INHIBITION OF NORADRENALINE UPTAKE BY DRUGS

Drug development in psychiatry: 50 years of failure and how to resuscitate it

The past 50 years have seen remarkable advances in the science of medicine. The pharmacological treatments of disorders such as hypertension, immune disorders, and cancer are fundamentally different from those used in the 1970s, and are now more often based on disorder-specific pathologies. The same cannot be said for psychiatric medicines: despite remarkable advances in neuroscience, very few innovative treatments have been developed in this field since the 1970s. For depression, schizophrenia, anxiety disorders, and ADHD, pharmacological classes of medicines discovered through serendipity in the 1950s are still used despite hundreds of billions of US dollars being spent on drug discovery attempts based on new neuroscience targets. This Personal View presents my opinion on the reasons innovation in psychiatric treatment has not progressed as well as in other disorders. As a researcher in the field, I offer suggestions as to how this situation must be rectified soon, as by most analyses mental illness is becoming a major health burden globally. Most of my evidence is referenced, but where I have unpublished knowledge gained from consulting with pharmaceutical companies, it is presented as an opinion.

The β3 -adrenoceptor agonist mirabegron increases human atrial force through β1 -adrenoceptors: an indirect mechanism?

BACKGROUND AND PURPOSE

Mirabegron has been classified as a β₃ -adrenoceptor agonist approved for overactive bladder syndrome. We investigated possible cardiac effects of mirabegron in the absence or presence of β-adrenoceptor subtype antagonists. In view of its phenylethanolamine structure, we investigated whether mirabegron has indirect sympathomimetic activity by using neuronal uptake blockers.

EXPERIMENTAL APPROACH

Right atrial trabeculae, from non-failing hearts, were paced and contractile force measured at 37°C. Single concentrations of mirabegron were added in the absence or presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), β₃ (L-748,337), β₁ (CGP 20712A), β₂ (ICI 118,551) -adrenoceptor antagonists, neuronal uptake inhibitors desipramine or phenoxybenzamine.

KEY RESULTS

Mirabegron significantly increased contractile force in human right atrium (1 μM, 7.6 ± 2.6%, n = 7; 10 μM, 10.2 ± 1.5%, n = 22 compared with (-)-isoprenaline P < 0.05). In the presence of IBMX, mirabegron (10 μM) caused a greater contraction. L-748,337 (100 nM) had no effect on the increase in contractile force caused by mirabegron (10 μM). In contrast, mirabegron (10 μM) reduced contractile force in the presence of CGP 20712A, which was not affected by L-748,337 (100 nM) or ICI 118,551 (50 nM). Mirabegron (10 μM) also reduced contractile force in the presence of desipramine or phenoxybenzamine.

CONCLUSIONS AND IMPLICATIONS

Mirabegron increases human atrial force through β₁ - but not β₃ -adrenoceptors. Desipramine and phenoxybenzamine block neuronal uptake and conceivably prevent mirabegron from releasing noradrenaline. A non-specific cardiodepressant effect is not mediated through β₃ (or β₂ )-adrenoceptors, consistent with lack of β₃ -adrenoceptor function on human atrial contractility.

Biophysical Approaches to the Study of LeuT, a Prokaryotic Homolog of Neurotransmitter Sodium Symporters

Ion-coupled secondary transport is utilized by multiple integral membrane proteins as a means of achieving the thermodynamically unfavorable translocation of solute molecules across the lipid bilayer. The chemical nature of these molecules is diverse and includes sugars, amino acids, neurotransmitters, and other ions. LeuT is a sodium-coupled, nonpolar amino acid symporter and eubacterial member of the solute carrier 6 (SLC6) family of Na(+)/Cl(-)-dependent neurotransmitter transporters. Eukaryotic counterparts encompass the clinically and pharmacologically significant transporters for γ-aminobutyric acid (GABA), glycine, serotonin (5-hydroxytryptamine, 5-HT), dopamine (DA), and norepinephrine (NE). Since the crystal structure of LeuT was first solved in 2005, subsequent crystallographic, binding, flux, and spectroscopic studies, complemented with homology modeling and molecular dynamic simulations, have allowed this protein to emerge as a remarkable mechanistic paradigm for both the SLC6 class as well as several other sequence-unrelated SLCs whose members possess astonishingly similar architectures. Despite yielding groundbreaking conceptual advances, this vast treasure trove of data has also been the source of contentious hypotheses. This chapter will present a historical scientific overview of SLC6s; recount how the initial and subsequent LeuT structures were solved, describing the insights they each provided; detail the accompanying functional techniques, emphasizing how they either supported or refuted the static crystallographic data; and assemble these individual findings into a mechanism of transport and inhibition.

