Stress — the neglected variable in experimental pharmacology and toxicology

Evaluation of Variability across Rat Acute Oral Systemic Toxicity Studies

Regulatory agencies rely upon rodent in vivo acute oral toxicity data to determine hazard categorization, require appropriate precautionary labeling, and perform quantitative risk assessments. As the field of toxicology moves toward animal-free new approach methodologies (NAMs), there is a pressing need to develop a reliable, robust reference data set to characterize the reproducibility and inherent variability in the in vivo acute oral toxicity test method, which would serve to contextualize results and set expectations regarding NAM performance. Such a data set is also needed for training and evaluating computational models. To meet these needs, rat acute oral LD₅₀ data from multiple databases were compiled, curated, and analyzed to characterize variability and reproducibility of results across a set of up to 2441 chemicals with multiple independent study records. Conditional probability analyses reveal that replicate studies only result in the same hazard categorization on average at 60% likelihood. Although we did not have sufficient study metadata to evaluate the impact of specific protocol components (eg, strain, age, or sex of rat, feed used, treatment vehicle, etc.), studies were assumed to follow standard test guidelines. We investigated, but could not attribute, various chemical properties as the sources of variability (ie, chemical structure, physiochemical properties, functional use). Thus, we conclude that inherent biological or protocol variability likely underlies the variance in the results. Based on the observed variability, we were able to quantify a margin of uncertainty of ±0.24 log₁₀ (mg/kg) associated with discrete in vivo rat acute oral LD₅₀ values.

Temporal factors alter effects of social housing conditions on responses to chemotherapy and hormone levels in a Shionogi mammary tumor model

OBJECTIVE

To identify possible hormonal factors involved in the differential responses to chemotherapy observed in our tumor model, we investigated if the timing among tumor cell injection, rehousing, and chemotherapy administration differentially affects levels of corticosterone (CORT), growth hormone (GH), and testosterone and tumor and host responses to chemotherapy.

METHODS

Mice were reared either individually (I) or in groups (G). At 2 to 4 months, mice were injected with tumor cells and retained in their original housing conditions or rehoused into different experimental groups (GG, IG, II, GI) either immediately (experiment 1) or 14 days later (experiment 2); chemotherapy was administered when tumors weighed approximately 0.8 g.

RESULTS

In experiment 1, IG and GG mice had better responses to chemotherapy than GI mice. Chemotherapy increased CORT levels in II mice and decreased GH levels in GI mice compared with those of their drug vehicle-treated counterparts. Under the temporal conditions of experiment 2, IG and GG mice lost the advantage seen in experiment 1 in terms of tumor and host responses to chemotherapy. Before chemotherapy administration, CORT levels in IG mice and GH levels in GI mice were higher than those in mice in all other housing conditions. At 1 day after chemotherapy, CORT levels were higher for chemotherapy-treated than for drug vehicle-treated IG mice, and at 5 days post chemotherapy, GH levels were higher in GI than in IG mice.

CONCLUSIONS

Temporal relationships among tumor cell injection, rehousing, and chemotherapy administration critically influence responses to chemotherapy; these effects may be mediated, in part, by alterations in hormone levels.

Effects of social housing condition on chemotherapeutic efficacy in a Shionogi carcinoma (SC115) mouse tumor model: influences of temporal factors, tumor size, and tumor growth rate

OBJECTIVE

The objective of this study was to investigate 1) whether social housing condition, tumor size, and tumor growth rate alter responses to chemotherapy and 2) whether the timing of tumor cell injection or chemotherapy initiation (relative to housing condition formation) influences tumor growth rate or the efficacy of chemotherapy.

METHODS

Mice were reared individually (I) or in groups (G). In experiment 1, mice were rehoused (IG or GI) or left in group housing (GG) immediately after tumor cell injection. In experiment 2, housing conditions (II, IG, GG, or GI) were formed when tumors weighed 1 g. Chemotherapy (adriamycin 4 mg/kg and cyclophosphamide 61.5 mg/kg IP) and exposure to acute novelty stress (15 min/d, 5 d/wk) were initiated 1 day after housing condition formation.

