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The Neurobiology of Social Recognition, Approach, and Avoidance Larry J. Young Introduction • Understanding how the brain processes social information and regulates social behavior helps us understand psychiatric disorders specifically affecting social behavior. • Animal models provide an opportunity for experimental manipulations that are not possible in human patients. • several rodent model systems that have proven particularly useful for understanding how the brain processes social information and regulates social behavior. • not necessarily models of any specific human condition • instead contribute to our understanding of the social brain. • The first model, the oxytocin knockout mouse, demonstrates the role of the neuropeptide oxytocin and the amygdala in the differential processing of social verses nonsocial information in the context of social recognition. • The second model, prairie and montane voles, has been the focus of a decade of research in social approach behaviors, or affiliation, and proven particularly useful in understanding the role of neuropeptides in facilitating social interest and attachment. • A third model uses conditioned defeat in hamsters to examine the neurochemical pathways involved in social avoidance as a consequence of adverse social experiences. Social Recognition and the Neural Processing of Social Stimuli • Several studies suggest that the brain has specific neural circuits involved in processing social information rather than nonsocial stimuli. • Human brain imaging studies have demonstrated that the brain processes social visual stimuli differently from nonsocial stimuli. • For example, the lateral fusiform gyrus is activated to a greater degree when subjects view faces than when viewing nonface objects • Studies in genetically engineered mice have provided a similar example of how the brain differentially processes social verses nonsocial information. • Social recognition in mice, unlike primates, is primarily based on olfactory cues. • During a brief social encounter, a male mouse will investigate a novel mouse by sniffing the head and anogenital region for approximately 1 min. • Presumably during this investigation, the mouse is collecting and storing information regarding the identity of the novel mouse. • If the male encounters the same mouse again, it will investigate the stimulus mouse for only a few seconds and then quickly engage in different behaviors. • This reduction in olfactory investigation after an initial exposure indicates that the male recognizes the stimulus mouse as familiar. • That is, a social memory was stored during the initial encounter and retrieved during the second encounter. • Behavioral studies using knockout mice have demonstrated that the neuropeptide oxytocin (OT) is essential for the expression of a social memory • Young et al. have used mice genetically engineered to lack a functional OT gene to investigate the role of OT in social behavior • One of the most intriguing phenotypes of the OT knockout mouse is that they fail to habituate to, or recognize, a stimulus mouse even after repeated exposures • This deficit in social memory is not due to problems with general olfactory processing because these mice habituate normally to nonsocial scents, such as a cotton ball scented with lemon extract • In addition, OT knockout mice appear to have normal general learning and memory abilities because they perform as well as normal mice in the Morris water maze, which quantifies performance on a spatial learning task. • The specific deficit in social recognition suggests that although general cognitive abilities and olfactory processing are intact, the processing of social stimuli is abnormal. • Social recognition in the OT knockout mouse can be fully restored by a single infusion of 1.0 ng OT into the brain just minutes before the initial social encounter. • infusion of a specific OT antagonist into the brain of wildtype mice prevents the expression of a social memory. • Injection of the OT after the initial exposure fails to restore social recognition, demonstrating the OT must be present during the initial processing of the social information, rather than for the retrieval of that information during subsequent exposures • Young et al. have used Fos immunocytochemistry to determine the brain areas that are activated during a social encounter in normal and OT knockout • Fos is the product of the immediate early gene c-fos, which is expressed when a neuron has been activated. • Fos immunoreactivity has been used extensively as a marker of neuronal activation during the expression of behavior. • In their experiment, normal and OT knockout mice were either left alone in their cages or presented with a social stimulus animal for 90 sec. • During such an exposure, both wildtype and OT knockout mice engaged in similar levels of olfactory investigation and other behaviors, but only the wildtype mice formed a social memory during the exposure. • One hour after the 90-sec exposure, the brains were harvested and processed for Fos