In-vivo evidence of a role for nitric oxide in regulating the activity of the norepinephrine transporter

We examined the role of nitric oxide (NO) in the regulation of neuronal uptake of norepinephrine (uptake-1) in rats under anesthesia. The effect on systolic blood pressure of two pressor drugs that work by different mechanisms, norepinephrine and angiotensin II, was explored in anesthetized rats under control conditions and after prevention of NO synthesis with Nw-nitro-L-arginine (L-NNA). The results showed that whereas the pressor effects of increasing doses of norepinephrine were potentiated by L-NNA, those of angiotensin II were not affected, which implied that NO was selectively involved in modulating the pressor effect of norepinephrine. To explore the mechanisms involved in this potentiation, we examined the effect of L-NNA on the pressor effect of tyramine, a purely-indirectly-acting sympathomimetic amine which enters nerve terminals thorough uptake 1 and liberates norepinephrine from storage vesicles. Increasing doses of tyramine produced pressor effects which, in contrast to those of norepinephrine, were significantly attenuated by pre-treatment with L-NNA. Similarly, pretreatment with cocaine, the classical inhibitor of uptake 1, significantly decreased the pressor effect of tyramine; however, the response to tyramine was then restored when L-NNA was administered, thus reversing the effect of cocaine. We conclude that NO plays a major role in the adrenergic system by enhancing the activity of uptake 1 in sympathetic nerve terminals. Blockade of uptake 1 by cocaine is also partly dependent on NO. The stimulus for the mobilization of the NO synthase pathway in adrenergic neurons and the subsequent steps involved in modulating uptake 1 deserve further exploration.

Respiratory distress syndrome and thrombotic, non-bacterial endocarditis after amitriptyline overdose

Three patients with documented or suspected amitriptyline poisoning developed a uniform clinical picture with progressive respiratory insufficiency, thrombocytopenia and evidence of embolization. Post-mortem examination revealed equally uniform changes: thrombotic, non-bacterial endocarditis (TNBE) with multiple embolization and marked pulmonary stiffness with extensive invasion of alveoli with connective tissue. The lung pathology and clinical picture of progressive respiratory insufficiency are in agreement with the respiratory distress syndrome (RDS) in the adult. The simultaneous occurrence of RDS and TNBE suggests a similar pathogenesis. Prophylactic measures are described.

A regulated interaction of syntaxin 1A with the antidepressant-sensitive norepinephrine transporter establishes catecholamine clearance capacity

Norepinephrine (NE) transporters (NETs) terminate noradrenergic synaptic transmission and represent a major therapeutic target for antidepressant medications. NETs and related transporters are under intrinsic regulation by receptor and kinase-linked pathways, and clarification of these pathways may suggest candidates for the development of novel therapeutic approaches. Syntaxin 1A, a presynaptic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein, interacts with NET and modulates NET intrinsic activity. NETs colocalize with and bind to syntaxin 1A in both native preparations and heterologous systems. Protein kinase C activation disrupts surface NET/syntaxin 1A interactions and downregulates NET activity in a syntaxin-dependent manner. Syntaxin 1A binds the NH(2) terminal domain of NET, and a deletion of this domain both eliminates NET/syntaxin 1A associations and prevents phorbol ester-triggered NET downregulation. Whereas syntaxin 1A supports the surface trafficking of NET proteins, its direct interaction with NET limits transporter catalytic function. These two contradictory roles of syntaxin 1A on NET appear to be linked and reveal a dynamic cycle of interactions that allow for the coordinated control between NE release and reuptake.

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Inhibitory effect of noradrenaline uptake inhibitors on contractions of rat aortic smooth muscle

1 The effects of noradrenaline (NA) uptake inhibitors on contractions induced by NA, high K+, and 12-O-tetradecanoylphorbol-13-acetate (TPA) in rat isolated aorta were investigated. 2 Protriptyline (0.3 microM) and amitriptyline (0.3 microM) produced an approximately parallel shift to the right in the dose-response curve to NA. Protriptyline (> 0.3 microM), amitriptyline (> 0.3 microM) and xylamine (0.01-1 microM) significantly reduced the maximal contractile response to NA. The IC50 values for inhibition of the contractile response to 3 microM NA were 1.58 microM for xylamine, 1.70 microM for amitriptyline and 2.57 microM for protriptyline. 3 Protriptyline and amitriptyline dose-dependently inhibited the high K+ (60 mM)-induced contraction (IC50 = 0.69 microM for protriptyline and IC50 = 3.15 microM for amitriptyline). In contrast, xylamine did not affect the high K(+)-induced contraction. 4 Protriptyline and amitriptyline dose-dependently inhibited TPA (1 microM)-induced contraction in calcium-free solution; xylamine (up to 30 microM) was without effect. Staurosporine (10 nM) completely inhibited the TPA- and NA-induced contraction. 5. Protriptyline (3 microM) and amitriptyline (3 microM) caused about 54% and 60% inhibition, respectively, of aortic contractions caused by endothelin-1 (10 nM) in the absence of endothelium. Xylamine (10 microM) was without effect. 6 Inhibitory effects of NA uptake inhibitors on contractions were independent of the presence of endothelium and were unaffected by the K+ channel blockers, tetraethylammonium ions (up to 3 mM) and glibenclamide (up to 30 microM). 7 These results indicate that tricyclic antidepressant drugs such as protriptyline and amitriptyline could act as both postsynaptic adrenoceptor antagonists and direct inhibitors of muscle contraction; whereas, xylamine, a structurally distinct NA uptake blocker might principally exert its action only at alpha-adrenoceptors on rat aortic smooth muscle.

