Cellular mechanisms of the augmenting response: short-term plasticity in a thalamocortical pathway

Abstract

Some thalamocortical pathways display an "augmenting response" when stimuli are delivered at frequencies between 7 and 14 Hz. Cortical responses to the first three stimuli of a series increase progressively in amplitude and are relatively stable thereafter. We have investigated the cellular mechanisms of the augmenting response using extracellular and intracellular recordings in vivo and in slices of the sensorimotor neocortex of the rat. Single stimuli to the ventrolateral (VL) nucleus of the thalamus generate EPSPs followed by feedforward IPSPs that hyperpolarize cells in layer V. A long-latency depolarization interrupts the IPSP with a peak at approximately 200 msec. A second VL stimulus delivered during the hyperpolarization and before the peak of the long-latency depolarization yields an augmenting response. The shortest latency for augmenting responses occurs in cells of layer V, and they appear in dendrites and somata recorded in upper layers approximately 5 msec later. Recordings in vitro show that some layer V cells have hyperpolarization-activated and deinactivated conductances that may serve to increase their excitability after IPSPs. Also in vitro, cells from layer V, but not from layer III, generated augmenting responses at the same stimulation frequencies that were effective in vivo. Control experiments indicated that neither paired-pulse depression of IPSPs nor presynaptically mediated facilitation can account for the augmenting response. Active dendritic conductances contribute to the spread of augmenting responses into upper layers by way of back-propagating fast spikes, which attenuate with repetition, and long-lasting spikes, which enhance in parallel with the augmenting response. In conclusion, we propose that the initiation of augmenting responses depends on an interaction between inhibition, intrinsic membrane properties, and synaptic interconnections of layer V pyramidal neurons.

Fig. 1.

Extracellular and intracellular measurements of the augmenting response in vivo. A, Intracellular recording from a layer V cell of the sensorimotor cortex in response to one stimulus delivered to VL (arrow). A short-latency EPSP was followed by a sharp IPSP, which was then interrupted by a long-latency depolarization (asterisk).B, A second VL stimulus delivered after 100 msec produced an augmented response that triggered two action potentials (truncated); the long-latency depolarization (asterisk) followed. C, Simultaneous intracellular (layer V cell) and extracellular recordings (1000 μm in depth) during VL stimuli. Paired stimuli were delivered at different intervals (100, 200, and 300 msec). An augmenting response was triggered when the second stimulus was delivered during the hyperpolarization preceding the long-latency potential (asterisk); responses at longer intervals were not augmented. D, Four pulses delivered at 10 Hz show the incremental nature of the augmenting response, which enhanced after the first three stimuli and reached a steady state by the third pulse.

Fig. 3.

Comparison of sequentially recorded intrasomatic, intradendritic, and extracellular potentials indicate that the augmenting response is initiated in layer V. Recordings were made along a single electrode track as trains of four stimuli (arrows) were delivered to VL at 10 Hz. Intracellular recordings were from a dendrite located in layer III (400 μm deep;top trace) and a soma in layer V of an intrinsically bursting cell (1400 μm deep; bottom trace). Themiddle traces show extracellular field potentials at various depths in the cortex (surface, 500, 1000, and 1500 μm). Note that the somatic layer V recording was phase-locked to the shortest latency component of the concurrently recorded field potential from layer V, whereas the upper layer dendrite was phase-locked to the longer-latency negativities in those layers. The long-latency depolarization, ∼175-200 msec after the last stimulus, occurred first in the layer V cell (dashed line witharrows).

Fig. 2.

The augmenting response in vivooccurs within a narrow range of interstimulus intervals and is initiated in layer V. A, Effect of paired-pulse VL stimulation as a function of interstimulus interval (circles and left y-axis; amplitude of second response as percentage of the first), and timing of the peak of the long-latency potential for 10 cases (bars andright y-axis). Note that the augmenting response begins at an interstimulus interval of 50 msec and is abolished at the peak of the long-latency potential (200 msec). B, Latency of the augmenting response (i.e., the start of the response to the second stimulus delivered at a 100 msec interstimulus interval in VL) when recorded extracellularly in layer V (1500 μm deep;n = 5), intracellularly from neuron somas in layer V (1000–1500 μm; n = 5), extracellularly from layer III (500 μm deep; n = 5), intracellularly from dendrites located in the upper layers (layer III dendrites;n = 5), or from somas in the upper layers (n = 5 layer III somas; 300–1000 μm). The shortest latencies were observed in cells located in layer V. All data expressed as mean ± SEM.

Fig. 4.

