The Sites of Origin of Brain Stem Neurotensin and Serotonin Projections to the Rodent Nucleus

The combined horseradish peroxidase retrograde transport-peroxidase-antiperoxidase immunohistochemical procedure was utilized in the present study to ascertain the sites of origin of serotonin and neurotensin projections to the rodent nucleus raphe magnus. The major serotonin inputs to the raphe magnus arise from the B-8 and B-9 groups of Dahlstrom and Fuxe (Dahlstrom, A., and K. Fuxe (1964) Acta Physiol. Stand. Suppl. 232 62: l-55), the nucleus reticularis paragigantocellularis, and the nucleus reticularis gigantocellularis pars (Y. Neurotensinergic projections to the raphe magnus originate predominantly from the periaqueductal gray, the nucleus solitarius, the dorsal and ventral parabrachial nuclei, and the nucleus cuneiformis. The periaqueductal gray and the nucleus paragigantocellularis were found to provide both a neurotensin and a serotonin projection to this raphe nucleus. The present results indicate that several brain stem nuclei, which have been implicated previously in endogenous analgesia mechanisms, provide serotonergic and neurotensinergic input to the nucleus raphe magnus. Reynolds (1969) and Mayer and Liebeskind (1974) were among the first investigators to demonstrate that potent analgesia could be produced by electrical brain stimulation in freely moving animals. Most of the early studies of centrally induced analgesia focused on the midbrain periaqueductal gray (PAG) from which stimulation-produced analgesia was best elicited. Subsequent studies have demonstrated that stimulation of the medullary nucleus raphe magnus (NRM), a source of serotonin neurons, also produces potent analgesia (Fields and Basbaum, 1978; Oliveras et al., 1979). Moreover, it has been shown that stimulation of the PAG alters the activity of cells in the NRM (Pomeroy and Behbehani, 1979) and that lesions of the NRM produce a pronounced decrease in nociceptive threshold (Proudfit, 1981). Both the analgesic action of opiates and electrical stimulation of the PAG are blocked by lesion of the dorsolateral part of the spinal cord (Basbaum et al., 1977). Since (I) the PAG sends very few direct projections to the spinal cord and (2) a PAG-NRM projection was demonstrated (Ruda, 1976; Gallager and Pert, 1978; Abols and Basbaum, 1981), it was proposed that the raphe-spinal pro’ This work was supported by Grant BNS 7906486 from the National Science Foundation and, in part, by Grant I ROl NS17401-01 from the National Institutes of Health. ‘Present address: Department of Veterinary Biology, School of Vet,erinary Medicine, University of Minnesota, St. Paul, MN 5.5108. jection is the critical link in the analgesia produced by PAG stimulation (Fields and Basbaum, 1978). Although the raphe-spinal system may play a role in PAG stimulation-produced analgesia, the role of the NRM in exogenous opiate action in still far from clear. Although systemic administration of opiates has been shown by some investigators to increase the discharge rate of raphe magnus neurons (Deakin et al., 1977; Oleson et al., 1978), most investigators interpret these results as indicating that raphe neurons are activated indirectly from the PAG (Fields and Basbaum, 1978). For instance, Proudfit (1981) has demonstrated recently that raphe magnus neurons are not involved directly in mediating the antinociceptive actions of opiates. In contrast Dickenson and co-workers (1979) reported that the rodent NRM is sensitive to the microinjection of opiates and that naloxone can reverse the analgesic action of NRM stimulation. Although some controversy exists, the majority of studies to date would suggest that opiates administered systemically do not act at the level of the raphe. It is probable that opiate compounds act at other CNS sites which, in turn, influence the NRM as indicated above. The present investigation is part of an ongoing study designed to elucidate the origin of several neurotransmitter and neuropeptide inputs to the NRM. This report deals with the brain stem origin of serotonergic and neurotensin projections to the NRM and the results are