Adaptive excitatory and inhibitory signaling in animal models of disease [electronic resource]

This dissertation studies mechanisms that mediate inhibitory and excitatory synaptic transmission in the healthy brain and in animal models of stroke and epilepsy. Stroke is a major cause of disability yet lacks pharmacotherapies for recovery (Donnan et al., 2008). During the repair phase, spontaneous cortical circuit plasticity and reorganization adjacent to the stroke site (peri-infarct) promote functional recovery (Carmichael, 2006; Dijkhuizen et al., 2001; Murphy and Corbett, 2009; Nudo et al., 1996). Elucidating mechanisms that target these endogenous brain repair processes could lead to new therapeutics with a broad treatment window. Inhibiting the post stroke increase in tonic (extrasynaptic) GABA signaling during the repair phase was reported to enhance functional recovery in mice, suggesting that GABA plays an important function in modulating brain repair (Clarkson et al., 2010). While tonic GABA appears to suppress brain repair after stroke, the role of phasic (synaptic) GABA during the repair phase is unknown. In Chapter 2, we report a post-synaptic increase in phasic GABA signaling within the peri-infarct cortex that is specific to layer 5 pyramidal neurons; we measured increased numbers of alpha-1 receptor subunit containing GABAergic synapses detected using array tomography, and an associated increased efficacy of spontaneous and miniature inhibitory post-synaptic currents in pyramidal neurons. In contrast to the reported effects of tonic inhibition, enhancing phasic GABA signaling in the recovery phase using zolpidem, an alpha-1 subunit positive allosteric modulator (Crestani et al., 2000), improved behavioral recovery. These data identify a novel role for phasic GABA signaling in brain repair, indicate zolpidem's potential to improve recovery, and underscore the necessity to distinguish the role of tonic and phasic inhibition in stroke recovery. Temporal lobe epilepsy (TLE) is the most common form of adult seizure disorder and is often associated with drug refractory epilepsy (Wiebe, 2000). GABAARs are thought to play a key role in the pathophysiology of many types of epilepsy, including TLE, and are the target site of benzodiazepines, commonly used as antiepileptic medications (Noebels et al., 2012a; Gonzalez and Brooks-Kayal, 2011). Previous studies have shown dentate granule cells display an increased pharmacological response to the central benzodiazepine receptor (CBR) antagonist flumazenil (FLZ) after status epilepticus (SE) in an animal model of temporal lobe epilepsy. It has been previously reported that in slices taken from pilocarpine-induced epileptic rats, hippocampal dentate granule cells demonstrate a FLZ induced reduction in mIPSC half width. This is in contrast to control animals, where FLZ application has no effect on dentate granule cell mIPSC kinetics. The mechanism(s) by which FLZ reduces mIPSC half width in SE tissue is not known, but one hypothesis is that an endogenous compound active at the CBR may be upregulated after SE. However, in Chapter 3 I performed preliminary experiments in animals with SE and did not find a consistent reduction in mIPSC parameters after SE previously reported (Leroy et al., 2004). My results demonstrate that FLZ is not acting as a pure negative allosteric modulator (NAM), but instead may act occasionally as a NAM and predominantly as a positive allosteric modulator (PAM) or that NAMs are expressed after seizures and actually weaken IPSCs. These experiments suggest that a small subset of dentate granule cells respond to FLZ with varying response profiles. Synaptic transmission requires a continuous supply of neurotransmitter for release. Although most types of neurons use direct reuptake to recycle released neurotransmitters, evidence indicates that glutamatergic synapses rely predominantly on astrocytes for generation and recycling of glutamate (Hertz, 1979). Although biochemical studies suggest that excitatory neurons are metabolically coupled with astrocytes to generate the glutamate necessary to maintain glutamatergic neurotransmission, direct electrophysiological evidence is lacking. In fact, a requirement for the cycle has only been demonstrated during epileptiform activity, a disease setting in which glutamate release is greatly increased (Bacci et al., 2002; Otsuki et al., 2005; Tani et al., 2010). The large distance between cell bodies and axon terminals limits the contribution of somatic sources to the pool of glutamate available for synaptic release and predicts that glutamine-glutamate cycle is synaptically localized. In Chapter 4, we investigated neurotransmitter release from isolated nerve terminals in brain slices by transecting hippocampal Schaffer collaterals and cortical layer I axons. Stimulating with alternating periods of high frequency (20 Hz) and rest (0.2 Hz), we identified an activity-dependent reduction in synaptic efficacy that correlated with reduced glutamate release. This was enhanced by inhibition of astrocytic glutamine synthetase and reversed or prevented by exogenous glutamine. Importantly, this activity dependence was also revealed with an in vivo derived natural stimulus both at network and cellular levels. These data provide direct electrophysiological evidence that an astrocyte-dependent glutamate-glutamine cycle is required to maintain active neurotransmission at excitatory terminals.

Advisor: Huguenard John primary advisor. Thesis advisor
Reimer Richard J. primary advisor. Thesis advisor
Buckmaster Paul S. advisor. Thesis advisor
Goodman Miriam Beth advisor. Thesis advisor
Maduke Merritt C.