Neuroprotective effect of weak static magnetic fields in primary neuronal cultures.

Low intensity static magnetic fields (SMFs) interact with various biological tissues including the CNS, thereby affecting key biological processes such as gene expression, cell proliferation and differentiation, as well as apoptosis. Previous studies describing the effect of SMFs on apoptotic cell death in several non-neuronal cell lines, emphasize the importance of such a potential modulation in the case of neurodegenerative disorders, where apoptosis constitutes a major route via which neurons degenerate and die. In this study, we examine the effect of SMFs on neuronal survival in primary cortical and hippocampal neurons that constitute a suitable experimental system for modeling the neurodegenerative state in vitro. We show that weak SMF exposure interferes with the apoptotic programing in rat primary cortical and hippocampal neurons, thereby providing protection against etoposide-induced apoptosis in a dose- and time-dependent manner. Primary cortical neurons exposed to SMF (50G) for 7days exhibited a 57.1±6.3% decrease in the percentage of cells undergoing apoptosis induced by etoposide (12µM), accompanied by a marked decrease in the expression of the pro-apoptotic markers: cleaved poly ADP ribose polymerase-1, cleaved caspase-3, active caspase-9 and the phospho-histone H2A variant (Ser139) by 41.0±5.0%, 81.2±5.0%, 72.9±6.4%, 42.75±2.9%, respectively, and by a 57.2±1.0% decrease in the extent of mitochondrial membrane potential collapse. Using the L-type voltage-gated Ca(2+) channel inhibitor nifedipine, which is selective to Ca(2+) influx through Cav1.2, we found that the anti-apoptotic effect of SMFs was mediated by Ca(2+) influx through these channels. Our findings demonstrating altered Ca(2+)-influx in response to thapsigargin stimulation in SMF-exposed cortical neurons, along with enhanced inhibition of KCl-induced Ca(2+)-influx through Cav1.2 channels and enhanced expression of Cav1.2 and Cav1.3 channels, allude to the involvement of voltage- and store-operated Ca(2+) channels in various aspects of the protective effect exerted by SMFs. These findings show the potential susceptibility of the CNS to weak SMF exposure and have implications for the design of novel strategies for the treatment and/or prevention of neurodegenerative diseases.

Analysis of volatile organic compounds in rats with dopaminergic lesion: Possible application for early detection of Parkinson's disease.

Parkinson's disease (PD) is characterized by dopaminergic (DA) neuron depletion. Early detection of PD may help in selecting the appropriate treatment. Biomarkers of PD have been suggested, however none of these is currently in clinical use. The aim of this study was to identify volatile organic compounds (VOCs) as early biomarkers of PD. Our hypothesis was that during PD progression, specific VOCs are generated that are linked to the biochemical pathways characterizing PD. These VOCs can be detected by GC-MS combined with solid-phase microextraction (SPME) technique. Three groups of rats were studied: DA-lesioned rats injected with 6-hydroxydopamine (HDA; 250µg/rat n=11); control rats injected with saline (n=9), and control rats injected with DSP-4 (n=8), a specific noradrenergic neuron toxin. Blood and striatal tissue homogenate were analyzed. In the blood, 1-octen-3-ol and 2-ethylhexanol were found at significantly higher concentrations in HDA versus sham rats. In the striatal homogenate 1-octen-3-ol and other four compounds were found at significantly lower concentrations in HDA versus sham rats. 1-Octen-3-ol is a cytotoxic compound. These results may lead to the development of an early diagnostic test for PD based on profiling of VOCs in body fluids.

Increased L-DOPA-derived dopamine following selective MAO-A or -B inhibition in rat striatum depleted of dopaminergic and serotonergic innervation.

