Effect of castration on the high affinity glutamate transporter in rat hypothalamic and cortical synaptosomes

High affinity transport of glutamic acid has been studied in cortical and hypothalamic synaptosomes from castrated male rats and compared to normal controls. For hypothalamic synaptosomes, both initial velocity of uptake of Va (apparent maximal velocity) were found to be about one-third lower in the castrated animals. Kt (glutamate concentration giving Va/2), however, was reduced by only 5%. Initial velocity of uptake in cortical synaptosomes was measured as a function of both sodium and glutamate concentration. Reductions in uptake subsequent to castration were found to be much less for cortical synaptosomes (2-15%) than for hypothalamic synaptosomes. Fit of these data to various models for the sodium dependence of transport resulted in the same minimal best fit model as that found for control animals. Thus castration does not alter the fundamental nature of the mechanism by which carrier, sodium and glutamate interact in the process of transport. However, quantitative changes were found to occur, as reflected in the best fit constants. These constants were used along with the rate equation for the minimal best fit model to calculate certain parameters which were then used to delineate the quantitative changes in the transporter following castration. A neuroregulatory role for glutamate in gonadotropin secretion has been recently proposed; the present study now provides additional information on the relationship between reproductive function and one aspect of glutamatergic synaptic function, namely, the high affinity transport system.