Effects of electrical stimulation of ventral septal area on firing rates of pyrogen-treated thermosensitive neurons in preoptic anterior hypothalamus from rabbits

Although there is considerable evidence supporting that fever evolved as a host defense response, it is important that the rise in body temperature would not be too high. Many endogenous cryogens or antipyretics that limit the rise in body temperature have been identified. Endogenous antipyretics attenuate fever by influencing the thermoregulatory neurons in the preoptic anterior hypothalamus (POAH) and in adjacent septal areas including ventral septal area (VSA). Our previous study showed that intracerebroventricular (I.C.V.) injection of interleukin-1beta (IL-1beta) affected electrophysiological activities of thermosensitive neurons in VSA regions, and electrical stimulation of POAH reversed the effect of IL-1beta. To further investigate the functional electrophysiological connection between POAH and VSA and its mechanisms in thermoregulation, the firing rates of thermosensitive neurons in POAH of forty-seven unit discharge were recorded by using extracellular microelectrode technique in New Zealand white rabbits. Our results show that the firing rates of the warm-sensitive neurons decreased significantly and those of the cold-sensitive neurons increased in POAH when the pyrogen (IL-1beta) was injected I.C.V. The effects of IL-1beta on firing rates in thermosensitive neurons of POAH were reversed by electrical stimulation of VSA. An arginine vasopressin (AVP) V1 antagonist abolished the regulatory effects of VSA on the firing rates in thermosensitive neurons of POAH evoked by IL-1beta. However, an AVP V2 antagonist had no effects. These data indicated that VSA regulates the activities of the thermosensitive neurons of POAH through AVP V1 but not AVP V2 receptor.     

Cyclic GMP alters the firing rate and thermosensitivity of hypothalamic neurons

The rostral hypothalamus, especially the preoptic-anterior hypothalamus (POAH), contains temperature-sensitive and -insensitive neurons that form synaptic networks to control thermoregulatory responses. Previous studies suggest that the cyclic nucleotide cGMP is an important mediator in this neuronal network, since hypothalamic microinjections of cGMP analogs produce hypothermia in several species. In the present study, immunohistochemisty showed that rostral hypothalamic neurons contain cGMP, guanylate cyclase (necessary for cGMP synthesis), and CNG A2 (an important cyclic nucleotide-gated channel). Extracellular electrophysiological activity was recorded from different types of neurons in rat hypothalamic tissue slices. Each recorded neuron was classified according to its thermosensitivity as well as its firing rate response to 2-100 microM 8-bromo-cGMP (a membrane-permeable cGMP analog). cGMP has specific effects on different neurons in the rostral hypothalamus. In the POAH, the cGMP analog decreased the spontaneous firing rate in 45% of temperature-sensitive and -insensitive neurons, an effect that is likely due to cGMP-enhanced hyperpolarizing K(+) currents. This decreased POAH activity could attenuate thermoregulatory responses and produce hypothermia during exposures to cool or neutral ambient temperatures. Although 8-bromo-cGMP did not affect the thermosensitivity of most POAH neurons, it did increase the warm sensitivity of neurons in other hypothalamic regions located dorsal, lateral, and posterior to the POAH. This increased thermosensitivity may be due to pacemaker currents that are facilitated by cyclic nucleotides. If some of these non-POAH thermosensitive neurons promote heat loss or inhibit heat production, then their increased thermosensitivity could contribute to cGMP-induced decreases in body temperature.     

