Effect of volatile general anesthetics and n-alcohols on glutamate-stimulated increases in calcium ion flux in hippocampal membrane vesicles

N-Methyl-D-aspartate (NMDA) receptor activity was determined by measurement of L-glutamate-stimulated increases in free intracellular calcium concentrations (Cai) in membrane vesicles isolated from mouse hippocampi. Incubation with anesthetic concentrations of the volatile general anesthetics, halothane, enflurane, and diethyl ether, inhibited NMDA responses. The n-alcohols, ethanol, propanol, butanol and pentanol inhibited NMDA receptor responses, but methanol was ineffective. The ability of volatile anesthetics and n-alcohols to inhibit NMDA responses appears to be unrelated to the ability of some of these agents to increase resting Cai since no correlation occurred between the two effects. The results of this study show that hippocampal NMDA receptors are sensitive to inhibition by volatile general anesthetics and n-alcohols. This effect may, in part, underlie their anesthetic effects in vivo.

Differences in ethanol sensitivity of brain NMDA receptors of long-sleep and short-sleep mice

Long-Sleep (LS) and Short-Sleep (SS) mice, selectively bred mice that differ in the duration of anesthesia produced by an acute dose of ethanol, were used to determine the possible association of differing ethanol sensitivity of brain NMDA receptors with differing sensitivity to the anesthetic effects of ethanol in vivo. NMDA receptor-mediated responses were determined by measurement of L-glutamate-stimulated increases in free intracellular calcium concentration (Cai) using the fluorescent indicator for Cai, Indo 1, in microsacs (a cell-free brain membrane vesicle preparation) isolated from hippocampi or cerebral cortices of the two mouse lines. In the absence of added drugs, NMDA responses did not differ between the two lines in hippocampal or cerebrocortical microsacs. However, a high concentration of ethanol (200 mM) inhibited NMDA responses in hippocampal microsacs from LS mice. In contrast, a moderate concentration of ethanol (50 mM) stimulated NMDA responses in hippocampal microsacs isolated from SS mice. In cerebrocortical microsacs, ethanol inhibited NMDA responses in the two lines to an equivalent degree. MK-801, a noncompetitive blocker of NMDA receptors, blocked NMDA responses at lower concentrations in hippocampal microsacs from LS mice than in SS mice, but produced a similar degree of inhibition of NMDA responses in cerebrocortical microsacs from the two lines. A high concentration of ethanol (200 mM) increased resting Cai in hippocampal microsacs from LS mice but not in hippocampal microsacs from SS mice, and increased resting Cai in cerebrocortical microsacs isolated from both lines of mice equally. The small change in resting Cai produced by MK-801 in cerebrocortical microsacs did not differ between the two lines.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol sensitivity of brain NMDA receptors in mice selectively bred for differences in response to the low-dose locomotor stimulant effects of ethanol

Brain NMDA receptor responses and their sensitivity to ethanol in vitro were determined in replicate lines of FAST and SLOW mice, selectively bred for differences in sensitivity to the locomotor stimulant effects of a low dose of ethanol. L-Glutamate-stimulated increases in the intracellular free calcium concentration (Cai) were determined in microsacs, a cell-free brain membrane preparation, isolated from hippocampus or cerebral cortex. Previous work showed that L-glutamate-stimulated increases in Cai in microsacs are mediated by activation of NMDA receptors. The concentration response for L-glutamate-stimulated increases in Cai did not differ between the lines in either hippocampal or cerebrocortical microsacs. Ethanol produced a concentration-dependent decrease in L-glutamate-stimulated increases in Cai in hippocampal and cerebrocortical microsacs from SLOW mice, but this effect of ethanol was reduced or absent in microsacs isolated from FAST mice. Resting Cai and the ability of a high ethanol concentration to increase resting Cai did not differ between the lines. These results suggest that differences in the sensitivity of brain NMDA receptors to the effects of ethanol determine, at least in part, differences in the locomotor stimulant effects of low doses of ethanol in FAST and SLOW mice. These differences are not due to ethanol effects on resting Cai.

