5-HT1A Activation Research Articles

Abstract 55: Chronic Loss of Serotonin Transporter Function Alters Platelet Adhesion and Spreading

Clinical observations indicate that patients taking selective serotonin reuptake inhibitors(SSRIs) have an increased risk of bleeding. However it remains unclear how SSRIs, which inhibit the serotonin transporter(SERT), modulate homeostasis and platelet activation. There are two primary mechanisms by which serotonin(5HT) affects platelet function: activation of the 5HT2A receptor and uptake by SERT. It has also been demonstrated that SERT regulates aIIbB3(GPIIbIIIa) function, the common signaling pathway in platelet activation and crucial for platelet aggregation. We sought to elucidate how the serotonergic system modulates aIIbB3 function by examining platelet spreading and attachment to immobilized fibrinogen. To model altered serotonergic tone, we used mice with a genetic deletion of SERT(SERT KO). As observed with SSRI use, the blood 5HT concentration in SERT KO mice is miniscule. We found that SERT KO mice display increased tail bleed time and thrombin time. Interestingly, SERT KO platelets showed reduced spreading but increased attachment on immobilized fibrinogen. 5HT(100nM) was able to rescue the reduced spreading on immobilized fibrinogen. To determine the mechanism, we examined platelet spreading with kentanserin(5HT2A antagonist) or citalopram(SSRI). We found that acute administration of citalopram did not alter spreading at physiologically relevant levels. Blockade of 5HT2A activation however, caused WT platelets to behave similarly to SERT KO with reduced spreading. Src is a tyrosine kinase that plays a major role in platelet spreading. Using in-cell western, P-Src(416) was decreased in SERT KO mice following spreading on immobilized. After addition of 5HT, P-Src(416) in SERT KO platelets was normalized to WT P-Src levels. Furthermore, treatment with ketanserin caused WT P-Src(416) to decreased to SERT KO P-Src(416) levels. These finding suggest that the peripheral serotonergic system is “recalibrated” in the absence of 5HT and leads to altered function of aIIbB3 via 5HT2A. From this data, we have elucidated a novel regulatory mechanism of 5HT for platelet fibrinogen binding that could regulate the recruitment of platelets by fibrinogen binding to a growing thrombus.

A physiologic role for serotonergic transmission in adult rat taste buds.

Of the multiple neurotransmitters and neuropeptides expressed in the mammalian taste bud, serotonin remains both the most studied and least understood. Serotonin is expressed in a subset of taste receptor cells that form synapses with afferent nerve fibers (type III cells) and was once thought to be essential to neurotransmission (now understood as purinergic). However, the discovery of the 5-HT1A serotonin receptor in a subset of taste receptor cells paracrine to type III cell suggested a role in cell-to-cell communication during the processing of taste information. Functional data describing this role are lacking. Using anatomical and neurophysiological techniques, this study proposes a modulatory role for serotonin during the processing of taste information. Double labeling immunocytochemical and single cell RT-PCR technique experiments documented that 5-HT1A-expressing cells co-expressed markers for type II cells, cells which express T1R or T2R receptors and release ATP. These cells did not co-express type III cells markers. Neurophysiological recordings from the chorda tympani nerve, which innervates anterior taste buds, were performed prior to and during intravenous injection of a 5-HT1A receptor antagonist. These experiments revealed that serotonin facilitates processing of taste information for tastants representing sweet, sour, salty, and bitter taste qualities. On the other hand, injection of ondansetron, a 5-HT3 receptor antagonist, was without effect. Collectively, these data support the hypothesis that serotonin is a crucial element in a finely-tuned feedback loop involving the 5-HT1A receptor, ATP, and purinoceptors. It is hypothesized that serotonin facilitates gustatory signals by regulating the release of ATP through ATP-release channels possibly through phosphatidylinositol 4,5-bisphosphate resynthesis. By doing so, 5-HT1A activation prevents desensitization of post-synaptic purinergic receptors expressed on afferent nerve fibers and enhances the afferent signal. Serotonin may thus play a major modulatory role within peripheral taste in shaping the afferent taste signals prior to their transmission across gustatory nerves.

