Spinal cord thermosensitivity: An afferent phenomenon?

Revisiting concepts of thermal physiology: understanding negative feedback and set-point in mammals, birds, and lizards

The thermoregulatory system of homeothermic endotherms operates to attain thermal equilibrium, that is no net loss or gain of heat, where possible, under a thermal challenge, and not to attain a set-point or any other target body temperature. The concept of a set-point in homeothermic temperature regulation has been widely misinterpreted, resulting in such confusion that some thermoregulation specialists have recommended that it be abandoned. But the set-point concept has enjoyed a resurgence in a different domain, lizard microclimate selection. We review the principles of thermoregulation in homeotherms, endorse a negative feedback system with independent set-points for individual thermo-effectors as its core mechanism, and address the misconceptions about homeothermic set-point. We also explore the concept of set-point range in lizard microclimate selection and conclude that there is substantial convergence between that concept and the set-points of homeothermic thermo-effectors, as thresholds. In neither homeothermic nor lizard thermoregulation is the concept of a unitary set-point appropriate. We review the problems of measuring the set-points for lizard microclimate selection. We do not believe that the set-point concept in thermoregulation should be abandoned just because it has been misinterpreted by some users. It is a valid concept, identifying the threshold body temperatures at which regulatory thermo-effectors will be activated, to aid in attaining thermal equilibrium.

Prostaglandin E2 production in the brainstem parabrachial nucleus facilitates the febrile response

Our body temperature is normally kept within a narrow range of 1°C. For example, if our body temperature rises, such as in a hot environment or due to strenuous exercise, our thermoregulatory system will trigger a powerful heat defense response with vasodilation, sweating, and lowered metabolism. During fever, which often involves body temperatures of up to 41°C, this heat defense mechanism is apparently inhibited; otherwise, the rising body temperature would be immediately combated, and fever would not be allowed to develop. New evidence suggests how and where this inhibition takes place. In two consecutive studies from Cheng et al. and Xu et al., it has been shown that prostaglandin E2, which generates fever by acting on thermosensory neurons in the preoptic hypothalamus, also acts on neurons in the brainstem parabrachial nucleus, which receive temperature information from temperature-activated spinal cord neurons and relay this information to the thermoregulatory center in the hypothalamus to either induce cold or heat defenses. By acting on the same type of prostaglandin E2 receptor that is critical for fever generation in the preoptic hypothalamus, the EP3 receptor, prostaglandin E2 inhibits the signaling of the heat-responsive parabrachial neurons, while stimulating the cold-responsive neurons. These novel findings thus show that prostaglandin E2, by binding to the same receptor subtype in the parabrachial nucleus as in the preoptic hypothalamus, adjusts the sensitivity of the thermosensory system in a coordinated manner to allow the development of febrile body temperatures.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

Corticospinal and spinal excitability during peripheral or central cooling in humans

Cold exposure can impair fine and gross motor control and threaten survival. Most motor task decrement is due to peripheral neuromuscular factors. Less is known about cooling on central neural factors. Corticospinal and spinal excitability were determined during cooling of the skin (T_sk) and core (T_co). Eight subjects (four female) were actively cooled in a liquid perfused suit for 90 min (2 °C inflow temperature), passively cooled for 7 min, and then rewarmed for 30 min (41 °C inflow temperature). Stimulation blocks included 10 transcranial magnetic stimulations [eliciting motor evoked potentials (MEPs) which indicate corticospinal excitability], 8 trans-mastoid electrical stimulations [eliciting cervicomedullary evoked potentials (CMEPs) which indicate spinal excitability] and 2 brachial plexus electrical stimulations [eliciting maximal compound motor action potentials (M_max)]. These stimulations were delivered every 30 min. Cooling for 90 min reduced T_sk to 18.2 °C while T_co did not change. At the end of rewarming T_sk returned to baseline while T_co decreased by 0.8 °C (afterdrop) (P < 0.001). Metabolic heat production was higher than baseline at the end of passive cooling (P = 0.01), and 7 min into rewarming (P = 0.04). MEP/M_max remained unchanged throughout. CMEP/M_max increased by 38% at end cooling (although increased variability at this time rendered the increase insignificant, P = 0.23) and 58% at end warming when T_co was 0.8 °C below baseline (P = 0.02). Cooling increased spinal excitability but not corticospinal excitability. Cooling may decrease cortical and/or supraspinal excitability which is compensated for by increased spinal excitability. This compensation is key to providing a motor task and survival advantage.