BACKGROUND AND PURPOSE: Selective MAO type B (MAO-B) inhibitors are effective in potentiation of the clinical effect of L-DOPA in Parkinson's disease, but dopamine (DA) is deaminated mainly by MAO type A (MAO-A) in rat brain. We sought to clarify the roles of MAO-A and MAO-B in deamination of DA formed from exogenous L-DOPA in rat striatum depleted of dopaminergic, or both dopaminergic and serotonergic innervations. We also studied the effect of organic cation transporter-3 (OCT-3) inhibition by decinium-22 on extracellular DA levels following L-DOPA. EXPERIMENTAL APPROACH: Striatal dopaminergic and/or serotonergic neuronal innervations were lesioned by 6-hydroxydopamine or 5,7-dihydroxytryptamine respectively. Microdialysate DA levels after systemic L-DOPA were measured after inhibition of MAO-A or MAO-B by clorgyline or rasagiline respectively. MAO subtype localization in the striatum was determined by immunofluorescence. KEY RESULTS: Rasagiline increased DA extracellular levels following L-DOPA to a greater extent in double- than in single-lesioned rats (2.8- and 1.8-fold increase, respectively, relative to saline treatment); however, clorgyline elevated DA levels in both models over 10-fold. MAO-A was strongly expressed in medium spiny neurons (MSNs) in intact and lesioned striata, while MAO-B was localized in glia and to a small extent in MSNs. Inhibition of OCT-3 increased DA levels in the double- more than the single-lesion animals. CONCLUSIONS AND IMPLICATIONS: In striatum devoid of dopaminergic and serotonergic inputs, most deamination of L-DOPA-derived DA is mediated by MAO-A in MSN and a smaller amount by MAO-B in both MSN and glia. OCT-3 plays a significant role in uptake of DA from extracellular space. Inhibitors of OCT-3 are potential future targets for anti-Parkinsonian treatments.

Alterations in striatal oxidative stress level produced by pharmacological manipulation of dopamine as shown by a novel synthetic marker molecule.

Oxidative stress (OS) is thought to participate in neurodegenerative diseases such as Parkinson's disease, but the contribution of dopamine metabolism and auto-oxidation to OS in Parkinson's and other diseases is not clear. Oxidative stress in rat striatum was measured by microdialysis using a novel synthetic compound composed of tyrosine and linoleic acid (LT), and determination of the oxidation products LT-OOH and LT-epoxy by HPLC-MS. Since LT is non-diffusible through the microdialysis membrane, the oxidized products formed in microdialyzate reflect oxidation state in the extracellular compartment. The extracellular oxidative stress (OS(ec)) was compared with intracellular oxidative stress (OS(ic)) as measured by tissue levels of oxidized and reduced glutathione and 7-ketocholesterol. Reserpinization caused an increase in OS(ic) but a reduction in OS(ec). Inhibition of both subtypes of monoamine oxidase (MAO-A and MAO-B) with tranylcypromine caused a reduction in both OS(ic) and OS(ec) whereas selective inhibition of MAO-A with clorgyline caused a reduction in Os(ic) but no change in OS(ec). A high dose (10 mg/kg) of amphetamine caused an increase in OS(ec) whereas a smaller dose (4 mg/kg) caused a reduction in OS(ec). Both doses of amphetamine reduced OS(ic). The present findings are consistent with a role of monoamine oxidase as well as dopamine auto-oxidation in production of striatal OS.

Modification of L-DOPA pharmacological activity by MAO inhibitors.

Dopamine behaves mainly as a MAO-A substrate in rodent brain, but selective inhibition of MAO-B results in an increased turning activity following L-DOPA administration in hemi-Parkinsonian rodents. Unilateral substantia nigra dopaminergic denervation results in serotonergic hyper-innervation which may increase the contribution of MAO-A in the denervated striatum. Possibly as a result of this, there was no change in striatal MAO-A activity when 95% of dopaminergic innervation was reduced by 6-hydroxydopamine, as assessed by apomorphine-induced turning activity. MAO-B as well as MAO-A may contribute to deamination of dopamine produced from L-DOPA.

Cardiovascular responses to combined treatment with selective monoamine oxidase type B inhibitors and L-DOPA in the rat.

