The distribution of thalamic projection neurons containing cholecystokinin messenger RNA, using in situ hybridization histochemistry and retrograde labeling.

The distribution of cholecystokinin (CCK) synthesizing neurons in the thalamus was studied using in situ hybridization histochemistry. The message coding for CCK is present at different levels of intensity in almost all neurons (95%) of the anterior, ventral, medial and lateral groups of nuclei, with the ventral nuclei having the highest grain density. Many neurons (90%) of the medial and dorsal lateral geniculate nuclei also contained high levels of CCK transcripts. Very few neurons were found to express CCK in the parafascicular and paraventricular nuclei (2% and 10%, respectively), whereas the other intralaminar and midline nuclei had intermediate levels of CCK transcripts (75% of the neurons). The ventral lateral geniculate nucleus and the reticular nucleus were completely devoid of signal. After injection of the fluorescent dye, Fluorogold, into several areas of the cortex and corpus striatum, almost all retrogradely labeled cells in the thalamus (except in the parafascicular nucleus) expressed the CCK gene.