Chapter 7: Glutamate and GABA

1. Chapter 7: Glutamate and GABA • Glutamate • An amino acid • Used throughout the body • Building proteins • Helps with energy metabolism • Also serve as NTs • excitatory

2. synthesis • Glutamine can be converted to glutamate via the enzyme glutaminase

3. Vesicular glutamate transporter • VGLUT1, VGLUT2, and VGLUT3 • Package glutamate into vesicles • Different parts of the brain use different VGLUTs • Not really known why

4. Reuptake • Excitatory amino acid transporter (EAAT1-EAAT5) • EAAT3 is main neuronal transporter • EAAT1 and EAAT2 are actually found on astrocytes • This relationship may occur because extracellular glutamate is dangerous • Spreading ischemia

5. 7.3 Cycling of glutamate and glutamine between glutamatergic neurons and astrocytes • Astrocytes breakdown glutamate • Into glutamine • via the enzyme glutamine synthetase • Then the astrocytes release the glutamine so it can be picked up by neurons and converted back to glutamate • This complex system may help prevent the toxicity of extracellular glutamate

6. Glutamate is the workhorse transmitter for excitatory signaling in the nervous system • Glutamate is found throughout the brain, so we won’t have specific pathways for this neurotransmitter • Involved in many behavioral and physiological functions, but perhaps the most important is synaptic plasticity • Changes in the strength of connections • Learning and memory

7. Receptors • Ionotropic glutamate receptors • 3 subtypes • AMPA • Named for the drug AMPA (a selective agonist of this receptor) • Most fast excitatory responses to glutamate occur through this receptor • Kainate • Named for the drug Kainic acid (a selective agonist) • NMDA • Named for N-methyl-D-aspartate (NMDA; selective agonist)

8. The AMPA and Kainate receptors mediate the flow of Na+ • Excitatory post synaptic potentials • NMDA receptors mediates Na+, but also Ca++ • CA++ works as a second messenger • Thus, NMDA receptors can directly activate a second messenger system

9. 7.4 All ionotropic glutamate receptor channels conduct Na+ ions into the cell

10. NMDA receptors • NMDA receptors require two different neurotransmitters to open the channel • 1) Glutamate • 2) Glycine or D-serine • Glycine (or D-serine) has its own binding site. • Thus glycine (or D-serine) is considered to be a co-agonist. • Usually the co-agonist binding site is occupied though, so the presence or absence of glutamate determines channel opening

11. NMDA receptor • There are two other binding sites on NMDA receptors that affect their function • Both of these receptor locations are inside the channel • Magnesium receptor • Mg++ • Phencyclidine receptor • PCP

12. Mg++ • When the cell membrane is at resting potential (-60 or -70 mv). • Mg++ binds to its location within the channel. • Thus, even if glutamate (and glycine or D-serine) bind to the receptor the ions cannot flow.

13. However, if the membrane becomes somewhat depolarized the Mg++ will leave its binding site and exit the channel. • Now the channel will allow the flow of ions if glutamate and the co-agonist are present.

14. NMDA receptor (coincidence detector) • Thus, the NMDA channel will open only if other receptors are active simultaneously • Two events must occur close together in time. • So channel will only open if • 1) glutamate is released onto NMDA receptor • 2) the cell membrane is depolarized by a different excitatory receptor.

15. PCP • This receptor recognizes • Phencyclidine (PCP) • Ketamine (Special K) • MK-801 (dizocilpine; a research drug) • Most of the behavioral effects of PCP and Ketamine are the result of antagonizing the NMDA receptor

16. NMDA receptors and learning and memory • Classical conditioning is based on the close timing of two events • Bell  Food • The NMDA receptor may be a biochemical mechanism that allows for these kinds of associations. • NMDA antagonism impairs learning and memory • The hippocampus has a high density of NMDA receptors • NMDA receptors are critically involved in synaptic plasticity • Long-term potentiation (LTP)

17. LTP • LTP is a persistent (at least 1 hour) increase in synaptic strength. • Produced by a burst of activity from the presynaptic neuron

18. LTP studies • Get a slice from the rat hippocampus and keep alive in Petri dish. • It is common to stimulate CA3 region which synapses with cells in CA1 • 100 stimulations in 1 second • Tetanic stimulation (tetanus) • Measure response of CA1 neurons • You can produce similar effects in other parts of the pathway, however.

