In Vivo Monitoring of Dopamine by Microdialysis with 1 min Temporal Resolution Using Online Capillary Liquid Chromatography with Electrochemical Detection.

Microdialysis is often applied to understanding brain function. Because neurotransmission involves rapid events, increasing the temporal resolution of in vivo measurements is desirable. Here, we demonstrate microdialysis with online capillary liquid chromatography for the analysis of 1 min rat brain dialysate samples at 1 min intervals. Mobile phase optimization involved adjusting the pH, buffer composition, and surfactant concentration to eliminate interferences with the dopamine peak. By analyzing electrically evoked dopamine transients carefully synchronized with the switching of the online LC sample valve, we demonstrate that our system has both 1 min sampling capabilities and bona fide 1 min temporal resolution. Evoked DA transients were confined to single, 1 min brain dialysate samples. After uptake inhibition with nomifensine (20 mg/kg i.p.), responses to electrical stimuli of 1 s duration were detected.

Intravenous self-administration of nomifensine in rats: implications for abuse potential in humans.

Rats acquired and maintained intravenous self-administration of nomifensine, a new antidepressant compound. Additional experiments implicated dopamine-containing neurons in this behavior. These findings, along with the marked pharmacological similarities between nomifensine and such drugs as cocaine and methylphenidate, indicate a potential for nomifensine abuse by humans.

Dopamine-independent locomotor actions of amphetamines in a novel acute mouse model of Parkinson disease.

Brain dopamine is critically involved in movement control, and its deficiency is the primary cause of motor symptoms in Parkinson disease. Here we report development of an animal model of acute severe dopamine deficiency by using mice lacking the dopamine transporter. In the absence of transporter-mediated recycling mechanisms, dopamine levels become entirely dependent on de novo synthesis. Acute pharmacological inhibition of dopamine synthesis in these mice induces transient elimination of striatal dopamine accompanied by the development of a striking behavioral phenotype manifested as severe akinesia, rigidity, tremor, and ptosis. This phenotype can be reversed by administration of the dopamine precursor, L-DOPA, or by nonselective dopamine agonists. Surprisingly, several amphetamine derivatives were also effective in reversing these behavioral abnormalities in a dopamine-independent manner. Identification of dopamine transporter- and dopamine-independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

Continuous stimulation of dopaminergic receptors by rotigotine does not interfere with the sleep-wake cycle in the rat.

Rotigotine is a new, non-ergoline dopamine receptor agonist developed for the treatment of Parkinson's disease in a transdermal formulation (Neupro ) to provide sustained drug delivery for 24 h with a once daily dosing. The aim of the present study was to determine whether or not continuous dopaminergic stimulation can interfere with the sleep-wake cycle. To achieve this, rotigotine was administered as a slow release formulation to provide stable plasma and brain levels over a period of 6 days and the sleep-wake cycle was evaluated in the freely-moving male rat using electroencephalagraphic recording. For comparison, the mixed dopamine/noradrenaline reuptake inhibitor nomifensine (16 mg/kg p.o.) was administered once daily for 6 days. In contrast to nomifensine, rotigotine (0.5 and 5 mg/kg s.c.) had no clear effects on the sleep-wake cycle. Nomifensine delayed the onset of rapid eye movement (REM) sleep and, to a lesser extent, also that of slow wave sleep (SWS). In addition, it increased the duration of waking time and decreased the duration of SWS and REM sleep. These effects were observed on all days and repeated administration lead neither to potentiation nor attenuation of the effects. It is concluded that a continuous dopaminergic stimulation of dopamine receptors by rotigotine may not only be beneficial for the treatment of the motor symptoms of Parkinson's disease but also have additional benefits by not compromising either sleep architecture or circadian rhythm.

Smaller effect of propofol than sevoflurane anesthesia on dopamine turnover induced by methamphetamine and nomifensine in the rat striatum: an in vivo microdialysis study.

