Hideho Arita

Abnormality of circadian rhythm accompanied by an increase in frontal cortex serotonin in animal model of autism.

Several clinical reports have indicated that autistic patients often show disturbance of the circadian rhythm, which may be related to dysfunction of the serotonergic system in the brain. Using rats exposed prenatally to valproic acid (VPA) as an animal model of autism, we examined locomotor activity and feeding under a reversed 12-h light/dark cycle, and found disturbance of the circadian rhythm characterized by frequent arousal during the light/sleep phase. In addition, measurement of brain serotonin (5-HT) level using in vivo microdialysis showed that the brain 5-HT level in VPA-exposed rats was significantly higher than that in control rats. These results suggest that a higher brain 5-HT level might be responsible for the irregular sleep/awake rhythm in autism.

Augmented brain 5-HT crosses the blood–brain barrier through the 5-HT transporter in rat

The present study re‐evaluated an existing notion that serotonin (5‐hydroxytryptamine; 5‐HT) could not cross the brain to the circulating blood via the blood–brain barrier (BBB). To elevate brain 5‐HT alone, 5‐hydroxytryptophan (5‐HTP; 30–75 mg/kg) was administrated intravenously to anaesthetized rats that had undergone gastrointestinal and kidney resections along with liver inactivation (organs contributing to increasing blood 5‐HT after 5‐HTP administration). A microdialysis method and HPLC system were used to determine the brain 5‐HT levels in samples collected from the frontal cortex. Blood 5‐HT levels were determined from whole blood, not platelet‐poor plasma, collected from the central vein. We found that blood 5‐HT levels showed a significant augmentation whenever brain 5‐HT levels were significantly elevated after the administration of 5‐HTP in those rats with the abdominal surgical procedures. This elevation was abolished after pretreatment with a selective serotonin reuptake inhibitor (fluoxetine; 10 mg/kg i.v.), although brain 5‐HT levels remained augmented. These results indicate that augmented brain 5‐HT can cross the BBB through the 5‐HT transporter from the brain to the circulating blood.

Generation of hiccup by electrical stimulation in medulla of cats

The present study has revealed that a hiccup-like response (a brief powerful inspiratory activity accompanied by glottic adduction) can be generated by electrical stimulation to a limited area within the medullary reticular formation of the cat. This finding indicates that there is a neuronal network coordinating the hiccup reflex within the lower brain stem.

Activation of the anterior prefrontal cortex and serotonergic system is associated with improvements in mood and EEG changes induced by Zen meditation practice in novices

To gain insight into the neurophysiological mechanisms involved in Zen meditation, we evaluated the effects of focused attention (FA) on breathing movements in the lower abdomen (Tanden) in novices. We investigated hemodynamic changes in the prefrontal cortex (PFC), an attention-related brain region, using 24-channel near-infrared spectroscopy during a 20-minute session of FA on Tanden breathing in 15 healthy volunteers. We found that the level of oxygenated hemoglobin in the anterior PFC was significantly increased during FA on Tanden breathing, accompanied by a reduction in feelings of negative mood compared to before the meditation session. Electroencephalography (EEG) revealed increased alpha band activity and decreased theta band activity during and after FA on Tanden breathing. EEG changes were correlated with a significant increase in whole blood serotonin (5-HT) levels. These results suggest that activation of the anterior PFC and 5-HT system may be responsible for the improvement of negative mood and EEG signal changes observed during FA on Tanden breathing.

Prolonged rhythmic gum chewing suppresses nociceptive response via serotonergic descending inhibitory pathway in humans.

