Question

The so far only known endogenous NMDA receptor antagonist is: a. glutamate b. glycine c. ketamine d. kynurenic acid e. quinolinic acid

Short answer

Answer: Kynurenic Acid

Step-by-step solution

Understanding NMDA receptors and antagonists

N-Methyl-D-aspartate (NMDA) receptors are specialized ion channels in neurons (nerve cells) that play important roles in synaptic plasticity and memory function. Antagonists are substances that block or reduce the activity of a receptor - in this case, the NMDA receptor. An endogenous NMDA receptor antagonist means it is produced within the body (as opposed to a drug that is introduced from outside the body).

Analyzing the possible answers

a. Glutamate: This is an excitatory neurotransmitter and is, in fact, an agonist (activator) of NMDA receptors, not an antagonist. b. Glycine: This is a co-agonist of the NMDA receptor, meaning it works with glutamate to activate the receptor. Thus, it is not an antagonist. c. Ketamine: While ketamine is an NMDA receptor antagonist, it is not an endogenous compound, as it is a synthetic drug used as an anesthetic and for other purposes. d. Kynurenic Acid: Kynurenic acid is an endogenous compound that has been shown to act as an NMDA receptor antagonist. e. Quinolinic Acid: Quinolinic acid is an endogenous compound and an agonist of the NMDA receptor, not an antagonist.

Identifying the correct answer

Based on our analysis, the correct answer to this question is (d) kynurenic acid, as it is the only endogenous NMDA receptor antagonist listed in the options.