Serotonin mediates cross-modal reorganization of cortical circuits

Abstract

Loss of one type of sensory input can cause improved functionality of other sensory systems. Whereas this form of plasticity, cross-modal plasticity, is well established, the molecular and cellular mechanisms underlying it are still unclear. Here, we show that visual deprivation (VD) increases extracellular serotonin in the juvenile rat barrel cortex. This increase in serotonin levels facilitates synaptic strengthening at layer 4 to layer 2/3 synapses within the barrel cortex. Upon VD, whisker experience leads to trafficking of the AMPA-type glutamate receptors (AMPARs) into these synapses through the activation of ERK and increased phosphorylation of AMPAR subunit GluR1 at the juvenile age when natural whisker experience no longer induces synaptic GluR1 delivery. VD thereby leads to sharpening of the functional whisker-barrel map at layer 2/3. Thus, sensory deprivation of one modality leads to serotonin release in remaining modalities, facilitates GluR1-dependent synaptic strengthening, and refines cortical organization.

Figure 1. VD Drives GluR1 into Layer 4 to Layer 2/3 Synapses of the Barrel Cortex in Juvenile Rats

(A) Experimental protocol (see text for details). (B) (Left) Synaptic responses (average of 50 consecutive trials) from infected with GFP-GluR1 expressing virus and noninfected neurons (held at −60 and +40 mV, as indicated) of juvenile rats (P21–P23) with intact or sutured eyes (VD). (Right) Graph of average rectification indices (RI: response at −60 mV/response at +40 mV) of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). Intact: n = 14, VD: n = 10. Scale bars are as indicated. *Statistical difference (p < 0.05. Student’s t test). Error bars indicate ± SEM. (C) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons infected with GFP-GluR1ct (labeled as GluR1 ct infected) expressing virus or noninfected in the barrel cortex of rats with or without VD were recorded by simultaneous whole-cell recordings. (Right) Graph of mean AMPARs-mediated transmission onto infected neurons (green), normalized to values obtained in noninfected neurons (white). Note synaptic depression of GFP-GluR1ct expressing neurons compared with nonexpressing neurons of animals with VD, while no difference in synaptic transmission was detected with intact animals. Intact: n = 9, VD: n = 10. Scale bars are as indicated. *Statistical difference (p < 0.05, Wilcoxon nonparametric test). Error bars indicate ± SEM. (D) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons in the barrel cortex of rats with intact or sutured eyes (VD). (Right) Graph of mean ratio of AMPARs-mediated currents to NMDA receptor-mediated currents (A/N ratio) of VD rats normalized to that of intact rats. Note that A/N ratio of VD rats (black) was larger than that of intact rats (white), suggesting that VD drives endogenous AMPARs into synapses. Intact: n = 17, VD: n = 13. Scale bars are as indicated. *Statistical difference (p < 0.05, Student’s t test). Error bars indicate ± SEM. (E) (Left) Synaptic responses from layer 2/3 to layer 2/3 pyramidal neurons (lateral pathway) infected with GFP-GluR1 expressing virus and noninfected neurons of juvenile VD rats (P21–P23). Note no difference of rectification between infected and noninfected neurons. (Right) Graph of average RI of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). n = 7. Scale bars are as indicated. Note that there was no statistical difference. Error bars indicate ± SEM. (F) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons infected with GFP-GluR1 expressing virus and noninfected neurons of juvenile VD rats with whiskers trimmed (P21–P23). Note no difference of rectification between infected and noninfected neurons. (Right) Graph of average RI of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). n = 6. Scale bars are as indicated. Note that there was no statistical difference. Error bars indicate ± SEM. (G) (Left) Evoked quantal EPSC responses at layer 4 to layer 2/3 synapses in the barrel cortex of VD and intact animals. (Right) The average amplitude of evoked quantal EPSC. Note that evoked quantal EPSC of VD animals was larger than intact animals. Intact animals: n = 191 events. VD animals: n = 294 events. Scale bars are as indicated. *Statistical difference (p < 0.001, Kolmogorov-Smirnov test). Error bars indicate ± SEM. (H) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons in the barrel cortex of rats with intact or sutured eyes (VD for 2 days). (Right) Graph of average RI of neurons from intact and VD animals. Intact animals: n = 12, VD: n = 10. Scale bars are as indicated. *Statistical difference (p < 0.05, Student’s t test). Error bars indicate ± SEM. See also Figure S1.

