Two 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 definite 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 neurones of Helix pomatia. Pflügers Arch 420:9-15]. The potentiation is mediated by the cAMP second messenger system and is triggered by 5-HT1-like type receptors. To understand the physiological and pharmacological significance of this phenomenon, we analysed the comparative effects of dopamine (DA) and 5-HT on voltage-operated Ca currents (Ica) in isolated, intracellularly perfused H. pomatia neurones in whole- cell patch-clamp experiments. Two types of effects of DA (1-10 mircoM) and 5-HT (1-10 microM) on Ica were observed in different neurones: reversible inhibition (by about 40% and 20% respectively) or reversible potentiation (up to 65% and 40% respectively) of current amplitude. Neurones insensitive to neurotransmitter application were also observed. Da could induce potentiation of ica only in the same neurones that were similarly sensitive to 5-ht. However, a similar correlation between inhibitory action of neurotransmitters on Ica was not observed. The potentiating effects of 5-HT and DA on Ica were not additive and were mimicked by intracellular cAMP (100 microM) or 20 microgram/ml of the catalytic subunit of protein kinase A. We established that the potentiating effects of neurotransmitters were mediated by two distinct receptors, as the DA receptor antagonist ergometrin (1 microM) selectively inhibited the enhancement of Ica by DA and did not affect the action of 5-HT in the same cell. A similar specificity was observed for the dopaminergic compound, 5-chlortryptamine (10 microM), whereas the classical neuroleptic fluphenazine (10 microM) effectively blocked the 5-HT-evoked effect without significantly changing the action of DA. Methiothepin, an antagonist of 5-HT1 and 5-HT2 receptors, blocked both 5-HT-and DA-evoked effects. The results point out a possible convergence of the two different receptors (5-HT1-like and D1) on the same cAMP-dependent system of phosphorylation in the up-regulation of L-type Ca channel activity in mollusc neurones.