D-tartrate alters uptake of [3H]dopamine into brain synaptic vesicles

The use of D-tartrate containing media for measuring uptake of catecholamines into brain synaptic vesicles alters the properties of transport. Absolute concentrations of inhibitors determined in competition studies should be viewed with caution.

The effect of nicotine on catecholaminergic storage vesicles

The present study examined the action of nicotine on the accumulation of [3H]dopamine into synaptic vesicles prepared from mouse cerebral cortex or bovine striatum. Nicotine was shown to be a weak inhibitor of [3H]dopamine accumulation, with an IC50 of approximately 0.2-0.4 mM. In addition, repeated nicotine administration (1.2 mg (-)-nicotine di-(+)tartrate/kg s.c., twice daily for 10 days) in vivo in BALB/cBy male mice did not alter the potency of reserpine in inhibiting [3H]dopamine accumulation into synaptic vesicles, nor did it change the slight shift induced by nicotine in the potency of reserpine in inhibiting [3H]dopamine accumulation. The present results show that nicotine is an inhibitor of vesicular dopamine accumulation at high concentrations.

Cerebral cortical concentrations of bioamines and their metabolites during arousal and after feeding in the little brown bat (Myotis lucifugus)

The concentrations of bioamines and their metabolites have been determined in March and April during arousal from hibernation in the cerebral cortex of the little brown bat (Myotis lucifugus). The patterns during arousal for dopamine and serotonin (5-HT) were similar with a significant fall in concentrations by 1 h of arousal, and an inverse relationship with their respective metabolites, 3,4-dihydroxyphenylacetic acid and 5-hydroxyindolacetic acid (5-HIAA). This suggests an acute release and metabolism of these bioamines with onset of arousal. During arousal, cerebral cortical concentrations of norepinephrine (NE) were not significantly changed. Levels of homovanillic acid were markedly depressed during hibernation and rose acutely with arousal. After arousal and 4 days of feeding in April, there was an increase in all bioamines and their metabolites studied except for NE. Of note is the marked decrease in the hibernating level of 5-HT and increase in its metabolite 5-HIAA from March to April, which may herald the natural termination of hibernation. Our results suggest that the brain of hibernators undergoes complex changes in the modulation of neurotransmitter systems which are consistent with both down- and up-regulation of neuronal activity in the maintenance of hibernation and the initiation of the arousal process.

Chronic L-deprenyl does not alter the restoration of striatal dopamine in MPTP-lesioned mice

The present study examined the effect of chronic L-deprenyl on dopaminergic terminal function after mouse striatal terminals were lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 2 x 30 mg/kg s.c.). In the MPTP-lesioned mice, the level of dopamine was decreased by 59% 1 week after MPTP administration and by 22% at 16 weeks. Chronic administration of L-deprenyl (0.1 mg/kg, once weekly for up to 16 weeks) did not alter striatal dopamine metabolism, although monoamine oxidase B activity was reduced by 50% during this 16-week period, and did not alter the rate of restoration of the level of striatal dopamine.

Uptake of acetyl-L-carnitine in the brain

Analysis in mouse brain slices of the uptake of acetyl-L-[N-methyl-14C]carnitine with time showed it to be concentrative, and kinetic analysis gave a Km of 1.92 mM and a Vmax of 1.96 mumol/min per ml, indicating the presence of a low-affinity carrier system. The uptake was energy-requiring and sodium-dependent, being inhibited in the presence of nitrogen (absence of O2), sodium cyanide, low temperature (4 degrees C), and ouabain, and in the absence of Na+. The uptake of acetyl-L-carnitine was not strictly substrate-specific; gamma-butyrobetaine, L-carnitine, L-DABA, and GABA were potent inhibitors, hypotaurine and L-glutamate were moderate inhibitors, and glycine and beta-alanine were only weakly inhibitory. In vivo, acetyl-L-carnitine transport across the blood-brain barrier had a brain uptake index of 2.4 +/- 0.2, which was similar to that of GABA. These results indicate an affinity of acetyl-L-carnitine to the GABA transport system.

Dopamine D1 receptor and dopamine D2 receptor binding activity changes during chronic administration of nicotine in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice

The effect of nicotine on MPTP-induced changes in striatal dopamine receptors binding activity was investigated. Dopamine D1 and D2 receptors were labeled with [3H]SCH-23390 and [3H]spiperone respectively in BALB/cBy mice. With administration of only MPTP, which caused more than an 80% decrease in striatal dopamine level, binding of 0.15 nM [3H]spiperone was increased by 37%; whereas 0.3 nM [3H]SCH-23390 binding was unchanged. With chronic nicotine treatment (0.4 mg/kg twice daily for 7-9 days), [3H]SCH-23390 binding activity was increased by 27% and [3H]spiperone binding activity was unchanged. When nicotine was administered after MPTP, their separate effects could be seen in that both the D1 and D2 dopamine receptor ligand binding activities were increased and that nicotine elevated the ratio of D1/D2 receptor binding activities in MPTP-treated mice.

