Physiology & Behavior 69 (2000) 455–461 0031-9384/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S0031-9384(99)00250-4 Pinealectomy does not affect the entrainment to light nor the generation of the circadian demand-feeding rhythms of rainbow trout F. Javier Sánchez–Vázquez a, *, Masayuki Iigo b , Juan Antonio Madrid a , Mitsuo Tabata c a Department of Physiology & Pharmacology, Faculty of Biology, University of Murcia, 30100-Murcia, Spain b Department of Anatomy, St. Marianna University School of Medicine, 2-16-1 Sugao, Miyamae-ku, Kawasaki 216-8511, Japan c Department of Biosciences, Teikyo University of Science & Technology, 2525 Uenohara, Yamanashi 409-0193, Japan Received 5 August 1999; received in revised form 1 December 1999; accepted 20 December 1999 Abstract The pineal organ and its secretory product melatonin are regarded as synchronizers of daily rhythms to the external light/dark (LD) cycle. In fish, the pineal organ acts as a direct photoreceptor, transducing light information into neural and humoral (melatonin) signals. In the present study, we investigate a possible role for the pineal organ and melatonin in the regulation of feeding rhythms of rainbow trout, Oncorhynchus mykiss. We used individual rainbow trout placed in an insulated room at constant temperature (14°C). Fish were self-fed ad lib by means of self-feeders coupled to a computer that continuously recorded demand-feeding activity. Before and after pine- alectomy, the fish were exposed to a LD cycle of 16:8 h and then constant light (LL) to test the effect of pinealectomy on demand-feeding rhythms. Feeding records revealed that trout fed exclusively during daytime (96% of feeding confined to the light phase), and that re- moval of the pineal organ did not disrupt this daily feeding profile, with synchronization to the LD cycle persisting. Moreover, the appear- ance of circadian feeding rhythms was not affected by pinealectomy: most of the operated fish free-ran with an average tau longer than 24 h. Plasma melatonin rhythms persisted in the pinealectomized trout, but with small amplitude. These results suggest that the pineal may not be the site of the pacemaker that controls feeding rhythms in trout, although further research is required to study the involvement of other photoperiod-transducing systems and melatonin (of nonpineal origin) in the regulation and expression of circadian rhythms in this species. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Pineal organ; Melatonin; Circadian rhythms; Feeding; Rainbow trout 1. Introduction Biological rhythms appear in most functions. Animals adapt and coordinate their behaviour and physiological functions to a daily schedule, which is synchronized to pre- dictable temporal changes in the environment (such as day– night or daily oscillations in temperature). Diel patterns are usually under the control of several endogenous pacemakers that form the so-called circadian system. In vertebrates, three brain structures (the pineal, the retina, and the supha- chiasmatic nuclei of the hypothalamus) are central pace- makers of the circadian system, although oscillators may be found in one, two, or three of these sites in different verte- brates, and each oscillator may have different strength, de- pending on the species. The pineal organ and its hormone melatonin are regarded as synchronizers of daily rhythms to the external light/dark (LD) cycle [1,2]. In birds, for in- stance, transplantation of the pineal into an arrhythmic, pi- neal-less recipient resulted in the regaining of circadian rhythmicity. Furthermore, the phase of the induced rhythm was that of the donor bird [3]. In fish, as in other nonmam- malian vertebrates, melatonin is primarily produced by the photoreceptor cells in the pineal organ, which acts as a pho- toneuroendocrine transducer and secretes melatonin into the blood [4]. As demonstrated in isolated pineals in vitro, most fish species possess a biological clock in the pineal that drives rhythmic melatonin secretion [2,5–7]. However, the pineal of salmonids lacks an endogenous oscillator capable of pro- ducing self-sustained melatonin rhythms, because rhythmic secretion does not persist under constant darkness in the rainbow trout, Oncorhynchus mykiss [8–10] or masu salmon, O. Masou [11]. Melatonin production by the salmonid pineal directly follows the light schedule, maintaining continuously high titters during the dark phase but decreased levels during the light phase, regardless of the duration of each phase. Although the lack of endogenous rhythmicity remains unex- plained, it has been suggested that the pineal may function * Corresponding author. Tel.: +349 68 367004 Fax: +34 968 363963 E-mail address: javisan@fcu.um.es