Esa Heinonen

Pharmacokinetic aspects of l-deprenyl (selegiline) and its metabolites

l‐Deprenyl (selegiline), an irreversible and selective inhibitor of monoamine oxidase type B (MAO‐B), is rapidly absorbed from the gastrointestinal tract and distributed into tissues. The reaction between MAO and selegiline takes place in two steps. The initial reversible reaction is followed by an irreversible reaction in which selegiline is bound covalently to the flavin part of the enzyme. Studies with positron emission tomography have shown retention of selegiline in brain areas with high MAO‐B activity, including striatal structures, hippocampus, thalamus, and substantia nigra. Inhibition of MAO‐B in vivo takes place rapidly; for example, platelet MAO is inhibited almost totally within the first 60 minutes after a single 10 mg oral dose of the drug. The recovery of MAO after inhibition depends on the organ and species in question. In rat brain the half‐life of recovery in the brain is approximately 8 to 12 days; in rat liver it is shorter, 1 to 3 days. Selegiline is metabolized into l‐(‐)‐desmethylselegiline, l‐(‐)‐methamphet‐amine, and l‐(‐)‐amphetamine mainly in the liver through the microsomal P‐450 system. The stereoselectivity of the metabolites is maintained; no racemic transformation takes place. All three main metabolites are found in human serum, cerebrospinal fluid, and urine, and l‐(‐)‐methamphetamine accounts for most of the metabolite pool. The metabolites are excreted mainly via urine. l‐(‐)‐Desmethylselegiline has been shown to be an irreversible inhibitor of MAO‐B in the rat and in humans.

Evaluation of the effects of a specific α2-adrenoceptor antagonist, atipamezole, on α1- and α2-adrenoceptor subtype binding, brain neurochemistry and behaviour in comparison with yohimbine

In the present study we evaluated the α1- and α2-adrenoceptor subtype binding, central α2-adrenoceptor antagonist potency, as well as effects on brain neurochemistry and behavioural pharmacology of two α2-adrenoceptor antagonists, atipamezole and yohimbine. Atipamezole had higher selectivity for α2- vs. α1-adrenoceptors than yohimbine regardless of the subtypes studied. Both compounds had comparable affinity for the α2A-, α2C- and α2B-adrenoceptors, but yohimbine had significantly lower affinity for the α2D-subtype. This may account for the fact that significantly higher doses of yohimbine than atipamezole were needed for reversal of α2-agonist (medetomidine) -induced effects in rats (mydriasis) and mice (sedation and hypothermia). The effect on central monoaminergic activity was estimated by measuring the concentrations of transmitters and their main metabolites in whole brain homogenate. At equally effective α2-antagonising doses in the rat mydriasis model, both drugs stimulated central noradrenaline turnover (as reflected by increase in metabolite levels) to the same extent. Atipamezole increased dopaminergic activity only slightly, whereas yohimbine elevated central dopamine but decreased central 5-hydroxytryptamine turnover rates. In behavioural tests, atipamezole (0.1–10 mg/kg) did not affect motor activity but stimulated food rewarded operant (FR-10) responding (0.03–3 mg/kg) whereas yohimbine both stimulated (1 mg/kg) and decreased (≥ 3 mg/kg) behaviour in a narrow dose range in these tests. In the staircase test, both antagonists increased neophobia, but in the two compartment test only yohimbine (≥ 3 mg/kg) decreased exploratory behaviour. The dissimilar effects of the antagonists on neurochemistry and behaviour are thought to be caused by non α2-adrenoceptor properties of yohimbine. In conclusion, the α2-antagonist atipamezole blocked all α2-adrenoceptor subtypes at low doses, stimulated central noradrenergic activity and had only slight effects on behaviour under familiar conditions, but increased neophobia. The low affinity for the α2D-adrenoceptor combined with its unspecific effects complicates the use of yohimbine as pharmacological tool to study α2-adrenoceptor physiology and pharmacology.

Longitudinal study of cerebrospinal fluid amyloid proteins and apolipoprotein E in patients with probable Alzheimer's disease

Levels of soluble amyloid beta protein (sAbeta), amyloid beta precursor protein (APP) and apolipoprotein E (apoE) were examined in cerebrospinal fluid (CSF) obtained twice, at baseline and after 3-year follow-up, from 25 patients with probable Alzheimers disease (AD). Levels of sAbeta and apoE from patients with the apoE4 allele decreased with time, whereas the levels were similar in patients without apoE4 allele. Changes of sAbeta and apoE concentrations correlated significantly with those of mini-mental state examination (MMSE) scores. Levels of sAbeta did not change with time in patients with mild dementia, whereas they decreased significantly in patients with moderate dementia. ApoE concentrations decreased in both groups whereas APP levels were similar. We conclude that measurements of CSF sAbeta and apoE levels may be helpful in monitoring progression of the disease.

