Acute administration of roflumilast enhances immediate recall of verbal word memory in healthy young adults.

The need for new and effective treatments for dementia remains indisputably high. Phosphodiesterase inhibitors (PDE-Is) have proven efficacy as cognitive enhancers based on their positive effects in numerous preclinical studies. Especially the PDE4 subfamily is of interest due to its expression in the hippocampus, the key structure for memory formation. The current study investigates the memory enhancing effects of the clinically approved PDE4-I roflumilast in a test battery including the Verbal Learning Task (VLT) combined with electroencephalography (EEG) recording. This acute study was conducted according to a double-blind, randomized, placebo-controlled, 4-way crossover design. Three capsulated dosages of roflumilast HCl (Daxas) and a placebo were administered in four study periods. Administration occurred 1 h before testing to reach maximal plasma concentrations. Memory performance was assessed using a 30 word Verbal Learning Task. The number of words recalled both immediately and after 45 min and 24 h were included as outcome measures. EEG was recorded during the cognitive tasks on the first day. Different event-related potentials (ERPs) were considered with special emphasis on P600, as this peak has been related to word learning. Memory performance was significantly improved after acute administration of 100 µg roflumilast. Specifically, immediate recall performance on the VLT increased 2-3 words, accompanied by an enhanced P600 peak during word presentation at the third learning trial. No side effects typical for PDE4-Is were reported for the lowest and effective dose of 100 µg roflumilast. The current proof-of-concept study shows for the first time the potential of low-dose roflumilast administration as a memory enhancer in humans.

Dosing time-dependent actions of psychostimulants.

The concept of the dosing time-dependent (DTD) actions of drugs has been used to describe the effects of diurnal rhythms on pharmacological responsiveness. Notwithstanding the importance of diurnal variability in drug pharmacokinetics and bioavailability, it appears that in the central nervous system (CNS), the DTD actions of psychotropic drugs involve diurnal changes in the CNS-specific expression of genes encoding for psychotropic drug targets and transcription factors known as clock genes. In this review, we focused our discussion on the DTD effects of the psychostimulants cocaine and amphetamines. Both cocaine and amphetamines produce differential lasting behavioral alterations, that is, locomotor sensitization, depending on the time of the day they are administered. This exemplifies a DTD action of these drugs. The DTD effects of these psychostimulants correlate with diurnal changes in the system of transcription factors termed clock genes, for example, Period 1, and with changes in the availability of certain subtypes of dopamine receptors, for example, D2 and D3. Diurnal synthesis and release of the pineal hormone melatonin influence the DTD behavioral actions of cocaine and amphetamines. The molecular mechanism of melatonin's effects on the responsiveness of CNS to psychostimulants appears to involve melatonin receptors and clock genes. It is proposed that the DTD characteristics of psychostimulant action and the contributions of the melatonergic system may have clinical implications that include treatments for the attention deficit hyperactivity disorder and possibly neurotoxicity/neuroprotection.

Dopamine receptor-mediated regulation of neuronal "clock" gene expression.

Using a transgenic mice model (i.e. "clock" knockouts), clock transcription factors have been suggested as critical regulators of dopaminergic behaviors induced by drugs of abuse. Moreover, it has been shown that systemic administration of psychostimulants, such as cocaine and methamphetamine regulates the striatal expression of clock genes. However, it is not known whether dopamine receptors mediate these regulatory effects of psychostimulants at the cellular level. Primary striatal neurons in culture express dopamine receptors as well as clock genes and have been successfully used in studying dopamine receptor functioning. Therefore, we investigated the role of dopamine receptors on neuronal clock gene expression in this model using specific receptor agonists. We found an inhibitory effect on the expression of mClock and mPer1 genes with the D2-class (i.e. D2/D3) receptor agonist quinpirole. We also found a generalized stimulatory effect on the expression of clock genes mPer1, mClock, mNPAS2 (neuronal PAS domain protein 2), and mBmal1 with the D1-class (i.e. D1) receptor agonist SKF38393. Further, we tested whether systemic administration of dopamine receptor agonists causes similar changes in striatal clock gene expression in vivo. We found quinpirole-induced alterations in mPER1 protein levels in the mouse striatum (i.e. rhythm shift). Collectively, our results indicate that the dopamine receptor system may mediate psychostimulant-induced changes in clock gene expression. Using striatal neurons in culture as a model, further research is needed to better understand how dopamine signaling modulates the expression dynamics of clock genes (i.e. intracellular signaling pathways) and thereby influences neuronal gene expression, neuronal transmission, and brain functioning.

