Tagging active neurons by soma-targeted Cal-Light

Verifying causal effects of neural circuits is essential for proving direct a circuit-behavior relationship. However, techniques for tagging only active neurons with high spatiotemporal precision remain at the beginning stages. Here we developed the soma-targeted Cal-Light (ST-Cal-Light) which selectively converts somatic calcium rise triggered by action potentials into gene expression. Such modification simultaneously increases the signal-to-noise ratio (SNR) of reporter gene expression and reduces the light requirement for successful labeling. Because of the enhanced efficacy, the ST-Cal-Light enables the tagging of functionally engaged neurons in various forms of behaviors, including context-dependent fear conditioning, lever-pressing choice behavior, and social interaction behaviors. We also targeted kainic acid-sensitive neuronal populations in the hippocampus which subsequently suppressed seizure symptoms, suggesting its applicability in controlling disease-related neurons. Furthermore, the generation of a conditional ST-Cal-Light knock-in (KI) mouse provides an opportunity to tag active neurons in a region- or cell-type specific manner via crossing with other Cre-driver lines. Thus, the versatile ST-Cal-Light system links somatic action potentials to behaviors with high temporal precision, and ultimately allows functional circuit dissection at a single cell resolution.

Microscale physiological events on the human cortical surface

Despite ongoing advancements in our understanding of the local single-cellular and network-level activity of neuronal populations in the human brain, extraordinarily little is known about their ‘intermediate’ microscale local circuit dynamics. Here, we utilized ultrahigh density microelectrode arrays and a rare opportunity to perform intracranial recordings across multiple cortical areas in human participants to discover three distinct classes of cortical activity that are not locked to ongoing natural brain rhythmic activity. The first included fast waveforms similar to extracellular single unit activity. The other two types were discrete events with slower waveform dynamics and were found preferentially in upper layers of the grey matter. They were also observed in rodents, non-human primates, and semi-chronic recordings in humans via laminar and Utah array microelectrodes. The rates of all three events were selectively modulated by auditory and electrical stimuli, pharmacological manipulation, and cold saline application and had small causal co-occurrences. These results suggest that with the proper combination of high resolution microelectrodes and analytic techniques it is possible to capture neuronal dynamics that lay between somatic action potentials and aggregate population activity and that understanding these intermediate microscale dynamics may reveal important details of the full circuit behavior in human cognition.

Subtype-Specific Dendritic Ca2+ Dynamics of Inhibitory Interneurons in the Rat Visual Cortex

The Ca2+ increase in dendrites that is evoked by the backpropagation of somatic action potentials (APs) is involved in the activity-dependent modulation of dendritic and synaptic functions that are location dependent. In the present study, we investigated dendritic Ca2+ dynamics evoked by backpropagating APs (bAPs) in four subtypes of inhibitory interneurons classified by their spiking patterns: fast spiking (FS), late spiking (LS), burst spiking (BS), and regular-spiking nonpyramidal (RSNP) cells. Cluster analysis, single-cell RT-PCR, and immunohistochemistry confirmed the least-overlapping nature of the grouped cell populations. Somatic APs evoked dendritic Ca2+ transients in all subtypes of inhibitory interneurons with different spatial profiles along the tree: constantly linear in FS and LS cells, increasing to a plateau in BS cells and bell-shaped in RSNP cells. The increases in bAP-evoked dendritic Ca2+ transients brought about by the blocking of A-type K+ channels were similar in whole dendritic trees of each subtype of inhibitory interneurons. However, in RSNP cells, the increases in the distal dendrites were larger than those in the proximal dendrites. On cholinergic activation, nicotinic inhibition of bAP-evoked dendritic Ca2+ transients was observed only in BS cells expressing cholecystokinin and vasoactive intestinal peptide mRNAs, with no muscarinic modulation in all subtypes of inhibitory interneurons. Cell subtype-specific differential spatial profiles and their modulation in bAP-evoked dendritic Ca2+ transients might be important for the domain-specific modulation of segregated inputs in inhibitory interneurons and differential control between the excitatory and inhibitory networks in the visual cortex.

