The Metabotropic Glutamate Receptor mGlu7 Activates Phospholipase C, Translocates Munc-13-1 Protein, and Potentiates Glutamate Release at Cerebrocortical Nerve Terminals

Abstract

At synaptic boutons, metabotropic glutamate receptor 7 (mGlu7 receptor) serves as an autoreceptor, inhibiting glutamate release. In this response, mGlu7 receptor triggers pertussis toxin-sensitive G protein activation, reducing presynaptic Ca 2؉ influx and the subsequent depolarization evoked release. Here we report that receptor coupling to signaling pathways that potentiate release can be seen following prolonged exposure of nerve terminals to the agonist L-(؉)-phosphonobutyrate, L-AP4. This novel mGlu7 receptor response involves an increase in the release induced by the Ca 2؉ ionophore ionomycin, suggesting a mechanism that is independent of Ca 2؉ channel activity, but dependent on the downstream exocytotic release machinery. The mGlu7 receptor-mediated potentiation resists exposure to pertussis toxin, but is dependent on phospholipase C, and increased phosphatidylinositol (4,5)-bisphosphate hydrolysis. Furthermore, the potentiation of release does not depend on protein kinase C, although it is blocked by the diacylglycerolbinding site antagonist calphostin C. We also found that activation of mGlu7 receptors translocate the active zone protein essential for synaptic vesicle priming, munc13-1, from soluble to particulate fractions. We propose that the mGlu7 receptor can facilitate or inhibit glutamate release through multiple pathways, thereby exerting homeostatic control of presynaptic function.Metabotropic glutamate receptors belong to the G proteincoupled receptors (GPCRs) 2 superfamily and their eight receptor subtypes (mGlu1-8 receptors) are classified into three major groups. Most group III mGlu receptors (mGlu4, -6, -7, and -8 receptors) are located within the presynaptic active zone (1) where they act as autoreceptors mediating feedback inhibition of glutamate release (2-4). The signaling mechanism initiated by mGlu7 receptors to inhibit neurotransmitter release involves the activation of G i/o proteins that inhibit Ca 2ϩ channels and adenylyl cyclase (5), and probably the release process itself (6). However, mGlu7 receptor signaling is not restricted to these pathways. Thus, transfected mGlu7 receptors expressed in cerebellar granule cells inhibits somatic Ca 2ϩ currents by a mechanism that involves the activation of phospholipase C (PLC) and the hydrolysis of phosphatidylinositol (4,5)-bisphosphate thereby generating inositol trisphosphate that releases Ca 2ϩ from intracellular stores and diacylglycerol (DAG) that activates protein kinase C (PKC) (7). However, one important question that remains to be resolved is whether endogenous mGlu7 receptors at synaptic sites also signal via PLC and if so, what effect such signaling has on release modulation.Phorbol esters, stable analogues of the endogenous product of PLC, DAG, potentiate synaptic transmission by increasing neurotransmitter release (8 -15). DAG signaling at synapses has long been thought to be mediated by PKC and both presynaptic K ϩ and Ca 2ϩ channels, as well as other proteins of the release machinery, have been identified as PKC substrates....

A restricted population of CB 1 cannabinoid receptors with neuroprotective activity

“…Synaptosomes were obtained from mouse striata as described previously (41) and used for immunomicroscopy analyses (as detailed later). Glutamate release was assayed in synaptosomal preparations from the P2 fraction with glutamate dehydrogenase, and the fluorescence of NADPH was followed by online fluorimetry (LS-50 luminescence spectrometer; PerkinElmer) (41). Stock solutions of WIN-55,212-2 were prepared in DMSO (final concentration in the assay, 0.1%).…”

Blocking Metabotropic Glutamate Receptor Subtype 7 (mGlu7) via the Venus Flytrap Domain (VFTD) Inhibits Amygdala Plasticity, Stress, and Anxiety-related Behavior

“…Supporting a key role in brain physiology, mice lacking mGlu7 were shown to have deficits in amygdala-dependent fear learning and aversive behavior (10), reduced anxiety-and depression-like behavior (11)(12)(13)(14), and alterations in stress responses, including elevated production of brain-derived neurotrophic factor in the hippocampus (15). At the synaptic level, mGlu7 appears to regulate presynaptic GABA and L-glutamate release, which may contribute to mGlu7's regulating role in emotion-relevant brain circuitry (14,16,17). This role is also supported by findings in mice using siRNA-induced knockdown of mGlu7, inducing changes in innate anxiety, physiological and behavioral stress responses, and fear learning (18).…”

Group III Metabotropic Glutamate Receptors: Pharmacology, Physiology and Therapeutic Potential

“…Similar effects of AMN082 on the release of glutamate and GABA have been described in the nucleus accumbens [150] and amygdala [144], although it was suggested that the increase in glutamate observed in these studies may have been secondary to the mGlu7-mediated decrease in GABA. However, a further study demonstrating enhanced release of glutamate from cerebrocortical nerve terminals following prolonged exposure of mGlu7 to L-AP4 does support a facilitatory role for this receptor in excitatory transmission [156]. Together, these data suggest that group III mGlu receptors may regulate the release of glutamate in a bimodal and activity-dependent manner.…”