In situ hybridization evidence for the synthesis of 5-HT1B receptor in serotoninergic neurons of anterior raphe nuclei in the rat brain

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

The nucleus accumbens 5-HTR₄-CART pathway ties anorexia to hyperactivity

In mental diseases, the brain does not systematically adjust motor activity to feeding. Probably, the most outlined example is the association between hyperactivity and anorexia in Anorexia nervosa. The neural underpinnings of this 'paradox', however, are poorly elucidated. Although anorexia and hyperactivity prevail over self-preservation, both symptoms rarely exist independently, suggesting commonalities in neural pathways, most likely in the reward system. We previously discovered an addictive molecular facet of anorexia, involving production, in the nucleus accumbens (NAc), of the same transcripts stimulated in response to cocaine and amphetamine (CART) upon stimulation of the 5-HT(4) receptors (5-HTR(4)) or MDMA (ecstasy). Here, we tested whether this pathway predisposes not only to anorexia but also to hyperactivity. Following food restriction, mice are expected to overeat. However, selecting hyperactive and addiction-related animal models, we observed that mice lacking 5-HTR(1B) self-imposed food restriction after deprivation and still displayed anorexia and hyperactivity after ecstasy. Decryption of the mechanisms showed a gain-of-function of 5-HTR(4) in the absence of 5-HTR(1B), associated with CART surplus in the NAc and not in other brain areas. NAc-5-HTR(4) overexpression upregulated NAc-CART, provoked anorexia and hyperactivity. NAc-5-HTR(4) knockdown or blockade reduced ecstasy-induced hyperactivity. Finally, NAc-CART knockdown suppressed hyperactivity upon stimulation of the NAc-5-HTR(4). Additionally, inactivating NAc-5-HTR(4) suppressed ecstasy's preference, strengthening the rewarding facet of anorexia. In conclusion, the NAc-5-HTR(4)/CART pathway establishes a 'tight-junction' between anorexia and hyperactivity, suggesting the existence of a primary functional unit susceptible to limit overeating associated with resting following homeostasis rules.

Autoradiographic Mapping of 5-HT(1B/1D) Binding Sites in the Rhesus Monkey Brain Using [carbonyl-C]zolmitriptan

Zolmitriptan is a serotonin 5-HT(1B/1D) receptor agonist that is an effective and well-tolerated drug for migraine treatment. In a human positron emission tomography study, [(11)C]zolmitriptan crossed the blood-brain barrier but no clear pattern of regional uptake was discernable. The objective of this study was to map the binding of [(11)C]zolmitriptan in Rhesus monkey brain using whole hemisphere in vitro autoradiography with [(11)C]zolmitriptan as a radioligand. In saturation studies, [(11)C]zolmitriptan showed specific (90%) binding to a population of high-affinity binding sites (Kd 0.95-5.06 nM). There was regional distribution of binding sites with the highest density in the ventral pallidum, followed by the external globus pallidus, substantia nigra, visual cortex, and nucleus accumbens. In competitive binding studies with 5-HT(1) receptor antagonists, [(11)C]zolmitriptan binding was blocked by selective 5-HT(1B) and 5-HT(1D) ligands in all target areas. There was no appreciable change in binding with the addition of a 5-HT(1A) receptor antagonist.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

The structure of the dorsal raphe nucleus and its relevance to the regulation of sleep and wakefulness

Serotonergic (5-HT) cells in the rat dorsal raphe nucleus (DRN) appear in topographically organized groups. Based on cellular morphology, expression of other neurotransmitters, afferent and efferent connections and functional properties, 5-HT neurons of the DRN have been grouped into six cell clusters. The subdivisions comprise the rostral, ventral, dorsal, lateral, caudal and interfascicular parts of the DRN. In addition to 5-HT cells, neurons containing γ-aminobutyric acid (GABA), glutamate, dopamine, nitric oxide and the neuropeptides corticotropin-releasing factor, substance P, galanin, cholecystokinin, neurotensin, somatostatin, vasoactive intestinal peptide, neuropeptide Y, thyrotropin-releasing hormone, growth hormone, leu-enkephalin, met-enkephalin and gastrin have been characterized in the DRN. Moreover, numerous brain areas have neurons that project to the DRN and express monoamines (norepinephrine, histamine), amino acids (GABA, glutamate), acetylcholine or neuropeptides (orexin, melanin-concentrating hormone, corticotropin-releasing factor and substance P) that directly or indirectly, through local circuits, regulate the activity of 5-HT cells. The 5-HT cells predominate along the midline of the rostral, dorsal and ventral subdivisions of the DRN and outnumber the non-5-HT cells occurring in the raphe nucleus. The GABAergic and glutamatergic neurons are clustered mainly in the lateral and dorsal subdivisions of the DRN, respectively. The 5-HT(1A) receptor is located on the soma and the dendrites of 5-HT neurons and at postsynaptic sites (outside the DRN). It is expressed, in addition, by non-5-HT cells of the DRN. The 5-HT(1B) receptor is located at presynaptic and postsynaptic sites (outside the boundaries of the DRN). It has been described also in the ventromedial DRN where it is expressed by non-5-HT cells. The 5-HT(2A) and 5-HT(2C) receptors are located within postsynaptic structures. At the level of the DRN the 5-HT(2A) and 5-HT(2C) receptor-containing cells are predominantly GABAergic interneurons and projection neurons. Within the boundaries of the DRN the 5-HT(3) receptor is expressed by, among others, glutamatergic interneurons. 5-HT(7) receptors in the DRN are not localized to serotonergic neurons but, at least in part, to GABAergic cells and terminals. The complex structure of the DRN may have important implications for neural mechanisms underlying 5-HT modulation of wakefulness and REM sleep.

