The effect of chronic treatment with antidepressant drugs on salbutamol-induced hypoactivity in rats

Possible neuroprotective mechanisms of action involved in the neurobehavioral property of naringin in mice

Flavonoids are naturally occurring bioactive phytochemical metabolites widely known to prevent and suppress several human diseases, and are important sources of therapeutic compounds from plants. Evidence derived from previous studies suggests that naringin, a neuroactive flavonoid possess functional beneficial neurobehavioral effects including anxiolytic, antidepressant and memory enhancing properties. However, literature search revealed that no studies have been carried out to evaluate the possible biochemical mechanisms involved in the neurobehavioral property of naringin alone following repeated treatment. Hence, this study was designed to evaluate the possible neuro-biochemical mechanisms involved in the neurobehavioral property of naringin following repeated administration in mice. The effects of naringin (2.5, 5 and 10 mg/kg), diazepam (2 mg/kg), imipramine (15 mg/kg) and donepezil (1 mg/kg) or vehicle on neurobehavioral and biochemical effects were evaluated in mice following repeated intraperitoneal injection for 7 consecutive days. Neurobehavioral activities consisting of open-field (locomotor), elevated-plus maze (anxiolytic), forced swim and social interaction (antidepressant and social preference), and Y-maze (memory enhancing) tests were assessed. Thereafter, brains levels of biomarkers of oxidative, nitrosative and cholinergic parameters were determined. Repeated treatment with naringin produced increased locomotor activity, and demonstrated antidepressant-like effects evidenced by decreased immobility time in forced swim test and increased % social preference in the social interaction test relative to controls. Also, naringin induced anxiolytic-like effect and increased cognitive performance in mice. Mechanistically, naringin significantly increased the activities of superoxide dismutase and catalase, and glutathione concentration relative to vehicle-controls. However, naringin significantly decreased malondialdehyde and nitrite contents, and reduced brain acetylcholinesterase activity in mice brains in a significant manner relative to controls. Taken together, these findings suggest that treatment with naringin might be useful to produce functional behavioral effects via mechanisms related to enhancement of cholinergic transmission, antioxidant defense systems, inhibition of lipid peroxidation and nitrosative processes.

Effect of repeated administration of clenbuterol on the regulation of beta-adrenoceptors in the central nervous system of the rat

The effect of the centrally acting beta-adrenoceptor agonist clenbuterol on beta-adrenergic responsiveness, beta-receptor density and N-protein coupling was studied in the rat cerebral cortex (which contains primarily beta 1-receptors) and cerebellum (containing mostly beta 2-receptors). The objective was to determine whether clenbuterol's effect on these variables was similar to that produced by standard antidepressants. When given to rats repeatedly, clenbuterol caused a decrease in beta-adrenergic responsiveness in slices from either the cerebral cortex or the cerebellum. The decreased beta-responsiveness in the cerebellum was associated with a decrease both in the density of beta-receptors and in receptor/N-protein coupling. In the cortex, only reduced receptor/N-protein coupling was observed by in vitro ligand-binding methods. However, when quantitative autoradiography was employed, clenbuterol treatment was found to reduce the binding of [125I]iodopindolol to beta 2-receptors throughout the brain, whereas binding to beta 1-receptors was not reduced. The down-regulation of beta 2-receptors by clenbuterol is due to its acting centrally as a beta 2-agonist. Although clenbuterol has about an equal affinity for beta 1-receptors and beta 2-receptors, no evidence was found for agonist activity of this drug at beta 1-receptors in the cerebral cortex. The strategies described here should be helpful in investigating important properties of centrally acting beta-agonists that might have potential as antidepressants.