Effect of anesthetic and convulsant barbiturates on N-methyl-D-aspartate receptor-mediated calcium flux in brain membrane vesicles

The effects of anesthetic and convulsant barbiturates on brain N-methyl-D-aspartate (NMDA) receptor function were examined in a cell-free membrane vesicle preparation from mouse hippocampus. Increases in intracellular free calcium concentrations (Cai) were determined using a fluorescent dye, Indo-1, after stimulation with the NMDA receptor agonist, L-glutamate. Anesthetic barbiturates inhibited NMDA responses in a concentration-dependent manner with a rank order of potency of secobarbital > amobarbital > pentobarbital > mephobarbital = phenobarbital >> barbital. However, the IC50 values for these barbiturates were larger than probable blood anesthetic concentrations. Barbiturates with both anesthetic and convulsant effects in mice [optical isomers of pentobarbital and secobarbital, 5-(2-cyclohexylideneethyl)-5-ethylbarbituric acid and (+/-)-dimethylbutylbarbituric acid] also reduced NMDA responses. Inhibition of NMDA responses by racemic pentobarbital or isomers of pentobarbital was noncompetitive. Resting Cai was altered by all barbiturates tested except secobarbital and barbital, but not in a consistent manner, suggesting that the effect of barbiturates on resting Cai is not related either to their effects on NMDA receptor responses or to their behavioral effects. These results show that anesthetic and convulsant barbiturates inhibit NMDA responses, but their anesthetic and convulsant activities may be primarily due to their effects on other brain targets.

Determination of the intravesicular ionized sodium concentration in a cell-free brain membrane vesicle preparation using the fluorescent indicator, SBFI

The intravesicular ionized Na concentration (Nai) was measured using the fluorescent Na indicator, SBFI, in microsacs, a cell-free brain vesicle preparation. SBFI fluorescence was monitored by a dual excitation-wavelength method at the same wavelengths commonly employed for Fura-2 determination of intracellular ionized calcium concentrations (Cai). Calibration of SBFI fluorescence was reliably performed in brain microsacs in situ. Resting Nai was dependent on the extravesicular Na concentration (Nao) and was about 36 mM in the presence of 120 mM extracellular Nao. In the presence of ouabain, an inhibitor of the plasma membrane Na/K-ATPase, Nai increased by 27 mM over 60 s. Nai was also increased by resuspension of microsacs in buffers of low free Ca concentrations (0 to 0.8 mM), indicating that the extravesicular Ca concentration (Cao) is an important regulator of Nai. Alkaloids active at voltage-sensitive Na channels, veratridine and aconitine, also increased Nai. These results demonstrate the presence of homeostatic mechanisms for neuronal Nai regulation and show that Nai can be measured in a cell-free brain vesicle preparation using SBFI.

Alteration of general anesthetic potency by agonists and antagonists of the polyamine binding site of the N-methyl-D-aspartate receptor

Anesthetic potency was examined in mice after pretreatment with various putative agonists and antagonists of the polyamine site of the N-methyl-D-aspartate (NMDA) receptor. Anesthetic potency was determined for ethanol and pentobarbital by measurement of duration of loss of righting reflex, and for the volatile anesthetics, halothane and diethyl ether, by measurement of the minimum alveolar concentration (MAC). The polyamines, spermine and spermidine, increased the duration of ethanol and pentobarbital anesthesia and reduced halothane MAC, but had no effect on diethyl ether MAC. Putative antagonists of the polyamine site, ifenprodil and arcaine, also increased the anesthetic potency of ethanol, but diaminodecane, an inverse agonist, was inactive. Concurrent pretreatment with spermine or ifenprodil reduced the ability of MK-801 to increase ethanol anesthesia duration, but did not alter the ability of CGS 19755 to increase ethanol anesthesia duration. Although this study did not rule out effects of polyamines on other neurochemical systems, these results suggest that spermine and spermidines could increase anesthetic potency by acting at a site on the NMDA receptor which negatively modulates the binding of MK-801. Results of this study also suggest that the anesthetic potency of ethanol and halothane is more closely linked to the activity of brain NMDA receptors than is that of pentobarbital or diethyl ether.

