Somatostatin receptors in the haematopoietic system.

Multiple interactions exist between the immune, haematopoietic, endocrine and nervous systems (1,2). The bi-directional communication between the immune/haematopoietic and nervous systems is mediated by complex mechanisms involving multiple soluble factors (e.g. neuropeptides, neurotrophic factors, neurotransmitters and cytokines) produced by each system (3±5). Examples of such factors are: the neurotransmitter neuropeptide Y (6) produced by megakaryocytes (7), substance P, which enhances the proliferation of primitive bone marrow cells and progenitors (4), and nerve growth factor (NGF), which contributes to differentiation of human basophils (8) and stimulates the release of inflammatory mediators from these cells (9). The bone marrow is the major site of haematopoiesis in adults. All blood cells descend from pluripotent haematopoietic stem cells, which develop in lineagecommitted progenitors. These committed progenitors expand and differentiate towards functional end cells. Haematopoiesis is controlled by a complex cytokine network in combination with cellular signals provided by cell-to-cell contact with stromal elements within the bone marrow. A number of studies have demonstrated that somatostatin inhibits proliferation of lymphoid and haematopoietic cells (10). However, little is known of the effects of somatostatin on primary haematopoietic stem cells and progenitor cells. This brief overview will focus on recent insights into the expression and functional significance of somatostatin receptors (SST) in the haematopoietic system.