Immunocytochemistry of enkephalin and serotonin distribution in restricted zones of the rostral trigeminal spinal subnuclei: Comparisons with subnucleus caudalis

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Abstract
The spinal trigeminal subnucleus caudalis processes nociceptive input from the head. However, physiological and behavioral studies in monkeys and humans indicate that painful stimuli from the central face and oral cavity also project through tigeminal nuclei rostral to the spinal subnucleus caudalis. Both enkephalin (ENK) and serotonin (5‐HT) are present in rostral trigeminal nuclei and these regions receive inputs from the raphe complex. Thus, it appears that elements of pain‐modulating circuitry proposed by Basbaum and Fields (Annu. Rev. Neurosci., 7:309–338. 1984) for the spinal and medullary dorsal horn may also exist in this region. In order to begin an exploration of this circuitry, the present study combines the techniques of retrograde transport of HRP from the ventral posteromedial thalamic nucleus (VPM) of the cat's thalamus to label trigeminothalamic relay cells. Secondarily, immunocytochemical techniques are employed to define the distribution patterns of ENK and 5‐HT cells and terminals in relationship to both labeled and nonlabeled neurons in each of the subnuclei of the spinal trigeminal nucleus. Trigeminothalamic relay cells were observed in laminae I and II, the magnocellular region, and the interstitial nucleus (IN) of subnucleus caudalis (Vc). ENK was found in axodendritic and axosomatic terminals, together with a population of small fusiform neurons in all these same areas except the magnocellular region. ENK axosomatic contacts innervated approximately 30% of labeled relay cells, chiefly in lamina I and the IN, or small unlabeled neurons in the same area. Serotonin activity occurred principally in lamina I and the IN and was confined almost exclusively to axodendritic terminals. Examination of subnucleus interpolaris (Vi) revealed relay cells distributed throughout the length of the nucleus and increasing in numbers at rostral levels. A rostral extension of the IN was found just ventrolateral to the main body of Vi and contained numerous labeled cells. The distribution of ENK activity was restricted to the ventral part of Vi and the IN and occurred in axodendritic and axosomatic terminals. These latter elements innervated 30–40% of labeled relay cells in Vi, particularly those located in the IN. Cells containing ENK generally resembled the fusiform cells found in Vc and were distributed in ventral Vi and the IN. Some ENK cells were larger, displayed several dendrites, and occurred only in the ventral Vi. Serotonin within Vi and Vc was confined principally to axodendritic terminals. It was most concentrated in the IN. Examination of the subnucleus oralis (Vo) revealed large numbers of relay cells distributed homogeneously but no evidence of either ENK or 5‐HT activity. These findings indicate that the Vi contains a circumscribed zone rich in both ENK and 5‐HT distributed in cellular elements quite similar to those found in Vc and the dorsal horn of the spinal cord. This suggests the presence of an additional important painprocessing center and supports electrophysiological studies documenting wide dynamic range and nociceptive‐specific units in this area whose activity is controlled by input from the raphe complex.