Melatonin receptor agonist ramelteon activates the extracellular signal-regulated kinase 1/2 in mouse cerebellar granule cells.

The melatonin receptors MT1 and MT2 take part in the regulation of the activity (i.e. phosphorylation) of extracellular-signal-regulated kinase (ERK1/2), an enzyme involved in neuroplasticity. Primary cultures of mouse and rat cerebellar granule cells (CGC), which express both MT1 and MT2 receptors, have been widely used as an in vitro model to study neuronal ERK1/2. A novel MT1/MT2 agonist, ramelteon, has recently become clinically available. In this study, we characterized its action on neuronal ERK1/2. We used CGC cultures prepared from the cerebella of wild-type mice (MT1/MT2 CGC) and MT1- and MT2-knockout (KO) mice (MT1 KO CGC and MT2 KO CGC, respectively), and we employed a Western blot assay to evaluate ERK1/2 phosphorylation. Ramelteon increased ERK1/2 phosphorylation not only in MT1/MT2 CGC but also in CGC expressing only one of the two melatonin receptors. In the MT1 KO CGC, the stimulatory effect of ramelteon was blocked by an MT2 antagonist, 4P-PDOT, whereas in the MT2 KO CGC, this effect of ramelteon was blocked by luzindole. Pertussis toxin treatment did not prevent ramelteon from activating ERK1/2 but pretreatment with a tyrosine kinase (Trk) inhibitor, K252a, did, suggesting that an activation of Trk may mediate melatonin-receptor dependent ERK1/2 activation. In conclusion, we showed for the first time that a clinically used MT1/MT2 agonist, ramelteon, is capable of activating neuronal ERK1/2.

Effects of cocaine in 5-lipoxygenase-deficient mice.

5-Lipoxygenase (5-LOX), along with 12-lipoxygenase and cyclooxygenases, metabolizes arachidonic acid into eicosanoids. In rodents, 12-lipoxygenase deficiency alters behavioral responses to cocaine. We used 5-LOX-deficient mice and their controls to investigate cocaine's actions. After repeated cocaine injections, the increase in locomotor activity was greater in 5-LOX-deficient mice. Since the 5-LOX pathway may regulate the levels/metabolism of arachidonoylethanolamide (AEA) we assayed the AEA levels in the striatum, the binding of the endogenous AEA to the cannabinoid receptor CB1R, and anandamide hydrolase (FAAH) activity in the striatum, hippocampus, and cortex. Striatal AEA levels decreased after repeated cocaine injections. Cocaine also decreased CB1R binding in all brain regions studied and the only significant differences between 5-LOX-deficient and control mice was the greater hippocampal FAAH activity in 5-LOX-deficient mice. Our results demonstrated that a 5-LOX deficiency alters sensitivity to repeated cocaine. It should be investigated whether a human 5-LOX gene polymorphism affects cocaine's actions.

Effect of fluoxetine and cocaine on the expression of clock genes in the mouse hippocampus and striatum.

Long-term drug-induced alterations in CNS gene expression may be responsible for some therapeutic effects, such as antidepressant action, as well as for psychopathological conditions, such as drug addiction and abuse. Transcription factors called "clock" genes can be affected by psychotropic drugs and may modify the expression pattern of other genes. In this study in mice, we investigated the delayed effects of single and repeated (i.e. 14 days) administration of the antidepressant fluoxetine and the psychostimulant cocaine on the brain expression of clock genes Period1, Period2, Period3, Clock, Bmal1, Cryptochrome1, Cryptochrome2, and NPAS2 (neuronal PAS domain protein 2), and their putative target gene, serotonin N-acetyltransferase. Mice were treated at ZT05 (lights on at 5:00 am; ZT00). Brain samples (i.e. hippocampus, striatum, and prefrontal cortex) were processed for a semi-quantitative mRNA assay. Repeated but not single treatment with either drug increased serotonin N-acetyltransferase expression in all areas tested. On the other hand, the expression of clock genes was differentially affected depending on the drug (i.e. fluoxetine and cocaine), treatment schedule (i.e. single and repeated), and brain area (i.e. hippocampus and striatum) tested. More pronounced changes were induced by repeated rather than single administrations of fluoxetine or cocaine. We propose that the effects of psychoactive drugs on clock transcription factors may mediate long-term drug-induced changes, possibly by regulating the expression of a second set of genes (i.e. clock-controlled genes).

