Expressions Of 5-HT—5-HT_2A R—PKC Pathway In Hypoglossal Nucleus In Normal Rats

Objective: Persistent compensatory increases in breathing movements, referred to as respiratory long-term facilitation (LTF), represent a respiratory neuroplasticity based on experience and adaptation to meet demands under physiological or pathological conditions. LTF induced by episodic hypoxia is mainly expressed in phrenic and XII nerve , and is 5-HT2A receptors (5-HT_2AR)-dependent. While previous studies were largely concentrated on electrophysiological recording of LTF expression, less is attended to morphological understanding of neuronal neurochemical components of 5-HT/5-HT_2AR system, especially in the hypoglossal nucleus, a nucleus that innervates upper airway dilator muscles, and is able to induce XII nerve LTF in humans with persistent snore during sleep. The present study wished to explore expressions of the 5-HT/5-HT_2AR system and its downstream protein kinase C (PKC) signaling in the hypoglossal nucleus in rats, to provide neurochemical knowledge for understanding functional XII nerve LTF expression. Methods: Using immuno-fluorescence and immuno-electronic approaches, we detected expressions of 5-HT/5-HT_2AR and serotonin transporter (SERT), and PKC/PKC substrates in the hypoglossal nucleus in adult rats. Results: Under the light microscope, 5-HT-immunoreactive (ir) boutons, in single or string-like profiles, were found in the hypoglossal nucleus. 5-HT_2AR immunoreactivity was expressed mainly along the cytoplasm membrane, and some in the processes. SERT, PKC, and PKC substrates were densely distributed within somas. Some processes were also SERT-, PKC-, and PKC substrates -ir. Under the electron microscope, 5-HT-ir terminals, indicated by immunoperoxidase reaction product, were found in contact with somas and dendrites in the hypoglossal nucleus. Asymmetric synapses were detectable between them. Immunogold particles indicative 5-HT_2AR, SERT, PKC, and PKC substrates immunoreactivity were visualized in somas and dendrites. In somas, immunogold particles were usually associated with the endoplasmic reticulum and the ribosome, and some in close association with the plasma membrane. In dendrites, particles were distributed more along the inner surface of the plasma membrane. Interestingly, immunogold particles were found in close association with the postsynaptic membrane, especially PKC and PKC substrates that were more frequently examined in this region. PKC substrates were also found expressed along the inner surface of the nuclear membrane. Conclusion: The present study provides the morphological evidence of existence of a 5-HT-5-HT_2AR-PKC pathway in the hypoglossal motorneurons, which may participate in modulating the tension of the upper airway muscle and XII nerve LTF expression, especially an individual being under insults, such as hypoxia, hypercapnia, and asphyxia. A postsynaptic mechanism is speculated through phosphorylation of postsynaptic proteins in contribution to respiratory neuroplasticity, because of the existence of PKC/PKC substrates at the postsynaptic density.