Chronopharmacology of analgesic effect and its tolerance induced by morphine in mice

Role of daytime variation in pharmaceutical effects of sufentanil, dezocine, and tramadol: A matched observational study

The role of daytime variation in the comprehensive pharmaceutical effects of commonly used opioid analgesics in clinical setting remains unclear. This study aimed to explore the differences in daytime variation among elective surgery patients who were scheduled to receive preemptive analgesia with equivalent doses of sufentanil, dezocine, and tramadol in the morning and afternoon. The analgesic effect was assessed by changes in the pressure pain threshold before and after intravenous administration of sufentanil, dezocine, and tramadol. Respiratory effects were evaluated using pulse oximetry, electrical impedance tomography, and arterial blood gas analysis. Other side effects, including nausea, sedation, and dizziness, were also recorded, and blood concentration was measured. The results showed that the analgesic effects of sufentanil, dezocine, and tramadol were significantly better in the morning than in afternoon. In the afternoon, sufentanil had a stronger sedative effect, whereas dezocine had a stronger inhibitory respiratory effect. The incidence of nausea was higher in the morning with tramadol. Additionally, significant differences in different side effects were observed among three opioids. Our results suggest that the clinical use of these three opioids necessitates the formulation of individualized treatment plans, accounting for different administration times, to achieve maximum analgesic effect with minimal side effects.

Molecular rhythm alterations in prefrontal cortex and nucleus accumbens associated with opioid use disorder

Severe and persistent disruptions to sleep and circadian rhythms are common in people with opioid use disorder (OUD). Preclinical evidence suggests altered molecular rhythms in the brain modulate opioid reward and relapse. However, whether molecular rhythms are disrupted in the brains of people with OUD remained an open question, critical to understanding the role of circadian rhythms in opioid addiction. Using subjects' times of death as a marker of time of day, we investigated transcriptional rhythms in the brains of subjects with OUD compared to unaffected comparison subjects. We discovered rhythmic transcripts in both the dorsolateral prefrontal cortex (DLPFC) and nucleus accumbens (NAc), key brain areas involved in OUD, that were largely distinct between OUD and unaffected subjects. Fewer rhythmic transcripts were identified in DLPFC of subjects with OUD compared to unaffected subjects, whereas in the NAc, nearly double the number of rhythmic transcripts was identified in subjects with OUD. In NAc of subjects with OUD, rhythmic transcripts peaked either in the evening or near sunrise, and were associated with an opioid, dopamine, and GABAergic neurotransmission. Associations with altered neurotransmission in NAc were further supported by co-expression network analysis which identified OUD-specific modules enriched for transcripts involved in dopamine, GABA, and glutamatergic synaptic functions. Additionally, rhythmic transcripts in DLPFC and NAc of subjects with OUD were enriched for genomic loci associated with sleep-related GWAS traits, including sleep duration and insomnia. Collectively, our findings connect transcriptional rhythm changes in opioidergic, dopaminergic, GABAergic signaling in the human brain to sleep-related traits in opioid addiction.

Can chronopharmacology improve the therapeutic management of neurological diseases?

The importance of circadian rhythm dysfunctions in the pathophysiology of neurological diseases has been highlighted recently. Chronopharmacology principles imply that tailoring the timing of treatments to the circadian rhythm of individual patients could optimize therapeutic management. According to these principles, chronopharmacology takes into account the individual differences in patients' clocks, the rhythmic changes in the organism sensitivity to therapeutic and side effects of drugs, and the predictable time variations of disease. This review examines the current literature on chronopharmacology of neurological diseases focusing its scope on epilepsy, Alzheimer and Parkinson diseases, and neuropathic pain, even if other neurological diseases could have been analyzed. While the results of the studies discussed in this review point to a potential therapeutic benefit of chronopharmacology in neurological diseases, the field is still in its infancy. Studies including a sufficiently large number of patients and measuring gold standard markers of the circadian rhythmicity are still needed to evaluate the beneficial effect of administration times over the 24-hour day but also of clock modulating drugs.

