EFFERENT SYNAPTIC CONNECTIONS OF DOPAMINERGIC-NEURONS GRAFTED INTO THE CAUDATE-NUCLEUS OF EXPERIMENTALLY-INDUCED PARKINSONIAN MONKEYS ARE DIFFERENT FROM THOSE OF CONTROL ANIMALS

This study investigated the question of whether grafted dopamine cells in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP )-treated monkeys form synapses and, if they do, whether their postsyn aptic targets were the same as those in control monkeys or in previous studies in rats. Electron-microscopic single immunostaining was perfo rmed for tyrosine hydroxylase on vibratome sections prepared from the head of the caudate nucleus of controls and MPTP-treated African green monkeys (Cercopithecus aethiops sabaeus) that received a graft. Furth ermore, correlated light- and electron-microscopic double immunostaini ng was carried out for tyrosine hydroxylase and calbindin in the same brain area of MPTP-treated plus grafted animals. In control monkeys, t he majority (97%) of dopamine boutons terminate on spines that were al so synaptic targets of immunonegative boutons forming asymmetric synap tic contacts: synaptic triads. In MPTP-treated, grafted animals, the m ajority of transplanted dopamine cells terminate on dendritic shafts ( 67%) and somata (32%), and only a few (1.33%) form axospine synapses. The results of the double immunostaining experiments indicated that th ese newly formed axosomatic and axodendritic synapses are associated w ith calbindin-immunoreactive, medium-sized, spiny striatonigral projec tion neurons. These observations indicate that: (1) dopamine from tran splanted embryonic tissue acts via synaptic contacts on host neurons; (2) the primary synaptic targets of transplanted dopamine cells are no t spines but dendrites and somata of host neurons; (3) these target ne urons are the same as in control animals; and (4) comparing these obse rvations with results of control and grafted rats, there are major spe cies differences between rats and monkeys in the dopamine innervation of both control and transplanted animals.