Influence of cold adaptation on the activity of thermoresponsive neurons in thalamus and midbrain of the rat

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

Difference in brown adipose tissue effector response and associated thermoresponsiveness of ventromedial hypothalamic (VMH) neurons of 21 degrees C vs. 4 degrees C acclimatized rats to scrotal thermal stimulation

The present study was designed (1) to determine if scrotal thermal stimulation would activate brown adipose tissue (BAT) thermogenesis, indicated by increases in interscapular BAT temperature (TIBAT) of cold acclimatized (CA, kept at 4 degrees C for 4 weeks) and room temperature acclimatized rats (RA, kept at 21 degrees C for 4 weeks) and (2) to compare the thermoresponsiveness of VMH neurons of both groups to scrotal heating and cooling. VMH extracellular activity was recorded in male RA and CA Sprague-Dawley rats when scrotal temperatures (Tsc) were changed between 5-40 degrees C via localized thermode (3 mm2) along with measurements of TscS and TIBATS. The CA-group showed significant increases in TIBATS during scrotal cooling compared to respective TIBATS of the RA-group. The ratio of VMH warm responsive (WRN), cold responsive (CRN) and temperature non-responsive (TNRN) neurons in the CA-group changed compared to that in the RA-group as a greater percentage of CRNs occurred in the CA-group. Also, the thermoresponsiveness of individual VMH CRNs of the CA rats was significantly increased compared to VMH CRNs of the RA-group. The results indicated that localized scrotal cooling of CA-rats (not RA-rats) can activate BAT thermogenesis. Furthermore, VMH CRNs increased their thermoresponsiveness with chronic cold exposure which may be an important neuronal adaptive response for the subsequent enhanced BAT thermogenic effector response seen in that group.

Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels

The present study evaluated the ability of humans to discriminate temperature decreases in the noxious and innocuous cold range. Two groups of five subjects detected changes in cold stimuli applied to the maxillary face. For five subjects, adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C were used, and thresholds for detecting temperature decreases were determined using an adaptive psychophysical paradigm. Visual analogue scale (VAS) ratings of cold and pain sensation were also recorded at 5-min intervals throughout each session. A second group of five subjects performed a similar detection task, but in this case using classical psychophysical techniques (method of constant stimuli) and adapting temperatures of 22 degrees, 16 degrees, 10 degrees and 6 degrees C. These subjects described the quality of the detected change in sensation, in addition to rating overall cold and pain sensation throughout the session. Detection thresholds were 0.27 degrees, 0.48 degrees, 4.8 degrees, 8.0 degrees and >10.0 degrees C for baselines of 22 degrees, 16 degrees, 10 degrees, 6 degrees and 0 degrees C, respectively, indicating that discrimination was better in the innocuous cool (22 degrees and 16 degrees C) than in the noxious and near-noxious cold (10-0 degrees C) range (P < 0.05). Tonic adapting temperatures of 22 degrees and 16 degrees C were always rated as cool but not painful, whereas adapting temperatures of 10 degrees and 6 degrees were sometimes and 0 degrees C usually rated as painful. Phasic temperature decreases from 22 degrees and 16 degrees C always produced cooling sensations, whereas decreases from baselines of 10 degrees and 6 degrees C produced primarily sensations of painful and non-painful prickle. These data suggest that different afferent channels mediate cool and noxious cold perception and add support to the hypothesis that noxious cold sensation is mediated by subdermal nociceptors.

Adaptive changes in thermoregulation and their neuropharmacological basis

Adaptive changes of the thermoregulatory system include morphological and functional modifications. The morphological modifications such as changes in body shape and insulation need time periods of months to years to develop, unless they are genetically fixed and appear seasonally. In general, they are preceded by functional modifications, including changes in capacity of the effector systems and changes in regulatory characteristics, which need much less time to develop. These early changes in regulatory characteristics, which can be defined as deviations in threshold and gain of the thermoregulatory responses, have been described and subdivided into short-term (minutes) and long-term (weeks) modifications. Evidence for the participation of monoaminergic brain stem systems in these modifications has been reviewed. On the basis of recent insights into the organization of the thermoregulatory system, and of evaluation of experimental evidence from electrophysiological, neuropharmacological, and neuroanatomical studies it can be concluded that these systems are involved in adaptive modifications. Receiving information from several sensory systems they seem to deliver additional modulatory signals, which may interfere with the processing of specific thermal information at several sites. Theoretically, the central monoamines may participate in the control of thermal input, in the central integration of thermal signals, and in modification of output signals to thermoregulatory effectors. Best documented is their modulatory action on thermosensitive and thermointegrative hypothalamic neurons. There, the monoamines 5-hydroxytryptamine and noradrenaline act as antagonists, which enhance or diminish the effects of thermal afferents mediated by other transmitters. Moreover, the antagonistic monoaminergic systems are interconnected and can influence each other at the level of lower brain stem. 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. These interrelations can explain also some cross-adaptive changes in the thermoregulatory threshold for shivering evoked by nonthermal factors such as food intake or long-distance running.

Analysis of switching neurons within the thermoafferent system

A total of 740 midbrain, thalamic, and hypothalamic neurons were tested as to their distribution and response/stimulus characteristics by means of thermal stimulation of the scrotal skin. The most frequent response type exhibited a basic discharge rate up to a threshold temperature and then switched to a maximum firing rate. The inverse behaviour was found in a low proportion. The temperature range in which the neurons switch from the 'low' to the 'high' state has been called the 'operating range' of the neuron. This study reveals, in contrast to former studies, that the 'operating range' is extremely narrow. This holds both for the individual neuron and for the populations studied. It is confirmed by simultaneous recordings and by cross correlation analysis that there is only one temperature threshold for all warm responsive neurons involved. It is concluded that according to the mean firing rate, temperature may only be discriminated as to below or above threshold. This means that a binary information is transmitted. For continuous temperature sensation and regulation either further neuronal types disposing of a continuous temperature/frequency characteristic or additional coding mechanisms have to be assumed. The major task of the switching neurons, integrating information from all over the body should be the generation of a trigger signal for warm defence.