Figure 2. VD Procedure for 2 Days Occludes LTP in the Barrel Cortex

(A) Experimental protocol. We sutured both eyes at P21. For LTP experiment, acute brain slices were prepared at P23. Then LTP at layer 4-2/3 synapses of the barrel cortex was induced by pairing postsynaptic depolarization (+20 mV) with presynaptic stimulation (5 Hz, 90 s). For the rectification experiment, we microinjected GFP-GluR1 expressing virus at P23, and whole-cell recordings were performed at P25. (B) Recordings were maintained for at least 40 min after pairing. (Left) The EPSC amplitude was normalized to the average baseline amplitude before pairing. Paired pathways (black circle) and control pathways (triangle) of intact and VD animals are shown. (Right) Mean amplitude between 30 and 40 min after induction was normalized to base line amplitude. While LTP was successfully induced in intact rats, LTP induction was prevented in animals with 2 days of VD, indicating that 2 days of VD saturates synaptic AMPARs contents and occludes LTP. Intact: n = 7, VD: n = 7. Scale bars are as indicated. *Statistical difference (Student’s t test). Error bars indicate ± SEM. (C) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons at P25 infected with GFP-GluR1 expressing virus at P23 and noninfected neurons of juvenile rats exposed to VD from P21. Note no difference of rectification between infected and noninfected neurons, indicating no further delivery of GluR1 after 2 days of VD. (Right) Graph of average RI of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). n = 7. Scale bars are as indicated. Note that there was no statistical difference. Error bars indicate ± SEM. See also Figure S2.

Figure 3. Serotonin Mediates VD-Driven Synaptic GluR1 Delivery in the Juvenile Rat Barrel Cortex

(A) Extracelluar serotonin levels in the layer 2/3 of the barrel cortex and the visual cortex of rats housed under control conditions (white) or VD (black) was measured by in vivo microdialysis. VD increased extracellular serotonin levels in layer 2/3 of the barrel cortex but not of the visual cortex. Barrel cortex; intact: n = 8, VD: n = 6, visual cortex; intact: n = 6, VD: n = 5. *Statistical difference (p < 0.05, Student’s t test). (B) Extracelluar dopamine levels in the layer 2/3 of the barrel cortex of rats with intact (white) and sutured eyes (black) was measured with in vivo microdialysis. VD did not increase extracellular dopamine levels in the layer 2/3 of the barrel cortex. Intact: n = 8, VD: n = 6. Note that there was no statistical difference. (C) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons infected with virus expressing GFP-GluR1 and noninfected neurons of juvenile VD rats (P21–P23) injected with ketanserin. Note no difference of rectification between infected and noninfected neurons. (Right) Graph of average RI of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). n = 8. Scale bars are as indicated. Note that there was no statistical difference. (D) Graph of A/N ratio of VD rats (P21–P23) injected with ketanserin normalized to that of VD rats injected with saline. Note that A/N ratio of VD rats injected with saline (white) was larger than that of VD rats injected with ketanserin (black), suggesting that VD-induced synaptic endogenous AMPARs delivery is mediated by serotonin signaling. n = 7. Scale bars are as indicated. *Statistical difference (p < 0.05, Student’s t test). (E) (Left) Pictures of tissue expressing Sh5HT2A (upper left) and neurons expressing Sh5HT2A (upper right), res-Sh5HT2A (lower left), and Sh5HT2A+res-Sh5HT2A (lower right). (Top right) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons infected with virus (expressing Sh5HT2A, a scramble construct, and Sh5HT2A+res-Sh5HT2A) and noninfected neurons of juvenile VD rats (P21–P23). Note knock down of the expression of 5HT2A reduced AMPA transmission in juvenile VD rats. This effect was rescued by the coexpression of res-Sh5HT2A. (Bottom Right) Graph of average amplitude of AMPA transmission of infected neurons (expressing Sh5HT2A, a scramble construct, and Sh5HT2A+res-Sh5HT2A) and noninfected neurons of VD rats. AMPA EPSC amplitude of infected neurons was normalized to those of noninfected neurons. n = 8 (Sh5HT2A), n = 10 (scramble), n = 8 (Sh5HT2A+res-Sh5HT2A). Scale bars are as indicated. *Statistical difference (p < 0.05, Wilcoxon nonparametric test). Error bars indicate ± SEM. See also Figure S3.

