DOPAMINE-IMMUNOREACTIVITY IN THE RAT MESENCEPHALIC TRIGEMINAL NUCLEUS- AN ULTRASTRUCTURAL ANALYSIS

The ultrastructure and distribution of dopaminergic boutons within the rat mesencephalic trigeminal (Me5) nucleus was examined with the use of electronmicroscopic immunocytochemistry. A total of 5102 boutons, c omprising axosomatic and axodendritic synaptic terminals as well as no n-synaptic boutons (or varicosities), located in the ventrocaudal port ion of Me5 was analysed. Approximately 20% of these boutons were dopam ine-immunoreactive. Morphological analysis showed that the dopaminergi c synaptic terminals, axodendritic as well as axosomatic, were exclusi vely of the S- and G-bouton type; they contained, respectively, small spherical vesicles or small pleomorphic vesicles in combination with l arge granular dense-cored vesicles. All dopaminergic varicosities in t he Me5 were of the G-bouton type. Quantitative analysis revealed that most of the dopaminergic synaptic terminals in the Me5 nucleus contact ed dendrites, while only a minority (12%) contacted Me5 somata. This d opaminergic somatic input comprised about half (52%) of the total axos omatic input on Me5 neurons. The present results and previous findings with respect to the prominent serotonergic component of the axosomati c input to Me5 neurons indicate that dopamine and serotonin account fo r most of the axosomatic input in the ventrocaudal part of the Me5 nuc leus. In fact, the present results seem to support previous observatio ns regarding the existence of a population of afferent neurons in whic h dopamine and serotonin are colocalized.