Roles of serotonin 5-HT3 receptor in the formation of dendrites and axons in the rat cerebral cortex: An in vitro study

The roles of serotonin in cell adhesion and migration, and cytoskeletal remodeling

Serotonin is well known as a neurotransmitter. Its roles in neuronal processes such as learning, memory or cognition are well established, and also in disorders such as depression, schizophrenia, bipolar disorder, and dementia. However, its effects on adhesion and cytoskeletal remodelling which are strongly affected by 5-HT receptors, are not as well studied with some exceptions for e.g. platelet aggregation. Neuronal function is strongly dependent on cell-cell contacts and adhesion-related processes. Therefore the role played by serotonin in psychiatric illness, as well as in the positive and negative effects of neuropsychiatric drugs through cell-related adhesion can be of great significance. In this review, we explore the role of serotonin in some of these aspects based on recent findings.

Role of serotonin 4 receptor in the growth of hippocampal neurons during the embryonic development in mice

Serotonin (5-HT) homeostasis is critical for the brain development which influences neurogenesis, neuronal migration, and circuit formation. Distinctive distribution patterns of serotonin receptors (5-HTRs) in the brain govern various physiological activities. Amongst the 5-HTRs, serotonin 4 receptor (5-HT4R) is widely expressed in embryonic forebrain and affects neuronal development, synaptogenesis, and behavior, but its specific role in brain development is still not completely understood. Therefore, in the present study, we addressed the roles of 5-HT4R in the growth of hippocampal neurons during the development of mice brain. We cultured hippocampal neurons of the mouse at embryonic day 18 and then treatment of 5-HT4R agonist RS67333 was employed. We found RS67333 significantly increased the axonal length, diameter and branching along with total dendritic length, number of primary dendrites and their branching. In addition, these effects were neutralized by the concomitant treatment of 5-HT4R antagonist GR125487, which confirmed the specific role of the 5-HT4R in the growth of axon and dendrites. Further, the treatment of RS67333 upregulated the mRNA expression of collapsin response mediator protein-2 (CRMP2) and non-phosphorylated CRMP2 (npCRMP2) together with neurotrophic factors (BDNF, NT-3, NGF) and TRK-A. Additionally, the current research findings reveal that the knockdown of CRMP2 inhibited RS67333-induced growth of the axons and dendrites, which indicates that CRMP2 is required for the 5-HT4R-mediated growth of the axons and dendrites. Overall, the findings of the present in vitro study enrich the understanding and provide insight roles of 5-HT4R in embryonic brain development by promoting the growth of hippocampal neurons.

Roles of the serotonin 5-HT4 receptor in dendrite formation of the rat hippocampal neurons in vitro

Serotonin (5-HT) is involved in various aspects of hippocampal development, although the specific roles of 5-HT receptors are poorly understood. We investigated the roles of 5-HT receptors in the dendrite formation of hippocampal neurons. We focused on the 5-HT4 receptor, which is coupled with Gs protein, and compared the effects with those of the Gi-coupled 5-HT1A receptor. Neurons from rat hippocampi at embryonic day 18 were dissociated and treated for 4 days with the 5-HT4 receptor agonist BIMU8 or the 5-HT1A receptor agonist 8-OH DPAT. The formation of primary dendrites and dendrite branching were promoted by BIMU8, whereas the dendrite branching was inhibited by 8-OH DPAT. BIMU8-induced promotion of dendrite formation was neutralized by concomitant treatment with the 5-HT4 receptor antagonist, confirming the specific actions of the 5-HT4 receptor. We then examined the signaling mechanisms underlying the actions of the 5-HT4 receptor by using a protein kinase A (PKA) inhibitor. The BIMU8-induced promotion of dendrite formation was reversed partially by the PKA inhibitor, suggesting involvement of PKA signaling downstream of the 5-HT4 receptor. Finally, we examined the contribution of brain-derived neurotrophic factor (BDNF) to the promotion of dendrite formation by BIMU8. Quantitative RT-PCR analysis showed that BIMU8 increased the BDNF mRNA expression and that treatment of cultured neurons with the TrkB antagonist reversed the BIMU8-induced increase in dendrite formation. In summary, the present study suggests a novel role for the 5-HT4 receptor in facilitation of dendrite formation in which intracellular signaling of PKA and the BDNF-TrkB system may be involved.

