It is hypothesized that postnatal changes occur in intrinsic membrane currents and in the direct responses to neurotransmitters in neonatal rat hypoglossal motoneurons. These changes may be responsible for differences in both the sub- and supra-threshold behaviors of these motoneurons as well as differences in the responses to various neurotransmitters. It is further hypothesized that the changes in the neurotransmitter responses may be, in part, correlated with changes in neurotransmitter receptor gene expression in these neonatal motoneurons. The first two specific aims investigate two inward voltage- and time- dependent ionic currents that are important in the sub- and supra- threshold firing behaviors of hypoglossal motoneurons. The first specific aim is to investigate the postnatal development of properties of the mixed cationic inward rectifier current activated on hyperpolarization, Ih. Single electrode voltage-clamping of hypoglossal motoneurons in conventional brainstem slices will be utilized to study the properties of this current. The second specific aim is to investigate the postnatal development of properties of the low-voltage-activated Ca2+ current. Whole-cell currents measured with patch-type electrodes in brainstem slices will be utilized to study the properties of this current. Specific aims three and four investigate two neurotransmitters that have direct excitatory effects on hypoglossal motoneurons; their release may be state-dependent (sleep vs. awake). The third specific aim is to investigate the postnatal development of the response to thyrotropin- releasing hormone (TRH). It will also be determined whether these changes correlate with TRH receptor mRNA expression in hypoglossal motoneurons. The fourth specific aim is to investigate the postnatal development of the response to norepinephrine (NE). It will also be determined whether these changes correlate with alpha1-adrenergic receptor mRNA receptor mRNA expression in hypoglossal motoneurons. This proposal focuses on in vitro studies of hypoglossal motoneurons because these motoneurons may have an important function in regulating upper airway patency particularly during sleep states. Results from these studies will provide new information concerning the normal and postnatal development of these important motoneurons. The upper airway has been suggested as a site for airway obstruction; therefore a comprehensive understanding of these motoneurons may provide new insights into certain pathologic states, including apnea of prematurity and Sudden Infant Death Syndrome.