Synthesis, transport, and metabolism of serotonin formed from exogenously applied 5-HTP after spinal cord injury in rats.
Spinal cord transection leads to elimination of brain stem-derived monoamine fibers that normally synthesize most of the monoamines in the spinal cord, including serotonin (5-hydroxytryptamine, 5-HT) synthesized from tryptophan by enzymes tryptophan hydroxylase (TPH, synthesizing 5-hydroxytryptophan, 5-HTP) and aromatic l-amino acid decarboxylase (AADC, synthesizing 5-HT from 5-HTP). Here we examine whether spinal cord caudal to transection remains able to manufacture and metabolize 5-HT. Immunolabeling for AADC reveals that, while most AADC is confined to brain stem-derived monoamine fibers in spinal cords from normal rats, caudal to transection AADC is primarily found in blood vessel endothelial cells and pericytes as well as a novel group of neurons (NeuN positive and GFAP negative), all of which strongly upregulate AADC with injury. However, immunolabeling for 5-HT reveals that there is no detectable endogenous 5-HT synthesis in any structure in the spinal cord caudal to a chronic transection, including in AADC-containing vessels and neurons, consistent with a lack of TPH. In contrast, when we applied exogenous 5-HTP (in vitro or in vivo), AADC-containing vessels and neurons synthesized 5-HT, which contributed to increased motoneuron activity and muscle spasms (long-lasting reflexes, LLRs), by acting on 5-HT2 receptors (SB206553 sensitive) located on motoneurons (TTX resistant). Blocking monoamine oxidase (MAO) markedly increased the sensitivity of the motoneurons (LLR) to 5-HTP, more than it increased the sensitivity of motoneurons to 5-HT, suggesting that 5-HT synthesized from AADC is largely metabolized in AADC-containing neurons and vessels. In summary, after spinal cord injury AADC is upregulated in vessels, pericytes, and neurons but does not endogenously produce 5-HT, whereas when exogenous 5-HTP is provided AADC does produce functional amounts of 5-HT, some of which is able to escape metabolism by MAO, diffuse out of these AADC-containing cells, and ultimately act on 5-HT receptors on motoneurons.
Centre for Neuroscience, University of Alberta, Edmonton, Alberta, Canada.
Yaqing
Y
Li
Lisa
L
Li
Marilee J
MJ
Stephens
Dwight
D
Zenner
Katherine C
KC
Murray
Ian R
IR
Winship
Romana
R
Vavrek
Glen B
GB
Baker
Karim
K
Fouad
David J
DJ
Bennett
D000818:Animals; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000235:genetics / Q000378:metabolism*; D001933:Brain Stem / Q000378:metabolism; D005260:Female; D009046:Motor Neurons / Q000378:metabolism; D009928:Organ Specificity; D020286:Pericytes / Q000378:metabolism; D051381:Rats; D012701:Serotonin / Q000096:biosynthesis* / Q000378:metabolism / Q000494:pharmacology; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D013119:Spinal Cord Injuries / Q000378:metabolism*
D012701:Serotonin; D001142:Aromatic-L-Amino-Acid Decarboxylases
5-HT2 receptor; 5-hydroxytryptophan; aromatic acid decarboxylase; blood vessels; monoamine oxidase; motoneurons; muscle spasms; pericytes; serotonin; spasticity; spinal cord injury