immunocytochemistry. • Wildtype and knockout mice displayed similar levels of Fos induction in the olfactory pathway, including the main and accessory olfactory bulbs, the piriform cortex, and the cortical amygdala; • however, whereas wildtype mice exhibited a significant elevation of Fos staining in the medial amygdala, OT knockout mice had no induction of Fos immunoreactivity in this region. • The bed nucleus of the stria terminalis and the medial preoptic area, which receive direct input from the medial amygdala, also failed to show a Fos induction in the OT knockout mice. • The medial amygdala receives olfactory input directly from the olfactory bulbs and is rich in OT. • Together, these observations suggest that the amygdala differentially processes social and nonsocial information, and this differential processing is dependent on the presence of OT. • The OT knockout mice displayed a robust induction of Fos immunoreactivity in the somatosensory cortex after the social exposure, whereas the wildtype mice did not. • This altered pattern of neural activation is consistent with the hypothesis that in the absence of OT, the brain uses alternate neural circuits to process social information. • Brain imaging studies with high-functioning autistic patients also suggest that the amygdala is involved in processing social information. • Functional magnetic resonance imaging (fMRI) was performed on healthy and autistic subjects to examine brain activity during the processing of facial expressions • Autistic subjects failed to display an activation of the left amygdala during this task, whereas the healthy subjects had significant activation of this region. The Neurobiology of Social Approach • Once the brain gathers and processes social information, it must decide how to react to the situation. • In other words, should the individual engage in social interactions, such as grooming, or attack or flee? • What is it about interacting with other individuals in a social context that is rewarding to most individuals? • A rodent about the size of a golden hamster known as a vole has provided an excellent system for understanding affiliative behavior as well as social attachment • There are several species of voles that inhabit various regions of North America, and these species display a range of social behaviors. • Prairie voles (Microtus ochrogaster), found naturally in the Midwestern United States, are highly social, form long-lasting social attachments with their mates, and are monogamous • Like humans, prairie voles seek social contact. In nature, these rodents live in colonial nests consisting of a mating pair and several generations of offspring. • Prairie voles prefer to spend much of their time in physical contact with another prairie vole, typically in a side-by-side posture referred to as huddling. • In large, naturalistic enclosures, prairie voles spend more than 50% of their time interacting or huddling with another prairie vole • In contrast to prairie voles, montane voles (M. montanus), which inhabit the Rocky Mountain region, appear to avoid social contact except for the purpose of mating. Montane voles do not form social attachments between mates. • Female montane voles rear their young in isolated nests and abandon their offspring after 2 to 3 weeks • In a similar naturalistic enclosure as described above, montane voles spent only around 5% of the time socially interacting with other montane voles • It is not clear whether the avoidance of social interactions in montane voles is due to cognitive processes similar to social anxiety or simply a lack of interest in taking part in social interactions. • Because prairie and montane voles are genetically very similar, yet so different socially, together they provide an excellent comparative model system for examining the brain mechanisms involved in promoting social contact. • Neuroanatomic, pharmacologic, and molecular studies have begun to provide clues as to why prairie voles seek out social contact whereas montane voles do not. • Two neuropeptides, oxytocin (OT) and arginine vasopressin (AVP), appear to play a critical role in the social behavior of prairie voles. • Vasopressin and OT are 9–amino acid peptides with a ring structure connected by a disulfide bond. • The peptides differ only at two amino acid residues and the OT and AVP genes are located adjacent to each other on the same chromosome • Both peptides are synthesized in neurons in the hypothalamus that project to the posterior pituitary and are released into the peripheral blood supply where they regulate functions such as blood pressure, urine concentration, uterine contraction, and lactation • These neuropeptides are also synthesized in separate hypothalamic and extrahypothalamic neurons that release the peptides independently within the brain to modulate a number of social behaviors • Oxytocin is involved in promoting maternal behavior, sexual receptivity, and affiliative behavior • Infusions of OT into the brains of male rats increase the amount of social interactions with other male rats • Vasopressin (or its nonmammalian