Spatial and Temportal Properties of Neurotransmission
The principal goal of our research is to understand how the temporal and spatial properties of neurotransmission contribute to the function of a neuronal network. The control on neurotransmitter (NT) release is a highly regulated process, as is the fate of NT once it is released from the presynaptic terminal. Each time NT is released, a transient occurs which consists of a rapid rise in the extracellular concentration of NT followed by a return to baseline concentration. In the brain the main excitatory neurotransmitter is glutamate and enables communication between neurons. When too much glutamate is released, however, the brain ceases to function properly and pathological events like seizures occur. We are interesting in a number of aspects regarding how neurotransmission occurs in a network and how glutamate, once released, is controlled.
Figure 1. Glutamate availability drives epileptiform activity in the cortex. A. Examples of glutamate biosensor signal changes in a cortical slice at 5 and 10 s ISIs indicate a large increase in the area of cortex recruited into the network with longer ISIs. Pseudo-color scale for the biosensor signal (inverse of the FRET ratio) is shown (inset in A). B. A similar, but more dramatic, change in the glutamate biosensor signal is seen as the ISI is increased from 5 to 30 s. C. Example evoked field potentials (bottom traces) and number of pixels above threshold for biosensor (top traces) for the two conditions in A. Both EFPs and glutamate signal increased as the ISI was increased from5 s (gray) to 10 s (black). D. A similar, but more dramatic, change in the glutamate biosensor signal is seen as the ISI is increased from 5 to 30 s. E. A marked prolongation of the evoked field potential (bottom traces) and the glutamate biosensor transient (top traces) is associated with increasing the ISI from5 s (gray) to 30 s (black). There was a direct correlation between the ISI and the area of cortex in which glutamate release is detected (F_17.00; p_0.0001). Due to photobleaching of the sensor, data were collected at 5 s ISI and one additional ISI only in each slice. Values were normalized to the 5 s ISI to permit slice-to-slice comparison.
Presynaptic Control of Glutamate Release
Specifically we aim to understand how presynaptic control of glutamate release and astrocytic reuptake mechanisms provide spatiotemporal control of glutamate transients. A main focus of work in the lab is understanding how pathological changes in astrocytes change NT transients and lead to pathological disease states such as epilepsy. Neurotransmission occurs when NT filled vesicles in the presynaptic terminal fuse with the plasma membrane. Once vesicular fusion occurs, molecules of NT are released into the synaptic cleft where they can bind post-synaptic ligand-gated ion channels, G-protein coupled receptors, and, depending on the NT, can be removed via active transport or enzymatic degradation. Using advanced glutamate imaging techniques, we were the first to describe the spatiotemporal properties of glutamate (the main excitatory NT in the central nervous system) transients in the brain slice preparation, where active glial reuptake exist similar to the intact brain (Dulla, et al, 2008 ). The specific time course and spatial pattern of extracellular NT transients are directly correlated with post-synaptic responses and likely affect many synaptic properties such as NT receptor open-time, desensitization, and activation of extra-synaptic receptors. The interaction between NT transients at different release sites contributes to more complex neuronal functions such as dendritic integration and synaptic plasticity, and may help enable input specificity.
Figure 2. Afferent glutamatergic input is altered in freeze lesioned cortex. A. Brightfield image overlaid with glutamate sensor imaging showing location of glutamate output. Site of stimulation is shown by white dot. Experiments were performed in 3mM kynurenic acid and 100 mM TBOA. B. The same experiment as in A. but performed in a brain slice from freeze-lesioned epileptogenic cortex.
Disease Processes and Altered Neuronal Networks
Our previous work has shown that the maintenance of stereotyped glutamate transients ensures proper neuronal network function and if perturbed can lead to pathological, epileptic network activity. The specific questions we aim to answer are: 1) how does brain injury alter NT transients and can these changes contribute to the progression of diseases such as epilepsy, 2) how does glutamate reuptake shape NT transients in healthy and injured brain, 3) how do specific modulators of pre-synaptic release (i.e. adenosine and GABAB receptor agonists/antagonists) shape both the temporal and spatial extent of NT transients. These areas of research are of great interest personally and remain relatively unexplored. Our skills in electrophysiology, imaging, biosensor methodologies, and custom analytical software provide us with the unique skill set needed to answer these questions.