Peripheral nerve stimulation increases serotonin and dopamine metabolites in rat spinal cord

Extracellular serotonin (5-HT), dopamine (DA), and their metabolites, 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA), were assessed in the rat lumbar spinal cord (L3-4) by in vivo microdialysis with high performance liquid chromatography and electrochemical detection (HPLC-ECD). Under urethane-chloralose anesthesia, basal levels of 5-HT and DA in the dialysates were approximately 1.0-1.2 pg/22 microliters sample, 5-HIAA, DOPAC, and HVA were constant at 322.6 +/- 14.9, 8.6 +/- 0.7, and 10.4 +/- 0.4 pg/22 microliters sample (mean +/- SE), respectively. Local application of 100 mM KCl via the dialysis probe increased the 5-HT and DA. Peripheral nerve stimulation that selectively excited the large (A-beta) or small (A-delta) myelinated fibres increased the metabolites. Excitation of the A-beta fibers increased the levels of 5-HIAA to 138%, DOPAC to 155%, and HVA to 143% of the controls. Stimulation of the A-delta fibers increased 5-HIAA to 121%, DOPAC to 120%, and HVA to 124% of the controls. The results suggest that non-nociceptive peripheral nerve stimulation may activate the descending 5-HT and DA systems in the spinal cord.