Biochemistry and pharmacology of moclobemide, a prototype RIMA.

RIMA is a term for reversible inhibitors of monoamine oxidase (MAO) with preference for MAO-A; moclobemide is a prototype of this new class of antidepressants and is a highly selective inhibitor of MAO-A in vitro. This inhibition is reversible by dialysis in vitro, which accounts for the dose-dependent duration of in vivo enzyme inhibition of 12-24 h. Moclobemide increases the content of serotonin, noradrenaline and dopamine in the brain, and decreases that of their deaminated metabolites. Its biochemical, neurological and behavioural effects indicate that it increases the extracellular concentration of the classic monoamine neurotransmitters/neuromodulators - in particular 5-HT. Potentiation of the cardiovascular effects of tyramine is less pronounced after taking moclobemide than after irreversible MAO-A inhibitors. Understanding of the physiological role of MAO and of the events that link inhibition of the enzyme with modulation of neuronal activities in the CNS remains incomplete. A major physiological role of intraneuronal MAO is to keep cytosolic amine concentration very low, to enable the neuronal monoamine carriers to produce a net inward transport of monoamines, and thereby to act as the first step in the termination of action of extracellular monoamines. MAO is likely to have a similar function in non-monoaminergic cells with respect to the monoamine carriers they contain. In addition to the classic monoamines, "trace" amines may become functionally active after MAO inhibition. An alternative role for MAO is that of a scavenger, preventing natural substrates from accumulating in monoaminergic neurons and interacting with storage, release, uptake and receptor function of monoamines.

Disappearance of supra-ependymal 5-HT axons in the rat forebrain after electrolytic and 5,6-DHT-induced lesions of the medial forebrain bundle.

The present study was intended to demonstrate the origin of supra-ependymal 5-hydroxytryptamine axons in the rat forebrain. Electrolytic lesions and injections of 5,6-dihydroxytryptamine (10 mug in 4 mul) were carried out unilaterally in and close to the medial forebrain bundle in the posterior hypothalamus of rats. Ten to 14 days later, terminal axons and formaldehyde-induced indolealkylamine fluorescence had virtually disappeared supra-ependymally in the lateral ventricles and interventricular foramina ipsilateral to the lesion if the indolealkylamine axons passing through the medial part of the medial forebrain bundle had been destroyed. No changes were observed, electron microscopically or fluorescence histochemically, in ventricles contralateral to the lesion. It is concluded that the supra-ependymal serotonergic nerve terminals in the lateral ventricles and interventricular foramina originate, uncrossed, from non-terminal axons passing through the medial forebrain bundle in the posterior hypothalamus.

Effect of bilateral decortication on nerve growth factor content in basal nucleus and neostriatum of adult rat brain.

The content of nerve growth factor (NGF) was measured by a sensitive two-site enzyme-linked immunosorbent assay in the basal nucleus and neostriatum at various times following bilateral suction lesions of the cerebral neocortex in adult rats. At 14 and 29 postlesion days, NGF levels in basal nucleus were significantly elevated by 270 and 126%, respectively. In addition, 29 days after cortical lesions, NGF content in the neostriatum was increased 145% above controls. Thus, enhanced NGF production occurs in both basal nucleus and neostriatum of adult rats in response to antero- and/or retrograde neuronal degeneration.

Nerve growth factor increases substance P, cholecystokinin and vasoactive intestinal polypeptide immunoreactivities in primary sensory neurones of newborn rats.

Immunoreactive substance P, somatostatin, gastrin/cholecystokinin and vasoactive intestinal polypeptide were studied in lumbar dorsal root ganglia of 14-day-old rats treated from day 2 to 11 of life with nerve growth factor. Increased staining intensity of neuronal cell bodies and processes for substance P, gastrin/cholecystokinin and vasoactive intestinal polypeptide was observed by immunohistochemistry indicating increased neuronal peptide concentrations. These results were supported by radioimmunoassays showing increased ganglionic levels of substance P and vasoactive intestinal polypeptide. Both techniques, however, failed to show a significant increase of somatostatin levels.

Substance P-, met-enkephalin- and somatostatin-like immunoreactivity distribution in the frog spinal cord.

