5-HT2 Agonist Research Articles

Rimegepant for the treatment of migraine.

Migraine is a common form of primary headache, affecting up to 1 in every 6 Americans. The pathophysiology is an intricate interplay of genetic factors and environmental influence and is still being elucidated in ongoing studies. The trigeminovascular system is now known to have a significant role in the initiation of migraines, including the release of pain mediators such as CGRP and substance P. Traditional treatment of migraine is usually divided into acute and preventive treatment. Acute therapy includes non-specific therapy, such as NSAIDs and other analgesics, which may provide relief in mild to moderate migraines. 5-HT1 agonists may provide relief in severe migraine, but are not universally effective and carry a significant side-effect profile with frequent redosing requirement. Prophylactic therapy may reduce the occurrence of acute migraine attacks in selected patients, but does not completely eliminate it. More recently, CGRP antagonism has been studied and shown to be effective in both abortion and prevention of migraine. Novel medications, targeting CGRP, divide into CGRP antibodies and receptor antagonists (gepants). Rimegepant, a second-generation gepant, has shown efficacy in several clinical trials in treating acute migraine. Ongoing trials are also evaluating its role in migraine prophylaxis, and results are promising. It is also generally safer for use than existing options, does not appear to increase the chance of developing chronic migraines, and carries a very tolerable side effects profile. It is a part of a growing arsenal in migraine treatment, and may present the silver bullet for treatment of this disease.

5-HT7 Receptors Regulate Excitatory-Inhibitory Balance in Mouse Spinal Cord Dorsal Horn.

Serotonergic receptors of the 5-HT7 type (5-HT7Rs) are widely expressed in the central nervous system (CNS), where they modulate several functions, such as pain. Behavioral experiments in vivo have shown both anti- and pro-nociceptive actions of 5-HT7Rs, although an analgesic effect seems to be prevalent. In the spinal cord dorsal horn, the mechanisms involved in 5-HT7R-mediated synaptic modulation are still poorly understood, especially those regarding the control of synaptic inhibition. The present study investigated the modulation exerted by 5-HT7Rs on dorsal horn excitatory and inhibitory synaptic circuits, by performing patch-clamp recordings from lamina II neurons in mouse spinal cord slices. Our results show that applying the selective 5-HT7 agonist LP-211 facilitates glutamatergic release by enhancing the frequency of spontaneous postsynaptic currents (sEPSCs) and increasing the peak amplitude of excitatory postsynaptic currents (EPSCs) evoked by dorsal root stimulation. The effects on sEPSCs were still observed in the presence of the 5-HT1A antagonist WAY-100635, while the 5-HT7 antagonist SB-269970 blocked them. LP-211 was also able to increase the release of gamma-aminobutyric acid (GABA) and glycine, as shown by the increase of spontaneous inhibitory currents (sIPSC) frequency and evoked inhibitory postsynaptic currents (IPSC) amplitude. LP-211 was proved to be more effective in potentiating synaptic inhibition as compared to excitation: consistently, 5-HT7R activation significantly enhanced the excitability of tonic firing neurons, mainly corresponding to inhibitory interneurons. Our data bring new insights into the mechanisms of synaptic modulation mediated by 5-HT7Rs in the dorsal horn. Stronger impact on synaptic inhibition supports the hypothesis that these receptors may play an anti-nociceptive role in the spinal cord of naïve animals.

Metabotropic 5-HT receptor-mediated effects in the human submucous plexus.

