Receptor Antagonist SB Research Articles

The effect of orexin-A on motor and cognitive functions in a rat model of Parkinson’s disease

Objective: The aim of this study was to evaluate the effect of orexin-A (OX-A) and the orexin-1 receptor antagonist SB-334867 (SB) on motor and cognitive functions in a rat model of Parkinson’s disease. Methods: Parkinson was induced by intracerebroventricular (i.c.v) injection of 6- hydroxydopamine (6-OHDA) (200 μg/rat). 72 h later, the treatment was initiated by i.c.v administration of SB (30 nmol/rat) and/or OX-A (0.3 nmol/rat) for 10 days. Motor functions were monitored using rotarod and hanging tests. Cognitive function was assessed by passive avoidance test (Shuttle box). Results: OX-A administration in 6-OHDA treated rats remarkably increased the time which animals run on rod (in rotarod test) and also the latency to fall (in hanging test) (P < 0.01 and P < 0.001, respectively). Conversely, administration of SB in 6-OHDA-treated rats decreased the mentioned indices (P < 0.05 for latency to fall). Administration of agonist and/or antagonist of orexin-1 receptors had no significant effect on 6-OHDA induced cognitive impairment in rats. Conclusion: Results of this study suggest that the orexinergic system might be involved in sensory-motor deficits of Parkinson’s disease.

Characterization of three serotonin receptors from the small white butterfly, Pieris rapae

Serotonin (5-hydroxytryptamine, 5-HT) plays a key role in modulating diverse physiological processes and behaviors in both protostomes and deuterostomes. These functions are mediated through the binding of serotonin to its receptors, which are recognized as potential insecticide targets. We investigated the sequence, pharmacology and tissue distribution of three 5-HT receptors (Piera5-HT1A, Piera5-HT1B, Piera5-HT7) from the small white butterfly Pieris rapae, an important pest of cultivated cabbages and other mustard family crops. Activation of Piera5-HT1A or Piera5-HT1B by 5-HT inhibited the production of cAMP in a dose-dependent manner. Stimulation of Piera5-HT7 with 5-HT increased cAMP level significantly. Surprisingly, with the exception of 5-methoxytryptamine, agonists including α-methylserotonin, 8-Hydroxy-DPAT and 5-carboxamidotryptamine activated these receptors poorly. The results are consistent with previous findings in Manduca sexta. All three receptors were blocked by methiothepin, but ketanserin and yohimbine were not effective. The selective mammalian 5-HT receptor antagonists SB 216641 and SB 269970 displayed potent inhibition effects on Piera5-HT1B and Piera5-HT7 respectively. The results we achieved here indicate that the pharmacological properties of Lepidoptera 5-HT receptors are quite different from those in other insects and vertebrates and may contribute to development of new selective pesticides. This study offers important information on three 5-HT receptors from P. rapae that will facilitate further analysis of the functions of 5-HT receptors in insects.

5-HT causes splanchnic venodilation.

Serotonin [5-hydroxytryptamine (5-HT)] causes relaxation of the isolated superior mesenteric vein, a splanchnic blood vessel, through activation of the 5-HT7 receptor. As part of studies designed to identify the mechanism(s) through which chronic (≥24 h) infusion of 5-HT lowers blood pressure, we tested the hypothesis that 5-HT causes in vitro and in vivo splanchnic venodilation that is 5-HT7 receptor dependent. In tissue baths for measurement of isometric contraction, the portal vein and abdominal inferior vena cava relaxed to 5-HT and the 5-HT1/7 receptor agonist 5-carboxamidotryptamine; relaxation was abolished by the 5-HT7 receptor antagonist SB-269970. Western blot analyses showed that the abdominal inferior vena cava and portal vein express 5-HT7 receptor protein. In contrast, the thoracic vena cava, outside the splanchnic circulation, did not relax to serotonergic agonists and exhibited minimal expression of the 5-HT7 receptor. Male Sprague-Dawley rats with chronically implanted radiotelemetry transmitters underwent repeated ultrasound imaging of abdominal vessels. After baseline imaging, minipumps containing vehicle (saline) or 5-HT (25 μg·kg-1·min-1) were implanted. Twenty-four hours later, venous diameters were increased in rats with 5-HT-infusion (percent increase from baseline: superior mesenteric vein, 17.5 ± 1.9; portal vein, 17.7 ± 1.8; and abdominal inferior vena cava, 46.9 ± 8.0) while arterial pressure was decreased (~13 mmHg). Measures returned to baseline after infusion termination. In a separate group of animals, treatment with SB-269970 (3 mg/kg iv) prevented the splanchnic venodilation and fall in blood pressure during 24 h of 5-HT infusion. Thus, 5-HT causes 5-HT7 receptor-dependent splanchnic venous dilation associated with a fall in blood pressure.NEW & NOTEWORTHY This research is noteworthy because it combines and links, through the 5-HT7 receptor, an in vitro observation (venorelaxation) with in vivo events (venodilation and fall in blood pressure). This supports the idea that splanchnic venodilation plays a role in blood pressure regulation.

