Balance between cortical 5-HT1A and 5-HT2 receptor function: hypothesis for a faster antidepressant action

Convergence of glycogen synthase kinase 3β and GR signaling in response to fluoxetine treatment in chronically stressed female and male rats

Accumulating evidence strongly suggest that impaired glucocorticoid receptor (GR) signaling is involved in stress-related mood disorders, and nominate GR as a potential target for antidepressants (ADs). It is known that different classes of ADs affects the GR action via modifying its phosphorylation, while the mechanism through which ADs alter GR phosphorylation targeted by GSK3β, a kinase modulated via serotonin neurotransmission, are unclear. On this basis, we investigated whether GSK3β-GR signaling could be a convergence point of fluoxetine action on brain function and behavior, by examining its effect on GSK3β targeted-GR phosphorylation on threonine 171 (pGR171), and expression of GR-regulated genes in the hippocampus of female and male rats exposed to chronic isolation stress. Stress induced sex-specific GSK3β-targeted phosphorylation of pGR171 in the nucleus of the hippocampus of stressed animals. Namely, while in females stress triggered coupled action of GSK3β-pGR171 signaling, in males changes in pGR171 levels did not correspond to GSK3β activity. On the other hand, fluoxetine managed to up-regulate this pathway in sex-unbiased manner. Furthermore, fluoxetine reverted stress-induced changes in most of the analyzed genes in males, CRH, 5-HT1a and p11, while in females its effect was limited to CRH. These data further suggest that pGR171 signaling affects cellular localization of GR in response to chronic stress and fluoxetine in both sexes. Collectively, our results describe a novel convergence point between GR signaling and GSK3β pathway in rat hippocampus in response to stress and fluoxetine in both sexes and its involvement in fluoxetine-regulated brain function in males.

5-HT2A receptor antagonist M100907 reduces serotonin synthesis: an autoradiographic study

The effects of the administration of the serotonin (5-HT)(2A) antagonist, M100907, on 5-HT synthesis rates, were evaluated using the α-[(14)C]methyl-l-tryptophan (α-MTrp) autoradiographic method. In the treatment study, M100907 (10mg/kg) was injected intraperitoneally 30 min before the α-MTrp injection (30 μCi over 2 min). A single dose of M100907 caused a significant decrease in the synthesis in the anterior olfactory nucleus, accumbens nucleus, frontal cortex, sensory-motor cortex, cingulate cortex, medial caudate-putamen, dorsal thalamus, substantia nigra, inferior collicus, raphe magnus nucleus, superior olive, and raphe pallidus nucleus. These data suggest that the terminal 5-HT(2A) receptors are involved in the regulation of 5-HT synthesis in the entire brain. Further, 5-HT synthesis is likely regulated by the 5-HT(2A) antagonistic property of M100907 in the cortices, anterior olfactory nucleus, caudate putamen, and nucleus accumbens.

Prolonged 5-HT1A/5-HT2A receptor cross-talk is absent prior to adulthood in rats

In adult rats, serotonin 1A (5-HT1A) receptor activation produces heterologous desensitization of serotonin 2A (5-HT2A) neuroendocrine function at 1 h that persists up to 72 h. This study determined whether prolonged 5-HT1A/5-HT2A cross-talk exists before sexual maturation. Adolescent male rats (postnatal day 39) received an injection with saline or (+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide [(+)8-OH-DPAT] 24 h before receiving a challenge injection of saline, (+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide, or (-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl [(-)DOI] o assess changes in basal, 5-HT1A, and 5-HT2A receptor-mediated hormone responses. Although homologous desensitization of 5-HT1A neuroendocrine responses was present at 24 h, heterologous desensitization of 5-HT2A neuroendocrine responses was not. These data suggest that 5-HT1A heterologous desensitization of 5-HT2A receptor function does not develop until adulthood, is more transient, or follows a different time course before maturation.

