Core Differences in Synaptic Signaling Between Primary Visual and Dorsolateral Prefrontal Cortex

Abstract

Neurons in primary visual cortex (V1) are more resilient than those in dorsolateral prefrontal cortex (dlPFC) in aging, schizophrenia and Alzheimer's disease. The current study compared glutamate and neuromodulatory actions in macaque V1 to those in dlPFC, and found striking regional differences. V1 neuronal firing to visual stimuli depended on AMPA receptors, with subtle NMDA receptor contributions, while dlPFC depends primarily on NMDA receptors. Neuromodulatory actions also differed between regions. In V1, cAMP signaling increased neuronal firing, and the phosphodiesterase PDE4A was positioned to regulate cAMP effects on glutamate release from axons. HCN channels in V1 were classically located on distal dendrites, and enhanced cell firing. These data contrast with dlPFC, where PDE4A and HCN channels are concentrated in thin spines, and cAMP-HCN signaling gates inputs and weakens firing. These regional differences may explain why V1 neurons are more resilient than dlPFC neurons to the challenges of age and disease.

Figure 1.

Recording paradigm. (A) The behavioral task used to map receptive fields (RFs) in primary visual cortex (V1). Monkeys passively viewed probe stimuli (10–20 Hz) flashed in and around the RF while detecting a small change in the luminance of the fixation spot. Responses to probe stimuli (white or black, indicated by black arrows) were used to assess neuronal responsivity during drug application. The grid was invisible to monkeys. (B) The oculomotor delayed response (ODR) task used to assess visual spatial working memory during recordings from the dorsolateral prefrontal cortex (dlPFC). The location of a brief, spatial cue had to be remembered over a delay period to guide a saccadic response to the remembered location. The position of the cue randomly varied over 8 spatial locations. (C) The recording locations in V1 (blue) and in dlPFC (pink). LS, lunate sulcus; AS, arcuate sulcus; PS, principal sulcus.

Figure 2.

Iontophoresis of the AMPA receptor antagonist, CNQX in V1. (A) Left: an example neuron showing the effects of increasing doses of CNQX (5–20 nA) on neuronal firing. The highest dose of CNQX (20 nA, dark green) significantly decreased the visual response (control vs. 20 nA, one-way ANOVA, F_1,1888 = 76.2378, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 20 nA, one-way ANOVA, F_1,1930 = 132.8902, P < 0.001). Right: another example neuron showing that lower dose of CNQX (10 nA, light green) significantly decreased the visual response (control vs. 10 nA, one-way ANOVA, F_1,2164 = 45.852, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 10 nA, one-way ANOVA, F_1,1764 = 75.4852, P < 0.001). The visual probe was presented from time 0 to 250 ms, indicated by the red bar. The area indicated by the gray bar was defined as the visual response and was used for statistical analysis. (B) The same cell as in (A) left, showing firing dynamics during application of increasing doses of CNQX. Visual responses are classified as “On responses” to a luminance increment (pink line), “Off responses” to a luminance decrement (purple line), and spontaneous firing (spont, gray line). Each dot shows the neuron’s response averaged across one block. Error bars indicate standard error of the mean (SEM). Vertical dashed lines separate differing drug conditions. (C) Population analysis (average of 14 neurons) of low doses of CNQX. Low doses of CNQX (5–25 nA) significantly decreased neuronal firing (control vs. CNQX, two-tailed paired t-test, tdep(13) = 5.114, P < 0.001, n = 14); firing significantly increased following removal of drug (recovery vs. CNQX, two-tailed paired t-test, tdep(13) = 2.652, P < 0.05, n = 14). Visual response from time 50 to 200 ms are selected for significance test (two-tailed paired t-test) in population analysis. Black, control; gold, low dose of CNQX; blue, recovery. Gray shading, SEM.

Figure 3.

