Age-related scopolamine effects on social and individual behaviour in rats

From adolescence to late aging: A comprehensive review of social behavior, alcohol, and neuroinflammation across the lifespan

The passage of time dictates the pace at which humans and other organisms age but falls short of providing a complete portrait of how environmental, lifestyle and underlying biological processes contribute to senescence. Two fundamental features of the human experience that change dramatically across the lifespan include social interactions and, for many, patterns of alcohol consumption. Rodent models show great utility for understanding complex interactions among aging, social behavior and alcohol use and abuse, yet little is known about the neural changes in late aging that contribute to the natural decline in social behavior. Here, we posit that aging-related neuroinflammation contributes to the insipid loss of social motivation across the lifespan, an effect that is exacerbated by patterns of repeated alcohol consumption observed in many individuals. We provide a comprehensive review of (i) neural substrates crucial for the expression of social behavior under non-pathological conditions; (ii) unique developmental/lifespan vulnerabilities that may contribute to the divergent effects of low-and high-dose alcohol exposure; and (iii) aging-associated changes in neuroinflammation that may sit at the intersection between social processes and alcohol exposure. In doing so, we provide an overview of correspondence between lifespan/developmental periods between common rodent models and humans, give careful consideration to model systems used to aptly probe social behavior, identify points of coherence between human and animal models, and point toward a multitude of unresolved issues that should be addressed in future studies. Together, the combination of low-dose and high-dose alcohol effects serve to disrupt the normal development and maintenance of social relationships, which are critical for both healthy aging and quality of life across the lifespan. Thus, a more complete understanding of neural systems-including neuroinflammatory processes-which contribute to alcohol-induced changes in social behavior will provide novel opportunities and targets for promoting healthy aging.

Understanding Central Nervous System Effects of Deliriant Hallucinogenic Drugs through Experimental Animal Models

Hallucinogenic drugs potently alter human behavior and have a millennia-long history of use for medicinal and religious purposes. Interest is rapidly growing in their potential as CNS modulators and therapeutic agents for brain conditions. Antimuscarinic cholinergic drugs, such as atropine and scopolamine, induce characteristic hyperactivity and dream-like hallucinations and form a separate group of hallucinogens known as "deliriants". Although atropine and scopolamine are relatively well-studied drugs in cholinergic physiology, deliriants represent the least-studied class of hallucinogens in terms of their behavioral and neurological phenotypes. As such, novel approaches and new model organisms are needed to investigate the CNS effects of these compounds. Here, we comprehensively evaluate the preclinical effects of deliriant hallucinogens in various animal models, their mechanisms of action, and potential interplay with other signaling pathways. We also parallel experimental and clinical findings on deliriant agents and outline future directions of translational research in this field.

Behavioral Animal Model of the Emotional Response to Tinnitus and Hearing Loss

Increased prevalence of emotional distress is associated with tinnitus and hearing loss. The underlying mechanisms of the negative emotional response to tinnitus and hearing loss remain poorly understood, and it is challenging to disentangle the emotional consequences of hearing loss from those specific to tinnitus in listeners experiencing both. We addressed these questions in laboratory rats using three common rodent anxiety screening assays: elevated plus maze, open field test, and social interaction test. Open arm activity in the elevated plus maze decreased substantially after one trial in controls, indicating its limited utility for comparing pre- and post-treatment behavior. Open field exploration and social interaction behavior were consistent across multiple sessions in control animals. Individual sound-exposed and salicylate-treated rats showed a range of phenotypes in the open field, including reduced entries into the center in some subjects and reduced locomotion overall. In rats screened for tinnitus, less locomotion was associated with higher tinnitus scores. In salicylate-treated animals, locomotion was correlated with age. Sound-exposed and salicylate-treated rats also showed reduced social interaction. These results suggest that open field exploratory activity is a selective measure for identifying tinnitus distress in individual animals, whereas social interaction reflects the general effects of hearing loss. This animal model will facilitate future studies of the structural and functional changes in the brain pathways underlying emotional distress associated with hearing dysfunction, as well as development of novel interventions to ameliorate or prevent negative emotional responses.