Desensitization of 5-HT1A receptors by 5-HT2A receptors in neuroendocrine neurons in vivo

An imbalance between serotonin-2A (5-HT2A) and 5-HT1A receptors may underlie several mood disorders. The present studies determined whether 5-HT2A receptors interact with 5-HT1A receptors in the rat hypothalamic paraventricular nucleus (PVN). The sensitivity of the hypothalamic 5-HT1A receptors was measured as oxytocin and adrenocorticotropic hormone (ACTH) responses to the 5-HT1A receptor agonist (+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide [(+)8-OH-DPAT] (40 microg/kg s.c.). The 5-HT(2A/2C) receptor agonist (-)DOI [(-)-1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane HCl] (1 mg/kg s.c.) injected 2 h prior to (+)8-OH-DPAT significantly reduced the oxytocin and ACTH responses to (+)8-OH-DPAT, producing a heterologous desensitization of the 5-HT1A receptors. Microinjection of the 5-HT2A receptor antagonist MDL100,907 [(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol; 0, 10, or 20 nmol, 15 min prior to (-)DOI] into the PVN dose-dependently prevented the desensitization of 5-HT1A receptors induced by the 5-HT2A receptor agonist (-)DOI. Double-label immunocytochemistry revealed a high degree of colocalization of 5-HT1A and 5-HT2A receptors in the oxytocin and corticotropin-releasing factor neurons of the PVN. Thus, activation of 5-HT2A receptors in the PVN may directly induce a heterologous desensitization of 5-HT1A receptors within individual neuroendocrine cells. These findings may provide insight into the long-term adaptation of 5-HT1A receptor signaling after changes in function of 5-HT2A receptors; for example, during pharmacotherapy of mood disorders.

Neuropharmacological study of hetero[2,1]benzothiazepine derivatives analogues of tianeptine

Neuropharmacological studies were conducted in mice with a number of hetero[2,1]benzothiazepine derivatives, analogues of tianeptine. Seven of the 12 compounds under study potentiated the actions of 5-hydroxytryptophan (5-HTP, 50 mg/kg i.p.) and/or antagonised the hypothermia induced by high doses of apomorphine. Moreover, some of them inhibited the head twitches induced by 5-HTP (250 mg/kg i.p.) and the stereotyped behaviour and/or climbing behaviour of low doses of apomorphine. These compounds also produced a slight inhibition of exploratory behaviour in the holeboard test. On the other hand, no significant muscle relaxant, anticonvulsant and anxiolytic activities were observed at any dose employed. Together, these data suggest that some of the compounds under study exert antidepressant and neuroleptic effects in mice with no muscle relaxant, anxiolytic and anticonvulsant activities.

Pharmacology of flibanserin

Flibanserin has preferential affinity for serotonin 5-HT(1A), dopamine D(4k), and serotonin 5-HT(2A) receptors. In vitro and in microiontophoresis, flibanserin behaves as a 5-HT(1A) agonist, a very weak partial agonist on dopamine D(4) receptors, and a 5-HT(2A) antagonist. In vivo flibanserin binds equally to 5-HT(1A) and 5-HT(2A) receptors. However, under higher levels of brain 5-HT (i.e., under stress), flibanserin may occupy 5-HT(2A) receptors in higher proportion than 5-HT(1A) receptors. The effects of flibanserin on adenylyl cyclase are different from those of buspirone and 8-OH-DPAT, two other purported 5-HT(1A) receptor agonists. Flibanserin reduces neuronal firing rate in cells of the dorsal raphe, hippocampus, and cortex with the CA1 region being the most sensitive in the brain. Flibanserin-induced reduction in firing rate in the cortex seems to be mediated through stimulation of postsynaptic 5-HT(1A) receptors, whereas the reduction of the number of active cells seems to be mediated through dopamine D(4) receptor stimulation. Flibanserin quickly desensitizes somatic 5-HT autoreceptors in the dorsal raphe and enhances tonic activation of postsynaptic 5-HT(1A) receptors in the CA3 region. Flibanserin preferentially reduces synthesis and extracellular levels of 5-HT in the cortex, where it enhances extracellular levels of NE and DA. Flibanserin displays antidepressant-like activity in most animal models sensitive to antidepressants. Such activity, however, seems qualitatively different from that exerted by other antidepressants. Flibanserin seems to act via direct or indirect stimulation of 5-HT(1A), DA, and opioid receptors in those animal models. Flibanserin does not display consistent effects in animal models of anxiety and seems to exert potential antipsychotic effects. Flibanserin may induce some sedation but does not induce observable toxic effects at pharmacologically relevant doses.

