Effects of lithium treatment on extracellular serotonin levels in the dorsal hippocampus and wet-dog shakes in the rat

Molecular actions and clinical pharmacogenetics of lithium therapy

Mood disorders, including bipolar disorder and depression, are relatively common human diseases for which pharmacological treatment options are often not optimal. Among existing pharmacological agents and mood stabilizers used for the treatment of mood disorders, lithium has a unique clinical profile. Lithium has efficacy in the treatment of bipolar disorder generally, and in particular mania, while also being useful in the adjunct treatment of refractory depression. In addition to antimanic and adjunct antidepressant efficacy, lithium is also proven effective in the reduction of suicide and suicidal behaviors. However, only a subset of patients manifests beneficial responses to lithium therapy and the underlying genetic factors of response are not exactly known. Here we discuss preclinical research suggesting mechanisms likely to underlie lithium's therapeutic actions including direct targets inositol monophosphatase and glycogen synthase kinase-3 (GSK-3) among others, as well as indirect actions including modulation of neurotrophic and neurotransmitter systems and circadian function. We follow with a discussion of current knowledge related to the pharmacogenetic underpinnings of effective lithium therapy in patients within this context. Progress in elucidation of genetic factors that may be involved in human response to lithium pharmacology has been slow, and there is still limited conclusive evidence for the role of a particular genetic factor. However, the development of new approaches such as genome-wide association studies (GWAS), and increased use of genetic testing and improved identification of mood disorder patients sub-groups will lead to improved elucidation of relevant genetic factors in the future.

Chronic lithium treatment decreases NG2 cell proliferation in rat dentate hilus, amygdala and corpus callosum

An increasing number of investigations suggest volumetric changes and glial pathology in several brain regions of patients with bipolar disorder. Lithium, used in the treatment of this disorder, has been reported to be neuroprotective and increase brain volume. Here we investigate the effect of lithium on the proliferation and survival of glial cells positive for the chondroitin sulphate proteoglycan NG2 (NG2 cells); a continuously dividing cell type implicated in remyelination and suggested to be involved in regulation of neuronal signaling and axonal outgrowth. Adult male rats were treated with lithium for four weeks and injected with the proliferation marker bromodeoxyuridine (BrdU) before or at the end of the treatment period. Immunohistochemical analysis of brain sections was performed to estimate the number of newly born (BrdU-labeled) NG2 cells and oligodendrocytes in hippocampus, basolateral nuclei of amygdala and corpus callosum. Lithium significantly decreased the proliferation of NG2 cells in dentate hilus of hippocampus, amygdala and corpus callosum, but not in the molecular layer or the cornu ammonis (CA) regions of hippocampus. The effect was more pronounced in the corpus callosum. No effect of lithium on the survival of newborn cells or the number of newly generated oligodendrocytes could be detected. Our results demonstrate that in both white and gray matter brain regions implicated in the pathophysiology of bipolar disorder, chronic lithium treatment significantly decreases the proliferation rate of NG2 cells; the major proliferating cell type of the adult brain.

Serotonin and dopamine in the parabrachial nucleus of rats during conditioned taste aversion learning

A microdialysis technique was used to monitor changes in serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and dopamine (DA) in the extracellular space of the parabrachial nucleus (PBN) of rats to estimate the contribution of these neurotransmitter systems to the acquisition of conditioned taste aversion (CTA). A significant (280%) enhancement of 5-HT was found immediately after saccharin drinking (CS). I.p. injection of unconditioned stimulus LiCl alone (after water drinking) also increased level of 5-HT (200%). However, when saccharin intake was followed by injection of LiCl (CS-US pairing), no change in 5-HT was observed. 5-HIAA and DA were unaffected by any of the above treatments. Thus in spite of elevation of 5-HT in PBN following saccharin consumption alone (CS) or LiCl administration alone (US) no changes in 5-HT occurred after pairing of both stimuli (CS-US). Our work demonstrates that participation of 5-HT in acquisition of CTA appears to be unlikely, and also DA appears not to be engaged in this acquisition at all. At the level of the PBN 5-HT participates mainly in CS and/or US stimuli processing, where this phenomenon has close relationship to other important physiological mechanisms, involved in behavioral control. Such as anxiety, alimentation intake.

