2.3. Stressor controllability
For manipulation of controllability, subjects were run in a triad
design. One subject of each triad received ES, a second received
yoked IS, and third received no tailshock (home cage control, HC).
Each rat was placed in a Plexiglas box (14 11 17 cm) with a
wheel mounted in the front of the box. The tail was secured to a
Plexiglas rod extending from the back of the box, and affixed with
two copper electrodes and electrode paste (Parker Laboratories).
Each tailshock session consisted of 80 trials of tailshock
(27 1.0 mA, 27 1.3 mA, 26 1.6 mA) on a variable interval 60-s
schedule (range ¼ 45e90 s). As in prior studies (Baratta et al.,
2007), the following procedure was used to insure that the ES rat
learned the operant response to terminate the tail shock. Initially,
the shock was terminated by a one-quarter turn of the wheel. The
response requirements were increased by a one-quarter turn when
three consecutive trials were completed in less than 5 s. Subsequent
latencies under 5 s increased the requirement by 50% up to a
maximum of four full turns. The requirement was reduced if the
trial was not completed in less than 5 s. If the requirement was not
reached in less than 30 s, the shock was terminated and the
requirement was reduced to one-quarter turn of the wheel. An
additional rationale for this procedure is that it maintains shocks at
durations sufficient to produce the usual behavioral effects of IS in
the yoked IS subject. Rats in the IS group received an identical
amount of shock but had no control over its termination.