The day-night differences in ERK1/2, GSK3β activity and c-Fos levels in the brain, and the responsiveness of various brain structures to morphine

As with other drugs or pharmaceuticals, opioids differ in their rewarding or analgesic effects depending on when they are applied. In the previous study, we have demonstrated the day/night difference in the sensitivity of the major circadian clock in the suprachiasmatic nucleus to a low dose of morphine, and showed the bidirectional effect of morphine on pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus depending on the time of administration. The main aim of this study was to identify other brain structures that respond differently to morphine depending on the time of its administration. Using immunohistochemistry, we identified 44 structures that show time-of-day specific changes in c-Fos level and activity of ERK1/2 and GSK3β kinases in response to a single dose of 1 mg/kg morphine. Furthermore, comparison among control groups revealed the differences in the spontaneous levels of all markers with a generally higher level during the night, that is, in the active phase of the day. We thus provide further evidence for diurnal variations in the activity of brain regions outside the suprachiasmatic nucleus indicated by the temporal changes in the molecular substrate. We suggest that these changes are responsible for generating diurnal variation in the reward behavior or analgesic effect of opioid administration.

Timing of Morphine Administration Differentially Alters Paraventricular Thalamic Neuron Activity

The paraventricular thalamic nucleus (PVT) is a brain region involved in regulating arousal, goal-oriented behaviors, and drug seeking, all key factors playing a role in substance use disorder. Given this, we investigated the temporal effects of administering morphine, an opioid with strongly addictive properties, on PVT neuronal function in mice using acute brain slices. Here, we show that morphine administration and electrophysiological recordings that occur during periods of animal inactivity (light cycle) elicit increases in PVT neuronal function during a 24-h abstinence time point. Furthermore, we show that morphine-induced increases in PVT neuronal activity at 24-h abstinence are occluded when morphine administration and recordings are performed during an animals' active state (dark cycle). Based on our electrophysiological results combined with previous findings demonstrating that PVT neuronal activity regulates drug-seeking behaviors, we investigated whether timing morphine administration with periods of vigilance (dark cycle) would decrease drug-seeking behaviors in an animal model of substance use disorder. We found that context-induced morphine-seeking behaviors were intact regardless of the time morphine was administered (e.g., light cycle or dark cycle). Our electrophysiological results suggest that timing morphine with various states of arousal may impact the firing of PVT neurons during abstinence. Although, this may not impact context-induced drug-seeking behaviors.

A role for the mu opioid receptor in the antidepressant effects of buprenorphine

Buprenorphine (BPN), a mixed opioid drug with high affinity for mu (MOR) and kappa (KOR) opioid receptors, has been shown to produce behavioral responses in rodents that are similar to those of antidepressant and anxiolytic drugs. Although recent studies have identified KORs as a primary mediator of BPN's effects in rodent models of depressive-like behavior, the role of MORs in BPN's behavioral effects has not been as well explored. The current studies investigated the role of MORs in mediating conditioned approach behavior in the novelty-induced hypophagia (NIH) test, a behavioral measure previously shown to be sensitive to chronic treatment with antidepressant drugs. The effects of BPN were evaluated in the NIH test 24h post-administration in mice with genetic deletion of the MOR (Oprm1^-/-) or KOR (Oprk1^-/-), or after pharmacological blockade with the non-selective opioid receptor antagonist naltrexone and selective MOR antagonist cyprodime. We found that behavioral responses to BPN in the NIH test were blocked in Oprm1^-/- mice, but not in Oprk1^-/- mice. Both cyprodime and naltrexone significantly reduced approach latency at doses experimentally proven to antagonize the MOR. In contrast the selective MOR agonist morphine and the selective KOR antagonist nor-BNI were both ineffective. Moreover, antinociceptive studies revealed persistence of the MOR antagonist properties of BPN at 24h post-administration, the period of behavioral reactivity. These data support modulation of MOR activity as a key component of BPN's antidepressant-like effects in the NIH paradigm.

Acute morphine affects the rat circadian clock via rhythms of phosphorylated ERK1/2 and GSK3β kinases and Per1 expression in the rat suprachiasmatic nucleus

BACKGROUND AND PURPOSE

Opioids affect the circadian clock and may change the timing of many physiological processes. This study was undertaken to investigate the daily changes in sensitivity of the circadian pacemaker to an analgesic dose of morphine, and to uncover a possible interplay between circadian and opioid signalling.

