G protein-coupled receptors may modulate neurotransmitter release from the presynaptic terminal by a number of mechanisms. Two main effector targets are of interest to our laboratory and are diagrammed below. Gbg (blue) targeting the release machinery associated with vesicle fusion and exocytosis. Also mechanisms leading to release of Ca2+ from presynaptic internal stores (orange)

Of these pathways we have focused most of our recent work on the mechanisms by which G proteins may inhibit neurotransmitter release using the 5-HT receptor mediated mechanism in the lamprey giant axon as a model system. In a collaborative effort with the laboratory of Heidi Hamm at Vanderbilt University Medical School and with Tom Martin at the University of Wisconsin, Madison, this has lead to a number of papers listed below, describing a previously unidentified target for the Gbg G protein subunits on the SNARE complex.

GPCR mediated modulation at the terminal
Regulation of transmitter release at the presynaptic terminal is vital to the plasticity of a neuron and the signaling network in which it functions. Modulation of exocytotic release by GPCRs is an important component of this regulation (Alford and Grillner, 1991; Nicoll and Alger, 1979; Dutar and Nicoll, 1988; Miller, 1998; Stark and Wasserman, 1972; Rudomin and Madrid, 1972). GPCRs are the largest known family of cell-surface receptors (>1,000 known) and respond to hormones, neurotransmitters, chemokines and sensory stimuli (Hamm, 1998). GPCRs are integral membrane proteins comprising seven hydrophobic regions that form membrane-spanning a helices, connected by extracellular and intracellular loops. The intracellular loops form the heterotrimeric G protein-binding domain. Heterotrimeric G proteins consist of 3 subunits: the a subunit (35kDa) family, the b subunit (35–36kDa) family and the g (6–10kDa) subunit family (Hamm and Gilchrist, 1996).

Blackmer T, Larsen EC, Bartleson C, Kowalchyk JA, Yoon E-J, Preininger AM, Alford S, Hamm HE, Martin TFJ. (2005) G protein βγ directly regulates SNARE protein fusion machinery for secretory granule exocytosis. Nature Neuroscience 8: 421-425 pubmed
Gerachshenko T, Blackmer T, Yoon EJ, Bartleson C, Hamm HE, Alford S. (2005) Gβγ acts at the C terminus of SNAP-25 to mediate presynaptic inhibition. Nature Neuroscience 8: 597-605. pubmed
Photowala H, Blackmer T, Schwartz E, Hamm HE, Alford S (2006) G protein βγ-subunits activated by serotonin mediate presynaptic inhibition by regulating vesicle fusion properties.Proc Natl Acad Sci U S A. 103:4281-4286. pubmed
Schwartz EJ, Blackmer T, Gerachshenko T, Alford S. (2007) Presynaptic G protein-coupled receptors regulate synaptic cleft glutamate via transient vesicle fusion. J. Neuroscience 27:5857-5868 pubmed
YoonYoon E-J, Gerachshenko T, Spiegelberg BD, Alford S, & Hamm HE. (2007) Gβγ regulates exocytosis by interfering with Ca2+-dependent binding of synaptotagmin to the SNARE complex. Molecular Pharmacology 72:1210-1219 pubmed

Following activation by a GPCR, heterotrimeric G pr