Potentiation of 5-methoxy-N,N-dimethyltryptamine-induced hyperthermia by harmaline and the involvement of activation of 5-HT1A and 5-HT2A receptors

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) and harmaline are serotonin (5-HT) analogs often abused together, which alters thermoregulation that may indicate the severity of serotonin toxicity. Our recent studies have revealed that co-administration of monoamine oxidase inhibitor harmaline leads to greater and prolonged exposure to 5-HT agonist 5-MeO-DMT that might be influenced by cytochrome P450 2D6 (CYP2D6) status. This study was to define the effects of harmaline and 5-MeO-DMT on thermoregulation in wild-type and CYP2D6-humanized (Tg-CYP2D6) mice, as well as the involvement of 5-HT receptors. Animal core body temperatures were monitored noninvasively in the home cages after implantation of telemetry transmitters and administration of drugs. Harmaline (5 and 15 mg/kg, i.p.) alone was shown to induce hypothermia that was significantly affected by CYP2D6 status. In contrast, higher doses of 5-MeO-DMT (10 and 20 mg/kg) alone caused hyperthermia. Co-administration of harmaline (2, 5 or 15 mg/kg) remarkably potentiated the hyperthermia elicited by 5-MeO-DMT (2 or 10 mg/kg), which might be influenced by CYP2D6 status at certain dose combination. Interestingly, harmaline-induced hypothermia was only attenuated by 5-HT1A receptor antagonist WAY-100635, whereas 5-MeO-DMT- and harmaline-5-MeO-DMT-induced hyperthermia could be suppressed by either WAY-100635 or 5-HT2A receptor antagonists (MDL-100907 and ketanserin). Moreover, stress-induced hyperthermia under home cage conditions was not affected by WAY-100635 but surprisingly attenuated by MDL-100907 and ketanserin. Our results indicate that co-administration of monoamine oxidase inhibitor largely potentiates 5-MeO-DMT-induced hyperthermia that involves the activation of both 5-HT1A and 5-HT2A receptors. These findings shall provide insights into development of anxiolytic drugs and new strategies to relieve the lethal hyperthermia in serotonin toxicity.

Interaction between μ-opioid and 5-HT1A receptors in the regulation of panic-related defensive responses in the rat dorsal periaqueductal grey.

A wealth of evidence indicates that the activation of 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal grey matter (dPAG) inhibits escape, a panic-related defensive behaviour. Results that were previously obtained with the elevated T-maze test of anxiety/panic suggest that 5-HT1A and μ-opioid receptors in this midbrain area work together to regulate this response. To investigate the generality of this finding, we assessed whether the same cooperative mechanism is engaged when escape is evoked by a different aversive stimulus electrical stimulation of the dPAG. Administration of the μ-receptor blocker CTOP into the dPAG did not change the escape threshold, but microinjection of the μ-receptor agonist DAMGO (0.3 and 0.5 nmol) or the 5-HT1A receptor agonist 8-OHDPAT (1.6 nmol) increased this index, indicating a panicolytic-like effect. Pretreatment with CTOP antagonised the anti-escape effect of 8-OHDPAT. Additionally, combined administration of subeffective doses of DAMGO and 8-OHDPAT increased the escape threshold, indicating drug synergism. Therefore, regardless of the aversive nature of the stimulus, μ-opioid and 5-HT1A receptors cooperatively act to regulate escape behaviour. A better comprehension of this mechanism might allow for new therapeutic strategies for panic disorder.

Effects of central activation of serotonin 5-HT2A/2C or dopamine D 2/3 receptors on the acute and repeated effects of clozapine in the conditioned avoidance response test.

Acute administration of clozapine (a gold standard of atypical antipsychotics) disrupts avoidance response in rodents, while repeated administration often causes a tolerance effect. The present study investigated the neuroanatomical basis and receptor mechanisms of acute and repeated effects of clozapine treatment in the conditioned avoidance response test in male Sprague-Dawley rats. 2,5-dimethoxy-4-iodo-amphetamine (DOI, a preferential 5-HT2A/2C agonist) or quinpirole (a preferential dopamine D2/3 agonist) was microinjected into the medial prefrontal cortex (mPFC) or nucleus accumbens shell (NAs), and their effects on the acute and long-term avoidance disruptive effect of clozapine were tested. Intra-mPFC microinjection of quinpirole enhanced the acute avoidance disruptive effect of clozapine (10mg/kg, sc), while DOI microinjections reduced it marginally. Repeated administration of clozapine (10mg/kg, sc) daily for 5days caused a progressive decrease in its inhibition of avoidance responding, indicating tolerance development. Intra-mPFC microinjection of DOI at 25.0 (but not 5.0) μg/side during this period completely abolished the expression of clozapine tolerance. This was indicated by the finding that clozapine-treated rats centrally infused with 25.0μg/side DOI did not show higher levels of avoidance responses than the vehicle-treated rats in the clozapine challenge test. Microinjection of DOI into the mPFC immediately before the challenge test also decreased the expression of clozapine tolerance. Acute behavioral effect of clozapine can be enhanced by activation of the D2/3 receptors in the mPFC. Clozapine tolerance expression relies on the neuroplasticity initiated by its antagonist action against 5-HT2A/2C receptors in the mPFC.

