The ultrastructure of serotonergic projections to the dorsal horn of the rat spinal cord has been investigated, using a highly specific polyclonal antiserum. The highest concentrations of immunoreactive profiles were found in lamina I and the outer part of lamina II (IIo). Intermediate concentrations were found in laminae III and IV, while the inner part of lamina II (IIi) was almost devoid of immunoreactivity. Whereas 60% of the profiles show at least one varicosity studded with synaptic vesicles, only one-fifth of the latter contributes to classical synapses, the remaining profiles being devoid of a facing postsynaptic density. The results are compared with those in the literature and our own results relative to other regions of the cord. It is concluded that the pauci-synaptic projections to the dorsal horn could correspond to a diffuse influence of serotonin, the targets for which are determined by the corresponding serotonergic receptors.

We have investigated with light and electron microscope immunocytochemistry the aminergic and peptidergic innervation of Onuf's nucleus in adult baboons. This nucleus, located in the ventrolateral part of the sacral spinal cord (S2 and S3), is considered to control urethral and anal sphincters and penile muscles. By comparison of intact and transected spinal cords, we have found that serotoninergic innervation has two origins: first, supraspinal, innervating the whole nucleus, with a possible predominance in the dorsal half; and second, intraspinal, corresponding to the ventral half of the nucleus. Thyrotropin-releasing hormone innervation appears largely coincident with serotonin, both in intact and transected spinal cords. Noradrenaline is exclusively of supraspinal origin, as attested by its disappearance below the level of the section. Substance P, calcitonin gene-related peptide, and Leu- and Met-enkephalin, which profusely innervate Onuf's nucleus, are on the contrary not affected by the transection. They most likely originate from the cord itself or the dorsal root ganglia. Thus, Onuf's nucleus innervation in the baboon arises both from supraspinal and intraspinal sources. The present study provides an anatomical basis for both voluntary and reflex controls of excretory and sexual functions in a primate. The same neurotransmitter (serotonin) according to its cell origin and discrete topography could exert different influences upon the same effector system.

The mapping of noradrenergic innervation was performed in transverse and longitudinal sections of the adult rat spinal cord using noradrenaline immunocytochemistry. Noradrenergic fibres and terminals were distributed in the dorsal horn (mainly in the superficial part), in the vicinity of the different groups of motoneurons, and concentrated in the intermediolateral cell column and around the central canal. The ultrastructural study showed principally axodendritic synapses in the ventral horn and in the intermediolateral cell column. Fewer axosomatic synapses were detected. In the dorsal horn, noradrenaline-innervation was predominantly non-synaptic. It is hypothesized that the noradrenergic modulation of nociception is not mediated through classical synapses. The concept of 'volume transmission' can explain such an influence. Conversely, noradrenaline may be involved in the control of locomotion and automatic functions through conventional synapses.