Chronic L-deprenyl-induced up-regulation of the dopamine uptake carrier

L-Deprenyl is an inhibitor of monoamine oxidase B and dopamine uptake. Chronic L-deprenyl (10 mg/kg i.p., twice weekly for 4 weeks) was shown to inhibit monoamine oxidase B activity by 89%, and also to induce an up-regulation of the [3H]mazindol binding site associated with the striatal dopamine uptake carrier. Scatchard analysis indicated a 56% increase in the maximal number of [3H]mazindol binding sites in chronic L-deprenyl animals, but no effect on the affinity of these binding sites. The ability of L-deprenyl to up-regulate the [3H]mazindol-associated dopamine uptake carrier appears to be a result of its role as a dopamine uptake inhibitor.

Action of nicotine on accumbens dopamine and attenuation with repeated administration

The behavioral and physiological effects of repeated nicotine administration are complex; sedation and hypothermia are present early but become attenuated while locomotor activity increases. Maximal blood levels and behavioral changes occur within 10 min of s.c. injection. We examined the effects of 10 nicotine injections (0.8 mg/kg) in 14 days on the levels of brain amines following challenge with either saline or nicotine on the 15th day. Dopamine, DOPAC, HVA, 3-methoxytyramine, norepinephrine, 5-hydroxytyramine, and 5-HIAA were measured in the frontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen, substantia nigra and ventral tegmental area. Ten minutes after nicotine was given to rats that had previously received only saline the levels of dopamine and its metabolite DOPAC indicated an increase in dopamine turnover in the nucleus accumbens. Of the areas examined the accumbens was the most sensitive to nicotine, with few significant amine changes in other regions. Twenty-four hours after the last nicotine injection the levels of dopamine and its metabolites indicated a sustained decrease in dopamine turnover in the accumbens induced by repeated administration. Following repeated nicotine a nicotine challenge still induced an acute increase in dopamine turnover in the accumbens, but the response was less than in animals not previously given nicotine. The results confirm earlier studies indicating that the accumbens is a major site of nicotine action.

Oxidative metabolism of cocaine: comparison of brain and liver

Norcocaine (NC) and N-hydroxynorcocaine (NHNC), products of the oxidative metabolism of cocaine, were examined in plasma, brain, and liver of mice injected intraperitoneally with cocaine. Plasma levels of NHNC were altered in vivo by inhibiting esterase activity with diazinon and chloral hydrate or activating esterase activity with phenobarbital, and activating the microsomal P-450 system with phenobarbital. Changes in plasma concentrations of NHNC resulted in similar changes in brain, which were often different from those in liver. After intracisternal administration of cocaine to mice, no appreciable amount of NC or NHNC could be detected in brain; the same results were obtained upon intracisternal and intraventricular administration to rats. Microsomal preparations from mouse brain were found to be considerably less active than those from liver in converting NC to NHNC. We conclude that the cerebral oxidative metabolism of cocaine is not appreciable and that most of the NC and NHNC found in the brain after systemic cocaine administration is derived from plasma rather than formed centrally by brain microsomal enzymes.

Ascorbic acid inhibits [3H]SCH-23390 binding to striatal dopamine D1 receptors

The present study describes the inhibition of [3H]SCH-23390 binding to striatal dopamine D1 receptors in the presence of ascorbic acid. Specific [3H]SCH-23390 binding was maximally inhibited by 0.1 mM ascorbic acid. As determined by Scatchard analysis the binding in the presence of 0.01, 0.1, or 10 mM ascorbic acid was consonant with non-competitive inhibition with a 26%, 38%, or 19% decrease, respectively, in the maximal number of binding sites; the affinity of these binding sites was not affected. Inhibition of [3H]SCH-23390 binding by ascorbic acid was reversible; striatal homogenates incubated with 0.1 mM ascorbic acid and subsequently washed free of ascorbic acid had the same Scatchard parameters as untreated preparations.

Effect of chronic oral nicotine on dopaminergic function in the MPTP-treated mouse

Although epidemiological studies have suggested a lower incidence of Parkinson's disease in cigarette smokers, repeated exposure to cigarette smoke or nicotine does not protect against neurotoxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Since there is some evidence that nicotinic antagonists, nicotine, and neurotransmitters may have tropic effects on neurite outgrowth, the present study examined the effects of chronic nicotine administration for 16 weeks (in drinking water; 5 mg/kg consumed per day) on the rate of terminal recovery after striatal lesioning with MPTP (2 x 30 mg/kg, s.c.). Terminal recovery, as measured by the rate of recovery in the level of striatal dopamine, was not affected by nicotine. Monoamine oxidase-B activity was not reduced by MPTP, nor did nicotine affect its activity in striatal homogenates.

