Development of direct GABAergic projections from the zona incerta to the somatosensory cortex of the rat.

The postnatal development of direct thalamocortical projections from the zona incerta of the ventral thalamus to the whisker representation area of the rat primary somatosensory cortex was investigated. Cytoarchitectonic analysis based on Nissl staining, cytochrome oxidase histochemistry and immunohistochemistry for glutamic acid decarboxylase, GABA, parvalbumin and calbindin D28K revealed that the zona incerta can be clearly distinguished from surrounding diencephalic structures from the day of birth. Moreover, four distinct anatomical subdivisions of this nucleus were identified: the rostral, dorsal, ventral and caudal. Of these, the ventral subdivision is by far the most conspicuous, containing the highest density of neurons, and the highest levels of cytochrome oxidase, glutamate decarboxylase, GABA, parvalbumin and calbindin D28K. In contrast, the dorsal, rostral and caudal subdivisions contain fewer cells, lower levels of glutamic acid decarboxylase and GABA and very few parvalbumin-positive and calbindin-positive neurons. Small injections of rhodamine coated microspheres or Fluoro-gold in the primary somatosensory cortex of animals at different stages of development revealed the existence of retrogradely labeled neurons in the rostral and dorsal subdivisions of the zona incerta from postnatal day 1. At this age, retrogradely labeled cells were also found in the ventral lateral, ventral posterior medial, posterior medial, centrolateral, ventral medial and magnocellular subdivision of the medial geniculate nuclei of the dorsal thalamus. The density of the incertocortical projection reaches its maximum between the first and second postnatal weeks, decreasing subsequently, until an adult pattern of labeling is achieved. Tracer injections combined with immunohistochemistry revealed that the majority of the incertocortical projection derives from GABAergic neurons, implying a potentially inhibitory role for the incertocortical projection. These results demonstrate that the rat trigeminal system contains parallel thalamocortical pathways of opposite polarity, emerging from both the dorsal (glutamatergic, excitatory) and ventral (GABAergic, inhibitory) thalamus since the day of birth. As such, these findings suggest that, contrary to the classical notion, not only the dorsal but also the ventral thalamus may play a special role in both cortical maturation and function.