Mechanisms of N-methyl-D-aspartate receptor inhibition by melatonin in the rat striatum

N-methyl-D-aspartate (NMDA) receptor activation comprises multiple regulatory sites controlling Ca2+ influx into the cell. NMDA-induced increases in intracellular [Ca(+2)] lead to nitric oxide (NO) production through activation of neuronal NO synthase (nNOS). Melatonin inhibits either glutamate or NMDA-induced excitation, but the mechanism of this inhibition is unknown. In the present study, the mechanism of melatonin action in the rat striatum was studied using extracellular single unit recording of NMDA-dependent neuronal activity with micro-iontophoresis. Melatonin inhibited neuronal excitation produced by either NMDA or L-arginine. The effects of both NMDA and L-arginine were blocked by nitro-L-arginine methyl ester, suggesting that nNOS participates in responses to NMDA. However, excitation of NMDA-sensitive neurones induced by the NO donor sodium nitroprusside was only slightly modified by melatonin. Melatonin iontophoresis also counteracted excitation induced by tris(2-carboxyethyl)phosphine hydrochloride, showing that the redox site of the NMDA receptor may be a target for melatonin action. The lack of effects of the membrane melatonin receptor ligands luzindole, 4-phenyl-2-propionamidotetralin and 5-methoxycarbonylamino-N-acetyltryptamine, and the nuclear melatonin ligand, CGP 52608, a thiazolidine dione, excluded the participation of known membrane and nuclear receptors for melatonin. The data suggest that inhibition of NMDA-dependent excitation by melatonin involves both nNOS inhibition and redox site modulation.     

依达拉奉防治大鼠颈动脉球囊成形术后血管再狭窄

目的 观察依达拉奉对大鼠颈动脉损伤后再狭窄的防治作用,并探讨其作用机制.方法 雄性SD大鼠用数字表法随机分为两组:对照组和治疗组各20只.用2F球囊对大鼠右侧颈总动脉内膜损伤法制作再狭窄模型.治疗组制模后给予依达拉奉3 mg/kg,静脉注射,每日2次.对照组损伤后给予等体积的生理盐水.损伤后1、3、7、14和28 d原位灌注固定取材,分析病理形态学改变及采用免疫组织化学方法观察基质金属蛋白酶-2(MMP-2)的表达.结果 对照组损伤侧动脉14和28 d内膜面积分别为(0.240±0.043)mm2和(0.420±0.063)mm2,内膜增生指数为0.52±0.06和0.67±0.07,狭窄率为30%±9%和54%±9%,治疗组内膜面积分别为(0.063±0.025)mm2和(0.116±0.023)mm2,内膜增生指数为0.24±0.07和0.38±0.05,狭窄率为8%±3%和16%±4%,与对照组同一时间点比较,治疗组内膜面积减少(F分别为50.488和81.119),内膜增生指数(F分别为41.743和48.122)和狭窄率减轻(F分别为24.221和76.453,均P<0.05);MMP-2的表达术后3、7、14 d增加,阳性细胞率(%)对照组分别为27.16±7.15、22.59±6.68、18.85±4.91,治疗组为15.57±3.62、12.91±1.88、11.96±3.12,治疗组3、7 d MMP-2表达较对照组下降,差异有统计学意义(F=8.359.P=0.028和F=7.781,P=0.032).结论 依达拉奉可抑制大鼠颈动脉损伤后再狭窄,这种作用与其抑制MMP-2的表达有关.     

Prevention of neointimal formation by a serine protease inhibitor, FUT-175, after carotid balloon injury in rats

BACKGROUND AND PURPOSE: In vivo and vitro studies revealed the activation of thrombin and the complement system in vascular lesion formation during the process of atherosclerosis, along with pathological proliferation of smooth muscle cells. We examined the effect of the synthetic serine protease inhibitor FUT-175 (developed as a potent inhibitor of thrombin and the complement system) on vascular lesions using balloon dilatation-induced neointimal formation in the carotid artery of rats. METHODS: Sprague-Dawley (SD) rats underwent balloon dilatation injury of the left carotid artery to induce neointimal formation. Three groups of these rats (n=8, each) were treated with daily intraperitoneal injections of 1 of the following doses of FUT-175: 0.5, 1.0, or 2.0 mg/d in 1 mL of saline for 7 consecutive days. The control group (n=8) was similarly treated with 1 mL of saline for 7 days. The injections were started immediately after balloon injury. Two weeks after the injury, the left carotid arteries were perfusion-fixed, and the areas of the neointimal and medial layer were analyzed under a microscope. RESULTS: A morphometric analysis revealed that there were significant differences in the intima-media ratio between the 4 groups treated with vehicle (saline) or a low, medium, or high dose of FUT-175 (1.45+/-0.11, 1.08+/-0.06, 0.71+/-0.04, or 0.32+/-0.04, respectively). This suppression was achieved in a dose-dependent manner by the administration of FUT-175 after balloon injury. In the histological study, it was demonstrated that FUT-175 suppresses the production of platelet-derived growth factor (PDGF)-BB in the neointima and the medial smooth muscle cell layer. CONCLUSIONS: After balloon injury activated proteases that were inhibited by FUT-175 were demonstrated to have an essential role in the development of the pathological thickening of the arterial wall.     