Volatile anesthetics accelerate dopamine turnover in the brain, especially when used in conjunction with psychotropic agents such as methamphetamine and nomifensine. The effect of intravenous propofol anesthesia on the extracellular dopamine concentrations is unclear. The aim of this study was to compare the effect of two anesthetics on the extracellular concentrations of dopamine and metabolites using an in vivo microdialysis model. Male Sprague Dawley rats were implanted with a microdialysis probe into the right striatum. The probe was perfused with modified Ringer's solution, and the dialysate was directly injected into a high-performance liquid chromatography system every 20 min. The rats were intraperitoneally administered saline, methamphetamine at 2 mg/kg, or nomifensine at 10 mg/kg. After treatment, the rats were anesthetized with intravenous propofol (20 mg/kg followed by 25 or 50 mg/kg/h) or inhalational sevoflurane (2.5%) for 1 h. Propofol showed no effect on the extracellular concentration of dopamine during anesthesia; however, propofol decreased the dopamine concentration after anesthesia in the high-dose group. Sevoflurane anesthesia increased the concentration of metabolites. Systemic administration of methamphetamine and nomifensine increased the extracellular concentration of dopamine. Sevoflurane anesthesia significantly enhanced the increase in the dopamine concentration induced by both methamphetamine and nomifensine, whereas propofol anesthesia showed no effect on the methamphetamine- and nomifensine-induced dopamine increase during anesthesia. The enhancing effect of psychotropic agent-induced acceleration of dopamine turnover was smaller for propofol anesthesia than for sevoflurane anesthesia.

Preferential increases in nucleus accumbens dopamine after systemic cocaine administration are caused by unique characteristics of dopamine neurotransmission.

In vivo voltammetry was used to investigate the preferential increase of extracellular dopamine in the nucleus accumbens relative to the caudate-putamen after systemic cocaine administration. In the first part of this study, cocaine (40 mg/kg, i.p.) was compared with two other blockers of dopamine uptake, nomifensine (10 mg/kg, i.p.) and 3beta-(p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylmethyl ester hydrochloride (RTI-76; 100 nmol, i.c.v.), to assess whether the inhibitory mechanism of cocaine differed in the two regions. All three drugs robustly increased electrically evoked levels of dopamine, and cocaine elevated dopamine signals to a greater extent in the nucleus accumbens. However, kinetic analysis of the evoked dopamine signals indicated that cocaine and nomifensine increased the K(m) for dopamine uptake whereas the dominant effect of RTI-76 was a decrease in V(max). Under the present in vivo conditions, therefore, cocaine is a competitive inhibitor of dopamine uptake in both the nucleus accumbens and caudate-putamen. Whether the preferential effect of cocaine was mediated by regional differences in the presynaptic control of extracellular DA that are described by rates for DA uptake and release was examined next by a correlation analysis. The lower rates for dopamine release and uptake measured in the nucleus accumbens were found to underlie the preferential increase in extracellular dopamine after cocaine. This relationship explains the paradox that cocaine more effectively increases accumbal dopamine despite identical effects on the dopamine transporter in the two regions. The mechanism proposed for the preferential actions of cocaine may also mediate the differential effects of psychostimulant in extrastriatal regions and other uptake inhibitors in the striatum.

Neurotensin affects hyperactivity but not stereotypy induced by pre and post synaptic dopaminergic stimulation.

The effects of intraventricular administration of neurotensin (0.9, 3.75 and 15.0 micrograms) on hyperactivity and stereotypy induced by either amphetamine (1 mg/kg), nomifensine (20 mg/kg), apomorphine (0.5 mg/kg) or N-n-propylnorapomorphine (0.5 mg/kg) were examined. Results indicate that for each drug treatment, the effects of neurotensin were identical: hyperactivity was significantly reduced while stereotypy remained unaffected. Results also revealed that neurotensin significantly increased the hypothermia induced by apomorphine and N-n-propylnorapomorphine. Possible mechanisms which could underly neurotensin's selective inhibitory action on hyperactivity produced by both pre and post synaptic dopaminergic stimulation are discussed.

Metabolism of nomifensine after oral and intravenous administration.

