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 fenestrated capillary loops of the hypothalamo-hypophysial portal system. In the hypothalamic paraventricular nucleus, CCK has been shown to coexist with corticoliberin (CRH). However, in the median eminence (ME) nothing is known 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 the ME and to describe, at the electron microscopic level, the vesicular pattern of distribution of CCK-IR in the pericapillary endings of the ME. The use of the elution-restaining procedure showed notable similarities between stainings directed against CCK or CRH, respectively, suggesting a colocalization of both peptides in the same terminals. This result was confirmed using a simultaneous double-staining procedure. At the electron microscope level, double immunogold staining procedure enabled us to observe a consistent localization of CCK-IR and CRH-IR over dense-cored vesicles. Most of the terminals were seen to contain both immunoreactivities which, in addition, were often present together in the same vesicles. However, some rare endings remained exclusively stained either for CCK or for CRH. Our results provide evidence for a concomitant release of CCK and CRH into the portal blood.