Lack of nigrostriatal pathology in a rat model of proteasome inhibition

Systemic administration of ubiquitin-proteasome system inhibitors to rodents has been reported to induce certain behavioral and neuropathological features of Parkinson's disease. The goal of this study was to replicate these observations by administering a proteasome inhibitor (PSI) to rats using McNaught and colleagues' protocol. No alterations in locomotor activity or striatal dopamine and its metabolites were observed. Differences in nigral dopaminergic cell number between proteasome inhibitor- and vehicle-treated rats and inclusion bodies were not found. Extending the time of survival after administration and using different solvents failed to alter results, indicating this proteasome inhibitor does not consistently produce the selective toxicity and pathological hallmarks characterizing Parkinson's disease.     

Failure of neuronal protection by inhibition of glial activation in a rat model of striatonigral degeneration

Previous studies in rodent models of neurodegenerative disorders have demonstrated that minocycline exerts neuroprotective effects unrelated to its antimicrobial action. The purpose of the present study was to analyze whether minocycline exhibits neuroprotective activity in a rat model of striatonigral degeneration (SND), the core pathology underlying levodopa-unresponsive parkinsonism associated with multiple system atrophy (MSA). We observed no significant effect of minocycline on locomotor impairment in double-lesioned SND rats. Minocycline significantly suppressed astroglial and microglial activation (P < 0.01); however, 3'5'-monophosphate-regulated phosphoprotein (DARPP 32) immunohistochemistry revealed no significant differences in striatal lesion volume of minocycline-treated versus untreated control SND rats. Furthermore, there was no protection of nigral dopaminergic neurons in the double-lesion model. We conclude that despite its astrocytic and microglial suppression, minocycline failed to attenuate lesion-induced neuronal damage in the SND rat model.     

Treatment of vasospasm by balloon angioplasty: experimental studies and clinical experiences

Abstract

The vasodilation mode and degree of the invasion caused by balloon angioplasty were experimentally examined. Assessment by light microscopy and scanning electron microscopy demonstrated that the invasion to the implanted arterial wall, taken from a patient who died from vasospasm, was minimized by the use of the balloon under the condition at 1 atm, 10 times for 10 seconds. Furthermore, we applied angioplasty to eight patients who developed severe vasospasm after subarachnoid haemorrhage, and five showed improvement in neuro-physiological (transcranial Doppler sonography), neuroradiological, and clinical examinations. In addition, blood vessels obtained from one patient who died 10 days after angioplasty, demonstrated similar findings to those of the experimental studies. It can be said that angioplasty will be one of the effective therapeutic methods to manage vasospasm when it is applied under the conditions mentioned above.     

Mechanisms of hyperbaric oxygen-induced neuroprotection in a rat model of subarachnoid hemorrhage.

Acute cerebral ischemia occurs after subarachnoid hemorrhage (SAH) because of increased intracranial pressure (ICP) and decreased cerebral perfusion pressure (CPP). The effect of hyperbaric oxygen (HBO) on physiological and clinical outcomes after SAH, as well as the expressions of hypoxia-inducible factor-1alpha (HIF-1alpha) and its target genes, such as BNIP3 and VEGF was evaluated. Eighty-five male SD rats (300 to 350 g) were randomly assigned to sham, SAH, and SAH+HBO groups. Subarachnoid hemorrhage was induced by endovascular perforation. Cortical cerebral blood flow (CBF), ICP, brain water content, brain swelling, neurologic function, and mortality were assessed. HBO (100% O2, 2.8 ATA for 2 h) was initiated at 1 h after SAH. Rats were sacrificed at 24 h to harvest tissues for Western blot or for histology. Apoptotic morphology accompanied by strong immunostaining of HIF-1alpha, VEGF, and BNIP3 were observed in the hippocampus and the cortex after SAH. Increased expressions of HIF-1alpha, VEGF, and BNIP3 were quantified by Western blot. HBO reduced the expressions of HIF-1alpha, VEGF, and BNIP3, diminished neuronal damage and improved CBF and neurologic function. HBO reduced early brain injury after SAH, probably by inhibition of HIF-1alpha and its target genes, which led to the decrease of apoptosis and preservation of the blood-brain barrier function.     

