The relationship between stereochemical and both, pharmacological and ADME-Tox, properties of the potent hydantoin 5-HT7R antagonist MF-8.

This study concerns synthesis and evaluation of pharmacodynamic and pharmacokinetic profile for all four stereoisomers of MF-8 (5-(4-fluorophenyl)-3-(2-hydroxy-3-(4-(2-methoxyphenyl)piperazin-1-yl)propyl)-5-methylimidazolidine-2,4-dione), the previously described, highly potent 5-HT7R ligand with antidepressant activity on mice. The combination of DFT calculations of 1H NMR chemical shifts with docking and dynamic simulations, in comparison to experimental screening results, provided prediction of the configuration for one of two present stereogenic centers. The experimental data for stereoisomers (MF-8A-MF-8D) confirmed the significant impact of stereochemistry on both, 5-HT7R affinity and antagonistic action, with Ki and Kb values in the range of 3-366 nM and 0.024-99 µM, respectively. We also indicated the stereochemistry-dependent influence of the tested compounds on P-glycoprotein efflux, absorption in Caco-2 model, metabolic pathway as well as CYP3A4 and CYP2C9 activities.

Sub-chronic vortioxetine (but not escitalopram) normalizes brain rhythm alterations and memory deficits induced by serotonin depletion in rats.

Major depressive disorder (MDD) is a chronic and disabling psychiatric disorder characterized by a wide range of signs/symptoms, including cognitive dysfunction. Vortioxetine (VOR) is a multimodal antidepressant drug with pro-cognitive actions in animal models and MDD patients. The VOR-mediated blockade of 5-HT3-R in a subpopulation of GABA interneurons enhances pyramidal neuron activity in rat medial prefrontal cortex, an effect possibly underlying its pro-cognitive action. Brain oscillations are involved in regulation of cognitive function. We therefore examined VOR effects on oscillatory activity in four brain areas of freely-moving rats (prelimbic cortex, PrL; nucleus accumbens, NAc; dorsal hippocampus, dHPC; paraventricular thalamic nucleus, PVA), in standard and in serotonin-depleted rats showing recognition memory deficits. 4-chloro-dl-phenylalanine (pCPA) markedly reduced low frequency oscillations (LFO, mainly 1 Hz oscillations) and enhanced theta oscillations in PrL and NAc. It also reduced gamma and high frequency oscillations (HFO) in PVA. Subchronic VOR and escitalopram (ESC) treatments had little effect on oscillatory activity in standard rats. However, VOR -but not ESC- prevented recognition memory deficits in 5-HT-depleted rats, and normalized LFO and theta powers in PrL and NAc. In parallel, VOR -but not ESC- prevented the deficit in PrL-dHPC gamma coherence, but not the decrease in gamma and HFO powers in PVA. Overall, this supports a prominent role of serotonergic neurotransmission on brain oscillatory activity, particularly in cortico-striatal pathways linked to short-term recognition memory. Further, VOR prevented pCPA-induced cognitive deficits by normalizing oscillatory activity at lower frequencies in the PrL-NAc pathway, also normalizing the PrL-dHPC coherence at gamma frequencies.

Risperidone stimulates food intake and induces body weight gain via the hypothalamic arcuate nucleus 5-HT2c receptor-NPY pathway.

