Role of various types of serotonin receptors in regulation of drinking behavior and salt appetite in vasopressin-deficient brattleboro rats

Serotonin 5-HT_1A receptor agonist 8-OH DPAT suppressed drinking behavior in Brattleboro and Wistar rats. 5-HT_1B agonist CGS-12066A and 5-HT_2A antagonist ketanserin did not affect drinking behavior in Brattleboro rats; 5-HT₃ antagonist ondansetron suppressed water consumption and 5-HT_1A agonist stimulated salt appetite in Brattleboro, but not in Wistar rats. Presumably, vasopressin regulates thirst and salt appetite by modulating sensitivity/density of various types of 5-HT receptors.     

Gender and the use of hormonal contraception in women are not associated with cerebral cortical 5-HT 2A receptor binding

Gender influences brain function including serotonergic neurotransmission, which may play a role in the well-known gender variations in vulnerability to mood and anxiety disorders. Even though hormonal replacement therapy in menopause is associated with globally increased cerebral 5-HT_2A receptor binding it is not clear if gender or use of hormonal contraception exhibits associations with 5-HT_2A receptor binding. We found no significant effect of gender on cortical 5-HT_2A receptor binding (P=0.15, n=132). When adjusting for the personality trait neuroticism, known to be positively correlated to frontolimbic 5-HT_2A receptor binding and to be more pronounced in women, again, the effect of gender was not significant (P=0.42, n=127). Also, the use of hormonal contraception (n=14) within the group of pre-menopausal women (total n=29) was not associated with cortical 5-HT_2A receptor binding (P=0.31). In conclusion, neither gender, nor the use of hormonal contraception in premenopausal women was associated with cortical 5-HT_2A receptor binding.     

Differential roles of hypothalamic serotonin receptor subtypes in the regulation of prolactin secretion in the turkey hen

In the turkey, exogenous serotonin (5-hydroxytryptamine, 5-HT) increases prolactin (PRL) secretion by acting through the dopaminergic (DAergic) system. In the present study, infusion of the 5-HT_2C receptor agonist, (R)(−)-DOI hydrochloride (DOI), into the third ventricle stimulates PRL secretion, whereas the 5-HT_1A receptor agonist, (+/−)-8-OH-DPAT hydrobromide (DPAT), inhibits PRL secretion. Using the immediate-early gene, c-fos, as an indicator of neuronal activity, in situ hybridization histochemistry showed preferential c-fos co-localization within tyrosine hydroxylase immunoreactive neurons (the rate limiting enzyme in DA synthesis) in the areas of the nucleus preopticus medialis (POM) and the nucleus premammillaris (PMM), in response to DPAT and DOI, respectively. To clarify the involvement of 5-HT_1A and 5-HT_2C receptors in PRL regulation, their mRNA expression was determined on hypothalamic tissue sections from birds in different reproductive stages. A significant difference in 5-HT_1A receptor was observed, with the POM of hypoprolactinemic short day and photorefractory birds showing the highest expression. 5-HT_2C receptors mRNA did not change during the reproductive cycle. The data presented support the notion that DA neurons in the PMM and POM mediate the stimulatory and inhibitory effects of 5-HT, respectively, on PRL secretion and the 5-HTergic system can both stimulate and inhibit PRL secretion.     

Selective decline of 5-HT1A receptor binding sites in rat cortex, hippocampus and cholinergic basal forebrain nuclei during aging

The effect of aging on 5-HT_1A receptor binding in several forebrain areas associated with the basal forebrain cholinergic system was investigated in rats of 3-, 24- and 30-months-old by receptor autoradiography and biochemical binding assay using [³H]8-OH-DPAT as a ligand. Autoradiographic measurements demonstrated a marked region-specific decline of ligand binding in: (i) regions of the basal forebrain cholinergic cell groups, i.e. the medial septum, diagonal band nuclei and magnocellular nucleus basalis, (ii) the frontal and parietal neocortex and (iii) the dentate gyrus of the hippocampus. No change or only a slight decrease of the 5-HT_1A receptor density was found in other areas investigated: the CA1 and CA3 sectors of hippocampus, the cingular and perirhinal cerebral cortex and the lateral septum. The autoradiographic findings were substantiated by the biochemical binding assay, which revealed a comparable loss of 5-HT_1A receptor in the hippocampus and neocortex at the age of 30 months. The results clearly show that with increasing age the decrement of 5-HT_1A receptor binding in the rat forebrain is remarkably region-selective and particularly affects the cholinergic cell groups that innervate cortex and hippocampus. This phenomenon appears to be especially significant in relation to the neuronal substrates underlying the age-related alterations of mood and cognition.     

