Serotonin regulation of behaviour via large-scale neuromodulation of serotonin receptor networks

Although we understand how serotonin receptors function at the single-cell level, what role different serotonin receptors play in regulating brain-wide activity and, in turn, human behaviour, remains unknown. Here, we developed transcriptomic-neuroimaging mapping to characterise brain-wide functional signatures associated with specific serotonin receptors: serotonin receptor networks (SRNs). Probing SRNs with optogenetics-fMRI and pharmacology in mice, we show that activation of dorsal raphe serotonin neurons differentially modulates the amplitude and functional connectivity of different SRNs, showing that receptors’ spatial distributions can confer specificity not only at the local, but also at the brain-wide, network-level. In humans, using resting state fMRI, different sets of SRNs are linked to different behavioural phenotypes. These results provide compelling evidence that heterogeneous brain-wide distributions of different serotonin receptor types may underpin behaviourally-distinct modes of serotonin regulation. This suggests that dorsal raphe serotonin neurons may regulate multiple aspects of human behaviour via modulation of large-scale receptor networks.

Maturation of prefrontal input to dorsal raphe increases behavioral persistence in mice

The ability to persist towards a desired objective is a fundamental aspect of behavioral control whose impairment is implicated in several behavioral disorders. One of the prominent features of behavioral persistence is that its maturation occurs relatively late in development. This is presumed to echo the developmental time course of a corresponding circuit within late-maturing parts of the brain, such as the prefrontal cortex, but the specific identity of the responsible circuits is unknown. Here, we describe the maturation of the projection from layer 5 neurons of the prefrontal cortex to the dorsal raphe nucleus in mice. We show using pathway-specific optogenetic stimulation that this connection undergoes a dramatic increase in synaptic potency between postnatal weeks 3 and 8, corresponding to the transition from juvenile to adult. We then show that this period corresponds to an increase in the behavioral persistence that mice exhibit in a foraging task. Finally, we use genetic targeting to selectively ablate this pathway in adulthood and show that mice revert to a behavioral phenotype similar to juveniles. These results suggest that the prefrontal to dorsal raphe pathway is a critical anatomical and functional substrate of the development and manifestation of behavioral control.

Hypocretin in locus coeruleus and dorsal raphe nucleus mediates inescapable footshock stimulation (IFS)-induced REM sleep alteration

Hypocretin (hcrt) is a stress-reacting neuropeptide mediating arousal and energy homeostasis. An inescapable footshock stimulation (IFS) could initiate the hcrt release from the lateral hypothalamus (LHA) and suppresses rapid eye movement (REM) sleep in rodents. However, the effects of the IFS-induced hcrts on REM-off nuclei, the locus coeruleus (LC) and dorsal raphe nucleus (DRN), remained unclear. We hypothesized that the hcrt projections from the LHA to LC or DRN mediate IFS-induced sleep disruption. Our results demonstrated that the IFS increased hcrt expression and the neuronal activities in the LHA, hypothalamus, brainstem, thalamus, and amygdala. Suppressions of REM sleep and slow wave activity during non-REM (NREM) sleep caused by the high expression of hcrts were blocked when a non-specific and dual hcrt receptor antagonist was administered into the LC or DRN. Furthermore, the IFS also caused an elevated innate anxiety, but was limitedly influenced by the hcrt antagonist. This result suggests that the increased hcrt concentrations in the LC and DRN mediate stress-induced sleep disruptions and might partially involve IFS-induced anxiety.

Separable Dorsal Raphe Dopamine Projections Mediate Sociability and Valence

Affiliative social connections facilitate well-being and survival in numerous species. Engaging in social interactions requires positive and negative motivational drive, elicited through coordinated activity across neural circuits. However, the identity, interconnectivity, and functional encoding of social information within these circuits remains poorly understood. Here, we focused on downstream projections of dorsal raphe nucleus (DRN) dopamine neurons (DRNDAT), which we previously implicated in ‘negative drive’-induced social motivation. We show that three prominent DRNDAT projections – to the bed nucleus of the stria terminalis (BNST), central amygdala (CeA), and posterior basolateral amygdala (BLP) – play separable roles in behavior, despite substantial collateralization. Photoactivation of the DRNDAT-CeA projection promoted social behavior and photoactivation of the DRNDAT-BNST projection promoted exploratory behavior, while the DRNDAT-BLP projection supported place avoidance, suggesting a negative affective state. Downstream regions showed diverse, region-specific, receptor expression, poising DRNDAT neurons to act through dopamine, neuropeptide, and glutamate transmission. Furthermore, we show ex vivo that the effect of DRNDAT photostimulation on downstream neuron excitability was predicted by baseline cell properties, suggesting cell-type-specific modulation. Collectively, these data indicate that DRNDAT neurons may bias behavior via precise modulation of cellular activity in broadly-distributed target structures.