Effect of CGS 19755, a competitive N-methyl-D-aspartate antagonist, on general anesthetic potency

CGS 19755 is a competitive N-methyl-D-aspartate (NMDA) receptor antagonist which penetrates the blood-brain barrier. The effect of pretreatment with subanesthetic doses of CGS 19755 on general anesthetic potency was determined in mice. Mice were pretreated with saline or CGS 19755 by intraperitoneal (IP) administration 30 min before IP administration of an anesthetic dose of ethanol or pentobarbital or measurement of the volatile anesthetic minimum alveolar concentration (MAC). CGS 19755 increased the duration of ethanol- and pentobarbital-induced loss of righting reflex in a dose-dependent manner. The highest dose of CGS 19755 tested, 50 mg/kg, increased duration of loss of righting reflex by about four- and twofold for ethanol and pentobarbital, respectively. CGS 19755 also decreased the MAC for halothane. However, CGS 19755 pretreatment had no effect on the MAC for diethyl ether. These results suggest that the potency of certain general anesthetic agents can be increased by antagonism of brain NMDA receptors.

N-methyl-D-aspartate increases cytoplasmic free calcium in mouse hippocampus

The effect of N-methyl-D-aspartate (NMDA) and L-glutamate on the concentration of intracellular free calcium (Cai) and on uptake of the calcium was determined in microsacs and synaptosomes isolated from mouse brain. L-Glutamate and NMDA increased Cai in hippocampal microsacs but had little or no effect on Cai in microsacs, isolated from cortex or cerebellum or in synaptosomes. N-Methyl-D-aspartate also increased uptake of calcium, measured using 45Ca into hippocampal microsacs. The EC50 values for NMDA-stimulated increases in Cai and uptake of calcium in microsacs were about 30 microM. Maximum responses were observed with 100 microM NMDA. Increases in Cai stimulated by NMDA were dependent on extracellular calcium, indicating that NMDA increased Cai in microsacs by increasing conductance through an NMDA receptor-operated cation channel, rather than by releasing calcium from intracellular stores. The NMDA antagonists, 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), 2-amino-5-phosphonopentanoic acid (AP-5), magnesium and zinc blocked responses to NMDA. This demonstrates NMDA-mediated effects on ion flux in a cell-free preparation from brain. This preparation may be useful for study of the in vitro effects of drugs or toxins on NMDA receptors in brain.

Effect of chronic ethanol treatment and selective breeding for sensitivity to ethanol on calcium release induced by inositol trisphosphate or ethanol from brain and liver microsomes

Our previous work showed that ethanol increases the resting intracellular free calcium concentration (CAi) in synaptosomes and releases calcium from an inositol trisphosphate (IP3)-insensitive calcium store of brain microsomes. In this report, we investigated the effects of chronic ethanol treatment and selective breeding for hypnotic sensitivity to ethanol on IP3 and ethanol-stimulated calcium release from brain and liver microsomes. Chronic ethanol treatment did not alter IP3-stimulated calcium release from brain microsomes or ethanol-stimulated calcium release from brain or liver microsomes. Chronic ethanol treatment increased the spontaneous release of calcium from brain but not liver microsomes. In microsomes isolated from cerebellum or cerebral cortex of long-sleep (LS) and short-sleep (SS) mice, ethanol and IP3 released calcium in a concentration dependent manner. The amount of calcium released by ethanol and IP3 was larger in microsomes isolated from cerebellum than microsomes from cerebral cortex. However, the amount of calcium released by ethanol and IP3 did not differ between the two lines in either brain area. These results do not support the idea that the hypnotic effects of ethanol are due to ethanol-induced calcium release from a nonmitochondrial intracellular calcium store in brain tissue. The development of ethanol tolerance and dependence also does not appear to be associated with altered ability of ethanol to release calcium from non-mitochondrial intracellular stores; however, effects of chronic ethanol exposure on spontaneous release of intracellular calcium could alter neuronal function in ethanol dependence.