Serotonin (5-HT) is an important mediator in the gastrointestinal tract, acting on different neuronal 5-HT receptors. The ionotropic 5-HT3 receptor mediates immediate but transient spike discharge in human enteric neurons. We studied the role of the metabotropic 5-HT1P , 5-HT4 , and 5-HT7 receptors to activate human submucous neurons. Neuroimaging using the voltage sensitive dye Di-8-ANEPPS was performed in submucous plexus preparations from human surgical specimens of the small and large intestine. We synthesized a new, stable 5-HT1P agonist, 5-benzyloxyhydrazonoindalpine (5-BOHIP). 5-HT evoked a fast and late-onset spike discharge in enteric neurons. The fast component was blocked by the 5-HT3 receptor antagonist cilansetron, while the remaining sustained response was significantly reduced by the 5-HT1P receptor antagonist 5-hydroxytryptophanyl-5-hydroxytryptophan amide (5-HTP-DP). The newly synthesized 5-HT1P agonist 5-BOHIP induced a slowly developing, long-lasting activation of submucous neurons, which was blocked by 5-HTP-DP. We could not demonstrate any 5-HT7 receptor-induced spike discharge based on the lack of response to 5-carboxamidotryptamine. Similarly, the 5-HT4 agonists 5-methoxytryptamine and prucalopride evoked no immediate or late-onset spike discharge. Our work demonstrated for the first time the presence of functional 5-HT1P receptors on human submucous neurons. Furthermore, we found no evidence for a role of 5-HT4 or 5-HT7 receptors in the postsynaptic activation of human submucous neurons by 5-HT.

The Effect of the 5-HT4 Agonist, Prucalopride, on a Functional Magnetic Resonance Imaging Faces Task in the Healthy Human Brain.

Depression is a common and often recurrent illness with significant negative impact on a global scale. Current antidepressants are ineffective for up to one third of people with depression, many of whom experience persistent symptomatology. 5-HT4 receptor agonists show promise in both animal models of depression and cognitive deficit. We therefore studied the effect of the 5-HT4 partial agonist prucalopride (1 mg daily for 6 days) on the neural processing of emotional faces in 43 healthy participants using a randomised placebo-controlled design. Participants receiving prucalopride were more accurate at identifying the gender of emotional faces. In whole brain analyses, prucalopride was also associated with reduced activation in a network of regions corresponding to the default mode network. However, there was no evidence that prucalopride treatment produced a positive bias in the neural processing of emotional faces. Our study provides further support for a pro-cognitive effect of 5-HT4 receptor agonism in humans. While our current behavioural and neural investigations do not suggest an antidepressant-like profile of prucalopride in humans, it will be important to study a wider dose range in future studies.

Serotonin-Mediated Activation of Serotonin Receptor Type 1 Oppositely Modulates Voltage-Gated Calcium Channel Currents in Rat Sensory Neurons Innervating Hindlimb Muscle.

Serotonin (5-HT) is a multifaceted neurotransmitter that has been described to play a role as a peripheral inflammatory mediator when released in ischemic or injured muscle. Dorsal root ganglia (DRG) neurons are key sensors of noxious stimuli that are released under inflammatory conditions or mechanical stress. Little information is available on the specific 5-HT receptor subtypes expressed in primary afferents that help regulate reflex pressor responses. In the present study, the whole-cell patch-clamp technique was employed to examine the modulation of voltage-gated calcium channel (CaV) 2.2 currents by 5-HT and to identify the 5-HT receptor subtype(s) mediating this response in acutely dissociated rat DRG neurons innervating triceps surae muscle. Our results indicate that exposure of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled DRG neurons to 5-HT can exert three modulatory effects on CaV currents: high inhibition, low inhibition, and enhancement. Both 5-HT-mediated inhibition responses were blocked after pretreatment with pertussis toxin (PTX), indicating that 5-HT receptors are coupled to CaV2.2 via Gα i/o protein subunits. Application of selective serotonin receptor type 1 (5-HT1) agonists revealed that modulation of CaV2.2 currents occurs primarily after 5-HT1A receptor subtype stimulation and minimally from 5-HT1D activation. Finally, the intrathecal administration of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), significantly (P < 0.05) decreased the pressor response induced by intra-arterial administration of lactic acid. This suggests that 5-HT1A receptors are expressed presynaptically on primary afferent neurons innervating triceps surae muscle. Our findings indicate that preferential stimulation of 5-HT1 receptors, expressed on thin fiber muscle afferents, serves to regulate the reflex pressor response to metabolic stimuli. SIGNIFICANCE STATEMENT: The monoamine serotonin (5-HT), released under ischemic conditions, can contribute to the development of inflammation that negatively affects the exercise pressor reflex. The 5-HT receptor subtype and signaling pathway that underlies calcium channel modulation in dorsal root ganglia afferents, innervating hindlimb muscles, are unknown. We show that 5-HT can either block (primarily via serotonin receptor type 1 (5-HT1)A subtypes) or enhance voltage-gated calcium channel (CaV2.2) currents. Our findings suggest 5-HT exhibits receptor subtype selectivity, providing a complexity of cellular responses.

