The effects of long-term administration of rubidium or lithium on reactivity to stress and on dopamine output in the nucleus accumbens in rats

Intracellular effects of lithium in aging neurons

Lithium therapy received approval during the 1970s, and it has been used for its antidepressant, antimanic, and anti-suicidal effects for acute and long-term prophylaxis and treatment of bipolar disorder (BPD). These properties have been well established; however, the molecular and cellular mechanisms remain controversial. In the past few years, many studies demonstrated that at the cellular level, lithium acts as a regulator of neurogenesis, aging, and Ca2+ homeostasis. At the molecular level, lithium modulates aging by inhibiting glycogen synthase kinase-3β (GSK-3β), and the phosphatidylinositol (PI) cycle; latter, lithium specifically inhibits inositol production, acting as a non-competitive inhibitor of inositol monophosphatase (IMPase). Mitochondria and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) have been related to lithium activity, and its regulation is mediated by GSK-3β degradation and inhibition. Lithium also impacts Ca2+ homeostasis in the mitochondria modulating the function of the lithium-permeable mitochondrial Na+-Ca2+exchanger (NCLX), affecting Ca2+ efflux from the mitochondrial matrix to the endoplasmic reticulum (ER). A close relationship between the protease Omi, GSK-3β, and PGC-1α has also been established. The purpose of this review is to summarize some of the intracellular mechanisms related to lithium activity and how, through them, neuronal aging could be controlled.

Clozapine but not lithium reverses aberrant tyrosine uptake in patients with bipolar disorder

RATIONALE

Availability of the dopamine and noradrenaline precursor tyrosine is critical for normal functioning, and deficit in tyrosine transport across cell membrane and the blood-brain barrier has been reported in bipolar disorder and schizophrenia. Clozapine and lithium are two psychoactive agents used to treat psychosis, mood disorders and suicidal behavior, but their mechanism of action remains largely unknown.

OBJECTIVE

To characterize immediate and delayed differences in tyrosine uptake between healthy controls (HC) and bipolar patients (BP) and see if these differences could be normalized by either clozapine, lithium or both. A second objective was to see if clozapine and lithium have additive, antagonistic or synergistic effects in this.

METHOD

Fibroblasts from five HC and five BP were incubated for 5 min or 6 h with clozapine, lithium, or combination of both. Radioactive labelled tyrosine was used to quantify tyrosine membrane transport.

RESULTS

There was significantly reduced tyrosine uptake at baseline in BP compared to HC, a deficit that grew with increasing incubation time. Clozapine selectively increased tyrosine uptake in BP and abolished the deficit seen under baseline conditions, while lithium had no such effect. Combination treatment with clozapine and lithium was less effective than when clozapine was used alone.

CONCLUSIONS

There was significant deficit in tyrosine transport in BP compared to HC that was reversed by clozapine but not lithium. Clozapine was more effective when used alone than when added together with lithium. Potential clinical implications of this will be discussed.

Mood and behavior regulation: interaction of lithium and dopaminergic system

Lithium is one of the most effect mood-stabilizing drugs prescribed especially for bipolar disorder. Lithium has wide range effects on different molecular factors and neural transmission including dopaminergic signaling. On the other hand, mesolimbic and mesocortical dopaminergic signaling is significantly involved in the pathophysiology of neuropsychiatric disorders. This review article aims to study lithium therapeutic mechanisms, dopaminergic signaling, and the interaction of lithium and dopamine. We concluded that acute and chronic lithium treatments often reduce dopamine synthesis and level in the brain. However, some studies have reported conflicting results following lithium treatment, especially chronic treatment. The dosage, duration, and type of lithium administration, and the brain region selected for measuring dopamine level were not significant differences in different chronic treatments used in previous studies. It was suggested that lithium has various mechanisms affecting dopaminergic signaling and mood, and that many molecular factors can be involved, including brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), β-catenin, protein kinase B (Akt), and glycogen synthase kinase-3 beta (GSK-3β). Thus, molecular effects of lithium can be the most important mechanisms of lithium that also alter neural transmissions including dopaminergic signaling in mesolimbic and mesocortical pathways.

Breast-Milk Rubidium and Other Trace Elements are Associated with Neurocognitive Development in Infants at Age of 8 Months

BACKGROUND

Trace elements may affect neurodevelopment. There is a lack of data on breast-milk rubidium (Rb) in relation to neurodevelopment in infants. The associations of copper (Cu), zinc (Zn) and strontium (Sr) with neurodevelopment in infants remain uncertain.

OBJECTIVES

We sought to evaluate the associations of breast-milk Rb (primary exposure), Cu, Zn, and Sr with neurodevelopment in infants at age 8 months.

METHODS

The study cohort included 117 breastfed infants. Breast-milk samples were collected at 42 days and 8 months postpartum. Breast-milk Rb, Zn, Cu, and Sr were measured by inductively coupled plasma mass spectrometer. Neurodevelopment was assessed at age 8 months. The primary outcomes were attention and working memory scores, as evaluated by the A-not-B task. Other outcomes included the Mental Development Index (MDI) and Psychomotor Development Index (PDI) as evaluated by the Bayley Scale of Infant Development III. Generalized linear models and restricted cubic spline regression were used to assess the associations between trace elements and neurodevelopment indices. Bonferroni correction was conducted on all data presented.

RESULTS

A nonlinear association was observed between breast-milk Rb at 42 days and infant's attention at age 8 months (nonlinearity P = 0.037). Positive associations were observed with infant MDI scores and breast-milk Rb at 42 days (β = 4.46; P = 0.06) and 8 months (β = 3.79; P = 0.009) postpartum. Breast-milk Zn at 42 days was positively associated with infant's attention (β = 0.31; P = 0.039). Sr at 42 days was positively correlated with attention (β = 0.18; P = 0.043) and MDI scores (β = 2.18; P = 0.015) at 8 months. Inverted U-shape associations were observed for breast-milk Cu at 42 days with infant attention and PDI scores. All associations were not significant after correction for multiple tests.

