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

Relationship between central behavioral effects and peripheral sympathetic neurotransmission functionality during acute cocaine withdrawal syndrome in adult rats

BACKGROUND

Acute cocaine withdrawal syndrome (ACWS) is characterized as a set of organic alterations triggered by abrupt discontinuation of chronic cocaine consumption, usually occurring at 24-40 hours after withdrawal. However, little is known about the relationship between central and peripheral sympathetic neurotransmission during ACWS.

OBJECTIVE AND METHODS

We investigated the mechanisms involved in central and peripheral sympathetic neurotransmission and how ACWS affects the sympathetic functionality. Cocaine was administered twice daily for 5 days in Wistar rats (at least 5 in each group): on the first and second day, 15 mg/kg/i.p.; third day, 20 mg/kg/i.p.; and finally in the last two days, 30 mg/kg/i.p. Subsequently, at 1, 24, 48 and 120 h after cocaine administration the following experiments were done: (i) at the central level, behavioral tests of open-field and elevated plus maze; and (ii) at the peripheral level, tests of catecholamine release, function of α2-adrenergic receptors (α2-ARs), imidazoline receptors (I(1,2)-Rs), L-type voltage-gated (Ca(v1.2)) Ca(2+) channels and α1-ARs.

RESULTS

During ACWS, rats showed hypolocomotion and exacerbation of anxiogenic-effects 24 h after cocaine withdrawal. Likewise, a decrease in the catecholamine release and activity of α2-ARs/I(1,2)-Rs at 24-48 h after cocaine withdrawal was observed. A decrease in Ca(v1.2) channels and α1-ARs function at 48 h after cocaine withdrawal was observed.

CONCLUSIONS

The relationship of central and peripheral sympathetic neurotransmission during ACWS possibly due to a failure in activation and/or inactivation of presynaptic α2-ARs/I(1,2)-Rs, may offer a potential target for attenuating ACWS.

Differential modulation of α-1 adrenoceptor subtypes by antidepressants in the rat brain

The aim of the present study was to examine the effect of chronic antidepressants treatment on the density of α₁-adrenoceptor (AR) subtypes in rat brain. Density of total α₁ and α(1A)- and α(1Β)-ARs was measured in cortex and cerebellum of rats treated with amitriptyline (AMI), desipramine (DMI) and fluoxetine (FLX), (10 mg/kg body wt), for 30 days, using [³H]prazosin in presence and absence of WB-4101. The density of cortical total α₁-ARs was significantly decreased with AMI (54%) and DMI (25%) treatment, without altering the affinity of the receptor. Fluoxetine did not alter the density of cortical α₁-ARs. The density of cortical α(1A)-ARs was also significantly decreased with AMI (85%) and DMI (50%) treatment, without affecting the affinity. The density of cerebellar total α₁-ARs was significantly decreased with AMI (37%), DMI (50%) and FLX (70%) treatment, without affecting the affinity for [³H]prazosin. The density of α(1A)-ARs was significantly decreased with AMI (67%), DMI (59%) and FLX (92%) treatment. α(1B)-AR density was decreased only with FLX (47%) and DMI (47%) treatment. Correspondingly the basal IP3 and NE (10 μM) stimulated IP3 levels were significantly decreased in AMI (47%), DMI (22%) and FLX (48%) treated rat cortex. The results suggest that chronic antidepressant (AD) treatment down-regulates the cortical and cerebellar total α₁-ARs in rat brain. However, α(1A) subtype is predominantly down-regulated by AMI and DMI, where as FLX affects cerebellar α(1A)-ARs. The region-specific and subtype specific down-regulation of α₁-ARs density, which occurs after prolonged AD treatment, may underline the therapeutic mechanism of action.

The effect of lithium on the adrenoceptor-mediated second messenger system in the rat brain

OBJECTIVE

Lithium remains the most widely used treatment for bipolar disorder; however, the molecular mechanisms underlying its therapeutic actions have not been fully elucidated. We studied the in-vivo effect of lithium on the density of alpha-adrenoceptor (alpha-AR) and beta-AR subtypes and linked second messenger systems in the rat brain.

