Looming Effects on Attentional Modulation of Prepulse Inhibition Paradigm

In a hazardous environment, it is fundamentally important to successfully evaluate the motion of sounds. Previous studies demonstrated "auditory looming bias" in both macaques and humans, as looming sounds that increased in intensity were processed preferentially by the brain. In this study on rats, we used a prepulse inhibition (PPI) of the acoustic startle response paradigm to investigate whether auditory looming sound with intrinsic warning value could draw attention of the animals and dampen the startle reflex caused by the startling noise. We showed looming sound with a duration of 120 ms enhanced PPI compared with receding sound with the same duration; however, when both sound types were at shorter duration/higher change rate (i.e., 30 ms) or longer duration/lower rate (i.e., more than 160 ms), there was no PPI difference. This indicates that looming sound-induced PPI enhancement was duration dependent. We further showed that isolation rearing impaired the abilities of animals to differentiate looming and receding prepulse stimuli, although it did not abolish their discrimination between looming and stationary prepulse stimuli. This suggests that isolation rearing compromised their assessment of potential threats from approaching objects and receding objects.

Long-Term Impacts of Post-weaning Social Isolation on Nucleus Accumbens Function

Adolescence is a period of incredible change, especially within the brain's reward circuitry. Stress, including social isolation, during this time has profound effects on behaviors associated with reward and other neuropsychiatric disorders. Because the Nucleus Accumbens (NAc), is crucial to the integration of rewarding stimuli, the NAc is especially sensitive to disruptions by adolescent social isolation stress. This review highlights the long-term behavioral consequences of adolescent social isolation rearing on the NAc. It will discuss the cellular and molecular changes within the NAc that might underlie the long-term effects on behavior. When available sex-specific effects are discussed. Finally by mining publicly available data we identify, for the first time, key transcriptional profiles induced by adolescence social isolation in genes associated with dopamine receptor 1 and 2 medium spiny neurons and genes associated with cocaine self-administration. Together, this review provides a comprehensive discussion of the wide-ranging long-term impacts of adolescent social isolation on the dopaminergic system from molecules through behavior.

Long-Term Behavioral Effects of Post-weaning Social Isolation in Males and Females

Adolescence is a developmental period associated with vast neural and behavioral changes which are accompanied by altered sensitivity to stimuli, both stressful and rewarding. Perturbations, especially stressful stimuli, during this period have been shown to alter behavior in adulthood. Social isolation rearing is one such perturbation. This review highlights the long-term behavioral consequences of adolescent social isolation rearing in rodents with a specific focus on anxiety- and addiction-related behaviors. Sex-specific effects are discussed where data are available. We then consider changes in monoaminergic neurotransmission as one possible mechanism for the behavioral effects described. This research on both normative and perturbed adolescent development is crucial to understanding and treating the increased vulnerability to psychiatric disorders seen in humans during this life stage.

Postweaning Isolation Rearing Alters the Adult Social, Sexual Preference and Mating Behaviors of Male CD-1 Mice

Objective: No study has comprehensively evaluated the effect of postweaning isolation on the social and sexual behaviors of a certain strain of rodents in ethology. The present study aims to explore how and to what extent isolation rearing after postweaning affects the social and sexual behaviors of male CD-1 mice in adulthood systematically.

Methods: Male CD-1 mice were randomly assigned to two groups: isolation reared (IS, one mouse per cage, n = 30) and group housed (GH, five mice per cage, n = 15). The mice underwent isolation rearing from postnatal day 23 to day 93. Then, social affiliation, recognition and memory were measured through selection task experiments. Social interaction under a home cage and novel environment were measured via resident–intruder and social interaction test, respectively. Furthermore, sexual preference, homosexual and heterosexual behaviors were measured.

Results: Our study found that postweaning isolation increased the social affiliation for conspecifics (p = 0.001), reduced social recognition (p = 0.042) and impaired social memory. As for social interaction, isolated mice showed a remarkably increased aggression toward the intruder male in a home cage or novelty environment. For instance, isolated mice presented a short attack latency (p < 0.001), high attack frequency (p < 0.001) and long attack duration (p < 0.001). In addition, isolated mice exhibited further social avoidance. Contrastingly, isolated mice displayed a reduced sexual preference for female (IS: 61.47 ± 13.80%, GH: 70.33 ± 10.06%, p = 0.038). As for heterosexual behavior, isolated mice have a short mating duration (p = 0.002), long mounting latency (p = 0.002), and long intromission latency (p = 0.015). However, no association was observed between postweaning isolation and homosexual behavior in male CD-1 mouse.

