Somatostatin Inhibits Depolarization-evoked Intracellular Ca2+ Increases in Cultured Mouse YFP Expressing Retinal Ganglion Cells

Abstract

Purpose: : Somatostatin (somatotropin releasing-inhibitory factor, SRIF) is a potent cyclic neuropeptide that is widely distributed in both the mammalian central nervous system and retina. SRIF has been shown to exhibit both neuromodulatory and transmitter-like properties in the retina by regulating cell excitability through modulation of voltage-dependent ion channels. More importantly, SRIF receptor 1 (SST)1 and SST4 have been localized to retinal ganglion cells in rodent retina, suggesting that these receptors might underlie the neuromodulatory and transmitter-like properties observed in retinal ganglion cells. We therefore tested whether SRIF could inhibit depolarization-evoked Ca²⁺ influx through voltage-dependent Ca²⁺ channels and alter exocytosis from cultured retinal ganglion cells.

Methods: : Neuron rich sandwich cultures were prepared from a transgenic mouse line (YFP-16) expressing YFP in ganglion cells. [Ca²⁺]_i levels from were measured from both ganglion cell processes and somatic regions using the long-wavelength cell permeant Ca²⁺ sensitive dye Rhod-2/AM. Synaptic terminal labeling was also monitored with the activity-dependent dye, Synaptored-C2 (FM 4-64) to assess transmitter release. Ganglion cells were further characterized using immunocytochemistry with antibodies to Thy-1, VGLUT2, SRIF receptors (SST1 and SST4).

Results: : SRIF inhibited the K⁺-evoked Ca²⁺ increase in ganglion cells in a dose-dependent manner (1-1000 nM), that recovered upon washout, suggesting that activation of SRIF suppressed voltage dependent Ca²⁺ influx in ganglion cells. SRIF, like Cd²⁺ also reduced destaining or activity-dependent dye loss from cultured retinal ganglion cells, suggesting that SRIF can suppress transmitter release from ganglion cells.

Conclusions: : These results suggest that SRIF inhibits voltage-dependent Ca²⁺ influx and transmitter release from retinal ganglion cells. It is possible that these inhibitory effects of SRIF contribute to the light modulated signaling of ganglion cell inputs to the brain, perhaps by helping ganglion cells to adapt to a wide range of intensities in the mammalian retina.

Keywords: ganglion cells • retina: proximal (bipolar, amacrine, and ganglion cells) • neuropeptides