Cholinergic transmission regulates extrajunctional acetylcholine receptors

To determine the role of ACh transmission in the regulation of extrajunctional ACh receptors, we compared the effect of postsynaptic cholinergic blockade with that of surgical denervation. Blockade of ACh transmission was produced in the soleus muscles of rats by continuous local infusion of α-bungarotoxin, delivered by implantable osmotic pumps. Extrajunctional ACh receptors were measured by an ¹²⁵I-α-BuTx binding method. Our results showed an increase of extrajunctional ACh receptors quantitatively equivalent to that resulting from surgical denervation. This denervation-like effect is attributed to elimination of (i) impulse-dependent ACh transmission (which normally triggers muscle usage), and (ii) spontaneous quantal and nonquantal ACh transmission. The influence of the nerve in regulating extrajunctional ACh receptors appears to be due to the sum of these forms of ACh transmission.