PET studies of cocaine inhibition of myocardial norepinephrine uptake

Positron emission tomography (PET), [11C]cocaine, and (-)-6-[18F]fluoronorepinephrine [(-)-6-[18F]NE] were used to determine the extent to which the binding of labeled cocaine in the baboon heart represents binding to the norepinephrine transporter and to characterize the functional consequences of cocaine administration on the norepinephrine transporter. Peak heart binding of [11C]cocaine was high (0.038-0.055%/g) and clearance was rapid (t1/2 from peak: 2.5-9 min) for both tracer doses and a pharmacological dose. The binding of a tracer dose of labeled cocaine could not be inhibited by desipramine, tomoxetine, cocaine, nomifensine, or benztropine. The behavior of a pharmacological dose of [11C]cocaine could not be distinguished from a tracer dose and also could not be inhibited by tomoxetine. However, pretreatment with cocaine profoundly inhibited norepinephrine uptake as assessed by (-)-6-[18F]NE. Recovery was slow with only 48% of the baseline (-)-6-[18F]NE uptake being recovered by 78 minutes after cocaine administration. [11C]Benzoylecgonine, a vasoactive metabolite of cocaine, showed negligible retention in heart. The results of this study (i.e., the rapid clearance of cocaine from the heart, the inability to inhibit cocaine binding with desipramine and tomoxetine, and its relatively long-lasting effects on norepinephrine uptake) reinforce the need to understand the link between cocaine pharmacokinetics and norepinephrine transporter function and its relationship to cardiotoxicity.

Preparation and pharmacological characterization of [76Br]-meta-bromobenzylguanidine ([76Br]MBBG)

[76Br]-meta-Bromobenzylguanidine ([76Br]MBBG) was prepared from the iodinated analog (MIBG) and [76Br]NH4 using a Cu(+)-assisted halogen exchange reaction. [76Br]MBBG was produced in a 60-65% radiochemical yield with a specific activity of 20 MBq/nmol. In rats, biodistribution kinetic studies showed a high uptake of [76Br]MBBG in heart tissues with its maximum of 5% ID/S at 2 h p.i.; whereas 4 h p.i., the maximum of the heart-to-lung concentration ratio of 8 was observed. Metabolic studies in rats indicated that [76Br]MBBG was rapidly metabolized in plasma. However in heart tissue, 25 h p.i., 85% of the radioactivity still represented unchanged radiotracer. Pharmacological studies in rats showed that the myocardial uptake of [76Br]MBBG was similar to that of norepinephrine. After pretreatment of the rats, the uptake of [76Br]MBBG was reduced 4 h p.i. to the following values: after desipramine (DMI) to 37%, after dexamethasone (DXM) to 88% and after 6-hydroxydopamine (6-OHDA) to 16%. These preliminary results suggest that [76Br]MBBG can be useful for the assessment of heart catecholamine reuptake disorders with PET.

Noradrenaline uptake inhibition increases melatonin secretion, a measure of noradrenergic neurotransmission, in depressed patients

Eight patients with endogenous depression who had received no antidepressant treatment for the previous year were treated with the noradrenaline (NA) uptake inhibitor, desipramine (DMI). Pre-treatment plasma melatonin concentrations were normal. After one day of DMI treatment plasma melatonin concentrations were increased but the response was impaired compared to normal subjects. The acute effect of DMI on plasma melatonin persisted after six weeks of treatment. These findings question the hypothesis that beta adrenoceptors are supersensitive in depression and that antidepressant drugs act by down-regulating these receptors.

Presynaptic modulation of the release of noradrenaline from electrically stimulated bicuspid valve leaflet of the rabbit heart