5-HT(1B) receptor regulation of serotonin (5-HT) release by endogenous 5-HT in the substantia nigra

Axonal release of serotonin (5-hydroxytryptamine, 5-HT) in the CNS is typically regulated by presynaptic 5-HT autoreceptors. Release of 5-HT in substantia nigra pars reticulata (SNr), a principal output from the basal ganglia, has seemed an interesting exception to this rule. The SNr receives one of the highest densities of 5-HT innervation in mammalian brain and yet negative feedback regulation of axonal 5-HT release by endogenous 5-HT has not been identified here. We explored whether we could identify autoregulation of 5-HT release by 5-HT(1B) receptors in rat SNr slices using fast-scan cyclic voltammetry at carbon-fiber microelectrodes to detect 5-HT release evoked by discrete stimuli (50 Hz, 20 pulses) paired over short intervals (1-10 s) within which any autoreceptor control should occur. Evoked 5-HT release exhibited short-term depression after an initial stimulus that recovered by 10 s. Antagonists for 5-HT(1B) receptors, isamoltane (1 microM) or SB 224-289 (1 microM), did not modify release during a stimulus train, but rather, they modestly relieved depression of subsequent release evoked after a short delay (< or =2 s). Release was not modified by antagonists for GABA (picrotoxin, 100 microM, saclofen, 50 microM) or histamine-H(3) (thioperamide, 10 microM) receptors. These data indicate that 5-HT release can activate a 5-HT(1B)-receptor autoinhibition of subsequent release, which is mediated directly via 5-HT axons and not via GABAergic or histaminergic inputs. These data reveal that 5-HT release in SNr is not devoid of autoreceptor regulation by endogenous 5-HT, but rather is under modest control which only weakly limits 5-HT signaling.

Estrogen decreases 5-HT1B autoreceptor mRNA in selective subregion of rat dorsal raphe nucleus: inverse association between gene expression and anxiety behavior in the open field

We have recently shown that estrogen decreases anxiety and increases expression of tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme for 5-HT synthesis. However, the effects of estrogen on 5-HT release and reuptake may also affect the overall availability of 5-HT in the forebrain. Estrogen has been previously shown to have no effect on the inhibitory 5-HT 1A autoreceptor (5-HT(1A)) in the rat dorsal raphe nuclei (DRN); however the regulation of the inhibitory 5-HT 1B autoreceptor (5-HT(1B)) in the midbrain raphe by estrogen has not yet been investigated. Therefore, we examined the effects of estrogen on 5-HT(1B) mRNA in the rat DRN, focusing on specific subregions, and whether 5-HT(1B) mRNA levels correlated with TPH2 mRNA levels and with anxiety-like behavior. Ovariectomized rats were treated for 2 weeks with estrogen or placebo, exposed to the open field test, and 5-HT(1A) and 5-HT(1B) mRNA was quantified by in situ hybridization histochemistry. Estrogen had no effect on 5HT(1A) mRNA in any of the DRN subregions examined, confirming a previous report. In contrast, estrogen selectively decreased 5-HT(1B) mRNA in the mid-ventromedial subregion of the DRN, where 5-HT(1B) mRNA was associated with higher anxiety-like behavior and inversely correlated with TPH2 mRNA levels. These results suggest that estrogen may reduce 5-HT(1B) autoreceptor and increase TPH2 synthesis in a coordinated fashion, thereby increasing the capacity for 5-HT synthesis and release in distinct forebrain regions that modulate specific components of anxiety behavior.

Cellular effects of swim stress in the dorsal raphe nucleus

Swim stress regulates forebrain 5-hydroxytryptamine (5-HT) release in a complex manner and its effects are initiated in the serotonergic dorsal raphe nucleus (DRN). The purpose of this study was to examine the effects of swim stress on the physiology of DRN neurons in conjunction with 5-HT immunohistochemistry. Basic membrane properties, 5-HT(1A) and 5-HT(1B) receptor-mediated responses and glutamatergic excitatory postsynaptic currents (EPSCs) were measured using whole-cell patch clamp techniques. Rats were forced to swim for 15min and 24h later DRN brain slices were prepared for electrophysiology. Swim stress altered the resting membrane potential, input resistance and action potential duration of DRN neurons in a neurochemical-specific manner. Swim stress selectively elevated glutamate EPSC frequency in 5-HT DRN neurons. Swim stress non-selectively reduced EPSC amplitude in all DRN cells. Swim stress elevated the 5-HT(1B) receptor-mediated inhibition of glutamatergic synaptic activity that selectively targeted 5-HT cells. Non-5-HT DRN neurons appeared to be particularly responsive to the effects of a milder handling stress. Handling elevated EPSC frequency, reduced EPSC decay time and enhanced a 5-HT(1B) receptor-mediated inhibition of mEPSC frequency selectively in non-5-HT DRN cells. These results indicate that swim stress has both direct, i.e., changes in membrane characteristics, and indirect effects, i.e., via glutamatergic afferents, on DRN neurons. These results also indicate that there are distinct local glutamatergic afferents to neurochemically specific populations of DRN neurons, and furthermore that these distinct afferents are differentially regulated by swim stress. These cellular changes may contribute to the complex effects of swim stress on 5-HT neurotransmission and/or the behavioral changes underlying the forced swimming test model of depression.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

Potentiation of transient receptor potential V1 functions by the activation of metabotropic 5-HT receptors in rat primary sensory neurons

5-Hydroxytryptamine (5-HT) is one of the major chemical mediators released in injured and inflamed tissue and is capable of inducing hyperalgesia in vivo. However, the cellular mechanisms of 5-HT-induced hyperalgesia remain unclear. Transient receptor potential V1 (TRPV1) plays a pivotal role in nociceptive receptors. In the present study, we determined whether 5-HT changes TRPV1 functions in cultured dorsal root ganglion (DRG) neurons isolated from neonatal rats, using Ca(2+) imaging and whole-cell patch-clamp techniques. In more than 70% of DRG neurons, 5-HT potentiated the increases of [Ca(2+)](i) induced by capsaicin, protons and noxious heat. Capsaicin-induced current and depolarizing responses, and proton-induced currents were also augmented by 5-HT. RT-PCR analysis revealed the expression of 5-HT(2A) and 5-HT(7) receptors in rat DRG neurons. Agonists for 5-HT(2A) and 5-HT(7) receptors mimicked the potentiating effect of 5-HT, and their antagonists decreased it. In DRG ipsilateral to the complete Freund's adjuvant-injected inflammation side, expression levels of 5-HT(2A) and 5-HT(7) mRNAs increased, and the potentiating effect of 5-HT was more prominent than in the contralateral control side. These results suggest that the PKC- and PKA-mediated signalling pathways are involved in the potentiating effect of 5-HT on TRPV1 functions through the activation of 5-HT(2A) and 5-HT(7) receptors, respectively. Under inflammatory conditions, the increases of the biosynthesis of these 5-HT receptors may lead to further potentiation of TRPV1 functions, resulting in the generation of inflammatory hyperalgesia in vivo.

Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Serotonin (5-HT) is an ancient chemical that plays a crucial functional role in almost every living organism. It regulates platelet aggregation, activation of immune cells, and contraction of stomach and intestinal muscles. In addition, serotonin acts as a neurotransmitter in the brain and the peripheral nervous system. These activities are initiated by the binding of serotonin to 15 or more receptors that are pharmacologically classified into seven groups, 5-HT1 through 5-HT7. Each group is further divided into subgroups of receptors that are homologous but are encoded by discrete genes. With the exception of the 5-HT3 receptor--a cation channel--all of the others are G protein-coupled receptors that potentially activate or inhibit a large number of biochemical cascades. This review will endeavor to compare and contrast such signaling pathways with special attention to their tissue-specific occurrence, their possible role in immediate effects on covalent modification of other proteins, and relatively slower effects on gene expression, physiology and behavior.

5-HT1B receptors and aggression: A review

The serotonergic (5-HT) system in the brain is involved in the modulation of offensive aggressive behavior. The dogma that activity of the 5-HT system is inversely related to aggression is obsolete now. Research on the status of the 5-HT system before, during and after the execution of aggression is ongoing but has not yet led to a clear picture about the actual functional role of the 5-HT system, the more because state versus trait aggression seems to play a pivotal role in the outcome. Pharmacological challenges pinpoint 5-HT(1A) and 5-HT(1B) receptors as key players in the modulation of offensive aggression. This review emphasizes in particular the role of postsynaptic 5-HT(1B) (hetero) receptors as a premier site to modulate offensive aggression. Modulation of the firing and 5-HT release of the serotonergic neuron, via presynaptic 5-HT(1A) (auto) receptors, presynaptic 5-HT(1B) (auto) receptors and serotonergic transporters, may also have striking influences on aggression under certain conditions. Therefore, it is hypothesized that postsynaptic 5-HT(1B) (hetero) receptors directly influence the executive, consummatory phases of agonistic behavior, whereas presynaptic serotonergic feedback systems are particularly useful in the introductory (appetitive) phases of the agonistic behavioral complex.

Expression analysis of genes responsible for serotonin signaling in the brain

To thoroughly understand the function and regulation of neurotransmitter systems in the brain, as well as the underlying disease mechanisms, it is important to comprehensively analyze the expression patterns of genes participating in such systems. Using functional annotated cDNA clones (FANTOM), we examined the gene expression patterns of the serotonin neurotransmitter system, which is involved in psychiatric diseases such as depression. We chose 24 gene products and visualized their endogenous localizations using in situ hybridization (ISH). We were able to fine-tune an automated ISH method to obtain high-resolution cell-based figures within 24 h. We also measured the amounts of mRNAs with quantitative RT-PCR. The outline of the in situ gene expression pattern viewed under low magnification agreed with the results of the RT-PCR. In the high-resolution view obtained with ISH, we could document novel localizations of the several genes critically related to serotonin activity.

Regional expression of 5-HT1B receptor mRNA in the human brain

The regional mRNA expression pattern of 5-HT(1B) receptors has been extensively characterized in the rodent and guinea pig brain, but a detailed mapping of the 5-HT(1B) receptor mRNA expression in the human brain has not previously been performed. In the present study, the mRNA expression of 5-HT(1B) receptors was analyzed using in situ hybridization histochemistry and whole hemisphere sections of the human postmortem brain. The mRNA expression was compared with the autoradiographic distribution of 5-HT(1B) receptors. High levels of mRNA expression were found in the striatum, cortex, lateral geniculate nucleus, and raphe nuclei. The expression was higher in ventral than in dorsal striatal regions and was absent from the substantia nigra and pallidum, where high levels of 5-HT(1B) receptors were found. A layer-specific expression pattern was observed in cortical regions. The results extend previous knowledge about the localization of the 5-HT(1B) receptor in the human brain. This study provides evidence of a mismatch of the regional expression of 5-HT(1B) receptor mRNA and the 5-HT(1B) receptor distribution in human brain, similar to what has been demonstrated in other species. This is in line with the localization of this receptor subtype in nerve terminals. The results give support to species differences in the cortical mRNA expression pattern of this receptor subtype.

Wheel running alters serotonin (5-HT) transporter, 5-HT1A, 5-HT1B, and alpha 1b-adrenergic receptor mRNA in the rat raphe nuclei

BACKGROUND

Altered serotonergic (5-HT) neurotransmission is implicated in the antidepressant and anxiolytic properties of physical activity. In the current study, we investigated whether physical activity alters factors involved in the regulation of central 5-HT neural activity.

METHODS

In situ hybridization was used to quantify levels of 5-HT transporter (5-HTT), 5-HT(1A), 5-HT(1B), and alpha(1b)-adrenergic receptor (alpha(1b) ADR) messenger ribonucleic acids (mRNAs) in the dorsal (DRN) and median raphe (MR) nuclei of male Fischer rats after either sedentary housing or 3 days, 3 weeks, or 6 weeks of wheel running.

RESULTS

Wheel running produced a rapid and lasting reduction of 5-HT(1B) mRNA in the ventral DRN. Three weeks of wheel running decreased 5-HTT mRNA in the DRN and MR and increased alpha(1b) ADR mRNA in the DRN. After 6 weeks of wheel running, 5-HTT mRNA remained reduced, but alpha(1b) ADR mRNA returned to sedentary levels. Serotonin(1A) mRNA was increased in the MR and certain DRN subregions after 6 weeks only.

CONCLUSIONS

Data suggest that the central 5-HT system is sensitive to wheel running in a time-dependent manner. The observed changes in mRNA regulation in a subset of raphe nuclei might contribute to the stress resistance produced by wheel running and the antidepressant and anxiolytic effects of physical activity.

