Reduced peripheral presynaptic adrenoceptor sensitivity following chronic antidepressant treatment in rats

Neuropharmacological profile of a selective sigma ligand, igmesine: a potential antidepressant

Igmesine is a selective sigma (sigma(1)) ligand that was reported to exert antidepressant action through an unknown mechanism of action. A number of neurochemical measures were taken in this study in efforts to understand its mode of action. Following 21-day drug treatments, the actions of igmesine on a number of neurochemical measures were investigated. Data obtained showed significant decreases in the densities of beta-adrenergic but not 5-HT(1A), sigma(1) and GABA(B) receptors in fluoxetine (18%), desipramine (DMI, 32%) and igmesine (20%)-treated groups when compared with control. Tyrosine hydroxylase (TH) activity was significantly (30-32%) reduced in all treated groups. Further, fluoxetine and DMI excluding the igmesine-treated groups showed 85 and 40% reductions in serotonin (5-HT) and noradrenaline (NE) neuronal uptake, respectively. Following acute treatment, igmesine lacked activity for monoamine oxidase (MAO) A or B (IC(50)>10 microM). In in vivo studies, at behaviorally active doses, igmesine showed weak effects on the NE uptake but lacked activity in altering 5-HT and DA synthesis or antagonizing selective drug-induced depletion of monoamine neuronal uptake. N-methyl-D-aspartate (NMDA)-induced increases in cGMP was blocked by igmesine indicating that igmesine may interfere with the NMDA receptor/nitric oxide synthase/cGMP pathway. Although it appears that part of the pharmacological actions of igmesine is mediated by the monoaminergic system, there is still need to explore other possible mechanisms of antidepressant action.

Effect of the selective noradrenergic reuptake inhibitor reboxetine on the firing activity of noradrenaline and serotonin neurons

Reboxetine is a non-tricyclic antidepressant with selective noradrenergic (NA) reuptake-blocking effects. The effects of acute and sustained administration of reboxetine, on the firing activity of locus coeruleus NA neurons and dorsal raphe 5-HT neurons, were assessed using in vivo extracellular unitary recording in rats anaesthetized with chloral hydrate. Reboxetine (0.1-1.25 mg/kg, i.v.) dose-dependently decreased the firing activity of NA neurons (ED50 = 480 +/- 14 microg/kg). A 2-day treatment with reboxetine at 1.25, 2.5, 5, or 10 mg/kg per day (using osmotic minipumps implanted subcutaneously) produced significant decreases of 52%, 68%, 81%, and 83%, respectively, of NA firing activity. When the reboxetine treatment (2.5 mg/kg per day) duration was prolonged to 7 days, a 66% decrease in NA firing activity was observed which further decreased to 80% after 21 days of treatment. In contrast, 5-HT neuron firing rate remained unaltered following short- and long-term reboxetine treatments. The suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of NA neurons was unchanged in long-term reboxetine-treated rats, but its effect on the firing activity of 5-HT neurons was blunted. The enhancement of NA firing activity by the 5-HT1A agonist 8-OH-DPAT was abolished in long-term reboxetine-treated rats, whereas, the inhibitory effect of the 5-HT2 agonist DOI was attenuated by about three-fold. In conclusion, sustained NA reuptake blockade by reboxetine lead to profound alterations in the function of NA neurons and of 5-HT receptors modulating their firing activity.

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.

Modelling the changes induced by chronic desipramine treatment on the factors governing the agonism at prejunctional alpha 2-adrenoceptors

1. The adaptational changes induced after chronic desipramine treatment on functional responsiveness of alpha 2-adrenoceptor activation were investigated in prostatic portions of the rat vas deferens. 2. For this purpose, clonidine and xylazine were studied for their effects on twitch contractions elicited by electrical field stimulation of prostatic portions removed 48 h after the last injection to the animals of vehicle or desipramine (10 mg kg-1, i.p.; 14 days). Operational model-fitting and the nested hyperbolic method were used to analyse the effects of irreversible receptor alkylation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 300 nM) on the alpha 2-adrenoceptor-mediated effects of clonidine, either in vehicle- or in desipramine-treated animals. 3. Treatment with desipramine decreased the potency (increased the EC50) of clonidine and xylazine by about 12 and 9 fold respectively. However, the treatment did not modify the maximal effect (alpha) elicited by either agonist. The estimates of apparent affinity for clonidine did not depend on the method of calculation as the 'null' method and the 'operational' method gave similar answers. Estimates of tau values for both agonists revealed that chronic desipramine treatment resulted in significant decreases in the efficacy of agonists. However, desipramine treatment was not associated with significant changes in the affinity constant for clonidine while for xylazine, the operational model provided a higher estimate of KA (lower affinity) after desipramine treatment. 4. The results indicate a large receptor reserve at prejunctional alpha 2-adrenoceptors which is modulated by chronic desipramine treatment. 5. The comparison of results obtained after chronic desipramine exposure with those by using EEDQ suggests that chronic desipramine treatment is not a useful experimental intervention for the purpose of estimating agonist affinities and efficacies.

