Ethanol disrupts fear conditioning in C57BL/6 mice

Ethanol has been demonstrated to disrupt numerous forms of learning. For example, ethanol disrupts fear conditioning in rats. Surprisingly, the opposite result was reported for mice. Because of the importance of mouse models in ethanol research and the predominance of transgenic mice generated on a C57BL/6 background, the present study examined the effects of acute ethanol administration on fear conditioning in C57BL/6 mice. Fear conditioning was chosen because of the apparent contradiction in results between mice and rats, because of its popularity in assessing forebrain-dependent learning and because the task examines two types of learning: (i) the hippocampus-dependent contextual learning and (ii) the hippocampus-independent conditioned stimulus-unconditioned stimulus learning. Dose-response curves were generated for ethanol (0.5, 1.0 and 1.5 g/kg) given on either training day, testing day, or both days. Ethanol, in a dose-dependent manner, disrupted fear conditioning when given on training day or given on both training and testing days. Ethanol given on testing day only did not disrupt fear conditioning. The present results demonstrate that ethanol disrupts fear conditioning in C57BL/6 mice.     

The effects of DHBE and MLA on nicotine-induced enhancement of contextual fear conditioning in C57BL/6 mice

Rationale Previous research indicates that nicotine administration enhances hippocampus-dependent forms of learning, including contextual fear conditioning. This effect is blocked by mecamylamine, a noncompetitive, broad-spectrum nicotinic receptor antagonist. Objectives The present study extends previous research by further characterizing the nicotinic acetylcholinergic receptor (nAChR) subtypes through which nicotine acts to enhance contextual fear conditioning. Methods C57BL/6J mice were trained with two conditioned stimulus (CS; 30 s, 85-dB white noise)–unconditioned stimulus (US; 2 s, 0.57-mA foot shock) pairings and tested 24 h later for contextual and cued fear conditioning. The effects of the α7 nAChR antagonist methyllycaconitine (MLA; 1.00, 10.00, and 20.00 mg/kg) and the effects of the α4β2 nAChR antagonist dihydro-beta-erythroidine (DHBE; 1.00, 3.00, and 6.00 mg/kg) on cued and contextual fear conditioning and on the enhancement of contextual fear conditioning by nicotine (0.25 mg/kg) were examined. Results We demonstrate that DHBE (all doses) administration attenuates the enhancing effect of nicotine on contextual fear conditioning, and MLA administration has no significant effect on the enhancement of contextual fear conditioning by nicotine. Conclusions The data suggest that non-α7 nAChRs (most likely α4β2 nAChRs) underlie the enhancement of contextual fear conditioning by nicotine.     

Neuronal nicotinic receptor ligands modulate chronic nicotine-induced ethanol consumption in C57BL/6J mice

Alcohol and nicotine are commonly abused drugs in humans and evidence suggests that neuronal nicotinic acetylcholine receptors (nAChRs) in the midbrain dopamine system are common targets for the neurobehavioral interactions between alcohol (ethanol) and nicotine. The present study examined the efficacy of nAChR ligands with different pharmacological profiles such as cytisine, lobeline and dihydro-β-erythroidine (DHβE) to modulate chronic nicotine-induced increase in ethanol intake by C57BL/6J mice, using a two-bottle choice procedure. After establishment of baseline ethanol preference (10%, v/v), animals received daily subcutaneous injections of saline, nicotine (0.4 mg/kg) or different doses of cytisine, lobeline or DHβE 15 min prior to nicotine, for 10 days. Ethanol and water were presented immediately after the last (saline or nicotine) injection and fluid levels were monitored for post 1 h and 2 h treatment. Compared to control, nicotine injection significantly increased mean ethanol intake over 10 days, at both post 1 h and 2 h. Pretreatment with cytisine (0.5, 1.5 or 3.0 mg/kg) or lobeline (4.0 or 10.0 mg/kg) significantly reduced nicotine-induced increase in ethanol intake post 1 h and 2 h, without affecting water consumption. DHβE (0.5 or 2.0 mg/kg) failed to suppress nicotine-induced ethanol intake across 2 h post injection. These results indicate that nAChR-mediated signaling is critical in regulating nicotine-induced ethanol drinking behaviors.     

