Bennett, A., Kwok, C. H., Devonshire, I. M., & Hathway, G. J. (2014). Postnatal maturation of endogenous opioid systems within the periaqueductal grey and spinal dorsal horn of the rat. PAIN, 155(1), 168-178. https://doi.org/10.1016/j.pain.2013.09.022