The metabolism of nomifensine was studied after single oral and intravenous administration and after 2 weeks of oral dosing. The three principal metabolites reached maximum plasma concentrations rapidly (in 1 to 1.5 hours) after nomifensine administration. Less than 10% was detected as a free, unconjugated form. All three metabolites were eliminated rapidly (elimination t1/2 values between 6.8 and 9.0 hours). Only very low concentrations of free metabolites were found in plasma after 24 hours of nomifensine administration. AUC values for free metabolites were between 0.27 to 0.46 hr X mumol/L after all nomifensine schedules. Two weeks of dosing had no significant influence on the elimination t1/2 or AUC values of the metabolites, indicating no change in the hydroxylation and methylation reactions. In addition, there were no changes in the conjugation reactions during prolonged nomifensine dosing. Nomifensine has a very short t1/2 and no tendency for accumulation after repeated doses. We conclude that nomifensine's clinical pharmacokinetic profile is not significantly changed by the kinetic behavior of its three main metabolites after the usual maintenance doses.

Disposition of nomifensine after acute and prolonged dosing.

The pharmacokinetics of nomifensine were studied after single oral and intravenous doses. The effect of prolonged oral dosing on the pharmacokinetics of nomifensine was also evaluated. Nomifensine was rapidly absorbed from the gastrointestinal tract. The peak concentration of free nomifensine (0.18 mumol/L) was reached at 1.13 hours after dosing. The highest concentration after the intravenous dose was 1.21 mumol/L. The elimination t1/2 after a single dose was about 4 hours regardless of the route of administration. Nomifensine was extensively distributed in body fluids and tissues, with an apparent volume of distribution of 8.69 L/kg. The AUC of free nomifensine after oral dosing was only 26.5% of that after intravenous infusion. Absorption from the gastrointestinal tract was complete, and the AUCs of total nomifensine were equal after all treatments. The main reason for limited bioavailability seems to be extensive first-pass metabolism during the absorption process. The AUC of free nomifensine decreased substantially (from 0.78 to 0.32 hr X mumol/L) and the elimination t1/2 was shortened (from 4.39 to 2.11 hours) after a 2-week dosing period. These effects suggest marked induction of the metabolizing enzymes. An increase in nomifensine dosage may be needed in some patients to maintain a full therapeutic effect.

Noradrenergic and dopaminergic effects of nomifensine in healthy volunteers.

Intravenous doses (100 mg in 20 minutes) of the antidepressant drug nomifensine, administered to male volunteers, increased heart rate and blood pressure, elevated the plasma levels of norepinephrine and its metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), and powerfully stimulated growth hormone release and inhibited the secretion of prolactin. Oral nomifensine, either as a single 100 mg dose or as a similar dose after 2 weeks' treatment with the drug (150 mg/day), caused none of the above effects. This was in line with the limited (less than 30%) oral bioavailability of the active, unconjugated form of the drug, estimated in the same subjects. MHPG in plasma was slightly but consistently reduced by the 2 weeks' treatment, suggesting reduced turnover of norepinephrine. The observed clinical effects of nomifensine are compatible with uptake inhibition and augmented release of norepinephrine and dopamine and possibly direct agonistic effects on dopamine receptors. Although nomifensine was withdrawn from the market because of immunologic complications, it serves as a model compound of a new pharmacologic class of antidepressants, devoid of many of the disturbing side effects of the tricyclic drugs.

The pharmacokinetics and bioavailability of nomifensine maleate in healthy men.

Two studies were conducted in normal male volunteers to establish the pharmacokinetic parameters for nomifensine maleate and to determine the bioavailability of the drug from the Merital capsule intended for U.S. marketing. Single oral doses of 25, 100, and 200 mg of nomifensine maleate as aqueous solutions were administered to 24 men in the open-label Latin-square design pharmacokinetic study. In the bioavailability study, 24 men received single oral 50 mg doses of nomifensine maleate in a capsule or as an aqueous solution. Plasma levels of nomifensine were determined by radioimmunoassay and urinary levels of total nomifensine and its metabolites were assayed by thin-layer chromatography. There was a proportional increase in the area under the curve (AUC) with increasing dose, while peak plasma levels and amounts of total nomifensine and its metabolites excreted in the urine rose as dose increased. The pharmacokinetics of nomifensine are considered linear over the dose range tested. Nomifensine maleate was equally bioavailable from the 50 mg aqueous solution and the Merital capsule formulation.

