Involvement of serotonin 2C receptor RNA editing in accumbal neuropeptide Y expression and behavioural despair

Serotonin 2C receptors (5‐HT_2CRs) are widely expressed in the central nervous system, and are associated with various neurological disorders. 5‐HT_2CR mRNA undergoes adenosine‐to‐inosine RNA editing at five sites within its coding sequence, resulting in expression of 24 different isoforms. Several edited isoforms show reduced activity, suggesting that RNA editing modulates serotonergic systems in the brain with causative relevance to neuropsychiatric disorders. Transgenic mice solely expressing the non‐edited 5‐HT_2CR INI‐isoform (INI) or the fully edited VGV‐isoform exhibit various phenotypes including metabolic abnormalities, aggressive behaviour, anxiety‐like behaviour, and depression‐like behaviour. Here, we examined the behavioural phenotype and molecular changes of INI mice on a C57BL/6J background. INI mice showed an enhanced behavioural despair in the forced swimming test, elevated sensitivity to the tricyclic antidepressant desipramine, and significantly decreased serotonin in the nucleus accumbens (NAc), amygdala, and striatum. They also showed reduced expression of neuropeptide Y (NPY) mRNA in the NAc. In addition, by stereotactic injection of adeno‐associated virus encoding NPY into the NAc, we demonstrated that accumbal NPY overexpression relieved behavioural despair. Our results suggest that accumbal NPY expression may be regulated by 5‐HT_2CR RNA editing, and its impairment may be linked to mood disorders.     

Brain‐derived neurotrophic factor expression is increased in the hippocampus of 5‐HT2C receptor knockout mice

Several studies have suggested a close interaction between serotonin (5‐HT) and BDNF; however, little is known of the specific relationship between BDNF and the 5‐HT_2C receptor. Therefore, in this study we investigated BDNF expression in 5‐HT_2C receptor knockout mice (5‐HT_2CKO). We also assessed functional consequences of any changes in BDNF using a behavioral test battery. Western blot analysis demonstrated a significant 2.2‐fold increase in the expression of the mature form of BDNF in 5‐HT_2CKO mice when compared with wild‐type controls (WT) in the hippocampus (P = 0.008), but not frontal cortex or striatum. No differences in the expression of the pro‐BDNF isoform were found, and the ratio of mature/pro BDNF was significantly increased in 5‐HT_2CKO (P = 0.003). BDNF mRNA expression in the hippocampus was not different between the genotypes. Hence, increased mature BDNF levels in 5‐HT_2CKO hippocampus are most likely due to increased extracellular cleavage rates of pro‐BDNF to its mature form. Protein expression of the BDNF receptor, tropomycin‐related receptor B (TrkB), was also unchanged in the hippocampus, frontal cortex and striatum. With repeated training in a 10‐day win‐shift radial arm maze task, 5‐HT_2CKO and WT showed similar decreases of the number of working memory and reference memory errors. In addition, no genotype specific differences were observed for passive or active avoidance learning. 5‐HT_2CKO showed modest locomotor hyperactivity but no differences in tests for anxiety, sensorimotor gating, or depressive‐like behaviors; however, in the tail suspension test 5‐HT_2CKO showed significantly reduced climbing (P < 0.05). In conclusion, loss of 5‐HT_2C receptor expression leads to a marked and selective increase in levels of the mature form of BDNF in the hippocampus. Despite this marked increase, 5‐HT_2CKO show only subtle behavioral changes. © 2010 Wiley‐Liss, Inc.     

Serotonin‐2C receptor agonists decrease potassium‐stimulated GABA release in the nucleus accumbens