The bicuspid (mitral) valves were obtained from male albino New Zealand rabbits. The noradrenaline (NA) content (12.93 +/- 1.14 nmol/g) of the valve tissue was determined by high pressure liquid chromatography (HPLC) combined with electrochemical detection. After incubation with tritiated NA for 45 min, the tissues were mounted in perfusion baths and superfused with Krebs solution at a constant perfusion rate. After a 90 min washing period, the tissues were stimulated three times (S1; S2; S3) at a frequency of 1 or 10 Hz, and the release of NA was expressed as the stimulus-induced overflow of radioactivity. Using a constant number of impulses, the release of NA was significantly higher when the frequency applied was 10 Hz than in the case of 1 Hz. The release of NA was inhibited by stimulating the presynaptic alpha 2-adrenoceptors with xylazine or by stimulating the presynaptic muscarinic receptors with oxotremorine. Yohimbine (1 microM) not only overcame the effect of the alpha 2-adrenoceptor stimulation caused by xylazine, but increased it over the control level, whereas atropine blocked the inhibitory effect of oxotremorine. It is concluded that the adrenergic nerves in the valve tissue release NA in a frequency-dependent fashion, and the release of NA can be modulated through presynaptic alpha 2- and muscarinic receptors. This is the first case that neurochemical evidence was obtained showing that NA is released from the mitral valve and is subject to presynaptic modulation.

Influence of amineptine on changes of blood pressure evoked by norepinephrine and dopamine

The influence of amineptine, an antidepressant currently employed having mainly selective dopaminergic neurochemical activity, on the pressor responses evoked by norepinephrine (NE) and dopamine (DA) was studied in anesthetized whole rats. Amineptine at doses of 0.5, 1.5, and 5.0 mg/kg/30 min infused into the femoral vein of the rat caused a dose-related inhibition of the pressor responses of NE and DA. The hypertensive responses of NE and DA augmented by pretreatment with reserpine, a catecholamine depletor, were also clearly depressed following the infusion of amineptine with a rate of 1.5 mg/kg/30 min. Furthermore, the pressor responses of NE and DA potentiated by pretreatment with debrisoquin, a sympathetic neuron blocker, were markedly diminished after pretreatment with the infusion of amineptine at the above same rate (1.5 mg/kg/30 min). These experimental results demonstrate that amineptine causes an inhibitory effect on the pressor responses evoked by NE and DA. It is thought that the amineptine effect may be due to the blockade of the peripheral adrenergic alpha-receptors in addition to the previously described uptake inhibition of dopamine.

Biphasic effect of tricyclic antidepressants on the release of norepinephrine from the adrenergic nerves of the rabbit heart

The release of norepinephrine (NE) from the right atrium of the rabbit heart was used as a model to investigate biphasic effects due to tricyclic antidepressants, similar to those clinically observed in the treatment of depression and known as "therapeutic window". Strips of the atrium were loaded with 3H-NE, and then superfused by Krebs solution. The basal release and the electrical stimulation evoked release of 3H-NE were measured in the presence and absence of four clinically used tricyclic antidepressants: imipramine, amitriptyline, desipramine and nortriptyline. In addition, guanethidine, an adrenergic neuron blocker, was also studied. At lower concentrations (0.5-10 microM) tricyclic antidepressants increased, whereas higher concentrations (50-100 microM), inhibited the evoked release of NE. This inhibition was not prevented by the alpha2 adrenoceptor antagonist yohimbine, excluding the possibility of alpha 2 adrenoceptor-mediated inhibition of NE release. In higher concentrations the tricyclic antidepressants increased the basal release of NE in a Ca-independent way. Secondary amine derivatives were more potent inhibitors of the evoked release, and enhance the resting basal release of NE to a greater extent than the tertiary ones. Similarly, guanethidine (1-50 microM) also decreased the evoked release and increased the basal release of NE in a concentration dependent manner. Yohimbine failed to counteract the inhibition caused by guanethidine and the increment of the basal release was Ca-independent. It is concluded that the effect of tricyclic antidepressants in potentiating the release of NE is masked by their adrenergic neuron blocking properties, i.e. they inhibit the release of NE.(ABSTRACT TRUNCATED AT 250 WORDS)

In-vivo quantitative imaging of catecholaminergic nerve terminals in brain and peripheral organs using positron emission tomography (PET)

Positron emission tomography (PET), has made possible quantitative imaging of the origins of positron-emitting isotopes, such as 11C and 18F, in intact animals and in humans. Lack of absolute specificity of enzymes, storage mechanisms and transporters allows 11C-or 18F-labelled "false transmitters" to be formed, stored and released from nerve terminals. Discussed are the assumptions, limitations, and advantages of 18F-6-fluoroDOPA, 18F-6-fluorodopamine, 18F-6-fluorometaraminol, and 11C-N-methyl-metaraminol (m-hydroxyephedrine) for PET imaging of dopaminergic nerve terminals in brain or sympathetic innervation in peripheral organs.

