Monoamine receptor sensitivity changes following chronic administration of MDL 72394, a site-directed inhibitor of monoamine oxidase

Reduced density of functional 5-HT1A receptors in the brain, medulla and spinal cord of monoamine oxidase-A knockout mouse neonates

Abnormally high brain 5-HT levels in monoamine oxidase-A knockout (MAO-A KO) mouse neonates raise the question of whether the distribution and density of the 5-HT1A receptors (5-HT1AR) expressed in the brain by postnatal day P7 are affected and, if so, whether the 5-HT1A autoreceptors in the dorsal raphe are modified in the same way as the postsynaptic 5-HT1AR present in raphe target structures. [3H]8-OH-DPAT binding and quantitative autoradiography were performed to answer these questions. Binding specificity was first confirmed in adult wild-type mice and rat brain sections. 5-HT1AR binding was then analyzed in four MAO-A mutant vs. five wild-type neonatal brains, from olfactory bulb to cervical cord. Among 12 structures expressing postsynaptic 5-HT1AR in wild-type neonates, the highest densities involved the retrosplenial cortex, entorhinal cortex, and septum (52-46 fmol/mg tissue); low densities occurred in the hippocampus and spinal cord (24 fmol/mg tissue); in addition, the raphe autoreceptor density was only 20 fmol/mg tissue. In mutants, the distribution of postsynaptic 5-HT1AR was unchanged, but an overall decrease in density occurred (-32% to -63%); the raphe autoreceptors decreased in mutants by at least -79%. Data are discussed with reference to the ectopic 5-HT uptake and accumulation reported to occur during the first 10 postnatal days in wild-type and MAO-A KO mice. As previously suggested to explain the raphe autoreceptor loss in 2-month-old MAO-A KO mice, the overall 5-HT1AR down-regulation in mutant pups probably results from extracellular 5-HT excess in both raphe and target structures. The greater the 5-HT excess, the more the functional receptor density decreases.

Altered presynaptic function in monoaminergic neurons of monoamine oxidase-A knockout mice

Monoamine oxidase-A knockout (MAO-A KO) mice have elevated brain serotonin (5-HT) and noradrenaline (NA) levels, and one would therefore anticipate increased monoamine release and compensatory changes in other aspects of presynaptic monoamine function. In this study we used voltammetry in brain slices from the locus coeruleus (LC), dorsal raphe (DRN) and striatum (CPu) in 7-week-old MAO-A KO and C3H control mice to measure stimulated monoamine efflux and its control by amine transporters and autoreceptors. In LC, peak NA efflux on stimulation (99 pulses, 100 Hz) was higher in MAO-A KO than C3H mice (938 +/- 58 nm cf. 511 +/- 42 nm; P < 0.001). The NA uptake half time (t(1/2)) was longer in MAO-A KO than in C3H mice (6.0 +/- 0.9 s cf. 1.9 +/- 0.3 s; P < 0.001) and the selective NA reuptake inhibitor desipramine (50 nm) had a smaller effect in MAO-A KO mice. NA transporter binding was significantly lower in the LC of MAO-A KO mice compared to C3H controls (P < 0.01) but not in the DRN. The alpha 2 agonist dexmedetomidine (10 nm) decreased stimulated NA efflux more in C3H than in MAO-A KO mice (73.3% cf. 29.6% inhibition, P < 0.001). In DRN, peak 5-HT efflux on stimulation (99 pulses, 100 Hz) was greater (P < 0.01) in MAO-A KO (262 +/- 44 nm) than C3H mice (157 +/- 16 nm). Moreover, 5-HT uptake t(1/2) was longer (P < 0.05) in MAO-A KO than in C3H mice (8.8 +/- 1.1 s cf. 4.9 +/- 0.6 s, P < 0.05) and the effect of citalopram (75 nm) was attenuated in MAO-A KOs. Serotonin transporter binding was also lower in both the DRN and LC of MAO-A KO mice. The 5-HT(1A) agonist 8-OH-DPAT (1 microm) decreased 5-HT efflux more in C3H than in MAO-A KO mice (38.3% inhibition cf. 21.6%, P < 0.001). In contrast, there were no significant differences between MAO-A KO and C3H mice in CPu dopamine efflux and uptake and the effect of the D(2/3) agonist quinpirole was similar in the two strains. In summary, MAO-A KO mice show major dysregulation of monoaminergic presynaptic mechanisms such as autoreceptor control and transporter kinetics.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Effect of long-term administration of antidepressant drugs on the 5-HT3 receptors that enhance the electrically evoked release of [3H]noradrenaline in the rat hippocampus

