Ultrastructural localization of immunoreactive neurotensin in the monkey superficial dorsal horn

Neurotensin, a tridecapeptide, has been proposed to have a role in sensory systems, especially those mediating pain. The light microscopic and ultrastructural localization of neurotensin immunoreactivity in neurons of the monkey spinal cord was studied with the aim of examining their synaptic interactions. At the light microscopic level, neurotensin-containing cells were located in laminae II and III and immunoreactive axons and terminals were found in laminae I, II, and III. Neurotensin-positive axons were mostly thin and unmyelinated and their boutons contained both clear and large granular vesicles. Boutons varied considerably in size (1-3 micron) and in their relative content of large granular vesicles, which appeared occasionally in presynaptic locations. In lamina I neurotensin-immunoreactive terminals formed synapses with cell bodies which varied both in size and subcellular features. Some large dendrites in lamina I were contacted by numerous neurotensin-positive axons and also unlabeled terminals. In lamina II boutons with neurotensin immunoreactivity formed synapses mostly with small unlabeled dendrites some of which contained vesicles. The present results together with recent anatomical and physiological findings suggest that spinal cord neurons which contain neurotensin synapse with cells in the superficial dorsal horn that receive either input from primary afferents conveying nociceptive information or form part of the spinothalamic tract, or both. The diversity observed both in the morphology of neurotensin-positive terminals and in their synaptic patterns may indicate that they arise from more than one type of dorsal horn cell.