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Effect of desipramine-induced blockade of neuronal uptake mechanisms on adrenoceptor-mediated responses in the guinea-pig colon

In order to clarify whether adrenoceptors in the guinea-pig distal colon are under sympathetic control, we assessed possible variations in the sensitivity to adrenoceptor agonists after blockade of neuronal catecholamine uptake mechanisms by desipramine (DMI). First, experiments were carried out to investigate the effects of DMI added in the organ bath on propulsion velocity, endogenous and [3H] prelabelled acetylcholine overflow, electrically evoked noradrenaline overflow and longitudinal smooth muscle tone. Secondly, we studied the effects of adrenoceptor agonists on the above parameters in untreated animals and in animals chronically treated with DMI. DMI added in the organ bath at concentrations equal to or higher than 30 nM inhibited all the parameters under study. Thus, when evaluating the effect of DMI on concentration-response curves to adrenoceptor agonists, concentrations which were per se inactive were used. DMI added in the organ bath at concentrations up to 30 nM potentiated the inhibitory effects of exogenous noradrenaline on propulsion velocity and acetylcholine overflow, but it did not affect the concentration-response curve to exogenous noradrenaline on longitudinal smooth muscle tone. Furthermore, 30 nM DMI inhibited propulsion velocity during sympathetic nerve stimulation. In preparations obtained from animals chronically treated with DMI, no significant change of propulsion velocity, endogenous and [3H] prelabelled acetylcholine overflow was found with respect to untreated animals. Nevertheless, in such preparations subsensitivity to isoprenaline (acting mainly on muscular beta-adrenoceptors) and clonidine (acting on neuronal alpha 2-adrenoceptors) and super-sensitivity to phenylephrine were observed. Electrically evoked noradrenaline overflow was enhanced, in a frequency-dependent way, by yohimbine and inhibited by clonidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Antidepressant treatment and chemical sympathectomy fail to modulate alpha 1-adrenoceptor sensitivity in mouse eye

The mydriatic response to alpha 1-adrenergic agonists was used as a functional index of postsynaptic alpha 1-adrenoceptors in mouse iris dilator muscle. Topical ocular application of methoxamine or phenylephrine caused dose-related mydriasis which was inhibited by pretreatment with prazosin or phentolamine. Chemical sympathectomy with topical 6-hydroxydopamine (6-OHDA) produced supersensitivity to phenylephrine but not methoxamine. Daily antidepressant treatment for 14 days with desipramine (10 mg/kg, i.p.), amitriptyline (10 mg/kg, i.p.), fluoxetine (2 mg/kg, i.p.), or moclobemide (40 mg/kg, i.p.) did not alter the response to methoxamine. Central alpha 1-adrenoceptors labelled with [3H]prazosin were similarly unaffected except for a modest downregulation produced by fluoxetine. These results demonstrate that postsynaptic alpha 1-adrenoceptors in mouse CNS and iris dilator muscle are refractory to manipulations known to alter their sensitivity in other tissues.

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)

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.

Effects of chronic antidepressant treatment on alpha 1- and alpha 2-adrenoceptors in the rat anococcygeus muscle

The effects of a single administration (48 hours) and of chronic (14 days) treatment with tricyclic (desipramine, nortryptiline) and nontricyclic (mianserin, nomifensine) antidepressant drugs on responses of the isolated anococcygeus muscle to the alpha 2-adrenoceptor agonist xylazine (inhibition of contraction to field stimulation at 1 Hz) and to the alpha 1-adrenoceptor agonist phenylephrine (contraction of the muscle) have been studied. Of the drugs used only desipramine and nortryptiline administered chronically reduced the responsiveness of the anococcygeus muscle to phenylephrine suggesting a desensitization of postsynaptic alpha 1-adrenoceptors. Long-term but not acute administration of antidepressants resulted in significant decrease in sensitivity of presynaptic alpha 2-adrenoceptors to xylazine. These results show that the adaptative changes of alpha-adrenoceptors in the rat anococcygeus muscle following long-term administration may depend on the efficiency to inhibit the neuronal uptake and the ability to antagonize alpha 1-adrenoceptors.