Some behavioural effects of antidepressant drugs are time-dependent

1. The effects of repeated administration of antidepressant drugs (imipramine, IMI and citalopram, CIT) on the beta- and alpha2-adrenergic as well as dopaminergic D3 receptors were compared with time-dependent changes in the receptor responsiveness after acute treatment. 2. Repeated treatment with IMI or CIT (administered at a dose of 10 mg/kg p.o. twice a day for 14 days) induced down-regulation of beta-adrenergic receptors, demonstrated by behavioural experiment using salbutamol-induced hypoactivity and by binding studies using [3H]CGP12177. The changes in alpha2-adrenergic receptors were studied using clonidine-induced hypoactivity, which was attenuated by repeated treatment with IMI or CIT. Behavioural responsiveness of dopamine D3 receptors was investigated using two doses of 7-OH-DPAT. This drug at a dose of 0.05 mg/kg s.c. induced locomotor hypoactivity (interpreted as a result of stimulation of presynaptic dopamine D3 receptors), which was reversed by repeated administration of IMI or CIT, while 7-OH-DPAT at a dose of 3 mg/kg s.c. (which stimulated postsynaptic dopamine D3 receptors) induced significant hyperactivity, which was markedly enhanced by repeated administration of antidepressant drugs. 3. The effect of acute administration of IMI or CIT measured 14 days after drug treatment were similar to the described above alterations at the level of alpha2 adrenoreceptors and presynaptic dopamine D3 receptors, i.e. the drugs attenuated clonidine-induced hypoactivity and reversed locomotor hypoactivity evoked by low dose of 7-OH-DPAT. To induce the down-regulation of beta-adrenergic receptors or up-regulation of the behavioural responsiveness of dopaminergic D3 postsynaptic receptors, the repeated administration of IMI or CIT was necessary. 4. Therefore it has been concluded that presynaptic dopaminergic D3 and alpha2-adrenergic receptors are more sensitive to the acute treatment with antidepressant drugs than postsynaptic D3 and beta-adrenergic receptors.

The effect of chronic administration of amitriptyline on the effects of subsequent electroconvulsive treatment on responsiveness of alpha 1-and beta-adrenoceptors in the rat cortical slices

Both antidepressant drugs and repeated electroconvulsive shock (ECS) produce adaptive changes in cerebral neurotransmitter systems. As in the clinical practice ECS is used almost always after therapeutical failure of pharmacotherapy, we investigated presently how chronic administration of an antidepressant amitriptyline affects the action of subsequent multiple ECS in rats. Amitriptyline differed from ECS and from other classical antidepressant in producing no beta-downregulation and potentiating the inhibitory effect of protein kinase C activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), on responses of alpha 1-adrenoceptor system to noradrenaline. The action of ECS on alpha 1-adrenoceptor system remained essentially unaffected by previous amitriptyline administration. Its downregulatory effect on responses of beta-adrenoceptor system to noradrenaline, and particularly to isoproterenol, were attenuated by previous drug treatment. The present results suggest that previous chronic administration of antidepressant drugs may alter the effect of subsequent ECS.

Reduction of responses to angiotensin II by antidepressant drugs

The effects of the antidepressant drugs desipramine, fluoxetine and tranylcypromine and the non-antidepressant control cocaine on angiotensin II function were determined in vivo by use of angiotensin-induced drinking in rats and in vitro using contractile responses of the rat uterus. The results of the drinking studies showed that the three antidepressants, but not cocaine, reduced the dipsogenic effects of angiotensin II. In vitro, all of the drugs reduced the effects of not only angiotensin but also acetylcholine and oxytocin on the uterus. The inhibition appeared to be non-competitive in all cases. These results indicate that the antidepressant drugs reduced the activity of angiotensin II, albeit non-selectively, and suggest that the previously reported effects of antidepressants on isoprenaline-induced drinking in rats reflect an action on angiotensin activity rather than a reduction of beta-adrenoceptor activity as previously suggested.

Reversal by imipramine of beta-adrenoceptor up-regulation induced in a chronic mild stress model of depression

Male Wistar rats were subjected to a chronic mild stress procedure involving different stress stimuli applied for 8 weeks. During this time the consumption of 1% sucrose solution was monitored at weekly intervals. After the first 3 weeks, when stressed animals displayed a reduction of sucrose consumption, the control and stressed groups were divided into subgroups receiving daily placebo or imipramine (10 mg/kg/day) treatment. After 5 weeks of treatment, 24 h after the last injection, the rats were killed and beta-adrenoceptor density and affinity in cortical membrane preparations and the accumulation of cyclic AMP in cortical slices stimulated with noradrenaline were assessed. While in stressed placebo-treated rats the sucrose consumption remained reduced, in the imipramine-treated group the level of consumption gradually returned to control values. The stressed placebo-treated rats also displayed an increase in cortical beta-adrenoceptor density (by 34%) with no changes in affinity, and an increase (22%) in the cyclic AMP response to noradrenaline in cortical slices. Imipramine, which in non-stressed rats did not affect sucrose intake but depressed the beta-adrenoceptor density and the cyclic AMP response, reversed the stress-induced decrease in sucrose consumption and the increase in the beta-adrenoceptor density; at physiological noradrenaline concentrations it also reduced the enhanced cyclic AMP response. The results suggest that the chronic mild stress procedure produces behavioral and biochemical changes consistent with a realistic model of depression in animals.