The distribution of substance P-, Met-enkephalin- and somatostatin-like immunoreactivity was studied in the thoracic spinal cord of the frog using immunohistochemical techniques. In fibres, probably nerve terminals, immunoreactivity was greatest in the grey matter (mainly dorsal horn), but it was also present in white matter regions. While substance P- and, perhaps, somatostatin-like immunoreactivity appeared to be contained in primary afferents, the presence of all 3 peptides in neuronal cells of the grey matter indicates the existence of a propriospinal peptidergic system.

Supra-ependymal serotoninergic nerves in mammalian brain: morphological, pharmacological and functional studies.

Supra-ependymal nerves in mammals (mainly rats) have been shown to contain serotonin (5-hydroxytryptamine, 5-HT) by combined Falck-Hillarp fluorescence histochemistry, ultrastructural monoamine cytochemistry and pharmacology as well as by immunohistochemistry and autoradiography. Supra-ependymal 5-HT cells do not occur. At least in rats, virtually all supra-ependymal nerves contain 5-HT and in our opinion the occasionally described non-5-HT supra-ependymal nerve cells and their processes contribute little to the supra-ependymal nerve plexus (with the possible exception of those cells above the median eminence). The cells of origin of the supra-ependymal 5-HT nerves are situated in raphe nuclei. The axons and terminals (varicosities) contain small and large dense core vesicles in both of which 5-HT is stored. A co-transmitter has not been found among the candidates investigated so far (leu- and met-enkephalin, substance P and gamma-aminobutyric acid (GABA)). The nerves possess uptake mechanisms specific for 5-HT and possibly GABA. Occasionally desmosome-like junctions are observed between 5-HT nerve terminals and ependymal cells but no true synapses. The function of these nerves is not known. They do not appear to regulate ciliary movement, but might influence the shape of ependymal cells.

Cholinergic but not monoaminergic denervation increases nerve growth factor content in the adult rat hippocampus and cerebral cortex.

We have investigated whether degeneration of basal forebrain cholinergic neurons is a potential trigger for increased NGF production in the adult rat brain. Electrolytic lesions of cholinergic neurons in the septum-diagonal band and in the nucleus basalis of Meynert induced a transient increase in NGF in the ventral hippocampus (+70%) and cerebral cortex (+125%), respectively. In contrast, selective aminergic denervation of the forebrain by electrolytic lesion of the medial forebrain bundle, did not increase NGF levels in hippocampus and cerebral cortex. Thus, a cholinergic mechanism appears to regulate NGF production in adult rat basal forebrain.

Effects of clonidine on the rate of noradrenaline turnover in discrete areas of the rat central nervous system.

Turnover of noradrenaline in various regions of the rat brain was estimated by the decrease in noradrenaline content and/or formaldehyde-induced catecholamine fluorescence after inhibition of noradrenaline biosynthesis with alpha-methyl-p-tyrosine. Clonidine (0.1 and 0.3 mg/kg p.o.) decelerated the decrease in noradrenaline content of the locus coeruleus, the nucleus of the solitary tract, the intermediolateral cell column and the ventral horn of the thoracic spinal cord, as measured in tissue punches of the respective regions with a sensitive radioenzymatic method. In all these central regions the clonidine-induced decrease in noradrenaline turnover was antagonized by yohimbine, but not by phenoxybenzamine, indicating mediation through central alpha 2-adrenoceptors, similar to the cardiovascular effects of clonidine. When given alone, both yohimbine and phenoxybenzamine accelerated the disappearance of noradrenaline after inhibition of its biosynthesis. The combined results of radioenzymatic assay and fluorescence histochemistry determinations demonstrated that clonidine markedly reduced noradrenaline turnover in central noradrenaline-containing nerve terminals, but had no effect on the cell bodies of the A1 and A2 cell groups. Noradrenaline turnover was, however, decreased in projection areas of the A1 and A2 cell groups, namely the intermediolateral cell column of the spinal cord and nucleus of the solitary tract, respectively. This observation argues against the existence of a neuronal feedback loop running from the projection areas to the cell bodies of the A1 and A2 cell groups and mediating inhibition of noradrenaline turnover. The effect of clonidine on noradrenaline turnover is, therefore, most likely the result of a local feedback inhibition through presynaptic alpha-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of moclobemide.

