DEVELOPMENTAL LOSS OF GABA-INDUCED AND GLYCINE-INDUCED DEPOLARIZATION AND CA2+ TRANSIENTS IN EMBRYONIC RAT DORSAL HORN NEURONS IN CULTURE

More than 90% of dorsal horn neurons from embryonic day 15-16 rats responded to the inhibitory amino acids GABA and glycine by a transient elevation of intracellular Ca2+ concentration ([Ca2+](i)) when maintained in culture for < 1 week. This [Ca2+](i) response has previously been shown to be due to depolarization andoltage-gated Ca2+ channels following activation of bicuculline-sensitive GABA(A) receptors and strychnine-sensitive glycine receptors. Both the number of cells responding to GABA and glycine and the amplitude of the [Ca2+](i) response diminished over time in culture. By 30 days in culture, none of the cells responded to GABA, muscimol or glycine by elevation of [Ca2+](i). The loss of the [Ca2+](i) response was not due to a change in the abundance or the properties of voltage-gated Ca2+ channels, since over the same period of time dorsal horn neurons showed a large increase in the amplitude of the [Ca2+](i) transient in response to 30 mM K+. Nor was the loss of the [Ca2+](i) response due to a loss of GABA and glycine receptors. Instead, the decrease in the [Ca2+](i) response over time paralleled a similar change in the electrophysiological responses. More than 90% of the neurons tested were depolarized in response to inhibitory amino acids during the first week in culture. After 30 days, all neurons tested responded to GABA and glycine with a hyperpolarization. These observations add support to the suggestion that GABA and glycine may excite dorsal horn neurons early in development and play a role in postmitotic differentiation.