The roles of monoaminergic neurotransmitters in thermoregulation

Recent studies of the organization of the thermoregulatory system and evaluation of experimental evidence from electrophysiological, neuropharmacological, and neuroanatomical studies suggest that the monoamines noradrenaline and 5-hydroxytryptamine are involved in modulations of thermoregulation rather than in thermoregulation per se: they do not seem to transfer specific thermal information but rather modulate the signals passing from thermosensors to thermoregulatory effectors. Theoretically, the central monoamines could be modulating the input from thermosensors, or the central integration of thermal signals, or the outflow of signals to thermoregulatory effectors. The modulatory action of the monoamines on thermosensitive and thermointegrative hypothalamic neurons is best documented. There, the monoamines 5-hydroxytryptamine and noradrenaline seem to act as antagonists, which enhance or diminish the effects of thermal afferents mediated by other transmitters. Moreover, the antagonistic monoaminergic systems are apparently interconnected and can influence each other at a lower brain stem level. The activity in central monoaminergic systems can also be modified by neurohumoral feedback mechanisms from the periphery. By means of these interrelations the vegetative responses of the organism can be corrected and optimized.     

Single neuron studies and their usefulness in understanding thermoregulation

Electrophysiological studies of hypothalamic thermosensitive neurons have been conducted for the past 25 years. These studies have greatly improved our understanding of the neural control of thermoregulation. They have added a sense of reality to black-box models, and they have fostered the development of neuronal models having a major effect on the predictions and conclusions made in thermoregulatory studies. Neuron studies not only provide an understanding of the synaptic and cellular basis of thermosensitivity, but they also permit morphological identifications of neurons and their pathways. Neuron studies have identified sites at which central temperature information is integrated with peripheral temperature information. In addition, these experiments provide functional explanations for the types of integration observed. Neuron studies also provide explanations for the central actions of a variety of neurochemicals important in thermoregulation. Finally, neuronal specificity studies have aided in restoring the view that thermoregulation is part of a complex homeostatic system in which various regulatory systems interact with each other.     

Neuronal activities related to thermoregulation

The hypothalamic thermoregulatory center was regarded as a black box some 20 years ago. Subsequent microelectrode exploration revealed the existence of two kinds of thermosensitive neurons in this area. Discharges of these neurons are now recorded not only from anesthetized animals but also from tissue explants, tissue slices, and unanesthetized animals as well. Neuronal responses produced by some stimuli have been compared to whole-body thermoregulatory responses. Parallelism between the two was found in the actions of chemicals, in integration of peripheral and central temperatures, in cortical influence, and in temperature effects on other hypothalamic functions, implicating specific key roles to thermosensitive neurons in thermoregulation.     

Effect of beta-endorphin on the thermal excitability of preoptic neurons in the unanesthetized rabbit

The opioid peptide, beta-endorphin (beta-E), will promote changes in body temperature when injected into the brain. It is possible that beta-E alters body temperature by affecting the activity of thermoregulatory neurons in the preoptic anterior hypothalamus (POAH). Single unit activity in the POAH was recorded in unanesthetized rabbits while radiant heat was applied to the dorsal skin. Beta-E was then microinjected into the POAH, and the peripheral heating was repeated. Seventy-seven percent of the POAH neurons were responsive to skin heating. Beta-E and equal excitatory and inhibitory effects on warm-excited and warm-inhibited neurons. Four of six warm-excited neurons were converted to warm-inhibited or unresponsive following beta-E injection. Six out of ten warm-inhibited neurons were converted to warm-excited or unresponsive by beta-E. Beta-E-induced shifts in thermal excitability of POAH neurons may be responsible for the ability of POAH injections of beta-E to elevate body temperature in the rabbit.     

2010 Carl Ludwig Distinguished Lectureship of the APS Neural Control and Autonomic Regulation Section: Central neural pathways for thermoregulatory cold defense

Central neural circuits orchestrate the homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the research leading to a model representing our current understanding of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for control of heat loss, and brown adipose tissue, skeletal muscle, and the heart for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific core efferent pathways within the central nervous system (CNS) that share a common peripheral thermal sensory input. The thermal afferent circuit from cutaneous thermal receptors includes neurons in the spinal dorsal horn projecting to lateral parabrachial nucleus neurons that project to the medial aspect of the preoptic area. Within the preoptic area, warm-sensitive, inhibitory output neurons control heat production by reducing the discharge of thermogenesis-promoting neurons in the dorsomedial hypothalamus. The rostral ventromedial medulla, including the raphe pallidus, receives projections form the dorsomedial hypothalamus and contains spinally projecting premotor neurons that provide the excitatory drive to spinal circuits controlling the activity of thermogenic effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a platform for further understanding of the functional organization of central thermoregulation.     