METHODS

The densities of alpha(1)-ARs, alpha(2)-ARs, and beta(1)-ARs and beta(2)-ARs in the cortex and cerebellum of rats treated with lithium (0.4%), orally, for 30 days were measured using [(3)H]prazosin, [(3)H]clonidine and [(3)H]CGP-12177, respectively. The activity of adenylyl cyclase (AC) and levels of inositol trisphosphate (IP3), both second messengers linked to these receptors, were estimated using [(3)H]ATP and [(3)H]myoinositol, respectively.

RESULTS

A significant decrease in the densities of cortical alpha(1)-ARs (85%, p < 0.0001), alpha(2)-ARs (50%, p < 0.0001), beta(1)-ARs (26%, p < 0.0001) and beta(2)-ARs (25%, p < 0.0001) was observed after lithium treatment. However, only the density of alpha(1)-ARs was significantly decreased (25%, p < 0.0001) in the cerebellum. The affinity of [(3)H]prazosin for cerebellar alpha(1)-ARs was increased. A small, but statistically significant, increase (19%, p < 0.0001) in the density of total beta-ARs was seen in the cerebellum, without altering the affinity of the radioligand for these receptors. Basal AC activity was not altered in the lithium-treated rat cortex. However, the norepinephrine-stimulated AC activity, which represents alpha(2)-AR-linked and beta-AR-linked AC, was significantly increased (66%, p < 0.0001). Both basal IP3 formation and norepinephrine-stimulated IP3, which represents alpha(1)-AR-linked phospholipase C activity, were significantly decreased (50%, p < 0.0001) in the lithium-treated rat cortex.

CONCLUSION

Our results suggest that long-term administration of lithium treatment downregulates the cortical, but not cerebellar, alpha(1)-ARs, alpha(2)-ARs, beta(1)-ARs and beta(2)-ARs. Thus, it may be concluded that lithium induces region-specific and differential functional downregulation of alpha-AR and beta-AR subtypes in the rat brain.

Increasing synaptic noradrenaline, serotonin and histamine enhances in vivo binding of phosphodiesterase-4 inhibitor (R)-[11C]rolipram in rat brain, lung and heart

Phosphodiesterase-4 (PDE4) is one of the main enzymes that specifically terminate the action of cAMP, thereby contributing to intracellular signaling following stimulation of various G protein-coupled receptors. PDE4 expression and activity are modulated by agents affecting cAMP levels. The selective PDE4 inhibitor (R)-rolipram labeled with C-11 was tested in vivo in rats to analyze changes in PDE4 levels following drug treatments that increase synaptic noradrenaline (NA), serotonin (5HT), histamine (HA) and dopamine (DA) levels. We hypothesized that increasing synaptic neurotransmitter levels and subsequent cAMP-mediated signaling would significantly enhance (R)-[(11)C]rolipram retention and specific binding to PDE4 in vivo. Pre-treatments were performed 3 h prior to tracer injection, and rats were sacrificed 45 min later. Biodistribution studies revealed a dose-dependent increase in (R)-[(11)C]rolipram uptake following administration of the monoamine oxidase (MAO) inhibitor tranylcypromine, NA and 5HT reuptake inhibitors (desipramine [DMI], maprotiline; and fluoxetine, sertraline, respectively), and the HA H(3) receptor antagonist (thioperamide), but not with DA transporter blockers GBR 12909, cocaine or DA D(1) agonist SKF81297. Significant increases in rat brain and heart reflect changes in PDE4 specific binding (total-non-specific binding [coinjection with saturating dose of (R)-rolipram]). These results demonstrate that acute treatments elevating synaptic NA, 5HT and HA, but not DA levels, significantly enhance (R)-[(11)C]rolipram binding. Use of (R)-[(11)C]rolipram and positron emission tomography as an index of PDE4 activity could provide insight into understanding disease states with altered NA, 5HT and HA concentrations.