AIMS: Many patients taking risperidone for the treatment of psychiatric disorders experience substantial body weight gain. Researchers have speculated that risperidone induces obesity by modulating central signals; however, the precise central mechanisms involved remain to be fully elucidated. METHODS: Twenty-four C57BL/6J mice were divided into four groups: a control group; a risperidone-treated group; a lorcaserin-treated group; and a combined risperidone + lorcaserin-treated group. The mice were received the corresponding treatments for 4 weeks, and their brains were collected for in situ hybridization analysis. A subset of C57BL/6J mice was administrated with risperidone or placebo, and brains were collected 60 minutes post-treatment for determination of c-fos activity. In addition, brains of NPY-GFP mice treated with or without risperidone were collected to perform colocalization of NPY and c-fos, as well as NPY and 5-HT2c receptor using immunohistochemistry. RESULTS: There was significantly elevated c-fos expression in the hypothalamic arcuate nucleus (Arc) of risperidone-treated mice. More than 68% c-fos-positive neurons were NPY-expressing neurons. Furthermore, in situ hybridization revealed that Arc NPY mRNA expression was significantly increased in the risperidone-treated group compared with control group. Moreover, we identified that 95% 5-HT2c receptors were colocalized with NPY positive neurons, and increased Arc NPY mRNA expression induced by risperidone was markedly reduced by cotreatment with lorcaserin, a specific 5-HT2c receptor agonist. CONCLUSION: Our findings provide critical insight into the mechanisms underlying antipsychotic-induced obesity, which may assist the development of therapeutic strategies to address metabolic side effects of risperidone.

Experimental analysis of the onset mechanism of TdP reported in an LQT3 patient during pharmacological treatment with serotonin-dopamine antagonists against insomnia and nocturnal delirium.

Torsade de pointes (TdP) occurred in a long QT syndrome type 3 (LQT3) patient after switching perospirone to blonanserin. We studied how their electropharmacological effects had induced TdP in the LQT3 patient. Perospirone hydrochloride (n = 4) or blonanserin (n = 4) of 0.01, 0.1, and 1 mg/kg, i.v. was cumulatively administered to the halothane-anesthetized dogs over 10 min. The low dose of perospirone decreased total peripheral vascular resistance, but increased heart rate and cardiac output, facilitated atrioventricular conduction, and prolonged J-Tpeakc. The middle dose decreased mean blood pressure and prolonged repolarization period, in addition to those observed after the low dose. The high dose further decreased mean blood pressure with the reduction of total peripheral vascular resistance; however, it did not increase heart rate or cardiac output. It tended to delay atrioventricular conduction and further delayed repolarization with the prolongation of Tpeak-Tend, whereas J-Tpeakc returned to its baseline level. Meanwhile, each dose of blonanserin decreased total peripheral vascular resistance, but increased heart rate, cardiac output and cardiac contractility in a dose-related manner. J-Tpeakc was prolonged by each dose, but Tpeak-Tend was shortened by the middle and high doses. These results indicate that perospirone and blonanserin may cause the hypotension-induced, reflex-mediated increase of sympathetic tone, leading to the increase of inward Ca2+ current in the heart except that the high dose of perospirone reversed them. Thus, blonanserin may have more potential to produce intracellular Ca2+ overload triggering early afterdepolarization than perospirone, which might explain the onset of TdP in the LQT3 patient.

Shuangxia decoction alleviates p-chlorophenylalanine induced insomnia through the modification of serotonergic and immune system.

As a Traditional Chinese Medicine (TCM), Shuangxia Decoction (SXD) has been used to treat insomnia in oriental countries for more than thousands of years and it presents remarkable clinical effects. However, its active pharmacological fraction and the mechanism of sedative-hypnotic effects have not been explored. In this paper, we investigated active pharmacological fraction and revealed the detailed mechanisms underlying the sedative-hypnotic effects of SXD. It showed that SXD water extract compared to ethanol extract possessed better sedative effects on locomotion activity in normal mice and increased sleep duration in subhypnotic dose of sodium pentobarbital-treated mice. SXD alleviated p-chlorophenylalanine (PCPA) -induced insomnia by increasing the content of 5-HT in cortex [F (4, 55) = 12.67], decreasing the content of dopamine (DA) and norepinephrine (NE). Furthermore, SXD enhanced the expression of 5-HT1A and 5-HT2A receptors in hypothalamic and reduced serum levels of IL-1,TNF-α [F (5, 36) = 15.58]. In conclusion, these results indicated that SXD produced beneficial sedative and hypnotic bioactivities mediated by regulating the serotonergic and immune system.

