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Departments of
Pharmacology and Toxicology (J.J.C., J.D., M.R.V.)
and
Anesthesia (M.R.V.), Indiana University School of Medicine,
Indianapolis, Indiana 46202
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 a
Kd value of 0.3 ± 0.2 nM
and 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
and
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 the
Bmax 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.
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