Pharmacology Biochemistry & Behavior, Vol. 32. pp. 797-800. ©Pergamon Press plc, 1989. Printed in the U.S.A. 0091-3057/89 $3.00 + .00 Effects of 8-OH-DPAT on Motor Activity in the Rat VIVEKA HILLEGAART,*5" MARIE-LOUISE WADENBERG:~ AND SVEN AHLENIUS* *Department of Neuropharmacology, Astra Research Centre, S-151 85 SOdertiilje, Sweden -~Departments of Psychology and Zoology, University of GOteborg, GOteborg, Sweden and ~cDepartment of Psychology, University of Stockholm, Stockholm, Sweden Received 7 April 1988 HILLEGAART, V., M.-L. WADENBERG AND S. AHLENIUS. Effects of 8-OH-DPAT on motoractivity in the rat. PHARMACOL BIOCHEM BEHAV 32(3) 797-800, 1989.--The administration of 8-OH-DPAT to rats produced a dose-dependent suppression of spontaneous locomotor activity in an open field arena. 8-OH-DPAT was administered in the dose range 12.5-1,600 txg'kg-~ SC. Vertical activity ("rearing") was more sensitive to the treatment than horizontal activity ("locomotion"), both in terms of potency and efficacy. The activity along the walls of the open field arena ("peripheral activity") was increased, and the rearing activity was decreased, relative to total horizontal activity and total activity, respectively. There were no effects by 8-OH-DPAT on treadmill locomotion. The rectal temperature was decreased by 8-OH-DPAT administration, not only in animals tested in the open field, but also in animals with an increased body temperature, produced by treadmill locomotion. Open field Motor activity Treadmill 8-OH-DPAT 5-Hydroxytryptamine Rat THE role of central 5-hydroxytryptamine (5oHT) in the regulation of motor mechanisms is complex. Thus, a depletion of brain and spinal 5-HT, as well as an increase in the availability of central 5-HT, can result in a decrease or an increase in motor activity depending on the experimental model used [see (9)]. The somata of 5-HT-containing neurons of the CNS are located in the lower brain stem and innervate the entire neuroaxis and forebrain via ascending and descending projections (7,25). Thus, there are numerous levels at which central 5-HT can affect motor behavior, from sensory perception, sensory-motor integration to motor effector mechanisms. As an example of the complexity encoun- tered, it was recently shown that the local application of 5-HT into the dorsal raphe produced a decrease, and application into the median raphe produced an increase, in rat locomotor activity (12). Central serotonergic mechanisms cannot only be subdivided according to localization of cell bodies and projections areas, but also according to receptor subtypes, possibly subserving distinct functions [see, e,g., (19)]. In addition to the general differentia- tion of 5-HT~ and 5-HT 2 receptors, the 5-HT t receptor has been further subdivided 5-HT ~ A, 5-HT~B, and 5-HT~¢ subtypes (15,18). 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) has been identified as a 5-HT agonist (14) with preference for the 5-HT~A binding site (17), and associated with particular functions (5, 8, 10, 20). In the present experiments we have investigated the effects of systemically administered 8-OH-DPAT on motor activity in an open field, and on treadmill locomotion, in rats. In agreement with observations in mice (26), but in contrast to previous observations in rats (27), we found a dose-dependent suppression of spontane- ous motor activity. Rectal temperature was also monitored in some of the animals observed in the open field and on the treadmill. METHOD Animals Adult male Wistar rats, 250-350 g (ALAB, Laboratorietj~inst AB, Sollentuna, Sweden), were used. The animals were housed under controlled conditions of temperature (20-21°C), humidity (55-65%), and light-dark cycle (12:12 hr, lights off at 06.00 hr), and arrived in the laboratory at least one week before being used in experiments. Drugs 8-Hydroxy-2-(di-n-propylamino)tetralin.HBr (8-OH-DPAT) (RBI, Wayland, MA). 8-OH-DPAT was dissolved in 0.9% NaC1 and injected subcutaneously in a volume of 2 ml'kg-~. Doses of 8-OH-DPAT were calculated on the salt. Controls received the saline vehicle. Apparatus The spontaneous motor activity was measured in photocell cages (700 × 700 mm). The photocells were sensitive to infrared light and observations were made in the dark. The apparatus allowed the registration of the following components: Locomotor activity (all horizontal activity as registered by a row of photocells, 40 mm above the floor of the cage), Rearing (vertical activity as registered by a second row of photocells, 125 mm above the floor), Peripheral activity (horizontal activity as registered by the first photocell in the bottom row, 40 mm from the wall). The relation of peripheral and rearing activities to locomotor activity 797