5-HT inhibits N-type but not L-type Ca2+ channels via 5-HT1A receptors in lamprey spinal neurons

5-HT is a potent modulator of locomotor activity in vertebrates. In the lamprey, 5-HT dramatically slows fictive swimming. At the neuronal level it reduces the postspike slow afterhyperpolarization (sAHP), which is due to apamin-sensitive Ca2+-dependent K+ channels (K-Ca). Indirect evidence in early experiments suggested that the sAHP reduction results from a direct action of 5-HT on K-Ca channels rather than an effect on the Ca2+ entry during the action potential [Wallen et al., (1989) J. Neurophysiol., 61, 759-768]. In view of the characterization of different subtypes of Ca2+ channels with very different properties, we now reinvestigate if there is a selective action of 5-HT on a Ca2+ channel subtype in dissociated spinal neurons in culture. 5-HT reduced Ca2+ currents from high voltage activated channels. N-type, but not L-type, Ca2+ channel blockers abolished this 5-HT-induced reduction. It was also confirmed that 5-HT depresses Ca2+ currents in neurons, including motoneurons, in the intact spinal cord. 8-OH-DPAT, a 5-HT1A receptor agonist, also inhibited Ca2+ currents in dissociated neurons. After incubation in pertussis toxin, to block G(i/o) proteins, 5-HT did not reduce Ca2+ currents, further indicating that the effect is caused by an activation of 5-HT1A receptors. As N-type, but not L-type, Ca2+ channels are known to mediate the activation of K-Ca channels and presynaptic transmitter release at lamprey synapses, the effects of 5-HT reported here can contribute to a reduction in both actions