Behavioral Effects of a Novel Benzofuranyl-Piperazine Serotonin-2C Receptor Agonist Suggest a Potential Therapeutic Application in the Treatment of Obsessive-Compulsive Disorder.

Selective serotonin reuptake inhibitors (SSRIs) are the only effective pharmacological treatments for obsessive–compulsive disorder (OCD). Nonetheless, their generally limited efficacy, side-effects, and delayed onset of action require improved medications for this highly prevalent disorder. Preclinical and clinical findings have suggested serotonin2C (5-HT2C) receptors as a potential drug target. Data in rats and mice are presented here on the effects of a novel 5-HT2C receptor agonist ((3S)-3-Methyl-1-[4-(trifluoromethyl)-7-benzofuranyl]-piperazine) (CPD 1) with high potency and full efficacy at 5-HT2C receptors and less potency and partial agonism at 5-HT2A and 5-HT2B receptors. Effects of CPD 1 on consummatory (schedule-induced polydipsia in rats) and non-consummatory behaviors (marble-burying and nestlet-shredding in mice) that are repetitive and non-habituating were studied. We also evaluated the effects of CPD 1 in rats with isoproterenol- and deprivation-induced drinking in rats to compare with the polydipsia studies. The SSRIs, fluoxetine, and chlomipramine decreased the high rates of drinking in rats engendered by a schedule of intermittent food delivery (schedule-induced polydipsia). The effects of fluoxetine, but not of d-amphetamine, were prevented by the selective 5-HT2C receptor antagonist SB242084. The 5-HT2C receptor agonists Ro 60-0175 and CPD 1 also decreased drinking, but unlike the SSRIs and Ro 60-0175, CPD 1 dose-dependently decreased excessive drinking without affecting lever press responses that produced food. The effects of CPD 1 were prevented by SB242084. CPD 1 also suppressed drinking induced by isoproterenol and by water deprivation without affecting normative drinking behavior. CPD 1, like fluoxetine, also suppressed marble-burying and nestlet-shredding in mice at doses that did not affect rotarod performance or locomotor activity. The behavioral specificity of effects of CPD 1 against repetitive and excessive behaviors suggests a potential therapeutic application in OCD.

Facilitation of Contextual Fear Extinction by Orexin-1 Receptor Antagonism Is Associated with the Activation of Specific Amygdala Cell Subpopulations

BackgroundOrexins are hypothalamic neuropeptides recently involved in the regulation of emotional memory. The basolateral amygdala, an area orchestrating fear memory processes, appears to be modulated by orexin transmission during fear extinction. However, the neuronal types within the basolateral amygdala involved in this modulation remain to be elucidated.MethodsWe used retrograde tracing combined with immunofluorescence techniques in mice to identify basolateral amygdala projection neurons and cell subpopulations in this brain region influenced by orexin transmission during contextual fear extinction consolidation.ResultsTreatment with the orexin-1 receptor antagonist SB334867 increased the activity of basolateral amygdala neurons projecting to infralimbic medial prefrontal cortex during fear extinction. GABAergic interneurons expressing calbindin, but not parvalbumin, were also activated by orexin-1 receptor antagonism in the basolateral amygdala.ConclusionsThese data identify neuronal circuits and cell populations of the amygdala associated with the facilitation of fear extinction consolidation induced by the orexin-1 receptor antagonist SB334867.

