Effects of long-term acetyl-L-carnitine administration in rats: I. increased dopamine output in mesocorticolimbic areas and protection toward acute stress exposure.

Acetyl-L-carnitine (ALCAR) is the acetyl ester of carnitine that has been reported to be beneficial in depressive disorders and Alzheimer's disease. A 7-day administration of ALCAR in rats increased dopamine and serotonin output in the nucleus accumbens shell and it prevented the development of escape deficit produced by acute exposure to unavoidable stress. No tolerance developed to this protective effect, which appeared to be mediated by (1) the activation of 5-HT(1A) receptors, as it was antagonized by the administration of WAY100635 30 min before stress exposure; and (2) a process of neuronal plasticity dependent on NMDA receptor activity, as subcutaneous dizocilpine infusion during ALCAR treatment prevented the development of the protective effect on stress. Chronic stress exposure maintains an escape deficit condition that is reverted by a long-term treatment with antidepressants, but the same condition was not modified by long-term ALCAR administration. Thus, ALCAR cannot be defined as an antidepressant.     

Chronic stress increases the binding of the A1 adenosine receptor agonist, [3H]cyclohexyladenosine, to rat hypothalamus

We previously reported an increase in the binding of [3H]cyclohexyladenosine ([3H]CHA) to brain membranes from the hypothalamus of rats sacrificed following three days of chronic stress in an around-the-clock intermittent footshock avoidance/escape paradigm ("sustained performance" stress). Here we report stress-induced increases in [3H]CHA binding to hypothalamic membranes from rats stressed for 14 days in that escape/avoidance paradigm, in rats exposed to repeated restraint (3 hr/day for 10 days) and in rats exposed to four days of "activity-stress." Data from saturation binding experiments indicate that this up-regulation was due to an increase in the apparent number of [3H]CHA binding sites without change in affinity for [3H]CHA. Neither restraint for one three-hr period nor one 15-min exposure to intermittent footshock resulted in significant changes in [3H]CHA binding to hypothalamic membranes. Our present data demonstrate small but consistent increases in the number of [3H]CHA binding sites in hypothalamic membranes from rats stressed in several different chronic stress models but no change by acute stress.     

Morphine sensitization as a model of mania: comparative study of the effects of repeated lithium or carbamazepine administration

Repeated unavoidable stress induces in rats decreased reactivity to avoidable stressors and an anhedonia-like condition that are reverted by long-term antidepressant treatments and regarded as models of core symptoms of depression. Morphine-sensitized rats present resilience to stress-induced behavioral deficits and, if hyporeactivity to stress models a depressive symptom, stress resistance can be regarded as a manic symptom. This hypothesis is supported by the observation that long-term lithium administration reinstates sensitivity to stress in sensitized rats. The first aim of the study was to examine the effects of carbamazepine, a standard antimanic treatment, on the stress resilience of sensitized rats, to further characterize morphine sensitization as a model of manic symptom. Carbamazepine administration abolished stress resilience but did not interfere with the expression of sensitization. The second aim of the study was to assess whether repeated carbamazepine treatment affected the dopaminergic and behavioral responses to a natural reward, a palatable food (vanilla sugar, VS), in non food-deprived sensitized and control rats and compare these possible effects with those of repeated lithium administration. Control and sensitized rats showed increased extraneuronal dopamine levels in the nucleus accumbens shell after VS consumption and competence to acquire an instrumental VS-sustained appetitive behavior (VAB). Repeated carbamazepine treatment abolished the dopaminergic response to VS consumption and disrupted the competence to acquire VAB in control rats. Lithium-treated rats showed a dopaminergic response to VS and easily acquired the appetitive behavior. In sensitized rats, neither carbamazepine nor lithium administration interfered with the dopaminergic response to VS and the acquisition of VAB. In summary, the effect of carbamazepine on the stress resilience of sensitized rats further supported the hypothesis that morphine sensitization might model some symptoms of mania. Moreover, in control rats carbamazepine treatment elicited an anhedonia-like condition that clearly distinguished the effects of this drug from those of lithium.     

