Spatially and temporally restricted chemoattractive and chemorepulsive cues direct the formation of the nigro-striatal circuit

Identifying cellular and molecular mechanisms that direct the formation of circuits during development is thought to be the key to reconstructing circuitry lost in adulthood to neurodegenerative disorders or common traumatic injuries. Here we have tested whether brain regions situated in and around the developing nigro-striatal pathway have particular chemoattractive or chemorepulsive effects on mesencephalic dopamine axons, and whether these effects are temporally restricted. Mesencephalic explants from embryonic day (E)12 rats were either cultured alone or with coexplants from the embryonic, postnatal or adult medial forebrain bundle region (MFB), striatum, cortex, brain stem or thalamus. Statistical analysis of axon growth responses revealed a potent chemoattraction to the early embryonic MFB (i.e. E12-15) that diminished (temporally) in concert with the emergence of chemoattraction to the striatum in the late embryonic period (i.e. E19+). Repulsive responses by dopaminergic axons were obvious in cocultures with embryonic brain stem and cortex, however, there was no effect by the thalamus. Such results suggest that the nigro-striatal circuit is formed via spatially and temporally distributed chemoattractive and chemorepulsive elements that: (i) orientate the circuit in a rostral direction (via brain stem repulsion); (ii) initiate outgrowth (via MFB attraction); (iii) prevent growth beyond the target region (via cortical repulsion); and (iv) facilitate target innervation (via striatal chemoattraction). Subsequent studies will focus on identifying genes responsible for these events so that their products may be exploited to increase the integration of neuronal transplants to the mature brain, or provide a means to (re)establish the nigro-striatal circuit in vivo.