BACKGROUND AND PURPOSE: Postural hypotension is a common side-effect of L-DOPA treatment of Parkinson's disease, and may be potentiated when L-DOPA is combined with selegiline, a selective inhibitor of monoamine oxidase B (MAO-B). Rasagiline is a new, potent and selective MAO-B inhibitor, which does not possess the sympathomimetic effects of selegiline. We have studied the effects of these selective MAO inhibitors, L-DOPA and dopamine on the cardiovascular system of the rat. EXPERIMENTAL APPROACH: Blood pressure and heart rate was measured in conscious rats following acute or chronic administration of rasagiline, selegiline and L-DOPA, by comparison with the selective MAO-A inhibitor clorgyline, or the MAO-A/B inhibitor tranylcypromine. Cardiovascular responses, catecholamine release, and modification of pressor response to dopamine were studied in pithed rats. KEY RESULTS: In conscious rats neither rasagiline nor selegiline caused significant potentiation of the effects of L-DOPA (50, 100, 150 mg.kg(-1)) on blood pressure or heart rate at doses which selectively inhibited MAO-B, but L-DOPA responses were potentiated by clorgyline and tranylcypromine. In rats treated twice daily for 8 days with L-DOPA and carbidopa, selegiline (5 mg.kg(-1)) but not rasagiline (0.2 mg.kg(-1)) caused a significant hypotensive response to L-DOPA and carbidopa, although both drugs caused similar inhibition of MAO-A and MAO-B. In pithed rats, selegiline but not rasagiline increased catecholamine release and heart rate, and potentiated dopamine pressor response at MAO-B selective dose. CONCLUSIONS AND IMPLICATIONS: The different responses to the two MAO-B inhibitors may be explained by the amine releasing effect of amphetamine metabolites formed from selegiline.

Attenuation of methamphetamine induced dopaminergic neurotoxicity by flupirtine: microdialysis study on dopamine release and free radical generation.

Flupirtine is a triaminopyridine derived centrally acting analgetic, which has been found to display neuroprotective effects in models of excitotoxic cell damage, global, and focal ischemia, but no direct interaction with any component of the N-methyl-D-aspartate (NMDA) and glutamate triggered Ca(2+)-channel. Additionally flupirtine shows potent antioxidant effects in isolated mitochondria and cell culture. Work in models of monoamine depletion and neuroleptic induced catalepsy in rats suggests a interaction of flupirtine with the dopaminergic neurotransmitter system as well. This prompted us to examine the effect of flupirtine on methamphetamine toxicity in mice and to investigate the influence on dopamine release and free radical formation in the rat striatum by microdialysis that may explain methamphetamine neurotoxicity. Pretreatment of C57-BL mice with flupirtine (4 x 10 mg/kg) significantly attenuated the striatal dopamine loss after methamphetamine application (4 x 5 mg/kg). In rats, a single injection of 10 mg/kg flupirtine reduced the methamphetamine induced striatal dopamine release by almost 50%, as measured by in vivo microdialysis. Flupirtine, however, did not influence the increase of free radical formation after methamphetamine infusion, which was assayed after infusion of salicylic acid by quantification of 2,3- and 2,5-dihydroxybenzoic acid. This suggests that other mechanisms rather than dopamine metabolism and autoxidation, may contribute to methamphetamine neurotoxicity.

Rasagiline [N-propargyl-1R(+)-aminoindan], a selective and potent inhibitor of mitochondrial monoamine oxidase B.

1. Rasagiline [N-propargyl-1R(+)-aminoindan], was examined for its monoamine oxidase (MAO) A and B inhibitor activities in rats together with its S(-)-enantiomer (TVP 1022) and the racemic compound (AGN-1135) and compared to selegiline (1-deprenyl). The tissues that were studied for MAO inhibition were the brain, liver and small intestine. 2. While rasagiline and AGN1135 are highly potent selective irreversible inhibitors of MAO in vitro and in vivo, the S(-) enantiomer is relatively inactive in the tissues examined. 3. The in vitro IC(50) values for inhibition of rat brain MAO activity by rasagiline are 4.43+/-0.92 nM (type B), and 412+/-123 nM (type A). The ED(50) values for ex vivo inhibition of MAO in the brain and liver by a single dose of rasagiline are 0.1+/-0.01, 0.042+/-0.0045 mg kg(-1) respectively for MAO-B, and 6.48+/-0.81, 2.38+/-0.35 mg kg(-1) respectively for MAO-A. 4. Selective MAO-B inhibition in the liver and brain was maintained on chronic (21 days) oral dosage with ED(50) values of 0.014+/-0.002 and 0.013+/-0.001 mg kg(-1) respectively. 5. The degree of selectivity of rasagiline for inhibition of MAO-B as opposed to MAO-A was similar to that of selegiline. Rasagiline was three to 15 times more potent than selegiline for inhibition of MAO-B in rat brain and liver in vivo on acute and chronic administration, but had similar potency in vitro. 6. These data together with lack of tyramine sympathomimetic potentiation by rasagiline, at selective MAO-B inhibitory dosage, indicate that this inhibitor like selegiline may be a useful agent in the treatment of Parkinson's disease in either symptomatic or L-DOPA adjunct therapy, but lack of amphetamine-like metabolites could present a therapeutic advantage for rasagiline.

