Which neurotransmitter is known to inhibit libido?

The neurotransmitter that is known to inhibit libido is serotonin. Serotonin plays a significant role in the regulation of mood, anxiety, and appetite, but it also has a well-established impact on sexual function. Increased levels of serotonin are often associated with reduced sexual desire and libido. This effect is particularly noted in individuals treated with selective serotonin reuptake inhibitors (SSRIs), which are commonly used as antidepressants; one of the side effects of these medications is a decrease in sexual desire.

In contrast, dopamine, often referred to as the "feel-good" neurotransmitter, is associated with reward and pleasure, and is known to enhance libido. Ephedrine, a sympathomimetic drug, primarily affects the adrenergic receptors, and while it can elevate energy levels, it does not directly inhibit libido. Histamine is involved in various physiological functions, including immune responses and neurotransmission, but it does not have a clear role in dampening sexual desire.

Thus, serotonin’s inhibitory effect on libido makes it the correct answer to the question.