Complementary Ca2+ Activity of Sensory Activated and Suppressed Layer 6 Corticothalamic Neurons Reflects Behavioral State

Layer 6 (L6) corticothalamic neurons project to thalamus, where they are thought to regulate sensory information transmission to cortex. However, the activity of these neurons during different behavioral states has not been described. Here, we imaged calcium changes in visual cortex L6 primary corticothalamic neurons with two-photon microscopy in head-fixed mice in response to passive viewing during a range of behavioral states, from locomotion to sleep. In addition to a substantial fraction of quiet neurons, we found sensory-activated and suppressed neurons, comprising two functionally distinct L6 feedback channels. Quiet neurons could be dynamically recruited to one or another functional channel, and the opposite, functional neurons could become quiet under different stimulation conditions or behavior states. The state dependence of neuronal activity was heterogeneous with respect to locomotion or level of alertness, although the average activity was largest during highest vigilance within populations of functional neurons. Interestingly, complementary activity of these distinct populations kept the overall corticothalamic feedback relatively constant during any given behavioral state. Thereby, in addition to sensory and non-sensory information, a constant activity level characteristic of behavioral state is conveyed to thalamus, where it can regulate signal transmission from the periphery to cortex

A multi-compartment model for interneurons in the dorsal lateral geniculate nucleus

GABAergic interneurons (INs) in the dorsal lateral geniculate nucleus (dLGN) shape the information flow from retina to cortex, presumably by controlling the number of visually evoked spikes in geniculate thalamocortical (TC) neurons, and refining their receptive field. The INs exhibit a rich variety of firing patterns: Depolarizing current injections to the soma may induce tonic firing, periodic bursting or an initial burst followed by tonic spiking, sometimes with prominent spike-time adaptation. When released from hyperpolarization, some INs elicit rebound bursts, while others return more passively to the resting potential. A full mechanistic understanding that explains the function of the dLGN on the basis of neuronal morphology, physiology and circuitry is currently lacking. One way to approach such an understanding is by developing a detailed mathematical model of the involved cells and their interactions. Limitations of the previous models for the INs of the dLGN region prevent an accurate representation of the conceptual framework needed to understand the computational properties of this region. We here present a detailed compartmental model of INs using, for the first time, a morphological reconstruction and a set of active dendritic conductances constrained by experimental somatic recordings from INs under several different current-clamp conditions. The model makes a number of experimentally testable predictions about the role of specific mechanisms for the firing properties observed in these neurons. In addition to accounting for the significant features of all experimental traces, it quantitatively reproduces the experimental recordings of the action-potential- firing frequency as a function of injected current. We show how and why relative differences in conductance values, rather than differences in ion channel composition, could account for the distinct differences between the responses observed in two different neurons, suggesting that INs may be individually tuned to optimize network operation under different input conditions

AP waveforms and I/O curves.

<p>Experimental (dashed colored lines) and simulated (thick, colored lines) AP waveforms for IN1/P1 (<b>A1</b>) and IN2/P2 (<b>A2</b>). The morphology did not significantly affect the simulated AP-waveform (simulations with three alternative morphologies are shown by thin, black lines). The simulated (thick, colored lines) and experimentally obtained (circles, two repetitions for each stimuli) I/O curves for P1/IN1 (<b>B1</b>) and P2/IN2 (<b>B2</b>) were in agreement. Simulations are also shown for alternative morphologies (thin, black lines), with (a) similar, (b) smaller and (c) larger membrane area compared to the original morphology (o). The slopes of the I/O curves decreased with membrane area, but the essential differences between P1 and P2 were preserved (<b>C</b>). I/O curves were defined as #spikes elicited throughout the stimulus period as a function of the amplitude of the injected current. Slopes of I/O curves were always calculated in the range between 2 and 15 elicited APs.</p

Mechanisms behind rebound bursting.