Intrinsic membrane properties of layer V cells in slices in vitro. A, Repetitively bursting cells in layer V fire bursts within the frequency range of augmenting responses (top). During negative intracellular current injections, they show voltage sags characteristic of hyperpolarization-activated currents (such asI_H, middle). After negative pulses, they generate rebound depolarizations typical of cells with low-threshold calcium currents (I_T). Spikes during the rebound burst are truncated. The graph(bottom) plots the current–voltage relationship of the bursting cell, as measured at the points indicated (a–d). B, Adapting regular-spiking cells in layer V show no evidence of hyperpolarization-activated currents. Traces and graph as in A.

Fig. 5.

Augmenting responses are generated by layer V cells in vitro. A, Intracellular recording from a regular-spiking (nonadapting) layer V cell in the sensorimotor neocortex. Paired stimuli were delivered at different intervals (15, 25, 50, 75, 100, 150, 200, 300, and 500 sec), and an augmenting response was most prominent at 75, 100, and 150 msec intervals (long traces). Note also an enhanced response at other intervals, coincident with an apparent depression of short-latency inhibition. The inset traces show a different layer V cell with a particularly strong augmenting response at an interval of 100 msec. B, Four stimuli delivered at 10 Hz to another layer V cell show the incremental nature of the augmenting response in the slice. C, Intracellular recordings from a regular-spiking layer III cell in the slice. Paired stimuli delivered at different intervals generated only depression after the second stimulus at all intervals up to 5 sec (shown are 25, 50, and 100 msec). Baseline V_m is −63 mV for A and B, and −65 mV for the cell inC.

Fig. 6.

Frequency-dependent depression of IPSPs is not responsible for the augmenting response. Effects of paired-pulse stimulation in vitro. Data are expressed as a percentage of the change in the amplitude (excluding action potentials in layer V cells) of the second response, compared with the first response, in layer V cells (closed circles; n = 3), layer III cells (open circles; n= 3), and isolated IPSPs (recorded in the presence of AP5 and CNQX) from layer V cells (closed squares;n = 3). Note that IPSPs do not depress selectively at the intervals at which augmenting responses are generated more prominently in layer V cells in vitro.

Fig. 7.

Presynaptic facilitation is not responsible for generating the augmenting response. A, Schematic diagram illustrating the location of the intracellular recording electrode in layer V and stimulating electrodes (S1 andS2) activating two independent pathways to the recorded cell in vitro. B, A stimulus to S1 was capable of priming the cell so that a subsequent stimulus to S2, 100 msec later, produced an augmented response (top trace) similar to the augmented response observed after two stimuli are delivered to S2 (bottom trace). This result suggests that presynaptic mechanisms are unlikely to be of primary importance in the mechanisms of initiation of augmenting responses. Spike amplitude is filtered. Baseline V_m is −63 mV.

Fig. 8.

Intracellular recording from dendrites located in layer III in vivo. A, Intracellularly injected current pulses produced progressively attenuating fast spikes and longer-lasting spikes. B, Paired pulses at a 100 msec interval (top), but not at 200 msec (bottom), produced an augmenting response consisting of a fast spike, a longer-lasting spike, and synaptic components.C, In response to a short train of VL stimulation at 10 Hz, the augmenting response recorded from another dendrite in layer III shows progressive attenuation of the fast spikes (arrows) and a strong enhancement of a longer-lasting spike (asterisks). The overlapping traces atright show the first (1), second (2), and third (3) responses of the dendrite at faster sweep speed. Note the enhancement of the long-lasting spike and attenuation of the fast spike.

Fig. 9.

Intracellular recordings from dendrites in layer III in vitro. A, Current pulses produced fast spikes and long-lasting spikes in intradendritic recordings from layer III. B, Synaptic stimulation in layer V applied at different intensities produce graded synaptic responses and all-or-none spike components (left traces). At 10 μA current intensity a threshold was observed for the induction of the fast spike. Shown are 12 trials at 0.1 Hz (right traces). Note that the fast spike appears on approximately half of those trials at this threshold stimulation. C, Paired stimuli at a range of interstimulus intervals (shown are 100 and 500 msec) produce an enhancement of the long-lasting spike and attenuation of the fast spike. The dashed line is the control response to the first stimulus, and the overlapping traces correspond to the second response delivered at different interstimulus intervals.

Fig. 10.

High-threshold, long-lasting spikes can be observed in somatic recordings of repetitively bursting cells in layer V. Intracellular recording from a layer V repetitively bursting cell during three levels of depolarizing constant current injection. Low current (0.2 nA) generated a mix of fast spikes and bursts, moderate current (0.4 nA) triggered rhythmic fast spikes, and strong current (1.6 nA) evoked attenuated fast spikes interspersed with fast-spike/slow-spike complexes.