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.     

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.     

异氟烷对化疗性大鼠异食癖恶心呕吐模型神经功能及呕吐相关神经递质的影响

摘要 目的：研究异氟烷预处理对化疗性大鼠异食癖恶心呕吐模型神经功能及呕吐相关神经递质的影响。方法：选用SD大鼠作为研究对象,腹腔注射顺铂以建立化疗性大鼠异食癖恶心呕吐模型（Model组）,腹腔注射等量生理研究作为对照组（Control）,在腹腔注射顺铂前1 h和12 h吸入异氟烷预处理作为异氟烷治疗组（Isoflurane）。记录腹腔注射顺铂0~12 h和12~24 h内各组大鼠摄入高岭土量;在腹腔注射顺铂0 h、12 h和24 h后,通过神经功能缺损评分法对各组大鼠神经功能进行评分;并在腹腔注射顺铂24 h后处死大鼠,收集大鼠回肠和延髓组织以检测5-羟色胺(5-Hydroxytryptamine,5-HT)、5-羟基-吲哚乙酸（5-hydroxy-indole acetic acid,5-HIAA）、色氨酸羟化酶（Tryptophan hydroxylase,TPH）以及单胺氧化酶（Monoamine oxidase,MAOA）含量。结果：与Control组相比,Model组和Isoflurane组大鼠在腹腔注射顺铂0~12 h,12~24 h以及0~24 h内摄入的高岭土量均显著升高（P<0.05）,并且Isoflurane组大鼠均显著Model组。三组大鼠在腹腔注射顺铂0 h、12 h和24 h后,神经功能评分均无显著差异（P>0.05）。与Control组大鼠相比,Model组和Isoflurane组大鼠在腹腔注射顺铂24 h后回肠/延髓组织内5-HT和TPH含量均显著升高,并且Isoflurane组大鼠显著低于Model组大鼠（P<0.05）;与Control组相比,Model组大鼠回肠和延髓组织中5-HIAA/5-HIT比值和MAOA含量均显著降低（P<0.05）;与Model组大鼠相比,Isoflurane组大鼠回肠5-HIAA含量、回肠/延髓5-HIAA/5-HT比值和MAOA含量均显著升高（P<0.05）。结论：异氟烷预处理可用于预防腹腔注射顺铂诱导的恶性呕吐,其机制可能与下降THP含量和提高MAOA含量,抑制5-HT合成以及促进5-HT代谢有关。     

The inhibition of tryptophan hydroxylase, not protein synthesis, reduces the brain trapping of α-methyl-l-tryptophan: an autoradiographic study

The effects of the tryptophan hydroxylase (TPH) inhibitor p-chlorophenylalanine (PCPA; 200mg/kg; 3 days), and of the protein synthesis inhibitor cycloheximide (CXM, 2mg/kg), on regional serotonin (5-HT) synthesis were studied using the alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) autoradiographic method. The objectives of these investigations were to evaluate the changes, if any, on 5-HT synthesis, as measured with alpha-MTrp method, following the inhibition of TPH by PCPA, or the inhibition of proteins synthesis by CXM. The rats were used in the tracer experiment approximately 24h after the last dose of PCPA was administered, and in the CXM experiments, they were used 30 min following a single injection of CXM. In both experiments, the control rats were injected with the same volume of saline (0.5 ml/kg; s.c.) and at the same times as the drug injections. The results demonstrate that trapping of alpha-MTrp, which is taken to be related to brain 5-HT synthesis, is drastically reduced (40-80%) following PCPA treatment. The inhibition of protein synthesis with CXM did not have a significant effect on the global brain trapping of alpha-MTrp and 5-HT synthesis. These findings suggest that the brain trapping of alpha-[14C]MTrp relates to brain 5-HT synthesis, but not to brain protein synthesis.     

