Lithium enhances long-term potentiation independently of hippocampal neurogenesis in the rat dentate gyrus

Is hippocampal atrophy a future drug target?

Hippocampus is the brain structure, vital for episodic and declarative memory. Atrophy of the human hippocampus is seen in a variety of psychiatric and neurological disorders e.g. recurrent depression, schizophrenia, bipolar disorder, post-traumatic stress disorder, epilepsy, head injury, and Alzheimer's disease (AD). Importantly, aging hippocampus also undergoes atrophy. In many instances, for example, AD, the atrophy precedes the development of symptoms while in others, there is a temporal relationship between atrophy and symptomatology. The presence of atrophied hippocampus is one of the most consistent features of many common psychiatric disorders. Several factors contribute to this atrophy. Stress is one of the most profound factors implicated and the mechanisms involve glucocorticoids, serotonin, excitatory amino acids etc. Hippocampal formation as a whole can undergo atrophy or its individual structural components e.g. apical dendrities can exhibit atrophy. Several drugs of unrelated classes have been shown to prevent atrophy indicating heterogenous manner in which hippocampal atrophy is produced. These include, tianeptine (affects structural plasticity in hippocampus and is an effective antidepressant); phenytoin (antiseizure and neuroprotective); fluoxetine (downregulates neurodegenerative enzyme and increases neuroprotective hippocampal S100 beta); lithium (neuroprotective and antiapoptotic); tricyclic antidepressants (increase hippocampal neurogenesis); antipsychotics (reduce hippocampal neuronal suppression); sodium valproate (increases neurogenesis) and mifepristone (antioxidant, neuroprotective and anti-glucocorticoid). Now the most important question is: to what extent does the hippocampal atrophy play a role in the genesis of symptoms of diseases or their progression? And if it does, can we achieve the same degree of prevention or reversal seen in experimental animals, in humans also. An even more important question is: whether the prevention of atrophy would be clinically useful in affecting disease, viz slowing its progression, reducing morbidity, complications or positively affecting the outcome of one or more of its clinically important aspects. If the answer to this is yes, we would have to know at what stage of the disease we use the drugs, dose, duration, follow-up and efficacy. The use of these drugs in the above mentioned conditions can not only test the potential of atrophy as a future drug target, but could also help in learning more about the hippocampus in both health and diseases.

Pharmacological evidence of cholinergic involvement in adult hippocampal neurogenesis in rats

In adult hippocampus, neural progenitor cells give rise to neurons throughout life, and the neurogenesis is modulated by various intrinsic and extrinsic factors. Recent reports showed that lesion of septal cholinergic nuclei projecting to hippocampus suppressed the survival of newborn cells in the dentate gyrus (DG) of hippocampus. Here, we studied whether pharmacological treatment to activate or inhibit the cholinergic system could modulate adult hippocampal neurogenesis. 5'-Bromo-2'-deoxyuridine (BrdU) was injected to label dividing cells before the drug treatment. Immunohistochemical analysis was performed in normal rats chronically treated with an acetylcholinesterase inhibitor donepezil or a muscarinic acetylcholine receptor blocker scopolamine for four weeks. Donepezil increased, but scopolamine decreased, the number of BrdU-positive cells in the DG as compared with the control. Neither drug altered the percentage of BrdU-positive cells that were also positive for a neuronal marker neuronal nuclei, nor net population of proliferative cells labeled with proliferating cell nuclear antigen. We also found that donepezil enhanced, and scopolamine suppressed, the expression level of phosphorylated cAMP response element binding protein (CREB), which is related to cell survival, in the DG. These results indicate that donepezil enhances and scopolamine suppresses the survival of newborn cells in the DG via CREB signaling without affecting neural progenitor cell proliferation and the neuronal differentiation. This is the first evidence that pharmacological manipulation of the cholinergic system can modulate adult hippocampal neurogenesis.

The extracellular signal-regulated kinase pathway: an emerging promising target for mood stabilizers

PURPOSE OF REVIEW

There exists a growing appreciation that, though not classical neurodegenerative disorders, severe mood disorders are associated with regional impairments of structural plasticity and cellular resilience. Exciting recent data suggest that synaptic plasticity probably is involved in mechanisms of actions of mood stabilizers and antidepressants. Notably, the extracellular signal-regulated kinase pathway is a critical 'plasticity pathway' in the brain. The present review summarizes neurobiological, pharmacological, and behavioral data on the role of the extracellular signal-regulated kinase pathway in regulating some of the symptoms of bipolar disorder and as a therapeutically relevant target for mood stabilizers.