Effect of a short-term in vitro exposure to the marine toxin domoic acid on viability, tumor necrosis factor-alpha, matrix metalloproteinase-9 and superoxide anion release by rat neonatal microglia.

BACKGROUND: The excitatory amino acid domoic acid, a glutamate and kainic acid analog, is the causative agent of amnesic shellfish poisoning in humans. No studies to our knowledge have investigated the potential contribution to short-term neurotoxicity of the brain microglia, a cell type that constitutes circa 10% of the total glial population in the brain. We tested the hypothesis that a short-term in vitro exposure to domoic acid, might lead to the activation of rat neonatal microglia and the concomitant release of the putative neurotoxic mediators tumor necrosis factor-alpha (TNF-alpha), matrix metalloproteinases-2 and-9 (MMP-2 and -9) and superoxide anion (O2-). RESULTS: In vitro, domoic acid [10 microM-1 mM] was significantly neurotoxic to primary cerebellar granule neurons. Although neonatal rat microglia expressed ionotropic glutamate GluR4 receptors, exposure during 6 hours to domoic acid [10 microM-1 mM] had no significant effect on viability. By four hours, LPS (10 ng/mL) stimulated an increase in TNF-alpha mRNA and a 2,233 % increase in TNF-alpha protein In contrast, domoic acid (1 mM) induced a slight rise in TNF-alpha expression and a 53 % increase (p < 0.01) of immunoreactive TNF-alpha protein. Furthermore, though less potent than LPS, a 4-hour treatment with domoic acid (1 mM) yielded a 757% (p < 0.01) increase in MMP-9 release, but had no effect on MMP-2. Finally, while PMA (phorbol 12-myristate 13-acetate) stimulated O2- generation was elevated in 6 hour LPS-primed microglia, a similar pretreatment with domoic acid (1 mM) did not prime O2- release. CONCLUSIONS: To our knowledge this is the first experimental evidence that domoic acid, at in vitro concentrations that are toxic to neuronal cells, can trigger a release of statistically significant amounts of TNF-alpha and MMP-9 by brain microglia. These observations are of considerable pathophysiological significance because domoic acid activates rat microglia several days after in vivo administration.

Neurogenesis and neuroprotection in the adult brain. A putative role for 5-lipoxygenase?

5-Lipoxygenase (5-LOX) and cyclooxygenase-2 (COX-2) are two enzymes that are critical for the synthesis of eicosanoids, the inflammatory metabolites of arachidonic acid. Both 5-LOX and COX-2 are expressed in the brain, including in CNS neurons. The physiologic role of these proteins in neuronal functioning is not clear. In non-neuronal tissues these two enzymes often assume similar roles: in addition to their function in inflammation, 5-LOX and COX-2 appear to be associated with cell proliferation, that is, with tumor growth. High 5-LOX expression has been noticed in the proliferating brain or pancreatic tumor cells; reduction in tumor cell proliferation and/or destruction of tumor cells was achieved with 5-LOX inhibitors. Proliferation of immature neurons/neuroblasts is an important component of mitotic neurogenesis. We investigated the role of 5-LOX in proliferation using cultures of human neuronal precursor cells, NT2. We found that these cells express 5-LOX mRNA and we used 3H-thymidine incorporation as a measure of cell proliferation; this was reduced by treating the cultures with 5-LOX inhibitor AA-861. We propose that the 5-LOX pathway plays a crucial role in mitotic neurogenesis. Additional studies should explore whether 5-LOX may participate in neurogenesis related pathologies and whether it should be considered a target for procedures aimed at altering neurogenesis for therapeutic purposes.

Fluoxetine increases the content of neurotrophic protein S100beta in the rat hippocampus.

Recent studies indicate that a protracted daily administration of the antidepressant fluoxetine to adult rats increases cell proliferation/neurogenesis in the hippocampus. It has been hypothesized that this action of fluoxetine might be mediated by neurotrophic factors. We hypothesized that glial S100beta could be such a factor, and using quantitative Western immunoblotting, we investigated the effect of a 21-day treatment of rats with fluoxetine (5 mg/kg), and found that fluoxetine increases the content of hippocampal S100beta.

5-Lipoxygenase is required for proliferation of immature cerebellar granule neurons in vitro.

