Cutaneous temperature and unit activity in the hypothalamic thermoregulatory centers

Circadian Oscillations in the Murine Preoptic Area Are Reset by Temperature, but Not Light

Mammals maintain their internal body temperature within a physiologically optimal range. This involves the regulation of core body temperature in response to changing environmental temperatures and a natural circadian oscillation of internal temperatures. The preoptic area (POA) of the hypothalamus coordinates body temperature by responding to both external temperature cues and internal brain temperature. Here we describe an autonomous circadian clock system in the murine ventromedial POA (VMPO) in close proximity to cells which express the atypical violet-light sensitive opsin, Opn5. We analyzed the light-sensitivity and thermal-sensitivity of the VMPO circadian clocks ex vivo. The phase of the VMPO circadian oscillations was not influenced by light. However, the VMPO clocks were reset by temperature changes within the physiological internal temperature range. This thermal-sensitivity of the VMPO circadian clock did not require functional Opn5 expression or a functional circadian clock within the Opn5-expressing cells. The presence of temperature-sensitive circadian clocks in the VMPO provides an advancement in the understanding of mechanisms involved in the dynamic regulation of core body temperature.

Role of the Preoptic Area in Sleep and Thermoregulation

Sleep and body temperature are tightly interconnected in mammals: warming up our body helps to fall asleep and the body temperature in turn drops while falling asleep. The preoptic area of the hypothalamus (POA) serves as an essential brain region to coordinate sleep and body temperature. Understanding how these two behaviors are controlled within the POA requires the molecular identification of the involved circuits and mapping their local and brain-wide connectivity. Here, we review our current understanding of how sleep and body temperature are regulated with a focus on recently discovered sleep- and thermo-regulatory POA neurons. We further discuss unresolved key questions including the anatomical and functional overlap of sleep- and thermo-regulatory neurons, their pathways and the role of various signaling molecules. We suggest that analysis of genetically defined circuits will provide novel insights into the mechanisms underlying the coordinated regulation of sleep and body temperature in health and disease.

Regulation of Body Temperature by the Nervous System

The regulation of body temperature is one of the most critical functions of the nervous system. Here we review our current understanding of thermoregulation in mammals. We outline the molecules and cells that measure body temperature in the periphery, the neural pathways that communicate this information to the brain, and the central circuits that coordinate the homeostatic response. We also discuss some of the key unresolved issues in this field, including the following: the role of temperature sensing in the brain, the molecular identity of the warm sensor, the central representation of the labeled line for cold, and the neural substrates of thermoregulatory behavior. We suggest that approaches for molecularly defined circuit analysis will provide new insight into these topics in the near future.

More than a marker: interaction between the circadian regulation of temperature and sleep, age-related changes, and treatment possibilities

The neurobiological mechanisms of both sleep and circadian regulation have been unraveled partly in the last decades. A network of brain structures, rather than a single locus, is involved in arousal state regulation, whereas the suprachiasmatic nucleus (SCN) has been recognized as a key structure for the regulation of circadian rhythms. Although most models of sleep regulation include a circadian component, the actual mechanism by which the circadian timing system promotes--in addition to homeostatic pressure--transitions between sleep and wakefulness remains to be elucidated. Little more can be stated presently than a probable involvement of neuronal projections and neurohumoral factors originating in the SCN. This paper reviews the relation among body temperature, arousal state, and the circadian timing system and proposes that the circadian temperature rhythm provides an additional signaling pathway for the circadian modulation of sleep and wakefulness. A review of the literature shows that increased brain temperature is associated with a type of neuronal activation typical of sleep in some structures (hypothalamus, basal forebrain), but typical of wakefulness in others (midbrain reticular formation, thalamus). Not only local temperature, but also skin temperature are related to the activation type in these structures. Warming of the skin is associated with an activation type typical of sleep in the midbrain reticular formation, hypothalamus, and cerebral cortex (CC). The decreasing part of the circadian rhythm in core temperature is mainly determined by heat loss from the skin of the extremities, which is associated with strongly increased skin temperature. As such, alterations in core and skin temperature over the day could modulate the neuronal activation state or "preparedness for sleep" in arousal-related brain structures. Body temperature may thus provide a third signaling pathway, in addition to synaptic and neurohumoral pathways, for the circadian modulation of sleep. A proposed model for the effects of body temperature on sleep appears to fit the available data better than previous hypotheses on the relation between temperature and sleep. Moreover, when the effects of age-related thermoregulatory alterations are introduced into the model, it provides an adequate description of age-related changes in sleep, including shallow sleep and awakening closer to the nocturnal core temperature minimum. Finally, the model indicates that appropriately timed direct (passive heating) or indirect (bright light, melatonin, physical activity) manipulation of the nocturnal profile of skin and core temperature may be beneficial to disturbed sleep in the elderly. Although such procedures could be viewed by researchers as merely masking a marker for the endogenous rhythm, they may in fact be crucial for sleep improvement in elderly subjects.