<p>All panels show responses to strong hyperpolarizing synaptic input (50 synapses, activated with 10 ms intervals, during 300 ms). Rebound bursts were not elicited by P1 (<b>A1</b>) or P2 (<b>A2</b>), starting from their normal resting potentials of -63 mV and -69 mV, respectively. From a -57 mV holding potential, P1 (<b>B1</b>) elicited a rebound burst following hyperpolarization, while P2 did not (<b>B2</b>). Interchanging either the <i>I_h</i> conductance (<b>C</b>) or the <i>Ca_T</i> conductance (<b>D</b>) between P1 and P2, leaving all other parameters the same, reduced the rebound response in P1, and increased it in P2. Interchanging both the <i>I_h</i> and <i>Ca_T</i> conductances between P1 and P2, also interchanged their bursting abilities completely (<b>E</b>). The scale bar applies to all panels. When conductance values were changed, the resting potential was kept at the original level by small compensatory current injections.</p

<i>I_h</i>-kinetics.

<p>The steady state values of <i>I_h</i>-activation (<b>A</b>), and activation time constant (<b>B</b>), fitted to data from three INs (plusses, circles and diamonds). A simulation of the <i>I_h</i>-current at different step potentials between -130 and -65 mV, with 5mV steps (<b>C</b>) gives similar traces to those seen experimentally (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002160#pcbi-1002160-g002" target="_blank">Figure 2C</a>).</p

Experimental data.

<p>Somatic voltage responses in IN1, resting at -63 mV (<b>A1</b>), and IN2, resting at -69 mV (<b>A2</b>) to 7 current pulses of different intensity. When IN1 was held at -57 mV, a strong hyperpolarizing current injection was followed by a rebound burst (<b>B1</b>). When IN2 was held at -58 mV, a strong hyperpolarizing current injection did not cause a burst (<b>B2</b>). Current injections were applied as 900 ms step pulses to the soma, with intensities as indicated below the traces. Two repetitions were made of each CC-experiment, whereof one is shown. The voltage scale bar applies to all panels (A-B). <i>I_h</i> was measured at different potentials from -130 mV with 5 mV steps, in three different neurons. Recordings are shown for one neuron (<b>C</b>).</p

Mechanisms behind initial bursts and regular spiking.

<p>All panels show responses to a 70 pA depolarizing current injections. As a reference, the responses in (<b>A</b>) use the original parameters for P1 and P2, and are the same as in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002160#pcbi-1002160-g004" target="_blank">Figure 4</a>. In P2, the initial burst and the regular spiking are separated by a pronounced afterhyperpolarization (<b>A1</b>), while in P1 the transition between the initial burst and the regular spiking is more gradual (<b>A2</b>). These characteristics were interchanged between P1 and P2 when <i>g_AHP</i> was interchanged (i.e., multiplied/divided by a factor 2.03 in P1/P2) between the two parameterizations (<b>B</b>). Initial bursts were eliminated when <i>g_CaT</i> was set to zero (<b>C</b>), and became stronger when <i>g_CaT</i> was increased by a factor 2 (<b>D</b>). Increasing <i>g_AHP</i> and <i>g_CaT</i> by a factor 2 gave rise to periodic bursting in both neurons (<b>E</b>). The scale bar applies to all panels. When conductance values were changed, the resting potential was kept at the original level by small compensatory current injections.</p

Morphology and ion channel kinetics.

<p>The same dLGN morphology (<b>A</b>) was used in all simulations. The steady state values of activation/inactivation variables (red/black full lines), along with the activation/inactivation time constants (red/black dotted lines) are plotted as a function of voltage for voltage dependent ion channels (<b>B–F</b>), and as a function of intracellular calcium concentration for calcium dependent ion channels (<b>G–H</b>). <i>Na</i> and <i>K_dr</i> kinetics are shown for the parameterization P1 of the model. With respect to this, P2 kinetics was shifted +2.3 mV and +3.2 mV for <i>Na</i> and <i>K_dr</i> respectively. For all other ion channels, the same kinetics applies to both parameterizations (P1 and P2).</p