- Email: [email protected]
Effects of medial preoptic area lesions on sleep and wakefulness in unrestrained rats
Neuroscience Letters, 114 (1990) 300-304 Elsevier Scientific Publishers Ireland Ltd.
300
NSL 06952
Effects of medial preoptic area lesions on sleep and wakefulness in unrestrained rats Samuel A. Asala*, Yasuhisa O k a n o , K a z u k i H o n d a a n d Shojiro Inou6 Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo (Japan) (Received 26 January 1990; Revised version received 22 February 1990; Accepted 23 February 1990)
Key' words. Brain temperature; Circadian rhythm; Medial preoptic area lesion; Rat; Sleep-wakefulness Bilateral radiofrequency lesions of the medial preoptic area (MPOA) in unrestrained male rats resulted in a significant decrease in slow wave sleep (SWS) in the light period throughout two postoperative weeks, although the night-active pattern of circadian rhythms was little affected. Both diurnal and nocturnal paradoxical sleep (PS) gradually increased after the lesions. Within one week, however, the daily amount of total sleep (SWS + PS) was recovered to the normal level, since the loss of diurnal SWS was compensated by an increase in nocturnal sleep at the expense of wakefulness. The MPOA lesions brought about a transient elevation of brain temperature, which lasted only for the diurnal period of the day of lesioning. It is speculated that the MPOA plays a definite role in the passage of sleep-regulatory information, especially concerning the circadian distribution of sleep.
Several studies have attempted to define the role of specific areas of the central nervous system in the induction and regulation of sleep and wakefulness (for reviews, see refs. 7, 10). On the basis of transsection studies in rats, the preoptic area (POA) has been proposed as a sleep center [11]. Electrical stimulation of the POA may result in sleep in cats [13], whereas electrolytic lesions of this area cause a disturbance of sleep [9]. Neuronal activities of the POA are modulated by local application of sleeppromoting substance [6]. However, all these studies deal with a wide region of the basal forebrain including the lateral POA (LPOA) but often not the medial POA (MPOA). Hence an essential site for sleep regulation has not been specified. Meanwhile, it is reported that the adrenergic and serotonergic systems in the MPOA are differentially involved in the modulation of body temperature and vigilance states [2, 3, 8]. In the present study, both acute and chronic effects of radiofrequency lesions of the MPOA were investigated in freely behaving rats. This lesion method can accurately and acutely destroy all components of a localized tissue area. A preliminary report appeared as an abstract form [1]. *Present address: Department of Human Anatomy, Faculty of Medicine, Ahmadu BeUo University, Zaria, Nigeria. Correspondence." S. Inou6, Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Kanda-Surugadai 2-3-10, Chiyoda-ku, Tokyo 101, Japan. 0304-3940/90/$ 03.50 ~') 1990 Elsevier Scientific Publishers Ireland Ltd.
301
Young male rats of the Sprague-Dawley strain, kept in our closed colony on a 12-h light and 12-h dark schedule (lights on: 08.00-20.00 h) in a constant air-conditioned environment of 25 + 1°C and 60 + 6% relative humidity with ~free access to rat chow, were used. At the age of 60-70 days, animals weighing 300-400 g were anesthetized with pentobarbital sodium and implanted with 3 cortical EEG electrodes and 2 nuchal electromyographic (EMG) electrodes according to the technique described in a previous paper [4]. They were also implanted with a thermistor electrode in the thalamus for recording brain temperature (Tbr). The animals were individually housed in a special cage which enabled continuous monitoring of EEG, E M G and Tbr. Lead wires of the electrodes were connected with a polygraph (Nihon-Kohden EEG-4317) via a slip ring fixed above the cage. Thus free movement of the rats was guaranteed. Each cage was placed in a sound-proof, electromagnetically shielded chamber under the same environmental conditions as above. The postural, feeding and other behaviors were video-monitored by color and infrared-ray TV systems. One week was allowed to recover from the surgery. After observering the establishment of circadian rhythms in sleep-waking behavior, baseline recordings were done for 24 h starting at the onset of the light period. EEG (bipolary registered between two combinations of the three electrodes), E