Structures and connections of enkephalin- and γ-aminobutyric acid-immunoreactive profiles in the gustatory region of the nucleus tractus solitarius: a light and electron microscopic study

GABA and enkephalin (ENK) are principle inhibitory transmitters in the rostral portion of the nucleus tractus solitarius (rNTS) for regulating the gustatory information. Although the existence of GABA- and ENK-immunoreactive (ir) profiles in the rNTS has been demonstrated, there are no morphological data revealing the connections between them. In the present study, using immunofluorescent and electron microscopic methods, we examined their relationship in the rNTS of rat. Results showed the following: (1) dense ENK-ir fibers and terminals and GABA-ir cell bodies, fibers, and terminals were observed in the rNTS; (2) ENK-ir terminals mainly make symmetric synapses with GABA-ir and immunonegative somas and dendrites; (3) co-existence of ENK/GABA-ir axon terminals and convergence of ENK- and GABA-ir terminals on one immunonegative soma or dendrite can also be observed. These results suggest that ENK should inhibit directly or excite indirectly (by blocking the inhibition of the GABAergic neurons) the gustatory neurons in the rNTS.