Voltage-gated Sodium Channels

Recently we have identified two mechanisms by which GPCRs strongly regulate 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.

We  are currently identifying the molecular pathways by which exo- and endogenous cannabinoids indirectly stabilize the inactivated states of VGSCs. The pathway is present in nearly all cortical neurons and strongly reduces excitability. Delineation of the physiological and pathophysiological effects of this pathway is a central goal of our work.

Intracellular Signaling

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.

Calcium-sensing Receptor

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+ 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.

Terminal Recordings

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.