（A） Stimulates behavior in the mouse and in humans， whereas adenosine stimulates behavior in humans only

　　（B） has mixed effects in the brain， whereas adenosine has only a stimulatory effect

　　（C） increases cyclic AMP concentrations in target neurons， whereas adenosine decreases such concentrations

　　（D） permits release of neurotransmitters when it is bound to adenosine receptors， whereas adenosine inhibits such release

　　（E） inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain， whereas adenosine inhibits only neuron firing

　　4. In response to experimental results concerning IBMX， Snyder et al contended that it is not uncommon for psychoactive drugs to have

　　（A） mixed effects in the brain

　　（B） inhibitory effects on enzymes in the brain

　　（C） close structural relationships with caffeine

　　（D） depressive effects on mouse locomotion

　　（E） the ability to dislodge caffeine from receptors in the brain

　　5. The passage suggests that Snyder et al believe that if the older theory concerning caffeine's effects were correct， which of the following would have to be the case？

　　Ⅰ。All neurotransmitters would increase the short-term concentration of cyclic AMP in target neurons.

　　Ⅱ。Substances other than caffeine that inhibit the production of phosphodiesterase would be stimulants.

　　Ⅲ。All concentration levels of caffeine that are high enough to produce stimulation would also inhibit the production of phosphodiesterase.

　　（A） Ⅰ only

　　（B） Ⅰ and Ⅱ only

　　（C） Ⅰand Ⅲ only

　　（D） Ⅱ and Ⅲ only

　　（E） Ⅰ，Ⅱ，and Ⅲ

　　6. According to Snyder et al， all of the following compounds can bind to specific receptors in the brain EXCEPT

　　（A） IBMX

　　（B） caffeine

　　（C） adenosine

　　（D） theophylline

　　（E） phosphodiesterase

　　7. Snyder et al suggest that caffeine's ability to bind to A1 and A2 receptors can be at least partially attributed to which of the following？

　　（A） The chemical relationship between caffeine and phosphodiesterase

　　（B） The structural relationship between caffeine and adenosine

　　（C） The structural similarity between caffeine and neurotransmitters

　　（D） The ability of caffeine to stimulate behavior

　　（E） The natural occurrence of caffeine and adenosine in the brain