Conclusion: Postweaning isolation increased the social affiliation, impaired the social cognition and considerably increased the aggression in social interaction of adult male CD-1 mice. Postweaning isolation induced a decreased sexual preference for female in adulthood. Postweaning isolation extended the latency to mate, thereby reducing mating behavior. No association was observed between isolation and homosexual behavior.

The Evidence for Altered BDNF Expression in the Brain of Rats Reared or Housed in Social Isolation: A Systematic Review

There is evidence that development and maintenance of neural connections are disrupted in major mental disorders, which indicates that neurotrophic factors could play a critical role in their pathogenesis. Stress is a well-established risk factor for psychopathology and recent research suggests that disrupted signaling via brain-derived neurotrophic factor (BDNF) may be involved in mediating the negative effects of stress on the brain. Social isolation of rats elicits chronic stress and is widely used as an animal model of mental disorders such as schizophrenia and depression. We carried out a systematic search of published studies to review current evidence for an altered expression of BDNF in the brain of rats reared or housed in social isolation. Across all age groups (post-weaning, adolescent, adult), majority of the identified studies (16/21) reported a decreased expression of BDNF in the hippocampus. There are far less published data on BDNF expression in other brain regions. Data are also scarce to assess the behavioral changes as a function of BDNF expression, but the downregulation of BDNF seems to be associated with increased anxiety-like symptoms. The reviewed data generally support the putative involvement of BDNF in the pathogenesis of stress-related mental illness. However, the mechanisms linking chronic social isolation, BDNF expression and the elicited behavioral alterations are currently unknown.

Down-Regulation of Hippocampal Genes Regulating Dopaminergic, GABAergic, and Glutamatergic Function Following Combined Neonatal Phencyclidine and Post-Weaning Social Isolation of Rats as a Neurodevelopmental Model for Schizophrenia

BACKGROUND

Dysfunction of dopaminergic, GABAergic, and glutamatergic function underlies many core symptoms of schizophrenia. Combined neonatal injection of the N-methyl-D-aspartate (NMDA) receptor antagonist, phencyclidine (PCP), and post-weaning social isolation of rats produces a behavioral syndrome with translational relevance to several core symptoms of schizophrenia. This study uses DNA microarray to characterize alterations in hippocampal neurotransmitter-related gene expression and examines the ability of the sodium channel blocker, lamotrigine, to reverse behavioral changes in this model.

METHODS

Fifty-four male Lister-hooded rat pups either received phencyclidine (PCP, 10mg/kg, s.c.) on post-natal days (PND) 7, 9, and 11 before being weaned on PND 23 into separate cages (isolation; PCP-SI; n = 31) or received vehicle injection and group-housing (2-4 per cage; V-GH; n = 23) from weaning. The effect of lamotrigine on locomotor activity, novel object recognition, and prepulse inhibition of acoustic startle was examined (PND 60-75) and drug-free hippocampal gene expression on PND 70.

RESULTS

Acute lamotrigine (10-15mg/kg i.p.) reversed the hyperactivity and novel object recognition impairment induced by PCP-SI but had no effect on the prepulse inhibition deficit. Microarray revealed small but significant down-regulation of hippocampal genes involved in glutamate metabolism, dopamine neurotransmission, and GABA receptor signaling and in specific schizophrenia-linked genes, including parvalbumin (PVALB) and GAD67, in PCP-SI rats, which resemble changes reported in schizophrenia.

CONCLUSIONS

Findings indicate that alterations in dopamine neurotransmission, glutamate metabolism, and GABA signaling may contribute to some of the behavioral deficits observed following PCP-SI, and that lamotrigine may have some utility as an adjunctive therapy to improve certain cognitive deficits symptoms in schizophrenia.

Escalation of cocaine self-administration in adulthood after social defeat of adolescent rats: role of social experience and adaptive coping behavior

BACKGROUND

The link between adolescent social stress and substance abuse is modeled in social defeat of adolescent male rats, at an age when social experiences are essential for neurobehavioral maturation.

OBJECTIVE

We investigated the role of social experience and social defeat stress during adolescence on social behavior and cocaine self-administration (CocSelfAd) in early adulthood.

METHODS

We manipulated social experience by housing male rats in pairs (PH) or singly (SH) on postnatal day (P) 21. In addition, rats were subjected to social defeat from P35-44. Social behavior was measured during the first and last social defeat in PH and SH adolescents and PH adults. After assessing the behavioral response to novelty and cocaine (P57-61), intrajugular catheters were implanted and CocSelfAd was analyzed.

