76. The primary purpose of the passage is to ____.

1. discuss a plan for investigation of a phenomenon that is not yet fully understood
2. present two explanations of a phenomenon and reconcile the differences between them
3. summarize two theories and suggest a third theory that overcomes the problems encountered in the first two
4. describe an alternative hypothesis and provide evidence and arguments that support it

77. According to Snyder et al., caffeine differs from adenosine in that caffeine ____.

1. stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only
2. has mixed effects in the brain, whereas adenosine has only a stimulatory effect
3. increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations
4. permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release

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

1. mixed effects in the brain
2. inhibitory effects on enzymes in the brain
3. close structural relationships with caffeine
4. depressive effects on mouse locomotion

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

1. phosphodiesterase
2. caffeine
3. adenosine
4. theophylline

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

1. The chemical relationship between caffeine and phosphodiesterase.
2. The structural relationship between caffeine and adenosine.
3. The structural similarity between caffeine and neurotransmitters.
4. The ability of caffeine to stimulate behavior.

(Reference keys.)