MouseMet Hot
The Hargreaves, hot plate and tail flick tests are all known as "Latency tests". This means that the rodent is introduced to the testing chamber and then subjected to a fixed stimulus. The time taken for them to react and withdraw is recorded. In 2014 we were asked by a number of researchers if we could develop a better (and cheaper) alternative as the acclimation time, for the rodent to explore the surroundings and settle down before the test could be performed, was unacceptably long.
The result was MouseMet Hot, a ramped thermal stimulus that can be applied in the same runs as MouseMet EvF, thereby reducing the acclimation times dramatically. It uses our own miniature thermal probe, just 2.5mm in diameter and developed using our 25 years experience with nociceptive threshold testing on larger animals (right up to camels!). The image below shows the progression from our "standard" probe on the left, with an 8mm heating bar, through to a handheld thermal probe for rats (4mm, held between my fingers) and finally the 2.5mm probe for use with mice and rats confined in runs.
The probe heats at 2.5°C/sec. It's mounted on a rotating arm so that, when brought into contact with the plantar surface, with a force of just 1g, heating is activated automatically. This force, we have found, provides consistent thermal transfer without premature withdrawal even when allodynia is present. We provide a calibration weight to check it and an adjustment knob to fine tune it to your technique with the mice or rats.
At threshold, the probe is removed and the thermal threshold displayed on the screen, along with a graph of the heating cycle.
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Resolution: 0.1°C
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Overall accuracy: ± 0.5°C
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Price: £1350 ex works, when bought on line with a credit card
Just as with MouseMet EvF, it's very important that the Hot probe stays in good thermal contact with the plantar surface, but without sliding or scratching across the surface (which might elicit touch-on responses). So our Hot probe has a domed tip and is flexibly mounted to compensate for any side to side hand tremor during the test. It's mounted on a rotating, spring-loaded arm to maintain the 1g application force against the paw, even if the tester's hand shakes up and down.
And, like every other thermal threshold system that Topcat makes, there is a thermal cutout which can be set to the temperature required by your protocol.
The system was first trialled by the University of Queensland in 2015 where it produced baseline thresholds of 50°C on mice and detected both heat allodynia (at 43°C) and partial analgesia.
The baseline results are shown here. Comparison with the Hargreaves test showed lower coefficient of variation (in addition to the substantial reduction in acclimation time).
"MouseMet Hot eliminates the 30-60 minute acclimation time we had with the Hargreaves, enabling testing of compounds with short duration of action" Dr Jennifer Deuis, University of Queensland
A big 3Rs contribution:
The development was a success, with many of our existing MouseMet EvF customers taking up the Hot option to reduce the acclimation time and stress to the mice. On the left here you can also see our Cold stimulus, currently under validation.