CONCLUSIONS

Our data suggest that Rb, Zn, Cu, and Sr in breast milk at certain concentrations are associated with neurodevelopment in breastfed infants. Further studies are warranted to validate the findings.

Lithium and Erectile Dysfunction: An Overview

Lithium has been a mainstay of therapy for patients with bipolar disorders for several decades. However, it may exert a variety of adverse effects that can affect patients' compliance. Sexual and erectile dysfunction has been reported in several studies by patients who take lithium as monotherapy or combined with other psychotherapeutic agents. The exact mechanisms underlying such side effects of lithium are not completely understood. It seems that both central and peripheral mechanisms are involved in the lithium-related sexual dysfunction. Here, we had an overview of the epidemiology of lithium-related sexual and erectile dysfunction in previous clinical studies as well as possible pathologic pathways that could be involved in this adverse effect of lithium based on the previous preclinical studies. Understanding such mechanisms could potentially open a new avenue for therapies that can overcome lithium-related sexual dysfunction and improve patients' adherence to the medication intake.

The Genetics of Response to and Side Effects of Lithium Treatment in Bipolar Disorder: Future Research Perspectives

Although the mood stabilizer lithium is a first-line treatment in bipolar disorder, a substantial number of patients do not benefit from it and experience side effects. No clinical tool is available for predicting lithium response or the occurrence of side effects in everyday clinical practice. Multiple genetic research efforts have been performed in this field because lithium response and side effects are considered to be multifactorial endophenotypes. Available results from linkage and segregation, candidate-gene, and genome-wide association studies indicate a role of genetic factors in determining response and side effects. For example, candidate-gene studies often report GSK3β, brain-derived neurotrophic factor, and SLC6A4 as being involved in lithium response, and the latest genome-wide association study found a genome-wide significant association of treatment response with a locus on chromosome 21 coding for two long non-coding RNAs. Although research results are promising, they are limited mainly by a lack of replicability and, despite the collaboration of consortia, insufficient sample sizes. The need for larger sample sizes and “multi-omics” approaches is apparent, and such approaches are crucial for choosing the best treatment options for patients with bipolar disorder. In this article, we delineate the mechanisms of action of lithium and summarize the results of genetic research on lithium response and side effects.

Nitric oxide and glutamate are contributors of anti-seizure activity of rubidium chloride: A comparison with lithium

The neuro-protective effects of rubidium and lithium as alkali metals have been reported for different central nervous system dysfunctions including mania and depression. The aim of this study was evaluating as well as comparing the effects of rubidium chloride (RbCl) and lithium chloride (LiCl) on different seizures paradigms in mice and determining the involvement of NMDA receptors and nitrergic pathway. To assess the seizures threshold, animals received intravenous pentylenetetrazole (PTZ, 0.5%; 1 mL/min). Male NMRI mice (6-8 weeks) received intraperitoneal (i.p.) injections of different doses of RbCl and LiCl. Doses greater than 10 mg/kg of RbCl showed a significant anticonvulsant activity 60 min after administration; the anticonvulsant effects of LiCl was observed at the doses more than 5 mg/kg and after 30 min in PTZ-induced seizure threshold. But, RbCl (10, 20 mg/kg, i.p) or LiCl (5, 10 mg/kg, i.p) injection did not induce protection against maximal electroshock (MES) or intraperitoneal injection of PTZ lethal dose (80 mg/kg)-induced seizure models. Pre-treatment with L-NAME (non-selective nitric oxide synthase (NOS) inhibitor, 10 mg/kg; i.p.) and 7-nitroindazole (selective neuronal NOS inhibitor, 30 mg/kg; i.p.) enhanced the anticonvulsive effects of both RbCl (5 mg/kg, i.p.) and LiCl (1 mg/kg, i.p.) in PTZ-induced seizure threshold model. Injection of MK-801 (NMDA receptor antagonist, 0.05 mg/kg; i.p.) before RbCl (5 mg/kg, i.p.; P < 0.001) and LiCl (1 mg/kg, i.p.; P < 0.001) administration increased the anti-seizure activity. But, treatment with L-arginine (precursor of nitric oxide, 100 mg/kg; i.p.) decreased the seizure threshold of both RbCl (20 mg/kg, i.p.; P < 0.001) and LiCl (10 mg/kg, i.p.; P < 0.001). Measurement of nitrite levels in hippocampus of animals revealed a remarkable reduction after treatment with RbCl (20 mg/kg, i.p; P < 0.05) and LiCl (10 mg/kg, i.p; P < 0.01). To conclude, rubidium may protect central nervous system against seizures in PTZ-induced seizures threshold model through NMDA/nitrergic pathways with a similarity to lithium effects in mice.

Neurobehavioral effects of lithium in the rat: Investigation of the effect/concentration relationships and the contribution of the poisoning pattern

Severity of lithium poisoning depends on the ingested dose, previous treatment duration and renal function. No animal study has investigated neurobehavioral differences in relation to the lithium poisoning pattern observed in humans, while differences in lithium pharmacokinetics have been reported in lithium-pretreated rats mimicking chronic poisonings with enhanced brain accumulation in rats with renal failure. Our objectives were: 1)-to investigate lithium-related effects in overdose on locomotor activity, anxiety-like behavior, spatial recognition memory and anhedonia in the rat; 2)-to model the relationships between lithium-induced effects on locomotion and plasma, erythrocyte, cerebrospinal fluid and brain concentrations previously obtained according to the poisoning pattern. Open-field, elevated plus-maze, Y-maze and sucrose consumption tests were used. In acutely lithium-poisoned rats, we observed horizontal (p<0.001) and vertical hypolocomotion (p<0.0001), increased anxiety-like behavior (p<0.05) and impaired memory (p<0.01) but no altered hedonic status. Horizontal (p<0.01) and vertical (p<0.001) hypolocomotion peaked more markedly 24h after lithium injection and was more prolonged in acute-on-chronically vs. acutely lithium-poisoned rats. Hypolocomotion in chronically lithium-poisoned rats with impaired renal function did not differ from acutely poisoned rats 24h after the last injection. Interestingly, hypolocomotion/concentration relationships best fitted a sigmoidal E_max model in acute poisoning and a linear regression model linked to brain lithium in acute-on-chronic poisoning. In conclusion, lithium overdose alters rat behavior and consistently induces hypolocomotion which is more marked and prolonged in repeatedly lithium-treated rats. Our data suggest that differences between poisoning patterns regarding lithium-induced hypolocomotion are better explained by the duration of lithium exposure than by its brain accumulation.

