Serotonergic Receptors and Effects in Hypoglossal and Laryngeal Motoneurons

Developmental profiles of neurotransmitter receptors in respiratory motor nuclei

We discuss the time course of postnatal development of selected neurotransmitter receptors in motoneurons that innervate respiratory pump and accessory respiratory muscles, with emphasis on other than classic respiratory signals as important regulatory factors. Functions of those brainstem motoneurons that innervate the pharynx and larynx change more dramatically during early postnatal development than those of spinal respiratory motoneurons. Possibly in relation to this difference, the time course of postnatal expression of distinct receptors for serotonin differ between the hypoglossal (XII) and phrenic motoneurons. In rats, distinct developmental patterns include a decline or increase that extends over the first 3-4 postnatal weeks, a rapid increase during the first 2 weeks, or a transient decline on postnatal days 11-14. The latter period coincides with major changes in many transmitters in brainstem respiratory regions that may be related to a brain-wide reconfiguration of sensorymotor processing resulting from eye and ear opening and beginning of a switch from suckling to mature forms of food seeking and processing. Such rapid neurochemical changes may impart increased vulnerability on the respiratory system. We also consider rapid eye movement sleep as a state during which some brain functions may revert to conditions typical of perinatal period. In addition to normal developmental processes, changes in the expression or function of neurotransmitter receptors may occur in respiratory motoneurons in response to injury, perinatal stress, or disease conditions that increase the load on respiratory muscles or alter the normal levels and patterns of oxygen delivery.

Respiratory plasticity following intermittent hypoxia: roles of protein phosphatases and reactive oxygen species

Plasticity is an important property of the respiratory control system. One of the best-studied models of respiratory plasticity is pLTF (phrenic long-term facilitation). pLTF is a progressive increase in phrenic motor output lasting several hours following acute exposure to intermittent hypoxia. Similar to many other forms of neuroplasticity, pLTF is pattern-sensitive; it is induced by intermittent, but not sustained hypoxia of similar cumulative duration. Our understanding of the cellular/synaptic mechanisms underlying pLTF has increased considerably in recent years. Here, we review accumulating evidence suggesting that the pattern-sensitivity of pLTF arises substantially from differential reactive oxygen species formation and subsequent protein phosphatase inhibition during intermittent compared with sustained hypoxia in or near phrenic motor neurons. A detailed understanding of the cellular/synaptic mechanisms of pLTF may provide the rationale for new pharmacological approaches in the treatment of severe ventilatory control disorders, such as obstructive sleep apnoea and respiratory insufficiency either following spinal cord injury or during neurodegenerative diseases such as amyotrophic lateral sclerosis.