Spinal and corticospinal excitability in response to reductions in skin and core temperature via whole-body cooling

Cold stress impairs fine and gross motor movements. Although peripheral effects of muscle cooling on performance are well understood, less is known about central mechanisms. This study characterized corticospinal and spinal excitability during surface cooling, reducing skin (Tsk) and core (Tes) temperature. Ten subjects (3 female) wore a liquid-perfused suit and were cooled (9°C perfusate, 90 min) and rewarmed (41°C perfusate, 30 min). Transcranial magnetic stimulation [eliciting motor evoked potentials (MEPs)], as well as transmastoid [eliciting cervicomedullary evoked potentials (CMEPs)] and brachial plexus [eliciting maximal compound motor action potentials (Mmax)] electrical stimulation, were applied at baseline, every 20 min during cooling, and following rewarming. Sixty minutes of cooling, reduced Tsk by 9.6°C (P<0.001) but Tes remained unchanged (P=0.92). Tes then decreased ~0.6℃ in the next 30 minutes of cooling (P<0.001). Eight subjects shivered. During rewarming, shivering was abolished, and Tsk returned to baseline while Tes did not increase. During cooling and rewarming, Mmax, MEP, and MEP/Mmax were unchanged from baseline. However, CMEP and CMEP/Mmax increased during cooling by ~85% and 79% (P<0.001) respectively, and remained elevated post-rewarming. Results suggest that spinal excitability is facilitated by reduced Tsk during cooling, and reduced Tes during warming, while corticospinal excitability remains unchanged. ClinicalTrials.gov ID NCT04253730 Novelty: • This is the first study to characterize corticospinal, and spinal excitability during whole body cooling, and rewarming in humans. • Whole body cooling did not affect corticospinal excitability. • Spinal excitability was facilitated during reductions in both skin and core temperatures.

Diminished Cold Avoidance Behaviours after Chronic Cold Exposure - Potential Involvement of TRPM8

Ambient temperature changes trigger plastic biological responses. Cold temperature is detected by the somatosensory system and evokes perception of cold together with adaptive physiological responses. We addressed whether chronic cold exposure induces adaptive adjustments of (1) thermosensory behaviours, and (2) the principle molecular cold sensor in the transduction machinery, transient receptor potential melastatin subtype 8 (TRPM8). Mice in two groups were exposed to either cold (6 °C) or thermoneutral (27 °C) ambient temperatures for 4 weeks and subjected to thermosensory behavioural testing. Cold group mice behaved different from Thermoneutral group in the Thermal Gradient Test: the former occupied a wider temperature range and was less cold avoidant. Furthermore, subcutaneous injection of the TRPM8 agonist icilin, enhanced cold avoidance in both groups in the Thermal Gradient Test, but Cold group mice were significantly less affected by icilin. Primary sensory neuron soma are located in dorsal root ganglia (DRGs), and western blotting showed diminished TRPM8 levels in DRGs of Cold group mice, as compared to the Thermoneutral group. We conclude that acclimation to chronic cold altered thermosensory behaviours, so that mice appeared less cold sensitive, and potentially, TRPM8 is involved.

Regulation of Body Temperature by the Nervous System

The regulation of body temperature is one of the most critical functions of the nervous system. Here we review our current understanding of thermoregulation in mammals. We outline the molecules and cells that measure body temperature in the periphery, the neural pathways that communicate this information to the brain, and the central circuits that coordinate the homeostatic response. We also discuss some of the key unresolved issues in this field, including the following: the role of temperature sensing in the brain, the molecular identity of the warm sensor, the central representation of the labeled line for cold, and the neural substrates of thermoregulatory behavior. We suggest that approaches for molecularly defined circuit analysis will provide new insight into these topics in the near future.