Selective 5-HT1B receptor agonist reduces serotonin synthesis following acute, and not chronic, drug administration: results of an autoradiographic study

The effects of acute and chronic administration of the serotonin (5-HT)1B agonist CP-93,129, on 5-HT synthesis rates were evaluated using the alpha-[14C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method. In the acute treatment study, CP-93,129 (7 mg/kg) was injected intraperitoneally 30 min before the alpha-MTrp injection (30 microCi over 2 min). A single dose of CP-93,129 caused a significant increase in the synthesis in the median raphe nucleus (MR) without a significant influence on the dorsal raphe nucleus (DR). There was a reduction in 5-HT synthesis in almost all of the projection areas. In the chronic treatment study, CP-93,129 was administered continuously (7 mg/kg/day) for 14 days using an osmotic minipump implanted subcutaneously. The chronic treatment with CP-93,129 did not produce a significant change in 5-HT synthesis in the raphe nuclei nor in the nerve terminal structures, except for the medial frontal bundle and the visual and sensory-motor cortices. The unaltered 5-HT synthesis rates in the chronic treatment study probably reflect a normalization of the synthesis as a result of the desensitization of 5-HT1B autoreceptors and/or heteroreceptors.

Analysis of the role of the 5-HT1B receptor in spatial and aversive learning in the rat

The present study examined the role of the 5-HT1B receptor in learning and memory. The ability of the 5-HT1B receptor agonist anpirtoline and the selective 5-HT1B receptor antagonist NAS-181 to affect spatial learning in the water maze (WM) and aversive learning in the passive avoidance (PA) task were examined in the rat. Anpirtoline (0.1-1.0 mg/kg, s.c.) caused a dose-dependent impairment of learning and memory in both the WM and PA tasks. NAS-181 (1.0-10 mg/kg, s.c.) failed to alter performance of the WM task, but produced a dose-dependent (0.1-20 mg/kg) facilitation of PA retention. Furthermore, treatment with NAS-181 (10 mg/kg) fully blocked the impairment of the WM and PA performance caused by anpirtoline (1.0 mg/kg). In contrast, NAS-181 (3.0-10 mg/kg) did not attenuate the spatial learning deficit and the impairment of PA retention caused by scopolamine (0.1 mg/kg in WM task, 0.3 mg/kg in PA task, s.c.), a nonselective muscarinic antagonist. Moreover, a subthreshold dose of scopolamine (0.1 mg/kg) blocked the facilitation of PA retention induced by NAS-181 (1.0-10 mg/kg). In addition, the behavioral disturbances (eg thigmotaxic swimming and platform deflections) induced by anpirtoline and scopolamine were analyzed in the WM task and correlated with WM performance. These results indicate that: (1) 5-HT1B receptor stimulation and blockade result in opposite effects in two types of cognitive tasks in the rat, and that (2) the 5-HT1B antagonist NAS-181 can facilitate some aspects of cognitive function, most likely via an increase of cholinergic transmission. These results suggest that 5-HT1B receptor antagonists may have a potential in the treatment of cognitive deficits resulting from loss of cholinergic transmission.

Activation of 5HT1B receptors inhibits glycinergic synaptic inputs to mammalian motoneurons during postnatal development

We investigated whether 5-HT(1B) receptor-mediated inhibition of evoked glycinergic inhibitory postsynaptic currents (eIPSCs) in hypoglossal motoneurons (HMs) changed postnatally. In HMs from postnatal days 2-3 (P2-3, neonate) and P10-11 (juvenile) rats bath application of 5-HT (10 microM) caused a not significantly different large reduction in eIPSC amplitude to 35.0+/-22.5% (mean+/-S.D.) and 35.4+/-10.6% of control; respectively. The dose-response relationship for the 5-HT(1B) receptor agonist, CP-93,129, revealed that the mean agonist concentration at half-maximal inhibition (IC(50)) was similar, 1.6 and 2.0 nM, respectively. Additionally, strong antibody labeling of 5-HT(1B) receptors in the hypoglossal motor nucleus was observed in neonates, juveniles and adults. These results demonstrate that over the postnatal period studied, 5-HT(1B) receptor-mediated inhibition of glycinergic eIPSCs is not age dependent.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

5-HT(1B) mrna regulation in two animal models of altered stress reactivity

BACKGROUND

Acute stress has profound effects on serotonergic activity, but it is not known whether alterations in the serotonin system can predispose individuals to exaggerated stress responses. We examined the regulation of 5-HT(1B) and 5-HT(1A) mRNA in two rodent models of differential sensitivity to stress: congenital learned helplessness (cLH) and handling and maternal separation (HMS).

METHODS

5-HT(1B) and 5-HT(1A) mRNAs in brain tissue sections were quantitated by in situ hybridization from control, stress-sensitive, and stress-resistant male rats in the HMS model and stress-sensitive and stress-resistant rats (both males and females) in the cLH model. Dorsal raphe nucleus, striatum, and hippocampus were examined.

RESULTS

The main result was that dorsal raphe 5-HT(1B) mRNA was substantially elevated (63-73%) in male rats in the stress-resistant group of both models compared with stress-sensitive animals. 5-HT(1B) mRNA in female rats did not differ between groups in the cLH model. There were no differences in 5-HT(1A) mRNA between HMS groups.

CONCLUSIONS

These findings suggest that 5-HT(1B) autoreceptor regulation is altered in animals with diminished stress reactivity. These results suggest that 5-HT(1B) autoreceptors in unstressed and acutely stressed animals differ, indicating the importance of state versus trait changes in serotonin function in animal models of anxiety and depression.

Ligands for the investigation of 5-HT autoreceptor function

The existence of multiple 5-HT autoreceptors in the central nervous system is now firmly established and they have been pharmacologically identified as belonging to the 5-HT(1A), 5-HT(1B), and 5-HT(1D) receptor subtypes. In addition, 5-HT(1F), 5-HT(5A), and 5-HT(7) receptors remain as potential candidates for additional autoreceptors. The emergence of selective ligands, such as SB-224289 (5-HT(1B) receptor antagonist), BRL 15572 (5-HT(1D) receptor antagonist), GR 127935 (a mixed 5-HT(1B/1D) receptor antagonist), LY 334370 (5-HT(1F) receptor agonist), and SB-269970 (5-HT(7) receptor antagonist), has aided the characterisation of 5-HT autoreceptors and has highlighted the complexity of mechanisms which modulate the release of 5-HT.