Argatroban抑制颈动脉球囊损伤后新生内膜的形成

目的 观察特异性凝血酶抑制因子argatroban对颈动脉损伤后新生内膜形成的影响,探讨血管损伤后内膜增生、引起管腔狭窄的可能机制。方法32只SD大鼠于颈动脉球囊损伤后,经皮下埋置的渗透性微泵注射argatroban,分为高剂量组:40mg/周,n=9;中剂量组:20mg/周,n=6;低剂量组:10mg/周,n=8。对照组同法注射同体积的生理盐水,n=9。持续静脉给药1周。2周后检测颈动脉冠状切面新生内膜面积、新生内膜与中膜的面积比（NI/M）。结果中、高剂量治疗组的NI/M分别为1.00±0.11和1.08±0.09,明显低于对照组的1.46±0.11（P<0.05）;免疫染色显示伤后1周治疗组新生内膜血小板衍生生长因子（PDGF）-BB表达较对照组弱。结论 凝血酶在血管损伤后新生内膜形成的病理机制中发挥重要作用;特异性凝血酶抑制因子argatroban可能有助于预防颈动脉狭窄血管成形术后发生的再狭窄。     

The effect of thrombin inhibition in a rat arterial thrombosis model.

The effect of heparin and the synthetic irreversible antithrombin D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone (FPRCH2Cl) was studied on FeCl3-induced thrombotic occlusion of rat carotid arteries. Thrombocytopenia prevented occlusion in five of 7 rats for the 60 min observation period after FeCl3 injury demonstrating platelet dependence in this model of thrombosis. Intravenous injection of heparin (250 units/kg) followed by continuous infusion (250 units/kg/hr) failed to prevent occlusion in four of 6 rats whereas intravenous FPRCH2Cl infusion prevented occlusion at a dose of 200 nmol/kg/min during infusion in 6/6 rats. These findings indicate that thrombin plays a principle role in the platelet-dependent process of arterial thrombosis in FeCl3-damaged rat carotid arteries. Neutralization of the thrombogenic stimulus in this model by the thrombin inhibitor FPRCH2Cl suggests selective thrombin inhibition may be useful in the treatment of arterial thrombosis.     

Mechanisms involved in the augmentation of arachidonic acid-induced free-radical generation from rat neutrophils following hypoxia-reoxygenation

Polymorphonuclear leukocytes are known to play an important role in hypoxia/ischemia and reoxygenation injury. The present study was undertaken to investigate the involvement of protein kinase C, calmodulin, and cyclic adenosine monophosphate in the augmentation of the free-radical generation observed after hypoxia-reoxygenation (H-R). Free-radical generation from the rat polymorphonuclear leukocytes was measured as the arachidonic acid (1-5x10(-5) M)-induced luminol-dependent chemiluminescence response, which was augmented following H-R. The increase in free-radical generation after H-R was completely blocked by the pretreatment of cells with PKC inhibitor H(7), whereas indomethacin (a cyclo-oxygenase inhibitor) or forskolin (an adenylate cyclase activator) failed to modulate the H-R-dependent response. However, W(7)-a calcium/calmodulin (Ca(2+)/CaM) antagonist-partially reduced the augmented free-radical generation observed in the H-R cells. Results obtained thus suggest the possible involvement of protein kinase C and calcium in the augmentation of the free-radical generation response following H-R.     

Treatment of vasospasm by balloon angioplasty: experimental studies and clinical experiences.

The vasodilation mode and degree of the invasion caused by balloon angioplasty were experimentally examined. Assessment by light microscopy and scanning electron microscopy demonstrated that the invasion to the implanted arterial wall, taken from a patient who died from vasospasm, was minimized by the use of the balloon under the condition at 1 atm, 10 times for 10 seconds. Furthermore, we applied angioplasty to eight patients who developed severe vasospasm after subarachnoid haemorrhage, and five showed improvement in neurophysiological (transcranial Doppler sonography), neuroradiological, and clinical examinations. In addition, blood vessels obtained from one patient who died 10 days after angioplasty, demonstrated similar findings to those of the experimental studies. It can be said that angioplasty will be one of the effective therapeutic methods to manage vasospasm when it is applied under the conditions mentioned above.     