The dopamine hypothesis of bipolar affective disorder: the state of the art and implications for treatment

Bipolar affective disorder is a common neuropsychiatric disorder. Although its neurobiological underpinnings are incompletely understood, the dopamine hypothesis has been a key theory of the pathophysiology of both manic and depressive phases of the illness for over four decades. The increased use of antidopaminergics in the treatment of this disorder and new in vivo neuroimaging and post-mortem studies makes it timely to review this theory. To do this, we conducted a systematic search for post-mortem, pharmacological, functional magnetic resonance and molecular imaging studies of dopamine function in bipolar disorder. Converging findings from pharmacological and imaging studies support the hypothesis that a state of hyperdopaminergia, specifically elevations in D2/3 receptor availability and a hyperactive reward processing network, underlies mania. In bipolar depression imaging studies show increased dopamine transporter levels, but changes in other aspects of dopaminergic function are inconsistent. Puzzlingly, pharmacological evidence shows that both dopamine agonists and antidopaminergics can improve bipolar depressive symptoms and perhaps actions at other receptors may reconcile these findings. Tentatively, this evidence suggests a model where an elevation in striatal D2/3 receptor availability would lead to increased dopaminergic neurotransmission and mania, whilst increased striatal dopamine transporter (DAT) levels would lead to reduced dopaminergic function and depression. Thus, it can be speculated that a failure of dopamine receptor and transporter homoeostasis might underlie the pathophysiology of this disorder. The limitations of this model include its reliance on pharmacological evidence, as these studies could potentially affect other monoamines, and the scarcity of imaging evidence on dopaminergic function. This model, if confirmed, has implications for developing new treatment strategies such as reducing the dopamine synthesis and/or release in mania and DAT blockade in bipolar depression.

Chronic lithium treatment rectifies maladaptive dopamine release in the nucleus accumbens

Chronic lithium treatment effectively reduces behavioral phenotypes of mania in humans and rodents. The mechanisms by which lithium exerts these actions are poorly understood. Pre-clinical and clinical evidence have implicated increased mesolimbic dopamine (DA) neurotransmission with mania. We used fast-scan cyclic voltammetry to characterize changes in extracellular DA concentrations in the nucleus accumbens (NAc) core evoked by 20 and 60 Hz electrical stimulation of the ventral tegmental area (VTA) in C57BL6/J mice treated either acutely or chronically with lithium. The effects of chronic lithium treatment on the availability of DA for release were assessed by depleting readily releasable DA using short inter-train intervals, or administering d-amphetamine acutely to mobilize readily releasable DA. Chronic, but not acute, lithium treatment decreased the amplitude of DA responses in the NAc following 60 Hz pulse train stimulation. Neither lithium treatment altered the kinetics of DA release or reuptake. Chronic treatment did not impact the progressive reduction in the amplitude of DA responses when, using 20 or 60 Hz pulse trains, the VTA was stimulated every 6 s to deplete DA. Specifically, the amplitude of DA responses to 60 Hz pulse trains was initially reduced compared to control mice, but by the fifth pulse train, there was no longer a treatment effect. However, chronic lithium treatment attenuated d-amphetamine-induced increases in DA responses to 20 Hz pulse trains stimulation. Our data suggest that long-term administration of lithium may ameliorate mania phenotypes by normalizing the readily releasable DA pool in VTA axon terminals in the NAc. Read the Editorial Highlight for this article on Page 520.

Anti-anhedonic activity of long-term lithium treatment in rats exposed to repeated unavoidable stress

Behavioural and neurochemical responses to palatable food exposure represent an index of hedonic competence. In rats, a palatable meal increases extra-neuronal dopamine levels in the nucleus accumbens shell (NAcS) that confers to it incentive salience and reinforcing value. Repeated stress exposure decreases dopamine output and impairs the NAcS dopaminergic response to palatable food and the competence to acquire a vanilla sugar (VS)-reinforced instrumental behaviour [VS-sustained appetitive behaviour (VAB)]. Moreover, chronic stress exposure disrupts reactivity to aversive stimuli. A 3-wk treatment with lithium, the gold-standard treatment in bipolar disorder, tonically reduces NAcS dopamine output and the reactivity to aversive stimuli. However, it does not affect the dopaminergic response to VS and the competence to acquire VAB. This study investigated whether repeated lithium administration is endowed with anti-anhedonic activity. The NAcS dopaminergic response to VS and the competence to acquire VAB and sucrose self-administration (SA), in terms of fixed-ratio (FR)1, FR5 and progressive ratio schedules of reinforcement, were studied in saline or lithium-treated groups of non-food-deprived rats exposed or not to repeated unavoidable stress. Chronic stress exposure impaired the NAcS dopaminergic response to VS, acquisition of VAB and sucrose SA, in terms of FR1 and FR5 schedules of reinforcement and breaking point score. Repeated lithium treatment restored these parameters to control group values, even when treatment began in rats already showing an anhedonia-like condition. Since the breaking point defines the reinforcement efficacy of a hedonic stimulus, the present data suggest that lithium treatment is endowed with anti-anhedonic activity in rats.

