor populations on phosphoinositide hydrolysis.

The interaction of multiple receptor populations on a common second messenger system is a critical aspect of cell function and may be involved in pathology. We studied the interactions of the 5-HT2, alpha 2-adrenergic and prostaglandin (PGI2) receptors on phosphoinositide (PI) turnover in human platelets. Serotonin and epinephrine (EPI) stimulated PI hydrolysis in a dose-dependent manner. The PI turnover response to serotonin was mediated by the 5-HT2 receptor. The PI response to EPI was mediated by alpha 2-adrenergic receptors. An additive PI turnover response was generated by the combination of 5-HT and EPI. The sum of the maximal responses to 5-HT (72.5 +/- 4.9%) and EPI (56.0 +/- 4.2%) approximated the maximal response (129.3 +/- 9.5) to the combination. Prostacyclin (PGI2) at 1 microgram/mL reduced PI turnover by 21.8 +/- 1.1%. The PI response to 5-HT and EPI was not significantly altered once the reduction in the baseline PI turnover by PGI2 is taken into account. Similarly, PGI2 did not reduce PI hydrolysis stimulated by a combination of 5-HT (0.2 mM) and EPI (0.1 mM) once the decrease in baseline was taken into account (p greater than 0.20). The summation of serotonin stimulation of PI turnover by a combination of both epinephrine and serotonin was blocked by either yohimbine or ketanserin. These studies indicate: (1) the pool of phospholipases appears to exceed the maximal capacity of the individual alpha 2-adrenergic and 5-HT2 receptor populations to activate this second messenger system. (2) inhibition of serotonin or epinephrine-stimulated PI turnover by prostacyclin is due to a lowering of basal PI turnover. Future studies should examine other cell systems to assess the generalizability of these findings regarding the differences in effects on a second messenger system when activated by one receptor population as opposed to two different receptor types.     

5-HT(1A) receptor mutant mice exhibit enhanced tonic, stress-induced and fluoxetine-induced serotonergic neurotransmission

Mutant mice that lack serotonin(1A) receptors exhibit enhanced anxiety-related behaviors, a phenotype that is hypothesized to result from impaired autoinhibitory control of midbrain serotonergic neuronal firing. Here we examined the impact of serotonin(1A) receptor deletion on forebrain serotonin neurotransmission using in vivo microdialysis in the frontal cortex and ventral hippocampus of serotonin(1A) receptor mutant and wild-type mice. Baseline dialysate serotonin levels were significantly elevated in mutant animals as compared with wild-types both in frontal cortex (mutant = 0.44 +/- 0.05 n M; wild-type = 0.28 +/- 0.03 n M) and hippocampus (mutant = 0.46 +/- 0.07 n M; wild-type = 0.27 +/- 0.04 n M). A stressor known to elicit enhanced anxiety-like behaviors in serotonin(1A) receptor mutants increased dialysate 5-HT levels in the frontal cortex of mutant mice by 144% while producing no alteration in cortical 5-HT in wild-type mice. There was no phenotypic difference in the effect of this stressor on serotonin levels in the hippocampus. Fluoxetine produced significantly greater increases in dialysate 5-HT content in serotonin(1A) receptor mutants as compared with wild-types, with two- and three-fold greater responses being observed in the hippocampus and frontal cortex, respectively. This phenotypic effect was mimicked in wild-types by pretreatment with the serotonin(1A) antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (p-MPPI). These results indicate that deletion of central serotonin(1A) receptors results in a tonic disinhibition of central serotonin neurotransmission, with a greater dysregulation of serotonin release in the frontal cortex than ventral hippocampus under conditions of stress or increased interstitial serotonin levels.     

Effect of antagonists of serotonin receptors on modulation with serotonin of synaptic activity of projectional neurons of rat amygdala dorsolateral nucleus

The work studies role of different receptor types of serotonin (5-hydroxytryptamine; 5-HT) in the process of synaptic activity modulation with 5-HT of rat dorsolateral amygdala projection neurons. The selective antagonist of 5-HT_1,2 receptors methylsergid maleate was shown to suppress the 5-HT inhibitory action on amplitude of the postsynaptic currents evoked by glutamate and GABA, whereas the antagonist of 5-HT_3,4 receptors SDZ202-557 produced no effect on the above-mentioned 5-HT action. The obtained action indicates that the 5-HT modulatory effect on the projectional neuron synaptic inputs is mediated by 5-HT receptors of the 1 and 2 types.     

