Eyeblink conditioning in the freely moving rat: square-wave stimulation as the unconditioned stimulus.

The freely-moving rat model of eyeblink conditioning has been gaining popularity as a model of associative learning. The most commonly used preparation uses subcutaneous electrodes to deliver the unconditioned stimulus (US), which has usually been a 60-Hz electrical shock. However, measurement of the unconditioned response (UR) has been controversial, because the US (SHOCK) interferes with electromyograph (EMG) activity if measured shortly after US delivery. Here, we present a modification of eyeblink conditioning in the freely-moving rat that uses a square-wave stimulation as the US. Videographic evidence demonstrated that use of stimulation allowed eyelid opening to be evaluated in EMG activity. A parametric study demonstrated that: (1) the stimulation US supported acquisition of the eyeblink conditioned response (CR); (2) stimulation intensity was reflected in UR amplitude; and; (3) UR amplitude decreased with acquisition of the eyeblink CR. The stimulation US fulfils the basic criteria for a US, while allowing delivery of the US through subcutaneous electrodes. Thus, the stimulation US is a viable alternative for eyeblink conditioning in the freely-moving rat preparation.