Characterization of κ opiate receptors in rat spinal cord-dorsal root ganglion cocultures and their regulation by chronic opiate treatment

We have investigated the expression and regulation of κ opiate receptors in rat spinal cord-dorsal root ganglion primary cocultures. The density of opiate receptors increased markedly during the differentiation of the cultures; after 10 days in vitro the number of [³H]diprenorphine binding sites reached 244 ± 47 fmol/mg protein. Most of the binding sites were of the κ type, representing about 65-80% of total opiate receptors, while μ sites were expressed at a lower density (ca. 20% of total opiate sites). Following this period of development, the number of κ and μ receptors did not change significantly. No detectable δ sites were observed at any time of culture (up to 4 weeks in vitro). Chronic opiate agonist treatment (24 h) of the cultured cells with either 10 μM U50488 (a selective κ agonist), or 1 μM etorphine (a nonselective opiate agonist), did not change the number of κ receptors and their binding affinity to [³H]diprenorphine. On the other hand, 50% of the μ receptor sites down-regulated following 24 h treatment with 1 μM etorphine. Chronic antagonist exposure (5 days) with 10 μM naloxone, markedly up-regulated the μ receptors (261% of control), whereas κ sites exhibited a much weaker upregulation (164% of control). These data demonstrate that κ opiate receptors are expressed at high concentration in spinal cord-dorsal root ganglion cocultures and that contrary to μ sites, κ receptor density is less susceptible to modulation by chronic opiate treatment. The results also suggest that postreceptor components are important in regulating the κ receptor function following prolonged opiate exposure.