5-HT1A receptor activation counteracts c-Fos immunoreactivity induced in serotonin neurons of the raphe nuclei after immobilization stress in the male rat

The serotoninergic system and the 5-HT_1A receptors have been involved in the brain response to acute stress. The aim of our study was evaluate the role of the 5-HT_1A receptors in serotoninergic cells of rostral and caudal raphe nuclei under acute immobilization in rats. Double immunocytochemical staining of 5-hydroxy-tryptamine and c-Fos protein and stereology techniques were used to study the specific cell activation in the raphe nuclei neurons in five groups (control group, immobilization group (immobilization lasting 1 h), DPAT group (8-OH-DPAT 0.3 mg/kg, s.c.), DPAT + IMMO group (8-OH-DPAT 0.3 mg/kg, s.c., 30′ prior acute immobilization) and WAY + DPAT + IMMO group (WAY-100635 0.3 mg/kg, s.c. and 8-OH-DPAT 0.3 mg/kg, s.c., 45′ and 30′, respectively, before immobilization). Our results showed an increase in the number of c-Fos immunoreactive nuclei in serotoninergic cells in both dorsal and median raphe nuclei in the immobilized group. The 8-OH-DPAT pretreatment counteracted the excitatory effects of the acute immobilization in these brain regions. In addition, WAY-100635 administration reduced the effect of 8-OH-DPAT injection, suggesting a selective 5-HT_1A receptor role. Raphe pallidus and raphe obscurus did not show any differences among experimental groups. We suggest that somatodendritic 5-HT_1A receptors in rostral raphe nuclei may play a crucial role in both mediating the consequences of uncontrollable stress and the possible beneficial effects of treatment with 5-HT_1A receptor agonists.     

5-HT2A receptor inactivation potentiates the acute antidepressant-like activity of escitalopram: involvement of the noradrenergic system

Evidence suggests that the serotonin 2A receptor (5-HT_2AR) modulates the therapeutic activity of selective serotonin reuptake inhibitors (SSRIs). Indeed, among the genetic factors known to influence the individual response to antidepressants, the HTR2A gene has been associated with SSRIs response in depressed patients. However, in these pharmacogenetic studies, the consequences of HTR2A gene polymorphisms on 5-HT_2AR expression or function are lacking and the precise role of this receptor is still matter of debate. This study examined the effect of 5-HT_2AR agonism or antagonism with DOI and MDL100907, respectively, on the serotonergic system and the antidepressant-like activity of the SSRI escitalopram in mouse. The 5-HT_2AR agonist DOI decreased the firing rate of 5-HT neurons in the dorsal raphe (DR) nucleus of 5-HT_2AR^+/+ anesthetized mice. This inhibitory response persisted in 5-HT_2CR^?/? but was completely blunted in 5-HT_2AR^?/? mutants. Moreover, the suppressant effect of DOI on DR 5-HT neuronal activity in 5-HT_2AR^+/+ mice was attenuated by the loss of noradrenergic neurons induced by the neurotoxin DSP4. Conversely, in 5-HT_2AR^+/+ mice, the pharmacological inactivation of the 5-HT_2AR by the selective antagonist MDL100907 reversed escitalopram-induced decrease in DR 5-HT neuronal activity. Remarkably, in microdialysis experiments, a single injection of escitalopram increased cortical extracellular 5-HT, but not NE, levels in awake 5-HT_2AR^+/+ mice. Although the addition of MDL100907 did not potentiate 5-HT neurotransmission, it allowed escitalopram to increase cortical NE outflow and consequently to elicit an antidepressant-like effect in the forced swimming test. These results suggest that the blockade of the 5-HT_2AR may strengthen the antidepressant-like effect of escitalopram by facilitating the enhancement of the brain NE transmission. They provide support for the use of atypical antipsychotics with SSRIs as a relevant antidepressant augmentation strategy.     

