Unequal reinforcement values from equally warm and cool temperatures in Drosophila

Abstract

Insects inhabit extreme temperature environments and have evolved mechanisms to survive there. When temperatures either rise or fall, small insects alter their behavior within seconds to avoid, in extreme situations, freezing or high temperature induced death. Preference tests on gradients indicate a strong preferred temperature for Drosophila around 24°C. Warm temperatures can be used as effective negative reinforcers for place memory. The preference of 24°C over 18°C on a temperature gradient suggests low temperature might also have reinforcing qualities.

The so-called heat-box was used to test the effectiveness of warm and cool temperatures as reinforcers. In this paradigm, single flies are allowed to run freely in a small chamber that is heated or cooled depending on that flies' behavior. If a fly goes to one-half of the chamber it warms and if the fly goes to the other half it cools. Using temperatures either nine degrees above or below the preferred 24°C, we found that both are effective reinforcers. Surprisingly, however, the 24/15°C pair induced a place preference for the cool associated half of the chamber. Systematic tests varying training duration and reinforcement temperature indicates the 15°C reinforcer reaches asymptotic memory levels of 0.2 while 33°C reinforcement plateaus at 0.4 on a scale from 0 to 1. To test for the conservation of molecular mechanisms underlying warm and cool induced memories, wild-type CS, rut-AC and white mutant flies were tested. As has been shown for warm temperature reinforcement, both rut-AC and white mutant flies are defective in place memory formation.

Finally, to determine whether the warm and cool reinforcers could serve equally well in reversal learning, flies were trained to avoid one half of a chamber for 2-6 minutes, then trained to avoid the other chamber half. This procedure was repeated for a total of four training sessions with place memories tested after each session. Using a 33°C reinforcer, place memories increase with each training session even though flies are trained to avoid a different chamber-half each session. Interestingly, the effectiveness of the 15°C reinforcer was lost after the first reversal training session. This perhaps indicates that the additional experience in the chamber with only limited temperature extremes allows flies to conclude the temperature changes in the small, dark chambers are sub-lethal, and can ignore their rise and fall. Future experiments will explore this possibility.

Together, the evidence indicates flies can use cool temperatures as reinforcement in forming place memories. Their effectiveness as reinforcers, however, are not equal as paired prolonged training and reversal training indicates. These cool induced memories depend on the rut-AC and white mutation, as warm induced memories do.