GABA A Receptors and GABAergic Interneurons in Chronic Temporal Lobe Epilepsy

Transcription

1 GABA A Receptors and GABAergic Interneurons in Chronic Temporal Lobe Epilepsy December 4, 2010 Carolyn R. Houser Professor, David Geffen School of Medicine at UCLA and VA Greater Los Angeles Healthcare System American Epilepsy Society Annual Meeting

2 Disclosure I have nothing to disclose. American Epilepsy Society Annual Meeting

3 The GABA system is vulnerable, reorganized and compromised in temporal lobe epilepsy. 3

4 What changes occur? Loss of GABA neurons? Reorganization of GABA neurons? Loss of GABA A receptor subunits? Reorganization of GABA A receptors? 4

5 1. Are GABA neurons lost in temporal lobe epilepsy? 5

6 Hilar neurons regulate dentate gyrus output. ER Houser and Vinters, Neuropathology of Epilepsy. In: Pediatric Epilepsy (Pellock et al., eds.) New York: Demos, 2007.

7 Hilar neuron loss is a major feature of classical hippocampal sclerosis. Swartz et al., Epilepsia 47: , 2006.

8 Hilar neuron loss is one of the most consistent pathological alterations in TLE. CA1 preserved Minimal cell loss Swartz et al., Epilepsia 47: , 2006.

9 Hilar neuron loss in human TLE Margerison and Corsellis, Brain 89: ,1966. Babb et al., Epilepsia 25: , Sagar and Oxbury, Ann. Neurol. 22: , de Lanerolle, et al., Brain Res. 495: , Sloviter, Hippocampus 4: , 1994.

10 Hilar neurons include numerous GABA neurons. Extension of Obenaus et al., J. Neurosci. 13: ,

11 Hilar GABA neurons are vulnerable to damage while other GABA neurons in the dentate are relatively resistant. Extension of Obenaus et al., J. Neurosci. 13: ,

12 Hilar GABA neurons have extensive axonal projections. Freund and Buzsaki, Hippocampus 6: ,

13 GABAergic terminals are abundant in TLE. Increased GAD in remaining axon terminals? Sprouting of remaining GABAergic axons and terminals? Control Pilocarpine 2 months Houser lab, unpublished. 13

14 2. Do GABAergic axons undergo reorganization in temporal lobe epilepsy? 14

15 Hilar somatostatin (GABA) neurons are vulnerable to damage. G G H H G H Control Pilocarpine treated Houser lab, unpublished.

16 Hilar somatostatin neurons innervate the outer molecular layer. Houser lab, unpublished. 16

17 Hilar somatostatin (GABA) neurons are depleted after status epilepticus. Houser lab, unpublished.

18 Time course of somatostatin changes M G Houser lab, unpublished. 18

19 Axons of remaining GABA neurons exhibit reorganization or sprouting. Davenport et al., Exp. Neurol. 109: , Houser and Esclapez, Epilepsy Res. 26: , (Review) Bausch, Epilepsy Behav. 7: , (Review) Thind et al., J. Comp. Neurol. 518: ,

20 3. Is there a decrease of GABA A receptor subunits in temporal lobe epilepsy? 20

21 Multiple changes in GABA A receptor subunits in the hippocampal formation in acquired epilepsy Differences among subunits & Differences among regions Differences among cell types 21

22 GABA A RECEPTOR SUBUNITS 30% Identity % Identity Slide from R.W. Olsen. 22

23 PHASIC INHIBITION TONIC INHIBITION Relatively low GABA affinity Mainly synaptic locations Potentiated by benzos High GABA affinity Nonsynaptic locations Modulated by neurosteroids Modified from Wallner, Hanchar & Olsen, PNAS 100: ,

24 subunit is most highly expressed in the dentate gyrus. Peng et al., J. Neurosci. 23: ,

25 subunit labeling is decreased in granule cells of pilocarpine treated mice. Peng et al., J. Neurosci. 24: ,

26 subunit decreases progressively. Peng et al., J. Neurosci. 24: ,

27 What are the functional effects of subunit loss in dentate granule cells? Decrease in tonic inhibition? Decrease in effects of modulators such as neurosteroids? 27

28 Tonic inhibition is maintained in dentate granule cells during the chronic period. Zhang et al., J. Neurosci. 27: ,

29 Modulation of tonic inhibition is impaired. Reduced modulation of tonic inhibition by neurosteroids is consistent with loss of the subunit. Zhang et al., J. Neurosci. 27: ,