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats

This study assessed the possible antinociceptive role of peripheral 5-HT₁ receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 μl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT₁ receptor agonists R(+)-UH-301 (5-HT_1A; 0.1–3 μg/paw), CGS-12066A (5-HT_1B; 0.01–0.3 μg/paw), GR46611 (5-HT_1B/1D; 0.3–10 μg/paw), BRL54443 (5-HT_1E/1F; 3–300 μg/paw) or LY344864 (5-HT_1F; 3–300 μg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 μg/paw) was significantly reduced by WAY-100635 (30–100 μg/paw; a 5-HT_1A receptor antagonist). Moreover, the antagonists GR55562 (30–100 μg/paw; 5-HT_1B/D) or SB224289 (30–100 μg/paw; 5-HT_1B) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 μg/paw) whereas GR55562 (30–100 μg/paw) or BRL15572 (30–100 μg/paw, 5-HT_1D) reduced the antinociceptive effect of GR46611 (0.3 μg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 μg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_1F and, probably, 5-HT_1E receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT₁ receptor agonists could be a therapeutic strategy to reduce inflammatory pain.     

Changes in 5-HT2A-mediated behavior and 5-HT2A- and 5-HT1A receptor binding and expression in conditional brain-derived neurotrophic factor knock-out mice

Changes in brain-derived neurotrophic factor (BDNF) expression have been implicated in the etiology of psychiatric disorders. To investigate pathological mechanisms elicited by perturbed BDNF signaling, we examined mutant mice with central depletion of BDNF (BDNF^2L/2LCk-cre). A severe impairment specific for the serotonin 2A receptor (5-HT_2AR) in prefrontal cortex was described previously in these mice. This is of much interest, as 5-HT_2ARs have been linked to neuropsychiatric disorders and anxiety-related behavior. Here we further characterized the serotonin receptor alterations triggered by BDNF depletion. 5-HT_2A ([³H]-MDL100907) and 5-HT_1A ([³H]-WAY100635) receptor autoradiography revealed site-specific alterations in BDNF mutant mice. They exhibited lower 5-HT_2A receptor binding in frontal cortex but increased binding in hippocampus. Additionally, 5-HT_1A receptor binding was decreased in hippocampus of BDNF mutants, but unchanged in frontal cortex. Molecular analysis indicated corresponding changes in 5-HT_2A and 5-HT_1A mRNA expression but normal 5-HT_2C content in these brain regions in BDNF^2L/2LCk-cre mice. We investigated whether the reduction in frontal 5-HT_2AR binding was reflected in reduced functional output in two 5-HT_2A-receptor mediated behavioral tests, the head-twitch response (HTR) and the ear-scratch response (ESR). BDNF^2L/2LCk-cre mutants treated with the 5-HT_2A receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) showed a clearly diminished ESR but no differences in HTR compared to wildtypes. These findings illustrate the context-dependent effects of deficient BDNF signaling on the 5-HT receptor system and 5-HT_2A-receptor functional output.     

Serotonin-1A receptors in the dorsal periaqueductal gray matter mediate the panicolytic-like effect of pindolol and paroxetine combination in the elevated T-maze

The β-adrenergic blocker and 5-HT_1A receptor antagonist pindolol has been combined with selective serotonin reuptake inhibitors (SSRIs) in patients with depressive and anxiety disorders to shorten the onset of the clinical action and/or increase the proportion of responders. The results of a previous study have shown that pindolol potentiates the panicolytic effect of paroxetine in rats submitted to the elevated T-maze (ETM). Since reported evidence has implicated the 5-HT_1A receptors of the dorsal periaqueductal gray matter (DPAG) in the panicolytic effect of antidepressants, rats treated with pindolol (5.0 mg/kg, i.p.) and paroxetine (1.5 mg/kg, i.p.) received a previous intra-DPAG injection of the selective 5-HT_1A antagonist, WAY-100635 (0.4 μg) and were submitted to the ETM. Pretreatment with WAY-100635 reversed the increase in escape latency, a panicolytic effect, determined by the pindolol-paroxetine combination. These results implicate the 5-HT_1A receptors of the DPAG in the panicolytic effect of the pindolol-paroxetine combination administered systemically. They also give further preclinical support for the use of this drug combination in the treatment of panic disorder.     