Mechanism of action of flibanserin in the learned helplessness paradigm in rats

The mechanism of action of flibanserin, a 5-HT(1A) receptor agonist and a 5-HT(2A) receptor antagonist, was investigated in learned helplessness in rats. The effect of flibanserin (32 mg/kg, i.p. 30 min before testing) on learned helplessness was not antagonized by the (a) 5-HT synthesis inhibitor parachlorophenylalanine (pCPA; 150 mg/kg p.o.x3 times), which reduced brain 5-HT by 89%; (b) 5-HT(1A) receptor antagonists (+/-)-N-tert-butyl-3-4-(2-ethoxyphenyl)piperazin-1yl-2-phenyl propionamide [WAY100135; 10 mg/kg, i.p. 30 min before flibanserin, or 40 mg/kg, s.c. 15 min before flibanserin] and tertatolol (2.5 and 5 mg/kg, i.p. 30 min before flibanserin); and (c) 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI; 3 mg/kg, s.c. simultaneously with flibanserin). The effect of flibanserin on learned helplessness was antagonized by the dopamine D(1) receptor antagonist [R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390; 0.1 mg/kg, i.p. 30 min before flibanserin) and by the opioid receptor antagonist naloxone (3 mg/kg, s.c. 15 min before flibanserin). Flibanserin (32 and 64 mg/kg) did not induce conditioned place preference. In conclusion, flibanserin improved rats' performance in the learned helplessness paradigm, by stimulating dopamine D1 and opioid receptors, probably indirectly, since flibanserin has a low affinity for these receptors.

The effects of hypothyroidism on 5-HT1A and 5-HT2A receptors and the serotonin transporter protein in the rat brain

The effects of hypothyroidism on 5-HTIA and 5-HT2A receptors and the serotonin transporter protein were studied in thyroidectomized male Wistar rats in two experimental groups: 1) animals kept on an iodine-free diet hypothyroid rats) and 2) animals kept on thyroxine (15 microg/kg) for 21 days (giving normal thyroid hormone levels. euthyroid animals). Sham-operated rats served as controls. Binding of [3H]8-OH-DPAT with 5-HTIA receptors and [3H]citalopram with the transporter protein in the hippocampus and midbrain showed no changes in hypothyroid rats as compared with controls. Conversely. there were significant decreases in [3H]ketanserin binding to 5-HT2A receptors in the frontal cortex in hypothyroid rats as compared with controls; this decrease was reversed by thyroxine treatment. Thus, losses of cortical 5-HT2A receptors appears to be the main consequence of hypothyroidism at the level of the serotonin system of the brain.

Selective breeding of 5-HT(1A) receptor-mediated responses: application to emotion and receptor action

Rat lines that were selectively bred for high (high DPAT-sensitive, HDS) or low (low DPAT-sensitive, LDS) hypothermic responses to the specific 5-HT(1A) receptor agonist, 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT), differ in receptor binding and certain behaviors related to anxiety and depression. After reviewing this literature, the present communication summarizes new experiments designed to clarify and extend the nature of the pharmacological and biochemical differences between the lines. A challenge with the 5-HT(2) receptor agonist, DOI, produced similar degrees of head shakes and skin crawls in the HDS and LDS rats, suggesting similar sensitivity of 5-HT(2A) and 5-HT(2C) receptors. In contrast, DOI-induced flat body posture (FBP), which has been linked to 5-HT(1A) receptor stimulation, was observed more readily in the HDS rats. The HDS and LDS rats exhibited similar degrees of increase in 8-OH-DPAT-stimulated [35S]GTPgammaS binding in several brain regions. This result suggests that the dramatic differences in hypothermia in HDS and LDS rats cannot be related to 5-HT(1A) receptor-mediated action on G proteins. Overall, these findings indicate that the selective breeding for 5-HT(1A)-mediated hypothermia has been fairly selective, and that differences in emotionally relevant behaviors between these two rat lines can strongly be associated with an unidentified component of the 5-HT(1A) receptor signaling pathway.

