Muscarinic agonist-mediated increases in serum corticosterone levels are abolished in m(2) muscarinic acetylcholine receptor knockout mice

Muscarinic acetylcholine receptors (M(1)-M(5)) regulate many key functions in the central and peripheral nervous system. Due to the lack of receptor subtype-selective ligands, however, the physiological roles of individual muscarinic receptor subtypes remain to be determined. In this study, we examined the effects of the muscarinic M(2)/M(4) receptor-preferring agonist [5R-(exo)]-6-[4-butylthio-1,2,5-thiadiazol-3-yl]-1-azabicyclo-[3.2.1]-octane (BuTAC) on serum corticosterone levels in M(2) and M(4) receptor single knockout (KO) and M(2,4) receptor double KO mice. Responses were compared with those obtained with the corresponding wild-type (WT) mice. BuTAC (0.03-0.3 mg/kg s.c.) dose dependently and significantly increased serum corticosterone concentrations in WT mice to 5-fold or greater levels compared with vehicle controls. In muscarinic M(2) and M(2,4) KO mice, however, BuTAC had no significant effect on corticosterone concentrations at doses of 0.1, 0.3, and 1 mg/kg s.c. In both WT and muscarinic M(4) KO mice increases in serum corticosterone concentrations induced by BuTAC (0.1 and 0.3 mg/kg) were not significantly different and were blocked by scopolamine. In summary, the muscarinic M(2,4)-preferring agonist BuTAC had no effect on corticosterone levels in mice lacking functional muscarinic M(2) receptors. These data suggest that the muscarinic M(2) receptor subtype mediates muscarinic agonist-induced activation of the hypothalamic-pituitary-adrenocortical axis in mice.

Comparative affinity of duloxetine and venlafaxine for serotonin and norepinephrine transporters in vitro and in vivo, human serotonin receptor subtypes, and other neuronal receptors

The blockade of serotonin (5-HT) and norepinephrine (NE) transporters in vitro and in vivo by the dual 5-HT/NE reuptake inhibitors duloxetine and venlafaxine was compared. Duloxetine inhibited binding to the human NE and 5-HT transporters with K(i) values of 7.5 and 0.8 nM, respectively, and with a K(i) ratio of 9. Venlafaxine inhibited binding to the human NE and 5-HT transporters with K(i) values of 2480 and 82 nM, respectively, and with a K(i) ratio of 30. Duloxetine inhibited ex vivo binding to rat 5-HT transporters and NE transporters with ED(50) values of 0.03 and 0.7 mg/kg, respectively, whereas venlafaxine had ED(50) values of 2 and 54 mg/kg, respectively. The depletion of rat brain 5-HT by p-chloramphetamine and depletion of rat hypothalamic NE by 6-hydroxydopamine was blocked by duloxetine with ED(50) values of 2.3 and 12 mg/kg, respectively. Venlafaxine had ED(50) values of 5.9 and 94 mg/kg for blocking p-chloramphetamine- and 6-hydroxydopamine-induced monoamine depletion, respectively. Thus, duloxetine more potently blocks 5-HT and NE transporters in vitro and in vivo than venlafaxine.

The novel 5-Hydroxytryptamine(1A) antagonist LY426965: effects on nicotine withdrawal and interactions with fluoxetine

LY426965 [(2S)-(+)-1-cyclohexyl-4-[4-(2-methoxyphenyl)-1-piperazinyl]2-methyl- 2-phenyl-1-butanone monohydrochloride] is a novel compound with high affinity for the cloned human 5-hydroxytryptamine (HT)(1A) receptor (K(i) = 4.66 nM) and 20-fold or greater selectivity over other serotonin and nonserotonin receptor subtypes. Both in vitro and in vivo studies indicate that LY426965 is a full antagonist and has no partial agonist properties. LY426965 did not stimulate [(35)S]guanosine-5'-O-(3-thio) triphosphate (GTPgammaS) binding to homogenates of cells expressing the cloned human 5-HT(1A) receptor in vitro but did inhibit 300 nM 5-HT-stimulated [(35)S]GTPgammaS binding with a K(i) value of 3.07 nM. After both p.o. and s.c. administration, LY426965 blocked the lower lip retraction, flat body posture, hypothermia, and increase in rat serum corticosterone induced by the 5-HT(1A) agonist 8-OH-DPAT (8-hydroxy-2-dipropylaminotetralin). In pigeons, LY426965 dose-dependently blocked the stimulus cue induced by 8-OH-DPAT but had no 8-OH-DPAT-like discriminative properties. LY426965 completely reversed the effects of nicotine withdrawal on the auditory startle reflex in rats. In microdialysis experiments, LY426965 administered together with fluoxetine significantly increased extracellular levels of serotonin above those achievable with fluoxetine alone. In electrophysiological studies, the administration of LY426965 produced a slight elevation of the firing rate of 5-HT neurons in the dorsal raphe nucleus of anesthetized rats and both blocked and reversed the effects of fluoxetine on 5-HT neuronal activity. These preclinical results indicate that LY426965 is a selective, full 5-HT(1A) antagonist that may have clinical use as pharmacotherapy for smoking cessation and depression and related disorders.

The selective norepinephrine reuptake inhibitor, LY368975, reduces food consumption in animal models of feeding

The compound, LY368975 ((R)-thionisoxetine) is a potent and selective inhibitor of the norepinephrine (NE) reuptake site. We evaluated the in vivo properties of LY368975 in various animal models. In mice, LY368975 prevented heart NE depletion by 6-hydroxydopamine with an ED50 of 1.22 mg/kg. In rats, orally administered LY368975 inhibited 3H-NE uptake into hypothalamic synaptosomes ex vivo with an ED50 of 2.5 mg/kg and 3H-tomoxetine binding to the NE transporter with an ED50 of 2.7 mg/kg. When rats were deprived of food for 18 hr, 10 mg/kg LY368975 was able to suppress food intake 1, 2 and 4 hr after reintroduction of the feed. In nonfasted rats trained to drink sweetened condensed milk, LY368975 produced a dose-dependent reduction in consumption with a 44% decrease at 3 mg/kg. At doses up to 10 mg/kg p.o., LY368975 produced no significant effects on locomotor activity suggesting the compound does not activate or sedate the animals at pharmacologically relevant doses. Therefore, LY368975 is an orally available and centrally active NE reuptake inhibitor that is capable of reducing food consumption in rodents. Compounds of this class may have use in the treatment of obesity and eating disorders.

Involvement of 5-HT2A receptors in the elevation of rat serum corticosterone concentrations by quipazine and MK-212

The possible involvement of 5-HT2A or 5-HT2C receptors in the elevation of serum corticosterone in rats by quipazine (2-(1-piperazinyl)quinoline maleate) and MK-212 (6-chloro-(1-piperazinyl)pyrazine), direct-acting 5-HT receptor agonists, was investigated by the use of two newly available receptor antagonists, SB 200646A (N-(1-methyl-5-indolyl)-N'-(3-pyridyl)urea) and MDL 100,907 (R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]- 4-piperidinemethanol). MDL 100,907 blocked the increase in serum corticosterone elicited by quipazine and MK-212 with ED50 values of 0.0028 and 0.0027 mg/kg, s.c., respectively. In contrast, SB 200646A only partially antagonized the serum corticosterone concentration increases by quipazine and MK-212 even at the highest dose tested, 40 mg/kg, i.p. Because published data show the affinities of MDL 100,907 and SB 200646A for 5-HT2C receptors to be nearly identical, whereas the affinity of MDL 100,907 for 5-HT2A receptors is 17500-fold higher than that of SB 200646A, our findings suggest that 5-HT2A receptors rather than 5-HT2C receptors mediate the serum corticosterone increases by both quipazine and MK-212.

