Induction Of Cross-tolerance Research Articles

Soluble N-Acetylgalactosamine-Modified Antigens Enhance Hepatocyte-Dependent Antigen Cross-Presentation and Result in Antigen-Specific CD8+ T Cell Tolerance Development.

Hepatocytes compose up to 80% of the total liver and have been indicated as important players in the induction of immunologic tolerance in this organ. We show that hepatocytes possess the molecular machinery required for the cross-presentation of extracellular antigens. Using a derivative of the model antigen ovalbumin (OVA) covalently modified with a polymer containing multiple N-acetylgalactosamine residues (pGal-OVA) that enhance extracellular antigen uptake by mimicking the glycome of apoptotic debris, we show efficient hepatocyte-dependent induction of cross-tolerance of both adoptively transferred OT-I cells and endogenous OVA-specific CD8+ T lymphocytes, for example inducing tolerance to OVA-expressing skin transplants. Our study confirms that hepatocytes are capable of inducing peripheral tolerogenesis and provides proof of concept that they may be a valuable candidate for in vivo targeted tolerogenic treatments.

Induction of cross-tolerance between protective effect of morphine and nicotine in 6-hydroxydopamine-induce neurotoxicity in SH-SY5Y human dopaminergic neuroblastoma cells

Purpose: Parkinson’s disease is a progressive neurodegenerative disease characterized by progressive and selective death of dopaminergic neurons. It has been reported that nicotine and morphine have protective roles during neuronal damage in Parkinson’s disease. In addition, the induction of cross-tolerance between their biological effects has been shown in numerous reports.Methods: Here, we investigated the effects of nicotine and morphine on 6-OHDA-induced neurotoxicity in human neuroblastoma SH-SY5Y cell line as an in vitro model of Parkinson’s disease. Cell damage was induced by 150 μM 6-OHDA and the cells viability was examined by MTT assay. Intracellular reactive oxygen species, calcium level, and mitochondrial membrane potential were determined by fluorescence spectrophotometer method. Biochemical markers of apoptosis were also evaluated by immunoblotting.Result: The data showed that morphine and nicotine prevent 6-OHDA- induced cell damage and apoptosis. However, the protective effects of nicotine were not observed in chronic morphine-pretreated cells. Morphine had no protective effects in chronic nicotine-incubated cells.Conclusion: A cross-tolerance between protective effects of morphine and nicotine was occurred in 6-OHDA-induced SH-SY5Y cell toxicity.

Long-term HIF-1α transcriptional activation is essential for heat-acclimation-mediated cross tolerance: mitochondrial target genes.

An important adaptive feature of heat acclimation (HA) is the induction of cross tolerance against novel stressors (HACT) Reprogramming of gene expression leading to enhanced innate cytoprotective features by attenuating damage and/or enhancing the response of "help" signals plays a pivotal role. Hypoxia-inducible factor-1α (HIF-1α), constitutively upregulated by HA (1 mo, 34°C), is a crucial transcription factor in this program, although its specific role is as yet unknown. By using a rat HA model, we studied the impact of disrupting HIF-1α transcriptional activation [HIF-1α:HIF-1β dimerization blockade by intraperitoneal acriflavine (4 mg/kg)] on its mitochondrial gene targets [phosphoinositide-dependent kinase-1 (PDK1), LON, and cyclooxygenase 4 (COX4) isoforms] in the HA rat heart. Physiological measures of cardiac HACT were infarct size after ischemia-reperfusion and time to rigor contracture during hypoxia in cardiomyocytes. We show that HACT requires transcriptional activation of HIF-1α throughout the course of HA and that this activation is accompanied by two metabolic switches: 1) profound upregulation of PDK1, which reduces pyruvate entry into the mitochondria, consequently increasing glycolytic lactate production; 2) remodeling of the COX4 isoform ratio, inducing hypoxic-tolerant COX4.2 dominance, and optimizing electron transfer and possibly ATP production during the ischemic and hypoxic insults. LON and COX4.2 transcript upregulation accompanied this shift. Loss of HACT despite elevated expression of the cytoprotective protein heat shock protein-72 concomitantly with disrupted HIF-1α dimerization suggests that HIF-1α is essential for HACT. The role of a PDK1 metabolic switch is well known in hypoxia acclimation but not in the HA model and its ischemic setting. Remodeling of COX4 isoforms by environmental acclimation is a novel finding.