A working model for the assessment of disruptions in social behavior among aged rats: The role of sex differences, social recognition, and sensorimotor processes

Aging results in a natural decline in social behavior, yet little is known about the processes underlying these changes. Engaging in positive social interaction is associated with many health benefits, including reduced stress reactivity, and may serve as a potential buffer against adverse consequences of aging. The goal of these studies was to establish a tractable model for the assessment of social behavior deficits associated with late aging. Thus, in Exp. 1, 1.5-, 3-, and 18-month-old male Fischer 344 (F344) rats were assessed for object investigation, and social interaction with a same-aged partner (novel/familiar), or a different-aged partner, thereby establishing working parameters for studies that followed. Results revealed that 18-month-old males exhibited reductions in social investigation and social contact behavior, with this age-related decline not influenced by familiarity or age of the social partner. Subsequently, Exp. 2 extended assessment of social behavior to both male and female F344 rats at multiple ages (3, 9, 18, and 24 months), after which a series of sensorimotor performance tests were conducted. In this study, both males and females exhibited late aging-related reductions in social interactions, but these changes were more pronounced in females. Additionally, sensorimotor performance was shown to be impaired in 24-month-olds, but not 18-month-olds, with this deficit more evident in males. Finally, Exp. 3 examined whether aging-related inflammation could account for declines in social behavior during late aging by administering naproxen (0, 7, 14, and 28 mg/kg; s.c.)-a non-steroidal anti-inflammatory drug-to 18-month-old females. Results from this study revealed that social behavior was unaffected by acute or repeated (6 days) naproxen, suggesting that aging-related social deficits in females may not be a consequence of a general aging-related inflammation and/or malaise. Together, these findings demonstrate that aging-related declines in social behavior are (i) specific to social stimuli and (ii) not indicative of a general state of aging-related debilitation. Thus, these findings establish working parameters for a highly tractable model in which the neural and hormonal mechanisms underlying aging-related declines in social behavior can be examined.

The potential role of dopamine D₃ receptor neurotransmission in cognition

Currently available treatments have limited pro-cognitive effects for neuropsychiatric disorders, such as schizophrenia, Parkinson's disease and Alzheimer's disease. The primary objective of this work is to review the literature on the role of dopamine D₃ receptors in cognition, and propose dopamine D₃ receptor antagonists as possible cognitive enhancers for neuropsychiatric disorders. A literature search was performed to identify animal and human studies on D₃ receptors and cognition using PubMed, MEDLINE and EMBASE. The search terms included "dopamine D₃ receptor" and "cognition". The literature search identified 164 articles. The results revealed: (1) D₃ receptors are associated with cognitive functioning in both healthy individuals and those with neuropsychiatric disorders; (2) D₃ receptor blockade appears to enhance while D₃ receptor agonism seems to impair cognitive function, including memory, attention, learning, processing speed, social recognition and executive function independent of age; and (3) D₃ receptor antagonists may exert their pro-cognitive effect by enhancing the release of acetylcholine in the prefrontal cortex, disinhibiting the activity of dopamine neurons projecting to the nucleus accumbens or prefrontal cortex, or activating CREB signaling in the hippocampus. These findings suggest that D₃ receptor blockade may enhance cognitive performance in healthy individuals and treat cognitive dysfunction in individuals with a neuropsychiatric disorder. Clinical trials are needed to confirm these effects.

Social interactions in adolescent and adult Sprague-Dawley rats: impact of social deprivation and test context familiarity

Interactions with peers become particularly important during adolescence, and age differences in social interactions have been successfully modeled in rats. To determine the impact of social deprivation on social interactions under anxiogenic (unfamiliar) or non-anxiogenic (familiar) test circumstances during ontogeny, the present study used a modified social interaction test to assess the effects of 5 days of social isolation or group housing on different components of social behavior in early [postnatal day (P) 28], mid (P35), or late (P42) adolescent and adult (P70) male and female Sprague-Dawley rats. As expected, testing in an unfamiliar environment suppressed social interactions regardless of age, housing, and sex. Social deprivation drastically enhanced all forms of social behavior in P28 animals regardless of test situation, whereas depriving older animals of social interactions had more modest effects and was restricted predominantly to play fighting -- an adolescent-characteristic form of social interactions. Social investigation -- more adult-typical form of social behavior was relatively resistant to isolation-induced enhancement and was elevated in early adolescent isolates only. These findings confirm that different forms of social behavior are differentially sensitive to social deprivation across ontogeny.