homologue, vasotocin) modulates social communication in frogs, birds and hamsters and social recognition in rats • In prairie voles, OT and AVP have been shown to modulate two specific aspect of social behavior. • First, OT or AVP infusions increase the amount of time that a vole spends socially engaged with a stimulus vole. • Specifically, these peptides increase the amount of time spent in olfactory investigation and huddling in a side-by-side posture with another animal • Second, these peptides are involved in the formation of the pair bond. Mating facilitates the formation of the pair bond in the monogamous prairie vole. • In the laboratory, pair bond formation is assessed in a threechambered testing arena by quantifying the amount of time the experimental animals spends during a 3-hour test with either the mate (tethered in one chamber) or with a novel animal (tethered in separate chamber). • Intracerebroventricular infusions of an OT antagonist into a female prairie vole before mating prevents the formation of a partner preference (Insel and Hulihan 1995), whereas OT injections actually facilitate partner preference formation in the absence of mating • Similar results have been obtained using AVP antagonists and agonists in male prairie voles Neuropeptide Receptors and Social Behavior • Both OT and AVP are present in all mammalian species, and prairie and montane voles appear to have similar levels of these peptides (Wang et al 1996). • So what explains the differences in affiliative behavior in these species? • The answer appears to lie within the regional expression of the receptors for these peptides within the brain. • Receptor autoradiography studies have demonstrated that prairie and montane voles have dramatically different distributions of OT and AVP receptors within the brain • prairie voles have high levels of OT receptor in the nucleus accumbens and the basolateral amygdala relative to montane voles, • montane voles have high levels of receptors in the lateral septum. • prairie voles have high densities of the V1a subtype of the AVP receptor in the ventral pallidum and the medial amygdala compared with montane voles, • montane voles have much higher levels of receptors in the lateral septum than do prairie voles. • One might predict that differential localization of receptors in brain might lead to the activation of different circuits upon peptide release and ultimately to different behavioral responses. • This appears to be the case. • Male prairie and montane voles were given identical infusions of 1.0 ng of AVP, and their behavioral response in an affiliation test was observed • Within 15 min after injection, prairie voles injected with AVP exhibited significantly higher levels of social interactions with a stimulus animal compared with prairie voles injected with artificial cerebrospinal fluid • In contrast, the injection of AVP had no impact on social interactions in montane voles. • Instead montane voles respond to AVP injections by exhibiting increased levels of nonsocial behaviors such as autogrooming • To demonstrate experimentally that there is a direct relationship between the behavioral response AVP and the specific pattern of V1a vasopressin receptors (V1aR), mice transgenic for the prairie vole vasopressin receptor were created • The transgene contained 2.2 kb of the 5-prime flanking region, the coding sequence, the intron, and 2.4 kb of the 3. flanking region of the prairie vole V1aR gene. • Young et al. included the 5-prime flanking region of the gene because this region is likely has the regulatory sequences that direct the expression of the gene in a region-specific manner. • Mice transgenic for the prairie vole V1aR gene expressed the V1aR in a pattern that was similar (but not identical) to that of prairie voles, but markedly different from that of nontransgenic mice • high levels of V1aR binding was detected in the olfactory bulb, thalamus, and cingulate cortex of both the transgenic mice and prairie voles, but not in the wildtype mice • These mice were then cannulated and injected with 1.0 ng of AVP into the lateral ventricles and tested in an affiliation test, as had been performed previously with the voles. • The transgenic mice, which share some of the regional distribution of AVP receptors with the prairie vole, responded to the AVP treatment by displaying increased affiliative behavior (Young et al 1999; Figure 3). • Nontransgenic littermates showed no increase in affiliative behavior after AVP injection. • The transgenic mice did not display elevated V1aR binding, compared with nontransgenic mice, in some of the areas that may be critical specific aspects of social behavior, such as the amygdala and ventral pallidum. • These mice also did not display partner preferences as prairie voles do. • Nonetheless, this is the first study to demonstrate that the regional distribution and density of a neurotransmitter or neuropeptide receptor is directly associated with the social behavior displayed by an individual. Neural Circuits of Affiliation • Through what neural mechanisms do OT and AVP promote