EXPERIMENTAL APPROACH

A time-dependent effect of morphine (1 mg·kg(-1) , i.p.) applied either during the day or during the early night was followed, and the levels of phosphorylated ERK1/2, GSK3β, c-Fos and Per genes were assessed by immunohistochemistry and in situ hybridization. The effect of morphine pretreatment on light-induced pERK and c-Fos was examined, and day/night difference in activity of opioid receptors was evaluated by [(35) S]-GTPγS binding assay.

KEY RESULTS

Morphine stimulated a rise in pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus (SCN) when applied during the day but significantly reduced both kinases when applied during the night. Morphine at night transiently induced Period1 but not Period2 in the SCN and did not attenuate the light-induced level of pERK1/2 and c-Fos in the SCN. The activity of all three principal opioid receptors was high during the day but decreased significantly at night, except for the δ receptor. Finally, we demonstrated daily profiles of pERK1/2 and pGSK3β levels in the rat ventrolateral and dorsomedial SCN.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that the phase-shifting effect of opioids may be mediated via post-translational modification of clock proteins by means of activated ERK1/2 and GSK3β.

Chronopharmacodynamics and chronopharmacokinetics of pethidine in mice

BACKGROUND

Many studies have demonstrated that the pharmacokinetics and pharmacodynamics of analgesic drugs vary according to the circadian time of drug administration. This study aims at determining whether the analgesic effect and pharmacokinetics of pethidine in male BALB/c mice are influenced by administration time.

METHODS

A hot-plate test was used to evaluate the analgesic effect after pethidine (20 mg/kg) or saline injection at different dosing times. Mouse blood samples were collected at different intervals after dosing at 9:00 am and 9:00 pm, and were determined via liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS

A significant 24-h rhythm was observed in the latency to thermal response at 30 min after dosing, with the peak during the dark phase and the nadir during the light phase. Tolerance to analgesic effect was produced after chronic pethidine injection at 9:00 am or 9:00 pm, and the recovery from tolerance was faster during the dark phase. The peak concentration (Cmax) and area under the concentration-time curve (AUC) of pethidine and its metabolite norpethidine were significantly higher during the dark phase than during the light phase, but the total serum clearance (CL/F) exhibited the opposite trend. The rhythm of drug plasma concentration was positively correlated with the analgesic effect.

CONCLUSION

These results suggest that the pharmacodynamics and pharmacokinetics of pethidine in mice vary significantly according to the dosing time, which implies that the time of administration should be considered in the rational clinical use of pethidine to maximise analgesia and minimise the adverse effects.

Chronopharmacodynamics of drugs in toxicological aspects: A short review for clinical pharmacists and pharmacy practitioners

For many decades, researchers are aware of the importance of circadian rhythm in physiological/biochemical properties and drug metabolism. Chronopharmacology is the study of how the effects of drugs vary with biological timing and endogenous periodicities. It has been attaching substantial attention in the last years. Chronopharmacodynamics mainly deals with the biochemical and physiological effects of drugs on the body, the mechanisms of drug action, the relationship between drug concentration and effect in relation to circadian clock. In this review, we will focus on mammalian circadian pharmacodynamics and discuss new chronotherapy approaches. Moreover, we will try to highlight the chronopharmacodynamics of cardiovascular drugs, anti-cancer drugs, analgesics and non-steroidal anti-inflammatory drugs (NSAIDs) and give some practical concerns for clinical pharmacists and pharmacy practitioners, concerning this issue.

Modulation of physiological reflexes by pain: role of the locus coeruleus

The locus coeruleus (LC) is activated by noxious stimuli, and this activation leads to inhibition of perceived pain. As two physiological reflexes, the acoustic startle reflex and the pupillary light reflex, are sensitive to noxious stimuli, this review considers evidence that this sensitivity, at least to some extent, is mediated by the LC. The acoustic startle reflex, contraction of a large body of skeletal muscles in response to a sudden loud acoustic stimulus, can be enhanced by both directly ("sensitization") and indirectly ("fear conditioning") applied noxious stimuli. Fear-conditioning involves the association of a noxious (unconditioned) stimulus with a neutral (conditioned) stimulus (e.g., light), leading to the ability of the conditioned stimulus to evoke the "pain response". The enhancement of the startle response by conditioned fear ("fear-potentiated startle") involves the activation of the amygdala. The LC may also be involved in both sensitization and fear potentiation: pain signals activate the LC both directly and indirectly via the amygdala, which results in enhanced motoneurone activity, leading to an enhanced muscular response. Pupil diameter is under dual sympathetic/parasympathetic control, the sympathetic (noradrenergic) output dilating, and the parasympathetic (cholinergic) output constricting the pupil. The light reflex (constriction of the pupil in response to a light stimulus) operates via the parasympathetic output. The LC exerts a dual influence on pupillary control: it contributes to the sympathetic outflow and attenuates the parasympathetic output by inhibiting the Edinger-Westphal nucleus, the preganglionic cholinergic nucleus in the light reflex pathway. Noxious stimulation results in pupil dilation ("reflex dilation"), without any change in the light reflex response, consistent with sympathetic activation via the LC. Conditioned fear, on the other hand, results in the attenuation of the light reflex response ("fear-inhibited light reflex"), consistent with the inhibition of the parasympathetic light reflex via the LC. It is suggested that directly applied pain and fear-conditioning may affect different populations of autonomic neurones in the LC, directly applied pain activating sympathetic and fear-conditioning parasympathetic premotor neurones.

