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.     

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.     

Serotonin (5‐HT) regulates neurite outgrowth through 5‐HT1A and 5‐HT7 receptors in cultured hippocampal neurons

Serotonin (5‐HT) production and expression of 5‐HT receptors (5‐HTRs) occur early during prenatal development. Recent evidence suggests that, in addition to its classical role as a neurotransmitter, 5‐HT regulates neuronal connectivity during mammalian development by modulating cell migration and neuronal cytoarchitecture. Given the variety of 5‐HTRs, researchers have had difficulty clarifying the specific role of each receptor subtype in brain development. Signalling mediated by the G‐protein‐coupled 5‐HT_1AR and 5‐HT₇R, however, has been associated with neuronal plasticity. Thus, we hypothesized that 5‐HT promotes neurite outgrowth through 5‐HT_1AR and 5‐HT₇R. The involvement of 5‐HT_1AR and 5‐HT₇R in the morphology of rat hippocampal neurons was evaluated by treating primary cultures at 2 days in vitro with 5‐HT and specific antagonists for 5‐HT_1AR and 5‐HT₇R (WAY‐100635 and SB269970, respectively). The stimulation of hippocampal neurons with 100 nM 5‐HT for 24 hr produced no effect on either the number or the length of primary neurites. Nonetheless, after 5HT₇R was blocked, the addition of 5‐HT increased the number of primary neurites, suggesting that 5HT₇R could inhibit neuritogenesis. In contrast, 5‐HT induced secondary neurite outgrowth, an effect inhibited by 1 μM WAY‐100635 or SB269970. These results suggest that both serotonergic receptors participate in secondary neurite outgrowth. We conclude that 5‐HT_1AR and 5‐HT₇R regulate neuronal morphology in primary hippocampal cultures by promoting secondary neurite outgrowth. © 2014 Wiley Periodicals, Inc.     

Cannabinoid receptor agonists upregulate and enhance serotonin 2A (5‐HT2A) receptor activity via ERK1/2 signaling

Recent behavioral studies suggest that nonselective agonists of cannabinoid receptors may regulate serotonin 2A (5‐HT_2A) receptor neurotransmission. Two cannabinoids receptors are found in brain, CB1 and CB2 receptors, but the molecular mechanism by which cannabinoid receptors would regulate 5‐HT_2A receptor neurotransmission remains unknown. Interestingly, we have recently found that certain cannabinoid receptor agonists can specifically upregulate 5‐HT_2A receptors. Here, we present experimental evidence that rats treated with a nonselective cannabinoid receptor agonist (CP 55,940, 50 µg/kg, 7 days) showed increases in 5‐HT_2A receptor protein levels, 5‐HT_2A receptor mRNA levels, and 5‐HT_2A receptor‐mediated phospholipase C beta (PLCβ) activity in prefrontal cortex (PFCx). Similar effects were found in neuronal cultured cells treated with CP 55,940 but these effects were prevented by selective CB2, but not selective CB1, receptor antagonists. CB2 receptors couple to the extracellular kinase (ERK) signaling pathway by Gα_i/o class of G‐proteins. Noteworthy, GP 1a (selective CB2 receptor agonist) produced a strong upregulation of 5‐HT_2A receptor mRNA and protein, an effect that was prevented by selective CB2 receptor antagonists and by an ERK1/2 inhibitor, PD 198306. In summary, our results identified a strong cannabinoid‐induced upregulation of 5‐HT_2A receptor signaling in rat PFCx. Our cultured cell studies suggest that selective CB2 receptor agonists upregulate 5‐HT_2A receptor signaling by activation of the ERK1/2 signaling pathway. Activity of cortical 5‐HT_2A receptors has been associated with several physiological functions and neuropsychiatric disorders such as stress response, anxiety and depression, and schizophrenia. Therefore, these results may provide a molecular mechanism by which activation of cannabinoid receptors might be relevant to the pathophysiology of some cognitive and mood disorders in humans. Synapse, 2013. © 2012 Wiley Periodicals, Inc.     

