Gene expression of thermosensitive TRP ion channels in the rat brain structures: Effect of adaptation to cold

Hypothalamic TRPM8 and TRPA1 ion channel genes in the regulation of temperature homeostasis at water balance changes

The role of the hypothalamic cold-sensitive ion channels - transient receptor potential melastatin 8 (TRPM8) and transient receptor potential ankyrin 1 (TRPA1) in homeostatic systems of thermoregulation and water-salt balance - is not clear. The interaction of homeostatic systems of thermoregulation and water-salt balance without additional temperature load did not receive due attention, too. On the models of water-balance disturbance, we tried to elucidate some aspect of these problems. Body temperature (Tbody), O2 consumption, CO2 excretion, electrical muscle activity (EMA), temperature of tail skin (Ttail), plasma osmolality, as well as gene expression of hypothalamic TRPM8 and TRPA1 have been registered in rats of 3 groups: control; water-deprived (3 days under dry-eating); and hyperhydrated (6 days without dry food, drinking liquid 4 % sucrose). No relationship was observed between plasma osmolality and gene expression of Trpm8 and Trpa1. In water-deprived rats, the constriction of skin vessels, increased fat metabolism by 10 % and increased EMA by 48 % allowed the animals to maintain Tbody unchanged. The hyperhydrated rats did not develop sufficient mechanisms, and their Tbody decreased by 0.8 °C. The development of reactions was correlated with the expression of genes of thermosensitive ion channels in the anterior hypothalamus. Ttail had a direct correlation with the expression of the Trpm8 gene, whereas EMA directly correlated with the expression of the Trpa1 gene in water-deprived group. The obtained data attract attention from the point of view of management and correction of physiological functions by modulating the ion channel gene expression.

Suppressive effects of a transient receptor potential melastatin 8 (TRPM8) agonist on hyperthermia-induced febrile seizures in infant mice

Background: Febrile seizures (FSs) are the most frequent type of seizures in infancy and childhood. Epileptiform discharges (EDs) on electroencephalogram at the time of first FS recurrence can increase the risk of epilepsy development. Therefore, inhibition of EDs is important. Recently, WS-3, a transient receptor potential melastatin 8 (TRPM8) agonist, reportedly suppressed penicillin G-induced cortical-focal EDs. However, the effects of TRPM8 agonists on FSs remain unknown. In this study, we aimed to clarify the effects of the TRPM8 agonist, and the absence of TRPM8 channels, on hyperthermia-induced FS by analyzing the fast ripple band.

Methods: Hyperthermia (43°C for 30 min) induced by a heating pad caused FSs in postnatal day 7 wild-type (WT) and TRPM8 knockout (TRPM8KO) mice. FSs were defined as EDs occurring during behavioral seizures involving hindlimb clonus and loss of the righting reflex. Mice were injected with 1% dimethyl sulfoxide or 1 mM WS-3 20 min before the onset of hyperthermia, and electroencephalograms; movies; and rectal, brain and heating pad temperatures were recorded.

Results: In wild-type mice, WS-3 reduced the fast ripple amplitude in the first FS without changing rectal and brain temperature thresholds. In contrast, the anti-FS effect induced by the TRPM8 agonist was not observed in TRPM8KO mice and, compared with wild-type mice, TRPM8 deficiency lowered the rectal and brain temperature thresholds for FSs, exacerbated the fast ripple amplitude, and prolonged the duration of the initial FS induced by hyperthermia.

Conclusion: Our findings suggest that TRPM8 agonists can be used to treat hyperthermia-induced FSs.

Effect of Long-Term Adaptation to Cold and Short-Term Cooling on the Expression of the TRPM2 Ion Channel Gene in the Hypothalamus of Rats