Epithelial 5-HT4 Receptors as a Target for Treating Constipation and Intestinal Inflammation.

Because of their importance in the regulation of gut functions, several therapeutic targets involving serotonin-related proteins have been developed or repurposed to treat motility disorders, including serotonin transporter inhibitors, tryptophan hydroxylase blockers, 5-HT3 antagonists, and 5-HT4 agonists. This chapter focuses on our discovery of 5-HT4 receptors in the epithelial cells of the colon and our efforts to evaluate the effects of stimulating these receptors. 5-HT4 receptors appear to be expressed by all epithelial cells in the mouse colon, based on expression of a reporter gene driven by the 5-HT4 receptor promoter. Application of 5-HT4 agonists to the mucosal surface causes serotonin release from enterochromaffin cells, mucus secretion from goblet cells, and chloride secretion from enterocytes. Luminal administration of 5-HT4 agonists speeds up colonic motility and suppresses distention-induced nociceptive responses. Luminal administration of 5-HT4 agonists also decreases the development of, and improves recovery from, experimental colitis. Recent studies determined that the prokinetic actions of minimally absorbable 5-HT4 agonists are just as effective as absorbable compounds. Collectively, these findings indicate that targeting epithelial receptors with non-absorbable 5-HT4 agonists could offer a safe and effective strategy for treating constipation and colitis.

Evaluation of Antiepileptic activity of Mosapride in Albino wistar rats

Serotonin causes a significant shift in the excitability of neurons and endogenous serotonin and drugs acting on serotonergic receptors play a role in pathogenesis of epilepsy. This study was done to study the effect of Mosapride, a serotonin receptor 5HT4 agonist, in animal models of epilepsy. Albino Wistar rats were divided into 5 groups with six animals in each group. Group 1 was control group, group 2 was standard group and group 3, 4 and 5 received test drug mosapride in low dose (3mg/kg), high dose (6mg/kg) and mosapride plus standard antiepileptic drug respectively. The antiepileptic efficacy was evaluated using Maximal Electroshock Seizure model (MES) and Pentylenetetrazole (PTZ) induced convulsions. Data was analysed using ANOVA followed by post hoc Tukeys test. Mosapride treated animals showed statistically significant decrease (p&lt;0.001) in the duration of flexion, hind limb extension and post ictal depression in MES model which was comparable to phenytoin group. In PTZ model, mosapride alone did not show any significant difference as compared to control group in terms of latency and duration of seizures (p&gt;0.05). The antiepileptic efficacy of mosapride is similar to phenytoin in MES model. However, in PTZ model mosapride did not show any beneficial antiepileptic effect

The Advantages and Disadvantages of Prokinetics

Prokinetics are medications that enhance gastrointestinal contractility; they improve the symptoms of patients with delayed gastrointestinal motility. Prokinetics have conventionally been used to stimulate gastrointestinal propulsion and to treat symptoms correlated with motility problems, including gastroparesis and constipation. 5-Hydroxytryptamine receptor 4 (5-HT4) agonists, such as cisapride, very effectively increased human gastrointestinal tract motility. However, cisapride sometimes induced serious tachyarrhythmia; the drug was thus withdrawn from the market. Thereafter, many prokinetics have been developed to treat delayed gastrointestinal motility. However, some exhibit serious side-effects. Recently, a new, highly selective serotonin receptor agonist, prucalopride, has been introduced; there is as yet no evidence of serious cardiac side- effects. The drug has been approved by the Food and Drug Administration to treat chronic constipation. Thus, recently introduced, highly selective agents appear to show promise as treatments for gastrointestinal dysmotility; there seem to be no serious side-effects.