颈动脉球囊损伤模型大鼠核转录因子κB变化与颈动脉血管内膜增生的关系☆

目的：新生内膜异常增殖是血管成形术后再狭窄的主要原因，但其机制目前还不清楚。观察大鼠颈动脉球囊损伤术后核因子κB表达的变化及其与动脉内膜增生和血管重塑的关系。 方法：实验于2007-01/05在深圳市人民医院动物实验中心完成。①实验材料：SPF级雄性SD大鼠36只，体质量350 g 左右。②实验方法：将大鼠一侧颈动脉行球囊损伤术作为实验组，另一侧颈动脉作为对照组，分别在颈总动脉球囊损伤后6 h，3，7，14，28 d 后麻醉并处死大鼠，留取两侧颈总动脉标本。③实验评估：应用组织形态学方法检测内膜增生并进行计算机图像分析；同时通过凝胶电泳迁移率实验测定核因子κB的活性。 结果：纳入大鼠36只，因造模失败和死亡排除6只，进入结果分析30只。①球囊损伤后，内膜面积在7，14，28 d 逐渐增厚，内膜/中层比率增长，与对照组比较，差异显著（P < 0.05）。两组的中膜面积无明显变化。血管重塑指数在损伤后6 h 最大，之后不断减小。②核因子κB在对照组几乎不表达。而球囊损伤后6 h 即可见核因子κB表达，并于14 d 达高峰，28 d 仍有较强表达。实验组核因子κB各时间点与对照组比较差异均有显著性(P < 0.05)。 结论：大鼠颈动脉球囊损伤术后，核因子κB被迅速激活并持续增加，可能是内膜增生、血管重塑的重要机制之一。     

Mechanisms of NMDA receptor inhibition by diarylamidine compounds

Pentamidine, diminazene and 4′,6‐diamidino‐2‐phenylindole (DAPI) are antiprotozoal diarylamidine compounds. In the present work, we have studied their action on native N‐methyl‐D‐aspartate (NMDA) receptors in rat hippocampal pyramidal neurons. All three compounds inhibited NMDA receptors at ?80 mV holding voltage with IC₅₀ of 0.41 ± 0.08, 13 ± 3 and 3.1 ± 0.6 μM, respectively. The inhibition by pentamidine was strongly voltage‐dependent, while that of DAPI was practically voltage‐independent. Inhibition by diminazene had both voltage‐dependent and voltage‐independent components. Diminazene and DAPI demonstrated tail currents and overshoots suggesting “foot‐in‐the‐door” mechanism of action. In contrast, pentamidine was partially trapped in the closed NMDA receptor channels. Such difference in the mechanism of action can be explained by the difference in the 3D structure of compounds. In the pentamidine molecule, two benzamidine groups are connected with a flexible linker, which allows the molecule to fold up and fit in the cavity of a closed NMDA receptor channel. Diminazene and DAPI, in contrast, have an extended form and could not be trapped.     

Alleviation of motor hyperactivity and neurochemical deficits by endocannabinoid uptake inhibition in a rat model of Huntington's disease

Recent studies have demonstrated a loss of cannabinoid CB1 receptors in the postmortem basal ganglia of patients affected by Huntington's disease (HD) and in transgenic mouse models for this disease. These studies have led to the notion that substances that increase the endocannabinoid activity, such as receptor agonists or inhibitors of endocannabinoid uptake and/or metabolism, might be useful in the treatment of hyperkinetic symptoms of this disease. In the present study, we employed a rat model of HD generated by bilateral intrastriatal injections of 3-nitropropionic acid (3-NP), a toxin that selectively damages striatal GABAergic efferent neurons. These rats exhibited biphasic motor disturbances, with an early (1-2 weeks) hyperactivity followed by a late (3-4 weeks) motor depression. Analysis of GABA, dopamine, and their related enzymes, glutamic acid decarboxylase and tyrosine hydroxylase, in the basal ganglia proved marked decreases compatible with the motor hyperkinesia. In addition, mRNA levels for CB1 receptor, neuronal-specific enolase, proenkephalin, and substance P decreased in the caudate-putamen of 3-NP-injected rats. There were also reductions in CB1 receptor binding in the caudate putamen, the globus pallidus, and, to a lesser extent, the substantia nigra. By contrast, mRNA levels for tyrosine hydroxylase in the substantia nigra remained unaffected. Interestingly, the administration of AM404, an inhibitor of endocannabinoid uptake, to 3-NP-injected rats attenuated motor disturbances observed in the early phase of hyperactivity. Administration of AM404 also tended to induce recovery from the neurochemical deficits caused by the toxin in GABA and dopamine indices in the basal ganglia. In summary, morphological, behavioral, and biochemical changes observed in rats intrastriatally lesioned with 3-NP acid were compatible with a profound degeneration of striatal efferent GABAergic neurons, similar to that occurring in the brain of HD patients. As expected, a loss of CB1 receptors was evident in the basal ganglia of these rats. However, the administration of substances that increase endocannabinoid activity, by inhibiting the uptake process, allowed an activation of the remaining population of CB1 receptors, resulting in a significant improvement of motor disturbances and neurochemical deficits. These observations might be relevant to the treatment of hyperkinetic symptoms in HD, a human disorder with unsatisfactory symptomatic treatment for most patients.     