Acute regulation and long-term modulation of presynaptic serotonin output

This contribution summarizes the present knowledge about the mechanisms involved in the regulation of presynaptic serotonin release. Under conditions of a permanently altered environmental, (psychosocial, drug-induced, nutritional etc.) input, each one of these mechanisms may be adaptively adjusted to the novel use-dependent requirements. Examples of such long-term alterations of the 5-HT-output pattern in distant projection fields of the raphe-neurons are the downregulation of cortical serotonin-transporters after long-term food restriction, the loss of serotonergic nerve terminals caused by substituted amphetamines, the serotonergic hyperinnervation of the frontal cortex seen after olfactory bulbectomy in rats, or the long lasting changes in the levels of tryptophanhydroxylase, of serotonin-transporter depression or in the density of serotonergic nerve terminals in distant projection fields caused by long-term antidepressant treatments. All these long-lasting alterations and imbalances of a previously established serotonin-output-pattern will not only affect the impact of individual regional networks on whole brain information processing but also the established synaptic connectivity in distant projection fields of the central serotonergic system.

Effect of acute treatment with YM992 on extracellular serotonin levels in the rat frontal cortex

(S)-2-[[(7-fluoroindan-4-yl)oxy]methyl]morpholine monohydrochloride (YM992) is a novel putative antidepressant exhibiting both selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibition and 5-HT(2A) receptor antagonism. In vivo microdialysis revealed that a single treatment with YM992 (3, 10, 30 mg/kg i.p.) dose-dependently increased extracellular 5-HT levels in the rat frontal cortex. Fluoxetine, citalopram and venlafaxine also produced significant increases in 5-HT levels at doses of 10-30 mg/kg. However, the increase in 5-HT levels induced by YM992 was significantly larger than increases elicited by these three compounds at 30 mg/kg. The combined administration of R-(+)-alpha-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidine-methanol (MDL100,907) (a selective 5-HT(2A) receptor antagonist) and citalopram produced no additional increase in 5-HT levels compared with citalopram treatment alone. YM992 moderately enhanced [3H]5-HT release from rat cerebral cortex synaptosomes using different mechanisms than p-chloroamphetamine. In comparison, 10-microM fluoxetine markedly induced 5-HT release in vitro, while citalopram and venlafaxine had no noticeable effect on release. YM992 produces a more robust increase of 5-HT levels acutely than other antidepressants in vivo and the effect may be due to 5-HT releasing properties of the drug.

Comparison of the effects of antidepressants and their metabolites on reuptake of biogenic amines and on receptor binding

