5-HT1B receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

5-Hydroxytryptamine (5-HT) inhibits transmitter release via activating GTP-binding proteins, but the target of 5-HT receptors in the nerve terminal is not determined. We addressed this question at the calyx of Held synapse in the brainstem slice of immature rats. Bath-application of 5-HT attenuated the amplitude of nerve-evoked excitatory postsynaptic currents (EPSCs) associated with an increase in the paired-pulse ratio, whereas it had no effect on the amplitude of spontaneous miniature EPSCs. The 5-HT_1B receptor agonist CP93129 mimicked the inhibitory effect of 5-HT, but the 5-HT_1A agonist (R)-(+)-8-hydroxy-DPAT (8-OHDPAT) had no effect. The 5-HT_1B receptor antagonist NAS-181 blocked the inhibitory effect of 5-HT. These results suggest that 5-HT activated 5-HT_1B receptors in calyceal nerve terminals, thereby inhibiting transmitter release. In direct whole-cell recordings from calyceal nerve terminals, 5-HT attenuated voltage-dependent Ca²⁺ currents, but had no effect on voltage-dependent K⁺ currents. When EPSCs were evoked by presynaptic Ca²⁺ currents during simultaneous pre- and postsynaptic recordings, the magnitude of the 5-HT-induced inhibition of Ca²⁺ currents fully explained that of EPSCs. Upon repetitive applications, 5-HT showed tachyphylaxis, with its effect on both EPSCs and presynaptic Ca²⁺ currents becoming weaker in the second application. 1,2-bis(o-aminophenoxy)ethane-N-N′-N′-N′- tetraacetic acid (BAPTA; 10 mm) loaded into the nerve terminal abolished this tachyphylaxis. The presynaptic inhibitory effect of 5-HT was prominent at postnatal day 5, but became weaker as animals matured. We conclude that activation of 5-HT_1B receptors inhibits voltage-gated Ca²⁺ channels, thereby inhibiting transmitter release at immature calyceal nerve terminals, and that 5-HT_1B receptors undergo Ca²⁺- dependent tachyphylaxis on repetitive activations.