Acute and long-term administration of citalopram desensitizes alpha2-adrenoceptors in the rat vas deferens

The aim of this study was to investigate the effect of citalopram, a selective serotonin reuptake inhibitor, on the sensitivity of rat vas deferens alpha2-adrenoceptors and to compare it with the effects of serotonin and the dual noradrenaline-serotonin uptake inhibitor duloxetine. To this end, we studied the inhibitory effect of the alpha2-adrenoceptor agonist bromoxidine on the electrically induced contraction of the vas deferens. Citalopram (1, 3 x 10(3) and 3 x 10(4) nM) applied in-vitro significantly attenuated the concentration-response inhibition induced by activation of alpha2-adrenoceptors on the electrically evoked contraction of the vas deferens (concentration of the agonist required to promote 50% of the maximal effect, EC50, for bromoxidine increased by 232%, 421% and 818%, respectively). Similarly, serotonin also attenuated the concentration-response inhibition mediated by presynaptic alpha2-adrenoceptors (96% increase in EC50). Acute and long-term systemic administration of citalopram and duloxetine also produced a loss in the sensitivity of alpha2-adrenoceptors to bromoxidine (EC50 for bromoxidine increased by 97% and 144%, respectively, after citalopram, and by 214% and 167% after duloxetine). In addition, we observed that an increased fraction of receptors was required to be occupied to yield 50% of the inhibitory effect of bromoxidine after long-term administration of citalopram and duloxetine (KE increased by 142% and 83%). These results are indicative of early-onset and persistent down-regulation of peripheral alpha2-adrenoceptors by citalopram, which may account for some of its side effects.

Effects of tramadol on α2-adrenergic receptors in the rat brain

In recent years, it has been postulated that tramadol, used mainly for the treatment of moderate to severe pain, might display a potential as an antidepressant drug. The present study investigated the effects of acute and repeated tramadol administration on the binding of [3H]RX 821002, a selective alpha2-adrenergic receptor ligand, in the rat brain. Male Wistar rats were used. Tramadol (20 mg/kg, i.p.) administered acutely (single dose), at 24 h after dosing, induced a significant decrease in the alpha2-adrenergic receptors in all brain regions studied. The most pronounced effects were observed in all subregions of the olfactory system, nucleus accumbens and septum, thalamus, hypothalamus, amygdala, and cerebral cortex. Repeated treatment with tramadol (20 mg/kg, i.p., once daily for 21 days) also induced statistically significant downregulation of [3H]RX 821002 binding sites in the rat brain. However, the effect--although statistically significant--was less pronounced than in the group treated acutely with the drug. Since drugs such as mianserin and mirtazapine are potent antagonists of central alpha2-adrenergic receptors and are effective antidepressants, it is tempting to suggest that, in addition to other alterations induced by tramadol, downregulation of these receptors may represent a potential antidepressant efficacy. On the other hand, one should be careful to avoid the treatment of chronic pain with tramadol in patients already receiving antidepressant drugs. Tramadol-induced downregulation of alpha2-adrenergic receptors--when combined with ongoing antidepressant therapy with drugs, which themselves inhibit serotonin reuptake or are antagonists of alpha2-adrenergic receptors--might cause threatening complications.

New insights into the mechanisms of antidepressant therapy

Depressive disorders are among the most frequent psychiatric diseases in the Western world with prevalence numbers between 9% and 18%. They are characterized by depressed mood, a diminished interest in pleasurable activities, feelings of worthlessness or inappropriate guilt, decrease in appetite and libido, insomnia, and recurrent thoughts of death or suicide. Among other findings, reduced activity of monoaminergic neurotransmission has been postulated to play a role in the pathogenesis of depression. Consistent with this hypothesis, most antidepressive drugs exert their action by elevating the concentration of monoamines in the synaptic cleft. However, it is not the enhancement of monoaminergic signaling per se, but rather long-term, adaptive changes that may underlie the therapeutic effect. These include functional and structural changes that are discussed later. In addition, in the last years, evidence has emerged that remissions induced in patients using lithium or electroconvulsive therapy are accompanied by structural changes in neuronal networks thereby affecting synaptic plasticity in various regions of the brain.