Synthesis and Pharmacology of the enantiomers of the potential atypical antipsychotic agents 5-OMe-BPAT and 5-OMe-(2,6-di-OMe)-BPAT.

The optically pure enantiomers of the potential atypical antipsychotic agents 5-methoxy-2-[N-(2-benzamidoethyl)-N-n-propylamino]tetralin (5-OMe-BPAT, 5) and 5-methoxy-2-{N-[2-(2,6-dimethoxy)benzamidoethyl]-N-n-propylamino}t etralin [5-OMe-(2,6-di-OMe)-BPAT, 6] were synthesized and evaluated for their in vitro binding affinities at alpha1-, alpha2-, and beta-adrenergic, muscarinic, dopamine D1, D2A, and D3, and serotonin 5-HT1A and 5-HT2 receptors. In addition, their intrinsic efficacies at serotonin 5-HT1A receptors were established in vitro. (S)- and (R)-5 had high affinities for dopamine D2A, D3, and serotonin 5-HT1A receptors, moderate affinities for alpha1-adrenergic and serotonin 5-HT2 receptors, and no affinity (Ki > 1000 nM) for the other receptor subtypes. (S)- and (R)-6 had lower affinities for the dopamine D2A and the serotonin 5-HT1A receptor, compared to (S)- and (R)-5, and hence showed some selectivity for the dopamine D3 receptor. The interactions with the receptors were stereospecific, since the serotonin 5-HT1A receptor preferred the (S)-enantiomers, while the dopamine D2A and D3 receptors preferred the (R)-enantiomers of 5 and 6. The intrinsic efficacies at the serotonin 5-HT1A receptor were established by measuring their ability to inhibit VIP-induced cAMP production in GH4ZD10 cells expressing serotonin 5-HT1A receptors. Both enantiomers of 5 behaved as full serotonin 5-HT1A receptor agonists in this assay, while both enantiomers of 6 behaved as weak partial agonists. The potential antipsychotic properties of (S)- and (R)-5 were evaluated by establishing their ability to inhibit d-amphetamine-induced locomotor activity in rats, while their propensity to induce extrapyramidal side-effects (EPS) in man was evaluated by determining their ability to induce catalepsy in rats. Whereas (R)-5 was capable of blocking d-amphetamine-induced locomotor activity, indicative of dopamine D2 receptor antagonism, (S)-5 even enhanced the effect of d-amphetamine, suggesting that this compound has dopamine D2 receptor-stimulating properties. Since both enantiomers also were devoid of cataleptogenic activity, they are interesting candidates for further exploring the dopamine D2/serotonin 5-HT1A hypothesis of atypical antipsychotic drug action.     

Functional characterization of the novel antipsychotic iloperidone at human D2, D3, alpha 2C, 5-HT6, and 5-HT1A receptors

Iloperidone has demonstrated an interesting monoamine receptor profile in radioligand binding studies, with nanomolar affinity for certain noradrenaline, dopamine, and serotonin receptors. In this study, the agonist/antagonist activity of iloperidone was determined in cell lines expressing recombinant human D(2A), D(3), alpha(2C), 5-HT(1A), or 5-HT(6) receptors. With the exception of 5-HT(6) receptors, these receptors are negatively coupled to cyclase. Thus, after stimulation with forskolin, the agonists dopamine (at D(2A) and D(3)), noradrenaline (at alpha(2C)), or 8-OH-DPAT (at 5-HT(1A)) induced a reduction in cAMP accumulation. Conversely, activation of the 5-HT(6) receptor by 5-HT led to an increase in cAMP accumulation. Iloperidone alone was devoid of significant agonist activity but inhibited the agonist response in all 5 cell lines in a surmountable and concentration-dependent fashion. Iloperidone was most potent at D(3) receptors (pK(B) 8.59 +/- 0.20; n = 6), followed by alpha(2C) (pK(B) 7.83 +/- 0.06; n = 15), 5-HT(1A) (pK(B) 7.69 +/- 0.18; n = 10), D(2A) (pK(B) 7.53 +/- 0.04; n = 11) and 5-HT(6) (pK(B) 7.11 +/- 0.08; n = 11) receptors.     