Study of mirtazapine antidepressant effects in rats

Mirtazapine is a widely used antidepressant and the aim of this study was to further investigate its antidepressant activity in rats. Thus, the efficacy of long-term mirtazapine treatment was assessed in three models of depressive symptoms induced by stress exposure: the acute escape deficit, the chronic escape deficit, and the stress-induced disruption of the acquisition of an appetitive behaviour sustained by a palatable food (vanilla sugar). Administration of mirtazapine for 2 wk prevented the escape deficit development induced by acute exposure to unavoidable stress. This protective effect was antagonized by the administration of a beta-adrenergic or a 5-HT1A receptor antagonist just before stress exposure; that is, mirtazapine effect was dependent on functional beta-adrenergic and 5-HT1A receptor systems. Repeated stress exposure indefinitely prolongs the condition of escape deficit and a 40-d mirtazapine treatment reversed this model of chronic escape deficit. In a Y-maze satiated rats learn to choose the arm baited with vanilla sugar, and exposure to stress during Y-maze training prevents this learning. Repeated mirtazapine administration completely antagonized the disrupting effect of chronic stress on the acquisition of this instrumental behaviour. We consider these effects to be crucial in the definition of antidepressant activity.     

Neurochemical and behavioral consequences of mild, uncontrollable shock: effects of PCPA

The present experiments examined the role of the serotonergic system in the behavioral deficit produced by uncontrollable shock. In Experiment 1: Establishment of model, the behavioral potential of the Sprague-Dawley rat was defined. When exposed to mild uncontrollable stress such as a 0.8 mA electric footshock, a significant percentage of rats developed a shock escape deficit which was evident when subsequently placed in a shock escape paradigm. Serotonin depletion was produced by chronic treatment with p-chlorophenylalanine. Biogenic amine levels and 5-HT levels were monitored in various brain areas using HPLC. Following chronic treatment with PCPA, the shock escape capability of the Sprague-Dawley rat was assessed. The severe depletion of 5-HT in various brain regions was highly correlated with a dramatic improvement in the shock escape scores. Thus, the detrimental effects of exposure to a mild course of inescapable shock can be prevented by chronic treatment with PCPA. These experiments implicate the serotonergic system as a possible mediator of the "learned helplessness" phenomenon.     

The influence of antidepressants on aggressive behavior in stressed rats: the role of dopamine.

The influence of dopamine (DA) receptor blockers (haloperidol, sulpiride) on electric footshock-induced fighting behavior and on the effect of antidepressants (imipramine, clomipramine, nomifensine, mianserine) was investigated in chronically stressed male Wistar rats. Exploratory activity in an open field was measured in the same groups of animals. The effect of chronic stress and antidepressants on DA utilization in the brain was also investigated. It was shown that 48 h after the last session of repeated stress (various unpredictable stressors over 16 days) the number of fighting attacks was significantly reduced. However in stressed rats treated chronically (for 14 days) with antidepressants the intensity of fighting was restored to control value. On the contrary, when the stressed rats, receiving antidepressants chronically, were pretreated with DA receptor blockers: haloperidol (0.5 mg/kg) or sulpiride (50 mg/kg) but also alpha 1-adrenergic receptor blocker - prazosin (3 mg/kg) the effect of antidepressants was abolished. Exploratory activity was not significantly reduced under influence of stress. Neither antidepressants nor sulpiride modified exploratory activity of stressed rats. Haloperidol and prazosin but not sulpiride decreased this activity of normal, stressed and antidepressant-treated rats. It is concluded that prolonged treatment with antidepressants counteracts the decrease in aggression induced by chronic stress and that DA mechanism participate in this effect of antidepressant drugs.     

Reversal of stress-induced memory changes by moclobemide: the role of neurotransmitters.

