Dye coupling between dorsal raphe neurones

Spike avalanches exhibit universal dynamics across the sleep-wake cycle

BACKGROUND

Scale-invariant neuronal avalanches have been observed in cell cultures and slices as well as anesthetized and awake brains, suggesting that the brain operates near criticality, i.e. within a narrow margin between avalanche propagation and extinction. In theory, criticality provides many desirable features for the behaving brain, optimizing computational capabilities, information transmission, sensitivity to sensory stimuli and size of memory repertoires. However, a thorough characterization of neuronal avalanches in freely-behaving (FB) animals is still missing, thus raising doubts about their relevance for brain function.

METHODOLOGY/PRINCIPAL FINDINGS

To address this issue, we employed chronically implanted multielectrode arrays (MEA) to record avalanches of action potentials (spikes) from the cerebral cortex and hippocampus of 14 rats, as they spontaneously traversed the wake-sleep cycle, explored novel objects or were subjected to anesthesia (AN). We then modeled spike avalanches to evaluate the impact of sparse MEA sampling on their statistics. We found that the size distribution of spike avalanches are well fit by lognormal distributions in FB animals, and by truncated power laws in the AN group. FB data surrogation markedly decreases the tail of the distribution, i.e. spike shuffling destroys the largest avalanches. The FB data are also characterized by multiple key features compatible with criticality in the temporal domain, such as 1/f spectra and long-term correlations as measured by detrended fluctuation analysis. These signatures are very stable across waking, slow-wave sleep and rapid-eye-movement sleep, but collapse during anesthesia. Likewise, waiting time distributions obey a single scaling function during all natural behavioral states, but not during anesthesia. Results are equivalent for neuronal ensembles recorded from visual and tactile areas of the cerebral cortex, as well as the hippocampus.

CONCLUSIONS/SIGNIFICANCE

Altogether, the data provide a comprehensive link between behavior and brain criticality, revealing a unique scale-invariant regime of spike avalanches across all major behaviors.

Fluoxetine treatment abolishes the in vitro respiratory response to acidosis in neonatal mice

Background

To secure pH homeostasis, the central respiratory network must permanently adapt its rhythmic motor drive to environment and behaviour. In neonates, it is commonly admitted that the retrotrapezoid/parafacial respiratory group of neurons of the ventral medulla plays the primary role in the respiratory response to acidosis, although the serotonergic system may also contribute to this response.

Methodology/Principal Findings

Using en bloc medullary preparations from neonatal mice, we have shown for the first time that the respiratory response to acidosis is abolished after pre-treatment with the serotonin-transporter blocker fluoxetine (25–50 µM, 20 min), a commonly used antidepressant. Using mRNA in situ hybridization and immunohistology, we have also shown the expression of the serotonin transporter mRNA and serotonin-containing neurons in the vicinity of the RTN/pFRG of neonatal mice.

Conclusions

These results reveal that the serotonergic system plays a pivotal role in pH homeostasis. Although obtained in vitro in neonatal mice, they suggest that drugs targeting the serotonergic system should be used with caution in infants, pregnant women and breastfeeding mothers.

Lateral habenula and hippocampus: a complex interaction raphe cells-mediated

The study has shown an excitatory influence exerted by lateral habenula (LH) on hippocampal pyramidal cells. The modulatory influence is paradoxically serotonine-mediated; in fact all LH stimulation effects were abolished by intrahippocampal iontophoretic methysergide application. The data suggest the involvement of dorsal raphe nucleus. In fact, the dorsal raphe nucleus stimulation caused on hippocampus an expected inhibitory effect antagonized by intrahippocampal iontophoretic methysergide application. In the context of this neural structure we have highlighted a disinhibitory relation between two types of cells: slow serotonergic efferent neurones and fast GABAergic interneurones. The disinhibitory hypothesis is also supported by the following experimental tests performed on both slow and fast raphe cells: a) LH stimulation at low and high frequencies; b) iontophoretic administration of NMDA and GABA; c) LH stimulation during intraraphe iontophoretic injection of 2-APV (NMDA antagonist) and bicuculline (GABA antagonist).