Evans Strain Research Articles

The role of sex on sign-tracking acquisition and outcome devaluation sensitivity in Long Evans rats

Cues that predict rewards can trigger reward-seeking behaviors but also can, in some cases, become targets of motivation themselves. One behavioral phenomenon that captures this idea is sign-tracking in which animals, including humans, interact with reward-predictive cues even though it is not necessary to do so. Sign-tracking in rats has been studied in the domain of motivation and in how motivated behaviors can or cannot become excessive and habit-like over time. Many prior studies look at sign-tracking examine this behavior in male subjects, but there are few papers that look at this behavior in female subjects. Moreover, it is unknown where there might be sex-related variation in how flexible sign-tracking is when faced with changing reward values. Therefore, we asked if there were sex differences in the acquisition of sign-tracking behavior and if there were any sex differences in how sensitive animals were in their sign-tracking following reward devaluation. In contrast to previous reports, we found that males and females show no differences in how they acquire sign-tracking and in ultimate sign-tracking levels following training. Additionally, we found no difference in how quickly males and females learned to devalue the food reward, and we found no differences in sign-tracking levels by sex following outcome devaluation. We believe that this is primarily due to our experiment being performed in the Long Evans strain but also believe that there are many other factors contributing to differences between our study and previous work.

Inter-individual differences in laboratory rats as revealed by three behavioural tasks

Stable inter-individual differences in behaviour and personality have been studied for several decades now. The aim of this study was to test the repeatability of behaviour of the Long Evans strain of laboratory rats in order to assess their inter-individual differences. Male laboratory rats (n = 36) were tested in a series of tasks (Open field test, Elevated plus maze test, and modified T-maze test) repeated over time to assess their personality traits. To evaluate the temporal stability of the behaviour, we calculated repeatability estimates of the examined traits. We also checked for a link in behavioural traits across these experiments, which would suggest the existence of a behavioural syndrome. We found stable inter-individual differences in behaviour. Interestingly, no link emerged between the tasks we studied and therefore we did not find support for a behavioural syndrome. The lack of behavioural correlations between these experiments suggests that the results derived from these tasks should be interpreted carefully, as these experiments may measure various behavioural axes. Moreover, the animals habituate to the apparatus. Consequently, behaviour in the Open field test and Elevated plus maze test is not fully consistent and repeatable across subsequent trials.

Performance of complete blood count (CBC) upon use of different anticoagulants in rats

Complete blood count (CBC) is used as an index of health status of human and different animals as well as to diagnose a variety of diseases. Therefore, there is a growing need of using the most suitable anticoagulant to obtain the most appropriate hemogram. The present study was designed to assess the effect of different anticoagulants viz. Heparin, Sodium Citrate and EDTA on complete blood count (CBC) in rat with a view to choosing the best suitable candidate among the common anticoagulants. A total of 30 samples out of which 10 were for each type anticoagulant were collected from 10 apparently healthy rats of Long Evans strain. From each rat 6 ml of blood was drawn and subsequently divided into three different test tubes with three different anticoagulants. The samples were analyzed for their complete blood count (TEC, TLC, Hb, Hct, DLC, absolute leukocyte count, Red Cell Indices, RDW-SD, RDWCV, Platelet, MPV, PCT and PDW) using Sysmex XT-1800i Auto hematological analyzer. Results showed a significant (p<0.05) decrease in TLC, absolute neutrophil, and platelet count, and a significant (p<0.05) higher level of relative lymphocyte count in Sodium Citrate treated sample than with Heparin and EDTA. No significant changes were observed for RBC or erythrocyte indices among the three different groups. It can be concluded that samples treated with Sodium Citrate results in significant change in blood parameters. Therefore, it is better to use Heparin and EDTA than Sodium Citrate for complete blood count (CBC).
 Asian J. Med. Biol. Res. March 2020, 6(1): 16-21

Influence of preservation length of the sample on the performance of complete blood count (CBC) in rats