Heterogeneous distribution of the serotonin 5‐HT1A receptor mRNA in chemically identified neurons of the mouse rostral brainstem: Implications for the role of serotonin in the regulation of wakefulness and REM sleep

The 5‐HT_1A receptor (5‐HT_1AR) plays a key role in the inhibitory influence of serotonin (5‐HT) on rapid eye movement (REM) sleep in rodents. However, the neuronal networks mediating such influence are mostly unknown, notably in the mouse. This led us to map 5‐HT_1AR mRNA, by in situ hybridization histochemistry (ISHH), and to characterize the neuronal phenotype of 5‐HT_1AR mRNA‐positive neurons by dual ISHH and ISHH combined with immunohistochemistry, throughout the mouse rostral brainstem, a pivotal region for the generation of REM sleep and cortical activation. 5‐HT_1AR mRNA was found in most 5‐HT neurons in the dorsal raphe (DR), the median raphe (MnR), the B9, and the interpeduncular (IP) nuclei. 5‐HT_1AR mRNA‐positive neurons were also identified in individualized clusters of γ‐aminobutyric acid (GABA)ergic neurons in the DR and in neurons of an undetermined phenotype in the MnR. In addition, 1) GABAergic neurons of the ventral portion of Gudden's dorsal tegmental nucleus (DTg), the IP, and the caudal portion of the deep mesencephalic nucleus (DpMe), and 2) glutamatergic neurons scattered in the caudal pontine reticular nucleus (PnC) and densely packed in the internal lateral parabrachial subnucleus (PBil) also expressed 5‐HT_1AR mRNA. In contrast, no specific 5‐HT_1AR‐related ISHH signal was generally detected in brainstem cholinergic and catecholaminergic neurons. These results emphasize the role of 5‐HT_1AR as an autoreceptor and the phenotypical heterogeneity of 5‐HT_1AR‐expressing neurons within the DR and the MnR in the mouse brain. They also provide a neuroanatomical basis for understanding the influence of 5‐HT_1AR on REM sleep and wakefulness. J. Comp. Neurol. 518:2744–2770, 2010. © 2010 Wiley‐Liss, Inc.     

Nigrostriatal upregulation of 5‐HT2A receptors correlates with motor dysfunction in progressive supranuclear palsy

A dysfunction of multiple neurotransmitter systems is assumed as a neurochemical basis of the akinetic‐rigid syndrome of progressive supranuclear palsy (PSP). In vitro studies have produced conflicting results on the serotoninergic system in PSP. We, therefore, studied the binding potential of the serotonin 2A (5‐HT_2A) receptor ligand [ 18 F]altanserin in 8 patients with clinically probable PSP and 13 healthy controls using positron emission tomography. We found an up‐regulation of 5‐HT_2A receptors in the substantia nigra and, to a lower degree, in the striatum, while neocortical 5‐ HT_2A receptor densities showed no changes upon partial‐volume correction. Nigral and striatal receptor changes were significantly correlated with patients' scores of motor dysfunction (UPDRS III, PSP‐rating scale) pointing to a functional relevance of the described findings. © 2009 Movement Disorder Society     

Adaptations in 5‐HT receptor expression and function: Implications for treatment of cognitive impairment in aging

Malfunction of the serotonin system may contribute to memory deficits during aging. We evaluated the 5‐HT₆ antagonist RO4368554 in two models of learning and memory in aged rats. Male rats (18 months) were assigned to two groups of equal cognitive performance. After 2 weeks of 5‐HT₆ antagonist RO4368554 (5 mg/kg, i.p.) treatment, rats showed significant improvement in object recognition and social discrimination compared with rats given chronic vehicle. Brains from these animals were examined for changes in plasticity‐associated proteins Ki‐67 and PCNA. No differences were seen between groups in any of these markers. We also measured mRNA expression of 5‐HT₆, along with 5‐HT_1A, 5‐HT_1B, and tryptophan hydroxylase‐2 mRNAs in 4‐month‐old and 24‐month‐old F344 rats. Decreases in 5‐HT_1B expression were observed in several forebrain regions in the old rats. These results demonstrate that 5‐HT₆ and 5‐HT_1B receptors are potential targets for treatment of age‐related memory disorders. © 2009 Wiley‐Liss, Inc.     

