Co-existence of TRH with mesotocin in the same axon terminals of the bullfrog pars nervosa as revealed by double labeling immunocytochemistry.

The co-existence of thyrotropin-releasing hormone (TRH)- and mesotocin (MT)-like immunoreactivities in single axon terminals in the frog pars nervosa was observed using a method combining pre-embedding peroxidase-antiperoxidase for TRH with post-embedding immunocolloidal gold staining for MT. Both TRH- and MT-like immunoreactivities were localized in the same large dense elementary granules 130-220 nm in diameter (170 nm mean diameter). A few axons contained only TRH-like immunoreactive small granular vesicles 80-120 nm in diameter (92 nm mean diameter). Axon terminals containing both TRH- and MT-like immunoreactive granules were in direct contact with the perivascular basal lamina of blood capillaries.