The present study investigated the effects of various classes of antidepressant drugs (10 mg/kg per day, s.c. during 21 days) on the electrically evoked release of [3H]noradrenaline and on its modulation by the 5-HT3 receptor agonist 2-methyl-5-hydroxy-tryptamine (2-methyl-5-HT) using preloaded rat hippocampal slices. Treatments with either fluoxetine, a selective serotonin (5-HT) reuptake inhibitor, or moclobemide, a reversible type A monoamine oxidase inhibitor, increased the evoked release of [3H]noradrenaline. These two antidepressant treatments did not change, however, the magnitude of the enhancing effect of 2-methyl-5-HT on the electrically evoked release of [3H]noradrenaline. Desipramine produced a much larger increase of the electrically evoked release of [3H]noradrenaline than fluoxetine or moclobemide, and desensitized the 5-HT3 receptors that modulate this release. Trimipramine, which like desipramine has a tricyclic structure but does not block the reuptake of noradrenaline or that of 5-HT, did not increase the evoked release of [3H]noradrenaline and did not desensitize the 5-HT3 receptors that enhance the release of [3H]noradrenaline. Maprotiline, a selective noradrenaline reuptake inhibitor, did not produce the same changes as desipramine, but maprotiline inhibited noradrenaline reuptake to a lesser extent (50%) than desipramine (80%). These results suggest that the high potency noradrenaline reuptake blocker desipramine desensitizes 5-HT3 receptors modulating [3H]noradrenaline release, but that this effect is not common to all antidepressant drugs.

The new generation of monoamine oxidase inhibitors

Irreversible and unspecific inhibitors of MAO were the first modern antidepressants, but after an initial success they fell into discredit due to adverse side effects. In the past two decades interest in MAO inhibitors has been renewed because of progress in basic research, a milestone being the finding that there are two subtypes of MAO, MAO-A and MAO-B. These are distinct proteins with high amino acid homology, coded by separate genes both located on the short arm of the human chromosome X. The enzyme subforms show different substrate specificities in vitro and different distributions within the central nervous system and in peripheral organs. In the central nervous system of man MAO-A seems to be mainly involved in the metabolism of 5 HT and noradrenaline, whereas 2-phenylethylamine and probably dopamine are predominantly deaminated by MAO-B. In the intestinal tract tyramine is mainly metabolized by MAO-A. These characteristics indicate distinct physiological functions of the two MAO-subforms. Several irreversible and reversible non-hydrazine inhibitors with relative selectivities for one of the MAO-subforms have been developed. They belong to various chemical classes with different modes of enzyme inhibition. These range from covalent mechanism based interaction (e.g. by propargyl- and allylamine derivatives) to pseudosubstrate inhibition (e.g. by 2-aminoethyl-carboxamides) and non-covalent interaction (e.g. by brofaromine, toloxatone and possibly moclobemide). The most important pharmacological effects of the new types of MAO inhibitors are those observed in neuropsychiatric disorders. The inhibitors of MAO-A show a favorable action in various forms of mental depression. The drugs seem to have about the same activity as other types of antidepressants, including tricyclic and related compounds as well as classical MAO inhibitors. The onset of action of the MAO-A inhibitors is claimed to be relatively fast. Other possible indications of these drugs include disorders with cognitive impairment, e.g. dementia of the Alzheimer type. In subjects with Parkinson's disease the MAO-B inhibitor L-deprenyl exerts a L-dopa-sparing effect, prolongs L-dopa action and seems to have a favorable influence regarding on-off disabilities. The action is in general transitory (months to several years). In addition L-deprenyl has been shown to delay the necessity for L-dopa treatment in patients with early parkinsonism. Whether the drug influence the progression of the disease is still a matter of debate. L-deprenyl also appears to have some antidepressant effect (especially in higher doses) and to exert a beneficial influence in other disorders, e.g. dementia of the Alzheimer type.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonergic measures in suicide brain: 5-HT1A binding sites in frontal cortex of suicide victims

The density of 5-HT1A binding using 3H-8-hydroxy-2-(di-n-propyl-amino) tetralin (8-OH-DPAT) as binding ligand, was studied in human frontal cortex of suicide victims and normal controls who died due to medical disease or accidentally. There was no difference in the maximum number of binding site (Bmax) or Kd (an inverse measure of affinity) of 5-HT1A receptor binding sites between normal controls and the entire group of suicide victims. However, nonviolent suicides had significantly higher Bmax (22-25%) compared to both controls and violent suicides. A negative correlation between age and Bmax of 5-HT1A binding sites was found in male controls but not in female controls or suicide victims. This relationship was less apparent among the male controls over age 60.

