chapter 4.6 Flashcards
Three classes of conventional small-molecule neurotransmitters:
amino acids, the monoamines, and acetylcholine.
Unconventional neurotransmitters
small-molecule neurotransmitter; mechanism of action are unusual.
Class of large-molecule neurotransmitters
neuropeptides
Amino acid neurotransmitters
glutamate, aspartate, glycine, and gamma-aminobutyric (GABA). The first three are common in the protein we consume, whereas GABA is synthesized by a simple modification of the structure of glutamate.
Glutamate
most prevalent excitatory neurotransmitter in the mammalian CNS.
GABA
most prevalent inhibitory neurotransmitter; however, it has excitatory effects at some synapses.
Monoamines
each is synthesized from a single amino acid.
Monoamine neurotransmitters
slightly larger than amino acid neurotransmitters and their effects tend to be more diffuse. Present in small groups of neurons whose cell bodies are located in the brain stem. These neurons often have highly branched axons with many varicosities from which monoamine neurotransmitters are diffusely released into the extracellular fluid.
Four types of monoamine neurotransmitters
dopamine, epinephrine, norepinephrine, and serotonin. Subdivided into two groups, the catecholamines (all except serotonin) and indolamines on the basis of their structures.
Catecholamines
each is synthesized from the amino acid tyrosine. Tyrosine is converted to L-dopa, which in turn is converted to dopamine. Neurons that release norepinephrine have an extra enzyme (not present in dopaminergic neurons), which converts the dopamine in them to norepinephrine. Neurons that release epinephrine have all the enzymes present in neurons that release norepinephrine, along with an extra enzyme that converts norepinephrine to epinephrine.
Serotonin
also called 5-HT; is synthesiszed from the amino acid tryptophan and is classed as an indolamine.
Noradrenergic
neurons that release norepinephrine.
Adrenergic
neurons that release epinephrine.
Acetylcholine
small-molecule neurotransmitter that is created by adding an acetyl group to a choline molecule. The neurotransmitter at neuromuscular junctions, at many of the synapses in the autonomic nervous system, and at synapses in several parts of the CNS.
Cholinergic
neurons that release acetylcholine.
Soluble-gas neurotransmitters
unconventional neurotransmitter; includes nitric oxide and carbon monoxide. Produced in the neural cytoplasm and immediately diffuse through the cell membrane into the extracellular fluid and then into nearby cells. Easily pass through cell membranes because they are soluble in lipids. Once inside another cell, they stimulate the production of a second messenger and are deactivated by being converted to other molecules.
Soluble-gas neurotransmitters and retrograde transmission
at some synapses, they transmit feedback signals from the postsynaptic neuron back to the presynaptic neuron. Function of retrograde transmission seems to be to regulate the activity of presynaptic neurons.
Endocannabinoids
unconventional neurotransmitter; similar to THC, the main psychoactive constituent of marijuana. Includes anandamide. Produced immediately before they are released and are synthesized from fatty compounds in the cell membrane; they tend to be released from the dendrites and cell bod; and they tend to have most of their effects on presynaptic neurons, inhibiting subsequent synaptic transmission.
Neuropeptide transmitters
five groups: pituitary peptides, hypothalamic peptides, brain-gut peptides, opioid peptides, and miscellaneous peptides.
pituitary peptides
contains neuropeptides that were first identified as hormones released by the pituitary.
hypothalamic peptides
contains neuropeptides that were first identified as hormones released by the hypothalamus.
brain-gut peptides
contains neuropeptides that were first discovered in the gut.
opioid peptides
contains neuropeptides that are similar in structure to the active ingredient of opium.
miscellaneous peptides
catch-all category that contains all of the neuropeptide transmitters that do not fit into one of the other four categories.
