Mb>2 b> binding and m2 receptors in experimental monkey neocortex
摘要
Mb>2 b> binding modulates presynaptic ACh release but is not altered in rodents with lesions that remove all cholinergic inputs to posterior cingulate cortex (PCC), including layer V where these axons terminate. Since Mb>2 b> binding and m2 receptors may be postsynaptic in primates, changes in Mb>2 b> binding in Alzheimer's disease (AD) may not be due to presynaptic, cholinergic changes. Unilateral cingulumotomy lesions were placed in 5 monkeys and PCC assayed with AF-DX 384 binding in competition with pirenzepine (PZ). Under autoradiographic conditions (i.e., 2 nM AF-DX 384; 100 nM PZ) PZ-insensitive binding is comprised of 65%m2 and 24%m4 binding based on published Kb>d b> values and m1-m5 receptor densities for human cortex. Immunohistochemistry for choline acetyltransferase (ChAT) showed that there are no intrinsic cholinergic neurons in PCC and that the lesion completely removed ChAT-ir axons. In 3 animals, the Kb>H b> was 4.97±1.5 nM and the Kb>L b> was 210±43. The density of PZ-insensitive AF-DX 384 binding was not changed. The Bb>max b> in control hemispheres was 164±31 fmol/mg protein and in ablated hemispheres it was 186±26. Autoradiography of PCC allows for laminar analysis of binding in control and ablated hemispheres. No changes in binding were detected in any layers, including layer V. Finally, reverse transcriptase-PCR was used to isolate m2 mRNA from control and ablated hemispheres. There were no changes in mRNA levels for this receptor in the ablated hemisphere. CONCLUSIONS: 1) Presynaptic Mb>2 b> binding and m2 receptors do not make a major contribution to total Mb>2 b> binding in primate PCC. 2) Most changes in Mb>2 b> binding in AD (increases and decreases) may be due to postsynaptic changes associated with cortical neuron degeneration not presynaptic, cholinergic afferents.