The serotonin 5‐HT_2C receptor has shown promise in vivo as a pharmacotherapeutic target for alcoholism. For example, recently, a novel 4‐phenyl‐2‐N,N‐dimethylaminotetralin (PAT) drug candidate, that demonstrates 5‐HT_2C receptor agonist activity together with 5‐HT_2A/2B receptor inverse agonist activity, was shown to reduce operant responding for ethanol after peripheral administration to rats. Previous studies have shown that the 5‐HT_2C receptor is found throughout the mesoaccumbens pathway and that 5‐HT_2C receptor agonism causes activation of ventral tegmental area (VTA) GABA neurons. It is unknown what effect 5‐HT_2C receptor modulation has on GABA release in the nucleus accumbens core (NAcc). To this end, microdialysis coupled to capillary electrophoresis with laser‐induced fluorescence was used to quantify extracellular neurotransmitter concentrations in the NAcc under basal and after potassium stimulation conditions, in response to PAT analogs and other 5‐HT_2C receptor modulators administered by reverse dialysis to rats. 5‐HT_2C receptor agonists specifically attenuated stimulated GABA release in the NAcc while 5‐HT_2C antagonists or inverse agonists had no effect. Agents with activity at 5‐HT_2A receptors had no effect on GABA release. Thus, in contrast to results reported for the VTA, current results suggest 5‐HT_2C receptor agonists decrease stimulated GABA release in the NAcc, and provide a possible mechanism of action for 5HT_2C‐mediated negative modulation of ethanol self‐administration. Synapse 69:78–85, 2015. © 2014 Wiley Periodicals, Inc.     

Serotonin2C receptors in the nucleus accumbens are involved in enhanced alcohol-drinking behavior

Dopamine and serotonin (5‐HT) in the nucleus accumbens (ACC) and ventral tegmental area of the mesoaccumbens reward pathways have been implicated in the mechanisms underlying development of alcohol dependence. We used a C57BL/6J mouse model with increased voluntary alcohol‐drinking behavior by exposing the mice to alcohol vapor for 20 consecutive days. In the alcohol‐exposed mice, the expression of 5‐HT_2C receptor mRNA increased in the ACC, caudate nucleus and putamen, dorsal raphe nucleus (DRN), hippocampus and lateral hypothalamus, while the protein level of 5‐HT_2C receptor significantly increased in the ACC. The expression of 5‐HT₇ receptor mRNA increased in the ACC and DRN. Contents of 5‐HT decreased in the ACC shell (ACC_S) and DRN of the alcohol‐exposed mice. The basal extracellular releases of dopamine (DA) and 5‐HT in the ACC_S increased more in the alcohol‐exposed mice than in alcohol‐naïve mice. The magnitude of the alcohol‐induced ACC_S DA and 5‐HT release in the alcohol‐exposed mice was increased compared with the control mice. Intraperitoneal (i.p.) administration or local injection into ACC_S of the 5‐HT_2C receptor antagonist, SB‐242084, suppressed voluntary alcohol‐drinking behavior in the alcohol‐exposed mice. But the i.p. administration of the 5‐HT₇ receptor antagonist, SB‐258719, did not have significant effects on alcohol‐drinking behavior in the alcohol‐exposed mice. The effects of the 5‐HT_2C receptor antagonist were not observed in the air‐exposed control mice. These results suggest that adaptations of the 5‐HT system, especially the upregulation of 5‐HT_2C receptors in the ACC_S, are involved in the development of enhanced voluntary alcohol‐drinking behavior.     

On the role of 5‐HT1A receptor gene in behavioral effect of brain‐derived neurotrophic factor

Experiments were made on a congenic AKR.CBA‐D13Mit76C (76C) mouse strain created by transferring a chromosome 13 fragment containing the 5‐HT_1A receptor gene from a CBA strain to an AKR background. It was shown that 76C mice differed from AKR mice by decreased 5‐HT_1A receptor and tryptophan hydroxylase‐2 (tph‐2) genes expression in the midbrain. Functional activity of 5‐HT_2A receptors and 5‐HT_2A receptor mRNA levels in the midbrain and hippocampus of 76C mice were decreased compared with AKR mice. Central brain‐derived neurotrophic factor (BDNF) administration (300 ng i.c.v.) reduced 5‐HT_1A and 5‐HT_2A receptor mRNA levels in the frontal cortex and tph‐2 mRNA level in the midbrain of AKR mice. However, BDNF failed to produce any effect on the expression of 5‐HT_1A, 5‐HT_2A, and tph‐2 genes in 76C mice but decreased functional activity of 5‐HT_2A receptors in 76C mice and increased it in AKR mice. BDNF restored social deficiency in 76C mice but produced asocial behavior (aggressive attacks towards young mice) in AKR mice. The data indicate that a small genetic variation altered the response to BDNF and show an important role of 5‐HT_1A receptor gene in the 5‐HT system response to BDNF treatment and in behavioral effects of BDNF. © 2014 Wiley Periodicals, Inc.     

