Synaptic properties of thalamic input to layers 2/3 and 4 of primary somatosensory and auditory cortices

Abstract

We studied the synaptic profile of thalamic inputs to cells in layers 2/3 and 4 of primary somatosensory (S1) and auditory (A1) cortices using thalamocortical slices from mice age postnatal days 10-18. Stimulation of the ventral posterior medial nucleus (VPM) or ventral division of the medial geniculate body (MGBv) resulted in two distinct classes of responses. The response of all layer 4 cells and a minority of layers 2/3 cells to thalamic stimulation was Class 1, including paired-pulse depression, all-or-none responses, and the absence of a metabotropic component. On the other hand, the majority of neurons in layers 2/3 showed a markedly different, Class 2 response to thalamic stimulation: paired-pulse facilitation, graded responses, and a metabotropic component. The Class 1 and Class 2 response characteristics have been previously seen in inputs to thalamus and have been described as drivers and modulators, respectively. Driver input constitutes a main information bearing pathway and determines the receptive field properties of the postsynaptic neuron, whereas modulator input influences the response properties of the postsynaptic neuron but is not a primary information bearing input. Because these thalamocortical projections have comparable properties to the drivers and modulators in thalamus, we suggest that a driver/modulator distinction may also apply to thalamocortical projections. In addition, our data suggest that thalamus is likely to be more than just a simple relay of information and may be directly modulating cortex.

Fig. 1.

Slice connectivity verified using flavoprotein autofluorescence (FA) imaging and photo-uncaging of glutamate. Two typical examples where 20 Hz electrical stimulation (150 μA) of ventral posterior medial nucleus (VPM) and ventral division of the medial geniculate body (MGBv) (red asterisks) resulted in FA activation in layer 4 and layers 2/3 of S1 (A) and A1 (B), respectively. Color scale represents the %Δf/f change in fluorescence. Examples of inward currents recorded from layers 2/3 cells located in S1 (C) and A1 (D) while photo-uncaging glutamate over VPM and MGBv, respectively. Insets: false-color maps of location and magnitude of inward currents. Each pixel corresponds to a locus of uncaging as seen in the main figure. M, medial; L, lateral; D, dorsal; V, ventral; R, rostral; C, caudal; Hipp, hippocampus; TRN, thalamic reticular nucleus; BG, basal ganglia; L1, layer 1; L2/3, layers 2/3; L4, layer 4; L5, layer 5; L6, layer 6.

Fig. 2.

Morphology and intrinsic properties of pyramidal neurons in layers 2/3. A: a biocytin-filled cell in layers 2/3 of S1. B: a biocytin-filled cell in layers 2/3 of A1. C: responses to positive (100 pA) and negative (−100 pA) current injection of a layers 2/3 pyramidal cell in S1. D: responses to positive (100 pA) and negative (−100 pA) current injection of a layers 2/3 pyramidal cell in A1. White arrows in A and B point to the cell bodies while black arrows highlight the location of the apical dendrites. Scale bars in A and B: 0.025 mm.

Fig. 3.

Examples of Class 2 and Class 1 responses from cells in layers 2/3 and layer 4 of S1. A: Class 2 response in layers 2/3. Ai: the cell responded with paired-pulse facilitation to VPM stimulation at 10 Hz. Increasing stimulation intensities produced increases in excitatory postsynaptic potential (EPSP) amplitudes. Aii: stimulation at 10 Hz (250 μA), in the presence of ionotropic glutamate receptor antagonists (DNQX and AP5), failed to produce any EPSPs. Aiii: high-frequency stimulation (HFS; 125 Hz, 250 μA) in the presence of DNQX and AP5 produced a slow and prolonged membrane depolarization (blue trace) that could be blocked with a cocktail of type 1 (LY367385) and type 5 (MPEP) metabotropic glutamate receptor antagonists (black trace). B: Class 1 response in layers 2/3. Bi: this cell responded with paired-pulse depression to VPM stimulation at 10 Hz. EPSP amplitude was unaffected by increases in stimulation intensities. Bii: stimulation at 10 Hz (250 μA), in the presence of DNQX and AP5, failed to produce any EPSPs. Biii: HFS (125 Hz, 250 μA) in the presence of DNQX and AP5 did not produce any membrane potential changes. C: Class 1 response in layer 4. Ci: this cell responded with paired-pulse depression to VPM stimulation at 10 Hz. EPSP amplitude was unaffected by increases in stimulation intensities after a threshold was reached. Cii: stimulation at 10 Hz (250 μA), in the presence of DNQX and AP5, failed to produce any EPSPs. Ciii: HFS (125 Hz, 250 μA) in the presence of DNQX and AP5 did not produce any membrane potential changes. Arrows represent timing of stimulation for all 10 Hz trials, and black bars represent the duration of stimulation in HFS trials. Excluding HFS trials, all traces represent the average of 10 sweeps. Scale bars in Ai, Bi, and Ci apply to Aii, Bii, and Cii, respectively.

Fig. 4.