Presynaptic 5-HT1B receptor-mediated serotonergic inhibition of glutamate transmission in the bed nucleus of the stria terminalis

Activation of neurons in the bed nucleus of the stria terminalis (BNST) plays a critical role in stress and anxiety-related behaviors. Previously, we have shown that serotonin (5-HT) can directly modulate BNST neuronal excitability by an action at postsynaptic receptors. In this study we built upon that work to examine the effects of 5-HT on excitatory neurotransmission in an in vitro rat BNST slice preparation. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs). These effects were mimicked by the 5-HT_1B/D receptor agonist, sumatriptan, and by the 5-HT_1B receptor selective agonist, CP93129. Conversely, the effects of 5-HT and sumatriptan could be blocked by the 5-HT_1B receptor-selective antagonist, GR55562. In contrast, the 5-HT_1A receptor agonist 8-OH DPAT or antagonist WAY 100635 could not mimic or block the effect of 5-HT on eEPSCs. Together, these data suggest that the 5-HT-induced attenuation of eEPSCs was mediated by 5-HT_1B receptor activation. Moreover, sumatriptan had no effect on the amplitude of the postsynaptic current elicited by pressure applied AMPA, suggesting a possible presynaptic locus for the 5-HT_1B receptor. Furthermore, 5-HT, sumatriptan and CP93129 all increased the paired pulse ratio of eEPSCs while they concomitantly decreased the amplitude of eEPSCs, suggesting that these agonists act to reduce glutamate release probability at presynaptic locus. Consistent with this observation, sumatriptan decreased the frequency of miniature EPSCs, but had no effect on their amplitude. Taken together, these results suggest that 5-HT suppresses glutamatergic neurotransmission in the BNST by activating presynaptic 5-HT_1B receptors to decrease glutamate release from presynaptic terminals. This study illustrates a new pathway by which the activity of BNST neurons can be indirectly modulated by 5-HT, and suggests a potential new target for the development of novel treatments for depression and anxiety disorders.     

Hypothalamic serotonin-1A receptor binding measured by PET predicts the plasma level of dehydroepiandrosterone sulfate in healthy women

Serotonin modulates the activity of the hypothalamic–pituitary–adrenal (HPA) axis particularly via the serotonin-1A receptor (5-HT_1A). Therefore, the rationale of this positron emission tomography (PET) study was to investigate the influence of the 5-HT_1A receptor distribution in the human brain on plasma levels of dehydroepiandrosterone sulfate (DHEAS) and cortisol in vivo. Eighteen healthy female were measured with PET and the selective 5-HT_1A receptor radioligand [carbonyl-¹¹C]WAY-100635. Nine a priori defined brain regions (hypothalamus, orbitofrontal cortex, amygdala, hippocampus, anterior and posterior cingulate cortices, dorsal raphe nucleus, retrosplenial cortex, and insula) and the cerebellum (reference region) were delineated on coregistered MR images. DHEAS and cortisol plasma levels were collected by blood sampling in the morning of the PET day. Linear regression analysis of DHEAS plasma level as dependent variable and hypothalamic 5-HT_1A receptor binding potential (BP) as independent variable showed a highly significant association (r = .691, p = .002). The hypothalamic 5-HT_1A BP predicted 47.7% of the variability in DHEAS plasma levels. Regressions were borderline significant (p < .01, Bonferroni corrected threshold <.0056) between 5-HT_1A BP in the anterior cingulate and orbitofrontal cortices and free cortisol levels. No significant associations between DHEAS or cortisol and the 5-HT_1A receptor BP in other investigated brain regions were found. In conclusion, the serotonergic system may influence the DHEAS plasma level by modulating CRH and ACTH release via hypothalamic 5-HT_1A receptors as reported for cortisol before. As disturbances of the HPA axis as well as changes of the 5-HT_1A receptor distribution have been reported in affective disorders, future studies should focus on these interactions.     

