INTRODUCTION

synthesized as a propeptide, which undergoes tissue-specific processing to produce one of two isoforms, somatostatin-14 (SST-14) or somatostatin-28 (SST-28). The biological effects of SST peptides are mediated by high affinity membrane receptors (SSTRs), all of which bind SST-14 and SST-28 with nanomolar activity. SSTRs have a broad expression pattern and the individual receptors have both overlapping and tissue-specific patterns of expression, with SSTR2 usually being the most widely expressed subtype (1,2). There is a very high degree of amino acid homology among members of the SSTR family (overall approximately 50% amino acid homology) (1). The sequence differences, which reside primarily in the intracellular and extracellular domains, are responsible for their distinct signaling properties. SSTRs elicit their cellular responses through G-protein-linked modulation of various second-mes- senger systems including adenylyl cyclase, Ca2þ and Kþ ion channels, Naþ=Hþ antiporter, guanylate cyclase, phospholipase C, phospholipase A2, mitogen-activated protein (MAP) kinase and serine, threonine, and phosphotyrosyl protein phosphatase (1). Second messengers used by any SSTR are often cell, tissue, and species specific. The net result of activation of one or more of those signaling mechanisms, in any given tissue, is down-regulation of many synthetic and secretory processes including secretion of growth factors, cellular proliferation, and differentiation (1). Cloning of the five known receptor subtypes resulted in the development of highly specific agonists such as octreotide and lantreotide (SSTR-2 agonists).

Knowledge of SST production, its physiologic function, and the distribution of SSTR’s in the human ocular and orbital tissues is very limited. To date, the distribution of SST-produ- cing cells was only studied in human retinas. SST immunoreactive cells were detected in ganglion cell layer (GCL) and inner nuclear layer (INL) (amacrine cells) as well as on the cell processes in the inner plexiform layer (IPL) and nerve fiber layer (NFL) in fetal and adult human retinas (3–6). The exact function of SST positive cells in retina is poorly understood. No published data exist on the presence and distribution of SSTproducing cells in other ocular, orbital, and related tissues.

SSTR agonists have been used for treatment of several hormone overproduction states and a variety of benign and malignant tumors (2). In spite of the lack of clear understanding of SSTR distribution in normal intraocular and intraorbital tissues, the antiproliferative and antiangiogenic properties of octreotide have been used in short clinical trials in the treatment of various diseases, including proliferative diabetic retinopathy (PDR), cystoid macular edema, thyroid orbitopathy, and pseudotumor cerebri in small numbers of patients (7–13).

We used reverse transcription polymerase chain reaction (RT-PCR) to study gene expression for all five SSTRs in cultured retinal pigment epithelium (RPE) cells and ocular tissues dissected from normal human eyes. In addition, we studied cell and tissue-specific distribution of SSTR1 and SSTR2 in normal human eyes, cultured RPE cells, extraocular muscles, orbital fat, arachnoid granulations (AG), and choroid plexus (CP) (14–16). Our results demonstrate that genes for SSTRs are expressed in all analyzed tissues with SSTR1 and SSTR2 genes being the most widely expressed, followed by SSTR4 gene (expressed in retina, choroids, and ciliary body=iris). SSTR3 and SSTR5 gene expression was detected only in the retina. Good correlation was detected in the distribution of SSTR1-ir and SSTR2-ir and SSTR1 and SSTR2 gene expression. Immunohistochemical data revealed SSTR1-ir and SSTR2-ir on most of cells derived from neural crest (i.e., stromal keratocytes, corneal endothelium, iris stroma, RPE cells, and choroidal melanocytes). These results are in concordance with the previously published data showing the presence of SSTR1 and SSTR2 (octreoscan, RT-PCR, immunohistochemical detection) on a variety of benign and malignant tumors originating from neural crest-derived cells (17). Although SSTR1 and SSTR2 are membrane-associated receptors, a significant amount of staining was also detected within the cytoplasm and in the perinuclear regions in many immunoreactive cells. Electron microscopic immunogold cytochemistry study done by Dournaud et al. (18) on rat brain neurons showed that the relative proportion of membranebound SSTR2 receptors (membrane versus intracytoplasmic

receptor molecules) correlates inversely with the density of SST innervation of the same neurons. Those data suggest ligand-induced down-regulation of the receptor density on the cell membrane through the receptor internalization process.

This communication summarizes our results and provides rationale for the possible use of SSTR agonists for diagnosis and treatment of certain ocular and orbital diseases.

INTRAOCULAR TISSUES (Figs. 1 and 2)

Cornea

Corneal epithelium did not show immunoreactivity for either SSTR1 or SSTR2. Moderate SSTR1-ir and SSTR-2-ir were observed on cell membrane and cytoplasm of stromal keratocytes. On the other hand, corneal endothelial cells showed strong punctate SSTR1-ir and SSTR2-ir in all specimens. The exact function of SSTR1 and SSTR2 in human cornea, especially corneal endothelium, remains unknown. Considering the complex role of SSTRs in the processes of extracellular fluid formation and absorption (19) in human kidney and possibly CP, it is possible that SSTR1 and SSTR2 are involved in the fluid homeostasis in the cornea and anterior chamber provided by corneal endothelial cells.

Iris, Trabecular Meshwork, Schlemm’s Canal, and

Ciliary Body

Weak SSTR1-ir was detected in sphincter and dilator iris muscles. No other cells were found to be SSTR1 immunoreactive. However, strong SSTR2-ir was present on fibrocytes and clump cells present in iris stroma and sphincter and dilator muscles as well as on endothelial cells within iris blood vessels. SSTR1-ir or SSTR2-ir could not be reliably assessed for the posterior iris pigment epithelium and pigmented epithelium of the ciliary processes secondary to heavy pigmentation. We were the first to detect moderate SSTR1-ir and SSTR2-ir on trabecular endothelial cells in uveal and corneoscleral meshwork as well as on endothelial cells lining the Schlemm’s