Stimulatory effects of ethanol in C57BL/6 mice

Although ethanol stimulation is well documented in several species including humans, there is some controversy about whether the stimulation occurs in the highly inbred mouse strain, C57BL/6. Since inbred mouse strains are frequently used to elucidate mechanisms for individual differences in reaction to alcohol, the present study was undertaken to more completely characterize the behavioral effects of ethanol and to help resolve some of the controversy regarding the drug's stimulatory effect on C57 mice. Activity of female C57BL/6cr mice was assessed in either a lighted or dark environment for 20 min after injections of water or ethanol at doses of 0.5, 1.0, 2.0, 4.0 g/kg. Elevated activity (stimulation) was observed in mice injected with relatively low ethanol doses and tested in the light. The 2.0 g/kg dose produced a transient elevation in activity which declined rapidly across time. Animals tested under the dark condition were not stimulated by the drug but had activity reductions to high doses of ethanol. The detection of ethanol-induced stimulation appears to be related to the performance of control mice rather than a light-related difference in ethanol sensitivity.     

Immunotoxic effects of short-term atrazine exposure in young male C57BL/6 mice.

The herbicide atrazine (ATR) is a very widely used pesticide; yet the immunotoxicological potential of ATR has not been studied extensively. Our objective was to examine the effect of ATR on selected immune parameters in juvenile mice. ATR (up to 250 mg/kg) was administered by oral gavage for 14 days to one-month-old male C57BL/6 mice. One day, one week, and seven weeks after the last ATR dose, mice were sacrificed, and blood, spleens, and thymuses were collected and processed for cell counting and flow cytometry. Thymus and spleen weights were decreased by ATR, with the thymus being more sensitive than the spleen; this effect was still present at seven days, but not at seven weeks after the last ATR dose. Similarly, organ cellularity was persistently decreased in the thymus and in the spleen, with the splenic, but not thymic cellularity still being depressed at seven weeks post ATR. Peripheral blood leukocyte counts were not affected by ATR. There were also alterations in the cell phenotypes in that ATR exposure decreased all phenotypes in the thymus, with the number of CD4(+)/CD8(+) being affected the least. At the higher doses, the decreases in the thymic T-cell populations were still present one week after the last ATR dose. In the spleen, the CD8(+) were increased and MHC-II(+) and CD19(+) cells were decreased one day after the last ATR dose. Also, ATR treatment decreased the number of splenic na?ve T helper and T cytotoxic cells, whereas it increased the percentage of highly activated cytotoxic/memory T cells. Interestingly, the proportion of mature splenic dendritic cells (DC; CD11c(high)), was also decreased and it persisted for at least one week, suggesting that ATR inhibited DC maturation. In the circulation, ATR exposure decreased CD4(+) lymphocytes at one day, whereas at seven days after the last ATR dose, in addition to the decrease in CD4(+) lymphocytes, the MHC-II(+) cells were also decreased at the 250 mg/kg dose. Thus, ATR exposure appears to be detrimental to the immune system of juvenile mice by decreasing cellularity and affecting lymphocyte distribution, with certain effects persisting long after exposure has been terminated.     

Modulation of ethanol drinking-in-the-dark by mecamylamine and nicotinic acetylcholine receptor agonists in C57BL/6J mice

Rationale  Recent reports describe a restricted access ethanol consumption paradigm where C57Bl/6J mice drink until intoxicated. Termed “drinking in the dark” (DID), this paradigm has been used as a model of binge drinking. Although neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in alcohol drinking in rats pre-trained to self-administer ethanol, their role in binge-like ethanol consumption is unknown. Objectives  To determine if nAChRs are involved in binge drinking as measured by the DID assay in C57Bl/6J mice. Materials and methods  Adult male C57Bl/6J mice were injected i.p. with nicotinic receptor antagonists including mecamylamine, hexamethonium, dihydro-β-erythroidine, and methyllycaconitine. Immediately following injection, mice were presented with 20% ethanol for 2 h in the DID assay to measure ethanol consumption. Nicotinic agonists including cytisine and nicotine were also evaluated. The effects of mecamylamine and nicotine on ethanol-induced dopaminergic neuronal activation in the VTA were evaluated via immunohistochemistry. Results  Mecamylamine dose dependently reduced ethanol consumption; whereas, the peripheral antagonist hexamethonium had no significant effect. Nicotinic agonists, cytisine and nicotine, reduced ethanol consumption. None of the effective nicotinic receptor drugs reduced sucrose drinking. Mecamylamine blocked ethanol activation of dopaminergic neurons while nicotine alone activated them without additional activation by ethanol. Conclusions  Neuronal nAChRs are involved in ethanol consumption in the DID paradigm. The effects of mecamylamine, nicotine, and cytisine on ethanol intake appear to be specific because they do not reduce sucrose drinking. Mecamylamine reduces alcohol consumption by blocking activation of dopaminergic neurons; whereas, nicotinic agonists may activate the same reward pathway as alcohol.     

