Temperature sensitivity of neurones in slices of the rat spinal cord

Spinal cord thermosensitivity: An afferent phenomenon?

We review the evidence for thermoregulatory temperature sensors in the mammalian spinal cord and reach the following conclusions. 1) Spinal cord temperature contributes physiologically to temperature regulation. 2) Parallel anterolateral ascending pathways transmit signals from spinal cooling and spinal warming: they overlap with the respective axon pathways of the dorsal horn neurons that are driven by peripheral cold- and warm-sensitive afferents. 3) We hypothesize that these ‘cold’ and ‘warm’ ascending pathways transmit all extracranial thermosensory information to the brain. 4) Cutaneous cold afferents can be activated not only by cooling the skin but also by cooling sites along their axons: we consider that this is functionally insignificant in vivo. 5) By a presynaptic action on their central terminals, local spinal cooling enhances neurotransmission from incoming ‘cold’ afferent action potentials to second order neurons in the dorsal horn; this effect disappears when the spinal cord is warm. 6) Spinal warm sensitivity is due to warm-sensitive miniature vesicular transmitter release from afferent terminals in the dorsal horn: this effect is powerful enough to excite second order neurons in the ‘warm’ pathway independently of any incoming sensory traffic. 7) Distinct but related presynaptic mechanisms at cold- and warm-sensitive afferent terminals can thus account for the thermoregulatory actions of spinal cord temperature.

Not just the brain: methamphetamine disrupts blood-spinal cord barrier and induces acute glial activation and structural damage of spinal cord cells

Acute methamphetamine (METH) intoxication induces metabolic brain activation as well as multiple physiological and behavioral responses that could result in life-threatening health complications. Previously, we showed that METH (9 mg/kg) used in freely moving rats induces robust leakage of blood-brain barrier, acute glial activation, vasogenic edema, and structural abnormalities of brain cells. These changes were tightly correlated with drug-induced brain hyperthermia and were greatly potentiated when METH was used at warm ambient temperatures (29°C), inducing more robust and prolonged hyperthermia. Extending this line of research, here we show that METH also strongly increases the permeability of the blood-spinal cord barrier as evidenced by entry of Evans blue and albumin immunoreactivity in T9-12 segments of the spinal cord. Similar to the blood-brain barrier, leakage of bloodspinal cord barrier was associated with acute glial activation, alterations of ionic homeostasis, water tissue accumulation (edema), and structural abnormalities of spinal cord cells. Similar to that in the brain, all neurochemical alterations correlated tightly with drug-induced elevations in brain temperature and they were enhanced when the drug was used at 29°C and brain hyperthermia reached pathological levels (>40°C). We discuss common features and differences in neural responses between the brain and spinal cord, two inseparable parts of the central nervous system affected by METH exposure.

Effect of capsaicin on thermoregulation: an update with new aspects

Capsaicin, a selective activator of the chemo- and heat-sensitive transient receptor potential (TRP) V1 cation channel, has characteristic feature of causing long-term functional and structural impairment of neural elements supplied by TRPV1/capsaicin receptor. In mammals, systemic application of capsaicin induces complex heat-loss response characteristic for each species and avoidance of warm environment. Capsaicin activates cutaneous warm receptors and polymodal nociceptors but has no effect on cold receptors or mechanoreceptors. In this review, thermoregulatory features of capsaicin-pretreated rodents and TRPV1-mediated neural elements with innocuous heat sensitivity are summarized. Recent data support a novel hypothesis for the role of visceral warmth sensors in monitoring core body temperature. Furthermore, strong evidence suggests that central presynaptic nerve terminals of TRPV1-expressing cutaneous, thoracic and abdominal visceral receptors are activated by innocuous warmth stimuli and capsaicin. These responses are absent in TRPV1 knockout mice. Thermoregulatory disturbance induced by systemic capsaicin pretreatment lasts for months and is characterized by a normal body temperature at cool environment up to a total dose of 150 mg/kg s.c. Upward differential shift of set points for activation vasodilation, other heat-loss effectors and thermopreference develops. Avoidance of warm ambient temperature (35°C, 40°C) is severely impaired but thermopreference at cool ambient temperatures (Tas) are not altered. TRPV1 knockout or knockdown and genetically altered TRPV1, TRPV2 and TRPM8 knockout mice have normal core temperature in thermoneutral or cool environments, but the combined mutant mice have impaired regulation in warm or cold (4°C) environments. Several lines of evidence support that in the preoptic area warmth sensitive neurons are activated and desensitized by capsaicin, but morphological evidence for it is controversial. It is suggested that these neurons have also integrator function. Fever is enhanced in capsaicin-desensitized rats and the inhibition observed after pretreatment with low i.p. doses does not support in the light of their warmth sensitivity the concept that abdominal TRPV1-expressing nerve terminals serve as nonthermal chemosensors for reference signals in thermoregulation.