Serotonin regulation of tachykinin biosynthesis in the rat neostriatum

Serotonin (5-HT) neurotransmission was altered to determine its role in regulating the biosynthesis of tachykinins in the neostriatum (NS). Depletion of 5-HT with subchronic p-chlorophenylalanine (pCPA) treatment decreased preprotachykinin (PPT, the prohormone precursor to SP) mRNA levels in the NS. By contrast, raising extracellular 5-HT levels with zimelidine (a 5-HT uptake inhibitor) or clorgyline (a monoamine oxidase inhibitor) resulted in increased levels of PPT mRNA. To determine whether 5-HT receptors played a role in mediating the changes in PPT mRNA, animals were treated with the 5-HT2 agonist DOI. This drug significantly increased both PPT mRNA and SP-like immunoreactivity in the NS. These results together indicate that neostriatal tachykinin biosynthesis is sensitive to alterations in 5-HT neurotransmission.

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.

Imipramine and tetrabenazine: effects on monoamine receptor binding sites and phosphoinositide hydrolysis

Treatment of rats for 21 days with tetrabenazine, a drug which depletes monoamines and is used behaviorally to screen for antidepressants, significantly decreased 5-HT2 receptor density, increased alpha 1-adrenoceptor density but did not alter beta-adrenoceptor density in homogenates of frontal cortices labeled with [3H]ketanserin, [3H]prazosin and [3H]dihydroalprenolol, respectively. These effects were not opposite to those of the antidepressant drug imipramine which decreased both 5-HT2 and beta-adrenoceptor density and did not alter alpha 1-adrenoceptor density. Some evidence for antagonistic interactions between the two drugs was found in that imipramine partially prevented the tetrabenazine-induced increase in alpha 1-adrenoceptor density and tetrabenazine partially prevented the imipramine-induced decrease in beta-adrenoceptor density. Neither drug altered phosphoinositide hydrolysis coupled to alpha 1-adrenoceptors. While the effects of tetrabenazine are frequently attributed to its reserpine-like action of depleting monoamines, these results provide the first indication that tetrabenazine alters 5-HT2 and beta-adrenoceptor density in a manner different from that of reserpine.

Antidepressant treatments, including electroconvulsive shock and the putative antidepressant sibutramine hydrochloride, do not alter [3H]-prazosin binding to rat cortical membranes

Treatment for 1, 3 or 10 days with sibutramine hydrochloride (3 mg/kg), desi pramine (10 mg/kg), amitriptyline (10 mg/kg) or pargyline (10 mg/kg) did not effect the number or affinity of α(1)-adrenoceptors as determined by [(3)H]-prazosin binding to rat cerebral cortical membranes. Similarly, neither single, nor repeated (five times over 10 days) electroconvulsive shocks, given under halothane anaesthesia had any effect on the parameters of [(3)H]-prazosin binding. These data suggest that the clinical actions of these treatments are unlikely to involve changes in α(1)-adrenoceptor function.

Chronic MAO A and MAO B inhibition decreases the 5-HT1A receptor-mediated inhibition of forskolin-stimulated adenylate cyclase

The effect of chronic administration of various monoamine oxidase (MAO) inhibitors on the ability of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to inhibit forskolin-stimulated adenylate cyclase activity was studied. Groups of 12 rats were given either saline, (E)-beta-fluoromethylene-m-tyrosine (MDL 72394 0.25 mg/kg p.o.), clorgyline (1 mg/kg p.o.), selegiline (1 mg/kg p.o.) or tranylcypromine (5 mg/kg p.o.) once a day for 21 days. Biochemical determinations were made 72 h after the final dose. MDL 72394 and tranylcypromine produced a nonselective inhibition of MAO but clorgyline and selegiline selectively inhibited MAO A and MAO B respectively. All treatments that inhibited MAO A also increased tissue levels of 5-HT. Chronic treatment with MDL 72394, clorgyline or tranylcypromine reduced the ability of 8-OH-DPAT to inhibit forskolin-stimulated adenylate cyclase activity. These data suggest that chronic nonselective and chronic MAO A inhibition causes a down-regulation of the 5-HT1A-mediated inhibition of forskolin-stimulated adenylate cyclase activity.