The stimulus properties of LSD in C57BL/6 mice

RATIONALE: Drug-induced stimulus control has proven to be a powerful tool for the assessment of a wide range of psychoactive drugs. Although a variety of species has been employed, the majority of studies have been in the rat. However, with the development of techniques which permit the genetic modification of mice, the latter species has taken on new importance. Lysergic acid diethylamide [LSD], the prototypic indoleamine hallucinogen, has not previously been trained as a discriminative stimulus in mice. OBJECTIVE: To demonstrate the feasibility of LSD-induced stimulus control in the mouse and to provide a preliminary characterization of the stimulus properties of LSD in that species. METHODS: Male C57BL/6 mice were trained using a left or right nose-poke operant on a fixed ratio 10, water reinforced task following the injection of lysergic acid diethylamide [LSD, 0.17 or 0.30 mg/kg, s.c.; 15 min pretreatment] or vehicle. RESULTS: Stimulus control was established in 6 of 16 mice at a dose of LSD of 0.17 mg/kg after 39 sessions. An increase in dose to 0.30 mg/kg for the remaining mice resulted in stimulus control in an additional 5 subjects. In the low dose group, subsequent experiments demonstrated an orderly dose-effect relationship for LSD and a rapid offset of drug action with an absence of LSD effects 60 min after injection. When LSD [0.17 mg/kg] was administered in combination with the selective 5-HT2A antagonist, M100907, LSD-appropriate responding was significantly but incompletely reduced to approximately 50%; concurrently, response rates declined significantly. In mice trained with a dose of LSD of 0.30 mg/kg, full generalization to the phenethylamine hallucinogen, [-]-2,5-dimethoxy-4-methylamphetamine [DOM] was observed. CONCLUSIONS: The present data demonstrate the feasibility of LSD-induced stimulus control in the mouse. The general features of stimulus control by LSD in the mouse closely resemble those observed in the rat but the present data suggest that there may be significant differences as well.     

Acquisition of lever pressing for cocaine in C57BL/6J mice: effects of prior Pavlovian conditioning

The purpose of this study was to determine (1) if C57BL/6J (C57) mice would lever-press for intravenous cocaine infusions in a limited-access paradigm without previously establishing the instrumental response with natural reinforcers and (2) if prior Pavlovian conditioning of cocaine to the response contingent stimulus complex used in the cocaine self-administration sessions would facilitate acquisition of lever responding for cocaine. After implanting jugular catheters, some mice received Pavlovian conditioning during which 12 passive cocaine infusions (0.1 or 1 mg/kg unit doses) were paired with the tone/light/pump sound stimulus complex used in the self-administration sessions. The remaining mice simply began the cocaine self-administration sessions for 0.1 or 1 mg/kg unit doses of cocaine. Twenty-seven of the 33 mice with patent catheters acquired stable lever responding within an average of 5 to 6 days without previously establishing the instrumental response with natural rewards. Prior Pavlovian pairing of cocaine with the response contingent stimulus complex used in the self-administration sessions did not influence the acquisition of cocaine self-administration at the highest cocaine dose (1 mg/kg). This conditioning procedure using the low cocaine dose (0.1 mg/kg/infusion) reduced the number of mice acquiring cocaine self-administration to 50%, and the number of mice developing stable response patterns was only 25%. The results establish that C57 mice can acquire cocaine self-administration over several unit doses in a limited-access paradigm without previously establishing the instrumental response with natural reinforcers. Furthermore, prior pairing of response contingent cues with cocaine via Pavlovian conditioning did not facilitate the acquisition of cocaine self-administration.     