Cortical alpha-adrenoceptor downregulation by tricyclic antidepressants in the rat brain

The aim of the present study was to examine the effect of chronic tricyclic antidepressants (TCAs) treatment on the density of alpha-adrenoceptors in the rat brain. Density of alpha1- and alpha2-adrenoceptors was measured in cortex and hippocampus of rats treated with imipramine (IMI, 5mg/kg body weight), desipramine (DMI, 10mg/kg body weight), clomipramine (CMI, 10mg/kg body weight) and amitriptyline (AMI, 10mg/kg body weight), for 40 days, using [3H]prazosin and [3H]clonidine, respectively. The density of cortical alpha1-adrenoceptors was significantly decreased with IMI (46%), DMI (21%), CMI (50%) and AMI (67%) treatment, without altering the affinity of the receptor. The density of cortical alpha2-adrenoceptors was also significantly decreased with DMI (69%), CMI (81%) and AMI (80%) treatment, without affecting the affinity for [3H]clonidine. The density of hippocampal alpha1-adrenoceptors was significantly decreased only with AMI treatment (47%), without affecting the affinity for [3H]prazosin. However, no change in hippocampal alpha2-adrenoceptor density was observed with any of these TCAs. The results suggest that chronic antidepressant (AD) treatment downregulates the cortical, but not hippocampal, alpha1- and alpha2-adrenoceptors in rat brain. The region-specific downregulation of alpha1- and alpha2-adrenoceptors density, which occur after prolonged AD treatment, may underline the therapeutic mechanism of action.

Intact noradrenaline transporter is needed for the sympathetic fine-tuning of cytokine balance

Earlier studies demonstrated that cytokine production is under the tonic control of noradrenaline. As the level and/or the duration of noradrenaline action is regulated by the noradrenaline transporter (NET), which is also a target of antidepressant treatment, we studied its role in the regulation of the cytokine response during inflammation. The endotoxin-evoked tumour necrosis factor-alpha (TNF-alpha) and interleukin-10 response was studied in genetically produced noradrenaline transporter-deficient (NET-KO) mice and by treatment with desipramine, a monoamine uptake-blocker antidepressant. NET-KO mice responded to endotoxin with significantly lower TNF-alpha and interleukin-10 production in comparison to their wild-type counterparts. Functional involvement of both alpha- and beta-adrenoceptors could be demonstrated in our model systems, using 7,8-methylenedioxy-14 alpha-hydroxy-alloberbane.HCl (CH-38083) and propranolol; however, the differences between the two phenotypes remained, suggesting a limited role of alpha-adrenoceptors in the observed changes. Acute treatment of both wild-type and NET-KO mice with desipramine significantly decreased the TNF-alpha response and significantly increased interleukin-10 production, indicating the role of an intact noradrenaline transporter in anti-inflammatory responses.

COMPARISON OF PERIPHERAL INHIBITORY EFFECTS OF CLOMIPRAMINE WITH SELECTIVE SEROTONIN RE-UPTAKE INHIBITORS ON CONTRACTION OF VAS DEFERENS:

PURPOSE

We compared the peripheral inhibitory effects of the tricyclic antidepressant clomipramine with those of various selective serotonin re-uptake inhibitors on the contractile response of the vas deferens.

MATERIALS AND METHODS

The contractile responses of 17 circular smooth muscle strips of human vas deferens to 10-4 M. norepinephrine were observed in the absence and presence of clomipramine, and the selective serotonin re-uptake inhibitors fluoxetine, sertraline and paroxetine. The intraluminal pressure response of rat vas deferens to electrical stimulation of the hypogastric nerve was measured in 5 rats in the central plus peripheral effect group before and after the intravenous injection of 4.2 mg./kg. clomipramine or 8.3 mg./kg. sertraline. The pressure response to each agent was also observed after the transection of all proximal sympathetic input to the hypogastric nerve in 5 animals in the peripheral effect group.

