Control of breathing in the golden-mantled ground squirrel (Spermophilus lateralis)

The Golden-mantled ground squirrel (Spermophilis lateralis) breaths continuously during euthermia and episodically during hibernation. How and why this conversion occurs is unknown. Breathing is continuously shaped into a precise pattern which appropriately matches ventilation to metabolic demands. In all mammals, sensory inputs from pulmonary mechanoreceptors (carried in the vagus nerve), and a specific cluster of neurons located in the pons (the pneumotaxic center, PC) play key roles in modulating this pattern. The present investigation was designed to determine how the influence of these two inputs changes as squirrels enter hibernation, and if changes in the integration of these inputs could be responsible for producing the episodic breathing pattern observed during hibernation. Ventilation in euthermic ground squirrels was critically dependent on intact vagus nerves. These animals did not breathe in the absence of vagal feedback. In anesthetized animals, on the other hand, ventilation continued post-vagotomy but the shape of individual breaths was altered. This suggests there is a powerful inhibition of breathing that is normally offset by vagal feedback, but which is removed by anesthesia. In hibernating animals, vagal feedback was even less critical, it increased the overall level of ventilation by increasing the length of breathing episodes. Glutamatergic processes utilizing NMDA type receptors were shown to be involved in the expression of sleep and sleep-like states of central activation. They were also involved in producing the ventilatory response to hypoxia in anaesthetized and unanaesthetized animals. Finally, they also depressed breathing frequency during sleep, anesthesia and hibernation. All of these effects are deduced to arise from glutamatergic processes outside the PC, however. Glutamatergic processes utilizing NMDA type receptors within the PC are deduced to assist in the termination of inspiration in anaesthetized animals in a similar fashion to vagal feedback as has been shown in other mammals. While removal of either vagal feedback or NMDA receptor-mediated processes in the PC had only modest effects on breathing pattern, removal of both produced an extreme prolongation of inspiration (apneusis) in euthermic squirrels. In hibernating animals, removal of both inputs converted episodic breathing into a pattern of evenly spaced breaths. This latter observation suggests that integration of vagal feedback with glutamatergic processes (perhaps within the PC) is responsible for clustering breaths into episodes during hibernation. How the function of these inputs is transformed from one of shaping individual breaths to one of shaping episodes of breaths remains unknown.Science, Faculty ofZoology, Department ofGraduat