The primary cilium is now considered to function as a fundamental, not rudimentary, structure for mechanical and chemical sensing by individual cells. Primary cilia in neurons express type III adenylyl cyclase (ACIII) and GPCRs for somatostatin (somatostatin receptor 3, SSTR3), serotonin, and melanin-concentrating hormone. The present immunohistochemical and electron microscopic study revealed an abundant occurrence of SSTR3-expressing solitary cilia in insulin-and growth hormone-secreting cells of the mouse. The SSTR3 immunoreactivity was restricted to the plasma membrane of cilia in both cell types, differing from previously reported immunohistochemical localization of SSTRs to cell bodies. The primary cilia in the islet cells were longer than those in the pituitary cells and extended for a long distance in the intercellular canalicules endowed with microvilli. No other endocrine organs were provided with the SSTR3-expressing primary cilia, while the primary cilia in these organs were frequently immunolabeled with ACIII antibody. Since the somatostatin inhibition of both insulin and GH release is regulated mainly by SSTR1 and SSTR5, the primary cilia expressing SSTR3 may be involved in a signaling which differs from that via other SSTR subtypes expressing in cell bodies.

Morphologists have long observed a single cilium, termed the primary cilium, in a variety of cells under light and electron microscopes. The primary cilium is an immotile cilium that is extended from the centriole—usually surrounded by the Golgi apparatus—in almost all cell types (38). The primary cilia in sensory cells serve specialized sensing functions, as represented by photoreceptor cells, auditory hair cells, and olfactory receptor cells. However, evidence is accumulating that raises the idea that the primary cilium in non-sensory cells functions as a sensory device at a cellular level. Long solitary cilia on epithelial cells in the collecting tubules of the kidney act as mechanoreceptors to detect and re-