The brainstem origin of monoaminergic projections to the spinal cord of the North American opossum: A study using fluorescent tracers and fluorescence histochemistry

Abstract

The retrograde transport of fluorescent markers has been combined with the glyoxylic acid and Falck-Hillarp techniques to identify the origin of monoamine axons within the spinal cord of the North American opossum. Catecholamine axons arise from neurons located within the ventrolateral medulla, dorsal to the superior olivary complex, within the dorsolateral and rostrolateral pons and within the periventricular nuclei of the hypothalamus. Such neurons are most numerous within the dorsolateral pons where they are found dorsal and lateral to the motor trigeminal nucleus, within the nucleus locus coeruleus pars alpha and adjacent reticular formation as well as within the ventral part of the nucleus locus coeruleus. Neurons containing the fluorescent marker and catecholamines were interspersed with others containing only the injected marker with the possible exception of the nucleus locus coeruleus. Spinal axons of the indoleamine type arise from neurons within the nuclei pallidus, obscurus and magnus raphae, the nucleus reticularis gigantocellularis, the nucleus reticularis gigantocellularis pars ventralis, the nucleus reticularis pontis pars ventralis and the nucleus dorsalis raphae. The latter nucleus only innervates rostral cervical levels. Most of the above areas also contain many non-indoleamine neurons which were labelled by the injected marker. This was particularly true of the nucleus magnus raphae and the adjacent nucleus reticularis pontis pars ventralis after injections of fluorescent markers into the superficial dorsal horn.