Neuropeptides

1
Neuropeptides

• Peptides consist of 2 or more amino acids (linked
  by peptide bonds)
• Peptides are synthesized in the soma and
  transported to axon terminal in vesicles
• Often formed from larger molecules (propeptides
  or prepropeptides)
• Peptides are released from all parts of the
  terminal button and after release are
  enzymatically degraded (typically no reuptake)
• Peptidases are enzymes that cleave peptides

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Neuropeptide Transmission
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Organizational Scheme

• Opioid peptides
• Enkephalins, endorphins, dynorphin
• Gut-brain peptides
• Substance P, NPY, CCK
• Hypothalamic releasing factors
• CRH
• Pituitary hormones
• Vasopressin, oxytocin
• Misc peptides
• Angiotensin II

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Opiates/Opioids

• Opiates such as morphine exert potent effects on
  human/animal behaviors
• Block pain perception (analgesia)
• Induce euphoria
• Reduce coughing (antitussive)
• Induce Sleepiness
• Cause Emesis
• Constipation (useful in limiting diarrhea)
• Endogenous opioids were suggested by binding of
  labeled morphine to brain cells
• Opiate is the drug class, opioids are endogenous
  peptides
• Opioids are produced during stressful events
• Opioid receptor activation is reversed by
  naloxone

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Opioid Peptide Precursors
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Opiate/Opioid Receptors

• Opiates are readily absorbed from the gut
• However, very little passes into brain
• Kidney is primary route of excretion (10 in
  feces)
• Biological assay for opiates
• Inhibition of contraction of guinea pig ileum to
  neural stimulation (ACh effect)
• Opiate binding is decreased by high Na levels
• Three classes of opioid receptors
• Mu sensitive to morphine and other alkaloids
• Delta binds the enkephalins
• Kappa agonist ketocyclazocine (induces
  hallucinations, dysphoria), dynorphin is also an
  agonist
• Naloxone is the primary antagonist

7
Opioid Receptor Function

• Mu receptors mediate analgesia, catalepsy, reward
• Mu-KO mice are insensitive to reinforcing and
  analgesic actions of opiates
• Opiates inhibit GABA fibers that inhibit
  accumbens DA
• Delta Role in olfaction, motor integration,
  cognition
• Kappa localized in hypothalamus, caudate, nuc
  accumbens, amygdala
• Suggested roles in feeding, psychosis, water
  balance
• Difficult to relate neural structures to
  receptors
• PAG analgesia
• RF sedating effects of opiates
• P reoptic region hypothermia to opiates

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Opioid Receptor Subtypes
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Grilly, D.M. 2002
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Binding affinities of the endogenous opioid
peptides to the cloned ?, ?, and ? receptors.
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Opiate/Opioid Actions

• Opiates act via G-protein linked receptors
• Opiate receptors regulate ion channels
• Increased K conductance, decreased Ca
  conductance
• Opiate receptors inhibit adenylyl cyclase
• Kappa receptor agonist dynorphin acts on VTA
  cells to reduce DA release in the accumbens
• Leads to dysphoria

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Glutamate

• Glutamate (glutamic acid) is an excitatory
  neurotransmitter
• Fast neurotransmission (ionotrophic)
• Glutamate interacts with four receptor types
• NMDA receptor controls a CA channel
• Activation by glutamine requires glycine binding
  and displacement of magnesium ions
• AMPA receptor controls sodium channels
• Kainate receptor controls sodium channels
• Metabotropic glutamate receptor

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Glutamine-Glutamate Cycling
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Brain VGLUTs Distribution
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Glutamate Receptors
http//www.webvision.med.utah.edu/GLU.html
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Behavioral Roles of Glutamate

• Primary NT for fast excitatory signaling
• Excitoxicity
• MSG kills retinal cells
• Kainate is a neurotoxin
• Role in neuron cell death after ischemia?
• Vision glutamate is released from photoreceptors
• Memory role in LTP
• In the lateral hypothalamus, glutamate elicits
  eating
• Glutamate may interact with brain dopamine to
  alter thought processes (role in schizophrenia)

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Glutamate in Photoreceptors
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Glutamate Excitotoxicity

• Exposure to glutamate activates NMDA receptors
• Swelling of nerve cells after exposure to
  glutamate (via NMDA) leads to cell burst
• Cells pull water in to handle ion influx
• Increased CA ions induces cell toxicity
• Damage to mitochondria
• Free radicals in cells (toxic)
• Membrane transporters on adjacent neurons/glial
  cells regulate synaptic levels of glutamate
• Gene KO of glutamate transporter leads to epilepsy