Distribution and characterization of tumor necrosis factor-alpha-like immunoreactivity in the murine central nervous system

Tumor necrosis factor-alpha (TNF alpha) is a protein released from macrophages during infection and inflammation. Recent studies suggest that it has several effects within the central nervous system, including generation of fever, enhancement of slow wave sleep, and stimulation of pituitary hormone secretion. We have proposed that TNF alpha may be synthesized by neurons in the CNS and used as a neuromodulator in the pathways involved in the central control of these activities. To test this hypothesis, we have used an antiserum raised against recombinant murine (rm) TNF alpha with an indirect immunoperoxidase technique to stain the murine CNS immunohistochemically. Western blot analysis of mouse brain homogenates revealed one band with electrophoretic mobility identical to that of rmTNF alpha. We identified TNF alpha-like immunoreactive (ir) neurons in the hypothalamus, in the bed nucleus of the stria terminalis, in the caudal raphe nuclei, and along the ventral pontine and medullary surface. TNF alpha ir innervation was widespread within the CNS, particularly in areas involved in autonomic and endocrine regulation, including the hypothalamus, amygdala, bed nucleus of the stria terminalis, parabrachial nucleus, dorsal vagal complex, nucleus ambiguus, and thoracic sympathetic preganglionic cell column. Our data suggest that TNF alpha may serve as a neuromodulator in central pathways involved in the regulation of the autonomic, endocrine and behavioral components of the acute-phase response to inflammation and infection.

Anatomical and electrophysiological evidence for a projection from the medial preoptic area to the 'mesencephalic and subthalamic locomotor regions' in the rat

There is considerable physiological evidence indicating that the medial preoptic area plays an important role in neural circuits mediating ingestive, thermoregulatory, and reproductive behaviors, all of which involve foraging. The current series of anatomical and electrophysiological experiments was therefore designed to characterize a direct projection from the medial preoptic area to a region in the zona incerta just dorsal to the subthalamic nucleus, which appears to lie within the 'subthalamic locomotor region', and to the pedunculopontine nucleus, which lies within the 'mesencephalic locomotor region'. First, implants of the fluorescent tracer True blue were placed in the pedunculopontine nucleus, and retrogradely labeled neurons were consistently found in dorsal regions of the medial preoptic nucleus, anteroventral preoptic nucleus, rostral tip of the medial preoptic area, lateral parts of the medial preoptic area, and median preoptic nucleus. Second, combined retrograde-immunostaining experiments indicated that a small number of galanin-stained neurons in the rostral tip of the medial preoptic area project to the pedunculopontine nucleus, whereas in nearby regions some galanin- or neurotensin-stained neurons in the lateral preoptic area, and some neurotensin-stained neurons in the substriatal gray appear to project to the pedunculopontine nucleus, as do some neurotensin- or corticotropin releasing factor (CRF)-stained cells in the bed nucleus of the stria terminalis. Third, injections of the anterograde tracer Phaseolus vulgaris leukoagglutinin (PHA-L) into various parts of the medial preoptic area all labeled axons with terminal boutons in the caudal zona incerta and pedunculopontine nucleus. Fourth, single-pulse stimuli were delivered to the zona incerta and pedunculopontine nucleus and the location of antidromically activated neurons in the medial preoptic area was mapped using extracellular recordings. Somewhat less than one-third of the cells recorded from in the medial preoptic area were antidromically activated from either site and some 14% were influenced from both sites. The application of a reciprocal collision test to a small number of neurons suggested that at least some neurons in the medial preoptic area may send collaterals to both sites. And fifth, injections of procaine into the zona incerta were shown to block the antidromic activation of medial preoptic neurons by single-pulse stimulation of the pedunculopontine nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of selective cutaneous denervation on hypothalamic thermosensitivity in rats