Ascorbic acid and striatal transport of [3H] 1-methyl-4-phenylpyridine (MPP+) and [3H] dopamine

The inhibition of uptake of [3H] dopamine and [3H] 1-methyl-4-phenylpyridine (MPP+) was examined in mouse striatal synaptosomal preparations. Kinetic analysis indicated that ascorbic acid is a noncompetitive inhibitor of [3H] MPP+ uptake. No inhibition of [3H] dopamine uptake is observed. The dopamine uptake blockers, GBR-12909, cocaine, and mazindol strongly inhibit (IC50 less than 1 uM) both [3H] dopamine and [3H] MPP+ transport. Nicotine, its metabolites, and other tobacco alkaloids are weak inhibitors (IC50 greater than 1 mM) except 4-phenylpyridine and lobeline, which are moderate inhibitors (IC50 = 3 to 40 uM) of both [3H] dopamine and [3H] MPP+ uptake. These similarities in potencies are in agreement with the suggestion that [3H] MPP+ and [3H] dopamine are transported by the same carrier. The differences observed in the alteration of dopaminergic transport and mazindol binding by ascorbic acid suggest that ascorbic acid's effects on [3H] MPP+ transport are related to translocation and/or dissociation processes occurring subsequent to the initial binding event.

Metaphit prevents locomotor activation induced by various psychostimulants and interferes with the dopaminergic system in mice

Metaphit, an isothiocyanate analog of phencyclidine and a proposed phencyclidine receptor acylator, inactivated the carrier involved in the neuronal uptake of dopamine in in vitro experiments with preparations of the striatum in the mouse. In ex vivo experiments 2 and 24 hr after the intravenous administration of metaphit, no changes were observed either in the binding of [3H]cocaine to striatal membranes or in the uptake of [3H]dopamine into synaptosomes or slices. In in vivo experiments 24 hr after pretreatment with metaphit, selective labelling of uptake sites for dopamine in the striatum of the mouse with [3H]GBR 12935 was unaffected. In these in vivo experiments, however, metaphit antagonized the locomotor stimulation induced by blockers of the uptake of dopamine (methylphenidate, mazindol, cocaine, GBR 12909) but not that induced by drugs that affect locomotion by other mechanisms (amphetamine, phencyclidine). Twenty-four hours after treatment with metaphit there was an increase in homovanillic acid in all regions of the brain studied (striatum, olfactory tubercle, cerebral cortex). There was no effect of metaphit on the disappearance rate of 3,4-dihydroxyphenylacetic acid and homovanillic acid from the striatum during the inhibition of monoamine oxidase with pargyline. If the increase in homovanillic acid reflected a greater rate of dopamine catabolism in metaphit-treated mice, it could explain the lack of locomotor stimulation of blockers uptake of the dopamine in these animals, resulting from a rapid breakdown of extracellularly accumulated dopamine.

Behavioral and biochemical effects of nicotine in an MPTP-induced mouse model of Parkinson's disease

This study examined the effects of nicotine on locomotor activity and on the level of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum and olfactory tubercle of mice that had been treated with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP significantly lowered the spontaneous locomotor activity 1-2 weeks and 2 months after 2 injections of MPTP (30 mg/kg SC, 24 hr apart) in young adult (3 months) and old mice (22-24 months old). The effect of nicotine on locomotion was biphasic; an initial stimulation of locomotor (0-5 min after nicotine) followed by a depressant period lasting from 5 to 20 min after injection. Tolerance to the depressant effect of nicotine developed after the 5th day of daily injections of nicotine (0.4 mg/kg SC, twice daily). Tolerance did not occur by day 8 to the initial stimulatory effect of nicotine. A similar effect of nicotine on locomotor activity was seen in mice treated with MPTP. The levels of DOPAC and HVA in the striatum were reduced by about 20% in the chronic nicotine-treated animals. The levels of DOPAC, DA, and HVA were reduced in the MPTP-treated mice; however, acute and chronic nicotine did not cause an additional change in the amine levels. The results suggest that nicotine has an influence on locomotor activity in MPTP-treated mice and that this effect is not due to changes in DA receptor activity in the striatum caused by chronic nicotine.

Concurrent lithium administration results in higher haloperidol levels in brain and plasma of guinea pigs

The effects of lithium (Li) on brain and plasma levels of concurrently administered haloperidol (HAL) were investigated. One group of guinea pigs (n = 12) was also treated with HAL for 11 days, but Li was added during the last 5 days of treatment. At the end of treatment, the HAL + Li group had significantly higher brain and plasma levels of HAL than the group treated with HAL alone. The correlation coefficient between plasma and brain HAL (0.97) indicated that plasma levels of HAL determine brain levels of this drug.

