Electrophysiological and receptor studies in rat brain: effects of clorgyline

Desensitization of alpha(2)-adrenoceptors which regulate noradrenaline synthesis and release after chronic treatment with clorgyline in the rat brain

The sensitivity of alpha(2)-adrenoceptors which regulate synthesis and release of noradrenaline was investigated in hippocampus, parietal cortex, and hypothalamus of rats treated with clorgyline. After administering a DOPA decarboxylase inhibitor, the in vivo tyrosine hydroxylase activity and the noradrenaline content were evaluated. Acute and chronic treatment with clorgyline led to both increases of noradrenaline levels and decreases of tyrosine hydroxylase activity, determined as the accumulation of DOPA. Whereas the alpha(2)-adrenoceptor agonist clonidine induced a similar reduction in tyrosine hydroxylase activity in the group subjected to the acute treatment and in the control group, it failed to do so after chronic clorgyline treatment. In hippocampal and cortical synaptosomes, a reduction in the sensitivity of alpha(2)-adrenoceptors which regulate [(3)H]noradrenaline release, reflected by the shift to the right of the concentration-effect curves for oxymetazoline, was also found after the repeated treatment. These results indicate a desensitization of alpha(2)-adrenoceptors after chronic treatment with clorgyline.

Stimulatory effect of harmane and other beta-carbolines on locus coeruleus neurons in anaesthetized rats

Harmane, harmaline and norharmane are beta-carboline related compounds which have been proposed to be endogenous ligands for imidazoline receptors. The effect of these compounds on the activity of locus coeruleus (LC) neurons was studied by extracellular recordings techniques. Intracerebroventricular administration of harmane and harmaline increased the firing rate of LC neurons. Systemic administration of efaroxan, a mixed alpha(2)-adrenoceptor/I(1)-imidazoline antagonist or vagotomy failed to modify the harmane effect. Furthermore, local applications of harmane and harmaline increased the firing rate of LC neurons in a dose-related manner. Finally, intravenous administration of norharmane also increased the activity of LC neurons. Our results demonstrate that beta-carbolines stimulate LC neuron activity and indicate that this stimulation occurs directly in the LC by a mechanism independent of I(1)- and I(2)-imidazoline receptors.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Preclinical profile of befloxatone, a new reversible MAO-A inhibitor

Befloxatone, a novel oxazolidinone derivative, is a potent, selective and reversible monoamine oxidase A (MAO-A) inhibitor in vitro (K1A = 1.9-3.6 nM) and ex vivo (ED50 MAO-A = 0.02 mg/kg, p.o.). It does not interact with a large number of receptors, monoamine transporters or other amine oxidases. Binding studies with [3H]-befloxatone in rat brain sections show that it labels with high affinity (Kd = 1.3 nM) a single population of sites with the pharmacological characteristics and regional distribution of MAO-A. In the rat brain, befloxatone (0.75 mg/kg, i.p.) increases tissue levels of monoamines and decreases levels of their deaminated metabolites. Acute administration of befloxatone (0.75 mg/kg, i.p.) induces an increase in extracellular striatal dopamine and cortical norepinephrine but not cortical serotonin levels in the rat. Befloxatone (1 mg/kg, i.p.) potently inhibits the firing rate of serotonergic neurons, partially decreases the firing of noradrenergic neurons and has no effect on the firing of dopaminergic neurons (a mirror image of its effects on monoamine release in terminal regions), suggesting that the relative effects of befloxatone on monoamine release may be governed by autoreceptor-mediated control of monoaminergic neurons at the cell body level. Befloxatone (0.03-0.3 mg/kg, p.o.) exhibits potent activity in behavioural models predictive of antidepressant activity. Befloxatone (up to 1.5 mg/kg, p.o.) does not potentiate the pressor effects of orally administered tyramine at centrally active doses and duodenal [3H]-befloxatone binding is displaced by increasing doses of orally administered tyramine (0.1-40 mg/kg, i.p.). These results suggest that befloxatone is a potent reversible MAO-A inhibitor with antidepressant potential and a wide safety margin with regard to the potentiation of the pressor effect of tyramine.

Purification and characterization of G proteins from human brain: modification of GTPase activity upon phosphorylation

Three G proteins from human brain membranes were purified to near homogeneity by conventional techniques including preparative electrophoresis. These G proteins were characterized by their ability to bind GTP, GDP and GTP analogs. Two of these proteins have molecular weights of 50,000 (G50) and 36,000 (G36), as determined on SDS-gels. G36 was ADP-ribosylated by pertussis toxin. Thus, G50 could represent a Gs alpha subunit, whereas G36 could be Gi alpha or Go alpha. G50 was phosphorylated by cAMP dependent protein kinase and protein kinase C. G36 was phosphorylated by a protein kinase independent of calcium and phospholipid, a proteolytic product of protein kinase C, analogous to protein kinase M. Phosphorylation of G36 by this protein kinase induced a dramatic decrease in its GTPase activity. The third G protein, of molecular weight 22,000 probably belongs to the group of monomeric G proteins possessing functional similarities with ras gene products. The regulation of G proteins involving calcium-dependent and independent pathways is delineated.