Potential mechanisms of action of lithium in bipolar disorder. Current understanding

Lithium has been used for over half a century for the treatment of bipolar disorder as the archetypal mood stabilizer, and has a wealth of empirical evidence supporting its efficacy in this role. Despite this, the specific mechanisms by which lithium exerts its mood-stabilizing effects are not well understood. Given the inherently complex nature of the pathophysiology of bipolar disorder, this paper aims to capture what is known about the actions of lithium ranging from macroscopic changes in mood, cognition and brain structure, to its effects at the microscopic level on neurotransmission and intracellular and molecular pathways. A comprehensive literature search of databases including MEDLINE, EMBASE and PsycINFO was conducted using relevant keywords and the findings from the literature were then reviewed and synthesized. Numerous studies report that lithium is effective in the treatment of acute mania and for the long-term maintenance of mood and prophylaxis; in comparison, evidence for its efficacy in depression is modest. However, lithium possesses unique anti-suicidal properties that set it apart from other agents. With respect to cognition, studies suggest that lithium may reduce cognitive decline in patients; however, these findings require further investigation using both neuropsychological and functional neuroimaging probes. Interestingly, lithium appears to preserve or increase the volume of brain structures involved in emotional regulation such as the prefrontal cortex, hippocampus and amygdala, possibly reflecting its neuroprotective effects. At a neuronal level, lithium reduces excitatory (dopamine and glutamate) but increases inhibitory (GABA) neurotransmission; however, these broad effects are underpinned by complex neurotransmitter systems that strive to achieve homeostasis by way of compensatory changes. For example, at an intracellular and molecular level, lithium targets second-messenger systems that further modulate neurotransmission. For instance, the effects of lithium on the adenyl cyclase and phospho-inositide pathways, as well as protein kinase C, may serve to dampen excessive excitatory neurotransmission. In addition to these many putative mechanisms, it has also been proposed that the neuroprotective effects of lithium are key to its therapeutic actions. In this regard, lithium has been shown to reduce the oxidative stress that occurs with multiple episodes of mania and depression. Further, it increases protective proteins such as brain-derived neurotrophic factor and B-cell lymphoma 2, and reduces apoptotic processes through inhibition of glycogen synthase kinase 3 and autophagy. Overall, it is clear that the processes which underpin the therapeutic actions of lithium are sophisticated and most likely inter-related.

Metabonomic and metallomic profiling in the amniotic fluid of malnourished pregnant rats

Epidemiology and studies in animal models have revealed that prenatal malnutrition is highly correlated with abnormal fetal neurodevelopment. We present here a combined metabonomic and metallomic profiling technique to associate the metabolic and trace-elemental composition variations of rat amniotic fluid (AF) in malnourished pregnant rats with the retardation of fetal rat neurodevelopment. The AF samples from three groups of pregnant Sprague-Dawley rats, which were fed either a normal diet, a low-protein diet, or "a famine diet", were subjected to GC/MS and ICP/MS combined with multivariate data analysis (MVDA). PCA scores plot of both GC/MS and ICP/MS data showed similar and unique metabolic signatures of AF in response to the different diets. Rats in the famine group released increased amounts of glycine, inositol, putrescine, and rubidium and decreased amounts of methionine, dopa, tryptophan, glutamine, zinc, cobalt, and selenium in the AF. These discriminable variations in the AF may indicate the abnormality of a number of metabolic pathways in fetal rats including the folate cycle and methionine pathway, the monoamine pathway, and tri-iodothyronine (T3) metabolism. The abnormalities may be the result of metabolites or elemental differences or a combination of both. This study demonstrates the potential of combining profiling of small-molecule metabolites and trace elements to broaden the understanding of biological variations associated with fetal neurodevelopment induced by environmental perturbation.

Strain differences in lithium attenuation of d-amphetamine-induced hyperlocomotion: a mouse model for the genetics of clinical response to lithium

Lithium attenuation of stimulant-induced hyperlocomotion is a rodent model that may be useful both to understand the mechanism of the therapeutic action of lithium and to develop novel lithium-mimetic compounds. To lay the foundation for future investigations into the neurobiology and genetics of lithium as a therapeutic agent, we studied the effect of lithium on d-amphetamine-induced hyperlocomotion in 12 (3 outbred) mouse strains. In our initial screening, mice received either (1) no drugs, (2) LiCl only, (3) d-amphetamine only, or (4) d-amphetamine and LiCl. Whereas there was no significant effect of LiCl alone on locomotion in any strain, there was a large degree of strain variation in the effects of LiCl combined with d-amphetamine. LiCl attenuated d-amphetamine-induced hyperlocomotion in C57BL/6J, C57BL/6Tac, Black Swiss, and CBA/J mice, whereas CD-1, FVB/NJ, SWR/J, and NIH Swiss mice, which were responsive to d-amphetamine, showed no significant effect of LiCl. d-Amphetamine-induced hyperlocomotion in the C3H/HeJ strain was increased by pretreatment with lithium. A subset of strains were treated for 4 weeks with lithium carbonate before the d-amphetamine challenge, and in each of these strains, lithium produced effects identical to those seen following acute administration. Strain responsiveness to lithium was not dependent upon the dose of either d-amphetamine or LiCl. Further, the results are not explained by brain lithium levels, which suggests that these behavioral responses to lithium are under the control of inherent genetic or other biological mechanisms specific to the effects of lithium on brain function.

