Function-Specific Serotonergic Neurons in the Control of Breathing and Body Temperature

Abstract

The control of respiration and body temperature involves neural circuits within the brainstem modulated by the neurotransmitter serotonin (5HT), though it is unclear precisely which serotonergic neurons are critical to these functions. Recent work from our laboratory and others has demonstrated considerable heterogeneity among serotonergic neurons – in their projection targets, expressed genes, and developmental origin – which we hypothesize reflects the subserving of distinct functions by distinct serotonergic neuron subtypes. More specifically, our laboratory has parsed molecular subtypes of serotonergic neurons by their co-expression of the pan-serotonergic gene Pet1 along with a marker gene. When partnered with intersectional genetic tools, selective access to each subtype for functional study is enabled. Through application of these tools, we have identified a subtype of serotonergic neuron, designated the Egr2-Pet1 subtype, which modulates the CO2 breathing reflex and projects to CO2 chemosensory brain centers. Here, we describe a new molecular subtype of serotonergic neuron, defined by Tachykinin 1 (Tac1) expression, that projects to respiratory motor nuclei. Upon silencing the Tac1-Pet1 subtype, we show that it, too, modulates the CO2 breathing reflex, but presumably by affecting respiratory motor output. Thus, two discrete subtypes of serotonergic neurons modulate breathing: one involved in sensory processing and the other involved in motor output, reflecting an unexpected division of serotonergic labor along the motor-sensory axis. In parallel, we tested the contribution of serotonergic neuron subtypes to body temperature modulation, as we have previously shown that en masse acute silencing of serotonergic neurons leads to hypothermia. We provide evidence that serotonergic neurons within the median raphe and raphe pallidus may contribute to this task. In these studies, we manipulated serotonergic neuron activity via hM4Di-mediated hyperpolarization using the intersectional allele RC::FPDi, for which parameters were optimized and defined. In summary, we have identified molecularly and functionally distinct subtypes of serotonergic neurons necessary for normal homeostatic regulation. Results may shed light on homeostatic disorders involving 5HT, such as sleep apnea and sudden infant death syndrome, as well as inform strategies for potentially minimizing homeostatic side effects of present 5HT-affecting therapeutics.