Dopamine-like action of nicotine: lack of tolerance and reverse tolerance

Rats with unilateral 6-hydroxydopamine lesions of the substantia nigra became briefly sedated and hypothermic after the acute injection of nicotine s.c. (0.4 or 0.8 mg/kg free base). When nicotine was repeated 5 days per week there was rapid tolerance for the sedation and slower tolerance for the hypothermia and the lesioned animals began to rotate ipsiversively after each injection. Stereotypic behavior was also noted. Rats injected with nicotine 5 days per week and nigrally lesioned on the 24th day rotated promptly on their first postoperative injection of nicotine. The nicotinic antagonist, mecamylamine (1.0 mg/kg i.p.), completely blocked the induced rotation. The appearance of rotation did not seem to depend on tolerance to sedation. The direction of rotation indicated enhancement of activity in the intact nigrostriatal system. However, 10 min after the acute injection of 0.8 mg/kg nicotine no change was found in the ratios of dopamine to its metabolites DOPAC and homovanillic acid in the substantia nigra, caudate-putamen, nucleus accumbens, olfactory tubercle, frontal cortex, or ventral tegmental area. Rats given 0.4 or 0.8 mg/kg nicotine 5 days per week and either lesioned prior to nicotine or lesioned during the third week rotated during the sixth week without any sign of tolerance. One day after the 30th injection in intact or lesioned rats the ratios of dopamine to its metabolites did not differ from those in saline controls on either the right or left side of any of the regions examined. There was no evidence of a change in dopamine metabolism after an acute challenge with nicotine or of a sustained change after repeated injection. The possibility remains that repeated nicotine modifies the dopaminergic response to nicotine without causing a sustained change in metabolism.

Nicotine-induced changes in the metabolism of specific brain proteins

The effect of acute and chronic nicotine on the metabolism of specific brain proteins was examined by measuring incorporation of labeled valine into protein, with densitometric scanning of proteins resolved by gel electrophoresis. Acute and chronic administration of nicotine (0.4 mg/kg per 30 min for 2 hours, s.c., or 0.5 mg/kg per 30 min for 5 days (Alzet mini-pump implanted subcutaneously] reduced incorporation of [14C]valine administered by approximately 6-7%. The results with chronic nicotine administration indicated a lack of tolerance for this effect of nicotine. Mecamylamine, a nicotinic ganglionic antagonist, does not seem to block the inhibition of protein synthesis. Small increases in protein content were observed in a high- and a low-molecular-weight region of SDS-polyacrylamide gel, used to separate proteins from newborn brain. In adult brain after chronic nicotine administration, selective increases and a decrease were seen in selective bands. Results are consonant with selective effects of nicotine on the synthesis or degradation of specific brain proteins.

Effect of ascorbic acid on the synaptosomal uptake of [3H]MPP+, [3H]dopamine, and [14C]GABA

The effects of ascorbic acid on the synaptosomal uptake of [3H]MPP+, [3H]dopamine, and [14C]GABA were examined in attempts to understand the mechanism of ascorbic acid attenuation of MPTP neurotoxicity. [3H]Dopamine uptake was increased at lower levels (0.01 and 0.1 mM) and decreased at higher levels (10 mM) of ascorbic acid. Ascorbic acid inhibited [3H]MPP+ uptake (IC50 = 0.1 mM) and [14C]GABA uptake (IC50 = 10 mM). Washout of ascorbic acid restored uptake of [3H]dopamine and [3H]MPP+, suggesting that ascorbic-acid-induced lipid peroxidation was not involved in the effect on uptake. In addition to the possible involvement of antioxidant mechanisms in the in vivo attenuation of the neurotoxicity of MPTP by ascorbic acid, the present results indicate a direct effect of ascorbic acid in inhibiting the uptake of MPP+ into dopaminergic nerve terminals.

Superoxide radical-mediated alteration of synaptosome membrane structure and high-affinity gamma-[14C]aminobutyric acid uptake

