Kazuyuki Kanosue

Neuronal circuitries involved in thermoregulation

The body temperature of homeothermic animals is regulated by systems that utilize multiple behavioral and autonomic effector responses. In the last few years, new approaches have brought us new information and new ideas about neuronal interconnections in the thermoregulatory network. Studies utilizing chemical stimulation of the preoptic area revealed both heat loss and production responses are controlled by warm-sensitive neurons. These neurons send excitatory efferent signals for the heat loss and inhibitory efferent signals for the heat production. The warm-sensitive neurons are separated and work independently to control these two opposing responses. Recent electrophysiological analysis have identified some neurons sending axons directly to the spinal cord for thermoregulatory effector control. Included are midbrain reticulospinal neurons for shivering and premotor neurons in the medulla oblongata for skin vasomotor control. As for the afferent side of the thermoregulatory network, the vagus nerve is recently paid much attention, which would convey signals for peripheral infection to the brain and be responsible for the induction of fever. The vagus nerve may also participate in thermoregulation in afebrile conditions, because some substances such as cholecyctokinin and leptin activate the vagus nerve. Although the functional role for this response is still obscure, the vagus may transfer nutritional and/or metabolic signals to the brain, affecting metabolism and body temperature.

Efferent projection from the preoptic area for the control of non-shivering thermogenesis in rats

1 To investigate the characteristics of efferent projections from the preoptic area for the control of non‐shivering thermogenesis, we tested the effects of thermal or chemical stimulation, and transections of the preoptic area on the activity of interscapular brown adipose tissue in cold‐acclimated and non‐acclimated anaesthetized rats. 2 Electrical stimulation of the ventromedial hypothalamic nucleus (VMH) elicited non‐shivering thermogenesis in the brown adipose tissue (BAT); warming the preoptic area to 41.5 °C completely suppressed the thermogenic response. 3 Injections of d,l‐homocysteic acid (DLH; 0.5 mm, 0.3 μl) into the preoptic area also significantly attenuated BAT thermogenesis, whereas injections of control vehicle had no effect. 4 Transections of the whole hypothalamus in the coronal plane at the level of the paraventricular nucleus induced rapid and large rises in BAT and rectal temperatures. This response was not blocked by pretreatment with indomethacin. The high rectal and BAT temperatures were sustained more than 1 h, till the end of the experiment. Bilateral knife cuts that included the medial forebrain bundle but not the paraventricular nuclei elicited similar rises in BAT and rectal temperatures. Medial knife cuts had no effect. 5 These results suggest that warm‐sensitive neurones in the preoptic area contribute a larger efferent signal for non‐shivering thermogenesis than do cold‐sensitive neurones, and that the preoptic area contributes a tonic inhibitory input to loci involved with non‐shivering thermogenesis. This efferent inhibitory signal passes via lateral, but not medial, hypothalamic pathways.

Role of the medullary raphé in thermoregulatory vasomotor control in rats

To investigate the involvement of the medullary raphé in thermoregulatory vasomotor control, we chemically manipulated raphé neuronal activity while monitoring the tail vasomotor response to preoptic warming. For comparison, neuronal activity in the rostral ventrolateral medulla (RVLM) was manipulated in similar experiments. Injections of d,l‐homocysteic acid (DLH; 0.5 mm, 0.3 μl) into a restricted region of the ventral medullary raphé suppressed the tail vasodilatation normally elicited by warming the preoptic area to 42 °C. DLH injection into the RVLM also suppressed the vasodilatation elicited by preoptic warming. Injection of bicuculline (0.5 mm, 0.3 μl) into the same raphé region suppressed the vasodilatation elicited by preoptic warming. Bicuculline injection into the RVLM did not suppress tail vasodilatation. These results suggest that neurones in both the medullary raphé and the RVLM are vasoconstrictor to the tail, but only those in the raphé receive inhibitory input from the preoptic area. That input might be direct and/or indirect (e.g. via the periaqueductal grey matter).

Neuronal networks controlling thermoregulatory effectors

Publisher Summary This chapter summarizes the present knowledge about neuronal networks controlling thermoregulation, mainly knowledge about the efferent pathways from the hypothalamus. The body temperature of a homeothermic animal is regulated by multiple behavioral and autonomic effector responses, and the thermoreceptors responsible for these responses are distributed throughout the body. They are present in not only the skin and the hypothalamus but also in other brain areas and deep in the bodycore. The rostra1 ventrolateral medulla of a cat contains neurons projecting to the spinal cord and presumably sending signals to the sympathetic. Direct recording of sympathetic nerve activities in humans has revealed that skin sympathetic nerves contain vasoconstrictor and sudomotor fibers and that cutaneous sympathetic nerve activity is not correlated with the changes in blood pressure. Animals with preoptic and anterior hypothalamic lesions thermoregulate behaviorally, as well as control animals do, even though they show severe deficits in autonomic regulation. The only part of the neocortex or limbic system that has been systematically investigated in regard to behavioral thermoregulation is the sulcal prefrontal cortex in the rat.

