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.

The influence of cooling and TRPM8 ion channel activation on the level of pro-inflammatory cytokines in normotensive and hypertensive rats.

The role of thermosensitive TRP ion channels in physiological processes in the whole organism is far from being clear. In present work we tried to understand the possible participation of the cold-sensitive ion channel TRPM8 in regulation of the pro-inflammatory cytokine level in blood, and to see if hypertension (widely spread disease) changes this relationship. Experiments were carried out on normotensive Wistar rats and rats with inherited stress-induced arterial hypertension. The effects of deep rapid and slow cooling with decrease in rectal temperature by 3°C, and activation of the skin TRPM8 ion channel by its agonist menthol (application of 1% L-menthol) on the concentration of pro-inflammatory cytokines (IL-6, IL-1ß and TNFα) in blood have been studied. The data have shown that the TRPM8 ion channel participates in regulation of pro-inflammatory cytokine in the whole organism, as well as exposure to cold. In normotensive animals the activation of the cold-sensitive TRPM8 ion channel in skin by its agonist menthol without any temperature shifts causes an increase (about two fold) of pro-inflammatory cytokines IL-6 and IL-1 ß in blood without any changes in the level of TNFα. The effect of cooling (slow or rapid) on IL-6 and IL-1ß is comparable with the effect of the TRPM8 activation. The changes in TNFα concentration were observed only at slow cooling. In hypertensive rats there were no changes in the level of pro-inflammatory cytokines under the effect of cooling or activation of the TRPM8 ion channel. This evidences for the decrease in TRPM8 activity under arterial hypertension. One of possible mechanisms of profound changes at arterial hypertension may be the alteration in activity of some ion channels, and TRPM8 is one of them.

5-HT2A receptors control body temperature in mice during LPS-induced inflammation via regulation of NO production.

G protein-coupled 5-HT2A receptors are involved in the regulation of numerous normal and pathological physiological functions. At the same time, its involvement in the regulation of body temperature (Tb) in normal conditions is obscure. Here we study the effect of the 5-HT2A receptor activation or blockade on Tb in sick animals. The experiments were carried out on adult C57BL/6 mouse males. Systemic inflammation and sickness were produced by lipopolysaccharide (LPS, 0.1mg/kg, ip), while the 5-HT2A receptor was stimulated or blocked through the administration of the receptor agonist DOI or antagonist ketanserin (1mg/kg), respectively. LPS, DOI or ketanserin alone produced no effect on Tb. However, administration of LPS together with a peripheral or central ketanserin injection reduced Tb (32.2°C). Ketanserin reversed the LPS-induced expression of inducible NO synthase in the brain. Consequently, an involvement of NO in the mechanism of the hypothermic effect of ketanserin in sick mice was hypothesized. Administration of LPS together with NO synthase inhibitor, l-nitro-arginine methyl ester (60mg/kg, ip) resulted in deep (28.5°C) and prolonged (8h) hypothermia, while administration of l-nitro-arginine methyl ester alone produced no effect on Tb. Thus, 5-HT2A receptors play a key role in Tb control in sick mice. Blockade of this GPCR produces hypothermia in mice with systemic inflammation via attenuation of LPS-induced NO production. These results indicate an unexpected role of 5-HT2A receptors in inflammation and NO production and have a considerable biological impact on understanding the mechanism of animal adaptation to pathogens and parasites. Moreover, adverse side effects of 5-HT2A receptor antagonists in patients with inflammation may be expected.

Effect of the sympathetic nervous system co-transmitters ATP and norepinephrine on thermoregulatory response to cooling.

The existence of co-transmitters of the sympathetic nervous system norepinephrine (NE) and ATP implies variations in the neuromodulator mechanisms of physiological processes. The role of ATP, as a transmitter of the peripheral part of sympathetic nervous system in the formation of thermoregulatory response is not clear. Whether ATP modulates any parameters of thermoregulatory response to cold; if yes, whether co-transmitters of sympathetic nervous system ATP and NE differently modulate thermoregulatory response and on which parameters of cold-defense response the influence of ATP is more pronounced. Experiments were carried out on rats. ATP (10(-6)), NE (10(-3)), and their mixture introduced iontophoretically into skin. Their effects on thermoregulatory parameters (temperature parameters, total oxygen consumption, carbon dioxide release, muscle activity, respiratory coefficient) were studied in thermoneutral conditions (without cold load) and under the cooling. In thermoneutral conditions both ATP and NE enhance total metabolism through increase in metabolic rate of lipids, NE effect being more expressed. It was shown that ATP and NE influence predominantly on the different components of the metabolic response to cold. ATP affects to the greatest extent on cold muscular thermogenesis by increasing shivering almost twofold and lowering its initiation temperature thresholds, whereas NE mainly promotes increase in non-shivering thermogenesis. When introducing the mixture of these biological substances the effect of NE is more expressed and the ATP effect is weakened. The obtained results allow to suggest that in vivo the NE effects can be more expressed when the sympathetic nervous system is stimulated by cold. Thus, NE and ATP being co-transmitters and predominantly acting on the different processes of cold thermogenesis (ATP on shivering and NE on non-shivering) may organize the certain sequence of cold defense responses.

Central 5-HT3 receptor-induced hypothermia is associated with reduced metabolic rate and increased heat loss.

Activation of central 5-HT(3) receptors by the selective agonist m-CPBG (1-(3-chlorophenyl)biguanide hydrochloride, 40 nM i.c.v.) produced stronger hypothermic effect in mice than activation of 5-HT(1A) receptors by their agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propilamino)tetralin) injected by the same route at an equimolar dose. The hypothermic effect of m-CPBG was realized by influence on both the heat production and the heat loss: oxygen consumption and CO(2) expiration were decreased; heat dissipation determined by the tail skin temperature was increased. The heat loss effect of 5-HT(3) receptors was significantly shorter than the decrease in metabolism indicating the prevalent role of heat production decrease in 5-HT(3) receptor-induced deep and long-lasing hypothermia. In addition, the decrease in the respiratory exchange ratio (RER) was shown suggesting that the activation of the 5-HT(3) receptors switched metabolism to prevalent use of lipids as the main energetic substrate. 5-HT(1A) receptor agonist 8-OH-DPAT (40 nM i.c.v.) produced less depressing effect on general metabolism: a decrease in oxygen consumption and CO(2) excretion began later and was not so deep as after m-CPBG administration. Heat-loss effect of 5-HT(1A) receptors activation was not observed. In contrast to m-CPBG effect, RER after 5-HT(1A) receptors activation raised immediately after injection and then gradually decreased to the values observed in m-CPBG-treated mice. Obtained results show that activation of central 5-HT(3) receptors are more effective in hypothermia induction due to marked decrease in thermogenesis and increase in heat loss.