Cerebral metabolic effects of monoamine oxidase inhibition in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine acutely treated monkeys

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces dopaminergic cell death in the substantia nigra pars compacta (SNpc) and clinical parkinsonism in humans and experimental animals. Pretreatment with monoamine oxidase inhibitors prevents this cell death and associated parkinsonism by blocking the oxidation of MPTP to a toxic intermediate. The 2-deoxyglucose method was used to study the acute effects of MPTP in the monkey brain and the effects of monoamine oxidase inhibition on local cerebral glucose utilization in both normal and MPTP-treated monkeys. MPTP administration alone caused a major increase in glucose utilization in the SNpc and smaller increases in some subnuclei within the ventral tegmental area in which eventual dopaminergic cell loss also occurs. Pretreatment with pargyline abolished these metabolic increases, a finding suggesting both that the oxidized product of MPTP generates the metabolic increases and that the increased glucose consumption may contribute to cell toxicity. On the other hand, in most cortical, thalamic, striatal, brainstem, and cerebellar areas MPTP alone caused reductions in glucose utilization, and pargyline failed to prevent these effects. Pargyline alone depressed metabolism in the locus coeruleus and a few other monoaminergic structures.

Modulation of vocal and nonvocal behavior in adult squirrel monkeys by selective MAO-A and MAO-B inhibition

The acute effects of monoamine oxidase inhibitors L-deprenyl (0.5-5.0 mg/kg), clorgyline (1.0-10.0 mg/kg), and milacemide (100-400 mg/kg) on the behavior of adult male squirrel monkeys were examined during brief social separations beginning 60 min after subcutaneous drug administration. All three drugs selectively reduced the rate of calling during social separation at doses which did not affect time spent in locomotion, nor the frequency of vigilance-checking. Deprenyl and milacemide, but not clorgyline, produced concurrent decreases in locomotion at the higher doses tested. At threshold doses, clorgyline, but not deprenyl or milacemide, increased call duration and decreased call peak frequency compared to vehicle control values. Plasma levels of MHPG were decreased by an optimal dose of clorgyline but not by deprenyl or milacemide, indicating that substrate specificity was maintained at the drug doses employed. We conclude that different MAO substrates mediate different aspects of vocal and nonvocal behavior in adult male squirrel monkeys.

Modification of alpha-2 presynaptic receptor activity and catecholamine release following chronic MAO inhibition

Neuronal release of noradrenaline from the isolated rat vas deferens has been studied following long-term MAO inhibition. Fractional release of 3H-noradrenaline in response to electrical field stimulation was reduced following chronic clorgyline treatment, but yohimbine equalised the fractional release to that of control animals. Endogenous release of noradrenaline was increased 1.8 fold in yohimbine-treated tissues from rats treated chronically with clorgyline, while tissue noradrenaline content increased 2.3 fold. Electrophysiological activity measured from the renal nerve of anaesthetised rats decreased following acute treatment with clorgyline. This effect was absent following pretreatment with yohimbine. From isolated tissue studies, there was no conclusive evidence for down-regulation of alpha 2-presynaptic receptors by long-term MAO inhibition. Acute MAO inhibition with clorgyline produces distinct physiological effects in the CNS, but not in peripheral sympathetic nerves.

Acute and long-term regulation of brain alpha 2-adrenoceptors after manipulation of noradrenergic transmission in the rat

The specific binding of [3H]clonidine (KD and Bmax) to rat brain membranes was used as a biochemical index to directly evaluate alpha 2-adrenoceptor changes after manipulation of synaptic noradrenaline (NA) pools or stimulation or blockade of the receptor. Acute (2 h) and prolonged (7 days) inhibition of NA synthesis with alpha-methyl-p-tyrosine (150 mg/kg) or acute (2 h) and chronic (14 days) treatment with reserpine (0.1-0.5 mg/kg) reduced the NA content by 15-90%, which also resulted in marked reductions (35-55%) of the KD values for [3H]clonidine in all brain regions studied. In contrast to alpha-methyl-p-tyrosine, chronic reserpine treatment did not alter the Bmax values for [3H]clonidine or [3H]UK 14304 in any brain region. In the hypothalamus and cerebral cortex, acute (2 h) and chronic (7-14 days) treatment with the monoamine oxidase (MAO) inhibitors clorgyline (1 mg/kg) or tranylcypromine (5 mg/kg) increased the content of NA by 6-100%, which led to marked reductions (20-50%) of Bmax without altering the KD values for [3H]clonidine. Similarly, prolonged (21 days) inhibition of NA neuronal uptake with cocaine or protriptyline (10 mg/kg) also resulted in decreases in Bmax (20-25%) with no alterations in KD in the hypothalamus. In various brain regions, chronic (14 days) but not short-term (1 day) treatment with clonidine (0.1 mg/kg) or yohimbine (10 mg/kg) resulted in decreases (30-40%) and increases (15-20%), respectively, in Bmax without altering the KD values for [3H]clonidine. The results indicate that drugs which deplete endogenous NA up-regulate alpha 2-adrenoceptors (increased affinity of [3H]clonidine binding sites) while drugs which increase the intraneuronal and/or synaptic NA pools down-regulate the receptors (decreased number of [3H]clonidine binding sites). These adaptive receptor changes appear to be dependent on NA availability.

