| Summary: | 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.
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