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First published online May 30, 2008
Journal of Experimental Biology 211, v-a (2008)
Copyright © 2008 The Company of Biologists Limited
doi: 10.1242/jeb.011627
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Outside JEB |
A COLD RESPONSE TO WARM BLOOD
University of Calgary
sahewitt{at}ucalgary.ca
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Living organisms face a constant battle to maintain a stable internal environment, known as homeostasis. As part of the homeostatic system, warm-blooded animals, including humans, have developed a thermoregulatory response to counteract changes in temperature. Variations in the ambient temperature initiate various behavioural and physiological responses that are necessary to maintain an internal temperature within a narrow range for the proper functioning of many cellular and molecular mechanisms. The pre-optic area in the hypothalamus is the thermosensory command centre for thermoregulation. It elicits a barrage of physiological responses in the periphery before a significant impact on core temperature can occur. However, it is unknown how signals from skin thermoreceptors are relayed through the spinal cord to eventually terminate in the pre-optic area. Kazuhiro Nakamura and Shaun Morrison from the Neurological Sciences Institute at the Oregon Health and Science University have identified a novel pathway in rats that responds to cold and stimulates the pre-optic area to initiate a thermoregulatory response.
Neurons transmit signals in a linear fashion from one neuron to the next, requiring physical connections called synapses. The authors set out to identify the specific brain region where thermal signals from the spinal cord are transmitted before terminating on neurons in the pre-optic area. First, they injected a fluorescent tracer into the pre-optic area. This tracer migrated across synapses into regions directly connected with the pre-optic area. Then, when the temperature was changed from 24°C to 4°C, a subset of neurons in a region called the parabrachial nucleus were `switched on' by the cold stimulus. The parabrachial nucleus receives a variety of signals involved in homeostatic responses but had not previously been shown to communicate directly with the pre-optic area.
Next the team showed that cold-responsive neurons in the parabrachial nucleus play a functional role in modulating thermoregulatory responses in the pre-optic area. The authors measured physiological parameters that are known to increase in response to skin cooling. This included measurements of metabolic activity in heat-generating brown adipose tissue, expired carbon dioxide, heart rate and blood pressure. They then used pharmacological inhibitors of neurotransmission within the parabrachial nucleus to see whether switching off the pathway affected the animal's physiological response to cold. They found that in response to cooling there was no longer an increase in these measured parameters. This implies that neurons in the lateral parabrachial nucleus play a crucial role in mediating the thermoregulatory response to a cold challenge.
Finally, the authors investigated whether the pathway responsible for the conscious perception of temperature, the spinothalamocortical pathway, which is relayed through the thalamus to the cortex, is also involved in the unconscious thermoregulatory response. By preventing neuronal signalling in areas of the thalamus that receive an abundance of spinothalamic projections, and measuring the same parameters as above, the authors established that the temperature perception and the thermoregulatory signalling pathways are distinct.
While cold-blooded organisms must acquire heat from the sun, an internal thermoregulatory response is a fundamental characteristic of warm-blooded animals. However, there are days when sunbathing seems a pleasant alternative even for the most warm blooded.
References
Nakamura, K. and Morrison, S. F. (2008). A thermosensory pathway that controls body temperature. Nat. Neurosci. 11,62 -71.[CrossRef][Medline]
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