Gastroparesis.

This review highlights recent research advances regarding the pathophysiology and treatment of gastroparesis. Differences in brain activity have been identified on functional MRI imaging in gastroparesis patients with nausea. Abdominal pain is common in patients with gastroparesis and does not correlate with the severity of gastric emptying delay, though may be associated with depression and anxiety. Autonomic dysfunction may play an important role in the pathophysiology of gastroparesis. There is increasing sentiment that gastroparesis should be considered a part of the same spectrum of gastric neuromuscular disorders. The risk of tardive dyskinesia with metoclopramide has likely been significantly overestimated historically. Endoscopic BoTox injection of the pylorus and gastric electrical stimulation remain controversial treatments for gastroparesis. New, highly selective 5-hydroxytryptamine 4 (5-HT4) agonists appear safe and may be effective in improving symptoms of gastric emptying. Long-term data assessing the use of gastric peroral endoscopic myotomy (G-POEM) for the treatment of refractory gastroparesis suggest durable clinical improvement. Altered central processing and autonomic dysfunction may be important factors in the pathogenesis of gastroparesis. While the risk of tardive dyskinesia appears much lower than historically advertised, there is increasing hope for novel therapeutics with the advent of new 5-HT4 agonists, neurokinin-1 receptor (N1KR) antagonists, and G-POEM.

Roles of 5-HT3 and 5-HT7 receptors in acute pruriceptive processing in mice

The roles of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing have been demonstrated using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the involvement of 5-HT in acute pruriceptive processing has not yet been elucidated in detail. Scratching events induced by chloroquine (CQ) were attenuated by the administration of milnacipran or mirtazapine, and these effects were reversed by a treatment with ondansetron, a 5-HT3 antagonist, or SB26970, a 5-HT7 antagonist. CQ-induced scratching events were also ameliorated by the intrathecal administration of 5-HT, SR572227A and RS56812 (5-HT3 agonists), and LP211 and LP44 (5-HT7 agonists), indicating the modulation of CQ-induced scratching events by 5-HT and noradrenaline. By contrast, histamine-induced scratching events were not markedly affected by the administration of 5-HT and 5-HT7 agonists, whereas 5-HT3 agonists exerted attenuating effects. Similarly, they were not clearly reversed by the administration of the 5-HT7 antagonist, unlike a 5-HT3 antagonist. Therefore, 5-HT is involved in the attenuating effects of milnacipran and mirtazapine on CQ- and histamine-induced scratching events, and 5-HT3 and 5-HT7 receptors play different roles in pruriceptive processing induced by histamine or CQ.

New Developments in Prokinetic Therapy for Gastric Motility Disorders.

Prokinetic agents amplify and coordinate the gastrointestinal muscular contractions to facilitate the transit of intra-luminal content. Following the institution of dietary recommendations, prokinetics are the first medications whose goal is to improve gastric emptying and relieve symptoms of gastroparesis. The recommended use of metoclopramide, the only currently approved medication for gastroparesis in the United States, is for a duration of less than 3 months, due to the risk of reversible or irreversible extrapyramidal tremors. Domperidone, a dopamine D2 receptor antagonist, is available for prescription through the FDA’s program for Expanded Access to Investigational Drugs. Macrolides are used off label and are associated with tachyphylaxis and variable duration of efficacy. Aprepitant relieves some symptoms of gastroparesis. There are newer agents in the pipeline targeting diverse gastric (fundic, antral and pyloric) motor functions, including novel serotonergic 5-HT4 agonists, dopaminergic D2/3 antagonists, neurokinin NK1 antagonists, and ghrelin agonist. Novel targets with potential to improve gastric motor functions include the pylorus, macrophage/inflammatory function, oxidative stress, and neurogenesis. In the current review, we discuss the use of pharmacological approaches with potential to enhance motor functions in the management of gastroparesis.

