In whole-cell patch clamp recordings made from non-sensory neurons acutely isolated from the spinal cord of Xenopus (stage 40-42) larvae, two forms of inhibition of the high voltage-activated (HVA) Ca2+ currents were produced by 5-HT. One was voltage dependent and associated with both slowing of the activation kinetics and shifting of the voltage dependence of the HVA currents. This inhibition was relieved by strong depolarizing prepulses. A second form of inhibition was neither associated with slowing of the activation kinetics nor relieved by depolarizing prepulses and was thus voltage independent. 2. In all neurons examined, 5-HT (1 microM) reversibly reduced 34 +/- 1.6 % (n = 102) of the HVA Ca2+ currents. In about 40 % of neurons, the inhibition was totally voltage independent. In another 5 %, the inhibition was totally voltage dependent. In the remaining neurons, inhibition was only partially (by around 40 %) relieved by a large depolarizing prepulse, suggesting that in these, the inhibition consisted of both voltage-dependent and -independent components. 3. By using selective channel blockers, we found that 5-HT acted on both N- and P/Q-type channels. However, whereas the inhibition of P/Q-type currents was only voltage independent, the inhibition of N-type currents had both voltage-dependent and -independent components. 4. The effects of 5-HT on HVA Ca2+ currents were mediated by 5-HT1A and 5-HT1D receptors. The 5-HT1A receptors not only preferentially caused voltage-independent inhibition, but did so by acting mainly on the omega-agatoxin-IVA-sensitive Ca2+ channels. In contrast, the 5-HT1D receptor produced both voltage-dependent and -independent inhibition and was preferentially coupled to omega-conotoxin-GVIA sensitive channels. This complexity of modulation may allow fine tuning of transmitter release and calcium signalling in the spinal circuitry of Xenopus larvae.
