Changes in serotonin-induced potentials during spinal cord development.
1. Motoneuron responses to serotonin (5-hydroxytryptamine, 5-HT), and the growth pattern of 5-HT projections into the ventral horn were studied in the isolated spinal cord of embryonic and neonatal rats. 2. 5-HT projections first appeared in lumbar spinal cord at days 16-17 of gestation (E16-E17) and were localized in the lateral and ventral funiculi. By E18, the projections had grown into the ventral horn, and at 1-2 days after birth they were in close apposition to motoneuron somata. 3. At E16-E17, slow-rising depolarizing potentials of 1-4 mV were recorded intracellularly in lumbar motoneurons in response to bath application of 5-HT. These potentials were not apparent after E18; at that time 5-HT generated long-lasting depolarizations with an average amplitude of 6 mV, and an increase of 11% in membrane resistance. Starting at E18, 5-HT also induced high-frequency fast-rising potentials that were blocked by antagonists of glutamate, gamma-aminobutyric acid, and glycine. 4. Motoneuron responses to 5-HT increased significantly after birth, when 5-HT produced an average depolarization of 19 mV and repetitive firing of action potentials. 5. Tetrodotoxin and high Mg2+ did not reduce the amplitude of the long-lasting depolarizations, which suggested that they were produced by direct action of 5-HT on motoneuron membrane. 6. At all developmental ages, 5-HT reduced the amplitude of dorsal root-evoked potentials. The suppressed responses were neither due to 5-HT-induced depolarization nor the result of a decrease in motoneuron excitability. 7. The pharmacological profile of 5-HT-induced potentials was studied with the use of various agonists and antagonists of 5-HT. The findings indicated that the actions of 5-HT on spinal neurons were mediated via multiple 5-HT receptor subtypes. 8. Our results suggested that 5-HT excited spinal neurons before 5-HT projections grew into the ventral horn. The characteristics of 5-HT-induced potentials changed, however, at the time when the density of 5-HT projections increased in the motor nuclei.
Department of Physiology, University of Wisconsin, Madison 53706.
L
L
Ziskind-Conhaim
B S
BS
Seebach
B X
BX
Gao
D000200:Action Potentials; D000375:Aging / Q000502:physiology; D000818:Animals; D000831:Animals, Newborn; D004594:Electrophysiology; D004622:Embryo, Mammalian / Q000502:physiology*; D005314:Embryonic and Fetal Development; D005071:Evoked Potentials; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology; D051381:Rats; D012701:Serotonin / Q000494:pharmacology*; D013116:Spinal Cord / Q000196:embryology* / Q000254:growth & development* / Q000502:physiology; D013126:Spinal Nerve Roots / Q000502:physiology; D009435:Synaptic Transmission
D012701:Serotonin