Delta-opioid and 5-HT3 receptor antagonist effects on ethanol reward and discrimination in C57BL/6 mice

The effects of the receptor antagonists MDL 72222 (MDL, 5-HT3) and naltrindole (delta-opioid) on ethanol reward and its discrimination were examined in ethanol-preferring C57BL/6 (C57) mice. MDL attenuated lever responding for 12% ethanol delivered on a fixed-ratio 8 reinforcement schedule at a dose that did not influence responding for water reward, thus confirming a previous report that ICS 205-930 reduced ethanol reward for Long-Evans rats. Our study in combination with the reduced ethanol consumption reported for C57 mice injected with odansetron indicates that 5-HT3 receptor systems are involved in mediating behavior directed toward obtaining ethanol as well as its consumption. By attenuating the rewarding effects of ethanol or of ethanol conditioned cues (e.g., the operant environment), 5-HT3 antagonists may be useful in the treatment of alcohol abuse. The 5-HT3 antagonist effects in this study are comparable with the effects of naltrexone on ethanol reward in C57 mice, although higher doses were required to reduce operant responding for ethanol reward. In contrast to the 5-HT3 antagonist and naltrexone effects, naltrindole, an antagonist with greater specificity for the delta-opioid receptor, was without effect on ethanol reward. This result and recent reports for rats and monkeys suggests that the general antagonists might be more efficacious in attenuating ethanol reward. Both MDL and naltrindole produced only slight reductions in the ethanol discriminative cue, suggesting that the rewarding and discriminative effects of ethanol are not likely mediated by identical neural mechanisms as previously suggested.     

Bimodal effects of MK-801 on locomotion and stereotypy in C57BL/6 mice

Rationale Systemic injection of the non-competitive NMDA (N-methyl-d-aspartate) receptor antagonist MK-801 (dizocilpine maleate) causes both increased locomotion in rodents and various stereotypic behaviors that are proposed to model certain aspects of schizophrenic symptoms in humans.Objectives This study presents a comprehensive characterization of the bimodal effects of MK-801 on locomotion and stereotypy in the C57BL/6 mouse strain, a strain commonly used for genetically modified mice.Results We found that it is important to analyze both locomotion and stereotypy in parallel, as MK-801-induced stereotypy results in abnormal movements that are recorded as locomotion by automated beam detection systems. Furthermore, it is important to analyze the bimodal effects of MK-801 over an extended time span, rather than the commonly used narrower time window, as at higher doses (e.g., above 0.3 mg/kg) the hyperlocomotion phase develops only after the stereotypic phase subsides. We also observed that the apparent dose–response curve is very sensitive to the particular time window chosen for analysis because MK-801 affects both the time course and maximum value of stimulated locomotion. We show that analyzing the absolute peak value of locomotion induced for each animal, rather than group-averaged time courses, provides a measure that is sensitive over a wider range of MK-801 doses. Interestingly, MK-801 even at a very low dose of 0.02 mg/kg suppressed rather than enhanced rearing behavior, differing in this regard from amphetamine.Conclusions The non-competitive NMDA receptor antagonist MK-801 induces a complex pattern of behavioral modification in mice with respect to both the time course and the dose–response relationship of behavioral changes. The results of this study provide a foundation and frame of reference for the growing interest in studying MK-801-induced behavior in mice.The first three authors contributed equally to this work.Co-corresponding authors: Guoping Zhao, Meilei Jin, Lei Yu     

Teratogenic effects of polychlorinated dibenzofurans in combination in C57BL/6N mice

Polychlorinated dibenzofurans (PCDFs) are highly toxic environmental contaminants which have been involved in several incidents of human poisoning. Two congeners, 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) and 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF), have been shown to persist in the tissues of victims of accidental ingestion from Japan and Taiwan. The teratogenicity of these compounds, both alone and in combination, was assessed in C57BL/6N mice. Pregnant mice were treated with 10 ml/kg corn oil containing no PCDFs, 4-PeCDF (0-30 micrograms/kg), HCDF (0-300 micrograms/kg), or a combination of the two on gestation Days 10-13, followed by necropsy on gestation Day 18. Maternal and fetal toxicity were assessed and selected soft tissues were examined for abnormalities. Both chemicals caused hydronephrosis and cleft palate in the absence of any overt toxicity. Hydronephrosis occurred at doses approximately fivefold lower than those causing cleft palate. The combination of 4-PeCDF and HCDF was additive for terata based on responses predicted by probit analysis. In addition, the combination of 2,3,4,5,3',4'-hexachlorobiphenyl (0-60 mg/kg), a structurally related compound also present in PCDF poisoning victims, and 4-PeCDF appears additive. Thus, these chemicals, which cause toxic effects similar to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin, are additive in the induction of fetal anomalies in the mouse.     