Thermoregulatory responses to cold during a 7-week acclimatization process in rabbits

A group of six rabbits reared at +20 degrees C ambient temperature was adapted to moderate cold by housing for seven weeks at +10 degrees C. Rectal and skin temperatures, metabolic heat production and respiratory evaporative heat loss were recorded continuously over 1 h for each animal on 3 days per week in the climatic chamber. There was no significant change either of rectal or of ear skin temperature during the acclimatization process. On the other hand, metabolic heat production was progressively reduced (20% in the 7th week). Slight changes of mean skin temperature and respiratory evaporative heat loss could not account for compensation. Therefore it must be concluded that both adaptive improvement of peripheral insulation and reduction of heat production were achieved during the acclimatization process. Both processes together ensure that deviations of core temperature are minimal. The possible origin of the functional adaptive effects is discussed. The results are in full agreement both with former neurophysiological results and with system-theoretical considerations of adaptive processes.

Thermoafferent signal processing in rats: an electrophysiological analysis of midbrain influences on thermoresponsive neurons in the ventrobasal thalamus

The influence of electrical midbrain stimulation on thalamic cells which respond to scrotal skin warming with steep increases in firing rate was studied in anaesthetized rats. Square-wave stimuli of 200 microseconds duration and 30-250 microA intensity at 15 Hz were applied in trains lasting about 1 min. At low scrotal temperatures stimulation of the nucleus raphe dorsalis, the adjacent central grey or the median raphe nucleus was nearly always followed by a rise in thalamic activity. Inhibition occurred only in 4 of 75 experiments. Often midbrain stimulation caused neuronal responses similar to those following scrotal warming. Single stimulation experiments and the combination of electrical stimulation and electrolytic lesions indicated local interactions between excitatory and inhibitory neuronal networks in midbrain. Poststimulus latencies revealed polysynaptic midbrain/thalamic connections. Direct projections with no more than a few synaptic interruptions were seen in about 10% of the experiments. It is concluded that medial midbrain neurons influence the transmission of peripheral thermal information to the specific thalamus and contribute to the typical switching form of thalamic responses to scrotal warming.

Threshold temperatures of diencephalic neurons responding to scrotal warming

Neurons in the rat's thalamus and hypothalamus abruptly change their firing rates from minimum to maximum, or vice versa, with increase in scrotal temperature of 1 degree C or less ("switching response"). The threshold temperature of the switching response was compared in pairs of neurons simultaneously recorded from the right and left thalamus or from the right thalamus and the left hypothalamus. In dynamic conditions in which scrotal skin was gradually warmed at rates of 1-7 degrees C/min, the threshold temperatures of each neuron pair differed by only 0.4 degree C or less. The threshold temperatures of static responses of each pair of neurons, which was determined with step changes in scrotal temperature, fell in a range less than 0.8 degree C. Scrotal warming produced desynchronization of cortical EEG which was monitored during unit recordings. The change in EEG occurred at about the same scrotal temperature at which the switching responses of the diencephalic neurons were elicited.

Effects of ethanol on thermoregulation

Clinical reports of accidental hypothermia in alcohol intoxicated individuals exposed to low ambient temperature ( Paton , 1983) have generally been borne out by experimental studies in healthy volunteers. Small doses of ethanol, given to human subjects at normal ambient temperature (Ta), have very little effect on body temperature but a combination of large dose, low Ta and vasodilatation provoked by strenuous exercise, causes a sharp fall in rectal temperature. In experimental animals, the use of relatively larger doses of alcohol and more extreme temperatures, both above and below the thermoneutral zone, has shown that the effect of ethanol is essentially poikilothermic, i.e. an impairment of adaptation to both heat and cold. This effect has been studied in greater detail, in relation to each of the basic thermoregulatory processes. Though small doses of alcohol may increase the metabolic rate under some circumstances, the most common effect at low Ta is inhibition of shivering and therefore reduction of thermogenesis. At the same time it tends to cause increased heat loss by cutaneous vasodilatation. This makes for a greater feeling of comfort in the cold exposed subjects but increases in rate of fall of core temperature. The combination of decreased thermogenesis and increased heat loss, despite falling body temperature, is suggestive of a lowering of the set-point of the thermoregulatory control mechanisms. Consistent with this is a slight increase in ventilatory heat loss after low doses of ethanol but larger doses cause respiratory depression, so that heat loss through the lungs is minor. However, at high Ta ethanol caused hyperthermia in experimental animals and shows enhanced lethality, so that impairment of thermoregulatory effector mechanisms seems to be at least as important as change in set-point. Studies of the effects of ethanol on electrophysiological activity of single neurons in the pre-optic area and anterior hypothalamus (POAH), biochemical activities of neuronal membranes, hypothalamic blood flow, conventional neurotransmitters, amino acid putative neurotransmitters, neuropeptides, prostaglandins and inorganic ions have all failed so far to yield a clear comprehensive picture of the mechanisms by which ethanol affects thermoregulation. In each case, contradictory evidence has been obtained concerning the consequences of ethanol administration, whether by oral, intraperitoneal, intravenous, intracerebroventricular, or direct local (POAH) route.(ABSTRACT TRUNCATED AT 400 WORDS)