G-protein types involved in calcium channel inhibition at a presynaptic nerve terminal

The inhibition of presynaptic calcium channels via G-protein-dependent seco nd messenger pathways is a key mechanism of transmitter release modulation. We used the calyx-type nerve terminal of the chick ciliary ganglion to exa mine which G-proteins are involved in the voltage-sensitive inhibition of p resynaptic N-type calcium channels. Adenosine caused a prominent inhibition of the calcium current that was totally blocked by pretreatment with pertu ssis toxin (PTX), consistent with an exclusive involvement of G(o)/G(i) in the G-protein pathway. Immunocytochemistry was used to localize these G-pro tein 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 t reated the terminals with ligands for a variety of G-protein-linked neurotr ansmitter 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 Gi family is th e sole transmitter-induced G-protein inhibitory pathway. Second, we examine d the kinetics of calcium channel inhibition by uncaging the nonselective a nd irreversible G-protein activator GTP gamma S, bypassing the receptors. A large fraction of the rapid GTP gamma-induced inhibition persisted, consis tent with a G(o)/G(i)-independent pathway. Immunocytochemistry identified G (q),G(11),G(12), and G(13) as potential PTX-insensitive second messengers a t this terminal. Thus, our results suggest that whereas neurotransmitter-me diated calcium channel inhibition is mainly, and possibly exclusively, via G(o)/G(i), other rapid PTX-insensitive G-protein pathways exist that may in volve novel, and perhaps transmitter-independent, activating mechanisms.