Differential effects of β-methylphenylethylamine and octopamine on contractile parameters of the rat gastrointestinal tract

This study tested the effects of β-methylphenylethylamine (β-MPEA) and octopamine on contractile parameters of the gastrointestinal tract in rats. We hypothesized that some of their effects result from interactions with trace amine (TA)-associated receptors or serotoninergic 5-hydroxytryptamine (5-HT) receptors. β-MPEA-induced contractions in rat gastric fundus strips under resting tonus conditions, but induced relaxation in preparations that were previously contracted with carbachol. Octopamine relaxed gastric fundus strips maintained at resting tonus or contracted with carbachol. The contractile effect of β-MPEA was reduced by cyproheptadine and methiothepin, antagonists of excitatory 5-HT receptors. The relaxing effect of β-MPEA on gastric fundus was insensitive to pretreatment with N-(3-ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl)benzamide (EPPTB) and tropisetron, antagonists of TA1 and 5-HT4 receptors, respectively. Both EPPTB and tropisetron inhibited the relaxant effects of octopamine on carbachol-contracted preparations. Contrarily, EPPTB did not reduce the relaxant effects of RO5263397 (TA1 agonist) or zacopride (5-HT4 agonist). Octopamine, but not β-MPEA, delayed the gastrointestinal transit of a liquid test meal in awaken rats. In isolated preparations of the small intestine under resting conditions, β-MPEA did not alter the basal tonus, but octopamine relaxed it. Intestinal preparations previously contracted with carbachol relaxed after the addition of octopamine and decreased the magnitude of their spontaneous rhythmic contractions in a tropisetron-dependent manner. Thus, β-MPEA and octopamine exerted pharmacological actions on the rat gastrointestinal tract. The excitatory effects of β-MPEA involved 5-HT receptors. Octopamine inhibited the rat gut contractility through the likely involvement of 5-HT4 and TA receptors. Overall, octopamine effectively inhibited rat gastrointestinal transit.

In vivo blockade of 5HT3 receptors in the infralimbic medial prefrontal cortex enhances fear extinction in a rat model of PTSD.

Objective(s):Treatments that reverse deficits in fear extinction are promising for the management of post-traumatic stress disorder (PTSD). 5-Hydroxytryptamine type 3 (5-HT3) receptor is involved involved in the extinction of fear memories. The present work aims to investigate the role of 5HT3 receptors in the infralimbic part of the medial prefrontal cortex (IL-mPFC) in extinction of conditioned fear in the single prolonged stress (SPS) model of PTSD in rats.Materials and Methods: The effect of SPS administration was evaluated on the freezing behavior in contextual and cued fear conditioning models. After the behavioral tests, levels of 5HT3 transcription in IL-mPFC were also measured in the same animals using the real-time RT-PCR method. To evaluate the possible role of local 5HT3 receptors on fear extinction, conditioned freezing was evaluated in another cohort of animals that received local microinjections of ondansetron (a 5HT3 antagonist) and ondansetron plus a 5HT3 agonist (SR 57227A) after extinction sessions. Results:Our findings showed that exposure to SPS increased the freezing response in both contextual and cued fear models. We also found that SPS is associated with increased expression of 5HT3 receptors in the IL-mPFC region. Ondansetron enhanced the fear of extinction in these animals and the enhancement was blocked by the 5HT3 agonist, SR 57227A.Conclusion:It seems that up-regulation of 5HT3 receptors in IL-mPFC is an important factor in the neurobiology of PTSD and blockade of these receptors could be considered a potential treatment for this condition.

