Responses to pain induce activity in antinociceptive pathways. This activity begins when pain information transmitted by the spinothalamic tract reaches the brainstem and thalamus (A above). Activation of periaqueductal gray and the nucleus raphe magnus induces endorphin and enkephalin release and binding to "opioid" receptor systems. Sympathetic and parasympathetic influences within the spinal cord facilitate inactivation of antinociceptive pathways. Most of the endorphin and enkephalin receptors (70%) are localized presynaptically, substantial pain signal attenuation occurs before information reaches the dorsal horn (B above). Such information may be further attenuated by enkephalin-induced dynorphin activity at the level the cord (C above).
Dynorphin activates -type opioid receptors localized on inhibitory interneurons, activation of which induces release of the inhibitory neurotransmitter GABA. The mechanism by which -opioid receptor activation limits spinal cord cellular activity may be by means of closure of N-type Ca2+ channels. Interaction of GABA with its receptor results in dorsal horn neuronal hyperpolarization thus impeding transmission of the pain information. Reduction of visceral pain may occur particularly by this approach.
Enkephalin binds to -type opioid receptors which appear on nociceptive neurons when they actively transmit pain information. Furthermore, these receptors are often localized on presynaptic vesicles that contain neurotransmitter and following release receptor protein is incorporated into presynaptic membrane. Active nociceptors, because of preferential binding, are therefore more sensitive than inactive nociceptive receptors to endogenous opiates. This idea may be relevant in explaining how opioid analgesics appear to relieve ongoing pain but do not prevent sensing of pain subsequent to new injuries.