Individual differences in temperature perception: Evidence of common processing of sensation intensity of warmth and cold

The longstanding question of whether temperature is sensed via separate sensory systems for warmth and cold was investigated by measuring individual differences in perception of nonpainful heating and cooling. Sixty-two subjects gave separate ratings of the intensity of thermal sensations (warmth, cold) and nociceptive sensations (burning/stinging/pricking) produced by cooling (29 degrees C) or heating (37 degrees C) local regions of the forearm. Stimuli were delivered via a 4 x 4 array of 8 mm x 8 mm Peltier thermoelectric modules that enabled test temperatures to be presented sequentially to individual modules or simultaneously to the full array. Stimulation of the full array showed that perception of warmth and cold were highly correlated (Pearson r = 0.83, p < 0.05). Ratings of nonpainful nociceptive sensations produced by the two temperatures were also correlated, but to a lesser degree (r = 0.44), and the associations between nociceptive and thermal sensations (r = 0.35 and 0.22 for 37 and 29 degrees C, respectively) were not significant after correction for multiple statistical tests. Intensity ratings for individual modules indicated that the number of responsive sites out of 16 was a poor predictor of temperature sensations but a significant predictor of nociceptive sensations. The very high correlation between ratings of thermal sensations conflicts with the classical view that warmth and cold are mediated by separate thermal modalities and implies that warm-sensitive and cold-sensitive spinothalamic pathways converge and undergo joint modulation in the central nervous system. Integration of thermal stimulation from the skin and body core within the thermoregulatory system is suggested as the possible source of this convergence.

Hyperthermia increases sensitivity of pulmonary C-fibre afferents in rats

This study was carried out to investigate whether an increase in tissue temperature alters the excitability of vagal pulmonary C-fibres. Single-unit afferent activities of 88 C-fibres were recorded in anaesthetized and artificially ventilated rats when the intrathoracic temperature (T(it)) was maintained at three different levels by isolated perfusion of the thoracic chamber with saline: control (C: approximately 36 degrees C), medium (M: approximately 38.5 degrees C) and high (H: approximately 41 degrees C), each for 3 min with 30 min recovery. Our results showed: (1) The baseline fibre activity (FA) of pulmonary C-fibres did not change significantly at M, but increased drastically (>5-fold) at H. (2) The C-fibre response to right-atrial injection of capsaicin (0.5 microg kg(-1)) was markedly elevated at H (deltaFA = 5.94 +/- 1.65 impulses s(-1) at C and 13.13 +/- 2.98 impulses s(-1) at H; P < 0.05), but not at M. Similar increases in the C-fibre responses to other chemical stimulants (e.g. adenosine, etc.) were found at H; all the enhanced responses returned to control in 30 min. (3) The C-fibre response to lung inflation was also significantly potentiated at H. In sharp contrast, there was no detectable change in either the baseline activity or the responses to lung inflation and deflation in 10 rapidly adapting pulmonary receptors and 10 slowly adapting pulmonary receptors at either M or H. (4) The enhanced C-fibre sensitivity was not altered by pretreatment with indomethacin or capsazepine, a selective antagonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor, but was significantly attenuated by ruthenium red that is known to be an effective blocker of all TRPV channels. (5) The response of pulmonary C-fibres to a progressive increase in T(it) in a ramp pattern further showed that baseline FA started to increase when T(it) exceeded 39.2 degrees C. In conclusion, a pronounced increase in the baseline activity and excitability of pulmonary C-fibres is induced by intrathoracic hyperthermia, and this enhanced sensitivity probably involves activation of temperature-sensitive ion channel(s), presumably one or more of the TRPV receptors, expressed on the C-fibre endings.

