NEUROCHEMICAL CHARACTERIZATION OF EXTRACELLULAR SEROTONIN IN THE ROSTRAL VENTROMEDIAL MEDULLA AND ITS MODULATION BY NOXIOUS STIMULI

Using in vivo microdialysis, we have characterized serotonin release f rom the rostral ventromedial medulla of the freely moving rat. Additio n of tetrodotoxin or removal of calcium from the dialysis solution dim inished the dialysate serotonin content, suggesting that spontaneous, calcium channel- and sodium channel-dependent neuronal release mechani sms contribute to the extracellular serotonin collected from the rostr al ventromedial medulla. Extracellular serotonin concentration was inc reased by depolarization (with 100 mM potassium) and by the local admi nistration of either a reuptake blocker (citalopram), a monoamine oxid ase inhibitor (pargyline), or amphetamine. Serotonin release was reduc ed significantly by 8-hydroxy-2-(di-n-propylamino) tetralin, suggestin g that serotonin(1A) receptors may regulate release from rostral ventr omedial medulla neurons. Because the basal serotonin concentration in the rostral ventromedial medulla was approximately twofold higher than that collected from the rostral ventrolateral medulla, a region that contains serotonin terminals but many fewer cell bodies, the possibili ty of release of serotonin from rostral ventromedial medulla neurons i s discussed. Finally, intraplantar formalin injection significantly in creased serotonin release, suggesting that this neurotransmitter contr ibutes to nociceptive modulation by regulating the outflow of the rost ral ventromedial medulla neurons.