Glycine conductance changes in chick spinal cord neurons developing in culture

Whole-cell glycine-activated currents were investigated in chick spinal cord neurons cultivated for up to three weeks. Based on the morphological and electrophysiological characteristics of neurons, two different types of nerve cells were distinguished during the first few days in culture. The first type consisted of "mature" nerve cells which appear to be motoneurons. They died by five to seven days in vitro. Immature neurons or neuroblasts constituted another type of nerve cell. They developed in culture and became differentiated neurons. Glycine-activated currents were elicited in both types of neurons during different periods in vitro. Sensitivity to glycine of "mature" neurons decreased from two to five days in vitro: ED50 for agonist action increased from 0.4 to 1.3 mM. The sensitivity of neuroblasts to this transmitter increased during differentiation: ED50 decreased from 1.4 to 0.12 mM on three to 14 days in vitro, respectively. Changes in glycine-activated conductance of these developing neurons were investigated later on. The conductance in differentiated neurons was markedly sensitive to membrane potential, while neuroblasts did not show such dependence. Voltage sensitivity was due to voltage-dependent kinetics of the ion channel. Desensitization kinetics of the glycine-activated currents were double-exponential. The time constant for the slow desensitizing component was dependent on glycine concentration, which was not the case for the fast component. The increase in glycine sensitivity of the neuroblasts was accompanied by deceleration of desensitization kinetics of the agonist-activated currents. A remarkable feature of the currents elicited in neuroblasts was their extremely long time course after rapid agonist removal from the cells. The properties of these long-term currents suggest that a large fraction of the receptors are desensitized, even during quite short applications of the transmitter. The presence of glycine in the culture medium did not affect the increase of neuronal sensitivity to the agonist. The block of spontaneous bioelectric activity by adding tetrodotoxin to the culture medium abolished developmental changes in glycine-activated conductance. Possible mechanisms for the changes in transmitter sensitivity of the neurons are considered.