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J Neurophysiol (May 14, 2008). doi:10.1152/jn.01134.2007
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Submitted on October 15, 2007
Accepted on May 9, 2008

Layer 2/3 synapses in monocular and binocular regions of tree shrew visual cortex express mAChR dependent long-term depression and long-term potentiation

Portia A McCoy1, Thomas T Norton2, and Lori L. McMahon3*

1 Neurobiology, University Alabama Birmingham, Birmingham, Alabama, United States
2 Vision Sciences, University of Alabama Birmingham, Brimingham, Alabama, United States
3 Neurobiology, University Alabama Birmingham, Birmingham, Alabama, United States; Physiology and Biophysics, UAB, Birmingham, Alabama, United States

* To whom correspondence should be addressed. E-mail: mcmahon{at}physiology.uab.edu.

Acetylcholine is an important modulator of synaptic efficacy and is required for learning and memory tasks involving the visual cortex. In rodent visual cortex, activation of muscarinic acetylcholine receptors (mAChRs) induces a persistent long-term depression (LTD) of transmission at synapses recorded in layer 2/3 of acute slices (Kirkwood, 1999; McCoy and McMahon, 2007). Although the rodent studies expand our knowledge of how the cholinergic system modulates synaptic function underlying learning and memory, they are not easily extrapolated to more complex visual systems. Here we used tree shrews for their similarities to primates, including a visual cortex with separate, defined regions of monocular and binocular innervation, to determine whether mAChR activation induces long-term plasticity. We find that the cholinergic agonist carbachol (CCh) not only induces long-term plasticity, but the direction of the plasticity depends upon the subregion. In the monocular region, CCh application induces LTD of the postsynaptic potential recorded in layer 2/3 that requires activation of m3 mAChRs and a signaling cascade that includes activation of ERK 1/2. In contrast, layer 2/3 postsynaptic potentials recorded in the binocular region express long-term potentiation (LTP) following CCh application that requires activation of m1 mAChRs and phospholipase C (PLC). Our results show that activation of mAChRs induces long-term plasticity at excitatory synapses in tree shrew visual cortex. However, depending on the ocular inputs to that region, variation exists as to the direction of plasticity, as well as to the specific mAChR and signaling mechanisms that are required.







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