A chemical is termed a neurohormone when it is:

A neurohormone is specifically defined by its production by neurons and its subsequent release into the bloodstream, where it acts on distant targets much like traditional hormones do. This distinction is important because it highlights the dual role of these substances in both the nervous system (where they are released) and the endocrine system (where they circulate and exert effects).

For clarity, while other options mention the involvement of neurons and endocrine glands, they do not precisely fit the definition of a neurohormone. The second option refers to a chemical that is released by an endocrine gland and affects neurons, which does not capture the essence of a neurohormone. The third option implies that neurohormones can be released from both neurons and endocrine glands, which again deviates from the correct definition, as neurohormones are exclusively released by neurons. Lastly, the fourth option suggests uncertainty in which system is responsible for the chemical's communication, which does not align with the established understanding of neurohormones functioning within the clearly defined frameworks of neuronal and endocrine signaling.