Effect of a hallucinogenic serotonin 5‐HT2A receptor agonist on visually guided,hippocampal‐dependent spatial cognition in C57BL/6J mice

By acting on serotonin 5‐HT_2A receptors (5‐HT_2ARs), serotonergic psychedelic drugs induce perceptual and visual hallucinations by increasing neuronal excitability and altering visual‐evoked neuronal responses. The present study was designed to examine whether the perceptual alterations induced by a serotonergic psychedelic drug would affect the integrity of hippocampal‐dependent, visually guided spatial cognition. phenylalkylamine hallucinogen TCB‐2 is a selective agonist of 5‐HT_2ARs. Mice received TCB‐2 (1.0 mg kg⁻¹, i.p.), and spatial behaviors and hippocampal electrophysiological responses were measured with water maze tasks and in vivo single‐unit recording, respectively. TCB‐2 did not affect visual cue approach behavior in the visible platform water maze, but increased the latency of trained mice to initiate goal‐directed swimming during a probe test in the hidden platform Morris water maze, which could be prevented by 5‐HT_2AR antagonist MDL 11,939. Interestingly, TCB‐2 did not affect the efficiency of the swim path or the proper use of distal visual cues during the probe test. Hippocampal place cell activity is considered to represent spatial and context‐specific episodic memory. Systemic TCB‐2 did not affect previously established place fields of CA1 neurons in mice exploring a familiar environment, or the remapping of place cells when the mice explored a novel environment. However, TCB‐2 impaired the long‐term stability of place fields for the novel environment initially encoded under the influence of TCB‐2, which could be prevented by 5‐HT_2AR antagonist MDL 11,939. Our data indicate that hallucinogenic 5‐HT_2AR agonist delays the initiation of spatial search behavior, but does not impair the use of visual cues to guide goal‐directed spatial behavior. Moreover, activation of 5‐HT_2ARs does not impair the coding and retrieval of spatial information, but impairs the long‐term stability of new formed place fields of CA1 neurons. © 2017 Wiley Periodicals, Inc.     

Chronic fluoxetine treatment accelerates kindling epileptogenesis in mice independently of 5‐HT2A receptors

Patients with epilepsy often have mood disorders, and these are commonly treated with antidepressant drugs. Although these drugs are often successful in mitigating depressive symptoms, how they affect the epileptogenic processes has been little studied. Recent evidence has demonstrated that treatment with selective serotonin reuptake inhibitor (SSRI) antidepressant drugs adversely promotes epileptogenesis, which may be of great concern considering the number of patients exposed to these drugs. This study investigated 5‐HT_2A receptor signaling as a potential mechanism driving the pro‐epileptogenic effects of the prototypical SSRI fluoxetine. Male homozygous 5‐HT_2A receptor knockout mice or wild‐type littermates (n = 9‐14/group) were treated with continuous fluoxetine (10 mg kg^?1d^?1, sc) or vehicle and subjected to electrical kindling of the amygdala. Compared to vehicle, fluoxetine treatment accelerated kindling epileptogenesis (P < .001), but there was no effect of genotype (P = .75), or any treatment x genotype interaction observed (P = .90). Of interest, fluoxetine treatment increased afterdischarge thresholds in both genotypes (P = .007). We conclude that treatment with fluoxetine promotes epileptogenesis in mice, but this effect is not mediated by 5‐HT_2A receptors. This suggests that antidepressants may accelerate the onset of acquired epilepsy in patients who have experienced epileptogenic cerebral insults.     

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.     

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.     

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.     

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.     

Serotonin excites hippocampal CA1 GABAergic interneurons at the stratum radiatum‐stratum lacunosum moleculare border

The hippocampus receives robust serotonergic innervation that is thought to control the excitability of both pyramidal cells and GABAergic interneurons. Previous work has addressed serotonergic regulation of pyramidal cells but considerable gaps remain in our understanding of how serotonin regulates different interneuron subclasses. 5‐HT_2A receptors (5‐HT_2ARs) appear to localize predominantly, if not solely, on interneurons in the hippocampus and have been implicated in the regulation of hippocampal function including mnemonic and novelty recognition processes. Interneurons are functionally diverse. Therefore in the current work, we have used a BAC transgenic mouse line expressing EGFP under the control of the 5‐HT_2AR promoter to identify the interneuron subtype(s) regulated by serotonin via 5‐HT_2ARs. We find that EGFP expression in this mouse identifies a group of interneurons that resides predominantly along the border of the stratum radiatum (SR) and stratum lacunosum moleculare (SLM) of the CA1 region. We then show that these cells are depolarized and excited by serotonin acting through 5‐HT_2ARs and appear to belong predominantly to the perforant pathway‐associated and Schaffer collateral/commissural pathway‐associated subtypes. These results indicate that serotonin interneurons expressing 5‐HT_2ARs are localized primarily along the SR‐SLM border of the CA1 region and represent a newly identified target for serotonin regulation in the hippocampus. © 2016 Wiley Periodicals, Inc.     