The present study is aimed to elucidate the possible involvement of the thermosensitive TRPM2 ion channel in changing of the temperature sensitivity of the hypothalamus after different cold exposures-long-term adaptation to cold and short-term cooling. Quantitative RT-PCR was used to study the expression of the gene of thermosensitive TRPM2 ion channel in the hypothalamus in the groups of control (kept for 5 weeks at +20 to +22 °C) and cold-adapted (5 weeks at +4 to +6 °C) rats, as well as in the groups of animals which were subjected to acute cooling (rapid or slow) with subsequent restoration of body temperature to the initial level. It has been shown that after long-term adaptation to cold, the decrease in the Trpm2 gene expression was observed in the hypothalamus, while a short-term cooling does not affect the expression of the gene of this ion channel. Thus, long-term adaptation to cold results in the decrease in the activity not only of the TRPV3 ion channel gene, as shown earlier, but also of the Trpm2 gene in the hypothalamus. The overlapping temperature ranges of the functioning of these ion channels and their unidirectional changes during the adaptation of the homoeothermic organism to cold suggest their functional interaction. The decrease in the Trpm2 gene expression may indicate the participation of this ion channel in adaptive changes in hypothalamic thermosensitivity, but only as a result of long-term cold exposure and not of a short-term cooling. These processes occurring at the genomic level are one of the molecular mechanisms of the adaptive changes.

Effect of Skin Ion Channel TRPM8 Activation by Cold and Menthol on Thermoregulation and the Expression of Genes of Thermosensitive TRP Ion Channels in the Hypothalamus of Hypertensive Rats

ISIAH (inherited stress-induced arterial hypertension) rats are characterized by high blood pressure and decreased Trpm8 gene expression in the anterior hypothalamus. Thermosensitive ion channel TRPM8 plays a critical role in the transduction of moderately cold stimuli that give rise to cool sensations. In normotensive animals, the activation of skin TRPM8 is known to induce changes in gene expression in the hypothalamus and induce alterations of thermoregulatory responses. In this work, in hypertensive rats, we studied the effects of activation of the peripheral TRPM8 by cooling and by application of a 1% menthol suspension on (1) the maintenance of body temperature balance and (2) mRNA expression of thermosensitive TRP ion channels in the hypothalamus. In these hypertensive animals, (1) pharmacological activation of peripheral TRPM8 did not affect the thermoregulatory parameters either under thermoneutral conditions or during cold exposure; (2) the expression of Trpm8 in the anterior hypothalamus approximately doubled (to the level of normotensive animals) under the influence of (a) slow cooling and (b) at pharmacological activation of the peripheral TRPM8 ion channel. The latter fact seems the quite important because it allows the proposal of a tool for correcting at least some parameters that distinguish a hypertensive state from the normotensive one.

Pit organ-based infrared discrimination sensitivity and signal transduction in the Burmese python (Python molurus bivitattus)

Burmese pythons (Python molurus bivitattus) use a unique infrared (IR) targeting system to acquire prey, avoid predators and seek thermoregulatory sites through detection of IR energy in the environment. Previous studies of sensitivity of the python IR system that relied on analysis of complex, natural behaviors lacked robust, reliable responses in animals habituated to experiments, and in vitro electrophysiological study failed to test behavioral function of the implicated protein thermoreceptor, TRPA1. The present study used conditioned discrimination procedures to analyze behavioral sensitivity and signal transduction in the python IR system. Pythons trained to behaviorally discriminate thermal stimuli averaged 70% correct choices, but failed to make correct choices when pit organs were physically occluded with IR-blocking material. The pythons exhibited greater sensitivity to thermal stimuli than previously reported, evident by correct choices that exceeded chance in response to a 14×10^-6Wcm^-2 irradiance contrast, or 0.5°C thermal differential. Finally, in a test of the behavioral role of the putative thermoreceptor protein TRPA1, despite pit organ treatment with a TRPA1 inhibitor, python performance exceeded chance and was similar to baseline discrimination and control trials. Collectively, the results suggest that the IR system is a high sensitivity, broad-spectrum thermosensor that may operate through different and/or multiple thermoreceptive proteins with overlapping spectral response profiles. The findings reported here provide a better understanding of the relationship between the brain, behavior and environment in driving survival and ecological success of the Burmese python, especially as an invasive megapredator in the southern United States.