Anti-serotonin action in combination with noradrenaline-stimulating action is important for inhibiting muricide in midbrain raphe-lesioned rats

The present study was designed to examine the possible involvement of both an anti-serotonin action and a catecholamine-stimulating action in the mechanism of the inhibition of the muricide in rats with lesions of the midbrain raphe. Serotonin antagonists, such as cyproheptadine (10 mg/kg), cinanserin (10 mg/kg) and pirenperone (1 mg/kg), given alone showed little suppression of muricide in rats with raphe lesions, although the first two drugs were inhibitory at very large doses. Methamphetamine showed no inhibition of muricide at 0.32 mg/kg (i.p.), but exerted a marked inhibition of muricide when combined with the above serotonin antagonists. In addition, the dose-response curve for cyproheptadine and cinanserin was shifted markedly to the left when combined with L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS) (100 mg/kg i.p.), but not with lisuride (0.32 mg/kg i.p.). Similarly, pirenperone produced a marked inhibition of muricide at doses of 0.32-1.8 mg/kg (i.p.) when combined with L-threo-DOPS, but not when combined with lisuride. These results suggest that the combination of an anti-serotonin action with noradrenergic activation is important for inhibiting muricide, at least in rats with raphe lesions. A similar mechanism also seems to be valid for the anti-muricidal effect of antidepressant drugs.

The athlete, cocaine, and lactic acidosis: a hypothesis

The muscular makeup of the sprint-trained athlete may make him especially susceptible to severe lactic acidosis from cocaine-induced seizures. Because of a high percentage of glycolytic muscle fibers (compared to the muscle fiber type of the endurance-trained athlete), the lactic acidosis and heat generated from muscular activity is much greater in the sprint-trained athlete than in the endurance-trained athlete. The role of cocaine in producing seizures and increasing glycolysis, both of which produce lactic acidosis, is discussed. The hypothesis is presented that the elite athlete may be at greater risk of death than the general population from lactic acidosis produced as a result of cocaine-induced seizures.

Reversal of the reserpine-induced ptosis by L-threo-3,4-dihydroxy-phenylserine (L-threo-DOPS), a (-)-norepinephrine precursor, and its potentiation by imipramine or nialamide

The effect of L-threo-DOPS on the reserpine-induced ptosis in mice and its modification by imipramine, a norepinephrine (NE) uptake inhibitor, or nialamide, a monoamineoxidase inhibitor, were studied. Intraperitoneal (i.p.) injection of L-threo-DOPS (800 mg/kg) significantly reduced the severity of the ptosis. This reversal of the ptosis by L-threo-DOPS was markedly potentiated by i.p. injection of either imipramine (2.5 mg/kg) or nialamide (30 mg/kg). Response to L-threo-DOPS was also significantly potentiated by intracerebroventricular (i.c.v.) injection of imipramine (10 micrograms). On the other hand, this treatment with imipramine (10 micrograms, i.c.v.) also significantly potentiated the reversal of the ptosis by NE (20 micrograms, i.c.v.), but the reversal by the subcutaneous (s.c.) injection of NE (1 and 3 mg/kg) was not affected. Reserpine (5 mg/kg, i.p.) markedly decreased the brain content of NE in mice, whereas L-threo-DOPS (400 mg/kg, i.p.) slightly restored it. Moreover, by the pretreatment with nialamide (30 mg/kg, i.p.), L-threo-DOPS produced a significant increase in the brain content of NE in reserpine-treated mice. These results suggested that L-threo-DOPS was capable of reversing the reserpine-induced ptosis due to the formation, at least in part of (-)-NE at the synaptic sites of central noradrenergic neurons.

Local anesthetics

Local anesthetics are remarkably useful agents that enhance patient comfort and improve patient compliance. Their use, however, requires an understanding of their action, proper dosages, potential risks, and treatment of reactions. We have presented the history, pharmacokinetics, action, risks of using, and ways in which agents are used to treat the most common agents, with notes on the special aspects of each agent. With the increased awareness that these are, indeed, not benign substances, we hope that serious reactions can be avoided by prophylactic measures and proper treatment in the early stages of toxicity.

Tetrabenazine-induced depletion of brain monoamines: characterization and interaction with selected antidepressants

The peripheral administration of tetrabenazine (TBZ) induces rapid depletion of brain regional concentrations of norepinephrine (NE), dopamine (DA) and serotonin (5-HT). With respect to both dosage and time, striatal DA was most sensitive to the effects of TBZ while hypothalamic NE was least affected. Pretreatment with the monoamine oxidase (MAO)-inhibitor, clorgyline (1-6 mg/kg) dose-dependently prevented the reduction of all three monoamines for up to 60 min after TBZ (3 mg/kg). The TBZ-induced depletion of cortical NE was also significantly antagonized by desmethylimipramine (DMI) but was of shorter duration (up to 30 min after TBZ). DMI, however, did not influence the effect of TBZ on striatal DA or hypothalamic 5-HT. The protective effects of both clorgyline and DMI were also evident under the conditions of the behavioral TBZ test utilizing high doses of TBZ (20 mg/kg).