A20 Is Critical for the Induction of Pam3CSK4-Tolerance in Monocytic THP-1 Cells

A20 functions to terminate Toll-like receptor (TLR)-induced immune response, and play important roles in the induction of lipopolysacchride (LPS)-tolerance. However, the molecular mechanism for Pam3CSK4-tolerance is uncertain. Here we report that TLR1/2 ligand Pam3CSK4 induced tolerance in monocytic THP-1 cells. The pre-treatment of THP-1 cells with Pam3CSK4 down-regulated the induction of pro-inflammatory cytokines induced by Pam3CSK4 re-stimulation. Pam3CSK4 pre-treatment also down-regulated the signaling transduction of JNK, p38 and NF-κB induced by Pam3CSK4 re-stimulation. The activation of TLR1/2 induced a rapid and robust up-regulation of A20, suggesting that A20 may contribute to the induction of Pam3CSK4-tolerance. This hypothesis was proved by the observation that the over-expression of A20 by gene transfer down-regulated Pam3CSK4-induced inflammatory responses, and the down-regulation of A20 by RNA interference inhibited the induction of tolerance. Moreover, LPS induced a significant up-regulation of A20, which contributed to the induction of cross-tolerance between LPS and Pam3CSK4. A20 was also induced by the treatment of THP-1 cells with TNF-α and IL-1β. The pre-treatment with TNF-α and IL-1β partly down-regulated Pam3CSK4-induced activation of MAPKs. Furthermore, pharmacologic inhibition of GSK3 signaling down-regulated Pam3CSK4-induced A20 expression, up-regulated Pam3CSK4-induced inflammatory responses, and partly reversed Pam3CSK4 pre-treatment-induced tolerance, suggesting that GSK3 is involved in TLR1/2-induced tolerance by up-regulation of A20 expression. Taken together, these results indicated that A20 is a critical regulator for TLR1/2-induced pro-inflammatory responses.

The phenomenon of cross tolerance in osmotically and ionically adapted rice (Oryza sativa L.) cell lines

In the present study, cell lines of local rice ( Oryza sativa L.) cv. Swat-1 were incrementally adapted to osmotic [20% Poly ethylene glycol (PEG)] and ion specific (20 mM LiCl) stresses to investigate the phenomenon of cross tolerance. The tolerances of adapted lines were stable up to six generation on stress free medium. Both adapted cell lines were highly tolerant to either osmotic or ionic stresses while, the relative growth rates (RGR) of unadapted cells line decreased to about 95% at 20 mM LiCl and 20% polyethylene glycol, respectively. On subjection to temperature stress, the relative growth rates of unadapted cells line completely ceased, in contrast, both the adapted lines were found to be highly tolerant to low (18°C) and high temperature (36°C) stresses, respectively. At optimum temperature (27°C), proline content of unadapted and LiCl adapted lines were comparable (2 μmoles g -1 fresh weight), while polyethylene glycol adapted cell line had 17 time higher proline (35 μmole g -1 fresh weight) content. Under temperature stresses, the proline level of both the adapted lines increased significantly but the relative rate of increase was greater in LiCl adapted line than PEG adapted line. The tolerance of adapted cell lines to opposite stresses (osmotic or ionic) and to temperature stress (low and high) tangibly revealed that the presence of common stressor in the environment is not simply the cause of induction of cross tolerance, rather it seems to be the adaptations of software that facilitates induction of this phenomenon. Key words: Cross tolerance, growth, LiCl, polyethyleneglycol (PEG), proline.