Mouse cortical synaptosomal structure and function are altered when exposed to hypoxanthine/xanthine oxidase (HPX/XOD)-generated active oxygen/free radical species. The structure of both the synaptic vesicle and plasma membrane systems are altered by HPX/XOD treatment. The alteration of synaptic vesicle structure is exhibited by a significant increase in the cumulative length of nonsynaptic vesicle membrane per nerve terminal. With respect to the nerve terminal plasma membrane, the length of the perimeter of the synaptosome is increased as the membrane pulls away from portions of the terminal in blebs. The functional lesion generated by HPX/XOD treatment results in a reduction in selective high-affinity gamma-[14C]aminobutyric acid (GABA) uptake. Kinetic analysis of the reduction in high-affinity uptake reveals that the Vmax is significantly altered whereas the Km is not. Preincubation with specific active oxygen/free radical scavengers indicates that the super-oxide radical is directly involved. This radical, most probably in the protonated perhydroxyl form, initiates lipid peroxidative damage of the synaptosomal membrane systems. Low-affinity [14C]GABA transport is unaltered by the HPX/XOD treatment. The apparent ineffectiveness of free radical exposure on low-affinity [14C]GABA transport coupled with its effectiveness in reducing high-affinity transport supports the idea that two separate and different amino acid uptake systems exist in CNS tissue, with the high-affinity being more sensitive (lipid-dependent) and/or more energy-dependent (Na+,K+-ATPase) than the low-affinity system.

Effect of nicotine and amphetamine on the neurotoxicity of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice

The present results show the potentiating effect of amphetamine on the ability of MPTP to destroy dopaminergic neurons in striatum of the mouse. A single injection of MPTP (8 mg/kg, retro-orbital) reduced the binding of [3H]mazindol, a marker for dopamine terminals, by 24%. When D-amphetamine (10 mg/kg, s.c.) was given 20 min prior to MPTP, the binding of [3H]mazindol, measured 3-5 days later, was reduced by 58%. It is proposed that the mechanism of this potentiation primarily involves an increased release of dopamine by D-amphetamine, and free radical-mediated processes. Although nicotine also releases dopamine from the striatum, no effect was observed when it was administered prior to MPTP. The lack of effect is probably related to short duration of action of nicotine and the modest effect on release of dopamine as compared to that of amphetamine.

Effect of amphetamine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in mice

Amphetamine has been shown to either potentiate or protect against MPTP neurotoxicity. The time course of changes in dopamine and its metabolites was examined after MPTP, amphetamine, or MPTP plus amphetamine administration. Results suggest that under conditions of granular depletion and release of dopamine by 10 mg/kg amphetamine, increased MPTP neurotoxicity occurs. Amphetamine injections at 2-5 mg/kg prevents the decline in dopamine possibly by blockade of the uptake of MPP+, rather than by an inhibition of monoamine oxidase.

Kinetics of [3H]MPP+ uptake in dopaminergic neurons of mouse: regional effects of MPTP neurotoxicity

In an effort to determine the specificity of MPTP/MPP+ toxicity with respect to the dopaminergic systems, the effect of prior MPTP treatment on [3H]MPP+ uptake in the striatum and olfactory tubercle of BALB/cBy mice was examined. Kinetic analysis of [3H]MPP+ uptake indicated a reduction of Vmax values in both striatum (49%, P less than 0.05) and olfactory tubercule (26%, P less than 0.05). MPTP treatment did not significantly alter the Km in either region, although MPP+ accumulates in both olfactory tubercle and striatum, these dopaminergic systems show different sensitivity to the neurotoxicity of MPTP/MPP+.

Structural requirements for cocaine congeners to interact with dopamine and serotonin uptake sites in mouse brain and to induce stereotyped behavior

We report here saturation analysis of [3H]cocaine binding in various mouse brain regions, and the necessary structure-activity relationships for cocaine congeners to inhibit Na+-dependent [3H]cocaine binding and [3H]dopamine uptake in the mouse striatum, and to inhibit [3H]cocaine binding that cannot be stimulated by Na+ and [3H]serotonin uptake in the mouse cerebral cortex. Generally similar structure-activity relationships were noted for all these processes. The ester linkage between the tropane and phenyl rings was not required for activity, in contrast to the configuration of the groups on C2, and to a lesser extent C3, in the tropane ring. Stereospecificity was evident from the differences between cocaine and (+)-pseudococaine, and between WIN 35,065-2 and WIN 35,065-3. There were remarkable differences between the above structure-activity relationships and those for local anesthetic activity of cocaine congeners, indicating that sodium channels were not labeled to a measurable extent with [3H]cocaine under the present conditions. Preliminary data indicated a significant correlation between the potencies of cocaine congeners in inhibiting the Na+-dependent binding of [3H]cocaine and their potencies in inducing stereotyped sniffing upon intraventricular administration.

Binding sites for [3H]cocaine in mouse striatum and cerebral cortex have different dissociation kinetics

[3H]Cocaine dissociates from its binding sites in the mouse cerebral cortex with a half-time of 25 s. The dissociation kinetics in the striatum is consonant with the presence of two populations of sites with dissociation half times of 2 s and 27 s, comprising 88% and 12%, respectively, of the total binding sites. On the basis of previous pharmacological characterization of [3H]cocaine binding, we propose that the slowly dissociating component represents the sites associated with 5-hydroxytryptamine (serotonin) uptake, and the rapidly dissociating component the 3,4-dihydroxyphenylethylamine (dopamine)-related sites. Evidence is presented that the extremely high dissociation rates do not preclude the measurement of [3H]cocaine binding by rapid filtration. The dissociation of [3H]cocaine from cerebrocortical membranes is slowed to a small but statistically significant extent by serotonin.

