Effects of Serotonin Terminal Lesions in the Retrotrapezoid Nucleus on Ventilatory Chemoreflexes

Abstract

Brain chemoreceptors play a vital role in regulating blood gas homeostasis in response to acute changes in CO2 and/or pH. Serotonergic (5-HT) neurons, which also produce and release other neuromodulatory peptides, have properties consistent with being central respiratory chemoreceptors. In addition, recent in vitro studies have highlighted important contributions of G-protein-coupled 5-HT receptors (5-HT2 and/or 5-HT7 receptors) to the chemosensitivity of additional putative central respiratory chemoreceptors within the retrotrapezoid nucleus (RTN) marked by the expression of Phox2b. However, the importance of raphe-derived neuromodulation of the RTN on eupneic breathing as well as hypoxic and hypercapnic ventilatory chemoreflexes in vivo is unclear. Here we hypothesized that unilateral ablation of serotonergic terminals within the RTN cause a time-dependent attenuation of ventilatory chemoreflexes in vivo. To test this hypothesis, 8-week-old Sprague-Dawley rats received unilateral stereotaxic injections of the 5-HT pre-synaptic terminal toxin 5,7-Dihydroxytryptamine (5,7-DHT; 2 mg/ml, 2 µl vol.) with or without fluorescent microbeads (for injection site localization) targeted to the RTN after pretreatment with desipramine (30mg/kg; IP) to limit off-target effects on other aminergic neurons/terminals. A control group received an equal volume injection of vehicle in the RTN. Breathing was measured via whole body plethysmography while exposed to room air, hypercapnia (7% CO2 challenge), or hypoxia (12% O2 challenge) 2-3 days (d) before and 3, 5, 7, 10, and 14d post-injection. Preliminary data showed that RTN-targeted injections reduced ventilatory chemoreflexes 3, 5, and 7d post-injection with slightly greater effects in the 5,7-DHT injected group. Ventilatory chemoreflexes returned to control levels by 14d post-injection. To confirm the lesion site within the RTN and determine the extent of 5-HT-specific lesioning, we have also begun to quantify 5-HT using immunofluorescent staining. However, additional preliminary data showed no differences in 5-HT when comparing 5,7-DHT and control groups. Although these studies are ongoing, our data suggest that unilateral RTN-targeted 5-HT terminal lesions reduce ventilatory chemoreflexes.