RESULTS

If chemotherapy was initiated when the tumor burden was undetectable (experiment 1), housing condition did not alter tumor response to chemotherapy, although IG mice lost the most weight and overall had the lowest probability of survival. If chemotherapy was initiated when tumors weighed 1 g (experiment 2), both tumor and host responses to chemotherapy were poorest for IG mice. Timing of tumor cell injection relative to housing condition formation also differentially influenced the rate of tumor growth in mice treated with the drug vehicle; in experiment 1, tumor growth rate was faster in GI and GG mice than in IG mice, whereas in experiment 2, the rate of tumor growth was faster in II mice than in GG and IG mice.

CONCLUSIONS

Altering the temporal relationships among social housing condition formation, tumor cell injection, and chemotherapy initiation differentially influences the rate of tumor growth and the efficacy of chemotherapy. Effects of housing condition are independent of tumor growth rate at chemotherapy initiation and, in terms of host responses, independent of tumor burden.

Effects of social defeat and of diazepam on behavior in a resident-intruder test in male DBA/2 mice

Social stress induces robust behavioral and physiological changes, some of which may alter the responsiveness to pharmacological agents, including diazepam (DZP). We used a resident-intruder paradigm to (1) develop a comprehensive ethogram of behavioral changes following social defeat (SD) in the socially reactive strain, DBA/2 male mice, (2) determine whether acute exposure of DBA/2 mice to low-dose DZP would induce flight or aggressive behavior, both of which have been observed in other rodent models and (3) to test whether prior social stress affects responses to DZP. Behavioral responses to a nonaggressive intruder (NAI) mouse 24 h post-SD were measured in resident subject mice exposed to DZP (0, 0.5, 2.0 mg/kg, ip) either prior to the resident-intruder test (Experiment 1) or immediately post-SD (Experiment 2); control mice were not defeated (NOSD). In general, SD mice displayed increased passive and active avoidance, defense, immobility, and risk assessment relative to NOSD mice. In Experiment 1, mice treated acutely with 0.5 mg/kg DZP had more approach and flight behavior, while those treated with 2.0 mg/kg DZP had more avoidance than vehicle-treated mice, independent of SD. In Experiment 2, acute DZP (2 mg/kg) induced effects 24 h later, possibly secondary to withdrawal. In a nonsocial context (Experiment 3), DZP increased exploratory activity.

Time-dependent effects of stressor application on metastasis of tumor cells in the lung and its regulation by an immunomodulator in mice

The effects of the timing of stressor application on transplanted tumor cells and its possible regulation by an immunomodulator was investigated. Male C57 BL/6N mice were subjected to rotational stressor for 7 days relative to tumor cell inoculation: stressor after inoculation of Lewis lung cancer cells, stressor during inoculation and stressor before inoculation. Stressor application and tumor cell inoculation induced transient decreases in body weight, particularly in mice stressed after inoculation. The mice exposed to the stressor during inoculation or before inoculation showed significant increases in the number of metastatic foci relative to control mice. Early administration of an immunomodulator, PSK, significantly attenuated the increase of metastatic foci in stressed mice. The weights of thymus gland and spleen at 14 days after inoculation were similar in the three stressor groups and the control group. Application of the stressor reduced NK cell activity of the normal mice as well as tumor bearing mice. The lowest pre-inoculation NK cell activity was observed in mice stressed for 7 days beginning on the day of inoculation. The NK cell activity decreased in the tumor bearing mice which were stressed at the time of tumor inoculation. Decreased NK cell activity was reversed at day 14 after tumor inoculation. The mice exposed to the stressor after inoculation showed lowest level of NK cell activity relative to mice exposed to the stressor before or during inoculation. The treatment of mice with PSK reduced these changes significantly. The present results suggest that the rotational stress reduces splenic NK cell activity, which may influence the magnitude of tumor metastasis, depending on the time of tumor cell injection. Further, administration of an immunomodulator may counteract the reduction of the NK cell activity.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Metastasis and neuroendocrine system in stressed mice

The effects of experimental stressors have been examined for their differential effects on primary tumor growth and spontaneous lung metastasis in syngeneic mice bearing the weakly immunogenic tumor Lewis lung carcinoma. The effects caused by the early weaning, physical restraint and foot-shock are in general small, and affected by a high variability. On the contrary, spatial disorientation reproducibly causes a specific increase in tumor metastases. The effects of spatial disorientation are sensitive to the inhibition by treatment with bromocryptine and guanethidine, and particularly by a central and peripheral adrenergic neuron blocking agent reserpine. The use of different lighting conditions and assay of urinary melatonin excretion reveal an association between pineal gland function and effects of spatial disorientation on metastasis.