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.     

Elevated 5-HT 2A receptors in postmortem prefrontal cortex in major depression is associated with reduced activity of protein kinase A

Previous human postmortem brain tissue research has implicated abnormalities of 5-HT receptor availability in depression and suicide. Although altered abundance of 5-HT 1A, 5-HT 2A, and 5-HT 2C receptors (5-HT_1A, 5-HT_2A, and 5-HT_2C) has been reported, the causes remain obscure. This study evaluated the availability of these three receptor subtypes in postmortem brain tissue specimens from persons with a history of major depression (MDD) and normal controls and tested the relationships to protein kinases A and C (PKA, PKC). Samples were obtained from postmortem brain tissue (Brodmann area 10) from 20 persons with a history of MDD and 20 matched controls as determined by a retrospective diagnostic evaluation obtained from family members. Levels of 5-HT_1A, 5-HT_2A, and 5-HT_2C receptor were quantitated via Western blot analyses. Basal and stimulated PKA and PKC activity were also determined. The depressed samples showed significantly increased 5-HT_2A receptor abundance relative to controls, but no differences in 5-HT_1A or 5-HT_2C receptors. Basal and cyclic AMP-stimulated PKA activity was also reduced in the depressed sample; PKC activity was not different between groups. 5-HT_2A receptor availability was significantly inversely correlated with PKC activity in controls, but with PKA activity in the depressed sample. Increased 5-HT_2A receptor abundance and decreased PKA activity in the depressed sample are consistent with prior reports. The correlation of 5-HT_2A receptor levels with PKA activity in the depressed group suggests that abnormalities of 5-HT_2A receptor abundance may depend on receptor uncoupling and heterologous regulation by PKA.     

Mirtazapine abolishes hyperthermia in an animal model of serotonin syndrome

Serotonin (5-HT) syndrome is a potentially fatal condition associated with various combinations of serotonergic drugs. Hyperthermia is the most serious symptom of this syndrome. Hyperthermia in 5-HT syndrome is reportedly the result of activation of 5-HT_2A receptors. Mirtazapine is a novel antidepressant and a potent 5-HT₂ receptor antagonistic. Although mirtazapine has been reported to cause 5-HT syndrome, the pharmacological profile of mirtazapine suggests that it improves hyperthermia in 5-HT syndrome. In the present study, we evaluated whether mirtazapine attenuates hyperthermia in a rat model of 5-HT syndrome. This model was induced by administration of tranylcypromine, a nonselective monoamine oxidase inhibitor, and fluoxetine, a selective serotonin reuptake inhibitor. Upon injection of these two drugs, the rectal temperature of the rats increased to over 40 °C. Pre- and post-administration of mirtazapine abolishes hyperthermia in this model of 5-HT syndrome. Post-administration of ritanserin, a 5-HT_2A receptor antagonist, completely inhibited hyperthermia and pre-administration of WAY100635, a 5-HT_1A receptor antagonist, significantly attenuated the ability of mirtazapine to abolish hyperthermia. The results of the present study suggest that mirtazapine inhibits hyperthermia in an animal model of 5-HT syndrome by blocking the activation of 5-HT_2A receptors, and that it partly inhibits hyperthermia by activating the 5-HT_1A receptors. The present study indicates that mirtazapine is unlikely to cause 5-HT syndrome and may be a useful drug for treating this condition.     

Receptor-genes cross-talk: effect of chronic 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin treatment on the expression of key genes in brain serotonin system and on behavior

