Three bulbospinal pathways from the rostral medulla of the cat: An autoradiographic study of pain modulating systems

Abstract

Small amounts of 3H‐leucine were injected into discrete regions in the rostral medulla of the cat. Descending projections from these sites were studied with autoradiographic methods. On the basis of differential projections to the medulla and spinal cord, three distinct regions were delineated. Nucleus reticularis gigantocellularis (Rgc), located dorsally in the medullary reticular formation, projects primarily to “motor” related sites, including cranial motor nuclei VI, VII, XII, nucleus intercalatus, and a part of the ipsilateral medial accessory olive. The projection to the spinal cord is primarily via the ipsilateral ventrolateral and contralateral ventral funiculi. The Rgc terminal field is in lamina VII and VIII ipsilateral and lamina VIII contralateral to the injection site. In contrast, nucleus raphe magnus, (NRM) located ventrally, in the midline of the rostral medulla projects primarily to structures with known nociceptive and/or visceral afferent input. These sites include the solitary nucleus, the dorsal motor nucleus (X) and the marginal and gelatinous layers of the spinal trigeminal nucleus caudalis. The projection to the spinal cord is bilateral, via the dorsolateral funiculus. Terminal fields are found in the marginal zone and the substantia gelatinosa of the dorsal horn, and more deeply in lamina V, medial VI and VII. Nucleus reticularis magnocellularis (Rmc), located lateral to NRM and ventral to Rgc, has an overlapping projection with NRM, but the projection is ipsilateral. This difference between Rmc and Rgc is correlated with cytoarchitectural features of the two regions. The possibility that the raphe‐spinal pathway in the DLF mediates opiate and brain stimulation‐produced analgesia is discussed.

Arrangement of neurons in the medullary reticular formation and raphe nuclei projecting to thoracic, lumbar and sacral segments of the spinal cord in the cat

“…Reticulo-spinal neurons arising from additional areas of the medullary reticular formation were also described, using the retrograde axonal transport of horseradish peroxidase (HRP), in the cat (Kuypers and Maisky 1975), and in the opossum (Martin et al 1979), the autoradiography method in the cat (Basbaum et al 1978) and the combined retrograde HRP transport-immunocytochemical method in the rat (Ross et al 1981). Both medullary raphe nuclei and the medullary reticular formation project to every level of the spinel cord (Kuypers et al 1962;Nyberg-Hansen 1965;Martin et al 1978).…”

“…Most of the reticulospinal fibers from the medulla descend in the lateral funiculus (Nyberg-Hansen 1965; Kuypers and Maisky 1977;Martin et al 1978;Tohyama et al 1979), including the dorsolateral funiculus (Basbaum et al 1978). Raphe-spinal fibers from the Rpa and Rm descend in the dorsolateral funiculus and from the Ro in the lateral portion of the anterior funiculus (Martin et al 1978) Reticulospinal and raphe-spinal fibers terminate on large parts of the spinal grey matter: in the lateral portion of the central grey matter around the somatomotor and visceromotor neurons, as well as in the dorsal horn (Dahlstrom and Fuxe 1964;Basbaum et al 1978;Martin et al 1979Martin et al , 1982Tohyama et al 1979).…”

“…Raphe-spinal fibers from the Rpa and Rm descend in the dorsolateral funiculus and from the Ro in the lateral portion of the anterior funiculus (Martin et al 1978) Reticulospinal and raphe-spinal fibers terminate on large parts of the spinal grey matter: in the lateral portion of the central grey matter around the somatomotor and visceromotor neurons, as well as in the dorsal horn (Dahlstrom and Fuxe 1964;Basbaum et al 1978;Martin et al 1979Martin et al , 1982Tohyama et al 1979). Recently, direct monosynaptic connection was described between the descending axon terminals arising from the Rpa and Rm, as well as the preganglionic neurons innervating the suprarenal gland (Zagon et al 1989).…”

Supraspinal contributions to hyperalgesia

“…Anatomically, this circuit is in place. Both the RVM and adjacent areas receive direct afferent input from the superficial spinal dorsal horn and in turn send descending projections through spinal funiculi that terminate in the superficial dorsal horn, completing a spinobulbospinal loop (20)(21)(22)(23). We review below recent studies that document that spinal transection, or inactivation of supraspinal sites, prevents the expression of secondary hyperalgesia in a variety of animal models of persistent inflammatory, neurogenic, or neuropathic pain, thus providing the functional context in support of the anatomy (see Table 1).…”

The antinociceptive action of etorphine in the dorsal horn is due to a direct spinal action and not to activation of descending inhibition

“…Ryall, unpublished observations) and are similar to those where others have shown that the microinjection of morphine causes behavioural analgesia (see Introduction). Furthermore, direct projections from some of these sites to the spinal cord have been reported (Basbaum, Clanton & Fields, 1978). Even if there were sites in the brainstem more sensitive to etorphine than those which we have studied the similarity of the ED50s in spinal and intact animals would suggest that such sites are unimportant for depression of dorsal horn responses when the opiate is given systemically.…”