G-Protein Types Involved in Calcium Channel Inhibition at a Presynaptic Nerve Terminal

Abstract

The inhibition of presynaptic calcium channels via G-protein-dependent second messenger pathways is a key mechanism of transmitter release modulation. We used the calyx-type nerve terminal of the chick ciliary ganglion to examine which G-proteins are involved in the voltage-sensitive inhibition of presynaptic N-type calcium channels. Adenosine caused a prominent inhibition of the calcium current that was totally blocked by pretreatment with pertussis toxin (PTX), consistent with an exclusive involvement of Go/Giin the G-protein pathway. Immunocytochemistry was used to localize these G-protein types to the nerve terminal and its transmitter release face. We used two approaches to test for modulation by other G-protein types. First, we treated the terminals with ligands for a variety of G-protein-linked neurotransmitter receptor types that have been associated with different G-protein families. Although small inhibitory effects were observed, these could all be eliminated by PTX, indicating that in this terminal the Gifamily is the sole transmitter-induced G-protein inhibitory pathway. Second, we examined the kinetics of calcium channel inhibition by uncaging the nonselective and irreversible G-protein activator GTPγS, bypassing the receptors. A large fraction of the rapid GTPγS-induced inhibition persisted, consistent with a Go/Gi-independent pathway. Immunocytochemistry identified Gq, G11, G12, and G13as potential PTX-insensitive second messengers at this terminal. Thus, our results suggest that whereas neurotransmitter-mediated calcium channel inhibition is mainly, and possibly exclusively, via Go/Gi, other rapid PTX-insensitive G-protein pathways exist that may involve novel, and perhaps transmitter-independent, activating mechanisms.

GTP-binding protein βγ subunits mediate presynaptic calcium current inhibition by GABA B receptor

“…At the calyx-type nerve terminal in chick ciliary ganglia, both the G o and G i subclasses are expressed together with other G proteins (28). Surprisingly, however, the calyx of Held terminal expressed only G o , whereas the postsynaptic cell expressed both G o and G i .…”

G protein βγ-subunits activated by serotonin mediate presynaptic inhibition by regulating vesicle fusion properties

“…However, this mechanism is not the only one by which G proteins alter release. Regulation of Ca 2ϩ channels is also important at the presynaptic terminal (44,45). Perhaps G␤␥ acts through parallel pathways, causing reduced Ca 2ϩ influx and transmitter release from the exocytosing vesicle.…”

A Syntaxin 1, Gαo, and N-Type Calcium Channel Complex at a Presynaptic Nerve Terminal: Analysis by Quantitative Immunocolocalization

“…N-type Ca channels are known to be inhibited via trimeric G-proteins, and this inhibition has been demonstrated directly in the presynaptic nerve terminal with both G-protein receptor ligands (Yawo and Chuhma, 1993;Takahashi et al, 1998;Mirotznik et al, 2000) and intracellular GTP analogs (Stanley and Mirotznik, 1997;Mirotznik et al, 2000). We have recently shown at the chick calyx presynaptic terminal that this inhibitory pathway uses a PTX-sensitive member of the G␣ O/i family, most likely G␣ O (Mirotznik et al, 2000).…”

“…We have recently shown at the chick calyx presynaptic terminal that this inhibitory pathway uses a PTX-sensitive member of the G␣ O/i family, most likely G␣ O (Mirotznik et al, 2000). This modulation pathway is of particular interest because of its potential for the rapid control of synaptic transmission (for review, see Hille, 1994;Ikeda and Dunlap, 1999;Jarvis and Zamponi, 2001b).…”