General Description

The stimulator Rodent Shocker Type 221 is specially designed for the electrical triggering of spasms in small experimental animals (e.g. mice & rats). Suitable electrodes are used to pass a sinusoidal 50Hz or 60Hz current pulse of variable intensity and duration through the brain of the animal.

The current flowing through the brain triggers synchronous excitation of the neurons which leads motorically to clonic and/or tonic convulsions, & loss of consciousness including loss of sensory and memory capacity. In this way the electrically induced seizure is similar to an epileptic attack and is therefore a standard model for the pharmacological testing of antiepileptic drugs.

Advantages:

• Power output up to 75 Watts (Rats)
• Choice of output voltage (250-500-750 Volt)
• Bargraph display of elapsed time from 0.1 to 9.9 seconds
• Permanent monitoring of output circuit
• Immediate shutdown on open output or excessive electrode resistance.

The Rodent Shocker is used to generate shock stimulation. In accordance with its intended use it produces output voltages and currents which are hazardous to humans (max. 750V, 0.5A)

Items Covered:

730105 Rodent Shocker Sine-Wave Generator with Foot Switch, 115VAC/60Hz

730106 Rodent Shocker Sine-Wave Generator with Foot Switch, 230VAC/50Hz

734946 Rodent Shocker RS Type 221/Low Current, 230VAC/50Hz

734947 Rodent Shocker RS Type 221/Low Current, 115VAC/60Hz

Description of Equipment

Front Panel

The front panel carries the controls and the indications for operating the equipment. At the top left is the mains switch with the built-in signal lamp. In the center is the indicator for shock current, time setting, elapsed time and current fault. 

The 5-pin socket marked "FOOT SWITCH" is used to connect the foot switch supplied with the equipment. The current pulse can then be triggered with the foot so that the hands remain free for handling the animal.

The shock electrodes are connected to the sockets marked "SHOCK OUTPUT." The sockets for (4mm banana plugs) are shrouded as protection against touching them. Use only electrodes fitted with suitable shrouded plugs. 

Figure 1. Form and duration of shock produced. The periodic time (T) is not variable; it corresponds to the supply frequency. For a frequency of 50Hz it is T = 20msec, for a frequency of 60Hz, T = 16.6msec.

Set-Up

The output passes through a special transformer circuit and is fully isolated, i.e. the output sockets have no electrical connection to the case, the ground contact of the mains supply cable or any other part of the circuit. This feature ensures the safety of the user when using the equipment. 

The output circuit of the rodent shocker is arranged as a constant current source. This means that the voltage increases automatically with the actual external resistance and the selected current actually flows through the electrodes connected to the unit. This increase is limited by the maximum available voltage.

The maximum output power produced by the rodent shocker is 75 W. The electrical power (P, Watts) is obtained by multiplying the voltage (U, Volts) with the current (I, Amps):

P (W) = U (V) x I (A)

With 75 Watt power output and a current demand of I = 100 mA (0.1A), for example, the maximum voltage available is 750 V, because:

U (V) = P (W) / I (A) = 75W / 0.1A = 750 V

The rodent shocker provides a choice of three different out put current ranges. The maximum voltage can then be derived as explained above.

Choice of current-voltage combinations:

Figures 3a to 3c show the output characteristics as a curve plotted against load resistance. The limits of the three ranges are show in Figure 2.

The most suitable current-voltage combination for a particular application depends largely on the animal used and on the method of applying the shock.

Shock electrodes with smaller contact surface generally have a higher contact resistance. When applying shock through ear electrodes there is also a higher resistance than when using eye electrodes. This in turn requires (by Ohm's Law) a higher voltage to ensure that the set shock current level is in fact applied. In general it is found that working with smaller animals (mice) requires smaller currents but higher voltages than when using larger animals (rats). 

One of the three current-voltage combinations is set automatically when the equipment is switched on. This combination can be selected on a switch inside the equipment. The combination setting can be changed at any time by holding down the key Current Setting and at the same time pressing the key MAX (V / mA). Each time the key is depressed the display changes to the next current/voltage combination. When the equipment is switched off the setting selected in this way is lost.

Figure 2: limitations of the current-voltage characteristics in the three available combinations for different load resistances.

Figure 3a.

Figure 3b.

Figure 3c.