Acute effects of atypical antidepressants on various receptors in the rat brain

Using in vitro receptor binding techniques, the effects of a number of different antidepressants on rat cerebral cortex receptors were investigated. In contrast to the tricyclic antidepressants, which potently inhibit several postsynaptic receptors, many atypical antidepressants have no or very little affinity for these receptors. Thus bupropion, amineptine, citalopram, fluoxetine, fluvoxamine and viloxazine are devoid of any activity, while amoxapine, doxepin and trazodone exert effects on a number of receptors. The implications of these receptor blocking effects are discussed.

On the metabolism of [3H]noradrenaline in different compartments of rat brain with respect to the role of catechol-O-methyltransferase

Rats were pretreated with either reserpine or desmethylimipramine, either alone or in combination with tropolone. At either 10 min or 1 h after the intraventricular injection of [3H]noradrenaline, in several brain regions the complete metabolic patterns were determined: normetanephrine; the glycol metabolites (methylated and nonmethylated) and their sulfate conjugates; and the acidic metabolites (methylated and non-methylated). A reserpine-induced increase in the turnover of [3H]noradrenaline caused a transient increase of the catechol glycol followed by elevated levels of the two glycol sulfates. The stimulated [3H]noradrenaline turnover if achieved by desmethylimipramine caused a transient increase of normetanephrine and initially lowered values of catechol glycols (both free and sulfated), which were followed by elevated levels. Drug-pretreated rats compensated for the inhibition of catechol-O-methyl-transferase by tropolone in different ways: Reserpine caused an early increase of the catechol glycol beyond the measurements in other treatment groups, whereas desmethylimipramine increased the nonmethylated carboxylic acid and glycol sulfates rather slowly to levels beyond those of other groups. The results support the existence of two compartments with a fast metabolism (an intraneuronal monoamine oxidase compartment and an extraneuronal catechol-O-methyltransferase compartment). In addition, there seems to exist another extra-neuronal space with a slow, monoamine oxidase-dependent noradrenaline turnover.

On the recovery of [3H]noradrenaline from different metabolic compartments of rat brain with respect to the role of catechol-O-methyltransferase

Rats were treated with reserpine, desmethyl-imipramine, or carrier, either alone or in combination with tropolone. Either 10 min (t1) or 1 h (t2) after intraventricular injection of [3H]noradrenaline, they were decapitated. The total 3H activity and the recovery of [3H]noradrenaline were determined in tissue extracts from various brain regions. Maximum total 3H activity was measured at t1 in all tropolone-treated rats; the mean sum of these results served as an estimate of the initial tissue concentration of [3H]noradrenaline. At t1, 40-50% of the sum of [3H]noradrenaline and its metabolites was recovered unchanged in normal rats; reserpine and DMI reduced the recovery to 18-27%. In all groups, the decline of [3H]noradrenaline was retarded after t1. Inhibition of catechol-O-methyltransferase by tropolone caused consistently elevated [3H]noradrenaline levels, but did not affect the metabolic rate after t1 when compared with similarly pretreated, but tropolone-free rats. Thus, if catechol-O-methyltransferase was inhibited during the injection of [3H]noradrenaline, a higher percentage of the amine had been taken up into spaces with a slow noradrenaline turnover. The maximum increase was seen when the neuronal uptake1 was inhibited by desmethylimipramine. This supported the hypothesis that an additional extraneuronal space exists, in addition to the known intraneuronal and extraneuronal compartments, which has a slow noradrenaline turnover. The tropolone effect on the noradrenaline recovery possibly shows that there might be a saturable "methylating system," similar to that described for the periphery, in which catechol-O-methyltransferase is linked to the extraneuronal uptake2.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of antidepressant drugs on histamine-H1 receptors in the brain

The histamine-H1 receptor blocking properties of a number of structurally different antidepressant drugs have been evaluated using a 3H-mepyramine binding assay and a guinea-pig ileum preparation. The tricyclic antidepressants all inhibited the histamine-H1 receptor. Some newer antidepressant drugs, such as zimelidine and nomifensine were devoid of activity while others, such as iprindole and mianserin were very potent. It is concluded that antagonistic effects on the histamine-H1 receptor is not associated with the therapeutic efficacy in depression, but may contribute to the sedative effects of the antidepressant drugs.

Protective effects of clonidine and verapamil in experimental amitriptyline poisoning in rabbits

The effects of various agents on acute amitriptyline-induced cardiotoxicity were investigated in conscious rabbits. Amitriptyline (50 mg/kg, IP) regularly produced consistent and reproducible electrocardiographic changes such as lengthening of Q-T intervals, widening of the QRS complex, arrhythmias of various types, convulsion, and severe hypotension. Prevention of these toxic manifestations by the use of various agents alone or in combination was explored. Clonidine (50 micrograms/kg, IV) and verapamil (0.25 mg/kg, IV) alone or in combination at lower doses (25 micrograms/kg plus 0.125 mg/kg, respectively) significantly reduced the lengthening of the QRS and Q-T intervals, restored sinus rhythm, maintained blood pressure at a constant level, and protected against a lethal dose of amitriptyline (75 mg/kg). These results suggest that verapamil, clonidine, and their combination appear to be promising antidotal agents in restoring cardiac function under these conditions.