The effects of aging and chronic fluoxetine treatment on circadian rhythms and suprachiasmatic nucleus expression of neuropeptide genes and 5‐HT1B receptors

Age‐related changes in circadian rhythms, including attenuation of photic phase shifts, are associated with changes in the central pacemaker in the suprachiasmatic nucleus (SCN). Aging decreases expression of mRNA for vasoactive intestinal peptide (VIP), a key neuropeptide for rhythm generation and photic phase shifts, and increases expression of serotonin transporters and 5‐HT_1B receptors, whose activation inhibits these phase shifts. Here we describe studies in hamsters showing that aging decreases SCN expression of mRNA for gastrin‐releasing peptide, which also modulates photic phase resetting. Because serotonin innervation trophically supports SCN VIP mRNA expression, and serotonin transporters decrease extracellular serotonin, we predicted that chronic administration of the serotonin‐selective reuptake inhibitor, fluoxetine, would attenuate the age‐related changes in SCN VIP mRNA expression and 5‐HT_1B receptors. In situ hybridization studies showed that fluoxetine treatment does not alter SCN VIP mRNA expression, in either age group, at zeitgeber time (ZT)6 or 13 (ZT12 corresponds to lights off). However, receptor autoradiographic studies showed that fluoxetine prevents the age‐related increase in SCN 5‐HT_1B receptors at ZT6, and decreases SCN 5‐HT_1B receptors in both ages at ZT13. Therefore, aging effects on SCN VIP mRNA and SCN 5‐HT_1B receptors are differentially regulated; the age‐related increase in serotonin transporter sites mediates the latter but not the former. The studies also showed that aging and chronic fluoxetine treatment decrease total daily wheel running without altering the phase of the circadian wheel running rhythm, in contrast to previous reports of phase resetting by acute fluoxetine treatment.     

Differential effects of serotonin (5‐HT)2 receptor‐targeting ligands on locomotor responses to nicotine‐repeated treatment

We verified the hypothesis that serotonin (5‐HT)₂ receptors control the locomotor effects of nicotine (0.4 mg kg⁻¹) in rats by using the 5‐HT_2A receptor antagonist M100907, the preferential 5‐HT_2A receptor agonist DOI, the 5‐HT_2C receptor antagonist SB 242084, and the 5‐HT_2C receptor agonists Ro 60‐0175 and WAY 163909. Repeated pairings of a test environment with nicotine for 5 days, on Day 10 significantly augmented the locomotor activity following nicotine administration. Of the investigated 5‐HT₂ receptor ligands, M100907 (2 mg kg⁻¹) or DOI (1 mg kg⁻¹) administered during the first 5 days in combination with nicotine attenuated or enhanced, respectively, the development of nicotine sensitization. Given acutely on Day 10, M100907 (2 mg kg⁻¹), Ro 60‐0175 (1 mg kg⁻¹), and WAY 163909 (1.5 mg kg⁻¹) decreased the expression of nicotine sensitization. In another set of experiments, where the nicotine challenge test was performed on Day 15 in animals treated repeatedly (Days: 1–5, 10) with nicotine, none of 5‐HT₂ receptor ligands administered during the second withdrawal period (Days: 11–14) to nicotine‐treated rats altered the sensitizing effect of nicotine given on Day 15. Our data indicate that 5‐HT_2A receptors (but not 5‐HT_2C receptors) play a permissive role in the sensitizing effects of nicotine, while stimulation of 5‐HT_2A receptors enhances the development of nicotine sensitization and activation of 5‐HT_2C receptors is essential for the expression of nicotine sensitization. Repeated treatment with the 5‐HT₂ receptor ligands within the second nicotine withdrawal does not inhibit previously established sensitization. Synapse 64:511–519, 2010. © 2010 Wiley‐Liss, Inc.     

5‐HT2A receptor blockade and 5‐HT2C receptor activation interact to reduce cocaine hyperlocomotion and fos protein expression in the caudate‐putamen

