Human neuroblastoma cells (SH-SY5Y) differentiated by retinoic acid exhibited high threshold-activated N-type Ca2+ currents, which were largely inhibited by the two enkephalins, DAGO and DPDPE, as well as by dopamine and somatostatin. The inhibitory effects were fully abolished after pretreatment of cells with pertussis toxin. After washing out the toxin, the inhibitory effects re-established with a time constant of about 16 h. The recovery of Ca2+ current inhibition was similar for all tested agonists. Unexpectedly, we observed a neurotransmitter-induced stimulation of Ca2+ currents in approximately 10% of all investigated cells during the recovery phase. Such a stimulatory effect by otherwise inhibitory receptors was never seen in control cells. It did also not occur when exogenous purified G-proteins of the Gi family were reconstituted via the patch pipette, suggesting that additional mechanisms may play a role in the appearance of stimulatory effects during the recovery phase after pertussis toxin pretreatment.