AVN-322 is a Safe Orally Bio-Available Potent and Highly Selective Antagonist of 5-HT6R with Demonstrated Ability to Improve Impaired Memory in Animal Models.

BACKGROUND: In recent years, 5-hydroxytryptamine subtype 6 receptor (5-HT6 receptor, 5- HT6R) has emerged as a promising therapeutic target for the treatment of neuropathological disorders, including Alzheimer's disease (AD) and schizophrenia. 5-HT6 receptors were hypothesized to be implicated in the processes of learning, memory, and cognition with 5-HT6R antagonists being effective in animal models of cognition and memory impairment. Several selective 5-HT6R ligands are currently undergoing clinical trials for treatment of AD. METHODS: We describe results of preclinical development of a novel and highly selective and potent 5- HT6R antagonist, AVN-322, as a clinical candidate for the treatment of AD to improve concurrent debilitation of memory and cognition in the AD patients, and schizophrenia as a substance with antipsychotic effect. In the manuscript, we present its in vitro and vivo efficacy, ADME, pharmacokinetics in animals and in humans, and toxicity. RESULTS: While having high binding affinity in medium picomolar range, the lead compound demonstrates substantially better selectivity index then the reference drug candidates currently being tested in clinical studies. AVN-322 showed high oral bioavailability and favorable blood-brain barrier (BBB) penetration. In vivo testing revealed its clear cognition enhancing effect. AVN-322 significantly restored both scopolamine- and MK-801-induced cognitive dysfunction and demonstrated antipsychotic potential. CONCLUSION: Taking into account its good safety profile and favorable pharmacokinetics, AVN-322 can be reasonably considered as a novel drug candidate for the treatment of neurological disorders such as AD and/or schizophrenia.

Carbon monoxide contributes to the constipating effects of granisetron in rat colon.

AIM: To investigate the mechanisms underlying the potential contribution of the heme oxygenase/carbon monoxide (HO/CO) pathway in the constipating effects of granisetron. METHODS: For in vivo studies, gastrointestinal motility was evaluated in male rats acutely treated with granisetron [25, 50, 75 µg/kg/subcutaneous (sc)], zinc protoporphyrin IX [ZnPPIX, 50 µg/kg/intraperitoneal (ip)] and hemin (50 µmol/L/kg/ip), alone or in combination. For in vitro studies, the contractile neurogenic response to electrical field stimulation (EFS, 3, 5, 10 Hz, 14 V, 1 ms, pulse trains lasting 10 s), as well as the contractile myogenic response to acetylcholine (ACh, 0.1-100 µmol/L) were evaluated on colon specimens incubated with granisetron (3 µmol/L, 15 min), ZnPPIX (10 µmol/L, 60 min) or CO-releasing molecule-3 (CORM-3, 100, 200, 400 µmol/L) alone or in combination. These experiments were performed under co-treatment with or without atropine (3 µmol/L, a muscarinic receptor antagonist) or NG-nitro-L-Arginine (L-NNA, 100 µmol/L, a nitric oxide synthase inhibitor). RESULTS: Administration of granisetron (50, 75 µg/kg) in vivo significantly increased the time to first defecation (P = 0.045 vs vehicle-treated rats), clearly suggesting a constipating effect of this drug. Although administration of ZnPPIX or hemin alone had no effect on this gastrointestinal motility parameter, ZnPPIX co-administered with granisetron abolished the granisetron-induced constipation. On the other hand, co-administration of hemin and granisetron did not modify the increased constipation observed under granisetron alone. When administered in vitro, granisetron alone (3 µmol/L) did not significantly modify the colon's contractile response to either EFS or ACh. Incubation with ZnPPIX alone (10 µmol/L) significantly reduced the colon's contractile response to EFS (P = 0.016) but had no effect on contractile response to ACh. Co-administration of ZnPPIX and atropine (3 µmol/L) abolished the ZnPPIX-mediated decrease in contractile response to EFS. Conversely, incubation with CORM-3 (400 µmol/L) alone increased both the contractile response to EFS at 10 Hz (10 Hz: 71.02 ± 19.16 vs 116.25 ± 53.70, P = 0.01) and the contractile response to ACh (100 µmol/L) (P = 0.012). Co-administration of atropine abolished the CORM-3-mediated effects on the EFS-mediated response. When granisetron was co-incubated in vitro with ZnPPIX, the ZnPPIX-mediated decrease in colon contractile response to EFS was lost. On the other hand, co-incubation of granisetron and CORM-3 (400 µmol/L) further increased the colon's contractile response to EFS (at 5 Hz: P = 0.007; at 10 Hz: P = 0.001) and to ACh (ACh 10 µmol/L: P = 0.001; ACh 100 µmol/L: P = 0.001) elicited by CORM-3 alone. L-NNA co-administered with granisetron and CORM-3 abolished the potentiating effect of CORM-3 on granisetron on both the EFS-induced and ACh-induced contractile response. CONCLUSION: Taken together, findings from in vivo and in vitro studies suggest that the HO/CO pathway is involved in the constipating effects of granisetron.