Dietary salbutamol and level of protein: effects on the acute stress response in pigs

Effect on the acute stress response of dietary inclusion of 3 ppm salbutamol (beta-2-adrenergic agonist) at two levels of protein were investigated in growing pigs (from 25 kg live weight). The trial comprised six litters (blocks) of four females allocated randomly to four treatment groups in accordance with a 2 x 2 factorial arrangement. The response to an open-field test and to an intruder were measured at 50 kg live weight. Salbutamol increased immobility and looking, reduced total exploration, and increased plasma ACTH after test. At high dietary levels of protein, salbutamol also increased the latency to attack. High dietary levels of protein reduced standing still, latency to contact a novel object and an intruder, and level of plasma cortisol before test. Moreover, high protein without salbutamol seemed to reduce the latency to attack an intruder. In conclusion, chronic treatment with salbutamol shifted the acute stress response in pigs toward a passive response, whereas high dietary level of protein provoked an active response, which may have consequences in pig production.

2-Phenylethylamine-induced changes in catecholamine receptor density: implications for antidepressant drug action

It is now established that (1) concentrations of 2-phenylethylamine (PEA) are greatly increased in brain following administration of monoamine oxidase inhibitor (MAOI) antidepressants; (2) PEA is a metabolite of the MAOI antidepressant phenelzine; and (3) PEA may be a neuromodulator of catecholamine activity. On the basis of these observations, the effects of long term increases in brain PEA on catecholamine receptors have been assessed. Both PEA and antidepressants induced a reduction in the behavioural response to the beta 2 adrenoceptor agonist salbutamol. Radioligand binding measurements revealed that 28 day administration of PEA in combination with the type B MAOI (-)-deprenyl results in a decrease in the density of beta 1 adrenoceptors but not beta 2 adrenoceptors in rat cerebral cortex and cerebellum. (-)-Deprenyl alone also induced a significant decrease in beta 1-adrenoceptors but when PEA was added to this treatment there was a further decrease in beta 1-adrenoceptor density. Only changes in beta 1 adrenoceptor density were evident following 28 day administration of MAOI antidepressants. PEA also induced a decrease in the density of D1-like dopamine (DA) receptors in the rat striatum. MAOI antidepressants induced a decrease in the density of both D1-like and D2-like DA receptors. These data are discussed in terms of a possible role of PEA-catecholamine interactions in antidepressant drug action.

Effects of chronic antidepressant treatment on beta-adrenoceptor subtype binding in the rat cerebral cortex and cerebellum

The effects of desipramine, tranylcypromine, and phenelzine on beta-adrenoceptor subtype binding were measured in rat cerebral cortex and cerebellum. The drugs were administered to male Sprague Dawley rats for 28 d sc via 2ML4 Alzet osmotic minipumps (desipramine HCl 10, tranylcypromine HCl 1, phenelzine sulfate 5, 10 mg kg-1/d). Binding of [3H]CGP 12177 was measured to determine total beta-adrenoceptor density in each brain region. The relative densities of beta 1- and beta 2-adrenoceptors were directly calculated from competition analysis using ICI 89406 to displace [3H]CGP 12177. beta 1-Adrenoceptor density was decreased in cortical tissue following each antidepressant treatment, but no effects on beta 2-adrenoceptor density were evident. With cerebellar tissue, despite a higher density of beta 2-adrenoceptors in this area, no effects of antidepressants on beta 2-adrenoceptor density were evident. The present data confirm and extend previous reports of beta 1- but not beta 2-adrenoceptor downregulation in brain following chronic antidepressant drug treatment.