Carbon dioxide and pH effects on temperature-sensitive and -insensitive hypothalamic neurons.

The preoptic-anterior hypothalamus (POAH) controls body temperature, and thermoregulatory responses are impaired during hypercapnia. If increased CO(2) or its accompanying acidosis inhibits warm-sensitive POAH neurons, this could provide an explanation for thermoregulatory impairment during hypercapnia. To test this possibility, extracellular electrophysiological recordings determined the effects of CO(2) and pH on the firing rates of both temperature-sensitive and -insensitive neurons in hypothalamic tissue slices from 89 male Sprague-Dawley rats. Firing rate activity was recorded in 121 hypothalamic neurons before, during, and after changing the CO(2) concentration aerating the tissue slice chamber or changing the pH of the solution bathing the tissue slices. Increasing the aeration CO(2) concentration from 5% (control) to 10% (hypercapnic) had no effect on most (i.e., 69%) POAH temperature-insensitive neurons; however, this hypercapnia inhibited the majority (i.e., 59%) of warm-sensitive neurons. CO(2) affected similar proportions of (non-POAH) neurons in other hypothalamic regions. These CO(2) effects appear to be due to changes in pH since the CO(2)-affected neurons responded similarly to isocapnic acidosis (i.e., normal CO(2) and decreased pH) but were not responsive to isohydric hypercapnia (i.e., increased CO(2) and normal pH). These findings may offer a neural explanation for some heat-related illnesses (e.g., exertional heat stroke) where impaired heat loss is associated with acidosis.     

Thermoregulatory responses to thermal stimulation of the preoptic anterior hypothalamus in the red fox (Vulpes vulpes)

The preoptic anterior hypothalamus (POAH) thermoregulatory controller can be characterized by two types of control, an adjustable setpoint temperature and changing POAH thermal sensitivity. Setpoint temperatures for shivering (T_shiver) and panting (T_pant) both increased with decreasing ambient temperature (T_a), and decreased with increasing T_a. The POAH controller is twice as sensitive to heating as to cooling. Metabolic rate (MR) increased during both heating and cooling of the POAH. Resting temperature of the POAH was lower than internal body temperature (T_b) at all temperatures. This indicates the presence of some form of brain cooling mechanism. Decreased T_b during POAH heating was a result of increased heat dissipation, while higher T_b during POAH cooling was a result of increased heat production and reduced heat dissipation. The surface temperature responses indicated that foxes can actively control heat flow from body surface. Such control can be achieved by increased peripheral blood flow and vasodilation during POAH heating, and reduced peripheral blood flow and vasoconstriction during POAH cooling. The observed surface temperature changes indicated that the thermoregulatory vasomotor responses can occur within l min following POAH heating or cooling. Such a degree of regulation can be achieved only by central neural control. Only surface regions covered with relatively short fur are used for heat dissipation. These thermoregulatory effective surface areas account for approximately 33% of the total body surface area, and include the area of the face, dorsal head, nose, pinna, lower legs, and paws.     

Interactions of reproductive steroids, osmotic pressure, and glucose on thermosensitive neurons in preoptic tissue slices