Blockade of striatal 5-HT2 receptors produces retrograde amnesia in rats

We have recently reported that intrastriatal administration of the serotonin (5-HT) releasing drug p-chloroamphetamine, and of 5-HT itself, produces a significant retention deficit of inhibitory avoidance. It is not known which of the 5-HT receptors are involved in the amnesic effect of serotonin. The present experiment was aimed at determining whether 5-HT2 receptors within the striatum are involved in memory consolidation. Ketanserine (0.5, 1.0, 2.0, or 4.0 ng) was infused bilaterally into the striatum of Wistar rats immediately after training of inhibitory avoidance, and retention of the task was measured 24 h later. A dose-dependent retention deficit was found. Together with the results from appropriate control groups, the results strongly suggest that striatal 5-HT2 receptors participate in memory consolidation of this aversive task.     

Alterations in 5-HT(1A) receptors and adenylyl cyclase response by trazodone in regions of rat brain

The in vivo effect of trazodone on the density of [(3)H]5-HT binding sites and 5-HT(1A) receptors and adenylyl cyclase (AC) response was studied in regions of rat brain. The chronic administration of trazodone (10 mg/Kg body wt, 40 days) resulted in a significant downregulation of [(3)H]5-HT binding sites and 5-HT(1A) receptors in cortex and hippocampus. Trazodone significantly (p < 0.0001) decreased the density of [(3)H]5-HT binding sites in cortex (42.6 +/- 3.6 fmol/mg protein, 65%) and hippocampus (12.6 +/- 1.6 fmol/mg protein, 87%) when compared to control values of 121.9 +/- 5.4 and 99.3 +/- 7.5 fmol/mg protein in these regions, respectively. Similarly there was a significant (p < 0.0001) decrease in the density of 5-HT(1A) receptors in both cortex (7.2 +/- 0.5 fmol/mg protein, 70%) and hippocampus (6.3 +/- 1.2 fmol/mg protein, 79%) when compared to control values of 24.2 +/- 2.1 and 30.6 +/- 3.7 fmol/mg protein, in these regions respectively. However, the affinity of [(3)H]5-HT to 5-HT binding sites (1.83 +/- 0.26 nM, p < 0.0001) and [(3)H]8-OH-DPAT to 5-HT(1A) receptors (0.60 +/- 0.06 nM, p < 0.05) was significantly decreased only in cortex when compared to the control K(d) values of 0.88 +/- 0.04 nM and 0.47 +/- 0.02 nM in these regions, respectively.The basal AC activity did not alter in treated rats, where as, the inhibition of forskolin-stimulated AC activity by 5-HT (10 microM) was significantly (p < 0.0001) decreased both in cortex (43%) and hippocampus (40%) when compared to control levels. In conclusion, chronic treatment with trazodone results in downregulation of 5-HT(1A) receptors in cortex and hippocampus along with concomitant increased AC response, suggesting the involvement of 5-HT(1A) receptor-mediated AC response in the mechanism of action of trazodone.     

Opposite effects of presynaptic 5-HT3 receptor activation on spontaneous and action potential-evoked GABA release at hippocampal synapses

5-HT(3) (serotonin type 3) receptors are targets of antiemetics, antipsychotics, and antidepressants and are believed to play a role in cognition. Nevertheless, contrasting results have been obtained with respect to their functions in the CNS and in the control of transmitter release. We used rat hippocampal neurons in single-neuron microcultures to identify the roles of presynaptic 5-HT(3) receptors at central synapses. 5-HT (10 microm) caused a transient > 10-fold increase in the frequency of miniature inhibitory postsynaptic currents without affecting amplitudes or kinetics. This effect was abolished by tropisetron (30 nm) and when Ca(2+) channels were blocked by 100 microm Cd(2+) it was mimicked and occluded when neurons were depolarized by 20 mm, but not 10 mm, K(+). Thus, activation of presynaptic 5-HT(3) receptors increased spontaneous GABA release by causing depolarization and opening of voltage-gated Ca(2+) channels. In microculture neurons, 5-HT transiently reduced action potential-evoked inhibitory autaptic currents by > 50%; this effect was blocked by tropisetron and mimicked by 20 mm, but not 10 mm, K(+). Miniature excitatory postsynaptic currents were not altered by 5-HT. Excitatory autaptic currents were tonically reduced, an effect attenuated by 5-HT(1A) antagonists. Thus, presynaptic 5-HT(3) receptors control GABA, but not glutamate, release and mediate opposite effects on spontaneous and action potential-dependent release.     

