Mayumi Nakamura

Relative importance of different surface regions for thermal comfort in humans

In a previous study, we investigated the contribution of the surface of the face, chest, abdomen, and thigh to thermal comfort by applying local temperature stimulation during whole-body exposure to mild heat or cold. In hot conditions, humans prefer a cool face, and in cold they prefer a warm abdomen. In this study, we extended investigation of regional differences in thermal comfort to the neck, hand, soles, abdomen (Experiment 1), the upper and lower back, upper arm, and abdomen (Experiment 2). The methodology was similar to that used in the previous study. To compare the results of each experiment, we utilized the abdomen as the reference area in these experiments. Thermal comfort feelings were not particularly strong for the limbs and extremities, in spite of the fact that changes in skin temperature induced by local temperature stimulation of the limbs and extremities were always larger than changes that were induced in the more proximal body parts. For the trunk areas, a significant difference in thermal comfort was not observed among the abdomen, and upper and lower back. An exception involved local cooling during whole-body mild cold exposure, wherein the most dominant preference was for a warmer temperature of the abdomen. As for the neck and abdomen, clear differences were observed during local cooling, while no significant difference was observed during local warming. We combined the results for the current and the previous study, and characterized regional differences in thermal comfort and thermal preference for the whole-body surface.

Estrogen in the medial preoptic nucleus of the hypothalamus modulates cold responses in female rats.

The present study examined the effect of the central administration of estrogen on responses to the cold. Estrogen or cholesterol was applied locally to the medial preoptic nucleus (MPO) or dorsomedial hypothalamic nucleus (DMH) of the hypothalamus in free-moving ovariectomized rats. Forty-eight hours after the application, rats had 2-h exposure at 10 or 25 degrees C. Body temperature (T(b)) and the tail surface temperature (T(tail)) were continuously measured by telemetry and thermography, respectively. The change of T(b) at 10 degrees C from the 25 degrees C baseline was higher in the estrogen application in the MPO than that in the cholesterol application; however, such difference was not observed in the DMH application. The uncoupling 1 protein mRNA level in the interscapular brown adipose tissue involved in non-shivering thermogenesis was not different between the estrogen and cholesterol applications in the MPO and DMH. T(tail) decreased in the cold, which was greater after the estrogen application in the MPO than after the cholesterol application. These results show that estrogen affects the MPO in female rats, changing T(b) in the cold. Moreover, suppression of heat loss from the tail may be involved in the mechanism.

Effects of alcohol on autonomic responses and thermal sensation during cold exposure in humans

We investigated the effects of alcohol on thermoregulatory responses and thermal sensations during cold exposure in humans. Eight healthy men (mean age 22.3+/-0.7 year) participated in this study. Experiments were conducted twice for each subject at a room temperature of 18 degrees C. After a 30-min resting period, the subject drank either 15% alcohol at a dose of 0.36 g/kg body weight (alcohol session) or an equal volume of distilled water (control session), and remained in a sitting position for another 60 min. Mean skin temperature continued to decrease and was similar in control and alcohol sessions. Metabolic rate was lower in the alcohol session, but the difference did not affect core temperature, which decreased in a similar manner in both alcohol and control sessions (from 36.9+/-0.1 degrees C to 36.6+/-0.1 degrees C). Whole body sensations of cold and thermal discomfort became successively stronger in the control session, whereas these sensations were both greatly diminished after drinking alcohol. In a previous study we performed in the heat, using a similar protocol, alcohol produced a definite, coordinated effect on all autonomic and sentient heat loss effectors. In the current study in the cold, as compared to responses in the heat, alcohol intake was followed by lesser alterations in autonomic effector responses, but increased changes in sensations of temperature and thermal discomfort. Overall, our results indicate that although alcohol influences thermoregulation in the cold as well as in the heat, detailed aspects of the influence are quite different.