The preoptic area contains thermosensitive neurons, thought to be important in thermoregulation, and steroid-sensitive neurons, thought to be involved in reproduction. The preoptic area also contains osmosensitive neurons, considered important in water balance, and glucosensitive neurons, thought to function in the regulation of glucose. If these various neurons belong to separate populations, one might predict that most osmosensitive, glucosensitive, and steroid-sensitive neurons constitute the population of temperature-insensitive neurons rather than thermosensitive neurons. To test this hypothesis, single unit activity was recorded in preoptic tissue slices prepared from male rats. In addition to temperature changes, neuronal responses were examined with various perfusion media containing testosterone or estradiol (30 pg/mL), low glucose (1.0 mM), and increased osmotic pressure (309 mosmol/kg). It was found that the steroid-sensitive, osmosensitive, and glucosensitive neurons were not confined to the temperature-insensitive neurons; but that nearly half of the thermosensitive neurons responded to these nonthermal stimuli. This lack of specificity was also observed between osmosensitive and glucosensitive neurons; however, most of the steroid-sensitive neurons were highly specific for either estradiol or testosterone. Although these findings do not suggest a strong functional specificity for preoptic neurons, they do support studies emphasizing interactions between regulatory systems.     

Central circuitries for body temperature regulation and fever

Body temperature regulation is a fundamental homeostatic function that is governed by the central nervous system in homeothermic animals, including humans. The central thermoregulatory system also functions for host defense from invading pathogens by elevating body core temperature, a response known as fever. Thermoregulation and fever involve a variety of involuntary effector responses, and this review summarizes the current understandings of the central circuitry mechanisms that underlie nonshivering thermogenesis in brown adipose tissue, shivering thermogenesis in skeletal muscles, thermoregulatory cardiac regulation, heat-loss regulation through cutaneous vasomotion, and ACTH release. To defend thermal homeostasis from environmental thermal challenges, feedforward thermosensory information on environmental temperature sensed by skin thermoreceptors ascends through the spinal cord and lateral parabrachial nucleus to the preoptic area (POA). The POA also receives feedback signals from local thermosensitive neurons, as well as pyrogenic signals of prostaglandin E(2) produced in response to infection. These afferent signals are integrated and affect the activity of GABAergic inhibitory projection neurons descending from the POA to the dorsomedial hypothalamus (DMH) or to the rostral medullary raphe region (rMR). Attenuation of the descending inhibition by cooling or pyrogenic signals leads to disinhibition of thermogenic neurons in the DMH and sympathetic and somatic premotor neurons in the rMR, which then drive spinal motor output mechanisms to elicit thermogenesis, tachycardia, and cutaneous vasoconstriction. Warming signals enhance the descending inhibition from the POA to inhibit the motor outputs, resulting in cutaneous vasodilation and inhibited thermogenesis. This central thermoregulatory mechanism also functions for metabolic regulation and stress-induced hyperthermia.     

Thermosensitivity of the preoptic region and the spinal cord in the golden hamster

1. 1.|The effects of thermal stimulation of the preoptic region (POAH) and the spinal cord on non-shivering thermogenesis (NST) and shivering were studied in euthermic golden hamsters.

2. 2.|Shivering intensity is suppressed by heating the POAH but is independent of spinal cord temperature. Therefore, NST in the interscapular brown adipose tissue does not suppress shivering.

3. 3.|NST is inhibited by heating of the POAH as well as of spinal cord. It is discussed that the control of NST by two different central thermosensitive areas is significant for thermoregulation during exercise.

Effects of temperature and neuroactive substances on hypothalamic neurones in vitro: possible implications for the induction of fever.

This paper reviews some of our findings which have shown the usefulness of in vitro methods in the study of hypothalamic neurones. (1) Membrane current analyses of dispersed neurones of the rat preoptic and anterior hypothalamus (POA) during thermal stimulation have revealed that warm-sensitive neurones are endowed with a non-inactivating Na+ channel having a high Q10 in the hyperthermic range (35-41 degrees C). (2) A brain slice study has shown that neurones in the organum vasculosum lamina terminalis (OVLT) region have much higher sensitivity to PGE2 than POA neurones. This provides further evidence of a critical role of the OVLT in translation of blood-borne cytokine signals into brain signals for fever induction. (3) Local application of IL-1 beta and IFN alpha altered the activity of thermosensitive (TS) neurones and glucose responsive (GR) neurones in vitro in an appropriate way to produce fever and anorexia. While the responses to IL-1 beta required the local release of prostaglandins, the responses to IFN alpha were found to be mediated by opioid receptor mechanisms. (4) The responses of POA TS neurones and VMH GR neurones to IL-1 beta but not those to IFN alpha, were reversibly blocked by alpha MSH, an endogenous antipyretic peptide. Thus, immune cytokines and their related neuroactive substances may affect hypothalamic TS and GR neurones thereby producing elaborately regulated changes in homeostatic functions such as thermoregulation (fever) and feeding (anorexia), which are considered as host defence responses.     

