Behavioural Inhibition and the Age at Social Isolation in Rats

Enriched Environment Procedures for Rodents: Creating a Standardized Protocol for Diverse Enrichment to Improve Consistency across Research Studies

Exposure to environmental enrichment has beneficial effects on learning and memory, diverse neurobiological effects, and promotes recovery of function after brain injury. The effect of enrichment is produced by a combination of increased social interaction, physical activity, spatial complexity, and novelty. Procedures in the literature have, however, been idiosyncratic with poor consistency in the manner or extent to which protocols provide consistent enrichment. We provide an environmental enrichment protocol that can be easily replicated with minor details determined locally so that animals across cohorts and cages all experience a comparable level of enrichment. Procedures are outlined to generate and use a daily pool of suitably varied objects using a standardized format, with objects systematically varied up to a 40-day continuous period. Together with using a large group of rats in a suitably-sized cage, and regular shifting of the position of food and water and cage location, these procedures have produced robust effects in different laboratories and rat strain, thereby improving comparisons within and across laboratories. Non-enriched comparisons can vary, but typically would include grouped animals in standard laboratory housing without objects and with stable food and water locations. Enrichment is a safe non-pharamacological tool to examine behavioral and neurobiological processes in animal models of the lifespan, brain dysfunction and injury.

Experience-dependent changes in neuronal processing in the nucleus accumbens shell in a discriminative learning task in differentially housed rats

Environmental enrichment is associated with enhanced learning of complex tasks, attenuated seeking of natural and drug rewards, and altered function of the nucleus accumbens (NAcc), a brain region involved in goal-directed behavior. For example, during acquisition of a discriminative learning task, neurons in the NAcc core subregion are more responsive to discrete, goal-directed movements in rats raised in an enriched condition (EC) relative to an isolated condition (IC), but as learning materialized, this enhanced responsiveness shifts to the cues that predict these movements. Here, we report that these results do not extend to NAcc shell: neuronal responses in this subregion are similar in EC and IC rats during goal-directed movement and the presentation of associative cues both during and after task acquisition. With experience in this task, however, the overall proportion of task-related neuronal responses in NAcc shell decreases. The response pattern of shell neurons is also sensitive to the presence of contextual cues: shell neuronal firing reveals a significant shift from a predominant excitatory to a predominant inhibitory profile in probe trials when the cue that predicts sucrose availability is absent. Collectively, these data suggest that NAcc shell neurons encode cues associated with natural reward, are less responsive during appetitive behavior in familiar conditions, and are insensitive to appetitive learning differences expressed in rats reared in different environmental conditions.

Relationships between the superior colliculus and hippocampus: Neural and behavioral considerations

Theories of superior collicular and hippocampal function have remarkable similarities. Both structures have been repeatedly implicated in spatial and attentional behaviour and in inhibitory control of locomotion. Moreover, they share certain electrophysiological properties in their single unit responses and in the synchronous appearance and disappearance of slow wave activity. Both are phylogenetically old and the colliculus projects strongly to brainstem nuclei instrumental in the generation of theta rhythm in the hippocampal EEC

On the other hand, close inspection of behavioural and electrophysiological data reveals disparities. In particular, hippocampal processing mainly concerns stimulus ambiguity, contextual significance, and spatial relations or other subtle, higher order characteristics. This requires the use of largely preprocessed sensory information and mediation of poststimulus investigation. Although collicular activity must also be integrated with that of “higher” centres (probably to a varying degree, depending on the nature of stimuli being processed and the task requirements), its primary role in attention is more “peripheral” and specific in controlling orienting/localisation via eye and body movements toward egocentrically labelled spatial positions. In addition, the colliculus may exert a nonspecific influence in alerting higher centres to the imminence of information potentially worthy of focal attention. Nevertheless, it is noteworthy that collicular and hippocampal lesions produce deficits on similar tasks, although the type of deficit is usually different (often opposite) in each case. Functional overlap between hippocampus and colliculus (i.e., strategically synchronised or mutually interdependent activity) is virtually certain vis-à-vis stimulus sampling, for example in the acquisition of information via vibrissal movements and visual scanning. In addition, insofar as stimulus significance is a factor in collicular orienting mechanisms, the hippocampus — cingulate – cortex — colliculus pathway may play a significant role, modulating collicular responsiveness and thus ensuring an attentional strategy appropriate to current requirements (stimulus familiarity, stage of learning). A tentative “reciprocal loop” model is proposed which bridges physiological and behavioural levels of analysis and which would account for the observed degree and nature of functional overlap between the superior colliculus and hippocampus.