RECENT FINDINGS

The extracellular signal-regulated kinase pathway is known to mediate neurotrophic actions and synaptic plasticity. Treatment with lithium and valproate activates the extracellular signal-regulated kinase pathway in cultured cells and in prefrontal cortex and hippocampus. In addition, lithium or valproate treatment promotes neurogenesis, neurite growth, and cell survival. The extracellular signal-regulated kinase pathway is also targeted by antipsychotics. Modulation of the central nervous system extracellular signal-regulated kinase pathway induces animal behavioral alterations reminiscent of manic symptoms; these complex behaviors probably depend on the effects of extracellular signal-regulated kinase on discrete brain regions and the presence of other interacting molecules.

SUMMARY

The extracellular signal-regulated kinase pathway may represent a novel target for the development of improved therapeutics for bipolar disorder.

Lithium attenuates stress-induced impairment of long-term potentiation induction

Stress impairs the induction of long-term potentiation in the hippocampus as well as hippocampus-dependent memory. Lithium, a classical mood stabilizer, is known to have beneficial effects on stress-induced impairment of spatial memory. In the present study, we investigated lithium effects on the impairment of long-term potentiation induction after exposure to acute immobilization stress. As previously reported, immobilization stress impaired long-term potentiation induction in the CA1 region of rat hippocampal slices. Treating the slices with 0.6 or 1 mM lithium attenuated impaired long-term potentiation induction in stressed animals. Lithium was without effect on long-term potentiation induction in unstressed animals or baseline synaptic responses in unstressed or stressed animals. These results demonstrate a protective effect of lithium against stress-induced impairment of long-term potentiation induction.

Mood-stabilizing drugs: are their neuroprotective aspects clinically relevant?

The possibility that there may be subtypes of bipolar disorder and the slow progress in understanding the therapeutic mechanism for approved mood-stabilizing drugs make the challenges of intelligent drug design seem daunting. Nonetheless, the numerous shortcomings in current pharmaco-therapy underscore the need to develop novel therapies. There are significant problems with currently approved mood-stabilizing drugs: 1. Up to 40% of patients fail to respond to monotherapy with either lithium or valproic acid. 2. Common use of polypharmacotherapy increases the side effects associated with treatment. 3. Treatment must continue for weeks to months for therapeutic effects to be greater than placebo. 4. Up to 60% of patients will discontinue therapy, which is somewhat attributable to unwanted side effects. Thus, it is critical that new medications without these problems be developed for bipolar disorder. The hypothesis that mood-stabilizing drugs are neuroprotective is an important first step in new drug development. To determine if the clinical efficacy of mood-stabilizing drugs is dependent on the neuroprotective or neurogenic properties of these medications, greater strides need to be made in relating findings from cell culture and animal models to human imaging and pathology. Mounting evidence supports the neuroprotective and neurogenic properties of lithium and valproic acid ina variety of cell-culture models. It is important for clinical, biochemical, and in vitro differences between these medications to be examined, not ignored,because these differences may reveal critical distinctions between the neural mechanisms of these drugs. Continuation of the in vitro work will aid in the understanding of the mechanism by which these drugs are neuroprotective,but such studies do not advance the understanding of whether these effects are critical for the clinical efficacy of these medications. In attempting to understand the in vivo effects of these medications, a variety of evidence supports the neuroprotective and neurogenic aspects of lithium and valproic acid in healthy rodents and animal models of gross brain insult. More work needs to be done to assess whether these effects occur in animal models for bipolar disorder. The proof of principle for supporting the claim that the neuroprotective or neurogenic properties are important clinically will come from longitudinal clinical studies that compare brain morphology and function before and during treatment. If enough evidence supports the hypothesis that the neuroprotective and neurogenic properties of mood-stabilizing drugs are important for their clinical efficacy, new medications that are more efficacious and have fewer side effects will be designed based on this discovery.

Lithium: Potential Therapeutics Against Acute Brain Injuries and Chronic Neurodegenerative Diseases

In addition to the well-documented mood-stabilizing effects of lithium in manic-depressive illness patients, recent in vitro and in vivo studies in rodents and humans have increasingly implicated that lithium can be used in the treatment of acute brain injuries (e.g., ischemia) and chronic neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, tauopathies, and Huntington's disease). Consistent with this novel view, substantial evidences suggest that depressive illness is not a mere neurochemical disease, but is linked to gray matter atrophy due to the reduced number/size of neurons and glia in brain. Importantly, neurogenesis, that is, birth/maturation of functional new neurons, continues to occur throughout the lifetime in human adult brains (e.g., hippocampus); the neurogenesis is impaired by multiple not-fully defined factors (e.g., aging, chronic stress-induced increase of glucocorticoids, and excitotoxicity), accounting for brain atrophy in patients with depressive illness and neurodegenerative diseases. Chronic treatment of lithium, in agreement with the delayed-onset of mood-stabilizing effects of lithium, up-regulates cell survival molecules (e.g., Bcl-2, cyclic AMP-responsive element binding protein, brain-derived neurotrophic factor, Grp78, Hsp70, and beta-catenin), while down-regulating pro-apoptotic activities (e.g., excitotoxicity, p53, Bax, caspase, cytochrome c release, beta-amyloid peptide production, and tau hyperphosphorylation), thus preventing or even reversing neuronal cell death and neurogenesis retardation.

