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

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.

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.

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.

The effects of clonidine, guanfacine and phenylephrine on the excitatory and inhibitory responses of the rat anococcygeus muscle

The effects of clonidine, guanfacine and phenylephrine on twitch responses and the basal tone of the rat anococcygeus muscle were investigated. Clonidine (10(-9)-3 x 10(-8) M) and guanfacine (10(-9)-10(-7) M) inhibited the twitch responses with the same potency, whereas phenylephrine (10(-9)-10(-7) M) was found ineffective. The inhibitory effect of clonidine and guanfacine was antagonized by yohimbine. Higher concentrations of clonidine and guanfacine increased the muscle tone and elicited inhibitory responses during field stimulation. Phenylephrine at concentrations greater than 10(-7) M also increased the muscle tone but induced biphasic responses. Clonidine (10(-7)-3 x 10(-5) M), guanfacine (3 x 10(-7)-3 x 10(-5) M) and phenylephrine (3 x 10(-7)-10(-5) M) caused concentration-dependent increases in the basal tone. The order of potency of these agonists in increasing the basal tone was clonidine > guanfacine > phenylephrine. Both yohimbine (10(-8)-10(-5) M) and prazosin (10(-9)-10(-7) M) antagonized these tonic contractions. Prazosin was found to be 39-, 122- and 83-fold more potent than yohimbine in antagonizing clonidine, guanfacine and phenylephrine-induced tonic contractions, respectively. Clonidine and guanfacine inhibited twitch responses through stimulation of presynaptic alpha-2 adrenoceptors. Postsynaptic alpha-1 adrenoceptors seem responsible for the contractile effects of clonidine, guanfacine and phenylephrine in the rat anococcygeus muscle.

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.

Alpha-adrenoceptors involved on the cardiovascular response induced by mianserin in the pithed rat

1. The effects of the antidepressant drug mianserin on the cardiovascular responses induced by preganglionic electrical stimulation, and i.v. infusion of the adrenergic agonists noradrenaline (NA, alpha 1 and alpha 2), phenylephrine (alpha 1) and xylazine (alpha 2) in the pithed normotensive rat were studied. 2. Mianserin inhibited in a dose-dependent manner the pressor effect caused by electrical stimulation of spinal cord (Th7-Th9) and the infusion of NA, phenylephrine and xylazine. Cocaine increased the inhibitory effect of mianserin on the pressor effect caused by electrical stimulation and NA. 3. Mianserin blocked the xylazine-induced inhibition of cardiac nerve stimulation effect. 4. These results suggest that mianserin blocks the NA uptake, and it is more effective in blocking presynaptic alpha 2- than postsynaptic alpha-adrenoceptors.