1. The present survey compares the effects of antidepressants and their principal metabolites on reuptake of biogenic amines and on receptor binding. The following antide-pressants were included in the study: the tricyclic antidepressants amitriptyline, dothiepin, and lofepramine and the atypical antidepressant bupropion, which all have considerable market shares in the UK and/or US markets; the selective serotonin reuptake inhibitors (SSRIs) citalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline; and the recently approved antidepressants venlafaxine and nefazodone. 2. Amitriptyline has similar in vitro reuptake inhibitory potencies for 5-HT and NA, whereas the metabolite nortriptyline is preferentially a NA reuptake inhibitor. Both amitriptyline and nortriptyline are also 5-HT2 receptor antagonists. 3. Dothiepin has equipotent 5-HT and NA reuptake inhibitory activity, whereas northiaden shows a slight selectivity for NA reuptake inhibition. Dothiepin and northiaden are also 5-HT2 receptor antagonists. The slow elimination rate of northiaden (36-46 hr) compared to dothiepin (14-24 hr) suggests that northiaden contributes significantly to the therapeutic effect of dothiepin. 4. Lofepramine is extensively metabolized to desipramine. Desipramine plays an important role in the antidepressant activity of lofepramine, as the plasma elimination half-life of lofepramine (4-6 hr) is much shorter than that of desipramine (24 hr). Both compounds are potent and selective inhibitors of NA reuptake. 5. The five approved SSRIs, citalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline, are potent 5-HT reuptake inhibitors, and the demethyl metabolites, norfluoxetine, demethylsertraline, and demethylcitalopram, also show selectivity. Paroxetine and sertraline are the most potent inhibitors of 5-HT reuptake, whereas citalopram is the most selective. Fluoxetine is the least selective and the metabolite of fluoxetine, norfluoxetine, is a more selective and more potent 5-HT reuptake inhibitor than the parent compound and has an extremely long half-life (7-15 compared to 1-3 days). Thus the metabolite plays an important role for the therapeutic effect of fluoxetine. Fluoxetine is also a 5-HT2C receptor antagonist. Demethylsertraline is a weaker and less selective 5-HT reuptake inhibitor in vitro than sertraline, but demethylsertraline has a very long half-life (62-104 hr) compared to the parent compound (24 hr) and it might play a role in the therapeutic effects of sertraline. Demethylcitalopram has about a 10 times lower 5-HT reuptake inhibitory potency in vitro than citalopram, and the elimination half-lives are approximately 1.5 and 2 days, respectively. 6. Bupropion and hydroxybupropion are weak inhibitors of biogenic amine reuptake. The mechanisms of action responsible for the clinical effects of bupropion are not fully understood, but it has been suggested that both dopaminergic and noradrenergic components play a role and that the hydroxybupropion metabolite contributes significantly to the antidepressant activity. 7. Venlafaxine and O-demethylvenlafaxine are weak inhibitors of 5-HT and NA reuptake, and the selectivity ratios are close to one. O-Demethylvenlafaxine is eliminated more slowly than venlafaxine (plasma half-lives of 5 and 11 hr, respectively), and it is likely that it contributes to the overall therapeutic effect of venlafaxin. 8. Nefazodone and alpha-hydroxynefazodone are equipotent 5-HT and NA reuptake inhibitors. Both compounds are also 5-HT2 receptor antagonists. Both parent compound and metabolite have short elimination half-lives.

Effect of fluoxetine on maximal electroshock seizures in mice: acute vs chronic administration

There are no definite reports regarding the effects of chronic fluoxetine on animal models of epilepsy. Since chronically administered fluoxetine, in comparison to acutely administered fluoxetine has different effects on CNS, the present study was undertaken to investigate the effect of acute and chronic fluoxetine pretreatment, on a median anticonvulsant dose (ED50) of phenytoin in male ICR albino mice. Additionally, the effects of fluoxetine pretreatment on median convulsive current (CC50) in the presence and absence of phenytoin were investigated and results were compared. The maximal electroshock seizure (MES) test was used to estimate the ED50of phenytoin. The electroshock threshold test was used to estimate CC50. ED50and CC50values were calculated by probit analysis. The effects of the chronic and acute fluoxetine groups on the ED50of phenytoin were significantly different (P<0.05), and on CC50this difference was not statistically significant. Chronic fluoxetine insignificantly increased the ED50of phenytoin and decreased the CC50while acute fluoxetine decreased the ED50of phenytoin and increased the CC50. Our results indicate that chronic fluoxetine does not have an antiepileptic property and it may have dubious proconvulsant properties, contrary to acute fluoxetine.

