Minoru Otokawa

Alteration in dopamine metabolism in the thermoregulatory center of exercising rats.

To examine the role of monoamines and amino acids in thermoregulation, we measured their concentrations in the preoptic area and anterior hypothalamus (PO/AH) in exercising rats, using an in vivo microdialysis technique. Body temperature (Tb) was monitored using a telemetry system. Tb increase by about 1.0 degrees C in the first 15 min of treadmill exercise (10 m/min; for 60 min), and was maintained thereafter at a steady high level possibly due to activation of the heat loss system. The levels of dopamine metabolites (3,4-dihydroxyphenylacetic acid and homovanillic acid) in the PO/AH significantly increased during exercise. However, exercise did not induce an increase in the level of either serotonergic substances (5-hydroxytryptamine and 5-hydroxyindoleacetic acid) or amino acids (aspartate and glutamate). Our data indicate that dopamine breakdown processes in the PO/AH are activated during exercise. Dopamine in the PO/AH may be involved in the heat loss mechanisms for thermoregulation when Tb rises during exercise.

Changes of body temperature and thermoregulatory responses of freely moving rats during GABAergic pharmacological stimulation to the preoptic area and anterior hypothalamus in several ambient temperatures.

Action of gamma-aminobutyric acid (GABA) in the preoptic area and anterior hypothalamus (PO/AH) has been implicated to regulate body temperature (T(b)). However, its precise role in thermoregulation remains unclear. Moreover, little is known about its release pattern in the PO/AH during active thermoregulation. Using microdialysis and telemetry techniques, we measured several parameters related to thermoregulation of freely moving rats during pharmacological stimulation of GABA in normal (23 degrees C), cold (5 degrees C), and hot (35 degrees C) ambient temperatures. We also measured extracellular GABA levels in the PO/AH during cold (5 degrees C) and heat (35 degrees C) exposure combined with microdialysis and high performance liquid chromatography (HPLC). Perfusion of GABA(A) agonist muscimol into the PO/AH increased T(b), which is associated with increased heart rate (HR), as an index of heat production in all ambient temperatures. Although tail skin temperature (T(tail)) as an index of heat loss increased only under normal ambient temperatures, its response was relatively delayed in comparison with HR and T(b), suggesting that the increase in T(tail) was a secondary response to increased HR and T(b). Locomotor activity also increased in all ambient temperatures, but its response was not extraordinary. Interestingly, thermoregulatory responses were different after perfusion of GABA(A) antagonist bicuculline at each ambient temperature. In normal ambient temperature conditions, perfusion of bicuculline had no effect on any parameter. However, under cold ambient temperature, the procedure induced significant hypothermia concomitant with a decrease in HR in spite of hyperactivity and increase of T(tail). It induced hyperthermia with the increase of HR but no additional change of T(tail) in hot ambient temperature conditions. Furthermore, the extracellular GABA level increased significantly during cold exposure. Its release was lower during heat exposure than in a normal environment. These results indicate that GABA in the PO/AH is an important neurotransmitter for disinhibition of heat production and inhibition of heat loss under cold ambient temperature. It is a neurotransmitter for inhibition of heat production under hot ambient temperature.

Functional role of the preoptic area and anterior hypothalamus in thermoregulation in freely moving rats

We recently reported that perfusion of tetrodotoxin (TTX) into the preoptic area and anterior hypothalamus (PO/AH), by using a microdialysis technique, induced an increase in body temperature (Tb) under normal and hot ambient temperatures (23 and 35 degrees C) in freely moving rats. However, the procedure had no effect on Tb under a cold ambient temperature (5 degrees C). The present study was designed to determine the mechanism(s) of increases in Tb after perfusion of TTX into the PO/AH, by measuring tail skin temperature (Ttail) as an index of heat loss, and heart rate (HR) and locomotor activity (Act) as indexes of heat production, under three ambient temperatures. Under normal ambient temperature (23 degrees C), perfusion of TTX induced significant hyperthermia with increased HR, Act and Ttail. In a hot environment (35 degrees C), perfusion of TTX induced a greater increase in Tb with increased HR but no change in Ttail and Act. In a cold environment (5 degrees C), perfusion of TTX had no effect on Tb with a slight increase in Act but no change in HR and Ttail. Our results suggest that the PO/AH may be involved in inhibition of heat production and excitation/inhibition of the tail vasomotor tone.

