ULTRASTRUCTURAL CHARACTERISTICS OF ENKEPHALIN-IMMUNOREACTIVE BOUTONS AND THEIR POSTSYNAPTIC TARGETS IN THE SHELL AND CORE OF THE NUCLEUS-ACCUMBENS OF THE RAT

The present study compared the ultrastructural morphology of enkephali n-immunoreactive boutons and their postsynaptic targets in different t erritories of the nucleus accumbens in the rat. The synaptic bouton pr ofiles were identified by antibodies directed against [leu5]enkephalin . Ninety-five percent of the synaptic contacts were symmetric in confi guration and the remaining 5% were asymmetric. Axosomatic contacts com prised 6% of all enkephalin-immunoreactive junctions and were distribu ted equally in all parts of the nucleus. Most (76%) synaptic terminals contacted dendrites but they contacted proportionally fewer dendrites in the shell (71%) than in the core (78%). Moreover, enkephalin-immun oreactive synaptic boutons in the shell (19%) and caudal enkephalin-ri ch areas (17%) of the core contacted twice as many spines than in the remaining parts of the core (8.5%). In the core, long pallidum-like de ndrites were occasionally found ensheathed in enkephalin-immunoreactiv e terminal boutons. We conclude that the differential arrangement of e nkephalinergic contacts in the shell and core could have important fun ctional consequences, especially when considered in relation to other known morphological and neurochemical differences between these region s.