In vivo electrophysiological assessment of the agonistic properties of flibanserin at pre- and postsynaptic 5-HT1A receptors in the rat brain

Flibanserin (BIMT 17) has been described as a 5-HT1A agonist with preferential affinity for postsynaptic 5-HT1A receptors and as a 5-HT2A antagonist. Indeed, using the forskolin-stimulated cAMP accumulation technique, flibanserin but not the 5-HT1A agonists buspirone and 8-OH-DPAT had agonistic activity at postsynaptic 5-HT1A receptors in the cerebral cortex. The present in vivo electrophysiological study investigated the agonistic properties of this novel compound in pre- and postsynaptic areas of the anesthetized rat brain using local microiontophoretic application and systemic administration. The inhibition induced by either local or intravenous administration of flibanserin was current- and dose-dependent. Based on the ability of 5-HT1A antagonists to block or reverse the inhibitory action of the compound, the effect of flibanserin was shown to be mediated via 5-HT1A receptors. In addition, as determined by the concurrent microiontophoretic application of flibanserin and 5-HT, flibanserin behaved as a full agonist in the dorsal raphe nucleus (DRN) and the medial prefrontal cortex (mPFC), but as a partial agonist in the CA3 region of the hippocampus. Based on neuronal responsiveness observed with the local microiontophoretic application of flibanserin, it was found that the agonist was most potent on 5-HT1A receptors in the hippocampus, followed by the mPFC and DRN (I.T50 values: 260, 1,260, and 1,365 nanocoulombs, respectively). However, based on the ED50 values obtained from intravenous administration of the drug, flibanserin was most potent in the DRN followed by the hippocampus and mPFC (ED50 values: 239, 1,414, and 2,984 micrograms/kg, respectively). Therefore, flibanserin presented a marked selectivity for postsynaptic 5-HT1A receptors when applied locally, but not when administered intravenously. It remains to be determined if flibanserin preferentially activates postsynaptic 5-HT1A receptors upon sustained systemic administration.

Specific [3H]8-OH-DPAT binding in brain regions of rats genetically predisposed to various defense behavior strategies

Distribution of 5-HT1A receptors was studied in rats genetically predisposed to two basic defense strategies--passive (freezing) or active (aggression) defensive behavior. Specific [3H]8-OH-DPAT binding was assayed in the brain structures of rat strains bred for 40 generations from Wistar stock for predisposition to freezing (catalepsy), and in wild rats bred for low and high aggression to humans. Considerable changes in [3H]8-OH-DPAT binding were found in the brain of rats with hereditary predisposition to catalepsy. A significant decrease in Bmax of specific receptor binding of [3H]8-OH-DPAT in the frontal cortex, and in the striatum as well as an increase in Kd in the hippocampus of cataleptic rats was shown. A clear-cut tendency to decrease of 5-HT1A receptor density was observed in the midbrain and hypothalamus of these rats. A comparison of wild Norway rats bred for aggressiveness against humans with those bred for the absence of affective aggressiveness showed a Bmax decrease without Kd change in the frontal cortex, hypothalamus, and amygdala of aggressive animals. It is hypothesized that 5-HT1A and probably 5-HT1A-like 5-HT7 serotonin receptors are involved in the mechanisms of both active and passive defense reactions, and the high expression of fear-induced defense is associated with their decrease in the frontal cortex. At the same time, the genetically determined preference for a certain defense behavior strategy depends either on the peculiarities of distribution of these receptor types in the brain regions or on some other types of serotonin receptors.

Long-term modulation of presynaptic 5-HT-output: experimentally induced changes in cortical 5-HT-transporter density, tryptophan hydroxylase content and 5-HT innervation density