M G and Tbr were polygraphically recorded at the paper speed of 0.5 mm/s. Tbr was also fed directly into a computer and numerically printed out at 3-min interals. At the end of the recording period, each rat was re-anesthetized with pentobarbital sodium and the MPOA was subjected to bilateral radiofrequency lesions by a lesion generator (Radionics RFG-4A) at the stereotaxic coordinates of 0.7 mm caudal, 0.6 mm lateral and 8.6 mm ventral to the bregma [12]. The lesion generator delivered microwaves of 500 kHz, 22 V and 7.5 mA, generating an electrode tip temperature of 70°C for 1 min. The whole procedure for this second operation was completed within 30 min, i.e. 08.00-08.30 h. Then the operated rats were returned to their own cages to continue the polygraphic recordings. On postlesion days 1 (day of operation), 7 and 14, EEG, EMG and Tbr were similarly recorded for 24 h. At the end of the experiment, each rat was killed with an overdose of pentobarbital sodium and perfused with 10% formol saline through the apex of the heart. The brain was then removed and sliced into 50-~tm-thick sections using a freezing sliding microtome. The sections were stained with 0.1% Cresyl violet and examined under a light microscope to ascertain the location of the lesions. Only rats whose brain histology revealed accurate placement of MPOA lesions were adopted in the present study. Sleep-waking states were visually classified on a large scale digitizer as slow wave sleep (SWS), paradoxical sleep (PS) and wakefulness according to our routinized criteria, numerically stored and processed in a computer system [5]. The data were statistically analyzed by Student's t-test. The MPOA lies adjacent to the 3rd ventricle and measures ca. 1 mm in its rostrocaudal dimension, ca. 1.2 mm in the transverse plane at its mid level and ca. 1.1 mm in its depth. The radiofrequency parameters used here were just sufficient to produce a complete lesion of the left and right MPOA. Fig. 1 shows a rostrocaudal reconstruction of the bilaterally lesioned MPOA and a photomicrographed example of a
-0.26~
302
L:Lateral POA M:Medial POA :Lesioned area
Fig. 1. Photomicrograph (left) indicating the bilateral lesions of the medial preoptic area (POA; arrows) and reconstruction (right) of typical lesion sites in the medial POA at approximate stereotaxiccoordinates [12] of 0.26, 0.4, 0.8 and 1.3 mm caudal to the bregma. 3V, 3rd ventricle; CC, corpus callosum; OX, optic chiasma; SCN, suprachiasmatic nucleus.
lesioned brain section. The lesion area became vacuolated and opened into the 3rd ventricle. The suprachiasmatic nucleus, fornix, anterior commissure, optic chiasma and L P O A remained intact (Fig. 1). In one rat, the left anterior commissure was partially damaged at level - 0 . 8 . In another rat, the right L P O A was partially lesioned also at the level - 0 . 8 . In both rats, however, the quantity of sleep parameters was not largely different from that of the other 6 successfully lesioned rats. Hence, the records of the eight rats were combined and averaged in this study. The effects of the complete bilateral lesions of the M P O A on sleep and wakefulness are summarized in Table I. The M P O A lesions exerted a slight but definite influence on the circadian pattern of rest-activity rhythms (see below), although the lesioned rats were night-active as before, exhibiting that 63-65% of their total sleep time was distributed in the daytime throughout the 2-week period. Observations of posture, movement and general behavior did not reveal any obvious change after lesioning.
303 TABLE I TOTAL TIME (rain) OF SLEEP AND WAKEFULNESS IN MEDIAL PREOPTIC AREALESIONED RATS (MEAN + S.E.M., n = 8) Parameter Wakefulness Light period Dark period Slow wave sleep (SWS) Light period Dark period Paradoxical sleep (PS) Light period Darkperiod Total sleep (SWS+ PS) Light period Dark period Light+dark periods
Baseline
Postlesion day I
Postlesionday 7 Postlesion day 14
211.3+ 8.0 476.6+ 15.6
284.0+13.8"* 477.1 __+11.2
237.4__+10.7 440.2 ___19.8
241.7+ 7.4* 444.8 + 14.1
451.7+ 3.7 216.5+11.6
387.9+12.8"* 215.6+ 8.9
419.5+11.2" 246.0__+17.6
404.9+ 7.1"* 238.2__+12.7
63.1 + 19.8 33.9__+ 3.0
73.4+ 4.1" 37.1+ 2.2
482.6+10.7 279.9-+ 19.8 762.5-+18.7
478.3_+7.5* 275.2+ 14.1 753.5+ 13.7
57.0+ 5.1 26.9+ 4.7 508.7+ 8.0 243.4__+15.6 752.1 -+ 18.7
48.8+ 5.0 27.2+ 3.1 436.7_+18.9"* 242.9+ 11.3 679.7+ 18.2"
*P < 0.02, **P < 0.01 as compared to the corresponding baseline (Student's t-test).