The performance of hematological tests deteriorates with the increase in the length of sample preservation. Therefore it has been an issue to characterize the maximum permissible period spent between blood collection and measurement to have the acceptable test report. From this view point, a study was undertaken to know about the effect of preservation length on complete blood count (CBC) in rat of Long Evans strain. A total of 30 samples were collected from 10 apparently healthy rats aged between 45-48 days and the blood samples were kept in commercial test tubes treated with EDTA. The test tubes containing whole blood samples were divided into three different groups based on preservation length and were allowed to keep at 4ºC for three different lengths of time viz. 2 hours, 4 hours and 6 hours until analysis. The samples were then analyzed for their complete blood count (TEC, TLC, Hb, PCV, DLC, Absolute Leukocyte Count, Red Cell Indices, RDW-SD, RDWCV, Platelet, MPV, PCT and PDW) using Sysmex XT-1800i auto hematological analyzer. Result showed that no significant change in CBC with the variation in preservation length. Based on these findings, it can be concluded that blood samples can be preserved for as long as 6 hours to have the same report obtainable when the samples are preserved at 4ºC in refrigerated condition for 2 or 4 hours.
 Asian J. Med. Biol. Res. March 2020, 6(1): 22-26

The effect of rat strain and stress exposure on performance in touchscreen tasks

Patients suffering from depression-associated cognitive impairments often recover incompletely after remission from the core symptoms of depression (lack of energy, depressed mood and anhedonia). This study aimed to set the basis for clinically relevant testing of cognitive impairments in a preclinical model of depression. Hence, we used the chronic mild stress (CMS) model of depression, which provokes the core symptom of anhedonia in a fraction of the stress exposed animals, while others remain resilient, and assessed the entire CMS groups' cognitive performance on the touchscreen operant platform. Specifically, we applied the pairwise discrimination (PD) and reversal task including a retention phase on Wistar and Long Evans controls and CMS exposed Long Evans rats. We observed differences between the albino Wistar and the pigmented Long Evans strain regarding performance in the PD and reversal task as well as in memory consolidation. CMS exposure did not alter learning and memory in the PD and reversal task, even though it altered affective behaviours in the elevated plus-maze and open field test. This is likely due to the heterogeneity of the CMS group, in which stress exposure elicited the expected range of phenotypes from anhedonic-like to resilient shown with the sucrose consumption test. Thus, our study suggests that pigmented rat strains, such as Long Evans, are superior to albino rats in the vision-based touchscreen studies. Furthermore, we propose investigation of the CMS subgroups in more complex, hippocampus-dependent tasks to refine a translational preclinical model of depression-induced cognitive impairments. Hence, this study increased awareness of strain-specific differences in touchscreen performance and added to the literature regarding the sensitivity of the PD and reversal task to stress-induced cognitive alterations.

Effect of lead acetate alone and in combination with whole milk (Star ship®) on body growth and liver functions in an experimentally induced lead toxicity in rat

The Effect of lead acetate alone and in combination with whole milk on body weight gain and some biochemical parameters were carried out on a total of 15 (15 days old) male weaning Long- Evans strain rats. The rats were randomly divided into three equal groups, each consisting of five rats. Rats of group A were kept as control (without giving any treatment), group B received lead acetate alone @ 6mg/ml drinking water and group C received lead acetate @ 6mg/ml plus whole milk (Star ship®) 150 mg/ml drinking water. The result showed that body weight gain of control group per week per rat was found to increase but in treated group B, the body weight gain was found to decrease most significantly (P< 0.01) on day 56 while in group C, body weight was reduced significantly (P< 0.05) on day 56. The reducing body weight gain was less in group C than group B. A most significantly (P< 0.01) increased SGOT and SGPT values were observed in Group B but in group C, those count increased significantly (P< 0.05) on day 56 of experiment. From the study it was concluded that treatment with lead acetate at low doses has adverse effects on body growth and liver functions in experimental animals.Asian J. Med. Biol. Res. June 2016, 2(2): 183-189

Glycemic, Insulinemic, Lipidemic and Antioxidant Status of nSTZ Rats after Chronic Administration of Cicer arietinum Extract