RESULTS

Clomipramine was about 100-fold more potent than sertraline, fluoxetine or paroxetine for inhibiting the norepinephrine induced contraction of human vasal muscle strips. The inhibitory effect of sertraline on rat intravasal pressure in the peripheral effect group was significantly lower than in the central plus peripheral effect group (p <0.05), while no significant difference was noted in the 2 groups regarding clomipramine. The effect of clomipramine was significantly higher than that of sertraline in the central plus peripheral and peripheral effect groups (p <0.01).

CONCLUSIONS

Differences in potency of the peripheral inhibitory effects of the selective serotonin re-uptake inhibitors and clomipramine may contribute to their differential effects on delaying ejaculatory latency in patients with premature ejaculation.

Antidepressant drug administration modifies the interactive relationship between alpha(2)-adrenergic sensitivity and levels of TNF in the rat brain

A reciprocally permissive interaction occurs between cellular responses elicited by the pleiotropic cytokine tumor necrosis factor-alpha (TNF) and alpha(2)-adrenergic receptor activation, such that each may adapt in response to modifications in the other's effects. Changes in presynaptic adrenergic sensitivity as well as neuronal sensitivity to TNF have been implicated in the mechanism of action of antidepressant drugs. The present study examines the influence of alpha(2)-adrenergic receptor activation on levels of TNF in regions of the brain associated with adrenergic function and the expression of mood. Additionally, the role of TNF as a neuromodulator is demonstrated by in vivo microinfusion of rrTNF proximal to the hippocampus. Administration to rats of an alpha(2)-adrenergic receptor agonist (clonidine) decreases levels of TNF in homogenates of rat locus coeruleus and hippocampus within 7.5 min. Chronic (14 days) administration of the antidepressant drugs desipramine or zimelidine transforms alpha(2)-adrenergic receptor-dependent decreases in TNF levels to increases in levels of TNF in the locus coeruleus. This transformation to an increase in total levels of TNF also occurs, although transiently, in the hippocampus following acute (1 day) antidepressant drug administration. The effect of TNF on presynaptic alpha(2)-adrenergic sensitivity was also investigated. Field stimulation of hippocampal slices from rats microinfused with rrTNF proximal to the hippocampus for 14 days demonstrates a decrease in fractional release of [3H]NE and an increase in alpha(2)-adrenergic autoreceptor sensitivity. These data demonstrate a mutual dependence between alpha(2)-adrenergic receptor activation and levels of TNF in the central nervous system that would culminate in an increase in neurotransmitter release following antidepressant drug administration.

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.

Enhanced alpha2A-autoreceptor reserve for clonidine induced by reserpine and cholinomimetic agents in the rat vas deferens

1. The adaptive changes in the functional parameters of the presynaptic alpha2A-adrenoceptors in rat vas deferens were examined after treatments with the monoamine depleter reserpine or with the direct/indirect cholinomimetic agents pilocarpine and neostigmine. 2. For this purpose, we studied the inhibition induced by the alpha2-adrenoceptor agonist clonidine on the twitch contraction of the vas deferens elicited by electrical field stimulation, in animals that had been treated with acute (single dose), short-term (for 4 days) and chronic (for 11 days) regimens of reserpine (0.25 mg kg(-1), s.c., every 48 h), pilocarpine (10 mg kg(-1), i.p., every 12 h) or neostigmine (0.1 mg kg(-1), i.p., every 12 h). The irreversible receptor alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 300 nM) was used to block partially the alpha2A-adrenoceptor-mediated effect of clonidine. 3. In control (untreated) animals, clonidine inhibited concentration-dependently the twitch response of the vas deferens (pEC50 = 8.66) with a maximal effect near 100%. The apparent affinity constant for clonidine was estimated with the nested hyperbolic methodology (pK(A) = 7.10). The analysis of the occupancy-effect relation for clonidine revealed a large receptor reserve at alpha2A-adrenoceptors. 4. Acute, short-term and chronic treatments with reserpine increased the sensitivity of alpha2A-adrenoceptors to clonidine (decreased the EC50) by about 3, 4 and 9 fold, respectively, and also increased the pool of receptor reserve for this agonist (decreased the K(E)) by 4, 10 and 10 fold, respectively. Receptor affinity values were not changed after treatments. 5. Short-term and chronic, but not acute, treatments with pilocarpine and neostigmine increased the sensitivity of alpha2A-adrenoceptors to clonidine (decreased the EC50) by about 3 and 2 fold, respectively, and also increased the pool of receptor reserve for this agonist (decreased the K(E)) by 2 and 3 fold, respectively. Receptor affinity values were not changed after these treatments. 6. These results indicate that an enhancement of the receptor reserve for clonidine might account for the supersensitivity of alpha2A-adrenoceptors induced by reserpine, pilocarpine or neostigmine treatments in the rat vas deferens.

