A digital tail-flick apparatus

IN THE evaluation of analgesic drugs, an animal model is often used. A mildly painful stimulus is applied under circumstances where the animal can escape at will, and the time is measured from the onset of the stimulus until escape behaviour is observed. A frequently used version of this general method is the 'tail-flick response ', in which heat is applied to the tail of a mouse or rat, and the time is measured until the animal flicks its tail away from the heat. Radiant heat is usually used in these devices, with an electric light bulb as a source and a lens to focus the heat on the animal's tail. In some instruments a photocell positioned beneath the tail causes an electric timer to stop when the tail flicks away. We have designed and built for our own use a tail-flick device which offers certain advantages over the focusedhoat type of instrument. It is compact, inexpensive, generates no wasted heat, has no optical parts, and is very simple to operate. We have used it in our research with very satisfactory results. Since application of heat for longer than 15 s may burn the tail, we have limited the timer readout to 19.9 s. Response times under 10 s are normally used. The heating wire consists of the filament of a microscope illuminating lamp (General Electric CM1493, rated at 6" 5 V, 2.75 A). The glass envelope is broken away and the base and filament are mounted in a socket beneath the upper surface of the box, with the filament protruding up through the top of the box into the groove for the mouse's tail. A current of about 0"9 A is used to heat the filament. The filament is mechanically durable and can withstand extensive use in this application.