DEVELOPMENT OF DIRECT GABAERGIC PROJECTIONS FROM THE ZONA INCERTA TO THE SOMATOSENSORY CORTEX OF THE RAT

The postnatal development of direct thalamocortical projections from t he zona incerta of the ventral thalamus to the whisker representation area of the rat primary somatosensory cortex was investigated. Cytoarc hitectonic analysis based on Nissl staining, cytochrome oxidase histoc hemistry and immunohistochemistry for glutamic acid decarboxylase, GAB A, parvalbumin and calbindin D28K revealed that the zona incerta can b e clearly distinguished from surrounding diencephalic structures from the day of birth. Moreover, four distinct anatomical subdivisions of t his nucleus were identified: the rostral, dorsal, ventral and caudal. Of these, the ventral subdivision is by far the most conspicuous, cont aining the highest density of neurons, and the highest levels of cytoc hrome oxidase, glutamate decarboxylase, GABA, parvalbumin and calbindi n D28K. In contrast, the dorsal, rostral and caudal subdivisions conta in fewer cells, lower levels of glutamic acid decarboxylase and GABA a nd very few parvalbumin-positive and calbindin-positive neurons. Small injections of rhodamine coated microspheres or Fluoro-gold in the pri mary somatosensory cortex of animals at different stages of developmen t revealed the existence of retrogradely labeled neurons in the rostra l and dorsal subdivisions of the zona incerta from postnatal day 1. At this age, retrogradely labeled cells were also found in the ventral l ateral, ventral posterior medial, posterior medial, centrolateral, ven tral medial and magnocellular subdivision of the medial geniculate nuc lei of the dorsal thalamus. The density of the incertocortical project ion reaches its maximum between the first and second postnatal weeks, decreasing subsequently, until an adult pattern of labeling is achieve d. Tracer injections combined with immunohistochemistry revealed that the majority of the incertocortical projection derives from GABAergic neurons, implying a potentially inhibitory role for the incertocortica l projection. These results demonstrate that the rat trigeminal system contains parallel thalamocortical pathways of opposite polarity, emer ging from both the dorsal (glutamatergic, excitatory) and ventral (GAB Aergic, inhibitory) thalamus since the day of birth. As such, these fi ndings suggest that, contrary to the classical notion, not only the do rsal but also the ventral thalamus may play a special role in both cor tical maturation and function.