Expression of calcitonin gene-related peptide, substance P and protein kinase C in cultured dorsal root ganglion neurons following chronic exposure to mu, delta and kappa opiates.

The mechanisms involved in morphine tolerance are poorly understood. It was reported by our group that calcitonin gene-related peptide (CGRP)-like immunoreactivity (IR) was increased in the spinal dorsal horn during morphine tolerance [Ménard et al. (1996) J. Neurosci. 16, 2342-2351]. More recently, we observed that it was possible to mimic these results in cultured dorsal root ganglion (DRG) neurons allowing for more detailed mechanistic studies [Ma et al. (2000) Neuroscience 99, 529-539]. The aim of the present series of experiments was to further validate the DRG cell culture model by establishing which subtypes of opioid receptors are involved in the induction of CGRP in cultured rat DRG neurons, and to examine the signaling pathway possibly involved in the induction of CGRP-like IR following repeated opiate treatments. Other neuropeptides known to be expressed in DRG neurons, such as substance P (SP), neuropeptide Y (NPY) and galanin, were investigated to assess specificity. Following treatment with any of the three opioid agonists (mu, DAMGO; delta, DPDPE; kappa, U50488H), the number of CGRP- and SP-IR cultured DRG neurons increased significantly, and in a concentration-dependent manner, with the effects of kappa agonist being less pronounced. NPY and galanin were not affected.Double-immunofluorescence staining showed that the three opioid receptors were co-localized with both CGRP- and SP-like IR.Protein kinase C (PKC)-like IR was found to be significantly increased following a repetitive treatment with DAMGO. Double-immunofluorescence staining showed the co-localization of PKCalpha with CGRP- and SP-IR in cultured DRG neurons. Moreover, a combined treatment with DAMGO and a PKC inhibitor (chelerythrine chloride or Gö 6976) was able to block the effects of the opioid on increased CGRP-like IR. These data suggest that the three opioid receptors may be involved in the induction of CGRP and SP observed following chronic exposure to opiates, and that PKC probably plays a role in the signaling pathway leading to the up-regulation of these neuropeptides. These findings further validate the DRG cell culture as a suitable model to study intracellular pathways that govern changes seen following repeated opioid treatments possibly leading to opioid tolerance.