Environmental enrichment reduces impulsivity during appetitive conditioning

Although environmental enrichment is presumed to enhance learning, appetitive behavior may also be altered by this experience: anticipatory responding for sucrose is reduced in environmentally enriched (EE) rats [van der Harst, J.E., Baars, A.M. and Spruijt, B.M. Standard housed rats are more sensitive to rewards than enriched housed rats as reflected by their anticipatory behaviour. Behav Brain Res 2003;142:151-156]. To assess the impact of differential environmental experience on learning and appetitive behavior, we trained 17 EE and socially isolated (SI) rats in a three-phase, operant-shaping procedure for sucrose reinforcement. In phase one, a feeder cue was paired with sucrose availability. In phase two, a nose poke to either one of two lit holes on the opposing wall activated the feeder cue. In phase three, the feeder cue was elicited by a poke to a single lit hole. While acquisition rates in phase one and phase two were similar, EE animals reached phase-three criteria [completion of 100 trials in 45 min and 15 or fewer bad pokes] faster than SI animals. These two groups showed similar session completion rates, reinforced and non-reinforced licking responses, and overall behavioral activity during phase three acquisition; however, SI rats performed more bad pokes (responses to the non-lit hole after nose-poke cue onset) and intertrial interval (ITI) pokes during this training period. Because all ITI (and presumably many bad) pokes were initiated before onset of nose-poke cue, this difference indicates greater anticipatory responding in SI animals. This experience-dependent alteration in appetitive behavior may explain, in part, the tendency of SI rats to show attenuated learning rates in appetitive contexts in which complex contingencies exist.

Differential environmental exposure alters NMDA but not AMPA receptor subunit expression in nucleus accumbens core and shell

Environmentally enriched (EE) rats show neurobehavioral differences relative to less stimulated, socially isolated (SI) littermates. Although experience-dependent cortical changes are presumed to underlie learning differences in these differentially housed animals, EE rats show reduced reward-seeking behavior and altered cytoarchitecture and dopaminergic function in the nucleus accumbens (NAcc), a brain area involved in adaptive, goal-directed activity. Given that glutamate and its interaction with dopamine regulate motivational and associative processing in this brain region, we assessed expression of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor and the GluR1 subunit of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor in the NAcc core and shell of EE and SI rats. Our results indicate fewer intensely stained NR1 immunopositive neurons in both core and shell of EE relative to SI rats. No such differences were observed in GluR1 staining. These results suggest that environmental experience alters NMDA but not AMPA receptor expression in NAcc. Increased expression of the NR1 subunit in the NAcc of SI rats may augment impulsivity and reward-seeking behavior relative to EE rats.

Effect of social isolation on stress-related behavioural and neuroendocrine state in the rat

The present study investigated the effects of post-weaning social isolation (SI) on behavioural and neuroendocrine reactivity to stress of male and female rats. Innate aspects of fear and anxiety were assessed in the open field and elevated plus maze tests. Spontaneous startle reflex and conditioned fear response were further investigated. The neuroendocrine response of isolates was examined by measuring basal and stress release of ACTH and corticosterone and by evaluating the mRNA expression of mineralocorticoid (MR) and glucocorticoid (GR) receptors using in situ hybridization. Locomotor activity in the open field was not modified by chronic SI. In males, but not females, SI produced an anxiogenic profile in the elevated plus maze. Male isolates showed a trend towards increased startle reflex amplitude relative to socially-reared controls. Moreover, SI in males produced alterations of the HPA axis functioning as reflected by higher basal levels of ACTH, and enhanced release of ACTH and corticosterone following stress. In contrast, startle response or HPA axis functioning were not altered in female isolates. Social isolates from both genders showed reduced contextual fear-conditioning. Finally, the mRNA expression of MR and GR was not modified by SI. The results of the present study suggest that chronic SI increases emotional reactivity to stress and produces a hyperfunction of the HPA axis in adult rats, particularly in males.

