5‐HT 1B 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 Ca 2+ currents, but had no effect on voltage‐dependent K + currents. When EPSCs were evoked by presynaptic Ca 2+ currents during simultaneous pre‐ and postsynaptic recordings, the magnitude of the 5‐HT‐induced inhibition of Ca 2+ currents fully explained that of EPSCs. Upon repetitive applications, 5‐HT showed tachyphylaxis, with its effect on both EPSCs and presynaptic Ca 2+ currents becoming weaker in the second application. 1,2‐bis(o‐aminophenoxy)ethane‐N‐N′‐N′‐N′‐tetraacetic acid (BAPTA; 10 m m ) 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 Ca 2+ channels, thereby inhibiting transmitter release at immature calyceal nerve terminals, and that 5‐HT 1B receptors undergo Ca 2+ ‐dependent tachyphylaxis on repetitive activations.