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Query: UMLS:C0016632 (Fox)
 1,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have studied the effect of dark rearing on the development of excitatory amino acid transmission in 6-week-old kittens. In normal kittens, the NMDA component of the visual response decreases between 3 and 6 weeks of age for cells located in layers IV, V, and VI (Fox et al., 1991). Dark rearing to 6 weeks of age prevents this decrease. Subsequent exposure to light allows the decrease to proceed. Ten days in the light after 6 weeks in the dark was sufficient to decrease the NMDA component of the visual response to the same levels seen in light-reared animals of the same age. Comparison of the effect of the non-NMDA antagonist 6-cyano-7-dinitroquinoxaline-2,3-dione with the NMDA antagonist aminophosphonovalerate showed that the changes were due to the relative contributions of NMDA and non-NMDA receptors to the visual response rather than the overall contribution of glutamate receptors. We also studied the receptive field properties of the cells in the various groups of kittens. Cells given 4 d in the light after 6 weeks in the dark showed increased direction selectivity but little change in response firing rate. After 10 d in the light, visual responses did show some recovery toward adult values, but neither average firing rates nor the proportion of direction-selective cells reached the levels found in normal 6-week-old animals, contrary to the suggestion that a short period in the light can reverse the effect of dark rearing completely. These results show that the decrease in the NMDA component of the visual response seen during normal development of the cortex is caused by visual experience. Changes in NMDA receptors and developmental events such as geniculocortical afferent segregation and acquisition of orientation tuning covary as a function of visual experience rather than age, strongly suggesting that NMDA receptors are involved in experience-dependent developmental processes.
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PMID:The effect of visual experience on development of NMDA receptor synaptic transmission in kitten visual cortex. 135 37

In this review, we have concentrated on the parallels between the cellular properties of the NMDA receptor and a variety of functional properties within sensory and motor systems. Of course, the NMDA channel exists within the cell in conjunction with a variety of other channels, including non-NMDA channels. Although the NMDA receptor is unique in a cellular sense--it is the only ligand-gated channel that is also voltage dependent and calcium permeable--it is not unique in a functional sense. A cell that has non-NMDA receptors and voltage-sensitive channels will also exhibit nonlinear behavior. Moreover, Buhrle & Sonnhof (1983) demonstrated some time ago that calcium flows into frog motor neurons through more than one type of calcium channel. The contribution to the inflow of calcium from NMDA channels may vary from cell to cell and could easily be a minor proportion of the total. Many authors have pointed out that the NMDA channel has a low conductance at a resting potential of -70 mV. However, many cells in the nervous system are depolarized from -70 mV by excitatory input. Thus, as pointed out above. NMDA receptors make a contribution to the tonic or spontaneous activity of cells in both visual cortex and spinal cord. In practice, many cells are probably working in a range of membrane potentials where the NMDA channels are always open to some extent. Even in the hippocampal slice where a substantial amount of afferent input is removed, NMDA receptors contribute to spontaneous activity (Sah et al 1989). Does the NMDA receptor act as a switch? Does it act as an AND gate? The suggestion that it may act as a switch comes from work on LTP in the hippocampus, which is readily produced by high-frequency stimulation and is abolished by APV. However, activation of the NMDA receptor is only the first in a sequence of reactions leading to LTP: In theory, switch-like behavior could also be produced by calcium-buffering systems within dendritic spines, or by enzymatic processes (Lisman 1985; Zador et al 1990). Fox & Daw (1992) have modeled the action of NMDA and non-NMDA receptors that are activated in parallel with each other, and shown that the occurrence of switch-like behavior depends on the relative density of NMDA versus non-NMDA receptors. Switch-like behavior is not seen in the visual cortex, but might be seen in the hippocampus if the relative density of NMDA receptors there was higher than in the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:The role of NMDA receptors in information processing. 846 Aug 91