Exp Brain Res (1998) 123:323±333  Springer-Verlag 1998 RESEARCH ARTICLE Csaba Leranth ´ John R. Sladek Jr. ´ Robert H. Roth D. Eugene Redmond Jr. Efferent synaptic connections of dopaminergic neurons grafted into the caudate nucleus of experimentally induced parkinsonian monkeys are different from those of control animals Received: 23 December 1997 / Accepted: 29 April 1998 C. Leranth ( ) ) Department of Obstetrics and Gynecology, Yale University, School of Medicine, P.O. Box 208063, FMB 328, New Haven, CT 06520±8063, USA e-mail: csaba.leranth@yale.edu, Fax: +1-203-785-7684 C. Leranth Section of Neurobiology, Yale University, School of Medicine, New Haven, CT 06520, USA J.R. Sladek Jr. Department of Neuroscience, University of Health Sciences, The Chicago Medical School, North Chicago, IL 60064, USA R.H. Roth Department of Pharmacology, Yale University, School of Medicine, New Haven, CT 06520, USA D.E. Redmond Jr. Departments of Psychiatry and Neurosurgery, Yale University, School of Medicine, New Haven, CT 06520 Abstract This study investigated the question of whether grafted dopamine cells in the striatum of 1-methyl-4-phe- nyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys form synapses and, if they do, whether their postsynaptic targets were the same as those in control monkeys or in previous studies in rats. Electron-microscopic single im- munostaining was performed for tyrosine hydroxylase on vibratome sections prepared from the head of the cau- date nucleus of controls and MPTP-treated African green monkeys (Cercopithecus aethiops sabaeus) that received a graft. Furthermore, correlated light- and electron-micro- scopic double immunostaining was carried out for tyro- sine hydroxylase and calbindin in the same brain area of MPTP-treated plus grafted animals. In control mon- keys, the majority (97%) of dopamine boutons terminate on spines that were also synaptic targets of immunonega- tive boutons forming asymmetric synaptic contacts: syn- aptic triads. In MPTP-treated, grafted animals, the major- ity 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 immu- nostaining experiments indicated that these newly formed axosomatic and axodendritic synapses are associated with calbindin-immunoreactive, medium-sized, spiny striatoni- gral projection neurons. These observations indicate that: (1) dopamine from transplanted embryonic tissue acts via synaptic contacts on host neurons; (2) the primary synap- tic targets of transplanted dopamine cells are not spines but dendrites and somata of host neurons; (3) these target neurons are the same as in control animals; and (4) com- paring these observations with results of control and graft- ed rats, there are major species differences between rats and monkeys in the dopamine innervation of both control and transplanted animals. Key words MPTP ´ Reinervation ´ Medium spiny neurons ´ Calbindin ´ Double immunostaining ´ African green monkey Introduction Intrastriatal grafts of dopamine (DA)-producing, embryon- ic, ventral mesencephalic tissue are able to compensate for motor or sensorimotor deficits induced by neurotoxic le- sions of the substantia nigra (Bakay 1992; Elsworth et al. 1996a,b; Redmond et al. 1986; Taylor et al. 1995). However, the operational mode of such grafts is still poor- ly understood. It seems clear from experiments with adre- nal medullary tissue that grafted cells may exert their func- tional effects through diffuse release of their active com- pounds into the cerebrospinal fluid or locally into the ex- tracellular fluid (Freed 1983). In the case of grafted em- bryonic DA-containing tissue, there is, in contrast, good evidence that the behavioral effects of the grafted tissue are directly dependent on the ability of the grafted nerve cells to establish new axonal networks in specific regions of the initially DA-denervated host (Freund et al. 1985; Kordower et al. 1995). Thus, this suggests that the grafted neurons function primarily through their afferent connec- tions via release of the DA transmitter onto receptive ele- ments in the host striatum (Bjorklund et al. 1980). These observations on intrastriatal grafts raise the important question of whether their functional effects are due to a re- establishment of specific synaptic contacts with the initial-