Both the 5‐HT_2A receptor (R) antagonist M100907 and the 5‐HT_2CR agonist MK212 attenuate cocaine‐induced dopamine release and hyperlocomotion. This study examined whether these drugs interact to reduce cocaine hyperlocomotion and Fos expression in the striatum and prefrontal cortex. We first determined from dose‐effect functions a low dose of both M100907 and MK212 that failed to alter cocaine (15 mg/kg, i.p.) hyperlocomotion. Subsequently, we examined whether these subthreshold doses given together would attenuate cocaine hyperlocomotion, consistent with a 5‐HT_2A/5‐HT_2CR interaction. Separate groups of rats received two sequential drug injections 5 min apart immediately before a 1‐h locomotion test as follows: (1) saline + saline, (2) saline + cocaine, (3) 0.025 mg/kg M100907 + cocaine, (4) 0.125 mg/kg MK212 + cocaine, or (5) cocktail combination of 0.025 mg/kg M100907 and 0.125 mg/kg MK212 + cocaine. Brains were extracted for Fos immunohistochemistry 90 min after the second injection. We next examined the effects of 0.025 mg/kg M100907 and 0.125 mg/kg MK212, alone and in combination, on spontaneous locomotor activity. While neither drug given alone produced any effects, the M100907/MK212 cocktail attenuated cocaine hyperlocomotion as well as cocaine‐induced Fos expression in the dorsolateral caudate‐putamen (CPu), but had no effect on spontaneous locomotion. The findings suggest that 5‐HT_2ARs and 5‐HT_2CRs interact to attenuate cocaine hyperlocomotion and Fos expression in the CPu, and that the CPu is a potential locus of the interactive effects between these 5‐HT₂R subtypes on behavior. Further research investigating combined 5‐HT_2AR antagonism and 5‐HT_2CR agonism as a treatment for cocaine dependence is warranted. Synapse, 2012. © 2012 Wiley Periodicals, Inc.     

Effect of glial cell line‐derived neurotrophic factor on behavior and key members of the brain serotonin system in mouse strains genetically predisposed to behavioral disorders

The effect of glial cell line‐derived neurotrophic factor (GDNF) on behavior and on the serotonin (5‐HT) system of a mouse strain predisposed to depressive‐like behavior, ASC/Icg (Antidepressant Sensitive Cataleptics), in comparison with the parental “nondepressive” CBA/Lac mice was studied. Within 7 days after acute administration, GDNF (800 ng, i.c.v.) decreased cataleptic immobility but increased depressive‐like behavioral traits in both investigated mouse strains and produced anxiolytic effects in ASC mice. The expression of the gene encoding the key enzyme for 5‐HT biosynthesis in the brain, tryptophan hydroxylase‐2 (Tph‐2), and 5‐HT_1A receptor gene in the midbrain as well as 5‐HT_2A receptor gene in the frontal cortex were increased in GDNF‐treated ASC mice. At the same time, GDNF decreased 5‐HT_1A and 5‐HT_2A receptor gene expression in the hippocampus of ASC mice. GDNF failed to change Tph2, 5‐HT_1A, or 5‐HT_2A receptor mRNA levels in CBA mice as well as 5‐HT transporter gene expression and 5‐HT_1A and 5‐HT_2A receptor functional activity in both investigated mouse strains. The results show 1) a GDNF‐induced increase in the expression of key genes of the brain 5‐HT system, Tph2, 5‐HT_1A, and 5‐HT_2A receptors, and 2) significant genotype‐dependent differences in the 5‐HT system response to GDNF treatment. The data suggest that genetically defined cross‐talk between neurotrophic factors and the brain 5‐HT system underlies the variability in behavioral response to GDNF. © 2013 Wiley Periodicals, Inc.     

Serotonin transporter, 5‐HT1A receptor,and behavior in DBA/2J mice in comparison with four inbred mouse strains

Prepulse inhibition (PPI), the reduction in acoustic startle produced when it is preceded by a weak prepulse stimulus, is impaired in schizophrenic patients. The DBA/2J mouse strain displayed deficient PPI and is therefore suggested as an experimental animal model for the loss of sensorimotor gating in schizophrenia. Brain serotonin (5‐HT) has been implicated in the pathophysiology of several psychiatric disorders, including major depressive disorder and schizophrenia. In the present study, behavior, 5‐HT transporter (5‐HTT) mRNA level, 5‐HT_1A receptor mRNA level, and 5‐HT_1A receptor density in the brain regions were studied in DBA/2J mice in comparison with four inbred mouse strains (CBA/Lac, C57BL/6, BALB/c, and ICR). A decrease in 5‐HTT mRNA level in the midbrain and a reduced density of 5‐HT_1A receptors in the frontal cortex without significant changes in 5‐HT_1A receptor mRNA level in DBA/2J mice were found. It was shown that, along with decreased PPI, DBA/2J mice demonstrated considerably reduced immobility in the tail suspension test and in the forced swim test. No significant interstrain differences in intermale aggression, or in light‐dark box and elevated plus‐maze tests, were found. The results suggested the involvement of decreased 5‐HTT gene expression and 5‐HT_1A receptor density in genetically defined PPI deficiency and showed a lack of any association between PPI deficiency and predisposition to aggressive, anxiety, and depressive‐like behaviors. © 2009 Wiley‐Liss, Inc.     