Iontophoresis of the general NMDA receptor antagonist, MK801 (MK), or the NMDA-NR2B selective antagonist, Ro25-6981 (Ro) in V1. (A) Left: an example neuron showing the effects of increasing doses of MK (15–35 nA) on neuronal firing. The highest dose of MK (35 nA, cyan) significantly decreased the visual response (control vs. 35 nA, one-way ANOVA, F_1,1614 = 63.413, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 35 nA, one-way ANOVA, F_1,1794 = 23.8717, P < 0.001). Right: another example neuron showing that higher dose of MK (50 nA, purple) decreased firing (control vs. 50 nA, one-way ANOVA, F_1,988 = 112.4641, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 50 nA, one-way ANOVA, F_1,1101 = 111.4134, P < 0.001). (B) The same cell as in (A) left, showing firing dynamics during application of increasing doses of MK. (C) Population analysis (average of 11 neurons) of low doses of MK. Low doses of MK (10–25 nA) did not change neuronal firing (control vs. low dose MK, two-tailed paired t-test, tdep(10) = 0.4223, P = 0.6817, n = 11). Black, control; gold, low dose of MK. (D) Population analysis (average of 16 neurons) to high doses of MK. Higher doses (35–50 nA) significantly decreased neuronal firing (control vs. high dose MK, two-tailed paired t-test, tdep(15) = 4.2606, P < 0.001, n = 16); firing significantly increased following removal of drug (recovery vs. high dose MK, two-tailed paired t-test, tdep(15) = 3.195, P < 0.01, n = 16). (E) Left: an example neuron showing the effects of increasing doses of ro (10–30 nA) on neuronal firing. The highest dose of Ro (30 nA, cyan) significantly decreased the visual response (control vs. 30 nA, one-way ANOVA, F_1,2017 = 45.8612, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 30 nA, one-way ANOVA, F_1,2041 = 16.0743, P < 0.001). Right: another example neuron showing that higher dose of ro (60 nA, dark purple) significantly decreased the visual response (control vs. 60 nA, one-way ANOVA, F_1,1949 = 13.1351, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 60 nA, one-way ANOVA, F₁₂₁₅₄ = 3.8558, P < 0.05). (F) The same cell as in (A) left, showing firing dynamics during application of increasing doses of ro. (G) Population analysis (average of 13 neurons) of low doses of ro. Low doses of ro (10–20 nA) did not change neuronal firing (control vs. low dose Ro, two-tailed paired t-test, tdep(12) =−0.8109, P = 0.4332, n = 13). Black, control; gold, low dose of ro. (H) Population analysis (average of 20 neurons) to high doses of ro. Higher doses (30–60 nA) significantly decreased neuronal firing (control vs. high dose Ro, two-tailed paired t-test, tdep(19) = 8.632, P < 0.001, n = 20); firing significantly increased following removal of drug (recovery vs. high dose Ro, two-tailed paired t-test, tdep(19) = 2.1528, P < 0.05, n = 20).

Figure 4.

Comparison of low doses of AMPA and NMDA-NR2B antagonists on the firing of V1 versus dlPFC neurons. (A) V1 neurons. Low doses (20–25 nA) of CNQX reduced firing to 58% of control values (P < 0.001, n = 10), while the same dose of Ro (n = 11) had no significant effect on V1 neuronal firing. (B) dlPFC Delay cells. Low doses (20–25 nA) of CNQX produced only a slight reduction in Delay cell firing (P < 0.05), while the same dose of Ro markedly reduced firing (P < 0.001). (C) dlPFC Cue cells. Low doses (20–25 nA) of CNQX or Ro both reduced the firing of dlPFC Cue cells, which fire only during presentation, similar to V1 neurons. The dlPFC data are from Wang et al. (2013). *P < 0.05, ***P < 0.001, n.s., nonsignificant.

Figure 5.

Iontophoresis of the cAMP analog, 8-Bromo-cAMP (8-Br), enhanced visual responses in V1. (A) An example neuron showing the effects of increasing doses of 8-Br (5–50 nA) on neuronal firing. The highest dose of 8-Br (50 nA, dark purple) significantly enhanced the visual response (control vs. 50 nA, one-way ANOVA, F_1,2075 = 37.64, P < 0.001); this increase was reduced when drug was removed (recovery vs. 50 nA, one-way ANOVA, F_1,1955 = 21.9319, P < 0.001). (B) The same cell as in (A), showing firing dynamics during application of increasing doses of 8-Br. (C) Population analysis (average of 14 neurons) of low doses of 8-Br. Low doses of 8-Br (10–20 nA) produced a small, nonsignificant increase in neuronal firing (control vs. low dose 8-Br, two-tailed paired t-test, tdep(13) = 2.0221, P = 0.069, n = 14). Black, control; gold, low dose of 8-Br. (D) Population analysis (average of 23 neurons) to high doses of 8-Br. Higher doses (30–50 nA) significantly increased neuronal firing (control vs. high dose 8-Br, two-tailed paired t-test, tdep(22) = 5.76, P < 0.001, n = 23); firing significantly decreased following removal of drug (recovery vs high dose 8-Br, two-tailed paired t-test, tdep(22) = 3.232, P < 0.01, n = 23). Black, control; Red, high dose of 8-Br; Blue, recovery. (E) Comparison of 8-Br on the firing of V1 neurons and dlPFC Delay cells. The effects of 8-Br on V1 neurons were opposite from those on dlPFC Delay cells. dlPFC low, low dose 8-Br in dlPFC; V1 low, low dose 8-Br in V1; V1 high, high dose 8-Br in V1. ***P < 0.001, †P = 0.069.