Maternal Inflammation Disrupts Fetal Neurodevelopment via Increased Placental Output of Serotonin to the Fetal Brain.

UNLABELLED: Maternal inflammation during pregnancy affects placental function and is associated with increased risk of neurodevelopmental disorders in the offspring. The molecular mechanisms linking placental dysfunction to abnormal fetal neurodevelopment remain unclear. During typical development, serotonin (5-HT) synthesized in the placenta from maternal l-tryptophan (TRP) reaches the fetal brain. There, 5-HT modulates critical neurodevelopmental processes. We investigated the effects of maternal inflammation triggered in midpregnancy in mice by the immunostimulant polyriboinosinic-polyribocytidylic acid [poly(I:C)] on TRP metabolism in the placenta and its impact on fetal neurodevelopment. We show that a moderate maternal immune challenge upregulates placental TRP conversion rapidly to 5-HT through successively transient increases in substrate availability and TRP hydroxylase (TPH) enzymatic activity, leading to accumulation of exogenous 5-HT and blunting of endogenous 5-HT axonal outgrowth specifically within the fetal forebrain. The pharmacological inhibition of TPH activity blocked these effects. These results establish altered placental TRP conversion to 5-HT as a new mechanism by which maternal inflammation disrupts 5-HT-dependent neurogenic processes during fetal neurodevelopment. SIGNIFICANCE STATEMENT: The mechanisms linking maternal inflammation during pregnancy with increased risk of neurodevelopmental disorders in the offspring are poorly understood. In this study, we show that maternal inflammation in midpregnancy results in an upregulation of tryptophan conversion to serotonin (5-HT) within the placenta. Remarkably, this leads to exposure of the fetal forebrain to increased concentrations of this biogenic amine and to specific alterations of crucially important 5-HT-dependent neurogenic processes. More specifically, we found altered serotonergic axon growth resulting from increased 5-HT in the fetal forebrain. The data provide a new understanding of placental function playing a key role in fetal brain development and how this process is altered by adverse prenatal events such as maternal inflammation. The results uncover important future directions for understanding the early developmental origins of mental disorders.

Molecular mechanisms of serotonergic action of the HIV-1 antiretroviral efavirenz.

