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） I and 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