RESULTS

Residents were less aggressive toward PH adolescent intruders compared to PH adult intruders. Adults were submissive and defensive when attacked, whereas PH adolescents froze. In the course of repeated defeats, adolescent PH rats increased freezing, while SH rats decreased freezing. Longer attack-induced freezing after repeated defeats predicted escalated CocSelfAd in adulthood. PH controls acquired CocSelfAd more slowly than PH defeated and SH rats. Defeated PH rats increased CocSelfAd during progressive ratio schedules of reinforcement and during a 24-h continuous access binge compared to PH controls and SH defeated rats.

CONCLUSIONS

Social defeat in adolescence of PH rats caused persistent increases in adult CocSelfAd. Adolescent PH rats coped with attacks adaptively by increasing freezing behavior after repeated social defeats, a measure that predicted CocSelfAd in adulthood.

Role of social encounter-induced activation of prefrontal serotonergic systems in the abnormal behaviors of isolation-reared mice

Isolation-reared male rodents show abnormal behaviors such as hyperlocomotion, aggressive behaviors, deficits of prepulse inhibition, and depression- and anxiety-like behaviors, but the neurochemical mechanism for the effects of psychological stress in these animals is not fully understood. This study examined the effects of social interactions between isolation-reared mice and intruder mice on brain monoaminergic systems. A cage was divided into two compartments by a mesh partition to prevent direct physical interactions. The 20-min encounter with an intruder elicited a restless and hyperexcitable state (hyperactivity) in male, but not in female, isolation-reared mice, whereas encounters with a sleeping intruder or a novel object did not. Although the encounter did not affect prefrontal neuronal-activity-marker c-Fos expression, dopamine (DA) levels, or serotonin (5-HT) levels in male group-reared mice or female isolation-reared mice, it increased prefrontal c-Fos expression, DA levels, and 5-HT levels in male isolation-reared mice. Furthermore, encounter-induced increases in c-Fos expression in the dorsal raphe nucleus and ventral tegmental area, but not in the nucleus accumbens shell, were much greater in isolation-reared than group-reared male mice. A 5-HT1A receptor agonist, a metabotropic glutamate 2/3 receptor agonist, and a gamma-aminobutyric acid A receptor agonist attenuated isolation-induced aggressive behaviors and encounter-induced hyperactivity, c-Fos expression in the prefrontal cortex and dorsal raphe nucleus, and increases in prefrontal 5-HT levels. These findings suggest that the prefrontal DA and 5-HT systems are activated by encounter stimulation in male isolation-reared mice, and the encounter-induced activation of 5-HT system triggers the induction of some abnormal behaviors in male isolation-reared mice. Furthermore, this study implies that the encounter stimulation-induced signal has a pharmacological significance.

Isolation rearing attenuates social interaction-induced expression of immediate early gene protein products in the medial prefrontal cortex of male and female rats

Early life adversity and stress in humans have been related to a number of psychological disorders including anxiety, depression, and addiction. The present study used isolation rearing, a well-characterized animal model of early life adversity, to examine its effects on social behavior and immediate early gene (IEG) expression produced by exposure to a novel social experience. Male and female rats were housed in same-sex groups or in isolation for 4 weeks beginning at weaning and were tested during late adolescence. The protein products of the IEGs c-fos and Arc, as well as the neurotrophic factor BDNF were assessed in medial prefrontal cortex (mPFC) subregions (anterior cingulate, prelimbic and infralimbic) using immunohistochemistry. Aggressive and non-aggressive behaviors during novel social exposure were also assessed. Exposure to a novel conspecific produced increases in Arc and c-fos activation in the mPFC of group reared animals in a sex- and subregion-dependent fashion compared to no social exposure controls, but this increase was blunted or absent in isolated animals. Isolates engaged in more social interactions and more aggressive behavior than group reared rats. Sex differences in some behaviors as well as in Arc and BDNF expression were observed. These results indicate that isolation rearing alters IEG activation in the mPFC produced by exposure to a novel conspecific, in addition to changing social behavior, and that these effects depend in part on sex.

