Resting and K+-evoked Release of Serotonin and Norephinephrine in Vivo from the Rat and Cat Spinal Cord

Using a spinal superfusion system, the release of serotonin (5-HT) and norepinephrine (NE) from rat and cat spinal cord was measured using a high performance liquid chromatograph with an electrochemical detector. Resting levels of 5-HT and NE in the chloralose-urethanized rat were of the order of 0.8 and 0.4 ng/ml, respectively. Potassium (40 or 80 mM, in excess) produced a dose-dependent increase in the levels of both monoamines in the superfusate. The substitution of cobalt blocked this evoked release. Depletion of spinal 5-HT or NE by the prior intrathecal administration of 5,6-dihydroxytryptamine (5,6-DHT) or 6-hydroxydopamine (6-OHDA) reduced the resting levels of 5-HT and NE, respectively, and prevented the increase produced by excess potassium. These experiments indicate the ability to reliably collect and measure monoamines which overflow from spinal terminals and which reflect the degree of activation of the spinal monoamine system.