Primary cultures of rat cerebellar granule neurons express 5-lipoxygenase, an enzyme from the inflammatory pathway of arachidonic acid. Outside the central nervous system (CNS) 5-lipoxygenase participates in cell proliferation. We hypothesized that 5-lipoxygenase is needed for proliferation of immature cerebellar granule neurons. Using cultures prepared from 7-day-old rat pups, we confirmed in vitro neurogenesis by immunocytolabeling with 2-bromo-5-deoxyuridine and beta-tubulin isotype III and quantified the rate of cell proliferation by assaying [3H]thymidine incorporation. We found that immature cerebellar granule neurons express large amounts of 5-lipoxygenase, and that treatment with a 5-lipoxygenase antisense, to reduce expression of this gene, decreased significantly (by 60%) the content of 5-lipoxygenase protein and effectively reduced cell proliferation. [3H]thymidine incorporation was significantly reduced by each of the three 5-lipoxygenase inhibitors we tested: AA-861 [2-(12-hydroxydodeca-5, 10-diynyl)-3,5,6-trimethyl-1,4-benzoquinone], MK-886 (C(27)H(33)ClNO(2)S.Na), and L-655,238 [alpha-penyl-3-(2-quinolinylmethoxy)-benzenemethanol]. Their anti-proliferative effect was reversible. We propose that neuronal expression of 5-lipoxygenase is crucial for neurogenesis in vitro, and possibly also in vivo.

Prolonged swim-test immobility of serotonin N-acetyltransferase (AANAT)-mutant mice.

Serotonin N-acetyltransferase (AANAT; EC 2.3.1.87) metabolizes serotonin into N-acetylserotonin (NAS). AANAT mRNA is expressed in the pineal gland and retina, and also in the rat brain. It was proposed that NAS could be a mediator of the antidepressant action of drugs, and it was shown that chronic but not acute treatment of rats with the antidepressant fluoxetine increases the content of AANAT mRNA in the rat hippocampus. Consequently, AANAT deficiency might be involved in the pathobiology of depression. C57BL/6J mice have a mutant AANAT gene and are considered AANAT-deficient, i.e., "knocked down" (compared with their normal counterparts, C3H/HeJ mice). In this study, we investigated whether AANAT mRNA is expressed in the brain of C57BL/6J and C3H/HeJ mice and whether those mice differ behaviorally, i.e., in a forced swimming test which is used to evaluate antidepressant drugs (such drugs shorten the time of immobility). We found that C3H/HeJ mice express in the brain normal AANAT mRNA, whereas C57BL/6J mice express mutated AANAT mRNA. The mutant, AANAT knocked down C57BL/6J mice displayed significantly longer times of immobility ("depression"). This difference was evident regardless of the circadian rhythm, i.e., both during the day and in the dark at night. Further studies are needed to fully characterize the behavioral significance of AANAT mutation and its possible link to depression.

Antidepressants alter cell proliferation in the adult brain in vivo and in neural cultures in vitro.

The action of antidepressants on cell proliferation (bromodeoxyuridine (BrdU) or [3H]thymidine incorporation) was studied in the adult rat hippocampus in vivo and in neural precursors (immature rat cerebellar granule cells) in vitro. In vivo, prolonged (21 days) but not acute (single) intraperitoneal treatment with fluoxetine (5 mg/kg) resulted in a 3.4-fold increase of bromodeoxyuridine-positive cells in the subgranular zone of the dentate gyrus. In cell cultures, at 1 and 10 days in vitro, 48-h fluoxetine exposure (1 microM, which is comparable to therapeutic plasma concentrations) reduced thymidine incorporation when initiated at 1 day in vitro, but increased cell proliferation when initiated at 10 days in vitro. Clomipramine and imipramine produced similar action in vitro; desipramine was ineffective.

5-Lipoxygenase and cyclooxygenase mRNA expression in rat hippocampus:early response to glutamate receptor activation by kainate.

Recent research has identified in central nervous system neurons the expression of two enzymes from the inflammatory pathway of the metabolism of arachidonic acid, the 5-lipoxygenase (5LOX) and the cyclooxygenase-2 (COX2). Expression of both enzymes appears to be upregulated during aging; upregulated 5LOX/COX2 expression in neurons may be responsible for the increased neuronal vulnerability to degeneration. Involvement of the excitatory neurotransmitter glutamate in aging-associated neurodegeneration has also been suggested. Stimulation of glutamate receptors by kainic acid (kainate) has been shown independently to affect the brain expression of 5LOX or COX2. Using a quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay to measure the contents of mRNAs we found 3h after kainate injection (intraperitoneally; 10 mg/kg) increased mRNA levels of 5LOX and COX2, but not that of COX1 in the hippocampus of rats. Pretreatment with the COX2 inhibitor NS-398 (9 mg/kg, 1h prior to kainate) inhibited the kainate-stimulated increase of 5LOX and COX2 mRNA levels. Our results indicate that hippocampal expression of both 5LOX and COX2 increases rather promptly when glutamate receptors are stimulated by kainate. The mechanism of how NS-398 inhibits this action of kainate should be further investigated.