Down-regulation of beta-adrenergic and dopaminergic receptors induced by 2-phenylethylamine

1. The effects of chronic administration (28 days s.c. via Alzet osmotic minipumps) of 2-phenylethylamine.HCl (10 mg kg-1 per day) and/or (-)-deprenyl.HCl (1 mg kg-1 per day) on dopamine and noradrenaline receptor subtypes have been measured in rat brain. 3H-CGP 12177 was used to label beta-adrenoceptors; 3H-spiperone and 3H-SCH 23390 were used to label D2-like and D1-like receptors. 2. Total cortical beta-adrenoceptor density was reduced by (-)-deprenyl but not 2-phenylethylamine alone. Combined administration of 2-phenylethylamine and (-)-deprenyl resulted in a significantly larger decrease than (-)-deprenyl alone. Subtype density analysis by competition experiments with ICI 89406 revealed that the (-)-deprenyl effect in cortex was due to a decrease in beta 1-adrenoceptor density. The combination of 2-phenylethylamine and (-)-deprenyl resulted in a significant decrease in both cortical beta 1- and cortical beta 2-adrenoceptors. Cerebellar beta-adrenoceptor density was not altered by the present drug treatments. The Kd values for total beta-adrenoceptor densities and Ki values for beta-adrenoceptor subtype densities were not altered by drug treatment in either cortex or cerebellum. 3. Administration of 2-phenylethylamine and of (-)-deprenyl resulted in a decrease in the density of D1-like 3H-SCH 23390 but not D2-like 3H-spiperone binding to dopamine receptors in the striatum. The effects of combined 2-phenylethylamine and (-)-deprenyl treatment on 3H-SCH 23390 binding were additive. These drug treatments did not alter Kd values for these binding sites. 4. The down-regulation of catecholamine receptors following chronically increased availability of 2-phenylethylamine may be due to the catecholamine releasing or uptake blocking effects of this amine. These effects may also be attributable to a direct neuromodulatory action of 2-phenylethylamine on catecholamine receptors. 5. The parallels between effects of increased 2-phenylethylamine availability and effects of administration of MAO inhibitor antidepressants on catecholamine receptor systems indicate that this substrate for MAO may mediate some of the effects of MAO inhibitor antidepressants.

Beta-adrenoceptors and antidepressants: possible 2-phenylethylamine mediation of chronic phenelzine effects

Effects of chronic administration of antidepressant drugs on beta-adrenoceptor function were assessed. Tricyclics (imipramine 30 mg/kg/day, desipramine 5 and 10 mg/kg/day) and monoamine oxidase inhibitors [(+/-)-tranylcypromine 1 mg/kg/day, phenelzine 5 and 10 mg/kg/day] were administered to Male Sprague-Dawley rats (n = 8), via Alzet 2ML2 osmotic minipumps for 28 days. Pumps were implanted subcutaneously in the interscapular region and replaced after 14 days. On days 21 and 22 motor-suppressant actions of the beta-adrenoceptor agonist salbutamol (3 mg/kg intraperitoneally [IP]) were assessed as a measure of beta-adrenergic receptor sensitivity. On day 28 the animals were killed and their brains used for measurement of drug levels and monoamine oxidase activity. Liver tissue was used to measure the trace amine 2-phenylethylamine. Each drug induced a decrease in the response to salbutamol. With phenelzine the decreased response to salbutamol was not observed at the lower dose. Differences in monoamine oxidase inhibition following phenelzine did not correspond to differential effects on functional beta-adrenergic sensitivity. Levels of 2-phenylethylamine, an endogenous amine that is also a metabolite of phenelzine, were significantly higher in the 10-mg/kg/day phenelzine group. These data suggest that 2-phenylethylamine may be one mediator of the chronic actions of phenelzine on beta-adrenoceptors.