The inhibition of tryptophan hydroxylase, not protein synthesis, reduces the brain trapping of alpha-methyl-L-tryptophan: an autoradiographic study

The effects of the tryptophan hydroxylase (TPH) inhibitor p-chlorophenylalanine (PCPA; 200mg/kg; 3 days), and of the protein synthesis inhibitor cycloheximide (CXM, 2mg/kg), on regional serotonin (5-HT) synthesis were studied using the alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) autoradiographic method. The objectives of these investigations were to evaluate the changes, if any, on 5-HT synthesis, as measured with alpha-MTrp method, following the inhibition of TPH by PCPA, or the inhibition of proteins synthesis by CXM. The rats were used in the tracer experiment approximately 24h after the last dose of PCPA was administered, and in the CXM experiments, they were used 30 min following a single injection of CXM. In both experiments, the control rats were injected with the same volume of saline (0.5 ml/kg; s.c.) and at the same times as the drug injections. The results demonstrate that trapping of alpha-MTrp, which is taken to be related to brain 5-HT synthesis, is drastically reduced (40-80%) following PCPA treatment. The inhibition of protein synthesis with CXM did not have a significant effect on the global brain trapping of alpha-MTrp and 5-HT synthesis. These findings suggest that the brain trapping of alpha-[14C]MTrp relates to brain 5-HT synthesis, but not to brain protein synthesis.     

The Distribution of Serotonergic Nerves in Microencephalic Rats Treated Prenatally with Methylazoxymethanol

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM) induces microencephaly in the offspring. In the present study of these microencephalic rats (MAM rats) we used quantitative autoradiography to investigate [³H] paroxetine binding sites, which are a selective marker of serotonin (5-HT) transporters (5-HTT). The binding in the accumbens, cortex, hippocampus, and dorsolateral thalamus was significantly increased in MAM rats, compared to the control rats, while there was a significant decrease in the dorsal raphe nucleus of the MAM rats. The levels of 5-HTT mRNA in the dorsal raphe nuclei were analyzed by in situ hybridization, which revealed a significant decrease in 5-HTT mRNA-positive neurons in the MAM rats compared to the control rats. The results imply serotonergic hyperinnervation in the cerebral hemispheres of MAM rats, while a target-dependent secondary degeneration of 5-HT neurons might be induced in the dorsal raphe nuclei of MAM rats.     

Serotoninergic neurotransmission is affected by n-3 polyunsaturated fatty acids in the rat

We explored the effects of chronic alpha-linolenic acid dietary deficiency on serotoninergic neurotransmission. In vivo synaptic serotonin (5-HT) levels were studied in basal and pharmacologically stimulated conditions using intracerebral microdialysis in the hippocampus of awake 2-month-old rats. We also studied the effects of reversion of the deficient diet on fatty acid composition and serotoninergic neurotransmission. A balanced (control) diet was supplied to deficient rats at different stages of development, i.e. from birth, 7, 14 or 21 days of age. We demonstrated that chronic n-3 polyunsaturated fatty acid dietary deficiency induced changes in the synaptic levels of 5-HT both in basal conditions and after pharmacological stimulation with fenfluramine. Higher levels of basal 5-HT release and lower levels of 5-HT-stimulated release were found in deficient than in control rats. These neurochemical modifications were reversed by supply of the balanced diet provided at birth or during the first 2 weeks of life through the maternal milk, whereas they persisted if the balanced diet was given from weaning (at 3 weeks of age). This suggests that provision of essential fatty acids is durably able to affect brain function and that this is related to the developmental stage during which the deficiency occurs.     

Optimal vitamin D spurs serotonin: 1,25-dihydroxyvitamin D represses serotonin reuptake transport (<Emphasis Type="Italic">SERT</Emphasis>) and degradation (<Emphasis Type="Italic">MAO-A</Emphasis>) gene expression in cultured rat serotonergic neuronal cell lines

Background

Diminished brain levels of two neurohormones, 5-hydroxytryptamine (5-HT; serotonin) and 1,25-dihydroxyvitamin D₃ (1,25D; active vitamin D metabolite), are proposed to play a role in the atypical social behaviors associated with psychological conditions including autism spectrum disorders and depression. We reported previously that 1,25D induces expression of tryptophan hydroxylase-2 (TPH2), the initial and rate-limiting enzyme in the biosynthetic pathway to 5-HT, in cultured rat serotonergic neuronal cells. However, other enzymes and transporters in the pathway of tryptophan metabolism had yet to be examined with respect to the actions of vitamin D. Herein, we probed the response of neuronal cells to 1,25D by quantifying mRNA expression of serotonin synthesis isozymes, TPH1 and TPH2, as well as expression of the serotonin reuptake transporter (SERT), and the enzyme responsible for serotonin catabolism, monoamine oxidase-A (MAO-A). We also assessed the direct production of serotonin in cell culture in response to 1,25D.