Figures 3a-3c: Output characteristic of the Rodent Shocker in the three current/voltage ranges. The shock current is plotted vertically, the total load resistance (electrode and body resistance) is shown horizontally.

Setting the shock current (intensity)

The shock current is set by rotating the control knob and is indicated on pressing the Current Setting key. The adjustment range depends on the current-voltage combination selected. NOTE: If you change the current with the control without pressing the Current Setting key at the same time, there is a time delay until the new value is indicated.

Setting the shock duration

The shock duration can be selected in the range from 0.1 to 9.9 seconds in 0.1 sec steps. The adjustment is made by means of a small screwdriver at the two selector switches below the Time key. Note: If you change the time setting you must press the Tim key in order to check the new time settings on the display and to enter the new value into the timer unit. 

During the actual shock you can monitor the elapsed time by pressing the Time Key; it is then indicated in digital form.

Triggering the shock and control of the timing

As a safety measure the rodent shocker is fitted with a trigger block. The shock can only be triggered when the block has been released by intentionally pressing the Release Key. The current status of the block is shown by the light signal (LED) in the center of the key

When triggering of a shock is unblocked the bar graph is visible. When triggering is blocked the bar graph is dark. 

The shock can be triggered:

• By pressing the Start Key
• By the foot switch which is plugged into the Foot Switch Socket.

The shock can be controlled in two ways, either in the trigger mode or in the hold mode. 

Trigger Mode: in the trigger mode the shock is started by briefly pressing the key and then continues for the selected duration (0.1 - 0.9 sec). Once it has been started the shock can be only interrupted by pressing the Release Key. The Start key has no effect once the shock has been triggered.

Hold Mode: with the unit in the hold mode the Start key has to be held down until the set time has elapsed, i.e. until the normal end of the shock has been reached. If the key is released while the shock is being produced the shock is interrupted.

In normal use it is advisable to work always in the "hold mode." The hold mode provides enhanced safety compared with trigger mode. If any fault occurs while the shock is being produced, you can react immediately by releasing the key and terminate the shock. Trigger mode does not have the same safety precautions.

The type of shock control (trigger or hold) can be selected inside the unit by means of a switch. The equipment is delivered set for hold mode.

Monitoring functions, fault recognition, fault indication

The output circuit of the rodent shocker is always being monitored. As soon as a fault is detected the output current is interrupted. There will be a double beep and a fault message. The fault report (signal light error flashing) is cancelled by pressing any key except the Start key.

The various possible faults and the equipment reaction are summarized below:

1-With open-circuit output:

• Electrodes are not connected
• Electrodes are not making proper contact
• Electrode resistance is too high (above 100kOhm)
• Break in electrode cable

2-The yellow light "ELECTRODE" is flashing and triggering the shock has been blocked. If you try to trigger a shock under these conditions there will be a beep for every attempt to press the Start key.

3-If the current is interrupted during the shock, e.g. through unreliable contact in the electrode circuit (bad connection) there is an immediate double beep and the shock is terminated. This is followed by the digital display flashing the final current reading for about 5 seconds. The display then changes back to the selected current "Set Current." The fault light "Error" continues to flash and indicates that there has been a fault. 

4-If the set current decreases by more than 50% during the set shock period, e.g. through an increase in electrode resistance, the "Error" light begins to flash. The shock is not terminated. After the end of the shock the final current reading can be checked by pressing the key "set/delivered current."

5-If the current increases by more than 10 mA during shock application the shock is terminated immediately and there is a double beep. The error light then flashes to indicate the presence of the fault condition.

This fault occurs when the current setting is increased during the shock application by turning the potentiometer, or if there is some malfunction in the equipment. 

In order to reduce the danger as far as possible you should give careful consideration to the use of the equipment and to include the working area and the entire surroundings into the safety assessment.

The working area must be carefully chosen. Carry out a safety check each time before using the equipment. Pay special attention to electrodes, electrode cables and plugs. Have a quick look at the equipment case. If there is any external damage to the case or if there are any signs that liquid may have found its way into the case, you should not use the equipment until after it has been factory checked. 

Safety Precautions

Below are summarized a number of points which you should observe when using the equipment.

Working area:

The working area must always be kept dry. There must be no water near the rodent shocker.

The work bench must not have any metal frame that can be touched and the bench top must have an insulated covering.

Water pipes and heating pipes must not be near the working area.

