Significant expression of prosaposin in the intermediate lobe of mouse pituitary gland

Prosaposin (PSAP) is a highly conserved glycoprotein in vertebrates. It is known to be transported into lysosomes and facilitates lysosomal hydrolysis. In addition, PSAP is secreted in various body fluids, including serum. Extracellular PSAP is known to function as a trophic factor for neurons, and recent studies have revealed that PSAP plays a pivotal role in dopaminergic neuron homeostasis. This study examined PSAP expression in the mouse pituitary gland, which is one of the principal sources of circulating hormones innervated by dopaminergic neurons. In situ hybridization showed that PSAP mRNA expression was high in the intermediate lobe (IL), whereas the expression was relatively low and sparse in the anterior (AL) and posterior lobes (PL). Immunohistochemical analyses showed that PSAP immunoreactivity was detected as fine, granular structures in the AL and IL. PSAP immunoreactivity was also observed in glial cells and the Herring bodies of the PL. The IL is innervated by axons from dopaminergic neurons in the periventricular hypothalamic area, and neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), are known to be highly expressed in the IL, where they support these neurons. The results of this study indicate that PSAP plays a pivotal role in the pituitary gland, particularly within the IL.