The ras-like rab3A protein is present in pinealocytes of the gerbil pineal gland

As part of our molecular and functional characterization of the compartment of synaptic-like microvesicles (SLMVs) in mammalian pinealocytes, we have now analyzed the pineal gland of the Mongolian gerbil for the presence of rab3 proteins. Members of this subfamily of small G proteins are thought to regulate the intracellular trafficking and/or membrane fusion of secretory vesicles. Immunostaining of serial semi-thin sections of plastic-embedded pineals with monoclonal antibodies which recognize rab3A revealed the occurrence of rab3A in pinealocytes throughout the gland. Rab3A immunoreactivity was markedly enriched in dilated pinealocyte process terminals known to contain accumulations of SLMVs. The latter could be labeled with the rab3 antibodies at the ultrastructural level when immunogold staining was performed. Our results lend further support to the hypothesis that mammalian pinealocytes are endowed with a population of SLMVs that serve secretory functions.