Binocular Form Deprivation Reactivates Ocular Dominance Plasticity In Adult Visual Cortex

Objective: Ocular dominance (OD) plasticity is a prominent feature of the mammalian visual cortex. Brief monocular deprivation (MD) induces a rapid shift in the OD of binocular neurons in the juvenile rat visual cortex but is ineffective in adults. Although such neural changes are most evident during development, adult cortical circuits also can be modified by a variety of manipulations. Elucidating the underlying mechanisms at the cellular and synaptic levels is an essential step in understanding neural plasticity in the mature animal. 1. Visual evoked potentals (VEPs) was applied to detect the OD shift in order to build the animal model whose visual cortex plasticity was reactivated by long time's binocular form deprivation (BFD) from adulthood. 2. Immunohistochemistry and immunoblotting were applied to investigate the changes of neural transmitter receptors in the visual cortex of adult rats manipulated by BFD. 3. Patch-clamp whole cell recording was applied to investigate the electrophysiological property of layer II-III neurons from visual cortex of adult rats manupilated by BFD. Investigation of the underlying mechanisms of how BFD reactivates OD plasticity in adult rat will help interpret the principle of adult visual cortex plasticity, and provide theoretical basis for the treatment of adult amblyopia.Methods: 1. A left craniotomy was performed over left visual cortex (centered at 7.0 mm posterior to bregma and 4 mm lateral to midline), keeping dura intact. The recording electrode was placed on the dura. The amplitude of P100 wave was used to assess the cortical response to visual stimulation. VEPs were amplified, filtered and averaged in synchrony with the stimulus. VEPs were recorded in the pigmented Long–Evans rats aged from postnatal week (PW) 3 to PW7 and recorded again after 3d, 5d or 7d's MD in the right eyes. Rats of PW7 were treated with BFD for 7d, 10d or 14d, and then left eyes were opened for 3d, 5d or 7d befor the next VEPs recording. 2. Normal Long–Evans rats aged from PW1 to PW9 and PW7 rats after 14d's BFD were decapitated and sections were stained with immunohistochemistry. Binocular visual cortices were dissected bilaterally after anesthetization and quantitative immunoblotting were performed. All immunoblots were performed with the experimenter blind to experimental conditions. For the immunohistochemistry and immunoblots, same primary antibodies were used, including anti-GABAA-α1, anti-NR2A, anti-NR2B and anti-GluR1. 3. The visual cortex slices were prepared from Long–Evans rats aged from PW1 to PW9 and PW7 rats after 14d's BFD. Patch-clamp whole cell recording techniques were adopted. Pre-synaptic stimulation was given at 0.5 mA through bipolar stimulating electrodes placed in layer IV. PSCs of layer II-III neurons were recorded by recording electrodes. Then GABAA-IPSCs were isolated by holding the membrane potential at 0 mV, which is the reversal potential for EPSCs. Likewise, EPSCs were isolated by holding the membrane potential at -60 mV, which is close to the reversal potential for IPSCs. Then NMDA-EPSCs were isolated by adding CNQX into artificial cerebrospinal fluid (ACSF) after EPSCs were obtained. Above current isolations were confirmed by pharmacological method.Results: 1. Brief (3 d) MD decreased C/I ratio (contralateral VEPs amplitude/ ipsilateral VEPs amplitude to the occluded eye) in rats from PW3, PW4 and PW5 but not in those from PW6 and PW7, confirming the absence of rapid OD plasticity in adults. However, when brief MD was preceded by 14 d of BFD, we observed a significant decreasing in the C/I ratio in rats from P7W, demonstrating the reactivation of rapid OD plasticity in adult rat. 2. Studies of immunohistochemistry and immunoblotting have shown that expression of GABAA-α1 reached its peak at PW5 and BFD induces a significant decrease in the level of GABAA-α1 receptors in adult visual cortex. Expression of NR2A and NR2B reached its peak at PW3 and BFD induces a significant increase in the level of NR2A receptors and a significant decrease in the level of NR2B in adult visual cortex. Expression of GluR1 reached its peak at PW5 and BFD did change the level of GluR1 receptors in adult visual cortex. 3. Input resistance (IR), resting membrane potential (RMP) and the peak value of PSCs were not changed in adult visual cortex treated with BFD. The peak value and 10-90% decaytime of GABA-IPSCs increased with age from PW1 in normal rat visual cortex, then reached its peak at P5W and kept a high level in adulthood. BFD statistically decreased the peak value of GABA-IPSCs but not the 10-90% decaytime. The peak value of NMDA-EPSCs and the NMDA-EPSCs/EPSCs ratio increased with age since PW1 in normal rat visual cortex and then reached its peak at PW3. The 10-90% decaytime of NMDA-EPSCs decreased with age. BFD had no effect on the peak value and 10-90% decaytime of EPSCs and NMDA-EPSCs, as well as the NMDA-EPSCs/EPSCs ratio.Conclusion: 1. BFD could successfully reactivate the OD plasticity in adult visual cortex which showed a similar response as that in junvenile rats, demonstrating that form deprivation could enhance the OD plasticity of rat visual cortex for a life time. 2. The decreased expression of GABAA-α1 receptors and the decreased peak value of GABA-IPSCs in visual cortex of adult rats after BFD suggest BFD affects inhibitory synaptic transmission through GABAA receptors, which might be the key to re-enter the cortical period and the possible mechenism for the reactivation of OD plasticity in adult visual cortex. 3. Our data demonstrate the molecular changes in inhibitory neurotransmitter receptors and the induced inhibitory currents observed in the adult visual cortex that we propose enable the reactivation of rapid ocular dominance plasticity...