Dorsomedial hypothalamus serotonin 1A receptors mediate a panic-related response in the elevated T-maze.

The dorsomedial hypothalamus (DMH) has long been associated with the regulation of escape, a panic-related defensive response. Previous evidence has shown that the activation of serotonin (5-HT) 1A and 2A receptors impairs escape behavior induced by the electrical stimulation of the same region. In this study we further explore the relationship of the DMH with defense by investigating the effects of 5-HT1A activation on escape behavior generated in male Wistar rats by an ethologically based aversive stimuli, exposure to one of the open arms of the elevated T-maze (ETM). Aside from escape, the ETM also allows the measurement of inhibitory avoidance, a defensive response associated with generalized anxiety disorder. To evaluate locomotor activity, after ETM measurements animals were submitted to an open field. Results showed that intra-DMH administration of the 5-HT1A receptor agonist 8-OH-DPAT inhibited escape expression. Local administration of the 5-HT1A antagonist WAY-100635 by its own was ineffective, but blocked the panicolytic-like effect of 8-OH-DPAT. Chronic (21 days) systemic treatment with imipramine potentiated the anti-escape effect of 8-OH-DPAT. No significant effects of treatment with 8-OH-DPAT or imipramine on avoidance latencies or the number of lines crossed in the open field were found. These results indicate that 5-HT1A receptors within the DMH may play a phasic inhibitory role on ETM escape expression. As previously proposed, facilitation of 5-HT1A-mediated neurotransmission in the DMH may be involved in the mechanism of action of anti-panic compounds.

Activation of serotonin 5-HT2A receptors inhibits high compulsive drinking on schedule-induced polydipsia.

Schedule-induced polydipsia (SIP) is an established model for studying compulsive behaviour in rats. Serotoninergic drugs effectively reduce compulsive drinking on SIP, and high compulsive drinker rats selected by SIP have shown differences in serotoninergic brain activity. However, the specific serotoninergic receptors that modulate compulsive SIP remain unclear. We investigated the functional role of serotonin 5-hydroxytryptamine 2A or C (5-HT2A/C) receptors in compulsive SIP behaviour. Rats were selected for low (LD) versus high drinking (HD) behaviour on SIP. The effects of the systemic administration of the selective serotonin reuptake inhibitor citalopram, selective norepinephrine reuptake inhibitor atomoxetine, serotonin 5-HT2A/C receptor agonist DOI hydrochloride ((±)-2,5-dimethoxy-4-iodoamphetamine), serotonin 5-HT2C receptor antagonist SB242084, serotonin 5-HT2A receptor antagonist ketanserin and M100907 were assessed on SIP. Subsequently, the effects of DOI were tested after the pre-administration of SB242084, ketanserin and M100907 on SIP. Citalopram and DOI reduced compulsive drinking in HD compared with LD rats on SIP. In contrast, SB242084 increased compulsive drinking in HD compared with LD rats on SIP. Atomoxetine, ketanserin and M100907 had no effect on SIP. The reduction in water intake produced by DOI was blocked by ketanserin and M100907, but not by SB242084 administration, in HD rats. These findings highlight the contribution of serotoninergic 5-HT2A/C receptors compared with noradrenergic mechanisms on SIP and reveal the "therapeutic" activation of serotonin 5-HT2A in the inhibition of the compulsive drinking behaviour in HD rats. Thus, it may represent a potentially new marker of vulnerability and provides additional insight for potential treatments on compulsive behaviours in neuropsychiatric populations.

5-HT₂ and 5-HT₇ receptor agonists facilitate plantar stepping in chronic spinal rats through actions on different populations of spinal neurons.

There is considerable evidence from research in neonatal and adult rat and mouse preparations to warrant the conclusion that activation of 5-HT2 and 5-HT1A/7 receptors leads to activation of the spinal cord circuitry for locomotion. These receptors are involved in control of locomotor movements, but it is not clear how they are implicated in the responses to 5-HT agonists observed after spinal cord injury. Here we used agonists that are efficient in promoting locomotor recovery in paraplegic rats, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT) (acting on 5-HT1A/7 receptors) and quipazine (acting on 5-HT2 receptors), to examine this issue. Analysis of intra- and interlimb coordination confirmed that the locomotor performance was significantly improved by either drug, but the data revealed marked differences in their mode of action. Interlimb coordination was significantly better after 8-OHDPAT application, and the activity of the extensor soleus muscle was significantly longer during the stance phase of locomotor movements enhanced by quipazine. Our results show that activation of both receptors facilitates locomotion, but their effects are likely exerted on different populations of spinal neurons. Activation of 5-HT2 receptors facilitates the output stage of the locomotor system, in part by directly activating motoneurons, and also through activation of interneurons of the locomotor central pattern generator (CPG). Activation of 5-HT7/1A receptors facilitates the activity of the locomotor CPG, without direct actions on the output components of the locomotor system, including motoneurons. Although our findings show that the combined use of these two drugs results in production of well-coordinated weight supported locomotion with a reduced need for exteroceptive stimulation, they also indicate that there might be some limitations to the utility of combined treatment. Sensory feedback and some intraspinal circuitry recruited by the drugs can conflict with the locomotor activation.

