Anatomical organization of the telencephalic connections of the parafascicular nucleus in adult and developing rats.

The parafascicular nucleus (PFN) of the rat, homologous to the human centre médian, is an intralaminar nucleus of the thalamus, classically considered as part of the ascending activating system. We have previously demonstrated that it is also connected to several subcortical nuclei. To obtain a more detailed picture of the connectivity of the PFN, the organization and the topography of the reciprocal parafascicular-telencephalic relationships were studied in both adult and developing rats, using anterograde and retrograde neuronal tracers. In the adult rat, the ascending parafascicular projections were densest to the striatum, dense to the frontal and least dense to cingulate cortex, and were strictly ipsilateral. They displayed a loose topography, with the more medial parafascicular neurons projecting to the medial frontal and cingulate cortex and medial striatum, and the more lateral neurons projecting to the lateral frontal cortex and lateral striatum. All these connections were already present at embryonic day 19. Parafascicular neurons projecting to the telencephalon in adult rats were mostly of the multipolar type, with a few bipolar neurons. In neonatal rats they showed a bipolar morphology at birth; they became mostly multipolar later on, with an increasing complexity of the dendritic arbor up to postnatal day 10. Neurons in the frontal cortex retrogradely labelled from the PFN were more numerous perinatally, and decreased as early as postnatal day 5. The telencephalic connections of the PFN were found to be more discrete and restricted than previously thought, thus suggesting a more specific functional role for the nucleus than cortical recruitment.