Long-term depression in rat visual cortex is associated with a lower rise of postsynaptic calcium than long-term potentiation

Two forms of use-dependent synaptic plasticity, called long-term potentiation (LTP) and long-term depression (LTD), can be elicited in the visual cortex following different paradigms of electrophysiological stimulation. These neurobiological phenomena often are considered as necessary components of models for the alteration in function of the nervous system that must occur at some level for the establishment and (or) maintenance of memory engrams, for learning processes, or for the consolidation of active neural connections and regression of inactive contacts in the developing brain. It has been postulated that for LTP and LTD to be produced in the hippocampus, activation of a particular subtype of excitatory amino acid receptor, the metabotropic receptor, is a critical requirement. Only recently has it become possible to test this hypothesis directly, as a new compound, (±)-α-methyl-4-carboxyphenylglycine (MCPG), has been introduced and the suggestion made that it selectively antagonizes the metabotropic receptor. This substance has been tested in the present study on responses recorded from slices of rat visual cortex and has been found both to block the activation of the metabotropic receptor and to interfere selectively with the form of synaptic plasticity called LTD. It thus appears from the experiments reported in this paper as though the metabotropic receptor subtype that is blocked by MCPG is required for the expression of LTD but not for the expression of LTP, in the visual cortex of adult rats.Key words: excitatory amino acids, long-term potentiation, long-term depression, visual cortex, (±)-α-methyl-4-carboxyphenylglycine (MCPG).

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Layer 2/3 Synapses in Monocular and Binocular Regions of Tree Shrew Visual Cortex Express mAChR-Dependent Long-Term Depression and Long-Term Potentiation

Journal of Neurophysiology ◽

10.1152/jn.01134.2007 ◽

2008 ◽

Vol 100 (1) ◽

pp. 336-345 ◽

Cited By ~ 20

Author(s):

Portia McCoy ◽

Thomas T. Norton ◽

Lori L. McMahon

Keyword(s):

Visual Cortex ◽

Learning And Memory ◽

Tree Shrew ◽

Long Term Potentiation ◽

Synaptic Function ◽

Muscarinic Acetylcholine Receptors ◽

Long Term Depression ◽

Term Potentiation ◽

Layer 2

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. 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 on 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 extracellular signal-regulated kinase (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. 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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