Hypothalamus-pituitary-adrenal modifications consequent to chronic stress exposure in an experimental model of depression in rats

The modifications in the hypothalamus-pituitary-adrenal (HPA) axis function induced by repeated unavoidable stress exposure, according to a standardized procedure used for inducing an experimental model of depression, were studied. Rats exposed to this procedure show hyporeactivity to both pleasurable and aversive stimuli and this condition is antagonized by the repeated administration of classical antidepressant drugs. We also studied whether imipramine administration during stress exposure would interfere with the possible modifications in the HPA axis. Rats were exposed to a 4-week stress procedure with and without imipramine treatment and then tested for escape, as compared with non-stressed control animals. Twenty-four hours later all rats were bled through a tail nick for plasma corticosterone measurement before and after dexamethasone (10 microg/kg) or corticotropin-releasing hormone (CRH, 1 microg/kg) administration. Rats were then killed, adrenals and thymus weighed, brain areas dissected out and frozen for glucocorticoid receptors (GRs) and corticotropin-releasing hormone receptor 1 (CRHR1) immunoblotting and for the assessment of hypothalamic corticotropin-releasing hormone levels.

RESULTS

Rats exposed to a 4-week unavoidable stress showed escape deficit and their basal plasma corticosterone levels were higher than those of control animals. Moreover, they had decreased response to dexamethasone administration, adrenal hypertrophy, and decreased GR expression in the hippocampus, hypothalamus, medial prefrontal cortex and pituitary. No significant modifications in CRHR1 expression were observed in the pituitary nor in different discrete brain areas. CRH levels in the hypothalamus and the plasma corticosterone response to CRH administration were found to be higher in stressed rats than in controls. Imipramine treatment offset all the behavioral and neurochemical stress-induced modifications. In conclusion, the present results strengthen the assumption that the escape/avoidance behavioral deficit induced by inescapable stress exposure is accompanied by steadily increased HPA activity, and that imipramine effect is strongly related to a normalization of HPA axis activity.

The behavioral actions of lithium in rodent models: leads to develop novel therapeutics

For nearly as long as lithium has been in clinical use for the treatment of bipolar disorder, depression, and other conditions, investigators have attempted to characterize its effects on behaviors in rodents. Lithium consistently decreases exploratory activity, rearing, aggression, and amphetamine-induced hyperlocomotion; and it increases the sensitivity to pilocarpine-induced seizures, decreases immobility time in the forced swim test, and attenuates reserpine-induced hypolocomotion. Lithium also predictably induces conditioned taste aversion and alterations in circadian rhythms. The modulation of stereotypy, sensitization, and reward behavior are less consistent actions of the drug. These behavioral models may be relevant to human symptoms and to clinical endophenotypes. It is likely that the actions of lithium in a subset of these animal models are related to the therapeutic efficacy, as well the side effects, of the drug. We conclude with a brief discussion of various molecular mechanisms by which these lithium-sensitive behaviors may be mediated, and comment on the ways in which rat and mouse models can be used more effectively in the future to address persistent questions about the therapeutically relevant molecular actions of lithium.

Chronic lithium chloride administration to rats elevates glucose metabolism in wide areas of brain, while potentiating negative effects on metabolism of dopamine D2-like receptor stimulation

RATIONALE AND OBJECTIVES

The regional cerebral metabolic rate for glucose (rCMRglc) can be imaged in vivo as a marker of brain functional activity. The effects of chronic lithium administration on baseline values of rCMRglc and values in response to administration of dopamine D2-like receptor agonists have not been examined in humans or rats. Knowing these effects may elucidate and localize the therapeutic action of lithium in bipolar disorder.

METHODS

In unanesthetized rats, we used the 2-deoxy-D-glucose (2-DG) technique to image the effects of a 6-week control diet or LiCl diet sufficient to produce a plasma lithium concentration therapeutically relevant to bipolar disorder, on rCMRglc at baseline and in response to the dopaminergic D2-like receptor agonist, quinpirole (1 mg/kg i.v.), or to i.v. saline.

RESULTS

Baseline rCMRglc was significantly elevated in 30 of 81 brain regions examined, in LiCl diet compared with control diet rats. Affected were visual and auditory structures, frontal cortex, amygdala, hippocampus, nucleus accumbens, caudate-putamen, interpeduncular nucleus, and substantia nigra. Acute quinpirole significantly decreased rCMRglc in four areas of the caudate-putamen in control diet rats, and in these and 19 additional brain areas in LiCl-fed rats.

CONCLUSIONS

In unanesthetized rats, chronic lithium administration widely upregulates baseline rCMRglc and potentiates the negative effects on rCMRglc of D2-like receptor stimulation. The baseline elevation may relate to lithium's reported ability to increase auditory and visual evoked responses in humans, whereas lithium's potentiation of quinpirole's negative effects on rCMRglc may be related to its therapeutic efficacy in bipolar disorder.

Lithium differs from anticonvulsant mood stabilizers in prefrontal cortical and accumbal dopamine release: role of 5-HT(1A) receptor agonism

Anticonvulsant mood stabilizers, e.g., valproic acid and carbamazepine, and atypical antipsychotic drugs (APDs), e.g., clozapine, quetiapine, olanzapine, risperidone, and ziprasidone, have been reported to preferentially increase dopamine (DA) release in rat medial prefrontal cortex (mPFC), an effect partially or fully inhibited by WAY100635, a selective 5-HT(1A) antagonist. These atypical APDs have themselves been reported to be effective mood stabilizers, although the importance of increased cortical DA release to mood stabilization has not been established. The purpose of the present study was to determine whether zonisamide, another anticonvulsant mood stabilizer, as well as lithium, a mood stabilizer without anticonvulsant properties, also increases prefrontal cortical DA release and, if so, whether this release is also inhibited by 5-HT(1A) antagonism. As with valproic acid and carbamazepine, zonisamide (12.5 and 25 mg/kg) increased DA release in the mPFC, but not the NAC, an increase abolished by WAY100635 (0.2 mg/kg). However, lithium (100 and 250 mg/kg) decreased DA release in the NAC, an effect also attenuated by WAY100635 (0.2 mg/kg). Lithium itself had no effect in the mPFC but the combination of WAY100635 (0.2 mg/kg) and lithium (100 and 250 mg/kg) markedly increased DA release in the mPFC. Furthermore, M100907 (0.1 mg/kg), a selective 5-HT(2A) antagonist, abolished this increase in DA release in the mPFC. These results indicate that not all mood-stabilizing agents but only those, which have anticonvulsant mood-stabilizing properties, increase DA release in the cortex, and that the effect is dependent upon 5-HT(1A) receptor stimulation. However, the combination of lithium and 5-HT(1A) blockade may result in excessive 5-HT(2A) receptor stimulation, relative to 5-HT(1A) receptor stimulation, both of which can increase prefrontal cortical DA release.

