Synaptic organization of afferent projections to the supramammillary nucleus of the rat.

We studied the fine structure of afferent terminals from the preoptic area, the nucleus of the diagonal band of Broca, the infralimbic cortex and the laterodorsal tegmental nucleus within the supramammillary nucleus (SUM) using the anterograde tracing method of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP). Injection of WGA-HRP into the preoptic area permitted ultrastructural recognition of many anterogradely labeled terminals in the SUM. Almost all labeled terminals (99%) contained clear round synaptic vesicles and formed asymmetric synaptic contacts (Gray's type I). About 86% of labeled terminals from the nucleus of the diagonal band were asymmetric (Gray's type I), whereas 14% contained pleomorphic synaptic vesicles and formed symmetric synaptic contacts (Gray's type II). Almost all labeled terminals from the infralimbic cortex were located in the ventral part of the SUM, and 95% of labeled terminals were Gray's type I. The majority of labeled terminals (90%) from the laterodorsal tegmental nucleus were Gray's type I, and the remaining (10%) were Gray's type II. The percentage of labeled terminals with dense-cored vesicles was very high in terminals from the preoptic area (70%), and low in terminals from the infralimbic cortex (19%). Labeled terminals in all cases contacted mainly intermediate-sized dendrites (0.5-1.0 micron diameter). All cases had only a few labeled axosomatic terminals. The cases of injections into the preoptic area and the diagonal band nucleus had some reciprocal connections at the ultrastructural level.