Thermosensitivity of large primary sensory neurons

Spontaneous activity originating in the injured nerve or the dorsal root ganglion (DRG) has been implicated in the development and maintenance of neuropathic pain. The inherent characteristics of spontaneous activity and the causal factors that modulate its firing pattern and frequency are not fully understood. We attempted to assess the thermosensitivity of spontaneous activity in dorsal root ganglion (DRG) neurons in normal rats and in rats with chronic compression of the DRG (CCD) in an in vitro nerve-DRG preparation. Extracellular, dorsal root recording from 66 spontaneously active CCD Abeta fibers indicate that: (1) decreasing bath temperature from 37 to 36-26 degrees C significantly decreased the firing rate (FR) in 85% (56/66) of fibers tested, of which 19 fibers (34%) responded to temperature change at physiological range (36-37 degrees C), whereas the remaining fibers responded at lower temperature levels (26-36 degrees C); (2) cooling of the DRG increased the FR in 12% (8/66) of fibers tested; (3) similarly, the firing rate of 21/26 spontaneously active Abeta fibers from normal rats was decreased following temperature decrease; (4) intracellular recordings from 38 normal neurons revealed that cooling the DRG significantly increased the action potential (AP) threshold, AP duration, AP amplitude and afterhyperpolarization (AHP) duration, but decreased AHP amplitude, maximal depolarizing and repolarizing rates. There was no significant change in the rheobase currents or the resting membrane potential. The present study indicates that large sensory neurons with myelinated axons are temperature dependent. It also suggests that maintenance of a stable temperature is critical for reliable characterization of spontaneous activity of sensory neurons.

Spinothalamic lamina I neurones selectively responsive to cutaneous warming in cats

1. In order to further characterize the role of lamina I as the source of central ascending neural pathways for thermoreception and thermoregulation, experiments were performed on anaesthetized cats to determine the quantitative response characteristics of warming-specific lumbosacral spinothalamic lamina I neurones. 2. We identified 10 neurones out of 474 that were selectively excited by cutaneous warming (Warm cells). Their thresholds were all in the range 35-37 degrees C at a baseline of 34.5 degrees C, and their discharge linearly encoded the temperature of graded, innocuous warming stimuli with a sensitivity of 2.1 Hz x degrees C(-1). 3. The stimulus-response function of the Warm cells plateaued at temperatures that were in the noxious heat range. 4. The Warm cells were distinguished from other classes of spinothalamic lamina I neurones by their peripheral inputs, central conduction velocities and level of ongoing activity. 5. The discharge of Warm cells compares well with the known human psychophysics of warm sensibility, and these neurones are likely to be crucial to discriminative thermoreception. Additionally, a role in thermoregulation, a defining feature of mammalian homeostasis, is suggested.

Intracellular recording of lamina X neurons in a horizontal slice preparation of rat lumbar spinal cord

A horizontal slice preparation of postnatal rat lumbar spinal cord has been developed which allows correlative observations of the morphology, electrophysiology, and receptor pharmacology of lamina X neurons. These slices better maintain afferent input and somatodendritic morphology and are amenable to subsequent immunohistochemical processing. Stable intracellular recordings obtained from postnatal day 14-45 animals reveal that a number of different intrinsic membrane conductances contribute to the regulation of excitability in lamina X neurons. In addition, lamina X neurons possess inhibitory GABAergic as well as excitatory glutamate and cholecystokinin receptors. This preparation will be useful in future studies designed to characterize developmental changes in the intrinsic membrane properties, synaptic profiles and neuropeptide responsiveness of lamina X neurons in the rat. Such a characterization is important given that lamina X represents a unique sexually dimorphic region that is a convergence site for somatic and visceral afferent inputs, which includes nociceptive information.

Nitric Oxide and Body Temperature Control

Pharmacological studies of thermoregulatory effector and neuronal responses indicate that nitric oxide (NO) may have differential roles in the control of body temperature and during fever. Histochemical analysis of site-specific changes in NO synthase activity in defined states of thermal stimulation appears a promising approach to unravel the underlying hypothalamic neuronal cytoarchitecture.

