| 第二段：
For many years, caffeine’s effects have been attri-
(20) buted to its inhibition of the production of phosphodi-
esterase, an enzyme that breaks down the chemical
called cyclic AMP.A number of neurotransmitters exert
their effects by first increasing cyclic AMP concentra-
tions in target neurons. Therefore, prolonged periods at
(25) the elevated concentrations, as might be brought about
by a phosphodiesterase inhibitor, could lead to a greater
amount of neuron firing and, consequently, to behav-
ioral stimulation. But Snyder et al point out that the
caffeine concentrations needed to inhibit the production
(30) of phosphodiesterase in the brain are much higher than
those that produce stimulation. Moreover, other com-
pounds that block phosphodiesterase’s activity are not
stimulants.
这段主要是阐述caffeine的老观点。最近想研究一下文章的脉络，此段先阐述老观点，然后再详细说明，但让我不解的是，L28用了个but表示转折，并引用代表新观点的Snyder等人的观点，指出咖啡因造成抑制phosphodiesterase的浓度，比它能产生兴奋的浓度要高很多----这不是和段落首句老观点：长久以来，咖啡因的作用被归为抑制phosphodieserase的合成 不谋而合吗？
gmat-rc公认逻辑严谨的，怎么会这样呢？难道我哪里理解错了？请大n指导。 | 
