Catecholaminergic innervation of the cat spinal cord

Abstract

The organization of catecholamine (CA)-containing nerve terminals in the cat spinal dorsal horn was examined in an immunocytochemical study employing antisera against tyrosine hydroxylase and dopamine-B-hydroxylase. Light microscopic analysis revealed that varicose axons were concentrated in laminae I, II & IV. Correlated ultrastructural analysis showed that these terminals usually formed single synapses with dendrites or somata, but not with other axon terminals. This suggests that descending catecholaminergic axons regulate sensory transmission through the dorsal horn via a postsynaptic action upon dorsal horn neurons.Using the retrograde tracer horseradish peroxidase to label particular groups of dorsal horn neurons, it was shown that the postsynaptic dorsal column (PSDC) pathway is a major projection target of CA-containing axons. Over 60% of these cells were found to have dopamine-Bhydroxylase immunostained varicosities closely apposed to their somata and/or proximal dendrites, and correlated electron microscopic analysis confirmed that many of these contacts were regions of synaptic association. In contrast, the cells of the spinocervical tract (SCT) did not receive a major innervation from these axons when viewed with the light microscope.The lateral cervical nucleus (LCN), the termination site of SCT cells, was found to possess a dense innervation from CA-containing axons. These fibres were 7 present throughout the nucleus and synapsed with dendrites and somata, including those of large cells in the lateral region, but not with other axon terminals. This suggests that catecholaminergic axons in the LCN regulate the activity of LCN neurons but not the terminals of SCT cells.It has been suggested that many catecholaminergic axons in the dorsal horn may contain neuropeptide Y (NPY), and an examination was made of NPY-immunoreactive axons to test this hypothesis. Light microscopy revealed a heavy concentration of NPY-positive profiles in laminae I-II but only low to moderate numbers in III-VI. Fine structural examination showed most of these profiles to be axon terminals but a few structures were observed which may have been NPY vesicle-containing dendrites. The postsynaptic targets of NPY terminals were mostly dendrites or somata, but many (27%) formed axo-axonic synaptic junctions. In laminae I-III, the postsynaptic axon terminals were sometimes the central boutons of glomeruli. These findings suggest that NPY axons regulate sensory transmission by a postsynaptic action upon dorsal horn neurons and a presynaptic action upon primary afferent terminals. NPY boutons often formed more than one synapse and some of these arrangements were confirmed to be triads. Clearly, NPY profiles are organized in a much more complex way than those containing CA, which suggests that co-1oca1ization between the two neurotransmitters in the spinal dorsal horn is, at best, limited.