Understanding the brain and behavior

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Understanding the brain and behavior

• The Neurobiology of Social Recognition, Approach,
  and Avoidance
• Larry J. Young

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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.

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• 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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Gene knock-out mouse From IDENTIGENE Mouse
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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.

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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

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• 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.

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• 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.

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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

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• 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

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• 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.

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• 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.

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• 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

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• What part of the brain controls social
  recognition using OT?
• Use Fos staining to see which brain neurons are
  activated

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Experiment

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

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Fos staining indicates neuron activation

• Wildtype mice amygdala activated
• OT knockout amygdala not activated

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• 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.

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• 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.

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Neuropeptides in social interest and attachment
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Montane vole Elizabeth Hadly Stanford University
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• 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?

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• 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.

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• 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

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• 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

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• 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

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• In a similar naturalistic enclosure as described
  above, montane voles spent only around 5 of the
  time socially interacting with other montane voles

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• 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.

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

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• Vasopressin and OT are 9amino 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

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• 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

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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

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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).

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• 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

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• 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?

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• 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

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• prairie voles have higher levels of OT receptor
  in the nucleus accumbens (the s brainpleasure
  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,

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

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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

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Prairie voles

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

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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

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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)

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

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• 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

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• 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.

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• 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.

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• 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.

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

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• 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.

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• 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.

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• 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

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• 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.

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• Young et al. tested this hypothesis using viral
  vector gene transfer to increase V1aR expression
  specifically in the ventral pallidum of male
  prairie voles.

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• 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 cells 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

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• 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.

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• 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

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• 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.

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• 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

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• agonist increases activity of target receptor
• antagonist lowers activity of target receptor

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• 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

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• 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.

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• 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

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• 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.

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• 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.

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• What causes the species differences in OT and AVP
  receptor expression?
• Young et al. investigated the molecular mechanisms

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• 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.

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• 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

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• 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.

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• 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.