Ultrastructural analysis of dynorphin B–immunoreactive cells and terminals in the superficial dorsal horn of the deafferented spinal cord of the rat

Abstract

Light microscopic studies have demonstrated important differences in the distribution of enkephalin and dynorphin cells and terminals in the dorsal horn. Most importantly, dynorphin neurons are located in regions almost exclusively associated with the transmission and/or control of nociceptive messages (laminae I, IIo, and V); enkephalin neurons, although located in the same regions, are also found in areas involved in the transmission of nonnociceptive messages, e.g., laminae IIi and III. To determine whether there are also differences in the synaptic organization of the two opioid peptides, we have examined the distribution of dynorphin B immunoreactivity at the ultrastructural level. The studies were performed in colchicine‐treated rats that underwent dorsal rhizotomy so that the relationship of dynorphin terminals and cells to primary afferent terminals could be established.Dynorphin B–immunoreactive cell bodies and dendrites in laminae I and IIo receive convergent primary and nonprimary afferent input, which suggests that dynorphin neurons receive a small‐diameter, nociceptive input. Dynorphin terminals predominantly contain round, agranular vesicles; some terminals also contain a few dense core vesicles. Most dynorphin terminals are presynaptic to unlabelled dendrites; both asymmetric and symmetrical axonal contacts were noted. Dynorphin‐immunoreactive boutons are also presynaptic to unlabelled cell bodies and spines. Twenty‐nine percent of dynorphin terminals were associated with axonal profiles, including degenerating primary afferent terminals; only rarely could a synaptic density be detected. Although some degenerating primary afferent terminals were clearly presynaptic to dynorphin‐immunoreactive terminals, in most cases, the polarity of the relationship between primary afferents and dynorphin terminals could not be established.These data indicate that synaptic interactions made by and with dynorphin‐immunoreactive cells and terminals in the superficial dorsal horn are not very different from those that were previously reported for enkephalin cells and terminals. Thus, it is unlikely that dynorphin terminals provide a significant presynaptic input to primary afferent fibers. On the other hand, the presence of a primary afferent input to dynorphin cell bodies and dendrites in the superficial dorsal horn suggests that dynorphin cells receive a direct input from small‐diameter, nociceptive primary afferents. That connection might contribute to the increased levels of dynorphin message and peptide that have been reported in rats experiencing a chronic inflammatory condition. The possibility that dynorphin neurons are associated more with the transmission than the control of nociceptive messages is discussed.