Acute ethanol withdrawal (hangover) and social behavior in adolescent and adult male and female Sprague-Dawley rats

BACKGROUND

The extensive use of alcohol during adolescence may be facilitated by an age-specific attenuation in sensitivity to adverse effects of ethanol. Adolescent rats are less sensitive than adults to some delayed effects of acute ethanol, including hangover-related anxiety on an elevated plus maze. The purpose of this study was to determine whether adolescent rats are also less sensitive than adults to hangover-related anxiogenesis when indexed in terms of social inhibition.

METHODS

Anxiety during ethanol hangover was indexed in adolescent and adult Sprague-Dawley rats of both genders by assessing the suppression in social behavior during a social interaction test. Animals were tested 18 hr after intraperitoneal administration of 0 g/kg (saline) or 4 g/kg ethanol (experiment 1) or at test intervals chosen on the basis of assessments of ethanol clearance time (experiment 2) to equate clearance-to-test intervals across age and gender (experiments 3-5).

RESULTS

Adults showed more hangover-related social suppression and slower postclearance recovery than adolescents. Sex differences were more pronounced in adults than adolescents, with males being more affected than females. Ethanol clearance time was considerably longer in adult males than in adolescent animals and adult females. In contrast to the modest decreases in social activity observed in adolescent animals shortly after ethanol clearance, adolescents showed a surprising increase in play fighting later in the recovery period- a hangover-related social facilitation that was not evident in adults.

CONCLUSIONS

The attenuated anxiety and increase in social interactions seen in adolescents during hangover are less likely to serve as deterrents for further drinking than the aversive increase in anxiety seen in adults. A facilitation of social interactions not only during a drinking episode, but also during the postalcohol recovery period, could help to establish a persisting cycle of drinking in at-risk adolescents, leading to dependency and a lifelong history of alcohol-related problems.

Ultrasonic vocalizations as an index of social memory in female mice

Ultrasonic vocalization (UV) as a measure of social memory was investigated in female mice. UVs emitted by a resident female in the presence of a same-sex partner were measured during a 3-min, pretest social interaction. In a second 3-min test session, mice were reexposed to the familiar partner or presented with a novel partner. In the first case, there was a decline in UVs emitted by resident mice when the intervals between the 2 sessions were 15, 30, or 60 min. After 24 hr, this effect disappeared. In contrast, with a novel female partner, the number of UVs remained unchanged. Scopolamine (0.05 mg/kg ip) disrupted this memory process: Drug-treated females did not show the expected decrease in UVs when reexposed to the familiar female after 30 min. This study provides behavioral and pharmacological evidence that ultrasonic calls can be used as a measure of social memory in female mice.

Social behavior and social motivation in adolescent rats: role of housing conditions and partner's activity

The present study investigated 1) the effects of individual and grouped housing on social investigation, social contact behavior, and play behavior in adolescent rats tested with low socially active (grouped) and high socially active (isolated) play partners; and 2) the effects of long-term (8 days) and short-term (24 h) isolation on social behavioral manifestations and social motivation in terms of preference or avoidance of play partners. Social isolation of adolescent rats activated play behavior and social behaviors different from play, but play was predominantly affected under the conditions of this study. Long-term isolation was more effective than short-term, and resulted in greater manifestation of play and social preference. Adolescent rats were able to modify their social behaviors in response to social activity of the play partner: in isolated animals exposed to low socially active group-housed partners, play behavior was transformed into social activities unrelated to play; exposure of group-housed adolescents to high socially active previously isolated partners resulted in an increase of play behavior. Testing that allowed avoidance of social contacts revealed a dissociation between manifestations of play behavior and social motivation: group-housed play partners of isolated animals showed elevated levels of play behavior but a tendency to avoid their isolated pairmates.