social interactions? • The differential distribution of OT and AVP receptors in prairie and montane vole brains provide some interesting clues. • Prairie voles have a high density of OT receptors in the nucleus accumbens, whereas montane voles have few receptors in this region (Figure 2). • Vasopressin receptors are concentrated in the ventral pallidum of the prairie vole but not of the montane vole. • Both the nucleus accumbens and the ventral pallidum are components of the mesolimbic dopamine reward system (McBride et al 1999). • Both regions receive dopamine projections from the ventral tegmental area and are thought to mediate the rewarding, or reinforcing, effects of both natural stimuli and drugs of abuse. • Infusions of psychostimulants into these regions of rats produce a conditioned place preference for the environment in which they received the injections (Gong et al 1996; • Depletion of dopaminergic projections to these regions prevents cocaine self- administration behavior in rodents • The high density of OT and AVP receptors in the dopamine reward systems of prairie voles, and the virtual lack thereof in montane voles, suggests that perhaps activation of these regions during social interactions is reinforcing for prairie voles, thus promoting social contact. • Young et al. tested this hypothesis using viral vector gene transfer to increase V1aR expression specifically in the ventral pallidum of male prairie voles. • Adeno-associated viral (AAV) vectors are an efficient means by which gene expression can be manipulated in the adult animal. As a parvovirus, AAV typically infects cells and inserts its own DNA into the host cell’s genome. • By deleting the AAV genes and replacing them with a gene of interest, it is possible to place any gene into the genome of the neurons surrounding the injection site • Young et al. constructed AAV vectors by placing the prairie vole V1aR gene sequence downstream of a neuron-specific enolase promoter, which directs expression in all neurons. • By injecting small amounts of the virus into the ventral pallidum, they were able to selectively increase the level of expression of the V1aR in this region • These AAV infusions result in an approximately 100% increase in V1aR expression, which persists for at least 4 months. • Male prairie voles that had artificially elevated V1aR in the ventral pallidum displayed elevated levels of social interactions with novel stimulus animals, as measured by olfactory investigation and huddling, compared with animals injected with the same virus in a control region, the caudate putamen • Male prairie voles with increased V1aR expression in the ventral pallidum, but not in the caudate putamen, developed a partner preference after cohabitating overnight, without mating, with a female • Thus V1aR activation in the ventral pallidum both increases social contact and facilitates social attachment. • A separate study demonstrated that OT receptor activation in the nucleus accumbens is necessary for the formation of social attachments in female prairie voles. • Infusions of a selective OT receptor antagonist into the nucleus accumbens prevented the formation of a partner preference after mating, but similar infusions into the caudate putamen had no effect • Infusions of the dopamine D2 agonist quipirole into the nucleus accumbens of the female prairie vole facilitated partner preference formation in absence of mating, whereas similar infusions of the D2 antagonist eticlopride prevented partner preference formation after mating • Although there is no direct evidence of a dopamine– peptide interaction, these studies are consistent with the hypothesis that in social species, OT and AVP may enhance the hedonic value of social interactions by activating the neural circuitry involved in reward and reinforcement. • These studies suggest the possibility that individual differences in neuropeptide receptor expression in the dopamine reward circuitry could underlie individual differences in personality traits in humans • There is some evidence to suggest that individual differences in dopamine systems are associated with social anxiety • Tiihonen reported that striatal dopamine reuptake site densities were markedly lower in patients with social phobia compared with age-and gendermatched comparison subjects. • A more recent brain imaging study used SPECT to examine the dopamine D2 receptor binding potential in the striatum of 10 subjects with generalized social phobia and 10 healthy comparison subjects (Schneier et al 2000). • This study reported significantly lower D2 receptor binding potentials in subjects with generalized social phobia compared with healthy control subjects. Genetic Mechanisms Affecting Affiliative Behavior • Young et al. have investigated the molecular mechanisms that result in the species differences in OT and AVP receptor expression. • The tissue-specific expression of a gene is determined by interactions of transcription factors with specific DNA sequences surrounding the gene, particularly in the 5- prime flanking region of the gene. • The region of the V1aR that