Results

Employing quantitative real-time PCR, we demonstrate that TPH-1/-2 mRNAs are 28- to 33-fold induced by 10 nM 1,25D treatment of cultured rat serotonergic neuronal cells (RN46A-B14), and the enhancement of TPH2 mRNA by 1,25D is dependent on the degree of neuron-like character of the cells. In contrast, examination of SERT, the gene product of which is a target for the SSRI-class of antidepressants, and MAO-A, which encodes the predominant catabolic enzyme in the serotonin pathway, reveals that their mRNAs are 51–59% repressed by 10 nM 1,25D treatment of RN46A-B14 cells. Finally, serotonin concentrations are significantly enhanced (2.9-fold) by 10 nM 1,25D in this system.

Conclusions

These results are consistent with the concept that vitamin D maintains extracellular fluid serotonin concentrations in the brain, thereby offering an explanation for how vitamin D could influence the trajectory and development of neuropsychiatric disorders. Given the profile of gene regulation in cultured RN46A-B14 serotonergic neurons, we conclude that 1,25D acts not only to induce serotonin synthesis, but also functions at an indirect, molecular-genomic stage to mimic SSRIs and MAO inhibitors, likely elevating serotonin in the CNS. These data suggest that optimal vitamin D status may contribute to improving behavioral pathophysiologies resulting from dysregulation of serotonergic neurotransmission.

     

5-HT(1B) receptor-mediated regulation of serotonin clearance in rat hippocampus in vivo

The 5-hydroxytryptamine (5-HT; serotonin) transporter (5-HTT) is important in terminating serotonergic neurotransmission and is a primary target for many psychotherapeutic drugs. Study of the regulation of 5-HTT activity is therefore important in understanding the control of serotonergic neurotransmission. Using high-speed chronoamperometry, we have demonstrated that local application of 5-HT(1B) antagonists into the CA3 region of the hippocampus prolongs the clearance of 5-HT from extracellular fluid (ECF). In the present study, we demonstrate that the 5-HT(1B) antagonist cyanopindolol does not produce this effect by increasing release of endogenous 5-HT or by directly binding to the 5-HTT. Dose-response studies showed that the potency of cyanopindolol to inhibit clearance of 5-HT was equivalent to that of the selective 5-HT reuptake inhibitor fluvoxamine. Local application of the 5-HT(1A) antagonist WAY 100635 did not alter 5-HT clearance, suggesting that the effect of cyanopindolol to prolong clearance is not via a mechanism involving 5-HT(1A) receptors. Finally, the effect of low doses of cyanopindolol and fluvoxamine to inhibit clearance of 5-HT from ECF was additive. These data are consistent with the hypothesis that activation of terminal 5-HT(1B) autoreceptors increases 5-HTT activity.     

Gender-dependent regulation of G-protein-gated inwardly rectifying potassium current in dorsal raphe neurons in knock-out mice devoid of the 5-hydroxytryptamine transporter

Agonists at G-protein-coupled receptors in neurons of the dorsal raphe nucleus (DRN) of knock-out mice devoid of the serotonin transporter (5-HTT(-/-)) exhibit lower efficacy to inhibit cellular discharge than in wild-type counterparts. Using patch-clamp whole-cell recordings, we found that a G-protein-gated inwardly rectifying potassium (GIRK) current is involved in the inhibition of spike discharge induced by 5-HT1A agonists (5-carboxamidotryptamine (5-CT) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT); 50 nM-30 microM) in both wild-type and 5-HTT(-/-) female and male mice. These effects were mimicked by 5'-guanylyl-imido-diphosphate (Gpp(NH)p; 400 microM) dialysis into cells with differences between genders. The 5-HTT(-/-) knock-out mutation reduced the current density induced by Gpp(NH)p in females but not in males. These data suggest that the decreased response of 5-HT1A receptors to agonists in 5-HTT(-/-) mutants reflects notably alteration in the coupling between G-proteins and GIRK channels in females but not in males. Accordingly, gender differences in central 5-HT neurotransmission appear to depend-at least in part-on sex-related variations in corresponding receptor-G protein signaling mechanisms.     