On the postlesion day 1, the amount of SWS was significantly reduced during the light period (Table I). The amount of diurnal PS also decreased but the change was statistically insignificant. These changes were largely ue to a considerable but statistically insignificant reduction of episode duration (data not shown). A significant reduction of diurnal SWS was still observable on postlesion days 7 and 14. Thus, the amount of wakefulness increased in the light period of these days. Both diurnal and nocturnal PS increased gradually after the lesions. Interestingly, the daily quantity of total sleep (SWS plus PS) was returned to the normal level within one week (Table I). This was mainly caused by a compensatory increase in nocturnal sleep at the expense of nocturnal wakefulness. Tb~ exhibited a circadian rhythm: average value in the light and dark period was 37.00__+ 0.14 °C (n = 7) and 37.82__+0.08°C (n = 7), respectively. Tbr was significantly elevated during the diurnal period of the lesion day (37.52 + 0.17°C, P < 0.02) but not in its nocturnal period (37.88__+ 0.10°C). The subsequent recordings revealed no difference from the baseline level, manifesting the normal circadian rhythmicity. Thus, the localized damage of the M P O A brought about a transient elevation of Tbr, similar to a previous study in cats [14], and an acute and chronic reduction of diurnal SWS. Since the thermal and sleep-waking reactions were separable, different neural mechanisms may be responsible for the modulation of temperature and sleep within the MPOA. This assumption is in agreement with that of Datta et al. [2, 3, 8], although our study suggested that the M P O A plays no crucial role in thermoregulation. Our finding on the disturbance of sleep after the bilateral lesions of the M P O A coincides with the previous studies dealing with a wide damage of the basal forebrain
304 [9, 1 1, 14]. However, c o n t r a r y to these reports, it should be noted that the chronic i m p a i r m e n t was limited only to d i u r n a l SWS in o u r M P O A - l e s i o n e d rats, a n d that the loss of d i u r n a l SWS was easily b a l a n c e d by a n o c t u r n a l r e b o u n d . Thus, the daily a m o u n t of total sleep was little affected by the lesions. This m e a n s that the g e n e r a t i o n of a fixed a m o u n t of daily sleep was n o t impaired, b u t that the circadian d i s t r i b u t i o n of sleep a n d wakefulness was definitely m o d u l a t e d . Such a n a l t e r n a t i o n c a n n o t be ascribed to the damage of the circadian clock itself, since the suprachiasmatic nucleus remained intact in these animals. Hence, it is speculated that the M P O A plays a n i m p o r t a n t but not p r i m a r y role both in the g e n e r a t i o n of sleep a n d in the passage of sleep-regulatory i n f o r m a t i o n to a n d from the other neural structures, especially c o n c e r n i n g the circadian d i s t r i b u t i o n of sleep. Dr. Samuel A. Asala was an awardee of a 1988 fellowship of the M a t s u m a e Intern a t i o n a l F o u n d a t i o n , Tokyo. I Asala, S.A., Okano, Y., Honda, K. and Inou6, S., Effect of medial preoptic area lesion on sleep in rats: a preliminary report, Jpn. J. Psychiat. Neurol., 43 (1989) 774--775. 2 Datta, S., Mohan Kumar, V., Chhina, G.S. and Singh, B., Effect of application of serotonin in medial preoptic area on body temperature and sleep-wakefulness,Ind. J. Exp. Biol., 25 (1987) 681-685. 3 Datta, S., Mohan Kumar, V., Chhina, G.S. and Singh, B., Interrelationship of thermal and sleep-wakefulness changes elicited from the medical preoptic area in rats, Exp. Neurol., 100 (1988) 40-50. 4 Honda, K. and Inou6, S., Establishment ofa bioassay method for the sleep-promotingsubstance, Rep. Inst. Med. Dent. Eng., 12 (1978) 81-85~ 5 Honda, K. and Inou6, S., Effects of sleep-promoting substance on sleep-wakingpatterns of male rats, Rep. Inst. Med. Dent. Eng., 15 (1981) 115 123. 6 lnokuchi, A. and Oomura, Y., Effects of sleep-promoting substance on rat hypothalamic neuron activity, Brain Res. Bull., 16 (1986) 429-433. 