The brainstem origin of preganglionic fibers to the sphenopalatine ganglion in rat was revealed by the aid of the retrograde axonal tracer True Blue (which does not traverse to a second order neuron) applied deep in the sphenopalatine ganglion or the Vidian nerve on one side. The majority of fibers originate in the ipsilateral lacrimo-muconasal nucleus in the ventrolateral rostral medulla oblongata and caudal pons. A smaller number of fibers originate more dorsomedially and caudally in the medullary reticular formation. After application to the ganglion a third small group of labelled neurons was found more rostrally in the brainstem, in the reticular formation ventrolateral to the caudal part of the dorsal raphe nucleus. Simultaneous visualization of catecholaminergic nerves revealed that the labelled neurons in the lacrimo-muconasal nucleus were heavily innervated by catecholaminergic fibers. It appears from previous studies that the preganglionic neurons may not be cholinergic. None of the labelled neurons in the brainstem stained positively for catecholamines. Thus, further studies are required to elucidate the transmitter(s) used in these neurons.
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