Abstract
Purpose: :
Dopamine modulates neural circuitry to optimise visual function. Here we use an anatomical method to chart light-responsiveness of retinal dopamine neurons following pathology in rodent models of retinal dystrophy.
Methods: :
RCS dystrophic rats and rd1 Mice of various ages were used along with appropriate controls. Rodents were sacrificed in darkness or following 90 minutes of constant white light (250µW/cm2). Retinal wholemounts were prepared and tissue processed for tyrosine hydroxylase (TH) and cfos immunohistochemistry. This technique allows visualisation of light-driven catecholaminergic activity, with a positive correlation between the appearance of nuclear cfos in TH cells and light-elicited dopamine turnover (Cameron et al., ARVO 2008 5791 / EJN in press). In order to visualise surviving photoreceptors, retinal wholemounts were co-labelled for either melanopsin or cone opsins.
Results: :
Light-induced nuclear cfos (of variable intensity) was found in the majority of TH cell nuclei in both non-dystrophic rats and wildtype mice. A small population of TH cells were displaced to the ganglion cell layer. The retinae of control animals killed in darkness contained cytosolic cfos. By 8 weeks of age in dystrophic rats the light-induced cfos-TH response was severely compromised, being almost absent by 3 months. At this age RCS dystrophic retinae also showed ectopic TH expression by inner nuclear layer (INL) cells directly adjacent to the classical TH cells. Light-independent cfos activity in TH negative cells of INL developed by 8 weeks in dark-treated RCS dystrophics and was concentrated into discrete foci by 5 months. In dystrophic rats aged 5 months and 2 years, light-induced nuclear cfos was observed in peripheral and central retina respectively. In rd1 mice, by 3 months of age there was a complete absence of light-induced nuclear cfos and unlike dystrophic rats, rd1 mice did not show foci of spontaneous activity in the dark. Surviving cones were found in all retinal degenerate animals and following light exposure there was robust nuclear cfos staining in melanopsin cells regardless of species, age or retinal dystrophy.
Keywords: dopamine • photoreceptors • plasticity