Adenosine A1 receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

At the calyx of Held synapse in brainstem slices of 5- to 7-day-old (P5–7) rats, adenosine, or the type 1 adenosine (A₁) receptor agonist N ⁶-cyclopentyladenosine (CPA), inhibited excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs. The A₁ receptor antagonist 8-cyclopentyltheophylline (CPT) had no effect on the amplitude of EPSCs evoked at a low frequency, but significantly reduced the magnitude of synaptic depression caused by repetitive stimulation at 10 Hz, suggesting that endogenous adenosine is involved in the regulation of transmitter release. Adenosine inhibited presynaptic Ca²⁺ currents ( I _pCa) recorded directly from calyceal terminals, but had no effect on presynaptic K⁺ currents. When EPSCs were evoked by I _pCa during simultaneous pre- and postsynaptic recordings, the magnitude of the adenosine-induced inhibition of I _pCa fully explained that of EPSCs, suggesting that the presynaptic Ca²⁺ channel is the main target of A₁ receptors. Whereas the N-type Ca²⁺ channel blocker ω-conotoxin attenuated EPSCs, it had no effect on the magnitude of adenosine-induced inhibition of EPSCs. During postnatal development, in parallel with a decrease in the A₁ receptor immunoreactivity at the calyceal terminal, the inhibitory effect of adenosine became weaker. We conclude that presynaptic A₁ receptors at the immature calyx of Held synapse play a regulatory role in transmitter release during high frequency transmission, by inhibiting multiple types of presynaptic Ca²⁺ channels.