Effect of chronic lithium and withdrawal from chronic lithium on presynaptic dopamine function in the rat

Bipolar affective disorders can be successfully treated with long-term use of the mood stabilizer lithium. However, discontinuation of lithium treatment is followed by a high incidence of manic episodes. In the present study, we attempted to identify neurobiological changes that might mediate this rebound mania. In vivo microdialysis in the anaesthetized rat and in situ hybridization histochemistry were used to study the effect of chronic lithium treatment and withdrawal from chronic lithium treatment on presynaptic dopamine (DA) function. Rats were maintained for 28 days on a lithium diet or control diet. The lithium-withdrawn treatment group had their lithium diet substituted for control diet from day 25 of the treatment period. Microdialysis probes were implanted in the shell of the nucleus accumbens and both basal extracellular DA levels and DA levels in the presence of the DA uptake inhibitor bupropion (1 microM) were collected. Basal DA levels did not differ between any of the treatment groups. However, during local perfusion of bupropion, the increase in DA was significantly attenuated in the lithium-treated animals compared to controls or lithium-withdrawn animals. In situ hybridization of DA transporter mRNA in the ventral tegmental area revealed no difference in the abundance of this mRNA in any of the groups. These data suggest that there is impaired DA release in rats during chronic lithium treatment, but DA release returns to normal levels on withdrawal from lithium treatment, and is therefore unlikely to underlie the rebound mania associated with lithium withdrawal.

Lithium and valproate attenuate dextroamphetamine-induced changes in brain activation

BACKGROUND

Previous studies have suggested that both lithium and valproate may decrease phosphoinositol second messenger system (PI-cycle) activity. There is also evidence that dextroamphetamine may increase PI cycle activity. It was previously demonstrated that dextroamphetamine administration in volunteers causes a region and task dependent decrease in brain activation in healthy volunteers. The current study assessed the effect of 14 days pretreatment with lithium and valproate on these dextroamphetamine-induced changes in regional brain activity in healthy volunteers.

METHODS

This was a double-blind, placebo-controlled, study in which volunteers received either 1000 mg sodium valproate (n = 12), 900 mg lithium (n = 9) or placebo (n = 12). Functional images were acquired using functional magnetic resonance imaging (fMRI) while subjects performed three cognitive tasks, a word generation paradigm, a spatial attention task and a working memory task. fMRI was carried out both before and after administration of dextroamphetamine (25 mg). Changes in the number of activated pixels and changes in the magnitude of the blood-oxygen-level-dependent (BOLD) signal after dextroamphetamine administration were then determined.

RESULTS

In keeping with previous findings dextroamphetamine administration decreased regional brain activation in all three tasks. Pretreatment with lithium attenuated changes in the word generation paradigm and the spatial attention task, while pretreatment with valproate attenuated the changes in the working memory task.

CONCLUSIONS

These results suggest that both lithium and valproate can significantly attenuate dextroamphetamine-induced changes in brain activity in a task dependent and region specific manner. This is the first human evidence to suggest that both lithium and valproate may have a similar effect on regional brain activation, conceivably via similar effects on PI-cycle activity.

Acquisition of a palatable-food-sustained appetitive behavior in satiated rats is dependent on the dopaminergic response to this food in limbic areas

Rats exposed to repeated unavoidable stress show decreased dopamine output in the nucleus accumbens shell (NAcS) and do not acquire vanilla sugar (VS)-sustained appetitive behavior (VAB). Rats treated with lithium for 3 weeks also show decreased NAcS dopamine output, yet they acquire VAB. Feeding a novel palatable food increases extraneuronal dopamine levels in the NAcS and medial prefrontal cortex (mPFC) in rats. In order to investigate the role of food-induced dopamine release in VAB acquisition, we studied by microdialysis the dopaminergic response in the NAcS and mPFC to the presentation and consumption of VS in satiated control rats, and in satiated rats exposed to repeated stress or lithium treatment. The dopaminergic response to VS was also studied in rats familiar with VS, or that had acquired VAB. In control rats, VS feeding was accompanied by increased dopamine output in the NAcS and mPFC, and one-trial habituation to this effect developed in the NAcS. Rats exposed to a 7-day stress showed reduced interest in VS pellets, and when fed VS they did not show a dopaminergic response in the NAcS and mPFC. Lithium-treated rats rapidly ate VS pellets and showed increased dopamine output in the NAcS and mPFC, with no habituation in the NAcS response. Rats familiar with VS and rats that had already learned VAB ate VS pellets. The first group showed a lower dopaminergic response to VS consumption than the control group, but the latter showed no dopaminergic response in the NAcS and mPFC. We propose that the limbic dopaminergic response to a novel palatable food plays a role in associative learning and that it is predictive of the competence to learn an appetitive behavior. Moreover, in our experimental conditions a phasic increase in mesolimbic dopamine no longer signals the VS stimulus once it has become a reinforcer in an appetitive task.