Actions of lofepramine, a new tricyclic antidepressant, and desipramine on electrophysiological and mechanical parameters of guinea pig atrial and papillary muscles

The effects of lofepramine on the isolated guinea pig atrial trabeculae were compared with those of desipramine. The preparations were taken from the same animal and mounted in the same tissue bath. All parameters were recorded in parallel. Lofepramine 10 microM was shown to exhibit minor changes in the transmembrane action potential duration only. The action potentials of the atrial trabeculae were prolonged, whereas they were shortened in the papillary muscles. Desipramine was about ten times more potent than lofepramine, but produced similar qualitative changes. Desipramine 10 microM and lofepramine 100 microM showed local anaesthetic properties: a decreased overshoot without a decreased resting potential, a decreased and rate-dependent Vmax, and a decrease in propagation velocity. After the addition of either drug in a lower concentration, a transient increase in force development and a concomitant increase in repolarization phase height (atrial trabeculum) or plateau length (papillary muscle) were recorded. The steady state effect on the force development was a decrease accompanied by a shortening of the action potential duration (papillary muscle). It is suggested that the action of lofepramine 100 microM and desipramine 10 microM on phase 0 of the action potential are produced by blockage of the fast sodium channel. The transient increase in developed force and the increase in repolarization phase height (atrial trabeculum) or plateau length (papillary muscle) could be caused by inhibition of the membrane re-uptake system for released noradrenaline. The steady state shortening and flattening of the plateau (papillary muscle) and the decrease in force development could be the cause of a block in the slow channel system.

Tricyclic antidepressant overdose: clinical and pharmacologic observations

Forty-seven patients with confirmed TCA overdoses were thoroughly evaluated with regard to clinical and electrocardiographic findings. In 22 patients plasma levels of the primary TCA and the desmethyl metabolite were determined. We found that both neurologic and cardiovascular complications were common, but independent of each other. While plasma levels of the parent of TCA compound correlated best with the neurologic disturbances, cardiac abnormalities were more related to the desmethyl metabolite level. Although plasma levels correlate with the clinical findings, sufficient overlap between normal and abnormal levels and findings exist that we do not feel that routine measurement of plasma levels is clinically useful at this time.

The pharmacology of zimelidine: a 5-HT selective reuptake inhibitor

Zimelidine (ZIM) and its main active metabolite norzimelidine (NZIM) have been shown to preferentially inhibit 5-hydroxytryptamine (5-HT) neuronal uptake both in vitro and in vivo while having much less effect on noradrenaline (NA) uptake. ZIM in vivo blocked the 5-HT uptake mechanism in the cerebral cortex, hippocampus, striatum, hypothalamus and spinal cord, thus indicating effects on both the ascending and descending 5-HT pathways. ZIM is devoid of a 5-HT releasing action, MAO-inhibitory properties and effects on dopamine (DA) uptake. ZIM failed to reduce NA turnover even in high doses, but markedly reduced 5-HT turnover in very low doses in the rat. ZIM also enhanced 5-HT mediated behaviours in mice in doses related to the inhibition of 5-HT uptake. In contrast to amitriptyline (AMI) and mianserin (MIAN), ZIM only in extremely high doses displayed a 5-HT receptor blocking action in vitro and failed to block 5-HT mediated behaviour. ZIM was practically devoid of action on histamine H1 and H2 receptors, and had also a neglible action on noradrenergic alpha 1- and alpha 2-receptors, and on beta-receptors. Unlike the tricyclic antidepressants (TAD's) ZIM had a negligible action on muscarinic receptors and failed to affect cholinergic induced activity. Long-term treatment with ZIM did not result in any attenuation of the 5-HT uptake blocking potency or the reduction of 5-HT turnover. This long-term treatment slightly reduced the number of beta-receptors in the brain. However, repeated ZIM-treatment induced a new 5-HT receptor binding site characterized by a low affinity and with a high number of binding sites and decreased the number of high affinity 5-HT receptor binding sites. Unlike the TAD's zimelidine failed to block the action of reserpine. Metabolic and behavioural interactions studies in mice showed that ZIM was devoid of any significant interactions with ethanol, barbiturates and benzodiazepines. It is concluded that ZIM markedly differs from both the TAD's and new antidepressants such as mianserin and nomifensine. ZIM seems preferentially to effect the presynaptic 5-HT reuptake mechanism while having a negligible action on noradrenergic, 5-HT, acetylcholine and histamine receptors in the brain.

