Down-regulation of μ-opioid receptors in rat and monkey dorsal root ganglion neurons and spinal cord after peripheral axotomy

Abstract

To understand the role of opioids and their receptors in chronic pain following peripheral nerve injury, we have studied the μ-opioid receptor in rat and monkey lumbar 4 and 5 dorsal root ganglion neurons and the superficial dorsal horn of the spinal cord under normal circumstances and after peripheral axotomy. Our results show that many small neurons in rat and monkey dorsal root ganglia, and some medium-sized and large neurons in rat dorsal root ganglia, express μ-opioid receptor-like immunoreactivity. Most of these neurons contain calcitonin gene-related peptide. The μ-opioid receptor was closely associated with the somatic plasmalemma of the dorsal root ganglion neurons. Both μ-opioid receptor-immunoreactive nerve fibers and cell bodies were observed in lamina II of the dorsal horn. The highest intensity of μ-opioid receptor-like immunoreactivity was observed in the deep part of lamina II. Most μ-opioid receptor-like immunoreactivity in the dorsal horn originated from spinal neurons. A few μ-opioid receptor-positive peripheral afferent terminals in the rat and monkey dorsal horn were calcitonin gene-related peptide-immunoreactive. In addition to pre- and post-junctional receptors in rat and monkey dorsal horn neurons, μ-opioid receptors were localized on the presynaptic membrane of some synapses of primary afferent terminals in the monkey dorsal horn. Peripheral axotomy caused a reduction in the number and intensity of μ-opioid receptor-positive neurons in the rat and monkey dorsal root ganglia, and of μ-opioid receptor-like immunoreactivity in the dorsal horn of the spinal cord. The decrease in μ-opioid receptor-like immunoreactivity was more pronounced in the monkey than in the rat dorsal root ganglia and spinal cord. It is probable that there was a parallel trans-synaptic down-regulation of μ-opioid-like immunoreactivity in local dorsal horn neurons of the monkey. These data suggest that one factor underlying the well known insensitivity of neuropathic pain to opioid analgesics could be due to a marked reduction in the number of μ-opioid receptors in the axotomized sensory neurons and in interneurons in the dorsal horn of the spinal cord.