Moclobemide is a reversible inhibitor of monoamine oxidase (MAO) with clear preference for the A type (so-called RIMA). The enzyme inhibition shows complex kinetics, and the molecular mechanism of interaction with the enzyme is not yet clear. Moclobemide increases the extracellular concentration of the monoamines in rat brain and decreases the level of their metabolites. Neither a loss nor a cumulation of activity has been observed after chronic treatment. Reversibility of MAO-A inhibition was demonstrated in vitro as well as in vivo. In various animal behavioral models, in particular in a novel model of stress-induced anhedonia, moclobemide was as effective as standard antidepressants. Moclobemide improves cognitive functions that are impaired in experimental situations. A neuroprotective action is seen in rats subjected to transient global ischemia/-hypoxia. Moclobemide lacks anticholinergic and other effects and only slightly increases the pressor effect of orally administered tyramine. Possible links between MAO-A inhibition and the various effects of moclobemide on brain function are discussed.

Ro 15-4513: partial inverse agonism at the BZR and interaction with ethanol.

The imidazobenzodiazepinone derivative Ro 15-4513 has the activity profile of a partial inverse (low efficacy) agonist at the benzodiazepine receptor (BZR). It reverses central nervous depressant effects of diazepam, and, in part, of phenobarbitone and ethanol in mice, rats and cats in behavioural, electrophysiological, and neurochemical paradigms. The interaction of Ro 15-4513 with barbiturates and ethanol is due to its inverse agonistic (negative allosteric modulatory) property at the BZR, as it was reversed by the selective BZR blocker flumazenil (Ro 15-1788). In the present experiment situations, other BZR partial inverse agonists in subconvulsant or overt convulsant doses were less effective against ethanol effects than Ro 15-4513. Possible mechanisms for this differential activity of BZR inverse agonists are discussed.

5-HT nerve terminals in the fourth ventricle of the rat brain: their identification and distribution studied by fluorescence histochemistry and electron microscopy.

The occurrence and distribution of supra-ependymal nerve terminals storing serotonin (5-HT) are described for the fourth ventricle of the rat brain. The nerve terminals were identified as monoaminergic 1) fluorescence-histochemically, by the presence of a varicose, formaldehyde-induced fluorescence (FIF) on the free surface of the ependyma, 2) electron microscopically, by the presence of electron dense (chromaffin) cores in small (50nm) and large (100 nm) vesicles found within the varicose regions of supra-ependymal nerve fibres, and 3) by the absence of both the FIF and chromaffin dense cores after treatment with reserpine. Moreover, the serotonergic nature of these nerve fibres could be concluded from 1) the yellow colour of the FIF, 2) the increased FIF after treatment with nialamide or reserpine+nialamide, 3) the diminished FIF and absence of chromaffin dense cores after treatment with p-CPA, and finally 4) the persistence of the FIF anc chromafin dense cores after treatment with alpha-MPT. A high density of 5-HT nerve terminals occurred throughout the floor of the fourth ventricle and on the floor and roof of the lateral recess. Few 5-HT nerve terminals occurred only on the roof of the fourth ventricle (velum medullare, lamina epithelialis of the tela chorioidea), and the surface of the choroid plexus epithelia was devoid of such nerves. Virtually all nerve terminals in the fourth ventricle appear to be serotonergic.

Absence of dopamine sensitive adenylate cyclase in the A10 region, the origin of mesolimbic dopamine neurones.

Dopamine (DA) failed to stimulate the adenylate cyclase of the mesolimbic A10 DA nerve cell body area, in contrast to tis activating effect in the nigrostriatal A9 DA cell body area. The enzyme was stimulated by GMPPNP (a GTP analog) and NaF. This indicates the absence in the A 10 cell area of DA receptors with functional coupling on adenylate cyclase, in contrast to the A9 cell area where such DA receptors are believed to be located on afferent axon terminals.

Fluorescence microscopical study of 5-hydroxytryptamine storage organelles in mepacrine-incubated blood platelets of beige mice (Chediak-Higashi syndrome).

The number and fluorescence intensity of fluorescent granules (5-HT storage organelles) of mepacrine-incubated blood platelets of beige mice (Chediak-Higashi syndrome) are decreased compared to those of control mice platelets. This indicates both quantitative and qualitative changes of the 5-HT organelles, namely a reduced number and a reduced storage capacity, respectively.