Light and electron microscopic localization of immunoreactive Leu- enkephalin in the monkey basal ganglia

In the mammalian central nervous system, high concentrations of Leu-enkephalin, an opioid pentapeptide, have been found within the globus pallidus and caudate-putamen, where opiate receptors also are found in abundance. Opiate analgesics and opioid peptides have been proposed to interact with known neurotransmitters in the neostriatum and globus pallidus. With the use of the peroxidase-antiperoxidase method, we examined the light microscopic and ultrastructural localization of immunoreactive Leu-enkephalin-containing neurons and axon terminals in the monkey caudate-putamen, globus pallidus, and substantia nigra. Only neostriatal neurons contained immunoreactive Leu-enkephalin. Labeled somata were medium size (12 to 20 pm) and were located primarily in the caudate nucleus and the ventral and medial portions of the putamen. Immunoreactive Leu-enkephalin fibers were present, in order of greatest density, in the outer segment of the globus pallidus, substantia nigra pars reticulata, and neostriatum. Fiber staining in the inner pallidal segment was sparse. At the electron microscopic level, labeled neostriatal somata had relatively large, slightly indented nuclei and scant cytoplasm. Primary dendrites were smooth and distal branches had numerous spines. Together, these features have been shown previously in the monkey by the Golgi-EM technique to distinguish spiny type 1 neurons. In the neostriatum and outer segment of the globus pallidus, small diameter unmyelinated (about 0.2 pm) and myelinated (about 1 pm) axons contained immunoreactive Leu-enkephalin. Labeled boutons were 0.5 to 1.5 pm in size and had small clear round and/or ovoid vesicles and also large granular vesicles. The majority of labeled terminals appeared to make symmetric contacts. In the neostriatum, they formed synapses with unlabeled somata which had cytologic features of spiny type 1 and aspiny type 1 neurons and with the primary and distal branches of unlabeled spiny (shafts) and aspiny dendrites. Few immunoreactive boutons synapsed with unlabeled dendritic spines. Positive boutons were found also to synapse upon the axon hillocks and axon initial segments of unlabeled somata. Other types of axoaxonic synapses were not found. Some immunoreactive Leu-enkephalin terminals formed synapses with cell bodies and dendrites which also were positively labeled for Leu-enkephalin. Within the globus pallidus, nearly one-half of all axon terminals that ensheath and synapse with pallidal dendrites contained immunoreactive Leu-enkephalin. From the anatomic observations, it appears likely that immunoreactive Leu-enkephalin is found within medium size spiny neurons (spiny type 1 in the monkey), which are known to have axon collaterals intrinsic to the caudate-putamen and to project to the globus pallidus and substantia nigra. The high density of enkephalin-positive terminals found in the latter two structures is consistent with the presence of a striatofugal enkephalin pathway which may have a direct influence on the efferent systems originating in the globus pallidus ' We wish to thank