SENSORY RESPONSIVENESS OF BRAIN NORADRENERGIC NEURONS IS MODULATED BYENDOGENOUS BRAIN-SEROTONIN

Previous results have indicated that application of serotonin (5-HT) o nto noradrenergic locus coeruleus (LC) neurons selectively attenuates the response of these cells to excitatory amino acids (EAAs). Other st udies revealed that certain sensory responses of LC neurons are mediat ed by EAA inputs. We examined the role of endogenous 5-HT in modulatin g sensory responses of LC neurons that are EAA-mediated. LC neurons re corded in rats pretreated with the serotonin (5-HT) depletor, p-chloro phenylalanine (PCPA), exhibited increased responsiveness to electrical stimulation of a rear footpad. Conversely, injection of the 5-HT prec ursor, 5-hydroxytryptophan (5-HTP), reversed this effect of PCPA and a ttenuated this sensory response of LC neurons in drug-naive animals. N either treatment altered the spontaneous discharge rate of LC neurons. These results are consistent with previous findings indicating that 5 -HT has potent but selective effects on EAA-mediated responses of LC n eurons, and in addition point to a possible functional role for endoge nous 5-HT in controlling sensory-evoked LC activity.