水合氯醛在C57BIM6小鼠麻醉中的应用研究

目的探索不同浓度的水合氯醛对C57BL／6小鼠的麻醉效果。方法将60只C57BL／6小鼠随机分为6组,A（2．5％、0．06mL／10g）、B（2．5％、0．1mL／10g）、C（5％、0．06mL／10g）、D（5％、0．1mL／10g）、E（10％、0．06mL/10g）、F（10％、0．1mL／10g）,分别进行腹腔注射水合氯醛麻醉实验,观察临床表现、诱导时间反维持时间。结果B、c、D和E组取得不同的麻醉效果,分别在3．34±0．48min、14．93±3．59rain、4．72±3．02min、9．20±1．51min进入麻醉状态,维持时间分别为45．74±1．94rain、44．18±8．64rain、179．25±3．99min、100．05±22．33rain。结论根据不同实验需要,选择水合氯醛恰当的浓度与剂量行腹腔注射麻醉,是安全有效的。     

Chronic effects of perfluorooctanesulfonate exposure on immunotoxicity in adult male C57BL/6 mice

A paucity of data exists to corroborate the few studies that report immune suppression after exposure to perfluorooctanesulfonate (PFOS). In this study, adult male C57BL/6 mice were exposed to PFOS daily via gavage for 60 days [0, 0.5, 5, 25, 50, or 125 mg/kg total administered dose (TAD)]. The results showed that liver mass was significantly increased at ≥5 mg PFOS/kg TAD and in a dose-dependent manner. Lymphocyte proliferation and natural killer cell activity were altered in male mice. Plaque forming cell (PFC) response was suppressed beginning at 5 mg/kg TAD. Based on the liver mass and PFC response, the no observed adverse effect level and lowest observed adverse effect level for male mice exposed PFOS for 60 days was 0.5 and 5 mg/kg TAD, respectively. Measured PFOS serum concentrations at these dose levels were 0.674 ± 0.166 and 7.132 ± 1.039 mg/l, respectively. These results indicate that PFOS exposure can affect the immunity function in mice at levels approximately 50-fold for highly exposed human populations.     

Subchronic effects of perfluorooctanesulfonate exposure on inflammation in adult male C57BL/6 mice

Previous studies indicate that exposure to perfluorooctanesulfonate (PFOS), a ubiquitous and highly persistent environmental contaminant, induces immunotoxicity in mice. However, few studies have specifically assessed the effects of PFOS on inflammation. This study utilized a standard 60‐day oral exposure period to assess the effects of PFOS on the response of inflammatory cytokines [tumor necrosis factor α (TNF‐α), interleukin‐1 β (IL‐1β), and interleukin‐6 (IL‐6)]. Adult male C57BL/6 mice were dosed daily by oral gavage with PFOS at 0, 0.0083, 0.0167, 0.0833, 0.4167, 0.8333 or 2.0833 mg/kg/day to yield a targeted Total Administered Dose (TAD) over 60 days of 0, 0.5, 1, 5, 25, 50, or 125 mg PFOS/kg, respectively. The percentage of peritoneal macrophages (CD11b⁺ cells) was significantly increased at concentrations ≥1 mg PFOS/kg TAD in a dose‐dependent manner. Ex vivo IL‐1β production by peritoneal macrophages was elevated substantially at concentrations of ≥5 mg PFOS/kg TAD. Moreover, PFOS exposure markedly enhanced the ex vivo production of TNF‐α, IL‐1β and IL‐6 by peritoneal and splenic macrophages when stimulated either in vitro or in vivo with lipopolysaccharide (LPS). The serum levels of these inflammatory cytokines observed in response to in vivo stimulation with LPS were elevated substantially by exposure to PFOS. PFOS exposure elevated the expression of pro‐inflammatory cytokines TNF‐α, IL‐1β, IL‐6, and proto‐oncogene, c‐myc, in the spleen. These data suggest that exposure to PFOS modulates the inflammatory response, and further research is needed to determine the mechanism of action. © 2010 Wiley Periodicals, Inc. Environ Toxicol, 2012.     

Different effects of apomorphine on locomotor activity in C57BL/6 and DBA/2 mice

The effects of apomorphine on spontaneous locomotor activity have been studied in two inbred strains of mice, the C57BL/6 and the DBA/2. In C57 mice low doses of apomorphine reduced motor activity, while higher doses produced hypermotility. In DBA mice the drug always depressed locomotor activity. The results have also been discussed in relation to the different sensitivities to morphine exhibited by the same two strains of mice.     