Interactions of serotonin (5‐HT)2 receptor‐targeting ligands and nicotine: Locomotor activity studies in rats

Male Wistar rats were used to verify the hypothesis that serotonin (5‐HT)_2A or 5‐HT_2C receptors may control the locomotor effects evoked by nicotine (0.4mg/kg). The 5‐HT_2A receptor antagonist (M100,907), the 5‐HT_2A receptor agonist (DOI), the 5‐HT_2C receptor antagonist (SB 242,084), and the 5‐HT_2C receptor agonists (Ro 60‐0175 and WAY 163,909) were used. M100,907 (0.5–2mg/kg) did not alter, while DOI (1 mg/kg) enhanced the nicotine‐induced hyperlocomotion. The effect of DOI was antagonized by M100,907 (1 mg/kg). SB 242,084 (0.25–1 mg/kg) augmented, while Ro 60‐0175 (1 and 3 mg/kg) and WAY 163,909 (1.5 mg/kg) decreased the overall effect of acute nicotine; effects of Ro 60‐0175 and WAY 163,909 were attenuated by SB 242,084 (0.125 mg/kg). In another set of experiments, M100,907 (2 mg/kg) on Day 10 attenuated, while DOI (0.1–1 mg/kg) enhanced the nicotine‐evoked conditioned hyperlocomotion in rats repeatedly (Days 1–5) treated with nicotine in experimental chambers. SB 242,084 (0.125 or 1 mg/kg) did not change, while Ro 60‐0175 (1 mg/kg) or WAY 163,909 (1.5 mg/kg) decreased the expression of nicotine‐induced conditioned hyperactivity. Only DOI (0.3 and 1 mg/kg) and SB 242,084 (1 mg/kg) enhanced the basal locomotion. The present data indicate that 5‐HT_2A receptors are significant for the expression of nicotine‐evoked conditioned hyperactivity. Conversely, 5‐HT_2C receptors play a pivotal role in the acute effects of nicotine. Pharmacological stimulation of 5‐HT_2A receptors enhances the conditioned hyperlocomotion, while activation of 5‐HT_2C receptors decreases both the response to acute nicotine and conditioned hyperactivity. Synapse 63:653–661, 2009. © 2009 Wiley‐Liss, Inc.     

Lack of serotonin 5‐HT2B receptor alters proliferation and network volume of interstitial cells of Cajal in vivo

Background Normal gastrointestinal motility requires intact networks of interstitial cells of Cajal (ICC). Interstitial cells of Cajal numbers are maintained by a balance between cell loss factors and survival/trophic/growth factors. Activation of 5‐HT_2B receptors expressed on ICC increases ICC proliferation in vitro. It is not known whether 5‐HT_2B receptors on ICC are activated in vivo. The aims of this study were to investigate if adult ICC proliferate, whether the proliferation of ICC in vivo is affected by knocking out the 5‐HT_2B receptor, and if alterations in proliferation affect ICC networks. Methods Proliferating ICC were identified by immunoreactivity for Ki67 in both the myenteric and deep muscular plexus regions of the jejunum in mice with a targeted insertion of a neomycin resistance cassette into the second coding exon of the htr2b receptor gene. Key Results Adult ICC do proliferate. The number of proliferating ICC was lower in the myenteric plexus region of Htr2b^?/? compared to Htr2b^+/+ mice. The volume of Kit‐positive ICC was 30% lower in the myenteric plexus region and 40% lower in the deep muscular plexus region in Htr2b^?/? mice where the number of ICC was also reduced. Conclusions & Inferences Interstitial cells of Cajal proliferate in adult mice and activation of 5‐HT_2B receptors results in increased proliferation of ICC in vivo. Furthermore, lack of 5‐HT_2B receptor signaling reduces the density of ICC networks in mature mice. These data suggest that 5‐HT_2B receptor signaling is required for maintenance of ICC networks, adding 5‐HT to the growing number of factors shown to regulate ICC networks.     