The noncompetitive N-methyl-D-aspartate antagonists, MK-801, phencyclidine and ketamine, increase the potency of general anesthetics

The potency of general anesthetics from different chemical classes was tested after pretreatment with subanesthetic doses of noncompetitive N-methyl-D-aspartate (NMDA) antagonists in mice. Changes in general anesthetic potency were assessed by determination of alteration of duration of loss of righting reflex for ethanol and pentobarbital and changes in the minimum alveolar concentration (MAC) for the volatile anesthetics, halothane and diethyl ether. The ability of the noncompetitive NMDA antagonists, MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclo-hepten-5,10-imine ], phencyclidine (PCP) and ketamine, to increase the potency of general anesthetics paralleled their potency as NMDA antagonists and their affinity for the PCP receptor site of the NMDA receptor-ionophore complex (MK-801 greater than PCP greater than ketamine). These results indicate that block of central NMDA receptors may contribute to the production of anesthesia by a variety of agents.

Decreased efficacy of inositol 1,4,5-trisphosphate to elicit calcium mobilization from cerebrocortical microsomes of aged rats

The effects of aging on the ability of brain microsomes to sequester calcium in response to ATP stimulation and to release calcium in response to inositol 1,4,5-trisphosphate (IP3) stimulation were examined. Calcium uptake and release were compared in microsomal preparations from the cerebral cortex, hippocampus, thalamus, and cerebellum of 3-, 16-, and 28-month-old male Fischer 344 rats. No age-related differences were found in the ability of brain microsomes to sequester calcium in response to ATP stimulation. A maximally effective concentration of IP3 (1 microM) released approximately 30% of the calcium sequestered by microsomes. This was observed in all brain regions and age groups studied except in the cerebral cortex, where the amount of calcium released by IP3 was reduced by 50% in the oldest age group. Concentration-response curves for IP3 in this brain region from 3- and 28-month-old rats confirmed that the maximally effective concentrations, the EC50 values, and the Hill coefficients did not differ with aging. These data indicate that the efficacy of IP3 is selectively diminished in the cerebral cortex of aged rats and that this age-related change may contribute to the attenuated responsiveness of neurons in this brain region to activation by phosphoinositide-coupled receptors.

Effect of anesthetics on calcium stores and membrane order of brain microsomes

The effects of anesthetic agents from different chemical classes and a nonanesthetic membrane-disordering agent, 2-[2-methoxyethoxy]ethyl-8-[cis-2-n-octylcyclopropyl]octanoate (A2C), on calcium stores of whole brain microsomes and on order of microsomal membranes were compared. Calcium release was determined by measurement of the extramicrosomal calcium concentration and membrane order by the fluorescence polarization of diphenylhexatriene (membrane core) and trimethylammonium-diphenylhexatriene (membrane "surface"). n-Alkanols (methanol, ethanol, propanol, butanol, pentanol, and hexanol), benzyl alcohol (10-100 mM), and diethyl ether (30-300 mM) released calcium from brain microsomes and decreased the surface and interior membrane order of microsomal membranes. Pentobarbital (0.05-1 mM) did not release calcium from microsomes and did not alter the order of brain microsomal membranes. Halogenated anesthetics (halothane, methoxyflurane, and enflurane), 4-phenyl-1-butanol, and A2C decreased membrane order but failed to release calcium from brain microsomes. Comparison of the effects of these agents on microsomal calcium release and order of microsomal membranes revealed that decreases in membrane order are unrelated to the calcium-mobilizing actions of anesthetic compounds. In addition, molecular size appeared to limit ability of anesthetic compounds to release calcium from microsomes. For n-alkanols, benzyl alcohol, and diethyl ether, the ability to release microsomal calcium was correlated with anesthetic potency. Our results demonstrate, for the first time, direct effects of anesthetic agents on intracellular calcium stores of brain tissue and indicate that these stores may be target sites for anesthetics.

