Voluntary exercise reduces the neurotoxic effects of 6-hydroxydopamine in maternally separated rats

Maternal separation has been associated with development of anxiety-like behaviour and learning impairments in adult rats. This has been linked to changes in brain morphology observed after exposure to high levels of circulating glucocorticoids during the stress-hyporesponsive period (P4-P14). In the present study, adult rats that had been subjected to maternal separation (180 min/day for 14 days) during the stress-hyporesponsive period, received unilateral infusions of a small dose of 6-hydroxydopamine (6-OHDA, 5 microg/4 microl saline) into the medial forebrain bundle. The results showed that voluntary exercise had a neuroprotective effect in both non-stressed and maternally separated rats in that there was a decrease in forelimb akinesia (step test) and limb use asymmetry (cylinder test). Maternal separation increased forelimb akinesia and forelimb use asymmetry and reduced the beneficial effect of exercise on forelimb akinesia. It also reduced exploratory behaviour, consistent with anxiety-like behaviour normally associated with maternal separation. Exercise appeared to reduce dopamine neuron destruction in the lesioned substantia nigra when expressed as a percentage of the non-lesioned hemisphere. However, this appeared to be due to a compensatory decrease in completely stained tyrosine hydroxylase-positive neurons in the contralateral, non-lesioned substantia nigra. In agreement with reports that maternal separation increases the 6-OHDA-induced loss of dopamine terminals in the striatum, there was a small increase in dopamine neuron destruction when expressed as a percentage of the non-lesioned hemisphere but there was no difference in dopamine cell number, suggesting that exposure to maternal separation did not exacerbate dopamine cell loss.