A SINGLE-CELL STUDY OF THE AXONAL PROJECTIONS ARISING FROM THE POSTERIOR INTRALAMINAR THALAMIC NUCLEI IN THE RAT

Thalamostriatal projections arising from the posterior intralaminar nu clei (PI; the parafascicular nucleus and the adjacent caudalmost part of the posterior thalamic group) were studied in rats by tracing the a xons of small pools of neurons labelled anterogradely with biocytin. T hirteen PI cells were also stained by juxtacellular application of the tracer. Relay cells of PI nuclei have a morphology that differs radic ally from the classical descriptions of the bushy cells which represen t the main neuronal type of the sensory thalamic relay nuclei. PI cell s have ovoid or polygonal somata of similar to 20-25 mu m, from which emerge four or five thick, long and poorly branched dendrites bearing spines and filamentous appendages; their dendritic domains extend for up to 1.5 mm. Before leaving the nucleus 20% of axons give off collate rals that ramify locally. All axons course through the thalamic reticu lar nucleus, where they emit one or two poorly branched collaterals, t raverse the globus pallidus, where they also distribute collaterals, a nd arborize massively in the striatum and sparsely in the cerebral cor tex. At the striatal level four or five collaterals leave the main axo n and terminate in patches scattered dorsoventrally within a rostrocau dally oriented slab. As revealed by calbindin D-28k immunohistochemist ry, only the matrix compartment receives terminations from Pi axons. T he cortical branch forms small terminal puffs centred upon layer VI of the motor cortex. Before entering the striatum some axons of the para fascicular nucleus give rise to descending collaterals that arborize i n the entopeduncular nucleus, in the subthalamic nucleus and in the vi cinity of the red nucleus. Other axons arising from the caudal part of the posterior group send descending branches only to the entopeduncul ar nucleus. These findings show that PI cells belong to a distinct cat egory of thalamic relay neurons which, beside their massive projection to the striatum, also distribute collaterals to other components of t he basal ganglia. Moreover, these results provide the first direct evi dence that virtually all PI cells project to both striatum and cerebra l cortex. Finally, it is proposed on the basis of morphological, histo chemical and hodological criteria that the caudal part of the posterio r thalamic group in the rat is homologous to the suprageniculate-limit ans nuclei of cats and primates.