Preclinical Evidence That 5-HT1B Receptor Agonists Show Promise as Medications for Psychostimulant Use Disorders

Background5-HT1B receptor agonists enhance cocaine intake during daily self-administration sessions but decrease cocaine intake when tested after prolonged abstinence. We examined if 5-HT1B receptor agonists produce similar abstinence-dependent effects on methamphetamine intake.MethodsMale rats were trained to self-administer methamphetamine (0.1 mg/kg, i.v.) on low (fixed ratio 5 and variable ratio 5) and high (progressive ratio) effort schedules of reinforcement until intake was stable. Rats were then tested for the effects of the selective 5-HT1B receptor agonist, CP 94,253 (5.6 or 10 mg/kg), or the less selective but clinically available 5-HT1B/1D receptor agonist, zolmitriptan (10 mg/kg), on methamphetamine self-administration both before and after a 21-day forced abstinence period during which the rats remained in their home cages.ResultsThe inverted U-shaped, methamphetamine dose-response function for intake on the fixed ratio 5 schedule was shifted downward by CP 94,253 both before and after abstinence. The CP 94,253-induced decrease in methamphetamine intake was replicated in rats tested on a variable ratio 5 schedule, and the 5-HT1B receptor antagonist SB 224,289 (10 mg/kg) reversed this effect. CP 94,253 also attenuated methamphetamine intake on a progressive ratio schedule both pre- and postabstinence. Similarly, zolmitriptan attenuated methamphetamine intake on a variable ratio 5 schedule both pre- and postabstinence, and the latter effect was sustained after each of 2 more treatments given every 2 to 3 days prior to daily sessions.ConclusionsUnlike the abstinence-dependent effect of 5-HT1B receptor agonists on cocaine intake reported previously, both CP 94,253 and zolmitriptan decreased methamphetamine intake regardless of abstinence. These findings suggest that 5-HT1B receptor agonists may have clinical efficacy for psychostimulant use disorders.

Role of Orexin-A in Hypertension and Obesity.

Hypertension is one of the most challenging health problems inducing cerebrovascular disease and high percentage of death when associated with diabetes, dyslipidemias, and obesity. Orexin/hypocretin is a peptide expressed by a small number of neurons of the dorsolateral hypothalamus, a brain feeding and autonomic "fight-or-flight" regulatory center. According to this function, orexin has been demonstrated to evoke cardiovascular responses, heart rate, hypertension, hyperarousal, hyperphagia, and obesity. The focus of this review is to provide an overview about the mechanism through which orexin regulates food intake and cardiovascular responses and its role in the pathogenesis of obesity and hypertension which could be of great interest to establish possible new therapies. In normal rats and mice, central administration of orexin increases food intake, blood pressure, and sympathetic nerve activity and these effects are blocked by selective orexin receptor antagonist SB-334867 or almorexant. Moreover, upregulation of orexin signaling, in combination with elevation of epinephrine and norepinephrine circulating levels, occurs in rats exposed to chronic stress, in models of spontaneous hypertension (SHR and BPH/2J Schlager mice) and in obese mice (ob/ob or mice fed with high fat diet). Therefore, hyperactivity of orexinergic neurons could be a factor in the development of obesity and essential hypertension. Because of their widespread projections to the brain regions involved in appetite and cardiovascular responses, as far down as sympathetic preganglionic neurons in the spinal cord, orexin evokes sympathetically mediated cardiovascular responses. Lasting upregulation of orexin signaling can lead to hyperphagia, obesity, and hypertensive state. Dual orexin receptor antagonists (DORAs) and selective orexin receptor antagonists (SORAs) have antihypertensive effects that could be of clinical use for regulation of food intake and hypertension, supporting the role of orexinergic neurons as critical checkpoint in the neurogenic control of metabolic and cardiovascular functions.

Hypocretin/Orexin Peptides Excite Rat Neuroendocrine Dopamine Neurons through Orexin 2 Receptor-Mediated Activation of a Mixed Cation Current

