canal in human eye. The potential role of these receptors in aqueous outflow is unknown at present.

Circular and longitudinal fibers of ciliary muscle showed intense cytoplasmic and membranous SSTR1-ir and SSTR2ir. Intense SSTR1-ir and SSTR2-ir was detected in the nonpigmented epithelium and on the endothelium of marginal capillaries of ciliary processes. An RT-PCR study done by Mori et al. (20) showed SSTR2 and SSTR4 gene expression in rat iris=ciliary body complex. Several experimental studies showed SST regulation of biochemical processes that are involved in aqueous fluid production in the nonpigmented epithelium of ciliary processes in rabbit eyes, that is cyclic AMP production and concentration of intracellular Ca2þ (21–23). The exact function of SSTR1 and SSTR2 in ciliary body and outflow structures in the human eye is not known; however, based on animal studies, it seems likely that they may be involved in the aqueous fluid production and=or absorption in human eyes as well. SSTR1-ir and SSTR2-ir were also detected on nonpigmented ciliary epithelium (i.e., site of aqueous fluid production), marginal capillaries of ciliary processes, and on endothelial cells in trabecular meshwork and in Schlemm’s canal (i.e., site of aqueous fluid absorption).

Retina, RPE, and Choroid

RT-PCR revealed gene expression for all five SSTRs in the retina. Expression of all five SSTRs in retina should not be surprising considering the complex cellular interactions among highly specialized cells present in the human retina. SSTR1-ir and SSTR2-ir were detected across all cell and fiber layers in retina. Both immunoreactivities were detected in the form of fine punctate staining on the membranes of outer and inner segments of rods and cones and individual cells in outer nuclear layer (ONL), INL, and GCL. Due to the high cell density within ONL and INL, morphology of individual cells could not be reliably assessed (i.e., amacrine cells, Muller cells, bipolar cells, etc.). Prominent SSTR1-ir and SSTR2-ir were also present in OPL, IPL, and NFL. Very intense SSTR1-ir and SSTR2-ir were abundantly present on the

membranes and in the cytoplasm of RPE cells. SSTR1 and SSTR2 seem to be coexpressed on the same cell types across all retinal layers. In addition, distinct punctate SSTR1-ir and SSTR2-ir were present on the membrane and in the cytoplasm of endothelial cells in retinal vessels including arterioles, venules, and capillaries. The same pattern of SSTR1-ir and SSTR2-ir was present within small and large choroidal blood vessels. SSTR4 gene expression was detected in the choroid as well.

The exact physiologic role of SSTR1 and SSTR2 in visual signal processing in immunoreactive human retinal cells is currently unknown. In the rabbit eye, sst2A-ir (alternative splicing variant of SSTR2 molecule) was detected only on the plasma membrane of rod bipolar cells, suggesting very specific function of this receptor molecule in the signal processing in the rabbit eye (24). In monkey retina, SSTR2-ir was primarily observed on cone photoreceptors but also in cell bodies in the INL and on the processes in the IPL (25). The exact physiologic function of SST-ir cells in complex processing of visual information along the visual pathway in primates is poorly understood at present. The relatively small number of SST positive cells in primate retinas (25) suggests that SST influences the overall retinal circuitry by modulating the release and=or action of other retinal neurotransmitters (e.g., dopamine or acetylcholine) on retinal cells. Our results show that SSTR1-ir and SSTR2-ir is much more widely distributed in human retinas than would be expected based on the distribution and connectivity of SST-ir cells in the retina. There are two possible explanations for this phenomenon: (a) once released by SST-ir cells, SST could diffuse across retinal layers in a radial and tangential manner affecting the retinal cells that do not directly synapse with SST-ir cells, and (b) some of the widely distributed receptors could be binding another ligand(s) like cortistatin as suggested by Siehler et al.(26). In addition, SSTR1-ir and SSTR2-ir are present in the same retinal layers in which SST-ir was observed previously (3–6), that is, GCL and INL suggesting that SST may have an autocrine effect on retinal ganglion cells and amacrine cells.

Intense SSTR1-ir and SSTR2-ir were observed on cell membranes and in cytoplasm of individual RPE cells in culture. Actively dividing RPE cells showed increased cytoplasmic SSTR1-ir and SSTR2-ir compared to nondividing RPE cells. The role(s) of SSTR1 and SSTR2 on RPE cells remains speculative at present. Somatostatin could be involved in the modulation of a variety of biochemical processes involved in ion transport mechanisms as well as in synthesis and release of a variety of cytokines (e.g., IGF-I and VEGF). Increase in SSTR1-ir and SSTR2-ir by actively dividing RPE cells suggests that SST could be critically involved in the regulation of their cell cycle.

Strong SSTR1-ir and SSTR2-ir presence on endothelial cells within retinal and choroidal blood vessels may have critical implications for the development of future treatment modalities for conditions characterized by proliferation of endothelial cells in neovascularization processes, such as proliferative diabetic retinopathy and choroidal neovascularization in age-related macular degeneration. Experimental studies using SST and octreotide showed an inhibitory effect on the proliferation of human and murine endothelial cells in culture (cell lines HUV- EC-C and HECa10) (27). Whether this antiproliferative effect is directly mediated by one of the signaling pathways used by some of the SSTRs (presumably SSTR1 and=or SSTR2) or by modulating the synthesis and release of some of the paracrine trophic factors [e.g., vascular endothelial growth factor (VEGF), insulin-like growth factor-1 (IGF-1)] remains unknown. One of those processes, activation of phosphotyrosine phosphatases, seems to be crucial in mediating the direct antiproliferative activity of SST and its analogs in several normal and malignant cell lines. The antiproliferative effect of SST is further potentiated by SST-induced inhibition of biological activity of paracrine and autocrine growth factors, including basic fibroblast growth factor (bFGF) and IGF-1 (27).