7 Jones, B.E,, Basic mechanisms of sleep-wake states. In M.H. Kryger, T. Roth and W.C. Dement (Eds.t, Principles and Practice of Sleep Medicine, Saunders, Philadelphia, 1989, pp. 121-138. 8 Mohan Kumar, V., Datta, S., Chhina, G.S. and Singh, B., Alpha adrenergic system in medical preoptic area involved in sleep-wakefulnessin rats, Brain Res. Bull., 16 (1986) 463-468. 9 McGinty, D.J. and Sterman, M.B., Sleep suppression after basal forebrain lesions in the cat, Science, 160(1968) 1253 1255. 10 McGinty, D. and Szymusiak, R., The basal forebrain and slow wave sleep: mechanistic and functional aspects. In A. Wauquier, C. Dugovic and M. Radulovacki (Eds.), Slow Wave Sleep: Physiological, Pathophysiological and Functional Aspects, Raven, New York, 1989, pp. 61 73. 11 Nauta, W.J.H., Hypothalamic regulation of sleep in rats. An experimental study, J. Neurophysiol., 9 (1946) 285 316. 12 Paximos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, 2nd edn., Academic Press, North Ryde, Australia, 1986, 26 pp. 13 Sterman, M.B. and Clemente, C.D., Forebrain inhibitory mechanisms:sleep patterns induced by basal forebrain stimulation in the behaving cat, Exp. Neurol., 6 (1962) 103-117. 14 Szymusiak, R. and McGinty, D., Sleep suppression following kainic acid-induced lesions of the basal forebrain, Exp. Neurol., 94 (1986) 598-614.
300
NSL 06952
Effects of medial preoptic area lesions on sleep and wakefulness in unrestrained rats Samuel A. Asala*, Yasuhisa O k a n o , K a z u k i H o n d a a n d Shojiro Inou6 Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo (Japan) (Received 26 January 1990; Revised version received 22 February 1990; Accepted 23 February 1990)
Key' words. Brain temperature; Circadian rhythm; Medial preoptic area lesion; Rat; Sleep-wakefulness Bilateral radiofrequency lesions of the medial preoptic area (MPOA) in unrestrained male rats resulted in a significant decrease in slow wave sleep (SWS) in the light period throughout two postoperative weeks, although the night-active pattern of circadian rhythms was little affected. Both diurnal and nocturnal paradoxical sleep (PS) gradually increased after the lesions. Within one week, however, the daily amount of total sleep (SWS + PS) was recovered to the normal level, since the loss of diurnal SWS was compensated by an increase in nocturnal sleep at the expense of wakefulness. The MPOA lesions brought about a transient elevation of brain temperature, which lasted only for the diurnal period of the day of lesioning. It is speculated that the MPOA plays a definite role in the passage of sleep-regulatory information, especially concerning the circadian distribution of sleep.
Several studies have attempted to define the role of specific areas of the central nervous system in the induction and regulation of sleep and wakefulness (for reviews, see refs. 7, 10). On the basis of transsection studies in rats, the preoptic area (POA) has been proposed as a sleep center [11]. Electrical stimulation of the POA may result in sleep in cats [13], whereas electrolytic lesions of this area cause a disturbance of sleep [9]. Neuronal activities of the POA are modulated by local application of sleeppromoting substance [6]. However, all these studies deal with a wide region of the basal forebrain including the lateral POA (LPOA) but often not the medial POA (MPOA). Hence an essential site for sleep regulation has not been specified. Meanwhile, it is reported that the adrenergic and serotonergic systems in the MPOA are differentially involved in the modulation of body temperature and vigilance states [2, 3, 8]. In the present study, both acute and chronic effects of radiofrequency lesions of the MPOA were investigated in freely behaving rats. This lesion method can accurately and acutely destroy all components of a localized tissue area. A preliminary report appeared as an abstract form [1]. *Present address: Department of Human Anatomy, Faculty of Medicine, Ahmadu BeUo University, Zaria, Nigeria. Correspondence." S. Inou6, Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Kanda-Surugadai 2-3-10, Chiyoda-ku, Tokyo 101, Japan. 0304-3940/90/$ 03.50 ~') 1990 Elsevier Scientific Publishers Ireland Ltd.
