Structure-function relation of isolated preBotC inspiratory networks in newborn rats

Breathing movements are initiated and controlled by the hypothesized pre-Botzinger Complex (preBotC) inspiratory center in the lower brainstem. Using established newborn rat brainstem-spinal cords and brainstem slices, this thesis examined primarily how preBotC activity is influenced by systematic variation of the rostrocaudal boundaries of these models or variation of superfusate Ca2+/K+ content. It was found that rostrocaudal extensions of respiratory brainstem marker nuclei and ventral brainstem surface markers are constant in 0-4 days-old rats. This enables, with the help of a novel brainstem reference atlas, the generation of preBotC-containing preparations with "calibrated" boundaries. Slices with the preBotC in the center and 500 or 600 pm thickness generate stable inspiratory (motor) rhythm in physiological (3 mM) K+ and show a high sensitivity to neuromodulators, in particular opioids.;In summary, this thesis provided novel information regarding the structure-function relationship of rhythmogenic preBotC inspiratory neuronal networks. The findings indicate that endogenous rhythm of the isolated preBotC depends on an extracellular Ca2+/K+ antagonism. The findings also suggest a (gradient-like) spatiochemical organization of regions adjacent to the preBotC, such that a small preBotC inspiratory-related oscillator generates eupnea under the predominant influence of caudal structures or thyrotropin-releasing hormone-like transmitters, but eupnea-sigh activity when the influence of rostral structures or substance-P-like transmitters predominates.;The rostrocaudal area sufficient (in thin slices) or necessary (in thicker slices) for a functional preBotC is <200 mum and <100 mum, respectively, centered at 0.5 mm caudal to the facial nucleus. Slices containing the preBotC plus caudal structures produce a eupnea-like burst pattern, whereas slices containing the preBotC plus rostral tissue generate a mixed eupnea-sigh pattern. Spontaneous arrest of inspiratory bursting in 3 mM K+ occurring after several hours ("in vitro apnea") is reversed by either elevated K+ or excitatory neuromodulators. Multineuronal multiphoton Ca2+ imaging revealed that preBotC neurons reconfigure their activity between the eupnea and the eupnea-sigh burst pattern induced upon in vitro apnea by thyrotropin-releasing hormone and substance-P, respectively. Depression of preBotC rhythms in 3 mM K+ in both the slice and en bloc model by elevation of superfusate Ca2+ above physiological (1.2 mM) levels was reversed by raised K+.