The major theme of our lab is rapid neuronal signaling in the mammalian brain and how this changes in disease states. Focusing mainly on the cerebral cortex, we study regulation of intrinsic excitability and synaptic transmission. A major target is the calcium sensing receptor (CaSR) which has been identified in neuronal somata and terminals.
Another interest has been heterogeneity of synaptic transmission at excitatory synapses and in particular the different roles of voltage-activated calcium channels (VACCs) at excitatory and inhibitory synapses.
Voltage-gated Sodium Channels
More recently we have begun to examine strong GPCR-mediated regulation of voltage-gated sodium channels (VGSCs). Since VGSCs constitute the keystone of neuronal excitability, characterization of their function is a major interest to neuroscientists. The sodium channel-dependent action potential provides a digital or “all or none” signal that has been shown to have different characteristics in various types of neurons. However, dynamic substantial regulation of sodium channels has not been described previously. Recently Glynis Mattheisen identified a new pathway, triggered by cinacalcet, by which VGSCs can be strongly and dynamically regulated by a G-protein mediated mechanism. The pathway is apparently ubiquitous (observed in all recordings in both acutely isolated and cultured neurons from neocortex and hippocampus). Clearly, a prevalent pathway that has high efficacy to inhibit VGSCs in the cortex is positioned to substantially impact neuronal excitability and important behaviors.
Characterization of the signaling pathway by which cinacalcet affects VGSCs is a goal of the lab. Candidate molecules include DAG, cAMP, and PIP2 which can be measured using genetically encoded sensors. The image of neocortical neurons fluorescing following infection with a green fluorescent protein variant was produced by Briana Knight.
Voltage-activated Calcium Channels
In earlier work we demonstrated differences in the fundamental properties of synaptic transmission at inhibitory and excitatory synapses, identifying voltage-activated calcium channels (VACCs) as physiological triggers for the majority of spontaneous release at inhibitory but not excitatory synapses. Moreover, at inhibitory synapses stochastic, synchronous openings of VACCs triggered the majority of spontaneous release. Timur Tsintsadze and Courtney Williams recently determined that this important difference also occurred in neocortical acute brain slices.
In collaboration with Dennis Weingarten and Henrique von Gersdorff we found this pattern to be wide ranging, holding for large and small synapses in the neocortex and brainstem. These findings indicated fundamental differences of the Ca2+ dependence of spontaneous release at excitatory and inhibitory synapses and heterogeneity of the mechanisms of release across the CNS.
The CaSR is a G-protein coupled receptor (GPCR) expressed in many tissues and its major role in the periphery is to sense and regulate external [Ca2+] ([Ca2+]o) in serum and tissue. The CaSR is expressed at nerve terminals of the nervous system and modulates evoked and spontaneous synaptic transmission differently. We showed CaSR agonists produced a graded inhibition of synaptic transmission in neo-cortical neurons. In contrast, CaSR agonists strongly stimulated spontaneous release at both excitatory and inhibitory synapses. The action on evoked release is explained by CaSR activation inhibiting a voltage-activated non-selective cation (NSC) channel in 80-90% of neocortical nerve terminals. It has also been hypothesized by Ren and colleagues that a similar pathway, localized to the neuronal soma, is responsible for Ca2+-dependent excitability. This is different to the classical idea that surface charge is the key determinant of Ca2+-dependent excitability. Briana Knight and Brian Jones are addressing the questions raised by these contrasting hypotheses.
Using a modified patch-clamp technique we have begun to characterize ion channels in neocortical nerve terminals. Potassium channels, including Ca2+-activated K channels and inward rectifiers, have been identified in neocortical terminals. Timur Tsintsadze is continuing to work on this in collaboration with Andy Ritzau-Jost and Stefan Hallermann.