Modulation by antidepressant drugs of CNS postsynaptic alpha 2-adrenoceptors mediating mydriasis in the rat

The modulation of central postsynaptic alpha 2-adrenoceptors mediating mydriasis in the pentobarbitone-anaesthetized rat was studied after the acute and short/long-term administration of antidepressant treatments (drugs, electroshock). The acute administration of cyclic antidepressant drugs (2.5 mg/kg, i.v.) resulted in different mydriatic effects (amitriptyline greater than protriptyline approximately imipramine greater than clomipramine greater than nortriptyline greater than desipramine approximately maprotiline) which were attenuated (17-55%) by idazoxan (1 mg/kg, i.v., 5 min) and reserpine (5 mg/kg, s.c., 18 h) indicating that, besides the well-known anticholinergic properties of some of these drugs, their mydriatic effects are due in part to activation of alpha 2-adrenoceptors (through endogenous noradrenaline). In contrast, the long-term (7-21 days) but not the short-term (1-4 days) administration of cyclic antidepressant drugs (2.5-10 mg/kg, i.p.), MAO inhibitors (1 mg/kg, i.p.), lithium (20 mg/kg, i.p.) and electroshock (150 mA, 63 Hz, 8 ms during 300 ms) resulted in dose- and time-dependent reductions of the dose-pupillary response curve for clonidine (ED50 increased 1.2-2.0-fold; Emax decreased by 9-29%) which indicated desensitization of postsynaptic alpha 2-adrenoceptors. In line with these findings, treatment for 7 days with clonidine (0.1-1 mg/kg, i.p.) or idazoxan (3-10 mg/kg, i.p.) led to an opposite modulation (down- and up-regulation) of the dose-pupillary response curve for clonidine. The main results demonstrate that cyclic antidepressant drugs, through indirect mechanisms which involve endogenous noradrenaline, can modulate the sensitivity of brain postsynaptic alpha 2-adrenoceptors mediating mydriasis in the rat.

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.

Decreased gamma-aminobutyric acid (GABA) modulatory effect on rat vas deferens neurotransmission after chronic administration of imipramine

1. Chronic (10 mg/kg, i.p., once daily for 14 days) but not acute (10 mg/kg, i.p., 24 hr) administration of imipramine resulted in a decrease in both the responsiveness and the sensitivity of the contractions of the isolated rat vas deferens elicited by field stimulation to GABA and (-)-baclofen. 2. In contrast, clonidine and isoproterenol effects were not altered by either treatment. 3. This study shows for the first time that GABA action in the peripheral nervous system is altered by chronic treatment with antidepressants, possibly by inducing changes in a postreceptor element.

Effects of monoamine oxidase inhibitors on levels of catechols and homovanillic acid in striatum and plasma

Levels of homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and dihydroxyphenylglycol (DHPG) in plasma and the striatium were measured after inhibition of monoamine oxidase type A (MAO-A) by clorgyline (4 mg/kg i.p.), MAO-B by (-)deprenyl (1 mg/kg i.p.), both MAO-A and MAO-B by nialamide (75 mg/kg i.p.) or peripheral neuronal MAO by debrisoquin (40 mg/kg i.p.). Levels of HVA in plasma decreased by about 60% after single doses of nialamide or clorgyline, by about 80% after repeated doses of nialamide, by about 40% after a single dose of debrisoquin and by about 50% after repeated doses of debrisoquin. The administration of clorgyline, nialamide or debrisoquin significantly decreased concentrations of DOPAC and DHPG in plasma, whereas (-)deprenyl did not affect levels of DHPG or HVA. None of the MAO inhibitors produced more than about 80% depression of levels of any of the deaminated metabolites. The results suggest that most of the HVA in plasma is derived from deamination of DA by MAO-A in peripheral neurons; that DOPAC in plasma is derived from cells outside the central nervous system; that DHPG in plasma is derived virtually exclusively from the metabolism of norepinephrine in sympathetic nerve endings and that residual levels of HVA after treatment with debrisoquin provide an improved but limited indication of central dopaminergic activity.

Modulation of noradrenaline release from cat cerebral arteries by presynaptic alpha 2-adrenoceptors. Effect of chronic treatment with desipramine and cocaine

1. Field electrical stimulation elicited an increase of the tritium efflux over the basal level from cat cerebral arteries previously incubated with (+/-) [3H]noradrenaline ([3H]NA). 2. This efflux was: (a) reduced by clonidine, NA or B-HT 920; (b) unaffected by methoxamine, prazosin and yohimbine (10(-6) M); (c) reduced by yohimbine (5 x 10(-6) M), and (d) increased by phentolamine. 3. The effect of clonidine was blocked by yohimbine. 4. The daily treatment with the neuronal uptake blockers desipramine (10 mg/kg, i.p.) or cocaine (10 mg/kg, i.p.) [during 12 days], antagonized the inhibitory action of clonidine totally or partially, respectively. 5. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors in these arteries, which modulate the NA release, and (2) that chronic treatment with desipramine or cocaine induces a subsensitivity of these alpha 2-receptors, which facilitates the NA release.

