Caffeine inhibits exercise-induced increase in tryptophan hydroxylase expression in dorsal and median raphe of Sprague±Dawley rats Baek-Vin Lim a , Mi-Hyun Jang a , Min-Chul Shin a , Hyun-Bae Kim a , Youn-Jung Kim a , Young-Pyo Kim a , Joo-Ho Chung a , Hong Kim b , Mal-Soon Shin b , Sung-Soo Kim b , Ee-Hwa Kim c , Chang-Ju Kim a, * a Kohwang Medical Research Institute, College of Medicine, Kyung Hee University, Dongdaemun-gu, Seoul, South Korea b Research Institute of Sports Science, Korea University, Sungbuk-gu, Seoul, South Korea c Department of Meridianology, College of Oriental Medicine, Semyung University, Chungbuk, South Korea Received 7 April 2001; received in revised form 23 May 2001; accepted 29 May 2001 Abstract Effect of caffeine on the expression of tryptophan hydroxylase (TPH), rate limiting enzyme of serotonin synthesis, in dorsal and median raphe was investigated via immunohistochemistry. In exercise groups, Sprague±Dawley rats were put on treadmill running for 30 min per day for 6 consecutive days. On the seventh day, animals of control-with-caffeine group were injected subcutaneously with 4 mg/kg caffeine, while control-without-caffeine group were injected with 0.9% NaCl, sacri®ced 2 h later. Exercise-with-caffeine group and exercise-without-caffeine group were injected with caffeine and NaCl, respectively; all-out time was determined 1 h after injection, and then sacri®ced. Caffeine increased all-out time in exercised rats, and inhibited the exercise-induced elevation in TPH expression. The suppressive effect of caffeine on TPH expression in exercised rats can be suggested as one possible ergogenic mechanism of caffeine. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Caffeine; Immunohistochemistry; Treadmill exercise; Tryptophan hydroxylase; Dorsal raphe; Median raphe Caffeine is one of the most widely consumed drugs in the world. There have been numerous reports that caffeine is an ergogenic aid; ingestion of the drug has been shown to increase endurance performance, particularly during prolonged periods of exercise lasting 30±120 min [11,12,18]. Caffeine is known to stimulate the central nervous system (CNS), to enhance neuromuscular transmis- sion, and to improve skeletal muscle contractility [20]. The exact mechanism of the stimulatory effect on CNS of caffeine is not clear, because caffeine exerts multiple effects at the cellular level. For instance, caffeine antagonizes adenosine [5] and g-amino butyric acid (GABA) receptor- mediated effects [15], inhibits phosphodiesterase [19], and stimulates Ca 21 release from the endoplasmic reticulum [8]. Davis and Bailey [7] reported that multiple neurological factors both central and peripheral, in¯uence fatigue during prolonged exercise. In the mammalian CNS, serotonin (5- hydroxytryptamine, 5-HT) is known to modulate body temperature, blood pressure, endocrine activity, appetite, sexual behavior, movement, emesis, and pain [14]. The central fatigue hypothesis suggests that increased concen- tration of 5-HT in the brain impairs CNS function during prolonged exercise, thereby resulting in deterioration of exercise performance [16]. It was also suggested that an increase or a decrease in brain 5-HT activity during prolonged exercise hastens or delays fatigue, respectively [7]. Tryptophan hydroxylase (TPH) catalyzes the rate-limit- ing step of serotonin biosynthesis in serotonergic neurons of the raphe nuclei [2]. As such, the TPH gene is a likely target in the modulation of serotonergic function [2]. It has been reported that TPH expression is modulated by several factors, such as immobilization and sound [1,4,17]. Although much evidence suggests that caffeine improves performance in endurance exercise, this phenomenon has not been investigated in relation to TPH expression in the Neuroscience Letters 308 (2001) 25±28 0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(01)01980-2 www.elsevier.com/locate/neulet * Corresponding author. Department of Physiology, College of Medicine, Kyung Hee University, 1 Hoigi-dong, Dongdaemun- gu, Seoul, 130-701, South Korea. Tel.: 182-2-961-0282; fax: 182-2-9642195. E-mail address: changju@khu.ac.kr (C.-J. Kim).