The morphology and distribution of neurons containing choline acetyltransferase in the adult rat spinal cord: An immunocytochemical study

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Abstract
A monoclonal antibody to choline acetyltransferase (ChAT), the acetyl‐choline (ACh)‐synthesizing enzyme, has been used to localize ChAT within neurons in immunocytochemical preparations of adult rat spinal cord. Morphological details of known cholinergic spinal neurons are presented in this study, and previously unidentified ChAT‐containing neurons are also described. Immunoreactions product was present within cell bodies, dendrites, axons, and axon terminals, thereby allowing comprehensive descriptions of the distribution of ChAT‐positive neurons and the interrelationships of their processes. In the ventral horn, ChAT‐positive motoneurons were located in the medial, central, and lateral motor columns, and their dendrites formed elaborate longitudinal and transverse ChAT‐positive bundles. These bundles were present throughout the rostrocaudal extent of the spinal cord. In the central gray matter, small ChAT‐positive cell bodies were clustered around the central canal. Small longitudinal fascicles of immunoreactive processes were also observed in this region adjacent to the ependymal layer. The intermediate gray matter of virtually the entire spinal cord was spanned by medium to large ChAT‐positive multipolar cells termed partition neurons. At autonomic spinal levels, partition neurons were intermingled with other immunoreactive cells that were identified as preganglionic sympathetic or parasympathetic neurons because of their locations and morphological characteristics. In the sympathetic system, four groups of ChAT‐positive neurons were observed; the principal intermediolateral nucleus (ILp) in the lateral horn, the central autonomic cell column (CA) dorsal to the central canal, the intercalated nucleus (IC) located between ILp and CA, and the funicular intermediolateral neurons (ILf) in the white matter lateral to the ILp. The dendrites of ILp and CA neurons formed substantial longitudinal bundles within each group, as well as transverse bundles between the groups that resembled the rungs of a ladder. ChAT‐positive cell bodies were also present in the dorsal horn, and those located in laminae III–V extended dendrites dorsally into a longitudinal plexus within lamina III. ChAT‐positive fibers and terminal‐like structures (puncta) were present throughout the spinal cord. In the vental horn, such structures often were closely associated with large and medium‐size motoneuronal somata, but were rarely observed contacting those of small motoneurons. In regions between the motor columns, ChAT‐positive puncta were associated with small ChAT‐negative neurons that, by virtue of their location, might have been Renshaw cells. The head of the dorsal horn also contained ChAT‐positive puncta, and some of them were shown to be presynaptic terminals by electron microscopy. Many of these terminals were concentrated in two hands, a major one in lamina I11 and a lesser one in lamina I. On the basis of the present results, it seems likely that cholinergic neurons are more numerous and widespread in the spinal cord than previously thought, thus suggesting more diverse cholinergic functions than classically described within this part of the CNS.