Neurochemical evidence for antagonism by olanzapine of dopamine, serotonin, alpha 1-adrenergic and muscarinic receptors in vivo in rats

The ability of the atypical antipsychotic drug candidate olanzapine to antagonize dopamine, serotonin, alpha-adrenergic and muscarinic receptors in vivo was assessed by various neurochemical measurements in rat brain. Olanzapine increased the concentrations of the dopamine metabolites DOPAC and HVA in striatum and nucleus accumbens. Olanzapine antagonized the pergolide-induced decrease of striatal DOPA concentrations in rats treated with gammabutyrolactone and NSD1015 and increased striatal 3-methoxytyramine concentrations in nomifensine-treated rats (but not after gammabutyrolactone administration), suggesting that olanzapine blocked terminal and somatodendritic autoreceptors on dopamine neurons. Inactivation of dopamine D1 and D2 receptors by EEDQ was antagonized by olanzapine. The ex vivo binding of the 5HT2 radioligand [3H]-ketanserin was inhibited by olanzapine treatment, as was quipazine-induced increases in MHPG-SO4, evidence suggesting that olanzapine antagonized 5HT2 receptors. At higher doses, olanzapine increased the concentrations of the norepinephrine metabolite, MHPG-SO4, probably by blocking alpha 1-adrenergic receptors. Olanzapine inhibited ex vivo binding of the muscarinic antagonist radioligand [3H]-pirenzepine and lowered concentrations of striatal, but not hippocampal, acetylcholine levels. The findings provide evidence that olanzapine antagonized dopamine, serotonin, alpha-adrenergic and muscarinic receptors in vivo, consistent with its high affinity for these receptor sites in vitro.

Serum corticosterone increases reflect enhanced uptake inhibitor-induced elevation of extracellular 5-hydroxytryptamine in rat hypothalamus

The increase in extracellular 5-hydroxytryptamine (5-HT) in rat hypothalamus following administration of fluoxetine, a 5-HT-uptake inhibitor, was enhanced by the injection of LY206130(1-[1-H-indol-4-yloxy]-3-[cyclohexylamino]-2-prop ano l maleate), a 5HT1A receptor antagonist, or by L-5-hydroxytryptophan (L-5-HTP), the 5-HT precursor. Elevation of serum corticosterone, measured as a functional output of hypothalamic 5-HT pathways, was greater in rats treated with fluoxetine plus LY206130 or with fluoxetine plus L-5-HTP than in rats treated with the agents alone. Synergism between effects of fluoxetine and L-5HTP has often been reported, but this is the first report of an increased functional effect when a 5-HT1A receptor antagonist is combined with a 5-HT uptake inhibitor to augment the increase in extracellular 5-HT.

(R)-thionisoxetine, a potent and selective inhibitor of central and peripheral norepinephrine uptake

Inhibitors of neuronal norepinephrine (NE) uptake are useful for the treatment of a variety of diseases including depression and urinary incontinence. In the present study, we synthesized and evaluated a novel analog of the potent and selective NE uptake inhibitor, nisoxetine. Thionisoxetine more potently inhibited the uptake of [3H]-NE into hypothalamic synaptosomes and [3H]-nisoxetine binding to the NE transporter than (R)-nisoxetine. The (R) enantiomer of this compound was significantly more potent than the (S) enantiomer, having a Ki of 0.20 nM in [3H]-nisoxetine binding. The (R) enantiomer was approximately 70-fold more potent in inhibiting [3H]-NE uptake when compared to [3H]-5HT uptake. In rats, (R)-thionisoxetine prevented hypothalamic NE depletion by 6-hydroxydopamine with an ED50 of 0.21 mg/kg. Depletion of NE in peripheral nerves was accomplished by the administration of metaraminol to rats. In this paradigm, (R)-thionisoxetine prevented the depletion of heart NE with an ED50 of 3.4 mg/kg and urethral NE with an ED50 of 1.2 mg/kg. Thus, (R)-thionisoxetine is a potent and selective inhibitor of NE uptake in both central and peripheral tissues.

Decreased hypothalamic epinephrine concentration by quipazine and other serotonin agonists in rats

Epinephrine concentrations in rat hypothalamus were decreased after the injection of quipazine, a direct-acting serotonin (5-HT) agonist. The decrease was statistically significant and dose-dependent from 0.1 to 10 mg/kg, s.c., was apparent within 1 hr, and persisted for 8 hr but not 24 hr. There was no decrease in epinephrine concentrations in rat medulla oblongata, a region containing epinephrine cell bodies. Epinephrine concentrations in rat hypothalamus were also decreased by 1-(m-trifluoromethylphenyl)-piperazine (TFMPP) and by 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), other direct-acting 5-HT agonists, and by d-fenfluramine, a 5-HT-releasing drug. The decrease evoked by quipazine was prevented by pretreatment with metergoline, ketanserin or LY53857 (6-methyl-1-[methylethyl]-ergoline-8-carboxylic acid 2-hydroxy-1-methyl-propyl ester), centrally acting 5-HT antagonists. The lowering of rat hypothalamic epinephrine concentrations by 8-OH-DPAT was prevented by pretreatment with pindolol, a centrally acting 5-HT1A receptor antagonist. These data suggest that serotonergic drugs affect epinephrine concentrations in rat hypothalamus.

Novel halogenated analogs of tomoxetine that are potent and selective inhibitors of norepinephrine uptake in brain

Halogenated analogs of the potent norepinephrine (NE) uptake inhibitor, tomoxetine, were synthesized and their affinities for the serotonin (5HT) and NE uptake sites evaluated. One of the most potent was the 2-iodo substituted analog (289306) that inhibited [3H]tomoxetine binding to rat cerebral cortex with a Ki of 0.37 nM. The compound also inhibited the uptake of [3H]NE into rat hypothalamic synaptosomes with a Ki of 3.5 nM. This analog was significantly less potent at the 5HT uptake site, as exhibited by a Ki of 25 nM in the inhibition of [3H]paroxetine binding and a Ki of 121 nM in [3H]5HT uptake. The resolved (R) enantiomer (303926) was 10 times more potent as a [3H]NE uptake inhibitor and 29 times more potent as an inhibitor of [3H]tomoxetine binding than the (S) enantiomer (303884). Administration of 289306 to rats prior to an i.c.v. injection of 6-hydroxydopamine prevented the depletion of hypothalamic NE and Epi with ED50 values of 0.28 and 0.47 mg/kg, respectively. Thus, 289306 was a potent inhibitor of NE uptake in vitro and in vivo. In addition, these compounds provide structures for potential ligands for the study of NE uptake sites by autoradiography, PET or SPECT imaging.

