Substance P is best described as which of the following?

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Substance P functions as a neurotransmitter in the pain pathway. It is released from the terminals of certain neurons in response to a signal, crosses the synaptic cleft, and binds to receptors on the postsynaptic neuron to propagate the pain signal. This local, synaptic signaling pattern is the defining feature of a neurotransmitter. It is not a hormone, which would travel through the bloodstream to affect distant targets, nor an enzyme, which would catalyze chemical reactions, nor a receptor, which would be the protein receiving a signal rather than the signaling molecule itself. As a neuropeptide, Substance P can also modulate pain signaling, and its action can be influenced by NK1 receptor antagonists used to dampen pain.