The effects of pre- and postweaning rearing conditions on latent inhibition and partial reinforcement extinction effect in male rats

Male rats were either handled daily or left undisturbed (nonhandled) during the first 21 days of life. At weaning (Day 22) animals in each early treatment condition were assigned to two housing conditions, isolated or grouped, creating four early/late treatment conditions: Handled-Grouped (HG), Handled-Isolated (HI), Nonhandled-Grouped (NHG), and Nonhandled-Isolated (NHI). At maturity, all animals were tested in the latent inhibition (LI, Experiment 1) and the partial reinforcement extinction effect (PREE, Experiment 2) paradigms. In LI, animals receiving prior nonreinforced preexposure to a stimulus (PE) show subsequently retarded conditioning to that stimulus as compared to nonpreexposed (NPE) controls. In the PREE, partially reinforced (PRF) animals exhibit higher resistance to extinction as compared to continuously reinforced (CRF) controls. LI and PREE were obtained in the HG condition, but were impaired in NHG and HI animals. The pattern of impairment was different in the two conditions. The absence of LI in NHG males was due to increased suppression in the PE group, whereas in HI males, was due to decreased suppression in the NPE group. Likewise, the attenuation of the PREE in NHG males stemmed primarily from decreased resistance to extinction in the PRF group, whereas in HI males, it resulted from increased resistance to extinction in the CRF group. The combination of nonhandling and isolation led to the reinstatement of both LI and PREE: normal LI and PREE were obtained in the NHI animals. These results demonstrate that early rearing experience interacts with later experience differentially depending upon the later experience.

Preoperative enrichment and behavioral recovery in rats with septal lesions

To assess the behavioral effects of preoperative differential housing male rats were placed in either enriched or isolated environments at weaning prior to receiving either sham operations or septal lesions when 57 days of age. Rats with septal lesions showed reduced habituation of ambulation and initially made fewer rears in an empty open field but made more object-contacts coupled with a lack of habituation in the object-filled field. Septal rats also showed severe impairments when tested in a 12-arm radial maze with 7 arms baited and 5 arms unbaited. Preoperative enrichment did not significantly affect these lesion-induced changes. Nevertheless, enrichment significantly lowered ambulation (but did not affect habituation) in the open field and increased the number of manipulatory relative to nonmanipulatory contacts. However, preoperatively enriched septal rats showed a deficit in spontaneous alternation (45%) in contrast to the high levels (83%) shown by intact enriched rats, whereas both intact and septal isolated rats showed similar levels of spontaneous alternation (68%). These results conflict with earlier reports that preoperative enrichment "protects" rats against the deficits produced by septal lesions.

Isolation, tactile startle and resting blood pressure in Long-Evans rats

The effect of social isolation, consisting of individual housing in the vivarium under standard conditions and imposed on male rats older than 90 days, was assessed on tactile startle reactivity, nociception, resting heart rate and arterial blood pressure, and on intercorrelations among these variables. Tactile startle was affected by stimulus intensity, repeated trials and time of testing relative to the light/dark cycle, but not by social isolation from 1 to 29 days. Hot-plate latency was not affected by individual housing from 12 to 43 days. Blood pressure and heart rate were negatively correlated with isolation from 9 to 58 days, showing a significant decline over increasing days of isolation. Although not affected directly by isolation, tactile startle was affected secondarily by differences in blood pressure produced by individual housing. Rats with lower resting pressures responded to low-intensity startle stimuli that were not effective in rats with higher pressures, indicating a threshold difference and suggesting the hypothesis that blood pressure acts to modulate sensory reactivity.

Stress adaptation and adrenal activity in isolated and crowded rats

Socially reared male control rats (Co) were compared with crowded reared rats (Cr) and isolated reared rats (I). We assayed the adaptation to moderate stress (activity and defecation rate in open-field test) and to intensive stress sensitivity to restraint ulcer), so as the basal adrenal activity measuring basal values of corticosterone (R.I.A.) and fresh weight of adrenal and thymus glands. Results show a great lack of adaptation to moderate and intensive stress in Cr and I reared rats which leads to a decrease of activity variables, an increase of defecation rates and a high sensitivity to restraint ulcers. The effect of crowding conditions show higher values than isolation. We also found, in isolated and crowded reared rats, an increase in the adrenal function with great values for the basal secretion of corticosterone and atrophy of the thymus gland, besides which, crowded reared animals showed adrenal hypertrophy.