Modification of dopamine release by selective inhibitors of MAO-B.

Chronic low dose deprenyl treatment in rats causes an increase in striatal extracellular dopamine level, without significant reduction in deaminated metabolite formation. This effect could be the result of increased endogenous levels of the MAO-B substrate beta-phenylethylamine, which is both a releaser of dopamine as well as an inhibitor of the neuronal membrane active dopamine uptake. In guinea pigs, however, striatal extracellular dopamine was not increased either by deprenyl or by clorgyline. Local infusion of the dopamine uptake inhibitor GBR-12909 caused a greater increase in striatal dopamine in microdialysate in rats than in guinea pigs. Intra-species differences in synaptic architecture or in density of dopamine transporter expression may account for these differences.

Selective monoamine oxidase subtype inhibition and striatal extracellular dopamine in the guinea-pig.

Striatal microdialysate levels of dopamine (DA) in conscious guinea-pigs were measured following acute (1 day) and chronic (21 days) treatment with deprenyl (2 and 0.25 mg kg(-1) s.c., respectively) or clorgyline (4 and 1 mg kg(-1) s.c., respectively), as well as by combination treatment using the same doses of the two inhibitors. These treatments caused selective inhibition of monoamine oxidase type B (MAO-B) or monoamine oxidase type A (MAO-A) respectively. Neither acute nor chronic treatments with deprenyl or clorgyline increased basal or KCl-induced DA levels. Acute and chronic clorgyline treatments were accompanied by significant reductions in striatal microdialysate 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). On the other hand, both acute and chronic deprenyl treatments were accompanied by significant increases in microdialysate HVA with no effect on DOPAC levels. Acute or chronic combined treatment with clorgyline and deprenyl increased tissue but not microdialysate DA levels. The combination treatment given chronically also reduced KCl-induced DA release but enhanced amphetamine-induced DA release. Microdialysate DA levels increased to a smaller extent in guinea-pig than in rat following local striatal infusion of GBR-12909 (100 microM). The difference between guinea-pigs and rats in the response to GBR-12909, could be the result of a lower dopaminergic innervation and/or density of DA transporter. This difference may explain why striatal microdialysate DA levels increased following chronic deprenyl treatment in the rat but not in the guinea-pig.

TV3326, a novel neuroprotective drug with cholinesterase and monoamine oxidase inhibitory activities for the treatment of Alzheimer's disease.

TV3326, [(N-propargyl-(3R) aminoindan-5-yl)-ethyl methyl carbamate] is a novel aminoindan derivative of the selective irreversible monoamine oxidase (MAO)-B inhibitor, rasagiline (N-propargyl-(1R)-aminoindan), possessing both cholinesterase (ChE) and MAO-inhibitory activity. In doses of 35-100 micromoles/kg administered orally to rats, it inhibits ChE by 25-40% and antagonises scopolamine-induced impairments in spatial memory. After daily administration of 75 micromoles/kg for 2 weeks, TV3326 does not show any motor stimulant effects but significantly reduces immobility in the forced swim test, an action consistent with that of known antidepressants. This could result from more than 70% inhibition of both MAO-A and B in the brain that occurs under these conditions, since it is not shared by the S-isomer, TV3279, which does not block MAO. TV3326 also shows selectivity for brain MAO, even after 2 months of daily administration, with little or no effect on the enzyme in the intestinal tract and liver. This reduces the likelihood of it producing the "cheese effect" if administered with tyramine-containing foods or beverages. TV3326 and TV3279 protect against ischemia-induced cytotoxicity in PC12 cells and reduce the oedema, deficits in motor function and memory after closed head injury in mice. These neuroprotective effects do not result from MAO inhibition. The pharmacological actions of TV3326 could be of clinical importance for the treatment of AD, and the drug is currently in development for this purpose.