Whereas experimentally induced long-term changes of postsynaptic mechanisms of 5-HT neurotransmission have been studied in great detail, much less is currently known about the effects of certain treatments on the presynaptic components governing the output of 5-HT in individual brain regions. This contribution summarizes the results of a series of experiments on the influence of different physiologic and pharmacologic manipulations on three different parameters of 5-HT presynapses, 5-HT transporter density, tryptophan hydroxylase content, and serotonin level in the rat frontal cortex. The combined measurement of several parameters of 5-HT presynapses allows to differentiate between treatments which exclusively affect the density of 5-HT transporters (long-term food restriction), which exclusively affect the level of tryptophan hydroxylase apoenzyme (imipramine treatment of olfactory bulbectomized rats) or which cause a parallel increase (bulbectomy, chronic administration of tranylcypramine to rats with chemical lesions of their cortical 5-HT innervation) or a parallel decrease (administration of p-chloroamphetamine) of both parameters, indicating treatment-induced changes in the density of 5-HT presynapses in the frontal cortex. Each of these changes may lead to an altered output of serotonergic afferences, and may therefore act to either potentiate or to attenuate the impact of serotonin-mediated effects on the activity of local networks located in a certain brain region.

Brain 5-HT1A receptor autoradiography and hypothermic responses in rats bred for differences in 8-OH-DPAT sensitivity

Three rat lines were selectively bred for high (HDS), random (RDS), or low (LDS) hypothermic responses to the specific 5-HT1A receptor agonist 8-OH-DPAT. Forty-five minutes after 8-OH-DPAT administration (0.5 mg/kg), body temperatures dropped 3-5 degrees C in HDS rats, yet this dose produced only about 1.2 degrees C and 0.7 degree C drops in RDS and LDS rats, respectively. To investigate the relationship of body temperature of 5-HT1A receptor binding sites, autoradiographic analyses of [3H]8-OH-DPAT binding to 5-HT1A receptors in brains of these rats were conducted. Significant differences in binding were found in specific limbic cortical projection sites, with the HDS line having the greatest density of sites. Body temperature responses correlated significantly with [3H]8-OH-DPAT receptor binding in only a few areas of frontal cortex. Binding in many other brain regions, including the anterior and posterior hypothalami (regions long associated with body temperature regulation) and the raphe showed no significant differences among the lines. [3H]Ketanserin binding to cortical 5-HT2 receptors did not differ among the lines, except in the cingulate and superficial frontal cortices where HDS exhibited higher binding. These data suggest that differences in 5-HT1A receptor number may contribute to the exaggerated hypothermic response to 8-OH-DPAT in HDS rats. These studies also suggest that genetic regulation of receptor density may be brain region specific which should encourage future studies of the mechanisms of 5-HT1A receptor activity in brain and the action of drugs affecting this receptor.

Partial role of 5-HT2 and 5-HT3 receptors in the activity of antidepressants in the mouse forced swimming test

The present study was designed to evaluate the roles of 5-HT2 and 5-HT3 receptors in the mouse forced swimming test, by using selective agonists and antagonists of 5-HT(2A/C) and 5-HT3 receptor sites. Agonists/antagonists and antidepressants were administered 45 min and 30 min, respectively, prior to testing. Pretreatment with (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) (4 mg/kg, i.p.) or 2-methyl-5-HT (4 mg/kg, i.p.) had no effect on the anti-immobility effects of any antidepressant tested. Prior administration of ritanserin (4 mg/kg, i.p.) or ketanserin (8 mg/kg, i.p.), on the other hand, potentiated the effects of sub-active doses of imipramine (8 mg/kg, i.p.) and desipramine (16 mg/kg, i.p.) but not of maprotiline (8 mg/kg, i.p.), fluoxetine (16 mg/kg, i.p.), citalopram (16 mg/kg, i.p.) or fluvoxamine (8 mg/kg, i.p.). Pretreatment with ondansetron (1 X 10(-5) mg/kg, i.p.) enhanced the antidepressant-like effects of sub-active doses of the selective serotonin reuptake inhibitors. The results of the present study suggested that, in the forced swimming test, the selective serotonin reuptake inhibitors act partially through 5-HT3 receptor sites, whereas the tricyclic antidepressants exert effects at 5-HT(2A/C) receptor sites. Anti-immobility effects of the selective noradrenaline reuptake inhibitor, maprotiline, do not seem to be mediated by 5-HT(2A/C) or 5-HT3 receptor function.