Comparison of desmethylsertraline with sertraline as a monoamine uptake inhibitor in vivo

1. Desmethylsertraline, a metabolite of the antidepressant drug sertraline, was compared with sertraline for its ability to produce effects characteristic of inhibitors of the serotonin transporter in vivo. Desmethylsertraline antagonized brain serotonin depletion by p-chloroamphetamine, a depletion dependent upon the serotonin transporter, being less potent than sertraline in rats but almost as potent as sertraline in mice. Desmethylsertraline was a weak antagonist of 6-hydroxydopamine-induced depletion of heart norepinephrine in mice; sertraline had no effect at the doses studied. 2. Desmethylsertraline decreased brain concentrations of 5-hydroxyindoleacetic acid (5HIAA) in rats as did sertraline, the duration of the effect after both drugs being at least 24 hrs but less than 48 hrs. 3. After sertraline injection, desmethylsertraline was present in rat brain at higher concentrations than the parent drug at 8 hrs and thereafter. 4. In rats, repeated injections of sertraline, at doses previously shown to diminish beta-adrenergic receptor-mediated responses, led to marked accumulation of desmethylsertraline in brain and to inhibition of the catecholamine transporters. 5. In mice, brain concentrations of desmethylsertraline were higher than those of parent drug within 7 hrs after sertraline injection and probably contributed importantly to the antagonism of p-chloroamphetamine effects. 6. These data show that desmethylsertraline is less potent than sertraline as a serotonin uptake inhibitor in vivo, as the in vitro data would have predicted, but that desmethylsertraline may nonetheless contribute to the prolonged inhibition of the serotonin transporter after sertraline administration, perhaps more in mice than in rats.

Fluoxetine at anorectic doses does not have properties of a dopamine uptake inhibitor

Although fluoxetine is a highly selective inhibitor of serotonin uptake in vitro and in vivo, some investigators have suggested that dopamine uptake inhibition may contribute to anorectic actions of fluoxetine. The present experiments were done to determine fluoxetine's effects in some animal protocols in which dopamine uptake inhibitors have characteristic actions. Mazindol prevented the depletion of striatal dopamine and its metabolites by amphetamine in iprindole-pretreated rats, but fluoxetine had no effect. Mazindol prevented the depletion of striatal dopamine and its metabolites by 6-hydroxydopamine injected intracerebroventricularly into rats, but fluoxetine had no effect. Mazindol enhanced the elevation of 3,4-dihydroxyphenylacetic acid concentration in rat brain after spiperone injection, but fluoxetine did not cause that effect. Fluoxetine did not mimic amfonelic acid in antagonizing the retention of alpha-methyl-m-tyramine invant striatum after the injection of alpha-methyl-m-tyrosine. These results show that fluoxetine, at doses that are effective in blocking the serotonin uptake carrier and causing anorexia, does not block the dopamine uptake carrier.

Effects of duloxetine, an antidepressant drug candidate, on concentrations of monoamines and their metabolites in rats and mice

Duloxetine, (+)-N-methyl-3-(1-naphthalenyloxy)-2-thiophenepropanamine, is an inhibitor of the serotonin and norepinephrine neuronal transporters (Wong et al., 1993). In mice, duloxetine antagonized the depletion of brain serotonin by p-chloroamphetamine (ED50 = 2.5 mg/kg, i.p.) and the depletion of heart norepinephrine by 6-hydroxydopamine (ED50 = 1.1 mg/kg, i.p.). Brain concentrations of 5-hydroxyindoleacetic acid were decreased by duloxetine at 2 hr after doses of 1, 3 and 10 mg/kg and at 1 to 8 hr (but not 24 hr) after a 10 mg/kg i.p. dose of duloxetine. Duloxetine antagonized norepinephrine depletion in frontal cortex, but not dopamine depletion in striatum, after treatment of mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. In rats, duloxetine decreased brain 5-hydroxyindoleacetic acid dose-dependently for up to 8 hr and decreased serotonin turnover measured by the accumulation of 5-hydroxytryptophan in rat hypothalamus after decarboxylase inhibition. In rats, duloxetine antagonized the depletion of brain serotonin by p-chloramphetamine and the depletion of norepinephrine and epinephrine in hypothalamus after i.c.v. injection of 6-hydroxydopamine. In vitro, duloxetine had little effect on either type A (serotonin as substrate) or type B (phenylethylamine as substrate) monoamine oxidase, IC50 concentrations being above 10(-5) M. These data extend evidence that duloxetine inhibits serotonin and norepinephrine transporters in vivo, actions that may lead to therapeutic efficacy in mental depression.

Evaluation of nefazodone as a serotonin uptake inhibitor and a serotonin antagonist in vivo

Nefazodone (2-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-ethyl- 2,4-dihydro-4-(2-phenoxyethyl)-3H-1,2,4-triazol-3-one) has been reported to be effective in the treatment of depression. Antagonism of serotonin type 2A (5HT2A) receptors, as well as inhibition of the serotonin (5HT) uptake carrier, has been suggested to contribute to the antidepressant action of nefazodone in vivo (Eison et al., 1990). Nefazodone weakly antagonized the quipazine-induced rise in rat serum corticosterone levels and the quipazine-induced increase in rat hypothalamic 3-methoxy-4-hydroxy-phenylglycol sulfate, suggesting blockade of 5HT2A receptors in vivo. Nefazodone, however, failed to antagonize the p-chloroamphetamine-induced depletion of mouse or rat brain 5HT, displaying a lack of effect on the 5HT uptake carrier. These data extend previous in vitro and in vivo data (Eison, et al. 1990) reporting nefazodone to be an antagonist at 5HT2A receptors, but fail to show inhibition of the 5HT uptake carrier in the same dose range.

Effect of 4-(4-methoxy-1,2,5-thiadiazol-3-yl)-1-methyl-1,2,3,6-tetrahydropyridine , an analog of MPTP, on mouse heart norepinephrine and brain catecholamines

An analog of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) with a thiadiazole substituent in place of the phenyl ring was compared to MPTP in mice for its ability to deplete striatal dopamine and its metabolites and norepinephrine in the frontal cortex and heart. One week after the last of 4 daily s.c. injections, MPTP at 20 mg/kg depleted mouse striatal dopamine, DOPAC and HVA as well as norepinephrine in the frontal cortex. One week after 4 daily s.c. doses of 4-(4-methoxy-1,2,5-thiadiazol-3-yl)-1-methyl-1,2,3,6-tetrahydro pyr idine (MZTP) at doses as high as 80 mg/kg, there was no effect on brain catecholamines. A single dose of MPTP (10 mg/kg s.c.) depleted heart norepinephrine concentration 24 h after injection. MZTP had no effect on heart norepinephrine at 10 mg/kg s.c., but did significantly deplete mouse heart norepinephrine 24 h after a dose of 20 mg/kg s.c.