Effects of collateral inhibition in a model of the immature rat cerebellar cortex: multineuron correlations

A model of the immature rat cerebellar cortex is used to simulate the effect of the inhibitory recurrent collateral axons of the Purkinje cells on the spike trains in the network. Inhibition induces an important overall change in the statistical characteristics of individual spike trains. It is also instrumental in producing a strong cooperativity between the different neurons. Moreover, a functional spatial anisotropy appears. A specific entropy index is used to analyze levels of information transfer between clustered and faraway neurons in the network. The formatting effect of recurrent collateral inhibition on spike trains and on network functional dynamics is studied by means of a model of the newborn rat cerebellar cortex. This immature structure has simpler morphological characteristics and fewer physiological parameters than the adult one. It is thus a good candidate for the comparison between experimental and theoretical data. The model network is made of 256 formal neurons (FN), arranged in a square lattice. Each neuron is coupled to its eight nearest neighbors by inhibitory links. All the parameters of the different elements of the model — in particular integration of inhibitory and excitatory inputs — are given anatomical and physiological values derived from biological data. Activities of single FNs and correlations between spatially distant ones are analyzed with classical statistical techniques as well as with a specific informational entropy method we introduce. Simulation results indicate that inhibition is instrumental in: (1) the transformation of the spike train characteristics. This includes a lengthening of the mean interspike interval as well as an overall change in the statistical distribution of intervals, with an emergence of long-lasting ones; (2) the functional structuration of the network. Inhibitory connections between nearest neighbors induce a strong cooperativity between FNs. Furthermore a clear spatial anisotropy occurs in the functioning of the network, with inhibitory effects extending beyond local connectivity in preferential directions. We propose an interpretation of this functional structuration in terms of the various routes followed by the inhibition, including relay effects. The parameters of the model (levels of activities, inhibition rules and connectivities) were varied in order to test the robustness of the above results. Finally, the results are compared with those obtained in an experimental situation.     

Time window for clinical effectiveness of mass evacuation in a rat balloon model mimicking an intraparenchymatous hematoma

The purpose of this study is to evaluate in a rat model if the early removal of an experimental intracerebral mass mimicking an extensive subcortical hematoma improves neurological outcome. Fifty six male Wistar rats were studied. A balloon was placed sterotactically at the level of the striatum. The balloon was inflated to 100 microl for periods of 10, 60 or 120 min (with 10 animals in each group). In 10 animals the balloon was not deflated and there were four sham operated cases. Neurological deficit was evaluated by a blinded observer by means of a clinical scale from 0 to 8 points at 24 and 72 h after inflation. Three additional animals at each inflation period were sacrificed after 6 h for pathological study with hematoxylin-eosin staining. Death rate was 9/10 animals who had permanent inflation, 4/10 in those with 2 h inflation, 2/10 for 1 h inflation and 0/10 for 10 min inflation (P<0.01 in chi square test). Many animals developed a particular clinical syndrome not previously described. Mean 72 h clinical scores (0-8 points) were 7.6 (S.D.: 1. 2) for the permanent inflation group, 4.4 (S.D.: 3.2) for 2 h of inflation, 2.3 (S.D.: 3.2) for 1 h and 0.4 (0.9) for 10 min of inflation (P<0.01 in Kruskal Wallis test). In the pathological study the rate of damaged neurons was significantly higher in the permanent than in transient inflation groups. In conclusion, in this balloon model evacuation of an extensive acute expanding subcortical (hematoma-like) mass must be performed within a limited time window to prevent the development of irreversible neurological deficits or death.     