Tumor necrosis factor induces GSK3 kinase–mediated cross-tolerance to endotoxin in macrophages

Endotoxin tolerance, a key mechanism for suppressing excessive inflammatory cytokine production, is induced by prior exposure of macrophages to Toll-like receptor (TLR) ligands. Induction of cross-tolerance to endotoxin by endogenous cytokines has not been investigated. Here we show that prior exposure to tumor necrosis factor (TNF) induced a tolerant state in macrophages, with less cytokine production after challenge with lipopolysaccharide (LPS) and protection from LPS-induced death. TNF-induced cross-tolerization was mediated by suppression of LPS-induced signaling and chromatin remodeling. TNF-induced cross-tolerance was dependent on the kinase GSK3, which suppressed chromatin accessibility and promoted rapid termination of signaling via the transcription factor NF-κB by augmenting negative feedback by the signaling inhibitors A20 and IκBα. Our results demonstrate an unexpected homeostatic function for TNF and a GSK3-mediated mechanism for the prevention of prolonged and excessive inflammation.

Endotoxin tolerance and cross-tolerance in mast cells involves TLR4, TLR2 and FcεR1 interactions and SOCS expression: perspectives on immunomodulation in infectious and allergic diseases

BackgroundThe study of the endotoxin tolerance phenomenon in light of the recently defined roles of mast cells and toll-like receptors as essential components of the innate immune response and as orchestrators of acquired immunity may reveal potentially useful mechanisms of immunomodulation of infectious and allergic inflammatory responses, such as sepsis or asthma. Here we evaluated the phenomenon of direct tolerance of endotoxins, as well as the induction of cross-tolerance and synergism by stimulation with toll-like receptor-2 (TLR2) and FcεR1 agonists, in murine mast cells prestimulated with lipopolysaccharide (LPS). Additionally, we evaluated some stimulatory and inhibitory signaling molecules potentially involved in these phenomena.MethodsMC/9 cells and primary bone marrow-derived mast cells obtained from C57BL/6 and TLR4-/- knock-out mice were sensitized to DNP-HSA (antigen) by incubation with DNP-IgE and were prestimulated with LPS for 18 hr prior to stimulation. Cultures were stimulated with LPS or Pam3Cys-Ser-(Lys)4 3HCl (P3C), a TLR2 agonist, individually or in combination with antigen. The production of IL-6 and TNFα, the phosphorylation of NFκB and p38 MAPK, and the expression of TLR4 and SOCS-1 and -3 were analyzed.ResultsWe found that production of TNFα and IL-6 in murine mast cells that have been pretreated with LPS and challenged with TLR4 (LPS) or -2 (P3C) agonists was reduced, phenomena described as endotoxin tolerance (LPS) and cross-tolerance (P3C), respectively. The expression of TLR4 was not affected by LPS pretreatment. Our results show that the FcεR1 agonist DNP-HSA (antigen) interacts synergistically with LPS or P3C to markedly enhance production of cytokines (TNFα and IL-6). This synergistic effect with LPS and P3C was also attenuated by LPS pretreatment and was mediated by TLR4. These results may be attributed to the reduction in phosphorylation of the mitogen-activated protein kinase (MAPK), p38, and the transcription factor NFκB, as well as to an increase in the expression of the suppressors of cytokine signaling (SOCS)-1 and -3 proteins in LPS-pretreated mast cells.ConclusionsThese findings can be explored with respect to the modulation of inflammatory responses associated with infectious and allergic processes in future studies.