Protection against amphetamine-induced neurotoxicity toward striatal dopamine neurons in rodents by LY274614, an excitatory amino acid antagonist

LY274614, 3SR,4aRS,6SR,8aRS-6-[phosphonomethyl]decahydr oisoquinoline-3- carboxylic acid, has been described as a potent antagonist of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. Here its ability to antagonize the prolonged depletion of dopamine in the striatum by amphetamine in iprindole-treated rats is reported. A single 18.4 mg/kg (i.p.) dose of (+/-)-amphetamine hemisulfate, given to rats pretreated with iprindole, resulted in persistent depletion of dopamine in the striatum 1 week later. This prolonged depletion of dopamine in the striatum was antagonized by dizocilpine (MK-801, a non-competitive antagonist of NMDA receptors) or by LY274614 (a competitive antagonist of NMDA receptors). The protective effect of LY274614 was dose-dependent, being maximum at 10-40 mgkg (i.p.). A 10 mg/kg dose of LY274614 was effective in antagonizing the depletion of dopamine in the striatum, when given as long as 8 hr prior to amphetamine but not when given 24 hr prior to amphetamine. Depletion of dopamine in the striatum was also antagonized when LY274614 was given after the injection of amphetamine; LY274614 protected when given up to 4 hr after but not when given 8 or 24 hr after amphetamine. The prolonged depletion of dopamine in the striatum in mice, given multiple injections of methamphetamine, was also antagonized dose-dependently and completely by LY274614. The data strengthen the evidence that the neurotoxic effect of amphetamine and related compounds toward nigrostriatal dopamine neurons involves NMDA receptors and that LY274614 is an NMDA receptor antagonist with long-lasting in vivo effects in rats.

MK801 antagonism of the prolonged depletion of striatal dopamine by amphetamine in iprindole-treated rats

The administration of amphetamine to rats pretreated with iprindole to inhibit the metabolism of amphetamine results in a long-lasting depletion of striatal dopamine and its metabolites, DOPAC and HVA. Pretreatment with MK801, a noncompetitive antagonist of the NMDA (N-methyl-D-aspartate) subclass of excitatory amino acid receptors, antagonized the depletion of striatal dopamine, DOPAC and HVA 3 days after a single dose of amphetamine in iprindole-treated rats. MK801 pretreatment was effective up to 4 hours but not at 8 or 24 hours in preventing amphetamine effects on striatal dopamine, DOPAC and HVA.

Potent in vivo but not in vitro inhibition of monoamine oxidase by 3-chloro-alpha-phenylpyrazinemethanol

3-Chloro-alpha-phenylpyrazinemethanol (3-CPM) inhibited monoamine oxidase (MAO) types A and B in vivo in mouse brain, heart and liver. The inhibition was dose-dependent at doses of 0.3-32 mg/kg i.p. and occurred within 1 h after the compound was injected. 3-CPM was a very weak inhibitor of mouse brain mitochondrial MAO activity in vitro, even when preincubated with the enzyme; MAO-A was inhibited only about 50% at a high concentration of 3-CPM (1 mM), and MAO-B was inhibited even less. After a 10 mg/kg i.p. dose of 3-CPM in mice, both MAO-A and MAO-B were inhibited at day 1, but activity had largely recovered within a few days in brain, liver and heart. 3-CPM at doses of 1, 3, 10 and 32 mg/kg i.p. caused dose-dependent antagonism of the depletion of striatal dopamine and of cortical norepinephrine by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. 3-CPM is therefore a potent inhibitor of MAO-A and of MAO-B in mice in vivo despite its weak effect on the enzyme in vitro. A metabolite of the drug may be involved in the in vivo effects.

Persistent depletion of striatal dopamine and cortical norepinephrine in mice by 1-methyl-4-phenyl-1,2,3,6-tetrahydro-3-pyridinol (MPTP-3-OL), an analog of MPTP

MPTP-3-ol injected s.c. once daily for 4 days resulted in a dose-dependent depletion of striatal dopamine and cortical norepinephrine one week after the last dose. MPTP-3-ol was approximately one-fourth as potent as MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in causing these effects. MPTP-3-ol was oxidized by monoamine oxidase in mouse brain in vitro and resulted in MPP+ (1-methyl-4-phenylpyridinium) formation in brain in vivo, both at about one-fourth the rates with MPTP. The in vitro metabolism of MPTP-3-ol was inhibited by deprenyl, a selective inhibitor of monoamine oxidase type B, and deprenyl pretreatment also blocked the depletion of striatal dopamine and cortical norepinephrine in vivo. Pretreatment with EXP 561, an inhibitor of catecholamine uptake, also prevented the dopamine- and norepinephrine-depleting effects of MPTP-3-ol. Thus, substitution of a hydroxy group on the 3-position of MPTP retains its neurotoxic potential toward catecholamine neurons but reduces potency by decreasing the rate of oxidation via monoamine oxidase type B.

Tissue concentrations of MPTP and MPP+ after administration of lethal and sublethal doses of MPTP to mice

Tissue concentrations of MPP+ (1-methyl-4-phenylpyridinium) were measured in Charles River CFW mice after administration of high doses of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) or MPP+ given subcutaneously or orally, to investigate the relationships between tissue concentrations and lethality resulting from these compounds. MPTP given subcutaneously led to much higher concentrations of MPP+ in brain and somewhat higher concentrations of MPP+ in peripheral tissues than did MPTP given orally. MPTP caused no lethality at oral doses up to 160 mg/kg whereas subcutaneous MPTP had an LD50 of 54 mg/kg. Deprenyl pretreatment antagonized the lethality of subcutaneous MPTP and reduced MPP+ concentrations in brain and in other tissues after MPTP injection. Deprenyl caused a greater and longer-lasting inhibition of MPTP oxidation than of phenylethylamine oxidation, especially in liver, although both compounds are thought to be oxidized by monoamine oxidase type B. The protective effect of deprenyl against MPTP lethality implicates MPP+ (or possibly some other metabolite) in the lethality after MPTP injection. The reduced lethality of MPTP when given orally and the relative lack of brain levels of MPTP or MPP+ after oral MPTP implicate the brain as a target organ in the lethality of MPTP. Lethality after MPP+ administration almost certainly does not involve the brain, since little or no MPP+ could be measured in brain after oral or subcutaneous dosing of MPP+.

Depletion of cardiac norepinephrine but not brain catecholamines by MPTP-N-oxide in mice

After a single dose in mice, MPTP-N-oxide caused a dose-dependent depletion of cardiac norepinephrine which was similar although less pronounced than that caused by MPTP itself. After repeated daily doses, MPTP-N-oxide depleted cardiac norepinephrine, but did not deplete norepinephrine in the frontal cortex or dopamine in the striatum of mice. MPTP-N-oxide differed from MPTP, which depleted cardiac norepinephrine, cortical norepinephrine and striatal dopamine after repeated daily doses, but was similar to MPP+, another metabolite of MPTP, which depleted only cardiac norepinephrine. These data suggest that MPTP-N-oxide may contribute to the peripheral catecholamine-depleting effects after MPTP injection.

A high dose of MPTP overcomes the protective effect of selegiline against dopaminergic neurotoxicity

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at 90 mg kg-1 s.c., a dose lethal in non-pretreated mice, was well tolerated in selegiline [-)-deprenyl)-pretreated mice and produced persistent depletion of striatal dopamine and its metabolites one week after the last of four daily injections. The protective effect of selegiline against dopaminergic neurotoxicity of MPTP can thus be overridden by a high dose of MPTP that would be lethal without selegiline pretreatment.