Hypocretin/Orexin (H/O) neurons of the lateral hypothalamus are compelling modulator candidates for the chronobiology of neuroendocrine output and, as a consequence, hormone release from the anterior pituitary. Here we investigate the effects of H/O peptides upon tuberoinfundibular dopamine (TIDA) neurons – cells which control, via inhibition, the pituitary secretion of prolactin. In whole cell recordings performed in male rat hypothalamic slices, application of H/O-A, as well as H/O-B, excited oscillating TIDA neurons, inducing a reversible depolarising switch from phasic to tonic discharge. The H/O-induced inward current underpinning this effect was post-synaptic (as it endured in the presence of tetrodotoxin), appeared to be carried by a Na+-dependent transient receptor potential-like channel (as it was blocked by 2-APB and was diminished by removal of extracellular Na+), and was a consequence of OX2R receptor activation (as it was blocked by the OX2R receptor antagonist TCS OX2 29, but not the OX1R receptor antagonist SB 334867). Application of the hormone, melatonin, failed to alter TIDA membrane potential or oscillatory activity. This first description of the electrophysiological effects of H/Os upon the TIDA network identifies cellular mechanisms that may contribute to the circadian rhythmicity of prolactin secretion.

Effects of 5-HT1A, 5-HT2A and 5-HT2C receptor agonists and antagonists on responding for a conditioned reinforcer and its enhancement by methylphenidate.

These experiments examined the effects of selective 5-HT1A, 5-HT2A and 5-HT2C receptor ligands on responding for a conditioned reinforcer (CRf). Effects of these ligands were measured under basal conditions and following elevated dopamine (DA) activity produced by the DA reuptake inhibitor methylphenidate. Water-restricted rats learned to associate a conditioned stimulus (CS) with water in operant chambers. Subsequently, two response levers were made available; responding on one lever delivered the CS (now a CRf), while responding on the second lever had no consequences. The effects of agonist and antagonists of 5-HT1A (8-hydroxy-2(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) and N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635)), 5-HT2A (DOI and M100907) and 5-HT2C (Ro60-0175 and SB242084) receptors on responding were examined alone, as well as in the presence of methylphenidate. Responding for a CRf was reduced by the agonists 8-OH-DPAT, DOI and Ro60-0175. 8-OH-DPAT also reduced responding for water and seemed to impair responding in a non-specific fashion. None of the receptor antagonists affected responding. Methylphenidate dose-dependently enhanced responding for a CRf, and this was attenuated by DOI and Ro60-0175. Conversely, the 5-HT2C receptor antagonist SB242084 potentiated the effect of methylphenidate. No evidence was found for a behaviourally selective effect of 5-HT1A receptor ligands on responding for a CRf. Activation of 5-HT2A receptors selectively inhibits responding for a CRf. 5-HT2C receptor ligands exerted bidirectional modulation of responding for a CRf, especially when DA activity was increased. This indicates that 5-HT2C receptor activity is an important modulator of DA-dependent reward-related behaviours.

Role of orexin receptor subtypes in the inhibitory effects of orexin-A on potassium chloride-induced increases in intracellular calcium ion levels in neurons derived from dorsal root ganglion of carrageenan-treated rats

We analysed the roles of orexin receptors in the effects of orexin-A on KCl-induced increases in intracellular calcium ion levels ([Ca2+]i) in C-fiber-like small neurons of rats with inflammation induced by intraplantar injection of carrageenan into the hind paw. Controls were treated with saline. Paw withdrawal and threshold forces in response to tactile stimuli were determined using von Frey filaments. [Ca2+]i in C-fiber-like neurons derived from dorsal root ganglia was visualised using a calcium fluorescence probe. Changes in neuronal [Ca2+]i were assessed as relative fluorescence intensity (F/F0). One day after carrageenan injection, the paw withdrawal response to tactile stimuli and the paw withdrawal threshold were increased and reduced, respectively. KCl loading of neurons from either carrageenan-treated or control rats increased F/F0 to about 2.0. KCl-induced increases in F/F0 of carrageenan-treated, but not control, rats were inhibited by orexin-A. The OX1 and OX2 receptor antagonist MK-4305, but not the OX1 receptor antagonist SB334867, counteracted the effects of orexin-A on the KCl-induced increase in F/F0. These results suggest that OX2, but not OX1 receptors mediate the inhibitory effect of orexin-A on KCl-induced increases in [Ca2+]i in C-fiber-like neurons of rats with inflammation.

An investigation into the inhibitory function of serotonin in diffuse noxious inhibitory controls in the neuropathic rat.