Central thermosensitivity and the integrative responses of hypothalamic neurons

1. Hypothalamic thermosensitive neurons are important in the integrative regulation of body temperature. In addition to receiving afferent thermal input, their activity correlates with the stimulation of thermoregulatory mechanisms.

2. While cold sensitivity is synaptically mediated, warm sensitivity depends on inherent cellular responses. Warm sensitive neurons may also respond to other homeostatic variations, hormones, and endogenous mediators.

3. In contrast to peripheral thermoresponsiveness, which depends on conductance changes that regulate membrane potential, hypothalamic warm sensitivity relies on regulating the prepotential phase of the action potential.

4. Additionally, the recent morphologic characterization of these neurons supports the criterion for warm sensitivity.

葛根素,生石膏配伍对致热原作用下猫POAH温敏神经元放电…

应用微电极细胞外记录技术,在３４只猫ＰＯＡＨ区记录了温敏神经元单位放电,研究中药葛根素和生石膏解热可能的中机制。致热原使４例热敏神经元放电频率减少;使１１例冷敏神经元放电频率增加,注射等量葛根素和生石膏能反转上述作用。致热原及二药对５例温度不敏感神经元放电无影响。结果显示,葛根素和生石膏是影响致热原作用下ＰＯＡＨ区温敏神经元的电活动而解热的。二者配伍使用对冷敏神经元放电频率的影响比单独使用作用强,     

Hypothalamic mechanisms in thermoregulation

Certain preoptic and rostral hypothalamic neurons are sensitive to changes in local preoptic temperature (Tpo). These neurons also receive much afferent input from peripheral thermoreceptors and control a variety of thermoregulatory responses. In thermode-implanted animals, preoptic warming increases the firing rate in warm-sensitive neurons and elicits heat loss responses such as panting and sweating. Preoptic cooling increases the firing rate in cold-sensitive neurons and elicits, first, heat retention responses (e.g., cutaneous vasoconstriction and thermoregulatory behavior), then heat production responses (e.g., shivering and nonshivering thermogenesis). It is likely that the preoptic thermosensitive neurons control these thermoregulatory responses because both respond similarly to changes in Tpo and skin temperature. Specifically, skin warming not only increases panting, skin blood flow, and the firing rate of warm-sensitive neurons, but also decreases the sensitivity of all these responses to Tpo changes. Skin cooling not only increases metabolic heat production, heat retention behavior, and the firing rate of cold-sensitive neurons, but also increases the hypothalamic thermosensitivity of all these responses. Low-firing warm-sensitive neurons receive little afferent input and are most sensitive to high Tpo. Many of these low-firing neurons probably serve in controlling heat loss responses. High-firing warm-sensitive neurons receive much excitatory afferent input and are usually sensitive only to low Tpo. These neurons probably exert their greatest influence on heat production responses, possibly by inhibiting and, thus, determining the thermosensitive characteristics of nearby cold-sensitive neurons.     