Spinal astrocytes contribute to the circadian oscillation of glutamine synthase, cyclooxygenase-1 and clock genes in the lumbar spinal cord of mice

Spinal astrocytes have key roles in the regulation of pain transmission. However, the relationship between astrocytes and the circadian system in the spinal cord remains poorly defined. In the current study, the circadian variations in the expression of several clock genes in the lumbar spinal cord of mice were examined by using real-time PCR. The expression of Period1, Period2 and Cryptochrome1 showed significant circadian oscillations, each gene peaking in the early evening. The expression of Bmal1 mRNA also exhibited a circadian pattern, peaking from around midnight to early morning. The mRNA levels of Cryptochrome2 were slightly, but not significantly altered. Molecules related to pain transmission were also investigated. The mRNA expression of glutamine synthase (GS), and cyclooxygenases (COXs), known to be involved in various spinal sensory functions, showed rhythmicity with a peak in the early evening, although the expression of the neurokinin-1 receptor, subunits of the N-methyl-d-aspartate receptor, and glutamate transporters did not change. In addition, we found that protein levels of GS and COX-1 were also high at midnight compared with midday. Furthermore, we examined the effect of intrathecal fluorocitrate (100pmol), an inhibitor of astrocytic metabolism, on the expression of oscillating genes in lumbar spinal cord. Fluorocitrate significantly suppressed astrocyte function. Furthermore, the circadian oscillation of clock gene expression and GS and COX-1 expression were suppressed. Together, these results suggest that a significant circadian rhythmicity of the expression of clock genes is present in the spinal cord and that the components of the circadian clock timed by astrocytes might contribute to spinal functions, including nociceptive processes.

Opioid chronopharmacology: influence of timing of infusion on fentanyl's analgesic efficacy in healthy human volunteers

Chronopharmacology studies the effect of the timing of drug administration on drug effect. Here, we measured the influence of 4 timing moments on fentanyl-induced antinociception in healthy volunteers. Eight subjects received 2.1 μg/kg intravenous fentanyl at 2 pm and 2 am, with at least 2 weeks between occasions, and 8 others at 8 am and 8 pm. Heat pain measurements using a thermode placed on the skin were taken at regular intervals for 3 hours, and verbal analog scores (VAS) were then obtained. The data were modeled with a sinusoid function using the statistical package NONMEM. The study was registered at trialregister.nl under number NTR1254. A significant circadian sinusoidal rhythm in the antinociceptive effect of fentanyl was observed. Variations were observed for peak analgesic effect, duration of effect, and the occurrence of hyperalgesia. A peak in pain relief occurred late in the afternoon (5:30 pm) and a trough in the early morning hours (5:30 am). The difference between the peak and trough in pain relief corresponds to a difference in VAS of 1.3–2 cm. Only when given at 2 am, did fentanyl cause a small but significant period of hyperalgesia following analgesia. No significant changes were observed for baseline pain, sedation, or the increase in end-tidal CO₂. The variations in fentanyl’s antinociceptive behavior are well explained by a chronopharmacodynamic effect originating at the circadian clock in the hypothalamus. This may be a direct effect through shared pathways of the circadian and opioid systems or an indirect effect via diurnal variations in hormones or endogenous opioid peptides that rhythmically change the pain response and/or analgesic response to fentanyl.

