2 DISTINCT RECEPTORS OPERATE THE CAMP CASCADE TO UP-REGULATE L-TYPE CA CHANNELS

Previously we have reported that serotonin's (5-hydroxytryptamine or 5 -HT) potentiating action on L-type Ca channels is present only in defi nite neurones from pedal ganglia of the mollusc Helix pomatia [Kostyuk PG, Lu kyanetz EA, Doroshenko PA (1992) Effects of serotonin and cAMP on calcium currents in different nevrones of Helix pomatia. Pflugers Arch 420:9-15]. The potentiation is mediated by the cAMP second messen ger system and is triggered by 5-HT1-like type receptors. To understan d the physiological and pharmacological significance of this phenomeno n, we analysed the comparative effects of dopamine (DA) and 5-HT on vo ltage-operated Ca currents (I-ca) in isolated, intracellularly perfuse d H. pomatia neurones in whole-cell patch-clamp experiments. Two types of effects of DA (1-10 mu M) and 5-HT (1-10 mu M) on I-ca were observ ed in different neurones: reversible inhibition (by about 40% and 20% respectively) or reversible potentiation (up to 65% and 40% respective ly) of current amplitude. Neurones insensitive to neurotransmitter app lication were also observed. DA could induce potentiation of I-ca, onl y in the same neurones that were similarly sensitive to 5-HT. However, a similar correlation between inhibitory action of neurotransmitters on I-ca was not observed. The potentiating effects of 5-HT and DA on I -ca were not additive and were mimicked by intracellular cAMP (100 mu M) or 20 mu g/ml of the catalytic subunit of protein kinase A. We esta blished that the potentiating effects of neurotransmitters were mediat ed by two distinct receptors, as the DA receptor antagonist ergometrin (1 mu M) selectively inhibited the enhancement of I-ca by DA and did not affect the action of 5-HT in the same eel. A similar specificity w as observed for the dopaminergic compound: 5-chlortryptamine (10 mu M) , whereas the classical neuroleptic fluphenazine (10 mu M) effectively blocked the 5-HT-evoked effect without significantly changing the act ion of DA. Methiothepin, an antagonist of 5-HT1 and 5-HT, receptors, b locked both 5-HT-and DA-evoked effects. The results point out a possib le convergence of the two different receptors (5-HT1-like and D-1) on the same cAMP-dependent system of phosphorylation in the up-regulation of L-type Ca channel activity in mollusc neurones.