Enhanced responsiveness to hypoxic panicogenic challenge in female rats in late diestrus is suppressed by short-term, low-dose fluoxetine: Involvement of the dorsal raphe nucleus and the dorsal periaqueductal gray

Background: Acute hypoxia, which is panicogenic in humans, also evokes panic-like behavior in male rats. Panic disorder is more common in women and susceptibility increases during the premenstrual phase of the cycle. Aims: We here investigated for the first time the impact of hypoxia on the expression of panic-like escape behavior by female rats and its relationship with the estrous cycle. We also evaluated functional activation of the midbrain panic circuitry in response to this panicogenic stimulus and whether short-term, low-dose fluoxetine treatment inhibits the hyper-responsiveness of females in late diestrus. Methods: Male and female Sprague Dawley rats were exposed to 7% O2. Females in late diestrus were also tested after short-term treatment with fluoxetine (1.75 or 10 mg/kg, i.p.). Brains were harvested and processed for c-Fos and tryptophan hydroxylase immunoreactivity in the periaqueductal gray matter (PAG) and dorsal raphe nucleus (DR). Results: Acute hypoxia evoked escape in both sexes. Overall, females were more responsive than males and this is clearer in late diestrus phase. In both sexes, hypoxia induced functional activation (c-Fos expression) in non-serotonergic cells in the lateral wings of the DR and dorsomedial PAG, which was greater in late diestrus than proestrus (lowest behavioral response to hypoxia). Increased responding in late diestrus (behavioral and cellular levels) was prevented by 1.75, but not 10 mg/kg fluoxetine. Discussion: The response of female rats to acute hypoxia models panic behavior in women. Low-dose fluoxetine administered in the premenstrual phase deserves further attention for management of panic disorders in women.

α-1 Adrenoceptor Activation in the Dorsal Raphe Nucleus Decreases Food Intake in Fasted Rats

The dorsal raphe (DR) nucleus is involved in a myriad of physiological functions, such as the control of sleep-wake cycle, motivation, pain, energy balance, and food intake. We have previously demonstrated that in ad libitum fed rats the intra-DR administration of phenylephrine, an α-1 receptor agonist, does not affect food intake, whereas clonidine, an α-2 receptor agonist, potently stimulates food intake. These results indicated that in fed rats an increased adrenergic tonus blocked food intake, since the activation of α-2 auto-receptors, which decreases pre-synaptic release of adrenaline/noradrenaline, affected food intake. Thus, in this study we assessed whether the response to adrenergic stimuli would differ after overnight fasting, a situation of low adrenergic activity in the DR. Intra-DR administration of adrenaline and noradrenaline blocked food intake evoked by overnight fasting. Similarly, phenylephrine administration decreased hunger-induced food intake. These changes in food intake were accompanied by changes in other behaviors, such as increased immobility time and feeding duration. On the other hand, intra-DR administration of clonidine did not affect food-intake or associated behaviors. These results further support the hypothesis that in fed animals, increased adrenergic tonus in DR neurons inhibiting feeding, while in fasted rats the adrenergic tonus decreases and favors food intake. These data indicate a possible mechanism through which adrenergic input to the DRN contributes to neurobiology of feeding.

Oscillatory population-level activity of dorsal raphe serotonergic neurons sculpts sleep structure

Dorsal raphe (DR) 5HT neurons are involved in regulating sleep/wake transitions. Previous studies demonstrated that single unit activity of DR 5HT neurons is high during wakefulness, decreases during non rapid eye movement (NREM) sleep, and ceases during rapid eye movement (REM) sleep. However, characteristics of the population level activity of DR 5HT neurons, which can influence the entire brain, are largely unknown. Here we measured population activities of 5 HT neurons in male and female mouse DR across the sleep/wake cycle by a ratiometric fiber photometry system. We found a slow oscillatory activity of compound intracellular Ca2+ signals during NREM sleep. The trough of concave 5HT activity increased along with sleep progression, but the 5HT activity level always returned to that seen in wake periods. When the trough reached the minimum level and remained there, REM sleep initiated. We also found a unique coupling of the oscillatory 5HT activity and EEG power fluctuation, suggesting that EEG fluctuation is a proxy for 5HT activity. Optogenetic activation of 5HT neurons during NREM sleep triggered a high EMG power and induced wakefulness. Optogenetic inhibition induced REM sleep or sustained NREM with an EEG power increase and EEG fluctuation. These manipulations demonstrated a causal role of DR 5HT neurons in sculpting sleep/wake structure. We also observed EEG fluctuations in human males during NREM sleep, implicating the existence of 5HT oscillatory activity in humans. We propose that NREM sleep is not a monotonous state, but that it is dynamically regulated by the oscillatory population activity of DR 5HT neurons.