5-HT INHIBITS CALCIUM CURRENT AND SYNAPTIC TRANSMISSION FROM SENSORY NEURONS IN LAMPREY

In the lamprey spinal cord, 5-hydroxytryptamine (5-HT) immunoreactivit y (ir) is present in the ventromedial plexus originating from intraspi nal neurons, ventrolateral column arising from the brainstem, and dors al column. The latter 5-HT system originates from small dorsal root ga nglion neurons. Combined Lucifer yellow intracellular labeling of the intraspinal sensory neurons, dorsal cells, and 5-HT immunohistochemist ry showed close appositions between 5-HT-ir fibers and dorsal cell axo ns. Application of 5-HT depressed monosynaptic EPSPs evoked in giant i nterneurons by stimulation of single dorsal cells, dorsal roots, or do rsal column without any detectable change in the input resistance of p ostsynaptic neurons. Furthermore, the amplitude of AMPA-evoked depolar izations in giant interneurons was unaffected by 5-HT. The lack of pos tsynaptic effects of 5-HT indicates that the decrease of the amplitude of sensory monosynaptic EPSPs by 5-HT is mediated by presynaptic mech anisms. The inhibition of monosynaptic EPSPs by 5-HT was not counterac ted by an antagonist of 5-HT1A receptors. 5-HT also reduced the amplit ude of the calcium current recorded in isolated dorsal cells and slowe d down its kinetics. The inhibition of calcium channels could represen t the mechanism mediating the depression of synaptic transmission at t he axonal level. These results show that activation of 5-HT receptors on dorsal cell axons as well as on other sensory neurons mediates inhi bition of sensory synaptic transmission to giant interneurons. In inta ct animals, 5-HT could be released from small 5-HT neurons in dorsal r oot ganglia, which thus may underlie direct sensory-sensory interactio ns.