Endocrine and neurochemical effects of (+)-PHNO, a dopamine D2 agonist

The naphthoxazine compound, (+)-PHNO, is a dopamine D2 receptor agonist which acts within the central nervous system. The effects of this drug on serum concentrations of corticosterone and prolactin and on brain concentrations of catecholamines and some of their metabolites were determined in male rats. Administration of (+)-PHNO in doses ranging from 3-300 micrograms/kg i.p. resulted in increased serum corticosterone, decreased serum prolactin and decreased concentrations of the dopamine metabolites, DOPAC and HVA, in the brain. At the higher doses of (+)-PHNO, concentrations of MHPG sulfate in the brain stem were increased and hypothalamic epinephrine concentrations were decreased. Pretreatment with centrally acting dopamine antagonists (spiperone or haloperidol) prevented the (+)-PHNO-induced changes in serum corticosterone, prolactin and brain catecholamines. In contrast, pretreatment with halopemide, a dopamine antagonist which penetrates poorly into the brain, was unable to block the effects of (+)-PHNO on serum corticosterone and brain catecholamines. These data show that (+)-PHNO, a dopamine agonist structurally unrelated to other dopamine agonists, acts centrally to affect serum corticosterone and brain catecholamines.

5'-thioadenosine derivatives as potent and selective inhibitors of histamine N-methyltransferase

Several new analogues of adenosine bearing a lipophilic side chain at the 5'-position have been synthesized and investigated for their ability to inhibit histamine N-methyltransferase (HNMT). The 5'-deoxy-5'-[4-(3-indolyl)but-1-yl]thio]adenosine (2e), exhibited a pI50 of 5.00 against guinea pig brain HNMT. Interestingly, the polar methyl sulphonium analogue (1c) was a more potent inhibitor of this enzyme (pI50 = 5.26). Both compounds were relatively ineffective inhibitors of rabbit adrenal phenylethanolamine N-methyltransferase (PNMT), rabbit lung indoleamine N-methyltransferase (INMT), and rat brain catechol O-methyltransferase (COMT). 5'-[N(4-phenylbutyl)]-amino-5'deoxyadenosine (2a) and 5'-[N-methyl,N-(4-phenylbutyl]-amino-5'deoxyadenosine (2b) also exhibited potent and selective inhibition against guinea pig brain HNMT. Results from kinetic studies indicate that the above compounds are inhibitors that compete for both the histamine and the S-adenosylmethionine (SAM) binding sites of HNMT. Compound 1c is one of the most potent adenosine analogue inhibitors of HNMT known.

Tissue concentrations of MPTP and MPP+ in relation to catecholamine depletion after the oral or subcutaneous administration of MPTP to mice

One hour after MPTP was given to mice at a dose of 30 mg/kg s.c., its concentration in tissues varied in the order kidney greater than liver greater than lung greater than brain greater than heart. When the same dose of MPTP was given orally, concentrations in most tissues were much lower at 1 hr than after s.c. administration, although the MPTP concentration in liver was only slightly lower. The concentrations of MPP+ (a metabolite of MPTP) at 1 hr were as high or higher than those of MPTP in all tissues except kidney, and MPP+ disappeared from the various tissues with half-lives from 3-20 hrs. The highest concentrations of MPP+, both absolute and relative to MPTP, were in heart. After oral administration of MPTP, no MPP+ was found in brain, and MPP+ concentrations in other tissues were lower than those after s.c. dosing. The depletion of heart norepinephrine was similar after MPTP administration by either route of administration even though MPTP and MPP+ concentrations in heart were lower after oral administration, suggesting that other metabolites of MPTP might also contribute to heart norepinephrine depletion.

Deprenyl antagonizes acute lethality of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

In Charles River CFW mice, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused lethality with an LD50 of 53.8 mg/kg s.c. In mice pretreated with deprenyl, no lethality occurred with MPTP doses up to 110 mg/kg s.c. MPTP alone at doses of 30 to 90 mg/kg s.c. caused marked salivation, licking and grooming, hyperlocomotion, hyperreactivity and convulsions during the 1st hr, followed by depression, continued salivation and respiratory distress at 2 to 3 hr and at longer times, with death occurring at the higher doses. In deprenyl-pretreated mice, MPTP produced only mild and transient effects. 1-Methyl-4-phenylpyridinium (MPP+) was more potent in causing lethality than was MPTP, and deprenyl did not affect its lethality. MPTP lethality was not antagonized by EXP 561 [4-phenyl-bicyclo-(2,2,2)octan-1-amine hydrochloride monohydrate], an uptake inhibitor that prevented the neurotoxic effects of a lower dose of MPTP on striatal dopamine and cortical norepinephrine neurons. In addition to deprenyl, other monoamine oxidase (MAO) inhibitors effective in inhibiting MAO-B (MD 240928 (R-3-[4-((3-chlorophenyl)methoxy)phenyl]-5-[(methylamino)methyl]-2- oxazolidinone methanesulfonate) and pargyline) protected against MPTP-induced lethality, but LY 51641 (N-[2-(o-chlorophenoxy)ethyl]cyclopropylamine hydrochloride) (a selective inhibitor of MAO-A) did not. The protective effect of deprenyl against MPTP-induced lethality was dose-dependent over a dose range of 0.01 to 10 mg/kg; in this range deprenyl inhibited MAO type B (MAO-B) in brain and liver. A 10-mg/kg i.p. dose of deprenyl antagonized MPTP-induced lethality as long as 14 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP+) effects on mouse heart norepinephrine

MPP+ (1-methyl-4-phenylpyridinium) mimicked MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in producing marked, dose-related depletion of cardiac norepinephrine after a single oral or subcutaneous dose in mice. MPP+ was approximately 4-fold more potent than MPTP in depleting norepinephrine, but the onset of depletion was not faster for MPP+ than for MPTP. The time courses of the effects of both compounds were similar to that for 6-hydroxydopamine, with maximum depletion occurring at 1 day, partial recovery at 2 and 4 days, and full recovery of norepinephrine concentrations at 1 week. Desipramine, over a dose range that completely prevented the depletion of cardiac norepinephrine by 6-hydroxydopamine at 24 hr, did not prevent cardiac norepinephrine depletion by either MPP+ or MPTP. In a short duration experiment, one or two doses of desipramine also failed to prevent heart norepinephrine depletion by MPP+ or by MPTP, although a slight antagonism was found. EXP 561 (4-phenylbicyclo[2,2,2]octan-1-amine hydrochloride monohydrate), another uptake inhibitor with possibly longer duration of action, also did not protect against norepinephrine depletion by a single dose of MPP+ or MPTP at a dose that prevented norepinephrine depletion by 6-hydroxydopamine. In mice given four daily doses of MPTP, EXP 561 prevented the depletion of norepinephrine in the frontal cortex and of dopamine in the striatum but not the depletion of norepinephrine in heart or spleen. Thus, both MPTP and MPP+ deplete norepinephrine in mouse heart, and this effect of the two compounds is resistant to antagonism by uptake inhibitors that antagonize the effects of MPTP on brain catecholamines.