Vascular alterations in rats with high blood pressure induced by social deprivation stress

While an activation of the opioid system has been found to play a role only in the triggering of the high blood pressure induced by brief (7-14 days) social deprivation stress in Wistar rats, factors responsible for the maintenance of the hypertension after long-term (30-35 days) isolation remained to be elucidated. To this aim, the effects of social deprivation stress on the functional and morphological features of blood vessels were studied. The tail artery, as a muscular vessel, and the aorta, as a large elastic vessel were used in these experiments. In ex vivo experiments, aorta and tail artery strips from rats isolated for 30-35 days were found to be hyperreactive to noradrenaline.

Changes in GABAergic function after chronic chemical stress

1. The function of the gamma-aminobutyric acid (GABA)-ergic system in certain areas of the rat brain was investigated after chronic chemical stress (exposure to either vapours 30 sec/day for 20 days). 2. GABA concentration, [3h] -GABA uptake and the activity of the synthesis enzyme glutamate decarboxylase (GAD) were measured. 3. Chronic stress: (a) reduced neuronal uptake of [3H] -GABA in the frontal cerebral cortex (43%) and increased non-neuronal uptake of [3H] -GABA in the hypothalamus (62%); (b) enhanced the activity of GAD (under subsaturating substrate concentration) in the frontal cortex (91%) and in the corpus striatum (69%); (c) did not modify GABA endogenous concentration; (d) did not affect the animals' body weight increase or produce any signs of toxicity. 4. The stimulation of GAD and reduction of [3H] -GABA neuronal uptake in the frontal cortex might suggest the stimulation of GABAergic neurotransmission induced by chronic stress in this area of the rat brain. Together with previous findings the frontal cortex would appear to be a key area in chronic stress processing.

Effects of cold-restraint and swim stress on convulsions induced by pentylenetetrazol and electroshock: influence of naloxone pretreatment

The influence of two stressogenic conditions, restraint at 4 degrees C for 30 min (cold-restraint stress; CRS) or swimming at 20 degrees C for 3 min (swim stress; SS), on nociception and on convulsions triggered by different agents was assessed in mice. In saline-pretreated mice CRS and SS caused analgesia (hot-plate test, 56 degrees C), delayed the onset of convulsions induced by pentylenetetrazol (PTZ, 100 mg/kg, IP) and aggravated convulsions elicited by maximal transcorneal electroshock (150 mA pulses at 60 Hz for 0.2 s). Pretreatment with naloxone (10 mg/kg, SC, 30 min prior to testing), which did not affect the responsiveness of nonstressed mice to the hot plate or to the convulsant treatments, attenuated the development of analgesia following CRS, but not SS, and further prolonged the latency to onset of PTZ-induced convulsions in both stressed groups. Thus the extent to which CRS and SS can each delay the onset of PTZ-triggered convulsion appears to be limited by activation of a proconvulsant opioid system. In contrast, naloxone pretreatment did not modify the effects of CRS or SS on the severity of electroshock-induced seizures. In conclusion, CRS and SS can each, simultaneously, exert anticonvulsant and proconvulsant influences on responsiveness to PTZ and electroshock, respectively. Also, both forms of stress can activate an opioid system modulating the onset of PTZ-induced seizures, which is distinct from that controlling nociception. These findings, together with those of other stress, convulsions and opioid systems, which depends on the characteristics of the stressogenic condition, species, convulsant agent and parameter considered.