Role of transient receptor potential vanilloid-1 in behavioral thermoregulation of the Mongolian gerbil Meriones unguiculatus

Ambient temperature considerably affects the physiology and behavior of mammals. Thermosensory and thermoregulatory abilities play an important role in the response to changing ambient temperature in endotherms. However, the molecular mechanisms of behavioral thermoregulation remain poorly understood. Transient receptor potential vanilloid-1 (TRPV1) is activated by changes in ambient temperature and is involved in acute thermoregulation. Here, we aimed to determine whether TRPV1 is involved in behavioral thermoregulation in wild rodents by conducting 2 experiments. In the first, 42 adult Mongolian gerbils (Meriones unguiculatus; 14 per treatment) were randomly assigned to 3 housing temperatures (4, 23, and 36°C) for 4 weeks. In the second, 20 gerbils (10 per treatment) were randomly injected with capsaicin (TRPV1 agonist) or AMG517 (TRPV1 antagonist). The results showed a significant decrease in food intake and non-shivering thermogenesis in the gerbils housed at 36°C. Additionally, there was a significant increase in the preference of gerbils housed at 4°C to low temperatures. The expression of TRPV1 protein in the brown adipose tissue (BAT) and liver was significantly positively correlated with that of protein kinase A (PKA). The expression of TRPV1 and PKA proteins in the BAT was positively correlated with the temperature preference of the gerbils. The gerbils injected with capsaicin preferred significantly lower temperatures than the control group gerbils. These findings suggest that TRPV1 and PKA are involved in behavioral thermoregulation in Mongolian gerbils.

Expression of the cold thermoreceptor TRPM8 in rodent brain thermoregulatory circuits

The cold- and menthol-activated ion channel transient receptor potential channel subfamily M member 8 (TRPM8) is the principal detector of environmental cold in mammalian sensory nerve endings. Although it is mainly expressed in a subpopulation of peripheral sensory neurons, it has also been identified in non-neuronal tissues. Here, we show, by in situ hybridization (ISH) and by the analysis of transgenic reporter expression in two different reporter mouse strains, that TRPM8 is also expressed in the central nervous system. Although it is present at much lower levels than in peripheral sensory neurons, we found cells expressing TRPM8 in restricted areas of the brain, especially in the hypothalamus, septum, thalamic reticular nucleus, certain cortices and other limbic structures, as well as in some specific nuclei in the brainstem. Interestingly, positive fibers were also found traveling through the major limbic tracts, suggesting a role of TRPM8-expressing central neurons in multiple aspects of thermal regulation, including autonomic and behavioral thermoregulation. Additional ISH experiments in rat brain demonstrated a conserved pattern of expression of this ion channel between rodent species. We confirmed the functional activity of this channel in the mouse brain using electrophysiological patch-clamp recordings of septal neurons. These results open a new window in TRPM8 physiology, guiding further efforts to understand potential roles of this molecular sensor within the brain.

Effect of Activation of Peripheral Ion Channel TRPM8 on Gene Expression of Thermosensitive TRP Ion Channels in the Hypothalamus. Comparison with the Effect of Cooling

Experiments on rats showed that activation of the peripheral ion channel TRPM8 with menthol and rapid cooling (decrease in core temperature by 3°C) led to 1.5-fold activation of the expression of TRPV3 ion channel gene in the posterior hypothalamus, but had no effect on the expression of this gene in the anterior hypothalamus. Neither stimulation of peripheral TRPМ8, nor acute cooling affected the expression of genes for other thermosensitive ion channels (TRPV1, TRPV2, TRPV4, TRPA1, and TRPМ8) in the hypothalamus. Enhanced expression of Trpv3 gene can indicate increased sensitivity of hypothalamic neurons in the range of TRPV3 ion channel functioning (31-39oC). The relationship between the changes in Trpv3 gene expression and the shift of thermoregulatory reaction thresholds is discussed. Our findings attest to the presence of a functional relationship between TRP ion channels of the peripheral nervous system and TRP channels in the central structures of the brain.

The Outwardly Rectifying Current of Layer 5 Neocortical Neurons that was Originally Identified as "Non-Specific Cationic" Is Essentially a Potassium Current

In whole-cell patch clamp recordings from layer 5 neocortical neurons, blockade of voltage gated sodium and calcium channels leaves a cesium current that is outward rectifying. This current was originally identified as a “non-specific cationic current”, and subsequently it was hypothesized that it is mediated by TRP channels. In order to test this hypothesis, we used fluorescence imaging of intracellular sodium and calcium indicators, and found no evidence to suggest that it is associated with influx of either of these ions to the cell body or dendrites. Moreover, the current is still prominent in neurons from TRPC1^-/- and TRPC5^-/- mice. The effects on the current of various blocking agents, and especially its sensitivity to intracellular tetraethylammonium, suggest that it is not a non-specific cationic current, but rather that it is generated by cesium-permeable delayed rectifier potassium channels.