301
Young male rats of the Sprague-Dawley strain, kept in our closed colony on a 12-h light and 12-h dark schedule (lights on: 08.00-20.00 h) in a constant air-conditioned environment of 25 + 1°C and 60 + 6% relative humidity with ~free access to rat chow, were used. At the age of 60-70 days, animals weighing 300-400 g were anesthetized with pentobarbital sodium and implanted with 3 cortical EEG electrodes and 2 nuchal electromyographic (EMG) electrodes according to the technique described in a previous paper [4]. They were also implanted with a thermistor electrode in the thalamus for recording brain temperature (Tbr). The animals were individually housed in a special cage which enabled continuous monitoring of EEG, E M G and Tbr. Lead wires of the electrodes were connected with a polygraph (Nihon-Kohden EEG-4317) via a slip ring fixed above the cage. Thus free movement of the rats was guaranteed. Each cage was placed in a sound-proof, electromagnetically shielded chamber under the same environmental conditions as above. The postural, feeding and other behaviors were video-monitored by color and infrared-ray TV systems. One week was allowed to recover from the surgery. After observering the establishment of circadian rhythms in sleep-waking behavior, baseline recordings were done for 24 h starting at the onset of the light period. EEG (bipolary registered between two combinations of the three electrodes), E M G and Tbr were polygraphically recorded at the paper speed of 0.5 mm/s. Tbr was also fed directly into a computer and numerically printed out at 3-min interals. At the end of the recording period, each rat was re-anesthetized with pentobarbital sodium and the MPOA was subjected to bilateral radiofrequency lesions by a lesion generator (Radionics RFG-4A) at the stereotaxic coordinates of 0.7 mm caudal, 0.6 mm lateral and 8.6 mm ventral to the bregma [12]. The lesion generator delivered microwaves of 500 kHz, 22 V and 7.5 mA, generating an electrode tip temperature of 70°C for 1 min. The whole procedure for this second operation was completed within 30 min, i.e. 08.00-08.30 h. Then the operated rats were returned to their own cages to continue the polygraphic recordings. On postlesion days 1 (day of operation), 7 and 14, EEG, EMG and Tbr were similarly recorded for 24 h. At the end of the experiment, each rat was killed with an overdose of pentobarbital sodium and perfused with 10% formol saline through the apex of the heart. The brain was then removed and sliced into 50-~tm-thick sections using a freezing sliding microtome. The sections were stained with 0.1% Cresyl violet and examined under a light microscope to ascertain the location of the lesions. Only rats whose brain histology revealed accurate placement of MPOA lesions were adopted in the present study. Sleep-waking states were visually classified on a large scale digitizer as slow wave sleep (SWS), paradoxical sleep (PS) and wakefulness according to our routinized criteria, numerically stored and processed in a computer system [5]. The data were statistically analyzed by Student's t-test. The MPOA lies adjacent to the 3rd ventricle and measures ca. 1 mm in its rostrocaudal dimension, ca. 1.2 mm in the transverse plane at its mid level and ca. 1.1 mm in its depth. The radiofrequency parameters used here were just sufficient to produce a complete lesion of the left and right MPOA. Fig. 1 shows a rostrocaudal reconstruction of the bilaterally lesioned MPOA and a photomicrographed example of a
-0.26~
302
L:Lateral POA M:Medial POA :Lesioned area
Fig. 1. Photomicrograph (left) indicating the bilateral lesions of the medial preoptic area (POA; arrows) and reconstruction (right) of typical lesion sites in the medial POA at approximate stereotaxiccoordinates [12] of 0.26, 0.4, 0.8 and 1.3 mm caudal to the bregma. 3V, 3rd ventricle; CC, corpus callosum; OX, optic chiasma; SCN, suprachiasmatic nucleus.
