Evidence for leucine-enkephalin immunoreactive neurons in the medulla which project to spinal cord in squirrel monkey

Evidence that dopamine-beta-hydroxylase immunoreactive neurons in the lateral reticular nucleus project to the spinal cord in the rat

The existence of noradrenergic projections from the lateral reticular nucleus (LRt) to the dorsal quadrant of cervical, thoracic, or lumbar spinal cord was investigated using a combined method of WGA-apo-HRP-gold retrograde tracing and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Preliminary retrograde tracing studies indicated that LRt neurons projecting to cervical, thoracic, or lumbar spinal cord were characteristically located near the perimeter of the LRt. Double-labeling experiments demonstrated that a portion of these peripherally-located, spinal-projecting neurons were DBH-immunoreactive. Double-labeled neurons were also located at the parvocellular division of the contralateral LRt in the thoracic injection cases. Double-labeled neurons were not observed at the subtrigeminal division in cervical, thoracic, or lumbar injection case. The results suggest the possibility that the noradrenergic LRt-spinal pathway might be involved in a variety of pain processing and cardiovascular regulatory functions in the rat.

Fluorescent double-label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat

Following injections of WGA-HRP into either the spinal cord or periaqueductal gray, labeled neurons were observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double-labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double-labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15-18 mu in their greatest transverse diameter. No double-labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN-spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions.

Direct evidence of an extensive GABAergic innervation of the spinal dorsal horn by fibres descending from the rostral ventromedial medulla

A long line of studies emphasizes the contribution of serotonergic fibres descending from the rostral ventromedial medulla in the control of spinal nociceptive information processing. A growing body of evidence, however, suggests that the relative contribution of serotonin to the mediation of spinal neuronal activity from the rostral ventromedial medulla may require re-evaluation. It has recently been substantiated that, in addition to the serotonergic fibres, the spinal dorsal horn receives an abundant non-serotonergic projection from the rostral ventromedial medulla. Furthermore, stimulation in the rostral ventromedial medulla could result in a powerful inhibition of nociceptive spinothalamic tract cells without any detectable serotonin release in the dorsal horn. After labelling raphe-spinal axons and axon terminals in the rat by iontophoretic injections of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin into the central region of the rostral ventromedial medulla (nucleus raphe magnus) and revealing GABA and glycine immunoreactivities of the labelled raphe-spinal terminals and their postsynaptic targets by postembedding immunocytochemical methods, here we demonstrate an extensive GABAergic projection from the rostral ventromedial medulla to the spinal dorsal horn. We show that the majority of the labelled raphe-spinal terminals in laminae I-IIo and IV-V contain GABA and some of the GABA-immunoreactive terminals are also immunoreactive for glycine. We also disclose that GABA-immunoreactive raphe-spinal terminals establish synaptic contacts primarily with GABA- and glycine-negative, presumably excitatory, spinal neurons, including Calbindin-D28k- as well as parvalbumin-immunoreactive cells in both laminae I-IIo and IV-V. The results suggest that volleys in fibres descending from the rostral ventromedial medulla may evoke GABA release from raphe-spinal terminals, and the released GABA, in some cases probably acting together with glycine, might play a crucial, as yet mostly unidentified, role in the inhibition of nociceptive information processing in the dorsal horn of the spinal cord.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Green fluorescent latex microspheres: a new retrograde tracer

The properties of a new fluorescent retrograde neuronal tracer, green fluorescent latex microspheres ("beads"), are described. Green beads, like the red beads that have been in use for several years, are readily transported by neurons in the mammalian central nervous system. After survival times ranging from 24 h to over one month they produce a distinct granular green fluorescence in the neuronal cytoplasm that is highly resistant to fading, when viewed using fluorescein filter sets. Green and red beads are transported with similar efficacy; cells labelled with either one or both tracers are easily distinguished. When injected into the brain, green beads, like red beads, show only minimal diffusion from the injection site. Because these two tracers differ only in their fluorescence, they should be ideal for double-labelling studies in a wide variety of systems and in developmental studies in which long-term retention of labels is required.

Enkephalin-immunoreactive neuronal projections from the medulla oblongata to the intermediolateral cell column: relationship to substance P-immunoreactive neurons

The present study investigated the ventral medullary distribution of enkephalin-immunoreactive neurons that project to the intermediolateral cell column and the relationship of these neurons to substance P-immunoreactive neurons. Neurons that projected to the intermediolateral cell column were identified by the presence of rhodamine-labeled microspheres within the neuronal cell body after an injection of the microspheres into the intermediolateral cell column of the third thoracic spinal cord segment. Enkephalin- and substance P-immunoreactivities were identified by dual-color immunohisto-chemistry. Enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were present in the raphe magnus, the nucleus reticularis magnocellularis pars alpha, the paragigantocellular reticular nucleus, and the parapyramidal region. These neurons were present throughout the rostrocaudal extent of each of these nuclei. However, in the raphe magnus the greatest number was present at more rostral levels of the nucleus. The morphology and distribution of enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were similar to those of enkephalin-immunoreactive neurons that were not observed to contain rhodamine-labeled microspheres. Substance P- and enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were present in similar distributions in each of the nuclei studied, except the raphe magnus. The raphe magnus contained more enkephalin- than substance P-immunoreactive neurons at rostral levels and more substance P-immunoreactive neurons than enkephalin-immunoreactive neurons at caudal levels. Coexistence of substance P- and enkephalin-immunoreactivities in ventral medullary neurons that projected to the intermediolateral cell column was rarely seen. These studies support the hypothesis that ventral medullary enkephalinergic neurons project to the intermediolateral cell column where they could act to modulate preganglionic sympathetic activity.