Studies on animals have shown that chronic stress is able to evoke behavioral changes such as locomotor activity deficit, decreased sleep, reduced food and water consumption and impaired memory. Chronic stress produces changes in concentrations of neurotransmitters, mainly in the hippocampus. The hippocampus is a vulnerable brain structure that is involved in learning and memory functions. In this study, we investigated the effects of chronic stress procedure and moclobemide in rats, and the influence of chronic stress on the levels of monoamines: noradrenaline (NE), dopamine (DA) and serotonin (5-HT) in the rat hippocampus [as well as their metabolites: dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA)]. It was found out that chronic 21-day stress caused worsening of memory: the well trained rats after stress procedure lost their ability to find food quickly. Because of many errors in finding the way, the time these animals needed was on average 2.4-times longer than that of the control group. Single, as well as prolonged (21 days) treatment with moclobemide (10 mg/kg/day) counteracted the deficit of memory induced by chronic stress. In stressed animals, we observed an increase in DA, decrease in DOPAC, 5-HT and 5-HIAA and decrease in NE levels. Moclobemide modulated the changes in the levels of neurotransmitters in the hippocampus, decreasing their turnover. The results demonstrate that moclobemide improves memory impaired by stress. They suggest also that moclobemide has a modulatory effect on stress-induced neurotransmitter changes which may be of importance for the protective effect of the drug with regard to memory impairment.     

Effects of stress on [3H]cyclohexyladenosine binding to rat brain membranes

The investigation of stress-induced changes in neuronal functioning is important to our understanding of mental disorders, stress-induced psychological impairment, and the emotional reactions of fear and anxiety. Data from previous animal studies have demonstrated various pituitary-adrenal responses to stress and also changes in brain neurotransmitters. We are investigating whether stress-induced neuroendocrine and brain monoamine changes are accompanied by concomitant changes in brain neurotransmitter and/or neuromodulator receptors. We have developed a behavioral paradigm of chronic stress which incorporates sustained stress, continuous performance, and disruption of sleep. Animals which are habituated to press a lever to receive food are trained in an active shock escape task. A matched set of animals housed in identical operant chambers but not exposed to footshock are used as comparative controls. [3H]Cyclohexyladenosine ([3H]CHA) (5-7 nM) binding to A1 adenosine receptors in hypothalamic membrane preparations from rats stressed for three days was fifteen percent higher than in matched controls. However, no differences in [3H]CHA binding were found in tissue preparations from frontal cortex, hippocampus, or striatum, when comparing stressed and matched control rats. Plasma corticosterone levels were higher in stressed rats than in matched controls.     

Effect of acute and chronic conditions of over-crowding on free choice ethanol intake in rats

Male albino rats of Wistar strain were exposed to overcrowding stress in two different groups for a period of seven days. One group of rats was kept under stress for six hours per day (acute stressed group) and the other group rats was kept under stress continuously (chronic stressed group). The effect of these acute and chronic stresses on voluntary alcohol (2% w/v) intake was monitored during the 7 days of stress exposure, and ethanol preference and total ethanol intake in terms of g/kg body weight were also studied. A significant increase in ethanol preference and ethanol intake was observed in one-day and 7 days chronic stressed group. No significant increase in ethanol intake was observed in acute stress. Thus a short lasting stressor may not increase ethanol-drinking behavior, whereas when animals were exposed to more intense stressor continuously for 7 days, an increase in voluntary drinking behavior may be seen.     

Effects of cocaine combined with a social cue on conditioned place preference and nucleus accumbens monoamines after isolation rearing in rats

Rationale

Social interaction during drug exposure can potentiate cocaine reward. Isolation rearing (ISO) during adolescence increases social interaction and may amplify this potentiation.

Objectives

The objectives of this study are to determine whether ISO alters conditioned place preference (CPP) for cocaine when combined with a social cue and to determine whether ISO alters the effects of cocaine when combined with social cue on nucleus accumbens shell (NAcS) dopamine (DA) and serotonin (5-HT).

Methods

Male and female rats were either ISO or group (GRP) reared for 4 weeks during adolescence. CPP was performed using a low dose of cocaine (2 mg/kg or saline) with or without exposure to a novel same-sex conspecific during conditioning. In vivo microdialysis was performed using the same parameters.