Regulation of c-Fos and NGF1-A by antidepressant treatments

The influence of antidepressant treatments on the expression of c-Fos and NGF-1A, two immediate early gene (IEG) transcription factors, was examined. Administration of electroconvulsive seizures (ECS), tranylcypromine, or imipramine, three different classes of antidepressants, increased the expression of c-Fos mRNA and immunoreactivity in rat frontal cortex, but the magnitude of the increase for each treatment differed and the effect of imipramine was preceded by inhibition of c-Fos expression. Expression of NGF-1A was increased by acute or chronic administration of ECS or tranylcypromine, and by chronic (21 d), but not acute, administration of imipramine. To study the mechanisms underlying these differences, we examined the neurotransmitter receptors that regulate the expression of c-Fos. ECS- and tranylcypromine induction of c-Fos immunoreactivity in frontal cortex was partially inhibited by pretreatment with specific antagonists for alpha 1-adrenergic, beta-adrenergic, and 5-HT2A/2C, but not D2-dopamine receptors. ECS induction of c-Fos was also inhibited by D1-dopamine and NMDA glutamate receptor antagonists, suggesting that the greater induction of c-Fos by ECS results from activation of these, and possibly other, neurotransmitter receptors. In the hippocampus, antagonism of tranylcypromine was similar to that in frontal cortex, except the D1-dopamine receptor antagonist also blocked the c-Fos response. In contrast, antagonism of the ECS response in hippocampus was only blocked by the NMDA receptor antagonist. The results demonstrate that ECS- and tranylcypromine induction of c-Fos is mediated by activation of several different neurotransmitter receptors, but that the exact pharmacological profile is different for each treatment and brain region.

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.

Four consecutive serines in the third intracellular loop are the sites for beta-adrenergic receptor kinase-mediated phosphorylation and desensitization of the alpha 2A-adrenergic receptor

During short term agonist exposure, the alpha 2A-adrenergic receptor (alpha 2AAR) undergoes rapid functional desensitization caused by phosphorylation of the receptor by the beta-adrenergic receptor kinase (beta ARK). This signal quenching is similar in nature to that found with a number of G-protein coupled receptors in which agonist-promoted desensitization is due to beta ARK phosphorylation; like these other receptors, the precise molecular determinants of the receptor required for beta ARK phosphorylation are not known. To delineate such a motif in the human alpha 2AAR (alpha 2C10), we constructed six mutated receptors consisting of deletions or substitutions of Ser-296-299 in the EESSSS sequence of the third intracellular loop of the receptor. These were expressed in Chinese hamster ovary and COS-7 cells, and agonist-promoted desensitization and receptor phosphorylation were assessed. Deletion of the EESSSS sequence and substitution of alanine for all four serines resulted in a total loss of phosphorylation and desensitization. Mutant receptors that retained two of the original serines (AASS and SSAA) underwent agonist-promoted phosphorylation of 55 +/- 7% and 57 +/- 8% of the phosphorylation found for wild type alpha 2C10. Additional substitution mutants (SSSA and SAAA) underwent 77 +/- 1% and 27 +/- 4% of wild type phosphorylation, respectively. Thus, substitution of alanine for each additional serine decreased overall phosphorylation as compared with wild type alpha 2C10 by approximately 25%, which is consistent with all 4 serines being phosphorylated. Mutated receptors that only partially phosphorylated (as compared with wild type) failed to undergo agonist-promoted desensitization. Thus, beta ARK-mediated phosphorylation of alpha 2C10 occurs at Ser-296-299 in the third intracellular loop, and this represents the critical step in rapid agonist-promoted desensitization. A number of other G-protein coupled receptors that undergo desensitization have a sequence motif similar to that which we have found for beta ARK-mediated phosphorylation of alpha 2C10, suggesting that these receptors may also be substrates for beta ARK.