Background: The aims of the study were to evaluate glycemic, insulinemic, lipidemic and antioxidant properties of C. arietinum in neonatal-streptozotocin (nSTZ) rats. Materials and methods: Seeds were collected from the commercially available sources of Dhaka city, identified from Bangladesh National Herbarium and absolute ethanol extract was prepared. A single iv injection of STZ were given to neonate rats of Long Evans strain and 12 weeks later an OGTT was done and rats with fasting glucose level above 7.5 mmol/L were selected. The rats were divided into four groups: i) Water control, ii) Glibenclamide (5 mg/kg bw), iii) C. arietinum 0.625 g/kg bw (CA Ext 1) and iv) 1.25 g/kg bw (CA Ext 2) treated. Body weight was measured weekly. Blood was collected by cutting the tail tip on 0 day and by decapitation on 28 day. Fasting serum glucose, insulin, lipid profiles, creatinine, ALT, MDA, GSH, hepatic glycogen were measured. HOMA B% and HOMA S% were calculated. The data were analyzed using appropriate tools. Results: A significant decrease of fasting glucose level was noticed on 28 day with CA Ext 2 compared to baseline (p<0.05); 26% and 18% decrease were found in comparison to water and glibenclamide treated groups respectively. Blood glucose lowering effect was associated with insulin lowering effect of CA Ext 2. Treatment with CA Ext 2 improved HOMAB%, and both treated groups improved HOMA IR of nSTZ diabetic rats. Total cholesterol was significantly decreased in comparison to water control on 28 day (p=0.014); triglycerides decreased by 11% and HDL increased by 4% respectively in CA Ext 2 group. Serum ALT and creatinine levels were remained unchanged by C. arietinum. A significant increase of reduced-GSH level was found in CA Ext 1 treated group (p=0.031). Conclusion: CA Ext 2 showed significant hypoglycemic and antilipidemic effects most likely through decreasing insulin resistance and improving insulin sensitivity. It also has antioxidant activity that reduces the oxidative changes induced by STZ administration.

Modulation of OCT3 expression by stress, and antidepressant-like activity of decynium-22 in an animal model of depression

The organic cation transporter-3 (OCT3) is a glucocorticoid-sensitive uptake mechanism that has been shown to regulate the bioavailability of monoamines in brain regions that are implicated in the pathophysiology of depression. In the present study, the relative impacts of acute stress alone and acute stress with a history of repeated stress (chronic+acute) were evaluated in two strains of rats: the stress-vulnerable Wistar–Kyoto (WKY) strain and the somewhat more stress-resilient Long–Evans (LE) strain. OCT3 mRNA was significantly upregulated in the hippocampus of LE rats 2h after exposure to acute restraint stress, but not in acutely-restrained rats with a history of repeated social defeat stress. WKY rats exhibited a very different pattern. OCT3 mRNA was unaffected by acute restraint stress alone but was robustly upregulated after repeated+acute stress. There was also a corresponding increase in cytosolic OCT3 protein following repeated+acute stress in WKY rats 3h after presentation of the acute stressor. These results are consistent with the hypothesis that altered expression of the OCT3 may play a role in stress coping, and strain differences in regulation of this expression may contribute to differences in physiological and behavioral responses to stress. Furthermore, the OCT3 inhibitor, decynium 22 (1 and 10μg/kg, i.p.) reduced immobility of WKY rats, but not that of LE rats, in the forced swim test, suggesting that blockade of the OCT3 has antidepressant-like effects. Since WKY rats also appear to be resistant to the behavioral effects of traditional antidepressants, this also suggests that OCT3 antagonism may be an alternative therapeutic strategy for the treatment of depression in individuals who do not respond to conventional antidepressants.

Hemorheology in experimental research: is it necessary to consider blood fluidity differences in the laboratory rat?