Vascular sympathetic neurotransmission and its serotonergic regulation are modified by chronic fluoxetine treatment

Given the interconnection between depressive and cardiovascular disorders, we investigated whether antidepressant treatment (fluoxetine) modifies the serotonergic influence on rat vascular noradrenergic outflow. Twelve-week-old male Wistar rats received fluoxetine treatment (10 mg/kg/day; p.o.) for 14 days; then, they were pithed and prepared for sympathetic stimulation. Vasopressor responses were obtained by electrical stimulation of the sympathetic outflow (0.1, 0.5, 1, and 5 Hz) or i.v. noradrenaline (NA; 0.01, 0.05, 0.1, and 0.5 μg/kg). In fluoxetine-treated group, the electrical-induced vasoconstrictions were lower compared to non-treated rats. Intravenous infusion of 5-HT (10 μg/kg/min) inhibited the sympathetically-induced vasoconstrictions. Only 5-CT, 8-OH-DPAT and L-694,247 (5-HT1/7, 5-HT1A and 5-HT1D agonists, respectively) mimicked 5-HT-induced inhibition, while α-methyl-5-HT (5-HT2 agonist) increased the vasopressor responses. The inhibitory effect of 5-HT was: a) no modified by SB269970 (5-HT7 antagonist); b) abolished by WAY-100,635 (5-HT1A antagonist) plus LY310762 (5-HT1D antagonist); and c) potentiated by ritanserin (5-HT2A receptor antagonist). The vasoconstrictions induced by exogenous NA were not modified by 5-CT but were increased by α-methyl-5-HT. Our results suggest that fluoxetine treatment decreases NA release at vascular level and changes 5-HT modulation on rat vascular noradrenergic neurotransmission, inducing sympatho-inhibition via prejunctional 5-HT1A/1D receptors, and sympatho-potentiation via pre and/or postjunctional 5-HT2A receptors.

Cardiovascular responses to putative chemoreceptor stimulation of embryonic common snapping turtles (Chelydra serpentina) chronically incubated in hypoxia (10% O2)

Developmental hypoxia has been shown to result in significant changes in cardiovascular development of American alligators and common snapping turtles. These include similar effects on cardiac mass and aspects of cardiovascular function. However, given the distant phylogenetic relationship between crocodilians and chelonians, we hypothesized that snapping turtles would also exhibit differences in the effects of developmental hypoxia on cardiovascular regulation. This hypothesis was based in part on prior studies that documented differences in plasticity of vagal tone on the heart between alligators and snapping turtles incubated in hypoxic conditions. To test this hypothesis, we investigated how 10% O2 exposure over final 80% of incubation altered the heart rate and blood pressure response to two chemical manipulations of the “chemoreflex” in common snapping turtles at 70% and 90% of incubation. NaCN injections produced a dose dependent bradycardia that was mediated by cholinergic receptor stimulation. This reflex was relatively unaffected by hypoxic incubation conditions in snapping turtle embryos. Injections of the 5-HT3 agonist phenylbiguanide (PBG) caused a pronounced bradycardia that decreased in intensity at 90% of incubation in embryos from the normoxic group while the heart rate response was unchanged in the hypoxic group. This differs from the previously reported diminished heart rate response of embryonic alligators incubated in 10% O2, suggesting plasticity in this chemoreflex response differs between the species. Our data also indicate the cardiovascular response is mediated by a secondary cholinergic receptor stimulation however the inability of ganglionic blockade to inhibit the PBG response leaves the location of the receptors antagonized by PBG in question in embryonic snapping turtles. Primarily, our findings refute the hypothesis that hypoxic incubation decreases the “chemoreflex’ response of snapping turtle embryos.

