Functional role of somatostatin receptors in neuroendocrine and immune cells.

During the last decade the concept of a narrow communication between the immune and classical neuroendocrine systems has been supported by cumulative evidence. One of the common links between the two systems is formed by the production of somatostatin (SS), the presence of SS receptors (SS-R) and the functional effects of SS on both endocrine and immune cells. While in the endocrine system SS-R activation is coupled to mainly inhibitory effects, both inhibitory and stimulatory effects of SS have been demonstrated on the function of immune cells (ie proliferation and secretion). Moreover, in contrast to endocrine cells (ie growth hormone (GH)-secreting pituitary cells) in which SS and its analogues inhibit GH secretion in the nanomolar range in a dose-dependent manner achieving maximal inhibitory effects at higher concentrations, biphasic effects of SS are generally found on the function of immune cells with inhibition at low (nanomolar) concentrations and absence of an effect at higher (micromolar) concentrations. Neuroendocrine cells often express multiple SS-R subtypes, which may be linked to specific functions. Scarce information is available so far on the SS-R subtype expression pattern as well as on the second messenger systems linked to SS-R activation in human lymphoid cells. The recent development of novel SS-R subtype-selective SS analogues will be helpful in unravelling the functional roles of the individual SS-R subtypes in SS-R-expressing human neuroendocrine and immune cells.