The k-opiate agonist U50488H decreases the entry of 45Ca into rat cortical synaptosomes by inhibiting N- but not L-type calcium channels

Abstract

The selective K-opiate agonist U50488H (1–100 μM) significantly reduced the uptake of 45Ca into cortical synaptosomes from the brain of the rat, in a time- and dose-dependent manner. In physiological medium, the maximum inhibition occurred after 2 min: this was approximately 55% (at 100 μM) and the IC 50 was 80 nM. Nifedipine (1 μM) had no significant effect on the influx of Ca 2+ in physiological medium (containing 5 mM K +), though, in fact, there was an approximately 20% decrease in the presence of 100 μ M of drug. Nifedipine, however, did cause a significant blockade of the entry of 45Ca in medium containing 10 or 15 mM K +, demonstrating that L-type channels on synaptosomes were operational under depolarising conditions. Under these depolarising conditions, there was an additive inhibitory effect on entry of 45Ca into synaptosomes when U50488H (1 μM) and nifedipine (1 μM) were incubated together. Treatment of synaptosomes with ω-conotoxin (ω-CgTx, 0.5 μM) resulted in a 35% reduction in the uptake of 45Ca. ω-Conotoxin (0.5 μM) or naloxone (20 μM) abolished the inhibitory effect of U50488H on the uptake of 45Ca, but naloxone did not alter the blockade of L-type Ca 2+ channels, caused by nifedipine. In conclusion, the data demonstrate that under depolarising conditions, there are functional L-type calcium channels on nerve endings in the CNS. Secondly, the results show that the K-opiate receptor agonist U50488H decreases [Ca 2+] i by preventing the entry of Ca 2+ into nerve terminals and that this occurs by closure of N- but not L-type Ca 2+ channels: this provides a biochemical rationale for the effects of k-opiate receptor agonists on release of neurotransmitter.