Regional saturation studies of [3H]naloxone binding in the naive, dependent and withdrawal states.

We have examined the saturation features (Kd and Bmax) of [3H]naloxone binding in rat brain regions in cytoarchitectonically intact tissues of naive, morphine dependent and precipitated withdrawal states using quantitative film autoradiography. Sixty-one saturation experiments in 13 regions of naive rat brains yielded monophasic Eadie-Hofstee plots with a mean (+/- S.D.) Kd of 1.87 +/- 0.87 nM and a mean Bmax (+/- S.D.) of 101 +/- 66 fmol/mg. The 61 Kd values in naive rats described a normal distribution of regional binding affinities that may reflect the biological variation of a single high affinity binding site. Similar studies in the morphine dependent and precipitated withdrawal states showed no apparent changes in either the Kd or Bmax of regional [3H]naloxone binding. The possibility that in vitro preincubation of tissue sections masked potential alterations in [3H]naloxone binding was examined in competition studies of both morphine and naloxone for 2.0 nM [3H]naloxone binding to striatal sections. Preincubation had no effect on the ability of either opiate agonist or antagonist to compete for [3H]naloxone binding in the naive, morphine dependent or precipitated withdrawal states. The regional data comparing Kd and Bmax for [3H]naloxone binding in naive, morphine dependent and precipitated withdrawal states eliminated the possibility that previous efforts to demonstrate alterations in opiate binding were confounded by homogenization and/or limitations of gross dissection. Our data suggest that either potential changes in antagonist binding in dependent and withdrawal states can only be demonstrated under strict in vivo conditions or that post-binding transduction mechanisms are the sites of adaptive changes in naloxone sensitivity in the states of opiate dependence and withdrawal.