葛根素、生石膏配伍对致热原作用下猫POAH温敏神经元放电的影响

应用微电极细胞外记录技术,在３４只猫ＰＯＡＨ区记录了温敏神经元单位放电,研究中药葛根素和生石膏解热可能的中枢机制。致热原使１４例热敏神经元放电频率减少;使１１例冷敏神经元放电频率增加。注射等量葛根素和生石膏能反转上述作用。致热原及二药对５例温度不敏感神经元放电无影响。结果显示,葛根素和生石膏是影响致热原作用下ＰＯＡＨ区温敏神经元的电活动而解热的。二者配伍使用对冷敏神经元放电频率的影响比单独使用作用强,提示在中枢水平二者有协同作用。     

The enigma of deep-body thermosensory specificity

Information provided by the analysis of peripheral cold and warm receptors may be considered a useful guide for assessing the specificity of thermal information originating in deep-body tissues. A wealth of data concerning the location of deep-body thermosensors and their neuronal correlates and modes of transduction permits the following theses to be proposed. 1. Unlike the peripheral warm and cold receptors, deep-body thermosensors are only in part represented by afferent fibers, mostly warm sensitive ones that are not character- ized in detail, as the source of thermal information outside the central nervous system (CNS). The more important thermal information generated in the CNS originates mainly from warm-sensitive neurons but contributions of cold-sensitive neurons are not definitely excluded. 2. Unlike the peripheral thermoreceptors, monomodality with respect to natural physical stimuli does not seem to be an essential property of deep-body thermosensors. By contrast, multimodality may underlie at least some of the multitude of interactions between thermoregulatory and other homeostatic control systems. 3. Temperature transduction seems to utilize molecular mechanisms that are also found in neurons that lack any thermosensory functions, and so the transduction mechanisms identified in warm-sensitive CNS neurons do not seem to be specific per se. 4. The observation of a multitude of temperature/response characteristics for thermosensitive CNS neurons has been helpful for categorizing these neurons, but there is no clear information that any one might be particularly relevant. 5. Originating from the peripheral cold and warm receptors two separate but interacting cold- and warm-signal pathways ascend multisynaptically to the hypothalamus as the highest level of thermoregulatory control, and to some extent go further to the sensory cortex. The signal contributed by a deep-body thermosensitive neuron, irrespective of its location, attains specificity by being fed properly into one of the two ascending thermosensory pathways. Received: 25 January 2000 / Accepted: 10 April 2000     

Effects of endogenous pyrogen and prostaglandin E2 on hypothalamic neurons in rat brain slices

We investigated the effects of endogenous pyrogen and prostaglandin E2 (PGE2) on the preoptic and anterior hypothalamic (POAH) neurons using brain slice preparations from the rat. Partially purified endogenous pyrogen did not change the activities of most of the neurons in the POAH region when applied locally through a micropipette attached to the recording electrode in proximity to the neurons. This indicates that partially purified endogenous pyrogen does not act directly on the neuronal activity in the POAH region. The partially purified endogenous pyrogen, applied into a culture chamber containing a brain slice, facilitated the activities in 24% of the total neurons tested, regardless of the thermal specificity of the neurons. Moreover, PGE2 added to the culture chamber facilitated 48% of the warm-responsive, 33% of the cold-responsive, and 29% of the thermally insensitive neurons. The direction of change in neuronal activity induced by partially purified endogenous pyrogen appears to be almost the same as that induced by PGE2 when these substances were applied by perfusion to the same neuron in the culture chamber. These results suggest that partially purified pyrogen applied to the perfusate of the culture chamber stimulates some constituents of brain tissue to synthesize and release prostaglandin, which in turn affects the neuronal activity of the POAH region.     

Arginine vasopressin as a central neurotransmitter

Anatomical and electrophysiological studies have revealed a widespread innervation of the brain by arginine vasopressin (AVP)-containing fibers. There is evidence that these central AVP pathways may be activated simultaneously with endocrine pathways. Stimulation of hypothalamic nuclei that contain AVP cell bodies causes changes in electrical activity of neurons in areas receiving AVP projections; in these same regions, release of immunoreactive AVP can be detected in response to appropriate stimuli or hypothalamic stimulation. These parts of the brain have also been shown to contain AVP receptors, and application of AVP to cells in these areas alters spontaneous activity or modifies the responses to other transmitters. AVP appears to act as a neurotransmitter involved in the central control of the cardiovascular, renal, and thermoregulatory systems. AVP may act centrally to coordinate autonomic and endocrine responses to homeostatic perturbations.