Pharmacological evidence that 5-HT1A/1B/1D, α2-adrenoceptors and D2-like receptors mediate ergotamine-induced inhibition of the vasopressor sympathetic outflow in pithed rats

The sympathetic nervous system that innervates the peripheral circulation is regulated by several mechanisms/receptors. It has been reported that prejunctional 5-HT1A, 5-HT1B, 5-HT1D, D2-like receptors and α2-adrenoceptors mediate the inhibition of the vasopressor sympathetic outflow in pithed rats. In addition, ergotamine, an antimigraine drug, displays affinity at the above receptors and may explain some of its adverse/therapeutic effects. Thus, the aims of this study were to investigate in pithed rats: (i) whether ergotamine produces inhibition of the vasopressor sympathetic outflow; and (ii) the major receptors involved in this effect. For this purpose, male Wistar pithed rats were pre-treated with gallamine (25mg/kg; i.v.) and desipramine (50µg/kg) and prepared to stimulate the vasopressor sympathetic outflow (T7–T9; 0.03–3Hz) or to receive i.v. bolus of exogenous noradrenaline (0.03–3µg/kg). I.v. continuous infusions of ergotamine (1 and 1.8μg/kgmin) dose-dependently inhibited the vasopressor responses to sympathetic stimulation but not those to exogenous noradrenaline. The sympatho-inhibition elicited by 1.8μg/kgmin ergotamine was (i) unaffected by saline (1ml/kg); (ii) partially antagonised by WAY 100635 (5-HT1A; 30μg/kg) and rauwolscine (α2-adrenoceptor; 300μg/kg), and (iii) dose-dependently blocked by GR 127935 (5-HT1B/1D; 100 and 300μg/kg) or raclopride (D2-like; 300 and 1000μg/kg), The above doses of antagonists did not modify per se the sympathetically-induced vasopressor responses. The above results suggest that ergotamine induces inhibition of the vasopressor sympathetic outflow by activation of prejunctional 5-HT1A, 5-HT1B/1D, α2-adrenoceptors and D2-like receptors in pithed rats.

Drosophila insulin-producing cells are differentially modulated by serotonin and octopamine receptors and affect social behavior.

A set of 14 insulin-producing cells (IPCs) in the Drosophila brain produces three insulin-like peptides (DILP2, 3 and 5). Activity in IPCs and release of DILPs is nutrient dependent and controlled by multiple factors such as fat body-derived proteins, neurotransmitters, and neuropeptides. Two monoamine receptors, the octopamine receptor OAMB and the serotonin receptor 5-HT1A, are expressed by the IPCs. These receptors may act antagonistically on adenylate cyclase. Here we investigate the action of the two receptors on activity in and output from the IPCs. Knockdown of OAMB by targeted RNAi led to elevated Dilp3 transcript levels in the brain, whereas 5-HT1A knockdown resulted in increases of Dilp2 and 5. OAMB-RNAi in IPCs leads to extended survival of starved flies and increased food intake, whereas 5-HT1A-RNAi produces the opposite phenotypes. However, knockdown of either OAMB or 5-HT1A in IPCs both lead to increased resistance to oxidative stress. In assays of carbohydrate levels we found that 5-HT1A knockdown in IPCs resulted in elevated hemolymph glucose, body glycogen and body trehalose levels, while no effects were seen after OAMB knockdown. We also found that manipulations of the two receptors in IPCs affected male aggressive behavior in different ways and 5-HT1A-RNAi reduced courtship latency. Our observations suggest that activation of 5-HT1A and OAMB signaling in IPCs generates differential effects on Dilp transcription, fly physiology, metabolism and social interactions. However the findings do not support an antagonistic action of the two monoamines and their receptors in this particular system.

