Gap junctions establish pathways of intercellular communication that coordinate processes such as embryogenesis, development, growth, differentiation and cellular response to injury. In the nervous system, gap junctions establish low-resistance channels that couple cells electrically, permitting a rapid and synchronous response to stimuli. Recent work using the in vitro brainstem-spinal cord and medullary slice preparations from mice have shown that gap junction blockade decreases respiratory frequency, suggesting that gap junctions may be critical for rhythmogenesis in this preparation. Electrical coupling has been identified in several respiratory-related neuronal pools including pre-Botzinger complex, genioglossal motor neurons, phrenic motorneurons, motorneurons of the nucleus ambiguus, neurons in locus coeruleus (LC) and neurons in the nucleus tractus solitarius(NTS). Of these sites, the PBC, LC and NTS are chemosensitive and appear to be important in central respiratory control based on both in vitro and in vivo studies. Congeners of glycyrrhetinic acid (e.g. carbenoxolone) have been shown to block gap junction communication by a mechanism that may involve conformational changes in connexin structure. The role of electrical coupling in central chemosensory function in vivo has not been examined. Here we show that unilateral, focal stimulation of the rostral NTS leads to a significant increase in ventilation in the conscious adult rat. Pharmacological blockade of gap junctions in the NTS with carbenoxolone decreased ventilation in young adults (about 7 weeks old) but not in older adults (about 14 weeks old). These findings suggest that electrical coupling at some sites involved in central respiratory control may give way to chemical synaptic transmission as the animal develops. In support of this hypothesis, we show that unilateral, focal perfusion of cobalt into the rostral NTS decreases ventilation about 60%, suggesting an important role of chemical synaptic transmission in the older adult. In this proposal we will examine 3 putative chemoreceptor sites. Two of these sites demonstrate electrical coupling between cells (the NTS and the LC) whereas the third chemosensitive site does not (the retrotrapezoid nucleus). Here we outline experiments designed to examine the changes in the relative contribution of electrical and chemical synaptic transmission in the NTS during development using the chronically instrumented, conscious rats.
