The cat locus coeruleus. Light and electron microscopic study of the neuronal somata.

The neuronal cell bodies of the locus coeruleus (LC) and subcoeruleus (SC) of the cat were investigated using Nissl and Golgi preparations, and electron microscopy. On the basis of morphological criteria--size and shape of cell body, branching pattern of dentrites, distribution of cytoplasmic organelles and number of axosomatic synapses--four types of neuronal perikarya were recognized in each region: medium-sized, small-sized and two groups of intermediate-sized neurons. The medium-sized neurons (30--50 micron) had an elongated cell body, thick dendrites with a moderate number of branchings, abundant organelles arranged in concentric rings around the nucleus and a moderate number of axosomatic synapses. They were found throughout the LC and SC and most probably correspond to the larger class of catecholaminergic neurons demonstrated by fluorescence histochemistry. The small neurons (10--25 micron) were also seen in both LC and SC and are believed to represent non-monoaminergic local interneurons. They displayed sparsely branching dendrites and a thin rim of cytoplasm containing few organelles. In the SC, some of these cells occurred in closely associated pairs. Ultrastructural analysis of such pairs revealed a close apposition (80--100 A degrees) of the cell membranes for long distances (up to 10 micron) and a narrowing of the intercellular space (30--40 A degrees) at some discrete points, perhaps indicative of an electrical interaction. The intermediate-sized neurons exhibited some regional morphological differences, but two distinct subgroups could be distinguished. One was characterized by a low number of axosomatic synapses, while the other exhibited a high number of such contacts. It may be assumed that the two subgroups of intermediate-sized neurons comprise catecholaminergic and indolaminergic neurons.