Serotonin inhibits Ca2+ currents in porcine melanotrophs by activating 5-HT1C and 5-HT1A receptors

1. We have investigated the effect of serotonin (5-HT) on Ca2+ currents in cultured porcine pituitary intermediate lobe (IL) cells. Electrophysiological recordings were performed in the whole-cell configuration of the patch-clamp technique. All membrane currents other than Ca2+ currents were blocked pharmacologically and by ionic substitution. 2. Two types of Ca2+ currents were recorded in IL cells, differing by their activation and inactivation properties. The first type of Ca2+ current was activated at membrane potentials more positive than -60 mV and had a transient time course during the 100 ms depolarizing voltage steps. The properties of this current correspond to those of the T-type or low-voltage-activated Ca2+ current. The second type of Ca2+ current had a threshold for activation between -30 and -20 mV and showed no sign of inactivation with time during the voltage steps. The properties of this current are similar to those of the L-type or high-voltage-activated Ca2+ current. 3. Current to voltage (I-V) relationships obtained either by conventional 100 ms voltage steps from a holding potential (VH) of -100 mV to various test potentials or by 800 ms voltage ramps from -100 to +50mV matched one another closely and showed two inward current humps corresponding to the activation of the T-type and L-type Ca2+ currents respectively. The ramp protocol was used to characterize the effect of 5-HT on the Ca2+ current I-V relationship. 4. 5-HT (100nM to 50 microM) reversibly inhibited the amplitude of the Ca2+ current triggered by 100 ms voltage jumps from a Vh of -100 mV to a test potential of 0 mV. 5. The effect of 5-HT was dose dependent with a threshold between 10 and 100 nM and a maximal effect at 10 microM. At a concentration of 10 microM, the average inhibition of Ca2+ current by 5-HT was 18.3 +/- 6.5% (n = 27). 5-HT inhibited Ba2+ current in a similar fashion. 6. When examining the effect of 5-HT on Ca2+ current I-V relationships, we observed a reversible inhibition of the high-threshold component corresponding to the L-type Ca2+ current. We never observed any effect of 5-HT on the T-type current. 7. The effect of 5-HT (10 microM) was antagonized to various extents by mianserin (1 microM) but not by ketanserin (0.1 microM), suggesting the involvement of 5-HT1C receptors.(ABSTRACT TRUNCATED AT 400 WORDS)