Rapid down regulation of beta-adrenoceptors by co-administration of desipramine and fluoxetine

Co-administration of desipramine and fluoxetine resulted in a 27% decline in cerebral cortical beta-adrenoceptor density after four days - a time point at which neither agent alone was effective. After 14 days, desipramine- and desipramine + fluoxetine-treated rats showed decreased receptor levels, with a greater decrement seen with the combined treatment. Fluoxetine, alone, had no affect on beta-adrenoceptor density at any time point examined. These effects are attributable to central serotonergic action since they were prevented by prior treatment with p-chlorophenylalanine. Cyproheptadine, a 5-HT2 antagonist, did not block these effects. Independent administration of fluoxetine and desipramine produced approximately 20% decrement in isoproterenol-stimulated cyclic AMP accumulation after four days of treatment. Co-administration of desipramine and fluoxetine resulted in a 35% decrement in cyclic AMP accumulation which was nearly additive with that produced by either drug alone. Consequently, the combination of a norepinephrine and serotonin uptake inhibitor may be an advantageous and rapid treatment for the alleviation of certain forms of depression.

Relation between brain monoamine oxidase (MAO) activity and the firing rate of locus coeruleus neurons

Utilizing a specific "low substrate concentration technique", intrasynaptosomal MAO-A and MAO-B activities within the rat brain noradrenaline system were studied. It was found that mainly MAO-A was localized intrasynaptosomally, whereas MAO-B contributed with less than 15% of the total intrasynaptosomal MAO activity, a phenomenon that was also observed within the central dopamine system. It is suggested that the intrasynaptosomal pool of MAO in the noradrenaline and the dopamine systems may reflect the density of innervation of the respective system throughout the brain. In addition, the effects of various selective monoamine oxidase (MAO) inhibitors on the noradrenergic intrasynaptosomal MAO activity as well as on the neuronal firing rate of noradrenaline containing cells in the locus coeruleus (LC) were investigated. Pretreatment with the MAO-A selective inhibitors clorgyline (10 mg/kg, i.p., 1 h) or (+)-FLA 336 (1 mg/kg, i.p., 1 h) caused a significant depression (40%) of mean spontaneous firing rate of LC neurones, randomly encountered throughout the LC. The MAO-B selective inhibitor pargyline (10 mg/kg, i.p., 1 h) was found to lack effect in this regard. However, pretreatment with (-)-deprenyl (10 mg/kg, i.p., 1 h), equally a selective MAO-B inhibitor, markedly suppressed the spontaneous firing rate of LC units. This inhibition by (-)-deprenyl was blocked by pretreatment with SK&F 525 A (50 mg/kg, i.p., 30 min), an inhibitor of microsomal drug metabolizing enzymes. Thus, the depression of LC units by (-)-deprenyl seems to be executed by a metabolite, e.g. l-amphetamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Clorgyline and desipramine alter the sensitivity of [3H]noradrenaline release to calcium but not to clonidine

Synaptosomes (P2) were prepared from cerebral cortices of control rats and from those which had received clorgyline (1 mg/kg/day for 21-28 days) or desipramine (10 mg/kg/day for 21-28 days). Following incubation with [3H]noradrenaline (500 nM/15 min, 37 degrees C), aliquots of the synaptosomes were gently filtered onto Whatman GF/A filters and superfused with Krebs buffer (pH 7.5, 37 degrees C) for a maximum period of 2 h. During this time, the basal efflux of tritiated materials (approximately 75% noradrenaline) together with K+-evoked release of the amine and metabolites, were measured. Chronic antidepressant drug regimens increased the K+-stimulated release, but its attenuation by clonidine was not altered. Thus, chronic antidepressant drug regimens do not apparently alter presynaptic alpha 2-adrenoceptors. These results suggest that the reported antidepressant drug induced decreases in [3H]clonidine binding, occur on sites which are postsynaptic to noradrenergic neurones. Following the chronic antidepressant drug regimens, the sensitivity of the [3H]noradrenaline release process to Ca2+ is significantly increased. This change may explain the enhanced K+-evoked release which follows the antidepressant drug regimens. It is proposed that this increased sensitivity of the [3H]noradrenaline release process may be an adaptation to the decrease in neuronal firing which have been reported following antidepressant drug treatments.