Which of these is a neuropeptide that is generally inhibitory and can reduce sensations of pain?
options: a) acetylcholine b) tryptophan c) norepinephrine d) enkephalins e) serotonin f) substance P g) histamine
The correct answer and explanation is:
Correct Answer: d) enkephalins
Explanation (300 words):
Enkephalins are a type of neuropeptide, which are small protein-like molecules used by neurons to communicate with each other. Enkephalins specifically function as inhibitory neurotransmitters in the central nervous system. They belong to a larger group called endorphins or endogenous opioids—chemicals produced naturally by the body that reduce pain and promote a sense of well-being.
Enkephalins bind to opioid receptors in the brain and spinal cord, especially the mu and delta opioid receptors, which are responsible for modulating pain perception. When enkephalins are released in response to painful stimuli, they inhibit the release of substance P, a neurotransmitter that transmits pain signals to the brain. By blocking this pathway, enkephalins effectively reduce the sensation of pain and promote analgesia (pain relief).
Other Options Explained:
- a) Acetylcholine: A neurotransmitter involved in muscle activation and parasympathetic nervous system functions. It is typically excitatory, not inhibitory.
- b) Tryptophan: An amino acid, not a neurotransmitter or neuropeptide. It is a precursor to serotonin but not directly involved in pain modulation.
- c) Norepinephrine: A neurotransmitter and hormone that plays a role in the fight-or-flight response. It can have both excitatory and inhibitory effects depending on the receptor, but it’s not primarily associated with pain reduction.
- e) Serotonin: A monoamine neurotransmitter that regulates mood, appetite, and sleep. It may play a role in modulating pain, but not as directly or strongly as enkephalins.
- f) Substance P: This is the opposite of enkephalins; it transmits pain signals rather than inhibits them.
- g) Histamine: A compound involved in immune responses, also linked with inflammation and sometimes pain enhancement, not reduction.
Thus, enkephalins are the correct answer because they are inhibitory neuropeptides that reduce pain through opioid receptor binding.