Efavirenz is highly effective at suppressing HIV-1, and the WHO guidelines list it as a component of the first-line antiretroviral (ARV) therapies for treatment-naïve patients. Though the pharmacological basis is unclear, efavirenz is commonly associated with a risk for neuropsychiatric adverse events (NPAEs) when taken at the prescribed dose. In many patients these NPAEs appear to subside after several weeks of treatment, though long-term studies show that in some patients the NPAEs persist. In a recent study focusing on the abuse potential of efavirenz, its receptor psychopharmacology was reported to include interactions with a number of established molecular targets for known drugs of abuse, and it displayed a prevailing behavioral profile in rodents resembling an LSD-like activity. In this report, we discovered interactions with additional serotonergic targets that may be associated with efavirenz-induced NPAEs. The most robust interactions were with 5-HT3A and 5-HT6 receptors, with more modest interactions noted for the 5-HT2B receptor and monoamine oxidase A. From a molecular mechanistic perspective, efavirenz acts as a 5-HT6 receptor inverse agonist of Gs-signaling, 5-HT2A and 5-HT2C antagonist of Gq-signaling, and a blocker of the 5-HT3A receptor currents. Efavirenz also completely or partially blocks agonist stimulation of the M1 and M3 muscarinic receptors, respectively. Schild analysis suggests that efavirenz competes for the same site on the 5-HT2A receptor as two known hallucinogenic partial agonists (±)-DOI and LSD. Prolonged exposure to efavirenz reduces 5-HT2A receptor density and responsiveness to 5-HT. Other ARVs such as zidovudine, nevirapine and emtricitabine did not share the same complex pharmacological profile as efavirenz, though some of them weakly interact with the 5-HT6 receptor or modestly block GABAA currents.

Effects of gonadectomy and serotonin depletion on inter-individual differences in anxiety-like behaviour in male Wistar rats.

Previous studies in Wistar rats suggest inter-individual differences in anxiety-like behaviour as assessed using the elevated plus maze (EPM), both between sexes and among males, to be abolished by serotonin depletion. To shed further light on the influence of sex steroids and serotonin - and on the interplay between the two - on proneness for EPM-assessed anxiety in males, outbred Wistar rats were divided into those with high and low anxiety, respectively, and exposed to gonadectomy or sham operation followed by administration of a serotonin synthesis inhibitor, para-chlorophenylalanine, or saline. Whereas gonadectomy enhanced anxiety-like behaviour in low anxiety rats so that these no longer differed in this regard from the high anxiety group, serotonin depletion reversed this effect, and also reduced anxiety in the low anxiety group regardless of gonadal state. A previously observed association between high anxiety-like behaviour and high expression of the serotonin-synthesizing enzyme tryptophan hydroxylase 2 (Tph2) in the raphe was confirmed in sham-operated animals but absent in gonadectomised rats, an ANCOVA revealing a significant interactive effect of baseline anxiety and gonadal state on Tph2 expression. It is suggested that androgens may contribute to upholding inter-individual differences in anxiety-like behaviour in male rats by interacting with serotonergic neurotransmission.

Synergistic effect between prelimbic 5-HT3 and CB1 receptors on memory consolidation deficit in adult male Sprague-Dawley rats: An isobologram analysis.

The serotonergic system has often been defined as a neuromodulator system, and is specifically involved in learning and memory via its various receptors. Serotonin is involved in many of the same processes affected by cannabinoids. The present study investigated the influence of bilateral post-training intra-prelimbic (PL) administrations of serotonergic 5-hydroxytryptamine type-3 (5-HT3) receptor agents on arachidonylcyclopropylamide (ACPA) (cannabinoid CB1 receptor agonist)-induced amnesia, using the step-through inhibitory avoidance (IA) task to assess memory in adult male Sprague-Dawley rats. The results indicated that sole intra-PL microinjection of ACPA (0.1 and 0.5 µg/rat) and 5-HT3 serotonin receptor agonist (m-Chlorophenylbiguanide hydrochloride, m-CPBG; 0.001, 0.01 and 0.1 µg/rat) impaired, whereas Y-25130 (a selective 5-HT3 serotonin receptor antagonist; 0.001 and 0.01 and 0.1 µg/rat) did not alter IA memory consolidation, by itself. Moreover, intra-PL administration of subthreshold dose of m-CPBG (0.0005 µg/rat) potentiated, while Y-25130 (0. 1 µg/rat) restored ACPA-induced memory consolidation deficit. The isobologram analysis showed that there is a synergistic effect between ACPA and m-CPBG on memory consolidation deficit. These findings suggest that 5-HT3 receptor mechanism(s), at least partly, play(s) a role in modulating the effect of ACPA on memory consolidation in the PL area.