Developmental Changes in Electrophysiological Properties and Synaptic Transmission in Rat Intracardiac Ganglion Neurons

We charted postnatal changes in the intrinsic electrophysiological properties and synaptic responses of rat intrinsic cardiac ganglion (ICG) neurons. We developed a whole-mount ganglion preparation of the excised right atrial ganglion plexus. Using intracellular recordings and nerve stimulation we tested the hypothesis that substantial transformations in the intrinsic electrical characteristics and synaptic transmission accompany postnatal development. Membrane potential ( Em) did not change but time constant (τ) and cell capacitance increased with postnatal development. Accordingly, input resistance ( Rin) decreased but specific membrane resistance ( Rm) increased postnatally. Comparison of the somatic active membrane properties revealed significant changes in electrical phenotype. All neonatal neurons had somatic action potentials (APs) with small overshoots and small afterhyperpolarizations (AHPs). Adult neurons had somatic APs with large overshoots and large AHP amplitudes. The range of AHP duration was larger in adults than in neonates. The AP characteristics of juvenile neurons resembled those of adults, with the exception of AHP duration, which fell midway between neonate and adult values. Phasic, multiply adapting, and tonic evoked discharge activities were recorded from ICG neurons. Most neurons displayed phasic discharge at each developmental stage. All neurons received excitatory synaptic inputs from the vagus or interganglionic nerve trunk(s), the strength of which did not change significantly with postnatal age. The changes in the electrophysiological properties of the postganglionic neuron suggest that increased complexity of parasympathetic regulation of cardiac function accompanies postnatal development.

Direct Depolarization and Antidromic Action Potentials Transiently Suppress Dendritic IPSPs in Hippocampal CA1 Pyramidal Cells

Whole-cell current-clamp recordings were made from distal dendrites of rat hippocampal CA1 pyramidal cells. Following depolarization of the dendritic membrane by direct injection of current pulses or by back-propagating action potentials elicited by antidromic stimulation, evoked γ-aminobutyric acid-A (GABAA) receptor-mediated inhibitory postsynaptic potentials (IPSPs) were transiently suppressed. This suppression had properties similar to depolarization-induced suppression of inhibition (DSI): it was enhanced by carbachol, blocked by dendritic hyperpolarization sufficient to prevent action potential invasion, and reduced by 4-aminopyridine (4-AP) application. Thus DSI or a DSI-like process can be recorded in CA1 distal dendrites. Moreover, localized application of TTX to stratum pyramidale blocked somatic action potentials and somatic IPSPs, but not dendritic IPSPs or DSI induced by direct dendritic depolarization, suggesting DSI is expressed in part in the dendrites. These data extend the potential physiological roles of DSI.

Action Potentials in the Dendrites of Retinal Ganglion Cells

Action potentials in the dendrites of retinal ganglion cells. The somas and dendrites of intact retinal ganglion cells were exposed by enzymatic removal of the overlying endfeet of the Müller glia. Simultaneous whole cell patch recordings were made from a ganglion cell’s dendrite and the cell’s soma. When a dendrite was stimulated with depolarizing current, impulses often propagated to the soma, where they appeared as a mixture of small depolarizations and action potentials. When the soma was stimulated, action potentials always propagated back through the dendrite. The site of initiation of action potentials, as judged by their timing, could be shifted between soma and dendrite by changing the site of stimulation. Applying QX-314 to the soma could eliminate somatic action potentials while leaving dendritic impulses intact. The absolute amplitudes of the dendritic action potentials varied somewhat at different distances from the soma, and it is not clear whether these variations are real or technical. Nonetheless, the qualitative experiments clearly suggest that the dendrites of retinal ganglion cells generate regenerative Na+ action potentials, at least in response to large direct depolarizations.