The role of 5-HT1B receptors in the regulation of serotonin cell firing and release in the rat brain

The release of 5-HT in terminal areas of the rodent brain is regulated by 5-HT1B receptors. Here we examined the role of 5-HT1B receptors in the control of 5-HT output and firing in the dorsal raphe nucleus (DR), median raphe nucleus (MnR) and forebrain of the rat in vivo. The local perfusion (30-300 microM) of the selective 5-HT1B receptor agonist CP-93,129 to freely moving rats decreased 5-HT release in the DR and more markedly in the MnR. Likewise, 300 microM CP-93,129 reduced 5-HT output in substantia nigra pars reticulata, ventral pallidum, lateral habenula and the suprachiasmatic nucleus. The effect of CP-93,129 was prevented by SB-224289, but not by WAY-100635, selective 5-HT1B and 5-HT1A receptor antagonists, respectively. SB-224289 did not alter dialysate 5-HT in any raphe nuclei. The intravenous administration of the brain-penetrant selective 5-HT1B receptor agonist CP-94,253 (0.5-2.0 mg/kg) to anesthetized rats decreased dialysate 5-HT in dorsal hippocampus and globus pallidus, increased it in MnR and left it unaltered in the DR and medial prefrontal cortex. SB-224289, at a dose known to block 5-HT1B autoreceptor-mediated effects (5 mg/kg), did not prevent the effect of CP-94,253 on MnR 5-HT. The intravenous administration of CP-94,253 (0.05-1.6 mg/kg) to anesthetized rats increased the firing rate of MnR, but not DR-5-HT neurons. The local perfusion of CP-94,253 in the MnR showed a biphasic effect, with 5-HT reductions at 0.3-3 microM and increase at 300 microM. These results suggest that 5-HT cell firing and release in midbrain raphe nuclei (particularly in the MnR) are under control of 5-HT1B receptors. The activation of 5-HT1B autoreceptors (possibly located on 5-HT nerve endings and/or varicosities within DR and MnR) reduces 5-HT release. The effects of higher concentrations of 5-HT1B receptor agonists seem more compatible with the activation of 5-HT1B heteroreceptors on inhibitory neurons.

Changes of the expression of 5-HT receptor subtype mRNAs in rat dorsal root ganglion by complete Freund's adjuvant-induced inflammation

By using the reverse transcriptase polymerase chain reaction technique, the expression of 5-hydroxytryptamine (5-HT) receptor subtype mRNAs in the rat lumbar dorsal root ganglion (DRG) was investigated following unilateral injection of complete Freund's adjuvant (CFA) into the rat hind paw. The results showed that 5-HT(1A), 5-HT(1B), 5-HT(1D), 5-HT(1F), 5-HT(2A), 5-HT(3), 5-HT(4), 5-HT(5A) and 5-HT(7) receptor subtypes were present in the rat lumbar DRG. CFA injection resulted in a significant increase in mRNA level of 5-HT(1A), 5-HT(1B), 5-HT(1F), 5-HT(2A), 5-HT(3), 5-HT(4) and 5-HT(7) receptor subtypes and a marked induction of 5-HT(2C) subtype mRNA in the DRG. The present results suggest the important roles for these 5-HT receptor subtypes in generating peripheral nociceptive signaling and provide evidence to elucidate the mechanism of 5-HT in nociception.

Serotonergic neuromodulation in the cerebellar cortex: cellular, synaptic, and molecular basis

The cerebellum, like most sensorimotor areas of the brain, receives a serotonergic innervation from neurons of the reticular formation. It is well established that local application of serotonin modulates the firing rate of cerebellar Purkinje cells in vivo and in vitro, but the mechanisms by which serotonin affects the cerebellar function are still poorly understood. Whereas interactions between serotonin, glutamate, and GABA have been reported to increase or decrease the firing frequency of Purkinje cells, there is little evidence for a modulation of excitatory and inhibitory synapses by serotonin in the cerebellar cortex. Changes in the intrinsic electrical properties of Purkinje cells upon application of serotonin have also been reported, but their impact on Purkinje cell firing is unclear. The recent finding that serotonin specifically modulates the activity of Lugaro cells, a class of inhibitory interneurons of the cerebellar cortex, offers new insights on the action of this neuromodulator. The peculiar axonal projection and specific interneuronal targets of the Lugaro cells suggest that the action of serotonin might occur upstream of Purkinje cells through a resetting of the computational properties of the cerebellar cortex. Understanding the mechanisms of the serotonergic modulation of the cerebellar cortex is of clinical relevance, as abnormal serotonin metabolism has been observed in animal models and pathological cases of motor disorders involving the cerebellum, and as chronic intravenous administration of L-5-hydroxytryptophan (5-HTP), a precursor of serotonin, was the first treatment shown to improve significantly cerebellar symptoms.

Multiple 5-HT(1) autoreceptor subtypes govern serotonin release in dorsal and median raphé nuclei

The present study investigated the possibility of multiple 5-HT(1) autoreceptor subtypes in the rostral raphé nuclei. Slices (350 microm) of rat dorsal or median raphé nucleus (DRN/MRN) were taken from male Wistar rats and superfused with artificial cerebrospinal fluid at 32 degrees C. Fast cyclic voltammetry at carbon fibre microelectrodes was used to monitor serotonin (5-HT) release following local electrical stimulation. In both DRN and MRN, 5-HT release on short trains was reduced by the selective 5-HT(1A) agonist 8-OH-DPAT (1 microM), an effect blocked by the selective 5-HT(1A) antagonist WAY 100635 (0.1 microM) but not by SB 216641 (0.05 and 0.2 microM) or BRL 15572 (0.5 microM), selective antagonists at the 5-HT(1B) and 5-HT(1D) receptors respectively. The selective 5-HT(1B) agonist CP 93129 (0.3 microM) also reduced 5-HT release in both nuclei. Its effect was blocked by SB 216641 but not by WAY 100635 or BRL 15572. The 5-HT(1D/1B) agonist sumatriptan (0.5 microM) decreased 5-HT release in both DRN and MRN. In DRN, the effect of sumatriptan was blocked by BRL 15572 but not by WAY 100635 or SB 216641. In MRN, the effect of sumatriptan was not blocked by any of the above antagonists. BRL 15572 increased 5-HT release on long stimulations in DRN and MRN while WAY 100635 had no effect. SB 216641 increased 5-HT release in MRN but not DRN. WAY 100635 potentiated the effect of SB 216641 in DRN but not MRN. The data suggest that 5-HT release in DRN is controlled by 5-HT(1A), 5-HT(1B) and 5-HT(1D) autoreceptors. 5-HT release in MRN is controlled by 5-HT(1A) and 5-HT(1B) autoreceptors and another, as yet unidentified mechanism.

