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

Oral Phenelzine Treatment Mitigates Metabolic Disturbances in Mice Fed a High-Fat Diet

Novel mechanisms and health benefits have been recently suggested for the antidepressant drug phenelzine (PHE), known as a nonselective monoamine oxidase inhibitor. They include an antilipogenic action that could have an impact on excessive fat accumulation and obesity-related metabolic alterations. We evaluated the metabolic effects of an oral PHE treatment on mice fed a high-fat diet (HFD). Eleven-week-old male C57BL/6 mice were fed a HFD and either a 0.028% PHE solution (HFD + PHE) or water to drink for 11 weeks. PHE attenuated the increase in body weight and adiposity without affecting food consumption. Energy efficiency was lower in HFD + PHE mice. Lipid content was reduced in subcutaneous fat pads, liver, and skeletal muscle. In white adipose tissue (WAT), PHE reduced sterol regulatory element-binding protein-1c and phosphoenolpyruvate carboxykinase mRNA levels, inhibited amine-induced lipogenesis, and did not increase lipolysis. Moreover, HFD + PHE mice presented diminished levels of hydrogen peroxide release in subcutaneous WAT and reduced expression of leukocyte transmigration markers and proinflammatory cytokines in visceral WAT and liver. PHE reduced the circulating levels of glycerol, triacylglycerols, high-density lipoprotein cholesterol, and insulin. Insulin resistance was reduced, without affecting glucose levels and glucose tolerance. In contrast, PHE increased rectal temperature and slightly increased energy expenditure. The mitigation of HFD-induced metabolic disturbances points toward a promising role for PHE in obesity treatment and encourages further research on its mechanisms of action. SIGNIFICANCE STATEMENT: Phenelzine reduces body fat, markers of oxidative stress, inflammation, and insulin resistance in high-fat diet mice. Semicarbazide-sensitive amine oxidase, monoamine oxidase, phosphoenolpyruvate carboxykinase, and sterol regulatory element-binding protein-1c are involved in the metabolic effects of phenelzine. Phenelzine could be potentially used for the treatment of obesity-related complications.

Involvement of alpha1-adrenergic receptors in tranylcypromine enhancement of nicotine self-administration in rat

RATIONALE

The mechanisms mediating tobacco addiction remain elusive. Nicotine, the psychoactive component in tobacco, is generally believed to be the main cause of reward and addiction. However, tobacco smoke contains thousands of constituents, some of which may interact with nicotine to enhance reward. It has previously been shown that monoamine oxidase (MAO) inhibition, known to result from smoking, can enhance nicotine self-administration. The aim of the present study was to evaluate the role of noradrenergic systems in mediating this enhancement of nicotine reward.

OBJECTIVE

The objective of this study was to test the hypothesis that MAO inhibitor pretreatment enhances nicotine self-administration by activation of noradrenergic pathways that regulate dopamine release in the nucleus accumbens (NAc).

METHODS

The effect of prazosin (0.0625-0.5 mg/kg, i.p.), a specific alpha1-adrenergic receptor antagonist, was examined on male rats pretreated with tranylcypromine (3 mg/kg), an irreversible inhibitor of MAO A and B. Acquisition of nicotine (10 mug kg(-1) inj(-1), i.v.) self-administration behavior was examined over a 5-day period. Nicotine (60 mug kg(-1) inj(-1), i.v.)-induced increase in NAc extracellular dopamine levels was examined by in vivo microdialysis in non-self-administering animals.

RESULTS

We have shown that (1) tranylcypromine enhances nicotine self-administration, (2) prazosin pretreatment blocks both the acquisition and the expression of nicotine self-administration, and (3) prazosin pretreatment diminishes nicotine-induced dopamine release in the NAc.

CONCLUSION

These data indicate that the stimulation of alpha1-adrenergic receptors is critical for tranylcypromine enhancement of nicotine reward and suggest a critical interplay between the noradrenergic and dopaminergic systems in tobacco addiction.

Tranylcypromine: new perspectives on an "old" drug

The irreversible inhibitor of monoamine oxidase, tranylcypromine, is a potent antidepressant, but its use is limited to special indications due to side effects and dietary restrictions. The antidepressant action of tranylcypromine is not completely explainable by its effects on monoamine oxidase. Tranylcypromine also leads to an increase in brain trace amines, which are believed to play a key role in the pathophysiology of depression. It also affects other pathophysiological pathways associated with depression. Tranylcypromine treatment leads to an up-regulation of GABA(B)-receptors and modulates the phospholipid metabolism, which is essential for normal brain function. These findings implicate that the efficacy of tranylcypromine as an antidepressant may be due to its multiple actions within the human brain.

