MOLECULAR-COMPONENTS OF THE EXOCYTOTIC MACHINERY IN THE RAT PITUITARY-GLAND

Several protein components that are essential for exocytotic membrane fusion in neurons have recently been identified. The expression and ce llular localization of such protein components were examined in the ra t pituitary gland. In situ hybridization using isoform-specific oligon ucleotide probes to different exocytotic protein messenger RNAs (mRNAs ) showed strong hybridization signal for synaptotagmin I, cysteine str ing protein (CSP), VAMP-2 (vesicle-associated membrane protein), cellu brevin, munc-18 (mammalian homologue of unc-18), SNAP-25a (synaptosoma l-associated protein of 25 kDa), syntaxin 1A, syntaxin 4, syntaxin 5, and alpha-SNAP (soluble NSF attachment protein) in the anterior and in termediate, but not in the posterior lobe of the pituitary. Moderate t o weak hybridization signal was detected for synaptotagmin III, SNAP-2 5b, and syntaxin 2 mRNA in the anterior and intermediate, but not in t he posterior lobe of the pituitary. Synaptotagmin II, VAMP-1, syntaxin 1B, or syntaxin 3 mRNA expression could not be detected in any part o f the pituitary gland. Immunofluorescence histochemistry in combinatio n with confocal laser microscopy revealed that synaptotagmin-, VAMP-, CSP-, NSF-, and alpha-SNAP-like immunoreactivities (-LI) were present in granules of cells in the anterior and intermediate lobe, whereas SN AP-25- and syntaxin-LI were primarily located to the plasma membrane. Synaptotagmin-, VAMP-, CSP-, NSF-, alpha-SNAP-, SNAP-25- and syntaxin- LI were all present in nerve fibers of the posterior lobe. Within cell s of the anterior lobe, colocalization could be demonstrated for synap totagmin I/II- and synaptotagmin III-LI with ACTH-, GH-, PRL- and TSH- , but not FSH- or LH-LI, whereas VAMP, CSP-, NSF-, alpha-SNAP-, SNAP-2 5- and syntaxin-LI were demonstrated in all hormone-containing cell ty pes of the anterior pituitary. The results show the presence of severa l protein components and their isoform-specific mRNAs in the rat pitui tary gland, suggesting that these proteins, similar to their roles in regulation of synaptic neurotransmitter release, may participate in ex ocytotic events in endocrine pituitary cells and in neurosecretory ner ve endings of the neurohypophysis.