Striatal susceptibility to a dopaminergic neurotoxin is independent of sex hormone effects on cell survival and DAT expression but is exacerbated by central aromatase inhibition

The aim of this study was to investigate further the hormone-dependent processes underlying sex differences in neurotoxic responses within the rat nigrostriatal dopaminergic (NSDA) pathway after partial lesioning with 6-OHDA, a state thought to mimic the early stages of Parkinson's disease where, in humans and animal models alike, males appear to be more susceptible. Contrary to our hypotheses, hormone manipulations (gonadectomy +/- oestrogen or androgen treatment) failed to alter survival of tyrosine hydroxylase immunoreactive cells in the substantia nigra pars compacta (SNc) after lesioning; this indicates that, unlike inherent sex differences in toxin-induced striatal dopamine depletion, sex differences in cell loss were not hormonally generated, and that hormone-dependent changes in dopamine depletion can occur independently of cell survival. In addition, hormonally induced changes in striatal expression of the dopamine transporter (DAT), an important factor for 6-OHDA toxicity, did not correlate with hormonal influences on striatal dopamine loss and, in males, central inhibition of aromatase prior to 6-OHDA infusion exacerbated striatal dopamine loss with no effect on SNc tyrosine hydroxylase-immunoreactive survival, suggesting locally generated oestrogen is neuroprotective. These results support the novel view that sex steroid hormones produced peripherally and centrally play a significant, sex-specific role within the sexually dimorphic NSDA pathway to modulate plastic, compensatory responses aimed at restoring striatal dopamine functionality, without affecting cell loss.