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De-sigh-phering the neural basis of sighing

The discovery of a specific brain circuit that may underlie sighing behaviour is reported online this week in Nature. The study finds that manipulation of two parallel pathways consisting of a few hundred neurons changes the frequency of sighing in rodents without affecting other aspects of breathing.

Deep, long breaths known as sighs are a crucial part of breathing that are associated with sadness, exhaustion or relief. Sighs occur spontaneously several times per hour in humans and dozens of times per hour in rodents during normal breathing. Sighing may help to re-inflate the tiny air sacs in the lungs, but until now the neural pathways involved in this behaviour have been elusive.

Mark Krasnow, Jack Feldman and colleagues identify approximately 230 neurons in the breathing control centre of the brain that express one of two genes (Nmb or Grp) and project to a small subset of receptors in a cluster of several thousand neurons that collectively generate respiratory rhythm (the preBötzinger complex or preBötC). They find that injecting either or both NMB or GRP neuropeptides into the preBötC substantially increased sighing in rats. Simultaneously inhibiting these pathways is shown to severely reduce or eliminate sighing. Finally, they show that neurons expressing these receptors are critical for inducing sighing when rats are exposed to an oxygen-deficient environment.

The work could be used to investigate the physiological benefits of sighing and to elucidate the full circuit – which may also integrate emotional input – responsible for transforming normal breaths into sighs.