Effects of baclofen on operant performance for food pellets and vegetable shortening after a history of binge-type behavior in non-food deprived rats

Operant performance of non-food deprived rats (n=8) was assessed under progressive ratio (PR) and concurrent PR-fixed ratio schedules of food pellet and/or vegetable shortening reinforcement. Post operant baselines, rats were matched and divided into 2 groups based upon the schedule of shortening availability: High restriction binge group (H, 1-hr home cage shortening access each week on Monday, Wednesday, and Friday) and Low restriction (L, 1-hr shortening access daily). Chow and water were continuously available; only access to the shortening was restricted. After 8 weeks, operant performance was reassessed. Lever pressing for shortening increased in the H rats for all schedules, but was either unaffected or decreased in the L rats. Pellet responding under the concurrent schedules increased for both groups. The effects of four dosages of (R)-baclofen (0.3-1.8 mg/kg, i.p.) on operant performance were also assessed. For both groups, 1.0 mg/kg baclofen significantly reduced shortening responding relative to saline for all schedules except one, but had no or minimal effect on pellet responding. This suggests a specific effect of baclofen on responding maintained by fat. These results indicate that intermittent episodes of bingeing on fat can increase the reinforcing efficacy of fat and that GABAB receptor activation can attenuate this effect.

Up-regulation of GABA(B) receptors by chronic administration of the GABA(B) receptor antagonist SCH 50,911

Chronic treatment of mice with the specific gamma-aminobutyric acid(B) (GABA(B)) receptor antagonist (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50,911) increased both the number of GABA(B) receptors in the whole brain (measured as [3H]CGP 54626 [S-(R,R)]-3-[[1-(3,4-dichlorophenyl)amino]-2-hydroxypropyl](cyclohexylmethyl)phosphinic acid hydrochloride binding) and the ability of baclofen to activate GABA(B) receptor coupled G-protein (measured as % reduction of the EC50 of baclofen to activate [35S]GTP(gamma)S binding). The results indicate that persistent blockade of GABA(B) receptors leads to their compensatory up-regulation and suggest that GABA(B) receptors are tonically activated by endogenous GABA.

Baclofen attenuates conditioned locomotion to cues associated with cocaine administration and stabilizes extracellular glutamate levels in rat nucleus accumbens

We have used cocaine-conditioned locomotion in rats as an animal model for cocaine-conditioned responses that contribute to drug craving and relapse in human addicts. The purpose of the present study was to examine the ability of the GABA(B) agonist, baclofen, to attenuate such associative responses. First, experiments were conducted to identify a dose range of baclofen that did not impede exploratory or spontaneous behavior. This dose range was used during testing for conditioned locomotion specific to a flashing light and metronome, which were previously associated with administration of cocaine (PAIRED group) or saline (UNPAIRED group). At 2.0 mg/kg, baclofen attenuated conditioned locomotion in PAIRED subjects to the level of UNPAIRED subjects receiving saline or 2.0 mg/kg baclofen. Considering the importance of glutamate transmission in the nucleus accumbens (NAc) during associative responses to reward-related stimuli, the effect of baclofen on extracellular levels of glutamate in the NAc was tested with microdialysis. Baclofen (2.0 mg/kg) did not alter basal glutamate levels. However, baclofen pretreatment prevented the predatory odor, 2,5-dihydro-2,4,5-trimethylthiazoline, from increasing glutamate levels. This is the first report of baclofen modulating extracellular levels of glutamate in the NAc. Baclofen may prove to have general utility for suppressing stimulus-evoked increases in NAc glutamate levels. This could explain its ability to prevent cocaine-conditioned responses. In summary, our results suggest that enhancing GABA(B) transmission inhibits cocaine-conditioned responses, possibly by suppressing glutamate transmission in the NAc. A better understanding of interactions between GABA and glutamate transmission in the NAc may lead to the development of pharmacotherapies for cocaine craving.