The in vitro genotoxic and cytotoxic effects of remeron on human peripheral blood lymphocytes.

Remeron (Mirtazapine) is an antidepressant drug which exerts its action by blocking presynaptic α-2-adrenergic receptors and postsynaptic serotonin types 2 and 3 receptors. In this in vitro analysis, human peripheral blood lymphocytes was treated by remeron (10, 25, 40 and 55 µg/mL) for 24 hours and 48 hours periods, then it was attempted to study of genotoxic and cytotoxic effects of the substance on human peripheral blood lymphocytes by some tests such as sister chromatid exchange (SCE), chromosomal abnormalities (CA) and micronucleus (MN) tests. Also proliferating effect of the substance was investigated. Remeron didn't significantly cause chromosomal abnormalities and sister chromatid exchange while caused micronucleus at 40 µg/mL concentration and 24 h periodic time and increased proliferation index of the both 24 and 48 hours treated cells was decreased in a concentration manner. Also, exposing to the remeron for 24 and 48 hours leaded to a decrease in mitotic index and nucleus division index in the cells by concentration dependent manner. These findings showed that remeron did not have significantly genotoxic effects on human peripheral blood lymphocytes while it showed cytotoxic effects on the cells, which is the first report on genotoxic and cytotoxic properties of remeron.

A 12-week subchronic intramuscular toxicity study of risperidone-loaded microspheres in rats.

Long-acting injectable formulations of antipsychotics have been an important treatment option to increase the compliance of the patient with schizophrenia by monitoring drug administration and identifying medication noncompliance and to improve the long-term management of schizophrenia. Risperidone, a serotoninergic 5-HT2 and dopaminergic D2 receptor antagonist, was developed to be a long-acting sustained-release formulation for the treatment of schizophrenia. In this study, 12-week subchronic toxicity study of risperidone-loaded microspheres (RMs) in rats by intramuscular injection with an 8-week recovery phase was carried out to investigate the potential subchronic toxicity of a novel long-acting sustained-release formulation. The results indicated that the dosage of 10-90 mg/kg of RM for 2 weeks did not cause treatment-related mortality. The main drug-related findings were contributed to the dopamine D2 receptor and α1-adrenoceptor antagonism of risperidone such as elevation of serum and pituitary prolactin levels and ptosis and changes in reproductive system (uterus, ovary, vagina, mammary gland, testis, seminal vesicle, epididymis, and prostate). In addition, foreign body granuloma in muscle at injection sites caused by poly-lactide-co-glycolide was observed. At the end of the recovery phase, these changes mostly returned to normal. The results indicated that RM had a good safety profile in rats.

Distribution of cells responsive to 5-HT6 receptor antagonist-induced hypophagia.