30 Expression of the 5 subunit is decreased in CA1 in chronic pilocarpinetreated rats. Houser and Esclapez, Hippocampus 13: ,

31 Similar preservation of GABAergic tonic current is observed by other groups. Scimemi et al., J. Neurosci. 25: , ( 5 CA1) Zhan and Nadler, J. Neurophysiol. 102: , (Dentate) Rajasekaran et al., Neurobiol. Dis. 40: , ( in Dentate) 31

32 GABA A receptor subunits that mediate tonic inhibition ( and 5) are particularly prone to decreased expression in principal cells in epileptic animals. Consistent with microarray studies Elliott, Miles and Lowenstein, J. Neurosci. 23: , Dingledine and Colleagues, NINDS Microarray Consortium, (unpublished findings) 32

33 Possible explanations for persistent tonic inhibition Maintained by: Low levels of the subunit? Receptors composed of other subunits that are associated with tonic inhibition, such as 5 or? Different receptors different subunit partnership? 33

34 4. Is there reorganization of GABA A receptor subunits in temporal lobe epilepsy? 34

35 4 and 2 subunits are increased in epileptic animals in the chronic period. Peng et al., J. Neurosci. 24: ,

36 Timecourse of Changes in Subunits Percentage intensity of labeling Days after status epilepticus Peng et al., J. Neurosci. 24: ,

37 Subunit Partnerships 4 2? 37

38 subunit is most concentrated at perisynaptic sites in dentate granule cells. δ α 4? γ 2? Wei et al., J. Neurosci 23: , 2003; Zhang et al., J. Neurosci. 27: ,

39 γ2 Cont γ2 is normally highly concentrated at synaptic contacts. Zhang et al., J. Neurosci. 27: ,

40 In pilocarpine treated mice, the γ2 subunit is increased at perisynaptic sites. A B γ2 Pilo C D Zhang et al., J. Neurosci. 27: ,

41 In pilocarpine treated mice, the γ2 subunit is increased at perisynaptic sites. CONT PILO Adapted from Zhang et al., J. Neurosci. 27: ,

42 Perisynaptic localization of the γ2 subunit increases at the expense of synaptic localization on granule cell dendrites. Zhang et al., J. Neurosci. 27: ,

43 Possible Scenario Following a decrease in the subunit, α4 forms an alternate partnership with the γ2 subunit at perisynaptic locations. γ2 containing receptors could contribute to tonic inhibition, but with altered pharmacology. Phasic inhibition at granule cell dendrites is decreased. 43

44 5. Is subunit expression altered in remaining interneurons in a temporal lobe epilepsy model? 44

45 subunit labeling is increased in interneurons of the dentate gyrus in pilocarpinetreated mice. Peng et al., J. Neurosci. 24: ,

46 subunit expression is increased in 1 labeled interneurons in epileptic mice. Peng et al., J. Neurosci. 24: ,

47 Do GABA neurons exhibit higher levels of tonic inhibition in pilocarpine treated mice? a 20 ms 20 pa W. Wei and I. Mody, unpublished. 47

48 48

49 X GABA 49

50 subunit changes extend beyond the dentate gyrus to other temporal lobe regions. Houser lab, unpublished. 50

51 Working Hypotheses Changes in subunit expression in the epileptic animals can increase the excitability of the dentate gyrus and other regions by altering tonic inhibition. This could occur through: Altered modulation and pharmacology of tonic inhibition in principal cells. Increased tonic inhibition of GABAergic interneurons. 51

52 SUMMARY For both GABA neurons and GABA A receptor subunits Loss occurs but is selective. Reorganization occurs but may result in altered circuitry and function. GABAergic function is compromised potentially leading to failure at times of increased demand, as at the onset of seizure activity. 52

53 HOUSER LAB Monique Esclapez Andre Obenaus Zechun Peng Nianhui Zhang Christine Huang Yliana Cetina Megan Wyeth COLLABORATORS Istvan Mody Richard Olsen Antonio Delgado Escueta Barbara Swartz CONTRIBUTORS Werner Sieghart Jean Marc Fritschy Support from the National Institute of Neurological Disorders and Stroke / NIH (NS & NS051311) and VA Medical Research Funds is gratefully acknowledged. 53

54 Yliana Cetina Christine Huang Megan Wyeth Nianhui Zhang Zechun Peng 54