Protection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine neurotoxicity by the antioxidant ascorbic acid

Administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 2 X 8 mg/kg retro-orbital) to BALB/cBy mice reduced [3H]mazindol binding to striatal membranes by 50%. Reactive oxygen derivatives have been suggested to be involved in MPTP neurotoxicity; therefore we examined the effects of ascorbic acid (an antioxidant). Ascorbic acid (100 mg/kg) given 20 min prior to MPTP administration appreciably prevented the reduction of [3H]mazindol binding. The involvement of oxidative processes in the mechanism of MPTP neurotoxicity may suggest a relationship to the etiology of Parkinson's disease, and the possible benefit of treatment with ascorbic acid.

Sodium-independent binding of [3H]cocaine in mouse striatum is serotonin related

There was a highly significant correlation between IC50 values of various drugs in inhibiting the Na+-independent [3H]cocaine binding in the mouse striatum and their values in inhibiting the synaptosomal uptake of [3H]serotonin. In contrast, there was no correlation between the inhibition of binding in the absence of Na+ and the inhibition of [3H]dopamine uptake. Lesioning of serotonergic nerve terminals with 5,7-dihydroxytryptamine reduced the Na+-independent [3H]cocaine binding, without affecting the Na+-dependent binding. These results indicate that the bulk of the Na+-independent [3H]cocaine binding in the mouse striatum is associated with serotonergic nerve terminals.

[3H]cocaine binding in brain is inhibited by Tris (hydroxymethyl) aminomethane

High-affinity binding of [3H]cocaine to membranes of mouse cerebral cortex is inhibited by Tris (hydroxymethyl) aminomethane, the buffer commonly used in receptor binding assays. This inhibition is not due to an effect of ionic strength in general. Comparison of binding in Tris buffer with that in sodium phosphate buffer indicates a more than 4-fold higher Kd in the former buffer, with no differences in the Bmax values.

Similarities and differences between high-affinity binding sites for cocaine and imipramine in mouse cerebral cortex

Previously we found close similarities between high-affinity binding sites for [3H]cocaine and those for [3H]imipramine in the mouse cerebral cortex in regard to their association with neuronal uptake of serotonin. In the present study we investigated whether the two ligands bind to the same site. The two ligands had the following high-affinity binding properties in common: localization in both synaptosomal and microsomal fractions; vulnerability to treatment with N-ethylmaleimide, trypsin, and phospholipase A2; and resistance to exposure to dithiothreitol. In contrast, cocaine binding in the cerebral cortex was more sensitive to heat inactivation than imipramine binding. In addition, the mechanism by which cocaine inhibited [3H]imipramine binding differed from that by which imipramine inhibited [3H]cocaine binding. These data suggest that the high-affinity binding sites for [3H]cocaine and [3H]imipramine in the cerebral cortex are distinct entities.

Binding of imipramine and cocaine to a model lipid membrane: comparison with binding to brain membranes

[3H]Imipramine and [3H]cocaine were concentrated at membranes of liposomes prepared from phosphatidylcholine, cholesterol, and dicetylphosphate. This "binding" has an apparent dissociation constant in the micromolar range and a density close to 2 pmol/micrograms of phosphatidylcholine. The potencies of various drugs in inhibiting the binding to liposomes correlated only weakly with those in inhibiting the high-affinity binding of [3H]imipramine and [3H]cocaine to brain membranes. However, there was a highly significant correlation between the potencies of drugs in inhibiting binding to liposomes and their lipophilic character, indicating the involvement of hydrophobic bonding. Although the amounts of phosphatidylcholine and cholesterol in brain preparations in assays for high-affinity binding to brain membranes were in the same range as those used in our assays with liposomes, the inhibition of the high-affinity binding to brain membranes was only weakly dependent upon the lipophilicity of the inhibiting drug. These results indicate that lipophilicity is but one of the factors in the complex binding interactions between lipophilic substances and integral brain membranes. In addition, the results are in agreement with the suggestion that phosphatidylcholine and cholesterol are not the primary sites of high-affinity binding [3H]imipramine and [3H]cocaine to brain membranes, although it cannot be ruled out that these lipids have different properties in natural biological membranes and in artificial liposome membranes.