Differential effects of antidepressants on GABAB and beta-adrenergic receptors in rat cerebral cortex

The effects of chronic administration of antidepressant drugs on beta-adrenergic and gamma-amino-butyric acid (GABA)B receptors have been assessed with radioligand binding. Tricyclics [imipramine (IMI), 30 mg/kg/day, and desmethylimipramine (DMI), 10 mg/kg day] or monoamine oxidase inhibitors [(+/-)-tranylcypromine (TCP), 1 mg/kg/day, and phenelzine (PLZ), 10 mg/kg/day] were administered to male Sprague-Dawley rats by constant infusion via Alzet 2ML4 osmotic minipumps for 28 days. Pumps were implanted s.c. in the interscapular region. On day 28 the animals were killed and their brains removed; [3H]GABA binding to GABAB receptors was measured in frontal cortex and the remaining cortical tissue was used to measure [3H]dihydroalprenolol ([3H]DHA) binding to beta-adrenoceptors. All drugs tested induced a significant decrease in density (Bmax) of [3H]DHA binding, although no significant changes in affinity (Kd) were observed. [3H]GABA binding was not altered significantly by chronic antidepressant treatment. TCP-treated animals showed a tendency towards increased [3H]-GABA binding, but the differences did not reach statistical significance. No effects on Kd were observed. These data do not support the proposal that an increase in the total population of cortical GABAB receptors is a common effect of chronic antidepressant treatment.

Reduction of salbutamol-induced drinking by putative and proven antidepressants

Salbutamol-induced drinking was determined in rats after 4 days of twice daily treatment with the conventional and putative antidepressants desipramine, clenbuterol or salbutamol. Drinking was also determined 21 h after a single dose of the abovementioned drugs. Both chronic and acute administration of the drugs resulted in a significant reduction in the salbutamol-induced fluid intake. The results suggest that these drugs decrease beta 2-adrenoceptor activity, but that repeated administration is not a prerequisite for the effect.

Differential effects of chronic antidepressants in behavioural tests of beta-adrenergic and GABAB receptor function

The effects of chronic administration of anti-depressant drugs (28 days SC via Alzet 2ML4 osmotic minipumps) on the functional sensitivity of beta-adrenergic and GABA receptors have been assessed. Phenelzine (10 mg/kg), tranylcypromine (1 mg/kg), imipramine (30 mg/kg) and desmethylimipramine (10 mg/kg) attenuated the motor-suppressant effects of salbutamol (3 mg/kg) observed at 21-22 days of drug administration. No changes in the motor-suppressant effects of the GABA prodrug progabide (50 mg/kg) or the GABAB agonist (+/-)-baclofen (5 mg/kg) were induced by these antidepressants. These findings extend and confirm previous reports of functional changes in beta-adrenergic receptors but not of GABAB receptors following chronic antidepressant treatment.

Behavioral indices of beta receptor subsensitivity after chronic treatment with viloxazine in the mouse

The aim of the experiments was to determine whether chronic pretreatment with viloxazine decreased the sensitivity of mice to the sedative effects of a beta agonist clenbuterol. Mice were subjected to chronic oral treatment with viloxazine (128 mg/kg twice daily) and then given a single administration of 32 mg/kg PO followed by clenbuterol (0.125 mg/kg IP) before being tested in a standard photocell activity meter. Imipramine, administered at the same doses in the same experimental conditions, was used as a comparison compound. The results showed that chronic but not acute viloxazine decreased the hypoactivity induced by clenbuterol, suggesting the induction of beta receptor subsensitivity. With imipramine the results were in the same direction but less clear. The findings are discussed in terms of the eventual specificity of the viloxazine effect to subsensitivity in beta-2 receptors.

Noradrenergic receptor interactions in feeding elicited by stimulation of the para-ventricular hypothalamus

Food intake and feeding behaviour were examined after the administration of noradrenaline (NA) or clonidine to the para-ventricular nucleus of the hypothalamus, or after systemic administration of clonidine. In 20-hr deprived animals all treatments dose-dependently reduced pellet consumption; however, at a low dose (2-5 micrograms) central clonidine increased eating time and bout length. In 4-hr deprived animals all treatments increased sucrose consumption. Clonidine (peripheral and central) increased feeding time but did not alter feeding rate; NA did not alter feeding time, but did increase feeding rate; NA also increased activity and decreased resting. The effects of NA on feeding rate, activity and resting were blocked by propranolol; however, the propranolol-NA combination increased feeding time. Thus, NA and clonidine increased feeding by different mechanisms, but after propranolol pretreatment the effects of NA were similar to those of clonidine. It is concluded that clonidine enhances feeding by inhibiting satiety and that the feeding stimulant effect of NA is mediated by a complex interaction of alpha- and beta-receptors.

