In situ hybridization demonstrating coexpression of urotensins I, II-alpha, and II-gamma in the caudal neurosecretory neurons of the carp, Cyprinus carpio.

In order to examine the coexpression of different urotensins in the same caudal neurosecretory neurons, in situ hybridization with synthetic oligonucleotide probes specific for mRNAs for urotensins I, II-alpha, and II-gamma (UI, UII-alpha, and UII-gamma) was applied to the caudal neurosecretory system of the carp, Cyprinus carpio, together with immunohistochemistry. Almost all identifiable caudal neurosecretory neurons were labeled with the oligonucleotide and cDNA probes for UI mRNA. Further, any two of the probes for UI, UII-alpha, and UII-gamma mRNAs labeled the same neurons in serial sections. These results suggest that essentially all caudal neurosecretory neurons of the carp coexpress UI, UII-alpha, and UII-gamma. Although most neurons were densely labeled with oligonucleotide probes, only a small portion of the neurons were intensely immunoreactive. This suggests that most caudal neurosecretory neurons actively synthesize all three hormones, but that in some neurons, all or some of the hormones are rapidly transported to the urophysis, resulting in low or undetectable immunoreactivity in the perikarya.