The role of dorsomedial and ventrolateral columns of the periaqueductal gray matter and in situ 5‐HT2A and 5‐HT2C serotonergic receptors in post‐ictal antinociception

The periaqueductal gray matter (PAG) consists in a brainstem structure rich in 5‐hydroxytryptamine (5‐HT) inputs related to the modulation of pain. The involvement of each of the serotonergic receptor subtypes found in PAG columns, such as the dorsomedial (dmPAG) and the ventrolateral (vlPAG) columns, regarding post‐ictal antinociception have not been elucidated. The present work investigated the participation of the dmPAG and vlPAG columns in seizure‐induced antinociception. Specifically, we studied the involvement of serotonergic neurotransmission in these columns on antinociceptive responses that follow tonic‐clonic epileptic reactions induced by pentylenetetrazole (PTZ), an ionophore GABA‐mediated Cl^‐ influx antagonist. Microinjections of cobalt chloride (1.0 mM CoCl₂/0.2 µL) into the dmPAG and vlPAG caused an intermittent local synaptic inhibition and decreased post‐ictal antinociception that had been recorded at various time points after seizures. Pretreatments of the dmPAG or the vlPAG columns with the nonselective serotonergic receptors antagonist methysergide (5.0 µg/0.2 µL) or intramesencephalic microinjections of ketanserin (5.0 µg/0.2 µL), a serotonergic antagonist with more affinity to 5‐HT_2A/2C receptors, decreased tonic‐clonic seizure‐induced antinociception. Both dmPAG and vlPAG treatment with either the 5‐HT_2A receptor selective antagonist R‐96544 (10 nM/0.2 µL), or the 5‐HT_2C receptors selective antagonist RS‐102221 (0.15 µg/0.2 µL) also decrease post‐ictal antinociception. These findings suggest that serotonergic neurotransmission, which recruits both 5‐HT_2A and 5‐HT_2C serotonergic receptors in dmPAG and vlPAG columns, plays a critical role in the elaboration of post‐ictal antinociception. Synapse, 68:16–30, 2014 . © 2013 Wiley Periodicals, Inc.     

The serotonergic anxiolytic buspirone attenuates circadian responses to light

Serotonergic drugs modify circadian responses to light, with agonists attenuating and some partial agonists or antagonists potentiating photic phase shifts. The anxiolytic buspirone is a 5‐HT_1A receptor partial agonist. Given that buspirone is used therapeutically to manage generalised anxiety disorder, it would be useful to understand if and how this drug may modify circadian responses to light, not only to help manage side effects, but also to examine its potential use as a chronobiotic. Here we examined behavioral and molecular responses to phase‐shifting light in mice and hamsters treated with buspirone. Phase advances to late subjective night light pulses in hamsters and wildtype mice were significantly attenuated by buspirone. 5‐HT_1A receptor knockout mice exhibited potentiated photic phase shifts when pretreated with buspirone. In wildtype mice, the attenuated phase shifts were accompanied by increased cFos expression in the suprachiasmatic nucleus, whereas potentiated phase shifts in knockouts were accompanied by increased phosphorylation of extracellular signal‐regulated kinase (ERK) and cyclic AMP response element‐binding protein (CREB), and decreased cFos expression. Attenuated photic phase shifts in buspirone‐treated hamsters were accompanied by decreased phosphorylation of ERK and CREB. Chronic buspirone treatment decreased the amplitude of wheel‐running rhythms, lengthened the duration of the active phase and advanced the phase angle of entrainment. Buspirone administration at midday produced non‐photic phase advances in wildtype but not 5‐HT_1A receptor knockout mice. These findings suggest that buspirone affected the circadian system in a manner similar to the 5‐HT_1A/7 agonist (±)‐8‐Hydroxy‐2‐dipropylaminotetralin hydrobromide, primarily through the 5‐HT_1A receptor, and suggest that therapeutic use of buspirone to manage anxiety may impact circadian function.     