Use of the Equipment:

If you have any suspicion that the equipment is damaged, do not use it until it has been checked thoroughly. 

If you notice any damage to the case, such as from being dropped, do not use the equipment until it has been checked thoroughly.

It is essential to protect the equipment against humidity.

Never switch on the equipment if any liquid has found its way inside it.

Never touch the conducting parts of the electrodes with both hands!

Ensure that the handle region of the electrodes is dry and free from electrode jelly or other conducting materials.

Never use any damaged electrodes or electrode cables.

Where possible wear insulating gloves, at least on the hand holding the animal.

Use only electrodes with shrouded plugs.

Never work with wet hands.

Starting Up, Functional Tests, Function Selection

Before starting

Before you connect the equipment to the electrical supply using the mains cable supplied, check that the supply voltage set on the equipment is set for your country.

Then switch on the equipment with the main switch marked Power. The mains lamp lights up immediately and the equipment performs a self-test, with all signal indications on the equipment flashing twice. The self-test is completed after about 4 seconds. This is followed by display of the current setting Set Current and indication of the current-voltage combination Max (V/mA) which has been set inside the equipment.

If the equipment behaves differently in any way, a fault has to be suspected and the equipment has to be checked by qualified service engineer.

Functional tests

If the equipment is OK, you should make yourself familiar with the equipment functions. Always remember the hazards.

Do Not Connect Electrodes to the Shock Output.

Check the set shock duration by pressing the Time Key. Use the small screwdriver supplied with the equipment and adjust the shock duration. It should be possible to vary the duration within the range 0.1 to 9.9 seconds in 0.1 second steps. Check this by suitably turning the two switches x0.1 and X1 with the screwdriver while holding down the Time Key. Then set the expected shock duration for your experiments, e.g. 0.5 seconds.

Next check the setting functions for shock current and maximum voltage (Max. V / mA).

Press the Current Setting key and at the same time turn the setting knob. The display shows the changes in the current setting. Next release the key and turn the knob again. You should see that indication of the changed setting is now delayed by 1 - 2 seconds.

The adjustment range of the shock current depends on the current setting of the current-voltage combination (Max V/mA). If you change this combination, there is a corresponding change in the current range and therefore in the actual current setting.

Now change the current-voltage combination by holding down the Current Setting key and briefly pressing the (Max V/mA) key. The appropriate signal lamp lights up to indicate that the next combination is selected. A the same time you can note the appropriate change in the current setting on the digital display. 

For this test you require a beaker of about 1 liter capacity and a shock electrode, e.g. an HSE eye electrode. "Stand" the electrode in the beaker and pour in saline solution until the conducting parts of the electrode are just immersed. The handle area of the electrode must be kept dry.

Set a current of about 50 mA as described above and release the shock blockage by pressing the Release Key. The signal light in the center of this key which was previously off now begins to flash. The display shows the marks of the bar graph. 

Now trigger a shock by pressing the Star Key (hold the key down until the end of the shock period) and note the display. The market near the right edge immediately changes from Set Current to Delivered Current, and the bar graph indication becomes shorter in accordance with the time setting and the elapsed time. The current reading should remain the same or vary only slightly during the duration of the shock.

At the beginning and at the end of the shock you can hear a clicking noise which is produced by the control relay.

After the end of the shock period the equipment returns to the initial condition: the marker is again on Set Current, the bar graph marks are again visible and the signal in the Release Key which was on continuously for the duration of the shock is now flashing again. The equipment is now ready for triggering the next shock.

Next connect the foot switch supplied to the socket marked Foot Switch and trigger a further shock, this time with the foot switch. Keep the foot switch depressed until the end of the shock period.

The instrument reactions should be the same as described above.

Next you should simulate a circuit interruption while the shock is being produced. First check that the electrode handle is dry, then produce a further shock and remove the electrode from the solution during the shock period.

You can immediately hear a double beep; the bar graph disappears from the display. The digital indication and the two markers Error and Delivered Current flash for about 5 seconds. The equipment then returns to the same state as before this test except that the two markers Error and Delivered Current continue to flash to indicate the fault.

Now press the "Set/Delivered Current" key to indicate the current which was flowing at the time the equipment recognized the fault. In the present case the reading should be 000 mA.