Results

ISO rats engaged in more social and aggressive behaviors during conditioning relative to GRP. Cocaine reduced social and aggressive behaviors in all rats. CPP was not influenced by rearing condition. Cocaine produced significant CPP, and a social cue produced CPP only in males. In contrast, the interaction of cocaine and a social cue on NAcS DA and 5-HT differed depending upon rearing condition. In isolates, cocaine-induced DA was attenuated, while cocaine plus a social cue produced potentiated DA and 5-HT.

Conclusions

Exposure to a low dose of cocaine in the presence of a social cue produced additive effects on CPP while producing synergistic effects on DA and 5-HT in the NAcS of ISO rats. The aversive effects of this compound stimulus may negate the rewarding effects in isolates.     

Acetylsalicylic acid accelerates the antidepressant effect of fluoxetine in the chronic escape deficit model of depression

Evidence has accumulated suggesting that major depression is associated with dysfunction of inflammatory mediators. Moreover, antidepressants show an anti-inflammatory action possibly related to their clinical efficacy. An improvement in psychiatric symptoms has been recently reported in patients treated with anti-inflammatory drugs for other indications. These data imply that inflammation may be involved in the pathogenesis of depression and that anti-inflammatory drugs may be used as an adjunctive therapy. The aim of the present study was to evaluate the behavioural effect of the co-administration of acetylsalicylic acid (ASA, 45 mg/kg or 22.5 mg/kg) and fluoxetine (FLX, 5 mg/kg) in the chronic escape deficit model of depression. The chronic escape deficit model is based on the modified reactivity of rats to external stimuli induced by exposure to unavoidable stress and allows evaluation of the capacity of a treatment to revert the condition of escape deficit. In this model, FLX alone needs to be administered for at least 3 weeks to revert this condition. Our results show that combined treatment of fluoxetine and ASA completely reverted the condition of escape deficit by as early as 7 days, the effect being already partially present after 4 days. The effect was maintained after 14 and 21 days of treatment. ASA alone was ineffective at any time tested and the effect of fluoxetine was significant only at 21 days. These results, together with clinical data from preliminary results, suggest that ASA might accelerate the onset of action of selective serotonin reuptake inhibitor antidepressants.     

Voluntary alcohol consumption alters stress-induced changes in dopamine-2 receptor binding in Wistar-Kyoto rat brain

The Wistar-Kyoto (WKY) rat has been proposed as an animal model of depressive behavior and exhibits hyper-responsiveness to stressful stimulation when compared to other rat strains. We have demonstrated that WKY rats consume 200% more alcohol under naïve conditions as compared to their outbred counterparts, Wistar (WIS) rats. The present study was designed to understand the influence of stress and alcohol consumption on central dopamine type-2 (D2) receptor sites in these two behaviorally distinct rat strains. The first part of this study examined the effects of chronic stress on alcohol consumption, while the second part examined the binding of [¹²⁵I]-Iodosulpiride to D2 receptors in control, stressed or stress and alcohol co-treated WKY compared to WIS rats. Exposure to chronic stress led to an increase in the amount of alcohol consumed by both rat strains, with WKY rats consuming significantly more alcohol than WIS rats with or without stress exposure. Quantitative autoradiography experiments showed that chronic stress increased D2 receptor binding in the caudate putamen (CPu), nucleus accumbens (NAc), substantia nigra (SN) and ventral tegmental area (VTA) of WKY rats, and reduced receptor binding in the CPu and SN of WIS rats. Compared to the stressed animals, WKY rats co-treated with stress and alcohol demonstrated a reduction in D2 receptor sites in the cell body regions (SN and VTA), while WIS rats showed no changes in receptor binding. The observed changes in D2 receptor sites may indicate altered DA neurotransmission following stress and alcohol exposure. Since stressed WKY rats consumed more alcohol, it is possible that consumption of alcohol reverses the stress-induced D2 receptor alterations in the cell body regions, suggestive of a self medicating phenotype.     

Chronic cold stress reduces the spontaneous activity of ventral tegmental dopamine neurons.

