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The distribution of cells belonging to the rat spinomesencephalic tract was studied by means of the retrograde transport of fluorescent dyes. Bilateral midbrain injections of cytoplasmic and nuclear tracers were made in order to evaluate the location of ipsilateral, contralateral, or bilaterally projecting cells. Spinal neurons with ascending projections to midbrain and descending propriospinal projections were identified by midbrain and spinal injections of different cytoplasmic labels. The locations of spinomesencephalic tract cells included seven regions of the spinal gray matter: marginal zone, lateral neck of the dorsal horn, nucleus proprius, the region around the central canal, the lateral cervical and spinal nuclei and the ventral horn. Cells projecting to the ipsilateral or contralateral midbrain had similar distributions and were frequently found in clusters with overlapping dendritic fields. Approximately 75% of spinomesencephalic cells projected to the contralateral midbrain. The largest contribution to the spinomesencephalic tract cell population was found in cervical cord segments 1-4. Cells with bilateral projections accounted for nearly 2% of all labeled cells, whereas 5% had both ascending and descending projections. Spinomesencephalic cells were found to have varying dendritic fields and morphology, e.g. fusiform, pyramidal, round/oval, and multipolar. The results of the present study lend further support to the view that the spinomesencephalic tract is a multi-component pathway with varied origins and projection targets.

The responses of 32 spinomesencephalic tract cells to intradermal capsaicin were examined in anesthetized monkeys. Wide dynamic range (n = 20) and nociceptive specific (n = 6) cells showed two types of excitatory responses to intradermal injection of capsaicin. The first excitatory response shown by the majority of wide dynamic range (n = 13) and nociceptive specific (n = 4) cells was consistent with their sensitization by capsaicin. The cells showed an acute and prolonged increase in ongoing activity with capsaicin injection. Responses to mechanical stimuli were substantially increased after capsaicin and an expansion of receptive field was often observed. The responses of the same cells to excitatory amino acid agonists applied locally by iontophoresis also increased. All cells showing sensitization were antidromically activated from periaqueductal gray regions dorsal to the sulcus limitans. Electrical stimulation at these sites did not affect the ongoing or evoked discharges of these cells. The second excitatory response of wide dynamic range (n = 5) and nociceptive specific (n = 1) cells was a novel pattern not consistent with sensitization. These cells nevertheless showed an acute and prolonged increase in background activity after capsaicin injection. Yet, there was no change or a decrease in responses to cutaneous stimuli, no evidence for change in receptive field size and no increase in responses to locally released excitatory amino acids. These cells projected to regions in the periaqueductal gray ventral to the sulcus limitans. Electrical stimulation at these sites produced a decrease in spontaneous activity of the same cell. Low threshold mesencephalic-projecting neurons (n = 6) showed a single inhibitory pattern (n = 4) of responses to capsaicin. The injection produced an acute decrease in spontaneous activity that was sustained for at least 30 min after injection. The responses to cutaneous stimuli and to excitatory amino acids were also substantially reduced. Low threshold cells were found that projected to both dorsal-lateral and ventral-lateral regions of the periaqueductal gray. In summary, three patterns of responses shown by primate spinomesencephalic tract cells to intradermal capsaicin appear dependent on the functional regions of the periaqueductal gray to which they project. These results suggest that inputs of spinomesencephalic tract neurons to the periaqueductal gray may evoke important components of the systemic response to the neurogenic hyperalgesia produced by intradermal capsaicin.

A study has been made of the involvement of spinal peptidergic neurons in ascending tracts at lumbar-sacral levels in rats, by combining the retrograde transport of a protein-gold complex with immunocytochemistry. Ten neuropeptides have been considered for their presence in the cells of origin of the following six ascending tracts, including some involved in pain transmission: the spinosolitary tract, the medial and lateral spinoreticular tracts, the spinomesencephalic tract, the spinothalamic tract and the postsynaptic dorsal column tract. Although there was overlap in the distribution of several of the types of peptidergic cells and some ascending tract cells only a very small percentage of long ascending tract cells were found to contain neuropeptides. Most (90%) of those peptidergic ascending tract cells, however, were clearly congregated in two distinct spinal regions: the lateral spinal nucleus and the region surrounding the central canal (including lamina X). Ascending tract cells in both of these regions contained a wide variety of neuropeptides. Immunoreactivities for a total of seven different peptides were seen. The lateral spinal nucleus had the highest percentage of neuropeptide containing ascending tract cells; cells of all the four populations of peptidergic neurons lying in this region were involved in supraspinal projections; they stained for vasoactive intestinal polypeptide, bombesin, substance P or dynorphin and their axons projected in the spinomesencephalic, spinoreticular and spinosolitary tracts. The region surrounding the central canal contained bombesin-, enkephalin-, cholecystokinin- and somatostatin-immunoreactive ascending tract cells; these cells were found at the origin of the spinothalamic, spinomesencephalic, spinoreticular and spinosolitary tracts. In this region only the cells staining for substance P were not involved in supraspinal projections. The peptidergic ascending tract cells in other spinal regions were few; they were found in either lamina I or lateral part of lamina V. Ascending tract lamina I cells reacted for dynorphin or vasoactive intestinal polypeptide and their axons projected in the spinosolitary and spinomesencephalic tracts. Ascending tract lamina V cells reacted for somatostatin and were found at the origin of the medial component of the spinoreticular tract. It is proposed that peptidergic ascending tract cells form minor but distinct subgroups within each ascending tract. Each of the ascending tracts are divisible into peptide- and nonpeptide-containing groups of cells which convey information in a parallel fashion.(ABSTRACT TRUNCATED AT 400 WORDS)