This study was designed to identify whether blood fluidity differs between commercially available laboratory rat strains. The hemorheological profiles of seven clinically healthy wild-type rat strains were analyzed to determine whether any diversity in blood fluidity might affect the outcome of cardiovascular studies. Study 1: 65 healthy adult rats (Lewis, Long-Evans, Hairless, Wistar and Fisher; mixed gender and comparable ages) were compared. In order to determine the greatest possible difference, the two strains with the greatest hematocrit (HCT) differences were selected for more detailed evaluation. Red blood cell (RBC) deformability (maximum elongation index, shear stress for half-maximal deformation of RBC; both P < 0.0001), and the effect of plasma protein concentration upon plasma viscosity (P < 0.0001) were different between Lewis and Long-Evans strains. Whole blood viscosity - although different at native HCT (P < 0.004) - was unaltered following HCT standardization of samples. Differences in RBC aggregation were statistically significant but these were small and may not be clinically relevant. Study 2: these 65 animals were compared with 21 animals (10-16 weeks old; both sexes) from mutant strains (Dahl SS/JrHsdMcwiCrl, n = 10; ZDF-Lepr(fa)/Crl, n = 11). In both mutant strains, plasma and whole blood viscosity were increased compared with commonly used strains at native and standardized HCT (P < 0.001). Unusually high RBC aggregation values were seen in the ZDF rat strain (P < 0.001). It was concluded that the variability in blood fluidity among clinically healthy adult laboratory rat strains was both statistically and clinically significant. A hemorheological profile should be added to a routine phenotyping process, since both variables can significantly influence study outcomes.

Assessment of attention in male and female Brattleboro rats using a self-paced five-choice serial reaction time task

The Brattleboro rat is a mutant variation of the Long–Evans strain that exhibits negligible central nervous system levels of vasopressin, a neuropeptide that may influence behavioral and cognitive processes. Compared to Long–Evans rats, Brattleboro rats exhibit diminished fear conditioning and have impairments in spatial memory and sensory gating. The present study sought to further evaluate the cognitive profile of vasopressin-deficient rats by studying attention in male and female Brattleboro and heterozygous rats using a self-paced version of the five-choice serial reaction time task. Male Brattleboro rats required significantly more sessions to meet the training criteria than those by heterozygotic and Long–Evans (wild type) rats. Female Brattleboro rats displayed significantly poorer attention accuracy compared to heterozygotic and Long–Evans rats. Taken together, the present findings add further evidence that vasopressin deficiency diminishes cognitive functioning.

HYPOLIPIDEMIC EFFECT OF GINGER EXTRACT IN VANASPATI FED RATS

The aim of the present study was to evaluate the hypolipidemic effect of ginger in vanaspati fed rats. A total of 18 rats of Long Evans strain weighing 200-250 g were randomly assigned to three groups: Group I, normal control; Group II, 5% vanaspati supplement and Group III, 5% vanaspati + ginger extract (300 mg/ 100ml/ kg b. wt./ day) orally. Serum lipid profile was measured at day 1st and day 49th. The administration of vanaspati augmented the total cholesterol, LDL-C, triglycerides levels and decreased the HDL-C level significantly (p &lt;0.05). Simultaneous administration of ginger extract significantly (p &lt;0.05) prevented the rise in total cholesterol, LDL-C(bad cholesterol), triglycerides levels and rise HDL(good cholesterol). In histopathological study, no significant changes were found in the liver and aorta of all treated groups as compared with control group. It is concluded that ginger extract showed hypolipidemic effect in vanaspati supplemented rats.DOI: http://dx.doi.org/10.3329/bjvm.v10i1-2.15652

Reinforced variability enhances object exploration in shy and bold rats

In an open-field test, the Long–Evans (LE) strain of rats was identified as “bold” and the PVG strain as “shy.” Some members of each strain then experienced 14 sessions of a common enrichment procedure, namely exposure to a series of novel objects (Exposed). Others in each strain were explicitly reinforced with food pellets for variable interactions with the same objects (Reinforced). Both experience and strain influenced object interactions. In particular, Reinforced rats interacted more variably with the objects – contacting, probing, pushing and so forth – than did the Exposed; and LEs interacted more variably than PVGs. Foraging proficiency in the same rats was then studied in a transfer-of-training test. Food pellets were hidden among never-before experienced objects and the rats were permitted to explore freely. Reinforced rats discovered and consumed more pellets than Exposed; and LEs discovered and consumed more than PVGs. Thus a bold genetic strain and reinforcement of variability independently contributed to successful foraging behavior.