Decrease in toll-like receptors 2 and 4 in the spleen of mouse with endotoxic tolerance

Down-regulation of toll-like receptor (TLRs) is present in animals with endotoxin tolerance. The spleen is the largest peripheral lymphatic organ in vivo, and it plays an important role in inflammatory reactivity. In this study we investigated TLR2 and TLR4 proteins in the spleen of endotoxic tolerant mice. The TLR2 and TLR4 proteins were quantitatively detected by Western blotting, and their locations in the spleen were observed by immunohistochemistry. IL1-beta mRNA in the spleen was analyzed by real-time quantitative PCR. The mice pretreated with 0.5 mg/kg/day of LPS for 14 days exhibited a decrease of TLR2 and TLR4, and presented endotoxin tolerance. In addition, a bolus injection of 10 mg/kg LPS for mice with endotoxic tolerance caused no alteration of TLR2 and TLR4 proteins and no change of IL1-beta mRNA expression in the spleen. TLR4 is the target of endotoxic tolerance induction. The induction of cross-tolerance of TLR2 following LPS challenge is present in vivo with endotoxin tolerance.

Exogenous 4-hydroxybenzoic acid and salicylic acid modulate the effect of short-term drought and freezing stress on wheat plants

Exogenous salicylic acid has been shown to confer tolerance against biotic and abiotic stresses. In the present work the ability of its analogue, 4-hydroxybenzoic acid to increase abiotic stress tolerance was demonstrated: it improved the drought tolerance of the winter wheat (Triticum aestivum L.) cv. Cheyenne and the freezing tolerance of the spring wheat cv. Chinese Spring. Salicylic acid, however, reduced the freezing tolerance of Cheyenne and the drought tolerance of Chinese Spring, in spite of an increase in the guaiacol peroxidase and ascorbate peroxidase activity. The induction of cross tolerance between drought and freezing stress was observed: drought acclimation increased the freezing tolerance of Cheyenne plants and cold acclimation enhanced the drought tolerance. The induction of drought tolerance in Cheyenne was correlated with an increase in catalase activity.

Involvement of hypothalamic pituitary adrenal axis on the analgesic cross-tolerance between morphine and nifedipine

Bidirectional cross-tolerance develops between opioids and Ca 2+ channel blockers relating to their antinociceptive effects; however, the role of hypothalamic pituitary adrenal (HPA) axis on this action has not been elucidated yet. We examined the analgesic cross-tolerance between morphine and nifedipine, a dihydropyridine calcium channel blocker, in intact and adrenalectomized (ADX) rats and also evaluated modification of HPA activity during this phenomenon. The tail-flick test was used to assess the nociceptive threshold. The plasma level of corticosterone, as a marker of HPA function, was measured by radioimmunoassay. Our results showed that, in sham operated rats which were chronically treated with morphine, nifedipine failed to affect nociceptive threshold but it could induce significant antinociceptive effect in ADX morphine treated animals. This effect was reversed by corticosterone replacement. Furthermore, morphine could not induce analgesic effect either in sham operated or in ADX animals that received chronic nifedipine. Chronic morphine inhibited the effect of nifedipine on corticosterone secretion but nifedipine treatment had no effect on morphine-induced corticosterone secretion. Based on these results, we can conclude that HPA axis is involved in the induction of cross-tolerance between morphine and nifedipine due to chronic morphine and not nifedipine treatment.

Immunostimulatory CpG-oligodeoxynucleotides (CpG-ODN) induce early hepatic injury, but provide a late window for protection against endotoxin-mediated liver damage