Mild hypohydration induced by exercise in the heat attenuates autonomic thermoregulatory responses to the heat, but not thermal pleasantness in humans

Hypohydration caused by exercise in the heat attenuates autonomic thermoregulation such as sweating and skin blood flow in humans. In contrast, it remains unknown if behavioral thermoregulation is modulated during hypohydration. We assume that thermal unpleasantness could drive the behavioral response, and would also be modulated during hypohydration. Nine healthy young men participated in the present study. Body and skin temperatures were monitored. Ratings of thermal sensation and pleasantness were conducted. After approximately 45 min rest at 27 degrees C, they performed 50-min cycling exercise, which was at the level of 40% of heart rate range at 35 degrees C (hypohydration trial) or at the level of 10% of heart rate range at 23 degrees C (control trial), respectively. Subjects returned to the rest at 27 degrees C, and the ambient temperature was then changed from 22 to 38 degrees C. Body weight decreased by 0.9+/-0.1% immediately after exercise in the hypohydration trial and 0.3+/-0.1% in the control trial. In the cold, no significant difference in thermal sensation or pleasantness was observed between trials. There was no significant difference in thermal pleasantness between trials in the heat, although thermal sensation in the heat (32.5-36 degrees C) was significantly lower in the hypohydration trial than in the control trial. In addition, laser Doppler flow of the skin and sweat rate were attenuated in the heat in the hypohydration trial. These results may indicate that mild hypohydration after exercise in the heat has no influence on behavioral responses to the heat.

Hyperosmolality in the plasma modulates behavioral thermoregulation in mice: the quantitative and multilateral assessment using a new experimental system.

We evaluated the effect of plasma hyperosmolality on behavioral thermoregulation in mice, using a new experimental system. The system consisted of Plexiglas box (dimensions: 50×12×19 cm) with five computer-controlled Peltier boards (dimensions: 10×10 cm) at the bottom. Experiments were conducted in two different settings of the system. An operant behavior setting: each board was first set to 39°C, and the right-end board was changed to 20°C for 1 min when a mouse moved to a specific position. A temperature mosaic setting: each board was randomly set to 15°C, 22°C, 28°C, 35°C, or 39°C with a 6-min interval, but each board temperature was different from the others at a given time point. Mice were injected subcutaneous (s.c.) isotonic or hypertonic saline (154 mM (IS group) or 2,500 mM (HS group), 10 ml/kg body wt), and exposed to either setting for 90 min. In the operant setting, the HS group showed fewer operant behavior counts than the IS group (11±5 and 25±4 counts, respectively; P<0.05) with greater increase in body temperature (1.6±0.4°C vs. 0.0±0.2°C, respectively; P<0.05). In the mosaic setting, the HS group selected the board temperature of 35°C more frequently than the other temperatures (P<0.05) with the same increase in body temperature. These results may suggest that plasma hyperosmolality modulates behavioral thermoregulatory response to heat and induce regulated hyperthermia.

The regional difference in temperature related sensations

It is unclear how our brain predicts reward and punishing outcome on perception of ambiguous stimuli in environment. To investigate relationship between perceptually ambiguous stimuli and reward processing on them, we examined reward-predicting brain activity on high and low coherency random dot motion stimuli by using fMRI. First, we trained subjects to establish a contingency between particular direction of motion stimuli and delivery of reward. Then, we presented high and low coherence motion stimuli, and asked subjects to judge direction of the motion. Juice or saliva was followed by stimulus direction associated with reward or neutral outcome, respectively. The data at the time of cue presentation showed that activation in the caudate was correlated with reward prediction based on stimulus direction that was influenced by coherence level, whereas reward-predicting activation based on subject’s performance was observed in the putamen. The results suggested that stimulus-based and perception-based reward prediction could be dissociated in the basal ganglia.

Menstrual cycle affects thermal regulation and comfort

Dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis is associated with vulnerability to depression and anxiety disorders. Daily repeated exposure to a stressor resulted in reduced HPA axis response to the same stressor, that is termed habituation. In this study, we established the animal model of vulnerability to repeated stress. We found that Fischer344 (F344) rats showed the deficit in habituation to repeated restraint stress such as suppressed inhibition of HPA axis and hypothalamic neuronal activity. In addition, F344 rats showed the decreased cell proliferation in hippocampus and increased anxiety behavior. Moreover, we found that microRNA (miR)-18a inhibited translation of glucocorticoid receptor (GR) mRNA in vitro and F344 rats showed the decreased expression of GR protein and increased expression of miR-18a in the hypothalamus. Thus, our results suggest that miR-18a-mediated downregulation of GR could be involved in vulnerability to stress.