Optic Nerve

Moderately strong SSTR1-ir and SSTR2-ir were observed in axons within the optic nerve (ON). No SSTR1-ir or SSTR2-ir could be detected on glial cells within the ON.

the distribution of SSTRs in orbital tissues. Since the identification of SSTRs on lymphocytes, orbital infiltration with mononuclear cells in TO has provided a rationale for receptor imaging with octreotide scintigraphy. Patients with active TO have a twoto three-fold increase in uptake of indiumoctreotide when compared to normals, which suggests that octreoscan could be used to monitor activity (stage) of the disease (28). Octreoscan seems to be particularly useful in moderately severe cases of TO in which it may be difficult to assess the activity of the disease by clinical examination. Consequently, octreoscan was also used to predict the response of TO to immunosuppressive treatment with corticosteroids. Patients with positive octreoscan (active disease) responded nicely to corticosteroid treatment, while those with negative scan did not (29). Several authors reported beneficial effects of octreotide or lantreotide treatment in TO (decrease in proptosis). The clinical effect could be explained by the fact that octreotide and other SST analogs inhibit IGF-1 action on orbital lymphocytes and fibrocytes, which may result in reduced glycosaminoglycan synthesis and collagen production (30). We detected very strong SSTR1-ir and SSTR2-ir on normal extraocular muscle (EOM) myocytes (all extraocular muscles analyzed), orbital fibroblasts, and lymphocytes. The exact function of those receptors on these cell types remains speculative. However, our results suggest that SSTRs could be involved in normal EOM physiology. In addition, the beneficial effect of SSTR analogs could be mediated at the level of individual muscle cells.

CHOROID PLEXUS AND ARACHNOID

GRANULATIONS (Fig. 4)

The production of cerebrospinal fluid (CSF) by CP and its absorption by AG are complex and poorly understood phenomena in humans (31). Recent work has demonstrated synthesis of numerous peptides and their receptors in CP (32). Peptide molecules and their receptors involved in the process of CSF absorption that takes place in the AG are largely unknown.

Figure 4 Intense SSTR1-ir (A) and SSTR2-ir (B) are present in the epithelial and vascular endothelial cells of CP in all samples analyzed. Strong SSTR1-ir (C) and SSTR2-ir (D) are evident on arachnoid cells, connective tissue, and dural layer cells in all analyzed A6 samples.

Octreotide has been used as an adjunctive treatment for idiopathic intracranial hypertension (IIH) without understanding its mechanism or site of action (33). By using immunohistochemistry, we detected intense membrane and cytoplasmic SSTR1 and SSTR2 immunoreactivity on all epithelial cells and vascular endothelial cells in capillaries within individual

villi of CP. Equally strong SSTR1 and SSTR2-ir were observed in arachnoid cells, connective tissue, and dural layer cells in AG samples.

The exact function of SST and its receptors in CSF homeostasis in humans is unknown at present; however, the abundance of SSTRs on CP and AG implies their involvement in the processes of CSF production and absorption. Cerebrospinal fluid dynamics might be affected locally at the level of CP and AG through SSTRs or systemically by modulating the levels or activity of circulating hormones [i.e., growth hormone (GH), IGF-1, insulin, leptin, etc.]. In addition, SSTR1 and SSTR2 might modulate blood flow in CP capillaries and filtration in CP epithelium.

The SST axis has a role in ion transport, extracellular fluid production, and absorption in different mammalian and human tissues (1). Recent work has demonstrated SST effects in kidney and in the eye. Intravenous injection of SST modulates glomerular filtration rate, renal plasma flow, urine volume, and water clearance in normal human kidney (34,35). The recent work of Balster et al. (19) demonstrated SSTR1 and SSTR2 gene expression as well as orderly segmental distribution of SSTR1 and SSTR2-ir along the glomerular and tubular system in normal human kidney tissue. Such data offer us a reasonable theoretical model for a similar role of the SST axis in CSF dynamics.

In the rabbit eye, SST inhibits aqueous production by stimulating adenylyl cyclase activity and release of intracellular Ca2þ in nonpigmented epithelium of ciliary processes (22,23,36). The best studied activity of SST and its analogs in humans is the inhibition of the release of GH from pituitary, which in turn results in the reduction of circulating levels of IGF-1 (37). This is of special interest because it is now apparent that a side-effect of recombinant IGF-1 treatment in children with GH receptor deficiency is a transient increase of intracranial pressure (ICP) (38–48). The increased ICP resolves with complete cessation of IGF-1 treatment or reduction of IGF-1 dose by 50%. In this regard, it is important to recognize that CP epithelium has one of the highest densities of IGF-1R in the human brain (49,50). These data suggest a role for high plasma IGF-1 levels in the