Final answer:
Histamine, synthesized from histidine, serves as a paracrine signaling molecule and neurotransmitter across various organisms.
Step-by-step explanation:
The molecule synthesized from the amino acid histidine, which functions as a paracrine signaling molecule and neurotransmitter in both invertebrates and vertebrates, is histamine.
Histamine plays a critical role in the body, including the regulation of physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. It is involved in the immune response during allergic reactions as a mediator of itching and inflammation.
Derived from histidine through a decarboxylation reaction, histamine's effects are mediated through its binding to histamine receptors, which are found in various tissues throughout the body. Its synthesis and release can be triggered by various stimuli, including allergens or injury. While commonly known for its role in allergic responses and its effects which can be counteracted by antihistamines, histamine also functions within the nervous system as a neurotransmitter.