Inflammatory 5-LOX mRNA and protein are increased in brain of aging rats.

5-Lipoxygenase (5-LOX) is the key enzyme in the synthesis of leukotrienes, inflammatory mediators of arachidonic acid. 5-LOX is also expressed in neurons (in particular in the hippocampus and the cerebellum), and it seems to be capable of promoting neurodegeneration. Recently, we observed greater 5-LOX mRNA content in the hippocampus of older (24 months) than younger (2 months) rats. In this study, we measured in the hippocampus and the cerebellum of younger and older male F344 rats the contents of: 5-LOX mRNA, FLAP (5-LOX activating protein) mRNA, and 5-LOX protein. By using a quantitative reverse transcription/polymerase chain reaction (PCR) (RT-PCR) with internal standards we found that 5-LOX but not FLAP mRNA content is greater (both in hippocampus and cerebellum) of older than younger rats. By using quantitative Western immunoblotting, we found a greater content of 5-LOX protein in the hippocampus and the cerebellum of older rats; we also established that the membrane/cytosol 5-LOX content ratio is larger in the brains of older than younger rats (statistically significant in the cerebellum). The latter can be considered an indication of 5-LOX translocation/activation during aging. Together these results suggest that aging increases both neuronal 5-LOX expression and protein translocation, and indicate that the 5-LOX system might play a significant role in the pathobiology of aging-associated neurodegenerative diseases.

New anti-inflammatory treatment strategy in Alzheimer's disease.

Numerous reports have indicated that patients suffering from inflammatory diseases (e.g., arthritis) who take anti-inflammatory medication have a reduced risk of developing Alzheimer's disease (AD). Thus, the first generation of anti-inflammatory cyclooxygenase (COX) inhibitors, such as aspirin and indomethacin, have been tested as potential therapeutics in AD. Because the inhibition of COX-1 is also known to cause tissue damage in the gastrointestinal system from the resultant reduced cytoprotection, selective COX-2 inhibitors are being investigated and tested clinically as potentially better therapeutics for AD patients. However, such drugs may also trigger unwanted effects; for example, the COX-2 inhibitors, which reduce the production of one type of eicosanoids, the prostaglandins, may increase the production of other eicosanoids; i.e., the leukotriene B4 (LTB4), which is one of the most potent endogenous chemotactic/inflammatory factors. LTB4 production is initiated by the enzyme 5-lipoxygenase (5-LOX). The expression of the 5-LOX gene is upregulated during neurodegeneration and with aging. In spite of the fact that 5-LOX and leukotrienes are major players in the inflammation cascade, their role in AD pathobiology/therapy has not been extensively investigated. We propose that the 5-LOX inflammatory cascade may take part in the process of aging-associated neurodegenerative diseases, and we point to the role of 5-LOX in neurodegeneration and discuss its relevance for anti-inflammatory therapy of AD.

Putative role of neuronal 5-lipoxygenase in an aging brain.

Aging is associated with increased incidence and/or severity of neurodegenerative pathologies. Oxygen-mediated events are being considered as possible mechanisms responsible for the increasing neuronal vulnerability. Lipoxygenases are enzymes that, as cyclooxygenases (COX), can insert oxygen into the molecule of arachidonic acid and thereby synthesize inflammatory eicosanoids: leukotrienes [due to 5-lipoxygenase (5-LOX) activity] and prostaglandins (via COX activity). It appears that 5-LOX is expressed in central nervous system neurons and may participate in neurodegeneration. 5-LOX-triggered cell death may be initiated by the enzymatic activity of 5-LOX but could also occur via the nonenzymatic actions of the 5-LOX protein; new data point to the possibility that 5-LOX protein exerts actions such as interaction with tyrosine kinase receptors, cytoskeletal proteins, and the nucleus. The expression of neuronal 5-LOX is susceptible to hormonal regulation, presumably due to the presence of hormone-responsive elements in the structure of the 5-LOX gene promoter. The expression of the 5-LOX gene and the activity of the 5-LOX pathway are increased in elderly subjects. One possible mechanism of such 5-LOX up-regulation implies the contribution of aging-associated hormonal changes: relative melatonin deficiency and/or hyperglucocorticoidemia. Thus, the 5-LOX pathway could become a promising target of neuroprotective therapies for the aging brain.