A pharmacological method to estimate the pKi of competitive inhibitors of agonist uptake processes in isolated tissues

An equation is derived from a mathematical model (proposed by Furchgott) which, under certain circumstances, estimates the pKI (-log dissociation constant) of a competitive inhibitor of agonist uptake by utilizing the sensitization of isolated tissues, to the substrate-agonist, by uptake inhibition. The method is theoretically more sound and appears to be improved by the use of potency-ratios of the substrate-agonist and an agonist which is not a substrate for the uptake process since this allows for the detection and correction of receptor and toxic effects of uptake inhibitors. The pKI values of cocaine, desmethylimipramine and imipramine for the neuronal uptake of norepinephrine were estimated by this method in guinea-pig tracheae and left atria. Also, the pKI values for 17 beta-oestradiol, corticosterone, clonidine and metanephrine for the extraneuronal uptake of isoproterenol were estimated in guinea-pig tracheae (and cat left atria for 17 beta-oestradiol). All estimates were consistent with literature pKI values obtained biochemically with radiolabelled substrates.

Effects of antidepressant drugs on different receptors in the brain

Radioligand receptor binding techniques were used to characterize the effects of different structural types of antidepressant drugs on neurotransmitter receptors. The tricyclic antidepressants more or less potently inhibited the binding to rat brain preparations of several different radiolabelled ligands [3H]WB4101, [3H]QNB, [3H]-d-LSD, [3H]mepyramine). The potency of the nontricyclic antidepressants varied greatly. Mianserin, potently displaced [3H]mepyramine, [3H]d-LSD and [3H]WB4101 while it was very weak on [3H]QNB-binding. Nomifensine and the specific 5-HT uptake inhibitors zimelidine and alaproclate had very low affinity for these receptors. All the antidepressants tested were practically devoid of activity on [3H]DHA binding, [3H]spiroperidol binding, [3H]flunitrazepam binding, [3H]muscimol binding and [3H]naloxone binding. The implications of these findings for biogenic amine theories of affective disorders are discussed.

Test-specific effects of the 5-HT reuptake inhibitors alaproclate and zimelidine on pain sensitivity and morphine analgesia

The effects of the specific 5-HT uptake inhibitors alaproclate and zimelidine, the 5-HT releasing compound p-chloroamphetamine (PCA) and the specific NA uptake inhibitor desipramine on pain sensitivity were examined in male rats using the hot-plate and tail-flick methods. The effects of alaproclate and zimelidine on 5-HT uptake mechanisms in the hypothalamus and spinal cord were also studied. Alaproclate, zimelidine, PCA and desipramine produced hypoalgesia in the hot-plate but not in the tail-flick test. Naloxone (1 mg/kg) failed to block the hypoalgesia produced by alaproclate and PCA in the hot-plate test. Zimelidine but not desipramine pretreatment blocked the analgetic action of PCA in the hot-plate test. Alaproclate significantly enhanced morphine analgesia in the hot-plate test but did not affect morphine analgesia in the tail-flick test. In contrast, zimelidine tended to enhance and significantly prolonged morphine analgesia in the tail-flick test but did not affect morphine analgesia in the hot-plate test. Zimelidine inhibited 5-HT uptake with equal potency in the hypothalamus and spinal cord, while alaproclate produced a greater inhibition of 5-HT uptake in the hypothalamus. These findings show test-specific effects after enhancement of central 5-HT neurotransmission. It is suggested that various aspects of pain sensitivity and morphine analgesia may involve different 5-HT pathways in the brain and spinal cord. Moreover, 5-HT pathways in the forebrain may mediate analgesia of a non-opiate type.

Decreased tyramine sensitivity after discontinuation of amitriptyline therapy. An index of pharmacodynamic half-life

A combined pharmacodynamic and pharmacokinetic approach was made to study the pharmacodynamic half-life (Pd1/2) of amitriptyline (AT). Six depressed patients were treated with 150 mg of amitriptyline as a single oral dose at night for six or more weeks. Decreased tyramine sensitivity (DTS), an index of this drug's pharmacological activity, was determined serially at various intervals after the last dose. Plasma concentrations of AT and nortriptyline (NT) were also estimated at above intervals. It was possible to detect DTS for 228-300 h after the last oral dose and the mean Pd1/2 of this decline of pharmacodynamic effect was observed to be 135 h. However, no measurable amount of AT or NT was present after 84 h and the mean elimination plasma half-life (t1/2) of AT and NT were 37.7 and 38.9 h, respectively. (In this study, pharmacokinetic parameters of NT were directly related with those of AT.) Prolonged pharmacodynamic effect of this drug after discontinuation should be borne in mind in order to avoid drug interactions and autonomic complications, especially after overdosage. Pd1/2, as assessed by DTS, correlated directly with the t1/2 (r = 0.91) and inversely with the plasma clearance rate (r = 0.60) of NT. DTS test can be used as an alternative technique to assess the biological activity of a drug which inhibits noradrenaline reuptake mechanism and/or blocks alpha-adrenoceptors at the peripheral neuronal sites, especially, where facilities to measure plasma concentrations of such drugs are limited.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).