Chronic inhibition of monoamine oxidase reduces noradrenaline release in rat vas deferens and anococcygeus muscle

Rats were treated once (acute) or once daily for 21 days (chronic) with clorgyline (2 mg/kg) or nialamide (50 mg/kg). (-)Deprenyl (1 mg/kg) was given for 21 days. One day after the last injection, vas deferens and anococcygeus muscles were removed and noradrenaline stores labelled with 3H-noradrenaline. Efflux of total tritium following electrical field stimulation was decreased by both acute and chronic treatment with clorgyline and nialamide, as well as by chronic treatment with deprenyl. Total tritium release from anococcygeus muscle was reduced by both acute and chronic treatment with clorgyline. Fractionation of the effluent showed that release of both free noradrenaline and metabolites was decreased by MAO inhibitor treatment in vivo, but this effect was not reproduced by in vitro incubation of the vas deferens with clorgyline (1 microM). By contrast to the effect of electrical field stimulation, release of 3H-noradrenaline induced by veratrine was increased by chronic treatment with both clorgyline and nialamide. Release of total tritium by depolarising concentrations of KCl was also increased by chronic clorgyline treatment. These results could be explained by a proportionally greater release of tritium from a cytoplasmic compartment following veratrine and KCl than electrical stimulation, since MAO inhibition increases cytoplasmic noradrenaline levels. Alternatively, release by electrical stimulation may be affected to a greater extent by presynaptic alpha 2-adrenoceptors, and presynaptic receptors may be stimulated by increased synaptic levels of free noradrenaline following MAO inhibition.

Chronic clonidine treatment produces desensitisation of post- but not presynaptic alpha 2-adrenoceptors

Rats were treated chronically with clonidine by osmotic minipump (500 micrograms/kg daily for 14 days). One day after the end of the infusion period, the animals were pithed, and alpha 2-presynaptic and -postsynaptic adrenoceptor responses evaluated using inhibition of cardioaccelerator response to sympathetic stimulation by guanabenz, and pressor response to guanabenz respectively. Chronic clonidine treatment had no effect on the presynaptic effect of guanabenz, but the postsynaptic response to this agonist was decreased. In a separate group of pithed rats treated chronically with clonidine, there was no alteration in the increment in plasma noradrenaline concentration produced by sympathetic stimulation, and no alteration in the yohimbine-induced enhancement of plasma noradrenaline response to sympathetic stimulation. These data argue against desensitisation of peripheral presynaptic alpha 2-adrenoceptors by chronic administration of alpha 2-adrenoceptor agonist. Sympathetic hyperactivity following withdrawal of chronic clonidine treatment may be mediated by down-regulation of postsynaptic alpha 2-adrenoceptors in the CNS.

Effect of chronic antidepressant treatment on noradrenergic modulation of [3H]dopamine release from rat nucleus accumbens and striatal slices

Unlike desipramine, a potent blocker of noradrenaline uptake, the antidepressant drugs, mianserin and citalopram did not, after chronic administration (28 days), attenuate the clonidine-induced inhibition of [3H]dopamine (DA) release from rat nucleus accumbens slices. Mianserin, like desipramine, failed to alter the isoproterenol-induced enhancement of [3H]DA release from rat nucleus accumbens and striatal slices. These findings suggest that antidepressant action does not necessarily involve desensitization of central alpha 2- or beta-adrenoceptors which influence neuronal transmission.

Inhibition of presynaptic alpha-2-adrenoceptor and opioid receptor agonist responses in the rat vas deferens by chronic imipramine treatment

