Immunohistochemical Localization of -, - and K-Opioid Receptors Within the Antinociceptive Brainstem Circuits

Three functionally linked units, periaqueductal gray (PAG), rostral ventromedial medulla (RVM), and spinal cord dorsal horn are thought to comprise supraspinal pain suppression system. As it has been suggested, the activation of neurons in PAG excites RVM-spinal cord projecting neurons which results in the inhibition of nociceptive cells in the spinal cord (1 –3 ). PAG-RVM-spinal cord descending pathway is one of the targets of opiates mediating antinociception. It has been hypothesized that the analgesic effects of opiates are mediated indirectly through inhibition of inhibitory GABAergic neurons within the PAG-RVM-spinal cord circuit (2 ,4 –6 ).