Radioautographic study of serotoninergic axon terminals in the rat trigeminal motor nucleus.

Serotonin (5-HT) axon terminals were demonstrated in the rat trigeminal motor nucleus (VmN) using radioautography following local injection of tritiated 5-HT. The light microscope radioautographs of the VmN showed silver grain aggregates typical of axonal varicosities having taken up the exogenous 5-HT. In electron microscope radioautographs, labeled 5-HT axon terminals took the form of unmyelinated enlargements, 0.3-2 micron in diameter (average 0.9 micron) containing small clear and large granular vesicles. Out of 182 labeled 5-HT profiles, only 4.9% showed a clear synaptic junction with dendritic branches, indicating the existence of non-synaptic as well as synaptic mechanisms for the 5-HT terminals within the VmN. An estimate of the density of 5-HT varicosities within the VmN was performed on histological radioautographs: the concentration of labeled terminals was evaluated at (1.8 +/- 0.38) X 10(6) per mm3.