Effects of caffeine on locomotor activity of horses: Determination of the no-effect threshold

Caffeine is the legal stimulant consumed most extensively by the human worl d population and may be found eventually in the urine and/or blood of race horses, The fact that caffeine is in foods led us to determine the highest no-effect dose (HNED) of caffeine on the spontaneous locomotor activity of horses and then to quantify this substance in urine until it disappeared. W e built two behavioural stalls equipped with juxtaposed photoelectric senso rs that emit infrared beams that divide the stall into nine sectors in a 't ic-tac-toe' fashion. Each time a beam was interrupted by a leg of the horse , a pulse was generated; the pulses were counted at 5-min intervals and sto red by a microcomputer. Environmental effects were minimized by installing exhaust fans producing white noise that obscured outside sounds. One-way ob servation windows prevented the animals from seeing outside. The sensors we re turned on 45 min before drug administration (saline control or caffeine) , The animals were observed for up to 8 h after i.v. administration of 2.0, 2.5, 3.0 or 5.0 mg caffeine kg(-1). The HNED of caffeine for stimulation o f the spontaneous locomotor activity of horses was 2.0 mg kg(-1). The quant ification of caffeine in urine and plasma samples was done by gradient HPLC with UV detection. The no-effect threshold should not be greater than 2.0 mug caffeine ml(-1) plasma or 5.0 mug caffeine ml(-1) urine. Copyright (C) 2001 John Wiley & Sons, Ltd.