Development of morphine-induced tolerance and withdrawal: involvement of the clock gene mPer2

The present study has been designed to assess specifically the involvement of the clock gene mPer2 in morphine-induced tolerance and withdrawal. At first, we checked the absence of initial differences in the expression of several gene transcripts involved in the development of morphine dependence in Per2(Brdm1) mutant mice and in their respective wild-type (WT) control littermates. Morphine-induced tolerance as well as precipitated withdrawal was then assessed in these mice. The Per2(Brdm1) mutant mice clearly developed less tolerance and showed attenuated withdrawal signs compared to WT. These results show that mPER2 is involved in morphine-induced tolerance and withdrawal.

Review Article: Chronobiology: influence of circadian rhythms on the therapy of severe pain

Modern pain therapy widely follows the WHO (World Health Organization) guidelines using a three-step ‘ladder’ for pain relief. This escalating step scheme includes the administration in the order nonopioids, mild opioids and strong opioids, and adjuvants at any step. Analgesics should be given ‘by the clock’ rather than ‘on demand’. However, the chronobiological parameters circadian pain rhythm, circadian efficacy of analgesics, and individual circadian need for analgesics are to be considered. The results of a multitude of studies in chronobiology are not consistent. Therefore, further studies with standardized protocols are needed that allow to assign more consistent rhythms to diseases, pain causes, and analgesic efficacy of opioids. In many cases, each patient perceives pain and its intensity individually during the time of day. By administration of analgesics over a constant or continuous dosage time fluctuations in pain perception and the outcomes of many studies in chronobiology are ignored that prove the influence of biological rhythms on the pharmacokinetic and pharmacodynamic aspects of analgesics. As different types of pain show different rhythms (highest pain intensities arising at different times of the day) analgesics should be dosed flexibly. It is also very important that drug therapy can be adjusted individually to the pain rhythm of the patient as well as to the type and cause of pain. In severe pain, therapy should be particularly careful. A flexible dosage depending on pain intensity and rapid dose adjustment are essentials of a modern pain therapy. Therefore, opioids that are flexible to use are better suited to treat the individual pain of the patient than rigid modified release oral or transdermal systems. J Oncol Pharm Practice (2010) 16: 81—87.

QSAR Study on the Antinociceptive Activity of Some Morphinans

Quantitative structure-activity relationship studies were performed to describe and predict the antinociceptive activity of 31 morphinan derivatives reported by the US Drug Evaluation Committee in 2005 and 2006. From these, three data sets were constructed and several models were calculated following the multiple linear regression and Leave-One-Out Cross-Validation (LOO-CV) tests. In general, these models achieved good descriptive power (approximately 92%) as well as predictive power (approximately 76%), but were unable to predict an external validation set of morphinan derivatives. When artificial neural networks were applied to these models, an improvement of the predictive and external validation values was obtained. It was observed that the results of the NN models are significantly better that those obtained by multiple linear regression. In spite that the problem under investigation can be handled adequately by a linear model, a neural network does bring slight improvements in the predictive power.

Effects of morphine on circadian rhythms of motor activity and body temperature in pig-tailed macaques

Previous studies of the effects of opiates on motor activity and body temperature in nonhuman primates have been limited in scope and typically only conducted with restrained animals. The present study used radio-telemetry devices to continuously measure activity and temperature in unrestrained pig-tailed macaques for 24 h following morphine administration. Two dose-response functions (0.56 to 5.6 mg/kg, i.m.) were determined, one with morphine administered at 9 a.m. and one with morphine administrated at 3 p.m. Under both the 9 a.m. or 3 p.m. administration schedules, body temperature and activity were increased acutely. Activity was also reduced the following morning after morphine administered at either time. In other regards, morphine's effects on both temperature and activity differed between 9 a.m. and 3 p.m. injection, including periods of decreased activity immediately after the acute increases after 9 a.m. but not 3 p.m. administration. Surprisingly, motor activity also increased 9-12 h post-injection following morphine administered at 9 a.m., but not at 3 p.m. These results clearly show an interaction between timing of morphine administration and effects on temperature and activity. These results also underscore the fact that single injections of drugs may have multiple and delayed effects on circadian rhythms in macaques.

