Immunogold localization of serotonin within synaptic terminals in the rat mesencephalic trigeminal nucleus.

Abstract

With the use of postembedding electron-microscopic immunogold cytochemistry, the vesicular distribution of serotonin within serotonergic synaptic terminals in the mesencephalic trigeminal nucleus was determined in order to obtain further insight into the mechanisms and functional significance of serotonin release to these jaw muscle spindle afferent neurons. Immunogold labelling was restricted to the previously described type I and type II terminals. The distribution of immunogold particles over the synaptic terminals indicated that serotonin was stored in small round or pleomorphic (RSV) vesicles and in large granular dense-cored (DSV) vesicles. The amount of serotonin present in the DSV vesicles, which are generally considered to contain colocalized neuropeptides, showed a high degree of variation. These DSV vesicles were usually located at some distance from the synaptic area of the terminals suggesting that they represent a nondirectly releasable vesicle pool. The serotonergic RSV vesicles were, in general, irregularly distributed over the terminals. However, in about 19% of the analyzed labelled synaptic terminals serotonergic RSV vesicles were clustered together near their release site at the synaptic cleft. These synaptic terminals may represent highly active serotonergic synapses with a high release frequency and a high reuptake level, resulting in a dense concentration of vesicles containing a high amount of serotonin near the synaptic cleft.