lesioned brain section. The lesion area became vacuolated and opened into the 3rd ventricle. The suprachiasmatic nucleus, fornix, anterior commissure, optic chiasma and L P O A remained intact (Fig. 1). In one rat, the left anterior commissure was partially damaged at level - 0 . 8 . In another rat, the right L P O A was partially lesioned also at the level - 0 . 8 . In both rats, however, the quantity of sleep parameters was not largely different from that of the other 6 successfully lesioned rats. Hence, the records of the eight rats were combined and averaged in this study. The effects of the complete bilateral lesions of the M P O A on sleep and wakefulness are summarized in Table I. The M P O A lesions exerted a slight but definite influence on the circadian pattern of rest-activity rhythms (see below), although the lesioned rats were night-active as before, exhibiting that 63-65% of their total sleep time was distributed in the daytime throughout the 2-week period. Observations of posture, movement and general behavior did not reveal any obvious change after lesioning.
303 TABLE I TOTAL TIME (rain) OF SLEEP AND WAKEFULNESS IN MEDIAL PREOPTIC AREALESIONED RATS (MEAN + S.E.M., n = 8) Parameter Wakefulness Light period Dark period Slow wave sleep (SWS) Light period Dark period Paradoxical sleep (PS) Light period Darkperiod Total sleep (SWS+ PS) Light period Dark period Light+dark periods
Baseline
Postlesion day I
Postlesionday 7 Postlesion day 14
211.3+ 8.0 476.6+ 15.6
284.0+13.8"* 477.1 __+11.2
237.4__+10.7 440.2 ___19.8
241.7+ 7.4* 444.8 + 14.1
451.7+ 3.7 216.5+11.6
387.9+12.8"* 215.6+ 8.9
419.5+11.2" 246.0__+17.6
404.9+ 7.1"* 238.2__+12.7
63.1 + 19.8 33.9__+ 3.0
73.4+ 4.1" 37.1+ 2.2
482.6+10.7 279.9-+ 19.8 762.5-+18.7
478.3_+7.5* 275.2+ 14.1 753.5+ 13.7
57.0+ 5.1 26.9+ 4.7 508.7+ 8.0 243.4__+15.6 752.1 -+ 18.7
48.8+ 5.0 27.2+ 3.1 436.7_+18.9"* 242.9+ 11.3 679.7+ 18.2"
*P < 0.02, **P < 0.01 as compared to the corresponding baseline (Student's t-test).
On the postlesion day 1, the amount of SWS was significantly reduced during the light period (Table I). The amount of diurnal PS also decreased but the change was statistically insignificant. These changes were largely ue to a considerable but statistically insignificant reduction of episode duration (data not shown). A significant reduction of diurnal SWS was still observable on postlesion days 7 and 14. Thus, the amount of wakefulness increased in the light period of these days. Both diurnal and nocturnal PS increased gradually after the lesions. Interestingly, the daily quantity of total sleep (SWS plus PS) was returned to the normal level within one week (Table I). This was mainly caused by a compensatory increase in nocturnal sleep at the expense of nocturnal wakefulness. Tb~ exhibited a circadian rhythm: average value in the light and dark period was 37.00__+ 0.14 °C (n = 7) and 37.82__+0.08°C (n = 7), respectively. Tbr was significantly elevated during the diurnal period of the lesion day (37.52 + 0.17°C, P < 0.02) but not in its nocturnal period (37.88__+ 0.10°C). The subsequent recordings revealed no difference from the baseline level, manifesting the normal circadian rhythmicity. Thus, the localized damage of the M P O A brought about a transient elevation of Tbr, similar to a previous study in cats [14], and an acute and chronic reduction of diurnal SWS. Since the thermal and sleep-waking reactions were separable, different neural mechanisms may be responsible for the modulation of temperature and sleep within the MPOA. This assumption is in agreement with that of Datta et al. [2, 3, 8], although our study suggested that the M P O A plays no crucial role in thermoregulation. Our finding on the disturbance of sleep after the bilateral lesions of the M P O A coincides with the previous studies dealing with a wide damage of the basal forebrain
