Which statement about Nucleosides is correct?

Question

BlackTom AI · Accepted Answer

Question restatement: Which statement about Nucleosides is correct?

Option 1: 'Nucleosides are usually obtained by chemical/enzymatic breakdown of polynucleic acids' 
- Here, the claim is that nucleosides come from the breakdown of nucleic acids. In biology, nucleic acids can be hydrolyzed to smaller units, and nucleotides can be dephosphorylated to nucleosides; thus, some nucleosides are indeed produced from polynucleic acids. However, the word 'usually' can be debated because nucleosides can also be formed de novo in metabolism or sourced from diet, and laboratory contexts often involve synthesis. The statement is not false by default, but it is not the most definitive or comprehensive description of nucleosides in physiology.

Option 2: 'Adenosine is a nucleoside neurotransmitter, serving as an excitatory neurotransmitter' 
- Adenosine is best described as a neuromodulator rather than a classic neurotransmitter, and its action is predominantly inhibitory in many synapses (via A1 receptors) and modulatory via A2A receptors, often producing sedative or anti-adrenergic effects rather than excitation. Additionally, adenosine’s general role is not “excitatory neurotransmitter”; this description mischaracterizes its net effect.

Option 3: 'Caffeine is an adenosine agonist, thus increases the action of adenosine and increases alertness' 
- This statement is the opposite of how caffeine works. Caffeine is a nonselective antagonist of adenosine receptors (especially A1 and A2A). By blocking adenosine signaling, caffeine reduces the inhibitory/suppressive effects of adenosine, leading to increased alertness. The claim that caffeine is an adenosine agonist is factually incorrect.

Option 4: 'Nucleoside modulates the release of other transmitters' 
- This option reflects a recognized concept: nucleosides (notably adenosine) act as neuromodulators that can modulate the release of various neurotransmitters such as dopamine, GABA, glutamate, and acetylcholine. Adenosine often exerts inhibitory control over neurotransmitter release through its receptor subtypes (e.g., A1 reducing release). This modulatory role aligns with established physiology and pharmacology, making the statement accurate.

Why the other options are not correct in plain terms:
- Option 2 mislabels adenosine’s role as an excitatory neurotransmitter, ignoring its predominant inhibitory/modulatory actions and the broader neuromodulatory context.
- Option 3 inaccurately describes caffeine as an agonist at adenosine receptors, when it is actually an antagonist, so its effect is to oppose adenosine signaling and promote wakefulness through disinhibition rather than by enhancing adenosine action.
- Option 1 is arguably true in a broad sense but is less precise about the physiological role and the typical context in which nucleosides function as neuromodulators; the statement about modulation of transmitter release (Option 4) captures a core, well-established function of nucleosides in the nervous system.

Putting it together: the statement that aligns with established neurochemical roles of nucleosides, especially adenosine as a neuromodulator influencing transmitter release, is captured in Option 4. The other options either misstate the physiological role of adenosine, mischaracterize caffeine’s pharmacology, or provide a description that is less central to the functional nature of nucleosides in neural signaling.

Which statement about Nucleosides is correct?单项选择题

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