&NA; Serotonergic (5‐HT) neurons are implicated in modulating nociceptive transmission. It is established that 5‐HT neuronal activity is enhanced by rhythmic behaviors such as chewing and locomotion in animals. We thus hypothesized that 5‐HT descending inhibitory pathways may be enhanced by rhythmic behavior of gum chewing in humans. To evaluate this idea, we examined nociceptive flexion reflex (NFR), while a subject chewed gum rhythmically for 20 min. NFR was elicited by electrical stimulation of the sural nerve, and the evoked potential was recorded from the biceps femoris muscle. Visual analogue scale (VAS) was also obtained. To assess 5‐HT activity, we determined 5‐HT levels quantitatively in platelet poor plasma (PPP) and whole blood (WB) using HPLC system. Both NFR area and VAS were significantly decreased at 5 min after the onset of chewing and these reductions persisted until cessation of chewing. There were no significant changes in NFR and VAS while resting without chewing. The PPP 5‐HT level increased significantly just after cessation of chewing and had returned to the pre‐chewing level by 30 min after cessation of chewing. The WB 5‐HT level obtained 30 min after cessation of chewing was significantly greater than the pre‐chewing level. Serotonin transporters have recently been discovered at the blood–brain barrier, suggesting that the rise in blood 5‐HT may possibly reflect an increase in 5‐HT level within the brain. The present results support our hypothesis that the rhythmic behavior of chewing suppresses nociceptive responses via the 5‐HT descending inhibitory pathway.

Ventral prefrontal cortex and serotonergic system activation during pedaling exercise induces negative mood improvement and increased alpha band in EEG.

This study evaluates a possible involvement of the prefrontal cortex (PFC) and serotonergic (5-HT) system in psychiatric and electroencephalography (EEG) changes during and after pedaling exercise (PE). The subjects performed PE for 15 min using a cycle ergometer. PE rate was kept at 60 rpm, and the work load (93+/-5.4 W) was decided for each subject before the experiment based on a Rating of Perceived Exertion of 12-13 for self-selected exercise intensity. Cerebral oxygenation in the PFC was assessed by concentration changes in oxygenated hemoglobin (oxyHb) using 24-channel near-infrared spectroscopy. We found that PE evoked a significant increase in oxyHb levels in the ventral PFC during PE as compared with that in the dorsal PFC. Subjects had a feeling of reduced negative mood accompanied by a tendency of increased vigor-activity after PE, as assessed by the Profile of Mood States (POMS) questionnaire. Because the ventral PFC is associated with mood state, we hypothesized that the observed mood changes may have been induced by the activation of the ventral PFC. As for EEG changes during and after PE, we found a significant increase in the relative powers of high-frequency alpha bands (10-13 Hz) during and after PE. A significant increase in whole blood 5-HT level was obtained after PE. Because cortical attenuation would be caused by the 5-HT-induced inhibition of the basal forebrain, we hypothesized that the observed EEG changes are linked with the increased blood 5-HT level or an augmentation of the 5-HT system in the brainstem.

Cortical arousal induced by microinjection of orexins into the paraventricular nucleus of the rat

Orexin-A is a neuropeptide which has been suggested to be involved in sleep and arousal mechanisms. Orexin-A, for example, stimulates arousal when administrated intracerebroventricularly to rats. We attempted to identify specific neural sites of orexin-A and orexin-B action. Orexin-A and orexin-B were microinjected into the medial parvocellular subdivision of the paraventricular nucleus (PVN) in anesthetized, spontaneously breathing rats, and cortical arousal and yawning responses were assessed. Cortical arousal responses were monitored with the electrocorticogram (ECoG), and yawning responses were evaluated by monitoring intercostal electromyograms as an index of inspiratory activity and digastric electromyograms as an indicator of mouth opening. We also measured blood pressure and heart rate during yawning responses, since yawning is accompanied by changes in autonomic activity. Microinjection of orexin-A into the PVN elicited an arousal shift in the ECoG to lower voltage and faster rhythms. This cortical arousal response was followed by a single large inspiration with mouth opening, i.e. a yawning response. On the other hand, microinjection of orexin-B into the PVN elicited an arousal shift in the ECoG without yawning responses. These results demonstrate that an orexin receptive site for triggering arousal/yawning responses exists in the PVN, and suggest that the PVN is involved in arousal mechanisms.

