Fluorometric comparisons of the retrograde axonal transport of true blue and diamidino yellow from the rat caudate-putamen.

The retrograde tracers true blue and diamidino yellow were injected into the caudate-putamen of adult rats which were allowed to survive for periods of 12 to 480 h after these injections. The retrograde axonal transport of these tracers was assessed by microphotometric measurement of the intensity of fluorescence in labelled neurones in the following three different sources of afferents to the caudate-putamen: the substantia nigra pars compacta; the parafascicular nucleus; and the dorsal raphe nucleus. It took respectively 12, 24 and 48 h for neurones in the substantia nigra, parafascicular nucleus and dorsal raphe nucleus to become retrogradely labelled. The average fluorescent intensity was greatest after 96-168 h but this highest value, and the subsequent decline, in fluorescence varied considerably between different tracers and different pathways. Neither axonal length nor neuronal activity correlate well with dynamics of build up and decline in fluorescence, but axonal collateralisation and diffuseness of terminals are associated with slower accumulation and more rapid removal of the tracers. Some neurones labelled with true blue retain their fluorescence better than those labelled with diamidino yellow but these differences are overshadowed by differences between different pathways in this respect.