Experience-Driven Axon Retraction in the Pharmacologically Inactivated Visual Cortex Does Not Require Synaptic Transmission Kana Watanabe, Yu Morishima, Masahito Toigawa, Yoshio Hata* Division of Integrative Bioscience, Institute of Regenerative Medicine and Biofunction, Tottori University Graduate School of Medical Sciences, Yonago, Japan Abstract Background: Experience during early postnatal development plays an important role in the refinement of specific neural connections in the brain. In the mammalian visual system, altered visual experiences induce plastic adaptation of visual cortical responses and guide rearrangements of afferent axons from the lateral geniculate nucleus. Previous studies using visual deprivation demonstrated that the afferents serving an open eye significantly retract when cortical neurons are pharmacologically inhibited by applying a c-aminobutyric acid type A receptor agonist, muscimol, whereas those serving a deprived eye are rescued from retraction, suggesting that presynaptic activity can lead to the retraction of geniculocortical axons in the absence of postsynaptic activity. Because muscimol application suppresses the spike activity of cortical neurons leaving transmitter release intact at geniculocortical synapses, local synaptic interaction may underlie the retraction of active axons in the inhibited cortex. Method and Findings: New studies reported here determined whether experience-driven axon retraction can occur in the visual cortex inactivated by blocking synaptic inputs. We inactivated the primary visual cortex of kittens by suppressing synaptic transmission with cortical injections of botulinum neurotoxin type E, which cleaves a synaptic protein, SNAP-25, and blocks transmitter release, and examined the geniculocortical axon morphology in the animals with normal vision and those deprived of vision binocularly. We found that afferent axons in the animals with normal vision showed a significant retraction in the inactivated cortex, as similarly observed in the muscimol-treated cortex, whereas the axons in the binocularly deprived animals were preserved. Conclusions: Therefore, the experience-driven axon retraction in the inactivated cortex can proceed in the absence of synaptic transmission. These results suggest that presynaptic mechanisms play an important role in the experience-driven refinement of geniculocortical axons. Citation: Watanabe K, Morishima Y, Toigawa M, Hata Y (2009) Experience-Driven Axon Retraction in the Pharmacologically Inactivated Visual Cortex Does Not Require Synaptic Transmission. PLoS ONE 4(1): e4193. doi:10.1371/journal.pone.0004193 Editor: Eric Warrant, Lund University, Sweden Received October 29, 2008; Accepted December 7, 2008; Published January 14, 2009 Copyright: 2009 Watanabe et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas- ‘Elucidation of neural network function in the brain’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 17023033) to Y.H.. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: yhata@med.tottori-u.ac.jp Introduction Experience during early postnatal development plays an important role in the development of brain function and the refinement of specific neural connections. In the mammalian visual system, monocular visual deprivation (MD) in early life induces a loss of cortical response to a deprived eye together with a significant retraction of afferent axons from the lateral geniculate nucleus (LGN) serving the deprived eye (ocular dominance plasticity) [1–4]. Previous studies showed that activity-dependent mechanisms should play a crucial role in experience-driven plasticity. For example, MD does not induce ocular dominance plasticity when the activity of cortical cells and their inputs from LGN are both blocked by tetrodotoxin [5]. Moreover, previous works have demonstrated a role of the activity of cortical cells themselves in controlling the direction of ocular dominance plasticity. When cortical cells are selectively inhibited pharmaco- logically by a c-aminobutyric acid type A (GABAA) receptor agonist, muscimol, leaving the activity of geniculocortical afferents intact, the open eye becomes less effective than the deprived eye [6–8]. Furthermore, active axons serving the open eye selectively retract and their cortical territory shrink, whereas axons from the deprived eye remain mostly intact [7,9]. Subsequent experiments revealed that presynaptic activity in the absence of postsynaptic activity, which is induced by visually evoked activity in afferent axons and cortical inhibition, plays an important role in the retraction of open eye afferents in the inhibited cortex [10]. These findings suggest a crucial role of the activity of presynaptic axons in the determination of whether the retraction can occur. In previous experiments, afferent axons might have retracted by a presynaptic mechanism in response to, for example, depletion of a possible trophic signal from cortical cells, which was induced by inhibiting cortical cells. Alternatively, the signal for retraction might be delivered only to active afferents as a result of a local synaptic PLoS ONE | www.plosone.org 1 January 2009 | Volume 4 | Issue 1 | e4193