Serotonin depletion produces long lasting increase in striatal glutamatergic transmission

The ability of serotonin (5-HT) to influence striatal glutamatergic transmission was examined by determining changes over time in glutamate extracellular levels, transporter expression and synaptosomal uptake in rats with lesion of serotonergic neurones. By 8 days after intraraphe injections of 5,7-dihydroxytryptamine, producing 80% decreases in striatal tissue 5-HT levels, no changes were observed in the glutamatergic transmission. When 5-HT depletion was almost complete (21 days post-lesion), high affinity glutamate uptake in striatal synaptosomal preparations was significantly increased (156% of control), although no changes in striatal GLT1, GLAST and EAAC1 mRNAs, and GLT1 protein were detected by in situ hybridization and immunohistochemistry. Meanwhile, the serotonin lesion produced large increases in basal extracellular levels of glutamate and glutamine (364% and 259%, respectively) determined in awake rats by in vivo microdialysis, whereas no change was observed in dopamine levels as compared with control rats. High potassium depolarization as well as L-trans-pyrrolidine-2,4-dicarboxylate, also induced larger increases in extracellular levels of glutamate in lesioned rats than in controls. Finally, similar changes in glutamate transmission were observed by 3 months post-lesion. These results suggest that 5-HT has a long lasting and tonic inhibitory influence on the striatal glutamatergic input, without affecting the basal dopaminergic transmission.     

Effect of a new potential psychotropic drug, 8-(trifluoromethyl)-1,2,3,4,5-benzopentathiepin-6-amine hydrochloride,on the expression of serotonin-related genes in the mouse brain

Investigation of molecular mechanisms underlying psychotropic drug action is the main aim of molecular psychopharmacology. Previously, a new synthetic varacin analog, 8-(trifluoromethyl)-1,2,3,4,5-benzopentathiepin-6-amine (TC-2153) was shown to produce anxiolytic and anticonvulsant effects in mice. This study investigated the effects of chronic TC-2153 administration on the expression of some serotonin-related genes in the mouse brain. The drug was administered (10 mg/kg, per os, 16 days) to adult male mice of the ASC (Antidepressant Sensitive Catalepsy) strain characterized by altered behavior and hereditary impairment of the brain serotonin system. Expression of genes encoding tryptophan hydroxylase 2 (TPH2), the key enzyme of serotonin synthesis, monoamine oxydase A (MAOA), the major serotonin-degrading enzyme, 5-HT transporter (SERT), and 5-HT_1A receptor was studied using quantitative RT-PCR. TC-2153 significantly reduced the 5-HT_1A receptor and MAOA mRNA levels in the midbrain, but did not have any effect on the expression of these genes in the frontal cortex and the hippocampus. The drug did not affect the expression of TPH2 and SERT in the midbrain. The results indicate that the brain 5-HT system is involved in the molecular basis of TC-2153 action.     

Immunohistochemical localization of tryptophan hydroxylase and serotonin transporter in the carotid body of the rat

It has been proposed that serotonin (5-HT) facilitates the chemosensory activity of the carotid body (CB). In the present study, we investigated mRNA expression and immunohistochemical localization of the 5-HT synthetic enzyme isoforms, tryptophan hydroxylase 1 (TPH1) and TPH2, and the 5-HT plasma membrane transport protein, 5-HT transporter (SERT), in the CB of the rat. RT-PCR analysis detected the expression of mRNA for TPH1 and SERT in extracts of the CB. Using immunohistochemistry, 5-HT immunoreactivity was observed in a few glomus cells. TPH1 and SERT immunoreactivities were observed in almost all glomus cells. SERT immunoreactivity was seen on nerve fibers with TPH1 immunoreactivity. SERT immunoreactivity was also observed in varicose nerve fibers immunoreactive for dopamine beta-hydroxylase, but not in nerve fibers immunoreactive for vesicular acetylcholine transporters or nerve terminals immunoreactive for P2X₃ purinoreceptors. These results suggest that 5-HT is synthesized and released from glomus cells and sympathetic nerve fibers in the CB of the rat, and that the chemosensory activity of the CB is regulated by 5-HT from glomus cells and sympathetic nerve fibers.     

