Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities

We describe a protocol to examine the development of opioid-induced hyperalgesia and tolerance in mice. Based on the measurement of thermal and mechanical nociceptive responses of naïve and morphine-treated animals, it allows to quantify the increase in pain sensitivity (hyperalgesia) and decrease in analgesia (tolerance) associated with chronic opiate administration.

The overall goal of the following experiment is to observe the development of opioid-induced hyperalgesia and analgesic tolerance in mice. This is achieved by measuring the thermal and mechanical nociceptive responses of naive mice after morphine administration over a time course on day zero as a second step. The nociceptive threshold is measured daily before the administration of morphine on the seventh day.

The analgesic effect of morphine is measured over a time course once again to reveal the development of analgesic tolerance. Results are obtained that show an increase in pain sensitivity as well as the development of analgesic tolerance. Following repeated morphine administration.

Though this method can provide insight into the development of pain hypersensitivity and, and analgesic tolerance in multi mice. Following chronic opiate treatment, it can also be applied to other opiates or genetically modified mice. Demonstrating the procedure will be a graduate student from my laboratory To minimize stress induced analgesia, mice are handled for five minutes every day and habituated until they freely enter into the restrainer.

Once habituated, begin the tail immersion test by setting the thermostat to 48 degrees Celsius. Gently introduce the mouse into the restrainer. Once properly restrained, dip the protruding two thirds end of its tail into the water bath and start the chronometer.

Discriminating between nociceptive and nociceptive response is a key element of this procedure. Proper arbitration and practice is critical for success. Monitor the animal's movement and stop the chronometer.

As soon as the mouse withdraws its tail from the hot water, record the withdrawal latency. Return the mouse to its cage and test the next animal until the end of the series maintaining the same order. Repeat the nociceptive response measurements two more times.

The nociceptive response latency in seconds for each mouse is determined as the mean value for three successive determinations. For the tail pressure test, first gently introduce the mouse into the restrainer and position its tail under the connic tip of the anal JMeter. Once in place, press the foot switch to apply uniformly increasing pressure onto the proximal part of the tail.

At the first sign of a nociceptive reaction, withdraw the tip. Nociceptive responses may include struggling squeaking or withdrawal of the tail. Record the current force in grams that elicits the nociceptive response.

Repeat this measure on the median and distal parts of the tail of the same mouse in 32nd intervals, return the animal to its home cage and repeat these steps until all mice are tested. First, define two groups of animals with comparable average nociceptive values. Next, measure and record the body weight of each animal.

Once the saline and morphine groups have been established, prepare a morphine solution in physiological saline for subcutaneous administration. Measure the nociceptive response latencies in both the tail immersion and tail pressure tests, and record these as time 0.0 for each group. Next, inject morphine or saline to each animal depending on their group.

After 30 minutes, begin recording nociceptive responses in each animal. Pay close attention to the cutoff values as mice are unlikely to respond to the noxious stimuli at this early time point. In the absence of a nociceptive reaction, a 25 second cutoff is used to prevent tissue damage maintaining the same test order.

Repeat the nociceptive measurements every 30 minutes until 3.5 hours Post injection for chronic morphine treatment. Obtain day one values by measuring nociceptive response of the animals on the TIT and TPT as demonstrated earlier. Prepare a fresh morphine solution.

Deliver morphine or saline injections according to group assignment and allow the animals to rest until the next day. The next day, measure nociceptive responses on the TIT and TPT before administering a second round of injections. Repeat these steps every day for six days.

On day seven, evaluate morphine induced analgesia. According to the time course paradigm demonstrated earlier in the tail immersion test, the maximal analgesic effect of morphine was reached after 30 minutes. However, in the tail pressure test, the maximum effect was reached after 60 minutes compared to saline injected controls.

These results show that daily morphine injections over a seven day treatment period induced a significant and progressive lowering of thermal and mechanical basal nociceptive values. After chronic treatment, the basal nociceptive value in morphine treated mice was significantly lower than that of saline injected control mice. Following acute morphine, the nociceptive response of the chronic morphine treated group significantly increased, but only slightly exceeded the basal nociceptive value of saline injected control.

Mice measured at 30 minutes in TIT and TPT and at 60 minutes in TIT here nociceptive threshold values measured 30 minutes after saline or morphine injection are plotted for day zero and day seven Afterwards development. This technique paved the way for researcher in the field of of pain research to explore the effect of pharmacological of genetic intervention in the development of pain hypersensitivity and analgesic tolerance induced by chronic morphine administration.