Gamma power abnormalities in a Fmr1-targeted transgenic rat model of fragile X syndrome.

Fragile X syndrome (FXS) is characteristically displayed intellectual disability, hyperactivity, anxiety, and abnormal sensory processing. Electroencephalography (EEG) abnormalities are also observed in subjects with FXS, with many researchers paying attention to these as biomarkers. Despite intensive preclinical research using Fmr1 knock out (KO) mice, an effective treatment for FXS has yet to be developed. Here, we examined Fmr1-targeted transgenic rats (Fmr1-KO rats) as an alternative preclinical model of FXS. We characterized the EEG phenotypes of Fmr1-KO rats by measuring basal EEG power and auditory steady state response (ASSR) to click trains of stimuli at a frequency of 10-80 Hz. Fmr1-KO rats exhibited reduced basal alpha power and enhanced gamma power, and these rats showed enhanced locomotor activity in novel environment. While ASSR clearly peaked at around 40 Hz, both inter-trial coherence (ITC) and event-related spectral perturbation (ERSP) were significantly reduced at the gamma frequency band in Fmr1-KO rats. Fmr1-KO rats showed gamma power abnormalities and behavioral hyperactivity that were consistent with observations reported in mouse models and subjects with FXS. These results suggest that gamma power abnormalities are a translatable biomarker among species and demonstrate the utility of Fmr1-KO rats for investigating drugs for the treatment of FXS.

Auditory Steady State Response; nature and utility as a translational science tool.

The auditory steady-state response (ASSR) has been used to detect auditory processing deficits in patients with psychiatric disorders. However, the methodology of ASSR recording from the brain surface has not been standardized in preclinical studies, limiting its use as a translational biomarker. The sites of maximal ASSR in humans are the vertex and/or middle frontal area, although it has been suggested that the auditory cortex is the source of the ASSR. We constructed and validated novel methods for ASSR recording using a switchable pedestal which allows ASSR recording alternatively from temporal or parietal cortex with a wide range of frequencies in freely moving rats. We further evaluated ASSR as a translational tool by assessing the effect of ketamine. The ASSR measured at parietal cortex did not show clear event-related spectral perturbation (ERSP) or inter-trial coherence (ITC) in any frequency bands or a change with ketamine. In contrast, the ASSR at temporal cortex showed clear ERSP and ITC where 40 Hz was maximal in both gamma-band frequencies. Ketamine exerted a biphasic effect in ERSP at gamma bands. These findings suggest that temporal cortex recording with a wide frequency range is a robust methodology to detect ASSR, potentially enabling application as a translational biomarker in psychiatric and developmental disorders.

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.