Following neuropathy α2-adrenoceptor-mediated diffuse noxious inhibitory controls (DNIC), whereby a noxious conditioning stimulus inhibits the activity of spinal wide dynamic range (WDR) neurons, are abolished, and spinal 5-HT7 receptor densities are increased. Here, we manipulate spinal 5-HT content in spinal nerve ligated (SNL) animals and investigate which 5-HT receptor mediated actions predominate. Using invivo electrophysiology we recorded WDR neuronal responses to von frey filaments applied to the hind paw before, and concurrent to, a noxious ear pinch (the conditioning stimulus) in isoflurane-anaesthetised rats. The expression of DNIC was quantified as a reduction in WDR neuronal firing in the presence of conditioning stimulus and was investigated in SNL rats following spinal application of (1) selective serotonin reuptake inhibitors (SSRIs) citalopram or fluoxetine, or dual application of (2) SSRI plus 5-HT7 receptor antagonist SB269970, or (3) SSRI plus α2 adrenoceptor antagonist atipamezole. DNIC were revealed in SNL animals following spinal application of SSRI, but this effect was abolished upon joint application of SSRI plus SB269970 or atipamezole. We propose that in SNL animals the inhibitory actions (quantified as the presence of DNIC) of excess spinal 5-HT (presumed present following application of SSRI) were mediated via 5-HT7 receptors. The anti-nociception depends upon an underlying tonic noradrenergic inhibitory tone via the α2-adrenoceptor. Following neuropathy enhanced spinal serotonin availability switches the predominant spinal 5-HT receptor-mediated actions but also alters noradrenergic signalling. We highlight the therapeutic complexity of SSRIs and monoamine modulators for the treatment of neuropathic pain.

Intra-LC microinjection of orexin type-1 receptor antagonist SB-334867 attenuates the expression of glutamate-induced opiate withdrawal like signs during the active phase in rats

Opiate withdrawal syndrome is temporally associated with the hyperactivity of locus coeruleus neurons. Previous studies have shown that this hyperactivity, at least in part, results from the activity of excitatory afferents which mainly include the orexinergic neurons of hypothalamus and glutamatergic neurons of paragigantocellularis (PGi) nucleus. The effect of intra LC orexin type 1 receptor antagonism was investigated on expression of glutamate-induced morphine withdrawal-like signs in rats. Regarding the involvement of both orexin and LC in modulation of circadian rhythm, experimental procedures were performed during the rest (day) and the active (night) phases. Male Wistar rats (250–300g) received escalating doses (6, 16, 26, 36, 46, 56, 66mg/kg, 2ml/kg) of morphine sulfate subcutaneously for 7days. Then, glutamate (100nM, 200nl) was microinjected into the LC region and the subsequent behavioral manifestations were visually monitored in both rest and active phases. SB-334867 (as a selective orexin type 1 receptor antagonist) was microinjected into the LC prior to glutamate administration. Results indicate that intra-LC microinjection of glutamate elicits morphine withdrawal-like behavioral signs in rats. It is noteworthy that this effect was significantly suppressed in rats pretreated with SB-334867 only during the active phase. It could be concluded that orexin-A plays a role in expression of glutamate-induced opiate withdrawal-like signs and differential orexinergic tone during the rest and active phases might explain the observed difference in activity of LC neurons.

Orexin‐A increases the firing activity of hippocampal CA1 neurons through orexin‐1 receptors

Orexins including two peptides, orexin-A and orexin-B, are produced in the posterior lateral hypothalamus. Much evidence has indicated that central orexinergic systems play numerous functions including energy metabolism, feeding behavior, sleep/wakefulness, and neuroendocrine and sympathetic activation. Morphological studies have shown that the hippocampal CA1 regions receive orexinergic innervation originating from the hypothalamus. Positive orexin-1 (OX1 ) receptors are detected in the CA1 regions. Previous behavioral studies have shown that microinjection of OX1 receptor antagonist into the hippocampus impairs acquisition and consolidation of spatial memory. However, up to now, little has been known about the direct electrophysiological effects of orexin-A on hippocampal CA1 neurons. Employing multibarrel single-unit extracellular recordings, the present study showed that micropressure administration of orexin-A significantly increased the spontaneous firing rate from 2.96 ± 0.85 to 8.45 ± 1.86 Hz (P < 0.001) in 15 out of the 23 hippocampal CA1 neurons in male rats. Furthermore, application of the specific OX1 receptor antagonist SB-334867 alone significantly decreased the firing rate from 4.02 ± 1.08 to 2.11 ± 0.58 Hz in 7 out of the 17 neurons (P < 0.05), suggesting that endogenous orexinergic systems modulate the firing activity of CA1 neurons. Coapplication of SB-334867 completely blocked orexin-A-induced excitation of hippocampal CA1 neurons. The PLC pathway may be involved in activation of OX1 receptor-induced excitation of CA1 neurons. Taken together, the present study's results suggest that orexin-A produces excitatory effects on hippocampal neurons via OX1 receptors. © 2016 Wiley Periodicals, Inc.

