PII S0031-9384(98)00036-5 Release of a Substance that Suppresses Spontaneous Motor Activity in the Brain by Physical Exercise KAZUO INOUE, HANAE YAMAZAKI, YASUKO MANABE, CHIAKI FUKUDA AND TOHRU FUSHIKI 1 Laboratory of Nutrition Chemistry, Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-01, Japan Received 23 June 1997; Accepted 19 January 1998 INOUE, K., H. YAMAZAKI, Y. MANABE, C. FUKUDA AND T. FUSHIKI. Release of a substance that suppresses spontaneous motor activity in the brain by physical exercise. PHYSIOL BEHAV 64(2) 185–190, 1998.—Injection of the cerebrospinal fluid (CSF) collected from rats fatigued by obligatory swimming into the cisterna magna of mice suppressed the spontaneous motor activity of the mice. The suppressive activity was abolished by heat denaturation of the CSF and was found in the CSF fraction with a molecular weight above 10,000 after ultrafiltration. These findings suggest the presence of a substance(s) released in a fatigued animal’s brain that suppresses its spontaneous motor activity and generates the sensation of fatigue. © 1998 Elsevier Science Inc. Exercise Central fatigue Spontaneous motor activity Cerebrospinal fluid Rats Mice THE sensation of fatigue (including physical and mental) during physical exercise may arise not only from periph- eral sources of exhaustion such as energy source depletion but also from the central regulation for overall homeostasis. The activities of 5-hydroxytryptamine (serotonin, 5-HT) neurons are related to central fatigue during physical exer- cise (1– 4,11,12). The modification of the endurance capac- ity by a 5-HT agonist or antagonist suggested the involve- ment of 5-HT neurons in the performance of physical exercise. The sensation of fatigue induced by exercise may be a signal that the body is overworking. The central nervous system (CNS) detects the state of overwork, assesses the level of exhaustion, and may also release signals to avoid further im- pairment. A signaling substance that reduces an animal’s mo- tivation to exercise and suppresses spontaneous motor activity may be released from the brain into the cerebrospinal fluid (CSF) surrounding the CNS, thus preventing the progression of impairment and preparing the milieu inte ´rieur for the recovery of the damaged regions. To investigate whether the brain of exercise-fatigued rats re- leases a substance(s) involved in the sensation of fatigue, we compared the spontaneous motor activity of mice injected with CSF collected from exercise-fatigued rats. MATERIALS AND METHODS Animals Male Sprague–Dawley rats (8 weeks old) and male Std ddY mice (5 weeks old) were used. All animals were maintained on an inverse 12-h light– dark cycle (1900 hours: light on/0700 hours: light off) for 1 week to become active during the experimental time. They were fasted overnight the day before the experiments. The care and treatment of the experimental animals conformed to the Kyoto University guidelines for the ethical treatment of labo- ratory animals. Swimming Protocol and CSF Sampling The rats were forced to complete eight sets of swimming exercise in a constant-current pool (7). Each exercise set consisted of 15 min of swimming and 5 min of rest. A weight equivalent to 2% of each rat’s body mass was attached to the tail in the last six sets. Immediately after the completion of the exercise schedule, the rats were anesthetized with pentobarbital, and CSF was collected from the cisterna magna after puncture of the atlantooccipital membrane with a 26-ga needle (6). CSF visibly contaminated by red blood cells was discarded. Uncontaminated CSF was centri- fuged to ensure removal of red blood cells. CSF was also collected from the sedentary rats which had not been subjected to exercise 1 To whom requests for reprints should be addressed. E-mail: d53765@sakura.kudpc.kyoto-u.ac.jp Physiology & Behavior, Vol. 64, No. 2, pp. 185–190, 1998 © 1998 Elsevier Science Inc. All rights reserved. Printed in the U.S.A. 0031-9384/98 $19.00 + .00 185