Chronic clorgyline treatment enhances release of norepinephrine following sympathetic stimulation in the rat

Differential modulation of alpha2-adrenoceptor subtypes in rat kidney by chronic desipramine treatment

The profile of [3H]RX821002 (2-methoxy idazoxan) binding to alpha2-adrenoceptor subtypes in rat kidney membranes was evaluated in controls and after chronic treatment with desipramine (10 mg/kg, i.p., every 12 h, 7 days) or clorgyline (2 mg/kg, i.p., every 24 h, 21 days). [3H]RX821002 recognized with high affinity (Kd=1.5+/-0.2 nM in controls) a single and saturable population of binding sites (Bmax=57+/-5 fmol/mg protein in controls). The competitions by (-)-adrenaline, the alpha2B-adrenoceptor selective drug ARC239 (2-[2-[4-(o-methoxyphenyl)-piperazin-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolindione) and the alpha2A-adrenoceptor selective drug BRL44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidaz ole) suggested the existence of both alpha2A- and alpha2B-adrenoceptors together with a non-adrenoceptor binding site. After chronic desipramine but not after chronic clorgyline treatments, the density (Bmax) of alpha2-adrenoceptors was increased (46%). In the presence of ARC239 (50 nM), the density of alpha2A-adrenoceptors increased (44%) in the desipramine-treated group without changes in the clorgyline-treated group. Conversely, in the presence of BRL44408 (100 nM), the density of alpha2B-adrenoceptors was not affected by the treatments. The selective upregulation of the alpha2A-adrenoceptor subtype following chronic desipramine administration is compatible with a differential location and function of the alpha2-adrenoceptor subtypes in the rat kidney.

The contribution by monoamine oxidase and catechol-O-methyltransferase to the total-body and pulmonary plasma clearance of catecholamines

To study the effects of inhibition of catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) on the removal of circulating catecholamines, anaesthetized rabbits were infused for 120 min with 3H-labelled noradrenaline, adrenaline and dopamine. Total-body plasma clearances (Cltot) and pulmonary fractional extractions (ERp) of the infused amines and the cardiac output of plasma (CO(p)) were determined under steady-state conditions at the end of each of two consecutive 60-min treatment periods. MAO and COMT were inhibited by treatment with pargyline (40 mg/kg) and tolcapone (3 mg/kg followed by 1.5 mg/kg given every 30 min), respectively. Two groups of animals were studied. Group I involved animals treated with tolcapone throughout and given pargyline at the beginning of the second treatment period. In group II, pargyline was given at the beginning of the first, and the treatment with tolcapone was started at the beginning of the second treatment period. As previous experiments had shown that COMT inhibition alone is without any effect on Cltot of the three catecholamines considered here, the results obtained in the first treatment period of group I can be taken to reflect control results. At the end of the first treatment period, Cltot of noradrenaline, adrenaline and dopamine (expressed as a percentage of CO(p)) was 88%, 85% and 142%, respectively, in group I (COMT inhibition) and 67%, 77% and 115%, respectively, in group II (MAO inhibition; P < 0.05 for the group difference regarding Cltot of noradrenaline and dopamine). MAO inhibition on top of COMT inhibition (group I) lowered Cltot of noradrenaline, adrenaline and dopamine by 23%, 12% and 26%, respectively, and COMT inhibition on top of MAO inhibition (group II) reduced Cltot of these catecholamines by 13%, 20% and 17%, respectively. At the end of the first treatment period, the pulmonary plasma clearance (Clp = ERp x CO(p)) of noradrenaline and dopamine was 13 and 25 ml kg-1 min-1, respectively, in group I and 12 and 28 ml kg-1 min-1, respectively, in group II. Clp of adrenaline did not differ from zero in either group. Clp of noradrenaline and dopamine was reduced by 74% and 70%, respectively, when both enzymes were inhibited in group I and by 70% and 67%, respectively, when both enzymes were inhibited in group II. Hence, inhibition of either MAO or COMT alone had little, if any, effect on the removal of noradrenaline, adrenaline and dopamine on passage through the systemic and pulmonary circulation. Combined inhibition of both MAO and COMT was highly effective in reducing the pulmonary clearance of noradrenaline and dopamine, but produced only minor decreases in the total-body clearance of all three catecholamines.

Enhancement of morphine analgesia by the alpha 2-adrenergic antagonist yohimbine

Although interactions between opioids and adrenergic agonists in the treatment of pain have been demonstrated in humans, the contribution of specific adrenergic receptors in this interaction remains to be clarified. In a double-blind, placebo-controlled study in male patients with postoperative dental pain, we investigated the effect of preoperative administration of the alpha 2-adrenergic antagonist, yohimbine, on analgesia produced by postoperative intravenous morphine. Although yohimbine by itself did not affect the pain, the overall analgesic effect of morphine was significantly enhanced in the presence of yohimbine. This report is the first to demonstrate that an alpha 2-adrenergic antagonist enhances opiate analgesia in humans.