Ethanol and inositol 1,4,5-trisphosphate release calcium from separate stores of brain microsomes

The effects of ethanol and inositol 1,4,5-trisphosphate (IP3) on releasable Ca stores were examined in microsomes isolated from mouse whole brain. Ca release was monitored by determination of changes in the extra-microsomal Ca concentration using Indo-1, a fluorescent Ca indicator. In the absence of ATP, ethanol released Ca from microsomes in a concentration-dependent manner, with a threshold for Ca release between 25 and 50 mM. Ethanol-induced release of microsomal Ca was reduced by approximately 50% after ATP-stimulated uptake of Ca, indicating that the ethanol-releasable pool was diminished by ATP-dependent uptake of Ca into an ethanol-insensitive microsomal pool. Release of Ca produced by ethanol was linear with concentration (up to 400 mM). By contrast, IP3-induced Ca release was saturable and was dependent on prior ATP-stimulated Ca uptake. Simultaneous addition of ethanol and IP3 produced additive responses. These results show that pharmacologically relevant concentrations of ethanol release Ca from an IP3-insensitive intracellular Ca store. Furthermore, our results demonstrate the existence of at least two releasable stores of Ca in brain microsomes.

Increased platelet intracellular calcium concentration in patients with bipolar affective disorders

Using the fluorescent indicator Fura 2, we measured the free intracellular calcium ion concentration in blood platelets of patients with untreated mania, bipolar depression, and unipolar depression; patients who had recovered from bipolar depression or mania; and age- and sex-matched controls. The baseline intracellular calcium ion concentration was significantly increased in platelets from patients with mania compared with controls. The free intracellular calcium ion concentration after stimulation with platelet-activating factor and thrombin was significantly higher in platelets of manic and bipolar depressed patients than in all other groups. The degree to which intracellular calcium ion concentration increased over baseline after stimulation was significantly lower in unipolar than in bipolar patients. These findings suggest that platelets of manic and depressed bipolar patients have a similar enhancement of intracellular calcium ion activity that is distinctly different from the decreased ability of platelets of unipolar patients to mobilize intracellular calcium in response to stimulation.

Antagonism of ethanol and pentobarbital actions by benzodiazepine inverse agonists: neurochemical studies

The benzodiazepine inverse agonist Ro 15-4513 has been shown to antagonize several behavioral effects of ethanol and to block the effects of ethanol on chloride flux across brain membranes. We used isolated mouse brain membranes to test whether Ro 15-4513 would reduce the effects of ethanol on membrane fluidity, voltage-dependent calcium channels, microsomal calcium release or binding of t-[35S]butylbicyclophosphorothionate. None of these actions of ethanol were altered by Ro 15-4513. The enhancement of gamma-aminobutyric acid (GABA)-activated chloride flux produced by ethanol or pentobarbital was antagonized partially by Ro 15-4513. Another inverse agonist, FG 7142, was more effective than Ro 15-4513 as an antagonist of ethanol actions on chloride flux. These results demonstrate that the ethanol antagonist action of Ro 15-4513 is specific for GABA-activated chloride flux and does not extend to other neurochemical actions of ethanol. The inverse agonist action (i.e., inhibition of GABA-activated chloride flux tested in the absence of ethanol) of Ro 15-4513 and FG 7142 was revealed by pretreatment of mice in vivo with ethanol. This raises the possibility that ethanol exposure increases the inverse agonist actions of Ro 15-4513 and related drugs and that these inverse agonist actions contribute to the ethanol antagonism observed in vivo and in vitro.

