• Formed by v-SNAREs, t-SNAREs, NSF, and SNAP-25, probably along with other proteins

1. fusion pore lasts only fraction of millisecond, then expands as contents of vesicle dumped into synaptic cleft, also possible for fusion pore to form briefly, allowing some neurotransmitter to enter cleft, then close again, pore may flutter open and closed several times

Local Ca²⁺ concentration

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• Ca²⁺ sensitive dyes flourece at a different wavelength when they bind Ca²⁺ ions, first used to detect intracellular Ca²⁺ was aequorin, chemical that gives jellyfish luminescent glow, inject neuron with dye to monitor Ca²⁺ distribution in cell
• During synaptic transmission, local very high concentrations of Ca²⁺ develop, enables Ca²⁺ to exert rapid effect, delay between Ca²⁺ entry and neurotransmitter release only 0.1 to 0.2 msec, not enough time for long distance diffusion or complex sequence of biochemical reactions

Tetanus and botulinum neurotoxins affect docking and fusion prevent release of neurotransmitter

• Tetanus toxin prevents release of neurotransmitter by inhibitory neurons in spinal cord, causes uncontrolable muscle contractions
• Botulinum prevents neurotransmitter release from spinal neurons causes flaccid paralysis

• Synaptic vesicle fuses with cell membrane and becomes part of it, measurement of membrane capacitance during and following synaptic transmission indicates increase in surface area of cell membrane
• Theoretical calculations indicate that if all vesicle membrane remained in cell membrane surface area increase should be much greater, therefore mechanism for removing membrane must exist
• Removed membrane recycled back to synaptic vesicles, patch of membrane that came from particular vesicle removed from synapse to specialized region where new vesicles are pinched off into cell, demonstrated by tagging v-SNARE proteins with flourescent antibodies
• 45-90 seconds to recycle synaptic vesicles

Two kinds of vesicles

1. clear small 40-50 nm, oval or spherical in shape

• some filled in cell body and transported to synapse, others filled from transmitter available in terminal
• reformed clear vesicles empty of transmitter
• new transmitter molecules synthesized in terminal and transported into vesicle
• ATP is expended to transport free protons (H+) into vesicle
• Proton gradient used to power movement of neurotransmitter into vesicle by transporter proteins in embedded in membrane

1. dense core 70-200 nm, solid looking dense core, spherical shape

• usually filled with peptide neurotransmitter
• peptide must be synthesized by Golgi apparatus in cell body
• synthesis and filling occur together
• filled vesicle moved by axonal transport down axon to release site

Non-spiking neurons cannot generate AP, present in retina, inner ear, lateral line of fish, and olfactory bulb, in invertebrates some sensory neurons and many interneurons involved in generating motor patterns don't spike

• AP not necessary for release of neurotransmitter, only facilitate necessary depolarization
• For non-spiking neurons release of neurotransmitter proportional to degree of depolarization, with even slight fluctuations having significant effect graded transmission,
• release in spiking cells is also graded to as indicated by presynaptic inhibition and by effects of experimental reduction in size of AP
• at some synapses both graded transmission and triggered release occur, for taste cells low level stimulation induces graded release, high level stimulation triggers AP

Hard to determine whether particular chemical serves as neurotransmitter at given synapse because synapses are not readily accessible for experimentation

Four criteria for identification of substance as neurotransmitter:

1. 1) must be synthesized in neuron from which released

• biochemical techniques used to demonstrate enzymatic pathways necessary synthesis of putative neurotransmitter

1. 2) must be stored in neuron from which it is released

• Anatomical labeling techniques such as immunohistochemistry may be used to determine the distribution of a putative transmitter, and in particular whether it is present in presynaptic terminals

1. 3) must be released from terminals of these neurons

• may be possible to collect and identify suspected neurotransmitter from vicinity of active synapses

1. 4) when applied exogenously, must mimic action of natural neurotransmitter

• ionophoresis may be used apply putative neurotransmitter to postsynaptic cell
• drugs known to block binding of natural neurotransmitter, should also block effects of iontophoretic application of chemical

Neurotransmitter chemicals

Most known or suspected neurotransmitters fall into three groups

1. 1) amino acids

• building blocks of proteins, possess an acid and an amine (NH₂ containing) chemical group
• many amino acids are neurotransmitters
• glutamate an excitatory neurotransmitter in vertebrate brain and in most excitatory neuromuscular junctions in insects
• aspartate also an excitatory neurotransmitter
• Gamma-aminobutyric acid (GABA) and glycine inhibitory

1. 2) Amines

• Contain amine group but lack acid group characterizing amino acids
• Histamine, octopamine, serotonin, octopamine important in invertebrate nervous systems
• Catacholamines amines containing catacol group (six carbon ring with 2 OH groups) main ones dopamine, epinephrine (adrenaline), norepinephrine (noradrenaline)
• serotonin and dopamine have influence on a variety of behaviors from mood to movement, neurons using them widespread
• norephinephrine released by sympathetic postganglionic neurons in autonomic NS, also important neurotransmitter in CNS

1. 3) neuropeptides

• small chains of amino acids, neurotransmitter role, discovered more recently
• substance P involved in conveying pain information in vertebrates
• endorphins important in amelieorating pain
• most are co-transmitters, substances released along with another neurotransmitter from a synapse, proctolin released along with glutamate at invertebrate neuromuscular junction

1. 4) other- a micellany of substances with no special chemical relationship

• acetylcholine- neurotransmitter at neuromuscular junctions of vertebrate somatic muscles, parasympathetic postganglionic neurons, and all preganglionic neurons in autonomic NS, also involved in insect nervous system, also may act presynaptically in vertebrate brain
• adenosine triphosphate (ATP)- coreleased with catacholamines, may have separate transmitter function, especially in autonomic NS
• the gases nitric oxide (NO) and carbon monoxide (CO), soluble gases neither stored nor released from vesicles, but synthesized as needed, and diffuses away, easily crossing cell membrane, effect is neuromodulatory, interact directly with biochemical machinery of target neuron rather than with a specific receptor

Chemically related neurotransmitters also have synthetic pathways in common, and some co-exist in the same neuron, in some cases co-transmitters in separate vesicles, in other cases in same vesicles (责任编辑：泉水)