The effect of altering input from cutaneous thermoreceptors of the face and trunk on the relationship between hypothalamic temperature (Thy) and heat production (HP) was studied in three rats. The signal from cutaneous receptors was altered in two ways: by altering skin temperature (Tsk) and by sectioning nerves supplying cutaneous receptors. It was found that when Tsk was lowered in normal rats Thy threshold for thermoregulatory HP was elevated, but the slope of the relationship between Thy and HP was not significantly altered. After the spinal nerves serving the trunk skin were sectioned, the slope was reduced and the threshold was elevated markedly at both test ambient temperatures (Ta), but Ta had essentially the same effect on the Thy vs. HP relationship after cutaneous denervation as before. Clearly, eliminating input from trunk cutaneous thermoreceptors has a different effect than does lowering or raising Tsk, but thermoregulation is being achieved by the same basic mechanism before and after cutaneous denervation. After the cranial nerves supplying the skin of the face were also sectioned, there was a further elevation in the Thy threshold for HP at Ta = 25 degrees C but no change at Ta = 15 degrees C. It is concluded that cutaneous denervation does not substantially interfere with the rat's ability to regulate its body temperature, and that the reduced Thy sensitivity and increased Thy threshold exhibited after cutaneous denervation is the result of input from intact warm- and cold-thermoreceptors located in the core and in tissues intermediate to core and skin.

Dynamic response of preoptic and hypothalamic neurons to scrotal thermal stimulation in rats

Activities of preoptic and anterior hypothalamic (POAH) neurons were recorded in anesthetized rats in response to scrotal thermal stimulation. Thirteen out of 54 neurons responsive to changes of scrotal temperature (Tscr) showed dynamic responses. Four of these neurons increased and 3 neurons decreased their firing rates responding dynamically to warming but not to cooling. Six other neurons were inhibited by scrotal cooling only. These dynamic responses were produced even by temperature changes as slow as 2 degrees C/min and only when the scrotum was warmed or cooled in the Tscr range above 35 degrees C. These dynamic responses are suggested to be a result of signal processing in supraspinal structures including POAH itself.

Effect of delta 9-tetrahydrocannabinol on hypothalamic thermosensitive units

Thermosensitive anterior hypothalamic neurons (pre-optic region) were studied in urethane and chloralose anesthetized cats in an attempt to characterize the hypothermic action of delta 9-THC at the neuronal level. One hundred and seventy-eight single neurons were isolated and subjected to thermal challenge, 66 were found to reproducibly alter firing frequency at a significant level (thermosensitivity (T.S.) greater than 0.75). Twenty-one of these units met the criteria for primary thermodetectors, 34 were heat-sensitive interneurons, and 11 were cold-sensitive interneurons. Administration of delta 9-THC (1.0-2.0 mg/kg i.v.) decreased the spontaneous firing and increased the T.S. of the primary thermodetector units. delta 9-THC also increased the spontaneous firing frequency as well as the T.S. of heat-sensitive interneurons, while decreasing both the T.S. and spontaneous firing of cold-sensitive interneurons. The decreased spontaneous firing of primary thermodetectors could result from altered facilitory or inhibitory influences converging on these cells. The increased thermosensitivity is consistent with the hypothesis that the pre-optic region modulates cannabinoid-induced hypothermia.

