The distribution of substance P-, enkephalin-and dynorphin-immunoreactive neurons in the medulla of the rat and their contribution to bulbospinal pathways

Abstract

This study examined the medullary distribution of peptide-containing neurons at the origin of bulbospinal pathways in the rat. Antisera directed against substance P, methionine-enkephalin-arg-glγ-leu and dynorphin B were used on sections in which spinally projecting brainstem neurons had been identified by the retrograde transport of a protein-gold complex that was injected into the spinal cord. Both the relative numbers and distribution of the different peptide-immunoreactive spinally projecting neurons differed. Methionine-enkephalin-immunoreactive neurons were twice as numerous as the substance P-immunoreactive cells and seven times more numerous than the dynorphin B-positive neurons. The methionine-enkephalin cells were found in all medullary raphénuclei, and in the ventromedial and ventrolateral medullary reticular formation. Caudally, the methionine-enkephalin cells were concentrated laterally; more rostrally, they were located more medially. Three major loci of methionine-enkephalin-immunoreactive cells were found: (1) the nucleus reticularis paragigantocellularis lateralis, at levels caudal to the facial nucleus, (2) the B3 cell group (nucleus raphémagnus and the nucleus reticularis magnocellularis, pars alpha) and the most rostral part of the B1 and B2 cell groups (nuclei raphépallidus and obscurus), (3) a dense cluster of cells that flanks the dorsal surface of the dorsal accessory olive (referred to as the nucleus interfascicularis hypoglossi, pars dorsalis). Substance P-like cells were seen in all raphénuclei except for the most anterior portion of the B3 cell group. Substance P-immunoreactive cells were also seen in both the ventromedial (nuclei reticularis ventralis and magnocellularis) and ventrolateral medulla (nucleus reticularis paragigantocellularis lateralis). Finally there was a dense concentration of substance P neurons in the nucleus interfascicularis hypoglossi, pars ventralis. The distribution of dynorphin-immunoreactive neurons differed significantly from that of methionine-enkephalin and substance P. Dynorphin cells were almost exclusively found in the ventrolateral medulla (nucleus reticularis paragigantocellularis lateralis), at all Ievels between the lateral reticular nucleus and the caudal pole of the facial nucleus. The proportion of each of these peptidergic-immunoreactive cells at the origin of bulbospinal pathways differed considerably. Substance P spinally projecting neurons were more numerous than methionine-enkephalin spinally projecting neurons. The majority of the substance P projection neurons were found in the Bl and B2 cell groups, in the adjacent ventromedial medulla and in the nucleus interfascicularis hypoglossi, pars ventralis. A few substance P projection neurons were also found in the caudal reaches of the B3 cell group. Methionine-enkephalin projection neurons were located in the rostral B1/B2 cell groups, B3 cell group and in the nucleus interfascicularis hypoglossi, pars dorsalis. None were found in caudal B1/B2 cell groups. Methionine-enkephalin projecting neurons were thus located rostral to the majority of the double-labelled substance P cells. Only scattered projection neurons were found among the methionine-enkephalin- and substance P-immunoreactive cells of the nucleus reticularis paragigantocellularis lateralis. In contrast to the large numbers of double-labelled methionine-enkephalin and substance P cells, no spinally projecting dynorphin neurons were found. We suggest that there are differential methionine-enkephalin 5-hydroxytryptamine inhibitory and substance P/5 hydroxytryptamine excitatory influences that issue from different medullary areas and are, respectively, exerted upon dorsal horn nociceptive neurons, and visceral/somatic motoneurons and intermediolateral preganglionic neurons.