Abstract 572: Serotonin and Selective Serotonin Reuptake Inhibitors in Platelet Function and RhoA Activation

Platelet hyperactivity leads to thrombotic disease, underlying cardiovascular (CV) pathologies including stroke and myocardial infarction. Clinical data suggest that selective serotonin reuptake inhibitors (SSRIs) have hemostatic effects, signifying a potentially novel mechanism to treat CV disease. SSRIs target the serotonin transporter (SERT), which maintains dense-granule serotonin (5HT) in platelets. Serotonin is also a platelet agonist, acting through 5HT2A receptors, yet the full role of 5HT in platelet physiology is not well understood. We hypothesized that SERT KO mice, mimicking chronic SSRI treatment, would exhibit a bleeding phenotype due to reduced platelet function. Consistent with previous reports, we demonstrated that SERT KO mice have decreased whole blood 5HT. There were comparable concentrations of the 5HT metabolite 5HIAA, suggesting that overall synthesis and metabolism of 5HT is unchanged. We next demonstrate that there is a bleeding phenotype in the SERT KO mice through a significantly increased tail-bleed time and increased thrombin time (TT) in whole blood. Additionally, SERT KO mice have reduced platelet retraction, a platelet function attributed to decreased RhoA activation. To determine if sub-chronic SSRI treatment alters platelet function, we treated both wild type and a hyperactive platelet (KI) mice with citalopram in drinking water for 6 days, one day exceeding the turnover rate of platelet production (15mg/kg/day). Activation of RhoA was examined to determine if reduced serotonylation leads to reduced platelet function. Collectively, these data suggest that genetic or pharmacological inhibition of SERT activity could lead to reduced platelet function. These findings may be due to a decrease in intracellular 5HT, as 5HT is still present for 5HT2A activation as determined by the presence of the metabolite. Future experiments will examine the mechanism of SSRI-mediated platelet activity attenuation, which may include reduced serotonylation of important signaling molecules.

5-HT1A/1B Receptors as Targets for Optimizing Pigmentary Responses in C57BL/6 Mouse Skin to Stress

Stress has been reported to induce alterations of skin pigmentary response. Acute stress is associated with increased turnover of serotonin (5-hydroxytryptamine; 5-HT) whereas chronic stress causes a decrease. 5-HT receptors have been detected in pigment cells, indicating their role in skin pigmentation. To ascertain the precise role of 5-HT in stress-induced pigmentary responses, C57BL/6 mice were subjected to chronic restraint stress and chronic unpredictable mild stress (CRS and CUMS, two models of chronic stress) for 21 days, finally resulting in abnormal pigmentary responses. Subsequently, stressed mice were characterized by the absence of a black pigment in dorsal coat. The down-regulation of tyrosinase (TYR) and tyrosinase-related proteins (TRP1 and TRP2) expression in stressed skin was accompanied by reduced levels of 5-HT and decreased expression of 5-HT receptor (5-HTR) system. In both murine B16F10 melanoma cells and normal human melanocytes (NHMCs), 5-HT had a stimulatory effect on melanin production, dendricity and migration. When treated with 5-HT in cultured hair follicles (HFs), the increased expression of melanogenesis-related genes and the activation of 5-HT1A, 1B and 7 receptors also occurred. The serum obtained from stressed mice showed significantly decreased tyrosinase activity in NHMCs compared to that from nonstressed mice. The decrease in tyrosinase activity was further augmented in the presence of 5-HTR1A, 1B and 7 antagonists, WAY100635, SB216641 and SB269970. In vivo, stressed mice received 5-HT precursor 5-hydroxy-l-tryptophan (5-HTP), a member of the class of selective serotonin reuptake inhibitors (fluoxetine; FX) and 5-HTR1A/1B agonists (8-OH-DPAT/CP94253), finally contributing to the normalization of pigmentary responses. Taken together, these data strongly suggest that the serotoninergic system plays an important role in the regulation of stress-induced depigmentation, which can be mediated by 5-HT1A/1B receptors. 5-HT and 5-HTR1A/1B may constitute novel targets for therapy of skin hypopigmentation disorders, especially those worsened with stress.

Two cases of mild serotonin toxicity via 5-hydroxytryptamine 1A receptor stimulation