Serotonin 2A receptor regulates microtubule assembly and induces dynamics of dendritic growth cones in rat cortical neurons in vitro

Serotonin (5-HT) regulates the development of cerebral cortex, but 5-HT receptors mediating the effects are poorly understood. We investigated roles of 5-HT2A receptor in dendritic growth cones using dissociation culture of rat cerebral cortex. Neurons at embryonic day 16 were cultured for 4 days and treated with 5-HT2A/2C receptor agonist (DOI) for 4h. DOI increased the size of growth cone periphery which was actin-rich and microtubule-associated protein 2-negative at the dendritic tip. The length increase of the growth cone periphery may be mediated by 5-HT2A receptor, because the 5-HT2A receptor antagonist reversed the effects of DOI. Moreover, the time-lapse analysis demonstrated the increase of morphological dynamics in dendritic growth cones by DOI. Next, to elucidate the mechanisms underlying the actions of 5-HT2A receptor in dendritic growth cones, we examined the cytoskeletal proteins, tyrosinated α-tubulin (Tyr-T; dynamic tubulin) and acetylated α-tubulin (Ace-T; stable tubulin). DOI increased the fluorescence intensity of Tyr-T, while decreased that of Ace-T in the dendritic growth cone periphery. These effects were reversed by the 5-HT2A receptor antagonist, suggesting that 5-HT2A receptor promotes microtubule dynamics. In summary, it was suggested that 5-HT2A receptor induces morphological changes and dynamics of dendritic growth cones through regulation of microtubule assembly.

High serotonin levels during brain development alter the structural input-output connectivity of neural networks in the rat somatosensory layer IV

Homeostatic regulation of serotonin (5-HT) concentration is critical for “normal” topographical organization and development of thalamocortical (TC) afferent circuits. Down-regulation of the serotonin transporter (SERT) and the consequent impaired reuptake of 5-HT at the synapse, results in a reduced terminal branching of developing TC afferents within the primary somatosensory cortex (S1). Despite the presence of multiple genetic models, the effect of high extracellular 5-HT levels on the structure and function of developing intracortical neural networks is far from being understood. Here, using juvenile SERT knockout (SERT^−/−) rats we investigated, in vitro, the effect of increased 5-HT levels on the structural organization of (i) the TC projections of the ventroposteromedial thalamic nucleus toward S1, (ii) the general barrel-field pattern, and (iii) the electrophysiological and morphological properties of the excitatory cell population in layer IV of S1 [spiny stellate (SpSt) and pyramidal cells]. Our results confirmed previous findings that high levels of 5-HT during development lead to a reduction of the topographical precision of TCA projections toward the barrel cortex. Also, the barrel pattern was altered but not abolished in SERT^−/− rats. In layer IV, both excitatory SpSt and pyramidal cells showed a significantly reduced intracolumnar organization of their axonal projections. In addition, the layer IV SpSt cells gave rise to a prominent projection toward the infragranular layer Vb. Our findings point to a structural and functional reorganization of TCAs, as well as early stage intracortical microcircuitry, following the disruption of 5-HT reuptake during critical developmental periods. The increased projection pattern of the layer IV neurons suggests that the intracortical network changes are not limited to the main entry layer IV but may also affect the subsequent stages of the canonical circuits of the barrel cortex.

The serotonin 5-HT3 receptor: a novel neurodevelopmental target

Serotonin (5-hydroxytryptamine, 5-HT), next to being an important neurotransmitter, recently gained attention as a key-regulator of pre- and postnatal development in the mammalian central nervous system (CNS). Several receptors for 5-HT are expressed in the developing brain including a ligand-gated ion channel, the 5-HT3 receptor. Over the past years, evidence has been accumulating that 5-HT3 receptors are involved in the regulation of neurodevelopment by serotonin. Here, we review the spatial and temporal expression patterns of 5-HT3 receptors in the pre- and early postnatal rodent brain and its functional implications. First, 5-HT3 receptors are expressed on GABAergic interneurons in neocortex and limbic structures derived from the caudal ganglionic eminence. Mature inhibitory GABAergic interneurons fine-tune neuronal excitability and thus are crucial for the physiological function of the brain. Second, 5-HT3 receptors are expressed on specific glutamatergic neurons, Cajal-Retzius cells in the cortex and granule cells in the cerebellum, where they regulate morphology, positioning, and connectivity of the local microcircuitry. Taken together, the 5-HT3 receptor emerges as a potential key-regulator of network formation and function in the CNS, which could have a major impact on our understanding of neurodevelopmental disorders in which 5-HT plays a role.