Direct activation of Ca2+ channels by palmitoyl carnitine, a putative endogenous ligand

1 Palmitoyl carnitine, a lipid metabolite which accumulates in cytoplasmic membranes during ischaemia, has been shown to resemble the Ca2+ channel activator, Bay K 8644, in K+-depolarized smooth muscle. Palmitoyl carnitine caused concentration-dependent (1-1000 mumol l-1) augmentations in the sensitivity to Ca2+ of K+-depolarized taenia preparations from the guinea-pig caecum. The (+/-)-isomer was equieffective with the (-)-isomer, whereas carnitine was ineffective and palmitic acid relaxed the tissues. The shift to the left of Ca2+ concentration-response curves induced by palmitoyl carnitine (100 mumol l-1) was additive with that of Bay K 8644 (1 mumol l-1). 2 The interactions of palmitoyl carnitine with the different classes of calcium-antagonist were similar to those seen with Bay K 8644. Schild plots of the calcium-antagonist effects of nifedipine were shifted to the right following preincubation of the taenia with palmitoyl carnitine (30-300 mumol l-1). The inhibitory effects of verapamil were especially sensitive to palmitoyl carnitine (100 mumol l-1). Whereas the potency of diltiazem as a calcium-antagonist was reduced by palmitoyl carnitine (100 mumol l-1), the inhibitory effects of the lipophilic class III calcium-antagonists, cinnarizine and flunarizine, were entirely resistant to palmitoyl carnitine (100 mumol l-1). 3 Although palmitoyl carnitine has detergent properties in high concentrations and lyses red blood cells, these effects were not Ca2+-dependent, nor were they modified by calcium-antagonists. Other detergents did not have selective interactions with Ca2+ channels. 4 Palmitoyl carnitine inhibited [3H]-nitrendipine, [3H]-verapamil and [3H]-diltiazem binding to rat cortical membranes with IC50 values (mumol l-1) of 120 +/- 1, 95 +/- 17 and 120 +/- 15 mumol l-1 respectively. The inhibition showed little temperature-dependence, in contrast to that of Bay K 8644, except for a small reduction in the IC50 value for [3H]-verapamil binding at 37 degrees C (42 +/- 5 mumol l-1). Palmitoyl carnitine interacted selectively with the Ca2+ channel, in that effects on ligand binding to alpha-adrenoceptors, beta-adrenoceptors and 5-HT1A receptors occurred only at 5-10 fold higher concentrations. 5 It is concluded that palmitoyl carnitine, at concentrations which have previously been shown to occur in the cytoplasm during myocardial ischaemia, may interact directly with Ca2+ channels and may therefore be considered as an endogenous modulator of channel function. The site of action differs from that of other agents.

Electroconvulsive shock but not antidepressant drugs increases alpha 1-adrenoceptor binding sites in rat brain

Treatment of rats with electroconvulsive shock once daily for 10-12 days increased the density of alpha 1-adrenoceptors labeled by [3H]prazosin in homogenates of frontal cerebral cortex. A single treatment did not affect [3H]prazosin binding. Repeated treatment with electroconvulsive shock did not appear to affect alpha 1-adrenoceptor binding in the hippocampus or hypothalamus. Treatment of rats with reserpine also increased [3H]prazosin binding in the frontal cortex. In contrast to electroconvulsive shock, three weeks administration of tricyclic antidepressant drugs, monoamine oxidase inhibitors, or atypical antidepressant drugs did not significantly affect [3H]prazosin binding sites in the frontal cortex. The affinities of antidepressant drugs for alpha 1-adrenoceptors ranged from about 50 nM for tricyclic and atypical antidepressants to about 100 microM for monoamine oxidase inhibitors. The high affinities of the tricyclic and atypical antidepressant drugs for alpha 1-adrenoceptors could contribute to clinical differences between these classes of drugs and monoamine oxidase inhibitors. Furthermore, the electroconvulsive shock-induced increase in alpha 1-adrenoceptors could contribute to differences in clinical effects between this treatment and antidepressant drugs.