Uptake of GABA and nipecotic acid in astrocytes and neurons in primary cultures: changes in the sodium coupling ratio during differentiation.

The coupling ratio between sodium and GABA or the GABA analog, (RS)- nipecotic acid, in neuronal and glial uptake of GABA or nipecotic acid was investigated as a function of the morphological differentiation of these cell types in primary cultures. Both in neurons and astrocytes a high-affinity uptake of GABA and nipecotic acid was observed regardless of the degree of differentiation. The values of Km were essentially identical in undifferentiated and differentiated cells. On the other hand, Vmax was significantly increased both in 10-day-old neurons compared to 1- and 3-day-old neurons and in astrocytes treated with dBcAMP compared to untreated cells, i.e., Vmax increased as a function of differentiation in both cell types. In neurons as well as in astrocytes the morphological differentiation resulted in an alteration in the coupling ratio between sodium and GABA. From calculated Hill coefficients it could be deduced that the coupling ratio between sodium and GABA was changed from 1 to 2 during differentiation. Similar results were obtained for (RS)-nipecotic acid.