MMP9 and Pain: In the early stages of neuropathic pain, a variety of neurotransmitter receptors (e.g., glutamate receptors, γ-aminobutyric acid receptors, and adrenoreceptors) are rapidly activated by signals released by the affected peripheral nerve endings and neurons in the dorsal horn (e.g., chemokines, neuregulin-1, matrix metalloproteinase-9, and colony stimulating factor-1) [28, 29].