The elucidation of the mechanisms regulating the properties of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors is important for understanding glutamatergic transmission. Here we report that qualitative as well as quantitative analysis of tritiated ligand binding to the AMPA receptor on thin frozen rat brain tissue sections reveals the existence of several mechanisms regulating the binding properties of AMPA receptors. Preincubation of tissue sections at 35°C results in a decreased amount of [3H]AMPA binding as compared to that measured following preincubation at 0°C. The decrease in binding appears to be mainly localized to cell bodies as evaluated by autoradiography, and could be due to proteolysis. Preincubation with calcium at 35°C produces increased levels of [3H]AMPA binding. The effect of calcium is mimicked by manganese and to a lesser extent by magnesium; it is concentration-dependent with a 50% effective concentration for calcium of ∼150 μM, time-dependent and temperature-dependent. The calcium-induced increase in [3H]AMPA binding is different among various brain structures, being larger in area CA1 of the hippocampus and in the superficial layers of the cerebral cortex. The effect of calcium is partly reduced by preincubation with the calpain inhibitor leupeptin and potentiated by preincubation with purified calpain II. The calcium-induced increase in [3H]AMPA binding is associated with a decrease in the binding of an antagonist of AMPA receptors, [3H]6-nitro-7-cyanoquinoxaline-2,3-dione. The results indicate that the binding properties of the AMPA receptor are rapidly regulated by calcium-dependent processes, and possibly by calcium-dependent proteases. They suggest that modulation of the binding properties involves changes in the configuration of the receptor, producing opposite changes in the affinities of the receptor for agonists and antagonists. Finally, these results strengthen the hypothesis that changes in the properties of AMPA receptors might underlie various forms of synaptic plasticity.