Dysfunction in brain serotonin (5-HT) system has been implicated in the psychopathology of anxiety, depression, drug addiction, and schizophrenia. The 5-HT_1A receptors play a central role in the control of 5-HTergic neurotransmission. There are some scarce data showing cross-regulation between 5-HT receptors. Here, we investigated whether interaction exists between 5-HT_1A receptor and genes encoding key members in brain 5-HT system. Chronic treatment with selective agonist of 5-HT_1A receptor 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (1.0 mg/kg i.p., 14 days) produced considerable decrease in hypothermic response to acute administration of 8-OH-DPAT in CBA/Lac mice indicating desensitization of 5-HT_1A receptors. The decrease in 5-HT_1A gene expression as well as decrease in the expression of gene encoding key enzyme in 5-HT synthesis, tryptophan hydroxylase-2 (TPH-2) in the midbrain, and the expression of the gene encoding 5-HT_2A receptor in the frontal cortex was shown. There were no significant changes in 5-HT transporter mRNA level in the midbrain. Despite considerable decrease in the expression of the genes encoding tryptophan hydroxylase-2, 5-HT_1A and 5-HT_2A receptors, chronic 8-OH-DPAT treatment failed to produce significant changes in 5-HT_1A-linked behavior (intermale aggression, open-field behavior, light-dark box, and pinch-induced catalepsy), suggesting compensatory and adaptive effect of genes suppression. The obtained data on the effect of 8-OH-DPAT-induced desensitization of 5-HT_1A receptors on 5-HT_1A, 5-HT_2A and TPH-2 gene expression demonstrated the role of 5-HT_1A receptor as indirect regulator of gene expression. The results provide the first evidence of receptor-key genes interaction in brain 5-HT system and may have profound implications in understanding the functioning of the brain neurotransmitter systems.     

Pharmacological,neurochemical, and behavioral profile of JB-788, a new 5-HT1A agonist

A novel pyridine derivative, 8-{4-[(6-methoxy-2,3-dihydro-[1,4]dioxino[2,3-b]pyridine-3-ylmethyl)-amino]-butyl}-8-aza-spiro[4.5]decane-7,9-dione hydrochloride, termed JB-788, was designed to selectively target 5-HT_1A receptors. In the present study, the pharmacological profile of JB-788 was characterized in vitro using radioligands binding tests and in vivo using neurochemical and behavioural experiments. JB-788 bound tightly to human 5-HT_1A receptor expressed in human embryonic kidney 293 (HEK-293) cells with a K_i value of 0.8 nM. Its binding affinity is in the same range as that observed for the (±)8-OH-DPAT, a reference 5HT_1A agonist compound. Notably, JB-788 only bound weakly to 5-HT_1B or 5-HT_2A receptors and moreover the drug displayed only weak or indetectable binding to muscarinic, α₂, β₁ and β₂ adrenergic receptors, or dopaminergic D₁ receptors. JB-788 was found to display substantial binding affinity for dopaminergic D₂ receptors and, to a lesser extend to α₁ adrenoreceptors. JB-788 dose-dependently decreased forskolin-induced cAMP accumulation in HEK cells expressing human 5-HT_1A, thus acting as a potent 5-HT_1A receptor agonist (E_max. 75%, EC₅₀ 3.5 nM). JB-788 did not exhibit any D₂ receptor agonism but progressively inhibited the effects of quinpirole, a D₂ receptor agonist, in the cAMP accumulation test with a K_i value of 250 nM. JB-788 induced a weak change in cAMP levels in mouse brain but, like some antipsychotics, transiently increased glycogen contents in various brain regions. Behavioral effects were investigated in mice using the elevated plus-maze. JB-788 was found to increase the time duration spent by animals in anxiogenic situations. Locomotor hyperactivity induced by methamphetamine in mouse, a model of antipsychotic activity, was dose-dependently inhibited by JB-788. Altogether, these results suggest that JB-788 displays pharmacological properties, which could be of interest in the area of anxiolytic and antipsychotic drugs.     

Central 5-HT3 receptor-induced hypothermia in mice: Interstrain differences and comparison with hypothermia mediated via 5-HT1A receptor

