Intrinsic properties of and thalamocortical inputs onto identified corticothalamic-VPM neurons

Abstract

Corticothalamic (CT) feedback plays an important role in regulating the sensory information that the cortex receives. Within the somatosensory cortex layer VI originates the feedback to the ventral posterior medial (VPM) nucleus of the thalamus, which in turn receives sensory information from the contralateral whiskers. We examined the physiology and morphology of CT neurons in rat somatosensory cortex, focusing on the physiological characteristics of the monosynaptic inputs that they receive from the thalamus. To identify CT neurons, rhodamine microspheres were injected into VPM and allowed to retrogradely transport to the soma of CT neurons. Thalamocortical slices were prepared at least 3 days post injection. Whole-cell recordings from labeled CT cells in layer VI demonstrated that they are regular spiking neurons and exhibit little spike frequency adaption. Two anatomical classes were identified based on their apical dendrites that either terminated by layer V (compact cells) or layer IV (elaborate cells). Thalamic inputs onto identified CT-VPM neurons demonstrated paired pulse depression over a wide frequency range (2-20 Hz). Stimulus trains also resulted in significant synaptic depression above 10 Hz. Our results suggest that thalamic inputs differentially impact CT-VPM neurons in layer VI. This characteristic may allow them to differentiate a wide range of stimulation frequencies which in turn further tune the feedback signals to the thalamus.

Keywords: Barrel cortex; layer VI; neuronal morphology.

Figure 1.

Bead labeling of corticothalamic neurons. Targeted injection of rhodamine microspheres into VPM stay confined to the nucleus (panel A left, scale bar=500 μm). Schematic of thalamic injection site (panel A right), LD=lateral dorsal nucleus, NRT=thalamic reticular nucleus, PO=posterior nucleus, VL=ventral lateral nucleus, VPL=ventral posterior lateral nucleus, ZI=zona inserta. After 2–3 days beads are retrogradely transported to the somata of CT cells in the upper half of layer VI in rat barrel cortex (panel B), laminae revealed using Hoechst nuclear staining, roman numerals indicate laminae (scale bar=50 μm). A bead-labeled CT neuron is targeted for whole-cell patch clamp recordings (panel C top), fluorescent illumination confirms retrograde labeling (panel C bottom).

Figure 2.

Firing properties of corticothalamic-VPM neurons. A typical response of a CT neuron to current steps of ± 0.10 nA pulses lasting 500 ms reveals the regular spiking phenotype (panel A). Neurons displayed linear I–V curves at maximum deviation from rest (solid line) and at 25 ms before stimulus offset in the subthreshold voltage range (panel B, population means and one standard error of the mean are plotted).

Figure 3.

Corticothalamic cells display little spike frequency adaptation. Panel A shows representative traces of neurons in response to 50, 100, and 250 pA current pulses lasting 500 ms. Panel B plots inter-spike interval (y-axis) vs. inter-spike interval number, regardless of the magnitude of the injected current the time between the action potentials is relatively constant. Each data point reflects responses from at least 10 neurons. Means and one standard error of the mean are plotted.

Figure 4.

Representative micrographs of corticothalamic-VPM neurons. Panel A shows a compact CT-VPM cell at low (left, scale bar=250 μm) and high magnification (right, scale bar=50 μm). Panel B is representative of our elaborate population with a cell body in layer VI and an apical dendrite extending towards more superficial layers (left, low magnification, scale bar=250 μm; right, high magnification, scale bar=50 μm). Arrows highlight CT-VPM cells in the low magnification images.

Figure 5.

Reconstruction of corticothalamic-VPM neurons. Panel A shows three representative compact cells and panel B displays three elaborate cells with their longer apical dendrites. Bars on right reflect extent of cortical laminae, scale bars = 50 μm.

Figure 6.

Evoked monosynaptic EPSPs are evident in corticothalamic-VPM neurons. Minimal stimulation (50 μA, 200 μs pulse) results in either failures (panel A) or evoked EPSPs (panel B). Repetitive stimulation reveals the consistency of the latency of the response with panel C showing the overlay of consecutive stimulus presentations, where approximately 50% of stimulus presentations result in failures. Stimulation with same intensity (100 μA) but with different pulse durations resulted in the same EPSP latency (panel D). CT-VPM neurons showed little variance in their latency to thalamic stimulation (panel E), where EPSP latency variance is plotted for each cell, values <10% are thought to be reflective of a monosynaptic input. A CT-VPM neuron recorded with low intracellular Cl⁻ (see Methods, panel F) at different holding potentials reveals prominent inhibition following the onset of a stimulus-evoked EPSP, dotted lines represent pre-stimulus membrane potential. Asterisks indicate stimulus artifact. The traces shown in panels A, B, C, D, and F are from the same cell, +100 μA current pulses of 200 μs duration were applied except where specified.

Figure 7.

Responses to trains of synaptic inputs reveal depressing synapses. Responses to single (panel A) stimulation of the thalamus vs. paired stimulation (panel B) reveal paired pulse depression. Trains of eight stimuli show depression with increased frequency of stimulation (panel C). All traces from the same cell, +1.0 mA current pulses of 5 ms duration were utilized which evoked an initial EPSP >1 mV. Paired pulse experiments revealed short-term synaptic depression (panel D), and trains of stimuli (8 pulses, panel E) also showed significant synaptic depression especially at higher stimulation frequencies. Means and one standard error of the mean are plotted (n = 15). Dotted line in panels E and F represent 100%.

Figure 8.

Postnatal development of cell intrinsic properties. Resting membrane potential (A) and input resistance (B) did not change over the 2-week experimental window. In contrast, action potential amplitude increased (C) and action potential rise time decreased (D) over the same period. Data represents means and one standard error of the mean.