Effect of low-dose treatment with selegiline on dopamine transporter (DAT) expression and amphetamine-induced dopamine release in vivo.

1. Chronic treatment with low doses of the selective monoamine oxidase (MAO) type B inhibitors selegiline [(-)-deprenyl] and rasagiline, causes elevation in extracellular level of 3,4-dihydroxyphenylethylamine (dopamine) in the rat striatum in vivo (Lamensdorf et al., 1996). The present study was carried out to determine whether this effect of selegiline could be the result of an inhibition of the high-affinity dopamine neuronal transport process. 2. Changes in activity of the dopamine transporter (DAT) in vivo following selegiline treatment were evaluated indirectly by microdialysis technique in the rat, from the change in striatal dopamine extracellular concentration following systemic amphetamine administration (4 mg kg(-1), i.p.). Striatal levels of the DAT molecule were determined by immunoblotting. Uptake of [3H]-dopamine was determined in synaptosomes from selegiline-treated animals. 3. Amphetamine-induced increase in striatal extracellular dopamine level was attenuated by one day and by chronic (21 days) treatment with selegiline (0.25 mg kg(-1), s.c.). 4. Striatal levels of DAT were elevated after 1 and 21 days treatment with selegiline, but were not affected by clorgyline, rasagiline, nomifensine or amphetamine. 5. The increase in DAT expression, and attenuation of amphetamine-induced dopamine release, were not accompanied by a change in [3H]-dopamine uptake in synaptosomes of selegiline-treated animals. 6. The results suggest that a reversible inhibition of dopamine uptake occurs following chronic low dose selegiline treatment in vivo which may be mediated by an increase in endogenous MAO-B substrates such as 2-phenylethylamine, rather than by the inhibitor molecule or its metabolites. Increased DAT expression appears to be a special property of the selegiline molecule, since it occurs after one low dose of selegiline, and is not seen with other inhibitors of MAO-A or MAO-B. The new DAT molecules formed following selegiline treatment appear not to be functionally active.

Nitric oxide and penile erection in streptozotocin-diabetic rats.

The purpose of this investigation was to study the time course, response to insulin and characteristics of erectile dysfunction in streptozotocin (STZ)-diabetic Sprague-Dawley rats, and the function of the NO-generating system in these animals. Copulation-induced and reflex erection were quantified in conscious Sprague-Dawley rats at different times after injection of STZ. The corporal vasodilatation response to nerve stimulation was studied by measuring the rise in corporal pressure in pithed rats following electrical stimulation of sacral spinal nerve roots. The activity of NO synthase was determined in corporal tissue by measuring the generation of [3H]citrulline from [3H]arginine. Copulation-induced erection was inhibited at 1 and 2 months after STZ treatment, but this could be prevented by a short (2-week) pretreatment with insulin. Reflex erection was inhibited at 1, 4, 6 and 9 months after STZ; at 6 months, this inhibition was also reversible by insulin pretreatment. Following pithing, the basal corporal pressure was elevated in diabetic rats. At 4 months after STZ, this increase was normalized by a 2-week, but not by a 1-week, pretreatment with insulin; however, at 9 months after STZ, insulin pretreatment did not normalize corporal pressure. The increase in corporal pressure caused by stimulation of sacral nerve roots in pithed rats was enhanced in diabetic animals. This enhancement was also normalized at 4 months, but not at 9 months, by 2 weeks of insulin treatment. The inhibition of the stimulation-induced increase in corporal pressure by NG-nitro-L-arginine methyl ester (5 mg/kg) was less following 9 months of diabetes, although NO synthase activity was normal in cavernosal tissue following 6-8 months of diabetes. In conclusion, STZ-induced diabetes caused changes in the erectile system that were initially reversible by a short insulin treatment, but which with time (more than 6 months) became irreversible. NO synthase activity in cavernosal tissue was normal, but the response to NG-nitro-L-arginine methyl ester was inhibited in long-term diabetes (9 months).

