Technical note A digital tail-flick apparatus* Keywords--Mouse tail-flick apparatus IN T H E 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. Operation of the instrument The mechanical construction of the instrument is shown in Fig. 1. A mouse is placed head first in the copper tube ( 1 2 5 m m • the mice willingly enter the tube, which is secured by a banana plug protruding from it. The bottom of the tube is inserted in its socket on the top of the box, and the mouse's tail is moistened (with a damp sponge or finger) and placed in the groove in the plastic block so that it contacts a filament which traverses the groove. The filament is at a potential of 12.6 V a.c.r.m.s., but no heating current is flowing through it. When the mouse's tail is in contact with the filament, a current which we find to be about 200 nA passes through the mouse and the copper tube, and is detected inside the instrument. This current causes the light-emitting diode L1 to light, signalling that the tail is in place. When L1 is oN, switch S1 is depressed by the experimenter. This switch applies current to the heating filament, resets the timer to zero, and permits the timer to run. When the mouse flicks its tail off the hot filament so that current no longer flows through the mouse, the timer stops, displaying the elapsed time in seconds and tenths of seconds. Releasing switch S1 then allows the wire to cool. We find that the small current passing through the mouse has no perceptible effect on it: no tail flick, or other response, is seen before heat is applied. We also find that the degree of moistening of the tail is noncritical within reasonable limits. Inexperienced personnel can quickly obtain reproducible results with this device. We adjusted the heating current, using R1, so that the typical tailflick time is 2 s for untreated mice. Morphine at doses around 1 mg/kg doubles the time, and the response may be delayed indefinitely by larger doses. * First received 15th May and in final form 22nd May 1974

Medical and Biological Engineering

July 1975

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. Electronic circuit

The circuit diagram is shown in Fig. 2. The heating filament is always at a potential of 12.6V a . c . r . m . s . When S1 (a double pole, normally open, momentary

Fig. 1 Tail-flick apparatus. Upper photograph shows overall view. Lower photograph shows detail of tail groove with heating wire protruding up into groove

597

contact pushbutton switch) is closed, current passes through the filament. The current is limited by a fixed resistor and a variable resistor R1. R1 is used to adjust the heat; it is located inside the box and is not changed once satisfactory performance is achieved. Current passing through the mouse is amplified by a type 741C operational amplifier, whose gain is adjusted with R2 (also located internally). The output of the amplifier is clamped with a 5' 1 V Zener diode to eliminate excursions outside the range of zero to 5.1 V. The amplifier output is detected by a 7413 Schmitt trigger, which produces a t.t.l.-compatible 60 Hz pulse train.

generated by a 74123 monostable when S1 is activated. This pulse is enabled by the output of the other half of the 74123 (i.e. the ' tail-in place' signal). Therefore, the counters can be cleared only when the tail is in place, preventing loss of data once the tail has been flicked. S! is slowed by the use of an R C network and a 7413 Schmitt trigger. This arrangement eliminates contact bounce and prevents the appearance of spurious counts when the switch is activated or deactivated. The unit is powered by a 12.6 V 2A transformer for the heating wire, two 9 V transistor radio batteries for the 741C amplifier, and a 5 V 1A ' logic' power supply

[ _ . . ~ ~o.o, ,F ('.k

-~..~

xt5

"

V~

"~

I

Jr'-. -~J

,

~

l

I

,ooo '"'"'4 x

,,v

_-4--

,,

',= :

~

I,=l

,v=o

I

I

:-~-'......... -~i

-I-==.F

,oo

J_

-

' I I

- -

I

I '~

,c.

s.

,t I

,

-

~S_>'~

,,,

f

'c'

l

,

L

7,pop

~

74,23

lr

l. 7,

,o ~1_'~._2 ~

~h

i

Tvr162

I 'c~

'

L,_I

[ 7447 ~

'c7 7447

I,I

I,.

I

Fig. 2 Circuit diagram This pulse train triggers a 74123 retriggerable monostable multivibrator, whose pulses are lengthened so that, with a 60 Hz input, the output is always oN. This output drives indicator L1 and enables the clear circuit for the counter. The 60 Hz pulse train also enters a 7492 divide-by-6 counter, which produces a 10 Hz output. If a similar instrument were constructed for use on 50 Hz power, the 7492 would be replaced by a 7490 counter, dividing the frequency by 5 instead of 6. This signal, if enabled by S1 via an AND gate, is counted by two 7490 decade counters. The otherwise-unused A flip-flop of the 7492 is used for the 10 s bit. The count is decoded and displayed on 7-segment readouts (Shelly 3105F-CN), with a 1.e.d. indicator for the 10 s digit. The counters are cleared by a single short pulse

598

(B & F Enterprises, Peabody, Massachusetts). Thi~ total construction cost was about US$50.00.

Acknowledgment--The author thanks Richard Schmidt for his skilful contributions to the assembly of this instrument. This work was supported by USPHS Grant No. 16538~)5. GILBERT R. HILLMAN

Division o f Biological and Medical Sciences Brown University Providence Rhode Island 02912 USA

Medical and Biological Engineering

July 1975