Serotonin innervation patterns differ among the various medullary motoneuronal groups involved in upper airway control

The purpose of this study was to test our hypothesis that the serotoninergic system plays a significant role in airway obstruction during sleep, by focusing on patterns of serotoninergic innervation of the medullary motoneurons involved in upper airway control. We used the combined techniques of retrograde labelling of motoneurons with unconjugated cholera toxin B and immunohistochemistry with antiserum against serotonin (5-HT). The retrograde tracers were injected into posterior cricoarytenoid (PCA), cricothyroid (CT), and genioglossal (GG) muscles of the cat. Motoneurons retrogradely labelled from PCA were identified ipsilateral to the injection site in the caudal part of nucleus ambiguus (NA). Serotonin immunoreactive terminals surrounded their somata and proximal dendrites, suggesting a strong influence of serotonin on the PCA-labelled motoneurons. Motoneurons retrogradely labelled from CT were located ipsilaterally in two distinct groups in the rostral NA and in the retrofacial nucleus (RFN). Selective peripheral nerve section revealed that the CT-labelled motoneurons in the NA had axons in the recurrent laryngeal nerve, whereas the other CT-labelled motoneurons in the RFN were innervated through the superior laryngeal nerve. In the RFN, the pattern of 5-HT innervation in relation to the CT-labelled motoneurons was analogous to that observed with the PCA-labelled motoneurons. In the NA, however, 5-HT terminals made few contacts with the CT-labelled motoneurons, although a dense network of 5-HT terminals was present in the surrounding region. In the GG-labelled motoneuron region of the hypoglossal nucleus, 5-HT terminals were apposed to distal dendrites, not to the soma, indicating less effect of serotonin on GG than on PCA activity. The present results demonstrated that the patterns of 5-HT innervation vary according to the type of motoneurons and their projections to the upper airway.

GABAergic inhibition of hiccup-like reflex induced by electrical stimulation in medulla of cats.

We hypothesize that the hiccup reflex is actively inhibited through GABA(B) receptor within central connections of the hiccup reflex arc. Because the hiccup-like reflex can be elicited by electrical stimulation to a limited area within the medullary reticular formation, the hiccup-evoking site (HES), electrical stimulation (50-100 microA, three train pulses at 20 Hz) was delivered to HES by means of a metal electrode containing 1.0 mM baclofen, in anesthetized spontaneously breathing cats. The evoked response was characterized by a brief powerful increase in diaphragmatic activity and a temporal suppression of the posterior cricoarytenoid muscle, laryngeal dilator, which corresponded to the fixed motor pattern of hiccup reflex. The hiccup-like response was rapidly suppressed after microinjection of baclofen (0.1-0.5 nmol) into HES, indicating that HES has GABA(B) receptors. In the other experiments, to histologically examine the inputs to the hiccup reflex arc, unconjugated cholera toxin subunit B (UCTB) was injected into HES. Following injections of UCTB, retrogradely labelled cells were found distributed in various areas of the lower brainstem. Among these areas, the nucleus raphe magnus (RM) is reported to have GABA-containing cells. It is thus hypothesized that RM is most likely to be the source of the GABAergic inhibitory inputs to the hiccup reflex arc.

Serotonergic cells in nucleus raphe pallidus provide tonic drive to posterior cricoarytenoid motoneurons via 5-hydroxytryptamine2 receptors in cats

Microinjection of serotonin and 5-hydroxytryptamine2 (5-HT2) agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) in nucleus ambiguus caused excitation of posterior cricoarytenoid (PCA) muscles of the larynx in anesthetized, spontaneously breathing cats. Intravenous administration of 5-HT2 antagonist ketanserin produced complete block of excitatory effect of DOM injection. Electrical stimulation of nucleus raphe pallidus caused excitation of PCA activity, that was blocked by pretreatment of ketanserin. These results indicate that serotonergic cells in the raphe pallidus provide tonic drive to medullary PCA motoneurons through 5-HT2 receptors.