Reduction of dopamine uptake and cocaine binding in mouse striatum by N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (NMPTP) (daily injections of 8 mg/kg for 5 days via tail vein) reduced [3H]dopamine uptake in striatal synaptosomes by 63% and reduced [3H]cocaine binding to striatal membranes by 61%. [3H]Cocaine binding was not affected in olfactory tubercle, suggesting a selective effect of NMPTP on the nigro-striatal but not on the mesolimbic dopaminergic system. The destruction of dopamine terminals in the striatum did not alter (up-regulate) [3H]spiroperidol binding. The results suggest that NMPTP causes a degenerative destruction of the striatal dopamine pathway and that NMPTP may be useful in developing a rodent model of Parkinson's disease.

Endogenous material in brain inhibiting [3H]nicotine and [3H]acetylcholine binding

The supernatant obtained from mouse brain homogenates contains material that inhibits the saturable binding of [3H]nicotine in mouse cerebral cortex. This inhibitory material was further purified by heat denaturation, ultrafiltration through an Amicon PM-10 membrane filter, and gel chromatography on Sephadex G-10. The material inhibited the binding of [3H]acetylcholine with the same potency as it did that of [3H]nicotine. It also had some affinity for the sites that specifically bind [3H]D-Ala, D-Leu enkephalin, but had much lower affinity for the binding sites for [3H]quinuclidinyl benzilate (QNB), [3H]spiroperidol, [3H]naloxone, or [3H]imipramine. Acid hydrolysis destroyed the activity. These preliminary results suggest the presence in brain of "nicotinelike" substances, one of which may be the endogenous ligand for the sites that specifically bind [3H]nicotine.

Imipramine does not act as a false transmitter in the rat hypothalamus

[3H]Imipramine was accumulated by rat hypothalamic slices mostly by a passive low-affinity process. Accumulated [3H]imipramine was released by 60 mM potassium, but the release was small and was not Ca2+-dependent. Imipramine induced some release of [3H]norepinephrine. None of these events were abolished by pretreatment of the rats with reserpine. These results do not favor the view that imipramine acts as a false transmitter in the rat hypothalamus. Rather, imipramine appears to be accumulated in an extragranular pool from which it releases extragranular norepinephrine.

A portion of [3H]cocaine binding in brain is associated with serotonergic neurons

Three lines of evidence are brought forward in support of an association in the brain cortex of some, but not all, of the cocaine binding sites with serotonergic nerve terminals. The first is based upon the significant correlation observed between the inhibition of cocaine binding by various drugs and the inhibition of neuronal uptake of serotonin in the mouse cerebral cortex. The second is based upon the demonstration of cocaine binding in human blood platelets, a model system for central serotonergic neurons. The third comes from experiments in which rats were treated with p-chloroamphetamine and 5,7-dihydroxytryptamine (serotonin neurotoxins), 6-hydroxydopamine (catecholamine neurotoxin), or p-chlorophenylalanine (inhibitor of tryptophan hydroxylase). Only the serotonin neurotoxins decreased the binding of [3H]cocaine in the rat cerebral cortex, but to a lower extent than the binding of [3]imipramine, which is known to be associated with serotonergic terminals. In contrast to the cocaine binding in the mouse cerebral cortex, the binding in the rat cerebral cortex included a considerable portion of low-affinity binding that was relatively unaffected by lesions of serotonergic neurons.

High- and low-affinity binding of [3H]imipramine in mouse cerebral cortex

Binding of [3H]imipramine in mouse cerebral cortex was found to be nonhomogeneous. Competition experiments, Scatchard analysis, and Hill plots are compatible with the existence of binding with high (nanomolar) and low (micromolar) affinity. Low-affinity binding could be eliminated by the use of low concentrations of imipramine as the competing ligand. In contrast to the high-affinity binding, the low-affinity binding was found to be unrelated to the neuronal uptake system for serotonin.

Effects of prenatal administration of nicotine on amino acid pools, protein metabolism, and nicotine binding in the brain

The effects of nicotine on brain protein metabolism and on the properties of the nicotine binding site were investigated in newborn animals exposed to nicotine during gestation. Brain protein synthesis rates measured in vivo were lower by 18% in newborn of treated animals. Protein degradation rates measured in vitro in the presence of nicotine were lower by 13%. The effect was specific for L-(-)nicotine, since D-(+)nicotine, nicotinic acid, or nicotinamide had no effect on degradation rates. Newborn brain amino acid levels, mainly nonessential amino acids and amino acids of putative neurotransmitter function, were changed somewhat; an increase in the level of taurine (13%), threonine (21%), serine (35%) and glycine (35%), and a decrease in lysine (14%) was observed in the offspring of nicotine treated animals (0.5 mg/kg, s.c., 2 x daily throughout gestation). These changes could not account for the decrease in protein metabolism. Nicotine binding was higher by 25% in the offspring of animals exposed to nicotine during gestation. No such increase was found after treatment of adult rats with nicotine, indicating that the properties of the nicotine binding site change with age.

