Understanding the brain and behavior

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.

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• 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.

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

• Social recognition in mice, unlike primates, is primarily based on olfactory cues.

• During the first social encounter, a male mouse will investigate a novel mouse by sniffing the head and anogenital region for approximately 1 min.

• 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.

Neuropeptide oxytocin

• Oxytocin is a neuropeptide

• Oxytocin induces maternal behavior, uterine contraction and lactation

• Young et al. used mice genetically engineered to lack a functional oxytocin (OT) gene to investigate the role of OT in social behavior

• OT knockout mouse 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

• 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

• What part of the brain controls social recognition using OT?

• Use Fos staining to see which brain neurons are activated

Experiment

• Normal and OT knockout mice were either – left alone in their cages, or – presented with a social stimulus animal for 90

sec.

Fos staining indicates neuron activation

• Wildtype mice: amygdala activated

• OT knockout: amygdala not activated

• The medial amygdala receives olfactory input directly from the olfactory bulbs and is rich in OT.

• Suggest that the amygdala differentially processes social and nonsocial information,

• Differential processing is dependent on the presence of OT.

• 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.

Neuropeptides in social interest and attachment

Montane vole Elizabeth Hadly Stanford University

• 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

• 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.

• Do oxytocin (OT) and arginine vasopressin (AVP) explain the differences in social behavior of 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

OT and AVP effects on social interaction

• OT or AVP infusions increase the amount of time that a vole spends in olfactory investigation and huddling in a side-by-side posture with another animal

OT and AVP effects on pair bonding

• The prairie vole, which is monogamous, forms a permanent pair bond after mating.

• In the laboratory, pair bond formation is assessed in a three chambered testing arena by quantifying the amount of time the experimental animals spends during a 3-hour test with either the mate (confined to one chamber) or with a novel animal (confined 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 facilitate partner preference formation, even in the absence of mating

• Similar results have been obtained using AVP antagonists and agonists in male prairie voles

• Both OT and AVP are present in all mammalian species, and prairie and montane voles appear to have similar levels of these peptides

• 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 show that prairie and montane voles have dramatically different distributions of OT and AVP receptors within the brain

• prairie voles have higher levels of OT receptor in the nucleus accumbens (the ‘s brain“pleasure center”) and the basolateral amygdala relative to montane voles

• prairie voles have higher densities of the V1a subtype of the AVP receptor in the ventral pallidum and the medial amygdala compared with montane voles,

• Differential localization of receptors in brain might lead to the activation of different circuits upon peptide release and ultimately to different behavioral responses.

Experiment

• Male prairie and montane voles were given either– 1.0 ng of AVP in artificial cerebrospinal fluid – artificial cerebrospinal fluid alone (control)

• Behavioral response: affiliation test

Prairie voles

• Prairie voles injected with AVP exhibited significantly higher levels of social interactions than control prairie voles injected with artificial cerebrospinal fluid

Montane voles

• 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

Do other species show the same effects of vasopressin?

• Young et al. created transgenic mice carrying the prairie vole vasopressin receptor

• Test if there is a direct relationship between the behavioral response to AVP and the specific pattern of V1a vasopressin receptors (V1aR)

• The transgene contained the regulatory sequences that direct the expression of the gene in a tissue-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

• 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 (Figure 3, next slide).

• Nontransgenic littermates showed no increase in affiliative behavior after AVP injection.

• The transgenic mice did not respond to vasopressin in exactly the same way as prairie voles.

• Mice 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.

• 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.

• How do the the differential distribution of OT and AVP receptors in prairie and montane vole brains promote social interactions?

• Prairie voles have a high density of OT receptors in the nucleus accumbens, whereas montane voles have few receptors in this region

• 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

• 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

• 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 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.

• 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

• agonist increases activity of target receptor

• antagonist lowers activity of target receptor

• Infusion of the dopamine D2 agonist quipirole into the nucleus accumbens of the female prairie vole – facilitated partner preference formation, even

in absence of mating

• 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.

• What causes the species differences in OT and AVP receptor expression?

• Young et al. investigated the molecular mechanisms

• 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 gene that encodes the 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 nucleotide 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 have a dramatic influence on both the pattern of receptor expression in the brain and in the social behavior of the organism.