The theta‐related firing activity of parvalbumin‐positive neurons in the medial septum‐diagonal band of broca complex and their response to 5‐HT1A Receptor stimulation in a rat model of Parkinson's disease

The parvalbumin (PV)‐positive neurons in the medial septum‐diagonal band of Broca complex (MS‐DB) play an important role in the generation of hippocampal theta rhythm involved in cognitive functions. These neurons in this region express a high density of 5‐HT_1A receptors which regulate the neuronal activity and consequently affect the theta rhythm. In this study, we examined changes in the theta‐related firing activity of PV‐positive neurons in the MS‐DB, their response to 5‐HT_1A receptor stimulation and the corresponding hippocampal theta rhythm, and the density of PV‐positive neurons and their co‐localization with 5‐HT_1A receptors in rats with 6‐hydroxydopamine lesions of the substantia nigra pars compacta (SNc). The lesion of the SNc decreased the rhythmically bursting activity of PV‐positive neurons and the peak frequency of hippocampal theta rhythm. Systemic administration of 5‐HT_1A receptor agonist 8‐OH‐DPAT (0.5–128 µg/kg, i.v.) inhibited the firing rate of PV‐positive neurons and disrupted rhythmically bursting activity of the neurons and the theta rhythm in sham‐operated and the lesioned rats, respectively. The cumulative doses producing inhibition and disruption in the lesioned rats were higher than that of sham‐operated rats. Furthermore, local application of 8‐OH‐DPAT (0.005 μg) in the MS‐DB also inhibited the firing rate of PV‐positive neurons and disrupted their rhythmically bursting activity in sham‐operated rats, while having no effect on PV‐positive neurons in the lesioned rats. The lesion of the SNc decreased the density of PV‐positive neurons in the MS‐DB, and percentage of PV‐positive neurons expressing 5‐HT_1A receptors. These results indicate that the lesion of the SNc leads to suppression of PV‐positive neurons in the MS‐DB and hippocampal theta rhythm. Furthermore, the lesion decreases the response of these neurons to 5‐HT_1A receptor stimulation, which attributes to dysfunction and/or down‐regulation of 5‐HT_1A receptor expression on these neurons. These changes may be involved in cognitive impairments of Parkinson's disease. © 2013 Wiley Periodicals, Inc.     

An immunocapture/scintillation proximity analysis of Gαq/11 activation by native serotonin (5‐HT)2A receptors in rat cortex: Blockade by clozapine and mirtazapine

Though transduction mechanisms recruited by heterologously expressed 5‐HT_2A receptors have been extensively studied, their interaction with specific subtypes of G‐protein remains to be directly evaluated in cerebral tissue. Herein, as shown by an immunocapture/scintillation proximity analysis, 5‐HT, the prototypical 5‐HT_2A agonist, DOI, and Ro60,0175 all enhanced [³⁵S]GTPγS binding to Gαq/11 in rat cortex with pEC₅₀ values of 6.22, 7.24 and 6.35, respectively. No activation of Go or Gs/olf was seen at equivalent concentrations of DOI. Stimulation of Gαq/11 by 5‐HT (30 μM) and DOI (30 μM) was abolished by the selective 5‐HT_2A vs. 5‐HT_2C/5‐HT_2B antagonists, ketanserin (pK_B values of 9.11 and 8.88, respectively) and MDL100,907 (9.82 and 9.68). By contrast, 5‐HT‐induced [³⁵S]GTPγS binding to Gαq/11 was only weakly inhibited by the preferential 5‐HT_2C receptor antagonists, RS102,221 (6.94) and SB242,084 (7.39), and the preferential 5‐HT_2B receptor antagonist, LY266,097 (6.66). The antipsychotic, clozapine, which had marked affinity for 5‐HT_2A receptors, blocked the recruitment of Gαq/11 by 5‐HT and DOI with pK_B values of 8.54 and 8.14, respectively. Its actions were mimicked by the “atypical” antidepressant and 5‐HT_2A receptor antagonist, mirtazapine, which likewise blocked 5‐HT and DOI‐induced Gαq/11 protein activation with pK_B values of 7.90 and 7.76, respectively. In conclusion, by use of an immunocapture/scintillation proximity strategy, this study shows that native 5‐HT_2A receptors in rat frontal cortex specifically recruit Gαq/11 and that this action is blocked by clozapine and mirtazapine. Quantification of 5‐HT_2A receptor‐mediated Gαq/11 activation in frontal cortex should prove instructive in characterizing the actions of diverse classes of psychotropic agent. Synapse 63:95–105, 2009. © 2008 Wiley‐Liss, Inc.     

