Retrograde axonal transport of 125I-tetanus toxin as a tool for tracing fiber connections in the central nervous system; connections of the rostral part of the rat neostriatum.

After previous studies had shown that 125I-tetanus toxin is taken up with high efficiency by all (adrenergic, sensory and motor)peripheral nerve terminals and that it is transported retrogradely by intraaxonal transport to the corresponding cell bodies, the present experiments were designed to investigate whether 125I-tetanus toxin could be used as a tool for retrograde tracing of fiber connections in the central nervous system and how the results obtained compared with other available retrograde tracing methods. Three to 12 h after injection of 125I-tetanus toxin into the rostral part of the nucleus caudatus of the rat, heavily labeled nerve cells were found in the substantia nigra zona compacta, in Forel field H2 and parts of the zona incerta, and in specific thalamic intralaminar nuclei (nucleus medialis pars lateralis and nucleus parafascicularis). Moderately labeled neurons were observed in the nucleus ventralis thalami and its dorsomedial part, in the nucleus raphe dorsalis and in the frontal, cingular and occipital cortex. Besides the labeling by retrograde transport, orthograde transport of radioactivity occurred in a striatonigral system. The high sensitivity and reliability of this method, even after injection of very small amounts of protein in a very small volume, represent important advantages over most of the commonly used tracing methods.