Abstract

The human SH-SY5Y neuroblastoma cell line displays morphological, neurochemical, and electrophysiological characteristics of sympathetic neurons. mu-Opioid receptors mediate inhibition of the N-type calcium current present in these cells. Here we have studied the effects of chronic incubation with morphine (1 microM for 3-7 days) in vitro on the inhibition of this current induced by mu-opioid agonists and noradrenaline. In untreated control cells the mu-opioid agonists and noradrenaline. In untreated control cells the mu-opioid agonists morphine (1 microM) and [D-Ala2,N-MePhe4,Gly-ol] enkephalin (DAMGO) (10 nM to 1 microM), and noradrenaline (10 nM to 10 microM) inhibited the calcium current to a similar extent. The maximal effects of DAMGO and noradrenaline were not additive. Chronic exposure to morphine had no effect on the maximum amplitude of the calcium current evoked or on its voltage sensitivity. However, the concentration-response curve to DAMGO was shifted to the right in a parallel manner, with a 7-fold increase in the IC50 value but no change in the maximum inhibition produced. In contrast, the maximum inhibition in response to morphine appeared to be substantially reduced. Noradrenaline inhibited the calcium current equally in untreated and morphine-tolerant cells. Thus, it is concluded that morphine-induced tolerance to inhibition of the N-type calcium current occurs at the single-cell level and is homologous to the mu-opioid receptor. Also, morphine appears to be an agonist of lower efficacy than DAMGO. The results are consistent with tolerance being due to a functional reduction in the mu-opioid receptor reserve, probably by disruption of the receptor/GTP-binding protein interaction.