Increased levels of 5‐HT1A receptor binding in ventral visual pathways in Parkinson's disease

Visual hallucinations are common in advanced Parkinson's disease (PD). The pathophysiology of visual hallucinations may involve enhanced serotonergic neurotransmission. The atypical antipsychotics clozapine and quetiapine, which have affinity for 5‐HT_2A and 5‐HT_1A receptors, are effective against visual hallucinations in PD. 5‐HT_2A receptors are increased in ventral visual pathways in PD patients with visual hallucinations, and we hypothesized that 5‐HT_1A receptors were also involved in visual hallucinations in PD. Autoradiographic binding using [³H]‐WAY‐100,635 and NAN‐190 was performed in brain sections from 6 PD patients with visual hallucinations, 6 PD patients without visual hallucinations, and 5 age‐matched controls. All PD subjects had been treated with L ‐dopa. Brain areas studied were the orbitofrontal, inferolateral temporal, and motor cortices, as well as the striatum, globus pallidus, substantia nigra, and thalamus. 5‐HT_1A‐binding levels were dramatically increased in the ventral visual pathways of all PD patients compared with controls (0 vs 11 and 0 vs 100 nmol/mg, respectively; both P < .05). There was no significant difference in 5‐HT_1A‐binding levels in PD patients with visual hallucinations compared with PD patients without visual hallucinations or with controls in any of the brain areas studied (P > .05). Gross abnormalities in 5‐HT_1A levels in ventral visual areas occurred in all PD patients exposed to L ‐dopa. However, as there was no difference in 5‐HT_1A‐binding levels between hallucinators and nonhallucinators, alterations in 5‐HT_1A receptor levels may not contribute specifically to visual hallucinations in PD. However, the discrete anatomical distribution of rises to the ventral visual areas suggests some role in predisposing to visual hallucinations. © 2012 Movement Disorder Society     

BDNF Val66met and 5‐HTTLPR polymorphisms predict a human in vivo marker for brain serotonin levels

Brain‐derived neurotrophic factor (BDNF) has been implicated in multiple aspects of brain function including regulation of serotonin signaling. The BDNF val66met polymorphism (rs6265) has been linked to aspects of serotonin signaling in humans but its effects are not well understood. To address this, we evaluated whether BDNF val66met was predictive of a putative marker of brain serotonin levels, serotonin 4 receptor (5‐HT₄) binding assessed with [¹¹C]SB207145 positron emission tomography, which has also been associated with the serotonin‐transporter‐linked polymorphic region (5‐HTTLPR) polymorphism. We applied a linear latent variable model (LVM) using regional 5‐HT₄ binding values (neocortex, amygdala, caudate, hippocampus, and putamen) from 68 healthy humans, allowing us to explicitly model brain‐wide and region‐specific genotype effects on 5‐HT₄ binding. Our data supported an LVM wherein BDNF val66met significantly predicted a LV reflecting [¹¹C]SB207145 binding across regions (P = 0.005). BDNF val66met met‐carriers showed 2–9% higher binding relative to val/val homozygotes. In contrast, 5‐HTTLPR did not predict the LV but S‐carriers showed 7% lower neocortical binding relative to LL homozygotes (P = 7.3 × 10^?6). We observed no evidence for genetic interaction. Our findings indicate that BDNF val66met significantly predicts a common regulator of brain [¹¹C]SB207145 binding, which we hypothesize reflects brain serotonin levels. In contrast, our data indicate that 5‐HTTLPR specifically affects 5‐HT₄ binding in the neocortex. These findings implicate serotonin signaling as an important molecular mediator underlying the effects of BDNF val66met and 5‐HTTLPR on behavior and related risk for neuropsychiatric illness in humans. Hum Brain Mapp, 36:313–323, 2015. © 2014 Wiley Periodicals, Inc .