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)

Acute and chronic effects of reserpine on biochemical and functional parameters of central and peripheral alpha 2-adrenoceptors

The specific binding of the agonist, [3H]UK 14304, and of the antagonist, [3H]RX 821002, to rat brain membranes, as well as clonidine-induced mydriasis, clonidine-induced inhibition and idazoxan-induced stimulation of brain 3,4-dihydroxyphenylalanine (DOPA) synthesis, and clonidine and UK 14304-induced inhibition of twitch responses in the vas deferens were used to evaluate the affinity and sensitivity of central and peripheral alpha 2-adrenoceptors after various treatments with reserpine. Treatment with reserpine (0.25 mg/kg s.c., every 48 h) for 4, 11 and 18 days induced consistent and significant increases in the affinity (KD values) of [3H]UK 14304 for the cortical alpha 2-adrenoceptor with no change in receptor density. Chronic treatment with reserpine also resulted in a greater affinity of (-)-adrenaline for the high-affinity state of the alpha 2-adrenoceptor when the catecholamine competed with the binding of [3H]RX 821002 to cortical membranes. In line with these radioligand binding data, various functional responses mediated by central and peripheral alpha 2-adrenoceptors were found to be potentiated after repeated treatment with reserpine. Thus, the inhibitory alpha 2-autoreceptor that modulates the synthesis of brain noradrenaline and the central postsynaptic inhibitory alpha 2-adrenoceptor that induces mydriasis displayed greater responses in vivo after chronic treatment with reserpine. Short-term and chronic treatments with reserpine also increased the sensitivity of peripheral presynaptic alpha 2-adrenoceptors in the vas deferens.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and chronic effects of cholinesterase inhibitors and pilocarpine on the density and sensitivity of central and peripheral alpha 2-adrenoceptors

The specific binding of the agonists [3H]clonidine and [3H]UK 14304 (bromoxidine) and of the antagonist [3H]RX 821002 (2-metoxy idazoxan) to rat brain membranes, as well as clonidine-induced mydriasis, clonidine-induced inhibition of brain (3,4-dihydroxyphenylalaninme) DOPA synthesis and clonidine-induced inhibition of twitch responses in the vas deferens, was used to evaluate the density and sensitivity of central and peripheral alpha 2-adrenoceptors after prolonged activation of the cholinergic system. Acute (12 h), short-term (4 days) or chronic (7-18 days) treatment with the cholinesterase inhibitors neostigmine (0.1 mg/kg), physostigmine (0.1 mg/kg) and diisopropylfluorophosphate (2 mg/kg) and with the muscarinic receptor agonist pilocarpine (10 mg/kg) did not alter the density of brain alpha 2-adrenoceptors. In contrast, various functional responses mediated by central and peripheral alpha 2-adrenoceptors were potentiated after the repeated treatments. Thus, the inhibitory alpha 2-autoreceptor that modulates the synthesis of brain noradrenaline and the central postsynaptic inhibitory alpha 2-adrenoceptor that induces mydriasis displayed greater responses in vivo after chronic treatment with neostigmine or pilocarpine. These treatments also increased the sensitivity of peripheral presynaptic alpha 2-adrenoceptors in the vas deferens. The results indicate that prolonged activation of central and peripheral cholinergic pathways results in up-regulation of alpha 2-adrenoceptor function without apparent increases in receptor density.

