Age-Dependent Decline in Supragranular Long-Term Synaptic Plasticity by Increased Inhibition During the Critical Period in the Rat Primary Visual Cortex

Abstract

Jang HJ, Cho KH, Kim HS, Hahn SJ, Kim MS, Rhie DJ. Age-dependent decline in supragranular long-term synaptic plasticity by increased inhibition during the critical period in the rat primary visual cortex. J Neurophysiol 101: 269 -275, 2009. First published October 29, 2008 doi:10.1152/jn.90900.2008. Supragranular longterm potentiation (LTP) and depression (LTD) are continuously induced in the pathway from layer 4 during the critical period in the rodent primary visual cortex, which limits the use of supragranular long-term synaptic plasticity as a synaptic model for the mechanism of ocular dominance (OD) plasticity. The results of the present study demonstrate that the pulse duration of extracellular stimulation to evoke a field potential (FP) is critical to induction of LTP and LTD in this pathway. LTP and LTD were induced in the pathway from layer 4 to layer 2/3 in slices from 3-wk-old rats when FPs were evoked by 0.1-and 0.2-ms pulses. LTP and LTD were induced in slices from 5-wk-old rats when evoked by stimulation with a 0.2-ms pulse but not by stimulation with a 0.1-ms pulse. Both the inhibitory component of FP and the inhibitory/excitatory postsynaptic potential amplitude ratio evoked by stimulation with a 0.1-ms pulse were greater than the values elicited by a 0.2-ms pulse. Stimulation with a 0.1-ms pulse at various intensities that showed the similar inhibitory FP component with the 0.2-ms pulse induced both LTD and LTP in 5-wk-old rats. Thus extracellular stimulation with shorter-duration pulses at higher intensity resulted in greater inhibition than that observed with longerduration pulses at low intensity. This increased inhibition might be involved in the age-dependent decline of synaptic plasticity during the critical period. These results provide an alternative synaptic model for the mechanism of OD plasticity.

Effects of Serotonin on the Induction of Long-term Depression in the Rat Visual Cortex

“…These findings suggest important roles for 5-HT in the regulation of the critical period in visual cortical plasticity. In a previous study, we reported an age-dependent decline of both LTP and LTD during the critical period in the rat visual cortex and the decrease in long-term synaptic plasticity was closely related to the degree of inhibitory influence [28]. In another study, we confirmed the inhibitory effect of 5-HT on LTP induction [29].…”

“…To further elucidate the role of 5-HT, the effect of 5-HT on LTD induction was investigated by bath application of 5-HT. In a previous report, we found that short-duration, high-intensity stimulation recruited more inhibition, and that LTD could not be induced in older animals with short-duration stimulation [28]. For example, LTD could be induced by 0.1-msec and 0.2-msec stimulation in 3-week-old rats, but only 0.2-msec stimulation could induce LTD in 5-week-old rats.…”

The Development of Phasic and Tonic Inhibition in the Rat Visual Cortex

“…In the visual cortex, inhibition is reportedly involved in the sculpting of columnar architecture [4], the processing of features of visual input [5], the regulation of the induction of long-term synaptic plasticity [6], and the regulation of the timing of critical period [7]. Since inhibition matures during postnatal development of the visual cortex [8,9], the effect of inhibition on these processes changes during development, and thus inhibitory maturation exerts its effect on the functional development of the visual cortex.…”

“…Since inhibition matures during postnatal development of the visual cortex [8,9], the effect of inhibition on these processes changes during development, and thus inhibitory maturation exerts its effect on the functional development of the visual cortex. For example, whereas inhibitory influence over a certain threshold level is necessary for the induction of long-term synaptic plasticity at the early postnatal developmental stage [10], further development of inhibition suppresses the induction of plasticity [6,11]. Furthermore, the kinetic features of inhibition are known to change during development of the visual cortex [12].…”

“…Since slow GABAAergic inhibition blocks N-methyl-D-aspartic acid receptors (NMDARs) more efficiently [13], changes in the kinetics of inhibition are important in regulating the activation of NMDARs, which is essential for the induction of many forms of synaptic plasticity [14][15][16]. Thus, it is crucial to investigate the developmental changes in inhibition in order to understand the underlying mechanisms for the developmental decline of long-term synaptic plasticity in the visual cortex [6,17].…”