Nitric oxide as a peripheral and central mediator in temperature regulation

In animals including humans nitric oxide (NO) serves as a biological messenger both peripherally at neuroeffector junctions and in the central nervous system where it modulates neuronal activity. Evidence for the involvement of NO in homeostatic control is accumulating also for temperature regulation in homeotherms. In the periphery an auxiliary role in the vasomotor control of convective heat transfer to heat dissipating surfaces and modulation of thermoregulatory heat generation, especially in brown adipose tissue as the site of nonshivering thermogenesis, are discussed as NO actions. At the central level a thermolytic role of NO in thermoregulation as well as in fever is assumed, however, experimental data opposing this view suggest that topical specificity may be important. At the level of single neurons, the observed interrelationships between thermosensitivity and responsiveness to NO are still not consistent enough to reconcile these data with the effects of NO-donors and inhibitors of NO-synthase on temperature regulation.

Spontaneous activity of preoptic neurons in slice preparations of the hypothalamus of European hamsters (Cricetus cricetus) and Wistar rats under different states of hypothermia

Hypothermia was induced in European hamsters (hibernators) and Wistar rats (nonhibernators), and changes in the firing rate and spike duration of extracellularly recorded action potentials were investigated in hypothalamic slices in vitro. At slice temperatures close to normal body temperature (37 +/- 3 degreesC), 32 and 57% of spontaneously active neurons in the medial preoptic area were classified as warm-sensitive in rats and hamsters, respectively. With decreasing slice temperature, the number of active neurons decreased progressively without a significant difference between rats and hamsters. At a slice temperature of 10 degreesC, 57% of all hypothalamic neurons in rats and 42% in hamsters were still spontaneously active. The average temperature at which activity ceased completely when the temperature was decreased further (the cut-off temperature) was 7.9 +/- 0.3 degreesC (n = 14) in rats but was significantly lower at 4.9 +/- 0.4 degreesC (n = 8) in hamsters (P < 0.001). Firing rates and temperature coefficients did not differ in their temperature dependence between rats and hamsters. Action potential duration increased with decreasing slice temperature in both species, but the increase in duration was significantly greater in rats.

Spinal neuronal thermosensitivity in vivo and in vitro in relation to hypothalamic neuronal thermosensitivity

In the spinal cord, temperature signals are generated which serve as specific inputs in the central nervous control of body temperature. Because of the spatially distinct organization of afferent and efferent neuronal systems at the spinal level, the afferent pathway for temperature signal transmission could be identified in vivo in the ascending, anterior and lateral tracts with a relationship of about 75:25% between warm and cold sensitive neuraxons. Analysis of spinal neuronal thermosensitivity in vitro on spinal cord tissue slices has been concerned, so far, with the superficial laminae of the dorsal horn as the site of origin of ascending nerve fibers conveying mostly temperature and pain signals, and with lamina X as a site of origin of afferent as well as efferent neurons. A relationship of about 95:5% between warm and cold sensitive neurons was found at the segmental level, indicating that warm sensitivity is the prevailing, primary property of spinal neurons, whereas cold sensitivity seems to be mainly generated by synaptic interaction as a secondary modality. Dynamic responses to temperature changes were frequently displayed in vitro at the spinal segmental level in lamina I + II but not in lamina X, even by neurons whose static activity was little influenced by local temperature. Dynamic thermosensitivity was found less frequently in ascending tract neuraxons and was not observed in hypothalamic neurons receiving temperature signal inputs from the spinal cord, and thus, does not seem to be relevant for the thermosensory function of spinal cord neurons, unlike peripheral warm and cold receptors. A majority of spinal warm sensitive neurons displayed both static and dynamic warm sensitivity as an inherent property after synaptic blockade. In the further analysis of spinal cord thermosensitivity, the in vitro approach permits application of the same electrophysiological and neuropharmacological methods as were established for the analysis of hypothalamic thermosensitivity. In addition, the topography of the spinal cord will provide additional structural and possibly histochemical information to characterize the functions of neurons independently of their thermal properties.