Astrocytes modulate baroreceptor reflex sensitivity at the level of the nucleus of the solitary tract

Abstract

Astrocytes play an important role in cardiovascular reflex integration at the level of the nucleus tractus solitarii (NTS). Existing reports from brainstem slice preparations suggest that astrocytes here respond to input from the solitary tract by increasing intracellular calcium. However, the physiological significance of this neuron-astrocyte signaling in vivo remains unknown. Here, we report that stimulation of the vagus nerve in an anesthetized rat induced rapid [Ca2+]i increases in astrocytes transduced to express calcium sensor GCaMP6. The receptors involved were determined using brainstem-derived astroglial cell cultures were loaded with [Ca2+] indicator Fura-2. 5-HT (10 {micro}M) caused robust increases in [Ca2+]i, and pharmacological interrogation revealed the expression of functional 5-HT2A receptors. This observation was confirmed in vivo: intravenous administration of ketanserin decreased the magnitude of [Ca2+]i responses, induced by vagal afferent simulation, by [~]50%. However, the response was completely blocked by topical application of the AMPA receptor antagonist CNQX alone. To investigate the role of astrocyte-neuron communication, the vesicular release in the NTS astrocytes was blocked by virally driven expression of a dominant-negative SNARE protein in vivo. This increased baroreflex sensitivity in awake animals, which was also observed in anesthetized animals after topical application of the P2Y1 receptor antagonist MDS-2500 to the NTS. We hypothesize that NTS astrocytes respond to incoming afferent release of glutamate and this response is modulated by 5-HT originating from vagal afferents or other sources. ATP is then released, which acts on inhibitory interneurons via P2Y1 receptors and thus modulates the expression of cardiovascular reflexes.\n\nSignificance statementCardiorespiratory nuclei in the brainstem integrate cardiovascular sensory information to optimise tissue perfusion and blood gas concentrations. We describe experimental evidence that NTS astrocytes participate in setting the baroreflex sensitivity by release of ATP acting on P2Y1 receptors on inhibitory interneurons. Activation of astrocytes is partly under control of 5-HT co-released with glutamate from vagal afferents, which allows modulation of autonomic response to high frequency/duration of afferent stimulation by monitoring extra-synaptic 5-HT acting on glial 5-HT2A receptors. This could represent a signaling pathway that is activated under pathological conditions and is responsible for baroreflex impairment in conditions that result in astrogliosis, for example from systemic inflammatory response or chronic hypoxia/hypercapnia.