Fine structure of noradrenergic terminals and their synapses in the rat spinal dorsal horn: an immunohistochemical study.

Noradrenergic fibers in the spinal dorsal horn originate from neurons in the A5-7 cell groups, and may participate in the modulation of pain. Here we studied the fine structure of noradrenergic terminals in the rat by immunohistochemistry using antiserum against dopamine-beta-hydroxylase (DBH). We also investigated the relationship between such terminals and primary afferent terminals. DBH-like immunoreactive terminals were found in lamina I and the outer layer of lamina II of the dorsal horn and they contained many clear round vesicles and some large granular vesicles. More than half of these terminals made synaptic contact with other neuronal elements with membrane specialization. Most of the postsynaptic structures of these terminals were small dendrites (69%); 28% were spines, and no synaptic contact was made with primary afferent terminals. These findings suggest that noradrenaline acts on the spinal dorsal horn neurons postsynaptically mainly via a direct synaptic mechanism.