Figure 6.

Iontophoresis of the HCN channel blocker, ZD7288 (ZD), decreased visual responses in V1. (A) An example neuron showing the effects of increasing doses of ZD (10–30 nA) on neuronal firing. The highest dose of ZD (30 nA, cyan) significantly decreased the visual response (control vs. 30 nA, one-way ANOVA, F_1,2594 = 101.9516, P < 0.001); this decrease was recovered when drug was removed (recovery vs. 30 nA, one-way ANOVA, F_1,3108 = 51.2565, P < 0.001). (B) The same cell as in (A), showing firing dynamics during application of increasing doses of ZD. (C) Population analysis (average of 30 neurons) of low doses of ZD. Low doses of ZD (5–15 nA) had no effect on neuronal firing (control vs. low dose ZD, two-tailed paired t-test, tdep(29) = 0.3612, P = 0.7206, n = 30). Black, control; Gold, low dose of ZD. (D) Population analysis (average of 35 neurons) to high doses of ZD. Higher doses (20–60 nA) significantly decreased neuronal firing (control vs. high dose ZD, two-tailed paired t-test, tdep(34) = 5.956, P < 0.001, n = 35); firing significantly increased following removal of drug (recovery vs. high dose ZD, two-tailed paired t-test, tdep(34) = 4.674, P < 0.001, n = 35). Black, control; red, high dose of ZD; blue, recovery. (E) Comparison of low dose of ZD on the firing of V1 neurons and dlPFC Delay cells. (F) Comparison of high dose of ZD on the firing of V1 neurons and dlPFC Delay cells. *P < 0.05, n.s., nonsignificant.

Figure 7.

PDE4A in monkey V1 is predominantly presynaptic. (A–F) PDE4A (red arrowheads) is expressed in glutamatergic-like terminal and preterminal (intervaricose) segments of axons (pink pseudocolored); compare with the nonglutamatergic-like ax2 (green-pseudocolored) in C and D. Note that PDE4A appears at nonsynaptic axonal membranes, and rarely perisynaptically (F). In addition to the plasma membrane, PDE4A is found on vesicular endomembranes (double red arrowheads in D and E). (G) Postsynaptic PDE4A is found in the spines and the shaft of dendrites (yellow-pseudocolored). In spines, PDE4A is perisynaptic and extrasynaptic, and associates with the spine apparatus. (H) Prevalence of PDE4A in various cellular profiles in layer III of V1 neuropil; expressed as percentage of a PDE4A profile (e.g., axon) per total PDE4A profiles (see quantitative assessment in Materials and Methods). Nondetermined (n.d.) are profiles that could not be unequivocally categorized. PDE4A is foremost expressed in terminal and preterminal axons. There is a smaller postsynaptic component in dendritic spines and shafts as well as a limited glial component. as, astrocyte; ax, axon; den, dendrite; sp, spine. Synapses are between arrows. Scale bars, 200 nm.

Figure 8.

HCN1 Channels in Monkey V1. (A–C) HCN1 channels (red arrowheads) are found in pyramidal neurons in layer III V1; black arrowheads point to the apical and basal dendrites (A). Channel expression along the pyramidal apical dendrite increases with distance from the soma (B), reaching maximal levels in the dendritic tufts that ramify into layer I (C). (D, E) Ultrastructurally, HCN1 channels are typically captured at the plasma membrane of dendritic shafts (yellow-pseudocolored); intracellular labeling likely represents channels en passant (double red arrowheads in D). (F) Channels were also found in spines, predominantly of the mushroom-type; compare with the nonreactive, thin-type sp2 (green-pseudocolored) in the same panel. (G, H) A presynaptic HCN1 channel component was detected in glutamatergic-like axons (pink-pseudocolored). Labeling appeared along nonsynaptic axonal membranes (G), and perisynaptically to asymmetric, excitatory-like synapses (H). as, astrocyte; ax, axon; den, dendrite; sp, spine. Synapses are between arrows. Scale bars, 10 μm (A–C); 200 nm (D–H).