Immunocytochemical studies of excitatory amino acid neurotransmitters and transporters in the spinal cord and nucleus submedius

Sammanfattning: The amino acid glutamate is today regarded as the main fast excitatory neurotransmitter in the central nervous system and a large body of evidence support that glutamate serves as a neurotransmitter in primary afferent terminals in the spinal cord. Previous evidence also suggests that the closely related amino acid aspartate may serve a similar role in some primary afferent terminals. However, the present quantitative analysis of aspartate immunogold labeling show that primary afferent terminals in both the superficial and deep laminae of the dorsal horn contain only low levels of aspartate. Further, whereas glutamate immunogold labeling density demonstrated a positive correlation with synaptic vesicle density in primary afferent terminals, no such correlation was evident in aspartate immunogold labeled sections. Thus, aspartate is likely to serve a metabolic role but not a neurotransmitter role in primary afferent terminals. Immunolabeling of spinal cord sections with antisera against the vesicular glutamate transporters VGLUT1 and VGLUT2 showed distribution patterns of immunolabeled varicosities consistent with an expression of mainly VGLUT2 in small caliber primary afferent fibers and of mainly VGLUT1 in large caliber primary afferent fibers. The latter suggestion was confirmed by demonstration of VGLUT1 in anterogradely labeled varicosities originating from large caliber primary afferent fibers. VGLUT1 was also detected in fibers of the corticospinal tract and in small varicosities in the spinal grey matter likely to originate from these fibers. VGLUT2 immunolabeled varicosities were distributed throughout the spinal gray matter. In addition to an origin from small caliber primary afferent fibers, such varicosities are likely to originate from intraspinal and bulbospinal neurons. The signals transmitted by primary afferent terminals are relayed through projections neurons via the thalamus to the cerebral cortex. In the context of pain, the trigeminal projection to the thalamic nucleus submedius is of particular interest, as it in contrast to other spino- an trigeminothalamic projections in rats include a significant component originating in lamina I. Glutamate and aspartate immunogold labeling demonstrate that the terminals of the trigeminal projection to the nucleus submedius are enriched in glutamate but contain only low levels of aspartate. Further, the glutamate signal in these terminals, but not that of aspartate, is positively correlated with the synaptic vesicle density. Thus, glutamate is likely to serve as a neurotransmitter in trigeminothalamic tract terminals in the nucleus submedius, whereas aspartate presumably serves metabolic functions in the same terminals. A neurotransmitter role of glutamate in these terminals is also substantiated by the localization of VGLUT2 in most such terminals. VGLUT1 in the nucleus submedius is primarily located in terminals likely to originate in the ventral lateral orbital cortex.