Ultrastructural colocalization of vesicular cholecystokinin and corticoliberin in the periportal nerve terminals of the rat median eminence

Cholecystokinin (CCK) is present in axon terminals distributed around the f enestrated capillary loops of the hypothalamo-hypophysial portal system. In the hypothalamic paraventricular nucleus, CCK has been shown to coexist wi th corticoliberin (CRH), However, in the median eminence (ME) nothing is kn own about the chemical phenotype of the CCK immunoreactive terminals. This study, carried out in the male rat, was designed to examine the possibility of coexistence of CCK immunoreactivity (CCK-IR) and CRH-IR in fibres of th e ME and to describe, at the electron microscopic level, the vesicular patt ern of distribution of CCK-IR in the pericapillary endings of the ME. The u se of the elution-restaining procedure showed notable similarities between stainings directed against CCK or CRH, respectively, suggesting a colocaliz ation of both peptides in the same terminals. This result was confirmed usi ng a simultaneous double-staining procedure. At the electron microscope lev el, double immunogold staining procedure enabled us to observe a consistent localization of CCK-IR and CRH-IR over dense-cored vesicles. Most of the t erminals were seen to contain both immunoreactivities which, in addition, w ere often present together in the same vesicles, However, some rare endings remained exclusively stained either for CCK or for CRH, Our results provid e evidence for a concomitant release of CCK and CRH into the portal blood.