Double-blind placebo-controlled multicenter evaluation of the efficacy and safety of nomifensine in depressed outpatients.

Outpatients with primary affective disorder-depression who scored at least 20 on the Hamilton Depression Rating Scale (HDRS) were randomly assigned to treatment for 1 month with nomifensine (100-200 mg/day) or placebo. Clinical laboratory and physical evaluations, including ECGs when feasible, revealed no clinically significant changes over the course of treatment. Nomifensine patients showed improvement compared to placebo on the HDRS total score endpoint analysis (p = .06) and the Cognitive Disturbance and Retardation factors (p less than or equal to .05). A better rate of improvement was seen with nomifensine on the Clinical Global Impressions severity of illness (p less than or equal to .05) and therapeutic index (p less than or equal to .05) components. No differences were seen between groups in the incidence of overall or specific side effects. Nomifensine thus appeared safe and superior to placebo on several key measures of depressive symptomatology in this multicenter study of depressed outpatients.

Double-blind comparison of the efficacy and safety of nomifensine maleate vs. placebo in depressed outpatients.

Nomifensine maleate in doses of 100-200 mg/day was compared to placebo in a double-blind study of 50 moderately to severely depressed outpatients seen in a private practice. Efficacy ratings over the 28-day treatment period revealed significant improvement in the nomifensine group on the Hamilton Depression Rating Scale and Clinical Global Impressions, as well as the Self-Rating Symptom Scale, Brief Psychiatric Rating Scale, and Hamilton Anxiety Scale. These differences in favor of nomifensine were seen as early as Day 7 of treatment and were generally consistent throughout the trial. No clinically meaningful changes in vital signs or laboratory values were found over the course of treatment. Only one side effect, a pruritic rash in one patient, was directly attributable to active drug treatment. The CGI Therapeutic Index revealed a very favorable ratio of clinical benefit to side effects for nomifensine maleate.

Multicenter double-blind comparison of nomifensine and imipramine for efficacy and safety in depressed outpatients.

Nomifensine, a tetrahydroisoquinoline antidepressant, was compared with imipramine in a 4-week multicenter double-blind study of depressed outpatients (100 on nomifensine, 56 on imipramine). Nomifensine was at least as effective as imipramine in reducing depressive symptoms at average doses of 150 mg/day. When significant differences did occur on Hamilton Depression Rating Scale scores, they favored nomifensine for improvement in cognitive symptoms and interest in work and activities. Early in treatment, nomifensine patients also showed a better relationship between clinical response and side effects. The proportions of patients experiencing at least one side effect or dropping out due to side effects were almost twice as high in the imipramine group. Dry mouth and sedating effects were 2-3 times more frequent among imipramine patients. Thus, nomifensine demonstrated clinical efficacy at least comparable with imipramine but with indications of a more favorable side effects profile.

Nomifensine in geriatric inpatients: a placebo-controlled study.

In a double-blind randomized group design, nomifensine (100 mg single daily dose) was compared to placebo in 100 geriatric inpatients (average age = 75 years). Patients in each group were categorized as depressed (Feighner criteria for primary depressive disorder and Hamilton Depression Rating Scale scores greater than or equal to 18) or non-depressed (no psychopathologic disorders and HDRS scores less than or equal to 7). Outcome measures included the HDRS and various tests of cognitive function. The nomifensine-treated depressed patients showed significant improvement over the placebo depressed patients early in and throughout treatment. Depressed patients also showed significant improvement on several cognitive tests with nomifensine treatment. Nomifensine was well tolerated in both depressed and nondepressed patients.

A double-blind comparative evaluation of the efficacy and safety of nomifensine, imipramine, and placebo in depressed geriatric outpatients.

