[医学类]

Until about five years ago, the very idea that peptide hormones might be made anywhere in the brain besides the hypothalamus was astounding. Peptide hormones, scientists thought, were made by endocrine glands and the hypothalamus was thought to be the brains’ only endocrine gland. What is more, because peptide hormones cannot cross the blood-brain barrier, researchers believed that they never got to any part of the brain other than the hypothalamus, where they were simply produced and then released into the bloodstream.

But these beliefs about peptide hormones were questioned as laboratory after laboratory found that antiserums to peptide hormones, when injected into the brain, bind in places other than the hypothalamus, indicating that either the hormones or substances that cross-react with the antiserums are present. The immunological method of detecting peptide hormones by means of antiserums, however, is imprecise. Cross-reactions are possible and this method cannot determine whether the substances detected by the antiserums really are the hormones, or merely close relatives. Furthermore, this method cannot be used to determine the location in the body where the detected substances are actually produced.

New techniques of molecular biology, however, provide a way to answer these questions. It is possible to make specific complementary DNA’s (c DNA’s) that can serve as molecular probes seek out the messenger RNA’s (mRNA’s) of the peptide hormones. If brain cells are making the hormones, the cells will contain these mRNA’s. If the products the brain cells make resemble the hormones but are not identical to them, then the c DNA’s should still bind to these mRNA’s, but should not bind as tightly as they would to m RNA’s for the true hormones. The cells containing these mRNA’s can then be isolated and their mRNA’s decoded to determine just what their protein products are and how closely the products resemble the true peptide hormones.

The molecular approach to detecting peptide hormones using cDNA probes should also be much faster than the immunological method because it can take years of tedious purifications to isolate peptide hormones and then develop antiserums to them. Roberts, expressing the sentiment of many researchers, states: “I was trained as an endocrinologist. But it became clear to me that the field of endocrinology needed molecular biology input. The process of grinding out protein purifications is just too slow.”

If, as the initial tests with cDNA probes suggest, peptide hormones really are made in brain in areas other than the hypothalamus, a theory must be developed that explains their function in the brain. Some have suggested that the hormones are all growth regulators, but Rosen’s work on rat brains indicates that this cannot be true. A number of other researchers propose that they might be used for intercellular communication in the brain.

1. Which of the following titles best summarizes the text?

[A] Is Molecular Biology the Key to Understanding Intercellular Communication in the Brain?

[B] Molecular Biology: Can Researchers Exploit Its Techniques to Synthesize Peptide Hormones?

[C] The Advantages and Disadvantages of the Immunological Approach to Detecting Peptide Hormones.

[D] Peptide Hormones: How Scientists Are Attempting to Solve Problems of Their Detection and to Understand Their Function?

2. The text suggests that a substance detected in the brain by use of antiserums to peptide hormones may

[A] have been stored in the brain for a long period of time.

[B] play no role in the functioning of the brain.

[C] have been produced in some part of the body other than the brain.

[D] have escaped detection by molecular methods.

3. According to the text, confirmation of the belief that peptide hormones are created in the brain in areas other than the hypothalamus would force scientists to

[A] reject the theory that peptide hormones are made by endocrine glands.

[B] revise their beliefs about the ability of antiserums to detect peptide hormones.

[C] invent techniques that would allow them to locate accurately brain cells that produce peptide hormones.

[D] develop a theory that account for the role played by peptide hormones in the brain.

4. Which of the following is mentioned in the text as a drawback of the immunological method of detecting peptide hormones?

[A] It cannot be used to detect the presence of growth regulators in the brain.

[B] It cannot distinguish between the peptide hormones and substances that are very similar to them.

[C] It uses antiserums that are unable to cross the blood-brain barrier.

[D] It involves a purification process that requires extensive training in endocrinology.

5. The idea that the field of endocrinology can gain from developments in molecular biology is regarded by Roberts with

[A] incredulity.

[B] derision.

[C] indifference.

[D] enthusiasm.

