200 Neuroscience Letters, 105 (1989) 200-204 Elsevier Scientific Publishers Ireland Ltd. NSL 06390 Modulation of mechanosensory threshold in Aplysia by serotonin, small cardioactive peptideB (SCPB), FMRFamide, acetylcholine, and dopamine Allen J. Billy* and Edgar T. Walters Department of Physiology and Cell Biology, University of Texas Medical School at Houston, Houston, TX 77225 (U.S.A.) (Received 21 June 1989;Accepted 26 June 1989) Key word~: Sensitization; Inhibition: Nociception; Excitability;Neuromodulation; Receptive field Mechanosensory threshold of tail sensory neurons in Aplysia was tested after injecting the tail with neu- romodulators known to affect defensivebehavior in this animal. Serotonin (5-HT) and small cardioactive peptide (SCPB) reduced peripheral threshold, while FMRFamide, acetylcholine (ACh), and dopamine increased threshold. FMRFamide and ACh also reduced spontaneous activity of sensory neurons. Gluta- mate and taurine had no effect. SCPBeffects persisted for 15 30 rain after washout. Functional similarities between peripheral and central effects of these neuromodulators support the hypothesis of coordinate modulation of both regions of the sensory neuron by the same set of modulators following noxious stimu- lation. Central components of tail sensory neurons display various plastic changes which enhance sensory signals within the CNS following noxious stimulation [20, 21]. This sensory enhancement contributes to general and site-specific sensitization of the tail withdrawal reflex. Noxious stimulation can also transiently inhibit Aplysia mechano- sensory neurons [6, 14]. Similar enhancement and inhibition of central sensory signals in tail, siphon, and head mechanosensory neurons can be produced by serotonin (5-HT), small cardioactive peptide (SCPB), FMRFamide, acetylcholine (ACh), and dopamine [1, 2, 5, 8, 9, 14, 15, 17-19, 21]. In contrast, little is known about modula- tion of peripheral sensory excitability in Aplysia, although peripheral modulation of mechanosensory neurons has been described in mammals [3] and arthropods [16]. Evidence for peripheral sensory modulation in Aplysia comes from observations that noxious cutaneous stimulation causes long-term depression of mechanosensory *Present address: Department of Anatomy, University of British Columbia, Vancouver, B.C. V6T IW5. Canada. Correspondence: E.T. Walters, Department of Physiology and Cell Biology, University of Texas Medical School at Houston, P.O. Box 20708, Houston, TX 77225, U.S.A. 0304-3940,/89/$ 03.50 (~?,1989 ElsevierScientificPublishers Ireland Ltd.