Chronic electroconvulsive shock and neurotransmitter receptors--an update

Electroconvulsive shock (ECS) produces many neurochemical alterations which may be related to its efficacy in the treatment of different psychiatric disorders. This review focuses particularly on experimental findings of CNS receptor changes in animals following chronic ECS and relates them to neurotransmitter and behavioral changes. Also, the pharmacological effect of other antidepressant treatment are compared. Possible mechanisms of action are discussed.

Helpless behavior (escape deficits) in streptozotocin-diabetic rats: resistance to antidepressant drugs

Using the learned helplessness model of depression in rats, the present study undertook to investigate the possibility of an impaired response to antidepressant drugs in diabetic animals. Experimental diabetes was induced by three intraperitoneal (IP) injections of streptozotocin (37.5, 37.5, 50 mg/kg, three days apart), four weeks before behavioral testing. Diabetic and non-diabetic rats were first exposed to 60 inescapable shocks. Forty-eight hours later and over three consecutive days, they were subjected to daily shuttle-box sessions for assessment of escape failures (helpless behavior). Twice daily (IP) injection of clomipramine (24 mg/kg), desipramine (24 mg/kg), imipramine (32 mg/kg) or clenbuterol (0.75 mg/kg) prevented escape deficits in the non-diabetic but not in the diabetic rats. However, this prevention was made possible in the diabetic rats by increasing the duration of the antidepressant treatment. Moreover, one week of insulin therapy restored operant escape responding to both the tricyclics and a beta-agonist. The inefficacy of clenbuterol (a central beta-agonist) in reversing helpless behavior in diabetic rats, along with the observation that triiodothyronine (T3) supplementation also restored the response to imipramine in the diabetic rats, suggests that thyroid-mediated alterations of central noradrenergic function might be a critical factor in the resistance or delayed response to antidepressants in experimental diabetes. These animal findings raise the possibility of a similar resistance to conventional antidepressants in depressed diabetic patients.

Chronic tranylcypromine treatment induces functional alpha 2-adrenoceptor down-regulation in rats

The effects of chronic (28 d Alzet 2002 osmotic mini-pumps) administration of antidepressant drugs on the functional sensitivity of alpha 2-adrenoceptors and on monoamine oxidase activity has been assessed. Tranylcypromine, 4-fluorotranylcypromine and clorgyline (0.5-1.0 mg kg-1) induced a decrease in the motor-suppressant effects of clonidine (50 micrograms kg-1) observed at 9-10 and 23-24 days of drug administration. These effects were associated with marked decreases in type A (clorgyline) or type A and B monoamine oxidase activity (tranylcypromine and 4-fluorotranylcypromine). The results indicate that alpha 2-adrenoceptor down-regulation is an early emergent feature of adaptive changes in noradrenaline systems induced by prolonged exposure to tranylcypromine.

Behavioral consequences of antidepressant treatment in rodents

This review discusses the effects of antidepressant drugs on behaviors that are changed during the clinical treatment of depression. We first consider whether there is a similar subjective state produced by antidepressant drugs that might be akin to the mood changes caused clinically by these drugs. We thus review the evidence that antidepressant drugs can produce a distinctive enough subjective state to serve as a discriminative stimulus, and then discuss the nature of the cue produced. Secondly, we discuss whether there is any evidence that antidepressants enhance the rewarding aspects of stimuli since this aspect of behavior is reported to change during the successful treatment of depression. In this section we review the effects on electrical brain stimulation, water and food intake, exploratory and social behaviors. Finally, because of the proposed role of stress in the aetiology of depression, we review the effects of antidepressants on the responses to acute and chronic stress.