Up-regulation of genes involved in cellular stress and apoptosis in a rat model of hippocampal degeneration

Changes in gene expression within the hippocampus induced by denervation after electrolytic fimbria-fornix lesion in rat were compared to morphological and biochemical alterations. Fimbria-fornix lesion results in degeneration of hippocampal cholinergic terminals as evidenced by a sustained (2 days to 1 month) decrease in cholineacetyltransferase (ChAT) activity (50%). These changes were accompanied by a decrease in growth associated protein 43 (GAP-43) immunoreactivity in all hippocampal layers 4 days after lesion followed by a subsequent increase and return to normal levels by 20 days postinjury. This increase in GAP-43 expression in the hippocampus between 7 to 20 days after lesion may reflect heterotypic sprouting. TUNEL-positive cells were revealed by in situ assay within the hippocampus at 10 days, but not at 3 days, after lesion. Two subtracted cDNA libraries from the dorsal hippocampus of control and injured rats (at 3 and 10 days postlesion) were constructed in order to search for new genes potentially implicated in degeneration/regeneration phenomena. We analysed 1,536 clones from each library by differential screening and found a total of 46 up-regulated genes. Among the 15 known genes, 6 coded for proteins involved in signal transduction pathways. The upregulation of growth arrest DNA damage induced gene (GADD153), brain-specific RING finger protein, JNK interacting protein (JIP-1), protein kinase A (PKA), and Na+K+ ATPase was studied by quantitative polymerase chain reaction (PCR). Two of these genes, GADD153 and JIP-1, have been previously shown to participate in cell modifications induced by stress and apoptosis.     

Mechanisms of erythropoietin-induced brain protection in neonatal hypoxia-ischemia rat model.

Erythropoietin, a hemotopoietic growth factor, has brain protective actions. This study investigated the mechanisms of Recombinant Human EPO (rhEPO)-induced brain protection in neonates. An established rat hypoxia-ischemia model was used by ligation of the right common carotid artery of 7-day-old pups, followed by 90 minute of hypoxia (8% 02 and 92% N2) at 37 degrees C. Animals were divided into three groups: control, hypoxia-ischemia, and hypoxia-ischemia plus rhEPO treatment. In rhEPO treated pups, 300 units rhEPO was administered intraperitoneally 24 hours before hypoxia. rhEPO treatment (300 units) was administered daily for an additional 2 days. ELISA and immunohistochemistry examined the expression of EPO and EPOR. Brain weight, morphology, TUNEL assay, and DNA laddering evaluated brain protection. rhEPO abolished mortality (from 19% to 0%) during hypoxia insult, increased brain weight from 52% to 88%, reduced DNA fragmentation, and decreased TUNEL-positive cells. Real-time RT-PCR, Western blot, and immunohistochemistry revealed an enhanced expression of heat shock protein 27 (HSP27) in ischemic brain hemisphere. Double labeling of TUNEL with HSP27 showed most HSP27 positive cells were negative to TUNEL staining. rhEPO reduces brain injury, especially apoptotic cell death after neonatal hypoxia-ischemia, partially mediated by the activation of HSP27.     

REM sleep inhibition by light in the albino rat

Two groups of rats were maintained on a 12 hr light-12 hr darkness (L12-D12) schedule. One was subjected to continuous light and the other to continuous darkness for 3 weeks and then returned to the original schedule. Rats placed in continuous light, initially showed a marked decrement in both rapid eye movement sleep (REMS) and slow wave sleep (SWS) time. A dissociation developed later between the sleep stages; REMS time remained below baseline but SWS time increased above it. In recovery (L12-D12), a temporary increase of REMS time above baseline was observed. During the first 2 weeks of continuous darkness REMS time increased 30% and REMS/total sleep (%-REMS) increased 17%. Both measures returned to baseline during the third week of darkness. In recovery (L12-D12), REMS decreased 37% and %-REMS decreased 18%. Changes in SWS time paralleled those in REMS time in direction but were proportionately smaller relative to baseline values. It is proposed that light stimulation inhibits REMS in the rat whereas darkness is the preferred and, whenever possible, the utilized condition for elaboration of REMS. Rats housed under long-term exposure to L12-D12 schedules develop and maintain a chronic REMS debt though daily REMS values return to normal. Prolonged darkness seems to be necessary for a complete REMS rebound. The possibility that light inhibits SWS as well as REMS is also considered. The effects of the lighting environment on the stages of sleep are interpreted within the larger context of the rat's life style.