SUBSTANCE-P AND ENKEPHALIN IMMUNOREACTIVITIES IN AXONAL BOUTONS PRESYNAPTIC TO PHYSIOLOGICALLY IDENTIFIED DORSAL HORN NEURONS - AN ULTRASTRUCTURAL MULTIPLE-LABELING STUDY IN THE CAT

A combination of intracellular electrophysiological recording and inje ction of horseradish peroxidase with ultrastructural immunocytochemist ry was used to investigate the synaptic interplay between substance P- and enkephalin-immunoreactive axonal boutons and three types of funct ionally characterized dorsal horn neurons in the cat spinal cord. The dorsal horn neurons were classified as nocicepti\e specific, wide dyna mic range and non-nociceptive based on their responses to innocuous an d noxious stimuli. Most of the nociceptive neurons (either nociceptive specific or wide dynamic range) contained enkephalin immunoreactivity , but none of the non-nociceptive neurons were enkephalin. Three types of immunoreactive boutons were found in contact with the characterize d dorsal horn neurons. These boutons were positive for either substanc e P, enkephalin, or substance P+enkephalin. Quantitative analysis reve aled that the percentages of substance P-immunoreactive boutons appose d to the cell bodies, proximal dendrites and distal dendrites of nocic eptive neurons were significantly higher than those of non-nociceptive neurons. Furthermore, the percentages of substance P+enkephalin-immun oreactive axonal boutons apposed to the distal dendrites of nociceptiv e neurons were significantly higher than those of non-nociceptive neur ons and the percentages of enkephalin-immunoreactive boutons apposed t o the cell bodies and proximal dendrites of nociceptive neurons were s ignificantly higher than in non-nociceptive neurons. Finally, neither enkephalin-immunoreactive nor substance P+enkephalin-immunoreactive bo utons were ever seen presynaptic to substance P-immunoreactive boutons . These results provide evidence of an anatomical substrate within the dorsal horn for the interaction of substance P-mediated with enkephal in-mediated mechanisms. The data support the idea that the modulation of nociceptive input in the dorsal horn by enkephalinergic neurons occ urs mainly via a postsynaptic mechanism, and thus suggest that dorsal horn enkephalinergic neurons participate in a local inhibitory Feedbac k loop in a distinct pathway from the previously postulated opioid-med iated depression of substance P release from primary afferent terminal s. (C) 1997 IBRO.