Activation of NMDA receptors is necessary for the recovery of cortical binocularity

Abstract

Classic experiments have indicated that monocular deprivation (MD) for a few days during a critical period of development results in a decrease in the strength of connections mediating responses to the deprived eye, leading to a dramatic breakdown of cortical neuron binocularity. Despite the substantial functional change in the visual cortex, recovery from the effects of MD can be obtained if binocular vision is promptly restored. While great efforts have been made to elucidate the mechanisms regulating loss of deprived eye function, the mechanisms that underlie the recovery of cortical binocularity are poorly understood. Here, we examined whether activation of the N-methyl-d-aspartate receptor (NMDAR) is required for the recovery of cortical binocularity by pharmacologically blocking the NMDAR using d,l-2-amino-5-phosphonopentanoic (APV). Ferrets (n = 10) were monocularly deprived for 6 days, and osmotic minipumps, filled with APV (5.6 mg/ml) or saline, were surgically implanted into the primary visual cortex. One day after surgery, the deprived eye was reopened, and the animals were allowed 24 h of binocular vision. Extracellular recordings showed that intracortical infusion of the NMDAR antagonist, APV, prevented recovery of cortical binocularity while preserving neuronal responsiveness. These findings provide an important new insight for a specific role of NMDARs in the recovery of cortical binocularity from the effects of MD.

Fig. 1.

Intracortical infusion of d,l-2-amino-5-phosphonopentanoic (APV) in the left primary visual cortex prevented recovery of deprived (right) eye responses for neurons located near the infusion site (A, 1–3.5 mm, n = 123 cells from 10 recording penetrations in 5 animals), whereas neurons located far from the infusion site (B, 5–7 mm, n = 117 cells from 11 recording penetrations) showed recovery of deprived eye responses. Saline treatment during restoration of binocular vision did not prevent recovery of deprived eye responses for neurons located close to the infusion site (C, 1–3.5 mm, n = 93 cells from 8 recording penetrations in 4 animals). Note that the ocular dominance profiles in B and C are similar to the ocular dominance distribution obtained from recordings in normally reared ferrrets (D, 189 cells from 16 recording penetrations in 5 animals). ODI, ocular dominance index; contra, contralateral; ipsi, ipsilateral; RE, right eye; LE, left eye. Error bars indicate SE. E: pie charts showing the percentage of binocular cells (ODIs from 0.2 to 0.79) and monocular cells (ODIs from 0.0 to 0.19, and 0.8 to 1.0) from APV and saline-treated animals and normally reared ferrets. The respective numbers of cells are also shown between brackets. Note the small percentage of binocular cells from recordings close to the APV infusion site compared with percentages of binocular cells from the other experimental groups. F: mean binocular indexes (BIs) plotted based on the proximity of the recordings to the infusion site (close: 1–3.5 mm, far: 5–7 mm) for APV and saline-treated animals. A BI value close to 0.1 indicates that the majority of the cells are driven exclusively by one eye or the other, whereas a BI shifted toward 0.5 suggests that several cells are driven by both eyes. Mean BI values from recordings close to the APV infusion site are significantly different from BI values obtained from saline and APV control sites (univariate ANOVA, F = 17.088, df = 2, P < 0.0001; Bonferroni test, P = 0.001 for both comparisons). Error bars indicate ±SE. Dashed line represents the average BI values of animals with normal visual experience (normally reared ferrets) and gray shaded area ±SE. G: mean ODI plotted by the distance of the recordings from the infusion site (rounded to the nearest 0.5 mm) for APV and saline-treated animals. APV-treated, number (n) of cells: 1.5 mm, n = 25; 2.5 mm, n = 65; 3.5 mm, n = 33; 5 mm, n = 27; 6 mm, n = 34; 7 mm, n = 56 cells. saline-treated, number (n) of cells: 1.5 mm, n = 24; 2.5 mm, n = 36; 3.5 mm, n = 33. Error bars indicate ±SE. Dashed line corresponds to the average ODI values of animals with normal visual experience (n = 189 cells) and gray shaded area ±SD.

Fig. 2.

A: photomicrograph of a ferret brain depicting the location of the cannula insertion in the visual cortex. The cannula pedestal, skull, and brain were carefully cut along the sagital plane. V1, primary visual cortex; d, dorsal; c, caudal. Scale bar 1 mm. B: camera lucida drawings of coronal sections (70 μm) through V1 of an animal after treatment with APV during binocular recovery of deprived eye responses. Sections are organized from rostral to caudal. Black dots indicate electrolytic lesions made during 2 electrode penetrations located near the minipump cannula (arrowheads, vertical black lines). The distance from the first section with a cannula-lesion track (1st) to the last section showing electrolytic lesions (12th) is indicated. d, dorsal; m, medial. Scale bar, 2 mm. C: superimposed sections shown in B depicting the lateral-medial distance between the cannula placement and the electrolytic lesions. d, dorsal; m, medial. Scale bar, 2 mm. D: diagram depicting the distance from the infusion and the recording sites based on the distances shown in B and C. E: photomicrograph of a Nissl-stained coronal section (highlighted in B, 2nd section, gray area) showing normal cortical laminae near the APV injection site. Arrowhead points to the mechanical lesion made by the insertion of the cannula into the ferret primary visual cortex. Scale bar, 500 μm.

Fig. 3.

Mean visual responses (A) and spontaneous activity (B) of V1 cortical cells for saline- and APV-treated animals based on the proximity of the recordings to the infusion site (close: 1–3.5 mm, far: 5–7 mm). Recordings in saline-treated animals were made at 1–3.5 mm from the infusion site. The number of spikes was averaged over the course of the 10 stimulus runs, and spontaneous activity was determined by recording in the absence of stimulation. Error bars indicate SE.