Heterogeneous binding of [3H]4-DAMP to muscarinic cholinergic sites in the rat brain: evidence from membrane binding and autoradiographic studies.

The present study shows that [3H]4-DAMP binds specifically, saturably, and with high affinity to muscarinic receptor sites in the rat brain. In homogenates of hippocampus, cerebral cortex, striatum, and thalamus, [3H]4-DAMP appears to bind two sub-populations of muscarinic sites: one class of high-affinity, low capacity sites (Kd less than 1 nM; Bmax = 45-152 fmol/mg protein) and a second class of lower-affinity, high capacity sites (Kd greater than 50 nM; Bmax = 263-929 fmol/mg protein). In cerebellar homogenates, the Bmax of [3H]4-DAMP binding sites was 20 +/- 2 and 141 +/- 21 fmol/mg protein for the high- and the lower-affinity site, respectively. The ligand selectivity profile for [3H]4-DAMP binding to its sites was similar for both the high- and lower-affinity sites; atropine = (-)QNB = 4-DAMP much greater than pirenzepine greater than AF-DX 116, although pirenzepine was more potent (16-fold) at the lower- than at the high-affinity sites. The autoradiographic distribution of [3H]4-DAMP sites revealed a discrete pattern of labeling in the rat brain, with the highest densities of [3H]4-DAMP sites present in the CA1 sub-field of Ammon's horn of the hippocampus, the dentate gyrus, the olfactory tubercle, the external plexiform layer of the olfactory bulb and layers I-II of the frontoparietal cortex. Although the distribution of [3H]pirenzepine sites was similar to that of [3H]4-DAMP sites in many brain regions, significant distinctions were apparent. Thus, both the ligand selectivity pattern of [3H]4-DAMP binding and the autoradiographic distribution of sites suggest that although the high-affinity [3H]4-DAMP sites may consist primarily of muscarinic-M3 receptors, the lower-affinity [3H]4-DAMP sites may be composed of a large proportion of muscarinic-M1 receptors.