Influence of 5-HT1A agonist on the feeding behavior of Coturnix japonica (Galliformes: Aves).

In this study, we investigate the effect of serotonin receptor 5-HT1A stimulation on the feeding behavior of quails (Coturnix japonica). The administration of 5-HT1A agonist, 8-OH-DPAT (0.05 to 5.0 mg/Kg) dose-dependently inhibited the food intake in normally fed quails. Greater inhibition was attained with 5.0 mg/kg (0.93 +/- 0.21 g vs. 5.83 +/- 0.25 g, P < 0.05, 2 h after food offer). A comparable response was obtained from previously fasted quails. At end of 2 h, a higher dose of 8-OH-DPAT induced more intense hypophagy (1.59 +/- 0.41 g vs. 6.85 +/- 1.04 g, P < 0.0001). Previous treatment with the antagonist 5-HT1A/beta-adrenergic, propranolol, failed to block the inhibitory action of 8-OH-DPAT, but instead, intensified it (controls, 5.22 +/- 1.09 g; 8-OH-DPAT, 1.41 +/- 0.19 g; propranolol + 8-OH-DPAT, 0.44 +/- 0.25 g, P < 0.01, for all comparisons). The administration of an isolated higher dose of propranolol induced a hypophagic action (controls, 4.5 +/- 0.8 g vs. propranolol, 2.0 +/- 0.2 g, P < 0.01). Current outcomes suggest a possible role of 5-HT1A receptor on the feeding behavior of quails, as opposed to mammals. On the other hand, the intensified hypophagy induced by previous administration of propranolol raises the hypothesis of a beta-adrenergic excitatory mechanism that controls the feeding behavior of quails.     

13-cis-Retinoic acid alters intracellular serotonin, increases 5-HT1A receptor, and serotonin reuptake transporter levels in vitro

In addition to their established role in nervous system development, vitamin A and related retinoids are emerging as regulators of adult brain function. Accutane (13-cis-retinoic acid, isotretinoin) treatment has been reported to increase depression in humans. Recently, we showed that chronic administration of 13-cis-retinoic acid (13-cis-RA) to adolescent male mice increased depression-related behaviors. Here, we have examined whether 13-cis-RA regulates components involved in serotonergic neurotransmission in vitro. We used the RN46A-B14 cell line, derived from rat embryonic raphe nuclei. This cell line synthesizes serotonin (5-hydroxytryptamine, 5-HT) and expresses the 5-HT(1A) receptor and the serotonin reuptake transporter (SERT). Cells were treated with 0, 2.5, or 10 microM 13-cis-RA for 48 or 96 hrs, and the levels of 5-HT; its metabolite, 5-hydroxyindoleacetic acid (5HIAA); 5-HT(1A) receptor; and SERT were determined. Treatment with 13-cis-RA for 96 hrs increased the intracellular levels of 5-HT and tended to increase intra-cellular 5HIAA levels. Furthermore, 48 hrs of treatment with 2.5 and 10 microM 13-cis-RA significantly increased 5-HT(1A) protein to 168.5 +/- 20.0% and 148.7 +/- 2.2% of control respectively. SERT protein levels were significantly increased to 142.5 +/- 11.1% and 119.2 +/- 3.6% of control by 48 hrs of treatment with 2.5 and 10 microM of 13-cis-RA respectively. Increases in both 5-HT(1A) receptor and SERT proteins may lead to decreased serotonin availability at synapses. Such an effect of 13-cis-RA could contribute to the increased depression-related behaviors we have shown in mice.     

Fenfluramine-induced pulmonary vasoconstriction: role of serotonin receptors and potassium channels.