Morphine-induced Straub tail reaction in mice treated with serotonergic compounds

Constitutively active 5-HT2 receptors have been suggested to contribute to motoneuronal excitability, muscle spasms and spasticity. Accordingly, 5-HT2C receptor inverse agonists have been demonstrated in pilot experiments to reduce spasticity in animal model of spasticity and patients with spinal cord injuries. Thus, 5-HT2C receptor inverse agonists may represent a novel class of anti-spasticity agents justifying a search for compounds with robust 5-HT2C receptor inverse agonist activity either among the existing medications or via a dedicated drug discovery program. Morphine-induced Straub tail response in mice is regarded as a model of transient spasticity that may be suitable for supporting such drug discovery efforts. Subcutaneous injection of morphine (10–60mg/kg) induced a dose-dependent Straub tail reaction in male Swiss mice with maximum response obtained 15–30min after the morphine administration. When given prior to morphine, 5-HT2B/2C receptor inverse agonists cyproheptadine (1–10mg/kg, i.p.) and SB206553 (0.3–3mg/kg, i.p.) diminished Straub tail reaction dose-dependently without affecting spontaneous locomotor activity. In contrast, 5-HT2B/2C receptor antagonist methysergide (1–5.6mg/kg, i.p.) and 5-HT2C receptor antagonist SB242084 (1–5.6mg/kg, i.p.) as well as 5-HT2A receptor inverse agonist pimavanserin (1–10mg/kg, i.p.) had no appreciable effects on Straub tail response. Taken together, the findings indicate that constitutive activity of 5-HT2B/2C receptor may be involved in the mechanisms of morphine-induced spasticity. Thus, morphine-induced Straub tail response may be evaluated further as a candidate higher throughput test to identify 5-HT2C receptor inverse agonists with anti-spasticity effects in vivo.

Behavioral, pharmacological and neuroanatomical analysis of serotonin 2C receptor agonism on maternal behavior in rats

As a highly motivated social behavior, maternal behavior in rats has been routinely used to study psychoactive drugs for clinical, neuroscience and pharmacological purposes. Recent evidence indicates that acute activation of serotonin 2C (5-HT2C) receptors causes a disruption of rat maternal behavior. The present study was designed to elucidate the behavioral, pharmacological mechanisms and neuroanatomical basis of this 5-HT2C effect. First, we replicated the finding that acute MK212 injection (2.0mg/kg, a highly selective 5-HT2C agonist) disrupts maternal behavior, especially on pup retrieval. Interestingly, this disruption was significantly attenuated by 4-h pup separation (a procedure putatively increased maternal motivation). MK212 also suppressed food retrieval, indicating that it has a general effect on motivated behaviors. Second, we showed that MK212 disrupts maternal behavior by specifically activating 5-HT2C receptor, as pretreatment with a 5-HT2C receptor antagonist SB242084 (0.6 and 1.0mg/kg) alleviated MK212-induced disruption on pup retrieval. Third, we microinjected MK212 into various brain regions implicated in the regulation of maternal behavior: nucleus accumbens shell (25, 75, 250ng/0.5μl/side), medial prefrontal cortex (25 and 250ng, 1, 2 and 5μg/0.5μl/side), and medial preoptic area (MPOA, 75ng, 1 and 5μg/0.5μl/side). Pup retrieval and other maternal responses were not affected by any of these manipulations. Finally, we used c-Fos immunohistochemistry to identify the central mechanisms of the acute and repeated MK212 effects on maternal behavior. Acute MK212 (2.0mg/kg) disrupted pup retrieval and concurrently decreased c-Fos expression in the ventral part of lateral septal nucleus (LSv), MPOA, dentate gyrus (DG) and dorsal raphe (DR), but increased it in the central amygdala (CeA). Five days of repeated MK212 (2.0mg/kg) treatment produced a persistent disruption of pup retrieval and only decreased c-Fos expression in the DR. These findings not only confirm a role of 5-HT2C receptor in rat maternal behavior, but also suggest that the coordinated 5-HT2C activity in various limbic (e.g., LSv, DG, CeA), hypothalamic regions (e.g., MPOA) and brainstem areas (e.g. DR), is likely involved in the mediation of important psychological processes (e.g. motor function, motivation) necessary for the normal expression of maternal behavior.