We propose the possibility of 5-hydroxytryptamine (5-HT)1A receptor involvement in mild serotonin toxicity. A 64-year-old woman who experienced hallucinations was treated with perospirone (8 mg/day). She also complained of depressed mood and was prescribed paroxetine (10 mg/day). She exhibited finger tremors, sweating, coarse shivering, hyperactive knee jerks, vomiting, diarrhea, tachycardia, and psychomotor agitation. After the discontinuation of paroxetine and perospirone, the symptoms disappeared. Another 81-year-old woman, who experienced delusions, was treated with perospirone (8 mg/day). Depressive symptoms appeared and paroxetine (10 mg/day) was added. She exhibited tachycardia, finger tremors, anxiety, agitation, and hyperactive knee jerks. The symptoms disappeared after the cessation of paroxetine and perospirone. Recently, the effectiveness of coadministrating 5-HT1A agonistic psychotropics with selective serotonin reuptake inhibitors (SSRIs) has been reported, and SSRIs with 5-HT1A agonistic activity have been newly approved in the treatment of depression. Perospirone is a serotonin–dopamine antagonist and agonistic on the 5-HT1A receptors. Animal studies have indicated that mild serotonin excess induces low body temperature through 5-HT1A, whereas severe serotonin excess induces high body temperature through 5-HT2A activation. Therefore, it could be hypothesized that mild serotonin excess induces side effects through 5-HT1A, and severe serotonin excess induces lethal side effects with hyperthermia through 5-HT2A. Serotonin toxicity via a low dose of paroxetine that is coadministered with perospirone, which acts agonistically on the 5-HT1A receptor and antagonistically on the 5-HT2A receptor, clearly indicated 5-HT1A receptor involvement in mild serotonin toxicity. Careful measures should be adopted to avoid serotonin toxicity following the combined use of SSRIs and 5-HT1A agonists.

Lateral/basolateral amygdala serotonin type-2 receptors modulate operant self-administration of a sweetened ethanol solution via inhibition of principal neuron activity

The lateral/basolateral amygdala (BLA) forms an integral part of the neural circuitry controlling innate anxiety and learned fear. More recently, BLA dependent modulation of self-administration behaviors suggests a much broader role in the regulation of reward evaluation. To test this, we employed a self-administration paradigm that procedurally segregates “seeking” (exemplified as lever-press behaviors) from consumption (drinking) directed at a sweetened ethanol solution. Microinjection of the nonselective serotonin type-2 receptor agonist, alpha-methyl-5-hydroxytryptamine (α-m5HT) into the BLA reduced lever pressing behaviors in a dose-dependent fashion. This was associated with a significant reduction in the number of response-bouts expressed during non-reinforced sessions without altering the size of a bout or the rate of responding. Conversely, intra-BLA α-m5HT only modestly effected consumption-related behaviors; the highest dose reduced the total time spent consuming a sweetened ethanol solution but did not inhibit the total number of licks, number of lick bouts, or amount of solution consumed during a session. In vitro neurophysiological characterization of BLA synaptic responses showed that α-m5HT significantly reduced extracellular field potentials. This was blocked by the 5-HT2A/C antagonist ketanserin suggesting that 5-HT2-like receptors mediate the behavioral effect of α-m5HT. During whole-cell patch current-clamp recordings, we subsequently found that α-m5HT increased action potential threshold and hyperpolarized the resting membrane potential of BLA pyramidal neurons. Together, our findings show that the activation of BLA 5-HT2A/C receptors inhibits behaviors related to reward-seeking by suppressing BLA principal neuron activity. These data are consistent with the hypothesis that the BLA modulates reward-related behaviors and provides specific insight into BLA contributions during operant self-administration of a sweetened ethanol solution.

The involvement of 5-HT-like receptors in the regulation of food intake in rainbow trout (Oncorhynchus mykiss)

It is known that in fish the serotonergic system is part of the neural network that controls feeding and that a pharmacologically induced increase in the brain 5-HT inhibits food intake. However, nothing is known about the 5-HT receptors involved in this inhibitory effect. In this study, we investigated the effects of several 5-HT1 and 5-HT2 receptor agonists on food intake in rainbow trout. In the first experiment, fish were injected i.p. or i.c.v. with two 5-HT1B receptor agonists, anpirtoline (2mg/kg, i.p.) and CP93129 (100 and 200μg/kg, i.c.v.). Neither of these treatments significantly altered food intake. In a second set of experiments, different groups of fish were injected i.p. (1mg/kg) or i.c.v. (30μg/kg) with the 5-HT1A receptor agonist 8-OH-DPAT. In both cases, administration of the 5-HT1A receptor agonist inhibited food intake. In a third set of experiments, we explored the effects of different 5-HT2 receptor agonists. Different groups of fish were injected i.p. or i.c.v. with the mixed 5-HT2B/2C agonist m-CPP (5mg/kg, i.p.), 5-HT2C agonist MK212 (60μg/kg, i.c.v.) and 5-HT2B agonist BW723C86 (50 and 100μg/kg, i.c.v.). Administration of the 5-HT2B/2C and 5HT2C receptor agonists significantly inhibited food intake. Administration of the lowest dose of the 5-HT2B receptor agonist did not have any significant effect, while administration of the highest dose induced a significant increase in food intake. Activation of the 5-HT1A-like (food intake inhibition) and 5-HT1B-like (no effect on food intake) receptors in the rainbow trout induced different effects on food intake from those observed in mammals. We conclude that in rainbow trout the anorexigenic actions of 5-HT are probably mediated by activation of 5-HT1A and 5-H2C-like receptors.