Neuronal intracellular calcium concentrations are altered by anesthetics: relationship to membrane fluidization

The effects of anesthetic agents selected from various chemical classes (halothane, diethylether, pentobarbital and n-alkanols) on intracellular ionized Ca concentrations (Cai) were examined in mouse whole brain synaptosomes. Cai were determined using fura-2, a fluorescent Ca indicator. Halothane (3-6 mM), diethylether (100 mM) and n-alkanols (ethanol, butanol, pentanol and hexanol) increased resting Cai by 20 to 70%. Pentobarbital did not alter resting Cai when examined over a wide range of concentrations (0.01-1 mM). The ability of the anesthetics to increase resting Cai was correlated with their membrane fluidizing actions. Depolarization of synaptosomes by the addition of 50 mM KCI increased Cai by about 25%. The values of Cai measured after depolarization were reduced by diethylether (10-50 mM) and pentobarbital (0.05-1 mm) but were increased or unaltered by halothane and n-alkanols, respectively. All anesthetic agents tested except halothane reduced the net depolarization-dependent increase in Cai. These results provide a mechanism for anesthetic-induced neuronal hyperpolarization and predict effects of these agents on neurotransmitter release.

Effect of chronic ethanol treatment and selective breeding for hypnotic sensitivity to ethanol on intracellular ionized calcium concentrations in synaptosomes

The effect of chronic ethanol treatment and of selective breeding for hypnotic sensitivity to ethanol on intracellular ionized calcium concentrations (Cai) were examined in mouse whole brain synaptosomes. Following treatment with a liquid diet for 7 days, resting Cai and KCl-stimulated increases in Cai were measured in synaptosomes isolated from chronic ethanol-treated and pair-fed animals. Ethanol (350-700 mM) increased resting Cai and reduced KCl-stimulated increases in Cai in synaptosomes isolated from pair-fed animals. Ethanol-induced changes in Cai were reduced in synaptosomes isolated from chronic ethanol-treated animals. The effect of ethanol on synaptosomal Cai in long-sleep (LS) and short-sleep (SS) mice, selectively bred for differential sensitivity to the hypnotic actions of acute ethanol, was also investigated. In the absence of ethanol, resting values of Cai and KCl-stimulated increases in Cai did not differ between the two lines of mice. Ethanol (200-600 mM) increased resting Cai and reduced depolarization-stimulated increases in Cai in both long-sleep and short-sleep mice to the same degree. Similarly, KCl-stimulated increases in Ca uptake did not differ in synaptosomes isolated from whole brains and cortices of LS and SS mice, in the absence of presence of ethanol. These findings demonstrate that tolerance develops to the effect of ethanol on neuronal Cai following chronic treatment. However, sensitivity to the hypnotic action of ethanol is not related to changes in neuronal Cai in LS and SS mice.

Effect of ethanol on intracellular ionized calcium concentrations in synaptosomes and hepatocytes

The effect of ethanol on intracellular ionized calcium concentrations (Cai) was studied in synaptosomes isolated from mouse whole brain and in hepatocytes isolated from rat liver. The fluorescent calcium chelator, fura-2, was used to quantitate Cai. Incubation of synaptosomes with ethanol (350-700 mM) increased resting Cai and decreased the effectiveness of KCl to raise Cai in a concentration-dependent manner. Ethanol produced an initial rapid (less than 10 sec) increase in resting Cai that remained elevated for at least 14 min in the presence of the drug. The increase in resting Cai was correlated with the inhibitory effect of ethanol on depolarization-induced increases in Cai. Resting Cai was dependent on the external calcium concentration (0-1 mM). However, the ethanol-induced increase in resting Cai (expressed as percent of control) did not differ in the presence of several extracellular calcium concentrations (0.01, 0.1, and 1 mM). Incubation of synaptosomes in a Na-free buffer resulted in a higher resting Cai and slightly enhanced the effect of ethanol to increase resting Cai. In contrast to these results in brain tissue, ethanol (30-600 mM) did not alter resting Cai or vasopressin-stimulated increases in Cai in hepatocytes. Our results suggest that the anesthetic effects of alcohols may be mediated, in part, by increased resting Cai and by decreased calcium influx through voltage-sensitive calcium channels. In addition, our findings suggest possible mechanisms by which ethanol increases resting Cai in neuronal tissue.