p-Chlorophenylalanine Changes Serotonin Transporter mRNA Levels and Expression of the Gene Product

Abstract: After a single intraperitoneal injection of the irreversible tryptophan hydroxylase inhibitor p -chlorophenylalanine (PCPA; 300 mg/kg), there was a rapid down-regulation of serotonin (5-HT) transporter mRNA levels in cell bodies. This change was significant at 1 and 2 days after PCPA administration within the ventromedial but not the dorsomedial portion of the dorsal raphe nucleus. Seven days after PCPA treatment, 5-HT transporter mRNA levels were significantly elevated compared with controls in both regions of the dorsal raphe nucleus. PCPA administration produced no change in the [³H]-citalopram binding and synaptosomal [³H]5-HT uptake in terminal regions at 2 and 7 days after treatment but significantly reduced both these parameters by ∼20% in the hippocampus and in cerebral cortex 14 days after PCPA administration. The striatum showed a lower sensitivity to this effect. No significant changes were observed in the levels of [³H]citalopram binding to 5-HT cell bodies in the dorsal raphe nucleus. In the same animals used for 5-HT transporter mRNA level measurements, levels of tryptophan hydroxylase mRNA in neurons of the ventromedial and dorsomedial portions of the dorsal raphe nucleus were increased 2 days after PCPA administration and fell to control levels 7 days after injection in the ventromedial region but not in the dorsomedial portion of the dorsal raphe nucleus, where they remained significantly higher than controls. Altogether, these results show that changes in 5-HT transporter mRNA are not temporally related to changes in 5-HT transporter protein levels. In addition, our results suggest that the 5-HT transporter and tryptophan hydroxylase genes are regulated by different mechanisms. We also provide further evidence that dorsal raphe 5-HT neurons are differentially regulated by drugs, depending on their location.     

Thyroid hormone plasmatic levels in rats treated with serotonin in acute and chronic way

Many experiments show that serotonin (5-HT) controls thyroidal function at hypothalamic level, inhibiting the TRH secretion. The majority of experiments are done in an acute way, consisting of a single serotonin dose injected intraperitoneally (ip) or intracerebroventricularly (ic) with the effect registered after a short time (usually 1 h) as in normal environmental conditions similar to the TSH stimulation test, that consists of transfer of the experimental animals from 30°C to 4°C for 30 min, thus inducing stimulation of the hypothalamus-hypophysis-thyroid axis. The aim of the present research was to study the correlation between 5-HT and the thyroidal function, measuring plasmatic thyroid hormone levels in rats ip treated in chronic (injected daily for 10 days with different doses of 5-HT), and in acute way (after 1 h from a single 2.0 mg/kg bw 5-HT dose) in normal environmental conditions to evidence the serotonin site action activity outside the blood-brain barrier. The results of the chronic experiment show an inhibitory effect of 5-HT, on T3 and T4 plasmatic level, only when it is injected at medium doses (0.2 and 0.4 mg/kg bw for T3, and 0.2 for T4), while the results of the acute experiment do not evidence any modification. These results show that in normal environmental conditions the outside 5-HT site action is active only when the 5-HT is injected chronically at defined doses, probably for a down-regulation phenomenon.     