The selective agonist of serotonin 5-HT₃ receptor 1-(3-chlorophenyl)biguanide hydrochloride (m-CPBG) administered intracerebroventricularly (40, 80 or 160 nmol) produced long-lasting dose-dependent hypothermic response in AKR/2J mice. m-CPBG (160 nmol i.c.v.) induced profound hypothermia (delta t = −4 °C) that lasted up to 7 h. m-CPBG (40 nmol i.c.v.)-induced hypothermia was attenuated by 5-HT₃ receptor antagonist ondansetron pretreatment. At the same time, intraperitoneal administration of m-CPBG in a wide range of doses (0.5, 1.0, 5.0 or 10.0 mg/kg) did not affect the body temperature. These findings indicate: (1) the implication of central, rather than peripheral 5-HT₃ receptor in the thermoregulation; (2) the inability of m-CPBG to cross blood–brain barrier in mice. The comparison of brain 5-HT₃-induced hypothermic reaction in six inbred mouse strains (DBA/2J, CBA/Lac, C57BL/6, BALB/c, ICR, AKR/J) was performed and two highly sensitive to m-CPBG strains (CBA/Lac and C57BL/6) were found. In the same six mouse strains the functional activity of 5-HT_1A receptor was studied. The comparison of hypothermic reactions produced by 5-HT_1A receptor agonist 8-OH-DPAT (1.0 mg/kg i.p.) and m-CPBG revealed significant correlation between 5-HT₃ and 5-HT_1A-induced hypothermia in five out of six investigated mouse strains. 5-HT_1A receptor antagonist p-MPPI pretreatment (1 mg/kg i.p.) diminished hypothermia produced by centrally administered m-CPBG (40 nmol i.c.v.). The data suggest the cross-talk between 5-HT_1A and 5-HT₃ receptors in the mechanism of 5-HT-related hypothermia.     

Morphology and distribution of neurons expressing serotonin 5-HT1A receptors in the rat hypothalamus and the surrounding diencephalic and telencephalic areas

Disorders of serotonergic neurotransmission are involved in disturbances of numerous hypothalamic functions including circadian rhythm, mood, neuroendocrine functions, sleep and feeding. Among the serotonin receptors currently recognized, 5-HT_1A receptors have received considerable attention due to their importance in the etiology of mood disorders. While previous studies have shown the presence of 5-HT_1A receptors in several regions of the rat brain, there is no detailed map of the cellular distribution of 5-HT_1A receptors in the rat diencephalon.In order to characterize the distribution and morphology of the neurons containing 5-HT_1A receptors in the diencephalon and the adjacent telencephalic areas, single label immunohistochemistry was utilized. Large, multipolar, 5-HT_1A-immunoreactive (IR) neurons were mainly detected in the magnocellular preoptic nucleus and in the nucleus of diagonal band of Broca, while the supraoptic nucleus contained mainly fusiform neurons. Medium-sized 5-HT_1A-IR neurons with triangular or round-shaped somata were widely distributed in the diencephalon, populating the zona incerta, lateral hypothalamic area, anterior hypothalamic nucleus, substantia innominata, dorsomedial and premamillary nuclei, paraventricular nucleus and bed nucleus of stria terminalis.The present study provides schematic mapping of 5-HT_1A-IR neurons in the rat diencephalon. In addition, the morphology of the detected 5-HT_1A-IR neural elements is also described. Since rat is a widely used laboratory animal in pharmacological models of altered serotoninergic neurotransmission, detailed mapping of 5-HT_1A-IR structures is pivotal for the neurochemical characterization of the neurons containing 5-HT_1A receptors.     

Selective immunolesion of cholinergic neurons leads to long-term changes in 5-HT2A receptor levels in hippocampus and frontal cortex

Although loss of cholinergic neurons in the basal forebrain is considered a key initial feature in Alzheimer's disease (AD), changes in other transmitter systems, including serotonin and 5-HT_2A receptors, are also associated with early AD. The aim of this study was to investigate whether elimination of the cholinergic neurons in the basal forebrain directly affects 5-HT_2A receptor levels. For this purpose intraventricular injection of the selective immunotoxin 192 IgG-Saporin was given to rats in doses of either 2.5 or 5 μg. The rats were sacrificed after 1, 2, 4 and 20 weeks. 5-HT_2A protein levels were determined by western techniques in frontal cortex and hippocampus. A significant 70% downregulation in frontal cortex and a 100% upregulation in hippocampus of 5-HT_2A receptor levels were observed 20 weeks after the cholinergic lesion when using the highest dose of 192 IgG-Saporin. Our results show that cholinergic deafferentation leads to decreased frontal cortex and increased hippocampal 5-HT_2A receptor levels. This is probably a consequence of the interaction between the serotonergic and the cholinergic system that may vary depending on the brain region.