Chronic lithium administration to rats selectively modifies 5-HT2A/2C receptor-mediated brain signaling via arachidonic acid

The effects of chronic lithium administration on regional brain incorporation coefficients k* of arachidonic acid (AA), a marker of phospholipase A2 (PLA2) activation, were determined in unanesthetized rats administered i.p. saline or 1 mg/kg i.p. (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), a 5-HT2A/2C receptor agonist. After injecting [1-(14)C]AA intravenously, k* (brain radioactivity/integrated plasma radioactivity) was measured in each of 94 brain regions by quantitative autoradiography. Studies were performed in rats fed a LiCl or a control diet for 6 weeks. In the control diet rats, DOI significantly increased k* in widespread brain areas containing 5-HT2A/2C receptors. In the LiCl-fed rats, the significant positive k* response to DOI did not differ from that in control diet rats in most brain regions, except in auditory and visual areas, where the response was absent. LiCl did not change the head turning response to DOI seen in control rats. In summary, LiCl feeding blocked PLA2-mediated signal involving AA in response to DOI in visual and auditory regions, but not generally elsewhere. These selective effects may be related to lithium's therapeutic efficacy in patients with bipolar disorder, particularly its ability to ameliorate hallucinations in that disease.

Increased extracellular serotonin level in rat hippocampus induced by chronic citalopram is augmented by subchronic lithium: neurochemical and behavioural studies in the rat

RATIONALE

A substantial number of patients do not respond sufficiently to antidepressant drugs and are therefore often co-medicated with lithium as an augmentation strategy. However, the neurochemical rationale behind this strategy needs to be further clarified.

OBJECTIVES

We examined the effect of chronic citalopram and subchronic lithium, alone or in combination, on (a) serum levels of citalopram and lithium, (b) animal behaviour and (c) hippocampal serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels. Furthermore, we examined the serum level of citalopram and hippocampal 5-HT following one acute citalopram injection.

METHODS

Microdialysis in the freely moving animals was used to determine hippocampal 5-HT and 5-HIAA. The animal behaviour was examined in the open field and forced swim test. RESULTS. We found that chronic administration of citalopram (20 mg/kg/24 h s.c.) significantly increased the 5-HT baseline relative to vehicle-treated rats. Addition of subchronic lithium (60 mmol/kg chow pellet p.o.) to chronic citalopram therapy further elevated the 5-HT levels. Moreover, we found acute citalopram (5 mg/kg s.c.) to increase the 5-HT level. The immobility time in the FST and the locomotion in the OF were unaffected by any treatments.

CONCLUSIONS

The present results support the assumption that increases in hippocampal 5-HT neurotransmission may be important in the augmentatory effect of lithium.

Effect of subchronic lithium treatment on citalopram-induced increases in extracellular concentrations of serotonin in the medial prefrontal cortex

We investigated the effect of citalopram [a selective serotonin (5-HT) reuptake inhibitor; SSRI] and MKC-242 (a selective 5-HT1A agonist), following treatment with subchronic lithium (p.o., 1 week) on extracellular 5-HT concentrations in the medial prefrontal cortex (mPFC). Acute treatment with citalopram (3 and 30 mg/kg) led to significant increases in extracellular 5-HT concentrations. The subchronic lithium group showed significantly higher basal levels of extracellular 5-HT than normal diet controls. Acute citalopram (3 and 30 mg/kg) treatment together with subchronic lithium treatment showed significant increases in the extracellular 5-HT concentrations, compared with citalopram treatment alone. Acute MKC-242 (1 mg/kg) treatment showed significant decreases in extracellular 5-HT concentrations, in both the normal diet and lithium diet groups to the same extent. The addition of lithium did not change the effect of the 5-HT1A agonist on extracellular 5-HT concentrations. This study suggests that lithium augmentation of the antidepressant effect of SSRI is mediated by the additional increases in extracellular 5-HT concentrations following the co-administrations of lithium and SSRI.