Central nervous system circuits that control body temperature

Maintenance of mammalian core body temperature within a narrow range is a fundamental homeostatic process to optimize cellular and tissue function, and to improve survival in adverse thermal environments. Body temperature is maintained during a broad range of environmental and physiological challenges by central nervous system circuits that process thermal afferent inputs from the skin and the body core to control the activity of thermoeffectors. These include thermoregulatory behaviors, cutaneous vasomotion (vasoconstriction and, in humans, active vasodilation), thermogenesis (shivering and brown adipose tissue), evaporative heat loss (salivary spreading in rodents, and human sweating). This review provides an overview of the central nervous system circuits for thermoregulatory reflex regulation of thermoeffectors.

Transcutaneous transmission of photobiomodulation light to the spinal canal of dog as measured from cadaver dogs using a multi-channel intra-spinal probe

The target level photobiomodulation (PBM) irradiances along the thoracic to lumbar segment of the interior spinal canal in six cadaver dogs resulting from surface illumination at 980 nm were measured. Following a lateral hemi-laminectomy, a flexible probe fabricated on a plastic tubular substrate of 6.325 mm diameter incorporating nine miniature photodetectors was embedded in the thoracic to lumbar segment of the spinal canal. Intra-spinal irradiances at the nine photodetector sites, spanning an approximate 8 cm length caudal to T13, were measured for various applied powers of continuous wave (CW) surface illumination at 980 nm with a maximal power of 10 W corresponding to a surface irradiance of 3.14 W/cm². The surface illumination conditions differed in skin transmission when the probe was off-contact with tissue and probe-skin contact when the skin was in place. For each condition of surface illumination, the beam was directed to respectively T13 (surface site 1), a spinal column site 4 cm caudal to T13 (surface site 5), and a spinal column site 8 cm caudal to T13 (surface site 9). Off-contact surface irradiation of 3.14 W/cm² at surface sites 1, 5, and 9 transmitted respectively 234.0 ± 120.7 μW/cm², 230.7 ± 178.3 μW/cm², and 130.2 ± 169.6 μW/cm² to the spinal canal without the skin, and respectively 35.7 ± 33.2 μW/cm², 50.9 ± 75.3 μW/cm², and 15.7 ± 16.3 μW/cm² with the skin. Transmission with skin was as low as 12% of the transmission without the skin. On-contact surface irradiation of 3.14 W/cm² at surface sites 1, 5, and 9 transmitted respectively 44.6 ± 43.1 μW/cm², 85.4 ± 139.1 μW/cm², and 22.0 ± 23.6 μW/cm² to the spinal canal. On-contact application increased transmission by a maximum of 67% comparing to off-contact application. The information gathered highlights the need to clinically consider the impact of skin transmission and on-contact application technique when attempting to treat spinal cord disease with PBM.

Peripheral thermoreceptors in innocuous temperature detection

The mammalian skin is innervated by cold-sensitive afferent neurons. These neurons exhibit ongoing activity at temperatures between ~10 and 42°C, are activated by innocuous cold stimuli, inhibited by warm stimuli and are mechanoinsensitive. Their axons are small-diameter myelinated (Aδ-) fibers in primates and unmyelinated (C-) fibers in nonprimate mammals. The mammalian skin is innervated by warm-sensitive afferent neurons. The density of innervation by these neurons is lower than that by cold-sensitive afferents. They exhibit ongoing activity between ~38 and 48°C, are activated by warm stimuli, inhibited by cold stimuli, and are mechanoinsensitive. Their axons are unmyelinated (C-) fibers. Cold-sensitive unmyelinated afferent neurons exhibit prominent cold sensitivity of their axons (in rats). The discharge pattern of the cutaneous cold-sensitive afferent neurons is fully preserved after nerve injury. Ongoing impulse activity and cold-evoked impulses originate ectopically at the nerve injury site. Deep somatic tissues and viscera are innervated by thermosensitive afferent neurons. Most are warm-sensitive and mechanoinsensitive and have unmyelinated axons. These afferent neurons have only rarely and incompletely been studied, e.g., in the upper gastrointestinal tract, the liver (both vagal afferents), the dorsal abdominal wall, and the skeletal muscle. Spinal cord warm sensitivity may be mediated by cutaneous afferent neurons with unmyelinated axons that are excited by spinal cord warming.