encodes the receptor protein is 99% identical between the prairie and montane vole; however, in the 5-prime flanking region of the prairie vole gene, there is a 428 bp expansion of a highly repetitive sequence located just over 700 bp upstream of the transcription start site • A similar sequence is also found in the same region of the V1aR of another highly social and monogamous species of vole, the alpine vole (M. pinetorum); however, the less social meadow vole (M. pennsylvanicus) V1aR gene does not contain this sequence. • Highly repetitive DNA sequences are unstable and subject to rapid mutation. • The human V1aR gene also has a highly repetitive sequence in the promoter that is quite variable among individuals (Thibonnier et al 2000). • These studies suggest that individual differences in regulatory DNA sequence upstream of a gene may actually have a dramatic influence on both the pattern of receptor expression in the brain and in the social behavior of the organism. Neurobiology of Social Anxiety and Avoidance in Rodents • Pharmacologic studies of social defeat in hamsters may provide a useful model for understanding some of the neurobiological mechanisms leading to social avoidance behaviors and therefore may provide an animal model with relevance to psychiatric conditions such as social anxiety. • Hamsters are territorial animals, and when allowed to establish a territory in a home cage, a male hamster will vigorously defend his territory with an aggressive attack. • The social defeat model involves placing a male hamster into the cage of an aggressive resident. During the 5-min exposure, the experimental hamster is defeated in an aggressive encounter. The exposure may be repeated several times. • This conditioned social defeat has a longlasting impact on the social behavior of the animal. • When the defeated hamster is subsequently exposed, in its own home cage, to a smaller, nonaggressive hamster, the defeated hamster will flee and display submissive behavior. • This submissive display lasts up to 30 days after the conditioned defeat training. • This defeat–exposure paradigm has the potential for pharmacologic manipulations to investigate the role of specific neurotransmitters in both the acquisition and the expression of the conditioned defeat. • In defeated animals, peripheral injection of a specific corticotropin releasing factor (CRF)1 receptor antagonist (CP-154,526) 1 hour before the exposure to a nonaggressive intruder did not reduce the level of submissive or defensive behaviors • Central infusions of a nonspecific CRF1/CRF2 receptor antagonist (D-Phe CRF(12– 41)) significantly reduced the display of submissive behavior to the small intruder. • These studies are consistent with a role for CRF and the CRF2 receptor in the expression of the social avoidance–submis-sive behaviors in the conditioned defeat model. • A large body of evidence suggests that CRF plays a key role in the regulation of the endocrine and behavioral responses to stress and has been implicated in the psychopathology of depression and anxiety disorders • A recent primate study using the orally administered CRF1 antagonist antalarmin investigated the effects of blocking CRF1 receptors on the behavioral response to a social stressor. • Rhesus monkeys given the drug displayed a decrease in anxiety-and fear-related behaviors when placed in close proximity to an unfamiliar male separated only by a Plexiglas screen • There is some evidence that specific neurotransmitter receptor systems may modulate social anxiety differently from other nonsocial forms of anxiety. • For example, Gonzalez examined the effects on anxiety of 5-HT1a and benzodiazepine receptor agonists injected into the basolateral amygdala as measured by the social interaction test and the elevated plus maze in rats • The social interaction test involves familiarizing a rat to a stimulus rat on 2 consecutive days for 5 min, followed by a 5-min exposure on the 3rd day, when social interactions were recorded. • Increases in olfactory investigation, pursuit, or grooming are interpreted as decreased anxiety levels. • In the elevated plus maze, the rat chooses between spending time in the closed, dark arms or the open, lighted arms. • Increases in the amount of time spent in the open arms indicate decreased anxiety levels. • Bilateral infusions of 50 or 200 ng of the 5HT1a agonist 8-OH-DPAT into the basolateral nucleus of the amygdala produced a 50% reduction in social interaction but had no significant effect on time spent in the open arms of the plus maze. • The effect of 8-OH-DPAT was blocked by the 5- HT1a receptor antagonist tertatolol. • In contrast, infusions of the benzodiazepine receptor agonist midazolam into the same region resulted in a doubling of time spent interacting socially but had no effect of plus maze behavior. • These results suggest that neurotransmitter systems acting on receptors in the basolateral amygdala may act preferentially to modulate social anxiety, but not nonsocial anxiety as measured by the elevated plus maze.
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