Origins of spinal cholinergic pathways in amphibians demonstrated by retrograde transport and choline acetyltransferase immunohistochemistry.

The existence of propriospinal cholinergic pathways and the origin of supraspinal cholinergic descending projections have been investigated in anuran and urodele amphibians. Retrograde tract tracing techniques with dextran amines injected in the spinal cord at different levels were combined with immunohistochemistry for choline acetyltransferase (ChAT). The analysis of the brachial, thoracic and lumbar spinal cord demonstrated that doubly labeled cells were present only close to the injection site. Thus, the participation of the spinal cholinergic cells in distant intersegmental connections is not present, or is very limited, in amphibians. In anurans, tracer applications to the brachial cord revealed cholinergic cells of origin of spinal projections located in four distinct brain nuclei. The most rostrally located cells were found bilaterally in the preoptic area, among the magnocellular cells. In the ipsilateral isthmic region, the laterodorsal tegmental nucleus also showed doubly labeled cells. Throughout the brainstem, abundant codistribution was observed but actual coexistence of the tracer and ChAT was only found in the nucleus of the solitary tract and the inferior reticular nucleus. In the case of the urodele, abundant codistribution between retrogradely labeled cells and ChAT-positive neurons in zones like the suprachiasmatic nucleus, the isthmic region and the rhombencephalic reticular formation was observed, but the only doubly labeled cells were the Mauthner neurons. The present results in amphibians contrast with previous data in mammals in which is striking the presence of a widespread intrinsic cholinergic innervation of the spinal cord and the virtual absence of cholinergic projections descending from the brainstem.