Ultrastructural characteristics of substance P-immunoreactive terminals in marginal division of rat striatum

In our previous work using immunocytochemical method combined with tract tracing techniques a new subdivision was described in the striatum of the rat. This "marginal division" is more densely filled with substance P, enkephalin and dynorphin B terminals than the rest of the striatum. In the present study, the synaptic organization of the substance P immunoreactive (SPIR) terminals in the marginal division of the rat striatum was studied using electron microscopy and immunocytochemistry for substance P (SP). Four major types of SPIR synapses were identified in the marginal division: axodendritic, axospinous, axo-axonal, and compound synapses. Axodendritic and axospinous synapses, in which the postsynaptic targets were small or large dendrites or spines, were the most common. A few axo-axonic synapses were observed as were several subtypes of compound synapses with more than two synaptic components. SPIR axon terminals formed the presynaptic components of all these synaptic types, but in one case an unlabeled bouton was observed making a synaptic connection onto a SPIR dendrite. Both symmetric and asymmetric SPIR synapses were observed in the marginal division. The vesicles in the SPIR presynaptic boutons were mostly pleomorphic although a few of them were round. The existence of asymmetric synapses, round synaptic vesicles and small postsynaptic dendrites distinguishes the ultrastructure of the marginal division from that of the other parts of the striatum. The complex characteristics of the synaptic organization in the marginal division implies that the SPIR terminals in the marginal division originate from a different source than those in the rest of the striatum. The complexity of the synaptic organization further suggests that the function of the marginal division is different from that of the rest of the striatum.