5HTR3A‐driven GFP labels immature olfactory sensory neurons

The ionotropic serotonin receptor, 5‐HT₃, is expressed by many developing neurons within the central nervous system. Since the olfactory epithelium continues to generate new olfactory sensory neurons (OSNs) throughout life, we investigated the possibility that 5‐HT₃ is expressed in the adult epithelium. Using a transgenic mouse in which the promoter for the 5‐HT_3a subunit drives expression of green fluorescent protein (GFP), we assessed the expression of this marker in the olfactory epithelium of adult mice. Both the native 5‐HT_3a mRNA and GFP are expressed within globose basal cells of the olfactory and vomeronasal epithelium in adult mice. Whereas the 5‐HT_3a mRNA disappears relatively quickly after final cell division, the GFP label persists for about 5 days, thereby labeling immature OSNs in both the main olfactory system and vomeronasal organ. The GFP‐labeled cells include both proliferative globose basal cells as well as immature OSNs exhibiting the hallmarks of ongoing differentiation including GAP43, PGP9.5, but the absence of olfactory marker protein. Some of the GFP‐labeled OSNs show characteristics of more mature yet still developing OSNs including the presence of cilia extending from the apical knob and expression of NaV1.5, a component of the transduction cascade. These findings suggest that 5‐HT_3a is indicative of a proliferative or developmental state, regardless of age, and that the 5‐HT_3AGFP mice may prove useful for future studies of neurogenesis in the olfactory epithelium. J. Comp. Neurol. 525:1743–1755, 2017. © 2016 Wiley Periodicals, Inc.     

Evaluation in monkey of two candidate PET radioligands, [11C]RX‐1 and [18F]RX‐2, for imaging brain 5‐HT4 receptors

The serotonin subtype‐4 (5‐HT₄) receptor, which is known to be involved physiologically in learning and memory, and pathologically in Alzheimer's disease, anxiety, and other neuropsychiatric disorders—has few radioligands readily available for imaging in vivo. We have previously reported two novel 5‐HT₄ receptor radioligands, namely [methoxy‐¹¹C](1‐butylpiperidin‐4‐yl)methyl 4‐amino‐3‐methoxybenzoate; [¹¹C]RX‐1), and the [¹⁸F]3‐fluoromethoxy analog ([¹⁸F]RX‐2), and in this study we evaluated them by PET in rhesus monkey. Brain scans were performed at baseline, receptor preblock or displacement conditions using SB 207710, a 5‐HT₄ receptor antagonist, on the same day for [¹¹C]RX‐1 and on different days for [¹⁸F]RX‐2. Specific‐to‐nondisplaceable ratio (BP_ND) was measured with the simplified reference tissue model from all baseline scans. To determine specific binding, total distribution volume (V_T) was also measured in some monkeys by radiometabolite‐corrected arterial input function after ex vivo inhibition of esterases from baseline and blocked scans. Both radioligands showed moderate to high peak brain uptake of radioactivity (2–6 SUV). Regional BP_ND values were in the rank order of known 5‐HT₄ receptor distribution with a trend for higher BP_ND values from [¹⁸F]RX‐2. One‐tissue compartmental model provided good fits with well identified V_T values for both radioligands. In the highest 5‐HT₄ receptor density region, striatum, 50–60% of total binding was specific. The V_T in receptor‐poor cerebellum reached stable values by about 60 min for both radioligands indicating little influence of radiometabolites on brain signal. In conclusion, both [¹¹C]RX‐1 and [¹⁸F]RX‐2 showed positive attributes for PET imaging of brain 5‐HT₄ receptors, validating the radioligand design strategy. Synapse 68:613–623, 2014 . © 2014 Wiley Periodicals, Inc.     

Acute and chronic effects of citalopram on 5‐HT1A receptor—Labeling by [18F]MPPF and—Coupling to receptors‐G proteins

