Somatostatin-induced regulation of SST(2A) receptor expression and cellsurface availability in central neurons: role of receptor internalization.

To investigate the effects of somatostatin (somatotropin release-inhibiting factor, SRIF) on the regulation of SST(2A) receptors in mammalian brain, we examined how blockade of SRIF release or stimulation by the SRIF analog [d-Trp(8)]-SRIF would affect the expression and cell surface availability of SST(2A) receptors in rat brain slices. First, we measured the intensity of SST(2A) immunoreactivity, using quantitative light microscopic immunocytochemistry, and levels of SST(2A) mRNA, using semiquantitative RT-PCR, under conditions of acute SRIF release blockade. Incubation of slices from the claustrum or basolateral amygdala, two regions previously shown to contain high concentrations of SST(2A) receptors, in Ca(2+)-free Ringer's for 40 min induced a decrease in the intensity of SST(2A) receptor immunoreactivity and concentration of SST(2A) mRNA as compared with control values obtained in Ca(2+)-supplemented Ringer's. These effects were counteracted in a dose-dependent manner by the addition of 10-100 nm [d-Trp(8)]-SRIF to the Ca(2+)-free medium. Furthermore, both of these effects were abolished in the presence of the endocytosis inhibitors phenylarsine oxide or hyperosmolar sucrose, suggesting that they were dependent on receptor internalization. Electron microscopic immunogold labeling confirmed the existence of an agonist-induced internalization of SST(2A) receptors in central neurons. At a high (10 microm), but not at a low (10 nm), concentration of agonist this internalization resulted in a significant decrease in cell surface receptor density, irrespective of the presence of Ca(2+) in the medium. Taken together, these results suggest that ligand-induced endocytosis is responsible for rapid transcriptional (increase in SST(2A) expression) and trafficking (loss of cell surface receptors) events involved in the control of the somatostatinergic signal.