Thermal sensory mechanisms involved in body temperature regulation

ce657bd6-99a0-4be1-bac5-be2e4475e4ae

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BBSRC
BBSRC
BBSRC
BBSRC
BBSRC

£2,008,990

May 31, 2014

May 31, 2017

In a cold environment mammals reduce blood flow to the skin in order to conserve heat, while in a hot environment sweating reduces body temperature. It is well established that the reduction in skin blood flow in a cold environment is mediated by noradrenergic sympathetic nerves, which innervate blood vessels and release noradrenaline in order to cause vasoconstriction in response to cold. Conversely, cholinergic sympathetic nerves innervate sweat glands to produce cooling in a hot environment. The heat and cold-sensitive mechanisms - the "thermostats" - which drive activity in these two divisions of the sympathetic nervous system are, however, unknown. In preliminary experiments we have found an ion channel in sympathetic neurons which is directly activated by cold, while a different ion channel expressed in parasympathetic neurons, which are cholinergic, is directly activated by warmth. The heat and cold-sensitive ion channels underlying these responses are novel thermosensory mechanisms, as they are not activated by any of the agonists for known thermally sensitive ion channels.

The discovery of these novel thermally sensitive mechanisms will now allow us to characterize their electrical properties. We will then proceed to determine their molecular basis using RNA sequencing, in which we will compare the mRNA expressed in specific cold-sensitive and warm-sensitive neuronal populations with similar populations which are not thermally sensitive, and so will determine which ion channel mRNAs are differentially expressed. Finally, we will clone and express the ion channels that we have identified in order to check that the properties of the cloned gene are the same as those expressed in thermally-sensitive neurons.

In parallel we will investigate the thermally sensitive mechanism which determines the mammalian core body temperature. Warmth-activated neurons are known to be present in the pre-optic nucleus of the anterior hypothalamus and to be important in regulating body temperature. We will isolate neurons from this region and will study their activation by thermal stimuli. Is the mechanism the same as the warmth-activated mechanism that we have discovered in cholinergic neurons? We will examine the mRNA expressed in these neurons using in situ hybridization, and if the same channels are expressed then we will have an answer to an important problem in biology, namely how mammals sense their core temperature. If the mechanism is not the same then we will isolate mRNA from populations of thermally sensitive neurons and will compare the mRNA abundance with that in adjacent non-thermally sensitive to clone the thermoregulatory gene in a similar approach to that outlined above.

Technical Abstract:
Seven thermally sensitive members of the TRP ion channel family have been cloned, but there is clear evidence that the mechanisms responsible for thermoregulation remain to be discovered, because mice null for each of the known thermo-TRPs maintain their bodily temperature normally. In preliminary studies we have found that ion channels in sympathetic neurons are activated by cold, and are therefore likely to mediate vasoconstriction in response to cold, while ion channels in parasympathetic neurons are activated by moderate warmth. Neither ion channel is activated by any of the known agonists for thermosensitive TRP ion channels. These are therefore novel thermosensory mechanisms.

Are cold-sensitive ion channels expressed in noradrenergic neurons of the sympathetic nervous system, which cause vasoconstriction, while warm-sensitive ion channels are expressed in cholinergic neurons, which activate sweat glands? The phenotypes of these two neuronal types can be switched by culture with growth factors - does thermal sensitivity also switch?

We will study cold and warm-sensitive ion channels electrophysiologically and will then determine their molecular basis by RNAseq of thermally sensitive and insensitive populations. Finally we will clone and express identified channels to confirm their thermally activated characteristics.

In a second and related area of research we will investigate the central mechanism of regulation of core body temperature. Neurons from the pre-optic area of the anterior hypothalamus will be isolated and their activation by warmth studied electrophysiologically as above. The warmth-activated mechanism we have discovered in cholinergic neurons is a likely candidate for the molecular basis of the mechanism, and we will use in situ hybridisation to determine whether the warmth-sensitive ion channel cloned above is expressed in pre-optic neurons. If not we will use RNAseq as above to clone the thermo-regulatory ion channel.

Potential Impact:
Who will benefit from this research?

The research is intended to be primarily curiosity-driven fundamental research which will elucidate the molecular mechanisms of how sensory neurons detect temperature. If the mechanism underlying mammalian thermoregulation can be elucidated then this will have substantial potential applications in controlling hyperthermia (e.g. during infection) and in treating hypothermia (e.g. in the elderly). We anticipate that novel thermo-sensitive ion channels detecting extremes of temperature may also play a role in pain sensation and in conditions such as Raynaud's syndrome and will therefore be targets of considerable interest for the pharmaceutical industry.

The general public has tremendous curiosity about science, as is shown by the number of invitations the applicant receives to give talks about my work and the general area of pain on radio and TV programmes, and in person to popular audiences. We are able to satisfy the natural curiosity of the general public, and they are therefore also beneficiaries of this work.

How will they benefit from this research?

An understanding of how thermo-sensitive ion channels and how they are modulated is of particular interest to pharmaceutical companies. An analogy can be drawn with the thermo-TRP channel TRPV1, because a substantial component of inflammatory hyperalgesia in vivo is thought to originate from modulation of the temperature threshold of TRPV1, as is shown by the abolition of heat hyperalgesia in TRPV1 knockout mice. Blockers of TRPV1 have been a major area of recent research in pharmaceutical companies, with more than 50 companies having a drug development programme in this area.

Timescales for developing innovations arising from this research may not be long. An analogy can be drawn with the ion channel TRPV1, the first thermo-TRP to be cloned, which was published in 1997. Its importance in inflammatory pain was recognised as soon as genetically deleted mice were shown in 2000 to have an absence of inflammatory hyperalgesia. Pharmaceutical companies immediately began the development of antagonists and 10 years later a number have entered clinical trials. A similar rapid timescale of development has been seen with TRPA1, which was cloned in 2004 and for which several antagonists have already entered clinical trials with a view to treating bronchospasm in asthmatics.