Rats anticipate damaged rungs on the elevated ladder: Applications for rodent models of Parkinson's disease

The present study examined rats' ability to anticipate undetectable wider gaps between rungs produced when they stepped on and dislodged damaged rungs while they traversed a slightly inclined elevated ladder. Rats in the first of three experiments reduced running speeds when they encountered four evenly spaced damaged rungs either always placed on the first or second half of the ladder (the break-a-way (BW) phase) but quickly recovered to their baseline (BL) levels when damaged rungs where replaced with intact rungs (the recovery phase). Rats previously exposed to damaged rungs over the first half of the ladder increased their speeds above BL on its second "safer" half during the recovery phase, a delayed "relief-like" positive contrast effect. In Experiment 2, other rats decreased their speeds more as they approached a single damaged rung at a fixed location when it occurred before than after the mid-point of the ladder. Although they quickly recovered to BL speeds on the portion of the ladder after the damaged rung or replaced intact rung, they never showed any "relief-like"/escape effects. Rats also reduced their likelihood of dislodging the damaged rung with a fore paw over extended BW training. In the third experiment rats encountered a more easily dislodged damaged rung that was signaled by a closer intact rung on half the trials. Under these conditions rats displayed a more reliable positive contrast "relief-like" effect. We discussed how traditional associative and cognitive theories of aversive conditioning account for these findings and their relationship to normal changes in dopamine production and possible effects of reduced production from the substantia nigra pars compacta (SNpc) in the Basal ganglia in rodent models of Parkinson's disease.