Connectional studies of the primate lateral geniculate nucleus: Distribution of axons arising from the thalamic reticular nucleus of Galago crassicaudatus

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Abstract
Anterograde and retrograde transport methods have been used to explore the interconnections between the thalamic reticular nucleus (TRN) and the dorsal lateral geniculate nucleus of Galago crassicaudatus. We first defined the region of the TRN, which is connected to the lateral geniculate nucleus, by examining the distribution of geniculo‐TRN axons, cortico‐TRN axons arising from area 17, and the location of TRN‐geniculate neurons. Following an intraocular injection of 3H‐proline/3 H‐leucine, trans‐synaptically transported protein is present bilaterally within the lateral portion of the caudal TRN. This same caudal and lateral region is also targeted by cortico‐TRN axons and contains neurons which project upon the lateral geniculate nucleu. Light microscopic anterograde transport methods were used to analyze the distribution of TRN‐geniculate axons. Our data reveal that all layers and interlaminar zones of the dorsal lateral geniculate nucleus contain TRN axon. Electron microscopic‐autoradiographic data support and extend our light microscopic findings by revealing labeled TRN terminals within all geniculate layers. These TRN profiles are the same size throughout the geniculate and exhibit morphological characteristics similar to Fl terminals described by others. That is, they possess predominantly pleomorphic vesicles, a dark cytoplasmic matrix, dark mitochondria, and symmetrical synaptic contacts. Two additional features of TRN terminals have been observed in some profiles. These include dense‐core vesicles and a dense, punctate cytoplasmic matrix, which is sometimes associated with the postsynaptic specialization. In addition to their morphology and size, the postsynaptic targets of TRN terminals are similar within the three sets (parvi‐, magno‐, and koniocellular) of geniculate layers. TRN profiles terminate upon dendrites of all sizes and somata. These findings suggest that the TRN modulates the retino‐geniculocortical pathway and that this modulation is occurring in all three streams.

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