The central 5-hydroxytryptamine (5-HT; serotonin) system is well established as an important regulator of appetite and continues to remain a focus of obesity research. While much emphasis has focussed on the 5-HT(2C) receptor (5-HT(2C)R) in 5-HT's anorectic effect, pharmacological manipulation of the 5-HT6 receptor (5-HT6R) also reduces appetite and body weight and may be amenable to obesity treatment. However, the neurological circuits that underlie 5-HT6R-induced hypophagia remain to be identified. Using c-fos immunoreactivity (FOS-IR) as a marker of neuronal activation, here we mapped the neuroanatomical targets activated by an anorectic dose of the 5-HT6R antagonist SB-399885 throughout the brain. Furthermore, we quantified SB-399855 activated cells within brain appetitive nuclei, the hypothalamus, dorsal raphe nucleus (DRN) and nucleus of the solitary tract (NTS). Our results reveal that 5-HT6R antagonist-induced hypophagia is associated with significantly increased neuronal activation in two nuclei with an established role in the central control of appetite, the paraventricular nucleus of the hypothalamus (PVH) and the NTS. In contrast, no changes in FOS-IR were observed between treatment groups within other hypothalamic nuclei or DRN. The data presented here provide a first insight into the neural circuitry underlying 5-HT6R antagonist-induced appetite suppression and highlight the PVH and NTS in the coordination of 5-HT6R hypophagia.

A 12-week intramuscular toxicity study of risperidone-loaded microspheres in Beagle dogs.

Long-acting formulations of antipsychotics are important treatment options to increase the compliance of schizophrenic patients. Risperidone, a 5-HT2 and dopaminergic D2 receptor antagonist, was developed as long-acting sustained-release microspheres with poly(lactide-co-glycolide) (PLGA) as a drug carrier for the treatment of schizophrenia. In the present study, the main objective is to determine the nonclinical safety profile of risperidone-loaded microspheres (RM) in Beagle dogs after intramuscular administration for 3 months, once in 2 weeks, followed by 8-week recovery phase. No animal death was found and no special toxicological findings were observed. The findings, such as hypoactivity, ptosis, increased heart rate, and elevated serum and pituitary prolactin levels, were observed and related to the pharmacological effects of risperidone. The changes in the reproductive system (uterus, ovary, vagina, cervix, and mammary gland) were considered secondary to the prolactin elevation, and the congestion of spleen was related to risperidone. The foreign body granulomas at injection sites might be caused by PLGA. At the end of recovery phase, the above changes mostly recovered to normal, and on administering 3 mg/kg dose level once in 2 weeks on Beagle dogs showed no observed adverse effect. Taken together, RM had exhibited the acceptable safety.

Neonatal serotonin (5-HT) depletion does not affect spatial learning and memory in rats.

BACKGROUND: Extensive previous research has suggested a role for serotonin (5-HT) in learning and memory processes, both in healthy individuals and pathological disorders including depression, autism and schizophrenia, most of which have a developmental onset. Since 5-HT dysfunction in brain development may be involved in disease etiology, the present investigation assessed the effects of neonatal 5-HT depletion on spatial learning and memory in the Morris water maze (MWM). METHODS: Three days old Sprague-Dawley rats were pretreated with desipramine (20 mg/kg) followed by an intraventricular injection of the selective 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 70 µg). Three months later rats were tested in the MWM. RESULTS: Despite a severe and permanent decrease (80-98%) in hippocampal, prefrontal and striatal 5-HT levels, treatment with 5,7-DHT caused no spatial learning and memory impairment. CONCLUSIONS: Limited involvement of chronic 5-HT depletion on learning and memory does not exclude the possibility that this neurotransmitter has an important neuromodulatory role in these functions. Future studies will be needed to identify the nature of the compensatory processes that are able to allow normal proficiency of spatial learning and memory in 5-HT-depleted rats.

Impaired reward learning and intact motivation after serotonin depletion in rats.