Role of serotonin 1A receptors in the median raphe nucleus on the behavioral consequences of forced swim stress

Despite the intense research on the neurobiology of stress, the role of serotonin (5-HT)1A receptors still remains to be elucidated. In the hippocampus, post-synaptic 5-HT1A receptors activation induces anxiolytic effects in animals previously exposed to stressful situations. However, little is known about somatodendritic 5-HT1A receptors in the median raphe nucleus (MRN). Therefore, the aim of this study was to investigate the role of 5-HT1A receptors located in the MRN in rats exposed to forced swim stress. After recovering from surgery, rats were forced to swim for 15 min in a cylinder. Intra-MRN injections of saline, 8-OH-DPAT (3 nmol/0.2 µL) and/or WAY-100635 (0.3 nmol/0.2 µL) were performed immediately before or after pre-exposure or 24 h later (immediately before test). Non-stressed rats received the same treatment 24 h or 10 min before test. Our data showed that 8-OH-DPAT increased latency to display immobility while decreasing time spent immobile in almost all experimental conditions. These effects were not prevented by previous treatment with WAY-100635. No effects of different treatments were described in non-stressed animals. Taken together, our data suggest that in addition to activation of 5-HT1A, 5-HT7 receptors may also be involved in the behavioural consequences of exposure to swim stress.

P.1.g.046 Activation of 5-HT1A and 5-HT2A/2C receptor in the dorsomedial hypothalamus attenuates escape behaviour

P.1.g.046 Activation of 5-HT1A and 5-HT2A/2C receptor in the dorsomedial hypothalamus attenuates escape behaviour

P.1.g.014 Vortioxetine, a novel multimodal antidepressant, modulates GABA and glutamate neurotransmission via serotonergic mechanisms

The role of 5-HT receptors in fluoxetine-induced nociception and antinociception in rats was assessed. Formalin produced a typical pattern of flinching and licking/lifting behaviors. Local peripheral ipsilateral, but not contralateral, pre-treatment with fluoxetine (0.3–3 nmol/paw) increased in a dose-dependent fashion 0.5% formalin-induced nociception. In contrast, intrathecal pretreatment with fluoxetine (0.3–3 nmol/rat) prevented nociception induced by formalin. The peripheral pronociceptive effect of fluoxetine was prevented by the 5-HT2A (ketanserin, 3–10 pmol/paw), 5-HT2B (3-(2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl)-2,4(1H,3H)-quinazolinedione(+) tartrate, RS-127445, 3–10 pmol/paw), 5-HT2C (8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulphonamido) phenyl-5-oxopentyl]1,3,8-triazaspiro[4.5] decane-2,4-dione hydrochloride, RS-102221, 3–10 pmol/paw), 5-HT3 (ondansetron, 3–10 nmol/paw), 5-HT4 ([1-[2-methylsulphonylamino ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate, GR-113808, 3–100 fmol/paw), 5-HT6 (4-iodo-N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]benzene-sulfonamide hydrochloride, SB-258585, 3–10 pmol/paw) and 5-HT7 ((R)-3-(2-(2-(4-methylpiperidin-1-yl) ethyl) pyrrolidine-1-sulfonyl) phenol hydrochloride, SB-269970, 0.3–1 nmol/paw), but not by the 5-HT1A (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate, WAY-100635, 0.3–1 nmol/paw), 5-HT1B/1D (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2’-methyl-4’-(5-methyl-1,2,4-oxadiazol-3-yl)-1,1’-biphenyl-4-carboxamide hydrochloride hydrate, GR-127935, 0.3–1 nmol/paw), 5-HT1B (1’-methyl-5-[[2’-methyl-4’-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydrospiro[furo[2,3-f]indole-3,4’-piperidine hydrochloride, SB-224289, 0.3–1 nmol/paw), 5-HT1D (4-(3-chlorophenyl)-α-(diphenylmethyl)-1-piperazineethanol hydrochloride, BRL-15572, 0.3–1 nmol/paw) nor 5-HT5A ((N-[2-(dimethylamino)ethyl]-N-[[4’-[[(2-phenylethyl)amino]methyl][1,1’-biphenyl]-4-yl]methyl]cyclopentanepropanamide dihydrochloride, SB-699551, 1–3 nmol/paw), receptor antagonists. In marked contrast, the spinal antinociceptive effect of fluoxetine was prevented by the 5-HT1A (WAY-100635, 0.3–1 nmol/rat), 5-HT1B/1D (GR-127935, 0.3–1 nmol/rat), 5-HT1B (SB-224289, 0.3–1 nmol/rat), 5-HT1D (BRL-15572, 0.3–1 nmol/rat) and 5-HT5A (SB-699551, 1–3 nmol/rat), but not by the 5-HT2A (ketanserin, 3–10 pmol/rat), 5-HT2B (RS-127445, 3–10 pmol/rat), 5-HT2C (RS-102221, 3–10 pmol/rat), 5-HT3 (ondansetron, 3–10 nmol/rat), 5-HT4 (GR-113808, 3–100 fmol/rat), 5-HT6 (SB-258585, 3–10 pmol/rat) nor 5-HT7 (SB-269970, 0.3–1 nmol/rat), receptor antagonists. These results suggest that fluoxetine produces nociception at the periphery by activating peripheral 5-HT2A/2B/2C/3/4/6/7 receptors. In addition, intrathecal fluoxetine produces antinociception by activation of spinal 5-HT1A/1B/1D/5A receptors.