Different toxicological profile of two COMT inhibitors in vivo: the role of uncoupling effects.

Summary. The toxicity profiles of entacapone and tolcapone, novel catechol-O-methyl-transferase (COMT) inhibitors, have been reported to differ from each other. It has also been shown that tolcapone, but not entacapone, is a potent uncoupler of oxidative phosphorylation in vitro at low micromolar concentrations. Signs of hepatotoxicity induced by tolcapone treatment have been previously reported in toxicological studies and in clinical use.The present study was designed to investigate the mechanism of hepatotoxicity of tolcapone and its possible relationship to uncoupling of oxidative phosphorylation in vivo. A 15-day oral toxicity study with entacapone or tolcapone (300 and 500 mg/kg/day) was carried out, and 2, 4-dinitrophenol (DNP), a known uncoupling agent of oxidative phosphorylation, served as a positive reference substance (20 mg/kg/day). No treatment related findings were observed in entacapone-treated rats. Clinical chemistry parameters regarding hepatocellular damage were increased in tolcapone and DNP-treated animals. The energy status measured as ATP/ADP ratio from the liver samples and energy charge (EC) in liver cell mitochondria were diminished both in tolcapone- and in DNP-treated rats. These signs together with clinical symptoms consisting of increased respiration, decreased activity and drowsiness, and elevation of the rectal body temperature observed in tolcapone- and DNP-treated animals suggest a relationship between the treatment and uncoupling of oxidative phosphorylation in vivo.

Dexmedetomidine alleviates ethanol withdrawal symptoms in the rat

The effect of dexmedetomidine, a selective alpha 2-adrenoceptor agonist, on ethanol withdrawal symptoms was studied in chronically ethanol-fed rats. After a 4-day ethanol intoxication period the rats were given s.c. injections of dexmedetomidine (3, 10, or 30 micrograms/kg) or saline (control group) at 10, 16, 22, and 39 h after the last dose of ethanol. The severity of ethanol withdrawal symptoms (rigidity, tremor, irritability, hypoactivity) was rated up to 58 h, blind to the treatments. The results showed that dexmedetomidine at doses 10 and 30 micrograms/kg significantly diminished the severity of the ethanol withdrawal reaction as measured by the sum score of the three most specific withdrawal signs (rigidity, tremor, and irritability). Dexmedetomidine at 10 micrograms/kg was the most effective dose, especially in the latter half of the withdrawal period (23-58 h after last dose of ethanol). The results suggest that dexmedetomidine in the treatment of ethanol withdrawal symptoms should be further studied.

Monoamine oxidase B inhibitor selegiline protects young and aged rat peripheral sympathetic neurons against 6-hydroxydopamine-induced neurotoxicity

Abstract Selegiline is a selective and irreversible monoamine B inhibitor with the capacity to increase the level of several antioxidative enzymes in rat brain. It can protect adrenergic neurons against injury induced by neurotoxins such as MPTP, DSP-4 and AF64A in animal studies. In addition, the protective action is not limited to catecholaminergic cells, as selegiline can also minimize the loss of developing motoneurons after axotomy. The aim of this study was to determine whether selegiline can protect peripheral catecholaminergic neurons against the neurotoxic effect of 6-OHDA. This kind of protective effect against 6-OHDA neurotoxicity has not been reported before. Wistar albino male rats aged 4 or 24 months were treated with selegiline or saline solution 1 h before 6-OHDA injection. At 2 weeks after the 6-OHDA injection, the superior cervical ganglia (SCG) and submandibular glands (SMG) were studied using catecholamine histofluorescence and immunohistochemistry for tyrosine hydroxylase (TH). The number of TH-positive cells in the SCG and the length and number of adrenergic nerve fibers in the SMG were quantified. Our findings showed that 6-OHDA caused a reduction of TH immunoreactivity and catecholamine histofluorescence in neuronal somata, as well as a decrease in the number and length of adrenergic nerve fibers in the submandibular gland. Selegiline pretreatment protected SCG neurons and their postganglionic nerve fibers in SMG against these changes in a dose-dependent manner. The mechanism through which selegiline exerts its neuroprotective effect is as yet unknown.

Comparative toxicological study on the hepatic safety of entacapone and tolcapone in the rat.