1. 【答案】D

【考点解析】这是一道中心主旨题。全文从头至尾讨论的是“peptide hormones”在人体内产生的部位，所以有关全文中心主旨内容的答案应该包含“peptide hormones”。从各段的主题句进行分析，第一至四段主要讲如何“detect”(探测)肽激素(peptide hormones)所产生的位置，第五段主要讲有关肽激素的“function”。可见本题的正确选项应该是D。考生在解题时一定要搞清楚原文所涉及的对象并且抓住每段的主题句以及它们之间的相互关系。

2. 【答案】C

【考点解析】这是一道审题定位题。根据题干中的“a substance detected”可把本题的正确选项迅速确定在第二段的尾句。该句所涉及的核心问题是“where”，所以本题的正确选项应该是C。考生在解题时一定要迅速而准确地进行审题定位。

3. 【答案】D

【考点解析】本题是一道细节推导题。通过本题题干中的“peptide hormones are created in the brain in areas other than the hypothalamus”可将本题的答案信息来源迅速确定在尾段的第一句。尾段第一句主要就肽激素的“function”进行论述，可见本题的正确选项应该是D，选项D中的“role”就等于原文中的“function”。考生在解题时一定要善于抓住主句中的重要信息。

4. 【答案】B

【考点解析】这是一道句间关系题。根据本题题干中的“the immunological method”可将本题的答案信息来源迅速确定在第二段的第二句，而本题的确切答题点在第二段的第三句的后半部分。从第二段第三句的后半部分可以推导出本题的正确答案是B。考生在解题时一定要注意一般概括句和具体陈述句之间的相互关系。

5. 【答案】D

【考点解析】本题是一道细节推导题。根据本题题干中的人名“Roberts”可将本题的答案迅速确定在倒数第二段引号部分的第二句话，即“But”一词引导的句子。从该句中的“needed”一词可以看出本题的正确选项应该是D。考生在解题时一定要学会深入理解原文的字面含义。

[参考译文]

肽激素除了下丘脑能制造，在大脑中任何其它的地方都能够制造。大约五年前仅这一想法本身就是令人惊诧的。科学家认为，肽激素是由内分泌腺制造的，而下丘脑被认为是大脑中唯一的内分泌腺。而且，由于肽激素无法穿过血脑障碍，研究人员认为它们从不曾到过除下丘脑以外的大脑任何其它部位，肽激素仅在下丘脑制造出来，然后被释放到血管中。

但是关于肽激素的这种观点已经遭到质疑。通过一次又一次的实验发现，肽激素的抗血清一旦被注射到大脑中，它就会在下丘脑以外的地方粘接起来。这就说明这些地方或是有肽激素存在，或是有与抗血清发生交叉反应的其他物质存在。但是，通过抗血清来检验肽激素的免疫学方法是不精确的。交叉反应可能会发生，而且以这种手段无法确认用抗血清检测的特质确实是肽激素还是仅是与其近似的亲缘物质。另外，这种方法不能用来确定被测物质在人体内产生的部位。

然而，分子生物学的新技术为解决这些问题提供了一个新途径。科学家可以制造出一种特别的互补DNA’S (cDNA’s)，作为分子探子查找出肽激素的信使RNA’S (mRNA’s)。如果脑细胞正在制造肽激素，那么它应该包含这些信使RNA’S。如果脑细胞制造的产品与肽激素相似但并不完全相同，那么这些互补cDAN’S仍然会和这些信使mRNA’S粘结，但不会象和真正肽激素的信使mRNA’S结合得那么紧密，这些包含信使mRNA’S的细胞能被分开。研究者可以将信使mRNA’S解码以确定其蛋白质产品究竟是什么及这些产品在多大程度上类似于真正的肽激素。

采用cDAN探子这一分子生物学方法检测肽激素同时也比免疫学方法快得多，因为如果用免疫学方法，分离肽激素需要几年枯燥乏味的提纯过程，然后还需培养出他们的抗血清。罗伯茨的一番话表达了许多研究人员的心声，他说：“我是作为一名内分泌学家接受训练的，但情况对我来说很清楚，内分泌学领域需要分子生物学的输入，靠碾磨来制造蛋白质纯化物的过程实在是太慢了。”

如果正如用cDNA探子所做的最初测试表明的那样，肽激素确实是由大脑中下丘脑以外的部位制造出来的，则有必要建立一套理论来解释它们在大脑中的作用。某些学者指出肽激素是生长调节剂，但罗森对老鼠大脑所作的实验表明事实并非如此。很多其它的研究人员指出肽激素或许被用于大脑内细胞与细胞间的信息传输。