Long-Term Provision of Environmental Resources Alters Behavior but not Physiology or Neuroanatomy of Male and Female BALB/c and C57BL/6 Mice.

Few studies have evaluated the long-term effects of providing environmental resources to mice. This consideration is important given that mice are often maintained in vivaria for months. We evaluated the effects of providing simple cage resources (wood wool, cotton nesting material, a plastic tunnel, and oat cereal) compared with standard housing (solid-bottom cage with hardwood chips) to group-housed adult male and female C57BL/6 and BALB/c mice (n = 20/sex/strain/group) over 6 mo to determine whether these resources had a lasting effect on animal physiology, anatomy, and behavior. Body weights increased in all groups over time but were proportionately higher in male and female BALB/c mice housed in resource-supplemented environments. Throughout the study, adding environmental resources had no effect on hematology and lymphocyte subsets, fecal corticoid metabolite levels, response to LPS injection, or dendritic spine length or density. Strain- or sex×environmentspecific changes occurred in dark-light activity and thermal nociceptive responses. Dominant agonistic behaviors, abnormal conspecific sexual behaviors, and social nonagonistic behaviors demonstrated sex and strain×environment interactions such that fewer maladaptive social behaviors were noted in mice that were provided with environmental resources. This association was particularly evident in male mice of both strains in resource-supplemented environments. A small but significant increase in brain weight:body weight ratios occurred in mice in resource-supplemented environments. Under the conditions evaluated here, consistent use of simple environmental resources had a positive long-term effect on the behavioral wellbeing of male and female BALB/c and C57BL/6 mice yet minimally affected other aspects of murine physiology and neuroanatomy.

Effects of single compared with pair housing on hypothalamic-pituitary-adrenal axis activity and low-dose heroin place conditioning in adult male Sprague-Dawley rats.

Whether social isolation of adult rats under standard laboratory conditions produces significant long-term alterations in behavior and physiology is unclear. In the present study, male Sprague-Dawley rats were singly or paired-housed for 10 wk. During this period, they were tested for acquisition, extinction, and reacquisition of heroin (0.3 mg/kg)-conditioned place preference. Fecal corticoid metabolite levels were analyzed several times throughout the period of housing, and food consumption and body weight were monitored. During place conditioning, heroin induced a significant increase in locomotor activity in both singly and pair housed rats, and the resulting place preference was similar in both groups. However, singly housed rats showed increased motor reactivity to heroin on reconditioning after extinction and displayed significant reacquisition of conditioned place preference, compared with pair-housed animals. Over the 10-wk period of the study, there were no differences in body weight or food consumption between groups. Mild significant increases in relative adrenal gland weight and decreases in relative brain weight were noted in singly housed animals compared with those paired. Significant decreases in nocturnal fecal corticoid metabolite output were noted in both groups, with loss of circadian variation in fecal corticoid levels over the course of the study. These data suggest that male Sprague-Dawley rats, irrespective of single or pair housing, develop reduced hypothalamic-pituitary-adrenal axis activity over time under standard laboratory housing conditions. Single housing can enhance both this effect and sensitivity to the stimulatory and rewarding actions of heroin after withdrawal.

Oral gavage in rats: animal welfare evaluation.

The effect of chronic daily orogastric gavage with water (5 mL/kg) on behavior and physiology was evaluated in male Sprague-Dawley rats. Treatment groups included: unmanipulated control, restraint control, dry gavage, and gavage, with all rats singly housed (n = 9 or 10 per group). In addition, a group of pair-housed rats (n = 18) was included to determine whether social housing affected response to gavage. Weekly body weights and food consumption were recorded as well as use of a nylon chew toy for enrichment. Feces were collected biweekly at the end of the light and dark phases for fecal corticoid metabolite determinations. After 28 d of treatment, animals underwent conditioned place preference testing to evaluate sensitivity to motivational properties of the anxiolytic drug chlordiazepoxide (5.6 mg/kg SC). Brain and paired adrenal gland weights were collected at necropsy. Week 2 total fecal corticosterone levels were elevated in all groups and attributed to a fire alarm accidentally tripped during building renovations. No differences occurred in body weight or food consumption between any groups. All groups used a nylon chew toy given for enrichment and demonstrated mild preference for the drug-associated chamber. Fecal weights and corticoid metabolite levels were similar between all groups at week 4 and showed normal diurnal variation. No biologically significant variations were noted in brain or paired adrenal gland to body weight ratios. We conclude that orogastric gavage of aqueous solutions at 5 mL/kg does not negatively affect the welfare of laboratory rats acclimated to handling.

Inactivation of the ventromedial prefrontal cortex mimics re-emergence of heroin seeking caused by heroin reconditioning.

The purpose of this study was to investigate the role of the ventromedial prefrontal cortex (vmPFC) in the expression of extinguished heroin seeking as measured by conditioned place preference (CPP) in males Sprague-Dawley rats (n=25). Heroin place conditioning (0.3mg/kg SC x 4 sessions) was followed by a test of preference 24h later, extinction (saline x 4 sessions), heroin reconditioning (saline or 1.0mg/kg x 1 session), and a second test of place preference 24h later. Fifteen minutes prior to this test, rats received intra-vmPFC infusions (bilateral, 0.5 microl/side) of a mixture of GABA(A) (muscimol; 0.03 nmol) and a GABA(B) (baclofen; 0.3 nmol) agonists, or vehicle. As expected on the basis of previous studies, reconditioning with heroin resulted in the re-emergence of a CPP. Importantly, inactivation of the vmPFC produced the same effect in animals that did not receive heroin on the session of reconditioning. These results indicate that the vmPFC modulates expression of extinguished heroin seeking and suggest that prefrontal inhibitory mechanisms are involved in relapse to drug seeking.

Reconditioning of heroin place preference requires the basolateral amygdala.

To investigate the role of the basolateral amygdala (BLA) in the reacquisition of heroin seeking, we studied the effect of BLA inactivation after heroin re-exposure in the presence of drug-conditioned cues. We employed a heroin conditioned place preference task [Leri F, Rizos Z, 2005. Reconditioning of drug-related cues: a potential contributor to relapse after drug re-exposure. Pharmacol Biochem Behav 2005;80:621-30.], where after initial conditioning and subsequent extinction, rats received a single reconditioning session (explicit compartment-heroin re-pairing), followed by a test of heroin seeking 24 h later. Rats were infused with GABA(A)/GABA(B) agonists (muscimol and baclofen, 0.03 and 0.3 nmol, respectively/0.3 microl) or vehicle, either 15 min or 6 h following the heroin reconditioning session. Animals that received vehicle infusions, whether they were given 15 min or 6 h following reconditioning, showed a significant preference for the heroin-paired compartment 24 h later. However, inactivation of the BLA 15 min post-reconditioning, but not 6 h following reconditioning, completely blocked the reacquisition of heroin seeking. These results suggest that the BLA plays an important role in a putative learning process initiated by drug re-exposure which may underlie the process of relapse.