Summary. Entacapone and tolcapone are novel COMT (catechol-O-methyltransferase) inhibitors indicated for the adjunctive treatment of Parkinsons disease (PD) in combination with levodopa. The marketing authorisation of tolcapone was suspended in the European Union (EU) in 1998 mainly due to severe abnormal hepatic reactions. This fact raised concern about the safety of COMT inhibitors in the treatment of parkinsonian patients.In order to investigate whether these COMT inhibitors exhibit different effects on the liver comparative toxicological studies were performed in the rat. Short term toxicological studies in rats at high oral doses of entacapone and tolcapone (200, 400 or 600 mg/kg daily) were carried out. Tolcapone (400 mg/kg/day or 600 mg/kg/day) increased mortality after only one week treatment and induced signs of toxicity such as a rise in body temperature, stimulation of respiration and rapid onset of rigor mortis after death. Entacapone did not show any adverse effects at the tested dose levels. In the histopathological examination liver cell necrosis was observed in the tolcapone (400 and 600 mg/kg/day) treated rats, but it revealed no treatment related signs of toxicity in entacapone-treated rats. We conclude that the toxicological profile of the two COMT inhibitors, entacapone and tolcapone, differ from each other, tolcapone – unlike entacapone – showed hepatotoxicity.

The interaction of L-deprenyl and scopolamine on spatial learning/memory in rats

SummaryL-Deprenyl, a specific MAO-B inhibitor, has been reported to improve learning/memory in some cognitive tests in aged rats. The present study investigated whether L-deprenyl could alleviate the spatial learning deficit induced by muscarinic blockade and aging in OFA rats. Scopolamine (0.25 mg/kg) impaired the acquisition of a water maze task in adult rats and increased their swimming speeds. L-Deprenyl (0.25 mg/kg, 14 days) had no effect on water maze performance in saline treated adult rats, but markedly alleviated the learning deficit induced by scopolamine and increased the time and distance of swimming in the training quadrant when the platform was removed (spatial probe trial). L-Deprenyl partly reduced the effect of scopolamine on speed of swimming. Nevertheless, administration of l-deprenyl (0.25 mg/kg, 14 days) had no effect on spatial learning/memory in aged rats. We suggest that the l-deprenyl-scopolamine interaction in the water maze test may be considered as a premise for further investigations of l-deprenyl as cognition enhancer.

Selegiline treatment and the extent of degenerative changes in brain tissue of patients with Alzheimer's disease

AbstractBackground: A beneficial effect of selegiline (l-deprenyl) in Alzheimers disease (AD) has been reported in several clinical studies. Methods: The brain tissue from 17 deceased patients, members of a double-blind clinical trial to assess the potential benefit of selegiline in AD, were analysed. Findings: In our study, the decrease in the Mini-Mental State Examination (MMSE) scores during the progress of the disease had been significantly influenced by selegiline treatment. Prior to death, the MMSE scores were significantly higher in those patients receiving selegiline than in those receiving placebo. However, according to our results, none of the lesions critical for AD diagnosis, such as counts of senile/neuritic plaques, neurofibrillary tangles or β-A4 load, were influenced by the selegiline treatment. Interpretation: In conclusion, according to our study, mechanisms other than neuronal degeneration seen as lesions critical for AD diagnosis are influenced by selegiline treatment, leading to the functional benefit found in AD.

Effects of monoamine oxidase inhibition by selegiline on concentrations of noradrenaline and monoamine metabolites in CSF of patients with Alzheimer's disease

SummaryA double-blind, cross-over trial with 12 patients with Alzheimers disease (AD) was carried out primarily to test the suitability of this design in the investigation of the clinical, effects of selegiline (10 mg/day) in AD. Cerebrospinal fluid (CSF) samples for the determination of concentrations of noradrenaline (NA) and several monoamine metabolites were collected at baseline and at the end of both four-week treatment periods (placebo and selegiline). The severity of dementia was assessed using Ferms and Gottfries-Bråne-Steen (GBS) dementia scales. The concentrations of the dopamine metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC) and the NA metabolites, 3,4-dihydroxyphenylglycol (DHPG), and 3-methoxy-4-hydroxyphenyl glycol (MHPG) decreased significantly during selegiline treatment. There was a clear trend of reduction in concentrations of homovanillic acid (HVA) during selegiline treatment, whereas the concentrations of NA, 5-hydroxyindoleacetic acid (5-HIAA), and tryptophan did not differ significantly. The study design was not suitable for the analysis of the clinical results as there was a significant carry-over effect in both scales. As only the first period data could be used in the analysis, there were no significant differences in the scores of Ferms or GBS scales, but clear positive trends could be detected in favour of selegiline.