Early postnatal development of the rat corticostriatal pathway: an anterograde axonal tracing study using biocytin pellets

The ontogenetic development of the neocortical projections to the striatum was studied in postnatal rats by sensitive anterograde tracing with biocytin. The developmental status of this mainly glutamatergic pathway is important as it plays a major role in regulation of striatal maturation and induction of excitotoxic striatal neurodegeneration resembling the type found in Huntington's disease. Biocytin pelletts were injected into the motorcortex caudal forelimb area of newborn rats and of rats aged 3, 6, 13, 27 and 61 days followed by sacrifice and visualisation of the tracer 24 h later, at P1, P7, P14, P28 and P62, respectively. Biocytin pellets were also injected into the motorcortex jaw-lip-tongue area and into the cingulate cortex of newborn and 6-day-old rats. The biocytin pellets produced intense anterograde labelling of corticofugal projection fibres from the injection sites, as well as a local Golgi-like labelling of neurons. From postnatal day 1 into adult age efferent fibres from the motorcortex caudal forelimb area displayed a progressively denser innervation of the ipsilateral and contralateral, dorsolateral striatum. The terminating fibres were most dense in the ipsilateral striatum. In the dorsolateral striatum the corticostriatal fibres displayed a patch/matrix-like arrangement from postnatal day 14 onwards. Injections into the motorcortex jaw-lip-tongue area at postnatal days 0 and 6 displayed a progressive, denser innervation of the ipsilateral and contralateral ventrolateral striatum. The cingulate corticostriatal fibre projection, was more developed at birth than the projection from the motorcortex and increased its fibre density in the ipsilateral dorsomedial striatum up to at least day 7. In conclusion: the ipsilateral corticostriatal projections from the rat motorcortex and cingulate areas are present in newborn rats. During postnatal life the pathway develops further and specialisation of the terminating fibres into a patch/matrix like arrangement can be identified by postnatal day 14.