Novel strategies for treatment-resistant depression

This article describes a highly selective constellation of the more unique strategies for managing the treatment-resistant patient. In light of the often-devastating toll that treatment-resistant depression takes on an individual's life, it behooves us to continue the search for more effective strategies for those patients that fail more traditional interventions. As each successive move down the treatment algorithm flowchart becomes necessary, the risk/benefit ratio may shift toward less well-substantiated, but still biologically informed, strategies. Although some of the more unusual treatments described in this article represent minimally charted territories, the more promising techniques are deserving of further careful exploration.

Biochemical profile of YM992, a novel selective serotonin reuptake inhibitor with 5-HT2A receptor antagonistic activity

YM992, (S)-2-[[(7-fluoro-4-indanyl)oxy]methyl]morpholine monohydrochloride, exhibited the biochemical profile of a selective serotonin (5-HT) reuptake inhibitor (SSRI) with 5-HT2A receptor antagonistic activity. YM922 showed the same high affinity as fluoxetine against the 5-HT reuptake site (Ki = 21 nM) and a similar affinity to that of crazodone against the 5-HT2A receptor (Ki = 86 nM). In other receptor binding studies, an affinity for the adrenergic alpha 1 receptor (Ki = 200 nM) and 5-HT2C receptor (Ki = 680 nM) was observed. In a monoamine uptake study, YM992 showed a selective 5-HT uptake inhibition (IC50 = 0.15 microM), but only very weakly inhibited both noradrenaline (NA) and dopamine (DA) uptake (IC50 = 3.1 microM (NA), > 10 microM (DA)). YM992 was also found to potently inhibit the aggregation of human platelets (IC50 = 1.9 microM), revealing antagonistic activity for the 5-HT2A receptor in vitro. Enhanced serotonergic neurotransmission, in particular that mediated by the 5-HT1A receptor, has recently been reported to be important in the long-term treatment of depressive disorders with antidepressants. In addition, some 5-HT1A receptor-mediated responses are known to be potentiated by co-administration of 5-HT2A receptor antagonists. Thus, YM992, having both selective 5-HT reuptake inhibition and 5-HT2A antagonistic activity, might show potent therapeutic activity as a novel antidepressant in comparison with conventional SSRIs.

BIMT 17, a 5-HT2A receptor antagonist and 5-HT1A receptor full agonist in rat cerebral cortex

In the search for antidepressant agents with a rapid onset of action, we have found that compound BIMT 17 (1-[2-[4-(3-trifluoromethylphenyl)piperazin-1- yl]ethyl]benzimidazol-[1H]-2-one) shows a good affinity for cerebral cortical 5-HT1A (pKi = 7.72) and 5-HT2A (pKi = 6.90) receptors, with no appreciable affinity for the other 5-HT receptor subtypes, including 5-HT2C. BIMT 17 reduced forskolin-stimulated cAMP accumulation in the cerebral cortex (pEC50 = 6.09) and in the hippocampus (pEC50 = 6.50), and antagonized 5-HT-induced phosphatidylinositol turnover (pKi = 6.96) in the cerebral cortex. The effect on cAMP accumulation was blocked by the 5-HT1A receptor antagonist tertatolol. Buspirone, 8-OH-DPAT and S 14671 (1-[2-(2-thenoylamino)ethyl]- 4[1-(7-methoxynaphtyl)]-piperazine), claimed to be 5-HT1A receptor agonists, did not reduce forskolin-stimulated cAMP formation in the cerebral cortex. On the basis of these data, it was concluded that BIMT 17 was the only compound that behaved as a full agonist with respect to the cAMP response in the cortex, while exerting concurrent agonism at 5-HT1A receptors and antagonism at 5-HT2A receptors. These characteristics might explain the peculiar behavior of BIMT 17 in mimicking the inhibitory action of 5-HT on the basal firing rate of the cortical neurons (see accompanying paper).