We have used the adenosine-stimulated adenylate cyclase of guinea-pig brain to examine the potency of diazepam as an adenosine uptake inhibitor. Diazepam at concentrations in the range 10–500 μM stimulates the production of cAMP in incubated slices of guinea-pig cerebral cortex, with maximal fivefold stimulations over basal levels by 200 μM diazepam. The increases can be largely (but not completely) blocked by the adenosine antagonist theophylline or by addition of excess adenosine deaminase to the system. It appears that the stimulation of cAMP production is due to a blockade of adenosine uptake which results in an increase in extracellular adenosine and concomitant activation of the adenosine receptor coupled to adenylate cyclase. Since the cAMP response to standard adenosine uptake blockers (dipyridamole, dilazep) can be completely blocked by theophylline or adenosine deaminase, a small component of the diazepam response cannot be explained by an adenosine effect. The concentration of diazepam at which the first significant cAMP increase occurs is 10 μM or slightly lower. This is significantly higher than the concentration of diazepam needed to saturate the pharmacologically characterized central nervous system receptors for benzodiazepines.
Characterization of VIP-sensitive adenylate cyclase in guinea pig brain
