Subcellular localization of preprogalanin messenger RNA in perikarya and axons of hypothalamo-posthypophyseal magnocellular neurons: an in situ hybridization study.

The subcellular compartmentalization and axonal transport of oxytocin and vasopressin messenger RNAs have recently been reported in the rat hypothalamo-posthypophyseal system using in situ hybridization. So far, no data are available concerning the intracellular distribution of co-localized peptide transcripts, for example of galanin, which is synthesized in the vasopressinergic magnocellular neurons of the rat and which is up-regulated in these neurons under different conditions, including salt loading and colchicine injection. In the present study, using non-radioactive in situ hybridization and immunohistochemistry at the light and electron microscope levels, preprogalanin messenger RNA and galanin-like immunoreactivity were localized in the hypothalamo-posthypophyseal system. After salt loading, preprogalanin transcripts were found throughout the perikaryal cytoplasm, especially in the peripheral cytoplasm and in the perinuclear area. Since immunohistochemistry also showed galanin-like immunoreactivity preferentially in the perinuclear area of control rats, galanin synthesis may occur mainly in this cytoplasmic domain. Preprogalanin messenger RNA was also clustered in dendrites containing rough endoplasmic reticulum. The use of a new in situ hybridization method involving tyramide signal amplification, based on catalysed reporter deposition, allowed visualization of preprogalanin messenger RNA in axonal projections running through the internal layer of the median eminence after salt loading, but not in control or in colchicine-injected animals. The negative results obtained after colchicine injection indicate that the mechanism of messenger RNA transport may require an intact cytoskeleton. The labelling was found in non-dilated axon segments as well as in a subset of axonal swellings in the rostral aspect of the median eminence, but was restricted to a few swellings in its caudal part, with no labelling in the posterior pituitary. Thus, preprogalanin messenger RNA was segregated in the axons. The functional significance of messenger RNAs' exportation into axons is not known, but our results suggest that this phenomenon may not be limited to the two principal magnocellular hormone messenger RNAs, but may also involve co-existing peptide messenger RNAs.