Loss of GluN2D subunit results in social recognition deficit, social stress, 5-HT2C receptor dysfunction, and anhedonia in mice

The N-methyl-d-aspartate (NMDA) receptor channel is involved in various physiological functions, including learning and memory. The GluN2D subunit of the NMDA receptor has low expression in the mature brain, and its role is not fully understood. In the present study, the effects of GluN2D subunit deficiency on emotional and cognitive function were investigated in GluN2D knockout (KO) mice. We found a reduction of motility (i.e., a depressive-like state) in the tail suspension test and a reduction of sucrose preference (i.e., an anhedonic state) in GluN2D KO mice that were group-housed with littermates. Despite apparently normal olfactory function and social interaction, GluN2D KO mice exhibited a decrease in preference for social novelty, suggesting a deficit in social recognition or memory. Golgi-Cox staining revealed a reduction of the complexity of dendritic trees in the accessory olfactory bulb in GluN2D KO mice, suggesting a deficit in pheromone processing pathway activation, which modulates social recognition. The deficit in social recognition may result in social stress in GluN2D KO mice. Isolation housing is a procedure that has been shown to reduce stress in mice. Interestingly, 3-week isolation and treatment with agomelatine or the 5-hydroxytryptamine-2C (5-HT2C) receptor antagonist SB242084 reversed the anhedonic-like state in GluN2D KO mice. In contrast, treatment with the 5-HT2C receptor agonist CP809101 induced depressive- and anhedonic-like states in isolated GluN2D KO mice. These results suggest that social stress that is caused by a deficit in social recognition desensitizes 5-HT2c receptors, followed by an anhedonic- and depressive-like state, in GluN2D KO mice. The GluN2D subunit of the NMDA receptor appears to be important for the recognition of individuals and development of normal emotionality in mice. 5-HT2C receptor antagonism may be a therapeutic target for treating social stress-induced anhedonia.This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’.

Serotonin7 receptors in the lateral habenular nucleus regulate depressive-like behaviors in the hemiparkinsonian rats

Preclinical studies indicate that serotonin7 (5-HT7) receptors may regulate depressive-like behaviors. Depression is a common symptom in Parkinson's disease (PD); however, its pathophysiology is unclear. Here we examined whether 5-HT7 receptors in the lateral habenular nucleus (LHb) involve in the regulation of PD-related depression. Unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta in rats induced depressive-like responses as measured by the sucrose preference and forced swim tests when compared to sham-operated rats. Intra-LHb injection of 5-HT7 receptor agonist AS19 (1, 2 and 4μg/rat) induced or increased the expression of depressive-like behaviors in sham-operated and the lesioned rats. Further, intra-LHb injection of 5-HT7 receptor antagonist SB269970 (1.5, 3 and 6μg/rat) produced antidepressant effects in the two groups of rats. However, the doses producing these effects in the lesioned rats were higher than those in sham-operated rats. Neurochemical results showed that intra-LHb injection of AS19 (4μg/rat) decreased dopamine and 5-HT levels in the medial prefrontal cortex, habenula and hippocampus in sham-operated and the lesioned rats; whereas SB269970 (6μg/rat) increased dopamine and 5-HT levels in these structures. In addition, noradrenaline levels in these structures were not changed after intra-LHb injection of AS19 or SB269970 in the two groups of rats. These findings suggest that activation or blockade of 5-HT7 receptors in the LHb may change the activity of LHb glutamate neurons, and then decreases or increases dopamine and 5-HT levels in the limbic and limbic-related brain regions, which are involved in the regulation of depressive-like behaviors.

Asenapine reduces anxiety-related behaviours in rat conditioned fear stress model.

