Local circuit neurons in the primate medial pulvinar nucleus

We studied the morphology of local circuit neurons (LCNs) of the medial pulvinar nucleus using Golgi-impregnated preparations in rhesus monkeys (Macaca, mulatta) and the immunostaining for GABA in cynomolgus monkeys (Macaco, fascicularis). There were three varieties of LCNs: tufted, radiating, and beaded. Tufted cells had long, thin, sparsely varicose dendritea that terminated in elaborate arbors which were about 200 mm in diameter. Radiating neurons had long dendrites that extended for up to 350 mm from the cell body and rarely terminated in tufts. Both tufted and radiating neurons had relatively small, rounded cell bodies (10-15 μ long axis). Beaded neurons had larger somata (15-20 μm long axis), shorter and thicker dendrites, and large varicosities. We measured approximately 200 each of GABA-immunoreactive and non-immunoreactive neurons from the same sections. GABA-immunoreactive neurons had long axes of 9.8-20 μm whereas non-immunoreactive neurons had long axes of 9.4-20 μm. As a population, non-immunoreactive neurons were larger (mean long axis: 15.9 ± 2.0 μm) than GABA-immunoreactive cells (mean long axis: 13.5 ± 1.9 μm). The same distribution of somatic ranges was also found for calbindin-immunoreactive and non-immunoreactive neurons. However, the ranges of somatic dimensions of parvalbumin-immunoreactive and non-immunoreactive neurons were identical. Thus it is likely that calbindin and parvalbumin represent different populations of pulvinar neurons. In sum, somatic dimensions of GABA-immunoreactive neurons can account for all three types of pulvinar local circuit neurons. However, not all the smallest cells (presumptive local circuit neurons), are GABAergic.