Three-dimensional analysis of the dendritic domains of on- and off-cells in the rostral ventromedial medulla.

The rostral ventromedial medulla contains two classes of physiologically defined neurons, on-cells and off-cells, that are implicated in nociceptive modulation. In a continuing effort to detail the neural circuitry that underlies the activity of these two distinct neuronal types, the three-dimensional somatodendritic morphology of on- and off-cells was studied in the cat and the rat. Following physiological characterization with intracellular recording, on- and off-cells were injected with horseradish peroxidase and sectioned in the coronal plane. Their somatodendritic arborizations were reconstructed with the aid of a computerized three-dimensional reconstruction program. There were no differences between on- and off-cells in any somatodendritic feature analyzed. On- and off-cells possess dendritic arbors that are elongated in the mediolateral axis relative to the dorsoventral and rostrocaudal axes. In the rat, the average dendritic extent was more than 1,400 microns mediolaterally but only about 800 microns in either the dorsoventral or rostrocaudal axis. All somatic and dendritic measures were greater for both cat cells than for any rat neuron. Dendrites and their tips were located predominantly in regions lateral to the soma. The center of total dendritic length was usually located within 150 microns of the soma, most frequently ventral and lateral to the soma. The present results demonstrate that the dendritic domains of neurons in the nuclei raphe magnus and reticularis paragigantocellularis pars alpha are not contained within cytoarchitechtonic boundaries. On- and off-cell dendrites are interdigitated throughout the mediolateral extent of the rostral ventromedial medulla. Since on- and off-cell dendritic domains occupy a restricted rostrocaudal plane, afferents to the rostral ventromedial medulla that are confined to a single coronal plane and those that collateralize at several levels are likely to play different roles in the control of on- and off-cell activity.