Novel polymorphisms within the Dlk1-Dio3 imprinted locus in rat: a putative genetic basis for strain-specific allelic gene expression

The imprinted iodothyronine deiodinase-III (Dio3) thyroid hormone metabolizing gene exhibits paternal expression in most fetal tissues, yet exhibits aberrant, maternal expression in the hippocampus in F1 offspring of Sprague Dawley (SD) × Brown Norway (BN) rats. The maternal hippocampal expression is associated with lower Dio3 mRNA levels specifically in the hippocampus. Here, we tested the hypothesis that genetic polymorphisms between the SD and BN parent strains cause this aberrant allelic Dio3 expression and contribute to behavioral sequelae of higher thyroid hormone levels locally in the hippocampus, including anxiety-related behavior. We mapped and sequenced the Dio3 gene and several previously unmapped regions in the Dlk1-Dio3 locus that could regulate imprinting of the Dio3 gene. In the Dio3 promoter we identified four novel polymorphisms between the BN and SD strains. Next we took advantage of the fact that the Long Evans (LE) strain exhibits identical polymorphisms as the SD strain in the region 5' and including the Dio3 gene. By reciprocally crossing LE and BN strains we tested the relationship among Dio3 promoter region polymorphisms and Dio3 mRNA expression in the hippocampus. Aberrant strain-specific hippocampal Dio3 allelic expression replicated in the LE-BN reciprocal crosses, suggesting that hippocampal-specific imprinting of the Dio3 gene is not the result of a unique genetic or epigenetic characteristic of the SD rat strain, or a unique epistatic interaction between SD and BN. To our knowledge no other studies have reported a genetic × epigenetic interaction of genetic origin in the brain.

Age-related changes in auditory temporal processing in the rat

Presbycusis, as the deterioration of hearing ability occurring with aging, can be manifested not only in a shift of hearing thresholds, but also in a deterioration of the temporal processing of acoustical signals, which may in elderly people result in degraded speech comprehension. In this study we assessed the age-related changes in the temporal processing of acoustical signals in the auditory system of pigmented rats (Long Evans strain). The temporal resolution was investigated in young adult (3–4 months) and old (30–34 months) rats by behavioral and electrophysiological methods: the rats' ability to detect and discriminate gaps in a continuous noise was examined behaviorally, and the amplitude-rate function was assessed for the middle latency response (MLR) to clicks. A worsening of the temporal resolution with aging was observed in the results of all tests. The values of the gap detection threshold (GDT) and the gap duration difference limen (GDDL) in old rats increased about two-fold in comparison with young adult rats. The MLR to a click train in old rats exhibited a significantly faster reduction in amplitude with an increasing stimulation rate in comparison with young adult rats. None of the age-related changes in the parameters characterizing temporal resolution (GDT, GDDL and MLR to a click train) correlated with the degree of the age-related hearing loss. However, the age-related changes in MLR amplitude-rate function correlated with the age-related changes in GDDL, but not with the changes in GDT. The behavioral and electrophysiological data clearly show that aging in rats is accompanied with a pronounced deficit in the temporal processing of acoustical signals that is associated with the deteriorated function of the central auditory system.