Glucocorticoids stimulate inflammatory 5-lipoxygenase gene expression and protein translocation in the brain.

In the brain, the expression of 5-lipoxygenase (5-LO), the enzyme responsible for the synthesis of inflammatory leukotrienes, increases during aging. Antiinflammatory drugs are currently being evaluated for the treatment of aging-associated neurodegenerative diseases such as Alzheimer's disease. Although generally considered antiinflammatory, glucocorticoids, whose production also increases during aging, are not particularly effective in this disease. In human monocytes, 5-LO mRNA content increases on exposure to the synthetic glucocorticoid dexamethasone, which prompted us to hypothesize that glucocorticoids might increase 5-LO expression in the brain as well. We treated rats for 10 days either with corticosterone (implanted subcutaneously) or with dexamethasone (injected daily); they were killed on day 10 after pellet implantation or 24 h after the 10th dexamethasone injection. We found increased levels of 5-LO mRNA and protein in hippocampus and cerebellum of glucocorticoid-treated rats; 5-LO-activating protein (FLAP) mRNA content was not affected. Using western immunobloting, we also observed the concurrent translocation of 5-LO protein from cytosol to membrane, an indication of its activation. Thus, glucocorticoid-mediated up-regulation of the neuronal 5-LO pathway may contribute to rendering an aging brain vulnerable to degeneration.

Chronic fluoxetine administration increases the serotonin N-acetyltransferase messenger RNA content in rat hippocampus.

BACKGROUND: It has been proposed that up-regulation of cyclic adenosine monophosphate response element binding protein is a common action of chronic antidepressant treatments that may regulate specific target genes in the hippocampus. We hypothesized that the serotonin N-acetyltransferase (AA-NAT; EC 2.3.1.87) gene is one such target. AA-NAT leads to formation of N-acetylserotonin from serotonin, and in the pineal gland, to melatonin synthesis. We investigated whether hippocampal AA-NAT expression can be modified by chronic administration of fluoxetine to rats. METHODS: Male Brown-Norway rats were administered 5 mg/kg fluoxetine or its vehicle either once (acute) or once daily for 21 days (chronic). They were sacrificed 18 hours after the last injection, and their hippocampi were processed for a quantitative reverse-transcription/polymerase-chain reaction assay of AA-NAT and cyclophilin (cyc) messenger (m)RNAs. The results are expressed as AA-NAT/cyc ratios. RESULTS: Chronic but not acute fluoxetine administration resulted in about a fivefold increase in hippocampal AA-NAT mRNA. CONCLUSIONS: Up-regulation of extrapineal, e.g., hippocampal, AA-NAT expression may play a role in mediating the therapeutic action of antidepressant drugs.

Primary cultures of rat cerebellar granule cells as a model to study neuronal 5-lipoxygenase and FLAP gene expression.

Aging is associated with chronic neurodegenerative diseases and increased brain vulnerability that may lead to a worse outcome from brain insults in elderly than in young subjects. Inflammation is one of the patholphysiological mechanisms of both chronic and acute neurodegeneration. Leukotrienes are inflammatory lipid mediators whose formation from arachidonic acid is initiated by 5-lipoxygenase (5-LO). 5-LO is also expressed in neurons and can be activated by brain injuries, whereas 5-LO inhibitors can provide neuroprotection. The expression of the 5-LO gene appears to be inhibited by the pineal hormone, melatonin, and stimulated by stress hormone glucocorticoids (e.g., corticosterone and the synthetic glucocorticoid dexamethasone). Melatonin deficiency and hyperglucocorticoidemia frequently develop with aging. We found that old or pinealectomized, i.e., melatonin-deficient rats are more susceptible to kainate-triggered excitotoxic limbic brain injury than the corresponding young or sham-pinealectomized controls, and that pinealectomy, aging, or glucocorticoid treatment result in an enhanced expression of 5-LO in limbic structures. We hypothesize that an aging brain is at a higher risk of neurodegeneration via aging-suppressed melatonin secretion and/or aging-increased glucocorticoid secretion and the resultant upregulation of 5-LO expression. Furthermore, we propose that suppressing the 5-LO expression and/or activity will increase the brain's resistance to injury. The results of our ongoing research are expected to elucidate the role of 5-LO in aging and neurodegeneration and to indicate neuroprotective therapies that would target the 5-LO pathway.