Dominant role of the cytosolic C-terminal domain of the rat 5-HT1B receptor in axonal-apical targeting

The 5-HT(1A) and 5-HT(1B) receptors for serotonin exhibit a different membrane localization to either soma and dendrites (5-HT(1A)R) or axons and terminals (5-HT(1B)R) of neurons in the CNS. The mechanisms responsible for their differential targeting were investigated previously by transfecting various 5-HT(1A)R/5-HT(1B)R chimeras in the epithelial Lilly pork kidney (LLC-PK1) cell line. This first study suggested that a specific targeting signal is located in the C-terminal portion (comprising the last two transmembrane and the cytoplasmic C-terminal domains) of the 5-HT(1A)and/or 5-HT(1B) receptors. In the present study, the role of the cytosolic C-terminal tail of the receptors was further investigated by transfecting truncated receptors and 5-HT(1A)R/5-HT(1B)R chimeras in both the epithelial LLC-PK1 cells and rat hippocampal neurons in primary culture. Confocal microscopic analysis of immunofluorescence with specific anti-5-HTR antibodies and anti-microtubule-associated protein 2 or anti-neurofilament 200k antibodies showed that substitution of the cytosolic C-terminal tail of the 5-HT(1B)R in the 5-HT(1A)R addressed the resulting chimera to the axon of neurons and to the apical domain of LLC-PK1 cells. Therefore, the short tail of the 5-HT(1B)R presents an apical targeting signal that can also act as an axonal targeting signal. In addition, a domain within the third intracytoplasmic loop of the 5-HT(1B)R, responsible for its Golgi sequestration in LLC-PK1 cells, appeared to act as another axonal targeting signal in hippocampal neurons.

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.

Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality?

The serotonergic cells of the dorsal raphé nucleus innervate much of the forebrain and are thought to be involved in the mechanism of action of antidepressants. Dysfunction of these cells might be involved in the neural mechanisms underlying depression and suicide. The traffic in pathways emanating from the dorsal raphé nucleus is controlled by 5-HT(1) autoreceptors. Until recently it was thought that the autoreceptors in the dorsal raphé nucleus were solely of the 5-HT(1A) subtype. In this article, we discuss evidence that the situation is more complex and that multiple 5-HT(1) subtypes govern different aspects of 5-HT function in the dorsal raphé nucleus presenting new therapeutic opportunities.

Organization of the corticotectal projection in hamsters with neonatally elevated levels of serotonin in the superior colliculus

A previous study from this laboratory showed that elevated serotonin (5-HT) levels in the hamsters superior colliculus (SC), induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth, resulted in an abnormally widespread distribution of the uncrossed retinotectal projection. The present study investigated whether the corticotectal projection in such animals was altered. Adult normal and 5,7-DHT-treated hamsters were injected with horseradish peroxidase (HRP) into occipital cortex and processed for anterograde tracing of corticotectal terminals in the SC. Quantitative analysis showed that normal and 5,7-DHT-treated hamsters were not significantly different in total labeling or in the gradient of labeling density within the SC. These data indicate that corticotectal axons achieve normal terminal fields after neonatal elevation of 5-HT in the SC.

5-HT1A and 5-HT1B receptors control the firing of serotoninergic neurons in the dorsal raphe nucleus of the mouse: studies in 5-HT1B knock-out mice

The characteristics of the spontaneous firing of serotoninergic neurons in the dorsal raphe nucleus and its control by serotonin (5-hydroxytryptamine, 5-HT) receptors were investigated in wild-type and 5-HT1B knock-out (5-HT1B-/-) mice of the 129/Sv strain, anaesthetized with chloral hydrate. In both groups of mice, 5-HT neurons exhibited a regular activity with an identical firing rate of 0.5-4.5 spikes/s. Intravenous administration of the 5-HT reuptake inhibitor citalopram or the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induced a dose-dependent inhibition of 5-HT neuronal firing which could be reversed by the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xane carboxamide (WAY 100635). Both strains were equally sensitive to 8-OH-DPAT (ED50 approximately 6.3 microgram/kg i.v.), but the mutants were less sensitive than wild-type animals to citalopram (ED50 = 0.49 +/- 0.02 and 0.28 +/- 0.01 mg/kg i.v., respectively, P < 0.05). This difference could be reduced by pre-treatment of wild-type mice with the 5-HT1B/1D antagonist 2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carbox yli c acid [4-methoxy-3-(4-methyl-piperazine-1-yl)-phenyl]amide (GR 127935), and might be accounted for by the lack of 5-HT1B receptors and a higher density of 5-HT reuptake sites (specifically labelled by [3H]citalopram) in 5-HT1B-/- mice. In wild-type but not 5-HT1B-/- mice, the 5-HT1B agonists 3-(1,2,5, 6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94253, 3 mg/kg i.v.) and 5-methoxy-3-(1,2,3, 6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 0.6 mg/kg i.v.) increased the firing rate of 5-HT neurons (+22.4 +/- 2.8% and +13.7 +/- 6.0%, respectively, P < 0.05), and this effect could be prevented by the 5-HT1B antagonist GR 127935 (1 mg/kg i.v.). Altogether, these data indicate that in the mouse, the firing of 5-HT neurons in the dorsal raphe nucleus is under both an inhibitory control through 5-HT1A receptors and an excitatory influence through 5-HT1B receptors.