Acquisition of an appetitive behavior prevents development of stress-induced neurochemical modifications in rat nucleus accumbens

In rats, exposure to chronic unavoidable stress produces a decrease in dopamine output in the nucleus accumbens shell that is accompanied by a decreased density of the dopamine transporter and an increased activity of the dopamine-D(1) receptor complex. These modifications have been hypothesized to be adaptive to decreased dopamine output in stressed rats. We investigated whether the learning of an appetitive behavior sustained by palatable food, which is associated with increased dopamine output in the nucleus accumbens shell as measured by microdialysis experiments, would affect the modifications induced by chronic stress exposure on dopamine transporter density and dopamine-D(1) receptor complex activity in the nucleus accumbens. Rats exposed to chronic unavoidable stress after acquisition of the appetitive behavior showed a higher dopamine extraneuronal release in the nucleus accumbens shell than that of stressed animals, and similar to that of control rats. Moreover, previous acquisition of the appetitive behavior prevented development of a stress-induced decrease in dopamine transporter density, measured by [(3)H]-WIN 35428 binding, a stress-induced increase in dopamine-D(1) receptor density, measured by binding of [(3)H]-SCH 23390, and SKF 38393-stimulated adenylyl cyclase activity in the nucleus accumbens. These results support the hypothesis that changes induced in pre- and postsynaptic dopaminergic transmission by chronic stress exposure are related to decreased dopamine output.

Sardinian alcohol-preferring and non-preferring rats show different reactivity to aversive stimuli and a similar response to a natural reward

Sardinian alcohol-preferring (sP) and non-preferring (sNP) rats were studied to ascertain whether some behavioral and/or neurochemical traits, beyond ethanol preference, differentiated the two lines. Spontaneous reactivity of Wistar, sP and sNP rats to aversive or pleasurable stimuli was examined in an avoidance test, an elevated plus maze test, and in response to palatable food presentation. As the mesocorticolimbic dopaminergic system plays a relevant role in the response to rewarding or aversive stimuli, extraneuronal dopamine levels and cocaine-induced dopamine accumulation in the nucleus accumbens shell (NAcS) and medial prefrontal cortex (mPFC) were studied by microdialysis in the three groups of rats. Moreover, rats were exposed to repeated unavoidable stress and their avoidance response and NAcS dopamine output were determined. Finally, the capacity of sP, sNP, and Wistar rats to learn a palatable food-sustained appetitive behavior was studied. The present study shows that, beyond ethanol preference, there are several behavioral and neurochemical distinctions between sP and sNP rats. The sP rats displayed an increased level of anxiety-like behavior and sNP rats showed a reduced avoidance of noxious stimuli, compared to Wistar rats. Moreover, in the NAcS and PFC, extraneuronal dopamine levels were higher in sP rats and lower in sNP rats compared to Wistar rats; cocaine-induced dopamine accumulation in the NAcS was higher in sP rats than in sNP and Wistar rats. However, sP and sNP rats showed a similar behavioral and neurochemical response to chronic unavoidable stress. Interestingly, they also showed similar behavioral and neurochemical responses to a natural rewarding stimulus and a similar ability to learn an appetitive behavior.

The efficacy of reboxetine in preventing and reverting a condition of escape deficit in rats

BACKGROUND

Different stress-induced experimental models of depression are currently used to study the efficacy and mechanism of action of classical or potential antidepressant compounds. We studied the effect of single and repeated administrations of reboxetine, an antidepressant that selectively inhibits noradrenaline reuptake, in the prevention and reversal of stress-induced escape deficit. Moreover, we examined the effect of chronic reboxetine on the stress-induced decrease of dopamine output in the nucleus accumbens shell (NAcS).

METHODS

Rats received a single or 21-day reboxetine administration before acute unavoidable stress exposure; 24 hours later their escape response was examined. Rats were exposed to repeated unavoidable stress for 21 days, with or without reboxetine treatment, and then were tested for escape; 2 days later they were implanted with microdialysis probes in the NAcS.

RESULTS

A single reboxetine administration showed a protective activity on stress-induced escape deficit development that significantly increased after 21 days of treatment. This effect was antagonized by propranolol, a selective beta-adrenergic antagonist. In rats exposed to chronic stress, a 21-day reboxetine treatment reinstated the avoidance response and NAcS dopamine output to control values.

CONCLUSIONS

In these stress-induced models, long-term reboxetine administration showed a protective activity similar to that of classical antidepressants.

Effects of long-term acetyl-L-carnitine administration in rats--II: Protection against the disrupting effect of stress on the acquisition of appetitive behavior

Long-term acetyl-L-carnitine (ALCAR) administration prevents the development of escape deficit produced by acute exposure to unavoidable stress. However, it does not revert the escape deficit sustained by chronic stress exposure. Rats exposed to chronic stress show a low dopamine (DA) output in the nucleus accumbens shell (NAcS) and do not acquire an appetitive behavior sustained by the earning of vanilla sugar (VS) made contingent on the choice of one of the two divergent arms of a Y-maze (VS-sustained appetitive behavior, VAB), while control rats consistently do. The present study shows that ALCAR treatment in rats exposed to a 7-day stress protocol prevented a decrease in DA output in the NAcS and medial prefrontal cortex (mPFC) of rats, and that it strengthened the DA response to VS consummation in the same two areas. Moreover, rats treated with long-term ALCAR or exposed to chronic stress while treated with ALCAR acquired VAB as efficiently as control rats. Moreover, VAB acquisition in stressed rats treated with ALCAR coincided with the reversal of the deficits in escape and in dopaminergic transmission in the NAcS. Thus, repeated ALCAR treatment preserved the DA response to VS in chronically stressed rats and this effect appeared to be predictive of the rat's competence to acquire VAB.