AL-1576, an aldose reductase inhibitor (ARI), did not prevent the decrease of norepinephrine turnover in diabetic rats

ONO-2235 [(E)-3-carboxymethyl-5-[(2E)-2-methyl-3-phenyl-propenylidene]rhodanine], an ARI, was reported to prevent significantly the decrease of norepinephrine (NE) turnover in three tissues of streptozotocin (STZ)-diabetic rats (1). To examine whether the partial restoration of NE turnover by ONO-2235 is related to its ARI activity, the effect of another ARI, AL-1576 [spiro(2,7-difluoro-9H-fluoren-9, 4'-imidazoline)-2'5'-dione], on NE turnover in STZ rats was investigated. STZ caused an accumulation of sorbitol in the lens and decreased NE turnover in interscapular brown adipose tissue (IBAT), heart and pancreas. AL-1576 totally prevented the accumulation of sorbitol in the lens but had no effect on the decreased NE turnover in all three tissues. These results suggest that the partial prevention of NE turnover decrease by ONO-2235 may not have been mediated by its ARI activity.

Different effects of monoamine oxidase inhibition on MPTP depletion of heart and brain catecholamines in mice

Pargyline, an inhibitor of monoamine oxidase type B (MAO-B), did not prevent the depletion of heart norepinephrine 24 hr after a single dose of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice. In mice killed 24 hr after the last of 4 daily doses of MPTP, the depletion of dopamine in the striatum and of norepinephrine in the frontal cortex was completely prevented by pargyline, but the depletion of heart norepinephrine was not prevented. These results with pargyline are the same as results obtained earlier with deprenyl, another selective inhibitor of MAO-B. The doses of pargyline and of deprenyl that were used resulted in almost complete inhibition of MAO-B activity (phenylethylamine as substrate) in brain, heart and liver of mice. Deprenyl did not inhibit MAO-A activity (serotonin as substrate) in brain, but pargyline caused some inhibition of MAO-A in brain. In heart and liver, serotonin was oxidized only at about 1/10 the rate of phenylethylamine oxidation, suggesting that MAO-B predominates in these tissues. Both pargyline and deprenyl caused some inhibition of serotonin deamination in heart and liver, suggesting that the oxidation may have been due partly to MAO-B. Experiments with selective MAO inhibitors in vitro showed that only about 20% of the oxidation of serotonin was occurring via MAO-B in heart and liver. The in vitro oxidation of MPTP by MAO in mouse brain, heart and liver was almost completely inhibited by pretreatment with either pargyline or deprenyl. Neither pargyline nor deprenyl had any significant effect on the concentrations of MPTP in brain or heart one-half hr after injection of MPTP into mice. The concentrations of the metabolite, MPP+ (1-methyl-4-phenyl-pyridinium), were markedly reduced in brain and in heart by pretreatment with either pargyline or deprenyl. The data suggest that MPP+ formation, which is necessary for the depletion of brain catecholamines after MPTP injection, may not be necessary for depletion of norepinephrine in heart. Since the oxidation of MPTP in vitro was inhibited more by pargyline or deprenyl pretreatment than was the appearance of MPP+ in vivo, the possibility exists that some MPP+ formation might occur by an enzyme other than MAO.

Comparative effects of 1-ethyl- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridines on catecholamine concentrations in mouse brain and heart

The effects of 1-ethyl-4-phenyl-1,2,3,6-tetrahydropyridine (EPTP) on striatal dopamine, cortical norepinephrine and cardiac norepinephrine concentrations in mice were compared to the effects of the 1-methyl analog (MPTP). One week after the last of four daily injections of MPTP into mice, persistent depletion of dopamine and its metabolites in the striatum and of norepinephrine in the frontal cortex was found. EPTP did not produce these effects even at doses up to 4 times those of MPTP. A single dose of MPTP depleted cardiac norepinephrine 24 hrs later in mice. EPTP and the corresponding pyridinium compound (EPP+) depleted cardiac norepinephrine to a similar extent as did MPTP. These results and earlier observations show differences between the characteristics of MPTP-induced depletion of brain catecholamines and those of MPTP-induced depletion of cardiac norepinephrine.

Effects of LY104119, a thermogenic weight-reducing compound, on norepinephrine concentrations and turnover in obese and lean mice

Dopamine (DA) accumulation in interscapular brown adipose tissue (IBAT), heart, and liver was linear for more than 60 min after injection of 1-cyclohexyl-2-mercapto-imidazole (CHMI), a DA beta-hydroxylase inhibitor, into mice and served as an index of the rate of synthesis of norepinephrine (NE) in these tissues. There was a marked diurnal rhythm of DA accumulation in all three tissues of mice fed ad libitum, the rhythm being dampened in obese mice in IBAT and liver but not in heart. LY104119 acutely decreased NE concentration in IBAT, heart and liver, but not in brain, and it also decreased DA accumulation after CHMI in IBAT, heart and liver. Chronic administration of LY104119 at doses that stimulated thermogenesis and reduced body weight decreased NE turnover in heart and liver but increased NE turnover in IBAT. These data suggest that the increased thermogenesis in IBAT after chronic LY104119 treatment may be mediated by increased noradrenergic tone.

Persistent depletion of striatal dopamine and its metabolites in mice by TMMP, an analogue of MPTP

TMMP (1-methyl-4-[methylpyrrol-2-yl]-1,2,3,6-tetrahydropyridine) mimicked MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in causing persistent depletion of striatal dopamine and its metabolites one week after the last of four daily subcutaneous injections in mice. MMPP (1-methyl-4-[1-methylpyrrol-2-yl]-4-piperidinol) produced no depletion of dopamine or its metabolites under these conditions. None of the three compounds affected dopamine or its metabolites when administered orally. TMMP was even more rapidly oxidized by type B monoamine oxidase in-vitro than was MPTP, but MMPP was a very poor substrate for the enzyme. The lack of neurotoxicity of MMPP toward nigrostriatal dopamine neurons when it was given orally or subcutaneously to mice contrasts with previously reported results in monkeys, in which case MMPP was reported to be neurotoxic.

Depletion of heart norepinephrine in mice by some analogs of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)

The effects of ten analogs of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) on heart norepinephrine concentration in mice were compared to those of MPTP itself. A single dose of MPTP (10 mg/kg s.c.) caused 73% depletion of heart norepinephrine 24 hrs after its injection. m-Hydroxy-MPTP caused an identical degree of depletion, and six other analogs caused statistically significant but lesser depletion. Three analogs caused no significant change in heart norepinephrine concentration. Since MPTP is oxidized by type B monoamine oxidase (MAO-B), the analogs were compared for their ability to be oxidized by mitochondrial MAO preparations from mouse brain and liver. MPTP and seven analogs were oxidized by MAO-B (oxidation was inhibited by deprenyl); three analogs were not oxidized or were oxidized only slowly and not by MAO-B. The ability of the compounds to deplete heart norepinephrine paralleled their ability to deplete striatal dopamine in the case of MPTP and seven analogs. Three analogs--the same three that were not substrates for MAO-B--depleted heart norepinephrine but not striatal dopamine. One conclusion is that the ability of an MPTP analog to be a substrate for MAO-B is a necessary but not sufficient requirement for depletion of striatal dopamine, but is not necessary for depletion of heart norepinephrine.