Figure 4. Serotonin Facilitates Synaptic GluR1 Delivery

(A) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons infected with GFP-GluR1 expressing virus and noninfected neurons of juvenile intact rats (expression during P21–P23) injected with DOI. Note increase of rectification in infected neurons compared with noninfected neurons. (Right) Graph of average RI of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). n = 8. Scale bars are as indicated. *Statistical difference (p < 0.05, Student’s t test). Error bars indicate ± SEM. (B) Graph of A/N ratio of intact rats injected with DOI normalized to that of intact rats injected with saline. Note that A/N ratio of intact rats injected with DOI (black) was larger than that of intact rats injected with saline (white), suggesting that increase of serotonin facilitates synaptic endogenous AMPARs delivery. Saline: n = 9, DOI: n = 8. Scale bars are as indicated. *Statistical difference (p < 0.05, Student’s t test). Error bars indicate ± SEM. (C) Experimental protocol (see text and Experimental Procedures). (D) (Left) Synaptic responses from layer 4 to layer 2/3 pyramidal neurons infected with GFP-GluR1 expressing virus and noninfected neurons of juvenile intact rats with mechanical stimulation of whiskers in the presence or absence of DOI. Note increase of rectification in infected neurons compared with noninfected neurons of animals with DOI but not without DOI. (Right) Graph of average RI of neurons expressing GFP-GluR1 (green), normalized to RI value of nearby noninfected cells (white). n = 8. Scale bars are as indicated. *Statistical difference (p < 0.05, Student’s t test). Error bars indicate ± SEM. (E) Synaptic plasticity was induced by pairing 1 Hz stimulation (3 min) with postsynaptic potential at −40 mV with (bottom left) or without DOI (top left). The EPSC amplitude was normalized to the average baseline amplitude before pairing. (Right) Mean amplitude between 30 and 40 min after induction was normalized to base line amplitude. Note that this protocol induced LTP in the presence of DOI (black), while no potentiation was observed in the absence of DOI (white). Control: n = 17, DOI: n = 8. *Statistical difference (p < 0.05, Student’s t test). Error bars indicate ± SEM. (F) Synaptic plasticity was induced by pairing 1 Hz stimulation (3 min) with postsynaptic potential at 0 mV with (bottom left) or without DOI (top left). The EPSC amplitude was normalized to the average baseline amplitude before pairing. (Right) Mean amplitude between 30 min and 40 min after induction was normalized to base line amplitude. Note that this protocol induced LTP both with (black) and without DOI (white). Control: n = 5, DOI: n = 7. Note that there was no statistical difference. Error bars indicate ± SEM. See also Figure S4.

Figure 5. Signaling Pathways of Serotonin-Induced Facilitation of Synaptic GluR1 Delivery

(A) VD increased phosphorylation of Ser845 of GluR1. The amount of GluR1 was used as a reference for the quantitative analysis. Phosphorylation level of VD rats was normalized to that of intact rats. n = 6 (intact), n = 5 (VD). (B) Ketanserin prevented VD-induced increase of phosphorylation at Ser845 of GluR1. The amount of GluR1 was used as a reference for the quantitative analysis. Phosphorylation level of VD animals with ketanserin was normalized to VD animals with saline. n = 9. (C) VD activated ERK (p42). Note that phospho-ERK was increased in VD rats compared with intact animals. The amount of total ERK was used as a reference for the quantitative analysis. Phosphorylation level of VD animals was normalized to that of intact animals. n = 6 (intact), n = 5 (VD). (D) Ketanserin prevented VD-induced activation of ERK. Note that VD-induced increase of phospho-ERK was decreased in the presence of ketanserin. The amount of ERK was used as a reference for the quantitative analysis. Phosphorylation level of VD rats with ketanserin was normalized to VD rats with saline. n = 9. *p < 0.05. Student’s t test. Error bars indicate ± SEM. See also Figure S5.

Figure 6. VD Sharpens Functional Whisker-Barrel Map

(A) Responses (number of spikes) in layer 2/3 neurons to deflection of first order surrounding whiskers (S1) are plotted normalized to response in principal whisker. Note that rats with VD (black) showed lower values in S1 than intact rats (white), indicating that map was sharpened in VD rats. n = 14 (intact), n = 14 (VD). (B) VD rats treated with ketanserin (black) showed higher values in S1 than nontreated VD animals (white), indicating VD-induced sharpening of whisker-barrel map was prevented in the presence of ketanserin. n = 10 (VD with saline), n = 13 (VD with ketanserin). (C) Rats with VD expressing GFP-GluR1ct (green) showed higher values in S1 than GFP expressing animals (white), indicating a less refined map in GFP-GluR1ct expressing animals. n = 12 (GFP), n = 12 (GFP-GluR1ct). *Statistical difference (Student’s t test). Error bars indicate ± SEM.