Indirect evidence for an association of 5-HT(1B) binding sites with retinal and geniculate axon terminals in the rat suprachiasmatic nucleus

The purpose of the present study was to investigate the possible cellular location of 5-HT(1B) receptors on retinal and geniculate afferents in the rat suprachiasmatic nucleus (SCN). Biocular enucleation significantly decreased 5-HT(1B) binding site labeling (35%), specifically in the ventral part of the SCN, while monocular enucleation produced a decrease of smaller magnitude (12%), limited to the ventral part of the contralateral SCN, these results being consistent with the known distribution of retinal afferents in the nucleus. By contrast, bilateral geniculate lesion did not induce any significant variation of 5-HT(1B) binding site labeling in the SCN. Previously, we reported that serotonin (5-HT) synthesis inhibition by parachlorophenylalanine increases 5-HT(1B) binding site labeling in the SCN. Using saturation studies, we have now demonstrated that this upregulation reflected an increase in the total number of 5-HT(1B) binding sites (+41% in the dorsal and +67% in the ventral part of the SCN). Furthermore, we evaluated the effects of bilateral geniculate lesion after 5-HT stores depletion in order to overcome problems of technical resolution limits. The magnitude of upregulation was significantly decreased (27%) after bilateral geniculate lesion, suggesting that part of the 5-HT(1B) receptor population was located on geniculate axon terminals within the SCN. The possible involvement of 5-HT(1B) receptors, according to their cellular locations evidenced in the present study, in photic and nonphotic entrainment of the circadian clock is discussed.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Cellular and subcellular localization of 5-hydroxytryptamine1B receptors in the rat central nervous system: immunocytochemical, autoradiographic and lesion studies

The localization of 5-hydroxytryptamine1B receptors in the rat central nervous system was investigated using anti-peptide antibodies that recognize a selective portion of the third intracytoplasmic loop of the receptor protein. At the light microscope level the densest 5-hydroxytryptamine1B receptor-like immunoreactivity was observed in ventral pallidum, globus pallidus, substantia nigra and dorsal subiculum. In addition, moderate immunoreactivity was found in the entopeduncular nucleus, the superficial gray layer of the superior colliculus, the caudate-putamen and the deep nuclei of the cerebellum. This distribution matched perfectly that previously described from radioligand binding studies. At the ultrastructural level, 5-hydroxytryptamine1B receptor-like immunoreactivity was associated with axons and axon terminals in the three areas examined: substantia nigra, globus pallidus and superficial gray layer of the superior colliculus. In all cases, immunostaining was located on the plasma membrane of unmyelinated axon terminals and in the cytoplasm close to the plasmalemma. Synaptic differentiations were never labelled but, in some cases, 5-hydroxytryptamine1B receptor-like immunoreactivity was found in their close vicinity. Injection of kainic acid into the neostriatum resulted in a marked decrease in receptor-like immunoreactivity in the globus pallidus and the substantia nigra, consistent with the location of 5-hydroxytryptamine1B receptors on terminals of striatopallidal and striatonigral fibres, respectively. A reduction in 5-hydroxytryptamine1B receptor-like immunoreactivity was also noted in the superficial gray layer of the superior colliculus after contralateral enucleation, as expected of the location of 5-hydroxytryptamine1B receptors on the terminals of retinocollicular fibres. In both lesion experiments, immunolabelled degenerating terminals were observed in the projection areas. Anterograde labelling experiments coupled with immunocytochemical detection further showed that 5-hydroxytryptamine1B receptors in the substantia nigra are located on axons of striatal neurons. These data provide anatomical support for the idea that 5-hydroxytryptamine1B receptors act as terminal receptors involved in presynaptic regulation of the release of various neurotransmitters, including 5-hydroxytryptamine itself.

5-HT1B receptor-mediated presynaptic inhibition of retinal input to the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain, and serotonin (5-HT) can modify the response of the SCN circadian oscillator to light. 5-HT1B receptor-mediated presynaptic inhibition has been proposed as one mechanism by which 5-HT modifies retinal input to the SCN (Pickard et al., 1996). This hypothesis was tested by examining the subcellular localization of 5-HT1B receptors in the mouse SCN using electron microscopic immunocytochemical analysis with 5-HT1B receptor antibodies and whole-cell patch-clamp recordings from SCN neurons in hamster hypothalamic slices. 5-HT1B receptor immunostaining was observed associated with the plasma membrane of retinal terminals in the SCN. 1-[3-(Trifluoromethyl)phenyl]-piperazine HCl (TFMPP), a 5-HT1B receptor agonist, reduced in a dose-related manner the amplitude of glutamatergic EPSCs evoked by stimulating selectively the optic nerve. Selective 5-HT1A or 5-HT7 receptor antagonists did not block this effect. Moreover, in cells demonstrating an evoked EPSC in response to optic nerve stimulation, TFMPP had no effect on the amplitude of inward currents generated by local application of glutamate. The effect of TFMPP on light-induced phase shifts was also examined using 5-HT1B receptor knock-out mice. TFMPP inhibited behavioral responses to light in wild-type mice but was ineffective in inhibiting light-induced phase shifts in 5-HT1B receptor knock-out mice. The results indicate that 5-HT can reduce retinal input to the circadian system by acting at presynaptic 5-HT1B receptors located on retinal axons in the SCN.

Induction of serotonin transporter by hypoxia in pulmonary vascular smooth muscle cells. Relationship with the mitogenic action of serotonin

-The increased delivery of serotonin (5-hydroxytryptamine, 5-HT) to the lung aggravates the development of hypoxia-induced pulmonary hypertension in rats, possibly through stimulation of the proliferation of pulmonary artery smooth muscle cells (PA-SMCs). In cultured rat PA-SMCs, 5-HT (10(-8) to 10(-6) mol/L) induced DNA synthesis and potentiated the mitogenic effect of platelet-derived growth factor-BB (10 ng/mL). This effect was dependent on the 5-HT transporter (5-HTT), since it was prevented by the 5-HTT inhibitors fluoxetine (10(-6) mol/L) and paroxetine (10(-7) mol/L), but it was unaltered by ketanserin (10(-6) mol/L), a 5-HT2A receptor antagonist. In PA-SMCs exposed to hypoxia, the levels of 5-HTT mRNA (measured by competitive reverse transcriptase-polymerase chain reaction) increased by 240% within 2 hours, followed by a 3-fold increase in the uptake of [3H]5-HT at 24 hours. Cotransfection of the cells with a construct of human 5-HTT promoter-luciferase gene reporter and of pCMV-beta-galactosidase gene allowed the demonstration that exposure of cells to hypoxia produced a 5.5-fold increase in luciferase activity, with no change in beta-galactosidase activity. The increased expression of 5-HTT in hypoxic cells was associated with a greater mitogenic response to 5-HT (10(-8) to 10(-6) mol/L) in the absence as well as in the presence of platelet-derived growth factor-BB. 5-HTT expression assessed by quantitative reverse transcriptase-polymerase chain reaction and in situ hybridization in the lungs was found to predominate in the media of pulmonary artery, in which a marked increase was noted in rats that had been exposed to hypoxia for 15 days. These data show that in vitro and in vivo exposure to hypoxia induces, via a transcriptional mechanism, 5-HTT expression in PA-SMCs, and that this effect contributes to the stimulatory action of 5-HT on PA-SMC proliferation. In vivo expression of 5-HTT by PA-SMC may play a key role in serotonin-mediated pulmonary vascular remodeling.

