Expression of receptors for the neuropeptide somatostatin was investigated in vitro in rat and human astrocytes, glioma cell lines, and solid human glial tumors that were all immunopositive for the astrocytic marker glial fibrillary acidic protein. After affinity labelling with a peptide‐gold conjugate of known biological activity, somatostatin‐binding sites could be visualized at the light‐ and electron‐microscopic level on the surface of glial cells. Glioma cells were generally labeled more strongly than were normal astrocytes and preferentially bound the ligand at their processes and not at their somata as were normal cells. Somatostatin transmembrane receptor (SSTR) subtype expression was probed by reverse transcription‐polymerase chain reaction: In rat and human cortical astrocytes and in one glioma cell line (U 118), a pattern of three subtypes (SSTR‐1, SSTR‐2, and SSTR‐4) was detected, whereas, in all other glioma cell lines and in six solid glial tumors investigated, the SSTR‐2 subtype was relatively stronger, expressed either alone or in combination with SSTR‐1; sometimes SSTR‐3 or SSTR‐4 was demonstrated in clearly reduced amounts. In astrocytes and gliomas, somatostatin reduced the levels of cyclic AMP elicited by the adenylate cyclase activator forskolin indicating that at least one of the receptor subtypes is negatively linked to adenylate cyclase. In contrast to other cell types, somatostatin did not inhibit the basal or the fetal calf serum‐stimulated proliferation of astrocytes, glioma cell lines, or glial tumors in culture. Thus, strong SSTR‐2 subtype expression characterizes glial tumors, but somatostatin is ineffective in inhibiting their growth.

Sympathetic nerves were visualized in sections from rat thymus by immunostaining of tyrosine hydroxylase, the rate‐limiting enzyme of catecholamine biosynthesis, and by glyoxylic acid‐induced fluorescence of catecholamines. Catecholaminergic nerve fibres were detected in close connection to thymic epithelial cells which therefore might be preferred target cells. To evaluate this, rat immunocytochemically defined, cultured thymic epithelial cells were investigated for adrenoceptors and adrenergic effects. In rat cultured thymic epithelial cells mRNA for β1‐ and β2‐adrenoceptors was detected by reverse transcription‐polymerase chain reaction by use of sequence‐specific primers. Specific, saturable binding to the cultivated cells was observed with the β‐adrenoceptor agonist CGP 12177. Adrenaline, noradrenaline or the β‐adrenoceptor agonist, isoprenaline, increased intracellular adenosine 3′: 5′‐cyclic monophosphate (cyclic AMP) levels in cultivated thymic epithelial cells dose‐dependently about 25 fold. The pharmacological properties revealed that this response was mediated by receptors of the β1‐ and the β2‐subtypes. The selective β3‐adrenoceptor agonist BRL 37344 had no effect on cyclic AMP levels. The increase in cyclic AMP was downregulated by preincubation with glucocorticoids like dexamethasone or cortisol which also changed the relative importance of β1‐/β2‐adrenoceptors to the response. Incubation with isoprenaline or the adenylate cyclase activator forskolin decreased basal and serum‐stimulated proliferation of thymic epithelial cells. However, adrenergic stimulation of thymic epithelial cells did not induce interleukin 1 production. Since thymic epithelial cells create a microenvironment which influences the maturation and differentiation of thymocytes to T‐lymphocytes, their observed capacity to respond to catecholamines provides novel evidence for the suggestion that adrenergic stimulation may interfere with the regulation of immune functions. British Journal of Pharmacology (1997) 120, 1401–1408; doi:

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We analyzed the internalization of the receptor subtype 2 (sst2) for the neuropeptide somatostatin in glioma cells at the ultrastructural level using an antibody against an extracellular amino acid sequence. Intact cells derived from solid human gliomas or those of the human glioma cell line U343 were receptor-labeled (a) by classical gold immunocytochemistry using a 15-nm gold-labeled second antibody, (b) directly with the sst2 antibody adsorbed to 5-nm colloidal gold, and (c) with the physiological ligand somatostatin conjugated to 5-nm colloidal gold. The receptor was predominantly internalized via uncoated vesicles budding from the cell membrane but only rarely via coated pits, which has been mostly reported for G-protein-coupled, seven transmembrane-domain receptors. In the presence of ligand and sst2 antibody vesicles, tubule-like structures, and multivesicular bodies were labeled in superficial and in perinuclear portions of the cells within the first 30 min. Lysosomal labeling was observed after 30 min and especially after an hour of internalization time. This internalization route is also used to study the directly labeled sst2 antibody or the labeled ligand. However, the late endosomal compartment appears to be reached more rapidly in these latter experiments.

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