The anorexic agent fenfluramine considerably increases the risk of primary pulmonary hypertension. The mechanism of this effect is unknown. The appetite-reducing action of fenfluramine is mediated by its interaction with the metabolism of serotonin [5-hydroxytryptamine (5-HT)] in the brain. We tested the hypothesis that the pulmonary vasoconstrictive action of fenfluramine is at least in part mediated by 5-HT receptor activation. In addition, we sought to determine whether pharmacological reduction of voltage-gated potassium (K(V)) channel activity would potentiate the pulmonary vascular reactivity to fenfluramine. Using isolated rat lungs perfused with Krebs-albumin solution, we compared the inhibitory effect of ritanserin, an antagonist of 5-HT(2) receptors, on fenfluramine- and 5-HT-induced vasoconstriction. Both 5-HT (10(-5) mol/l) and fenfluramine (5 x 10(-4) mol/l) caused significant increases in perfusion pressure. Ritanserin at a dose (10(-7) mol/l) sufficient to inhibit >80% of the response to 5-HT reduced the response to fenfluramine by approximately 50%. A higher ritanserin dose (10(-5) mol/l) completely abolished the responses to 5-HT but had no more inhibitory effect on the responses to fenfluramine. A pharmacological blockade of K(V) channels by 4-aminopyridine (3 x 10(-3) mol/l) markedly potentiated the pulmonary vasoconstrictor response to fenfluramine but was without effect on the reactivity to 5-HT. These data indicate that the pulmonary vasoconstrictor response to fenfluramine is partly mediated by 5-HT receptors. Furthermore, the pulmonary vasoconstrictor potency of fenfluramine is elevated when the K(V)-channel activity is low. This finding suggests that preexisting K(V)-channel insufficiency may predispose some patients to the development of pulmonary hypertension during fenfluramine treatment.     

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.     

Evidence for the Presence of Serotonin Receptors Negatively Coupled to Adenylate Cyclase in the Rabbit Iris-Ciliary Body

Serotonin has no obvious effect on basal cyclic AMP levels but reduces the forskolin-, isoproterenol-, and vasoactive intestinal peptide-induced stimulation of cyclic AMP levels in a dose-dependent manner. Serotonergic, cholinergic, muscarinic, alpha-adrenergic, and dopaminergic antagonists have no effect on the serotonin response. Topical application of a serotonin/pargyline solution to the living eye causes desensitisation of the serotonin response in the iris-ciliary body, an observation confirming the presence of specific serotonergic receptors linked to adenylate cyclase. The 5-HT1A [5-hydroxytryptamine (serotonin) type 1A] receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin and buspirone mimic the serotonin response in reducing the forskolin-stimulated cyclic AMP levels, as do the indole derivatives 5-methoxytryptamine, 5-hydroxtryptophan, and tryptamine. However, the ineffectiveness of the 5-HT1A agonist ipsapirone and the inability of spiroxatrine to block the serotonin response show that classical 5-HT1A receptors are not involved. The serotonin response is blocked by pertussis toxin and is insensitive to the phosphodiesterase inhibitor theophylline, which indicates the involvement of an inhibitory guanine regulatory protein in the coupling of the serotonin receptor to the adenylate cyclase catalytic unit.     

Effect of serotonin on the basal and gonadotrophin-releasing hormone-induced release of luteinizing hormone from rat pituitary glands in vitro

The effect of serotonin (5-HT) on the basal and gonadotrophin-releasing hormone (GnRH)-stimulated release of luteinizing hormone (LH) was studied in rat adenohypophysis in vitro. Anterior pituitary glands from ovariectomized rats were incubated for 1h in the presence of different doses of 5-HT (0.01 to 3 mumol/l). Serotonin added to the culture medium slightly dimished the basal release of LH and markedly inhibited the release of LH induced by GnRH. Responsiveness to GnRH (3 nmol/l) was significantly reduced, in a dose-dependent manner, by the simultaneous treatment of glands with 5-HT. Maximal inhibition to 65% of the response obtained with GnRH alone, was attained with 1 mumol/l 5-HT. The EC50 value was estimated to be about 1.9 X 10(-7) M. The inhibitory effect of 5-HT was evident within 30 min of incubation. Furthermore, 5-HT appear to exert a short-lasting action, since the rate of basal and GnRH-induced release of LH was reduced during the first hour of incubation, but after 2h the suppressive effects of 5-HT were no longer apparent. Methysergide, a serotonin receptor blocking agent, partially antagonized the inhibitory effect of 5-HT on LH release, either basal or GnRH-stimulated. This suggests that a receptor-mediated component may be involved in the mechanism of 5-HT action. The present results indicate that 5-HT can affect the release of LH by acting directly at the pituitary gland level.     