Warm and cold signals from the preoptic area : which contribute more to the control of shivering in rats?

1. To find out whether the thermosensitive neurones in the preoptic area that control shivering are predominantly warm or cold sensitive, we tested the effects of injecting the excitatory amino acid L‐glutamate at various sites in and adjacent to the preoptic area of anaesthetized rats shivering at ambient temperatures of 15‐21 degrees C. 2. L‐Glutamate injections (0.2 mM in 0.5‐1.0 microliter), as well as preoptic warming and electrical stimulation, suppressed shivering, whereas control saline injections had no effect. Effective sites were restricted to the anterior part of the preoptic area, and a tenfold lower concentration of L‐glutamate did not influence shivering. 3. Injections of procaine (0.2 M) into the sites where L‐glutamate suppressed shivering did not affect strong shivering activity, but facilitated shivering in three out of seven cases when shivering was weak or absent at higher ambient temperatures (25‐30 degrees C). 4. L‐Glutamate injections, as well as preoptic warming and electrical stimulation, also elicited vasodilatation of the paw skin and the tail. Procaine elicited vasoconstriction when it was applied during vasodilatation induced by local preoptic warming. 5. These results indicate that the contribution of the preoptic area to the control of shivering and vasomotion is influenced more by signals from warm‐sensitive neurones than by signals from cold‐sensitive neurones.

Fos activation in hypothalamic neurons during cold or warm exposure: projections to periaqueductal gray matter.

The hypothalamus, especially the preoptic area, plays a crucial role in thermoregulation, and our previous studies showed that the periaqueductal gray matter is important for transmitting efferent signals to thermoregulatory effectors in rats. Neurons responsible for skin vasodilation are located in the lateral portion of the rostral periaqueductal gray matter, and neurons that mediate non-shivering thermogenesis are located in the ventrolateral part of the caudal periaqueductal gray matter. We investigated the distribution of neurons in the rat hypothalamus that are activated by exposure to neutral (26 degrees C), warm (33 degrees C), or cold (10 degrees C) ambient temperature and project to the rostral periaqueductal gray matter or caudal periaqueductal gray matter, by using the immunohistochemical analysis of Fos and a retrograde tracer, cholera toxin-b. When cholera toxin-b was injected into the rostral periaqueductal gray matter, many double-labeled cells were observed in the median preoptic nucleus in warm-exposed rats, but few were seen in cold-exposed rats. On the other hand, when cholera toxin-b was injected into the caudal periaqueductal gray matter, many double-labeled cells were seen in a cell group extending from the dorsomedial nucleus through the dorsal hypothalamic area in cold-exposed rats but few were seen in warm-exposed rats. These results suggest that the rostral periaqueductal gray matter receives input from the median preoptic nucleus neurons activated by warm exposure, and the caudal periaqueductal gray matter receives input from neurons in the dorsomedial nucleus/dorsal hypothalamic area region activated by cold exposure. These efferent pathways provide a substrate for thermoregulatory skin vasomotor response and non-shivering thermogenesis, respectively.

Leg stiffness adjustment for a range of hopping frequencies in humans

The purpose of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies. Ten male subjects performed in place hopping on two legs, at three frequencies (1.5, 2.2, and 3.0Hz). Leg stiffness, joint stiffness and touchdown joint angles were calculated from kinetic and/or kinematics data. Electromyographic activity (EMG) was recorded from six leg muscles. Leg stiffness increased with an increase in hopping frequency. Hip and knee stiffnesses were significantly greater at 3.0Hz than at 1.5Hz. There was no significant difference in ankle stiffness among the three hopping frequencies. Although there were significant differences in EMG activity among the three hopping frequencies, the largest was the 1.5Hz, followed by the 2.2Hz and then 3.0Hz. The subjects landed with a straighter leg (both hip and knee were extended more) with increased hopping frequency. These results suggest that over the range of hopping frequencies we evaluated, humans adjust leg stiffness by altering hip and knee stiffness. This is accomplished by extending the touchdown joint angles rather than by altering neural activity.

