Specific binding of the muscarinic antagonist [3H]quinuclidinyl benzilate is not associated with preganglionic motor neurons in the dorsal motor nucleus of the vagus.

The present study evaluates the binding of [3H]quinuclidinyl benzilate, [3H]QNB, as a measure of cholinergic muscarinic binding in six areas of the rat medulla oblongata associated with the cranial nerves. In an experimental group, the right vagus nerve was severed in the neck in order to determine whether the specific muscarinic binding sites might be located on cells that contribute efferent fibers to the vagus nerve. The level of activity of choline acetyltransferase (ChAT) also was determined in the same six areas. Additional experiments utilizing the retrograde transport of toxic ricin, a 60,000 dalton agglutinin that acts as a potent ribosomal toxin, was carried out to further evaluate localization of specific muscarinic binding in the DMN after destruction of the preganglionic efferent cells. These results support the conclusion that specific binding of the muscarinic antagonist [3H]QNB observed in the DMN of the vagus of the rat is not associated with the large cells that contribute efferent fibers into the vagus nerve. We suggest that the specific cholinergic muscarinic binding is located on interneuronal cell surfaces, on afferent terminals of local circuit neurons, or on afferent terminals of long projection axons which arise from neurons in the brainstem, hypothalamus, or forebrain.