SOMATOSENSORY AFFERENT INPUTS RELEASE 5-HT AND NA FROM THE SPINAL-CORD

Endogenous serotonin (5-HT) and noradrenaline (NA) release by somatose nsory afferent inputs was investigated at the level of the spinal cord using in vivo microdialysis technique combined with high performance liquid chromatography and electrochemical detection (HPLC-ECD). Select ive stimulation of large myelinated A beta afferent fibers significant ly increased 5-HT release to 151.1 +/- 10.1% of the control, but did n ot affect NA release. However, selective stimulation of small myelinat ed A delta fibers released NA rather than 5-HT. The NA level enhanced to 128.8 +/- 6.4% of the control after A delta fibers were stimulated with the intensity of 6 times threshold. Stimulation of unmyelinated C fibers unavoidably excited the A beta and A delta afferent fibers, ca using both 5-HT and NA release from the spinal cord. The results sugge st that both innocuous and noxious information may activate serotonerg ic descending pathways. The noradrenergic descending pathways are only triggered by noxious inputs transmitted by small afferent fibers.