An impaired immunologic response to infection has been recognized as a major defect in the pathogenesis of sepsis and multi-organ failure. Sepsis-associated liver dysfunction and damage are main determinants for the course of the disease. CpG-motif-containing DNA-sequences (CpG-ODN) were previously shown to confer protection in models of infection by stimulating both innate and specific immune responses. Herein, we studied the effect of CpG-ODN in lipopolysaccharide (LPS)-associated hepatotoxicity. Sprague Dawley rats pre-treated at day 6 with either CpG-ODN or inert DNA were challenged with E. coli LPS and subsequently studied for liver injury at 6 and 16 h using in vivo fluorescence microscopy and immunohistochemistry. Western blot protein analysis served for assessment of expression of TLR4, TNF receptor-associated factor 6 (TRAF6), NFkappaB and caspase-3. To evaluate CpG-ODN effects during non-septic conditions, additional animals were solely exposed to CpG-ODN and studied after 1 and 6 days. CpG-ODN application induced marked hepatic microcirculatory deterioration and liver dysfunction at day 1, however, with almost complete recovery to normal at day 6. Interestingly, CpG-ODN pre-treatment decreased LPS-induced leukocyte-endothelial cell interaction, sinusoidal perfusion failure and caspase-3-dependent apoptotic cell death. Although Kupffer cell phagocytic activity was not affected, CpG-ODN pre-treatment in LPS-challenged animals attenuated hepatic protein expression of TRAF6 and NFkappaB and increased TLR4 by almost 100%. CpG-containing DNA-sequences induce early hepatic injury, but mediate long-term protection against LPS hepatotoxicity. The mechanism of protection is based on the induction of cross-tolerance, probably via inhibition of the downstream TRAF6-NFkappaB signaling pathway and upregulation of the TLR4 surface receptor.

CD8α–11b+ dendritic cells but not CD8α+ dendritic cells mediate cross-tolerance toward intestinal antigens

Cross-presentation is a critical process by which antigen is displayed to CD8 T cells to induce tolerance. It is believed that CD8alpha+ dendritic cells (DCs) are responsible for cross-presentation, suggesting that the CD8alpha+ DC population is capable of inducing both cross-priming and cross-tolerance to antigen. We found that cross-tolerance against intestinal soluble antigen was abrogated in C57BL/6 mice lacking mesenteric lymph nodes (MLNs) and Peyer patches (PPs), whereas mice lacking PPs alone were capable of developing CD8 T-cell tolerance. CD8alpha-CD11b+ DCs but not CD8alpha+ DCs in the MLNs present intestinal antigens to relevant CD8 T cells, while CD8alpha+ DCs but not CD8alpha-CD11b+ DCs in the spleen exclusively cross-present intravenous soluble antigen. Thus, CD8alpha-CD11b+ DCs in the MLNs play a critical role for induction of cross-tolerance to dietary proteins.

Dependence of bacterial protein adhesins on toll-like receptors for proinflammatory cytokine induction.

Toll-like receptors (TLRs) are important signal transducers that mediate inflammatory reactions induced by microbes through pattern recognition of virulence molecules such as lipopolysaccharide (LPS) and lipoproteins. We investigated whether proinflammatory cytokine responses induced by certain bacterial protein adhesins may also depend on TLRs. In differentiated THP-1 mononuclear cells stimulated by LPS-free recombinant fimbrillin (rFimA) from Porphyromonas gingivalis, cytokine release was abrogated by monoclonal antibodies (MAbs) to CD14 and TLR4 but not to TLR2. Similar experiments using anti-beta2 integrin MAbs suggested that beta2 integrins (CD11/CD18) also play a role in cytokine induction by rFimA or native fimbriae. Minor fimbriae (distinct from the fimA-encoded major fimbriae) of P. gingivalis induced proinflammatory cytokine release in a CD14- and TLR2-dependent mode. Cytokine induction by BspA, a leucine-rich repeat protein from Bacteroides forsythus, depended heavily on CD14 and TLR2. We also found that the ability of the streptococcal protein AgI/II to stimulate cytokine release depended partially on CD14 and TLR4, and the AgI/II segment that possibly interacts with these receptors was identified as its N-terminal saliva-binding region. When THP-1 cells were exposed to rFimA for 24 h, surface expression of CD14 and CD18 was decreased and the cells became hyporesponsive to cytokine induction by a second challenge with rFimA. However, tolerance induction was abolished when the THP-1 cells were pretreated with rFimA in the presence of either anti-CD14 MAb or anti-TLR4 MAb. Induction of cross-tolerance between rFimA and LPS correlated with downregulation of the pattern recognition receptors involved. Our data suggest that the CD14-TLR2/4 system is involved in cytokine production and tolerance induction upon interaction with certain proinflammatory bacterial protein adhesins.