Shuttle-box deficits induced by inescapable shocks in rats: reversal by the beta-adrenoreceptor stimulants clenbuterol and salbutamol

Beta-adrenoreceptor stimulants such as salbutamol and clenbuterol have been reported to be effective in depressive states and to share many actions with classical antidepressants in animals. To further explore the antidepressant activity of these drugs, we investigated their effects in rats subjected to helplessness training. Rats were first exposed to inescapable shock pre-treatment (60 shocks, 15 sec duration, 1 mA every minute +/- 15 sec) and 48 hr later, shuttle-box training (30 trials/day, ITI: 30 sec) was initiated in order to evaluate escape and avoidance deficits. Rats pretreated with inescapable shocks exhibited escape and avoidance deficits when tested for subsequent responding in a shuttle-box. The deficits are particularly marked at the third training session. Daily IP injections of clenbuterol (total daily dose: 0.5 and 0.75 mg/kg) and salbutamol (16 and 24 mg/kg) prevented escape deficits as did daily injections of classical antidepressants such as desipramine (16 and 24 mg/kg/day) and clomipramine (16 and 24 mg/kg/day). These data extend previous results bearing on the similarity of action of beta receptor stimulants and tricyclic antidepressants and further support the notion of a close relationship between noradrenergic function, more especially beta-adrenoreceptors, and "helpless" behavior.

Chronic treatment with the potential antidepressant drug rolipram: the effect on the behavioural responses to adrenergic and dopaminergic receptor agonists with some biochemical correlates

We studied the effect of acute and chronic treatment with rolipram, a potential antidepressant drug, on the behavioural responses induced by adrenergic and dopaminergic receptor agonists in mice and rats, and on (3H)prazosin and (3H)dihydroalprenolol binding to cortical membranes and whole brain noradrenaline and dopamine utilization in rats. Chronic, but not acute, administration of rolipram potentiated a behavioural response mediated through central alpha 1-adrenoceptors, attenuated an alpha 2-adrenoceptor-mediated response and inhibited a beta-adrenoceptor-mediated response. Neither treatment affected the behavioural responses to dopaminergic stimulants. Repeated treatment with rolipram decreased the density of cortical (3H)dihydroalprenolol, but not (3H)prazosin bindings sites, and reduced brain noradrenaline, but not dopamine utilization. These results suggest that chronic administration of rolipram induces the down-regulation of the central beta- and alpha 2-adrenoceptors and enhances the responsiveness of the central alpha 1-adrenoceptors with no apparent changes in the alpha 1-adrenoceptor density.

Adaptive and differential changes on beta- and alpha 2-adrenoreceptors mediated hyperthermia after chronic treatment with antidepressant drugs in the rat kept at high ambient temperature

We found previously that the beta-agonist clenbuterol and the alpha-agonist clonidine produced hyperthermia in rats kept at high ambient temperature, which effects were mediated by beta- and alpha 2-adrenoceptors, respectively. In the present paper this observation was used for testing the responsiveness of beta- or alpha 2-adrenoceptors, changed by a pharmacological manipulation, i.e., by chronic treatment with antidepressants. The animals were pretreated with desipramine, imipramine or amitriptyline twice a day for 1 or 2 weeks. All antidepressants significantly attenuated the clenbuterol-induced hyperthermia after 2 weeks of treatment. The effect of desipramine was stronger than that of the other antidepressants and appeared as little as 1 week after the treatment. The hyperthermic effect of clonidine was significantly reduced by repeated treatment with desipramine, increased after 2 weeks administration of imipramine, whereas amitriptyline produced no significant changes. In conclusion, these data suggest that, after repeated treatment, the antidepressants tested produce an adaptive decrease in function of beta-adrenoceptors while the same drugs exert differential effects on alpha 2-receptors. Moreover, clenbuterol induced hyperthermia may be a useful test for examining possible functional changes in beta-adrenoceptor sensitivity.

Behavioral evidence for beta-adrenoceptor subsensitivity after subacute antidepressant/alpha 2-adrenoceptor antagonist treatment

The behavioral consequences of beta-adrenoceptor subsensitivity were investigated by determining whether a physiological response that is mediated by beta-receptors, isoproterenol-induced drinking (IID), would be reduced by subacute antidepressant/alpha 2-antagonist treatment. The coadministration of typical (e.g., imipramine) or atypical (e.g., mianserin) antidepressants with yohimbine or piperoxan twice daily for four consecutive days reduced IID. Both the time course as well as the magnitude of beta-adrenoceptor subsensitivity could be behaviorally demonstrated. In addition, the reduction in IID observed after coadministration of imipramine with yohimbine was a centrally mediated effect since it was observed after systemic (subcutaneous) and central (intraventricular) administration of isoproterenol. These results provide evidence that IID is an appropriate behavioral model to demonstrate beta-adrenoceptor subsensitivity following subacute antidepressant/alpha 2-antagonist treatment.