Increased striatal dopamine production from L-DOPA following selective inhibition of monoamine oxidase B by R(+)-N-propargyl-1-aminoindan (rasagiline) in the monkey.

Striatal extracellular fluid concentrations of dopamine and metabolites in response to direct striatal administration of two L-DOPA boluses administered sequentially were determined in three rhesus monkeys during halothane anesthesia. Whereas in an initial microdialysis run, generation of dopamine was less following the second L-DOPA bolus than the first, in a subsequent run, in which the selective MAO-B inhibitor R(+)-N-propargyl-1-aminoindan (rasagiline) was administered systemically (0.2 mg/kg s.c.) between the two L-DOPA boluses, generation of dopamine was greater following the second bolus.

Increased survival of dopaminergic neurons by rasagiline, a monoamine oxidase B inhibitor.

Both deprenyl and rasagiline (R(+)-N-propargyl-1-aminoindane mesylate), at a concentration of 1-10 microM, increased survival in vitro of rat E14 mesencephalic dopaminergic neurons that had been primed with 10% serum for 12 h (p < 0.05). Rasagiline, but not deprenyl, also increased total neuronal (MAP2-positive) survival (p < 0.05) Under serum-free conditions, rasagiline, but not deprenyl, retained its neuroprotective action on dopaminergic neurones. GABAergic neurons were not affected by either deprenyl or rasagiline. Clorgyline, an MAO-A inhibitor, did not exert any of these effects. The protective action of rasagiline on dopaminergic neurons, even under stringent serum-free conditions, is striking, and warrants further investigation for a role in the treatment of Parkinson's disease.

Effect of single and repeated administration of fluvoxamine on noradrenaline release in rat brain.

In vivo microdialysis in conscious rats was used to evaluate the effect of acute and chronic administration of fluvoxamine on extracellular levels of noradrenaline, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex. A single administration of fluvoxamine (12 mg/kg i.p.) during dialysate collection caused a significant delayed decrease in 5-HIAA and a mild increase in noradrenaline with no change in HVA and DOPAC levels. Chronic administration of fluvoxamine (12 mg/kg i.p. daily for 3 weeks, last dose 24 h prior to microdialysis) or a single dose 24 h prior to microdialysis had no effect on noradrenaline, DOPAC and HVA levels in the frontal cortex; 5-HIAA levels were significantly decreased 24 h after a single dose, but increased following long term treatment. Tissue concentrations of 5-HT and 5-HIAA in the frontal cortex showed a mild (though not significant) increase in rats chronically treated with fluvoxamine. Release of noradrenaline in the frontal cortex may be enhanced initially by a 5-HT uptake inhibitor, but this effect is not seen following drug washout.

Reduced striatal tyrosine hydroxylase activity is not accompanied by change in responsiveness of dopaminergic receptors following chronic treatment with deprenyl.

Deprenyl is the only selective monoamine oxidase B (MAO-B) inhibitor that is in clinical use for the treatment of Parkinson's disease. Our previous studies showed that chronic treatment of rats with low (MAO-B selective) doses of deprenyl inhibited dopamine (DA) re-uptake and enhanced DA release in the striatum. These changes could affect DA synthesis rate by activation of negative feedback loops. Chronic deprenyl treatment has also been suggested to cause down-regulation of release-modulating DA receptors. The effects of chronic and acute treatment with deprenyl on ex vivo striatal tyrosine hydroxylase activity were therefore studied, by determination of steady-state tissue level of DOPA following administration of NSD-1015 (100 mg/kg i.p.). In addition, we assessed changes in the in vivo sensitivity of dopaminergic receptors from the reduction in DOPA extracellular level after systemic apomorphine administration (2.5 mg/kg s.c.), following elevation of microdialysate DOPA by systemic or local aromatic amino acid decarboxylase inhibition with NSD-1015. Chronic treatment with deprenyl (0.25 mg/kg s.c. daily for 21 days) caused a significant reduction in tyrosine hydroxylase activity to 60% of control, with no change in the apomorphine-induced reduction of microdialysate DOPA and DOPAC. The reduction in tyrosine hydroxylase activity is compatible with our previous results showing an increase in striatal DA extracellular level following chronic treatment with deprenyl. The increased extracellular striatal DA level could reduce tyrosine hydroxylase activity through activation of a negative feedback loop, by activation of either presynaptic or postsynaptic DA receptors.

