Effects of MPTP on lever-pressing for light extinction in rats

Rats were daily treated for seven days with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) at a dose of 20 mg/kg/day, i.p. Seven days after treatment withdrawal, the rats were individually tested in a brightly lit apparatus containing two levers: an active lever allowing periods of darkness, and an inactive one. The test was performed over two consecutive days, in 20-min sessions. While control rats had a higher number of total active lever pressings than inactive lever pressings, this was not the case for MPTP-treated rats. Control rats decreased their useless active lever pressings and inactive lever pressings across the two sessions, but MPTP-treated rats did not do either. The absence of the differential effect in rats injected with MPTP may be due to a reduction in reinforcement mechanisms caused by the mild depletion of dopamine in the striatum.