The role of preoptic area and anterior hypothalamus and median raphe nucleus on thermoregulatory system in freely moving rats

To clarify the role of the preoptic area and anterior hypothalamus (PO/AH) on thermoregulatory system and the effects of serotonergic innervation from the median raphe nucleus (MRN) on body temperature (Tb), we perfused tetrodotoxin (TTX) solution into the PO/AH or MRN by using a microdialysis technique at different ambient temperatures (5, 23 and 35 degrees C) in freely moving rats. Tb was continuously monitored by using a telemetry system. In the MRN, perfusion of TTX solution induced significant hypothermia in the normal environment, a greater decrease in Tb during cold exposure and had no effect on Tb during heat exposure. In the PO/AH, perfusion of TTX solution induced significant hyperthermia in normal environment, a greater increase in Tb during heat exposure and had no effect on Tb during cold exposure. Our results indicate that the PO/AH regulates mainly heat loss or inhibits the loci regulating heat production. Furthermore, heat production appears to be regulated by other loci receiving serotonergic innervation from the MRN.

Monoamines, amino acids and acetylcholine in the preoptic area and anterior hypothalamus of rats: measurements of tissue extracts and in vivo microdialysates

A microbore column high-performance liquid chromatography (HPLC) system was used to measure neurotransmitters in tissue extracts and in vivo microdialysates obtained from the preoptic area (PO) and anterior hypothalamus (AH) of rats. The extracts contained norepinephrine, epinephrine, 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine, 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT), aspartate, glutamate, GABA, acetylcholine (ACh) and choline. The microdialysates obtained from the PO and AH of freely moving rats contained all of these substances except for norepinephrine, epinephrine, dopamine, and 5-HT. During collection of microdialysate from the PO and AH, core body temperature and locomotor activity were simultaneously measured by means of telemetry. The locomotor activity and body temperature increased during the night. This was accompanied by increased levels of 5-HIAA. The results suggest that serotonergic neuronal mechanisms in the PO and AH may be involved in hypothalamic regulation of spontaneous behaviors and body temperature.

Effects of ambient light on body temperature regulation in resting and exercising rats.

We investigated the effects of environmental light and darkness on thermoregulation during both daytime and nighttime by monitoring body temperature (T(b)) and physical activity of rats using a telemetry system. Experiments were performed in both resting and exercising rats. In resting rats, lights-off during the daytime resulted in an increase in both T(b) and activity. Conversely, during the nighttime, T(b) decreased with the lights-on stimulus despite the fact that the activity was left unchanged. Treadmill exercise (10 m/min) always increased T(b) from the basal resting level. In both daytime and nighttime, exercising rats exhibited a persistent T(b)-rise when lights were on. However, in the lights-off condition at nighttime, the T(b) of exercising rats increased to a level significantly higher than that of exercising rats with the lights-on. Our results suggest that light at nighttime causes the suppression of T(b) in both resting and exercising rats.

Effect of chronic cold exposure on noradrenergic modulation in the preoptic area of thermoregulation in freely moving rats

For this study, we compared the thermoregulatory involvement of noradrenaline (NA) in the medial preoptic area (mPOA) of non-cold acclimated rats to that of cold-acclimated rats. We quantified the release of NA in the mPOA during 3 h cold (5 degrees C) exposure in room-temperature-acclimated rats (RA group, kept at 23 degrees C for 2 weeks) and cold-acclimated rats (CA group, kept at 5 degrees C for 2 weeks). We concurrently monitored the core body temperature (Tc), heart rate (HR), and tail skin temperature (Tt). Cold exposure significantly increased Tc and HR, and decreased Tt in both groups. However, the cold-induced increase of the extracellular NA levels in mPOA was observed only in the RA group: not in the CA group. To elucidate these different results in NA levels further, and to evaluate participation of the mPOA in thermoregulation in the cold, we measured Tc, HR, and Tt during perfusion of alpha-adrenoceptor antagonist phenoxybenzamine during cold exposure (5 degrees C). This pharmacological procedure induced marked hypothermia, with decreases in HR only in the RA group; no changes were observed in Tc or any thermoregulatory parameter in the CA group. These results suggest that NA in the mPOA modulates heat production in response to acute cold stress in the RA group. However, this thermoregulatory action of NA in the mPOA was attenuated in the CA group.