On the interaction between nicotine and cycloheximide

This study examined whether the nicotine-induced reversal of the amnestic effect of cycloheximide is due to an interaction between nicotine and cycloheximide on brain protein synthesis, and whether it involves the sites in brain that saturably bind [3H]nicotine. Nicotine did not reverse the cycloheximide-induced inhibition of protein synthesis, both in in vivo (intact animal) and in vitro (brain slice), suggesting that on-going protein synthesis is not necessarily involved in memory consolidation. The nicotine binding sites were not affected by in vivo or in vitro treatment with cycloheximide in the presence or absence of nicotine.

Comparison of the properties of central and peripheral binding sites for cocaine

Cocaine and its analogs bound saturably to membranes of brain and liver of mice. The binding sites on membranes of liver had a lower affinity for cocaine than those of brain. In addition, there were striking differences between the two tissues in regard to the relative potencies of cocaine analogs in competing with [3H]cocaine for binding. In comparison with the binding sites in brain, those in liver had only moderate stereospecificity, and they discriminated less between the centrally active compounds and the centrally inert analogs.

Strain differences in opiate receptors in mouse brain

Various opiate ligands were bound to brain membranes of mice of the Recombinant Inbred System. The specific binding of low levels of [3H]naloxone, [3H]dihydromorphine and [3H]ethylketocyclazocine was disturbed in a similar fashion among the inbred strains, and in a pattern different from that observed for [3H](D-Ala2,D-Leu5)-enkephalin. The results indicate that the inbred strains differ in mu- and delta-type binding and support the concept of multiple opiate receptors in mouse brain.

Binding of [3H]cocaine in mouse brain: kinetics and saturability

Kinetic experiments indicate that association of [3H]-cocaine to its binding site in brain occurs rapidly (seconds). Dissociation of membrane-bound cocaine is also rapid, with a dissociation half-life in seconds; this raises the question of whether membrane-bound cocaine is released during the time required for rapid filtration of tissue-containing filters. Results from experiments with increasing numbers of filter-washes indicate that there is no significant loss of [3H]-cocaine saturably bound to brain membranes within the time-scale of the rapid filtration procedure. In addition, saturation analysis of binding data obtained with the filtration procedure and with the centrifugation method give similar estimates of the affinity (dissociation constant: 0.8 microM) and the maximal binding (5 pmol/mg of protein) of cocaine. However, the nonspecific binding and the experimental error in the saturable binding are considerably greater in centrifugation assays than in filtration assays.

Noncholinergic, saturable binding of (+/-)-[3H]nicotine to mouse brain

(+/-)-[3H]Nicotine was bound saturably to crude particulate, and synaptosomal-mitochondrial fraction from mouse brain. Scatchard and Hill plots of the binding data are in agreement with the existence of two independent classes of binding sites with high (Kd of 0.1-0.4 microM) and low(Kd is 20 microM) affinities, although negative cooperativity or a two-step model of ligand-receptor interaction cannot be ruled out. Nicotinic or muscarinic agonists and antagonists had little or no affinity for the nicotine binding sites, suggesting that nicotine binds in brain to noncholinergic sites. The binding did not display stereospecificity; this is consistent with the similarity in the pharmacological effects of (-)- and (+)-nicotine. Our results indicate that binding studies with [3H]nicotine should be interpreted with extreme caution.

Endogenous peptide(s) inhibiting [3H]cocaine binding in mouse brain

The supernatant fraction of centrifuged homogenate of brain tissue contains material that inhibits the saturable binding of [3H]cocaine to crude mouse brain membranes. This material was subjected to heat treatment to remove protein; further purification was achieved by filtering through an Amicon UM-10 membrane ultrafilter and gel filtration of the ultrafiltrate on Sephadex G-25. Sensitivity to acid hydrolysis and peptidase action indicates that the inhibitory activity resides in peptide material with low molecular weight. The partially purified inhibitor has similar effects to that of cocaine on the specific binding of various ligands to opiate and nonopiate receptors in mouse brain membranes.

Cerebral uptake of nicotine and of amino acids

Nicotine is among the compounds that enter the brain very rapidly (blood-flow-limited). It also leaves the brain rapidly; by five minutes, 90% exits, an exit somewhat slower than that of water. In spite of rapid exit, brain levels remain higher than levels in blood over a wide range of blood concentrations. Nicotine enters the fetal circulation from the maternal blood; it enters fetal brain but to a smaller extent than adult brain. Nicotine entry is different from that of amino acid: No interaction of amino acid transport and nicotine could be detected. Most close analogs have no effect on nicotine uptake, but at higher concentrations nicotine uptake is saturable. Nicotine and morphine mutually inhibit each other's uptake. The results suggest an uptake compartment (lipid space) for nicotine shared by morphine.