Pharmacology of reversible and selective inhibitors of monoamine oxidase type A

The concept of reversible type A monoamine oxidase (MAO-A) inhibitors as effective antidepressant drugs with a minimal side effect profile has been vindicated in practice. Despite this, the pharmacological basis for their actions is unclear. Studies with the irreversible inhibitor clorgyline have shown that chronic but not acute treatment of rats leads to a significant enhancement of noradrenaline release from peripheral sympathetic nerves and cerebral cortex together with a more effective inhibition of MAO-A, as shown by reduction in levels of deaminated metabolites in cortical microdialysis fluid. Reversible inhibitors, however, do not have a cumulative effect on MAO inhibition and may have different effects on noradrenaline release. Reversible inhibitors did not produce the acute reduction in sympathetic nerve activity seen with clorgyline, which may be one factor in explaining their milder side effect profile. Other aspects of the pharmacology of reversible and irreversible selective inhibitors of MAO-A are reviewed.

Modification of cerebral cortical noradrenaline release by chronic inhibition of MAO-A

Chronic treatment of rats with clorgyline (1 mg/kg i.p. daily for 21 days) caused a highly significant increase in the concentration of noradrenaline in microdialysate from the frontal cortex of the awake animal. Acute (one injection, 2 mg/kg) or subacute (1 mg/kg daily for 3 days) treatment did not lead to a significant increase in microdialysate noradrenaline. Concentrations of deaminated metabolites (DPHG, MHPG, DOPAC) in the microdialysate decreased with time of treatment, reaching a minimum after 21 days.

Chronic inhibition of monoamine oxidase type A increases noradrenaline release in rat frontal cortex

Chronic but not acute treatment of rats with MAO inhibitors, as with other antidepressant drugs, has been shown to down-regulate the number of cerebro-cortical beta-adrenoceptors. In order to establish whether this effect is associated with an increase in cortical noradrenaline release, rats were treated for 1, 3 or 21 days with clorgyline (2 mg/kg i.p. single injection; 1 mg/kg i.p. repeated injections), and the frontal cortex was then perfused by microdialysis in the awake animal. Control animals were injected with saline. The concentration of noradrenaline in the microdialysate increased only slightly after 1 or 3 days of clorgyline treatment but increased fourfold over control levels after 21 days treatment. Yohimbine (20 mumol/l) added to the perfusing solution caused a similar degree of enhancement in microdialysate noradrenaline concentration in all groups of rats. Tetrodotoxin (10 mumol/l) reduced noradrenaline concentration to low levels in all groups of animals, but noradrenaline was still detectable in the microdialysate in rats treated with clorgyline for 21 days. Concentrations of the deaminated metabolites dihydroxyphenylacetic acid, dihydroxyphenylglycol and methoxy-hydroxyphenylglycol were lowest after the 21 day clorgyline treatment. Determination of enzyme activity ex vivo showed that MAO-A was inhibited more than 95% by all clorgyline treatments with less than 10% inhibition of MAO-B. The results indicate that cerebrocortical noradrenaline release increases gradually during chronic MAO inhibition. This may be the result of more complete inhibition of the enzyme with time, not detectable by the ex vivo assay, but shown by the progressive reduction in metabolite levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in purinergic responses of the rabbit isolated central ear artery after chronic electrical stimulation in vivo

1. The effect of chronic (4-16 days) electrical stimulation (5 Hz, 0.3 ms, 4-10 V) of the great auricular nerve in vivo on sympathetic cotransmission in the rabbit isolated central ear artery was examined. 2. Chronic stimulation had no significant effect on frequency-dependent (4-60 Hz) neurogenic contractions or contractile responses induced by exogenous noradrenaline (0.1-300 microM). 3. In contrast, contractions induced by exogenous alpha, beta-methylene ATP (10.0 microM) were significantly decreased in preparations from 16-day stimulated animals in comparison with sham-operated, 4-day and 8-day chronically stimulated animal groups. 4. It is concluded that chronic electrical stimulation of nerves supplying the ear artery may lead to the selective alteration of postjunctional P2x-purinoceptor mechanisms, while the effects mediated by post-junctional alpha 1-adrenoceptors remain unchanged.

Effects of clorgyline on the release of noradrenaline from rat vas deferens

Release of endogenous noradrenaline (NA) and functional activity of presynaptic alpha 2-adrenoceptors were measured in isolated rat vas deferens following acute (2 mg/kg) or chronic (21 * 1 mg/kg daily, i.p.) treatment with clorgyline. Noradrenaline tissue content was higher in rats treated chronically with clorgyline than in those treated acutely. In vitro experiments done in the presence of 1 microM desipramine and 0.5 microM yohimbine showed that NA release, elicited by electrical field stimulation, was higher in chronic clorgyline rats than in controls, while no significant difference was found between acute clorgyline and control rats. Yohimbine enhanced evoked release of NA in all treatment groups, provided that desipramine was present in the Krebs solution, and the enhancement was non significantly higher in chronic clorgyline than in acute clorgyline and control rats. Efflux of 3,4-dihydroxyphenylglycol was lower in chronic clorgyline rats than in other groups. No difference was found between the treatment groups in a 1 min [3H]NA uptake into the tissue, nor in the ability of desipramine (1 microM) to block the uptake. The results indicate that following chronic treatment with clorgyline, evoked release of NA increased, and there was no reduction in the ability of the presynaptic alpha 2-adrenoceptor inhibitory mechanism to reduce nerve stimulation induced release of NA.

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)

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.