The neurobiology of social play behavior in rats

Social play behavior is one of the earliest forms of non-mother-directed social behavior appearing in ontogeny in mammalian species. During the last century, there has been a lot of debate on the significance of social play behavior, but behavioral studies have indicated that social play behavior is a separate and relevant category of behavior. The present review provides a comprehensive survey of studies on the neurobiology of social play behavior. Evidence is presented that opioid and dopamine systems play a role in the reward aspect of social play behavior. The role of cholinergic, noradrenergic and opioid systems in attentional processes underlying the generation of social play behavior and the involvement of androgens in the sexual differentiation of social play behavior in rats is summarized. It is concluded that there is not only behavioral, but also neurobiological evidence to suggest that social play behavior represents a separate category of behavior, instead of a precursor for adult social, sexual or aggressive behavior.

Cholinergic modulation of a decrement in social investigation following repeated contacts between mice

Social recognition has been inferred from a decline in olfactory investigation of conspecific intruders during repeated or protracted confrontation with a resident rat. A stimulus-response relationship defined by lack of response remains somewhat ambiguous. Since it is likely that behavior continues to be emitted by the resident animal, how behavior reorganizes as the resident becomes familiar with an intruder represents an important issue in the characterization of recognition. We examined the decline in olfactory investigation of ovariectomized females by adult male mice. The duration and frequency of olfactory investigation was measured during four 1 minute confrontations with 10-min intertrial intervals (Training trials). If the same female was presented in each trial, investigation declined to less than 50% of initial levels. Aggressive behavior gradually increased with repeated trials. No decline in investigation or increased aggression was measured when females were changed in each trial. Administration of doses of scopolamine (0.16-1.0 mg/kg, IP) blocked decrements in olfactory investigation in repeated confrontations and significantly reduced aggression. Co-administration of heptylphysostigmine (0.32-5.0 mg/kg, IP) reversed scopolamine's effects on olfactory investigation but not aggression. Acetylcholinesterase inhibitors heptylphysostigmine, galanthamine (0.63-2.5 mg/kg, IP) and tacrine (0.63-10.0 mg/kg, IP) all enhanced the rate of decrement of olfactory investigation when administered alone, but had differential effects on aggression. The decline in investigation corresponds to criteria for habituation. Increased responsivity expressed as aggression indicates recognition may also be characterized as a change in behavioral strategy dependent on the sexual and social status of the stimulus animal. Pharmacological data support a role for acetylcholine release in the development of social recognition as an olfactory memory, or through modulation of olfactory perception.

Evaluation of the neurotoxic potential of chemicals in animals

This review provides a scientific view on how to evaluate effectively the neurotoxic potential of chemicals in order to provide adequate safeguards for human health. Detection of compounds that may cause direct, persistent, adverse effects on the nervous system should be given the most critical attention. Evaluation of the neurotoxic potential of a chemical should include descriptions of functional and morphological effects as well as the determination of the dose response, no-observed-effect level, time course and reversibility of effects. Evaluation of the nervous system in the context of standard toxicity studies that use a variety of species and study durations are appropriate screening tests (Tier 1) for the detection of potential neurotoxicity. Studies specifically designed to assess neurotoxicity (Tier 2) should be performed with chemicals for which there is an indication of neurotoxic potential and where the available data are inadequate for risk assessment. Tier 2 studies should evaluate the function and structure of the nervous system by comprehensive clinical examinations and neuropathological assessment. These studies may be conducted in conjunction with standard toxicity studies so that any potential neurotoxicity can be interpreted in the context of other systemic toxicity. More specific neurotoxicity tests (Tier 3) may be necessary for advanced characterization of discovered neurotoxicants.

