Effects of opiates on synaptosomal calmodulin and calcium uptake

Inactivation of depolarization-induced calcium uptake in rat brain synaptosomes

The inactivation of depolarization-induced Ca uptake into rat brain synaptosomes was demonstrated biochemically by comparing 45Ca fluxes after various intervals of predepolarization achieved by abruptly increasing [K+]o. The chemical composition of the medium was maintained throughout the predepolarization and Ca uptake steps. Under these conditions, inactivation was dependent on depolarization, i.e., basal unstimulated Ca uptake in the presence of 5 mM [K+]o did not inactivate. Inactivation of stimulated Ca uptake was dependent on the predepolarization interval, moderately dependent on [Ca]o and relatively independent of membrane potential, i.e., [K+]o and ions such as Ni2+ and Co2+ that blocked Ca uptake. Both cinnarizine and lidoflazine blocked stimulated Ca uptake in a concentration-dependent manner without affecting the % inactivation. Although the amount of stimulated uptake increased greatly between 10 and 30 degrees C, the % inactivation was unaffected by temperature. These findings suggest that inactivation of the presynaptic Ca uptake is an intrinsic property of the channel independent of calcium uptake.

Morphine inhibition of calcium fluxes, neurotransmitter release and protein and lipid phosphorylation in brain slices and synaptosomes

Morphine (1-100 microM) was found to inhibit several concomitant events in brain slices and synaptosomes which are augmented by depolarizing agents. Thus, 45Ca2+ uptake, amino acid neurotransmitter release, increases in 3',5' cyclic AMP levels and 32Pi incorporation to proteins and lipids induced by veratrine (25 microM) and by potassium (56 mM), were each inhibited in a dose related manner. These inhibitory actions of morphine were all prevented by naloxone (1 microM). Evidence is presented that morphine binding to a receptor on the synaptic membrane affects intracellular mechanisms involved in neurotransmitter release possibly via a second messenger system. An enhancing action of GTP on the inhibitory influences of morphine suggests that its actions are mediated at least in part, via a coupling of the receptor to adenyl cyclase in the outer membrane. This is supported by its inhibitory action on the capacity of depolarizing agents to increase cyclic AMP levels.

Chronic ethanol administration inhibits calmodulin-dependent Ca++ uptake in synaptosomal membranes

Chronic ethanol administration inhibits ATP-dependent Ca++ uptake in a preparation of synaptic membranes prepared from mice following 1, 4 and 7 days of ethanol exposure in a liquid diet. Addition of calmodulin (2.5 micrograms) to membranes from mice receiving the control diet produced a slight stimulation of ATP dependent Ca++ uptake. Membranes from ETOH treated mice exhibited reduced capacity to take up Ca++ in ATP-dependent fashion. When calmodulin was added to membranes isolated from mice receiving ETOH on Days 1, 4 and 7 ATP-dependent Ca++ uptake was significantly stimulated (p less than 0.01) compared to (1) ETOH treated membranes in absence of calmodulin, and (2) control membranes. Behavioral tolerance as estimated by bar holding technique was found to be 25, 65 and 91 percent complete for Days 1, 4 and 7 respectively. These studies demonstrate that continued exposure of mice to ethanol via consumption of an ethanol containing liquid diet inhibits one of the mechanisms involving the cytosolic buffering of intracellular Ca++ in nerve terminals. This biochemical effect seen in parallel with the development of tolerance to ethanol impairment of bar holding suggests that increased cytosolic Ca++ may aid in central nervous system adaptation to ethanol.

Effects of morphine and aging on brain calmodulin levels in mice

Calmodulin levels were determined in brain areas (cerebellum, cortex, diencephalon, medulla + pons and striatum) of immature, mature and old mice one hour after they were injected with morphine (10 mg/kg, s.c.) and 3 days after morphine pellet (75 mg) implantation (during tolerance). Acutely, morphine caused significant (two-fold) increases in calmodulin in striatal regions from immature and old mice, and increased calmodulin after morphine administration to mature mice occurred only in the cerebellum. After tolerance to morphine had developed, calmodulin levels in all brain regions were the same as those from placebo-implanted mice.

Effects of aging and morphine administration on calmodulin and calmodulin-regulated enzymes in striata of mice

Male ICR mice, young (25-days old), mature (3-months old), and old (22 months), were injected with morphine sulfate (10 mg/kg, s.c.) or were implanted with morphine pellets (75 mg). Controls received saline injections or placebo pellets. One hour after injections and 72 h after pellet implantations, the mice were decapitated and striatal regions were removed for the following analyses: calmodulin (CaM) levels via radioimmunoassay and activities of cyclic nucleotide phosphodiesterases, adenylate and guanylate cyclases, and Ca2+, Mg2+-ATPase. Acute morphine treatment produced the following: (1) increases in calmodulin levels in the young and old mice while having no effect on mature levels; (2) increases in activities of guanylate cyclase of mature mice while decreasing those of the old mice; (3) no effects on activity of adenylate cyclase; (4) decreased activity of cyclic AMP-phosphodiesterase in young mice only; (5) decreased activity of Ca2+, Mg2+-ATPase in the old mice only. The only changes found in striata from morphine-tolerant mice when compared with age-matched controls were elevations in cyclic GMP-phosphodiesterase activities in all three age groups. Differences in control values of the three age groups were as follows: CaM levels, mature greater than old greater than young; Ca2+, Mg2+-ATPase activity, old greater than mature-young. The results indicate age-induced changes in cellular regulation and biochemical responses to morphine.

Calcium channel activity in rat brain synaptosomes: effects of neuroleptics and other factors regulating phosphorylation and transmitter release

Neuroleptic drugs inhibit depolarization-induced Ca uptake in nerve endings, having IC50 values in the micromolar range. Dopamine and a variety of other substances including opiates and PGE1 are inactive. The effect is probably not mediated by the interaction of the neuroleptics with calmodulin, which itself is a potent inhibitor of stimulated Ca uptake. Dibutyryl cyclic AMP, but not fluoride, increases K+-stimulated Ca uptake. Phosphatidic acid, which is an intermediate in transmitter-stimulated phosphatidylinositol turnover, acts as a Ca ionophore in nerve endings and enhances K+-stimulated Ca uptake at a relatively low concentration. Carbamyl choline, a known stimulator of phosphatidylinositol turnover, did not, however, cause a significant increase in K+-stimulated Ca uptake. Treatment of the nerve ending fraction with relatively small amounts of phospholipase A2 greatly inhibited depolarization-induced Ca uptake, demonstrating the importance of phospholipids for the functioning of the potential-dependent Ca channel in nerve endings. These studies suggest that the regulation of voltage-sensitive Ca channels in nerve endings may be one mechanism controlling transmitter release.

Depolarization-induced increase in synaptosomal membrane calcium monitored by chlorotetracycline fluorescence

Chlorotetracycline (CT) was used as a fluorescent probe for membrane calcium with intact synaptosomes. The net increase in fluorescence intensity at 520 nm, which is a measure of membrane-bound Ca, increases with increasing Ca, saturating in the millimolar range. Membrane Ca can also be detected in the absence of added external Ca. Potassium-induced depolarization of synaptosomes leads to an increase in membrane Ca, reaching a new steady-state value within 5 min. Neither opiates nor phenytoin affected synaptosomal membrane Ca. Relatively high concentrations of chlorotetracycline increased depolarization-induced uptake of 45Ca into synaptosomes. The data suggest that the Ca-CT complex binds to synaptic plasma membranes, and that depolarization-induced Ca influx results in increased Ca binding to the internal surface of the plasma membrane and/or other internal membranes.