Tolerance to or facilitation of pharmacological effects induced by chronic treatment with the beta-adrenergic stimulant clenbuterol

The effect in mice of chronic administration of the beta-adrenergic agonist clenbuterol was studied using four pharmacological tests, two of which are predictive of antidepressant activity in man. The suppressive effect of acute administration of the beta-adrenergic agonist in two tests unrelated to antidepressant activity, locomotor activity and investigation of food, was attenuated after chronic treatment. However, in the two tests which are predictive of antidepressant activity, antagonism of reserpine induced hypothermia and potentiation of yohimbine toxicity, the effect of clenbuterol was greater following chronic treatment.

Repeated treatment with antidepressant drugs prevents salbutamol-induced hypoactivity in rats

We have found earlier that a number of antidepressant drugs, administered repeatedly, prevent the salbutamol-induced hypoactivity in rats. Our further experiments show that a similar effect is produced by repeated, but not acute, treatment with other antidepressants: clomipramine, mianserin and nialamide. Two non-antidepressant psychotropic drugs, haloperidol and diazepam, were inactive after repeated administration. These results seem to support our earlier hypothesis that prevention of the salbutamol-induced hypoactivity may be regarded as functional evidence at the behavioral level for the subsitivity of beta-adrenoceptors.

Chronic treatment with electroconvulsive shock prevents the salbutamol-induced hypoactivity in rats

Several lines of evidence (binding studies, reduced responsiveness of brain adenylate cyclase to noradrenergic stimulation) indicate that chronic treatment with electroconvulsive shock (ECS) induces down-regulation of central beta-adrenoceptors. The effect of acute and chronic (10 days) treatment with ECS on salbutamol-induced suppression of exploratory activity in rats has been examined. This effect was prevented by chronic but not by acute treatment with ECS. Chronic treatment with ECS did not affect exploratory activity. The salbutamol-induced hypoactivity is mediated through central beta-adrenoceptors (antagonistic effect of (-)-propranolol but not (+)-propranolol or practolol), so the results may be regarded as functional evidence at the behavioral level for the down-regulation of beta-adrenoceptors produced by chronic treatment with ECS.

Repeated treatment with antidepressant drugs increases the behavioural response to apomorphine

The effect of repeated treatment (twice a day for 14 days) with antidepressant drugs (AD): imipramine, amitriptyline, zimelidine, citalopram and mianserin on the behavioural response to apomorphine in rats (open field test) was investigated. AD studied, given alone in a single dose or repeatedly, do not change the rats behaviour. A repeated but not single-dose treatment with AD facilitates the behaviour stimulation induced by apomorphine. This facilitation is observed 2 hours after the last dose of imipramine, zimelidine, citalopram and mianserin but 72 hours after the last dose of amitriptyline. The results presented suggest that the AD given repeatedly are able to increase the responsiveness of the brain DA system, probably the mesolimbic one.

Stimulation of beta-adrenergic receptors and spontaneous motor activity in mice

The effects of 3 beta-adrenergic agonists (clenbuterol, isoproterenol and salbutamol) on the spontaneous motor activity of mice were studied. The present research indicated that motor activity was significantly decreased 30 minutes after IP injection of either clenbuterol (0.06 mg/kg), isoproterenol (0.5 mg/kg) or salbutamol (2 mg/kg). Hypomotility induced by clenbuterol was also significantly antagonized by propranolol in doses ranging from 1 to 8 mg/kg and by penbutolol in doses from 0.03 to 0.5 mg/kg. However, practolol, which does not cross the blood brain barrier, did not antagonize the effect of clenbuterol. Therefore, it may be hypothesized that beta adrenergic agonists decrease motor activity by a central mechanism. It was also found that tachyphylaxis or resistance to treatment, observed in cardiovascular and bronchopulmonary systems with beta-adrenergic agonists, developed after 7 injections of clenbuterol (0.25 mg/kg IP, twice daily) in the behavioral model of spontaneous motor activity in mice.