Enhancement of hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by repeated lithium treatment in rats

Hippocampal cholinergic neuronal activity is reported to be regulated, at least partly, through serotonin1A (5-HT1A) receptors. Chronic lithium treatment has been shown to alter both behavioral and neurochemical responses mediated by postsynaptic 5-HT1A receptors. We investigated whether long-term lithium treatment affects central cholinergic neurotransmission through 5-HT1A receptor-mediated pathways. Changes in acetylcholine (ACh) release induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, in the rat hippocampus were measured using a microdialysis technique and a radioimmunoassay for ACh. Administration of lithium for 21 days resulted in a serum lithium concentration of 1.03 mM and caused little change in density or affinity of [3H]8-OH-DPAT binding sites in the hippocampus. The local application of 8-OH-DPAT into the hippocampus of lithium treated rats increased the ACh efflux in both the absence and the presence of physostigmine, a cholinesterase (ChE) inhibitor, in the perfusion fluid. The basal ACh efflux of lithium treated rats was not different from that of the control rats under normal conditions, but was significantly higher than that of the controls when ChE was inhibited. These results demonstrate that chronic lithium treatment increases spontaneous ACh release in the hippocampus under conditions of ChE inhibition, but not under normal conditions, and enhances cholinergic neurotransmission through 5-HT1A receptor-mediated pathways, and suggest that activation of 5-HT1A receptor function by lithium is related to the enhancement of hippocampal cholinergic neurotransmission.     

Regionally specific effects of diazepam on brain serotonin metabolism in rats: Sustained effects following repeated administration

The effects of single (1mg/kg) and repeated (1mg/kg 2¹ daily for 4 days) diazepam administration are investigated on brain regional 5-hydroxytryptamine (5-HT; serotonin) and 5-hydroxy indoleacetic acid (5-HIAA) concentration in rats. Daily treatment decreased food intakes but body weights did not decrease. Administration of diazepam (1mg/kg) to 4 day sahne injected rats on the 5th day decreased 5-HT levels in the hippocampus and increased it in the hypothalamus. 5-HIAA levels were increased in the striatum and decreased in the hypothalamus. 4 day diazepam injected rats injected with sahne on the 5th day also exhibited silmilar changes of 5-HT and 5-HIAA. Cortical levels of 5-HIAA were also smaller in these rats. Administration of diazepam to 4 day diazepam injected rats again decreased 5-HT in the hippocampus and 5-HIAA in the hypothalamus. 5-HT and 5-HIAA were both decreased in the striatum. Regionally specific effects of diazepam on brain serotonin metabolism are discussed in relation to their possible functions.     

Serotonin receptor subtypes required for ventilatory long-term facilitation and its enhancement after chronic intermittent hypoxia in awake rats

Respiratory long-term facilitation (LTF), a serotonin-dependent, persistent augmentation of respiratory activity after episodic hypoxia, is enhanced by pretreatment of chronic intermittent hypoxia (CIH; 5 min 11-12% O2-5 min air, 12 h/night for 7 nights). The present study examined the effects of methysergide (serotonin 5-HT1,2,5,6,7 receptor antagonist), ketanserin (5-HT2 antagonist), or clozapine (5-HT2,6,7 antagonist) on both ventilatory LTF and the CIH effect on ventilatory LTF in conscious male adult rats to determine which specific receptor subtype(s) is involved. In untreated rats (i.e., animals not exposed to CIH), LTF, induced by five episodes of 5-min poikilocapnic hypoxia (10% O2) separated by 5-min normoxic intervals, was measured twice by plethysmography. Thus the measurement was conducted 1-2 days before (as control) and approximately 1 h after systemic injection of methysergide (1 mg/kg ip), ketanserin (1 mg/kg), or clozapine (1.5 mg/kg). Resting ventilation, metabolic rate, and hypoxic ventilatory response (HVR) were unchanged, but LTF ( approximately 18% above baseline) was eliminated by each drug. In CIH-treated rats, LTF was also measured twice, before and approximately 8 h after CIH. Vehicle, methysergide, ketanserin, or clozapine was injected approximately 1 h before the second measurement. Neither resting ventilation nor metabolic rate was changed after CIH and/or any drug. HVR was unchanged after methysergide and ketanserin but reduced in four of seven clozapine rats. The CIH-enhanced LTF ( approximately 28%) was abolished by methysergide and clozapine but only attenuated by ketanserin (to approximately 10%). Collectively, these data suggest that ventilatory LTF requires 5-HT2 receptors and that the CIH effect on LTF requires non-5-HT2 serotonin receptors, probably 5-HT6 and/or 5-HT7 subtype(s).