Modulation of alpha 1-adrenoceptors and functional consequences in the bisected rat vas deferens following chronic inhibition of neuronal noradrenaline uptake

1. The adaptational changes induced after chronic inhibition of neuronal noradrenaline uptake on both functional responsiveness of alpha 1-adrenoceptor activation and [3H]-prazosin binding were investigated in prostatic and epididymal portions of the rat vas deferens. 2. Contractile concentration-response curves to phenylephrine and saturation isotherms of [3H]-prazosin binding to homogenates of each of the portions of the bisected rat vas deferens were determined 48 h after the last injection of desipramine, nomifensine or nisoxetine (10 mg kg-1; i.p. for 14 days). 3. Treatment with both nomifensine and nisoxetine decreased the potency (pD2) of phenylephrine by about 10 and 8 fold respectively in the epididymal portion. However, administration of desipramine only reduced the potency of the alpha 1-adrenoceptor agonist by about 1.8 fold. None of the treatments modified the maximal effect (Emax) elicited by phenylephrine in this portion of the vas deferens. In the prostatic portion only the treatment with nomifensine (1.4 fold) and nisoxetine (1.8 fold) decreased the potency of phenylephrine; the maximal contraction elicited by the agonist after the treatments was also reduced. 4. Chronic treatment with either nomifensine or nisoxetine did not change the KD for [3H]-prazosin binding in either epididymal or prostatic membranes. However, these two treatments resulted in a significant decrease in the [3H]-prazosin Bmax in membranes in both portions of rat vas deferens. The reduction in density of alpha 1-adrenoceptors was higher in the epididymal than the prostatic half. Desipramine reduced the Bmax only in the epididymal portion. 5. These results indicate that differential regulation of ax-adrenoceptors in either portion of the rat vas deferens could result from a greater degree of activation of these receptors in the epididymal half after chronic inhibition of neuronal noradrenaline uptake. The different functional consequences of the loss of alpha l-adrenoceptors in each portion seems to be explained on the basis of a different relationship between the occupancy of the receptor and the response elicited.

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.

Two phases of contractile response in rat isolated vas deferens and their regulation by adenosine and alpha-receptors

Contractions to transmural electrical stimulation and exogenous norepinephrine were recorded in isolated longitudinal segments of rat vas deferens. Electrical stimulation for 30 s produced a biphasic contraction in the vas deferens consisting of a rapid, transient response (Phase I), followed by a slowly developing, sustained contraction (Phase II). N6-Cyclohexyladenosine (CHA), a selective adenosine1 (A1)-receptor agonist, attenuated in a concentration-dependent manner the Phase I contractile response, while having little effect on the Phase II response. In contrast, 2-(phenylamino)adenosine (CV-1808), a selective adenosine2 (A2)-receptor agonist had little effect on either contractile phase. CHA did not inhibit the contraction to exogenous norepinephrine, suggesting that A1-receptors were located at a presynaptic site. The relatively selective alpha 2-receptor agonist clonidine produced the same pattern of contractile inhibition as CHA. The inhibitory effect of CHA on the Phase I contractile response in the vas deferens could be antagonized by the selective A1-receptor antagonist 8-cyclopentyltheophylline, while the selective alpha 2-receptor antagonist idazoxan preferentially antagonized the inhibitory effect of clonidine on the Phase I response. Both the Phase I and Phase II contractile responses were reduced by the selective alpha 1-adrenoceptor antagonist prazosin and the ATP analog alpha, beta-methylene adenosine triphosphate (alpha, beta-methylene ATP), suggesting that norepinephrine and ATP are coreleased as neurotransmitters for both responses. The results of the present study demonstrate that in the rat vas deferens the presynaptic inhibitory effects of adenosine is mediated by the A1-receptor subtype, and that both A1- and alpha 2-receptor agonists exert a selective inhibitory effect on the Phase I contractile response to electrical stimulation.

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.

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.