DEVELOPMENTAL-CHANGES IN SEROTONERGIC RECEPTOR-MEDIATED MODULATION OFEMBRYONIC CHICK MOTONEURONS IN-VITRO

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 br acket a critical period in spinal cord growth. The resting membrane po tential of chick motoneurons did not change significantly between E12 and E18, but there was a significant decrease in neuronal input resist ance. A small inward rectification was present in cells of both ages, although a lower proportion of E12 motoneurons exhibited inward rectif ication 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 fir ing. Bath application of serotonin (5-HT) and its agonists 5-carboxami do-tryptamine (5-CT, a 5-HT1 agonist) and alpha-methyl 5-HT (a 5-HT2 a gonist) produced hyperpolarizing responses accompanied by decreased in put 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 moto neuronal excitability were tested using depolarizing current pulses. I n most cases, serotonergic agonists caused a decrease in firing freque ncy during the hyperpolarizing response in E12 neurons. At E18, bath a pplication 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 contr ibute to modulation of chick motoneuron excitability and appear to rev erse the polarity of their effects on membrane potential after a criti cal period in development of the spinal cord. (C) 1997 Elsevier Scienc e B.V.