Selective modifications in the nucleus accumbens of dopamine synaptic transmission in rats exposed to chronic stress

Stressful events are accompanied by modifications in dopaminergic transmission in distinct brain regions. As the activity of the neuronal dopamine (DA) transporter (DAT) is considered to be a critical mechanism for determining the extent of DA receptor activation, we investigated whether a 3-week exposure to unavoidable stress, which produces a reduction in DA output in the nucleus accumbens shell (NAcS) and medial prefrontal cortex (mPFC), would affect DAT density and DA D1 receptor complex activity in the NAcS, mPFC and caudate-putamen (CPu). Rats exposed to unavoidable stress showed a decreased DA output in the NAcS accompanied by a decrease in the number of DAT binding sites, and an increase in the number of DA D1 binding sites and Vmax of SKF 38393-stimulated adenylyl cyclase. In the mPFC, stress exposure produced a decrease in DA output with no modification in DAT binding or in DA D1 receptor complex activity. Moreover, in the CPu stress exposure induced no changes in DA output or in the other neurochemical variables examined. This study shows that exposure to a chronic unavoidable stress that produces a decrease in DA output in frontomesolimbic areas induced several adaptive neurochemical modifications selectively in the nucleus accumbens.

Long-term behavioral and neurochemical effects of chronic stress exposure in rats

Rats exposed to acute unavoidable stress develop a deficit in escaping avoidable aversive stimuli that lasts as long as unavoidable stress exposure is repeated. A 3-week exposure to unavoidable stress also reduces dopamine (DA) output in the nucleus accumbens shell (NAcS). This study showed that a 7-day exposure to unavoidable stress induced in rats an escape deficit and a decrease in extraneuronal DA basal concentration in the NAcS. Moreover, animals had reduced DA and serotonin (5-HT) accumulation after cocaine administration in the medial pre-frontal cortex (mPFC) and NAcS, compared with control animals. After a 3-week exposure to unavoidable stress, escape deficit and reduced DA output in the NAcS were still significant at day 14 after the last stress administration. In the mPFC we observed: (i) a short-term reduction in DA basal levels that was back to control values at day 14; (ii) a decrease in DA accumulation at day 3 followed by a significant increase beyond control values at day 14; (iii) a significant reduction in 5-HT extraneuronal basal levels at day 3, but not at day 14. Finally, a significant decrease in 5-HT accumulation following cocaine administration was present in the NAcS and mPFC at day 3, but not at day 14. In conclusion, a long-term stress exposure induced long-lasting behavioral sequelae associated with reproducible neurochemical modifications.

Dopamine output in the nucleus accumbens shell is related to the acquisition and the retention of a motivated appetitive behavior in rats

Chronic stress exposure consistently impairs the reactivity to aversive and pleasurable stimuli in rats; these behavioral modifications are associated with a decrease in dopamine output in the nucleus accumbens shell (NAcS). However, when rats that have already acquired an appetitive behavior are exposed to chronic stress, they develop an impaired reactivity to avoidable aversive stimuli while retaining the appetitive behavior. The dissociation between these two behavioral traits was used to study whether the decreased dopaminergic activity in the NAcS was connected to either of the two deficits. Dopamine output was studied through microdialysis as dopamine accumulation following re-uptake inhibition by cocaine. When rats that had previously acquired the appetitive behavior were exposed to chronic stress, they showed a dopaminergic transmission in the NAcS similar to that of controls and significantly higher than that of chronically stressed animals. Thus, dopamine output in the NAcS was consistently associated to the acquisition and maintenance of appetitive behavior, while the expression of a deficit in avoidance appeared to be independent of it.

Acquisition of an appetitive behavior reverses the effects of long-term treatment with lithium in rats

Rats exposed to a long-term treatment with lithium chloride develop a deficit of avoidance accompanied by a reduction in the basal levels of extraneuronal dopamine and in dopamine accumulation in the nucleus accumbens shell after acute uptake inhibition. Such a condition is similar to that of an experimental model of depression induced by exposing rats to a chronic stress procedure. Rats exposed to chronic stress are also unable to acquire an appetitive behavior sustained by a highly palatable food. Thus, it was studied whether rats fed a diet containing lithium would develop an appetitive behavior induced by a pure hedonic stimulus. Rats on the lithium diet developed a clear-cut escape deficit condition accompanied by a decreased dopamine output in the nucleus accumbens shell; nevertheless, they learned the appetitive behavior within a similar period to controls. The development of the appetitive behavior coincided with the recovery of the capacity to avoid a noxious stimulus and with the return of the dopaminergic transmission in the nucleus accumbens shell to values similar to those of control rats. It may be concluded that the mechanism of action underlying the behavioral and neurochemical sequelae of a chronic stress is distinct from that of the analogous effects produced by lithium.

Long-term lithium administration abolishes the resistance to stress in rats sensitized to morphine

Morphine sensitized rats appear protected from the sequelae of an unavoidable stress: when exposed to stress (after a 7-day morphine wash-out) and then tested for escape, they perform like naive animals. This protection appears similar to that induced by chronic imipramine treatment, as it is antagonized by the inhibition of D(1)-dopamine receptors before exposure to unavoidable stress. Repeated unavoidable stress induces in rats a condition characterized by hyporeactivity to noxious stimuli and reverted by long-term antidepressant treatments, and this state is regarded as an experimental model of depression. The resistance to stress in morphine sensitized rats could be considered as the behavioral counterpart of the sensitivity to stress in control rats, i.e. as a model of mania. The aim of the present study was to validate such a putative model by studying whether the resistance to stress induced by morphine sensitization would respond to a long-term administration of lithium, the reference antimanic drug. Long-term lithium treatment induces in rats a condition of hyporeactivity to noxious stimuli, accompanied by decreased levels of dopamine in the nucleus accumbens shell. In morphine sensitized rats chronic lithium abolished the resistance to stress, but it did not modify the D(1)-dopamine receptor mediated response to morphine, nor did it modify the levels of extraneuronal dopamine in the nucleus accumbens shell. Thus, lithium treatment abolished the resistance to stress in morphine sensitized rats, conferring predictive validity to the paradigm. Moreover, it did so through a mechanism which appeared to be independent of D(1)-dopamine receptor activity.