304 [9, 1 1, 14]. However, c o n t r a r y to these reports, it should be noted that the chronic i m p a i r m e n t was limited only to d i u r n a l SWS in o u r M P O A - l e s i o n e d rats, a n d that the loss of d i u r n a l SWS was easily b a l a n c e d by a n o c t u r n a l r e b o u n d . Thus, the daily a m o u n t of total sleep was little affected by the lesions. This m e a n s that the g e n e r a t i o n of a fixed a m o u n t of daily sleep was n o t impaired, b u t that the circadian d i s t r i b u t i o n of sleep a n d wakefulness was definitely m o d u l a t e d . Such a n a l t e r n a t i o n c a n n o t be ascribed to the damage of the circadian clock itself, since the suprachiasmatic nucleus remained intact in these animals. Hence, it is speculated that the M P O A plays a n i m p o r t a n t but not p r i m a r y role both in the g e n e r a t i o n of sleep a n d in the passage of sleep-regulatory i n f o r m a t i o n to a n d from the other neural structures, especially c o n c e r n i n g the circadian d i s t r i b u t i o n of sleep. Dr. Samuel A. Asala was an awardee of a 1988 fellowship of the M a t s u m a e Intern a t i o n a l F o u n d a t i o n , Tokyo. I Asala, S.A., Okano, Y., Honda, K. and Inou6, S., Effect of medial preoptic area lesion on sleep in rats: a preliminary report, Jpn. J. Psychiat. Neurol., 43 (1989) 774--775. 2 Datta, S., Mohan Kumar, V., Chhina, G.S. and Singh, B., Effect of application of serotonin in medial preoptic area on body temperature and sleep-wakefulness,Ind. J. Exp. Biol., 25 (1987) 681-685. 3 Datta, S., Mohan Kumar, V., Chhina, G.S. and Singh, B., Interrelationship of thermal and sleep-wakefulness changes elicited from the medical preoptic area in rats, Exp. Neurol., 100 (1988) 40-50. 4 Honda, K. and Inou6, S., Establishment ofa bioassay method for the sleep-promotingsubstance, Rep. Inst. Med. Dent. Eng., 12 (1978) 81-85~ 5 Honda, K. and Inou6, S., Effects of sleep-promoting substance on sleep-wakingpatterns of male rats, Rep. Inst. Med. Dent. Eng., 15 (1981) 115 123. 6 lnokuchi, A. and Oomura, Y., Effects of sleep-promoting substance on rat hypothalamic neuron activity, Brain Res. Bull., 16 (1986) 429-433. 7 Jones, B.E,, Basic mechanisms of sleep-wake states. In M.H. Kryger, T. Roth and W.C. Dement (Eds.t, Principles and Practice of Sleep Medicine, Saunders, Philadelphia, 1989, pp. 121-138. 8 Mohan Kumar, V., Datta, S., Chhina, G.S. and Singh, B., Alpha adrenergic system in medical preoptic area involved in sleep-wakefulnessin rats, Brain Res. Bull., 16 (1986) 463-468. 9 McGinty, D.J. and Sterman, M.B., Sleep suppression after basal forebrain lesions in the cat, Science, 160(1968) 1253 1255. 10 McGinty, D. and Szymusiak, R., The basal forebrain and slow wave sleep: mechanistic and functional aspects. In A. Wauquier, C. Dugovic and M. Radulovacki (Eds.), Slow Wave Sleep: Physiological, Pathophysiological and Functional Aspects, Raven, New York, 1989, pp. 61 73. 11 Nauta, W.J.H., Hypothalamic regulation of sleep in rats. An experimental study, J. Neurophysiol., 9 (1946) 285 316. 12 Paximos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, 2nd edn., Academic Press, North Ryde, Australia, 1986, 26 pp. 13 Sterman, M.B. and Clemente, C.D., Forebrain inhibitory mechanisms:sleep patterns induced by basal forebrain stimulation in the behaving cat, Exp. Neurol., 6 (1962) 103-117. 14 Szymusiak, R. and McGinty, D., Sleep suppression following kainic acid-induced lesions of the basal forebrain, Exp. Neurol., 94 (1986) 598-614.