Receptor-dependent regulation of dendrite formation of noradrenaline and dopamine in non-GABAergic cerebral cortical neurons

The present study characterized the receptor-dependent regulation of dendrite formation of noradrenaline (NA) and dopamine (DA) in cultured neurons obtained from embryonic day 16 rat cerebral cortex. Morphological diversity of cortical dendrites was analyzed on various features: dendrite initiation, dendrite outgrowth, and dendrite branching. Using a combination of immunocytochemical markers of dendrites and GABAergic neurons, we focused on the dendrite morphology of non-GABAergic neurons. Our results showed that (1) NA inhibited the dendrite branching, (2) β adrenergic receptor (β-AR) agonist inhibited the dendrite initiation, while promoted the dendrite outgrowth, (3) β1-AR and β2-AR were present in all the cultured neurons, and both agonists inhibited the dendrite initiation, while only β1-AR agonist induced the dendrite branching; (4) DA inhibited the dendrite outgrowth, (5) D1 receptor agonist inhibited the dendrite initiation, while promoted the dendrite branching. In conclusion, this study compared the effects of NA, DA and their receptors and showed that NA and DA regulate different features on the dendrite formation of non-GABAergic cortical neurons, depending on the receptors.

The role of serotonin in axon and dendrite growth

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays multiple roles in the enteric, peripheral, and central nervous systems (CNS). Although its most prominent biological function is as a signal transmission messenger from pre- to postsynaptic neurons, other roles such as shaping brain development and regulating neurite growth have also been described. Here, we review the less well-studied role of 5-HT as a modulator of neurite growth. 5-HT has been shown to regulate neurite growth in multiple systems and species, including in the mammalian CNS. 5-HT predominantly appears to suppress neurite growth, but depending on the model system and 5-HT receptor subtype, in rare cases, it may promote neurite outgrowth and elongation. Failure of axon regeneration in the adult mammalian CNS is a major problem in multiple diseases, and understanding how 5-HT receptors signal opposing effects on neurite growth may lead to novel neuroregenerative therapies, by targeting either 5-HT receptors or their downstream signaling pathways.

Neonatal fluoxetine exposure affects the action potential properties and dendritic development in cortical subplate neurons of rats

Selective serotonin reuptake inhibitor (SSRI)-type antidepressants might be given to depressive pregnant women and the developing fetuses are thus exposed to these drugs. Since serotonin plays important roles in the maturation of the nervous system, early SSRI exposure might influence the fetal brain development. To test this hypothesis, we treated the neonatal rat pups with fluoxetine (Flx) from the day of birth to postnatal day (P) 4, comparable to the third trimester of human gestation, and observed the physiological and morphological features of subplate neurons (SPns), a group of cells important for early cortical development and vulnerable to neonatal neural insults. Using whole-cell patch-clamp recording technique, we examined the passive membrane properties and characteristics of action potential (AP). In SPns of Flx-treated rats, the rheobase for generating an AP was increased and the width of APs was reduced, especially in the falling phase. In the morphological aspect, the dendritic remodeling of SPns including dendritic branching, elongation and pruning were affected by early Flx treatment. Together, our results demonstrate that the teratogenic effect of early SSRI exposure on the structure and function of developing SPns and these changes may lead to undesired brain activity and distorted behaviors later in life.

Differential roles of calcitonin family peptides in the dendrite formation and spinogenesis of the cerebral cortex in vitro

We examined roles of calcitonin family peptides in the initial stages of dendrite formation and the maturation of dendritic spines in the rat cerebral cortex in vitro. Embryonic day 18 cortical neurons were dissociated and cultured for 2-3days in the presence of calcitonin gene-related peptide (CGRP), calcitonin, amylin or adrenomedullin. The treatment of cortical neurons with CGRP promoted the formation of primary dendrites of non-GABAergic neurons. In contrast, the treatment with amylin and adrenomedullin for 3days inhibited the dendritic elongation of non-GABAergic neurons. Calcitonin had no effect on the initial dendrite formation. Next, we examined roles of the peptides in the spine formation. Embryonic day 16 cortical neurons were cultured for 14days and then treated acutely with CGRP, amylin or adrenomedullin for 24h. The density of filopodia, puncta/stubby spines and spines were increased by the CGRP treatment, whereas decreased by amylin. Therefore, CGRP and amylin showed opposite effects on the formation of dendritic filopodia, puncta and spines. Adrenomedullin had no effects on the spine formation. In conclusion, the present study showed that calcitonin family peptides have differential effects both in the dendrite formation during the initial stages and the spine formation of cortical neurons in vitro.

Subtype specific roles of serotonin receptors in the spine formation of cortical neurons in vitro

Dendritic spines are postsynaptic structures which are formed from filopodia. We examined roles of serotonin (5-HT) receptors in the spine formation. Embryonic rat cortical neurons were cultured for 10 or 14 days and treated by 5-HT receptor agonists for 24 h. At 11 days in vitro, 5-HT(1A) agonist increased filopodia density, whereas 5-HT(2A/2C) agonist increased the density of puncta and spines. At 15 days in vitro, 5-HT(1A) agonist decreased the density of puncta and spines, whereas 5-HT(2A/2C) agonist decreased filopodia density with increase of spines. In conclusion, the present study shows 5-HT receptors have subtype-specific effects on the spine formation.