Molecular mechanisms that control initiation and termination of physiological depolarization-evoked transmitter release

Abstract

Ca 2؉ is essential for physiological depolarization-evoked synchronous neurotransmitter release. But, whether Ca 2؉ influx or another factor controls release initiation is still under debate. The time course of ACh release is controlled by a presynaptic inhibitory G protein-coupled autoreceptor (GPCR), whose agonist-binding affinity is voltage-sensitive. However, the relevance of this property for release control is not known. To resolve this question, we used pertussis toxin (PTX), which uncouples GPCR from its Gi/o and in turn reduces the affinity of GPCR toward its agonist. We show that PTX enhances ACh and glutamate release (in mice and crayfish, respectively) and, most importantly, alters the time course of release without affecting Ca 2؉ currents. These effects are not mediated by G␤␥ because its microinjection into the presynaptic terminal did not alter the time course of release. Also, PTX reduces the association of the GPCR with the exocytotic machinery, and this association is restored by the addition of agonist. We offer the following mechanism for control of initiation and termination of physiological depolarization-evoked transmitter release. At rest, release is under tonic block achieved by the transmitter-bound high-affinity presynaptic GPCR interacting with the exocytotic machinery. Upon depolarization, the GPCR uncouples from its G protein and consequently shifts to a low-affinity state toward the transmitter. The transmitter dissociates, the unbound GPCR detaches from the exocytotic machinery, and the tonic block is alleviated. The free machinery, together with Ca 2؉ that had already entered, initiates release. Release terminates when the reverse occurs upon repolarization.G protein-coupled receptor ͉ neurotransmitter release ͉ pertussis toxin ͉ presynaptic receptors C a 2ϩ influx is essential for physiological depolarizationinduced neurotransmitter (NT) release (1, 2). A broader, Ca 2ϩ voltage, hypothesis suggests that two factors control release: Ca 2ϩ and G protein-coupled receptors (GPCRs), whose agonist-binding affinity is voltage-dependent (3). The mechanism suggested for this control is as follows. (i) At resting potential and rest concentration (nMs) of transmitter, the release machinery (SNARE proteins and synaptotagmin) is under tonic block imposed by the transmitter-bound high-affinity (nMs) GPCR. (ii) Depolarization shifts the GPCR to a lowaffinity state ( Ms), resulting in rapid transmitter dissociation (it should be emphasized that at this stage release of NT did not occur yet, and the concentration of NT in the synapse is still in the nM range). (iii) The unbound GPCR detaches from the release machinery to relieve the tonic block. The free-release machinery together with Ca 2ϩ , which had already entered, initiates release. (iv) Upon repolarization, release terminates because the receptor returns to its high-affinity state and the tonic block is reinstated.Much of this suggested mechanism was supported experimentally (3-8) by using mainly the cholinergic neuromuscular junction (N...

Regulation of neuronal communication by G protein‐coupled receptors

“…Depolarization has been suggested to inhibit all three of these receptors [180], which should lead to enhanced neurotransmitter release. Evidence in support of this hypothesis has recently been obtained from studies of neurotransmitter release in which pertussis toxin (PTX)‐induced uncoupling of inhibitory autoreceptors from their G proteins exerts effects independently of changes in Ca 2+ release [181]. Theoretically, this direct voltage control of synaptic function could contribute to synaptic plasticity [180].…”

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

“…As autoreceptors, group II and III mGluRs are localized mostly just outside, at the annulus of the synaptic cleft, although they have been reported to exist also inside the synaptic cleft (Petralia et al, 1996;Shigemoto et al, 1997;Tamaru et al, 2001). These receptors regulate glutamate neurotransmission through various pathways, including activation of presynaptic K + channels (Anwyl, 1999), inhibition of presynaptic Ca 2+ channels (Anwyl, 1999;Robbe et al, 2002a), and direct interaction with the release machinery (Kupchik et al, 2008(Kupchik et al, , 2011b.…”

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

“…Group II mGlu receptor activation depresses glutamate release in critical regions such as the mPFC (Otani et al, 1999, 2002; Huang and Hsu, 2008; Walker et al, 2015), dorsal striatum (Lovinger and McCool, 1995; Kahn et al, 2001), NAc (Manzoni et al, 1997; Robbe et al, 2002a,b), central amygdala (Neugebauer et al, 2000), and BNST (Grueter and Winder, 2005). A variety of mechanisms might contribute to inhibition of neurotransmitter release by group II mGlu receptors, including inhibition of voltage-gated calcium channels (Anwyl, 1999; Robbe et al, 2002a; Kupferschmidt and Lovinger, 2015), interference with vesicle fusion and release machinery (Kupchik et al, 2008, 2011), and activation of presynaptic potassium channels (Anwyl, 1999). …”