The dopamine (DA) neurons in the ventral tegmental area and medial substantia nigra (VTA/mSN) projecting to the limbic forebrain and prefrontal cortex have long been postulated to play a major role in cognitive and behavioral effects of stress. In this study, the effects of a chronic stressor (prolonged exposure to cold) on the spontaneous activity of DA neurons in the VTA/mSN were examined. Extracellular single-unit recordings of DA neurons were performed in rats following a 17-day continuous exposure to a cold (4 degrees C) environment. Compared to controls, cold-exposed rats displayed 64% fewer spontaneously active DA neurons. The average spike activity (average firing rate, average spikes fired in bursts) of the DA cells that remained active in the cold-exposed rats did not differ significantly from controls. However, a significantly larger proportion of those cells showed excessive burst activity, compared to the DA cell population in controls. These results show that chronic stress can lead to the cessation of spontaneous activity in a subpopulation of VTA/mSN DA cells. These changes may indicate that unlike acute stress, which can potently activate the mesolimbic/mesocortical DA systems, chronic stress leads to an adaptive reduction in the number of active DA cells, perhaps altering the response of these systems to subsequent stressors.     

Neurotoxic effects of chronic restraint stress in the striatum of methamphetamine-exposed rats

Rationale Stress is a common experience in drug abusers. Methamphetamine (METH) is an abused psychostimulant that damages dopamine and serotonin terminals through pro-oxidant mechanisms and glutamate-mediated excitotoxicity. Both METH and stress increase dopamine and glutamate release in the striatum. Since dopamine inhibits striatal glutamate release and METH depletes dopamine, stress-induced glutamate release may be disinhibited after METH exposure. Objective We examined if repeated stress would worsen excitotoxic damage to the striatum after METH pretreatment. Materials and methods In vivo microdialysis was used to examine stress-induced striatal glutamate release in rats pre-exposed to METH (7.5 mg/kg × 4 injections) or saline. The effects on striatal DA, serotonin, DAT, SERT, and spectrin proteolysis produced by chronic restraint stress (CRS, 6 h/day for 21 days) in the presence or absence of corticosterone synthesis inhibition by metyrapone (50 mg/kg) beginning 7 days after METH were also examined. Results Stress-induced glutamate release was augmented in rats pre-exposed to METH. CRS 7 days after METH enhanced METH-induced DAT depletions from 23 to 44% in the nonstressed versus stressed rats, respectively. Striatal SERT and serotonin tissue content were decreased by 51 and 36%, respectively, in rats exposed to both METH and CRS but was unchanged by either treatment alone. Spectrin proteolysis was increased by 53% in rats treated with both METH and CRS but was unaffected by either treatment alone. Metyrapone blocked the effects of CRS on METH-induced depletions of SERT but not DAT. Conclusions Exposure to chronic stress depleted striatal dopamine and serotonin terminal markers possibly through excitotoxic mechanisms in METH-treated rats.     

Intracortical glutamate injection produces helpless-like behavior in the rat

Acute injection of glutamate into frontal neocortex of naive rats produced a subsequent deficit in escape performance behavior that was similar to that produced by exposure to uncontrollable shock. The behavioral deficit was dose-related. The behavioral deficit was similar in time-course to that produced by 15 min (but not 40 min) of exposure to learned helplessness induction. Unlike learned helplessness produced by exposure to inescapable shock, the behavioral deficit produced by intracortical glutamate injection was not prevented by chronic intraperitoneal administration of imipramine.     

Chronic stress reduces fighting behavior of rats: the effect of antidepressants

The effect of chronic stress (14 various unpredictable stressors over 16 days) on electric footshock-induced fighting behavior of pairs of male Wistar rats was studied. The influence of antidepressant drugs (imipramine, desmethylimipramine, nomifensine, clomipramine, mianserine and doxepine) administered chronically (1 h before the stressor) on the aggressive behavior was also investigated in control and in stressed rats. Moreover, the effect of chronic stress on noradrenaline (NA) utilization in the brain was estimated in control and in antidepressant-treated rats. It was demonstrated that, in rats submitted to repeated unpredictable stress, the fighting behavior was significantly reduced 48 and 72 h after the last stressor. NA utilization in the brain was decreased 72 h after the stress termination. Prolonged treatment with antidepressant drugs restored the intensity of fighting behavior in stressed rats to control value as well as normalized NA utilization in the brain. It is suggested that antidepressant drugs may counteract the affective aggression deficit induced by chronic stress.     

