A STEREOLOGICAL STUDY OF SUBSTANTIA-NIGRA IN YOUNG AND OLD RHESUS-MONKEYS

The number of pigmented and non-pigmented neurons in the substantia ni gra (SN) of 10 old and six young female Macaca mulatta monkeys and in three old alpha male monkeys were estimated using new stereological ce ll counting methods. No systematic right-left differences were noted, nor were old animals different from young ones with respect to SN volu me (68.9 mm(3) vs. 62.8 mm(3)) or absolute number of nerve cells (320, 000 vs. 312,000). However, the total number of pigmented neurons was a bout eight times higher in old animals compared with young ones (166,0 00 vs. 21,400) while the total number of non-pigmented SN neurons was less than half in old animals compared with young ones (139,000 vs. 28 5,000). These differences create difficulties in generalizing experime ntal results from the rhesus animal model to man. It seems unlikely th at a simple correlation can be made between pigmented and tyrosine hyd roxylase (TH) positive neurons in SN in monkeys. Instead of estimating the total number of pigmented and non-pigmented cells, only SN neuron s positive for TH using immunohistochemical techniques might be used a s an indicator of the total number of dopaminergic neurons in SN in mo nkeys.