Differential effects of acute and short-term lithium administration on dialysate glutamate and GABA levels in the frontal cortex of the conscious rat

In the present study, we employed in vivo microdialysis in the frontal cortex of the awake rat to investigate the effects of acute and short-term (twice daily, 3 days) lithium chloride administration (1, 2, and 4 meq/kg, s.c.) on local dialysate glutamate and GABA levels. Acute lithium (1 meq/kg) failed to influence cortical glutamate levels while the higher (2 and 4 meq/kg) doses increased (+38 +/- 6% of basal levels) and reduced (-27 +/- 4%) cortical glutamate levels, respectively. Cortical GABA levels were affected by acute lithium only at the highest 4 meq/kg dose (+62 +/- 6%). Furthermore, these effects were prevented by tetrodotoxin (1 microM) and low-calcium (0.2 mM) medium perfusion. Following short-term administration, lithium increased (+58 +/- 4%) cortical dialysate glutamate levels at the 1 meq/kg dose, was ineffective at 2 meq/kg, while the effect of the 4 meq/kg dose was similar to that observed after acute administration. Interestingly, intracortical perfusion with the GABA(B) receptor antagonist CGP 35348 (100 microM) reversed the acute lithium (4 meq/kg)-induced decrease in glutamate levels. Taken together, these findings indicate a differential dose and duration dependent effect of lithium on cortical dialysate glutamate levels involving both a direct enhancement and an indirect inhibition that is mediated via an activation of local GABA(B) receptor. These findings may be relevant for the therapeutic effects of the drug.

The effect of acute citalopram on extracellular 5-HT levels is not augmented by lithium: an in vivo microdialysis study

A substantial number of patients do not respond sufficiently to antidepressant drugs and are therefore often co-medicated with lithium as an augmentative strategy. Since lithium has been demonstrated to affect 5-HT neurotransmission, we examined the effect of acute and subchronic lithium on 5-HT levels after a challenge with citalopram. We found that subchronic administration of lithium increases extracellular 5-HT baseline level and decreases the extracellular 5-HIAA baseline. However, we found no evidence that the effect of acute citalopram on extracellular 5-HT levels is augmented by acute or subchronic lithium.

Pharmacokinetics of mirtazapine and lithium in healthy male subjects

A substantial proportion of patients diagnosed with depression and treated with antidepressants show no or insufficient response. In such patients, lithium is often added to the antidepressant for augmentation. The present study investigated the possible drug-drug interaction between mirtazapine and lithium in 12 healthy male subjects in a randomized, double-blind, placebo-controlled two-period cross-over design. Subjects meeting the inclusion and exclusion criteria were randomly assigned to one of two groups. After an overnight fast, they received either a single oral dose of 600 mg lithium carbonate (16 meq Li+) for 10 days at 08.00 h and a single oral dose of 30 mg mirtazapine at 21.00 h on day 9 or the same number (n = 4) of placebo capsules and and a single oral dose of 30 mg mirtazapine at 21.00 h on day 9. At pre-defined times, blood samples were drawn for the measurement of mirtazapine plasma concentrations and lithium serum concentrations. In addition, psychometric tests were performed and the safety and tolerability of the drugs were assessed. The results indicate that mirtazapine does not alter the pharmacokinetics of lithium and vice versa. In addition, the combination of mirtazapine and lithium appeared to be safe and well-tolerated. Extensive psychometric testing after the administration of mirtazapine did not reveal any differences on any tests in subjects on lithium and placebo, respectively.

Enhancement of hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by repeated lithium treatment in rats

Hippocampal cholinergic neuronal activity is reported to be regulated, at least partly, through serotonin1A (5-HT1A) receptors. Chronic lithium treatment has been shown to alter both behavioral and neurochemical responses mediated by postsynaptic 5-HT1A receptors. We investigated whether long-term lithium treatment affects central cholinergic neurotransmission through 5-HT1A receptor-mediated pathways. Changes in acetylcholine (ACh) release induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, in the rat hippocampus were measured using a microdialysis technique and a radioimmunoassay for ACh. Administration of lithium for 21 days resulted in a serum lithium concentration of 1.03 mM and caused little change in density or affinity of [3H]8-OH-DPAT binding sites in the hippocampus. The local application of 8-OH-DPAT into the hippocampus of lithium treated rats increased the ACh efflux in both the absence and the presence of physostigmine, a cholinesterase (ChE) inhibitor, in the perfusion fluid. The basal ACh efflux of lithium treated rats was not different from that of the control rats under normal conditions, but was significantly higher than that of the controls when ChE was inhibited. These results demonstrate that chronic lithium treatment increases spontaneous ACh release in the hippocampus under conditions of ChE inhibition, but not under normal conditions, and enhances cholinergic neurotransmission through 5-HT1A receptor-mediated pathways, and suggest that activation of 5-HT1A receptor function by lithium is related to the enhancement of hippocampal cholinergic neurotransmission.

Key words: Acetylcholine (ACh), hippocampus, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), lithium, serotonin1A (5-HT1A) receptor.