Characterization of the cyclic 3′,5′-nucleotide phosphodiesterase activity associated with synaptosomal plasma membranes and synaptic junctions

Some characteristics of the cyclic 3′,5′-nucleotide phosphodiesterase (phosphodiesterase) activity associated with the synaptosomal plasma membrane (synaptic membrane) and the synaptic junction fractions of rat brain are reported. Kinetic analysis revealed that only one type of phosphodiesterase activity, with a K_m of 2 · 10^?4 M for cyclic AMP, is associated with both fractions. The specific activities of the phosphodiesterase in synaptic membranes and synaptic junctions have been estimated at 23.4 nmol/min per mg protein and 22.5 nmol/min per mg protein, respectively. The synaptic junction-associated activity undergoes a 30% stimulation by Ca²⁺ while no Ca²⁺ sensitivity of the synaptic membrane-associated activity could be detected. Cytochemical studies performed on the synaptic membrane fraction demonstrated a predominant localization of phosphodiesterase activity over postsynaptic densities, while dense deposits were sometimes observed over the synaptic cleft region.