Research reportEnvironmental enrichment effects on the neurobehavioral profile of selective outbred trait anxiety rats
Introduction
Since Hall [1] first demonstrated that defecation scores in an open-field apparatus could distinguish anxiety levels in rat lines, investigations of anxiety-like behavior in rodent models have revealed significant neural, endocrine, and behavioral responses associated with an animal's innate level of anxiety [2]. Currently, studies of anxiety-like behavior in rodents use a number of tasks including the elevated plus maze [3], [4], forced swim test [5], social interaction test [6] and novel environment exposure test [7], [8]. In addition, a number of studies have described a selectively-bred line of rats that show low (LAB) and high anxiety-like behavior (HAB) on the elevated plus maze [9]. HAB animals exhibit increased CRF mRNA expression in hypothalamic nuclei following prenatal stress [10] and increased stress susceptibility [11] as well as hyperactivity along the hypothalamic pituitary adrenal (HPA) axis following stress [12]. Moreover, HAB rats also show elevated Fos immunoreactivity in the hypothalamus following anxiety provoking stimuli [13] and increased expression of the neuropeptide cocaine and amphetamine regulated transcript (CART) in the hypothalamus [14] which may contribute to increased sensitivity to amphetamine-induced locomotion [15]. These findings suggest that individual differences in anxiety-like behavior involve activation along the HPA axis that may contribute to psychostimulant drug vulnerability.
It remains to be seen whether early environments can modify individual differences in both anxiety and drug vulnerability. Postnatal environments that are enriched (e.g., toys, wheels) can influence an animal's coping skills [16], [36] as well as its vulnerability to drugs of abuse such as cocaine [17], morphine [18] and amphetamine [19], [20]. Conflicting findings exist on enriched environment's (EE) impact on stress with some reporting no difference in corticosterone levels at baseline or after stressor [21] and others indicating a reduction in Sprague-Dawley rats [22] or even elevated levels in enriched Wistar rats [23]. Konkle et al. [24] indicated that a minimum of 40 days in EE, particular strain and type of test impacted HPA and behavioral responses to stress but not reward, with EE rats showing greater flexibility in the face of stress. EE rats that show diminished response to psychostimulants display alterations in dopamine synthesis and turnover, and dopamine transporter (DAT) [20], [25], [26]. Differences in D2 are also observed in anxiety and social dominance models [27], [28]. Further, a role for neurotrophins in the reversal of amphetamine sensitivity in EE rats has been reported [25] as well as increases in dendritic branching and hippocampal neurogenesis [29]. However, there are few studies examining whether enriched environment can modulate stress as a major vulnerability factor for amphetamine sensitivity.
In order to determine whether hippocampal neurogenesis or neuroplasticity in the dorsal striatum contributes to the effects of EE on anxiety-like behavior and amphetamine susceptibility in high anxiety rats, we measured an indirect marker of BDNF activity, the high-affinity neurotrophin intra-cellular signaling by phosphorylation of the tropomyosin-related kinase B receptor (TrkB). cAMP-dependent TrkB phosphorylation has been demonstrated to mediate the structural and functional changes in synaptic development and plasticity associated with BDNF [30]. Further, since numerous studies on anxiety [28] and amphetamine sensitization [31] implicate D2 receptor as a mechanism, we also explored whether D2 protein levels may contribute to environmental enrichment's modulation of the stress response and amphetamine-induced locomotion. We used the percent open arm time on the EPM as a behavioral screen to phenotype low and high anxiety Long Evans rats. We selectively bred non-sibling pairs of low (above median split/upper quartile, greater %OA time) and high (below median split/lower quartile) then housed same-anxiety groups of unrelated male offspring in enriched (EE), social (SE) or isolated environments (IE). We subsequently tested the ability of EE to correct anxiety-like behavior on the elevated plus maze, novelty place preference, amphetamine sensitivity and the stress-induced adrenocortical response following a novel environment stressor.
Section snippets
Subjects
A total of 165 Long–Evans rats were used in the current study. The stock parental lines (N = 85) were purchased from Charles River Breeding Labs (Wilmington, MA), consisting of adult males (n = 40) and adult females (n = 45) (250–275 g). The subsequent outbred generational lines were bred at the University of Massachusetts Boston where they received standard handling, care and husbandry. Animals were housed in pairs, unless pregnant, and given free access to food and water, while maintained in a
Generational anxiety screening
A reduction in anxiety-like behavior (i.e. increased percent time on open arms) was observed in the LAn phenotype across multiple generations, while increased anxiety-like behavior (i.e. decreased percent time on open arms) was observed in the HAn phenotype across multiple generations. Two-way ANOVA indicated a significant main effect of Trait [F(1,69) = 101.5, p < 0.0001] as well as a Trait × Generation interaction effect [F(3,69) = 8.776, p < 0.0001], with F3 and F5 generations of LAn and F5 generation
Discussion
Our findings demonstrate that Long Evans male rats selectively bred (unrelated pairs) for extreme anxiety on the elevated plus maze show increased divergence across generations and that the enriched environment (EE) can interact with trait anxiety to modify behavioral as well as physiological and neural responses. In the current study, animals were placed in select environments shortly after weaning, a critical period of neural development. Developmental timing of and duration of exposure to EE
Conclusion
Taken together, we report that phenotyping outbred animals based on response on the EPM and mating selectively thereafter resulted in an increase in extreme anxiety responses on the EPM across generations (i.e., for F3 and F5; see Fig. 1). This, however, did not extend to the 3rd generation animals reared in select housing presented in the current work. We did not find trait differences in anxiety-like behavior or exploration along the elevated plus maze, novelty place preference or locomotor
Acknowledgements
The authors gratefully acknowledge Elizabeth Boates, Mitzi Sweeney and Dr. Nichole Neugebauer for assistance with animal maintenance and husbandry, technical help and comments on the manuscript. STD was supported by Award Number P20MD002290 from the National Institute on Minority Health and Health Disparities (Celia Moore, Ph.D., P.I.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Minority Health and
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2023, Behavioural Brain ResearchCitation Excerpt :All animals used were bred from nine generations of unrelated Trait ALB mating pairs (HAn males and HAn females/LAn males and LAn females), determined by quartile analysis of time spent on the open arms in the EPM (e.g., lower quartile, less time spent on OA, HAn; upper quartile, more time spent on OA, LAn), in the animal facilities of the University of Massachusetts Boston in Boston, Massachusetts (For details, see [34]. Previous work in our lab [34] has demonstrated that successive generational lines of animals starting at filial 3 (4th generational line) resemble the parental line (filial 0, F0) when screened on the EPM and this effect was observed for filial 5 generational line animals as well [33]. Accordingly, in the current study, we selected females (N = 65) screened on the EPM that shared similar Trait ALB to the F0 parental line.
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2021, NeuroscienceCitation Excerpt :Moreover, D2R blockade in the ventral striatum induces hyperalgesia following the lateral hypothalamus chemical stimulation (Moradi et al., 2015). In agreement with other investigation that reported the enhanced DA concentration (Segovia et al., 2010) and D2R upregulation (Ravenelle et al., 2013) following the EE exposure in the ventral striatum, our results indicate an enhancement in the DA content and receptors’ expression. In this sense, we interpreted the increased pain threshold in EE groups as a result of the DA system’s modulations.