Quantitative electron microscopic study of immunoreactive somatostatin axons in the rat neostriatum

Various features of immunoreactive somatostatin axons including bouton size, synaptic length, the type of synapse formed (symmetric or asymmetric) and postsynaptic target, were examined at the ultrastructural level in the caudate nucleus. These features were compared to those of unlabeled axons in the surrounding caudate neuropil. Results showed that immunoreactive somatostatin axons make relatively short-surfaced, symmetric contacts, mostly with dendritic shafts whereas the majority of unlabeled axons form long-surfaced, asymmetric synapses with dendritic spines. Observations indicate that immunoreactive somatostatin axons belong to a sparse and homogeneous population of axons, have features corresponding to those of intrinsic caudate neurons, and synapse with caudate spiny cells. These findings are consistent with earlier speculation that immunoreactive somatostatin axons in caudate arise from a population of aspiny interneurons which have previously been identified to contain the peptide.