GABAergic dysfunction in mood disorders

The authors review the available literature on the preclinical and clinical studies involving GABAergic neurotransmission in mood disorders. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter present almost exclusively in the central nervous system (CNS), distributed across almost all brain regions, and expressed in interneurons modulating local circuits. The role of GABAergic dysfunction in mood disorders was first proposed 20 years ago. Preclinical studies have suggested that GABA levels may be decreased in animal models of depression, and clinical studies reported low plasma and CSF GABA levels in mood disorder patients. Also, antidepressants, mood stabilizers, electroconvulsive therapy, and GABA agonists have been shown to reverse the depression-like behavior in animal models and to be effective in unipolar and bipolar patients by increasing brain GABAergic activity. The hypothesis of reduced GABAergic activity in mood disorders may complement the monoaminergic and serotonergic theories, proposing that the balance between multiple neurotransmitter systems may be altered in these disorders. However, low GABAergic cortical function may probably be a feature of a subset of mood disorder patients, representing a genetic susceptibility. In this paper, we discuss the status of GABAergic hypothesis of mood disorders and suggest possible directions for future preclinical and clinical research in this area.

GABA(B) receptor agonists for the treatment of drug addiction: a review of recent findings

A growing preclinical and clinical literature suggests that GABA(B) receptor agonists promote abstinence and reduce the use of cocaine, heroin, alcohol and nicotine. The purpose of this paper is to critically review these data. GABA(B) receptor agonists, such as baclofen, appear to reduce the reinforcing effects of abused drugs in animal models under multiple experimental procedures. This occurs at doses that have little effect on responding for other positive reinforcers such as food or water. We review evidence that these potential therapeutic effects may be mediated by modulation of mesolimbic dopamine neurons. This review also examines the preliminary clinical data from studies of the efficacy of baclofen for treatment of cocaine, alcohol, and nicotine dependence. We suggest that these preliminary data provide a rationale for conducting more systematic studies of the effects of GABA(B) receptor agonists as treatment for drug abuse. This line of research may also improve our understanding of the neurochemical mechanisms underlying the drug dependence process.

GABA modulation of cocaine self-administration

Recent experiments suggest that GABA compounds produce a clinically relevant modulation of cocaine reinforcement. This review summarizes the results of a number of studies that examined the effect of the GABAB agonist baclofen on cocaine self-administration using a variety of schedules of reinforcement. The results demonstrate that baclofen causes a dramatic reduction in cocaine self-administration which does not appear to be accounted for by a general disruption of behavior. However, the effect is dependent on the unit injection dose of cocaine and on the response requirements of the schedule. These results predict that in a clinical setting any potential therapeutic effect of baclofen will interact with the cost and availability of cocaine.

GABA function in mood disorders: an update and critical review

Over the past twenty years, several lines of evidence from preclinical and clinical studies has accumulated suggesting that a GABA deficit may be involved in mood disorders, particularly in depression, and that increasing GABAergic neurotransmission may exert an antidepressant effect and perhaps a mood stabilizing effect. Given that GABA has an inhibitory effect on biogenic amine neurotransmitters such as norepinephrine and serotonin and this inhibition may be involved in local circuits and interneurons, it has been suggested that the hypothesis of a GABA deficit in mood disorders does not compete with but complements the well-established hypotheses of alterations in noradrenergic and serotonergic function in mood disorders. In this paper, we systematically reviewed the results from preclinical and clinical studies of GABA function in the pathophysiology of mood disorders and in the mechanism of action of mood stabilizers, antidepressants and electroconvulsive therapy. We also discussed the unifying theory of the neurochemistry of mood disorders, which integrates the GABA hypothesis into the biogenic amine hypotheses, and indicated future directions for research.

Involvement of GABAB receptor systems in experimental depression: baclofen but not bicuculline exacerbates helplessness in rats

There are two gamma-aminobutyric acid (GABA) hypotheses of the antidepressants action: an increase in GABAA neurotransmission or a decrease in GABAB neurotransmission may contribute to action of antidepressants. In this study, involvement of GABAA and GABAB receptor systems was examined in the learned helplessness paradigm in rats. Rats were injected with bicuculline or baclofen for 14 days. On day 14, the rats were subjected to 15 inescapable shocks. On day 15, they underwent the 40-trial escape test. Baclofen exacerbated the escape failures in the rats subjected to the inescapable shocks, although baclofen had no effects in the animals without shock pre-treatment. Bicuculline failed to influence the escape failures in the rats with the 15-shock pre-treatment. These results suggest that the long-term increase in GABAB neurotransmission but not the long-term attenuation of GABAA neurotransmission may be related to helplessness in rats.