Interaction of surfactant protein A with lipopolysaccharide and regulation of inflammatory cytokines in the THP-1 monocytic cell line.

Pulmonary surfactant protein A (SP-A) is involved in innate immunity in the lung. In this study we investigated the interaction of SP-A with different serotypes of lipopolysaccharide (LPS) on the regulation of inflammatory cytokines in vitro. In the human monocytic cell line, THP-1, combining SP-A with lipid A or rough LPS further enhanced lipid A- or rough LPS-stimulated tumor necrosis factor alpha (TNF-alpha) mRNA levels, while SP-A-elicited increases in TNF-alpha mRNA levels were partially neutralized. In contrast, the combination of smooth LPS and SP-A resulted in additive effects on TNF-alpha mRNA levels. We also demonstrated that there was cross-tolerance between SP-A and LPS in THP-1 cells. Pretreatment of THP-1 cells with LPS modestly inhibited the response of these cells to subsequent challenge with SP-A, with regard to the production of TNF-alpha, whereas there was no or little effect on the production of interleukin-1beta (IL-1beta) and IL-8. Conversely, pretreatment of THP-1 cells with SP-A markedly increased the response to subsequent challenge with LPS with regard to the production of IL-1beta and IL-8, although the production of TNF-alpha was modestly decreased. However, a synergistic stimulatory effect was observed when the two agents were added simultaneously to the cells. NF-kappaB formation was downregulated in SP-A- but not in LPS-induced tolerant cells. These results suggested that SP-A exhibits different interactions with distinct serotypes of LPS. In addition, SP-A is different from LPS with regard to the induction of cross-tolerance, and these actions may be mediated, at least in part, through different mechanisms.

Cellular lipid composition influences stress activation of the yeast general stress response element (STRE).

The heat inducibility of the yeast heat-shock response (HSR) pathway has been shown to be critically dependent on the level of unsaturated fatty acids present in the cell. Here the inducibility by heat or salt of the independently regulated general stress response (GSR) pathway is shown to be affected in the same way. An increase in the percentage of unsaturated fatty acids in heat- or salt-acclimated cells correlated with a decrease in the induction of a general stress-response-promoter-element (STRE)-driven reporter gene by either stress. Despite inducing reporter gene expression, sorbic acid treatment did not confer salt cross-tolerance on the cells. This failure correlated with a failure to increase the percentage of unsaturated fatty acids in the cells, suggesting that GSR pathway induction, in the absence of lipid changes, is insufficient for the induction of cross-tolerance. Cells grown with fatty acid supplements under anaerobic conditions provided further evidence for a potential role for lipids in the acquisition of stress resistance. These cells contained different fatty acid profiles depending on the fatty acid supplement supplied, exhibited differential sensitivity to both heat and salt stress, but had not undergone STRE induction. These results suggest that heat- and salt-stress induction of the GSR are sensitive to the level of unsaturated fatty acids present in the cell and that stress cross-tolerance may be a lipid-mediated phenomenon. Given that an increased level of unsaturated fatty acids also down-regulates heat induction of the HSR pathway, these observations lead to the provocative hypothesis that lipid modifications, rather than HSR or GSR pathway induction, are a major contributor to the induced heat and salt tolerance of yeast cells.

Induction of tolerance with intranasal administration of human cartilage gp-39 in DBA/1 mice: amelioration of clinical, histologic, and radiologic signs of type II collagen-induced arthritis.

