Several compounds which are not true neurotransmitters can facilitate or inhibit synaptic activity.
¨Biogenic amine
transmitters
Serotonin- active in descending pathways
Norepinephrine- inhibits pain transmission
between 1st &2nd order neurons
¨Neuroactive peptides
Substance P- from small-diameter primary afferent
neurons
Enkephalins - opiod active in descending pathways
ß-endorphin- opiod endogenous to CNS