The influence of stress on convulsive parameters in the mouse

Mice exposed to the stress of conspecific aggression for 10 min showed shorter latencies to convulsions induced by pentylenetetrazol but not by pilocarpine. This effect was short lived and was not influenced by pretreatment with naltrexone (5 mg/kg, SC). The onset of pilocarpine-induced convulsions in stressed mice was reduced by the opioid antagonist. Aggression stress did not change the incidence, duration or severity of convulsions triggered by the chemoconvulsants or electroshock. The results differ widely from those obtained using other stressogenic models such as cold-restraint or swim stress. This suggests that alterations of convulsive parameters and the involvement of opioid mechanisms in their mediation are critically dependent on the characteristics of the stressogenic procedure employed.

Effects of stress on tumor growth and metastasis in mice bearing Lewis lung carcinoma

The progression of Lewis lung carcinoma has been examined in mice under the stress of different housing and experimental conditions. The maintenance of the animals in a low stress environment decreased the weight of spontaneous lung metastases in comparison with conventional housing. The handling of mice in the low stress environment for intraperitoneal saline administration increased metastasis formation, whereas the application of a psychological stressor (spatial disorientation) to these animals increased both primary tumor growth and metastasis formation. These results indicate that psychological and experimental stressors can modulate, presumably via neuroendocrine mechanisms, the host's antitumor responses which can control metastases and primary tumor independently from each other.

Changes in lymphocyte beta-adrenergic receptor density and noradrenaline content of the spleen are early indicators of immune reactivity in acute experimental allergic encephalomyelitis in the Lewis rat

Neural activity in the spleen in the pre-clinical stage of experimental allergic encephalomyelitis (EAE) mirrored that of animals immunized with complete Freund's adjuvant (CFA) alone. In response to immune challenge the splenic noradrenaline content fell significantly, accompanied by an increase in lymphocyte beta-receptor density. The response of the pituitary-adrenal axis was reflected in increased circulating levels of corticosterone in both experimental groups; this was amplified by the stress of clinical signs leading to a 2-fold increase in animals with EAE. The increased, potentially immunosuppressive concentration of noradrenaline in the spleen during the acute stage may also represent a recovery mechanism and indicate how neuroimmune interactions could influence the relapsing-remitting nature of chronic inflammatory demyelinating disease.

Restraint-induced analgesia in the CD-1 mouse: interactions with morphine and time of day

The tail-flick response of adult male CD-1 mice was used to assess the analgesic properties of restraint alone and in combination with morphine during the diurnal and nocturnal periods. Mice were restrained in conical metal devices that allowed a change in position from supine to prone but not from front to back. Restraint induced an analgesia equipotent to a 2.5 mg/kg dose of morphine within 0.5 h of its initiation. Although habituation occurred over the restraint period a pronounced analgesia was still evident at the end of the 3 h test period. The habituation to restraint-induced analgesia was more rapid at night. Although the basal tail-flick latency to thermal stimulation was decreased during the nocturnal period the time of day did not alter the degree of analgesia induced by either restraint or morphine. Morphine induced a dose- and time-dependent analgesia during both the diurnal and nocturnal periods and this analgesia was potentiated by restraint stress only during the nocturnal period. Naloxone at high doses (10.0 or 20.0 mg/kg) blocked the analgesia induced by morphine but did not totally block the analgesia induced either by restraint or morphine plus restraint. These data suggest the potentiation of an opiate effect by stress may depend on habituation or tolerance to the stressor.

Hypotensive effect of naloxone on high blood pressure induced by stress in the rat

A naloxone-reversible enhancement of systolic blood pressure (BP) was induced in rats by application of three different types of stressor, i.e. intense light and sound, cold and foot-shock. In the case of labile high BP provoked by short-term isolation, the opiate antagonist naloxone (1 mg/Kg, i.p.) was also found to reverse hypertension. Naltrexone (2.5 mg/Kg, i.p.) also diminished high BP readings of briefly isolated rats. Conversely, blockade of the opiate receptor with naloxone did not alter elevated BP in cases of established hypertension (spontaneously hypertensive rats, deoxycorticosterone (DOCA)-salt rats and long-term isolated rats). These data can be taken as an evidence of opioid involvement at the onset of high BP readings induced by stress. However, once hypertension becomes established, the opioid system appears to recover its silent features.