Examples of Class 2 and Class 1 responses from cells in layers 2/3 and layer 4 of A1. A: Class 2 response in layers 2/3. Ai: the cell responded with paired-pulse facilitation to MGBv stimulation at 10 Hz. Increasing stimulation intensities produced increases in EPSP amplitudes. Aii: stimulation at 10 Hz (250 μA), in the presence of ionotropic glutamate receptor antagonists (DNQX and AP5), failed to produce any EPSPs. Aiii: HFS (125 Hz, 200 μA) in the presence of DNQX and AP5 produced a slow and prolonged membrane depolarization (blue trace) that could be blocked with a cocktail of type 1 (LY367385) and type 5 (MPEP) metabotropic glutamate receptor antagonists (black trace). B: Class 1 response in layers 2/3. Bi: this cell responded with paired-pulse depression to MGBv stimulation at 10 Hz. EPSP amplitude was unaffected by increases in stimulation intensities. Bii: stimulation at 10 Hz (250 μA), in the presence of DNQX and AP5, failed to produce any EPSPs. Biii: HFS (125 Hz, 250 μA) in the presence of DNQX and AP5 did not produce any membrane potential changes. C: Class 1 response in layer 4. Ci: this cell responded with paired-pulse depression to MGBv stimulation at 10 Hz. EPSP amplitude was unaffected by increases in stimulation intensities after a threshold was reached. Cii: stimulation at 10 Hz (250 μA), in the presence of DNQX and AP5, failed to produce any EPSPs. Ciii: HFS (125 Hz, 200 μA) in the presence of DNQX and AP5 did not produce any membrane potential changes. Arrows represent timing of stimulation for all 10 Hz trials, and black bars represent the duration of stimulation in HFS trials. Excluding HFS trials, all traces represent the average of 10 sweeps. Scale bars in Ai, Bi, and Ci apply to Aii, Bii, and Cii, respectively.

Fig. 5.

Examples of Mixed responses from cells in layers 2/3 of S1 and A1. A: the short-term plasticity profile of a Mixed response recorded from S1 changed depending on the intensity of stimulation in VPM. At lower stimulation intensities, the cell responded with paired-pulse depression, whereas at higher stimulation intensities, it responded with paired-pulse facilitation. Increasing stimulation intensities produced increases in EPSP amplitudes. Bi: stimulation at 10 Hz (250 μA) in the presence of ionotropic receptor antagonists (DNQX and AP5) failed to produce any EPSPs. Bii: HFS (125 Hz, 200 μA) in the presence of DNQX and AP5 produced a slow and prolonged membrane depolarization (blue trace) that could be blocked with a cocktail of type 1 (LY367385) and type 5 (MPEP) metabotropic glutamate receptor antagonists (black trace). C: voltage-clamp recordings in a cell of A1 while stimulating MGBv at 10 Hz. At lower stimulation intensities, the cell responded with paired-pulse depression, whereas at higher stimulation intensities, it responded with paired-pulse facilitation. Increasing stimulation intensities produced increases in excitatory postsynaptic current (EPSC) amplitudes. Arrows represent timing of stimulation for all 10 Hz trials, and the black bar represents the duration of stimulation in the HFS trial. Excluding the HFS trial, all traces represent the average of 10 sweeps. D: changes in the E2/E1 ratio across stimulation intensities. Lines represent averages within each type of cells from both S1 and A1 (Error bars are SE). Whereas the E2/E1 ratios of cells exhibiting pure Class 1 and 2 responses remained either above or below 1 across stimulation intensities, for cells exhibiting Mixed responses, there was a transition from below 1 to above 1 as stimulation intensity increased. Wilcoxon's 1-sample rank sum tests: *P < 0.05.

Fig. 6.

Summary of response properties. A: proportions of cells with Classes 1 and 2 and Mixed responses found in layers 2/3 of S1 and A1. B: average 1st EPSP amplitude at minimal stimulation intensity. C: normalized 1st EPSP amplitudes for all cells across stimulation intensity bins for S1 (Ci) and A1 (Cii). Increases in stimulation intensity did not produce increases in EPSP amplitude for cells exhibiting Class 1 responses in layers 2/3 and layer 4 of both cortices once a threshold was reached (typically around 50 μA). On the other hand, gradual increases in stimulation intensity resulted in gradual increases in EPSP amplitudes in cells exhibiting Class 2 and Mixed responses in layers 2/3 of both cortices. D: 3D scatter plot of 1st EPSP amplitude, E2/E1 ratio at minimal stimulation intensity, and slope of the normalized amplitude across stimulation intensities (50–300 μA) for all cells with Class 1 and Class 2 responses in both cortices. All error bars represent SE. Mann-Whitney tests: *P < 0.05, **P < 0.001; NS, not significant.

Fig. 7.

Laminar positions and response latencies. A: scatter plots of the relationship between response latency and laminar position for layers 2/3 cells in S1 (Ai) and A1 (Aii). B: scatter plots of the relationship between 1st EPSP amplitude at minimal stimulation intensity and response latency for layers 2/3 neurons in S1 (Bi) and A1 (Bii).

Fig. 8.

Anterograde tracing of thalamic biotinylated dextran amine (BDA) injections. A: anterograde labeling of axons and boutons in barrel field of S1 after injection of BDA in VPM (inset). Asterisks represent individual barrels. Bi: BDA-labeled axons and boutons in layers 2/3 of S1. Highlighted area in Bi seen at higher magnification in Bii. Ci: BDA-labeled axons and boutons in layer 4 of S1. Highlighted area in Ci seen at higher magnification in Cii. D: histogram of bouton area in layers 2/3 and layer 4 of S1 (μm²). E: anterograde labeling of axons and boutons in A1 after injection of BDA in MGBv (inset). Fi: BDA-labeled axons and boutons in layers 2/3 of A1. Highlighted area in Fi seen at higher magnification in Fii. Gi: BDA-labeled axons and boutons in layer 4 of A1. Highlighted area in Gi seen at higher magnification in Gii. H: histogram of bouton area in layers 2/3 and layer 4 of A1 (μm²). Scale bars: A and E, 0.125 mm; A and E insets, 0.25 mm; Bi, Ci, Fi, and Gi, 20 μm; Bii, Cii, Fii, and Gii, 5 μm.