Comparison of the effects of two 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analogs, 1-methyl-4-(2-thienyl)-1,2,3,6-tetrahydropyridine and 1-methyl-4-(3-thienyl)-1,2,3,6-tetrahydropyridine, on monoamine oxidase in vitro and on dopamine in mouse brain

1-Methyl-4-(2-thienyl)-1,2,3,6-tetrahydropyridine (2-MTTP) and 1-methyl-4-(3-thienyl)-1,2,3,6-tetrahydropyridine were compared with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) with respect to their interactions with MAO (monoamine oxidase) in vitro and their ability to produce persistent depletion of striatal dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, in mice. Both 2-MTTP and 1-methyl-4-(3-thienyl)-1,2,3,6-tetrahydropyridine were like MPTP in being potent, competitive inhibitors of MAO-A (MAO type A) and weak, noncompetitive inhibitors of MAO-B (MAO type B) in vitro. 2-MTTP resulted in persistent depletion of striatal dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid 1 week after the last of four daily injections to mice although 2-MTTP was less than one-fourth as potent as MPTP. The other isomer, 1-methyl-4-(3-thienyl)-1,2,3,6-tetrahydropyridine, failed to deplete dopamine or its metabolites in mouse striatum. Dopamine and its metabolites were also depleted in mouse nucleus accumbens by 2-MTTP and by MPTP; however, norepinephrine in frontal cortex was depleted by MPTP but not by 2-MTTP. The depletion of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid by 2-MTTP was prevented by pretreatment with deprenyl, a selective inhibitor of MAO-B, or with EXP 561, a dopamine uptake inhibitor, just as the depletion by MPTP was prevented. Mouse brain MAO oxidized 2-MTTP in vitro less rapidly than it oxidized MPTP; deprenyl was a potent inhibitor of the oxidation of both substrates, suggesting that MAO-B oxidizes both 2-MTTP and MPTP.(ABSTRACT TRUNCATED AT 250 WORDS)

Depletion of norepinephrine in mouse heart by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mimicked by 1-methyl-4-phenylpyridinium (MPP+) and not blocked by deprenyl

A single dose of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice caused 75-87% depletion of heart norepinephrine (NE) concentration 24 hrs later. MPP+ (1-methyl-4-phenylpyridinium) caused similar depletion of heart NE. The effect of MPTP was not blocked by pretreatment with deprenyl, an inhibitor of type B monoamine oxidase (MAO-B). Also, deprenyl pretreatment did not prevent the depletion of heart NE after 4 daily doses of MPTP, even though in the same mice deprenyl pretreatment did prevent depletion of dopamine in the striatum and of NE in the frontal cortex. Apparently the depletion of heart NE by MPTP, unlike the depletion of brain catecholamines, does not require that MPTP be metabolized by MAO-B and can be mimicked by systemic injection of MPP+.

Characterization of the neurotoxic potential of m-methoxy-MPTP and the use of its N-ethyl analogue as a means of avoiding exposure to a possible Parkinsonism-causing agent

1-Methyl-4-(3-methoxyphenyl)-1,2,3,6-tetrahydropyridine (2) produced persistent depletion of striatal dopamine in mice after four daily injections, although it was less potent than 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP has been implicated as a cause of Parkinsonism in drug abusers who inadvertently self-administered it and in industrial chemists who were exposed to it. Our results suggest that the m-methoxy compound has the same neurotoxic potential to cause destruction of nigrostriatal dopamine neurons that would lead to Parkinsonian symptoms in humans. In contrast, 1-ethyl-4-(3-methoxyphenyl)-1,2,3,6-tetrahydropyridine (11) had no effects on striatal dopamine in mice, even at doses 8 times those of MPTP. A method of preparing 11 and using it as an intermediate in the synthesis of potential analgesic drugs, thus avoiding a potentially neurotoxic intermediate, is described.

MD 240928 and harmaline: opposite selectivity in antagonism of the inactivation of types A and B monoamine oxidase by pargyline in mice

In mice treated 24 hrs earlier with pargyline (20 mg/kg i.p), both type A and type B monoamine oxidase (MAO-A and MAO-B) were partially inactivated in brain, heart and liver. The abilities of two short-acting, reversible inhibitors of MAO to antagonize that inactivation were compared. Pretreatment with MD 240928 at doses of 10, 20 or 30 mg/kg i.p. antagonized the inactivation of type B MAO but did not alter the inactivation of type A MAO in all three tissues. In contrast, pretreatment with harmaline at a dose of 10 mg/kg i.p. antagonized the inactivation of type A MAO but did not alter the inactivation of type B MAO. Antagonism of the pargyline-induced inactivation is interpreted as being due to the transient selective inhibition of MAO-A by harmaline and of MAO-B by MD 240928, preventing the mechanism-based inactivation of those enzymes by pargyline. The selective protection by harmaline is in agreement with earlier results with that compound in rats; the selective protection by MD 240928 is the first report of selective protection against MAO-B inactivation.

Comparison of 1-methyl-4-(p-chlorophenyl)-1,2,3,6-tetrahydropyridine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and p-chloroamphetamine as monoamine depletors

1-Methyl-4-(p-chlorophenyl)-1,2,3,6-tetrahydropyridine (PC-MPTP) failed to mimic the effects of MPTP in producing persistent depletion of striatal dopamine and its metabolites in mice one week after the last of four daily doses. MPTP was given at 20 mg/kg, whereas PC-MPTP was given at doses up to 80 mg/kg. In rats, p-chloroamphetamine (PCA) given in a single dose of 0.1 mmole/kg (20.6 mg/kg) i.p. caused marked depletion of hypothalamic serotonin and 5-hydroxyindoleacetic acid (5HIAA) concentration at 6 hrs, and the depletion persisted at 1 day and at 1 week. In contrast, PC-MPTP given at an equimolar dose failed to affect either serotonin or 5HIAA concentration at these times. Apparently the addition of a p-chloro substituent to MPTP eliminates its neurotoxicity to striatal dopamine neurons, and replacement of the aminoisopropyl side chain of PCA with a 1-methyl-1,2,3,6-tetrahydropyridin-4-yl group eliminates its neurotoxicity to brain serotonin neurons.

Influence of selective, reversible inhibitors of monoamine oxidase on the prolonged depletion of striatal dopamine by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) hydrochloride injected s.c. at 20 mg/kg once daily for four days resulted in marked depletion of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in mouse striatum one week after the last dose. Pretreatment with MD 240928, (R)-[4-((3-chlorophenyl)-methoxy)phenyl]-5-[(methylamino)methyl]-2- oxazolidinone methanesulfonate, prevented the depletion of striatal dopamine, DOPAC and HVA, whereas pretreatment with harmaline did not. MD 240928 selectively inhibited type B not type A monoamine oxidase (MAO), whereas harmaline selectively inhibited type A MAO in mouse striatum. Acutely after injection of harmaline, DOPAC and HVA concentrations were decreased in mouse striatum; these changes were not produced by MD 240928. The acute changes in dopamine metabolites reveal that MAO-A not MAO-B is responsible for the oxidation of dopamine in mouse striatum. Protection against the neurotoxic effects of MPTP by MD 240928 but not by harmaline indicates that prevention of dopamine oxidation is not the mechanism of the protective effect; instead the protection probably is due to prevention of MPTP metabolism by MAO-B, this metabolism having been shown to occur by other workers. The results with these reversible, competitive inhibitors of the two types of MAO are in agreement with previously reported results from studies using irreversible inhibitors of MAO.