Analysis of the mechanisms underlying the antinociceptive effect of epicatechin in diabetic rats

AimsThe purpose of this study was to investigate the antinociceptive effect of epicatechin as well as the possible mechanisms of action in diabetic rats. Main methodsRats were injected with streptozotocin to produce hyperglycemia. The formalin test was used to assess the nociceptive activity. Key findingsAcute pre-treatment with epicatechin (0.03–30mg/kg, i.p.) prevented formalin-induced nociception in diabetic rats. Furthermore, daily or every other day treatment for 2weeks with epicatechin (0.03–30mg/kg, i.p.) also prevented formalin-induced nociception in diabetic rats. Acute epicatechin-induced antinociception was prevented by l-NAME (Nω-nitro-l-arginine methyl ester hydrochloride, 1–10mg/kg, non-selective nitric oxide synthesis inhibitor), 7-nitroindazole (0.1–1mg/kg, selective neuronal nitric oxide synthesis inhibitor), ODQ (1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one, 0.2–2mg/kg, guanylyl cyclase inhibitor) or glibenclamide (1–10mg/kg, ATP-sensitive K+ channel blocker). Moreover, epicatechin (3mg/kg)-induced antinociception was fully prevented by methiothepin (0.1–1mg/kg, serotonergic receptor antagonist), WAY-100635 (0.03–0.3mg/kg, selective 5-HT1A receptor antagonist) or SB-224289 (0.03–0.3mg/kg, selective 5-HT1B receptor antagonist). In contrast, BRL-15572 (0.03–0.3mg/kg, selective 5-HT1D receptor antagonist) only slightly prevented the antinociceptive effect of epicatechin. Naloxone (0.1–1mg/kg, opioid antagonist) did not modify epicatechin's effect. SignificanceData suggest the involvement of the nitric oxide–cyclic GMP–K+ channel pathway as well as activation of 5-HT1A and 5HT1B, and at a lesser extent, 5-HT1D, but not opioid, receptors in the antinociceptive effect of epicatechin in diabetic rats. Our data suggest that acute or chronic treatment with epicatechin may prove to be effective to treat nociceptive hypersensitivity in diabetic patients.

Improving the Treatment of Schizophrenia: Role of 5-HT Receptors in Modulating Cognitive and Extrapyramidal Motor Functions

Patients with schizophrenia exhibit various clinical symptoms including positive and negative symptoms, neurocognitive impairments and mood disturbances. Although a series of second generation antipsychotics (SGAs) (e.g., risperidone, olanzapine and quetiapine) have been developed in the past two decades, clinical reports do not necessarily show advantages over first generation antipsychotics (FGAs) in the treatment of schizophrenia, especially in their efficacy against cognitive impairment and ability to cause extrapyramidal side effects (EPS). Recently, several lines of studies have revealed therapeutic roles of 5-HT receptors in modulating cognitive impairments and extrapyramidal motor disorders. Specifically, inhibition of 5-HT1A, 5-HT3 and 5-HT6 receptors or activation of 5-HT4 receptors alleviates cognitive impairments (e.g., deficits in learning and memory). In addition, stimulation of 5-HT1A receptors or inhibition of 5-HT3 and 5-HT6 receptors as well as 5-HT2A/2C receptors can ameliorate extrapyramidal motor disorders. Thus, controlling the activity of 5-HT1A, 5-HT3 or 5-HT6 receptors seems to provide benefits by both alleviating cognitive impairments and reducing antipsychotic-induced EPS. This article reviews the functional roles and mechanisms of 5-HT receptors in the treatment of schizophrenia, focusing on the serotonergic modulation of cognitive and extrapyramidal motor functions, and illustrates future therapeutic strategies.