Human cartilage glycoprotein 39 (HC gp-39) was recently identified as a candidate autoantigen in the pathogenesis of rheumatoid arthritis. In the present studies, we investigated the capacity of HC gp-39 to interfere in clinical disease induced by an unrelated autoantigen, type II collagen (CII), by the induction of cross-tolerance. DBA-1j/Bom mice were immunized with bovine CII/complete Freund's adjuvant and were given intraperitoneal booster injections of CII on day 21. Tolerance was induced via the intranasal pathway with either the disease-inducing antigen (CII), a control antigen (ovalbumin), or HC gp-39 either before priming with CII or near the day of the booster injection. Arthritis was monitored visually, and joint pathology was examined histologically and radiologically. In addition, CII antibody levels in serum were analyzed by enzyme-linked immunosorbent assay. In contrast to treatment before priming, intranasal application of HC gp-39 after immunization markedly suppressed disease activity and prevented joint destruction, whereas application of ovalbumin or CII was ineffective. Interference of HC gp-39 with the immune response to CII was demonstrated by decreased anti-CII antibody levels. The combined data indicate that intranasal treatment with HC gp-39 may trigger modulatory or regulatory mechanisms that interfere with the expression of disease in murine collagen-induced arthritis. HC gp-39 is the first cross-tolerance-inducing protein in arthritis that down-modulates a spectrum of disease features when given in a semitherapeutic protocol.

Effects of naltrexone and cross-tolerance to morphine in a learned helplessness paradigm.

Opiates have been implicated in learned helplessness (LH), a phenomenon known to be related to opiate stress-induced analgesia (SIA). In the present study, we investigated the role of opiates in the induction of LH and SIA under different conditions. Adult female Wistar rats were trained either by receiving 60 inescapable 1-mA footshocks (IS group, N = 114) or by confinement in the shock box (control or NS group, N = 92). The pain threshold of some of the animals was immediately evaluated in a tail-flick test while the rest were used 24 h later in a shuttle box experiment to examine their escape performance. The opiate antagonist naltrexone (0 or 8 mg/kg, ip) and the previous induction of cross-tolerance to morphine by the chronic administration of morphine (0 or 10 mg/kg, sc, for 13 days) were used to identify opiate involvement. Analysis of variance revealed that only animals in the IS group demonstrated antinociception and an escape deficit, both of which were resistant to the procedures applied before the training session. However, the escape deficit could be reversed if the treatments were given before the test session. We conclude that, under our conditions, induction of the LH deficit in escape performance is not opiate-mediated although its expression is opiate-modulated.

Determination of halothane-induced sleeping time in the rat: effect of prior administration of centrally active drugs.

A method is described for the determination of halothane-induced sleeping time in the rat. 2 The sleeping time exhibited a diurnal variation which was due, at least in part, to a change in the sensitivity of the central nervous system (CNS) to the anaesthetic. 3 Tolerance to halothane did not develop in rats repeatedly exposed to the anaesthetic over a period of over 48 hours. 4 Repeated sleeping time determinations have been used to follow changes in the sensitivity of the CNS to the anaesthetic occurring with time. 5 A tolerance to halothane was induced by pretreatment of rats with doses of amylobarbitone, pentobarbitone or meprobamate sufficient to keep animals anaesthetized for approximately 12 hours. This tolerance was followed by a period of halothane-hypersensitivity. 6 Halothane-tolerant animals awakened with higher brain halothane concentrations and were also tolerant to intracerebroventricularly administered pentobarbitone. 7 Halothane-hypertensive rats awakened with lower brain halothane concentrations and were also hypersensitivity to intracerebroventricularly administered pentobarbitone. 8 The possibility that the induction of cross-tolerance to halothane may be indicative of a drug's potential to produce dependence is discussed.

Induction of Cross-Tolerance for Contact Sensitivity by a Non-Immunogenic Chemical

Abstract 1,2-dichloro-4-nitrobenzene (DcNB), a non-immunogenic chemical, when fed to guinea pigs 21 days before attempted contact sensitization to 2,4-dinitro-1-chlorobenzene (DNCB), induced tolerance with respect to contact sensitivity to DNCB. The tolerance was specific since DCNB induced no tolerance to citraconic anhydride under similar experimental conditions. Guinea pigs which were contact sensitive to DNCB did not cross-react to DCNB. The theoretical and potential practical implications of these findings are discussed.