Effect of long-term treatment with selective monoamine oxidase A and B inhibitors on dopamine release from rat striatum in vivo.

Acute inhibition of monoamine oxidase B (MAO-B) in the rat does not affect striatal dopamine (DA) metabolism, but chronic MAO-B inhibition with deprenyl has been reported to increase the release of striatal DA, as shown using in vitro techniques. To see whether chronic MAO-B inhibition also causes an increase in DA release in vivo, rats were treated for 21 days with either deprenyl (0.25 mg/kg), TVP-1012 [R(+)-N-propargyl-1-aminoindan mesylate; 0.05 mg/kg], an irreversible inhibitor of MAO-B that is not metabolized to amphetamines, clorgyline (0.2 mg/kg), or saline (all doses once daily by subcutaneous injection). Concentric 4-mm-long microdialysis probes were implanted in the left striatum under pentobarbital/chloral hydrate anesthesia on day 21, and microdialysate DA, 3,4, dihydroxyacetic acid (DOPAC), and 4-hydroxy-3-methoxyphenyl acetic acid (HVA) were determined in the conscious animals on day 22. Baseline levels of DA were as follows: control, 0.34 +/- 0.04 (n = 13); deprenyl, 0.88 +/- 0.10 (n = 8, p < 0.01); TVP-1012, 0.94 +/- 0.20 (n = 7, p < 0.01); clorgyline, 0.90 +/- 0.12 (n = 7, p < 0.01) pmol/20 min. Levels of DOPAC and HVA were reduced only in the clorgyline-treated group. The incremental release of DA induced by depolarizing concentration of K+ (100 mM bolus of KCl in perfusate) was significantly greater in clorgyline- and deprenyl-treated rats and elevated (nonsignificantly) in TVP-1012-treated rats. Chronic treatment with the MAO-B inhibitors reduced striatal MAO-B activity by 90%, with 15% (TVP-1012) or 40% (deprenyl) inhibition of MAO-A. Clorgyline inhibited MAO-A by 95%, with 30% inhibition of MAO-B. A single dose of deprenyl (0.25 mg/kg, 24 h before microdialysis) had no significant effect on striatal efflux of DA. The results show that DA metabolism was reduced only by clorgyline, whereas neuronal release of DA was enhanced by both MAO-A and MAO-B inhibitors on chronic administration. The enhanced DA release by chronic MAO-B inhibition does not appear to be dependent on production of amphetamine-like metabolites of the inhibitor. Possible mechanisms for the release-enhancing effect of the MAO-B inhibitors include elevation in levels of endogenous beta-phenylethylamine, or an inhibition of DA reuptake, which develops only on chronic administration, because both deprenyl and TVP-1012 have only very weak effects on amine uptake in acute experiments.

Possible role for endothelins in penile erection.

The effects of endothelins (endothelin-1 and endothelin-3) on penile erection were studied in vivo in the pithed rat. Penile erectile responses were evaluated by measurements of intracorporal pressure via a needle inserted into one corpus cavernosum. Injection of endothelins (0.1-10 micrograms/kg i.v.) was followed by a rapid rise in corporal pressure. The responses to endothelin-1 and endothelin-3 were equivalent at the lower doses, but at 10 micrograms/kg the response to endothelin-3 was greater than that to endothelin-1. NG-Nitro-L-arginine-methyl ester (20 mg/kg) but not indomethacin (10 mg/kg) antagonized the rise in corporal pressure caused by submaximal doses of endothelins. Electrical nerve stimulation-induced increase in corporal pressure was reduced after 5 micrograms/kg endothelin-1 but not after the same dose of endothelin-3. The results indicate that in corporal vasculature endothelin-3 has a predominantly vasodilator effect, while endothelin-1 has a vasodilator effect at lower doses and a vasoconstrictor effect at higher doses. The vasodilator action of endothelins is probably mediated by activation of endothelin ETB receptors and local release of nitric oxide. Thus, endothelins may participate in initiation and/or maintenance of penile erection.