Thermoregulatory control of sympathetic fibres supplying the rat's tail

We investigated the thermoregulatory responses of sympathetic fibres supplying the tail in urethane‐anaesthetised rats. When skin and rectal temperatures were kept above 39 °C, tail sympathetic fibre activity was low or absent. When the trunk skin was cooled episodically by 2–7 °C by a water jacket, tail sympathetic activity increased in a graded fashion below a threshold skin temperature of 37.8 ± 0.6 °C, whether or not core (rectal) temperature changed. Repeated cooling episodes lowered body core temperature by 1.3–3.1 °C, and this independently activated tail sympathetic fibre activity, in a graded fashion, below a threshold rectal temperature of 38.4 ± 0.2 °C. Tail blood flow showed corresponding graded vasoconstrictor responses to skin and core cooling, albeit over a limited range. Tail sympathetic activity was more sensitive to core than to trunk skin cooling by a factor that varied widely (24‐fold) between animals. Combined skin and core cooling gave additive or facilitatory responses near threshold but occlusive interactions with stronger stimuli. Unilateral warming of the preoptic area reversibly inhibited tail sympathetic activity. This was true for activity generated by either skin or core cooling. Single tail sympathetic units behaved homogeneously. Their sensitivity to trunk skin cooling was 0.3 ± 0.08 spikes s−1°C−1 and to core cooling was 2.2 ± 0.5 spikes s−1°C−1. Their maximum sustained firing rate in the cold was 1.82 ± 0.35 spikes s−1.

Neurons of the rat preoptic area and the raphe pallidus nucleus innervating the brown adipose tissue express the prostaglandin E receptor subtype EP3

The major effector organ for thermogenesis during inflammation or experimental pyrogen‐induced fever in rodents is the brown adipose tissue (BAT). Prostaglandin E2 (PGE2) microinjection into the medial preoptic area (POA) of rats leads to hyperthermia through an increase in BAT thermogenesis and induces pyrogenic signal transmission towards the raphe pallidus nucleus (RPa), a brainstem nucleus known to contain sympathetic premotor neurons for BAT control. The medial POA has a high expression of prostaglandin E receptor subtype EP3 (EP3R) on POA neurons, suggesting that these EP3R are main central targets of PGE2 to mediate BAT thermogenesis. To reveal central command neurons that contain EP3R and polysynaptically project to the BAT, we combined EP3R immunohistochemistry with the detection of transneuronally labelled neurons that were infected after injection of pseudorabies virus into the BAT. Neurons double‐labelled with EP3R and viral surface antigens were particularly numerous in two brain regions, the medial POA and the RPa. Of all medial POA neurons that became virally infected 71 h after BAT inoculation, about 40% expressed the EP3R. This subpopulation of POA neurons is the origin of a complete neuronal chain that connects potential PGE2‐sensitive POA neurons with the BAT. As for the efferent pathway of pyrogenic signal transmission, we hypothesize that neurons of this subpopulation of EP3R expressing POA neurons convey their pyrogenic signals towards the BAT via the RPa. We additionally observed that two‐thirds of those RPa neurons that polysynaptically project to the interscapular BAT also expressed the EP3R, suggesting that RPa neurons themselves might possess prostaglandin sensitivity that is able to modulate BAT thermogenesis under febrile conditions.

Effect of midbrain stimulations on thermoregulatory vasomotor responses in rats.

1 Efferent projections eliciting vasodilatation when the preoptic area is warmed were investigated by monitoring tail vasomotor responses of ketamine‐anaesthetized rats when brain areas were stimulated electrically (0.2 mA, 200 μs, 0 Hz) or with the excitatory amino acid D,L‐homocysteic acid (1 mM, 0.3 μM). 2 Both stimulations elicited vasodilatation when applied within a region extending from the most caudal part of the lateral hypothalamus to the ventrolateral periaqueductal grey matter (PAG) and the reticular formation ventrolateral to the PAG. 3 Vasodilatation elicited by preoptic warming was suppressed when either stimulation was applied within the rostral part of the ventral tegmental area (VTA). 4 Sustained vasodilatation was elicited by knife cuts caudal to the VTA, and vasodilatation elicited by preoptic warming was suppressed by cuts either rostral to the VTA or in the region including the PAG and the reticular formation ventrolateral to it. 5 These results, together with the results of earlier physiological and histological studies, suggest that warm‐sensitive neurones in the preoptic area send excitatory signals to vasodilatative neurones in the caudal part of the lateral hypothalamus, ventrolateral PAG and reticular formation, and send inhibitory signals to vasoconstrictive neurones in the rostral part of the VTA.