Neuroneogenese

Research into neurogenesis, i.e., the growth of new neurons in the adult brain, is leaving the area of pure basic science and gaining relevance for clinical disciplines such as psychopharmacology and molecular psychiatry. Neurogenesis is proposed to play a crucial role in psychiatric disorders which exhibit degenerative alterations, neural maldevelopment, and changes in neural plasticity as potentially important pathophysiological factors. Especially in dementia, drug addiction, and schizophrenic and affective psychoses, disruption of adult neurogenesis could thus represent a considerable pathogenetic element. Interestingly, several psychotropic drugs (e.g., antidepressants, atypical antipsychotics) are able to modify neurogenesis significantly. Further elucidation of the importance and implications of neurogenesis may concomitantly result in better understanding of the etiopathogenesis of mental disorders and increased knowledge of the mechanisms of action of psychotropic substances. Furthermore, this may support the development of promising innovative therapeutic approaches in clinical practice.

Separate and concomitant use of lamotrigine, lithium, and divalproex in bipolar disorders

Expert consensus emphasizes the need for better recognition and accurate diagnosis of bipolar disorder. Current research on lithium, divalproex, and lamotrigine provides new insight into the effective management of this illness. Advances in identifying the mechanism of action of mood stabilization has focused on signaling pathways within the cell that are associated with neurotrophic effects. Clinical research has led to confirmatory evidence of the efficacy of lithium in all phases of bipolar disorder, with the greatest effects seen in the treatment and prevention of mania. Compared to divalproex, lithium also has been found to have greater efficacy in the prevention of suicide. Lamotrigine has emerged as a first line treatment for bipolar depression, which is an area of weakness for other mood stabilizers. Oral loading of divalproex leads to rapid stabilization of mania without imposing a greater adverse effect burden than conventional dosing. Because no agent is universally effective in all phases of the illness, combination therapy with two or more agents often is the best option.

[Mechanisms of psychotropics' action in relation to CNS neurogenesis]

Newly available psychotropics seem to put the practical psychiatry to a stage of reform. In addition, the recent advancements in the study of neurogenesis in the adult brain force the change of the therapeutic strategy of mental disorders. The fact that the central nervous tissues can repair even after the maturation and that the replacement of neurons continues during adulthood will alter our understanding about their pathogenesis. The action of several psychiatric medications such as antidepressants, mood stabilizers, antipsychotics, and electroconvulsive therapy is converging at neurogenesis and/or neuroprotection. When the validity of the "neurogenesis/neuroprotection hypothesis" of psychiatric medication will be proved in psychiatric practice, we can establish a more rational and more effective treatment of mental disorders.

Lithium selectively increases neuronal differentiation of hippocampal neural progenitor cells both in vitro and in vivo

Lithium has been demonstrated to increase neurogenesis in the dentate gyrus of rodent hippocampus. The present study was undertaken to investigate the effects of lithium on the proliferation and differentiation of rat neural progenitor cells in hippocampus both in vitro and in vivo. Lithium chloride (1-3 mM) produced a significant increase in the number of bromodeoxyuridine (BrdU)-positive cells in high-density cultures, but did not increase clonal size in low-density cultures. Lithium chloride at 1 mM (within the therapeutic range) also increased the number of cells double-labeled with BrdU antibody and TuJ1 (a class III beta-tubulin antibody) in high-density cultures and the number of TuJ1-positive cells in a clone of low-density cultures, whereas it decreased the number of glial fibrillary acidic protein-positive cells in both cultures. These results suggest that lithium selectively increased differentiation of neuronal progenitors. These actions of lithium appeared to enhance a neuronal subtype, calbindin(D28k)-positive cells, and involved a phosphorylated extracellular signal-regulated kinase and phosphorylated cyclic AMP response element-binding protein-dependent pathway both in vitro and in vivo. These findings suggest that lithium in therapeutic amounts may elicit its beneficial effects via facilitation of neural progenitor differentiation toward a calbindin(D28k)-positive neuronal cell type.