Decrease in hypothalamic epinephrine concentration and other neurochemical changes produced by quinpirole, a dopamine agonist, in rats

Quinpirole, (4 aR-trans)-4, 4a, 5, 6, 7, 8, 8a, 9-octahydro-5-propyl-1 H-pyrazolo [3, 4-g]quinoline, is a dopamine agonist selective for the D2 subtype of dopamine receptors. In rats, quinpirole at doses of 0.3 mg/kg i.p. and higher decreased hypothalamic epinephrine concentrations. The doses required for this effect are only slightly higher than the minimum doses that decreased the concentration of dopamine metabolites in cerebral hemispheres. At higher doses of quinpirole (2-3 mg/kg i.p.), dopamine concentration was increased and norepinephrine concentration was decreased in hypothalamus, and MHPG sulfate (the norepinephrine metabolite) concentration was increased in brain stem and in hypothalamus. All of these neurochemical effects of quinpirole were blocked by pretreatment with spiperone, a dopamine antagonist. The effects were not produced by SKF 38393, a selective D1 agonist, or by the dextrorotatory enantiomer of quinpirole, which lacks D2 agonist activity. The data support the interpretation that quinpirole, by activating D2 receptors, results in a decrease in dopamine metabolites, a decrease in hypothalamic epinephrine concentration, and an increased conversion of norepinephrine to MHPG sulfate in rat brain probably through enhanced norepinephrine release.

Effects of amfonelic acid, alpha-methyltyrosine, Ro 4-1284 and haloperidol pretreatment on the depletion of striatal dopamine by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) given in 4 daily s.c. injections to mice resulted in marked depletion of striatal dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) 1 week after the last dose. Pretreatment of mice with alpha-methyltyrosine or Ro 4-1284 to deplete dopamine prior to MPTP injection did not prevent these effects of MPTP, nor did pretreatment with haloperidol, a dopamine receptor antagonist. The depletion of striatal dopamine by MPTP in mice therefore differs from the persistent depletion of striatal dopamine by amphetamine in iprindole-treated rats, which is prevented by pretreatment with either alpha-methyltyrosine or haloperidol. The depletion of dopamine, DOPAC and HVA in mouse striatum by MPTP was totally prevented by pretreatment with amfonelic acid, an inhibitor of dopamine uptake. This finding suggests that these effects of MPTP, like those of amphetamine in iprindole-treated rats, are dependent upon a functional transport system into the dopamine neuron.

Pergolide elevation of MHPG sulphate concentration in rat hypothalamus blocked by spiperone and mimicked by other dopamine agonists

Pergolide increased the concentration of MHPG sulphate (3-methoxy-4-hydroxy-phenylethylene glycol sulphate) in rat hypothalamus, and the increase was prevented by pretreatment with spiperone, a dopamine antagonist. An increase in hypothalamic MHPG sulphate concentration similar to that caused by pergolide was found after injection of quinpirole, a 'partial ergoline' that is a selective D2 agonist not affecting alpha-adrenoceptors, and by (-)-N-propylnorapomorphine, a dopamine agonist not related to the ergolines. Although the increase in MHPG sulphate concentration produced by pergolide had earlier been assumed to result from blockage of alpha-adrenoceptors, the present data indicate that it is an effect produced by dopamine D2 receptor stimulation.

Inhibition of types A and B monoamine oxidase by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was studied as an inhibitor of type A monoamine oxidase (MAO) acting on [14C]serotonin as substrate and of type B MAO acting on [14C]phenylethylamine as substrate. MPTP was a reasonably potent (Ki = 9 microM), competitive, reversible inhibitor of MAO-A from rat brain in vitro. MPTP given at a 30-mg/kg i.p. dose antagonized the irreversible inactivation of MAO-A in rat brain by pargyline, indicating that it inhibited MAO-A in vivo. At that same dose, MPTP prevented the conversion of dopamine released by Ro 4-1284 to 3,4-dihydroxyphenylacetic acid and attenuated its conversion to homovanillic acid. Because dopamine is mainly deaminated by MAO-A, at least in rodent brain, inhibition of MAO-A by MPTP might play some part in its production of persistent effects on striatal dopamine neurons such as protection of intraneuronal, extragranular dopamine from deamination. MPTP was less potent as an inhibitor of MAO-B from rat brain in vitro (Ki = 106 microM). In contrast to the inhibition of MAO-A, the inhibition of MAO-B by MPTP showed noncompetitive kinetics, was not fully reversible by dialysis and was time dependent. The characteristics of MAO-B inhibition are like those of a kcat inhibitor, which is acted upon by an enzyme to produce a reactive product that can covalently attach to the enzyme or other macromolecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) neurotoxicity to striatal dopamine neurons in mice

MPTP given to mice in 4 daily doses (20 mg/kg s.c.) resulted in 56-70% depletion of striatal dopamine 1 week after the last dose. Pretreatment with deprenyl or MD 240928, selective inhibitors of monoamine oxidase type B, or with amfonelic acid or nomifensine, selective inhibitors of dopamine uptake, prevented the depletion of striatal dopamine. In contrast, pretreatment with alpha-methyl-tyrosine, Ro 4-1284 or haloperidol did not prevent the depletion of striatal dopamine by MPTP. The results are compatible with the view that dopamine itself is not involved in the neurotoxic effect of MPTP but that MPP+, a metabolite of MPTP formed by MAO-B and accumulated by the dopamine uptake carrier, is responsible for the neurotoxicity.

Altered behavioral response to a D2 agonist, LY141865, in spontaneously hypertensive rats exhibiting biochemical and endocrine responses similar to those in normotensive rats

LY141865, a dopamine agonist selective for D2 dopamine receptors, caused hypomotility at low doses (0.01 and 0.1 mg/kg) and hypermotility at high doses (1 and 10 mg/kg) after i.p. injection into normotensive rats (WKY). In age-matched hypertensive rats (SHR), LY141865 caused hypomotility (not hypermotility) at all of these doses. The basal locomotor activity was higher in SHR than in WKY, but striatal concentrations of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, and of serotonin and its metabolite, 5-hydroxyindoleacetic acid, were not different between the two groups of rats. The specific binding of a dopamine receptor radioligand, tritiated pergolide, in striatum and mesolimbic regions, did not differ in SHR compared with WKY. In contrast to the lack of locomotor stimulation in SHR, other dopaminergic responses to LY141865 occurred in SHR as well as WKY. For instance, LY141865 decreased striatal and mesolimbic concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid, increased striatal and mesolimbic concentrations of acetylcholine, decreased hypothalamic concentrations of epinephrine, increased serum corticosterone concentration and decreased serum prolactin concentration in SHR as in WKY. Because radioligand-labeled dopamine receptors and several LY141865 responses mediated by dopamine receptors did not differ appreciably in SHR compared with WKY, the lack of behavioral hypermotility in response to LY141865 in SHR may be due to abnormalities in some synaptic mechanisms beyond dopamine receptor activation that are involved in the expression of increased locomotion in response to the dopamine agonist.

Antagonism by tomoxetine of the depletion of norepinephrine and epinephrine in rat brain by alpha-methyl-m-tyrosine

Tomoxetine hydrochloride, a potential antidepressant drug, antagonized the alpha-methyl-m-tyrosine-induced depletion of hypothalamic epinephrine and norepinephrine in rats. The findings suggest that tomoxetine, like several uptake-inhibiting antidepressant drugs that have been examined, inhibits uptake into epinephrine neurons as well as norepinephrine neurons in brain.