The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaq ueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is po stulated that the PAG exerts this inhibitory effect on spinal nociceptive f unctions through the activation of descending serotonergic and noradrenergi c pathways that arise from the rostral ventromedial medulla (RVM) and ponti ne noradrenergic nuclei. To investigate the neuroanatomical substrate of th is functional link between the PAG and RVM, as well as the pontine noradren ergic nuclei in the rat, we labelled axons that project from the ventrolate ral PAG to various regions of the pons and medulla oblongata using the ante rograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstra ted that some of PAG efferents really do terminate in the RVM and pontine n oradrenergic nuclei, but a substantial proportion of them project to the in termediate subdivision of the pontobulbar reticular formation. Combining th e axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemis try, we also found that, in contrast to previous results, PAG efferents mak e relatively few appositions with serotonin- and tyrosine-hydroxylase-immun oreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the v entrolateral PAG may activate a complex pontobulbar neuronal assembly inclu ding neurons in the intermediate subdivision of the pontobulbar reticular f ormation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunor eactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulb ar neural circuitry, then, may mediate the PAG-evoked activities towards th e spinal dorsal horn resulting in the inhibition of spinal nociceptive func tions.