Specific responses of rat raphé neurones to skin temperature

1. The responses of single neurones in the nuclei raphés magnus, medianus, dorsalis and pontis to changes in skin temperature were recorded in rats anaesthetized with urethane. Skin temperature was altered by means of a water-perfused jacket. 2. Of 210 neurones studied, thirty-five were specifically excited by warming the skin whilst twenty were cold responsive. The greatest proportion of cells responding to skin temperature were in the nucleus raphé magnus, whilst few neurones in the raphé dorsalis and pontis were influenced. 3. The warm units had peak activity at a mean skin temperature of 37.7 degrees C whilst the cold cells had a corresponding maximal rate at 29.0 degreet C. Mechanical and noxious peripheral stimulation, blood pressure changes and temperatures other than that of skin did not affect the neurones. 4. The neurones influenced by skin temperature were histologically verified as being within the raphé system. 5. LSD inhibited all neurones tested, indicating that the cells were serotonergic. 6. The responses to skin temperature were unchanged in rats with midcollicular sections suggesting an ascending thermal system. 7. The results suggest that any involvement of 5-HT in central thermo-regulation is in terms of an afferent thermal pathway mediated by serotonergic raphé neurones.

Thermosensitive midbrain neurons in the cat

Responses of single neurons in the midbrain of cats anesthetized with chloral hydrate were studied during manipulations of midbrain temperature produced with a bilateral water-perfused thermode. Temperatures of the thermodes and the anterior hypothalamus were monitored while single neuron activity was recorded between the thermodes and correlated with the midbrain temperature. Q10's and thermal coefficients were calculated from the estimated temperature at the neuron itself. A surprisingly high percentage (72%) of the 72 neurons recorded in the caudal paramedian midbrain of 11 cats were thermoresponsive. Most of these were heat sensitive and exhibited a variety of frequency/temperature curves. Explorations of more rostral regions of the midbrain in 9 cats yielded only 18% thermoresponsive units out of the 99 neurons sampled. We suggest that the concentrated pool of warm-sensitive neurons in the caudal midbrain is part of an extensive system of brain stem thermosensors which are involved in establishing and controlling normal brain temperature.

Morphine hyperthermia in the rat: an action on the central thermostats

The mechanism underlying the hyperthermic response to low doses of morphine has been investigated in rats. Doses of morphine sulfate less than 10 mg/kg i.p. caused a rise in body temperature accompanied by vasoconstriction of the cutaneous blood vessels of the tail. This hyperthermia, unlike the hypothermia following higher doses of morphine was not blocked by naloxone nor did tolerance develop to the response. Injections directly into the hypothalamus suggested that, as with the fall in temperature after high doses of morphine, the hyperthermic effect is also due to an action on the preoptic/anterior hypothalamic thermoregulatory centers. Experiments measuring thermoregulatory behavior showed that rats delayed escaping from a heat load after low doses of morphine even though their core temperature was rising. These results suggest that low doses of morphine raise the set point of the central thermostats in rats resulting in a hyperthermia mediated, at least in part, by decreased cutaneous heat loss.

The effect of spinal and skin temperatures on the firing rate and thermosensitivity of preoptic neurones

1. In anaesthetized rabbits, preoptic single unit activity was recorded while preoptic, spinal cord and skin temperatures were independently manipulated.2. The units that were insensitive to preoptic temperature were characterized by low firing rates and also by a very low incidence of extrahypothalamic thermosensitivity.3. Thirty-seven units having positive coefficients to preoptic temperature were tested for their response to spinal or skin temperature. Of these, twenty-two units responded to extrahypothalamic temperature, seventeen with positive thermal coefficients. In addition, the incidence of extrahypothalamic thermosensitivity generally increased among the higher firing units.4. Twenty-two units had negative coefficients for preoptic temperature and were tested for their extrahypothalamic thermosensitivities. Of these, sixteen units had dual thermosensitivities, ten with negative coefficients for the extrahypothalamic temperatures. In addition, there was no correlation between the incidence of extrahypothalamic thermosensitivity and the level of firing rate.5. In the units having positive coefficients for preoptic temperature, an increased firing rate, due to extrahypothalamic temperature, generally resulted in a decreased preoptic thermosensitivity. Conversely, a decreased firing rate usually resulted in an increased preoptic thermosensitivity.6. In the units having negative coefficients for preoptic temperature, an increased firing rate, due to extrahypothalamic temperature, usually increased the preoptic thermosensitivity; while a decreased firing rate tended to decrease the sensitivity to preoptic temperature.