Further characterization of the predictive validity of the Brattleboro rat model for antipsychotic efficacy

Our laboratory and others have reported that Brattleboro (BRAT) rats, a Long Evans (LE) strain with a single gene mutation, have inherent deficits in prepulse inhibition (PPI) homologous to those observed in schizophrenia patients and that these deficits are reversed by antipsychotic drugs (APDs). To further evaluate the potential predictive validity of BRAT rat PPI for APDs, we compared the effects of acute subcutaneous administration of the typical APD chlorpromazine to that of three psychotropic drugs without antipsychotic efficacy, the antidepressant imipramine, the anxiolytic diazepam and the anticonvulsant mood stabilizer valproic acid on male and female BRAT rat PPI. Male and female BRAT rats exhibited baseline (saline treatment) PPI that was not different from each other (21.1% and 21.3%, respectively) and low compared with those historically exhibited by LE rats (approximately 59%). Chlorpromazine facilitated PPI in male and female BRAT rats, whereas imipramine, diazepam, and valproic acid had no significant effect on PPI. These results suggest that PPI in the BRAT rat responds specifically to drugs with APD efficacy but not psychotropic drugs of different therapeutic families.

Amphetamine effects on startle gating in normal women and female rats

Dopamine agonists disrupt prepulse inhibition (PPI) of startle in male rodents. In humans, this is observed only in some studies. We reported that PPI was disrupted by D: -amphetamine in men, but only among those with high basal PPI levels. Here, amphetamine effects on PPI were tested in normal women and female rats. Acoustic startle and PPI were tested in normal women after placebo or 20 mg amphetamine, in a double-blind, crossover design, and in female rats after vehicle or 4.5 mg/kg amphetamine. Rats were from Sprague-Dawley (SD) and Long Evans (LE) strains that differ significantly in gene expression in PPI-regulatory circuitry, including levels of nucleus accumbens (NAC) catechol-O-methyl transferase (COMT) mRNA. Amphetamine was bioactive in humans based on quantitative autonomic and self-rating measures, but did not significantly change startle magnitude or PPI across all subjects. Amphetamine's effects on PPI in women correlated significantly (p < 0.0008) with placebo PPI levels (reducing PPI only in women whose basal PPI levels exceeded the sample median) and with measures of novelty and sensation seeking. Amphetamine decreased PPI in SD rats that have relatively low NAC COMT gene expression and increased PPI in LE rats that have relatively high NAC COMT gene expression. The dopaminergic regulation of PPI in humans is related to basal levels of sensorimotor gating and to specific personality traits in normal men and women. In rats, the effects of amphetamine on PPI differ significantly in strains with low vs. high NAC COMT expression.

Inhibitory effect of dimethylthiourea on rat urinary bladder inflammation produced by 6-hydroxydopamine application

The present study was to investigate 6-hydroxydopamine (6-OHDA)-induced inflammatory response and underlying mechanisms in the urinary bladder in anesthetized male rats of Long–Evans strain. The magnitude of inflammation was evaluated by morphometric analysis of the relative number of leaky blood vessels expressed by the area density of India ink-labeled blood vessels in whole mount specimens. Light and scanning electron microscopies were employed to study the changes in histologic structure and endothelial ultrastructure of bladder wall. Local injection of 6-OHDA to lumen of urinary bladder induced a dose-dependent increase in plasma leakage. Following application of vehicle, 5 mg/kg 6-OHDA, and 10 mg/kg 6-OHDA, area densities of India ink-labeled leaky vessels were 5.65 ± 3.72% ( N = 6), 22.63 ± 5.12% ( N = 6), and 35.02 ± 11.25% ( N = 6), respectively. Inflammatory response was completely abolished by pretreatment alone with dimethylthiourea (DMTU), a hydroxyl radical scavenger, and was also attenuated by pretreatment with L-732,138, a NK1 receptor antagonist. 6-OHDA caused edema formation and venular endothelial gap formation in bladder tissue. It is concluded that 6-OHDA induced inflammation in the rat urinary bladder, the response of which was dose-dependently increased and free radicals and tachykinins were involved in the inflammatory process.