Involvement of GABAB receptor systems in action of antidepressants. II: Baclofen attenuates the effect of desipramine whereas muscimol has no effect in learned helplessness paradigm in rats

Involvement of GABAergic systems in action of antidepressants was examined in the learned helplessness paradigm in rats. Rats were treated with desipramine, baclofen or muscimol for 14 days. On day 14, the rats were subjected to 90 inescapable shocks. On day 15, the rats received the 40-trial escape test. The inescapable shocks induced the subsequent increase in escape failures in the escape test. Desipramine dose-dependently improved the increased escape failures induced by the inescapable shocks. Baclofen attenuated the escape failures-improving effect of desipramine, although baclofen had no effects on the increased escape failures when it was injected alone. Muscimol at any dose failed to influence the increased escape failures. Therefore, it is suggested that the long-term decrease in GABAB neurotransmission may be involved in the action of antidepressants. Our present results do not support the hypothesis that activation of GABAA receptors may contribute to the action of antidepressants.

Involvement of GABA(B) receptor systems in action of antidepressants: baclofen but not bicuculline attenuates the effects of antidepressants on the forced swim test in rats

Involvement of GABAergic systems in action of antidepressants was examined in the forced swim test in rats. Rats were forced to swim in a cylinder for 15 min on day 1 and for 5 min on day 2. Desipramine, mianserin and buspirone, administered after the 15-min swim session on day 1 and before the 5-min swim test on day 2, dose-dependently decreased the duration of immobility in the swim test on day 2. Baclofen attenuated the decreased duration of immobility induced by desipramine, mianserin and buspirone in the swim test, although baclofen did not affect the duration of immobility when it was injected alone. Muscimol dose-dependently decreased the duration of immobility in the swim test on day 2. Bicuculline antagonized the decreased duration of immobility induced by muscimol. However, bicuculline failed to antagonize the decreased duration of immobility induced by desipramine, mianserin and buspirone. These results suggest that GABA(B) but not GABA(A) receptor systems may be involved in action of antidepressants.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Chronic administration of the antidepressant phenelzine and its N-acetyl analogue: effects on GABAergic function

The MAO inhibitor phenelzine (2-phenylethylhydrazine; PLZ) is used widely in psychiatry for the treatment of depression and panic disorder. Its N-acetyl metabolite, N2-acetylphenelzine (N2AcPLZ) is a reasonably potent nonselective inhibitor of monoamine oxidase (MAO) that causes elevation in brain levels of the biogenic amines. In the studies reported here, PLZ (0.05 mmol/kg/day), N2AcPLZ (0.10 mmol/kg/day) or vehicle were administered to male rats for 28 days s.c. with Alzet minipumps, and their effects on GABAergic function were examined. Whole brain concentrations of gamma-aminobutyric acid (GABA) were significantly elevated in the PLZ but not in the N2AcPLZ-treated group. PLZ was found to inhibit the anabolic enzyme glutamic acid decarboxylase (GAD) and, to a greater extent, the catabolic enzyme GABA transaminase (GABA-T). The results of these investigations suggest that the free hydrazine moiety in PLZ is crucial to producing the elevated levels of GABA, probably through inhibition of GABA-T. Despite the considerable increase in whole brain GABA levels in the PLZ-treated rats, there were no significant differences in GABAA or benzodiazepine receptor binding parameters (KD or Bmax) between the groups as measured using 3H-muscimol and 3H-flunitrazepam in radioligand binding assays.

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.

Effects of the antidepressant/antipanic drug phenelzine on GABA concentrations and GABA-transaminase activity in rat brain

The effects of long-term (28-day) administration of several antidepressant/antipanic drugs [imipramine, desipramine, tranylcypromine and phenelzine (PLZ)] on gamma-aminobutyric acid-tranaminase (GABA-T) activity and GABA levels were investigated in rat frontal cortex. Of the drugs investigated, only PLZ inhibited GABA-T and elevated GABA levels. Additional short-term experiments were conducted with PLZ, and they demonstrated a dose-dependent inhibition of GABA-T in rat whole brain. Time-response studies on inhibition of GABA-T in whole brain demonstrated that at a dose of PLZ of 15 mg/kg i.p. inhibition of GABA-T remained relatively constant from 1 to 8 hr and that the enzyme was still inhibited by 23% at 24 hr after PLZ administration.

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.