Animal models of schizophrenia

Developing reliable, predictive animal models for complex psychiatric disorders, such as schizophrenia, is essential to increase our understanding of the neurobiological basis of the disorder and for the development of novel drugs with improved therapeutic efficacy. All available animal models of schizophrenia fit into four different induction categories: developmental, drug-induced, lesion or genetic manipulation, and the best characterized examples of each type are reviewed herein. Most rodent models have behavioural phenotype changes that resemble 'positive-like' symptoms of schizophrenia, probably reflecting altered mesolimbic dopamine function, but fewer models also show altered social interaction, and learning and memory impairment, analogous to negative and cognitive symptoms of schizophrenia respectively. The negative and cognitive impairments in schizophrenia are resistant to treatment with current antipsychotics, even after remission of the psychosis, which limits their therapeutic efficacy. The MATRICS initiative developed a consensus on the core cognitive deficits of schizophrenic patients, and recommended a standardized test battery to evaluate them. More recently, work has begun to identify specific rodent behavioural tasks with translational relevance to specific cognitive domains affected in schizophrenia, and where available this review focuses on reporting the effect of current and potential antipsychotics on these tasks. The review also highlights the need to develop more comprehensive animal models that more adequately replicate deficits in negative and cognitive symptoms. Increasing information on the neurochemical and structural CNS changes accompanying each model will also help assess treatments that prevent the development of schizophrenia rather than treating the symptoms, another pivotal change required to enable new more effective therapeutic strategies to be developed.

CRF2 null mutation increases sensitivity to isolation rearing effects on locomotor activity in mice

BACKGROUND

Developmental stressors are consistently reported to increase risk for certain neuropsychiatric disorders including schizophrenia, depression, and post-traumatic stress disorder. Recent clinical evidence supports a "double-hit" hypothesis of genetic vulnerability interacting with developmental challenges to modulate this risk. Early life stressor effects on behavior may be modulated in part by alterations in corticotropin releasing factor (CRF) signaling via two known receptors, CRF(1) and CRF(2). One extant hypothesis is that CRF(2) activation may modulate long-term adaptive responses after homeostatic challenge. As such, loss of CRF(2) activity via genetic variance may increase sensitivity to the long-term effects of developmental stress.

METHODS

We tested the hypothesis that CRF(2) function may mitigate the behavioral effects of isolation rearing, predicting that loss of CRF(2) function increases sensitivity to this developmental challenge. Using the behavioral pattern monitor (BPM), we examined exploratory behavior and locomotor patterns in adult CRF(2) wild-type (WT) and gene knockout (KO) mice reared socially or in isolation.

RESULTS

Isolation housing produced robust increases in the amount of locomotor activity and investigatory holepoking, and altered the temporal distribution of activity in CRF(2) KO but not CRF(2) WT mice. Isolation housing significantly increased rearing behavior and altered spatial patterns of locomotor activity regardless of genotype.

CONCLUSIONS

Loss of CRF(2) function increased sensitivity to the effects of chronic social isolation on exploratory locomotor behavior. Thus, CRF(2) activation appears to mitigate isolation rearing effects on exploratory behavior. Further research assessing the interaction between CRF(2) function and developmental challenges is warranted.

Enhanced neurotensin neurotransmission is involved in the clinically relevant behavioral effects of antipsychotic drugs: evidence from animal models of sensorimotor gating

To date, none of the available antipsychotic drugs are curative, all have significant side-effect potential, and a receptor-binding profile predictive of superior therapeutic ability has not been determined. It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather from an imbalance between several interacting systems. Targeting neuropeptide neuromodulator systems that concertedly regulate all affected neurotransmitter systems could be a promising novel therapeutic approach for schizophrenia. A considerable database is concordant with the hypothesis that antipsychotic drugs act, at least in part, by increasing the synthesis and release of the neuropeptide neurotensin (NT). In this report, we demonstrate that NT neurotransmission is critically involved in the behavioral effects of antipsychotic drugs in two models of antipsychotic drug activity: disrupted prepulse inhibition of the acoustic startle response (PPI) and the latent inhibition (LI) paradigm. Blockade of NT neurotransmission using the NT receptor antagonist 2-[[5-(2,6-dimethoxyphenyl)-1-(4-(N-(3-dimethylaminopropyl)-N-methylcarbamoyl)-2-isopropylphenyl)-1H- pyrazole-3-carbonyl]-amino]-adamantane-2-carboxylic acid, hydrochloride (SR 142948A) prevented the normal acquisition of LI and haloperidol-induced enhancement of LI. In addition, SR 142948A blocked the PPI-restoring effects of haloperidol and the atypical antipsychotic drug quetiapine in isolation-reared animals deficient in PPI. We also provide evidence of deficient NT neurotransmission as well as a left-shifted antipsychotic drug dose-response curve in isolation-reared rats. These novel findings, together with previous observations, suggest that neurotensin receptor agonists may represent a novel class of antipsychotic drugs.