DYNORPHIN-IMMUNOREACTIVE TERMINALS IN THE RAT NUCLEUS-ACCUMBENS - CELLULAR SITES FOR MODULATION OF TARGET NEURONS AND INTERACTIONS WITH CATECHOLAMINE AFFERENTS

Dynorphin facilitates conditioned place aversion and reduces locomotor activity through mechanisms potentially involving direct activation o f target neurons or release of catecholamines from afferents in the nu cleus accumbens. We examined the ultrastructural substrates underlying these actions by combining immunoperoxidase labeling for dynorphin 1- 8 and immunogold silver labeling for the catecholamine synthesizing en zyme, tyrosine hydroxylase (TH). The two markers were simultaneously v isualized in single coronal sections through the rat nucleus accumbens . By light microscopy, dynorphin immunoreactivity was seen as patches of immunoreactive varicosities throughout all rostrocaudal levels of t he nucleus accumbens. The dynorphin-immunoreactive terminals identifie d by electron microscopy ranged from 0.2 to 1.5 mu m in cross-sectiona l diameter, contained numerous small (30-40 nm) clear vesicles, as wel l as one or more large (80-100 nm) dense core vesicles. From the dynor phin-immunoreactive terminals quantitatively examined in single sectio ns, 74% (173/370) showed symmetric synaptic junctions mainly with larg e unlabeled dendrites. Of the dynorphin-immunoreactive terminals formi ng identifiable Synapses, approximately 30% contacted more than one de ndritic target. In addition, single dendrites frequently received conv ergent input from more than one dynorphin-labeled terminal. Irrespecti ve of their dendritic associations, dynorphin-immunoreactive terminals also frequently showed close appositions with other axons and termina ls; these included unlabeled (41%), TH-labeled (10%) or dynorphin-labe led axons (14%). In contrast to dynorphin-immunoreactive terminals, TH -labeled terminals formed primarily symmetric synapses with small dend rites and spines or lacked recognizable specializations in the plane o f section analyzed. In some cases, single dendrites were postsynaptic to both dynorphin and TH-immunoreactive terminals. We conclude that dy norphin-immunoreactive terminals potently modulate, and most likely in hibit, target neurons in both subregions of the rat nucleus accumbens. This modulatory action could attenuate or potentiate incoming catecho lamine signals on more distal dendrites of the accumbens neurons. The findings also suggest potential sites for presynaptic modulatory inter actions involving dynorphin and catecholamine or other transmitters in apposed terminals. (C) 1994 Wiley-Liss, Inc.