Cholinergic blockade and response timing in rats

The effects of central cholinergic blockade on the temporal regulation of behaviour were studied with a two-level DRL schedule. Five-month-old male Wistar rats had to press lever A and then wait for a minimum of 5 s before pressing lever B to obtain the reinforcer (sweetened milk). After a stable baseline performance, subjects were injected in random order with the general cholinergic blocker, scopolamine, 0.15 and 0.5 mg/kg, the peripheral cholinergic blocker, methylscopolamine, 0.15 and 0.5 mg/kg, and a combination of the cholinesterase inhibitor, physostigmine, 0.2 mg/kg, and scopolamine, 0.5 mg/kg. Each drug treatment was separated by 2 days of saline treatment. Results showed that scopolamine at 0.5 mg/kg significantly impaired the temporal regulation of the A-B response sequence: the median A-B inter-response time (IRT) was shortened and the coefficient of variation of the A-B IRT distribution was increased, thus revealing a loss in the sensitivity to time. This disruption of accurate timing behaviour lowered efficiency. The drug changed neither the duration of the B-A interval nor the A-B response rate, but significantly increased the rate of the superfluous B-B sequences. Methylscopolamine was without effects and physostigmine totally or partially reversed all the scopolamine effects. These results suggest that scopolamine at 0.5 mg/kg specifically affected the mechanism(s) underlying response timing, and that the effects were not secondary to changes in activity or motivation. They partly corroborate data obtained in other procedures and support the idea that the central cholinergic system is involved in the temporal regulation of behaviour.

Pre- and post-synaptic cholinergic dysfunction in aged rodent brain regions: new findings and an interpretative review

Age-related impairment of dynamic aspects of central cholinergic neurotransmission has been indicated by many studies of aged rodents, but the regional distribution of cholinergic deficits and the relative contribution of presynaptic hypofunction and reduced acetylcholine release, loss of synaptic integrity or loss of muscarinic receptors remains unclear. This study therefore compared choline acetyltransferase activity (as a structural marker) and sodium-dependent high affinity choline uptake (which reflects both ongoing cholinergic neuronal activity and structural integrity) in the hippocampus, cortex and straitum of male C57BL mice at 3-4, 10-12 or 28-32 months of age. To evaluate the relationship of changes in muscarinic receptors to presynaptic alterations, binding of the antagonist 3H-quinuclidinyl benzilate was compared in membranes prepared from each of these brain regions. High affinity choline uptake was significantly reduced in all three brain regions by 28-32 months of age. This trend was already evident by 10-12 months of age, especially in hippocampus and cortex. By contrast, choline acetyltransferase activity was unchanged in striatum and actually increased in hippocampus and cortex of aged mice. Muscarinic binding was reduced significantly only in striatum and this effect was significant by 10-12 months of age. This decrease in antagonist binding was accompanied by a small but significant reduction in the relative proportion of high affinity agonist sites as defined by carbachol displacement. The impairment of high affinity choline uptake in the absence of a parallel reduction of choline acetyltransferase activity suggests a decline of ongoing cholinergic activity rather than loss of terminal integrity as the basis of presynaptic deficits in aging. This functional decline may be exacerbated by reduction of muscarinic receptors in striatum. Despite considerable literature support for the hypothesis that cholinergic mechanisms are impaired with age, several controversies leave important issues unresolved. Therefore, the present results are discussed in the context of a critical review with emphasis on dynamic properties of presynaptic function which require analysis in experimental animal models. The impact of normal aging on brain cholinergic systems is distinguished from the neurodegenerative changes in Alzheimer disease in that presynaptic function is compromised with a relative preservation of the integrity of innervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Difference in scopolamine sensitivity during ageing in rats: dichotomy between behavioural repertoire and learning performances

The effects of different doses of scopolamine in adult and aged rats were compared in two behavioural experiments. The first experiment involved visual reversal learning, while the second experiment investigated individual and social behavioural patterns. Results showed that the same dose of scopolamine was more effective in impairing the reverse discrimination in aged than in adult rats. In contrast, low doses which were effective in modifying behavioural patterns in adults, were inactive in aged rats. This difference in scopolamine sensitivity (supersensitivity versus hyposensitivity), observed between the two types of behaviour, is discussed in relation to age-related functional receptor adaptation, selective changes in muscarinic receptors in the different brain areas and dopaminergic- cholinergic interactions.