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Developmental changes in serotonergic receptor-mediated modulation of embryonic chick motoneurons in vitro
T Hayashi1, B Mendelson, K D Phelan
1Department of Anatomy, University of Arkansas for Medical Sciences, Little Rock 72205-7199, USA.
Insights
Spinal cord development in embryonic chicks shows a critical period where serotonin
Area of Science:
- Neuroscience
- Developmental Biology
- Spinal Cord Research
Background:
- Motoneuron excitability is crucial for motor control.
- Developmental changes in the embryonic spinal cord are not fully understood.
- Serotonergic pathways play a significant role in neural development.
Purpose of the Study:
- To investigate developmental changes in chick motoneuron properties.
- To examine the effects of serotonin on motoneuron excitability during development.
- To identify the roles of specific serotonin receptor subtypes.
Main Methods:
- Intracellular recordings from embryonic chick spinal cord slices (E12 and E18).
- Assessment of resting membrane potential, input resistance, and rectification.
- Analysis of responses to depolarizing current pulses and serotonergic agonists (5-HT, 5-CT, alpha-methyl 5-HT).
Main Results:
- Neuronal input resistance decreased from E12 to E18.
- E12 motoneurons showed spike adaptation; E18 motoneurons exhibited tonic firing.
- Serotonergic agonists caused hyperpolarization and decreased input resistance at E12.
- Serotonergic agonists induced depolarization and increased input resistance at E18.
- Both 5-HT1 and 5-HT2 receptors were involved, with reversed effects post-development.
Conclusions:
- Chick motoneuron excitability undergoes significant changes during a critical developmental period.
- Serotonergic modulation of motoneuron excitability reverses polarity between E12 and E18.
- Both 5-HT1 and 5-HT2 receptors mediate these developmental changes in excitability.
Abstract:
Intracellular recordings were obtained from antidromically identified motoneurons in an embryonic chick spinal cord slice preparation at two developmental stages (embryonic days 12 and 18, E12 and E18) which bracket a critical period in spinal cord growth. The resting membrane potential of chick motoneurons did not change significantly between E12 and E18, but there was a significant decrease in neuronal input resistance. A small inward rectification was present in cells of both ages, although a lower proportion of E12 motoneurons exhibited inward rectification compared to E18 motoneurons. Injection of depolarizing current pulses revealed that most E12 motoneurons exhibited spike adaptation, while the majority of E18 motoneurons showed high frequency tonic firing. Bath application of serotonin (5-HT) and its agonists 5-carboxamido-tryptamine (5-CT, a 5-HT1 agonist) and alpha-methyl 5-HT (a 5-HT2 agonist) produced hyperpolarizing responses accompanied by decreased input resistance in all E12 motoneurons studied. The same three agonists produced depolarizing responses and increased input resistance in all E18 motoneurons studied. The effects of serotonergic agonists on motoneuronal excitability were tested using depolarizing current pulses. In most cases, serotonergic agonists caused a decrease in firing frequency during the hyperpolarizing response in E12 neurons. At E18, bath application of 5-HT, 5-CT or alpha-methyl 5-HT produced an increase in firing frequency in all motoneurons during the depolarizing response. Our results indicate that both 5-HT1 and 5-HT2 receptor subtypes contribute to modulation of chick motoneuron excitability and appear to reverse the polarity of their effects on membrane potential after a critical period in development of the spinal cord.