Investigating Opioid‐induced Respiratory Depression and Analgesia Using Larval Zebrafish

Introduction Opioid drugs produce both analgesia and severe respiratory depression which can be lethal. In 2019, over 50,000 North Americans died of an opioid overdose. Our understanding on how to differentiate between these two effects is limited due to a lack of simple animal models. Therefore, we aim to understand respiratory depression and analgesia by opioids using novel zebrafish models, allowing for drug screening. Using these models, we tested a variety of drugs including serotoninergic agonists, glutamatergic modulators, and calcium channel activators, for their ability to reverse opioid-induced respiratory depression. Methods We have established phenotype-based approaches using in-vivo zebrafish models of respiratory depression and analgesia by fentanyl. To quantify respiratory depression, we measured respiratory activity by quantifying mandible movements in 12-14 day post-fertilization larvae. To quantify analgesia, we measured the escape swimming response to nociceptive stimuli such as formalin or allyl isothiocyanate (AITC) combined with fentanyl to induce analgesia. We then introduced various candidate drugs to measure their effects on respiratory depression and analgesia. Results Respiratory rate was normalized to each fish's baseline (baseline=100%). Zebrafish strains were found to be differentially sensitive to opioids, where Tübingen (TU) fish did not respond to fentanyl (1µM, p=0.4240, n=9) and AB fish showed significant respiratory rate depression (p<0.001, n=11). AB and TU crosses did not show respiratory depression (n=7). In AB zebrafish, a dose-dependent decrease in respiratory rate was observed with 1μM and 3µM fentanyl (p=0.01, n=11 and p<0.001, n=27, respectively). The opioid antagonist naloxone (20µM) and selective antagonist CTAP (4μM) reversed respiratory depression by fentanyl (p<0.001, n=9 and p=0.049, n=11, respectively). Respiratory depression by 1μM fentanyl was reversed by the AMPA receptor positive allosteric modulator CX614 (5μM, p<0.001, n=9) but not the serotoninergic 5-HT4 agonist BIMU8 (10μM, n=21). Calcium channel activators nefiracetam (1µM) and FLP-64176 (1µM) reversed respiratory depression by fentanyl (1µM, p=0.005, n=11 and3µM, p<0.001, n=15, respectively). Formalin (0.075%) and AITC (100µM) significantly increased swimming velocity (p=0.005, n=21 and p<0.001, n=23, respectively), which was reduced by fentanyl (4μM, p<0.001, n=12and 6µM, p=0.004, n=14, respectively). FLP-64176 reversed fentanyl analgesia when administered with formalin (p<0.001, n=10). Discussion: Our models show that respiratory depression and analgesia by opioids can be measured in zebrafish larvae. The strain differences point to a potential genetic protection which we are investigating using quantitative real-time polymerase chain reaction (qPCR). Our novel zebrafish models can be used to investigate the effects of opioids, as well as for drug screening to find ways to treat respiratory depression while preserving analgesia.

P.306 An exploration of the pro-cognitive potential of the 5-HT4 agonist, prucalopride, in the healthy human brain

P.306 An exploration of the pro-cognitive potential of the 5-HT4 agonist, prucalopride, in the healthy human brain

Investigation of cardiorespiratory effects of the selective 5-HT4 agonist BIMU-8 in etorphine-immobilised goats (Capra aegagrus hircus) in a randomized, blinded and controlled trial.

Opioid-induced respiratory compromise remains a significant challenge in etorphine-immobilised wildlife. Serotonergic agonists offer a potential avenue for preventing or treating opioid-induced respiratory compromise. We therefore aimed to determine whether the selective 5-hydroxytryptamine receptor 4 (5-HT4) agonist, BIMU-8, reverses opioid-induced respiratory compromise in etorphine-immobilised goats. Seven healthy adult goats were immobilised with etorphine, then treated with BIMU-8 or sterile water 5 minutes later in a randomised, prospective cross-over study. Cardiorespiratory variables were measured at 1-minute intervals from 4 minutes before etorphine to 15 minutes after its administration. Arterial blood gas analyses were also performed before and after etorphine administration and the respective treatments. Intravenous injection of BIMU-8 attenuated etorphine-induced respiratory compromise, as indicated by improvements, compared to baseline and between treatments, in respiratory rate (fR ), peripheral arterial blood oxygen saturation (SpO2 ), partial pressure of arterial oxygen (PaO2 ) and the alveolar-arterial oxygen partial pressure gradient (P(A-a)O2 ). BIMU-8 caused an increase in heart rate and a temporary decrease in arterial blood pressure. Mild movements and slight muscle spasm occurred but BIMU-8 did not reverse immobilisation. Our results indicate that BIMU-8 may be a potential drug candidate for the treatment, or prevention, of etorphine-induced respiratory compromise in immobilised ungulates.