Reversal of stress-induced deficit in aggression by monoamine oxidase inhibitors

In the present study we investigated the effect of two monoamine oxidase (MAO) inhibitors: moclobemide (selective, reversible inhibitor of MAO-type A) or selegiline (selective irreversible inhibitor of MAO-type B) on electric footshock-induced fighting behavior in normal (unstressed) and chronically stressed (14 various stressors over 16 days) rats. In rats exposed to chronic stress the number of fighting attacks was reduced by about 75%. Prolonged (once a day, for 14 days) treatment with moclobemide (50 mg/kg/day) or selegiline (2 mg/kg/day) counteracted the deficit in aggression induced by chronic stress. The findings of the present study demonstrate that the selective MAO inhibitors, moclobemide and selegiline, protect against "behavioral depression" induced by the chronic stress similarly to other classes of antidepressant drugs.     

Influence of different antidepressant drugs on the effect of chronic variable stress on restraint-induced dopamine release in frontal cortex.

The aim of this study was to evaluate the influence of an early chronic variable stress procedure (CVS) associated or not with repeated administration of various antidepressants on cortical restraint-induced dopamine (DA) release in vivo. Animals were subjected to the CVS schedule and one day after submitted to persistent administration with vehicle, desipramine (DMI, 10 mg/kg, i.p.), fluoxetine (FLU, 10 mg/kg, i.p.) or phenelzine (PHE; 10 mg/kg, i.p.) and later on exposed to a 60-min restraint period. In addition, we also explored the effect of acute administration of these antidepressants on cortical DA overflow in response to restraint in CVS treated rats. A higher increase in cortical DA release in response to restraint was observed in CVS animals as compared with those without previous CVS. Persistent, but not acute, administration with DMI, FLU and PHE blocked the sensitized output induced by restraint following CVS exposure.     

5-HT1b receptors in an animal model of depression

After exposure to a 0.8 mA course of uncontrollable shocks, Sprague-Dawley rats can be differentiated into two distinct groups defined in term of their performance in a shock escape paradigm. Learned helpless (LH) rats do not learn to escape a controllable shock while non-helpless (NLH) rats learn this response as quickly as naive controls (NC) rats do. The current experiments were designed to extend our studies of 5-HT receptors in these three groups of rats. The major finding in the present study concerned post-synaptic 5-HT receptor effects in the cortex, hippocampus, septum and hypothalamus of LH rats. These included an up regulation of 5-HT1b receptors in the cortex, hippocampus and septum in LH rats. In contrast, 5-HT1b receptors in the hypothalamus of LH rats were down-regulated. These results implicate serotonergic mechanisms in the behavioral deficit caused by uncontrollable shock with a limbic-hypothalamic circuit serving as a center for adaptation to stress.     

Females do not express learned helplessness like males do.

Women are more likely than men to suffer from stress-related mental disorders, such as depression. In the present experiments, we identified sex differences in one of the most common animal models of depression, that of learned helplessness. Male and female rats were trained to escape a mild footshock each day for 7 days (controllable stress). Each rat was yoked to another rat that could not escape (uncontrollable stress), but was exposed to the same amount of shock. One day later, all stressed rats and unstressed controls were tested on a more difficult escape task in a different context. Most males exposed to uncontrollable stress did not learn to escape and were therefore helpless. In contrast, most females did learn to escape on the more difficult escape task, irrespective of whether they had been exposed to controllable or uncontrollable stress. The sex differences in helplessness behavior were not dependent on the presence of sex hormones in adulthood, because neither ovariectomy of females nor castration of males abolished them. The absence of helplessness in females was neither dependent on organizational effects of testosterone during the day of birth, because masculinized females did not express helplessness as adults. Thus, sex differences in helplessness behavior are independent of gonadal hormones in adulthood and testosterone exposure during perinatal development. Learned helplessness may not constitute a valid model for depressive behavior in women, at least as reflected by the response of female rats to operant conditioning procedures after stressful experience.