Nomifensine, imipramine, and placebo were compared in 61 depressed geriatric outpatients over a 35-day period. At average daily doses of 150 mg, nomifensine and imipramine were significantly more effective than placebo in reducing symptoms of depression in this sample of elderly depressed patients. Nomifensine and imipramine were generally comparable in clinical effect; 78% of the nomifensine-treated patients were rated as improved at the end of treatment as compared with 64% of imipramine and 20% of placebo patients. The findings suggest a more favorable side effect profile for nomifensine, which was associated with a lower frequency of sedating and anticholinergic effects than was seen in the imipramine group.

Pooling 12 nomifensine studies for efficacy generalizability.

Twelve parallel group, randomized, double-blind studies of nomifensine's safety and efficacy in the treatment of depressed patients were combined into three pools according to common protocols. This approach permitted evaluation of 1) efficacy results for studies with moderate-sized pools of patients, 2) the degree to which efficacy was generalizable to depressed patients in the general population, and 3) the conditions under which pooled active vs. active (imipramine vs. nomifensine) studies could be regarded as pivotal in support of efficacy. Results showed that nomifensine's superiority over placebo was generalizable to patients with a wide range of characteristics, including age 60 years or older. An appropriate statistical profile of more pronounced nomifensine responders would include patients with a duration of present episode less than 4 months who are acutely depressed, exhibit more severe symptoms, and have been previously hospitalized or treated with other psychotropic medications. A comprehensive assessment and power analysis of the pooled active vs. active studies provided strong evidence for comparability of nomifensine and imipramine.

Acute therapy of depression.

The acute therapy (the initial 8 weeks of treatment) of depression (including the whole spectrum of "less than major," "major," and "more than major") has been reviewed comparing the old tricyclics with the new generations (especially mianserin, moclobemide, and the serotonin selective reuptake inhibitors (SSRIs), i.e., citalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline). The Hamilton Rating Scale for Depression has been used to measure clinical efficacy. Statistically, the method of meta-analysis has been applied. The results showed that the SSRIs and moclobemide are equal to the tricyclics. Mianserin is inferior to tricyclics as well as to SSRIs. The antidepressive profile of the SSRIs is nonsedation but still with anxiolytic effects. The safety profile of the SSRIs is much more benign than that of the tricyclics.

Pharmacodynamics of nomifensine: a review of studies in healthy subjects.

A review of pharmacodynamic studies of nomifensine in healthy subjects indicates that nomifensine caused neither negative nor positive (euphoric) mood alterations and led to no impairments in various tests of performance and to some improvement in vigilance, attention, and psychomotor performance. Nomifensine was shown to alter the waking EEG in a manner similar to desimipramine. In marked contrast to nomifensine's pharmcokinetics, its CNS effects were shown to reach a maximum after 6 hours and to last for more than 8 hours. The sleep EEG was not altered significantly. Biochemical models confirmed nomifensine's noradrenergic and dopaminergic mode of action.

Ethanol effects on self-administration of alfentanil, cocaine, and nomifensine in rhesus monkeys.

A common form of polydrug use is that of cocaine and ethanol. The identification of an ethanol-cocaine combination product, cocaethylene, with properties in common with cocaine, has led to speculation that this metabolite may contribute to the co-abuse of cocaine and ethanol. In order to determine whether ethanol pretreatments selectively altered cocaine's reinforcing potency, ethanol pretreatments were given to monkeys trained to press levers and receive IV infusions of several doses of cocaine or alfentanil. In addition, nomifensine, a drug which has a mechanism of action similar to cocaine's, was evaluated in the presence and absence of ethanol in monkeys with the cocaine baseline history. Ethanol, in doses ranging from 100 to 1780 mg/kg, given 10 min before the 130-min session, had no effect on responding maintained by alfentanil. These doses also had no significant effect on cocaine-maintained responding, although the potency of cocaine as a reinforcer was increased following administration of 1000 mg/kg ethanol in two of the four subjects. The potency of nomifensine as a reinforcer was significantly increased by 1000 mg/kg ethanol, but again, this enhancement was limited to the same two subjects. These data indicate that, in this paradigm, cocaethylene did not selectively modify cocaine's reinforcing potency, but there appear to be individual differences with respect to ethanol's ability to stimulate rates of drug-maintained responding.