Serotonin enhances depolarizing spontaneous fluctuations, excitability, and ongoing activity in isolated rat DRG neurons via 5-HT4 receptors and cAMP-dependent mechanisms

Ongoing activity in nociceptors, a driver of spontaneous pain, can be generated in dorsal root ganglion neurons in the absence of sensory generator potentials if one or more of three neurophysiological alterations occur – prolonged depolarization of resting membrane potential (RMP), hyperpolarization of action potential (AP) threshold, and/or increased amplitude of depolarizing spontaneous fluctuations of membrane potential (DSFs) to bridge the gap between RMP and AP threshold. Previous work showed that acute, sustained exposure to serotonin (5-HT) hyperpolarized AP threshold and potentiated DSFs, leading to ongoing activity if a separate source of maintained depolarization was present. Cellular signaling pathways that increase DSF amplitude and promote ongoing activity acutely in nociceptors are not known for any neuromodulator. Here, isolated DRG neurons from male rats were used to define the pathway by which low concentrations of 5-HT enhance DSFs, hyperpolarize AP threshold, and promote ongoing activity. A selective 5-HT4 receptor antagonist blocked these 5-HT-induced hyperexcitable effects, while a selective 5-HT4 agonist mimicked the effects of 5-HT. Inhibition of cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), attenuated 5-HT's hyperexcitable effects, but a blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels had no significant effect. 5-HT4-dependent PKA activation was specific to DRG neurons that bind isolectin B4 (a nonpeptidergic nociceptor marker). 5-HT's effects on AP threshold, DSFs, and ongoing activity were mimicked by a cAMP analog. Sustained exposure to 5-HT promotes ongoing activity in nonpeptidergic nociceptors through the Gs-coupled 5-HT4 receptor and downstream cAMP signaling involving both PKA and EPAC.

Serotonin and vasotocin function in territoriality

This ethopharmacological investigation comprised a long-term field study that examined the function of serotonergic and vasotonergic systems in territoriality. Adult territorial and non-territorial (silent) male coquí frogs (Eleutherodactylus coqui) were injected (IP) with either arginine vasotocin (AVT) or one of two serotonin agonists, 5-HT2A/2C selective agonist, (±) DOI – [(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane], or 2) the 5-HT1A selective agonist, 8-OH-DPAT – [(±)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene]. Control groups received saline injections. Each male received two injections. Following the first injection, whether AVT or a 5HT agonist, the male was observed so that behavior could be documented prior to the second injection, which consisted of the other drug class. All frogs were marked, placed back in the exact location as captured, and observed for all behaviors and vocalizations. Territoriality in E. coqui includes several behavioral components: movement into a calling site, presentation of dominant physical displays, emitting advertisement calls, and defense a territory (including the use of physical force and/or aggressive vocalizations). This investigation found that particular territorial behaviors were significantly influenced by 5HT and AVT action. Initiation of advertisement calling is activated by AVT and suppressed by 5HT, calling rate is affected by 5HT activation, presentation of dominant physical displays are activated by AVT and repressed by 5HT activation, and movement associated with activation of territorial behavior is stimulated by AVT. These data suggested that both 5HT and AVT have a profound impact on territoriality and are two fundamental neuroendocrine systems that govern territorial behavior in social systems.