The effects of chronic imipramine administration on agonist responses in rat isolated smooth muscle preparations were investigated. The administration of 20 mg/kg imipramine two times a day for 4 and 11 days resulted in an equivalent subsensitivity (approximately 8-fold) of clonidine-induced inhibition of electrically evoked contractions in the rat vas deferens (presynaptic alpha 2-adrenoceptor response). Imipramine (4 days) resulted in a marked inhibition of the ability of [D-Ala2, D-Leu5] enkephalin to decrease electrically evoked contractions of the vas deferens (presynaptic opioid receptor response) but did not significantly affect the carbachol-induced increase in electrically evoked contractions (muscarinic receptor response). In the absence of cocaine the contractile effects of norepinephrine and tyramine in the vas deferens were, respectively, potentiated and inhibited, following imipramine (4 days), suggesting a decrease in the activity of the neuronal uptake mechanism. When determined in the presence of cocaine, the potency of the postsynaptic effects of norepinephrine in the vas deferens (alpha 1-adrenoceptor response) was not significantly altered by imipramine (4 days). With regard to other postsynaptic receptors, imipramine (4 days) decreased slightly the potency of phenylephrine in the aorta (alpha 1-adrenoceptor response) and increased slightly the potency of carbachol in the trachea (muscarinic receptor response) and the potency of serotonin in the rat aorta (5HT2-receptor response). Thus, chronic imipramine administration decreased the potency of presynaptic alpha 2- and opioid agonist responses in the vas deferens but caused very little or no changes in the potencies of agonists at postsynaptic sites.

Activation and desensitization of presynaptic alpha 2-adrenoceptors after inhibition of neuronal uptake by antidepressant drugs in the rat vas deferens

The isolated field-stimulated vas deferens of the rat (0.1 Hz, 3 ms, 30-40 V) was used to study the relationship between the in vivo inhibition of neuronal uptake of noradrenaline (NA) by cyclic antidepressant drugs and the subsequent activation/desensitization of presynaptic alpha 2-adrenoceptors. Receptor activation was indirectly measured by quantifying the ability of each drug to inhibit basal twitch responses after their acute administration. Receptor desensitization was also indirectly measured by quantifying the ability of the drugs to reduce the inhibitory effects of selective alpha 2-adrenoceptor agonists on the electrically-induced twitch responses after their long-term administration. The acute in vivo administration of desipramine and other antidepressants (0.5-10 mg kg-1; i.p.; 2 h) resulted in dose-dependent inhibitions of the basal twitch responses which were rapidly reversed to control values by idazoxan (10-5 M). In vitro, desipramine and other antidepressants also inhibited in a concentration-dependent manner (10(-9)-10(-5) M) the twitch responses. In rats pretreated 12 h earlier with reserpine (1 mg kg-1; i.p.) or oxypertine (4 mg kg-1; i.p.), desipramine (10 mg kg-1; 2 h) did not induce inhibition of the basal twitch responses or it induced a smaller effect, respectively. For the various antidepressants the degree of inhibition of the basal twitch responses (desipramine greater than protriptyline greater than nortriptyline greater than maprotiline = imipramine greater than amitriptyline greater than viloxazine greater than iprindole much greater than zimelidine) was highly correlated (r = 0.914) with the potency for blockade of [3H]-NA uptake into rat brain synaptosomes. Clonidine and xylazine inhibited in a concentration-dependent manner (10(-9)-10(-6) M) the twitch responses. The long-term (7-14 days) administration of antidepressants or cocaine (10 mg kg-1, i.p.) resulted in significant decreases in sensitivity to clonidine or xylazine. Short-term (3 days) treatment with desipramine did not reduce the sensitivity to clonidine. The results indicate that the acute in vivo inhibition of NA neuronal uptake by antidepressants leads to the activation (through endogenous NA) of presynaptic inhibitory alpha 2-adrenoceptors which results in inhibition of the twitch responses. In contrast, prolonged in vivo inhibition of NA reuptake is followed by a slow desensitization process of the same receptors which results in a reduction of sensitivity to clonidine.

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

Plasma catecholamine levels were determined in pithed rats during electrical stimulation of the entire sympathetic nervous system. In animals treated chronically with clorgyline (1 mg/kg daily for 21 days) the increment in plasma norepinephrine concentration during stimulation was greater than in control animals, whereas a single dose of clorgyline (2 mg/kg 2 h before pithing), which produced the same degree of inhibition of arterial MAO type A and a similar increase in arterial norepinephrine content, had no effect on the plasma norepinephrine response to stimulation. Injection of yohimbine (1 mg/kg) produced the same degree of enhancement of plasma norepinephrine response to stimulation in chronically treated and control animals, showing that the overall gain of the alpha 2-adrenoceptor inhibitory loop in vascular sympathetic nerves was not affected. Plasma epinephrine concentration during electrical stimulation was also increased by chronic but not by acute clorgyline treatment. Chronic clorgyline treatment did not significantly affect the total systemic metabolic clearance rate of infused norepinephrine, thus the increased plasma norepinephrine response to stimulation reflects an increased release rate from sympathetic neurons. In rats treated chronically with clorgyline, the pressor response to norepinephrine in the presence of yohimbine (0.3 mg/kg) was significantly reduced, whereas the pressor response to guanabenz was unchanged. There was also no change in the guanabenz-induced inhibition of the tachycardic response to electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)