Early chronic lead exposure reduces exploratory activity in young C57BL/6J mice

Research has suggested that chronic low‐level lead exposure diminishes neurocognitive function in children. Tests that are sensitive to behavioral effects at lowest levels of lead exposure are needed for the development of animal models. In this study we investigated the effects of chronic low‐level lead exposure on exploratory activity (unbaited nose poke task), exploratory ambulation (open field task) and motor coordination (Rotarod task) in pre‐adolescent mice. C57BL/6J pups were exposed to 0 ppm (controls), 30 ppm (low‐dose) or 230 ppm (high‐dose) lead acetate via dams’ drinking water administered from birth to postnatal day 28, to achieve a range of blood lead levels (BLLs) from not detectable to 14.84 µg dl^–1). At postnatal day 28, mice completed behavioral testing and were killed (n = 61). BLLs were determined by inductively coupled plasma mass spectrometry. The effects of lead exposure on behavior were tested using generalized linear mixed model analyses with BLL, sex and the interaction as fixed effects, and litter as the random effect. BLL predicted decreased exploratory activity and no threshold of effect was apparent. As BLL increased, nose pokes decreased. The C57BL/6J mouse is a useful model for examining effects of early chronic low‐level lead exposure on behavior. In the C57BL/6J mouse, the unbaited nose poke task is sensitive to the effects of early chronic low‐level lead exposure. This is the first animal study to show behavioral effects in pre‐adolescent lead‐exposed mice with BLL below 5 µg dl^–1. Copyright © 2014 John Wiley & Sons, Ltd.     

Biotransformation of mexidol in inbred BALB/C and C57BL/6 mice

The pharmacokinetics and biotransformation of mexidol was studied in BALB/C and C57BL/6 mice. The blood plasma contains dealkylated metabolites, and the urine contains glucuronoconjugated derivatives of the drug. The process of glucuronoconjugation more intensively proceeds in C57BL/6 mice than in BALB/C mice.     

水溶性米诺地尔对C57BL/6小鼠毛发生长影响的研究

目的 以C57BL/6小鼠为动物模型,观察水溶性米诺地尔(Water-soluble Minoxidi,WMD)洗剂对毛发生长及毛发生长周期的影响.方法 采用松稀:蜡(1:1)混合物脱毛法诱导小鼠体毛由休止期进入生长期.脱毛次日,实验组小鼠每日分别以1%WMD、1.5%WMD和3%WMD洗剂擦洗脱毛区,连续21 d,记录小鼠皮肤颜色变化过程及新生毛发生长状况评分分值,并检测WMD对新生毛长、新生毛重、真皮厚度及毛囊数目的影响.结果 与对照组小鼠相比,给药组小鼠皮肤颜色提前24h由粉红色变为黑色,推迟48h由黑色变为灰色;除毛重外,实验组小鼠毛长、真皮厚度及毛囊数目在统计学上均与模型组小鼠具有显著性差异,且1.5%WMD组小鼠结果最为明显.结论 WMD可诱导和延长毛发生长期,推迟毛发退行期,促进C57BL/6小鼠毛发生长.     

Effect of traxanox on antibody formation in C57BL/6 mice

C57BL/6 mice, lower responders to sheep red blood cells (SRBC), were intraperitoneally immunized with 5 X 10(8) SRBC on day 0. Traxanox (10 and 30 mg/kg) administered orally on days 0 and 1 potentiated the production of spleen- and thymus-rosette forming cells (RFC) assessed on day 7. The production of hemolytic plaque forming cells (HPFC) to SRBC in the spleen of the syngeneic recipient mice assessed on day 4 was inhibited by the transfer of spleen-RFC obtained from the vehicle-treated donor mice, but not by that obtained from the traxanox (30 mg/kg)-treated donor mice. The same results were obtained in the thymectomized-recipient mice. The activity of the spleen-RFC obtained from the vehicle-treated donor mice was abolished by treatment with anti-Thy 1.2 or anti-Lyt 2.2 antibody and complement. On the other hand, the activity of the spleen-RFC obtained from the traxanox-treated donor mice was abolished by treatment with anti-Lyt 1.2 antiserum and complement. Traxanox (3 and 30 mg/kg) also caused the induction of the Thy 1.2-positive RFC in the spleen of the thymectomized mice. These results suggest that traxanox has a capacity to potentiate the immune responses to SRBC in C57BL/6 mice by the induction of Lyt 1.2-positive cells (helper T cells).