A robust GTP-induced shift in alpha(2)-adrenoceptor agonist affinity in tissue sections from rat brain.

A method is presented for monitoring the coupling of the alpha(2)-adrenoceptor, as well as other receptors, to their G proteins using the GTP-induced shift in agonist affinity states. In tissue sections GTP, but not ATP, induces a robust decrease in agonist affinity of greater than 100-fold, which is much larger than previously found in membrane preparations. A sensitive and easy procedure to monitor the extent of coupling is to compare the amount of [(3)H]RX821002 binding remaining in the presence of 100 nM brimonidine in the absence and presence of 100 microM GTP. This method should be especially applicable for determining the extent of coupling of receptors to their G proteins in multiple brain regions using autoradiographic procedures.