Defining cortical frequency tuning with recurrent excitatory circuitry

Abstract

Neurons in the recipient layers of sensory cortices receive excitatory input from two major sources: the feedforward thalamocortical and recurrent intracortical inputs. To address their respective functional roles, we developed a new method for silencing cortex by competitively activating GABA(A) while blocking GABA(B) receptors. In the rat primary auditory cortex, in vivo whole-cell recording from the same neuron before and after local cortical silencing revealed that thalamic input occupied the same area of frequency-intensity tonal receptive field as the total excitatory input, but showed a flattened tuning curve. In contrast, excitatory intracortical input was sharply tuned with a tuning curve that closely matched that of suprathreshold responses. This can be attributed to a selective amplification of cortical cells' responses at preferred frequencies by intracortical inputs from similarly tuned neurons. Thus, weakly tuned thalamocortical inputs determine the subthreshold responding range, whereas intracortical inputs largely define the tuning. Such circuits may ensure a faithful conveyance of sensory information.

Figure 1

Specific silencing of local intracortical connections with a cocktail pharmacological method. (a) Tone-evoked field potentials recorded in A1 before (top) and after (bottom) cortical injection of muscimol (1 mM), baclofen (1 mM), SCH50911 (1.5mM) or a cocktail of muscimol (1 mM) and SCH50911 (1.5 mM). Small arrow marks the onset of tone stimulus. (b) Effective blocking of cortical spikes by the muscimol and SCH50911 (4mM: 6mM) cocktail in both layer 4 and layer 6 within a horizontal distance of 500 μm from the injection site (see Methods). Multi-unit tone-evoked spikes were detected by extracellular recordings. Red dot line indicates 90% reduction in spike count. (c) Example excitatory synaptic TRFs of A1 neurons obtained shortly after muscimol injection (left) and cocktail injection (right). Each small trace represents the response (recorded at −70mV) to a tone of a particular frequency and intensity. (d) Average bandwidth of synaptic TRF measured at 60dB (BW60) in A1 injected with muscimol, cocktail, or vehicle solution (ACSF). Bar is s.d. (e) Spike TRF (average of four repetitions) for a recording site in the MGBv before and after cortical injection of the cocktail. Color represents the number of spikes evoked by a tone stimulus. (f) Percentage change in the bandwidth and spike count for tone evoked spikes (measured at 60 dB) in the MGBv before and after cortical cocktail application (n = 6 sites). Bar = s.d.

Figure 2

Changes in excitatory synaptic TRF after local cortical silencing. (a) Excitatory (left) and inhibitory (right) synaptic currents evoked by a tone of 1.5 kHz and 70 dB before (gray) and after (black) cocktail application. (b) Left, synaptic currents (average of five repeats) evoked by a tone of 1.9 kHz and 70 dB recorded at different holding potentials. Right, I –V curves (V is corrected) for synaptic currents averaged within a 20–22.5 ms window after the stimulus onset (black) and 0–1 ms window after the response onset (red). (c) Morphology of this recorded cell. Bar, 20 μm. (d) TRF of excitatory synaptic currents before (average of two repeats) and after (four repeats) silencing. Blue dots mark the responses at the intensity threshold (20 dB). The color maps show the average amplitudes. Number in the bracket indicates the original scale before correction. Bottom, I–IV, the rising phase of average synaptic response to a 1.3 kHz tone at 60 dB (I/II) or a 5.6 kHz tone at 20 dB (III/IV) before (I/III) and after (II/IV) cortical silencing. (e) Color map of onset latencies of evoked excitatory currents. (f) Onset latencies (at 70 dB) before (blue) and after (red) cocktail application. Triangle represents the difference. (g) Amplitudes of responses before (blue) and after (red) cocktail application at 70 dB. (h) Tuning curves of excitatory currents at four different tone intensities. The black line represents the tuning curve of subtracted responses (before minus after).

Figure 3

Intracortical inputs are more sharply tuned than thalamocortical inputs. (a–h) Change in excitatory synaptic TRF in another four cells. a, c, e, g, Top, color maps represent the excitatory synaptic TRF before and after cocktail application. Bottom, kinetics of the rising phase of synaptic currents (1–IV). The curved line outlines the boundary of synaptic TRF before (blue) and after (red) cocktail application (V). Data are presented in the same way as in Fig. 2. b, d, f, h, Top, excitatory synaptic currents evoked by tones (at 70 dB) of different frequencies before and after cocktail application. The amplitudes of currents after application are corrected. Bottom, excitatory tuning curves at 70 dB before (blue) and after (red) cortical silencing. Data are presented in a similar manner as in Fig. 2. Black lines are for the subtracted inputs. (i) Half-peak bandwidths of tuning curves at 70 dB for total excitatory inputs (before), thalamic inputs (after) and intracortical inputs (subtracted). Data points from the same cell are connected with lines (n = 5, paired t-test, * P < 0.01). (j) Average ratio of onset latency, intensity threshold of excitatory synaptic TRF, bandwidth at 10 dB above the intensity threshold (TRF BW), and half-peak bandwidth of tuning curve at 70 dB (half-peak BW) between after and before values. Bar = s.d.

Figure 4

Similarly tuned intracortical inputs sharpen the frequency tuning curve in layer 4. (a) Membrane potential responses to tones of various frequencies and intensities, recorded under current clamp from a representative A1 neuron. Blue dashed line delineates the boundary for the TRF of tone-evoked membrane depolarizations. The yellow and red lines indicate the frequency range (at 70 dB) for subthreshold and spike responses respectively. (b) Left, frequency range for subthreshold (yellow) and spike (red) responses at 70 dB of 10 cells detected with current-clamp recording. Middle, percentage frequency range for spike responses within the range for membrane potential responses. Each cross represents one cell. The square represents the average of all cells (± s.d.). Right, for 24 cells in which both voltage-clamp and current-clamp recordings were obtained, the frequency range (at 70 dB) of membrane depolarizations (ordinate) matches well with that of excitatory synaptic currents (abscissa). (c) Left, normalized tuning curves after thresholding within the estimated suprathreshold response range for total excitatory input (black), thalamocortical (red) and intracortical (blue) input in a cell. Right, average bandwidths at 50% and 80% peak amplitude of “suprathreshold” tuning curves (n = 5 cells, paired t-test, * P < 0.03). Bar = s.d.