Temperature Receptors in the Central Nervous System

For half a century, it has been known that thermo sensitive regions of the rostral brain stem are important in thermoregulation (61). Figure 1 indicates that a variety of thermoregulatory responses can be elicited by changing the tempera­ ture of the preoptic area and anterior hypothalamus (POI AH). POI AH warming evokes heat-loss responses, and POIAH cooling evokes heat-production re­ sponses. If POIAH temperature is changed slightly above or below normal, there are changes in heat-retention responses such as skin blood flow and thermoregulatory behavior. Figure 1 also describes the synaptic organization of POI AH thermosensitive neurons (4). While most neurons are temperature insensitive, warm-sensitive neurons have firing rates that increase with warm­ ing or decrease with cooling. Conversely, the firing rates of cold-sensitive neurons increase with cooling or decrease with warming. Many central thermo­ sensitive neurons also receive synaptic inputs from skin and spinal thermore­ ceptive pathways (4, 7). This indicates that such neurons are capable of thermal integration, and it increases the likelihood that these neurons function in thermoregulation. As summarized in Figure 1, the evidence for central thermosensitivity comes from thermoregulatory studies during thermal stimulation of discrete neural areas and from electrophysiological studies of temperature-sensitive neurons. In this review, these studies are presented first as an overview of the comparative aspects of central thermo sensitivity in vertebrates. The second half of this review focuses on the properties and synaptic organization of POIAH thermosensitive neurons.