Selective serotonin reuptake inhibitors take several weeks to produce their maximal therapeutic antidepressant effect. This delay has been attributed to the gradual desensitization of somatodendritic serotonin 5‐HT_1A autoreceptors. We evaluated adaptive changes of 5‐HT_1A receptors after acute and chronic citalopram challenges in rat. Small animal positron emission tomography trial and quantitative ex vivo autoradiography studies using [¹⁸F]MPPF were employed, as well as in vitro 8‐OH‐DPAT‐stimulated [³⁵S]‐GTPγS binding assay. Additionally, 5‐HT_1A receptor knock‐out mice were used to assess the specificity of [¹⁸F]MPPF. Acute treatment with citalopram did not alter [¹⁸F]MPPF binding in dorsal raphe nucleus (DR), frontal cortex, or hippocampus. The absence of [¹⁸F]MPPF binding in the brain of 5‐HT_1A knock‐out mice demonstrates the specificity of MPPF for 5‐HT_1A receptor brain imaging, but the high affinity of [¹⁸F]MPPF compared to 5‐HT suggests that it would only be displaced by dramatic increases in extracellular 5‐HT. Chronic citalopram did not modify 5‐HT_1A receptor density in any of the brain regions studied. In addition, this treatment did not modify 8‐OH‐DPAT‐stimulated [³⁵S]‐GTPγS binding in DR, although a significant increase was observed in frontal cortex and hippocampus. [¹⁸F]MPPF appears to be an efficient radioligand to quantify specifically 5‐HT_1A receptor density in brain imaging. The delayed therapeutic efficacy of citalopram did not appear to be linked to either a downregulation of 5‐HT_1A receptors or to a 5‐HT_1A receptor‐G protein decoupling process in serotonergic neurons, but to increased functional sensitivity of postsynaptic 5‐HT_1A receptors. Synapse 63:106–116, 2009. ©2008 Wiley‐Liss, Inc.     

Hippocampal serotonin‐1A receptor function in a mouse model of anxiety induced by long‐term voluntary wheel running

We have recently demonstrated that, in C57/Bl6 mice, long‐term voluntary wheel running is anxiogenic, and focal hippocampal irradiation prevents the increase in anxiety‐like behaviors and neurobiological changes in the hippocampus induced by wheel running. Evidence supports a role of hippocampal 5‐HT_1A receptors in anxiety. Therefore, we investigated hippocampal binding and function of 5‐HT_1A receptors in this mouse model of anxiety. Four weeks of voluntary wheel running resulted in hippocampal subregion‐specific changes in 5‐HT_1A receptor binding sites and function, as measured by autoradiography of [³H] 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin binding and agonist‐stimulated binding of [³⁵S]GTPγS to G proteins, respectively. In the dorsal CA1 region, 5‐HT_1A receptor binding and function were not altered by wheel running or irradiation. In the dorsal dentate gyrus and CA2/3 region, 5‐HT_1A receptor function was decreased by not only running but also irradiation. In the ventral pyramidal layer, wheel running resulted in a decrease of 5‐HT_1A receptor function, which was prevented by irradiation. Neither irradiation nor wheel running affected 5‐HT_1A receptors in medial prefrontal cortex or in the dorsal or median raphe nuclei. Our data indicate that downregulation of 5‐HT_1A receptor function in ventral pyramidal layer may play a role in anxiety‐like behavior induced by wheel running. Synapse 67:648–655, 2013. © 2013 Wiley Periodicals, Inc.     

A 5‐HT4‐receptor activation‐induced neural plasticity enhances in vivo reconstructs of enteric nerve circuit insult

Background It was recently reported that some 5‐HT₄‐receptor agonists increased neuronal numbers and length of neurites in enteric neurons developing in vitro from immunoselected neural crest‐derived precursors. We aimed to explore a novel approach in vivo to reconstruct the enteric neural circuitry that mediates a fundamental distal gut reflex. Methods The neural circuit insult was performed in guinea pigs by rectal transection and subsequent end‐to‐end one layer anastomosis. A 5‐HT₄‐receptor agonist, mosapride citrate (10–100 μmol L^?1) (applied for a patent) was applied locally at the anastomotic site. Key Results Mosapride promoted the regeneration of the neural circuit in the impaired myenteric plexus and the recovery of the defecation reflex in the distal gut. Furthermore, mosapride generated neurofilament (NF)‐, 5‐HT₄‐receptor‐ and 5‐bromo‐2′‐deoxyuridine (BrdU)‐positive cells and surprisingly formed neural network in the newly formed granulation tissue at the anastomotic site 2 weeks after enteric nerve circuit insult. Possible neural stem cell markers, anti‐distal less homeobox 2 (DLX2)‐ and p75‐positive and NF‐positive cells increased during the same time period. All actions by mosapride were inhibited by the specific 5‐HT₄‐receptor antagonist, GR113808 (10 μmol L^?1). Conclusions & Inferences These results indicate that activation of enteric neural 5‐HT₄‐receptors promotes reconstruction of an enteric neural circuit leading to the recovery of the defecation reflex in the distal gut, and that this reconstruction involves possibly neural stem cells. These findings indicate that treatment with 5‐HT₄ agonists could be a novel therapy for generating new enteric neurons to rescue aganglionic gut disorders.