Glucocorticoid Is Involved in Food-Entrainable Rhythm of μ-Opioid Receptor Expression in Mouse Brainstem and Analgesic Effect of Morphine

The repeated manipulation of feeding schedule has a marked influence on the chronopharmaological aspects of many drugs. In this study, we investigated the role of endogenous glucocorticoid in the mechanism by which restricting the feeding time modulates the analgesic effect of morphine. Male ICR mice were housed under a light-dark cycle (light on from 07:00 to 19:00) with food and water ad libitum or under repeated time-restricted feeding (feeding time from 09:00 to 17:00) for 2 weeks before the experiment. Under the ad libitum feeding, mRNA levels of mu-opioid receptor and its binding capacity in mouse brainstem increased around the early dark phase, following the 24-h variation in circulating glucocorticoid levels. As a consequence, potent analgesic effects of morphine were observed in mice injected with the drug during the dark phase. Daily restricted feeding modulated the time-dependency of mu-opioid receptor function, accompanied by the alteration of the rhythm in circulating glucocorticoid levels. Under the time-restricted feeding, potent analgesic effects of morphine were found in mice injected with the drug during the light phase. Because the manipulation of feeding schedule was unable to produce the food-entrainable rhythm in the expression of mu-opioid receptor in the brainstem of adrenalectomized mice, endogenous rhythm of glucocorticoid secretion seems to be involved in the mechanism by which the time-restricted feeding modulates the analgesic effects of morphine.

Glucocorticoid Hormone Regulates the Circadian Coordination of μ-Opioid Receptor Expression in Mouse Brainstem

The 24-h variation in glucocorticoid secretion from the adrenal cortex is observed not only in nocturnally active rodents but also in diurnally active humans. Although the cyclic change in circulating glucocorticoid levels is thought to influence the efficacy and/or toxicity of many drugs, the mechanism underlying the influence remains poorly understood. In this study, we demonstrate that the 24-h variation in circulating glucocorticoid levels modulates the analgesic effect of morphine by regulating the expression of the mu-opioid receptor. Significant time-dependent variations in the mRNA levels of the mu-opioid receptor and its binding capacity were observed in mouse brainstem. The analgesic effect of morphine was enhanced by administering the drug when mu-opioid receptor levels were increased. However, corticotrophin-releasing hormone (CRH)-deficient mice, disrupting the 24-h rhythm of glucocorticoid secretion, showed no significant time-dependent variation in the expression of the mu-opioid receptor. As a consequence, there was no significant dosing time-dependent difference in the analgesic effect of morphine in CRH-deficient mice. A single administration of corticosterone significantly induced the expression of the mu-opioid receptor in the CRH-deficient mouse brainstem and also enhanced the analgesic effect of morphine. These findings suggest a mechanism underlying the time-dependent variation in mu-opioid receptor function and provide clues to select the most appropriate time of day for administration of morphine.

Influence of Feeding Schedule on 24-h Rhythm of Hepatotoxicity Induced by Acetaminophen in Mice

The influence of feeding schedule on the chronopharmacological aspects of acetaminophen (APAP) was examined in mice housed under 12-h light/dark cycle (lights on from 7:00 AM to 7:00 PM) with food and water ad libitum feeding (ALF) or under repeated time-restricted feeding (feeding time between 9:00 AM and 5:00 PM) for 2 weeks before the experiment. For the ALF group, there was a significant 24-h rhythm of mortality after APAP (600 mg/kg i.p.) injection. Peak mortality was observed after APAP injection at 9:00 PM and 1:00 AM, and nadir mortality was observed after drug injection at 9:00 AM. Hepatotoxicity after APAP (300 mg/kg i.p.) injection at 9:00 PM was significantly more severe than that after drug injection at 9:00 AM. Immunohistochemical staining using anti-APAP antibody 2 h after APAP injection was detected in centrilobular hepatocytes after drug injection at 9:00 PM but not after drug injection at 9:00 AM. CYP2E1 activity and hepatic glutathione (GSH) levels in untreated mice showed significant 24-h rhythms associated with APAP toxicity rhythm. The reduction in hepatic GSH levels after APAP injection at 9:00 PM was greater than that after drug injection at 9:00 AM. On the other hand, manipulation of the feeding schedule modified APAP hepatotoxicity rhythmicity, CYP2E1 activity, and GSH levels in the liver. Manipulation of the feeding schedule and choosing the most appropriate time of the day for drug administration may help to achieve rational chronopharmacology of some drugs including APAP in specific experimental and clinical situations.