 After you have carried out all test procedures as described above and repeated them if necessary, you should be sufficiently familiar with the equipment behavior and there should be no surprised when you are actually using the equipment. However, never forget the high output energy produced by the equipment!

Setting the equipment status.

When the equipment has just been switched on it sets itself to a definite status. This can be changed to suit special requirements, using a 4-way switch inside the equipment. The switch is near the top edge of the circuit board located close to the mains switch. 

When the equipment is supplied it is set to the functions marked with arrows unless it is otherwise specified.

Table 1: Settings of the selector switch inside the equipment

Off = Switch Off (Down)

ON = Switch On (Up)

X = Any Switch Position, No Effect

To change the setting you must open the case. Pull out the mains plug. Then release the two screws at the sides of the front frame. Now you can remove the front frame and also the upper part of the case. Use a small screwdriver or some other tool which is not too pointed (tweezers) to set the switches to the required position according to the table above. Then close the case again before connecting the equipment to the mains supply.

Experiment

The mouse is held with one hand by the fur on its neck. An insulating glove should be worn on this hand to protect the operator.

Shock can be induced either by applying moistened electrodes to the eyes, or by application to the ear passage (here the use of commercial EEG / ECG contact jelly is advisable to ensure good current conduction). The shock is triggered with the foot switch. The foot switch must only be actuated when the electrodes are securely applied to the animal (eye or ear) under light pressure.

Application of 7.5 or 10 mA, depending on the sensitivity of the animal strain, through ear electrodes leads to short-duration clonic convulsion characterized by uncontrolled jumps.

Increasing the current to 20 - 25mA results immediately in tonic convulsion with interruption of respiration; the hind limbs of the animal are fully extended. This convulsion changes in certain animals to clonic seizure after a few seconds; in other animals it results directly in death. Further increase in current leads to tonic seizure and death in the case of all animals.

Drugs which can be used to treat this condition can prevent mortality, tonic convulsions and eventually also clonic seizure.

Practical Details

Preliminary evaluation of the current which just causes death. In all animals of this strain at the time of the experiment. This is the current setting for the experiment.

Application of different doses of the test drug to groups of animals.

After an appropriate time for the test drug to become effective, production of convulsions at the current setting determined in the preliminary experiments.

Protocol to cover the observed symptoms (clonic and tonic convulsions, death).

Preparation of dose-response curve and evaluation of ED50 for all three symptomatic parameters.

Example

Dose-response curves of diphenylhydantoin.

Groups of 10 mice receive 1 - 5 - 10 - 50 mg/kg orally. After 1 hour convulsions are produced (25 mA through ear electrodes). It is evident that mortality is reduced at lower doses (I), tonic convulsion at higher doses (II), and clonic convulsions at still higher doses (III).

Figure 6. Dose-Response curve for diphenylhydantoin.

Electrodes

Eye-electrodes

Eye-electrodes are metal electrodes covered with cloth or leather. They must be moistened with saline solution before use and pressed firmly against the eyes to make good contact and ensure passage of the shock current into the body. Care must be taken not to use excessive amounts of saline solution. It is important to prevent excess solution moistening the fur away from the electrodes; the resulting salt bridge on the skin is electrically conducting and would short-circuit the shock current so that only part of the current passes into the body.

Ear-electrodes

When ear electrodes are employed either electrode jelly (ECG paste) or saline solution may be used. For safety reasons it is advisable to wear an insulating glove on the hand holding the animal. With saline solution it is again important to avoid forming a liquid bridge between the two applications points. Successful shock is achieved, depending on the animal (mouse or rat), with amplitudes between 25 and 50 mA and shock durations between 0.2 and 0.5 sec.

Successful shock parameters

The following table summarizes a number of settings which have proved successful in different laboratories. 

Maintenance and Cleaning

The Rodent Shocker does not require any special maintenance or attention. It is however recommended to check the equipment and in particular the electrodes (cables, plugs) for external faults and insulation damage each time before use.

If you find or suspect that any liquid has found its way into the case, do not switch it on and use it only after it has been checked thoroughly by a qualified service engineer. In an emergency if not service engineer is available, you must wait at least until the liquid has dried up. Do not use the equipment immediately but first carry out a functional test. 

Cleaning: pull out the mains supply plug! Case and front panel can be cleaned by rubbing with a moist cloth. Do not use any organic solvents. Take care to prevent any liquid passing inside the case.