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Long-Term Potentiation

• LTP requires
• activation of synapses and depolarization of
  postsynaptic membrane
• LTP involves
• release of glutamate
• activation of NMDA receptors in depolarized
  membrane
• entry of calcium ions
• Feedback signal onto presynaptic neuron (NO)?
• New AMPA receptors in the postsynaptic membrane

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Gamma-Aminobutyric Acid

• GABA is synthesized from glutamate in axon
  terminals
• GABA neurons act to dampen neuronal activity
• GABA receptors
• GABAA Ionotropic control of a Cl- channel
  (IPSPs)
• GABAB Metabotropic control of a K channel
  (IPSPs) autoreceptors
• GABAC Metabotropic control of a Cl- channel
  (IPSPs)

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CNS Distribution of GABA

• Widespread GABA distribution (60-80 of brain
  cells may have GABA receptors)
• Heavy localization in the limbic system
• Amygdala GABA acts to limit emotional/stress
  responses
• Is there an endogenous ligand for GABA receptors?
• Diazepam Binding Inhibitor (DBI) may inactivate
  GABA receptors (induces anxiety) 
• FG7142, caused sweating, nausea, palpitations,
  tightness in the chest, restlessness, tremors,
  and feelings of worry and impending doom- this
  drug is an inverse agonist for the GABA receptor

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GABA Pathways

• GABA is stored within vesicles
• GABA is subject to reuptake into axon terminals
• GAD is a marker for GABA
• Allows for mapping of GABA pathways in brain
  (spinal cord, cerebellum, limbic system,10-40 of
  cortex)
• GABA found in PNS
• Regulates heart, reproductive, bladder function

Feldman et al., 1997
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GABAA Receptors

• GABAA binding sites
• Benzodiazepine site (increases channel
  frequency) anxiolytic
• Barbiturate site (increases channel duration)
• Steroid site (increases channel frequency)
• Picrotoxin site (inactivates the GABAA receptor)
• GABAA receptors are comprised of subunits
• ?(6),?(4),?(2)
• Anxiolytic action of BZs requires ?2 subunit
• Subunit configuration may change during chronic
  treatment (tolerance)

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GABA and Behavior

• Reduced GABA function leads to seizures
• GABA agonists for seizure control
• Role of GABA in anxiety
• Benzodiazepines potentiate GABA action
• Alcohol and GABA
• Ethanol enhances GABAA chloride currents
• Mice can be bred for different sleep times to
  ETOH
• LS show GABA-ETOH potentiation
• SS mice do not

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Animal Models of Anxiety

• Conflict Tests These train a hungry/thirsty rat
  to do something to get food or water, then
  occasionally shock them for performing this
  response
• Geller-Seifter test hungry rats are trained to
  bar-press for food reward, then intermittently
  shocked when they press the bar
• Vogel conflict test Thirsty rats lick a metal
  tube to get water occasionally they also get
  shocked
• Elevated Plus Maze Two arms are dark, two light
  rodents prefer the dark, especially after being
  frightened somehow
• Defensive Burying Rats and mice bury objects
  they don't like
• An electrified prod place in the home cage is
  quickly buried in bedding
• Mice do the same thing to unfamiliar marbles
• Social Interaction Test Simply measures amount
  of time spent by two strangers in social
  interactions (works only in male rats)
• Sensitivity and Validity of these tests
• All are sensitive to human anxiolytics, including
  alcohol, barbiturates
• None are sensitive to neuroleptics or anti-
  depressants

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Anxiolytic Drugs

• The minimum laboratory dose represents the lowest
  dose producing significant anti-conflict effects
  in an animal model (Geller-Seifter paradigm)
• Average clinical daily dose represents dose found
  effective in treating patients in over 100
  studies.
• Redrawn from data in Sepinwall Cook, 1975.

http//salmon.psy.plym.ac.uk/../year2/psy221anxiet
y/psy221anxiety_frames_container.htm
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Benzodiazepine Issues

• Benzodiazepine (BZ) drugs act to facilitate the
  action of GABA at the GABAA receptor
• BZ effects on anxiety are blocked by GABA
  antagonists
• BZ actions also include
• Muscle relaxation
• Sedation
• Anticonvulsant action
• BZ therapy concerns
• Tolerance with prolonged use
• Sedation
• Memory disorder (anterograde memory disorder)