That warm fuzzy feeling: brain serotonergic neurons and the regulation of emotion.

Whether lying on the beach in the midday sun on a Caribbean island, grabbing a few minutes in the sauna or spa after work, or sitting in a hot bath or Jacuzzi in the evening, we often associate feeling warm with a sense of relaxation and well-being. Even 'working up a good sweat', exercising or performing manual labour in the garden can have its rewards. Although we take these feelings for granted, convergent lines of evidence suggest that sensations of 'warmth' may alter neural circuits controlling cognitive function and mood, including serotonergic circuits, in addition to those directly involved in thermoregulatory cooling. One mechanism through which sensations of warmth may modulate neural circuits controlling cognitive function and mood is the activation of temperature-activated transient receptor potential (TRP) ion channels, including TRPv3 and TRPv4 which are active in the non-noxious thermal range, 27-42 degrees C, and subsequent activation of a subpopulation of brainstem serotonergic neurons. In this article, we explore the hypothesis that a subpopulation of serotonergic neurons are thermosensitive and form part of a thermoafferent pathway regulating physiology and behaviour. We also propose the novel hypothesis that dysregulation of this thermosensitive population of serotonergic neurons plays an important role in stress-related neuropsychiatric disorders, including anxiety and affective disorders.