5-HT1A receptor-regulated signal transduction pathways in brain

Serotonin is an influential monoamine neurotransmitter that signals through a number of receptors to modulate brain function. Among different serotonin receptors, the serotonin 1A (5-HT1A) receptors have been tied to a variety of physiological and pathological processes, notably in anxiety, mood, and cognition. 5-HT1A receptors couple not only to the classical inhibitory G protein-regulated signaling pathway, but also to signaling pathways traditionally regulated by growth factors. Despite the importance of 5-HT1A receptors in brain function, little is known about how these signaling mechanisms link 5-HT1A receptors to regulation of brain physiology and behavior. Following a brief summary of the known physiological and behavioral effects of 5-HT1A receptors, this article will review the signaling pathways regulated by 5-HT1A receptors, and discuss the potential implication of these signaling pathways in 5-HT1A receptor-regulated physiological processes and behaviors.     

Evidence for a spinal serotonergic control of the peripheral inflammation in the rat

We investigated the effect of serotonergic agonists and antagonists injected intrathecally by direct punction of the spinal cord at the lumbar level (between L5-L6) on peripheral inflammatory edema. Edema was induced by carrageenan injected subcutaneously in one hindpaw 30 min after spinal treatments. Serotonin (0.1, 1, 10 pmol) caused a graded-inhibition of the inflammatory paw edema. The corticosteroid inhibitor aminoglutethimide (100 mg/kg, p.o. 1.5 h before spinal treatment) did not modify this effect. The 5-HT1A agonist buspirone and the 5-HT1B/1D agonist sumatriptan (0.1, 1.0 and 10 nmol) also inhibited paw edema. The 5-HT1,2 antagonist methysergide (10 and 100 pmol) enhanced edema, but higher doses ( 4 and 8 nmol) diminished edema. NAN-190 (5-HT1 antagonist; 1 and 10 nmol) increased paw edema, while ritanserin (5-HT2 antagonist; 1 nmol) inhibited paw edema. Ondansetron (5-HT3 antagonist; up to 10 nmol) did not affect edema, but metoclopramide (5-HT3 antagonist / 5-HT4 agonist; 5, 10 and 30 pmol) inhibited edema. These data suggest that a tonic release of serotonin in the spinal cord may occurs during ongoing peripheral inflammation, modulating the neurogenic component of edema either by an inhibitory action on 5-HT1 receptors or by a stimulatory action on 5-HT2 receptors. A disfunction in such mechanism may be involved in the pathophysiology of certain types of headaches or migraine, which seem to depend on neurogenic vasodilation, and may also help to explain the therapeuthic effectiveness of some serotonergic agents in these conditions.     

Roles of 5-HT receptors in the release and action of secretin on pancreatic secretion in rats

5-Hydroxytryptamine (serotonin, 5-HT) is a hormone and neurotransmitter regulating gastrointestinal functions. 5-HT receptors are widely distributed in gastrointestinal mucosa and the enteric nervous system. Duodenal acidification stimulates not only the release of both 5-HT and secretin but also pancreatic exocrine secretion. We investigated the effect of 5-HT receptor antagonists on the release of secretin and pancreatic secretion of water and bicarbonate induced by duodenal acidification in anesthetized rats. Both the 5-HT(2) receptor antagonist ketanserin and the 5-HT(3) receptor antagonist ondansetron at 1-100 microg/kg dose-dependently inhibited acid-induced increases in plasma secretin concentration and pancreatic exocrine secretion. Neither the 5-HT(1) receptor antagonists pindolol and 5-HTP-DP nor the 5-HT(4) receptor antagonist SDZ-205,557 affected acid-evoked release of secretin or pancreatic secretion. None of the 5-HT receptor antagonists affected basal pancreatic secretion or plasma secretin concentration. Ketanserin or ondansetron at 10 microg/kg or a combination of both suppressed the pancreatic secretion in response to intravenous secretin at 2.5 and 5 pmol x kg(-1) x h(-1) by 55-75%, but not at 10 pmol x kg(-1) x h(-1). Atropine (50 microg/kg) significantly attenuated the inhibitory effect of ketanserin on pancreatic secretion but not on the release of secretin. These observations suggest that 5-HT(2) and 5-HT(3) receptors mediate duodenal acidification-induced release of secretin and pancreatic secretion of fluid and bicarbonate. Also, regulation of pancreatic exocrine secretion through 5-HT(2) receptors may involve a cholinergic pathway in the rat.