19. Box 7.1 Role of Glutamate Receptors in Long-Term Potentiation (Part 4)

20. Synaptic activity at test pulse • Test pulse elicits release of a small amount glutamate from CA3 axons onto CA1 • Glutamate binds to AMPA receptors and NMDA receptors • Channel does not open because membrane not depolarized enough to dislodge Mg++

21. Synaptic activity during tetanus • More glutamate is released • Prolonged activation of AMPA • Depolarization dissociates Mg++ • NMDA channel opens • Na+ enters • More importantly Ca++ enters

22. Ca++ Works as a second messenger • Increases the sensitivity of receptors to glutamate • Inserts more AMPA receptors into the membrane. • May also produce presynaptic changes that increase glutamate release from terminal button • Retrograde messenger • Nitric oxide? • These effects increase the strength of the synapse

23. Doogie mouse • Genetically engineered to have a more efficient NMDA receptor. • Also may have more NMDA receptors than normal mice • Show enhanced LTP • Show improved learning and memory • Enhanced fear conditioning • Learn Morris water maze more quickly

24. Object recognition • Novel-object-recognition • Explore 2 objects for 5 minutes • Wait 1hour, 1 day, 3 days, or 1 week • Present 2 objects 1 novel and 1 familiar • Animals tend to prefer to examine the novel object • Can only have this preference if they remember

25. 7.8 Enhanced memory shown by Doogie mice in the novel-object-recognition task

26. Metabotropic glutamate receptors • mGluR1-mGluR8 • Some serve as autoreceptors • mGluR1 has been implicated in movement • Knockout mice • No mGluR1 – inactivity • MGluR1 only in cerebellum • Normal movement • Implies mGluR1 in cerebellum is required for normal movement

27. High levels of glutamate can be toxic • Injection of monosodium glutamate (MSG) caused retinal damage in mice • Lucas and Newhouse (1957) • Olney (1969) showed MSG causes brain damage in young mice • Now known that glutamate causes lesions in any brain area when injected directly into that area. Young or old

28. Excitotoxicity hypothesis • Excitotoxicity hypothesis • The damage produced by exposure to glutamate is caused by a prolonged depolarization of receptive neurons • Studied in cultured nerve cells • Strong activation of NMDA receptors most readily causes cell death • Though AMPA and Kainate activation can also cause cell death • If both NMDA and non NMDA receptors are activated by substantial amounts of glutamate there is a large percentage of cell death in a few hours.

29. Necrosis vs Apoptosis • When many cells die in a few hours this is called necrosis • Cells burst (called lysis) due to swelling • However, there can also be delayed responses that can continue for hours after initial exposure • Apoptosis (programmed cell death). • No lysis • Do not spill contents into extracellular space.

30. Apoptosis occurs normally during development • Selective pruning • Can also be elicited by ingesting toxins • Domoic acid • Excitatory amino acid contained in marine algae. • When marine animals eat this algae they concentrate the toxin • If consumed by humans can lead to neurological problems • Headache • Dizziness • Muscle weakness • Mental confusion • Loss of short-term memory • Regulated for humans, but can affect other wild life • Dolphins • Sea birds

31. Ischemia • Ischemia occurs whent there is disruption of blood flow to brain (or part of the brain). • Massive release of glutamate in affected area • Prolonged NMDA receptor activation • In animal studies treatment with NMDA antagonists reduces damage • Clinical studies with humans not so successful • Some researchers are considering drugs that block the co-agonist location • Glycine blockers • Fewer side effects (than drugs like PCP)

32. y-aminobutyric acid (GABA) = gamma-aminobutyric acid • Synthesis • Precursor • Glutamate • Enzyme • Glutamic acid decarboxylase (GAD)

33. Drugs that block GABA synthesis • Reduce GABA levels • Cause convulsions • Provides evidence that inhibitory effects of GABA are important in controlling brain excitability

34. Transporters • Vesicular GABA transporter (VGAT) • Puts GABA into synaptic vesicles • GABA transporters • GAT-1, GAT-2, and GAT-3 • GAT-1 primary neuronal GABA transporter • Drugs that block GABA transporters • Prevent seizures • GAT-1 blocker is most studied • tiagabine (Gabatril) • Used as a treatment for epilepsy

35. Breakdown of GABA • GABA aminotransferase (GABA-T) • Converts GABA into • Glutamate • Succinate • In astrocytes • Glutamate is converted to glutamine • By glutamine synthetase • Then released to be recycled by cells just like for Glutamate cells • Drugs that block GABA-T • Used as anticonvulsants

36. GABA receptors • Only two • GABAA (ionotropic) • GABAB (metabotropic) • GABAA is the receptor that is relevant to us

37. GABAA • GABAA controls a chloride channel • Notice that there are additional binding sites • Picrotoxin (antagonist) • Blocks the channel if occupied • Causes convulsions • Synthetic versions used to be used to induce convulsions to treat depression

38. GABAA • Benzodiazepines • valium • Barbiturates • These drugs enhance the effectiveness of GABA. • Open the Cl- channel more effectively. • Alcohol works similarly at the GABA channel • These drugs tend to reduce anxiety • anxiolytic