A comparative study of cytoarchitecture and serotonergic afferents in the suprachiasmatic nucleus of primates ( Cebus apella and Callithrix jacchus) and rats (Wistar and Long Evans strains)

The suprachiasmatic nucleus, an essential diencephalic component of the circadian timing system, plays a role in the generation and modulation of behavioral and neuroendocrine rhythms in mammals. Its cytoarchitecture, neurochemical and hodological characteristics have been investigated in various mammalian species, particularly in rodents. In most species, two subdivisions, based on these aspects and considered to reflect functional specialization within the nucleus, can be recognized. Many studies reveal a typical dense innervation by serotonergic fibers in this nucleus, mainly in the ventromedial area, overlapping the retinal afferents. However, a different pattern occurs in certain animals, which lead us to investigate the distribution of serotonergic afferents in the suprachiasmatic nucleus of the Capuchin monkey, Cebus apella, compared to the marmoset, Callithrix jacchus, and two Rattus norvegicus lines (Long Evans and Wistar), and to reported findings for other mammalian species. Our morphometric data show the volume and length of the suprachiasmatic nucleus along the rostrocaudal axis to be greatest in C. apella > C. jacchus > Long Evans ≥ Wistar rats, in agreement with their body sizes. In C. apella, however, the serotonergic terminals occupy only some 10% of the nucleus' area, less than the 25% seen in the marmoset and rats. The distribution of the serotonergic fibers in C. apella does not follow the characteristic ventral organization pattern seen in the rodents. These findings raise questions concerning the intrinsic organization of the nucleus, as well as regarding the functional relationship between serotonergic input and retinal afferents in this diurnal species.

Sexual dimorphism in hybrids rats

Laboratory rat strains descend from Wistar rats as a consequence of artificial selection. Previously we reported that the medial posterior division of the bed nucleus of the stria terminalis (BSTMP) was sexually dimorphic in Wistar and Long–Evans strains while the medial anterior division (BSTMA) and the locus coeruleus (LC) only showed sex differences in the ancestor Wistar strain. The lateral posterior division (BSTLP) was isomorphic in both strains. The present work studies the number of neurons in the BSTMP, BSTMA, BSTLP and LC of male and female Wistar and Long–Evans rats (F 0) and their hybrid F 1 and F 2 generations. The BSTMP is sexually dimorphic in the F 0, F 1 and F 2 generations while sex differences in the LC are only seen in F 0 Wistar rats but not in the F 0 Long–Evans or the F 1 and F 2 hybrid generations. Sex differences in the BSTMA are seen in F 0 Wistar but not in F 0 Long–Evans rats and completely disappear in the F 2 generations. The number of neurons in the LC of both males and females decreased in heterozygotic individuals (F 1) but increased in homozygotic (F 2). However, the number of neurons in the BSTMP changes significantly over the generations, although the ratio of neurons (female/male) is stable and unaffected in homo- or heterozygosis. Thus, the mechanism that regulates the neuronal female/male ratio would be different from the one that controls the number of neurons. The facts that sex differences in the BSTMP are not affected by homo- or heterozygosis and that they are seen in several mammalian orders suggest the existence of a “fixed” type of brain sex differences in the Mammalia Class.

Collagen changes in the cochlea of aged Fischer 344 rats

Hearing function in the Fischer 344 (F344) albino inbred strain of rats deteriorates with aging faster than in other strains, in spite of the small hair cell loss in old F344 animals [Popelar, J., Groh, D., Pelanova, J., Canlon, B., Syka, J., 2005. Age-related changes in cochlear and brainstem auditory function. Neurobiol. Aging, in press.]. This study was aimed at elucidating the structural changes in the inner ear of this rat strain during aging. Cochlear histopathology was examined in 20–24-month-old F344 rats and compared with that of young F344 rats (4 months) and of old rats of the Long–Evans (LE) strain. Hematoxylin/eosin staining in aged F344 rats showed degenerative changes in the organ of Corti, consisting of a damaged layer of marginal cells, reduced vascularization of the stria vascularis and a distorted tectorial membrane detached from the organ of Corti. Age-related changes in collagen distribution were observed with Masson's trichrome staining in the spiral ligament of old F344 rats. The results of immunohistochemical staining for type II collagen revealed a marked decrease in collagen fibers in the area connecting the spiral ligament and stria vascularis and a decrease in area IV fibrocytes in old F344 but not in LE rats. These findings may contribute to an explanation of the substantial hearing loss found in old F344 rats.