Asenapine is an atypical antipsychotic that is currently available for the treatment of schizophrenia and bipolar I disorder. Although the atypical antipsychotics clozapine and olanzapine are effective for depression and anxiety in schizophrenia, as demonstrated by animal model studies, this has not been clarified for asenapine. Therefore, we compared the effects of asenapine in the conditioned fear stress model with those of clozapine and olanzapine. Rats were individually fear conditioned using electrical foot shock in a Skinner box. Approximately 24 h later, individual animals were returned to the same Skinner box (without electrical shock) and their freezing behaviour was observed for 5 min. Animals were treated with asenapine, clozapine, olanzapine, the 5-HT1A receptor partial agonist buspirone, or the 5-HT2C receptor antagonist SB242084 at 30 min before freezing behaviour assessment. The 5-HT1A receptor antagonist WAY100635 or the 5-HT2C receptor agonist Ro60-0175 was also used concomitantly with asenapine. The effects of asenapine, clozapine, and olanzapine on serotonin release in the rat hippocampus were also measured using in vivo microdialysis. Asenapine reduced freezing behaviour, while neither clozapine nor olanzapine reduced freezing behaviour. Buspirone and SB242084 also reduced freezing behaviour. The effect of asenapine in reducing freezing behaviour was not altered by the concomitant administration of WAY100635 or Ro60-0175. Both asenapine and clozapine, but not olanzapine, increased serotonin release in the rat hippocampus. Asenapine may have superior therapeutic effect on anxiety symptoms than other agents, although the underlying mechanism of its anxiolytic activity remains unknown.

5-HT7 Receptors Are Not Involved in Neuropeptide Release in Primary Cultured Rat Trigeminal Ganglion Neurons.

Migraine is a common but complex neurological disorder. Its precise mechanisms are not fully understood. Increasing indirect evidence indicates that 5-HT7 receptors may be involved; however, their role remains unknown. Our previous in vivo study showed that selective blockade of 5-HT7 receptors caused decreased serum levels of calcitonin gene-related peptide (CGRP) in the external jugular vein following electrical stimulation of the trigeminal ganglion (TG) in an animal model of migraine. In the present study, we used an in vitro model of cultured TG cells to further investigate whether 5-HT7 receptors are directly responsible for the release of CGRP and substance P from TG neurons. We stimulated rat primary cultured TG neurons with capsaicin or potassium chloride (KCl) to mimic neurogenic inflammation, resulting in release of CGRP and substance P. 5-HT7 receptors were abundantly expressed in TG neurons. Greater than 93% of 5-HT7 receptor-positive neurons co-expressed CGRP and 56% co-expressed substance P. Both the capsaicin- and KCl-induced release of CGRP and substance P were unaffected by pretreatment of cultured TG cells with the selective 5-HT7 receptor agonist AS19 and antagonist SB269970. This study demonstrates for the first time that 5-HT7 receptors are abundantly co-expressed with CGRP and substance P in rat primary TG neurons and suggests that they are not responsible for the release of CGRP and substance P from cultured TG neurons evoked by capsaicin or KCl.

Lateral hypothalamic area orexin-A influence the firing activity of gastric distension-sensitive neurons and gastric motility in rats

The orexins system consists of two G-protein coupled receptors (the orexin-1 and the orexin-2 receptor) and two neuropeptides, orexin-A and orexin-B. Orexin-A is an excitatory neuropeptide that regulates arousal, wakefulness and appetite. Recent studies have shown that orexin-A may promote gastric motility. We aim to explore the effects of orexin-A on the gastric -distension (GD) sensitive neurons and gastric motility in the lateral hypothalamic area (LHA), and the possible regulation by the paraventricular nucleus (PVN). Extracellular single unit discharges were recorded and the gastric motility was monitored by administration of orexin-A into the LHA and electrical stimulation of the PVN. There were GD neurons in the LHA, and administration of orexin-A to the LHA could increase the firing rate of both GD-excitatory (GD-E) and GD-inhibited (GD-I) neurons. The gastric motility was significantly enhanced by injection of orexin-A into the LHA with a dose dependent manner, which could be completely abolished by pre-treatment with orexin-A receptor antagonist SB334867. Electrical stimulation of the PVN could significantly increase the firing rate of GD neurons responsive to orexin-A in the LHA as well as promote gastric motility of rats. However, those effects could be partly blocked by pre-treatment with SB334867 in the LHA. It is suggested that orexin-A plays an important role in promoting gastric motility via LHA. The PVN may be involved in regulation of LHA on gastric motility.