Inhibition of fast phase calcium uptake and endogenous norepinephrine release in rat brain region synaptosomes by ethanol

The effects of ethanol on fast phase calcium (Ca2+) uptake and endogenous norepinephrine release were assessed simultaneously in KCl-depolarized synaptosomes isolated from rat hypothalamus, brainstem and cerebellum. Incubation of brain regional synaptosomes with ethanol resulted in a concentration-dependent inhibition of Ca2+ uptake after 1 s of depolarization. Hypothalamic synaptosomes were most sensitive to the inhibitory effect of ethanol on voltage-dependent Ca2+ uptake and brainstem synaptosomes were least sensitive. Endogenous norepinephrine release from synaptosomes was not altered by addition of ethanol in vitro at any of the concentrations examined (25-200 mM). Chronic ethanol administration resulted in an adaptation to the inhibitory effect of ethanol on Ca2+ uptake into hypothalamic synaptosomes but did not alter the inhibitory effect of ethanol on Ca2+ uptake into brainstem or cerebellar synaptosomes. Fast phase, voltage-dependent norepinephrine release was inhibited by ethanol added in vitro but only in synaptosomes isolated from hypothalami and cerebella of chronically treated animals. Brain regional norepinephrine concentrations were unaltered by chronic ethanol administration. These results suggest that chronic ethanol treatment may alter the coupling of Ca2+ entry with norepinephrine release in some noradrenergic neurons. Effects of ethanol on synaptosomal Ca2+ entry and norepinephrine release differ depending on the brain region.

Correlation of rates of calcium entry and release of endogenous norepinephrine in rat brain region synaptosomes

Voltage-dependent 45Ca2+ uptake and endogenous norepinephrine (NE) release were measured simultaneously in synaptosomes isolated from rat hypothalamus, brainstem, and cerebellum at 1, 3, 5, 15, and 30 s. In synaptosomes depolarized by 125 mM KCl, 45Ca2+ uptake and NE release exhibited fast and slow components. Rates of NE release and 45Ca2+ uptake were fastest from 0 to 1 s. NE release and 45Ca2+ uptake rates from 1 to 5 s were less than 15% of 0-1 s rates. Both resting (5 mM KCl) and depolarization-induced (125 mM KCl) NE release paralleled 45Ca2+ uptake from 1 to 30 s. Voltage-dependent NE release was approximately 1% and 2% of total synaptosomal NE content at 1- and 30-s measurement intervals, respectively, and did not differ between the three brain regions studied. Calcium and potassium dependence studies showed that NE release was stimulated by increased potassium and that depolarization-induced NE release was dependent on the presence of external calcium. These results show that calcium-dependent NE release from synaptosomes is correlated with calcium entry. Both processes exhibit fast and slow temporal components.

45Ca2+ uptake into rat whole brain synaptosomes unaltered by dihydropyridine calcium antagonists

Voltage-dependent 45Ca2+ uptake into rat whole brain synaptosomes was measured after 3-s KCl-induced depolarization to investigate possible inhibitory effects of calcium antagonists, nitrendipine, nimodipine, and nisoldipine. At a Ca2+ concentration of 1.2 mM, nitrendipine, in concentrations ranging from 0.1 nM to 10 microM, had no effect on 45Ca2+ uptake. When the Ca2+ concentration was lowered to 0.06 and 0.12 mM, nitrendipine, 10 microM, inhibited 45Ca2+ uptake in response to 109 mM KCl depolarization. However, in a separate concentration response study, nitrendipine, nimodipine, and nisoldipine, 0.1 nM to 10 microM, failed to alter the uptake of 45Ca2+ (0.06 mM Ca2+) into 30 mM KCl-depolarized synaptosomes. The high concentrations of these agents required to depress 45Ca2+ uptake indicate that the dihydropyridine calcium antagonists are considerably less potent in brain tissue than in peripheral tissue.