Aside from the well-known influence of serotonin (5-hydroxytryptamine, 5-HT) on emotional regulation, more recent investigations have revealed the importance of this monoamine in modulating cognition. Parachlorophenylalanine (PCPA) depletes 5-HT by inhibiting tryptophan hydroxylase, the enzyme required for 5-HT synthesis and, if administered at sufficiently high doses, can result in a depletion of at least 90% of the brain's 5-HT levels. The present study assessed the long-lasting effects of widespread 5-HT depletions on two tasks of cognitive flexibility in Long Evans rats: effort discounting and reversal learning. We assessed performance on these tasks after administration of either 250 or 500 mg/kg PCPA or saline (SAL) on two consecutive days. Consistent with a previous report investigating the role of 5-HT on effort discounting, pretreatment with either dose of PCPA resulted in normal effortful choice: All rats continued to climb tall barriers to obtain large rewards and were not work-averse. Additionally, rats receiving the lower dose of PCPA displayed normal reversal learning. However, despite intact motivation to work for food rewards, rats receiving the largest dose of PCPA were unexpectedly impaired relative to SAL rats on the pretraining stages leading up to reversal learning, ultimately failing to approach and respond to the stimuli associated with reward. High performance liquid chromatography (HPLC) with electrochemical detection confirmed 5-HT, and not dopamine, levels in the ventromedial frontal cortex were correlated with this measure of associative reward learning.

Adverse effects of serotonin depletion in developing zebrafish.

In this study, p-chlorophenylalanine (pCPA), an inhibitor of tryptophan hydroxylase (the rate limiting enzyme of serotonin synthesis), was used to reduce serotonin (5HT) levels during early development in zebrafish embryos. One day old dechorionated embryos were treated with 25 µM pCPA for 24h and subsequently rescued. Immunohistological studies using a 5HT antibody confirmed that 5HT neurons in the brain and spinal cord were depleted of transmitter by 2 days post fertilization (dpf). Twenty four hours after pCPA exposure embryos were unable to burst swim and were nearly paralyzed. Movement began to improve at 4 dpf, and by 7 dpf, larvae exhibited swimming activity. Rescued larvae continued to grow in rostrocaudal length over 5 days post-rescue, but their length was always 16-21% below controls. Surprisingly, both groups displayed the same number of myotomes. To examine whether hypertonicity of myotomes in treated embryos played a role in their shorter rostrocaudal lengths, 1 dpf embryos were exposed to a combination of 25 µM pCPA and 0.6 mM of the sodium channel blocker ethyl 3-aminobenzoate methanesulfonate (MS-222). After a 24 hour exposure, the embryos exhibited the same rostrocaudal length as control embryos suggesting that myotome hypertonicity plays a major role in the decreased axial length of the treated larvae. In addition, pCPA treated 2 dpf embryos exhibited abnormal notochordal morphology that persisted throughout recovery. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed to determine the relative levels of the serotonin 1A receptor (5HT(1A)) transcript and the serotonin transporter (SERT) transcript in the brain and spinal cord of control and treated embryos. Transcripts were present in both brain and spinal cord as early as 1 dpf and reached maximal concentrations by 3 dpf. Embryos treated with pCPA demonstrated a decrease in the concentration of 5HT(1A) transcript in both brain and spinal cord. While SERT transcript levels remained unaffected in brain, they were decreased in spinal cord. Five days subsequent to pCPA rescue, 5HT(1A) transcript concentrations remained decreased in brain while SERT transcript levels were elevated in both regions. These findings suggest that reduction of 5HT during early zebrafish development may have an adverse effect on body length, notochordal morphology, locomotor behavior, and serotonin message-related expression.

Contributions of serotonin in addiction vulnerability.

The serotonin (5-hydroxytryptamine; 5-HT) system has long been associated with mood and its dysregulation implicated in the pathophysiology of mood and anxiety disorders. While modulation of 5-HT neurotransmission by drugs of abuse is also recognized, its role in drug addiction and vulnerability to drug relapse is a more recent focus of investigation. First, we review preclinical data supporting the serotonergic raphe nuclei and their forebrain projections as targets of drugs of abuse, with emphasis on the effects of psychostimulants, opioids and ethanol. Next, we examine the role of 5-HT receptors in impulsivity, a core behavior that contributes to the vulnerability to addiction and relapse. Finally, we discuss evidence for serotonergic dysregulation in comorbid mood and addictive disorders and suggest novel serotonergic targets for the treatment of addiction and the prevention of drug relapse.