The superior cervical ganglion (SCG) is the largest of the sympathetic chain ganglia which control a number of autonomic cardiovascular reflexes via neural activity in the postganglionic nerve trunk. In addition to the large principal neurons, these ganglia contain a minority population of smaller cells, the small intensely fluorescent or SIF cells, so named because of their intense fluorescence following treatments which reveal the presence of endogenous catecholamines (mainly dopamine in the rat). The physiological functions of the SIF cells are largely unknown and various roles have been proposed including (i) dopaminergic interneuron, which modulates ganglionic transmission, (ii) endocrine function, since many of them have a close association with the vasculature and (iii) chemosensory function, similar to that of the arterial chemoreceptors which sense blood gases and pH. Understanding the physiological role of SIF cells has been hampered by their small size, sparse distribution and relative inaccessibility, all of which render microelectrode electrophysiological studies difficult. In this thesis these limitations were overcome by use of (i) dissociated cell cultures of the rat SCG, in which growth conditions favoured SIF cell survival but not that of the principal neurons, and (ii) the novel high resolution patch clamp/whole cell recording technique which is ideal for the study of the electrophysiology of small cells. The ionic currents, which underlie many basic electrophysiological processes, were characterized in 5-16 day old cultures of SIF cells obtained from the SCG of neonatal rats. The main methodology consisted of whole cell recording under voltage clamp conditions, which permit the study of membrane ionic currents. Five main ionic currents were identified in all of the SIF cells ( > 100) studied: (i) a fast transient inward Na+ current, sensitive to the well-known blocker of voltage-gated Na+ channels i.e. tetrodotoxin or TTX; (ii) the delayed rectifier outward K+ current that is found in a variety of cell types; (iii) a Ca2+- activated outward K+ current, sensitive to Ca2+ channel blockers; (iv) a transient inward Ca2+ current which appears to be carried by N-type Ca2+ channels and (v) a slower, sustained inward Ca2+ current which appears to be carried by L-type Ca2+ channels. In addition a third type of outward K+ current, the fast transient K+ current or lA, was found in SIF cells obtained from 3-7 day old rats, but not from 1 day old rats. It therefore appears that this lA current, which is known to modulate firing frequency in neurons develops rapidly in vivo during the first postnatal week. This broad repertoire of ion channels in SIF cells suggests several possible sites for modulation by various agents including neurotransmitters, neuromodulators, or other chemosensitive agents. Since SIF cells were recently proposed to have arterial chemoreceptor function similar to glomus cells, the effect of one such stimulus, i.e. an acidic (intracellular) pH, was tested. It has recently been suggested that a decrease in intracellular pH is part of the pathway responding to extracellular stimuli in the glomus cell (Stea, Alexander and Nurse, in press). Acidification of the SIF cell's cytoplasm with the K+/H+ ionophore nigericin resulted in a suppression of both the fast inward Na+ current as well as the outward K+ current. However, these effects do not appear to beunique to SIF cells and therefore the possibility of a chemoreceptor role m the cardiovascular system requires further study. In summary, the characterization of the various ionic currents in SIF cells resulting from this thesis provides the necessary background which should eventually resolve not only the question of the physiological role of SIF cells in autonomic ganglia, but also help to understand the underlying mechanisms responsible for SIF cell function. / Thesis / Master of Science (MS)

In adult vertebrates, cardiovascular regulation is accomplished by numerous systems with neural, hormonal and local components responsible for the majority of regulation. These regulatory components work in concert to maintain the essential function of blood perfusion to adult tissues. Given the essential nature of this function it is therefore surprising that the development of cardiovascular regulation during gestation is poorly understood. The majority of what is known is based on a single vertebrate model, the fetal lamb. The fetal lamb has been used in multiple studies due to the clear clinical applications and has been pivotal in understanding the onset of regulation in developing vertebrates. However, study on the fetal lamb is limited to the latter 40% of gestation and has the added complication of an in-utero developmental strategy. Therefore the primary focus of this dissertation was to characterize basic cardiovascular regulation in the chicken embryo to provided the needed information for it's use an alternative to the fetal lamb. Developing chicken embryos rely on both alpha and beta adrenergic tones to maintain normal heart rate and arterial blood pressure during incubation. However, on day 21, just prior to hatch, these animals lose both tones on arterial pressure suggesting the onset of adult regulation. Cholinergic tone, however, was absent throughout chicken development indicating that it must mature during the neonatal life. Adult cardiovascular reflexes become apparent late in chicken development with a clear baroreflex specifically operating initially on day. However, an adult response to changes in ambient gas tension was absent during incubation suggesting embryos possess unique regulatory systems that are absent in adult chickens. This mechanism is comprised entirely of adrenergic systems with no cholinergic action during change in ambient gas tension. Similar developmental patterns were determined in embryos of the desert tortoise suggesting fundamental differences between in-utero and ex-utero developing vertebrates.