Regulation of Opioid Receptors in Rat Sensory Neurons in Culture

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Abstract
To determine whether opioid receptors in sensory neurons are regulated by chronic exposure to opioids, we assessed the binding of various opioid ligands to membranes derived from isolated rat dorsal root ganglia neurons grown in culture. Equilibrium binding of [3H]diprenorphine onto membranes from cells grown for 13–15 days revealed a saturable binding site with aKd value of 0.3 ± 0.2 nmand an approximate Bmax value of 1300 ± 200 fmol/mg of protein. [3H]Diprenorphine binding increased 3-fold from 1–15 days in culture. The μ receptors represent ∼70 ± 11% of the [3H]diprenorphine binding sites, as indicated by saturation binding of [3H]DAMGO. The κ and δ receptors represent ∼10 ± 3% and ∼5 ± 2% of the [3H]diprenorphine binding sites, respectively. Preexposure of neurons to 10 μm naloxone for 48 hr up-regulated the receptors by 40%, whereas incubation with 100 nm to 10 μm DAMGO for 48 hr resulted in a significant decrease in theBmax value of opioid receptors, with a maximum reduction of 70%. The identification of a high level of opioid receptors expressed in isolated sensory neurons and their modulation by opioids demonstrates that cultured sensory neurons are an excellent model with which to study opioid receptor regulation.