Responses of anterior hypothalamic-preoptic neurons to direct thermal stimulation during wakefulness and sleep

382

Brain Research, 269 (1983) 382-385 Elsevier

Responses of anterior hypothalamic-preoptic neurons to direct thermal stimulation during wakefulness and sleep P . L PARMEGGIANI, A. AZZARONI, D. CEVOLANI and G. FERRARI Istituto di Fisiologia umana, Universita di Bologna, Piazza di Porta San Donato 2, 1-40127 Bologna (Italy) (Accepted February 22nd, 1983) Key words: cat - - anterior hypothalamic-preoptic region - - thermoresponsive neurons - - wakefulness - - sleep

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.

Previous studies showed that thermoregulatory responses to anterior hypothalamic-preoptic (AH-PO) warming 4-6 or cooling I are absent during the stage of desynchronized sleep (DS). The present study was aimed at clarifying at unitary level the changes in responsiveness of A H - P O thermoregulatory structures occurring in relation to sleep processes. Three adult cats were used. Chronic implantation of extradural screw E E G and depth dipolar PGO

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electrodes, and of a unilateral thermode (1.5 mm in diameter, water perfused) and an adjacent thermistor (Yellow Springs) in the A H - P O region was carried out under general pentobarbital sodium (35 mg/kg i.p.) and local procaine (1%) anesthesia. A metal frame to be fitted to a Kopf stereotaxic apparatus was fixed on the skull and two trephine holes were opened in the bone ipsilaterally and contralaterally with respect to the thermode to uncover the dura

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Fig. 1. Changes in unit activity of the anterior hypothalamic-preoptic region in response to direct thermal stimulation during wakefulness and sleep. Warming during wakefulness elicits both synchronized sleep with PGO and a decrease in firing rate (A). A decrease in firing rate is associated with spontaneous occurrence of synchronized sleep with PGO (B). Abbreviations in this and the following figures: DS, desynchronized sleep; EEG, electroencephalogram; PGO: ponto-geniculo-occipital waves; SS, synchronized sleep; Tthy, temperature of the hypothalamic thermode; UA, unit activity; W, wakefulness. 0006-8993/83/$03.00 ~ 1983 Elsevier Science Publishers B.V.

383 which had been pretreated with trypsin. Unit activity was monitored by an A C preamplifier provided with a high impedance F E T probe placed close to the microelectrode, displayed on an oscilloscope. Analyzed spikes were separated from those of nearby neurons by means of a window discriminator with variable upper and lower thresholds, the output of which was a standard square pulse. Rate counts (spikes/s) were obtained by feeding the standard pulse output into a frequency meter connected to the polygraph also re-

over the deep brain region to be explored. The asymmetrical implantation of the thermode minimized damage to the A H - P O region and showed the existence of consensual responses of contralateral units. Tungsten microelectrodes (electrical impedance at 1000 Hz between 1 and 4 Mr2) were used for recording negative or positive-negative spikes from single neurons. Microelectrode penetrations (sliding device driven by a remotely controlled electro-mechanical microdrive) were carried out through the dura,

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Fig. 2, Changes in unit activity of the anterior hypothalamic-preoptic region in response to direct thermal stimulation during wakefulness and sleep, Cooling (A) and warming (B) during wakefulness elicit an increase and a decrease in firing rate, respectively. Warming during synchronized sleep elicits a decrease in firing rate (C). In contrast, a decrease in firing rate is observed during DS on cooling (D).

384 cording the other variables. Experimental sessions in a sound proof room lasted 6-8 h at 22-25 °C ambient temperature. The animal was restrained in a loose canvas bag and its head was held rigidly but atraumatically in the Kopf apparatus (cf. ref. 2). Several sessions were allowed for the animal to adapt to experimental conditions. When necessary, spontaneous sleep was ensured during recording sessions by sleep deprivation induced in the unrestrained animal by exposure to low ambient temperature (0 °C) during the previous 15 h. Histological control of the site of origin of recorded unit activity was carried out after microelectrolysis around the microelectrode tip, performed at the lowest point of successful penetrations. Of the 57 units selected in the present study, 79% showed consistent changes in firing rate in relation to wakefulness and sleep stages. In particular, during DS the firing rate increased and decreased in 56 and 23%, respectively (cf. ref. 3). In some cases thermal stimulation influenced the E E G , inducing synchronized sleep (SS) on warming and wakefulness (W) on cooling. The observed changes in unit activity were not always directly dependent on the thermal stimulus, but rather related to the E E G changes elicited by the same stimulus (Fig. 1). Therefore, in order to classify units as thermoresponsive, regardless of their function as receptors or interneurons, it was

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necessary to assess that responses to thermal stimuli were similar in W and SS, and unrelated to any significant E E G change. On this basis, 49% of the units characterized by activity related to E E G patterns were considered to be directly affected by thermal stimulation (33% ipsilateral and 16% contralateral with respect to the thermode), During W and SS, the most common responses (82%) were an increase and a decrease in firing rate on cooling and warming, respectively (Fig. 2). Opposite responses were observed in 18% of the units (Fig. 3). At present, any hypothesis on the functional significance of such neurons in the thermoregulatory network is unwarranted. During DS, responses to thermal stimulation were absent (78%) or the firing rate of units changed in relation to warming or cooling (22%). Concerning the latter group of units, a reversal of rate increase-decrease responses to cold-warm stimuli from SS to DS was found in some cases (Fig. 2), which supports the hypothesis of an unspecific thermochemical effect in DS. Otherwise, DS was characterized by attenuation and/or variability (from rate increase to rate decrease and vice versa in response to constant stimulus iteration during the DS episode) of unit responses to thermal stimulation. Such a variability could be considered as the result of a casual coincidence of thermal stimulation with spontaneous

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385 changes in firing rate, which are typical of DS. In conclusion.,..the results of this study point to a basic change in the functional A H - P O responsiveness to thermal stimulation during D S with respect to W and SS. This change seems to be inconsistent with h o m e o thermic regulation. Concerning h o m e o t h e r m i c regulation during W and SS, G l o t z b a c h and Hellerl

showed quantitative differences in the rate of metabolic responses to A H - P O cooling b e t w e e n these E E G states. F u r t h e r research is necessary in o r d e r to assess analogous differences at unitary level.

1 Glotzbach, S. F. and Heller, H. C., Central nervous regulation of body temperature during sleep, Science, 194 (1976) 537-539. 2 Orem, J. and Dement, W. C., Neurophysiological substrates of the changes in respiration during sleep. In E. D. Weitzman (Ed.), Advances in Sleep Research, Spectrum, New York, 1976. 3 Parmeggiani, P. L. and Franzini, C., On the functional significance of subcortical single unit activity during sleep, Electroencephalogr. clin. Neurophysiol., 34 (1973) 495-508. 4 Parmeggiani, P. L., Franzini, C. and Lenzi, P., Respiratory

frequency as a function of preoptic temperature during sleep, Brain Research, 111 (1976) 253-260. Parmeggiani, P. L., Franzini, C., Lenzi, P. and Zamboni, G., Threshold of respiratory responses to preoptic heating during sleep in freely moving cats, Brain Research, 52 (1973) 189-201. Parmeggiani, P. L., Zamboni, G., Cianci, T., Calasso, M., Absence of thermoregulatory vasomotor responses during fast wave sleep in cats, Electroencephalogr. clin. Neurophysiol., 42 (1977) 372-380.

This work was s u p p o r t e d by G r a n t 81.00305.04 from the National R e s e a r c h Council, R o m e , Italy.