Biomedical subjects
Publications and source records attributed to A Azzaroni.
The experimental evidence on the behavioral state-dependent compartmentalization of temperature in the central nervous system of three homeothermic species has been reviewed to address the question of how selective brain cooling influences hypothalamic temperature regulation.
In cats, the behavioral state-dependent negative correlation of the pontine-hypothalamic temperature difference, an indicator of selective brain cooling, with the hypothalamic-ear pinna temperature difference, which is an indicator of heat loss from the heat exchangers of the head, is suppressed after bilateral common carotid ligature. Behavioral state-dependent selective brain cooling may underlie a thermal feedback mechanism differentiating the relative influences of hypothalamic and extra-hypothalamic thermoreceptors on the thermoregulatory system during quiet wakefulness and NREM sleep.
There are systemic and selective mechanisms for brain cooling in mammals. The difference between the temperatures of the vertebral and the carotid blood perfusing the brain is determined by selective heat loss and is, therefore, a quantitative indicator of the intensity of selective brain cooling. Across the wake-sleep cycle systemic and selective brain cooling are affected by state-dependent autonomic changes. In REM sleep selective brain cooling is impaired.
The role of cutaneous and respiratory heat loss for selective brain cooling in different species is discussed and new experimental results from a comparative study are summarized. In three species (cat, rabbit and rat) the difference between pontine and hypothalamic temperatures was studied as a function of head heat exchanger vasomotion appraised by the difference between hypothalamic and ear pinna (cats and rabbits) or nasal mucosa (rats) temperatures during the behavioral states of wakefulness and slow wave sleep at an ambient temperature of 24+/-1 degrees C. The results show that: (i) the pontine-hypothalamic temperature difference is an useful indicator of selective brain cooling since it is positive and inversely correlated with the hypothalamic-ear pinna temperature difference in cats and rabbits and with the hypothalamic-nasal mucosa temperature difference in rats; (ii) respiratory heat loss prevails quantitatively over cutaneous heat loss in maintaining this difference.
The influence of posture and tonic vasoconstrictor sympathetic outflow on systemic (ear pinna-environment) and selective (carotid rete-venous plexus) heat exchange underlying brain cooling was studied in cats chronically implanted with EEG and EMG electrodes, and transducers that measured hypothalamic, pontine and ear pinna temperatures across the ultradian wake-sleep cycle in a thermoneutral environment. Transmural pressure on heat exchanger vasculature was varied by keeping the animal's head above or at heart level. The vasoconstrictor sympathetic outflow to heat exchanger vasculature was varied both by keeping the animal's abdomen cool or warm and by means of bilateral common carotid ligature. The results show that a rise in transmural pressure enhances selective brain cooling and weakens systemic brain cooling. An increase in tonic vasoconstrictor sympathetic outflow decreases both systemic and selective brain cooling.
The relationship between duration of the ultradian sleep cycle and intensity of tonic vasoconstrictor sympathetic outflow to thermoregulatory heat exchangers was studied in cats chronically implanted with EEG and EMG electrodes, and temperature transducers. Vasoconstrictor sympathetic outflow to heat exchangers was indirectly appraised by measuring the difference between hypothalamic temperature and ear pinna temperature. The vasoconstrictor sympathetic outflow was varied experimentally by means of thermal and vascular manipulations. The results show that the duration of the stage of synchronized sleep, but not that of the stage of desynchronized sleep of the ultradian sleep cycle, is correlated positively with the intensity of tonic vasoconstrictor sympathetic outflow to heat exchangers in the thermal zone of vasomotor regulation of body temperature.
The mechanisms underlying hypothalamic temperature (Thy) changes across the ultradian wake-sleep cycle were analyzed in cats chronically implanted with EEG and EMG electrodes, and transducers that measured Thy and pontine temperature (Tp). The influence of artificially induced changes in (i) systemic blood temperature, (ii) heat loss from the specific heat exchangers of the head, and (iii) carotid artery occlusion, on Thy, Tp and ear pinna temperature (Ts) during waking (W), synchronized sleep (SS) and desynchronized sleep (DS) were assessed in animals maintained in a thermoneutral environment. The results show that the decrease in Thy during SS is dependent on increased heat loss from heat exchangers (ear pinna), whereas the increase in Thy during DS is due to an alteration in the arterial blood perfusion of the circle of Willis receiving an increased supply of warm vertebral blood that replaces a decrease in supply of cool carotid blood.
The thermosensitivity of anterior hypothalamic-preoptic neurons was studied in cats during the waking-sleeping cycle. Direct cooling and warming of the anterior hypothalamic-preoptic region was accomplished with water-perfused thermodes. Neuronal thermosensitivity was determined by means of the linear regression analysis of firing rate changes vs anterior hypothalamic-preoptic temperature changes. A total of 117 neurons were classified as thermosensitive during wakefulness and synchronized sleep (20.1% of the studied neurons). Cold-sensitive neurons outnumbered warm-sensitive neurons by 3.7:1. The homeothermic states, wakefulness and synchronized sleep, are characterized by similar frequency distributions of neuronal thermosensitivity, although variable changes in single neuron thermosensitivity are state-dependent. Such changes underlie the quantitative differences in homeothermic regulation between these states. The impairment of thermoregulation during desynchronized sleep is characterized by a different frequency distribution of neuronal thermosensitivity resulting from both a drop in the responsiveness to thermal stimulation of a majority of neurons and a reversal in the sensitivity to cooling and warming of a minority of neurons. In conclusion, only the frequency distribution of thermosensitivity in the neuronal population is indicative of changes in the thermoregulation paradigm across behavioral states.
Thermal responsiveness of anterior hypothalamic-preoptic (AH-PO) neurons was studied in unanesthetized sleeping cats carrying chronically implanted electrodes and thermistors for recording EEG, EMG and AH-PO temperature. Activity of AH-PO neurons was recorded using tungsten microelectrodes during atraumatic stereotaxic head restraint in a sound-attenuated chamber at 22-25 degrees C. Direct cooling or warning of the AH-PO region was accomplished with bilaterally positioned water-perfused thermodes. 110 thermosensitive neurons (77% cold-sensitive and 23% warm-sensitive) were sampled during wakefulness and synchronized sleep. In contrast, the changes in spontaneous firing rate shown by AH-PO neurons during desynchronized sleep were often associated with a strong depression or outright suppression of their responsiveness to AH-PO thermal stimulation.
The responses of anterior hypothalamic-preoptic units to direct thermal stimulation were studied during wakefulness and sleep in cats. Seventy-nine percent of the selected units showed changes in firing rate in relation to wakefulness and sleep stages. Forty-nine percent of the units characterized by activity related to EEG patterns were found to be responsive to thermal stimulation in wakefulness and synchronized sleep. Unit responses to thermal stimulation were either absent or inconsistent in desynchronized sleep.
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The effects of hippocampal electrical stimulation on unit responses to clicks in the primary auditory area were quantitatively studied in the cat. Differences in number of spikes per response, spike distribution within the response and constancy of responses were statistically tested in the absence and in the presence of hippocampal stimulation, respectively. The results show that statistically significant changes are produced in the firing pattern of about 50% of neurons responding to clicks.
Differences between sequences of evoked unit responses may concern the average number of spikes per response, the spike distribution inside the response, the "regularity" of such responses, etc. "Regularity" is defined and measured on the basis of the "distances" between consecutive responses. Each response can be characterized by a set of numbers allowing to represent it as a point in a multidimensional space. Distances between consecutive points are then measured in a euclidean way. Differences are eventually evaluated by proper statistical tests.
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Explore the source record for details and available documents.