Endothelial expression of the 5-hydroxytryptamine1B antimigraine drug receptor in rat and human brain microvessels

In addition to triggering vasoconstriction of peripheral blood vessels, which led to its discovery as a circulating neurohormone 50 years ago, serotonin (5-hydroxytryptamine) acts as a neurotransmitter/ modulator in the central nervous system and regulates local cerebral blood flow and vascular permeability through direct and indirect effects on intraparenchymal microvessels. Among the various 5-hydroxytryptamine receptors which mediate these effects, particular attention has been paid to the 5-hydroxytryptamine1B and 5-hydroxytryptamine1D subtypes, as the preferred targets of modern antimigraine agents. Immunoelectron microscopic labeling of the 5-hydroxytryptamine1B receptor in rat brain parenchyma has revealed a distinct localization to the endothelium of microvessels, which was predominantly cytoplasmic as opposed to membrane-bound, contrary to that on preterminal unmyelinated axons [Riad et al. (1997) Soc. Neurosci. Abstr. 23, 1214]. Similar observations have now been made in human cortical tissue, in which the expected localization of the vascular 5-hydroxytryptamine1B receptor to periarteriolar myocytes was also confirmed. Such a dual localization in human brain microvessels suggests that the 5-hydroxytryptamine1B receptor might mediate opposite effects, vasodilatory and contractile, depending upon its activation by circulating or centrally released 5-hydroxytryptamine. It raises new possibilities as regards 5-hydroxytryptamine effects on human brain microvessels in health and disease, and notably the triggering of migraine headache.

Presynaptic inhibition by 5-HT1B receptors of glutamatergic synaptic inputs onto serotonergic caudal raphe neurones in rat

1. Autonomous, pacemaker-like activity of serotonergic raphe neurones and its autoregulation by somatodendritic 5-HT1A receptors are well described, but little is known of synaptic inputs onto raphe neurones or their modulation. Therefore, we recorded unitary excitatory postsynaptic currents (EPSCs) in caudal raphe neurones (raphe obscurus and pallidus) following local electrical stimulation in a neonatal rat brainstem slice preparation; most neurones (79 %; n = 72/91) recovered following post hoc immunohistochemistry were tryptophan hydroxylase-immunoreactive, indicating that they were serotonergic. 2. Evoked EPSCs occurred at relatively constant latency with variable amplitude and apparent 'failures' at fixed suprathreshold stimulus intensity. At -60 mV, EPSCs were wholly due to CNQX-sensitive, non-NMDA glutamate receptors; at depolarized potentials, a small AP-5-sensitive NMDA component was often observed. 3. EPSCs were potently and reversibly inhibited by 5-HT with an EC50 of 0.1 microM. This effect was mimicked by 5-HT1B agonists (CP-93,129 and anpirtoline), but not by a 5-HT1A agonist (8-OH-DPAT), indicating that 5-HT1B receptors mediate the inhibition of EPSCs. 4. Multiple lines of evidence indicate that inhibition of EPSCs by 5-HT was mediated presynaptically. First, currents evoked by exogenous glutamate application were unaffected by 5-HT and/or 5-HT1B agonists. In addition, the frequency of spontaneous glutamatergic miniature EPSCs was diminished by CP-93,129 and paired-pulse facilitation of EPSCs was enhanced by 5-HT. Finally, the 5-HT1B receptor agonists that blocked synaptic transmission had no effect on resting membrane properties of raphe neurones. 5. These data indicate that serotonergic caudal raphe neurones receive glutamatergic inputs that are inhibited by presynaptic 5-HT1B receptors; inhibition of excitatory synapses onto raphe cells may represent a novel mechanism for autoregulation of serotonergic neuronal activity by 5-HT.

5-HT4 receptor involvement in the serotonin-enhanced dopamine efflux from the substantia nigra of the freely moving rat: a microdialysis study

The functional regulation by serotonin (5-HT) receptors of the 5-HT-enhanced dopamine (DA) release from the rat substantia nigra (SN) was investigated using in vivo microdialysis. Exogenously administered or extracellularly enhanced 5-HT (by means of intranigral citalopram perfusion) (both 1 microM for 1 h) significantly increased nigral DA efflux to 165% and 145%, respectively. Intranigral administration of pindolol (10 microM, 3 h), a 5-HT1A/1B receptor antagonist which is clinically used in order to block 5-HT1A/1B autoreceptors, did not affect DA levels but significantly increased nigral 5-HT levels to 135%. Co-perfusion of this antagonist with 5-HT (1 microM, 1 h) did not abolish the 5-HT-induced DA release from the SN as DA was increased to 166%. Local application of the 5-HT1A/1B receptor agonist, CP 93129 (1 microM, 1 h), increased DA release from the SN to 4770% whereas 5-HT release was significantly decreased to 75%. Co-perfusion of the 5-HT1A/1B receptor antagonist, pindolol, with this agonist only partly abolished the CP 93129-induced DA release whereas the CP 93129-induced decrease in nigral 5-HT release was completely abolished. Administration of the 5-HT2A/2C receptor antagonist, ketanserin (50 microM, 3 h), significantly increased DA to 143% and 5-HT release to 363%. Co-perfusion of this antagonist with 5-HT still caused an increase in nigral DA release to 214%. Intranigral perfusion of the 5-HT4 receptor antagonist, RS 39604 (10 microM, 3 h), did not affect DA levels but significantly decreased nigral 5-HT levels to 74%. Co-perfusion of this antagonist with 5-HT was able to prevent the 5-HT-enhanced DA efflux from the SN. From this study it can be concluded that the 5-HT-enhanced (and possibly the citalopram-induced) nigral DA release is 5-HT4 receptor mediated.